Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

111 files changed, 54941 insertions, 0 deletions

diff --git a/compiler/rustc_middle/Cargo.toml b/compiler/rustc_middle/Cargo.toml
new file mode 100644
index 000000000..008d2c709
--- /dev/null
+++ b/compiler/rustc_middle/Cargo.toml
@@ -0,0 +1,40 @@
+[package]
+name = "rustc_middle"
+version = "0.0.0"
+edition = "2021"
+
+[lib]
+doctest = false
+
+[dependencies]
+rustc_arena = { path = "../rustc_arena" }
+bitflags = "1.2.1"
+either = "1.5.0"
+gsgdt = "0.1.2"
+tracing = "0.1"
+rustc-rayon = { version = "0.4.0", optional = true }
+rustc-rayon-core = { version = "0.4.0", optional = true }
+polonius-engine = "0.13.0"
+rustc_apfloat = { path = "../rustc_apfloat" }
+rustc_attr = { path = "../rustc_attr" }
+rustc_feature = { path = "../rustc_feature" }
+rustc_hir = { path = "../rustc_hir" }
+rustc_target = { path = "../rustc_target" }
+rustc_macros = { path = "../rustc_macros" }
+rustc_data_structures = { path = "../rustc_data_structures" }
+rustc_query_system = { path = "../rustc_query_system" }
+rustc_errors = { path = "../rustc_errors" }
+rustc_graphviz = { path = "../rustc_graphviz" }
+rustc_index = { path = "../rustc_index" }
+rustc_serialize = { path = "../rustc_serialize" }
+rustc_ast = { path = "../rustc_ast" }
+rustc_span = { path = "../rustc_span" }
+chalk-ir = "0.80.0"
+smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+rustc_session = { path = "../rustc_session" }
+rustc_type_ir = { path = "../rustc_type_ir" }
+rand = "0.8.4"
+rand_xoshiro = "0.6.0"
+
+[features]
+rustc_use_parallel_compiler = ["rustc-rayon", "rustc-rayon-core"]
diff --git a/compiler/rustc_middle/README.md b/compiler/rustc_middle/README.md
new file mode 100644
index 000000000..de58f546c
--- /dev/null
+++ b/compiler/rustc_middle/README.md
@@ -0,0 +1,3 @@
+For more information about how rustc works, see the [rustc dev guide].
+
+[rustc dev guide]: https://rustc-dev-guide.rust-lang.org/
diff --git a/compiler/rustc_middle/benches/lib.rs b/compiler/rustc_middle/benches/lib.rs
new file mode 100644
index 000000000..237751bcb
--- /dev/null
+++ b/compiler/rustc_middle/benches/lib.rs
@@ -0,0 +1,54 @@
+#![feature(test)]
+
+extern crate test;
+
+use test::Bencher;
+
+// Static/dynamic method dispatch
+
+struct Struct {
+    field: isize,
+}
+
+trait Trait {
+    fn method(&self) -> isize;
+}
+
+impl Trait for Struct {
+    fn method(&self) -> isize {
+        self.field
+    }
+}
+
+#[bench]
+fn trait_vtable_method_call(b: &mut Bencher) {
+    let s = Struct { field: 10 };
+    let t = &s as &dyn Trait;
+    b.iter(|| t.method());
+}
+
+#[bench]
+fn trait_static_method_call(b: &mut Bencher) {
+    let s = Struct { field: 10 };
+    b.iter(|| s.method());
+}
+
+// Overhead of various match forms
+
+#[bench]
+fn option_some(b: &mut Bencher) {
+    let x = Some(10);
+    b.iter(|| match x {
+        Some(y) => y,
+        None => 11,
+    });
+}
+
+#[bench]
+fn vec_pattern(b: &mut Bencher) {
+    let x = [1, 2, 3, 4, 5, 6];
+    b.iter(|| match x {
+        [1, 2, 3, ..] => 10,
+        _ => 11,
+    });
+}
diff --git a/compiler/rustc_middle/src/arena.rs b/compiler/rustc_middle/src/arena.rs
new file mode 100644
index 000000000..b94de537d
--- /dev/null
+++ b/compiler/rustc_middle/src/arena.rs
@@ -0,0 +1,108 @@
+/// This higher-order macro declares a list of types which can be allocated by `Arena`.
+///
+/// Specifying the `decode` modifier will add decode impls for `&T` and `&[T]` where `T` is the type
+/// listed. These impls will appear in the implement_ty_decoder! macro.
+#[macro_export]
+macro_rules! arena_types {
+    ($macro:path) => (
+        $macro!([
+            [] layout: rustc_target::abi::LayoutS<'tcx>,
+            [] fn_abi: rustc_target::abi::call::FnAbi<'tcx, rustc_middle::ty::Ty<'tcx>>,
+            // AdtDef are interned and compared by address
+            [decode] adt_def: rustc_middle::ty::AdtDefData,
+            [] steal_thir: rustc_data_structures::steal::Steal<rustc_middle::thir::Thir<'tcx>>,
+            [] steal_mir: rustc_data_structures::steal::Steal<rustc_middle::mir::Body<'tcx>>,
+            [decode] mir: rustc_middle::mir::Body<'tcx>,
+            [] steal_promoted:
+                rustc_data_structures::steal::Steal<
+                    rustc_index::vec::IndexVec<
+                        rustc_middle::mir::Promoted,
+                        rustc_middle::mir::Body<'tcx>
+                    >
+                >,
+            [decode] promoted:
+                rustc_index::vec::IndexVec<
+                    rustc_middle::mir::Promoted,
+                    rustc_middle::mir::Body<'tcx>
+                >,
+            [decode] typeck_results: rustc_middle::ty::TypeckResults<'tcx>,
+            [decode] borrowck_result:
+                rustc_middle::mir::BorrowCheckResult<'tcx>,
+            [decode] unsafety_check_result: rustc_middle::mir::UnsafetyCheckResult,
+            [decode] code_region: rustc_middle::mir::coverage::CodeRegion,
+            [] const_allocs: rustc_middle::mir::interpret::Allocation,
+            [] region_scope_tree: rustc_middle::middle::region::ScopeTree,
+            // Required for the incremental on-disk cache
+            [] mir_keys: rustc_hir::def_id::DefIdSet,
+            [] dropck_outlives:
+                rustc_middle::infer::canonical::Canonical<'tcx,
+                    rustc_middle::infer::canonical::QueryResponse<'tcx,
+                        rustc_middle::traits::query::DropckOutlivesResult<'tcx>
+                    >
+                >,
+            [] normalize_projection_ty:
+                rustc_middle::infer::canonical::Canonical<'tcx,
+                    rustc_middle::infer::canonical::QueryResponse<'tcx,
+                        rustc_middle::traits::query::NormalizationResult<'tcx>
+                    >
+                >,
+            [] implied_outlives_bounds:
+                rustc_middle::infer::canonical::Canonical<'tcx,
+                    rustc_middle::infer::canonical::QueryResponse<'tcx,
+                        Vec<rustc_middle::traits::query::OutlivesBound<'tcx>>
+                    >
+                >,
+            [] dtorck_constraint: rustc_middle::traits::query::DropckConstraint<'tcx>,
+            [] candidate_step: rustc_middle::traits::query::CandidateStep<'tcx>,
+            [] autoderef_bad_ty: rustc_middle::traits::query::MethodAutoderefBadTy<'tcx>,
+            [] query_region_constraints: rustc_middle::infer::canonical::QueryRegionConstraints<'tcx>,
+            [] type_op_subtype:
+                rustc_middle::infer::canonical::Canonical<'tcx,
+                    rustc_middle::infer::canonical::QueryResponse<'tcx, ()>
+                >,
+            [] type_op_normalize_poly_fn_sig:
+                rustc_middle::infer::canonical::Canonical<'tcx,
+                    rustc_middle::infer::canonical::QueryResponse<'tcx, rustc_middle::ty::PolyFnSig<'tcx>>
+                >,
+            [] type_op_normalize_fn_sig:
+                rustc_middle::infer::canonical::Canonical<'tcx,
+                    rustc_middle::infer::canonical::QueryResponse<'tcx, rustc_middle::ty::FnSig<'tcx>>
+                >,
+            [] type_op_normalize_predicate:
+                rustc_middle::infer::canonical::Canonical<'tcx,
+                    rustc_middle::infer::canonical::QueryResponse<'tcx, rustc_middle::ty::Predicate<'tcx>>
+                >,
+            [] type_op_normalize_ty:
+                rustc_middle::infer::canonical::Canonical<'tcx,
+                    rustc_middle::infer::canonical::QueryResponse<'tcx, rustc_middle::ty::Ty<'tcx>>
+                >,
+            [] all_traits: Vec<rustc_hir::def_id::DefId>,
+            [] privacy_access_levels: rustc_middle::middle::privacy::AccessLevels,
+            [] foreign_module: rustc_session::cstore::ForeignModule,
+            [] foreign_modules: Vec<rustc_session::cstore::ForeignModule>,
+            [] upvars_mentioned: rustc_data_structures::fx::FxIndexMap<rustc_hir::HirId, rustc_hir::Upvar>,
+            [] object_safety_violations: rustc_middle::traits::ObjectSafetyViolation,
+            [] codegen_unit: rustc_middle::mir::mono::CodegenUnit<'tcx>,
+            [decode] attribute: rustc_ast::Attribute,
+            [] name_set: rustc_data_structures::fx::FxHashSet<rustc_span::symbol::Symbol>,
+            [] hir_id_set: rustc_hir::HirIdSet,
+
+            // Interned types
+            [] tys: rustc_data_structures::intern::WithStableHash<rustc_middle::ty::TyS<'tcx>>,
+            [] predicates: rustc_middle::ty::PredicateS<'tcx>,
+            [] consts: rustc_middle::ty::ConstS<'tcx>,
+
+            // Note that this deliberately duplicates items in the `rustc_hir::arena`,
+            // since we need to allocate this type on both the `rustc_hir` arena
+            // (during lowering) and the `librustc_middle` arena (for decoding MIR)
+            [decode] asm_template: rustc_ast::InlineAsmTemplatePiece,
+            [decode] used_trait_imports: rustc_data_structures::fx::FxHashSet<rustc_hir::def_id::LocalDefId>,
+            [decode] is_late_bound_map: rustc_data_structures::fx::FxIndexSet<rustc_hir::def_id::LocalDefId>,
+            [decode] impl_source: rustc_middle::traits::ImplSource<'tcx, ()>,
+
+            [] dep_kind: rustc_middle::dep_graph::DepKindStruct,
+        ]);
+    )
+}
+
+arena_types!(rustc_arena::declare_arena);
diff --git a/compiler/rustc_middle/src/dep_graph/dep_node.rs b/compiler/rustc_middle/src/dep_graph/dep_node.rs
new file mode 100644
index 000000000..2d095438f
--- /dev/null
+++ b/compiler/rustc_middle/src/dep_graph/dep_node.rs
@@ -0,0 +1,435 @@
+//! Nodes in the dependency graph.
+//!
+//! A node in the [dependency graph] is represented by a [`DepNode`].
+//! A `DepNode` consists of a [`DepKind`] (which
+//! specifies the kind of thing it represents, like a piece of HIR, MIR, etc.)
+//! and a [`Fingerprint`], a 128-bit hash value, the exact meaning of which
+//! depends on the node's `DepKind`. Together, the kind and the fingerprint
+//! fully identify a dependency node, even across multiple compilation sessions.
+//! In other words, the value of the fingerprint does not depend on anything
+//! that is specific to a given compilation session, like an unpredictable
+//! interning key (e.g., `NodeId`, `DefId`, `Symbol`) or the numeric value of a
+//! pointer. The concept behind this could be compared to how git commit hashes
+//! uniquely identify a given commit. The fingerprinting approach has
+//! a few advantages:
+//!
+//! * A `DepNode` can simply be serialized to disk and loaded in another session
+//!   without the need to do any "rebasing" (like we have to do for Spans and
+//!   NodeIds) or "retracing" (like we had to do for `DefId` in earlier
+//!   implementations of the dependency graph).
+//! * A `Fingerprint` is just a bunch of bits, which allows `DepNode` to
+//!   implement `Copy`, `Sync`, `Send`, `Freeze`, etc.
+//! * Since we just have a bit pattern, `DepNode` can be mapped from disk into
+//!   memory without any post-processing (e.g., "abomination-style" pointer
+//!   reconstruction).
+//! * Because a `DepNode` is self-contained, we can instantiate `DepNodes` that
+//!   refer to things that do not exist anymore. In previous implementations
+//!   `DepNode` contained a `DefId`. A `DepNode` referring to something that
+//!   had been removed between the previous and the current compilation session
+//!   could not be instantiated because the current compilation session
+//!   contained no `DefId` for thing that had been removed.
+//!
+//! `DepNode` definition happens in the `define_dep_nodes!()` macro. This macro
+//! defines the `DepKind` enum. Each `DepKind` has its own parameters that are
+//! needed at runtime in order to construct a valid `DepNode` fingerprint.
+//! However, only `CompileCodegenUnit` and `CompileMonoItem` are constructed
+//! explicitly (with `make_compile_codegen_unit` cq `make_compile_mono_item`).
+//!
+//! Because the macro sees what parameters a given `DepKind` requires, it can
+//! "infer" some properties for each kind of `DepNode`:
+//!
+//! * Whether a `DepNode` of a given kind has any parameters at all. Some
+//!   `DepNode`s could represent global concepts with only one value.
+//! * Whether it is possible, in principle, to reconstruct a query key from a
+//!   given `DepNode`. Many `DepKind`s only require a single `DefId` parameter,
+//!   in which case it is possible to map the node's fingerprint back to the
+//!   `DefId` it was computed from. In other cases, too much information gets
+//!   lost during fingerprint computation.
+//!
+//! `make_compile_codegen_unit` and `make_compile_mono_items`, together with
+//! `DepNode::new()`, ensures that only valid `DepNode` instances can be
+//! constructed. For example, the API does not allow for constructing
+//! parameterless `DepNode`s with anything other than a zeroed out fingerprint.
+//! More generally speaking, it relieves the user of the `DepNode` API of
+//! having to know how to compute the expected fingerprint for a given set of
+//! node parameters.
+//!
+//! [dependency graph]: https://rustc-dev-guide.rust-lang.org/query.html
+
+use crate::mir::mono::MonoItem;
+use crate::ty::TyCtxt;
+
+use rustc_data_structures::fingerprint::Fingerprint;
+use rustc_hir::def_id::{CrateNum, DefId, LocalDefId};
+use rustc_hir::definitions::DefPathHash;
+use rustc_hir::HirId;
+use rustc_query_system::dep_graph::FingerprintStyle;
+use rustc_span::symbol::Symbol;
+use std::hash::Hash;
+
+pub use rustc_query_system::dep_graph::{DepContext, DepNodeParams};
+
+/// This struct stores metadata about each DepKind.
+///
+/// Information is retrieved by indexing the `DEP_KINDS` array using the integer value
+/// of the `DepKind`. Overall, this allows to implement `DepContext` using this manual
+/// jump table instead of large matches.
+pub struct DepKindStruct {
+    /// Anonymous queries cannot be replayed from one compiler invocation to the next.
+    /// When their result is needed, it is recomputed. They are useful for fine-grained
+    /// dependency tracking, and caching within one compiler invocation.
+    pub is_anon: bool,
+
+    /// Eval-always queries do not track their dependencies, and are always recomputed, even if
+    /// their inputs have not changed since the last compiler invocation. The result is still
+    /// cached within one compiler invocation.
+    pub is_eval_always: bool,
+
+    /// Whether the query key can be recovered from the hashed fingerprint.
+    /// See [DepNodeParams] trait for the behaviour of each key type.
+    pub fingerprint_style: FingerprintStyle,
+
+    /// The red/green evaluation system will try to mark a specific DepNode in the
+    /// dependency graph as green by recursively trying to mark the dependencies of
+    /// that `DepNode` as green. While doing so, it will sometimes encounter a `DepNode`
+    /// where we don't know if it is red or green and we therefore actually have
+    /// to recompute its value in order to find out. Since the only piece of
+    /// information that we have at that point is the `DepNode` we are trying to
+    /// re-evaluate, we need some way to re-run a query from just that. This is what
+    /// `force_from_dep_node()` implements.
+    ///
+    /// In the general case, a `DepNode` consists of a `DepKind` and an opaque
+    /// GUID/fingerprint that will uniquely identify the node. This GUID/fingerprint
+    /// is usually constructed by computing a stable hash of the query-key that the
+    /// `DepNode` corresponds to. Consequently, it is not in general possible to go
+    /// back from hash to query-key (since hash functions are not reversible). For
+    /// this reason `force_from_dep_node()` is expected to fail from time to time
+    /// because we just cannot find out, from the `DepNode` alone, what the
+    /// corresponding query-key is and therefore cannot re-run the query.
+    ///
+    /// The system deals with this case letting `try_mark_green` fail which forces
+    /// the root query to be re-evaluated.
+    ///
+    /// Now, if `force_from_dep_node()` would always fail, it would be pretty useless.
+    /// Fortunately, we can use some contextual information that will allow us to
+    /// reconstruct query-keys for certain kinds of `DepNode`s. In particular, we
+    /// enforce by construction that the GUID/fingerprint of certain `DepNode`s is a
+    /// valid `DefPathHash`. Since we also always build a huge table that maps every
+    /// `DefPathHash` in the current codebase to the corresponding `DefId`, we have
+    /// everything we need to re-run the query.
+    ///
+    /// Take the `mir_promoted` query as an example. Like many other queries, it
+    /// just has a single parameter: the `DefId` of the item it will compute the
+    /// validated MIR for. Now, when we call `force_from_dep_node()` on a `DepNode`
+    /// with kind `MirValidated`, we know that the GUID/fingerprint of the `DepNode`
+    /// is actually a `DefPathHash`, and can therefore just look up the corresponding
+    /// `DefId` in `tcx.def_path_hash_to_def_id`.
+    pub force_from_dep_node: Option<fn(tcx: TyCtxt<'_>, dep_node: DepNode) -> bool>,
+
+    /// Invoke a query to put the on-disk cached value in memory.
+    pub try_load_from_on_disk_cache: Option<fn(TyCtxt<'_>, DepNode)>,
+}
+
+impl DepKind {
+    #[inline(always)]
+    pub fn fingerprint_style(self, tcx: TyCtxt<'_>) -> FingerprintStyle {
+        // Only fetch the DepKindStruct once.
+        let data = tcx.query_kind(self);
+        if data.is_anon {
+            return FingerprintStyle::Opaque;
+        }
+        data.fingerprint_style
+    }
+}
+
+macro_rules! define_dep_nodes {
+    (<$tcx:tt>
+    $(
+        [$($attrs:tt)*]
+        $variant:ident $(( $tuple_arg_ty:ty $(,)? ))*
+      ,)*
+    ) => (
+        #[macro_export]
+        macro_rules! make_dep_kind_array {
+            ($mod:ident) => {[ $($mod::$variant()),* ]};
+        }
+
+        /// This enum serves as an index into arrays built by `make_dep_kind_array`.
+        #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Encodable, Decodable)]
+        #[allow(non_camel_case_types)]
+        pub enum DepKind {
+            $($variant),*
+        }
+
+        fn dep_kind_from_label_string(label: &str) -> Result<DepKind, ()> {
+            match label {
+                $(stringify!($variant) => Ok(DepKind::$variant),)*
+                _ => Err(()),
+            }
+        }
+
+        /// Contains variant => str representations for constructing
+        /// DepNode groups for tests.
+        #[allow(dead_code, non_upper_case_globals)]
+        pub mod label_strs {
+           $(
+                pub const $variant: &str = stringify!($variant);
+            )*
+        }
+    );
+}
+
+rustc_dep_node_append!([define_dep_nodes!][ <'tcx>
+    // We use this for most things when incr. comp. is turned off.
+    [] Null,
+
+    // We use this to create a forever-red node.
+    [] Red,
+
+    [anon] TraitSelect,
+
+    // WARNING: if `Symbol` is changed, make sure you update `make_compile_codegen_unit` below.
+    [] CompileCodegenUnit(Symbol),
+
+    // WARNING: if `MonoItem` is changed, make sure you update `make_compile_mono_item` below.
+    // Only used by rustc_codegen_cranelift
+    [] CompileMonoItem(MonoItem),
+]);
+
+// WARNING: `construct` is generic and does not know that `CompileCodegenUnit` takes `Symbol`s as keys.
+// Be very careful changing this type signature!
+pub(crate) fn make_compile_codegen_unit(tcx: TyCtxt<'_>, name: Symbol) -> DepNode {
+    DepNode::construct(tcx, DepKind::CompileCodegenUnit, &name)
+}
+
+// WARNING: `construct` is generic and does not know that `CompileMonoItem` takes `MonoItem`s as keys.
+// Be very careful changing this type signature!
+pub(crate) fn make_compile_mono_item<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    mono_item: &MonoItem<'tcx>,
+) -> DepNode {
+    DepNode::construct(tcx, DepKind::CompileMonoItem, mono_item)
+}
+
+pub type DepNode = rustc_query_system::dep_graph::DepNode<DepKind>;
+
+// We keep a lot of `DepNode`s in memory during compilation. It's not
+// required that their size stay the same, but we don't want to change
+// it inadvertently. This assert just ensures we're aware of any change.
+#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+static_assert_size!(DepNode, 18);
+
+#[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
+static_assert_size!(DepNode, 24);
+
+pub trait DepNodeExt: Sized {
+    /// Construct a DepNode from the given DepKind and DefPathHash. This
+    /// method will assert that the given DepKind actually requires a
+    /// single DefId/DefPathHash parameter.
+    fn from_def_path_hash(tcx: TyCtxt<'_>, def_path_hash: DefPathHash, kind: DepKind) -> Self;
+
+    /// Extracts the DefId corresponding to this DepNode. This will work
+    /// if two conditions are met:
+    ///
+    /// 1. The Fingerprint of the DepNode actually is a DefPathHash, and
+    /// 2. the item that the DefPath refers to exists in the current tcx.
+    ///
+    /// Condition (1) is determined by the DepKind variant of the
+    /// DepNode. Condition (2) might not be fulfilled if a DepNode
+    /// refers to something from the previous compilation session that
+    /// has been removed.
+    fn extract_def_id(&self, tcx: TyCtxt<'_>) -> Option<DefId>;
+
+    /// Used in testing
+    fn from_label_string(
+        tcx: TyCtxt<'_>,
+        label: &str,
+        def_path_hash: DefPathHash,
+    ) -> Result<Self, ()>;
+
+    /// Used in testing
+    fn has_label_string(label: &str) -> bool;
+}
+
+impl DepNodeExt for DepNode {
+    /// Construct a DepNode from the given DepKind and DefPathHash. This
+    /// method will assert that the given DepKind actually requires a
+    /// single DefId/DefPathHash parameter.
+    fn from_def_path_hash(tcx: TyCtxt<'_>, def_path_hash: DefPathHash, kind: DepKind) -> DepNode {
+        debug_assert!(kind.fingerprint_style(tcx) == FingerprintStyle::DefPathHash);
+        DepNode { kind, hash: def_path_hash.0.into() }
+    }
+
+    /// Extracts the DefId corresponding to this DepNode. This will work
+    /// if two conditions are met:
+    ///
+    /// 1. The Fingerprint of the DepNode actually is a DefPathHash, and
+    /// 2. the item that the DefPath refers to exists in the current tcx.
+    ///
+    /// Condition (1) is determined by the DepKind variant of the
+    /// DepNode. Condition (2) might not be fulfilled if a DepNode
+    /// refers to something from the previous compilation session that
+    /// has been removed.
+    fn extract_def_id<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Option<DefId> {
+        if self.kind.fingerprint_style(tcx) == FingerprintStyle::DefPathHash {
+            Some(tcx.def_path_hash_to_def_id(DefPathHash(self.hash.into()), &mut || {
+                panic!("Failed to extract DefId: {:?} {}", self.kind, self.hash)
+            }))
+        } else {
+            None
+        }
+    }
+
+    /// Used in testing
+    fn from_label_string(
+        tcx: TyCtxt<'_>,
+        label: &str,
+        def_path_hash: DefPathHash,
+    ) -> Result<DepNode, ()> {
+        let kind = dep_kind_from_label_string(label)?;
+
+        match kind.fingerprint_style(tcx) {
+            FingerprintStyle::Opaque => Err(()),
+            FingerprintStyle::Unit => Ok(DepNode::new_no_params(tcx, kind)),
+            FingerprintStyle::DefPathHash => {
+                Ok(DepNode::from_def_path_hash(tcx, def_path_hash, kind))
+            }
+        }
+    }
+
+    /// Used in testing
+    fn has_label_string(label: &str) -> bool {
+        dep_kind_from_label_string(label).is_ok()
+    }
+}
+
+impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for () {
+    #[inline(always)]
+    fn fingerprint_style() -> FingerprintStyle {
+        FingerprintStyle::Unit
+    }
+
+    #[inline(always)]
+    fn to_fingerprint(&self, _: TyCtxt<'tcx>) -> Fingerprint {
+        Fingerprint::ZERO
+    }
+
+    #[inline(always)]
+    fn recover(_: TyCtxt<'tcx>, _: &DepNode) -> Option<Self> {
+        Some(())
+    }
+}
+
+impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for DefId {
+    #[inline(always)]
+    fn fingerprint_style() -> FingerprintStyle {
+        FingerprintStyle::DefPathHash
+    }
+
+    #[inline(always)]
+    fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
+        tcx.def_path_hash(*self).0
+    }
+
+    #[inline(always)]
+    fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
+        tcx.def_path_str(*self)
+    }
+
+    #[inline(always)]
+    fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
+        dep_node.extract_def_id(tcx)
+    }
+}
+
+impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for LocalDefId {
+    #[inline(always)]
+    fn fingerprint_style() -> FingerprintStyle {
+        FingerprintStyle::DefPathHash
+    }
+
+    #[inline(always)]
+    fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
+        self.to_def_id().to_fingerprint(tcx)
+    }
+
+    #[inline(always)]
+    fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
+        self.to_def_id().to_debug_str(tcx)
+    }
+
+    #[inline(always)]
+    fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
+        dep_node.extract_def_id(tcx).map(|id| id.expect_local())
+    }
+}
+
+impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for CrateNum {
+    #[inline(always)]
+    fn fingerprint_style() -> FingerprintStyle {
+        FingerprintStyle::DefPathHash
+    }
+
+    #[inline(always)]
+    fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
+        let def_id = self.as_def_id();
+        def_id.to_fingerprint(tcx)
+    }
+
+    #[inline(always)]
+    fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
+        tcx.crate_name(*self).to_string()
+    }
+
+    #[inline(always)]
+    fn recover(tcx: TyCtxt<'tcx>, dep_node: &DepNode) -> Option<Self> {
+        dep_node.extract_def_id(tcx).map(|id| id.krate)
+    }
+}
+
+impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for (DefId, DefId) {
+    #[inline(always)]
+    fn fingerprint_style() -> FingerprintStyle {
+        FingerprintStyle::Opaque
+    }
+
+    // We actually would not need to specialize the implementation of this
+    // method but it's faster to combine the hashes than to instantiate a full
+    // hashing context and stable-hashing state.
+    #[inline(always)]
+    fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
+        let (def_id_0, def_id_1) = *self;
+
+        let def_path_hash_0 = tcx.def_path_hash(def_id_0);
+        let def_path_hash_1 = tcx.def_path_hash(def_id_1);
+
+        def_path_hash_0.0.combine(def_path_hash_1.0)
+    }
+
+    #[inline(always)]
+    fn to_debug_str(&self, tcx: TyCtxt<'tcx>) -> String {
+        let (def_id_0, def_id_1) = *self;
+
+        format!("({}, {})", tcx.def_path_debug_str(def_id_0), tcx.def_path_debug_str(def_id_1))
+    }
+}
+
+impl<'tcx> DepNodeParams<TyCtxt<'tcx>> for HirId {
+    #[inline(always)]
+    fn fingerprint_style() -> FingerprintStyle {
+        FingerprintStyle::Opaque
+    }
+
+    // We actually would not need to specialize the implementation of this
+    // method but it's faster to combine the hashes than to instantiate a full
+    // hashing context and stable-hashing state.
+    #[inline(always)]
+    fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint {
+        let HirId { owner, local_id } = *self;
+
+        let def_path_hash = tcx.def_path_hash(owner.to_def_id());
+        let local_id = Fingerprint::from_smaller_hash(local_id.as_u32().into());
+
+        def_path_hash.0.combine(local_id)
+    }
+}
diff --git a/compiler/rustc_middle/src/dep_graph/mod.rs b/compiler/rustc_middle/src/dep_graph/mod.rs
new file mode 100644
index 000000000..c8b3b52b0
--- /dev/null
+++ b/compiler/rustc_middle/src/dep_graph/mod.rs
@@ -0,0 +1,140 @@
+use crate::ty::{self, TyCtxt};
+use rustc_data_structures::profiling::SelfProfilerRef;
+use rustc_query_system::ich::StableHashingContext;
+use rustc_session::Session;
+
+#[macro_use]
+mod dep_node;
+
+pub use rustc_query_system::dep_graph::{
+    debug::DepNodeFilter, hash_result, DepContext, DepNodeColor, DepNodeIndex,
+    SerializedDepNodeIndex, WorkProduct, WorkProductId,
+};
+
+pub use dep_node::{label_strs, DepKind, DepKindStruct, DepNode, DepNodeExt};
+pub(crate) use dep_node::{make_compile_codegen_unit, make_compile_mono_item};
+
+pub type DepGraph = rustc_query_system::dep_graph::DepGraph<DepKind>;
+pub type TaskDeps = rustc_query_system::dep_graph::TaskDeps<DepKind>;
+pub type TaskDepsRef<'a> = rustc_query_system::dep_graph::TaskDepsRef<'a, DepKind>;
+pub type DepGraphQuery = rustc_query_system::dep_graph::DepGraphQuery<DepKind>;
+pub type SerializedDepGraph = rustc_query_system::dep_graph::SerializedDepGraph<DepKind>;
+pub type EdgeFilter = rustc_query_system::dep_graph::debug::EdgeFilter<DepKind>;
+
+impl rustc_query_system::dep_graph::DepKind for DepKind {
+    const NULL: Self = DepKind::Null;
+    const RED: Self = DepKind::Red;
+
+    fn debug_node(node: &DepNode, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{:?}(", node.kind)?;
+
+        ty::tls::with_opt(|opt_tcx| {
+            if let Some(tcx) = opt_tcx {
+                if let Some(def_id) = node.extract_def_id(tcx) {
+                    write!(f, "{}", tcx.def_path_debug_str(def_id))?;
+                } else if let Some(ref s) = tcx.dep_graph.dep_node_debug_str(*node) {
+                    write!(f, "{}", s)?;
+                } else {
+                    write!(f, "{}", node.hash)?;
+                }
+            } else {
+                write!(f, "{}", node.hash)?;
+            }
+            Ok(())
+        })?;
+
+        write!(f, ")")
+    }
+
+    fn with_deps<OP, R>(task_deps: TaskDepsRef<'_>, op: OP) -> R
+    where
+        OP: FnOnce() -> R,
+    {
+        ty::tls::with_context(|icx| {
+            let icx = ty::tls::ImplicitCtxt { task_deps, ..icx.clone() };
+
+            ty::tls::enter_context(&icx, |_| op())
+        })
+    }
+
+    fn read_deps<OP>(op: OP)
+    where
+        OP: for<'a> FnOnce(TaskDepsRef<'a>),
+    {
+        ty::tls::with_context_opt(|icx| {
+            let Some(icx) = icx else { return };
+            op(icx.task_deps)
+        })
+    }
+}
+
+impl<'tcx> DepContext for TyCtxt<'tcx> {
+    type DepKind = DepKind;
+
+    #[inline]
+    fn with_stable_hashing_context<R>(&self, f: impl FnOnce(StableHashingContext<'_>) -> R) -> R {
+        TyCtxt::with_stable_hashing_context(*self, f)
+    }
+
+    #[inline]
+    fn dep_graph(&self) -> &DepGraph {
+        &self.dep_graph
+    }
+
+    #[inline(always)]
+    fn profiler(&self) -> &SelfProfilerRef {
+        &self.prof
+    }
+
+    #[inline(always)]
+    fn sess(&self) -> &Session {
+        self.sess
+    }
+
+    #[inline(always)]
+    fn fingerprint_style(&self, kind: DepKind) -> rustc_query_system::dep_graph::FingerprintStyle {
+        kind.fingerprint_style(*self)
+    }
+
+    #[inline(always)]
+    fn is_eval_always(&self, kind: DepKind) -> bool {
+        self.query_kind(kind).is_eval_always
+    }
+
+    fn try_force_from_dep_node(&self, dep_node: DepNode) -> bool {
+        debug!("try_force_from_dep_node({:?}) --- trying to force", dep_node);
+
+        // We must avoid ever having to call `force_from_dep_node()` for a
+        // `DepNode::codegen_unit`:
+        // Since we cannot reconstruct the query key of a `DepNode::codegen_unit`, we
+        // would always end up having to evaluate the first caller of the
+        // `codegen_unit` query that *is* reconstructible. This might very well be
+        // the `compile_codegen_unit` query, thus re-codegenning the whole CGU just
+        // to re-trigger calling the `codegen_unit` query with the right key. At
+        // that point we would already have re-done all the work we are trying to
+        // avoid doing in the first place.
+        // The solution is simple: Just explicitly call the `codegen_unit` query for
+        // each CGU, right after partitioning. This way `try_mark_green` will always
+        // hit the cache instead of having to go through `force_from_dep_node`.
+        // This assertion makes sure, we actually keep applying the solution above.
+        debug_assert!(
+            dep_node.kind != DepKind::codegen_unit,
+            "calling force_from_dep_node() on DepKind::codegen_unit"
+        );
+
+        let cb = self.query_kind(dep_node.kind);
+        if let Some(f) = cb.force_from_dep_node {
+            f(*self, dep_node);
+            true
+        } else {
+            false
+        }
+    }
+
+    fn try_load_from_on_disk_cache(&self, dep_node: DepNode) {
+        let cb = self.query_kind(dep_node.kind);
+        if let Some(f) = cb.try_load_from_on_disk_cache {
+            f(*self, dep_node)
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/hir/map/mod.rs b/compiler/rustc_middle/src/hir/map/mod.rs
new file mode 100644
index 000000000..47b04c33e
--- /dev/null
+++ b/compiler/rustc_middle/src/hir/map/mod.rs
@@ -0,0 +1,1405 @@
+use crate::hir::{ModuleItems, Owner};
+use crate::ty::{DefIdTree, TyCtxt};
+use rustc_ast as ast;
+use rustc_data_structures::fingerprint::Fingerprint;
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::svh::Svh;
+use rustc_data_structures::sync::{par_for_each_in, Send, Sync};
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::def_id::{CrateNum, DefId, LocalDefId, CRATE_DEF_ID, LOCAL_CRATE};
+use rustc_hir::definitions::{DefKey, DefPath, DefPathHash};
+use rustc_hir::intravisit::{self, Visitor};
+use rustc_hir::*;
+use rustc_index::vec::Idx;
+use rustc_middle::hir::nested_filter;
+use rustc_span::def_id::StableCrateId;
+use rustc_span::symbol::{kw, sym, Ident, Symbol};
+use rustc_span::Span;
+use rustc_target::spec::abi::Abi;
+
+fn fn_decl<'hir>(node: Node<'hir>) -> Option<&'hir FnDecl<'hir>> {
+    match node {
+        Node::Item(Item { kind: ItemKind::Fn(sig, _, _), .. })
+        | Node::TraitItem(TraitItem { kind: TraitItemKind::Fn(sig, _), .. })
+        | Node::ImplItem(ImplItem { kind: ImplItemKind::Fn(sig, _), .. }) => Some(&sig.decl),
+        Node::Expr(Expr { kind: ExprKind::Closure(Closure { fn_decl, .. }), .. })
+        | Node::ForeignItem(ForeignItem { kind: ForeignItemKind::Fn(fn_decl, ..), .. }) => {
+            Some(fn_decl)
+        }
+        _ => None,
+    }
+}
+
+pub fn fn_sig<'hir>(node: Node<'hir>) -> Option<&'hir FnSig<'hir>> {
+    match &node {
+        Node::Item(Item { kind: ItemKind::Fn(sig, _, _), .. })
+        | Node::TraitItem(TraitItem { kind: TraitItemKind::Fn(sig, _), .. })
+        | Node::ImplItem(ImplItem { kind: ImplItemKind::Fn(sig, _), .. }) => Some(sig),
+        _ => None,
+    }
+}
+
+#[inline]
+pub fn associated_body<'hir>(node: Node<'hir>) -> Option<BodyId> {
+    match node {
+        Node::Item(Item {
+            kind: ItemKind::Const(_, body) | ItemKind::Static(.., body) | ItemKind::Fn(.., body),
+            ..
+        })
+        | Node::TraitItem(TraitItem {
+            kind:
+                TraitItemKind::Const(_, Some(body)) | TraitItemKind::Fn(_, TraitFn::Provided(body)),
+            ..
+        })
+        | Node::ImplItem(ImplItem {
+            kind: ImplItemKind::Const(_, body) | ImplItemKind::Fn(_, body),
+            ..
+        })
+        | Node::Expr(Expr { kind: ExprKind::Closure(Closure { body, .. }), .. }) => Some(*body),
+
+        Node::AnonConst(constant) => Some(constant.body),
+
+        _ => None,
+    }
+}
+
+fn is_body_owner<'hir>(node: Node<'hir>, hir_id: HirId) -> bool {
+    match associated_body(node) {
+        Some(b) => b.hir_id == hir_id,
+        None => false,
+    }
+}
+
+#[derive(Copy, Clone)]
+pub struct Map<'hir> {
+    pub(super) tcx: TyCtxt<'hir>,
+}
+
+/// An iterator that walks up the ancestor tree of a given `HirId`.
+/// Constructed using `tcx.hir().parent_iter(hir_id)`.
+pub struct ParentHirIterator<'hir> {
+    current_id: HirId,
+    map: Map<'hir>,
+}
+
+impl<'hir> Iterator for ParentHirIterator<'hir> {
+    type Item = (HirId, Node<'hir>);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.current_id == CRATE_HIR_ID {
+            return None;
+        }
+        loop {
+            // There are nodes that do not have entries, so we need to skip them.
+            let parent_id = self.map.get_parent_node(self.current_id);
+
+            if parent_id == self.current_id {
+                self.current_id = CRATE_HIR_ID;
+                return None;
+            }
+
+            self.current_id = parent_id;
+            if let Some(node) = self.map.find(parent_id) {
+                return Some((parent_id, node));
+            }
+            // If this `HirId` doesn't have an entry, skip it and look for its `parent_id`.
+        }
+    }
+}
+
+/// An iterator that walks up the ancestor tree of a given `HirId`.
+/// Constructed using `tcx.hir().parent_owner_iter(hir_id)`.
+pub struct ParentOwnerIterator<'hir> {
+    current_id: HirId,
+    map: Map<'hir>,
+}
+
+impl<'hir> Iterator for ParentOwnerIterator<'hir> {
+    type Item = (LocalDefId, OwnerNode<'hir>);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.current_id.local_id.index() != 0 {
+            self.current_id.local_id = ItemLocalId::new(0);
+            if let Some(node) = self.map.tcx.hir_owner(self.current_id.owner) {
+                return Some((self.current_id.owner, node.node));
+            }
+        }
+        if self.current_id == CRATE_HIR_ID {
+            return None;
+        }
+        loop {
+            // There are nodes that do not have entries, so we need to skip them.
+            let parent_id = self.map.def_key(self.current_id.owner).parent;
+
+            let parent_id = parent_id.map_or(CRATE_HIR_ID.owner, |local_def_index| {
+                let def_id = LocalDefId { local_def_index };
+                self.map.local_def_id_to_hir_id(def_id).owner
+            });
+            self.current_id = HirId::make_owner(parent_id);
+
+            // If this `HirId` doesn't have an entry, skip it and look for its `parent_id`.
+            if let Some(node) = self.map.tcx.hir_owner(self.current_id.owner) {
+                return Some((self.current_id.owner, node.node));
+            }
+        }
+    }
+}
+
+impl<'hir> Map<'hir> {
+    pub fn krate(self) -> &'hir Crate<'hir> {
+        self.tcx.hir_crate(())
+    }
+
+    pub fn root_module(self) -> &'hir Mod<'hir> {
+        match self.tcx.hir_owner(CRATE_DEF_ID).map(|o| o.node) {
+            Some(OwnerNode::Crate(item)) => item,
+            _ => bug!(),
+        }
+    }
+
+    pub fn items(self) -> impl Iterator<Item = ItemId> + 'hir {
+        self.tcx.hir_crate_items(()).items.iter().copied()
+    }
+
+    pub fn module_items(self, module: LocalDefId) -> impl Iterator<Item = ItemId> + 'hir {
+        self.tcx.hir_module_items(module).items()
+    }
+
+    pub fn par_for_each_item(self, f: impl Fn(ItemId) + Sync + Send) {
+        par_for_each_in(&self.tcx.hir_crate_items(()).items[..], |id| f(*id));
+    }
+
+    pub fn def_key(self, def_id: LocalDefId) -> DefKey {
+        // Accessing the DefKey is ok, since it is part of DefPathHash.
+        self.tcx.definitions_untracked().def_key(def_id)
+    }
+
+    pub fn def_path_from_hir_id(self, id: HirId) -> Option<DefPath> {
+        self.opt_local_def_id(id).map(|def_id| self.def_path(def_id))
+    }
+
+    pub fn def_path(self, def_id: LocalDefId) -> DefPath {
+        // Accessing the DefPath is ok, since it is part of DefPathHash.
+        self.tcx.definitions_untracked().def_path(def_id)
+    }
+
+    #[inline]
+    pub fn def_path_hash(self, def_id: LocalDefId) -> DefPathHash {
+        // Accessing the DefPathHash is ok, it is incr. comp. stable.
+        self.tcx.definitions_untracked().def_path_hash(def_id)
+    }
+
+    #[inline]
+    pub fn local_def_id(self, hir_id: HirId) -> LocalDefId {
+        self.opt_local_def_id(hir_id).unwrap_or_else(|| {
+            bug!(
+                "local_def_id: no entry for `{:?}`, which has a map of `{:?}`",
+                hir_id,
+                self.find(hir_id)
+            )
+        })
+    }
+
+    #[inline]
+    pub fn opt_local_def_id(self, hir_id: HirId) -> Option<LocalDefId> {
+        if hir_id.local_id == ItemLocalId::new(0) {
+            Some(hir_id.owner)
+        } else {
+            self.tcx
+                .hir_owner_nodes(hir_id.owner)
+                .as_owner()?
+                .local_id_to_def_id
+                .get(&hir_id.local_id)
+                .copied()
+        }
+    }
+
+    #[inline]
+    pub fn local_def_id_to_hir_id(self, def_id: LocalDefId) -> HirId {
+        self.tcx.local_def_id_to_hir_id(def_id)
+    }
+
+    /// Do not call this function directly. The query should be called.
+    pub(super) fn opt_def_kind(self, local_def_id: LocalDefId) -> Option<DefKind> {
+        let hir_id = self.local_def_id_to_hir_id(local_def_id);
+        let def_kind = match self.find(hir_id)? {
+            Node::Item(item) => match item.kind {
+                ItemKind::Static(_, mt, _) => DefKind::Static(mt),
+                ItemKind::Const(..) => DefKind::Const,
+                ItemKind::Fn(..) => DefKind::Fn,
+                ItemKind::Macro(_, macro_kind) => DefKind::Macro(macro_kind),
+                ItemKind::Mod(..) => DefKind::Mod,
+                ItemKind::OpaqueTy(..) => DefKind::OpaqueTy,
+                ItemKind::TyAlias(..) => DefKind::TyAlias,
+                ItemKind::Enum(..) => DefKind::Enum,
+                ItemKind::Struct(..) => DefKind::Struct,
+                ItemKind::Union(..) => DefKind::Union,
+                ItemKind::Trait(..) => DefKind::Trait,
+                ItemKind::TraitAlias(..) => DefKind::TraitAlias,
+                ItemKind::ExternCrate(_) => DefKind::ExternCrate,
+                ItemKind::Use(..) => DefKind::Use,
+                ItemKind::ForeignMod { .. } => DefKind::ForeignMod,
+                ItemKind::GlobalAsm(..) => DefKind::GlobalAsm,
+                ItemKind::Impl { .. } => DefKind::Impl,
+            },
+            Node::ForeignItem(item) => match item.kind {
+                ForeignItemKind::Fn(..) => DefKind::Fn,
+                ForeignItemKind::Static(_, mt) => DefKind::Static(mt),
+                ForeignItemKind::Type => DefKind::ForeignTy,
+            },
+            Node::TraitItem(item) => match item.kind {
+                TraitItemKind::Const(..) => DefKind::AssocConst,
+                TraitItemKind::Fn(..) => DefKind::AssocFn,
+                TraitItemKind::Type(..) => DefKind::AssocTy,
+            },
+            Node::ImplItem(item) => match item.kind {
+                ImplItemKind::Const(..) => DefKind::AssocConst,
+                ImplItemKind::Fn(..) => DefKind::AssocFn,
+                ImplItemKind::TyAlias(..) => DefKind::AssocTy,
+            },
+            Node::Variant(_) => DefKind::Variant,
+            Node::Ctor(variant_data) => {
+                // FIXME(eddyb) is this even possible, if we have a `Node::Ctor`?
+                assert_ne!(variant_data.ctor_hir_id(), None);
+
+                let ctor_of = match self.find(self.get_parent_node(hir_id)) {
+                    Some(Node::Item(..)) => def::CtorOf::Struct,
+                    Some(Node::Variant(..)) => def::CtorOf::Variant,
+                    _ => unreachable!(),
+                };
+                DefKind::Ctor(ctor_of, def::CtorKind::from_hir(variant_data))
+            }
+            Node::AnonConst(_) => {
+                let inline = match self.find(self.get_parent_node(hir_id)) {
+                    Some(Node::Expr(&Expr {
+                        kind: ExprKind::ConstBlock(ref anon_const), ..
+                    })) if anon_const.hir_id == hir_id => true,
+                    _ => false,
+                };
+                if inline { DefKind::InlineConst } else { DefKind::AnonConst }
+            }
+            Node::Field(_) => DefKind::Field,
+            Node::Expr(expr) => match expr.kind {
+                ExprKind::Closure(Closure { movability: None, .. }) => DefKind::Closure,
+                ExprKind::Closure(Closure { movability: Some(_), .. }) => DefKind::Generator,
+                _ => bug!("def_kind: unsupported node: {}", self.node_to_string(hir_id)),
+            },
+            Node::GenericParam(param) => match param.kind {
+                GenericParamKind::Lifetime { .. } => DefKind::LifetimeParam,
+                GenericParamKind::Type { .. } => DefKind::TyParam,
+                GenericParamKind::Const { .. } => DefKind::ConstParam,
+            },
+            Node::Crate(_) => DefKind::Mod,
+            Node::Stmt(_)
+            | Node::PathSegment(_)
+            | Node::Ty(_)
+            | Node::TypeBinding(_)
+            | Node::Infer(_)
+            | Node::TraitRef(_)
+            | Node::Pat(_)
+            | Node::Local(_)
+            | Node::Param(_)
+            | Node::Arm(_)
+            | Node::Lifetime(_)
+            | Node::Block(_) => return None,
+        };
+        Some(def_kind)
+    }
+
+    pub fn find_parent_node(self, id: HirId) -> Option<HirId> {
+        if id.local_id == ItemLocalId::from_u32(0) {
+            Some(self.tcx.hir_owner_parent(id.owner))
+        } else {
+            let owner = self.tcx.hir_owner_nodes(id.owner).as_owner()?;
+            let node = owner.nodes[id.local_id].as_ref()?;
+            let hir_id = HirId { owner: id.owner, local_id: node.parent };
+            Some(hir_id)
+        }
+    }
+
+    pub fn get_parent_node(self, hir_id: HirId) -> HirId {
+        self.find_parent_node(hir_id)
+            .unwrap_or_else(|| bug!("No parent for node {:?}", self.node_to_string(hir_id)))
+    }
+
+    /// Retrieves the `Node` corresponding to `id`, returning `None` if cannot be found.
+    pub fn find(self, id: HirId) -> Option<Node<'hir>> {
+        if id.local_id == ItemLocalId::from_u32(0) {
+            let owner = self.tcx.hir_owner(id.owner)?;
+            Some(owner.node.into())
+        } else {
+            let owner = self.tcx.hir_owner_nodes(id.owner).as_owner()?;
+            let node = owner.nodes[id.local_id].as_ref()?;
+            Some(node.node)
+        }
+    }
+
+    /// Retrieves the `Node` corresponding to `id`, returning `None` if cannot be found.
+    #[inline]
+    pub fn find_by_def_id(self, id: LocalDefId) -> Option<Node<'hir>> {
+        self.find(self.local_def_id_to_hir_id(id))
+    }
+
+    /// Retrieves the `Node` corresponding to `id`, panicking if it cannot be found.
+    pub fn get(self, id: HirId) -> Node<'hir> {
+        self.find(id).unwrap_or_else(|| bug!("couldn't find hir id {} in the HIR map", id))
+    }
+
+    /// Retrieves the `Node` corresponding to `id`, panicking if it cannot be found.
+    #[inline]
+    pub fn get_by_def_id(self, id: LocalDefId) -> Node<'hir> {
+        self.find_by_def_id(id).unwrap_or_else(|| bug!("couldn't find {:?} in the HIR map", id))
+    }
+
+    pub fn get_if_local(self, id: DefId) -> Option<Node<'hir>> {
+        id.as_local().and_then(|id| self.find(self.local_def_id_to_hir_id(id)))
+    }
+
+    pub fn get_generics(self, id: LocalDefId) -> Option<&'hir Generics<'hir>> {
+        let node = self.tcx.hir_owner(id)?;
+        node.node.generics()
+    }
+
+    pub fn item(self, id: ItemId) -> &'hir Item<'hir> {
+        self.tcx.hir_owner(id.def_id).unwrap().node.expect_item()
+    }
+
+    pub fn trait_item(self, id: TraitItemId) -> &'hir TraitItem<'hir> {
+        self.tcx.hir_owner(id.def_id).unwrap().node.expect_trait_item()
+    }
+
+    pub fn impl_item(self, id: ImplItemId) -> &'hir ImplItem<'hir> {
+        self.tcx.hir_owner(id.def_id).unwrap().node.expect_impl_item()
+    }
+
+    pub fn foreign_item(self, id: ForeignItemId) -> &'hir ForeignItem<'hir> {
+        self.tcx.hir_owner(id.def_id).unwrap().node.expect_foreign_item()
+    }
+
+    pub fn body(self, id: BodyId) -> &'hir Body<'hir> {
+        self.tcx.hir_owner_nodes(id.hir_id.owner).unwrap().bodies[&id.hir_id.local_id]
+    }
+
+    pub fn fn_decl_by_hir_id(self, hir_id: HirId) -> Option<&'hir FnDecl<'hir>> {
+        if let Some(node) = self.find(hir_id) {
+            fn_decl(node)
+        } else {
+            bug!("no node for hir_id `{}`", hir_id)
+        }
+    }
+
+    pub fn fn_sig_by_hir_id(self, hir_id: HirId) -> Option<&'hir FnSig<'hir>> {
+        if let Some(node) = self.find(hir_id) {
+            fn_sig(node)
+        } else {
+            bug!("no node for hir_id `{}`", hir_id)
+        }
+    }
+
+    pub fn enclosing_body_owner(self, hir_id: HirId) -> LocalDefId {
+        for (parent, _) in self.parent_iter(hir_id) {
+            if let Some(body) = self.find(parent).map(associated_body).flatten() {
+                return self.body_owner_def_id(body);
+            }
+        }
+
+        bug!("no `enclosing_body_owner` for hir_id `{}`", hir_id);
+    }
+
+    /// Returns the `HirId` that corresponds to the definition of
+    /// which this is the body of, i.e., a `fn`, `const` or `static`
+    /// item (possibly associated), a closure, or a `hir::AnonConst`.
+    pub fn body_owner(self, BodyId { hir_id }: BodyId) -> HirId {
+        let parent = self.get_parent_node(hir_id);
+        assert!(self.find(parent).map_or(false, |n| is_body_owner(n, hir_id)));
+        parent
+    }
+
+    pub fn body_owner_def_id(self, id: BodyId) -> LocalDefId {
+        self.local_def_id(self.body_owner(id))
+    }
+
+    /// Given a `LocalDefId`, returns the `BodyId` associated with it,
+    /// if the node is a body owner, otherwise returns `None`.
+    pub fn maybe_body_owned_by(self, id: LocalDefId) -> Option<BodyId> {
+        self.get_if_local(id.to_def_id()).map(associated_body).flatten()
+    }
+
+    /// Given a body owner's id, returns the `BodyId` associated with it.
+    pub fn body_owned_by(self, id: LocalDefId) -> BodyId {
+        self.maybe_body_owned_by(id).unwrap_or_else(|| {
+            let hir_id = self.local_def_id_to_hir_id(id);
+            span_bug!(
+                self.span(hir_id),
+                "body_owned_by: {} has no associated body",
+                self.node_to_string(hir_id)
+            );
+        })
+    }
+
+    pub fn body_param_names(self, id: BodyId) -> impl Iterator<Item = Ident> + 'hir {
+        self.body(id).params.iter().map(|arg| match arg.pat.kind {
+            PatKind::Binding(_, _, ident, _) => ident,
+            _ => Ident::empty(),
+        })
+    }
+
+    /// Returns the `BodyOwnerKind` of this `LocalDefId`.
+    ///
+    /// Panics if `LocalDefId` does not have an associated body.
+    pub fn body_owner_kind(self, def_id: LocalDefId) -> BodyOwnerKind {
+        match self.tcx.def_kind(def_id) {
+            DefKind::Const | DefKind::AssocConst | DefKind::InlineConst | DefKind::AnonConst => {
+                BodyOwnerKind::Const
+            }
+            DefKind::Ctor(..) | DefKind::Fn | DefKind::AssocFn => BodyOwnerKind::Fn,
+            DefKind::Closure | DefKind::Generator => BodyOwnerKind::Closure,
+            DefKind::Static(mt) => BodyOwnerKind::Static(mt),
+            dk => bug!("{:?} is not a body node: {:?}", def_id, dk),
+        }
+    }
+
+    /// Returns the `ConstContext` of the body associated with this `LocalDefId`.
+    ///
+    /// Panics if `LocalDefId` does not have an associated body.
+    ///
+    /// This should only be used for determining the context of a body, a return
+    /// value of `Some` does not always suggest that the owner of the body is `const`,
+    /// just that it has to be checked as if it were.
+    pub fn body_const_context(self, def_id: LocalDefId) -> Option<ConstContext> {
+        let ccx = match self.body_owner_kind(def_id) {
+            BodyOwnerKind::Const => ConstContext::Const,
+            BodyOwnerKind::Static(mt) => ConstContext::Static(mt),
+
+            BodyOwnerKind::Fn if self.tcx.is_constructor(def_id.to_def_id()) => return None,
+            BodyOwnerKind::Fn if self.tcx.is_const_fn_raw(def_id.to_def_id()) => {
+                ConstContext::ConstFn
+            }
+            BodyOwnerKind::Fn if self.tcx.is_const_default_method(def_id.to_def_id()) => {
+                ConstContext::ConstFn
+            }
+            BodyOwnerKind::Fn | BodyOwnerKind::Closure => return None,
+        };
+
+        Some(ccx)
+    }
+
+    /// Returns an iterator of the `DefId`s for all body-owners in this
+    /// crate. If you would prefer to iterate over the bodies
+    /// themselves, you can do `self.hir().krate().body_ids.iter()`.
+    pub fn body_owners(self) -> impl Iterator<Item = LocalDefId> + 'hir {
+        self.tcx.hir_crate_items(()).body_owners.iter().copied()
+    }
+
+    pub fn par_body_owners<F: Fn(LocalDefId) + Sync + Send>(self, f: F) {
+        par_for_each_in(&self.tcx.hir_crate_items(()).body_owners[..], |&def_id| f(def_id));
+    }
+
+    pub fn ty_param_owner(self, def_id: LocalDefId) -> LocalDefId {
+        let def_kind = self.tcx.def_kind(def_id);
+        match def_kind {
+            DefKind::Trait | DefKind::TraitAlias => def_id,
+            DefKind::TyParam | DefKind::ConstParam => self.tcx.local_parent(def_id),
+            _ => bug!("ty_param_owner: {:?} is a {:?} not a type parameter", def_id, def_kind),
+        }
+    }
+
+    pub fn ty_param_name(self, def_id: LocalDefId) -> Symbol {
+        let def_kind = self.tcx.def_kind(def_id);
+        match def_kind {
+            DefKind::Trait | DefKind::TraitAlias => kw::SelfUpper,
+            DefKind::TyParam | DefKind::ConstParam => self.tcx.item_name(def_id.to_def_id()),
+            _ => bug!("ty_param_name: {:?} is a {:?} not a type parameter", def_id, def_kind),
+        }
+    }
+
+    pub fn trait_impls(self, trait_did: DefId) -> &'hir [LocalDefId] {
+        self.tcx.all_local_trait_impls(()).get(&trait_did).map_or(&[], |xs| &xs[..])
+    }
+
+    /// Gets the attributes on the crate. This is preferable to
+    /// invoking `krate.attrs` because it registers a tighter
+    /// dep-graph access.
+    pub fn krate_attrs(self) -> &'hir [ast::Attribute] {
+        self.attrs(CRATE_HIR_ID)
+    }
+
+    pub fn rustc_coherence_is_core(self) -> bool {
+        self.krate_attrs().iter().any(|attr| attr.has_name(sym::rustc_coherence_is_core))
+    }
+
+    pub fn get_module(self, module: LocalDefId) -> (&'hir Mod<'hir>, Span, HirId) {
+        let hir_id = HirId::make_owner(module);
+        match self.tcx.hir_owner(module).map(|o| o.node) {
+            Some(OwnerNode::Item(&Item { span, kind: ItemKind::Mod(ref m), .. })) => {
+                (m, span, hir_id)
+            }
+            Some(OwnerNode::Crate(item)) => (item, item.spans.inner_span, hir_id),
+            node => panic!("not a module: {:?}", node),
+        }
+    }
+
+    /// Walks the contents of the local crate. See also `visit_all_item_likes_in_crate`.
+    pub fn walk_toplevel_module(self, visitor: &mut impl Visitor<'hir>) {
+        let (top_mod, span, hir_id) = self.get_module(CRATE_DEF_ID);
+        visitor.visit_mod(top_mod, span, hir_id);
+    }
+
+    /// Walks the attributes in a crate.
+    pub fn walk_attributes(self, visitor: &mut impl Visitor<'hir>) {
+        let krate = self.krate();
+        for info in krate.owners.iter() {
+            if let MaybeOwner::Owner(info) = info {
+                for attrs in info.attrs.map.values() {
+                    for a in *attrs {
+                        visitor.visit_attribute(a)
+                    }
+                }
+            }
+        }
+    }
+
+    /// Visits all item-likes in the crate in some deterministic (but unspecified) order. If you
+    /// need to process every item-like, and don't care about visiting nested items in a particular
+    /// order then this method is the best choice.  If you do care about this nesting, you should
+    /// use the `tcx.hir().walk_toplevel_module`.
+    ///
+    /// Note that this function will access HIR for all the item-likes in the crate.  If you only
+    /// need to access some of them, it is usually better to manually loop on the iterators
+    /// provided by `tcx.hir_crate_items(())`.
+    ///
+    /// Please see the notes in `intravisit.rs` for more information.
+    pub fn visit_all_item_likes_in_crate<V>(self, visitor: &mut V)
+    where
+        V: Visitor<'hir>,
+    {
+        let krate = self.tcx.hir_crate_items(());
+
+        for id in krate.items() {
+            visitor.visit_item(self.item(id));
+        }
+
+        for id in krate.trait_items() {
+            visitor.visit_trait_item(self.trait_item(id));
+        }
+
+        for id in krate.impl_items() {
+            visitor.visit_impl_item(self.impl_item(id));
+        }
+
+        for id in krate.foreign_items() {
+            visitor.visit_foreign_item(self.foreign_item(id));
+        }
+    }
+
+    /// This method is the equivalent of `visit_all_item_likes_in_crate` but restricted to
+    /// item-likes in a single module.
+    pub fn visit_item_likes_in_module<V>(self, module: LocalDefId, visitor: &mut V)
+    where
+        V: Visitor<'hir>,
+    {
+        let module = self.tcx.hir_module_items(module);
+
+        for id in module.items() {
+            visitor.visit_item(self.item(id));
+        }
+
+        for id in module.trait_items() {
+            visitor.visit_trait_item(self.trait_item(id));
+        }
+
+        for id in module.impl_items() {
+            visitor.visit_impl_item(self.impl_item(id));
+        }
+
+        for id in module.foreign_items() {
+            visitor.visit_foreign_item(self.foreign_item(id));
+        }
+    }
+
+    pub fn for_each_module(self, mut f: impl FnMut(LocalDefId)) {
+        let crate_items = self.tcx.hir_crate_items(());
+        for module in crate_items.submodules.iter() {
+            f(*module)
+        }
+    }
+
+    #[cfg(not(parallel_compiler))]
+    #[inline]
+    pub fn par_for_each_module(self, f: impl Fn(LocalDefId)) {
+        self.for_each_module(f)
+    }
+
+    #[cfg(parallel_compiler)]
+    pub fn par_for_each_module(self, f: impl Fn(LocalDefId) + Sync) {
+        use rustc_data_structures::sync::{par_iter, ParallelIterator};
+        par_iter_submodules(self.tcx, CRATE_DEF_ID, &f);
+
+        fn par_iter_submodules<F>(tcx: TyCtxt<'_>, module: LocalDefId, f: &F)
+        where
+            F: Fn(LocalDefId) + Sync,
+        {
+            (*f)(module);
+            let items = tcx.hir_module_items(module);
+            par_iter(&items.submodules[..]).for_each(|&sm| par_iter_submodules(tcx, sm, f));
+        }
+    }
+
+    /// Returns an iterator for the nodes in the ancestor tree of the `current_id`
+    /// until the crate root is reached. Prefer this over your own loop using `get_parent_node`.
+    pub fn parent_iter(self, current_id: HirId) -> ParentHirIterator<'hir> {
+        ParentHirIterator { current_id, map: self }
+    }
+
+    /// Returns an iterator for the nodes in the ancestor tree of the `current_id`
+    /// until the crate root is reached. Prefer this over your own loop using `get_parent_node`.
+    pub fn parent_owner_iter(self, current_id: HirId) -> ParentOwnerIterator<'hir> {
+        ParentOwnerIterator { current_id, map: self }
+    }
+
+    /// Checks if the node is left-hand side of an assignment.
+    pub fn is_lhs(self, id: HirId) -> bool {
+        match self.find(self.get_parent_node(id)) {
+            Some(Node::Expr(expr)) => match expr.kind {
+                ExprKind::Assign(lhs, _rhs, _span) => lhs.hir_id == id,
+                _ => false,
+            },
+            _ => false,
+        }
+    }
+
+    /// Whether the expression pointed at by `hir_id` belongs to a `const` evaluation context.
+    /// Used exclusively for diagnostics, to avoid suggestion function calls.
+    pub fn is_inside_const_context(self, hir_id: HirId) -> bool {
+        self.body_const_context(self.enclosing_body_owner(hir_id)).is_some()
+    }
+
+    /// Retrieves the `HirId` for `id`'s enclosing method, unless there's a
+    /// `while` or `loop` before reaching it, as block tail returns are not
+    /// available in them.
+    ///
+    /// ```
+    /// fn foo(x: usize) -> bool {
+    ///     if x == 1 {
+    ///         true  // If `get_return_block` gets passed the `id` corresponding
+    ///     } else {  // to this, it will return `foo`'s `HirId`.
+    ///         false
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// ```compile_fail,E0308
+    /// fn foo(x: usize) -> bool {
+    ///     loop {
+    ///         true  // If `get_return_block` gets passed the `id` corresponding
+    ///     }         // to this, it will return `None`.
+    ///     false
+    /// }
+    /// ```
+    pub fn get_return_block(self, id: HirId) -> Option<HirId> {
+        let mut iter = self.parent_iter(id).peekable();
+        let mut ignore_tail = false;
+        if let Some(node) = self.find(id) {
+            if let Node::Expr(Expr { kind: ExprKind::Ret(_), .. }) = node {
+                // When dealing with `return` statements, we don't care about climbing only tail
+                // expressions.
+                ignore_tail = true;
+            }
+        }
+        while let Some((hir_id, node)) = iter.next() {
+            if let (Some((_, next_node)), false) = (iter.peek(), ignore_tail) {
+                match next_node {
+                    Node::Block(Block { expr: None, .. }) => return None,
+                    // The current node is not the tail expression of its parent.
+                    Node::Block(Block { expr: Some(e), .. }) if hir_id != e.hir_id => return None,
+                    _ => {}
+                }
+            }
+            match node {
+                Node::Item(_)
+                | Node::ForeignItem(_)
+                | Node::TraitItem(_)
+                | Node::Expr(Expr { kind: ExprKind::Closure { .. }, .. })
+                | Node::ImplItem(_) => return Some(hir_id),
+                // Ignore `return`s on the first iteration
+                Node::Expr(Expr { kind: ExprKind::Loop(..) | ExprKind::Ret(..), .. })
+                | Node::Local(_) => {
+                    return None;
+                }
+                _ => {}
+            }
+        }
+        None
+    }
+
+    /// Retrieves the `HirId` for `id`'s parent item, or `id` itself if no
+    /// parent item is in this map. The "parent item" is the closest parent node
+    /// in the HIR which is recorded by the map and is an item, either an item
+    /// in a module, trait, or impl.
+    pub fn get_parent_item(self, hir_id: HirId) -> LocalDefId {
+        if let Some((def_id, _node)) = self.parent_owner_iter(hir_id).next() {
+            def_id
+        } else {
+            CRATE_DEF_ID
+        }
+    }
+
+    /// Returns the `HirId` of `id`'s nearest module parent, or `id` itself if no
+    /// module parent is in this map.
+    pub(super) fn get_module_parent_node(self, hir_id: HirId) -> LocalDefId {
+        for (def_id, node) in self.parent_owner_iter(hir_id) {
+            if let OwnerNode::Item(&Item { kind: ItemKind::Mod(_), .. }) = node {
+                return def_id;
+            }
+        }
+        CRATE_DEF_ID
+    }
+
+    /// When on an if expression, a match arm tail expression or a match arm, give back
+    /// the enclosing `if` or `match` expression.
+    ///
+    /// Used by error reporting when there's a type error in an if or match arm caused by the
+    /// expression needing to be unit.
+    pub fn get_if_cause(self, hir_id: HirId) -> Option<&'hir Expr<'hir>> {
+        for (_, node) in self.parent_iter(hir_id) {
+            match node {
+                Node::Item(_)
+                | Node::ForeignItem(_)
+                | Node::TraitItem(_)
+                | Node::ImplItem(_)
+                | Node::Stmt(Stmt { kind: StmtKind::Local(_), .. }) => break,
+                Node::Expr(expr @ Expr { kind: ExprKind::If(..) | ExprKind::Match(..), .. }) => {
+                    return Some(expr);
+                }
+                _ => {}
+            }
+        }
+        None
+    }
+
+    /// Returns the nearest enclosing scope. A scope is roughly an item or block.
+    pub fn get_enclosing_scope(self, hir_id: HirId) -> Option<HirId> {
+        for (hir_id, node) in self.parent_iter(hir_id) {
+            if let Node::Item(Item {
+                kind:
+                    ItemKind::Fn(..)
+                    | ItemKind::Const(..)
+                    | ItemKind::Static(..)
+                    | ItemKind::Mod(..)
+                    | ItemKind::Enum(..)
+                    | ItemKind::Struct(..)
+                    | ItemKind::Union(..)
+                    | ItemKind::Trait(..)
+                    | ItemKind::Impl { .. },
+                ..
+            })
+            | Node::ForeignItem(ForeignItem { kind: ForeignItemKind::Fn(..), .. })
+            | Node::TraitItem(TraitItem { kind: TraitItemKind::Fn(..), .. })
+            | Node::ImplItem(ImplItem { kind: ImplItemKind::Fn(..), .. })
+            | Node::Block(_) = node
+            {
+                return Some(hir_id);
+            }
+        }
+        None
+    }
+
+    /// Returns the defining scope for an opaque type definition.
+    pub fn get_defining_scope(self, id: HirId) -> HirId {
+        let mut scope = id;
+        loop {
+            scope = self.get_enclosing_scope(scope).unwrap_or(CRATE_HIR_ID);
+            if scope == CRATE_HIR_ID || !matches!(self.get(scope), Node::Block(_)) {
+                return scope;
+            }
+        }
+    }
+
+    pub fn get_foreign_abi(self, hir_id: HirId) -> Abi {
+        let parent = self.get_parent_item(hir_id);
+        if let Some(node) = self.tcx.hir_owner(parent) {
+            if let OwnerNode::Item(Item { kind: ItemKind::ForeignMod { abi, .. }, .. }) = node.node
+            {
+                return *abi;
+            }
+        }
+        bug!(
+            "expected foreign mod or inlined parent, found {}",
+            self.node_to_string(HirId::make_owner(parent))
+        )
+    }
+
+    pub fn expect_owner(self, id: LocalDefId) -> OwnerNode<'hir> {
+        self.tcx.hir_owner(id).unwrap_or_else(|| bug!("expected owner for {:?}", id)).node
+    }
+
+    pub fn expect_item(self, id: LocalDefId) -> &'hir Item<'hir> {
+        match self.tcx.hir_owner(id) {
+            Some(Owner { node: OwnerNode::Item(item), .. }) => item,
+            _ => bug!("expected item, found {}", self.node_to_string(HirId::make_owner(id))),
+        }
+    }
+
+    pub fn expect_impl_item(self, id: LocalDefId) -> &'hir ImplItem<'hir> {
+        match self.tcx.hir_owner(id) {
+            Some(Owner { node: OwnerNode::ImplItem(item), .. }) => item,
+            _ => bug!("expected impl item, found {}", self.node_to_string(HirId::make_owner(id))),
+        }
+    }
+
+    pub fn expect_trait_item(self, id: LocalDefId) -> &'hir TraitItem<'hir> {
+        match self.tcx.hir_owner(id) {
+            Some(Owner { node: OwnerNode::TraitItem(item), .. }) => item,
+            _ => bug!("expected trait item, found {}", self.node_to_string(HirId::make_owner(id))),
+        }
+    }
+
+    pub fn expect_variant(self, id: HirId) -> &'hir Variant<'hir> {
+        match self.find(id) {
+            Some(Node::Variant(variant)) => variant,
+            _ => bug!("expected variant, found {}", self.node_to_string(id)),
+        }
+    }
+
+    pub fn expect_foreign_item(self, id: LocalDefId) -> &'hir ForeignItem<'hir> {
+        match self.tcx.hir_owner(id) {
+            Some(Owner { node: OwnerNode::ForeignItem(item), .. }) => item,
+            _ => {
+                bug!("expected foreign item, found {}", self.node_to_string(HirId::make_owner(id)))
+            }
+        }
+    }
+
+    pub fn expect_expr(self, id: HirId) -> &'hir Expr<'hir> {
+        match self.find(id) {
+            Some(Node::Expr(expr)) => expr,
+            _ => bug!("expected expr, found {}", self.node_to_string(id)),
+        }
+    }
+
+    #[inline]
+    fn opt_ident(self, id: HirId) -> Option<Ident> {
+        match self.get(id) {
+            Node::Pat(&Pat { kind: PatKind::Binding(_, _, ident, _), .. }) => Some(ident),
+            // A `Ctor` doesn't have an identifier itself, but its parent
+            // struct/variant does. Compare with `hir::Map::opt_span`.
+            Node::Ctor(..) => match self.find(self.get_parent_node(id))? {
+                Node::Item(item) => Some(item.ident),
+                Node::Variant(variant) => Some(variant.ident),
+                _ => unreachable!(),
+            },
+            node => node.ident(),
+        }
+    }
+
+    #[inline]
+    pub(super) fn opt_ident_span(self, id: HirId) -> Option<Span> {
+        self.opt_ident(id).map(|ident| ident.span)
+    }
+
+    #[inline]
+    pub fn opt_name(self, id: HirId) -> Option<Symbol> {
+        self.opt_ident(id).map(|ident| ident.name)
+    }
+
+    pub fn name(self, id: HirId) -> Symbol {
+        self.opt_name(id).unwrap_or_else(|| bug!("no name for {}", self.node_to_string(id)))
+    }
+
+    /// Given a node ID, gets a list of attributes associated with the AST
+    /// corresponding to the node-ID.
+    pub fn attrs(self, id: HirId) -> &'hir [ast::Attribute] {
+        self.tcx.hir_attrs(id.owner).get(id.local_id)
+    }
+
+    /// Gets the span of the definition of the specified HIR node.
+    /// This is used by `tcx.def_span`.
+    pub fn span(self, hir_id: HirId) -> Span {
+        self.opt_span(hir_id)
+            .unwrap_or_else(|| bug!("hir::map::Map::span: id not in map: {:?}", hir_id))
+    }
+
+    pub fn opt_span(self, hir_id: HirId) -> Option<Span> {
+        fn until_within(outer: Span, end: Span) -> Span {
+            if let Some(end) = end.find_ancestor_inside(outer) {
+                outer.with_hi(end.hi())
+            } else {
+                outer
+            }
+        }
+
+        fn named_span(item_span: Span, ident: Ident, generics: Option<&Generics<'_>>) -> Span {
+            if ident.name != kw::Empty {
+                let mut span = until_within(item_span, ident.span);
+                if let Some(g) = generics
+                    && !g.span.is_dummy()
+                    && let Some(g_span) = g.span.find_ancestor_inside(item_span)
+                {
+                    span = span.to(g_span);
+                }
+                span
+            } else {
+                item_span
+            }
+        }
+
+        let span = match self.find(hir_id)? {
+            // Function-like.
+            Node::Item(Item { kind: ItemKind::Fn(sig, ..), .. })
+            | Node::TraitItem(TraitItem { kind: TraitItemKind::Fn(sig, ..), .. })
+            | Node::ImplItem(ImplItem { kind: ImplItemKind::Fn(sig, ..), .. }) => sig.span,
+            // Constants and Statics.
+            Node::Item(Item {
+                kind:
+                    ItemKind::Const(ty, ..)
+                    | ItemKind::Static(ty, ..)
+                    | ItemKind::Impl(Impl { self_ty: ty, .. }),
+                span: outer_span,
+                ..
+            })
+            | Node::TraitItem(TraitItem {
+                kind: TraitItemKind::Const(ty, ..),
+                span: outer_span,
+                ..
+            })
+            | Node::ImplItem(ImplItem {
+                kind: ImplItemKind::Const(ty, ..),
+                span: outer_span,
+                ..
+            })
+            | Node::ForeignItem(ForeignItem {
+                kind: ForeignItemKind::Static(ty, ..),
+                span: outer_span,
+                ..
+            }) => until_within(*outer_span, ty.span),
+            // With generics and bounds.
+            Node::Item(Item {
+                kind: ItemKind::Trait(_, _, generics, bounds, _),
+                span: outer_span,
+                ..
+            })
+            | Node::TraitItem(TraitItem {
+                kind: TraitItemKind::Type(bounds, _),
+                generics,
+                span: outer_span,
+                ..
+            }) => {
+                let end = if let Some(b) = bounds.last() { b.span() } else { generics.span };
+                until_within(*outer_span, end)
+            }
+            // Other cases.
+            Node::Item(item) => match &item.kind {
+                ItemKind::Use(path, _) => path.span,
+                _ => named_span(item.span, item.ident, item.kind.generics()),
+            },
+            Node::Variant(variant) => named_span(variant.span, variant.ident, None),
+            Node::ImplItem(item) => named_span(item.span, item.ident, Some(item.generics)),
+            Node::ForeignItem(item) => match item.kind {
+                ForeignItemKind::Fn(decl, _, _) => until_within(item.span, decl.output.span()),
+                _ => named_span(item.span, item.ident, None),
+            },
+            Node::Ctor(_) => return self.opt_span(self.get_parent_node(hir_id)),
+            Node::Expr(Expr { kind: ExprKind::Closure(Closure { fn_decl_span, .. }), .. }) => {
+                *fn_decl_span
+            }
+            _ => self.span_with_body(hir_id),
+        };
+        Some(span)
+    }
+
+    /// Like `hir.span()`, but includes the body of items
+    /// (instead of just the item header)
+    pub fn span_with_body(self, hir_id: HirId) -> Span {
+        match self.get(hir_id) {
+            Node::Param(param) => param.span,
+            Node::Item(item) => item.span,
+            Node::ForeignItem(foreign_item) => foreign_item.span,
+            Node::TraitItem(trait_item) => trait_item.span,
+            Node::ImplItem(impl_item) => impl_item.span,
+            Node::Variant(variant) => variant.span,
+            Node::Field(field) => field.span,
+            Node::AnonConst(constant) => self.body(constant.body).value.span,
+            Node::Expr(expr) => expr.span,
+            Node::Stmt(stmt) => stmt.span,
+            Node::PathSegment(seg) => {
+                let ident_span = seg.ident.span;
+                ident_span
+                    .with_hi(seg.args.map_or_else(|| ident_span.hi(), |args| args.span_ext.hi()))
+            }
+            Node::Ty(ty) => ty.span,
+            Node::TypeBinding(tb) => tb.span,
+            Node::TraitRef(tr) => tr.path.span,
+            Node::Pat(pat) => pat.span,
+            Node::Arm(arm) => arm.span,
+            Node::Block(block) => block.span,
+            Node::Ctor(..) => self.span_with_body(self.get_parent_node(hir_id)),
+            Node::Lifetime(lifetime) => lifetime.span,
+            Node::GenericParam(param) => param.span,
+            Node::Infer(i) => i.span,
+            Node::Local(local) => local.span,
+            Node::Crate(item) => item.spans.inner_span,
+        }
+    }
+
+    pub fn span_if_local(self, id: DefId) -> Option<Span> {
+        if id.is_local() { Some(self.tcx.def_span(id)) } else { None }
+    }
+
+    pub fn res_span(self, res: Res) -> Option<Span> {
+        match res {
+            Res::Err => None,
+            Res::Local(id) => Some(self.span(id)),
+            res => self.span_if_local(res.opt_def_id()?),
+        }
+    }
+
+    /// Get a representation of this `id` for debugging purposes.
+    /// NOTE: Do NOT use this in diagnostics!
+    pub fn node_to_string(self, id: HirId) -> String {
+        hir_id_to_string(self, id)
+    }
+
+    /// Returns the HirId of `N` in `struct Foo<const N: usize = { ... }>` when
+    /// called with the HirId for the `{ ... }` anon const
+    pub fn opt_const_param_default_param_hir_id(self, anon_const: HirId) -> Option<HirId> {
+        match self.get(self.get_parent_node(anon_const)) {
+            Node::GenericParam(GenericParam {
+                hir_id: param_id,
+                kind: GenericParamKind::Const { .. },
+                ..
+            }) => Some(*param_id),
+            _ => None,
+        }
+    }
+}
+
+impl<'hir> intravisit::Map<'hir> for Map<'hir> {
+    fn find(&self, hir_id: HirId) -> Option<Node<'hir>> {
+        (*self).find(hir_id)
+    }
+
+    fn body(&self, id: BodyId) -> &'hir Body<'hir> {
+        (*self).body(id)
+    }
+
+    fn item(&self, id: ItemId) -> &'hir Item<'hir> {
+        (*self).item(id)
+    }
+
+    fn trait_item(&self, id: TraitItemId) -> &'hir TraitItem<'hir> {
+        (*self).trait_item(id)
+    }
+
+    fn impl_item(&self, id: ImplItemId) -> &'hir ImplItem<'hir> {
+        (*self).impl_item(id)
+    }
+
+    fn foreign_item(&self, id: ForeignItemId) -> &'hir ForeignItem<'hir> {
+        (*self).foreign_item(id)
+    }
+}
+
+pub(super) fn crate_hash(tcx: TyCtxt<'_>, crate_num: CrateNum) -> Svh {
+    debug_assert_eq!(crate_num, LOCAL_CRATE);
+    let krate = tcx.hir_crate(());
+    let hir_body_hash = krate.hir_hash;
+
+    let upstream_crates = upstream_crates(tcx);
+
+    let resolutions = tcx.resolutions(());
+
+    // We hash the final, remapped names of all local source files so we
+    // don't have to include the path prefix remapping commandline args.
+    // If we included the full mapping in the SVH, we could only have
+    // reproducible builds by compiling from the same directory. So we just
+    // hash the result of the mapping instead of the mapping itself.
+    let mut source_file_names: Vec<_> = tcx
+        .sess
+        .source_map()
+        .files()
+        .iter()
+        .filter(|source_file| source_file.cnum == LOCAL_CRATE)
+        .map(|source_file| source_file.name_hash)
+        .collect();
+
+    source_file_names.sort_unstable();
+
+    let crate_hash: Fingerprint = tcx.with_stable_hashing_context(|mut hcx| {
+        let mut stable_hasher = StableHasher::new();
+        hir_body_hash.hash_stable(&mut hcx, &mut stable_hasher);
+        upstream_crates.hash_stable(&mut hcx, &mut stable_hasher);
+        source_file_names.hash_stable(&mut hcx, &mut stable_hasher);
+        if tcx.sess.opts.unstable_opts.incremental_relative_spans {
+            let definitions = tcx.definitions_untracked();
+            let mut owner_spans: Vec<_> = krate
+                .owners
+                .iter_enumerated()
+                .filter_map(|(def_id, info)| {
+                    let _ = info.as_owner()?;
+                    let def_path_hash = definitions.def_path_hash(def_id);
+                    let span = resolutions.source_span[def_id];
+                    debug_assert_eq!(span.parent(), None);
+                    Some((def_path_hash, span))
+                })
+                .collect();
+            owner_spans.sort_unstable_by_key(|bn| bn.0);
+            owner_spans.hash_stable(&mut hcx, &mut stable_hasher);
+        }
+        tcx.sess.opts.dep_tracking_hash(true).hash_stable(&mut hcx, &mut stable_hasher);
+        tcx.sess.local_stable_crate_id().hash_stable(&mut hcx, &mut stable_hasher);
+        // Hash visibility information since it does not appear in HIR.
+        resolutions.visibilities.hash_stable(&mut hcx, &mut stable_hasher);
+        resolutions.has_pub_restricted.hash_stable(&mut hcx, &mut stable_hasher);
+        stable_hasher.finish()
+    });
+
+    Svh::new(crate_hash.to_smaller_hash())
+}
+
+fn upstream_crates(tcx: TyCtxt<'_>) -> Vec<(StableCrateId, Svh)> {
+    let mut upstream_crates: Vec<_> = tcx
+        .crates(())
+        .iter()
+        .map(|&cnum| {
+            let stable_crate_id = tcx.stable_crate_id(cnum);
+            let hash = tcx.crate_hash(cnum);
+            (stable_crate_id, hash)
+        })
+        .collect();
+    upstream_crates.sort_unstable_by_key(|&(stable_crate_id, _)| stable_crate_id);
+    upstream_crates
+}
+
+fn hir_id_to_string(map: Map<'_>, id: HirId) -> String {
+    let id_str = format!(" (hir_id={})", id);
+
+    let path_str = || {
+        // This functionality is used for debugging, try to use `TyCtxt` to get
+        // the user-friendly path, otherwise fall back to stringifying `DefPath`.
+        crate::ty::tls::with_opt(|tcx| {
+            if let Some(tcx) = tcx {
+                let def_id = map.local_def_id(id);
+                tcx.def_path_str(def_id.to_def_id())
+            } else if let Some(path) = map.def_path_from_hir_id(id) {
+                path.data.into_iter().map(|elem| elem.to_string()).collect::<Vec<_>>().join("::")
+            } else {
+                String::from("<missing path>")
+            }
+        })
+    };
+
+    let span_str = || map.tcx.sess.source_map().span_to_snippet(map.span(id)).unwrap_or_default();
+    let node_str = |prefix| format!("{} {}{}", prefix, span_str(), id_str);
+
+    match map.find(id) {
+        Some(Node::Item(item)) => {
+            let item_str = match item.kind {
+                ItemKind::ExternCrate(..) => "extern crate",
+                ItemKind::Use(..) => "use",
+                ItemKind::Static(..) => "static",
+                ItemKind::Const(..) => "const",
+                ItemKind::Fn(..) => "fn",
+                ItemKind::Macro(..) => "macro",
+                ItemKind::Mod(..) => "mod",
+                ItemKind::ForeignMod { .. } => "foreign mod",
+                ItemKind::GlobalAsm(..) => "global asm",
+                ItemKind::TyAlias(..) => "ty",
+                ItemKind::OpaqueTy(..) => "opaque type",
+                ItemKind::Enum(..) => "enum",
+                ItemKind::Struct(..) => "struct",
+                ItemKind::Union(..) => "union",
+                ItemKind::Trait(..) => "trait",
+                ItemKind::TraitAlias(..) => "trait alias",
+                ItemKind::Impl { .. } => "impl",
+            };
+            format!("{} {}{}", item_str, path_str(), id_str)
+        }
+        Some(Node::ForeignItem(_)) => format!("foreign item {}{}", path_str(), id_str),
+        Some(Node::ImplItem(ii)) => match ii.kind {
+            ImplItemKind::Const(..) => {
+                format!("assoc const {} in {}{}", ii.ident, path_str(), id_str)
+            }
+            ImplItemKind::Fn(..) => format!("method {} in {}{}", ii.ident, path_str(), id_str),
+            ImplItemKind::TyAlias(_) => {
+                format!("assoc type {} in {}{}", ii.ident, path_str(), id_str)
+            }
+        },
+        Some(Node::TraitItem(ti)) => {
+            let kind = match ti.kind {
+                TraitItemKind::Const(..) => "assoc constant",
+                TraitItemKind::Fn(..) => "trait method",
+                TraitItemKind::Type(..) => "assoc type",
+            };
+
+            format!("{} {} in {}{}", kind, ti.ident, path_str(), id_str)
+        }
+        Some(Node::Variant(ref variant)) => {
+            format!("variant {} in {}{}", variant.ident, path_str(), id_str)
+        }
+        Some(Node::Field(ref field)) => {
+            format!("field {} in {}{}", field.ident, path_str(), id_str)
+        }
+        Some(Node::AnonConst(_)) => node_str("const"),
+        Some(Node::Expr(_)) => node_str("expr"),
+        Some(Node::Stmt(_)) => node_str("stmt"),
+        Some(Node::PathSegment(_)) => node_str("path segment"),
+        Some(Node::Ty(_)) => node_str("type"),
+        Some(Node::TypeBinding(_)) => node_str("type binding"),
+        Some(Node::TraitRef(_)) => node_str("trait ref"),
+        Some(Node::Pat(_)) => node_str("pat"),
+        Some(Node::Param(_)) => node_str("param"),
+        Some(Node::Arm(_)) => node_str("arm"),
+        Some(Node::Block(_)) => node_str("block"),
+        Some(Node::Infer(_)) => node_str("infer"),
+        Some(Node::Local(_)) => node_str("local"),
+        Some(Node::Ctor(..)) => format!("ctor {}{}", path_str(), id_str),
+        Some(Node::Lifetime(_)) => node_str("lifetime"),
+        Some(Node::GenericParam(ref param)) => format!("generic_param {:?}{}", param, id_str),
+        Some(Node::Crate(..)) => String::from("root_crate"),
+        None => format!("unknown node{}", id_str),
+    }
+}
+
+pub(super) fn hir_module_items(tcx: TyCtxt<'_>, module_id: LocalDefId) -> ModuleItems {
+    let mut collector = ItemCollector::new(tcx, false);
+
+    let (hir_mod, span, hir_id) = tcx.hir().get_module(module_id);
+    collector.visit_mod(hir_mod, span, hir_id);
+
+    let ItemCollector {
+        submodules,
+        items,
+        trait_items,
+        impl_items,
+        foreign_items,
+        body_owners,
+        ..
+    } = collector;
+    return ModuleItems {
+        submodules: submodules.into_boxed_slice(),
+        items: items.into_boxed_slice(),
+        trait_items: trait_items.into_boxed_slice(),
+        impl_items: impl_items.into_boxed_slice(),
+        foreign_items: foreign_items.into_boxed_slice(),
+        body_owners: body_owners.into_boxed_slice(),
+    };
+}
+
+pub(crate) fn hir_crate_items(tcx: TyCtxt<'_>, _: ()) -> ModuleItems {
+    let mut collector = ItemCollector::new(tcx, true);
+
+    // A "crate collector" and "module collector" start at a
+    // module item (the former starts at the crate root) but only
+    // the former needs to collect it. ItemCollector does not do this for us.
+    collector.submodules.push(CRATE_DEF_ID);
+    tcx.hir().walk_toplevel_module(&mut collector);
+
+    let ItemCollector {
+        submodules,
+        items,
+        trait_items,
+        impl_items,
+        foreign_items,
+        body_owners,
+        ..
+    } = collector;
+
+    return ModuleItems {
+        submodules: submodules.into_boxed_slice(),
+        items: items.into_boxed_slice(),
+        trait_items: trait_items.into_boxed_slice(),
+        impl_items: impl_items.into_boxed_slice(),
+        foreign_items: foreign_items.into_boxed_slice(),
+        body_owners: body_owners.into_boxed_slice(),
+    };
+}
+
+struct ItemCollector<'tcx> {
+    // When true, it collects all items in the create,
+    // otherwise it collects items in some module.
+    crate_collector: bool,
+    tcx: TyCtxt<'tcx>,
+    submodules: Vec<LocalDefId>,
+    items: Vec<ItemId>,
+    trait_items: Vec<TraitItemId>,
+    impl_items: Vec<ImplItemId>,
+    foreign_items: Vec<ForeignItemId>,
+    body_owners: Vec<LocalDefId>,
+}
+
+impl<'tcx> ItemCollector<'tcx> {
+    fn new(tcx: TyCtxt<'tcx>, crate_collector: bool) -> ItemCollector<'tcx> {
+        ItemCollector {
+            crate_collector,
+            tcx,
+            submodules: Vec::default(),
+            items: Vec::default(),
+            trait_items: Vec::default(),
+            impl_items: Vec::default(),
+            foreign_items: Vec::default(),
+            body_owners: Vec::default(),
+        }
+    }
+}
+
+impl<'hir> Visitor<'hir> for ItemCollector<'hir> {
+    type NestedFilter = nested_filter::All;
+
+    fn nested_visit_map(&mut self) -> Self::Map {
+        self.tcx.hir()
+    }
+
+    fn visit_item(&mut self, item: &'hir Item<'hir>) {
+        if associated_body(Node::Item(item)).is_some() {
+            self.body_owners.push(item.def_id);
+        }
+
+        self.items.push(item.item_id());
+
+        // Items that are modules are handled here instead of in visit_mod.
+        if let ItemKind::Mod(module) = &item.kind {
+            self.submodules.push(item.def_id);
+            // A module collector does not recurse inside nested modules.
+            if self.crate_collector {
+                intravisit::walk_mod(self, module, item.hir_id());
+            }
+        } else {
+            intravisit::walk_item(self, item)
+        }
+    }
+
+    fn visit_foreign_item(&mut self, item: &'hir ForeignItem<'hir>) {
+        self.foreign_items.push(item.foreign_item_id());
+        intravisit::walk_foreign_item(self, item)
+    }
+
+    fn visit_anon_const(&mut self, c: &'hir AnonConst) {
+        self.body_owners.push(self.tcx.hir().local_def_id(c.hir_id));
+        intravisit::walk_anon_const(self, c)
+    }
+
+    fn visit_expr(&mut self, ex: &'hir Expr<'hir>) {
+        if matches!(ex.kind, ExprKind::Closure { .. }) {
+            self.body_owners.push(self.tcx.hir().local_def_id(ex.hir_id));
+        }
+        intravisit::walk_expr(self, ex)
+    }
+
+    fn visit_trait_item(&mut self, item: &'hir TraitItem<'hir>) {
+        if associated_body(Node::TraitItem(item)).is_some() {
+            self.body_owners.push(item.def_id);
+        }
+
+        self.trait_items.push(item.trait_item_id());
+        intravisit::walk_trait_item(self, item)
+    }
+
+    fn visit_impl_item(&mut self, item: &'hir ImplItem<'hir>) {
+        if associated_body(Node::ImplItem(item)).is_some() {
+            self.body_owners.push(item.def_id);
+        }
+
+        self.impl_items.push(item.impl_item_id());
+        intravisit::walk_impl_item(self, item)
+    }
+}
diff --git a/compiler/rustc_middle/src/hir/mod.rs b/compiler/rustc_middle/src/hir/mod.rs
new file mode 100644
index 000000000..211a61471
--- /dev/null
+++ b/compiler/rustc_middle/src/hir/mod.rs
@@ -0,0 +1,182 @@
+//! HIR datatypes. See the [rustc dev guide] for more info.
+//!
+//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/hir.html
+
+pub mod map;
+pub mod nested_filter;
+pub mod place;
+
+use crate::ty::query::Providers;
+use crate::ty::{DefIdTree, ImplSubject, TyCtxt};
+use rustc_data_structures::fingerprint::Fingerprint;
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::{par_for_each_in, Send, Sync};
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::*;
+use rustc_query_system::ich::StableHashingContext;
+use rustc_span::{ExpnId, DUMMY_SP};
+
+/// Top-level HIR node for current owner. This only contains the node for which
+/// `HirId::local_id == 0`, and excludes bodies.
+///
+/// This struct exists to encapsulate all access to the hir_owner query in this module, and to
+/// implement HashStable without hashing bodies.
+#[derive(Copy, Clone, Debug)]
+pub struct Owner<'tcx> {
+    node: OwnerNode<'tcx>,
+    hash_without_bodies: Fingerprint,
+}
+
+impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for Owner<'tcx> {
+    #[inline]
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        let Owner { node: _, hash_without_bodies } = self;
+        hash_without_bodies.hash_stable(hcx, hasher)
+    }
+}
+
+/// Gather the LocalDefId for each item-like within a module, including items contained within
+/// bodies.  The Ids are in visitor order.  This is used to partition a pass between modules.
+#[derive(Debug, HashStable, Encodable, Decodable)]
+pub struct ModuleItems {
+    submodules: Box<[LocalDefId]>,
+    items: Box<[ItemId]>,
+    trait_items: Box<[TraitItemId]>,
+    impl_items: Box<[ImplItemId]>,
+    foreign_items: Box<[ForeignItemId]>,
+    body_owners: Box<[LocalDefId]>,
+}
+
+impl ModuleItems {
+    pub fn items(&self) -> impl Iterator<Item = ItemId> + '_ {
+        self.items.iter().copied()
+    }
+
+    pub fn trait_items(&self) -> impl Iterator<Item = TraitItemId> + '_ {
+        self.trait_items.iter().copied()
+    }
+
+    pub fn impl_items(&self) -> impl Iterator<Item = ImplItemId> + '_ {
+        self.impl_items.iter().copied()
+    }
+
+    pub fn foreign_items(&self) -> impl Iterator<Item = ForeignItemId> + '_ {
+        self.foreign_items.iter().copied()
+    }
+
+    pub fn definitions(&self) -> impl Iterator<Item = LocalDefId> + '_ {
+        self.items
+            .iter()
+            .map(|id| id.def_id)
+            .chain(self.trait_items.iter().map(|id| id.def_id))
+            .chain(self.impl_items.iter().map(|id| id.def_id))
+            .chain(self.foreign_items.iter().map(|id| id.def_id))
+    }
+
+    pub fn par_items(&self, f: impl Fn(ItemId) + Send + Sync) {
+        par_for_each_in(&self.items[..], |&id| f(id))
+    }
+
+    pub fn par_trait_items(&self, f: impl Fn(TraitItemId) + Send + Sync) {
+        par_for_each_in(&self.trait_items[..], |&id| f(id))
+    }
+
+    pub fn par_impl_items(&self, f: impl Fn(ImplItemId) + Send + Sync) {
+        par_for_each_in(&self.impl_items[..], |&id| f(id))
+    }
+
+    pub fn par_foreign_items(&self, f: impl Fn(ForeignItemId) + Send + Sync) {
+        par_for_each_in(&self.foreign_items[..], |&id| f(id))
+    }
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    #[inline(always)]
+    pub fn hir(self) -> map::Map<'tcx> {
+        map::Map { tcx: self }
+    }
+
+    pub fn parent_module(self, id: HirId) -> LocalDefId {
+        self.parent_module_from_def_id(id.owner)
+    }
+
+    pub fn impl_subject(self, def_id: DefId) -> ImplSubject<'tcx> {
+        self.impl_trait_ref(def_id)
+            .map(ImplSubject::Trait)
+            .unwrap_or_else(|| ImplSubject::Inherent(self.type_of(def_id)))
+    }
+}
+
+pub fn provide(providers: &mut Providers) {
+    providers.parent_module_from_def_id = |tcx, id| {
+        let hir = tcx.hir();
+        hir.get_module_parent_node(hir.local_def_id_to_hir_id(id))
+    };
+    providers.hir_crate_items = map::hir_crate_items;
+    providers.crate_hash = map::crate_hash;
+    providers.hir_module_items = map::hir_module_items;
+    providers.hir_owner = |tcx, id| {
+        let owner = tcx.hir_crate(()).owners.get(id)?.as_owner()?;
+        let node = owner.node();
+        Some(Owner { node, hash_without_bodies: owner.nodes.hash_without_bodies })
+    };
+    providers.local_def_id_to_hir_id = |tcx, id| {
+        let owner = tcx.hir_crate(()).owners[id].map(|_| ());
+        match owner {
+            MaybeOwner::Owner(_) => HirId::make_owner(id),
+            MaybeOwner::Phantom => bug!("No HirId for {:?}", id),
+            MaybeOwner::NonOwner(hir_id) => hir_id,
+        }
+    };
+    providers.hir_owner_nodes = |tcx, id| tcx.hir_crate(()).owners[id].map(|i| &i.nodes);
+    providers.hir_owner_parent = |tcx, id| {
+        // Accessing the local_parent is ok since its value is hashed as part of `id`'s DefPathHash.
+        tcx.opt_local_parent(id).map_or(CRATE_HIR_ID, |parent| {
+            let mut parent_hir_id = tcx.hir().local_def_id_to_hir_id(parent);
+            if let Some(local_id) =
+                tcx.hir_crate(()).owners[parent_hir_id.owner].unwrap().parenting.get(&id)
+            {
+                parent_hir_id.local_id = *local_id;
+            }
+            parent_hir_id
+        })
+    };
+    providers.hir_attrs =
+        |tcx, id| tcx.hir_crate(()).owners[id].as_owner().map_or(AttributeMap::EMPTY, |o| &o.attrs);
+    providers.source_span =
+        |tcx, def_id| tcx.resolutions(()).source_span.get(def_id).copied().unwrap_or(DUMMY_SP);
+    providers.def_span = |tcx, def_id| {
+        let def_id = def_id.expect_local();
+        let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+        tcx.hir().opt_span(hir_id).unwrap_or(DUMMY_SP)
+    };
+    providers.def_ident_span = |tcx, def_id| {
+        let def_id = def_id.expect_local();
+        let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+        tcx.hir().opt_ident_span(hir_id)
+    };
+    providers.fn_arg_names = |tcx, id| {
+        let hir = tcx.hir();
+        let def_id = id.expect_local();
+        let hir_id = hir.local_def_id_to_hir_id(def_id);
+        if let Some(body_id) = hir.maybe_body_owned_by(def_id) {
+            tcx.arena.alloc_from_iter(hir.body_param_names(body_id))
+        } else if let Node::TraitItem(&TraitItem {
+            kind: TraitItemKind::Fn(_, TraitFn::Required(idents)),
+            ..
+        }) = hir.get(hir_id)
+        {
+            tcx.arena.alloc_slice(idents)
+        } else {
+            span_bug!(hir.span(hir_id), "fn_arg_names: unexpected item {:?}", id);
+        }
+    };
+    providers.opt_def_kind = |tcx, def_id| tcx.hir().opt_def_kind(def_id.expect_local());
+    providers.all_local_trait_impls = |tcx, ()| &tcx.resolutions(()).trait_impls;
+    providers.expn_that_defined = |tcx, id| {
+        let id = id.expect_local();
+        tcx.resolutions(()).expn_that_defined.get(&id).copied().unwrap_or(ExpnId::root())
+    };
+    providers.in_scope_traits_map =
+        |tcx, id| tcx.hir_crate(()).owners[id].as_owner().map(|owner_info| &owner_info.trait_map);
+}
diff --git a/compiler/rustc_middle/src/hir/nested_filter.rs b/compiler/rustc_middle/src/hir/nested_filter.rs
new file mode 100644
index 000000000..6896837aa
--- /dev/null
+++ b/compiler/rustc_middle/src/hir/nested_filter.rs
@@ -0,0 +1,31 @@
+use rustc_hir::intravisit::nested_filter::NestedFilter;
+
+/// Do not visit nested item-like things, but visit nested things
+/// that are inside of an item-like.
+///
+/// Notably, possible occurrences of bodies in non-item-like things
+/// include: closures/generators, inline `const {}` blocks, and
+/// constant arguments of types, e.g. in `let _: [(); /* HERE */];`.
+///
+/// **This is the most common choice.** A very common pattern is
+/// to use `visit_all_item_likes_in_crate()` as an outer loop,
+/// and to have the visitor that visits the contents of each item
+/// using this setting.
+pub struct OnlyBodies(());
+impl<'hir> NestedFilter<'hir> for OnlyBodies {
+    type Map = crate::hir::map::Map<'hir>;
+    const INTER: bool = false;
+    const INTRA: bool = true;
+}
+
+/// Visits all nested things, including item-likes.
+///
+/// **This is an unusual choice.** It is used when you want to
+/// process everything within their lexical context. Typically you
+/// kick off the visit by doing `walk_krate()`.
+pub struct All(());
+impl<'hir> NestedFilter<'hir> for All {
+    type Map = crate::hir::map::Map<'hir>;
+    const INTER: bool = true;
+    const INTRA: bool = true;
+}
diff --git a/compiler/rustc_middle/src/hir/place.rs b/compiler/rustc_middle/src/hir/place.rs
new file mode 100644
index 000000000..83d3b0100
--- /dev/null
+++ b/compiler/rustc_middle/src/hir/place.rs
@@ -0,0 +1,117 @@
+use crate::ty;
+use crate::ty::Ty;
+
+use rustc_hir::HirId;
+use rustc_target::abi::VariantIdx;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub enum PlaceBase {
+    /// A temporary variable.
+    Rvalue,
+    /// A named `static` item.
+    StaticItem,
+    /// A named local variable.
+    Local(HirId),
+    /// An upvar referenced by closure env.
+    Upvar(ty::UpvarId),
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub enum ProjectionKind {
+    /// A dereference of a pointer, reference or `Box<T>` of the given type.
+    Deref,
+
+    /// `B.F` where `B` is the base expression and `F` is
+    /// the field. The field is identified by which variant
+    /// it appears in along with a field index. The variant
+    /// is used for enums.
+    Field(u32, VariantIdx),
+
+    /// Some index like `B[x]`, where `B` is the base
+    /// expression. We don't preserve the index `x` because
+    /// we won't need it.
+    Index,
+
+    /// A subslice covering a range of values like `B[x..y]`.
+    Subslice,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct Projection<'tcx> {
+    /// Type after the projection is applied.
+    pub ty: Ty<'tcx>,
+
+    /// Defines the kind of access made by the projection.
+    pub kind: ProjectionKind,
+}
+
+/// A `Place` represents how a value is located in memory.
+///
+/// This is an HIR version of [`rustc_middle::mir::Place`].
+#[derive(Clone, Debug, PartialEq, Eq, Hash, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct Place<'tcx> {
+    /// The type of the `PlaceBase`
+    pub base_ty: Ty<'tcx>,
+    /// The "outermost" place that holds this value.
+    pub base: PlaceBase,
+    /// How this place is derived from the base place.
+    pub projections: Vec<Projection<'tcx>>,
+}
+
+/// A `PlaceWithHirId` represents how a value is located in memory.
+///
+/// This is an HIR version of [`rustc_middle::mir::Place`].
+#[derive(Clone, Debug, PartialEq, Eq, Hash, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct PlaceWithHirId<'tcx> {
+    /// `HirId` of the expression or pattern producing this value.
+    pub hir_id: HirId,
+
+    /// Information about the `Place`.
+    pub place: Place<'tcx>,
+}
+
+impl<'tcx> PlaceWithHirId<'tcx> {
+    pub fn new(
+        hir_id: HirId,
+        base_ty: Ty<'tcx>,
+        base: PlaceBase,
+        projections: Vec<Projection<'tcx>>,
+    ) -> PlaceWithHirId<'tcx> {
+        PlaceWithHirId { hir_id, place: Place { base_ty, base, projections } }
+    }
+}
+
+impl<'tcx> Place<'tcx> {
+    /// Returns an iterator of the types that have to be dereferenced to access
+    /// the `Place`.
+    ///
+    /// The types are in the reverse order that they are applied. So if
+    /// `x: &*const u32` and the `Place` is `**x`, then the types returned are
+    ///`*const u32` then `&*const u32`.
+    pub fn deref_tys(&self) -> impl Iterator<Item = Ty<'tcx>> + '_ {
+        self.projections.iter().enumerate().rev().filter_map(move |(index, proj)| {
+            if ProjectionKind::Deref == proj.kind {
+                Some(self.ty_before_projection(index))
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Returns the type of this `Place` after all projections have been applied.
+    pub fn ty(&self) -> Ty<'tcx> {
+        self.projections.last().map_or(self.base_ty, |proj| proj.ty)
+    }
+
+    /// Returns the type of this `Place` immediately before `projection_index`th projection
+    /// is applied.
+    pub fn ty_before_projection(&self, projection_index: usize) -> Ty<'tcx> {
+        assert!(projection_index < self.projections.len());
+        if projection_index == 0 { self.base_ty } else { self.projections[projection_index - 1].ty }
+    }
+}
diff --git a/compiler/rustc_middle/src/infer/canonical.rs b/compiler/rustc_middle/src/infer/canonical.rs
new file mode 100644
index 000000000..200de9079
--- /dev/null
+++ b/compiler/rustc_middle/src/infer/canonical.rs
@@ -0,0 +1,363 @@
+//! **Canonicalization** is the key to constructing a query in the
+//! middle of type inference. Ordinarily, it is not possible to store
+//! types from type inference in query keys, because they contain
+//! references to inference variables whose lifetimes are too short
+//! and so forth. Canonicalizing a value T1 using `canonicalize_query`
+//! produces two things:
+//!
+//! - a value T2 where each unbound inference variable has been
+//!   replaced with a **canonical variable**;
+//! - a map M (of type `CanonicalVarValues`) from those canonical
+//!   variables back to the original.
+//!
+//! We can then do queries using T2. These will give back constraints
+//! on the canonical variables which can be translated, using the map
+//! M, into constraints in our source context. This process of
+//! translating the results back is done by the
+//! `instantiate_query_result` method.
+//!
+//! For a more detailed look at what is happening here, check
+//! out the [chapter in the rustc dev guide][c].
+//!
+//! [c]: https://rust-lang.github.io/chalk/book/canonical_queries/canonicalization.html
+
+use crate::infer::MemberConstraint;
+use crate::ty::subst::GenericArg;
+use crate::ty::{self, BoundVar, List, Region, Ty, TyCtxt};
+use rustc_index::vec::IndexVec;
+use rustc_macros::HashStable;
+use smallvec::SmallVec;
+use std::iter;
+use std::ops::Index;
+
+/// A "canonicalized" type `V` is one where all free inference
+/// variables have been rewritten to "canonical vars". These are
+/// numbered starting from 0 in order of first appearance.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub struct Canonical<'tcx, V> {
+    pub max_universe: ty::UniverseIndex,
+    pub variables: CanonicalVarInfos<'tcx>,
+    pub value: V,
+}
+
+pub type CanonicalVarInfos<'tcx> = &'tcx List<CanonicalVarInfo<'tcx>>;
+
+/// A set of values corresponding to the canonical variables from some
+/// `Canonical`. You can give these values to
+/// `canonical_value.substitute` to substitute them into the canonical
+/// value at the right places.
+///
+/// When you canonicalize a value `V`, you get back one of these
+/// vectors with the original values that were replaced by canonical
+/// variables. You will need to supply it later to instantiate the
+/// canonicalized query response.
+#[derive(Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub struct CanonicalVarValues<'tcx> {
+    pub var_values: IndexVec<BoundVar, GenericArg<'tcx>>,
+}
+
+/// When we canonicalize a value to form a query, we wind up replacing
+/// various parts of it with canonical variables. This struct stores
+/// those replaced bits to remember for when we process the query
+/// result.
+#[derive(Clone, Debug)]
+pub struct OriginalQueryValues<'tcx> {
+    /// Map from the universes that appear in the query to the universes in the
+    /// caller context. For all queries except `evaluate_goal` (used by Chalk),
+    /// we only ever put ROOT values into the query, so this map is very
+    /// simple.
+    pub universe_map: SmallVec<[ty::UniverseIndex; 4]>,
+
+    /// This is equivalent to `CanonicalVarValues`, but using a
+    /// `SmallVec` yields a significant performance win.
+    pub var_values: SmallVec<[GenericArg<'tcx>; 8]>,
+}
+
+impl<'tcx> Default for OriginalQueryValues<'tcx> {
+    fn default() -> Self {
+        let mut universe_map = SmallVec::default();
+        universe_map.push(ty::UniverseIndex::ROOT);
+
+        Self { universe_map, var_values: SmallVec::default() }
+    }
+}
+
+/// Information about a canonical variable that is included with the
+/// canonical value. This is sufficient information for code to create
+/// a copy of the canonical value in some other inference context,
+/// with fresh inference variables replacing the canonical values.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable, HashStable)]
+pub struct CanonicalVarInfo<'tcx> {
+    pub kind: CanonicalVarKind<'tcx>,
+}
+
+impl<'tcx> CanonicalVarInfo<'tcx> {
+    pub fn universe(&self) -> ty::UniverseIndex {
+        self.kind.universe()
+    }
+
+    pub fn is_existential(&self) -> bool {
+        match self.kind {
+            CanonicalVarKind::Ty(_) => true,
+            CanonicalVarKind::PlaceholderTy(_) => false,
+            CanonicalVarKind::Region(_) => true,
+            CanonicalVarKind::PlaceholderRegion(..) => false,
+            CanonicalVarKind::Const(..) => true,
+            CanonicalVarKind::PlaceholderConst(_, _) => false,
+        }
+    }
+}
+
+/// Describes the "kind" of the canonical variable. This is a "kind"
+/// in the type-theory sense of the term -- i.e., a "meta" type system
+/// that analyzes type-like values.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable, HashStable)]
+pub enum CanonicalVarKind<'tcx> {
+    /// Some kind of type inference variable.
+    Ty(CanonicalTyVarKind),
+
+    /// A "placeholder" that represents "any type".
+    PlaceholderTy(ty::PlaceholderType),
+
+    /// Region variable `'?R`.
+    Region(ty::UniverseIndex),
+
+    /// A "placeholder" that represents "any region". Created when you
+    /// are solving a goal like `for<'a> T: Foo<'a>` to represent the
+    /// bound region `'a`.
+    PlaceholderRegion(ty::PlaceholderRegion),
+
+    /// Some kind of const inference variable.
+    Const(ty::UniverseIndex, Ty<'tcx>),
+
+    /// A "placeholder" that represents "any const".
+    PlaceholderConst(ty::PlaceholderConst<'tcx>, Ty<'tcx>),
+}
+
+impl<'tcx> CanonicalVarKind<'tcx> {
+    pub fn universe(self) -> ty::UniverseIndex {
+        match self {
+            CanonicalVarKind::Ty(kind) => match kind {
+                CanonicalTyVarKind::General(ui) => ui,
+                CanonicalTyVarKind::Float | CanonicalTyVarKind::Int => ty::UniverseIndex::ROOT,
+            },
+
+            CanonicalVarKind::PlaceholderTy(placeholder) => placeholder.universe,
+            CanonicalVarKind::Region(ui) => ui,
+            CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.universe,
+            CanonicalVarKind::Const(ui, _) => ui,
+            CanonicalVarKind::PlaceholderConst(placeholder, _) => placeholder.universe,
+        }
+    }
+}
+
+/// Rust actually has more than one category of type variables;
+/// notably, the type variables we create for literals (e.g., 22 or
+/// 22.) can only be instantiated with integral/float types (e.g.,
+/// usize or f32). In order to faithfully reproduce a type, we need to
+/// know what set of types a given type variable can be unified with.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable, HashStable)]
+pub enum CanonicalTyVarKind {
+    /// General type variable `?T` that can be unified with arbitrary types.
+    General(ty::UniverseIndex),
+
+    /// Integral type variable `?I` (that can only be unified with integral types).
+    Int,
+
+    /// Floating-point type variable `?F` (that can only be unified with float types).
+    Float,
+}
+
+/// After we execute a query with a canonicalized key, we get back a
+/// `Canonical<QueryResponse<..>>`. You can use
+/// `instantiate_query_result` to access the data in this result.
+#[derive(Clone, Debug, HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub struct QueryResponse<'tcx, R> {
+    pub var_values: CanonicalVarValues<'tcx>,
+    pub region_constraints: QueryRegionConstraints<'tcx>,
+    pub certainty: Certainty,
+    /// List of opaque types which we tried to compare to another type.
+    /// Inside the query we don't know yet whether the opaque type actually
+    /// should get its hidden type inferred. So we bubble the opaque type
+    /// and the type it was compared against upwards and let the query caller
+    /// handle it.
+    pub opaque_types: Vec<(Ty<'tcx>, Ty<'tcx>)>,
+    pub value: R,
+}
+
+#[derive(Clone, Debug, Default, HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub struct QueryRegionConstraints<'tcx> {
+    pub outlives: Vec<QueryOutlivesConstraint<'tcx>>,
+    pub member_constraints: Vec<MemberConstraint<'tcx>>,
+}
+
+impl QueryRegionConstraints<'_> {
+    /// Represents an empty (trivially true) set of region
+    /// constraints.
+    pub fn is_empty(&self) -> bool {
+        self.outlives.is_empty() && self.member_constraints.is_empty()
+    }
+}
+
+pub type Canonicalized<'tcx, V> = Canonical<'tcx, V>;
+
+pub type CanonicalizedQueryResponse<'tcx, T> = &'tcx Canonical<'tcx, QueryResponse<'tcx, T>>;
+
+/// Indicates whether or not we were able to prove the query to be
+/// true.
+#[derive(Copy, Clone, Debug, HashStable)]
+pub enum Certainty {
+    /// The query is known to be true, presuming that you apply the
+    /// given `var_values` and the region-constraints are satisfied.
+    Proven,
+
+    /// The query is not known to be true, but also not known to be
+    /// false. The `var_values` represent *either* values that must
+    /// hold in order for the query to be true, or helpful tips that
+    /// *might* make it true. Currently rustc's trait solver cannot
+    /// distinguish the two (e.g., due to our preference for where
+    /// clauses over impls).
+    ///
+    /// After some unification and things have been done, it makes
+    /// sense to try and prove again -- of course, at that point, the
+    /// canonical form will be different, making this a distinct
+    /// query.
+    Ambiguous,
+}
+
+impl Certainty {
+    pub fn is_proven(&self) -> bool {
+        match self {
+            Certainty::Proven => true,
+            Certainty::Ambiguous => false,
+        }
+    }
+}
+
+impl<'tcx, R> QueryResponse<'tcx, R> {
+    pub fn is_proven(&self) -> bool {
+        self.certainty.is_proven()
+    }
+}
+
+impl<'tcx, R> Canonical<'tcx, QueryResponse<'tcx, R>> {
+    pub fn is_proven(&self) -> bool {
+        self.value.is_proven()
+    }
+
+    pub fn is_ambiguous(&self) -> bool {
+        !self.is_proven()
+    }
+}
+
+impl<'tcx, R> Canonical<'tcx, ty::ParamEnvAnd<'tcx, R>> {
+    #[inline]
+    pub fn without_const(mut self) -> Self {
+        self.value = self.value.without_const();
+        self
+    }
+}
+
+impl<'tcx, V> Canonical<'tcx, V> {
+    /// Allows you to map the `value` of a canonical while keeping the
+    /// same set of bound variables.
+    ///
+    /// **WARNING:** This function is very easy to mis-use, hence the
+    /// name!  In particular, the new value `W` must use all **the
+    /// same type/region variables** in **precisely the same order**
+    /// as the original! (The ordering is defined by the
+    /// `TypeFoldable` implementation of the type in question.)
+    ///
+    /// An example of a **correct** use of this:
+    ///
+    /// ```rust,ignore (not real code)
+    /// let a: Canonical<'_, T> = ...;
+    /// let b: Canonical<'_, (T,)> = a.unchecked_map(|v| (v, ));
+    /// ```
+    ///
+    /// An example of an **incorrect** use of this:
+    ///
+    /// ```rust,ignore (not real code)
+    /// let a: Canonical<'tcx, T> = ...;
+    /// let ty: Ty<'tcx> = ...;
+    /// let b: Canonical<'tcx, (T, Ty<'tcx>)> = a.unchecked_map(|v| (v, ty));
+    /// ```
+    pub fn unchecked_map<W>(self, map_op: impl FnOnce(V) -> W) -> Canonical<'tcx, W> {
+        let Canonical { max_universe, variables, value } = self;
+        Canonical { max_universe, variables, value: map_op(value) }
+    }
+}
+
+pub type QueryOutlivesConstraint<'tcx> =
+    ty::Binder<'tcx, ty::OutlivesPredicate<GenericArg<'tcx>, Region<'tcx>>>;
+
+TrivialTypeTraversalAndLiftImpls! {
+    for <'tcx> {
+        crate::infer::canonical::Certainty,
+        crate::infer::canonical::CanonicalVarInfo<'tcx>,
+        crate::infer::canonical::CanonicalVarKind<'tcx>,
+    }
+}
+
+TrivialTypeTraversalImpls! {
+    for <'tcx> {
+        crate::infer::canonical::CanonicalVarInfos<'tcx>,
+    }
+}
+
+impl<'tcx> CanonicalVarValues<'tcx> {
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.var_values.len()
+    }
+
+    /// Makes an identity substitution from this one: each bound var
+    /// is matched to the same bound var, preserving the original kinds.
+    /// For example, if we have:
+    /// `self.var_values == [Type(u32), Lifetime('a), Type(u64)]`
+    /// we'll return a substitution `subst` with:
+    /// `subst.var_values == [Type(^0), Lifetime(^1), Type(^2)]`.
+    pub fn make_identity(&self, tcx: TyCtxt<'tcx>) -> Self {
+        use crate::ty::subst::GenericArgKind;
+
+        CanonicalVarValues {
+            var_values: iter::zip(&self.var_values, 0..)
+                .map(|(kind, i)| match kind.unpack() {
+                    GenericArgKind::Type(..) => {
+                        tcx.mk_ty(ty::Bound(ty::INNERMOST, ty::BoundVar::from_u32(i).into())).into()
+                    }
+                    GenericArgKind::Lifetime(..) => {
+                        let br =
+                            ty::BoundRegion { var: ty::BoundVar::from_u32(i), kind: ty::BrAnon(i) };
+                        tcx.mk_region(ty::ReLateBound(ty::INNERMOST, br)).into()
+                    }
+                    GenericArgKind::Const(ct) => tcx
+                        .mk_const(ty::ConstS {
+                            ty: ct.ty(),
+                            kind: ty::ConstKind::Bound(ty::INNERMOST, ty::BoundVar::from_u32(i)),
+                        })
+                        .into(),
+                })
+                .collect(),
+        }
+    }
+}
+
+impl<'a, 'tcx> IntoIterator for &'a CanonicalVarValues<'tcx> {
+    type Item = GenericArg<'tcx>;
+    type IntoIter = ::std::iter::Cloned<::std::slice::Iter<'a, GenericArg<'tcx>>>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.var_values.iter().cloned()
+    }
+}
+
+impl<'tcx> Index<BoundVar> for CanonicalVarValues<'tcx> {
+    type Output = GenericArg<'tcx>;
+
+    fn index(&self, value: BoundVar) -> &GenericArg<'tcx> {
+        &self.var_values[value]
+    }
+}
diff --git a/compiler/rustc_middle/src/infer/mod.rs b/compiler/rustc_middle/src/infer/mod.rs
new file mode 100644
index 000000000..38868c210
--- /dev/null
+++ b/compiler/rustc_middle/src/infer/mod.rs
@@ -0,0 +1,32 @@
+pub mod canonical;
+pub mod unify_key;
+
+use crate::ty::Region;
+use crate::ty::{OpaqueTypeKey, Ty};
+use rustc_data_structures::sync::Lrc;
+use rustc_span::Span;
+
+/// Requires that `region` must be equal to one of the regions in `choice_regions`.
+/// We often denote this using the syntax:
+///
+/// ```text
+/// R0 member of [O1..On]
+/// ```
+#[derive(Debug, Clone, HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub struct MemberConstraint<'tcx> {
+    /// The `DefId` and substs of the opaque type causing this constraint.
+    /// Used for error reporting.
+    pub key: OpaqueTypeKey<'tcx>,
+
+    /// The span where the hidden type was instantiated.
+    pub definition_span: Span,
+
+    /// The hidden type in which `member_region` appears: used for error reporting.
+    pub hidden_ty: Ty<'tcx>,
+
+    /// The region `R0`.
+    pub member_region: Region<'tcx>,
+
+    /// The options `O1..On`.
+    pub choice_regions: Lrc<Vec<Region<'tcx>>>,
+}
diff --git a/compiler/rustc_middle/src/infer/unify_key.rs b/compiler/rustc_middle/src/infer/unify_key.rs
new file mode 100644
index 000000000..f2627885d
--- /dev/null
+++ b/compiler/rustc_middle/src/infer/unify_key.rs
@@ -0,0 +1,162 @@
+use crate::ty::{self, Ty, TyCtxt};
+use rustc_data_structures::unify::{NoError, UnifyKey, UnifyValue};
+use rustc_span::def_id::DefId;
+use rustc_span::symbol::Symbol;
+use rustc_span::Span;
+use std::cmp;
+use std::marker::PhantomData;
+
+pub trait ToType {
+    fn to_type<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
+}
+
+#[derive(PartialEq, Copy, Clone, Debug)]
+pub struct UnifiedRegion<'tcx>(pub Option<ty::Region<'tcx>>);
+
+#[derive(PartialEq, Copy, Clone, Debug)]
+pub struct RegionVidKey<'tcx> {
+    pub vid: ty::RegionVid,
+    pub phantom: PhantomData<UnifiedRegion<'tcx>>,
+}
+
+impl<'tcx> From<ty::RegionVid> for RegionVidKey<'tcx> {
+    fn from(vid: ty::RegionVid) -> Self {
+        RegionVidKey { vid, phantom: PhantomData }
+    }
+}
+
+impl<'tcx> UnifyKey for RegionVidKey<'tcx> {
+    type Value = UnifiedRegion<'tcx>;
+    #[inline]
+    fn index(&self) -> u32 {
+        self.vid.as_u32()
+    }
+    #[inline]
+    fn from_index(i: u32) -> Self {
+        RegionVidKey::from(ty::RegionVid::from_u32(i))
+    }
+    fn tag() -> &'static str {
+        "RegionVidKey"
+    }
+}
+
+impl<'tcx> UnifyValue for UnifiedRegion<'tcx> {
+    type Error = NoError;
+
+    fn unify_values(value1: &Self, value2: &Self) -> Result<Self, NoError> {
+        Ok(match (value1.0, value2.0) {
+            // Here we can just pick one value, because the full constraints graph
+            // will be handled later. Ideally, we might want a `MultipleValues`
+            // variant or something. For now though, this is fine.
+            (Some(_), Some(_)) => *value1,
+
+            (Some(_), _) => *value1,
+            (_, Some(_)) => *value2,
+
+            (None, None) => *value1,
+        })
+    }
+}
+
+impl ToType for ty::IntVarValue {
+    fn to_type<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        match *self {
+            ty::IntType(i) => tcx.mk_mach_int(i),
+            ty::UintType(i) => tcx.mk_mach_uint(i),
+        }
+    }
+}
+
+impl ToType for ty::FloatVarValue {
+    fn to_type<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        tcx.mk_mach_float(self.0)
+    }
+}
+
+// Generic consts.
+
+#[derive(Copy, Clone, Debug)]
+pub struct ConstVariableOrigin {
+    pub kind: ConstVariableOriginKind,
+    pub span: Span,
+}
+
+/// Reasons to create a const inference variable
+#[derive(Copy, Clone, Debug)]
+pub enum ConstVariableOriginKind {
+    MiscVariable,
+    ConstInference,
+    ConstParameterDefinition(Symbol, DefId),
+    SubstitutionPlaceholder,
+}
+
+#[derive(Copy, Clone, Debug)]
+pub enum ConstVariableValue<'tcx> {
+    Known { value: ty::Const<'tcx> },
+    Unknown { universe: ty::UniverseIndex },
+}
+
+impl<'tcx> ConstVariableValue<'tcx> {
+    /// If this value is known, returns the const it is known to be.
+    /// Otherwise, `None`.
+    pub fn known(&self) -> Option<ty::Const<'tcx>> {
+        match *self {
+            ConstVariableValue::Unknown { .. } => None,
+            ConstVariableValue::Known { value } => Some(value),
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+pub struct ConstVarValue<'tcx> {
+    pub origin: ConstVariableOrigin,
+    pub val: ConstVariableValue<'tcx>,
+}
+
+impl<'tcx> UnifyKey for ty::ConstVid<'tcx> {
+    type Value = ConstVarValue<'tcx>;
+    #[inline]
+    fn index(&self) -> u32 {
+        self.index
+    }
+    #[inline]
+    fn from_index(i: u32) -> Self {
+        ty::ConstVid { index: i, phantom: PhantomData }
+    }
+    fn tag() -> &'static str {
+        "ConstVid"
+    }
+}
+
+impl<'tcx> UnifyValue for ConstVarValue<'tcx> {
+    type Error = (ty::Const<'tcx>, ty::Const<'tcx>);
+
+    fn unify_values(&value1: &Self, &value2: &Self) -> Result<Self, Self::Error> {
+        Ok(match (value1.val, value2.val) {
+            (ConstVariableValue::Known { .. }, ConstVariableValue::Known { .. }) => {
+                bug!("equating two const variables, both of which have known values")
+            }
+
+            // If one side is known, prefer that one.
+            (ConstVariableValue::Known { .. }, ConstVariableValue::Unknown { .. }) => value1,
+            (ConstVariableValue::Unknown { .. }, ConstVariableValue::Known { .. }) => value2,
+
+            // If both sides are *unknown*, it hardly matters, does it?
+            (
+                ConstVariableValue::Unknown { universe: universe1 },
+                ConstVariableValue::Unknown { universe: universe2 },
+            ) => {
+                // If we unify two unbound variables, ?T and ?U, then whatever
+                // value they wind up taking (which must be the same value) must
+                // be nameable by both universes. Therefore, the resulting
+                // universe is the minimum of the two universes, because that is
+                // the one which contains the fewest names in scope.
+                let universe = cmp::min(universe1, universe2);
+                ConstVarValue {
+                    val: ConstVariableValue::Unknown { universe },
+                    origin: value1.origin,
+                }
+            }
+        })
+    }
+}
diff --git a/compiler/rustc_middle/src/lib.rs b/compiler/rustc_middle/src/lib.rs
new file mode 100644
index 000000000..ef06c457b
--- /dev/null
+++ b/compiler/rustc_middle/src/lib.rs
@@ -0,0 +1,106 @@
+//! The "main crate" of the Rust compiler. This crate contains common
+//! type definitions that are used by the other crates in the rustc
+//! "family". Some prominent examples (note that each of these modules
+//! has their own README with further details).
+//!
+//! - **HIR.** The "high-level (H) intermediate representation (IR)" is
+//!   defined in the `hir` module.
+//! - **MIR.** The "mid-level (M) intermediate representation (IR)" is
+//!   defined in the `mir` module. This module contains only the
+//!   *definition* of the MIR; the passes that transform and operate
+//!   on MIR are found in `rustc_const_eval` crate.
+//! - **Types.** The internal representation of types used in rustc is
+//!   defined in the `ty` module. This includes the **type context**
+//!   (or `tcx`), which is the central context during most of
+//!   compilation, containing the interners and other things.
+//!
+//! For more information about how rustc works, see the [rustc dev guide].
+//!
+//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/
+//!
+//! # Note
+//!
+//! This API is completely unstable and subject to change.
+
+#![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(allocator_api)]
+#![feature(array_windows)]
+#![feature(assert_matches)]
+#![feature(backtrace)]
+#![feature(box_patterns)]
+#![feature(core_intrinsics)]
+#![feature(discriminant_kind)]
+#![feature(exhaustive_patterns)]
+#![feature(get_mut_unchecked)]
+#![feature(generic_associated_types)]
+#![feature(if_let_guard)]
+#![feature(map_first_last)]
+#![feature(negative_impls)]
+#![feature(never_type)]
+#![feature(extern_types)]
+#![feature(new_uninit)]
+#![feature(once_cell)]
+#![feature(let_chains)]
+#![feature(let_else)]
+#![feature(min_specialization)]
+#![feature(trusted_len)]
+#![feature(type_alias_impl_trait)]
+#![feature(associated_type_bounds)]
+#![feature(rustc_attrs)]
+#![feature(half_open_range_patterns)]
+#![feature(control_flow_enum)]
+#![feature(associated_type_defaults)]
+#![feature(trusted_step)]
+#![feature(try_blocks)]
+#![feature(try_reserve_kind)]
+#![feature(nonzero_ops)]
+#![feature(unwrap_infallible)]
+#![feature(decl_macro)]
+#![feature(drain_filter)]
+#![feature(intra_doc_pointers)]
+#![feature(yeet_expr)]
+#![feature(const_option)]
+#![recursion_limit = "512"]
+#![allow(rustc::potential_query_instability)]
+
+#[macro_use]
+extern crate bitflags;
+#[macro_use]
+extern crate rustc_macros;
+#[macro_use]
+extern crate rustc_data_structures;
+#[macro_use]
+extern crate tracing;
+#[macro_use]
+extern crate smallvec;
+
+#[cfg(test)]
+mod tests;
+
+#[macro_use]
+mod macros;
+
+#[macro_use]
+pub mod query;
+
+#[macro_use]
+pub mod arena;
+#[macro_use]
+pub mod dep_graph;
+pub mod hir;
+pub mod infer;
+pub mod lint;
+pub mod metadata;
+pub mod middle;
+pub mod mir;
+pub mod thir;
+pub mod traits;
+pub mod ty;
+
+pub mod util {
+    pub mod bug;
+    pub mod common;
+}
+
+// Allows macros to refer to this crate as `::rustc_middle`
+extern crate self as rustc_middle;
diff --git a/compiler/rustc_middle/src/lint.rs b/compiler/rustc_middle/src/lint.rs
new file mode 100644
index 000000000..2f45222de
--- /dev/null
+++ b/compiler/rustc_middle/src/lint.rs
@@ -0,0 +1,443 @@
+use std::cmp;
+
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_errors::{Diagnostic, DiagnosticId, LintDiagnosticBuilder, MultiSpan};
+use rustc_hir::HirId;
+use rustc_index::vec::IndexVec;
+use rustc_query_system::ich::StableHashingContext;
+use rustc_session::lint::{
+    builtin::{self, FORBIDDEN_LINT_GROUPS},
+    FutureIncompatibilityReason, Level, Lint, LintExpectationId, LintId,
+};
+use rustc_session::Session;
+use rustc_span::hygiene::MacroKind;
+use rustc_span::source_map::{DesugaringKind, ExpnKind};
+use rustc_span::{symbol, Span, Symbol, DUMMY_SP};
+
+/// How a lint level was set.
+#[derive(Clone, Copy, PartialEq, Eq, HashStable, Debug)]
+pub enum LintLevelSource {
+    /// Lint is at the default level as declared
+    /// in rustc or a plugin.
+    Default,
+
+    /// Lint level was set by an attribute.
+    Node(Symbol, Span, Option<Symbol> /* RFC 2383 reason */),
+
+    /// Lint level was set by a command-line flag.
+    /// The provided `Level` is the level specified on the command line.
+    /// (The actual level may be lower due to `--cap-lints`.)
+    CommandLine(Symbol, Level),
+}
+
+impl LintLevelSource {
+    pub fn name(&self) -> Symbol {
+        match *self {
+            LintLevelSource::Default => symbol::kw::Default,
+            LintLevelSource::Node(name, _, _) => name,
+            LintLevelSource::CommandLine(name, _) => name,
+        }
+    }
+
+    pub fn span(&self) -> Span {
+        match *self {
+            LintLevelSource::Default => DUMMY_SP,
+            LintLevelSource::Node(_, span, _) => span,
+            LintLevelSource::CommandLine(_, _) => DUMMY_SP,
+        }
+    }
+}
+
+/// A tuple of a lint level and its source.
+pub type LevelAndSource = (Level, LintLevelSource);
+
+#[derive(Debug, HashStable)]
+pub struct LintLevelSets {
+    pub list: IndexVec<LintStackIndex, LintSet>,
+    pub lint_cap: Level,
+}
+
+rustc_index::newtype_index! {
+    #[derive(HashStable)]
+    pub struct LintStackIndex {
+        const COMMAND_LINE = 0,
+    }
+}
+
+#[derive(Debug, HashStable)]
+pub struct LintSet {
+    // -A,-W,-D flags, a `Symbol` for the flag itself and `Level` for which
+    // flag.
+    pub specs: FxHashMap<LintId, LevelAndSource>,
+
+    pub parent: LintStackIndex,
+}
+
+impl LintLevelSets {
+    pub fn new() -> Self {
+        LintLevelSets { list: IndexVec::new(), lint_cap: Level::Forbid }
+    }
+
+    pub fn get_lint_level(
+        &self,
+        lint: &'static Lint,
+        idx: LintStackIndex,
+        aux: Option<&FxHashMap<LintId, LevelAndSource>>,
+        sess: &Session,
+    ) -> LevelAndSource {
+        let (level, mut src) = self.get_lint_id_level(LintId::of(lint), idx, aux);
+
+        // If `level` is none then we actually assume the default level for this
+        // lint.
+        let mut level = level.unwrap_or_else(|| lint.default_level(sess.edition()));
+
+        // If we're about to issue a warning, check at the last minute for any
+        // directives against the warnings "lint". If, for example, there's an
+        // `allow(warnings)` in scope then we want to respect that instead.
+        //
+        // We exempt `FORBIDDEN_LINT_GROUPS` from this because it specifically
+        // triggers in cases (like #80988) where you have `forbid(warnings)`,
+        // and so if we turned that into an error, it'd defeat the purpose of the
+        // future compatibility warning.
+        if level == Level::Warn && LintId::of(lint) != LintId::of(FORBIDDEN_LINT_GROUPS) {
+            let (warnings_level, warnings_src) =
+                self.get_lint_id_level(LintId::of(builtin::WARNINGS), idx, aux);
+            if let Some(configured_warning_level) = warnings_level {
+                if configured_warning_level != Level::Warn {
+                    level = configured_warning_level;
+                    src = warnings_src;
+                }
+            }
+        }
+
+        // Ensure that we never exceed the `--cap-lints` argument
+        // unless the source is a --force-warn
+        level = if let LintLevelSource::CommandLine(_, Level::ForceWarn(_)) = src {
+            level
+        } else {
+            cmp::min(level, self.lint_cap)
+        };
+
+        if let Some(driver_level) = sess.driver_lint_caps.get(&LintId::of(lint)) {
+            // Ensure that we never exceed driver level.
+            level = cmp::min(*driver_level, level);
+        }
+
+        (level, src)
+    }
+
+    pub fn get_lint_id_level(
+        &self,
+        id: LintId,
+        mut idx: LintStackIndex,
+        aux: Option<&FxHashMap<LintId, LevelAndSource>>,
+    ) -> (Option<Level>, LintLevelSource) {
+        if let Some(specs) = aux {
+            if let Some(&(level, src)) = specs.get(&id) {
+                return (Some(level), src);
+            }
+        }
+        loop {
+            let LintSet { ref specs, parent } = self.list[idx];
+            if let Some(&(level, src)) = specs.get(&id) {
+                return (Some(level), src);
+            }
+            if idx == COMMAND_LINE {
+                return (None, LintLevelSource::Default);
+            }
+            idx = parent;
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct LintLevelMap {
+    /// This is a collection of lint expectations as described in RFC 2383, that
+    /// can be fulfilled during this compilation session. This means that at least
+    /// one expected lint is currently registered in the lint store.
+    ///
+    /// The [`LintExpectationId`] is stored as a part of the [`Expect`](Level::Expect)
+    /// lint level.
+    pub lint_expectations: Vec<(LintExpectationId, LintExpectation)>,
+    pub sets: LintLevelSets,
+    pub id_to_set: FxHashMap<HirId, LintStackIndex>,
+}
+
+impl LintLevelMap {
+    /// If the `id` was previously registered with `register_id` when building
+    /// this `LintLevelMap` this returns the corresponding lint level and source
+    /// of the lint level for the lint provided.
+    ///
+    /// If the `id` was not previously registered, returns `None`. If `None` is
+    /// returned then the parent of `id` should be acquired and this function
+    /// should be called again.
+    pub fn level_and_source(
+        &self,
+        lint: &'static Lint,
+        id: HirId,
+        session: &Session,
+    ) -> Option<LevelAndSource> {
+        self.id_to_set.get(&id).map(|idx| self.sets.get_lint_level(lint, *idx, None, session))
+    }
+}
+
+impl<'a> HashStable<StableHashingContext<'a>> for LintLevelMap {
+    #[inline]
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        let LintLevelMap { ref sets, ref id_to_set, ref lint_expectations } = *self;
+
+        id_to_set.hash_stable(hcx, hasher);
+        lint_expectations.hash_stable(hcx, hasher);
+
+        hcx.while_hashing_spans(true, |hcx| sets.hash_stable(hcx, hasher))
+    }
+}
+
+/// This struct represents a lint expectation and holds all required information
+/// to emit the `unfulfilled_lint_expectations` lint if it is unfulfilled after
+/// the `LateLintPass` has completed.
+#[derive(Clone, Debug, HashStable)]
+pub struct LintExpectation {
+    /// The reason for this expectation that can optionally be added as part of
+    /// the attribute. It will be displayed as part of the lint message.
+    pub reason: Option<Symbol>,
+    /// The [`Span`] of the attribute that this expectation originated from.
+    pub emission_span: Span,
+    /// Lint messages for the `unfulfilled_lint_expectations` lint will be
+    /// adjusted to include an additional note. Therefore, we have to track if
+    /// the expectation is for the lint.
+    pub is_unfulfilled_lint_expectations: bool,
+    /// This will hold the name of the tool that this lint belongs to. For
+    /// the lint `clippy::some_lint` the tool would be `clippy`, the same
+    /// goes for `rustdoc`. This will be `None` for rustc lints
+    pub lint_tool: Option<Symbol>,
+}
+
+impl LintExpectation {
+    pub fn new(
+        reason: Option<Symbol>,
+        emission_span: Span,
+        is_unfulfilled_lint_expectations: bool,
+        lint_tool: Option<Symbol>,
+    ) -> Self {
+        Self { reason, emission_span, is_unfulfilled_lint_expectations, lint_tool }
+    }
+}
+
+pub fn explain_lint_level_source(
+    lint: &'static Lint,
+    level: Level,
+    src: LintLevelSource,
+    err: &mut Diagnostic,
+) {
+    let name = lint.name_lower();
+    match src {
+        LintLevelSource::Default => {
+            err.note_once(&format!("`#[{}({})]` on by default", level.as_str(), name));
+        }
+        LintLevelSource::CommandLine(lint_flag_val, orig_level) => {
+            let flag = match orig_level {
+                Level::Warn => "-W",
+                Level::Deny => "-D",
+                Level::Forbid => "-F",
+                Level::Allow => "-A",
+                Level::ForceWarn(_) => "--force-warn",
+                Level::Expect(_) => {
+                    unreachable!("the expect level does not have a commandline flag")
+                }
+            };
+            let hyphen_case_lint_name = name.replace('_', "-");
+            if lint_flag_val.as_str() == name {
+                err.note_once(&format!(
+                    "requested on the command line with `{} {}`",
+                    flag, hyphen_case_lint_name
+                ));
+            } else {
+                let hyphen_case_flag_val = lint_flag_val.as_str().replace('_', "-");
+                err.note_once(&format!(
+                    "`{} {}` implied by `{} {}`",
+                    flag, hyphen_case_lint_name, flag, hyphen_case_flag_val
+                ));
+            }
+        }
+        LintLevelSource::Node(lint_attr_name, src, reason) => {
+            if let Some(rationale) = reason {
+                err.note(rationale.as_str());
+            }
+            err.span_note_once(src, "the lint level is defined here");
+            if lint_attr_name.as_str() != name {
+                let level_str = level.as_str();
+                err.note_once(&format!(
+                    "`#[{}({})]` implied by `#[{}({})]`",
+                    level_str, name, level_str, lint_attr_name
+                ));
+            }
+        }
+    }
+}
+
+pub fn struct_lint_level<'s, 'd>(
+    sess: &'s Session,
+    lint: &'static Lint,
+    level: Level,
+    src: LintLevelSource,
+    span: Option<MultiSpan>,
+    decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a, ()>) + 'd,
+) {
+    // Avoid codegen bloat from monomorphization by immediately doing dyn dispatch of `decorate` to
+    // the "real" work.
+    fn struct_lint_level_impl<'s, 'd>(
+        sess: &'s Session,
+        lint: &'static Lint,
+        level: Level,
+        src: LintLevelSource,
+        span: Option<MultiSpan>,
+        decorate: Box<dyn for<'b> FnOnce(LintDiagnosticBuilder<'b, ()>) + 'd>,
+    ) {
+        // Check for future incompatibility lints and issue a stronger warning.
+        let future_incompatible = lint.future_incompatible;
+
+        let has_future_breakage = future_incompatible.map_or(
+            // Default allow lints trigger too often for testing.
+            sess.opts.unstable_opts.future_incompat_test && lint.default_level != Level::Allow,
+            |incompat| {
+                matches!(incompat.reason, FutureIncompatibilityReason::FutureReleaseErrorReportNow)
+            },
+        );
+
+        let mut err = match (level, span) {
+            (Level::Allow, span) => {
+                if has_future_breakage {
+                    if let Some(span) = span {
+                        sess.struct_span_allow(span, "")
+                    } else {
+                        sess.struct_allow("")
+                    }
+                } else {
+                    return;
+                }
+            }
+            (Level::Expect(expect_id), _) => {
+                // This case is special as we actually allow the lint itself in this context, but
+                // we can't return early like in the case for `Level::Allow` because we still
+                // need the lint diagnostic to be emitted to `rustc_error::HandlerInner`.
+                //
+                // We can also not mark the lint expectation as fulfilled here right away, as it
+                // can still be cancelled in the decorate function. All of this means that we simply
+                // create a `DiagnosticBuilder` and continue as we would for warnings.
+                sess.struct_expect("", expect_id)
+            }
+            (Level::ForceWarn(Some(expect_id)), Some(span)) => {
+                sess.struct_span_warn_with_expectation(span, "", expect_id)
+            }
+            (Level::ForceWarn(Some(expect_id)), None) => {
+                sess.struct_warn_with_expectation("", expect_id)
+            }
+            (Level::Warn | Level::ForceWarn(None), Some(span)) => sess.struct_span_warn(span, ""),
+            (Level::Warn | Level::ForceWarn(None), None) => sess.struct_warn(""),
+            (Level::Deny | Level::Forbid, Some(span)) => {
+                let mut builder = sess.diagnostic().struct_err_lint("");
+                builder.set_span(span);
+                builder
+            }
+            (Level::Deny | Level::Forbid, None) => sess.diagnostic().struct_err_lint(""),
+        };
+
+        // If this code originates in a foreign macro, aka something that this crate
+        // did not itself author, then it's likely that there's nothing this crate
+        // can do about it. We probably want to skip the lint entirely.
+        if err.span.primary_spans().iter().any(|s| in_external_macro(sess, *s)) {
+            // Any suggestions made here are likely to be incorrect, so anything we
+            // emit shouldn't be automatically fixed by rustfix.
+            err.disable_suggestions();
+
+            // If this is a future incompatible that is not an edition fixing lint
+            // it'll become a hard error, so we have to emit *something*. Also,
+            // if this lint occurs in the expansion of a macro from an external crate,
+            // allow individual lints to opt-out from being reported.
+            let not_future_incompatible =
+                future_incompatible.map(|f| f.reason.edition().is_some()).unwrap_or(true);
+            if not_future_incompatible && !lint.report_in_external_macro {
+                err.cancel();
+                // Don't continue further, since we don't want to have
+                // `diag_span_note_once` called for a diagnostic that isn't emitted.
+                return;
+            }
+        }
+
+        // Lint diagnostics that are covered by the expect level will not be emitted outside
+        // the compiler. It is therefore not necessary to add any information for the user.
+        // This will therefore directly call the decorate function which will in turn emit
+        // the `Diagnostic`.
+        if let Level::Expect(_) = level {
+            let name = lint.name_lower();
+            err.code(DiagnosticId::Lint { name, has_future_breakage, is_force_warn: false });
+            decorate(LintDiagnosticBuilder::new(err));
+            return;
+        }
+
+        explain_lint_level_source(lint, level, src, &mut err);
+
+        let name = lint.name_lower();
+        let is_force_warn = matches!(level, Level::ForceWarn(_));
+        err.code(DiagnosticId::Lint { name, has_future_breakage, is_force_warn });
+
+        if let Some(future_incompatible) = future_incompatible {
+            let explanation = match future_incompatible.reason {
+                FutureIncompatibilityReason::FutureReleaseError
+                | FutureIncompatibilityReason::FutureReleaseErrorReportNow => {
+                    "this was previously accepted by the compiler but is being phased out; \
+                         it will become a hard error in a future release!"
+                        .to_owned()
+                }
+                FutureIncompatibilityReason::FutureReleaseSemanticsChange => {
+                    "this will change its meaning in a future release!".to_owned()
+                }
+                FutureIncompatibilityReason::EditionError(edition) => {
+                    let current_edition = sess.edition();
+                    format!(
+                        "this is accepted in the current edition (Rust {}) but is a hard error in Rust {}!",
+                        current_edition, edition
+                    )
+                }
+                FutureIncompatibilityReason::EditionSemanticsChange(edition) => {
+                    format!("this changes meaning in Rust {}", edition)
+                }
+                FutureIncompatibilityReason::Custom(reason) => reason.to_owned(),
+            };
+
+            if future_incompatible.explain_reason {
+                err.warn(&explanation);
+            }
+            if !future_incompatible.reference.is_empty() {
+                let citation =
+                    format!("for more information, see {}", future_incompatible.reference);
+                err.note(&citation);
+            }
+        }
+
+        // Finally, run `decorate`. This function is also responsible for emitting the diagnostic.
+        decorate(LintDiagnosticBuilder::new(err));
+    }
+    struct_lint_level_impl(sess, lint, level, src, span, Box::new(decorate))
+}
+
+/// Returns whether `span` originates in a foreign crate's external macro.
+///
+/// This is used to test whether a lint should not even begin to figure out whether it should
+/// be reported on the current node.
+pub fn in_external_macro(sess: &Session, span: Span) -> bool {
+    let expn_data = span.ctxt().outer_expn_data();
+    match expn_data.kind {
+        ExpnKind::Inlined
+        | ExpnKind::Root
+        | ExpnKind::Desugaring(DesugaringKind::ForLoop | DesugaringKind::WhileLoop) => false,
+        ExpnKind::AstPass(_) | ExpnKind::Desugaring(_) => true, // well, it's "external"
+        ExpnKind::Macro(MacroKind::Bang, _) => {
+            // Dummy span for the `def_site` means it's an external macro.
+            expn_data.def_site.is_dummy() || sess.source_map().is_imported(expn_data.def_site)
+        }
+        ExpnKind::Macro { .. } => true, // definitely a plugin
+    }
+}
diff --git a/compiler/rustc_middle/src/macros.rs b/compiler/rustc_middle/src/macros.rs
new file mode 100644
index 000000000..0e85c60a3
--- /dev/null
+++ b/compiler/rustc_middle/src/macros.rs
@@ -0,0 +1,232 @@
+#[macro_export]
+macro_rules! bug {
+    () => ( $crate::bug!("impossible case reached") );
+    ($msg:expr) => ({ $crate::util::bug::bug_fmt(::std::format_args!($msg)) });
+    ($msg:expr,) => ({ $crate::bug!($msg) });
+    ($fmt:expr, $($arg:tt)+) => ({
+        $crate::util::bug::bug_fmt(::std::format_args!($fmt, $($arg)+))
+    });
+}
+
+#[macro_export]
+macro_rules! span_bug {
+    ($span:expr, $msg:expr) => ({ $crate::util::bug::span_bug_fmt($span, ::std::format_args!($msg)) });
+    ($span:expr, $msg:expr,) => ({ $crate::span_bug!($span, $msg) });
+    ($span:expr, $fmt:expr, $($arg:tt)+) => ({
+        $crate::util::bug::span_bug_fmt($span, ::std::format_args!($fmt, $($arg)+))
+    });
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Lift and TypeFoldable/TypeVisitable macros
+//
+// When possible, use one of these (relatively) convenient macros to write
+// the impls for you.
+
+#[macro_export]
+macro_rules! CloneLiftImpls {
+    (for <$tcx:lifetime> { $($ty:ty,)+ }) => {
+        $(
+            impl<$tcx> $crate::ty::Lift<$tcx> for $ty {
+                type Lifted = Self;
+                fn lift_to_tcx(self, _: $crate::ty::TyCtxt<$tcx>) -> Option<Self> {
+                    Some(self)
+                }
+            }
+        )+
+    };
+
+    ($($ty:ty,)+) => {
+        CloneLiftImpls! {
+            for <'tcx> {
+                $($ty,)+
+            }
+        }
+    };
+}
+
+/// Used for types that are `Copy` and which **do not care arena
+/// allocated data** (i.e., don't need to be folded).
+#[macro_export]
+macro_rules! TrivialTypeTraversalImpls {
+    (for <$tcx:lifetime> { $($ty:ty,)+ }) => {
+        $(
+            impl<$tcx> $crate::ty::fold::TypeFoldable<$tcx> for $ty {
+                fn try_fold_with<F: $crate::ty::fold::FallibleTypeFolder<$tcx>>(
+                    self,
+                    _: &mut F
+                ) -> ::std::result::Result<$ty, F::Error> {
+                    Ok(self)
+                }
+            }
+
+            impl<$tcx> $crate::ty::visit::TypeVisitable<$tcx> for $ty {
+                fn visit_with<F: $crate::ty::visit::TypeVisitor<$tcx>>(
+                    &self,
+                    _: &mut F)
+                    -> ::std::ops::ControlFlow<F::BreakTy>
+                {
+                    ::std::ops::ControlFlow::CONTINUE
+                }
+            }
+        )+
+    };
+
+    ($($ty:ty,)+) => {
+        TrivialTypeTraversalImpls! {
+            for <'tcx> {
+                $($ty,)+
+            }
+        }
+    };
+}
+
+#[macro_export]
+macro_rules! TrivialTypeTraversalAndLiftImpls {
+    ($($t:tt)*) => {
+        TrivialTypeTraversalImpls! { $($t)* }
+        CloneLiftImpls! { $($t)* }
+    }
+}
+
+#[macro_export]
+macro_rules! EnumTypeTraversalImpl {
+    (impl<$($p:tt),*> TypeFoldable<$tcx:tt> for $s:path {
+        $($variants:tt)*
+    } $(where $($wc:tt)*)*) => {
+        impl<$($p),*> $crate::ty::fold::TypeFoldable<$tcx> for $s
+            $(where $($wc)*)*
+        {
+            fn try_fold_with<V: $crate::ty::fold::FallibleTypeFolder<$tcx>>(
+                self,
+                folder: &mut V,
+            ) -> ::std::result::Result<Self, V::Error> {
+                EnumTypeTraversalImpl!(@FoldVariants(self, folder) input($($variants)*) output())
+            }
+        }
+    };
+
+    (impl<$($p:tt),*> TypeVisitable<$tcx:tt> for $s:path {
+        $($variants:tt)*
+    } $(where $($wc:tt)*)*) => {
+        impl<$($p),*> $crate::ty::visit::TypeVisitable<$tcx> for $s
+            $(where $($wc)*)*
+        {
+            fn visit_with<V: $crate::ty::visit::TypeVisitor<$tcx>>(
+                &self,
+                visitor: &mut V,
+            ) -> ::std::ops::ControlFlow<V::BreakTy> {
+                EnumTypeTraversalImpl!(@VisitVariants(self, visitor) input($($variants)*) output())
+            }
+        }
+    };
+
+    (@FoldVariants($this:expr, $folder:expr) input() output($($output:tt)*)) => {
+        Ok(match $this {
+            $($output)*
+        })
+    };
+
+    (@FoldVariants($this:expr, $folder:expr)
+     input( ($variant:path) ( $($variant_arg:ident),* ) , $($input:tt)*)
+     output( $($output:tt)*) ) => {
+        EnumTypeTraversalImpl!(
+            @FoldVariants($this, $folder)
+                input($($input)*)
+                output(
+                    $variant ( $($variant_arg),* ) => {
+                        $variant (
+                            $($crate::ty::fold::TypeFoldable::try_fold_with($variant_arg, $folder)?),*
+                        )
+                    }
+                    $($output)*
+                )
+        )
+    };
+
+    (@FoldVariants($this:expr, $folder:expr)
+     input( ($variant:path) { $($variant_arg:ident),* $(,)? } , $($input:tt)*)
+     output( $($output:tt)*) ) => {
+        EnumTypeTraversalImpl!(
+            @FoldVariants($this, $folder)
+                input($($input)*)
+                output(
+                    $variant { $($variant_arg),* } => {
+                        $variant {
+                            $($variant_arg: $crate::ty::fold::TypeFoldable::fold_with(
+                                $variant_arg, $folder
+                            )?),* }
+                    }
+                    $($output)*
+                )
+        )
+    };
+
+    (@FoldVariants($this:expr, $folder:expr)
+     input( ($variant:path), $($input:tt)*)
+     output( $($output:tt)*) ) => {
+        EnumTypeTraversalImpl!(
+            @FoldVariants($this, $folder)
+                input($($input)*)
+                output(
+                    $variant => { $variant }
+                    $($output)*
+                )
+        )
+    };
+
+    (@VisitVariants($this:expr, $visitor:expr) input() output($($output:tt)*)) => {
+        match $this {
+            $($output)*
+        }
+    };
+
+    (@VisitVariants($this:expr, $visitor:expr)
+     input( ($variant:path) ( $($variant_arg:ident),* ) , $($input:tt)*)
+     output( $($output:tt)*) ) => {
+        EnumTypeTraversalImpl!(
+            @VisitVariants($this, $visitor)
+                input($($input)*)
+                output(
+                    $variant ( $($variant_arg),* ) => {
+                        $($crate::ty::visit::TypeVisitable::visit_with(
+                            $variant_arg, $visitor
+                        )?;)*
+                        ::std::ops::ControlFlow::CONTINUE
+                    }
+                    $($output)*
+                )
+        )
+    };
+
+    (@VisitVariants($this:expr, $visitor:expr)
+     input( ($variant:path) { $($variant_arg:ident),* $(,)? } , $($input:tt)*)
+     output( $($output:tt)*) ) => {
+        EnumTypeTraversalImpl!(
+            @VisitVariants($this, $visitor)
+                input($($input)*)
+                output(
+                    $variant { $($variant_arg),* } => {
+                        $($crate::ty::visit::TypeVisitable::visit_with(
+                            $variant_arg, $visitor
+                        )?;)*
+                        ::std::ops::ControlFlow::CONTINUE
+                    }
+                    $($output)*
+                )
+        )
+    };
+
+    (@VisitVariants($this:expr, $visitor:expr)
+     input( ($variant:path), $($input:tt)*)
+     output( $($output:tt)*) ) => {
+        EnumTypeTraversalImpl!(
+            @VisitVariants($this, $visitor)
+                input($($input)*)
+                output(
+                    $variant => { ::std::ops::ControlFlow::CONTINUE }
+                    $($output)*
+                )
+        )
+    };
+}
diff --git a/compiler/rustc_middle/src/metadata.rs b/compiler/rustc_middle/src/metadata.rs
new file mode 100644
index 000000000..c8e78747d
--- /dev/null
+++ b/compiler/rustc_middle/src/metadata.rs
@@ -0,0 +1,26 @@
+use crate::ty;
+
+use rustc_hir::def::Res;
+use rustc_macros::HashStable;
+use rustc_span::symbol::Ident;
+use rustc_span::Span;
+
+/// This structure is supposed to keep enough data to re-create `NameBinding`s for other crates
+/// during name resolution. Right now the bindings are not recreated entirely precisely so we may
+/// need to add more data in the future to correctly support macros 2.0, for example.
+/// Module child can be either a proper item or a reexport (including private imports).
+/// In case of reexport all the fields describe the reexport item itself, not what it refers to.
+#[derive(Copy, Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct ModChild {
+    /// Name of the item.
+    pub ident: Ident,
+    /// Resolution result corresponding to the item.
+    /// Local variables cannot be exported, so this `Res` doesn't need the ID parameter.
+    pub res: Res<!>,
+    /// Visibility of the item.
+    pub vis: ty::Visibility,
+    /// Span of the item.
+    pub span: Span,
+    /// A proper `macro_rules` item (not a reexport).
+    pub macro_rules: bool,
+}
diff --git a/compiler/rustc_middle/src/middle/codegen_fn_attrs.rs b/compiler/rustc_middle/src/middle/codegen_fn_attrs.rs
new file mode 100644
index 000000000..45d33a165
--- /dev/null
+++ b/compiler/rustc_middle/src/middle/codegen_fn_attrs.rs
@@ -0,0 +1,146 @@
+use crate::mir::mono::Linkage;
+use rustc_attr::{InlineAttr, InstructionSetAttr, OptimizeAttr};
+use rustc_span::symbol::Symbol;
+use rustc_target::spec::SanitizerSet;
+
+#[derive(Clone, TyEncodable, TyDecodable, HashStable, Debug)]
+pub struct CodegenFnAttrs {
+    pub flags: CodegenFnAttrFlags,
+    /// Parsed representation of the `#[inline]` attribute
+    pub inline: InlineAttr,
+    /// Parsed representation of the `#[optimize]` attribute
+    pub optimize: OptimizeAttr,
+    /// The `#[export_name = "..."]` attribute, indicating a custom symbol a
+    /// function should be exported under
+    pub export_name: Option<Symbol>,
+    /// The `#[link_name = "..."]` attribute, indicating a custom symbol an
+    /// imported function should be imported as. Note that `export_name`
+    /// probably isn't set when this is set, this is for foreign items while
+    /// `#[export_name]` is for Rust-defined functions.
+    pub link_name: Option<Symbol>,
+    /// The `#[link_ordinal = "..."]` attribute, indicating an ordinal an
+    /// imported function has in the dynamic library. Note that this must not
+    /// be set when `link_name` is set. This is for foreign items with the
+    /// "raw-dylib" kind.
+    pub link_ordinal: Option<u16>,
+    /// The `#[target_feature(enable = "...")]` attribute and the enabled
+    /// features (only enabled features are supported right now).
+    pub target_features: Vec<Symbol>,
+    /// The `#[linkage = "..."]` attribute and the value we found.
+    pub linkage: Option<Linkage>,
+    /// The `#[link_section = "..."]` attribute, or what executable section this
+    /// should be placed in.
+    pub link_section: Option<Symbol>,
+    /// The `#[no_sanitize(...)]` attribute. Indicates sanitizers for which
+    /// instrumentation should be disabled inside the annotated function.
+    pub no_sanitize: SanitizerSet,
+    /// The `#[instruction_set(set)]` attribute. Indicates if the generated code should
+    /// be generated against a specific instruction set. Only usable on architectures which allow
+    /// switching between multiple instruction sets.
+    pub instruction_set: Option<InstructionSetAttr>,
+    /// The `#[repr(align(...))]` attribute. Indicates the value of which the function should be
+    /// aligned to.
+    pub alignment: Option<u32>,
+}
+
+bitflags! {
+    #[derive(TyEncodable, TyDecodable, HashStable)]
+    pub struct CodegenFnAttrFlags: u32 {
+        /// `#[cold]`: a hint to LLVM that this function, when called, is never on
+        /// the hot path.
+        const COLD                      = 1 << 0;
+        /// `#[rustc_allocator]`: a hint to LLVM that the pointer returned from this
+        /// function is never null and the function has no side effects other than allocating.
+        const ALLOCATOR                 = 1 << 1;
+        /// An indicator that function will never unwind. Will become obsolete
+        /// once C-unwind is fully stabilized.
+        const NEVER_UNWIND              = 1 << 3;
+        /// `#[naked]`: an indicator to LLVM that no function prologue/epilogue
+        /// should be generated.
+        const NAKED                     = 1 << 4;
+        /// `#[no_mangle]`: an indicator that the function's name should be the same
+        /// as its symbol.
+        const NO_MANGLE                 = 1 << 5;
+        /// `#[rustc_std_internal_symbol]`: an indicator that this symbol is a
+        /// "weird symbol" for the standard library in that it has slightly
+        /// different linkage, visibility, and reachability rules.
+        const RUSTC_STD_INTERNAL_SYMBOL = 1 << 6;
+        /// `#[thread_local]`: indicates a static is actually a thread local
+        /// piece of memory
+        const THREAD_LOCAL              = 1 << 8;
+        /// `#[used]`: indicates that LLVM can't eliminate this function (but the
+        /// linker can!).
+        const USED                      = 1 << 9;
+        /// `#[ffi_returns_twice]`, indicates that an extern function can return
+        /// multiple times
+        const FFI_RETURNS_TWICE         = 1 << 10;
+        /// `#[track_caller]`: allow access to the caller location
+        const TRACK_CALLER              = 1 << 11;
+        /// #[ffi_pure]: applies clang's `pure` attribute to a foreign function
+        /// declaration.
+        const FFI_PURE                  = 1 << 12;
+        /// #[ffi_const]: applies clang's `const` attribute to a foreign function
+        /// declaration.
+        const FFI_CONST                 = 1 << 13;
+        /// #[cmse_nonsecure_entry]: with a TrustZone-M extension, declare a
+        /// function as an entry function from Non-Secure code.
+        const CMSE_NONSECURE_ENTRY      = 1 << 14;
+        /// `#[no_coverage]`: indicates that the function should be ignored by
+        /// the MIR `InstrumentCoverage` pass and not added to the coverage map
+        /// during codegen.
+        const NO_COVERAGE               = 1 << 15;
+        /// `#[used(linker)]`: indicates that LLVM nor the linker can eliminate this function.
+        const USED_LINKER               = 1 << 16;
+        /// `#[rustc_deallocator]`: a hint to LLVM that the function only deallocates memory.
+        const DEALLOCATOR               = 1 << 17;
+        /// `#[rustc_reallocator]`: a hint to LLVM that the function only reallocates memory.
+        const REALLOCATOR               = 1 << 18;
+        /// `#[rustc_allocator_zeroed]`: a hint to LLVM that the function only allocates zeroed memory.
+        const ALLOCATOR_ZEROED          = 1 << 19;
+    }
+}
+
+impl CodegenFnAttrs {
+    pub const EMPTY: &'static Self = &Self::new();
+
+    pub const fn new() -> CodegenFnAttrs {
+        CodegenFnAttrs {
+            flags: CodegenFnAttrFlags::empty(),
+            inline: InlineAttr::None,
+            optimize: OptimizeAttr::None,
+            export_name: None,
+            link_name: None,
+            link_ordinal: None,
+            target_features: vec![],
+            linkage: None,
+            link_section: None,
+            no_sanitize: SanitizerSet::empty(),
+            instruction_set: None,
+            alignment: None,
+        }
+    }
+
+    /// Returns `true` if `#[inline]` or `#[inline(always)]` is present.
+    pub fn requests_inline(&self) -> bool {
+        match self.inline {
+            InlineAttr::Hint | InlineAttr::Always => true,
+            InlineAttr::None | InlineAttr::Never => false,
+        }
+    }
+
+    /// Returns `true` if it looks like this symbol needs to be exported, for example:
+    ///
+    /// * `#[no_mangle]` is present
+    /// * `#[export_name(...)]` is present
+    /// * `#[linkage]` is present
+    pub fn contains_extern_indicator(&self) -> bool {
+        self.flags.contains(CodegenFnAttrFlags::NO_MANGLE)
+            || self.export_name.is_some()
+            || match self.linkage {
+                // These are private, so make sure we don't try to consider
+                // them external.
+                None | Some(Linkage::Internal | Linkage::Private) => false,
+                Some(_) => true,
+            }
+    }
+}
diff --git a/compiler/rustc_middle/src/middle/dependency_format.rs b/compiler/rustc_middle/src/middle/dependency_format.rs
new file mode 100644
index 000000000..e079843bf
--- /dev/null
+++ b/compiler/rustc_middle/src/middle/dependency_format.rs
@@ -0,0 +1,28 @@
+//! Type definitions for learning about the dependency formats of all upstream
+//! crates (rlibs/dylibs/oh my).
+//!
+//! For all the gory details, see the provider of the `dependency_formats`
+//! query.
+
+use rustc_session::config::CrateType;
+
+/// A list of dependencies for a certain crate type.
+///
+/// The length of this vector is the same as the number of external crates used.
+/// The value is None if the crate does not need to be linked (it was found
+/// statically in another dylib), or Some(kind) if it needs to be linked as
+/// `kind` (either static or dynamic).
+pub type DependencyList = Vec<Linkage>;
+
+/// A mapping of all required dependencies for a particular flavor of output.
+///
+/// This is local to the tcx, and is generally relevant to one session.
+pub type Dependencies = Vec<(CrateType, DependencyList)>;
+
+#[derive(Copy, Clone, PartialEq, Debug, HashStable, Encodable, Decodable)]
+pub enum Linkage {
+    NotLinked,
+    IncludedFromDylib,
+    Static,
+    Dynamic,
+}
diff --git a/compiler/rustc_middle/src/middle/exported_symbols.rs b/compiler/rustc_middle/src/middle/exported_symbols.rs
new file mode 100644
index 000000000..631fd09ec
--- /dev/null
+++ b/compiler/rustc_middle/src/middle/exported_symbols.rs
@@ -0,0 +1,72 @@
+use crate::ty::subst::SubstsRef;
+use crate::ty::{self, Ty, TyCtxt};
+use rustc_hir::def_id::{DefId, LOCAL_CRATE};
+use rustc_macros::HashStable;
+
+/// The SymbolExportLevel of a symbols specifies from which kinds of crates
+/// the symbol will be exported. `C` symbols will be exported from any
+/// kind of crate, including cdylibs which export very few things.
+/// `Rust` will only be exported if the crate produced is a Rust
+/// dylib.
+#[derive(Eq, PartialEq, Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
+pub enum SymbolExportLevel {
+    C,
+    Rust,
+}
+
+impl SymbolExportLevel {
+    pub fn is_below_threshold(self, threshold: SymbolExportLevel) -> bool {
+        threshold == SymbolExportLevel::Rust // export everything from Rust dylibs
+          || self == SymbolExportLevel::C
+    }
+}
+
+/// Kind of exported symbols.
+#[derive(Eq, PartialEq, Debug, Copy, Clone, Encodable, Decodable, HashStable)]
+pub enum SymbolExportKind {
+    Text,
+    Data,
+    Tls,
+}
+
+/// The `SymbolExportInfo` of a symbols specifies symbol-related information
+/// that is relevant to code generation and linking.
+#[derive(Eq, PartialEq, Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
+pub struct SymbolExportInfo {
+    pub level: SymbolExportLevel,
+    pub kind: SymbolExportKind,
+    pub used: bool,
+}
+
+#[derive(Eq, PartialEq, Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
+pub enum ExportedSymbol<'tcx> {
+    NonGeneric(DefId),
+    Generic(DefId, SubstsRef<'tcx>),
+    DropGlue(Ty<'tcx>),
+    NoDefId(ty::SymbolName<'tcx>),
+}
+
+impl<'tcx> ExportedSymbol<'tcx> {
+    /// This is the symbol name of an instance if it is instantiated in the
+    /// local crate.
+    pub fn symbol_name_for_local_instance(&self, tcx: TyCtxt<'tcx>) -> ty::SymbolName<'tcx> {
+        match *self {
+            ExportedSymbol::NonGeneric(def_id) => tcx.symbol_name(ty::Instance::mono(tcx, def_id)),
+            ExportedSymbol::Generic(def_id, substs) => {
+                tcx.symbol_name(ty::Instance::new(def_id, substs))
+            }
+            ExportedSymbol::DropGlue(ty) => {
+                tcx.symbol_name(ty::Instance::resolve_drop_in_place(tcx, ty))
+            }
+            ExportedSymbol::NoDefId(symbol_name) => symbol_name,
+        }
+    }
+}
+
+pub fn metadata_symbol_name(tcx: TyCtxt<'_>) -> String {
+    format!(
+        "rust_metadata_{}_{:08x}",
+        tcx.crate_name(LOCAL_CRATE),
+        tcx.sess.local_stable_crate_id().to_u64(),
+    )
+}
diff --git a/compiler/rustc_middle/src/middle/lang_items.rs b/compiler/rustc_middle/src/middle/lang_items.rs
new file mode 100644
index 000000000..cc9706f2d
--- /dev/null
+++ b/compiler/rustc_middle/src/middle/lang_items.rs
@@ -0,0 +1,61 @@
+//! Detecting language items.
+//!
+//! Language items are items that represent concepts intrinsic to the language
+//! itself. Examples are:
+//!
+//! * Traits that specify "kinds"; e.g., `Sync`, `Send`.
+//! * Traits that represent operators; e.g., `Add`, `Sub`, `Index`.
+//! * Functions called by the compiler itself.
+
+use crate::ty::{self, TyCtxt};
+
+use rustc_hir::def_id::DefId;
+use rustc_hir::LangItem;
+use rustc_span::Span;
+use rustc_target::spec::PanicStrategy;
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Returns the `DefId` for a given `LangItem`.
+    /// If not found, fatally aborts compilation.
+    pub fn require_lang_item(self, lang_item: LangItem, span: Option<Span>) -> DefId {
+        self.lang_items().require(lang_item).unwrap_or_else(|msg| {
+            if let Some(span) = span {
+                self.sess.span_fatal(span, &msg)
+            } else {
+                self.sess.fatal(&msg)
+            }
+        })
+    }
+
+    pub fn fn_trait_kind_from_lang_item(self, id: DefId) -> Option<ty::ClosureKind> {
+        let items = self.lang_items();
+        match Some(id) {
+            x if x == items.fn_trait() => Some(ty::ClosureKind::Fn),
+            x if x == items.fn_mut_trait() => Some(ty::ClosureKind::FnMut),
+            x if x == items.fn_once_trait() => Some(ty::ClosureKind::FnOnce),
+            _ => None,
+        }
+    }
+
+    pub fn is_weak_lang_item(self, item_def_id: DefId) -> bool {
+        self.lang_items().is_weak_lang_item(item_def_id)
+    }
+}
+
+/// Returns `true` if the specified `lang_item` must be present for this
+/// compilation.
+///
+/// Not all lang items are always required for each compilation, particularly in
+/// the case of panic=abort. In these situations some lang items are injected by
+/// crates and don't actually need to be defined in libstd.
+pub fn required(tcx: TyCtxt<'_>, lang_item: LangItem) -> bool {
+    // If we're not compiling with unwinding, we won't actually need these
+    // symbols. Other panic runtimes ensure that the relevant symbols are
+    // available to link things together, but they're never exercised.
+    match tcx.sess.panic_strategy() {
+        PanicStrategy::Abort => {
+            lang_item != LangItem::EhPersonality && lang_item != LangItem::EhCatchTypeinfo
+        }
+        PanicStrategy::Unwind => true,
+    }
+}
diff --git a/compiler/rustc_middle/src/middle/limits.rs b/compiler/rustc_middle/src/middle/limits.rs
new file mode 100644
index 000000000..acced0492
--- /dev/null
+++ b/compiler/rustc_middle/src/middle/limits.rs
@@ -0,0 +1,85 @@
+//! Registering limits:
+//! * recursion_limit,
+//! * move_size_limit,
+//! * type_length_limit, and
+//! * const_eval_limit
+//!
+//! There are various parts of the compiler that must impose arbitrary limits
+//! on how deeply they recurse to prevent stack overflow. Users can override
+//! this via an attribute on the crate like `#![recursion_limit="22"]`. This pass
+//! just peeks and looks for that attribute.
+
+use crate::bug;
+use crate::ty;
+use rustc_ast::Attribute;
+use rustc_session::Session;
+use rustc_session::{Limit, Limits};
+use rustc_span::symbol::{sym, Symbol};
+
+use std::num::IntErrorKind;
+
+pub fn provide(providers: &mut ty::query::Providers) {
+    providers.limits = |tcx, ()| Limits {
+        recursion_limit: get_recursion_limit(tcx.hir().krate_attrs(), tcx.sess),
+        move_size_limit: get_limit(
+            tcx.hir().krate_attrs(),
+            tcx.sess,
+            sym::move_size_limit,
+            tcx.sess.opts.unstable_opts.move_size_limit.unwrap_or(0),
+        ),
+        type_length_limit: get_limit(
+            tcx.hir().krate_attrs(),
+            tcx.sess,
+            sym::type_length_limit,
+            1048576,
+        ),
+        const_eval_limit: get_limit(
+            tcx.hir().krate_attrs(),
+            tcx.sess,
+            sym::const_eval_limit,
+            1_000_000,
+        ),
+    }
+}
+
+pub fn get_recursion_limit(krate_attrs: &[Attribute], sess: &Session) -> Limit {
+    get_limit(krate_attrs, sess, sym::recursion_limit, 128)
+}
+
+fn get_limit(krate_attrs: &[Attribute], sess: &Session, name: Symbol, default: usize) -> Limit {
+    for attr in krate_attrs {
+        if !attr.has_name(name) {
+            continue;
+        }
+
+        if let Some(s) = attr.value_str() {
+            match s.as_str().parse() {
+                Ok(n) => return Limit::new(n),
+                Err(e) => {
+                    let mut err =
+                        sess.struct_span_err(attr.span, "`limit` must be a non-negative integer");
+
+                    let value_span = attr
+                        .meta()
+                        .and_then(|meta| meta.name_value_literal_span())
+                        .unwrap_or(attr.span);
+
+                    let error_str = match e.kind() {
+                        IntErrorKind::PosOverflow => "`limit` is too large",
+                        IntErrorKind::Empty => "`limit` must be a non-negative integer",
+                        IntErrorKind::InvalidDigit => "not a valid integer",
+                        IntErrorKind::NegOverflow => {
+                            bug!("`limit` should never negatively overflow")
+                        }
+                        IntErrorKind::Zero => bug!("zero is a valid `limit`"),
+                        kind => bug!("unimplemented IntErrorKind variant: {:?}", kind),
+                    };
+
+                    err.span_label(value_span, error_str);
+                    err.emit();
+                }
+            }
+        }
+    }
+    return Limit::new(default);
+}
diff --git a/compiler/rustc_middle/src/middle/mod.rs b/compiler/rustc_middle/src/middle/mod.rs
new file mode 100644
index 000000000..8dc68b1f5
--- /dev/null
+++ b/compiler/rustc_middle/src/middle/mod.rs
@@ -0,0 +1,37 @@
+pub mod codegen_fn_attrs;
+pub mod dependency_format;
+pub mod exported_symbols;
+pub mod lang_items;
+pub mod lib_features {
+    use rustc_data_structures::fx::FxHashMap;
+    use rustc_span::{symbol::Symbol, Span};
+
+    #[derive(HashStable, Debug)]
+    pub struct LibFeatures {
+        /// A map from feature to stabilisation version.
+        pub stable: FxHashMap<Symbol, (Symbol, Span)>,
+        pub unstable: FxHashMap<Symbol, Span>,
+    }
+
+    impl LibFeatures {
+        pub fn to_vec(&self) -> Vec<(Symbol, Option<Symbol>)> {
+            let mut all_features: Vec<_> = self
+                .stable
+                .iter()
+                .map(|(f, (s, _))| (*f, Some(*s)))
+                .chain(self.unstable.iter().map(|(f, _)| (*f, None)))
+                .collect();
+            all_features.sort_unstable_by(|a, b| a.0.as_str().partial_cmp(b.0.as_str()).unwrap());
+            all_features
+        }
+    }
+}
+pub mod limits;
+pub mod privacy;
+pub mod region;
+pub mod resolve_lifetime;
+pub mod stability;
+
+pub fn provide(providers: &mut crate::ty::query::Providers) {
+    limits::provide(providers);
+}
diff --git a/compiler/rustc_middle/src/middle/privacy.rs b/compiler/rustc_middle/src/middle/privacy.rs
new file mode 100644
index 000000000..751c7f464
--- /dev/null
+++ b/compiler/rustc_middle/src/middle/privacy.rs
@@ -0,0 +1,64 @@
+//! A pass that checks to make sure private fields and methods aren't used
+//! outside their scopes. This pass will also generate a set of exported items
+//! which are available for use externally when compiled as a library.
+
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_macros::HashStable;
+use rustc_query_system::ich::StableHashingContext;
+use rustc_span::def_id::LocalDefId;
+use std::hash::Hash;
+
+/// Represents the levels of accessibility an item can have.
+///
+/// The variants are sorted in ascending order of accessibility.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, HashStable)]
+pub enum AccessLevel {
+    /// Superset of `AccessLevel::Reachable` used to mark impl Trait items.
+    ReachableFromImplTrait,
+    /// Exported items + items participating in various kinds of public interfaces,
+    /// but not directly nameable. For example, if function `fn f() -> T {...}` is
+    /// public, then type `T` is reachable. Its values can be obtained by other crates
+    /// even if the type itself is not nameable.
+    Reachable,
+    /// Public items + items accessible to other crates with the help of `pub use` re-exports.
+    Exported,
+    /// Items accessible to other crates directly, without the help of re-exports.
+    Public,
+}
+
+/// Holds a map of accessibility levels for reachable HIR nodes.
+#[derive(Debug, Clone)]
+pub struct AccessLevels<Id = LocalDefId> {
+    pub map: FxHashMap<Id, AccessLevel>,
+}
+
+impl<Id: Hash + Eq> AccessLevels<Id> {
+    /// See `AccessLevel::Reachable`.
+    pub fn is_reachable(&self, id: Id) -> bool {
+        self.map.get(&id) >= Some(&AccessLevel::Reachable)
+    }
+
+    /// See `AccessLevel::Exported`.
+    pub fn is_exported(&self, id: Id) -> bool {
+        self.map.get(&id) >= Some(&AccessLevel::Exported)
+    }
+
+    /// See `AccessLevel::Public`.
+    pub fn is_public(&self, id: Id) -> bool {
+        self.map.get(&id) >= Some(&AccessLevel::Public)
+    }
+}
+
+impl<Id> Default for AccessLevels<Id> {
+    fn default() -> Self {
+        AccessLevels { map: Default::default() }
+    }
+}
+
+impl<'a> HashStable<StableHashingContext<'a>> for AccessLevels {
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        let AccessLevels { ref map } = *self;
+        map.hash_stable(hcx, hasher);
+    }
+}
diff --git a/compiler/rustc_middle/src/middle/region.rs b/compiler/rustc_middle/src/middle/region.rs
new file mode 100644
index 000000000..c886175c6
--- /dev/null
+++ b/compiler/rustc_middle/src/middle/region.rs
@@ -0,0 +1,443 @@
+//! This file declares the `ScopeTree` type, which describes
+//! the parent links in the region hierarchy.
+//!
+//! For more information about how MIR-based region-checking works,
+//! see the [rustc dev guide].
+//!
+//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/borrow_check.html
+
+use crate::ty::TyCtxt;
+use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_hir as hir;
+use rustc_hir::Node;
+use rustc_macros::HashStable;
+use rustc_query_system::ich::StableHashingContext;
+use rustc_span::{Span, DUMMY_SP};
+
+use std::fmt;
+use std::ops::Deref;
+
+/// Represents a statically-describable scope that can be used to
+/// bound the lifetime/region for values.
+///
+/// `Node(node_id)`: Any AST node that has any scope at all has the
+/// `Node(node_id)` scope. Other variants represent special cases not
+/// immediately derivable from the abstract syntax tree structure.
+///
+/// `DestructionScope(node_id)` represents the scope of destructors
+/// implicitly-attached to `node_id` that run immediately after the
+/// expression for `node_id` itself. Not every AST node carries a
+/// `DestructionScope`, but those that are `terminating_scopes` do;
+/// see discussion with `ScopeTree`.
+///
+/// `Remainder { block, statement_index }` represents
+/// the scope of user code running immediately after the initializer
+/// expression for the indexed statement, until the end of the block.
+///
+/// So: the following code can be broken down into the scopes beneath:
+///
+/// ```text
+/// let a = f().g( 'b: { let x = d(); let y = d(); x.h(y)  }   ) ;
+///
+///                                                              +-+ (D12.)
+///                                                        +-+       (D11.)
+///                                              +---------+         (R10.)
+///                                              +-+                  (D9.)
+///                                   +----------+                    (M8.)
+///                                 +----------------------+          (R7.)
+///                                 +-+                               (D6.)
+///                      +----------+                                 (M5.)
+///                    +-----------------------------------+          (M4.)
+///         +--------------------------------------------------+      (M3.)
+///         +--+                                                      (M2.)
+/// +-----------------------------------------------------------+     (M1.)
+///
+///  (M1.): Node scope of the whole `let a = ...;` statement.
+///  (M2.): Node scope of the `f()` expression.
+///  (M3.): Node scope of the `f().g(..)` expression.
+///  (M4.): Node scope of the block labeled `'b:`.
+///  (M5.): Node scope of the `let x = d();` statement
+///  (D6.): DestructionScope for temporaries created during M5.
+///  (R7.): Remainder scope for block `'b:`, stmt 0 (let x = ...).
+///  (M8.): Node scope of the `let y = d();` statement.
+///  (D9.): DestructionScope for temporaries created during M8.
+/// (R10.): Remainder scope for block `'b:`, stmt 1 (let y = ...).
+/// (D11.): DestructionScope for temporaries and bindings from block `'b:`.
+/// (D12.): DestructionScope for temporaries created during M1 (e.g., f()).
+/// ```
+///
+/// Note that while the above picture shows the destruction scopes
+/// as following their corresponding node scopes, in the internal
+/// data structures of the compiler the destruction scopes are
+/// represented as enclosing parents. This is sound because we use the
+/// enclosing parent relationship just to ensure that referenced
+/// values live long enough; phrased another way, the starting point
+/// of each range is not really the important thing in the above
+/// picture, but rather the ending point.
+//
+// FIXME(pnkfelix): this currently derives `PartialOrd` and `Ord` to
+// placate the same deriving in `ty::FreeRegion`, but we may want to
+// actually attach a more meaningful ordering to scopes than the one
+// generated via deriving here.
+#[derive(Clone, PartialEq, PartialOrd, Eq, Ord, Hash, Copy, TyEncodable, TyDecodable)]
+#[derive(HashStable)]
+pub struct Scope {
+    pub id: hir::ItemLocalId,
+    pub data: ScopeData,
+}
+
+impl fmt::Debug for Scope {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.data {
+            ScopeData::Node => write!(fmt, "Node({:?})", self.id),
+            ScopeData::CallSite => write!(fmt, "CallSite({:?})", self.id),
+            ScopeData::Arguments => write!(fmt, "Arguments({:?})", self.id),
+            ScopeData::Destruction => write!(fmt, "Destruction({:?})", self.id),
+            ScopeData::IfThen => write!(fmt, "IfThen({:?})", self.id),
+            ScopeData::Remainder(fsi) => write!(
+                fmt,
+                "Remainder {{ block: {:?}, first_statement_index: {}}}",
+                self.id,
+                fsi.as_u32(),
+            ),
+        }
+    }
+}
+
+#[derive(Clone, PartialEq, PartialOrd, Eq, Ord, Hash, Debug, Copy, TyEncodable, TyDecodable)]
+#[derive(HashStable)]
+pub enum ScopeData {
+    Node,
+
+    /// Scope of the call-site for a function or closure
+    /// (outlives the arguments as well as the body).
+    CallSite,
+
+    /// Scope of arguments passed to a function or closure
+    /// (they outlive its body).
+    Arguments,
+
+    /// Scope of destructors for temporaries of node-id.
+    Destruction,
+
+    /// Scope of the condition and then block of an if expression
+    /// Used for variables introduced in an if-let expression.
+    IfThen,
+
+    /// Scope following a `let id = expr;` binding in a block.
+    Remainder(FirstStatementIndex),
+}
+
+rustc_index::newtype_index! {
+    /// Represents a subscope of `block` for a binding that is introduced
+    /// by `block.stmts[first_statement_index]`. Such subscopes represent
+    /// a suffix of the block. Note that each subscope does not include
+    /// the initializer expression, if any, for the statement indexed by
+    /// `first_statement_index`.
+    ///
+    /// For example, given `{ let (a, b) = EXPR_1; let c = EXPR_2; ... }`:
+    ///
+    /// * The subscope with `first_statement_index == 0` is scope of both
+    ///   `a` and `b`; it does not include EXPR_1, but does include
+    ///   everything after that first `let`. (If you want a scope that
+    ///   includes EXPR_1 as well, then do not use `Scope::Remainder`,
+    ///   but instead another `Scope` that encompasses the whole block,
+    ///   e.g., `Scope::Node`.
+    ///
+    /// * The subscope with `first_statement_index == 1` is scope of `c`,
+    ///   and thus does not include EXPR_2, but covers the `...`.
+    pub struct FirstStatementIndex {
+        derive [HashStable]
+    }
+}
+
+// compilation error if size of `ScopeData` is not the same as a `u32`
+static_assert_size!(ScopeData, 4);
+
+impl Scope {
+    /// Returns an item-local ID associated with this scope.
+    ///
+    /// N.B., likely to be replaced as API is refined; e.g., pnkfelix
+    /// anticipates `fn entry_node_id` and `fn each_exit_node_id`.
+    pub fn item_local_id(&self) -> hir::ItemLocalId {
+        self.id
+    }
+
+    pub fn hir_id(&self, scope_tree: &ScopeTree) -> Option<hir::HirId> {
+        scope_tree
+            .root_body
+            .map(|hir_id| hir::HirId { owner: hir_id.owner, local_id: self.item_local_id() })
+    }
+
+    /// Returns the span of this `Scope`. Note that in general the
+    /// returned span may not correspond to the span of any `NodeId` in
+    /// the AST.
+    pub fn span(&self, tcx: TyCtxt<'_>, scope_tree: &ScopeTree) -> Span {
+        let Some(hir_id) = self.hir_id(scope_tree) else {
+            return DUMMY_SP;
+        };
+        let span = tcx.hir().span(hir_id);
+        if let ScopeData::Remainder(first_statement_index) = self.data {
+            if let Node::Block(ref blk) = tcx.hir().get(hir_id) {
+                // Want span for scope starting after the
+                // indexed statement and ending at end of
+                // `blk`; reuse span of `blk` and shift `lo`
+                // forward to end of indexed statement.
+                //
+                // (This is the special case alluded to in the
+                // doc-comment for this method)
+
+                let stmt_span = blk.stmts[first_statement_index.index()].span;
+
+                // To avoid issues with macro-generated spans, the span
+                // of the statement must be nested in that of the block.
+                if span.lo() <= stmt_span.lo() && stmt_span.lo() <= span.hi() {
+                    return span.with_lo(stmt_span.lo());
+                }
+            }
+        }
+        span
+    }
+}
+
+pub type ScopeDepth = u32;
+
+/// The region scope tree encodes information about region relationships.
+#[derive(TyEncodable, TyDecodable, Default, Debug)]
+pub struct ScopeTree {
+    /// If not empty, this body is the root of this region hierarchy.
+    pub root_body: Option<hir::HirId>,
+
+    /// Maps from a scope ID to the enclosing scope id;
+    /// this is usually corresponding to the lexical nesting, though
+    /// in the case of closures the parent scope is the innermost
+    /// conditional expression or repeating block. (Note that the
+    /// enclosing scope ID for the block associated with a closure is
+    /// the closure itself.)
+    pub parent_map: FxIndexMap<Scope, (Scope, ScopeDepth)>,
+
+    /// Maps from a variable or binding ID to the block in which that
+    /// variable is declared.
+    var_map: FxIndexMap<hir::ItemLocalId, Scope>,
+
+    /// Maps from a `NodeId` to the associated destruction scope (if any).
+    destruction_scopes: FxIndexMap<hir::ItemLocalId, Scope>,
+
+    /// Identifies expressions which, if captured into a temporary, ought to
+    /// have a temporary whose lifetime extends to the end of the enclosing *block*,
+    /// and not the enclosing *statement*. Expressions that are not present in this
+    /// table are not rvalue candidates. The set of rvalue candidates is computed
+    /// during type check based on a traversal of the AST.
+    pub rvalue_candidates: FxHashMap<hir::HirId, RvalueCandidateType>,
+
+    /// If there are any `yield` nested within a scope, this map
+    /// stores the `Span` of the last one and its index in the
+    /// postorder of the Visitor traversal on the HIR.
+    ///
+    /// HIR Visitor postorder indexes might seem like a peculiar
+    /// thing to care about. but it turns out that HIR bindings
+    /// and the temporary results of HIR expressions are never
+    /// storage-live at the end of HIR nodes with postorder indexes
+    /// lower than theirs, and therefore don't need to be suspended
+    /// at yield-points at these indexes.
+    ///
+    /// For an example, suppose we have some code such as:
+    /// ```rust,ignore (example)
+    ///     foo(f(), yield y, bar(g()))
+    /// ```
+    ///
+    /// With the HIR tree (calls numbered for expository purposes)
+    ///
+    /// ```text
+    ///     Call#0(foo, [Call#1(f), Yield(y), Call#2(bar, Call#3(g))])
+    /// ```
+    ///
+    /// Obviously, the result of `f()` was created before the yield
+    /// (and therefore needs to be kept valid over the yield) while
+    /// the result of `g()` occurs after the yield (and therefore
+    /// doesn't). If we want to infer that, we can look at the
+    /// postorder traversal:
+    /// ```plain,ignore
+    ///     `foo` `f` Call#1 `y` Yield `bar` `g` Call#3 Call#2 Call#0
+    /// ```
+    ///
+    /// In which we can easily see that `Call#1` occurs before the yield,
+    /// and `Call#3` after it.
+    ///
+    /// To see that this method works, consider:
+    ///
+    /// Let `D` be our binding/temporary and `U` be our other HIR node, with
+    /// `HIR-postorder(U) < HIR-postorder(D)`. Suppose, as in our example,
+    /// U is the yield and D is one of the calls.
+    /// Let's show that `D` is storage-dead at `U`.
+    ///
+    /// Remember that storage-live/storage-dead refers to the state of
+    /// the *storage*, and does not consider moves/drop flags.
+    ///
+    /// Then:
+    ///
+    ///   1. From the ordering guarantee of HIR visitors (see
+    ///   `rustc_hir::intravisit`), `D` does not dominate `U`.
+    ///
+    ///   2. Therefore, `D` is *potentially* storage-dead at `U` (because
+    ///   we might visit `U` without ever getting to `D`).
+    ///
+    ///   3. However, we guarantee that at each HIR point, each
+    ///   binding/temporary is always either always storage-live
+    ///   or always storage-dead. This is what is being guaranteed
+    ///   by `terminating_scopes` including all blocks where the
+    ///   count of executions is not guaranteed.
+    ///
+    ///   4. By `2.` and `3.`, `D` is *statically* storage-dead at `U`,
+    ///   QED.
+    ///
+    /// This property ought to not on (3) in an essential way -- it
+    /// is probably still correct even if we have "unrestricted" terminating
+    /// scopes. However, why use the complicated proof when a simple one
+    /// works?
+    ///
+    /// A subtle thing: `box` expressions, such as `box (&x, yield 2, &y)`. It
+    /// might seem that a `box` expression creates a `Box<T>` temporary
+    /// when it *starts* executing, at `HIR-preorder(BOX-EXPR)`. That might
+    /// be true in the MIR desugaring, but it is not important in the semantics.
+    ///
+    /// The reason is that semantically, until the `box` expression returns,
+    /// the values are still owned by their containing expressions. So
+    /// we'll see that `&x`.
+    pub yield_in_scope: FxHashMap<Scope, Vec<YieldData>>,
+
+    /// The number of visit_expr and visit_pat calls done in the body.
+    /// Used to sanity check visit_expr/visit_pat call count when
+    /// calculating generator interiors.
+    pub body_expr_count: FxHashMap<hir::BodyId, usize>,
+}
+
+/// Identifies the reason that a given expression is an rvalue candidate
+/// (see the `rvalue_candidates` field for more information what rvalue
+/// candidates in general). In constants, the `lifetime` field is None
+/// to indicate that certain expressions escape into 'static and
+/// should have no local cleanup scope.
+#[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
+pub enum RvalueCandidateType {
+    Borrow { target: hir::ItemLocalId, lifetime: Option<Scope> },
+    Pattern { target: hir::ItemLocalId, lifetime: Option<Scope> },
+}
+
+#[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
+pub struct YieldData {
+    /// The `Span` of the yield.
+    pub span: Span,
+    /// The number of expressions and patterns appearing before the `yield` in the body, plus one.
+    pub expr_and_pat_count: usize,
+    pub source: hir::YieldSource,
+}
+
+impl ScopeTree {
+    pub fn record_scope_parent(&mut self, child: Scope, parent: Option<(Scope, ScopeDepth)>) {
+        debug!("{:?}.parent = {:?}", child, parent);
+
+        if let Some(p) = parent {
+            let prev = self.parent_map.insert(child, p);
+            assert!(prev.is_none());
+        }
+
+        // Record the destruction scopes for later so we can query them.
+        if let ScopeData::Destruction = child.data {
+            self.destruction_scopes.insert(child.item_local_id(), child);
+        }
+    }
+
+    pub fn opt_destruction_scope(&self, n: hir::ItemLocalId) -> Option<Scope> {
+        self.destruction_scopes.get(&n).cloned()
+    }
+
+    pub fn record_var_scope(&mut self, var: hir::ItemLocalId, lifetime: Scope) {
+        debug!("record_var_scope(sub={:?}, sup={:?})", var, lifetime);
+        assert!(var != lifetime.item_local_id());
+        self.var_map.insert(var, lifetime);
+    }
+
+    pub fn record_rvalue_candidate(
+        &mut self,
+        var: hir::HirId,
+        candidate_type: RvalueCandidateType,
+    ) {
+        debug!("record_rvalue_candidate(var={var:?}, type={candidate_type:?})");
+        match &candidate_type {
+            RvalueCandidateType::Borrow { lifetime: Some(lifetime), .. }
+            | RvalueCandidateType::Pattern { lifetime: Some(lifetime), .. } => {
+                assert!(var.local_id != lifetime.item_local_id())
+            }
+            _ => {}
+        }
+        self.rvalue_candidates.insert(var, candidate_type);
+    }
+
+    /// Returns the narrowest scope that encloses `id`, if any.
+    pub fn opt_encl_scope(&self, id: Scope) -> Option<Scope> {
+        self.parent_map.get(&id).cloned().map(|(p, _)| p)
+    }
+
+    /// Returns the lifetime of the local variable `var_id`, if any.
+    pub fn var_scope(&self, var_id: hir::ItemLocalId) -> Option<Scope> {
+        self.var_map.get(&var_id).cloned()
+    }
+
+    /// Returns `true` if `subscope` is equal to or is lexically nested inside `superscope`, and
+    /// `false` otherwise.
+    ///
+    /// Used by clippy.
+    pub fn is_subscope_of(&self, subscope: Scope, superscope: Scope) -> bool {
+        let mut s = subscope;
+        debug!("is_subscope_of({:?}, {:?})", subscope, superscope);
+        while superscope != s {
+            match self.opt_encl_scope(s) {
+                None => {
+                    debug!("is_subscope_of({:?}, {:?}, s={:?})=false", subscope, superscope, s);
+                    return false;
+                }
+                Some(scope) => s = scope,
+            }
+        }
+
+        debug!("is_subscope_of({:?}, {:?})=true", subscope, superscope);
+
+        true
+    }
+
+    /// Checks whether the given scope contains a `yield`. If so,
+    /// returns `Some(YieldData)`. If not, returns `None`.
+    pub fn yield_in_scope(&self, scope: Scope) -> Option<&[YieldData]> {
+        self.yield_in_scope.get(&scope).map(Deref::deref)
+    }
+
+    /// Gives the number of expressions visited in a body.
+    /// Used to sanity check visit_expr call count when
+    /// calculating generator interiors.
+    pub fn body_expr_count(&self, body_id: hir::BodyId) -> Option<usize> {
+        self.body_expr_count.get(&body_id).copied()
+    }
+}
+
+impl<'a> HashStable<StableHashingContext<'a>> for ScopeTree {
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        let ScopeTree {
+            root_body,
+            ref body_expr_count,
+            ref parent_map,
+            ref var_map,
+            ref destruction_scopes,
+            ref rvalue_candidates,
+            ref yield_in_scope,
+        } = *self;
+
+        root_body.hash_stable(hcx, hasher);
+        body_expr_count.hash_stable(hcx, hasher);
+        parent_map.hash_stable(hcx, hasher);
+        var_map.hash_stable(hcx, hasher);
+        destruction_scopes.hash_stable(hcx, hasher);
+        rvalue_candidates.hash_stable(hcx, hasher);
+        yield_in_scope.hash_stable(hcx, hasher);
+    }
+}
diff --git a/compiler/rustc_middle/src/middle/resolve_lifetime.rs b/compiler/rustc_middle/src/middle/resolve_lifetime.rs
new file mode 100644
index 000000000..9b2f44567
--- /dev/null
+++ b/compiler/rustc_middle/src/middle/resolve_lifetime.rs
@@ -0,0 +1,54 @@
+//! Name resolution for lifetimes: type declarations.
+
+use crate::ty;
+
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::ItemLocalId;
+use rustc_macros::HashStable;
+
+#[derive(Clone, Copy, PartialEq, Eq, Hash, TyEncodable, TyDecodable, Debug, HashStable)]
+pub enum Region {
+    Static,
+    EarlyBound(/* index */ u32, /* lifetime decl */ DefId),
+    LateBound(ty::DebruijnIndex, /* late-bound index */ u32, /* lifetime decl */ DefId),
+    Free(DefId, /* lifetime decl */ DefId),
+}
+
+/// A set containing, at most, one known element.
+/// If two distinct values are inserted into a set, then it
+/// becomes `Many`, which can be used to detect ambiguities.
+#[derive(Copy, Clone, PartialEq, Eq, TyEncodable, TyDecodable, Debug, HashStable)]
+pub enum Set1<T> {
+    Empty,
+    One(T),
+    Many,
+}
+
+impl<T: PartialEq> Set1<T> {
+    pub fn insert(&mut self, value: T) {
+        *self = match self {
+            Set1::Empty => Set1::One(value),
+            Set1::One(old) if *old == value => return,
+            _ => Set1::Many,
+        };
+    }
+}
+
+pub type ObjectLifetimeDefault = Set1<Region>;
+
+/// Maps the id of each lifetime reference to the lifetime decl
+/// that it corresponds to.
+#[derive(Default, HashStable, Debug)]
+pub struct ResolveLifetimes {
+    /// Maps from every use of a named (not anonymous) lifetime to a
+    /// `Region` describing how that region is bound
+    pub defs: FxHashMap<LocalDefId, FxHashMap<ItemLocalId, Region>>,
+
+    /// Set of lifetime def ids that are late-bound; a region can
+    /// be late-bound if (a) it does NOT appear in a where-clause and
+    /// (b) it DOES appear in the arguments.
+    pub late_bound: FxHashMap<LocalDefId, FxHashSet<LocalDefId>>,
+
+    pub late_bound_vars: FxHashMap<LocalDefId, FxHashMap<ItemLocalId, Vec<ty::BoundVariableKind>>>,
+}
diff --git a/compiler/rustc_middle/src/middle/stability.rs b/compiler/rustc_middle/src/middle/stability.rs
new file mode 100644
index 000000000..414912dd0
--- /dev/null
+++ b/compiler/rustc_middle/src/middle/stability.rs
@@ -0,0 +1,591 @@
+//! A pass that annotates every item and method with its stability level,
+//! propagating default levels lexically from parent to children ast nodes.
+
+pub use self::StabilityLevel::*;
+
+use crate::ty::{self, DefIdTree, TyCtxt};
+use rustc_ast::NodeId;
+use rustc_attr::{self as attr, ConstStability, Deprecation, Stability};
+use rustc_data_structures::fx::FxHashMap;
+use rustc_errors::{Applicability, Diagnostic};
+use rustc_feature::GateIssue;
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::{self as hir, HirId};
+use rustc_middle::ty::print::with_no_trimmed_paths;
+use rustc_session::lint::builtin::{DEPRECATED, DEPRECATED_IN_FUTURE, SOFT_UNSTABLE};
+use rustc_session::lint::{BuiltinLintDiagnostics, Level, Lint, LintBuffer};
+use rustc_session::parse::feature_err_issue;
+use rustc_session::Session;
+use rustc_span::symbol::{sym, Symbol};
+use rustc_span::Span;
+use std::num::NonZeroU32;
+
+#[derive(PartialEq, Clone, Copy, Debug)]
+pub enum StabilityLevel {
+    Unstable,
+    Stable,
+}
+
+/// An entry in the `depr_map`.
+#[derive(Copy, Clone, HashStable, Debug, Encodable, Decodable)]
+pub struct DeprecationEntry {
+    /// The metadata of the attribute associated with this entry.
+    pub attr: Deprecation,
+    /// The `DefId` where the attr was originally attached. `None` for non-local
+    /// `DefId`'s.
+    origin: Option<LocalDefId>,
+}
+
+impl DeprecationEntry {
+    pub fn local(attr: Deprecation, def_id: LocalDefId) -> DeprecationEntry {
+        DeprecationEntry { attr, origin: Some(def_id) }
+    }
+
+    pub fn external(attr: Deprecation) -> DeprecationEntry {
+        DeprecationEntry { attr, origin: None }
+    }
+
+    pub fn same_origin(&self, other: &DeprecationEntry) -> bool {
+        match (self.origin, other.origin) {
+            (Some(o1), Some(o2)) => o1 == o2,
+            _ => false,
+        }
+    }
+}
+
+/// A stability index, giving the stability level for items and methods.
+#[derive(HashStable, Debug)]
+pub struct Index {
+    /// This is mostly a cache, except the stabilities of local items
+    /// are filled by the annotator.
+    pub stab_map: FxHashMap<LocalDefId, Stability>,
+    pub const_stab_map: FxHashMap<LocalDefId, ConstStability>,
+    pub depr_map: FxHashMap<LocalDefId, DeprecationEntry>,
+    /// Mapping from feature name to feature name based on the `implied_by` field of `#[unstable]`
+    /// attributes. If a `#[unstable(feature = "implier", implied_by = "impliee")]` attribute
+    /// exists, then this map will have a `impliee -> implier` entry.
+    ///
+    /// This mapping is necessary unless both the `#[stable]` and `#[unstable]` attributes should
+    /// specify their implications (both `implies` and `implied_by`). If only one of the two
+    /// attributes do (as in the current implementation, `implied_by` in `#[unstable]`), then this
+    /// mapping is necessary for diagnostics. When a "unnecessary feature attribute" error is
+    /// reported, only the `#[stable]` attribute information is available, so the map is necessary
+    /// to know that the feature implies another feature. If it were reversed, and the `#[stable]`
+    /// attribute had an `implies` meta item, then a map would be necessary when avoiding a "use of
+    /// unstable feature" error for a feature that was implied.
+    pub implications: FxHashMap<Symbol, Symbol>,
+}
+
+impl Index {
+    pub fn local_stability(&self, def_id: LocalDefId) -> Option<Stability> {
+        self.stab_map.get(&def_id).copied()
+    }
+
+    pub fn local_const_stability(&self, def_id: LocalDefId) -> Option<ConstStability> {
+        self.const_stab_map.get(&def_id).copied()
+    }
+
+    pub fn local_deprecation_entry(&self, def_id: LocalDefId) -> Option<DeprecationEntry> {
+        self.depr_map.get(&def_id).cloned()
+    }
+}
+
+pub fn report_unstable(
+    sess: &Session,
+    feature: Symbol,
+    reason: Option<Symbol>,
+    issue: Option<NonZeroU32>,
+    suggestion: Option<(Span, String, String, Applicability)>,
+    is_soft: bool,
+    span: Span,
+    soft_handler: impl FnOnce(&'static Lint, Span, &str),
+) {
+    let msg = match reason {
+        Some(r) => format!("use of unstable library feature '{}': {}", feature, r),
+        None => format!("use of unstable library feature '{}'", &feature),
+    };
+
+    if is_soft {
+        soft_handler(SOFT_UNSTABLE, span, &msg)
+    } else {
+        let mut err =
+            feature_err_issue(&sess.parse_sess, feature, span, GateIssue::Library(issue), &msg);
+        if let Some((inner_types, ref msg, sugg, applicability)) = suggestion {
+            err.span_suggestion(inner_types, msg, sugg, applicability);
+        }
+        err.emit();
+    }
+}
+
+/// Checks whether an item marked with `deprecated(since="X")` is currently
+/// deprecated (i.e., whether X is not greater than the current rustc version).
+pub fn deprecation_in_effect(depr: &Deprecation) -> bool {
+    let is_since_rustc_version = depr.is_since_rustc_version;
+    let since = depr.since.as_ref().map(Symbol::as_str);
+
+    fn parse_version(ver: &str) -> Vec<u32> {
+        // We ignore non-integer components of the version (e.g., "nightly").
+        ver.split(|c| c == '.' || c == '-').flat_map(|s| s.parse()).collect()
+    }
+
+    if !is_since_rustc_version {
+        // The `since` field doesn't have semantic purpose without `#![staged_api]`.
+        return true;
+    }
+
+    if let Some(since) = since {
+        if since == "TBD" {
+            return false;
+        }
+
+        if let Some(rustc) = option_env!("CFG_RELEASE") {
+            let since: Vec<u32> = parse_version(&since);
+            let rustc: Vec<u32> = parse_version(rustc);
+            // We simply treat invalid `since` attributes as relating to a previous
+            // Rust version, thus always displaying the warning.
+            if since.len() != 3 {
+                return true;
+            }
+            return since <= rustc;
+        }
+    };
+
+    // Assume deprecation is in effect if "since" field is missing
+    // or if we can't determine the current Rust version.
+    true
+}
+
+pub fn deprecation_suggestion(
+    diag: &mut Diagnostic,
+    kind: &str,
+    suggestion: Option<Symbol>,
+    span: Span,
+) {
+    if let Some(suggestion) = suggestion {
+        diag.span_suggestion_verbose(
+            span,
+            &format!("replace the use of the deprecated {}", kind),
+            suggestion,
+            Applicability::MachineApplicable,
+        );
+    }
+}
+
+fn deprecation_lint(is_in_effect: bool) -> &'static Lint {
+    if is_in_effect { DEPRECATED } else { DEPRECATED_IN_FUTURE }
+}
+
+fn deprecation_message(
+    is_in_effect: bool,
+    since: Option<Symbol>,
+    note: Option<Symbol>,
+    kind: &str,
+    path: &str,
+) -> String {
+    let message = if is_in_effect {
+        format!("use of deprecated {} `{}`", kind, path)
+    } else {
+        let since = since.as_ref().map(Symbol::as_str);
+
+        if since == Some("TBD") {
+            format!("use of {} `{}` that will be deprecated in a future Rust version", kind, path)
+        } else {
+            format!(
+                "use of {} `{}` that will be deprecated in future version {}",
+                kind,
+                path,
+                since.unwrap()
+            )
+        }
+    };
+
+    match note {
+        Some(reason) => format!("{}: {}", message, reason),
+        None => message,
+    }
+}
+
+pub fn deprecation_message_and_lint(
+    depr: &Deprecation,
+    kind: &str,
+    path: &str,
+) -> (String, &'static Lint) {
+    let is_in_effect = deprecation_in_effect(depr);
+    (
+        deprecation_message(is_in_effect, depr.since, depr.note, kind, path),
+        deprecation_lint(is_in_effect),
+    )
+}
+
+pub fn early_report_deprecation<'a>(
+    lint_buffer: &'a mut LintBuffer,
+    message: &str,
+    suggestion: Option<Symbol>,
+    lint: &'static Lint,
+    span: Span,
+    node_id: NodeId,
+) {
+    if span.in_derive_expansion() {
+        return;
+    }
+
+    let diag = BuiltinLintDiagnostics::DeprecatedMacro(suggestion, span);
+    lint_buffer.buffer_lint_with_diagnostic(lint, node_id, span, message, diag);
+}
+
+fn late_report_deprecation(
+    tcx: TyCtxt<'_>,
+    message: &str,
+    suggestion: Option<Symbol>,
+    lint: &'static Lint,
+    span: Span,
+    method_span: Option<Span>,
+    hir_id: HirId,
+    def_id: DefId,
+) {
+    if span.in_derive_expansion() {
+        return;
+    }
+    let method_span = method_span.unwrap_or(span);
+    tcx.struct_span_lint_hir(lint, hir_id, method_span, |lint| {
+        let mut diag = lint.build(message);
+        if let hir::Node::Expr(_) = tcx.hir().get(hir_id) {
+            let kind = tcx.def_kind(def_id).descr(def_id);
+            deprecation_suggestion(&mut diag, kind, suggestion, method_span);
+        }
+        diag.emit();
+    });
+}
+
+/// Result of `TyCtxt::eval_stability`.
+pub enum EvalResult {
+    /// We can use the item because it is stable or we provided the
+    /// corresponding feature gate.
+    Allow,
+    /// We cannot use the item because it is unstable and we did not provide the
+    /// corresponding feature gate.
+    Deny {
+        feature: Symbol,
+        reason: Option<Symbol>,
+        issue: Option<NonZeroU32>,
+        suggestion: Option<(Span, String, String, Applicability)>,
+        is_soft: bool,
+    },
+    /// The item does not have the `#[stable]` or `#[unstable]` marker assigned.
+    Unmarked,
+}
+
+// See issue #38412.
+fn skip_stability_check_due_to_privacy(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+    if tcx.def_kind(def_id) == DefKind::TyParam {
+        // Have no visibility, considered public for the purpose of this check.
+        return false;
+    }
+    match tcx.visibility(def_id) {
+        // Must check stability for `pub` items.
+        ty::Visibility::Public => false,
+
+        // These are not visible outside crate; therefore
+        // stability markers are irrelevant, if even present.
+        ty::Visibility::Restricted(..) | ty::Visibility::Invisible => true,
+    }
+}
+
+// See issue #83250.
+fn suggestion_for_allocator_api(
+    tcx: TyCtxt<'_>,
+    def_id: DefId,
+    span: Span,
+    feature: Symbol,
+) -> Option<(Span, String, String, Applicability)> {
+    if feature == sym::allocator_api {
+        if let Some(trait_) = tcx.opt_parent(def_id) {
+            if tcx.is_diagnostic_item(sym::Vec, trait_) {
+                let sm = tcx.sess.parse_sess.source_map();
+                let inner_types = sm.span_extend_to_prev_char(span, '<', true);
+                if let Ok(snippet) = sm.span_to_snippet(inner_types) {
+                    return Some((
+                        inner_types,
+                        "consider wrapping the inner types in tuple".to_string(),
+                        format!("({})", snippet),
+                        Applicability::MaybeIncorrect,
+                    ));
+                }
+            }
+        }
+    }
+    None
+}
+
+/// An override option for eval_stability.
+pub enum AllowUnstable {
+    /// Don't emit an unstable error for the item
+    Yes,
+    /// Handle the item normally
+    No,
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Evaluates the stability of an item.
+    ///
+    /// Returns `EvalResult::Allow` if the item is stable, or unstable but the corresponding
+    /// `#![feature]` has been provided. Returns `EvalResult::Deny` which describes the offending
+    /// unstable feature otherwise.
+    ///
+    /// If `id` is `Some(_)`, this function will also check if the item at `def_id` has been
+    /// deprecated. If the item is indeed deprecated, we will emit a deprecation lint attached to
+    /// `id`.
+    pub fn eval_stability(
+        self,
+        def_id: DefId,
+        id: Option<HirId>,
+        span: Span,
+        method_span: Option<Span>,
+    ) -> EvalResult {
+        self.eval_stability_allow_unstable(def_id, id, span, method_span, AllowUnstable::No)
+    }
+
+    /// Evaluates the stability of an item.
+    ///
+    /// Returns `EvalResult::Allow` if the item is stable, or unstable but the corresponding
+    /// `#![feature]` has been provided. Returns `EvalResult::Deny` which describes the offending
+    /// unstable feature otherwise.
+    ///
+    /// If `id` is `Some(_)`, this function will also check if the item at `def_id` has been
+    /// deprecated. If the item is indeed deprecated, we will emit a deprecation lint attached to
+    /// `id`.
+    ///
+    /// Pass `AllowUnstable::Yes` to `allow_unstable` to force an unstable item to be allowed. Deprecation warnings will be emitted normally.
+    pub fn eval_stability_allow_unstable(
+        self,
+        def_id: DefId,
+        id: Option<HirId>,
+        span: Span,
+        method_span: Option<Span>,
+        allow_unstable: AllowUnstable,
+    ) -> EvalResult {
+        // Deprecated attributes apply in-crate and cross-crate.
+        if let Some(id) = id {
+            if let Some(depr_entry) = self.lookup_deprecation_entry(def_id) {
+                let parent_def_id = self.hir().get_parent_item(id);
+                let skip = self
+                    .lookup_deprecation_entry(parent_def_id.to_def_id())
+                    .map_or(false, |parent_depr| parent_depr.same_origin(&depr_entry));
+
+                // #[deprecated] doesn't emit a notice if we're not on the
+                // topmost deprecation. For example, if a struct is deprecated,
+                // the use of a field won't be linted.
+                //
+                // With #![staged_api], we want to emit down the whole
+                // hierarchy.
+                let depr_attr = &depr_entry.attr;
+                if !skip || depr_attr.is_since_rustc_version {
+                    // Calculating message for lint involves calling `self.def_path_str`.
+                    // Which by default to calculate visible path will invoke expensive `visible_parent_map` query.
+                    // So we skip message calculation altogether, if lint is allowed.
+                    let is_in_effect = deprecation_in_effect(depr_attr);
+                    let lint = deprecation_lint(is_in_effect);
+                    if self.lint_level_at_node(lint, id).0 != Level::Allow {
+                        let def_path = with_no_trimmed_paths!(self.def_path_str(def_id));
+                        let def_kind = self.def_kind(def_id).descr(def_id);
+
+                        late_report_deprecation(
+                            self,
+                            &deprecation_message(
+                                is_in_effect,
+                                depr_attr.since,
+                                depr_attr.note,
+                                def_kind,
+                                &def_path,
+                            ),
+                            depr_attr.suggestion,
+                            lint,
+                            span,
+                            method_span,
+                            id,
+                            def_id,
+                        );
+                    }
+                }
+            };
+        }
+
+        let is_staged_api = self.lookup_stability(def_id.krate.as_def_id()).is_some();
+        if !is_staged_api {
+            return EvalResult::Allow;
+        }
+
+        let stability = self.lookup_stability(def_id);
+        debug!(
+            "stability: \
+                inspecting def_id={:?} span={:?} of stability={:?}",
+            def_id, span, stability
+        );
+
+        // Only the cross-crate scenario matters when checking unstable APIs
+        let cross_crate = !def_id.is_local();
+        if !cross_crate {
+            return EvalResult::Allow;
+        }
+
+        // Issue #38412: private items lack stability markers.
+        if skip_stability_check_due_to_privacy(self, def_id) {
+            return EvalResult::Allow;
+        }
+
+        match stability {
+            Some(Stability {
+                level: attr::Unstable { reason, issue, is_soft, implied_by },
+                feature,
+                ..
+            }) => {
+                if span.allows_unstable(feature) {
+                    debug!("stability: skipping span={:?} since it is internal", span);
+                    return EvalResult::Allow;
+                }
+                if self.features().active(feature) {
+                    return EvalResult::Allow;
+                }
+
+                // If this item was previously part of a now-stabilized feature which is still
+                // active (i.e. the user hasn't removed the attribute for the stabilized feature
+                // yet) then allow use of this item.
+                if let Some(implied_by) = implied_by && self.features().active(implied_by) {
+                    return EvalResult::Allow;
+                }
+
+                // When we're compiling the compiler itself we may pull in
+                // crates from crates.io, but those crates may depend on other
+                // crates also pulled in from crates.io. We want to ideally be
+                // able to compile everything without requiring upstream
+                // modifications, so in the case that this looks like a
+                // `rustc_private` crate (e.g., a compiler crate) and we also have
+                // the `-Z force-unstable-if-unmarked` flag present (we're
+                // compiling a compiler crate), then let this missing feature
+                // annotation slide.
+                if feature == sym::rustc_private && issue == NonZeroU32::new(27812) {
+                    if self.sess.opts.unstable_opts.force_unstable_if_unmarked {
+                        return EvalResult::Allow;
+                    }
+                }
+
+                if matches!(allow_unstable, AllowUnstable::Yes) {
+                    return EvalResult::Allow;
+                }
+
+                let suggestion = suggestion_for_allocator_api(self, def_id, span, feature);
+                EvalResult::Deny {
+                    feature,
+                    reason: reason.to_opt_reason(),
+                    issue,
+                    suggestion,
+                    is_soft,
+                }
+            }
+            Some(_) => {
+                // Stable APIs are always ok to call and deprecated APIs are
+                // handled by the lint emitting logic above.
+                EvalResult::Allow
+            }
+            None => EvalResult::Unmarked,
+        }
+    }
+
+    /// Checks if an item is stable or error out.
+    ///
+    /// If the item defined by `def_id` is unstable and the corresponding `#![feature]` does not
+    /// exist, emits an error.
+    ///
+    /// This function will also check if the item is deprecated.
+    /// If so, and `id` is not `None`, a deprecated lint attached to `id` will be emitted.
+    ///
+    /// Returns `true` if item is allowed aka, stable or unstable under an enabled feature.
+    pub fn check_stability(
+        self,
+        def_id: DefId,
+        id: Option<HirId>,
+        span: Span,
+        method_span: Option<Span>,
+    ) -> bool {
+        self.check_stability_allow_unstable(def_id, id, span, method_span, AllowUnstable::No)
+    }
+
+    /// Checks if an item is stable or error out.
+    ///
+    /// If the item defined by `def_id` is unstable and the corresponding `#![feature]` does not
+    /// exist, emits an error.
+    ///
+    /// This function will also check if the item is deprecated.
+    /// If so, and `id` is not `None`, a deprecated lint attached to `id` will be emitted.
+    ///
+    /// Pass `AllowUnstable::Yes` to `allow_unstable` to force an unstable item to be allowed. Deprecation warnings will be emitted normally.
+    ///
+    /// Returns `true` if item is allowed aka, stable or unstable under an enabled feature.
+    pub fn check_stability_allow_unstable(
+        self,
+        def_id: DefId,
+        id: Option<HirId>,
+        span: Span,
+        method_span: Option<Span>,
+        allow_unstable: AllowUnstable,
+    ) -> bool {
+        self.check_optional_stability(
+            def_id,
+            id,
+            span,
+            method_span,
+            allow_unstable,
+            |span, def_id| {
+                // The API could be uncallable for other reasons, for example when a private module
+                // was referenced.
+                self.sess.delay_span_bug(span, &format!("encountered unmarked API: {:?}", def_id));
+            },
+        )
+    }
+
+    /// Like `check_stability`, except that we permit items to have custom behaviour for
+    /// missing stability attributes (not necessarily just emit a `bug!`). This is necessary
+    /// for default generic parameters, which only have stability attributes if they were
+    /// added after the type on which they're defined.
+    ///
+    /// Returns `true` if item is allowed aka, stable or unstable under an enabled feature.
+    pub fn check_optional_stability(
+        self,
+        def_id: DefId,
+        id: Option<HirId>,
+        span: Span,
+        method_span: Option<Span>,
+        allow_unstable: AllowUnstable,
+        unmarked: impl FnOnce(Span, DefId),
+    ) -> bool {
+        let soft_handler = |lint, span, msg: &_| {
+            self.struct_span_lint_hir(lint, id.unwrap_or(hir::CRATE_HIR_ID), span, |lint| {
+                lint.build(msg).emit();
+            })
+        };
+        let eval_result =
+            self.eval_stability_allow_unstable(def_id, id, span, method_span, allow_unstable);
+        let is_allowed = matches!(eval_result, EvalResult::Allow);
+        match eval_result {
+            EvalResult::Allow => {}
+            EvalResult::Deny { feature, reason, issue, suggestion, is_soft } => report_unstable(
+                self.sess,
+                feature,
+                reason,
+                issue,
+                suggestion,
+                is_soft,
+                span,
+                soft_handler,
+            ),
+            EvalResult::Unmarked => unmarked(span, def_id),
+        }
+
+        is_allowed
+    }
+
+    pub fn lookup_deprecation(self, id: DefId) -> Option<Deprecation> {
+        self.lookup_deprecation_entry(id).map(|depr| depr.attr)
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/basic_blocks.rs b/compiler/rustc_middle/src/mir/basic_blocks.rs
new file mode 100644
index 000000000..78080fcd5
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/basic_blocks.rs
@@ -0,0 +1,147 @@
+use crate::mir::graph_cyclic_cache::GraphIsCyclicCache;
+use crate::mir::predecessors::{PredecessorCache, Predecessors};
+use crate::mir::switch_sources::{SwitchSourceCache, SwitchSources};
+use crate::mir::traversal::PostorderCache;
+use crate::mir::{BasicBlock, BasicBlockData, Successors, START_BLOCK};
+
+use rustc_data_structures::graph;
+use rustc_data_structures::graph::dominators::{dominators, Dominators};
+use rustc_index::vec::IndexVec;
+
+#[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable, TypeFoldable, TypeVisitable)]
+pub struct BasicBlocks<'tcx> {
+    basic_blocks: IndexVec<BasicBlock, BasicBlockData<'tcx>>,
+    predecessor_cache: PredecessorCache,
+    switch_source_cache: SwitchSourceCache,
+    is_cyclic: GraphIsCyclicCache,
+    postorder_cache: PostorderCache,
+}
+
+impl<'tcx> BasicBlocks<'tcx> {
+    #[inline]
+    pub fn new(basic_blocks: IndexVec<BasicBlock, BasicBlockData<'tcx>>) -> Self {
+        BasicBlocks {
+            basic_blocks,
+            predecessor_cache: PredecessorCache::new(),
+            switch_source_cache: SwitchSourceCache::new(),
+            is_cyclic: GraphIsCyclicCache::new(),
+            postorder_cache: PostorderCache::new(),
+        }
+    }
+
+    /// Returns true if control-flow graph contains a cycle reachable from the `START_BLOCK`.
+    #[inline]
+    pub fn is_cfg_cyclic(&self) -> bool {
+        self.is_cyclic.is_cyclic(self)
+    }
+
+    #[inline]
+    pub fn dominators(&self) -> Dominators<BasicBlock> {
+        dominators(&self)
+    }
+
+    /// Returns predecessors for each basic block.
+    #[inline]
+    pub fn predecessors(&self) -> &Predecessors {
+        self.predecessor_cache.compute(&self.basic_blocks)
+    }
+
+    /// Returns basic blocks in a postorder.
+    #[inline]
+    pub fn postorder(&self) -> &[BasicBlock] {
+        self.postorder_cache.compute(&self.basic_blocks)
+    }
+
+    /// `switch_sources()[&(target, switch)]` returns a list of switch
+    /// values that lead to a `target` block from a `switch` block.
+    #[inline]
+    pub fn switch_sources(&self) -> &SwitchSources {
+        self.switch_source_cache.compute(&self.basic_blocks)
+    }
+
+    /// Returns mutable reference to basic blocks. Invalidates CFG cache.
+    #[inline]
+    pub fn as_mut(&mut self) -> &mut IndexVec<BasicBlock, BasicBlockData<'tcx>> {
+        self.invalidate_cfg_cache();
+        &mut self.basic_blocks
+    }
+
+    /// Get mutable access to basic blocks without invalidating the CFG cache.
+    ///
+    /// By calling this method instead of e.g. [`BasicBlocks::as_mut`] you promise not to change
+    /// the CFG. This means that
+    ///
+    ///  1) The number of basic blocks remains unchanged
+    ///  2) The set of successors of each terminator remains unchanged.
+    ///  3) For each `TerminatorKind::SwitchInt`, the `targets` remains the same and the terminator
+    ///     kind is not changed.
+    ///
+    /// If any of these conditions cannot be upheld, you should call [`BasicBlocks::invalidate_cfg_cache`].
+    #[inline]
+    pub fn as_mut_preserves_cfg(&mut self) -> &mut IndexVec<BasicBlock, BasicBlockData<'tcx>> {
+        &mut self.basic_blocks
+    }
+
+    /// Invalidates cached information about the CFG.
+    ///
+    /// You will only ever need this if you have also called [`BasicBlocks::as_mut_preserves_cfg`].
+    /// All other methods that allow you to mutate the basic blocks also call this method
+    /// themselves, thereby avoiding any risk of accidentaly cache invalidation.
+    pub fn invalidate_cfg_cache(&mut self) {
+        self.predecessor_cache.invalidate();
+        self.switch_source_cache.invalidate();
+        self.is_cyclic.invalidate();
+        self.postorder_cache.invalidate();
+    }
+}
+
+impl<'tcx> std::ops::Deref for BasicBlocks<'tcx> {
+    type Target = IndexVec<BasicBlock, BasicBlockData<'tcx>>;
+
+    #[inline]
+    fn deref(&self) -> &IndexVec<BasicBlock, BasicBlockData<'tcx>> {
+        &self.basic_blocks
+    }
+}
+
+impl<'tcx> graph::DirectedGraph for BasicBlocks<'tcx> {
+    type Node = BasicBlock;
+}
+
+impl<'tcx> graph::WithNumNodes for BasicBlocks<'tcx> {
+    #[inline]
+    fn num_nodes(&self) -> usize {
+        self.basic_blocks.len()
+    }
+}
+
+impl<'tcx> graph::WithStartNode for BasicBlocks<'tcx> {
+    #[inline]
+    fn start_node(&self) -> Self::Node {
+        START_BLOCK
+    }
+}
+
+impl<'tcx> graph::WithSuccessors for BasicBlocks<'tcx> {
+    #[inline]
+    fn successors(&self, node: Self::Node) -> <Self as graph::GraphSuccessors<'_>>::Iter {
+        self.basic_blocks[node].terminator().successors()
+    }
+}
+
+impl<'a, 'b> graph::GraphSuccessors<'b> for BasicBlocks<'a> {
+    type Item = BasicBlock;
+    type Iter = Successors<'b>;
+}
+
+impl<'tcx, 'graph> graph::GraphPredecessors<'graph> for BasicBlocks<'tcx> {
+    type Item = BasicBlock;
+    type Iter = std::iter::Copied<std::slice::Iter<'graph, BasicBlock>>;
+}
+
+impl<'tcx> graph::WithPredecessors for BasicBlocks<'tcx> {
+    #[inline]
+    fn predecessors(&self, node: Self::Node) -> <Self as graph::GraphPredecessors<'_>>::Iter {
+        self.predecessors()[node].iter().copied()
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/coverage.rs b/compiler/rustc_middle/src/mir/coverage.rs
new file mode 100644
index 000000000..efa946452
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/coverage.rs
@@ -0,0 +1,186 @@
+//! Metadata from source code coverage analysis and instrumentation.
+
+use rustc_macros::HashStable;
+use rustc_span::Symbol;
+
+use std::cmp::Ord;
+use std::fmt::{self, Debug, Formatter};
+
+rustc_index::newtype_index! {
+    /// An ExpressionOperandId value is assigned directly from either a
+    /// CounterValueReference.as_u32() (which ascend from 1) or an ExpressionOperandId.as_u32()
+    /// (which _*descend*_ from u32::MAX). Id value `0` (zero) represents a virtual counter with a
+    /// constant value of `0`.
+    pub struct ExpressionOperandId {
+        derive [HashStable]
+        DEBUG_FORMAT = "ExpressionOperandId({})",
+        MAX = 0xFFFF_FFFF,
+    }
+}
+
+impl ExpressionOperandId {
+    /// An expression operand for a "zero counter", as described in the following references:
+    ///
+    /// * <https://github.com/rust-lang/llvm-project/blob/rustc/13.0-2021-09-30/llvm/docs/CoverageMappingFormat.rst#counter>
+    /// * <https://github.com/rust-lang/llvm-project/blob/rustc/13.0-2021-09-30/llvm/docs/CoverageMappingFormat.rst#tag>
+    /// * <https://github.com/rust-lang/llvm-project/blob/rustc/13.0-2021-09-30/llvm/docs/CoverageMappingFormat.rst#counter-expressions>
+    ///
+    /// This operand can be used to count two or more separate code regions with a single counter,
+    /// if they run sequentially with no branches, by injecting the `Counter` in a `BasicBlock` for
+    /// one of the code regions, and inserting `CounterExpression`s ("add ZERO to the counter") in
+    /// the coverage map for the other code regions.
+    pub const ZERO: Self = Self::from_u32(0);
+}
+
+rustc_index::newtype_index! {
+    pub struct CounterValueReference {
+        derive [HashStable]
+        DEBUG_FORMAT = "CounterValueReference({})",
+        MAX = 0xFFFF_FFFF,
+    }
+}
+
+impl CounterValueReference {
+    /// Counters start at 1 to reserve 0 for ExpressionOperandId::ZERO.
+    pub const START: Self = Self::from_u32(1);
+
+    /// Returns explicitly-requested zero-based version of the counter id, used
+    /// during codegen. LLVM expects zero-based indexes.
+    pub fn zero_based_index(self) -> u32 {
+        let one_based_index = self.as_u32();
+        debug_assert!(one_based_index > 0);
+        one_based_index - 1
+    }
+}
+
+rustc_index::newtype_index! {
+    /// InjectedExpressionId.as_u32() converts to ExpressionOperandId.as_u32()
+    ///
+    /// Values descend from u32::MAX.
+    pub struct InjectedExpressionId {
+        derive [HashStable]
+        DEBUG_FORMAT = "InjectedExpressionId({})",
+        MAX = 0xFFFF_FFFF,
+    }
+}
+
+rustc_index::newtype_index! {
+    /// InjectedExpressionIndex.as_u32() translates to u32::MAX - ExpressionOperandId.as_u32()
+    ///
+    /// Values ascend from 0.
+    pub struct InjectedExpressionIndex {
+        derive [HashStable]
+        DEBUG_FORMAT = "InjectedExpressionIndex({})",
+        MAX = 0xFFFF_FFFF,
+    }
+}
+
+rustc_index::newtype_index! {
+    /// MappedExpressionIndex values ascend from zero, and are recalculated indexes based on their
+    /// array position in the LLVM coverage map "Expressions" array, which is assembled during the
+    /// "mapgen" process. They cannot be computed algorithmically, from the other `newtype_index`s.
+    pub struct MappedExpressionIndex {
+        derive [HashStable]
+        DEBUG_FORMAT = "MappedExpressionIndex({})",
+        MAX = 0xFFFF_FFFF,
+    }
+}
+
+impl From<CounterValueReference> for ExpressionOperandId {
+    #[inline]
+    fn from(v: CounterValueReference) -> ExpressionOperandId {
+        ExpressionOperandId::from(v.as_u32())
+    }
+}
+
+impl From<InjectedExpressionId> for ExpressionOperandId {
+    #[inline]
+    fn from(v: InjectedExpressionId) -> ExpressionOperandId {
+        ExpressionOperandId::from(v.as_u32())
+    }
+}
+
+#[derive(Clone, PartialEq, TyEncodable, TyDecodable, Hash, HashStable, TypeFoldable, TypeVisitable)]
+pub enum CoverageKind {
+    Counter {
+        function_source_hash: u64,
+        id: CounterValueReference,
+    },
+    Expression {
+        id: InjectedExpressionId,
+        lhs: ExpressionOperandId,
+        op: Op,
+        rhs: ExpressionOperandId,
+    },
+    Unreachable,
+}
+
+impl CoverageKind {
+    pub fn as_operand_id(&self) -> ExpressionOperandId {
+        use CoverageKind::*;
+        match *self {
+            Counter { id, .. } => ExpressionOperandId::from(id),
+            Expression { id, .. } => ExpressionOperandId::from(id),
+            Unreachable => bug!("Unreachable coverage cannot be part of an expression"),
+        }
+    }
+
+    pub fn is_expression(&self) -> bool {
+        matches!(self, Self::Expression { .. })
+    }
+}
+
+impl Debug for CoverageKind {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        use CoverageKind::*;
+        match self {
+            Counter { id, .. } => write!(fmt, "Counter({:?})", id.index()),
+            Expression { id, lhs, op, rhs } => write!(
+                fmt,
+                "Expression({:?}) = {} {} {}",
+                id.index(),
+                lhs.index(),
+                if *op == Op::Add { "+" } else { "-" },
+                rhs.index(),
+            ),
+            Unreachable => write!(fmt, "Unreachable"),
+        }
+    }
+}
+
+#[derive(Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct CodeRegion {
+    pub file_name: Symbol,
+    pub start_line: u32,
+    pub start_col: u32,
+    pub end_line: u32,
+    pub end_col: u32,
+}
+
+impl Debug for CodeRegion {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        write!(
+            fmt,
+            "{}:{}:{} - {}:{}",
+            self.file_name, self.start_line, self.start_col, self.end_line, self.end_col
+        )
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub enum Op {
+    Subtract,
+    Add,
+}
+
+impl Op {
+    pub fn is_add(&self) -> bool {
+        matches!(self, Self::Add)
+    }
+
+    pub fn is_subtract(&self) -> bool {
+        matches!(self, Self::Subtract)
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/generic_graph.rs b/compiler/rustc_middle/src/mir/generic_graph.rs
new file mode 100644
index 000000000..f3621cd99
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/generic_graph.rs
@@ -0,0 +1,69 @@
+use gsgdt::{Edge, Graph, Node, NodeStyle};
+use rustc_hir::def_id::DefId;
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+
+/// Convert an MIR function into a gsgdt Graph
+pub fn mir_fn_to_generic_graph<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'_>) -> Graph {
+    let def_id = body.source.def_id();
+    let def_name = graphviz_safe_def_name(def_id);
+    let graph_name = format!("Mir_{}", def_name);
+    let dark_mode = tcx.sess.opts.unstable_opts.graphviz_dark_mode;
+
+    // Nodes
+    let nodes: Vec<Node> = body
+        .basic_blocks()
+        .iter_enumerated()
+        .map(|(block, _)| bb_to_graph_node(block, body, dark_mode))
+        .collect();
+
+    // Edges
+    let mut edges = Vec::new();
+    for (source, _) in body.basic_blocks().iter_enumerated() {
+        let def_id = body.source.def_id();
+        let terminator = body[source].terminator();
+        let labels = terminator.kind.fmt_successor_labels();
+
+        for (target, label) in terminator.successors().zip(labels) {
+            let src = node(def_id, source);
+            let trg = node(def_id, target);
+            edges.push(Edge::new(src, trg, label.to_string()));
+        }
+    }
+
+    Graph::new(graph_name, nodes, edges)
+}
+
+fn bb_to_graph_node(block: BasicBlock, body: &Body<'_>, dark_mode: bool) -> Node {
+    let def_id = body.source.def_id();
+    let data = &body[block];
+    let label = node(def_id, block);
+
+    let (title, bgcolor) = if data.is_cleanup {
+        let color = if dark_mode { "royalblue" } else { "lightblue" };
+        (format!("{} (cleanup)", block.index()), color)
+    } else {
+        let color = if dark_mode { "dimgray" } else { "gray" };
+        (format!("{}", block.index()), color)
+    };
+
+    let style = NodeStyle { title_bg: Some(bgcolor.to_owned()), ..Default::default() };
+    let mut stmts: Vec<String> = data.statements.iter().map(|x| format!("{:?}", x)).collect();
+
+    // add the terminator to the stmts, gsgdt can print it out separately
+    let mut terminator_head = String::new();
+    data.terminator().kind.fmt_head(&mut terminator_head).unwrap();
+    stmts.push(terminator_head);
+
+    Node::new(stmts, label, title, style)
+}
+
+// Must match `[0-9A-Za-z_]*`. This does not appear in the rendered graph, so
+// it does not have to be user friendly.
+pub fn graphviz_safe_def_name(def_id: DefId) -> String {
+    format!("{}_{}", def_id.krate.index(), def_id.index.index(),)
+}
+
+fn node(def_id: DefId, block: BasicBlock) -> String {
+    format!("bb{}__{}", block.index(), graphviz_safe_def_name(def_id))
+}
diff --git a/compiler/rustc_middle/src/mir/generic_graphviz.rs b/compiler/rustc_middle/src/mir/generic_graphviz.rs
new file mode 100644
index 000000000..11ac45943
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/generic_graphviz.rs
@@ -0,0 +1,173 @@
+use rustc_data_structures::graph::{self, iterate};
+use rustc_graphviz as dot;
+use rustc_middle::ty::TyCtxt;
+use std::io::{self, Write};
+
+pub struct GraphvizWriter<
+    'a,
+    G: graph::DirectedGraph + graph::WithSuccessors + graph::WithStartNode + graph::WithNumNodes,
+    NodeContentFn: Fn(<G as graph::DirectedGraph>::Node) -> Vec<String>,
+    EdgeLabelsFn: Fn(<G as graph::DirectedGraph>::Node) -> Vec<String>,
+> {
+    graph: &'a G,
+    is_subgraph: bool,
+    graphviz_name: String,
+    graph_label: Option<String>,
+    node_content_fn: NodeContentFn,
+    edge_labels_fn: EdgeLabelsFn,
+}
+
+impl<
+    'a,
+    G: graph::DirectedGraph + graph::WithSuccessors + graph::WithStartNode + graph::WithNumNodes,
+    NodeContentFn: Fn(<G as graph::DirectedGraph>::Node) -> Vec<String>,
+    EdgeLabelsFn: Fn(<G as graph::DirectedGraph>::Node) -> Vec<String>,
+> GraphvizWriter<'a, G, NodeContentFn, EdgeLabelsFn>
+{
+    pub fn new(
+        graph: &'a G,
+        graphviz_name: &str,
+        node_content_fn: NodeContentFn,
+        edge_labels_fn: EdgeLabelsFn,
+    ) -> Self {
+        Self {
+            graph,
+            is_subgraph: false,
+            graphviz_name: graphviz_name.to_owned(),
+            graph_label: None,
+            node_content_fn,
+            edge_labels_fn,
+        }
+    }
+
+    pub fn set_graph_label(&mut self, graph_label: &str) {
+        self.graph_label = Some(graph_label.to_owned());
+    }
+
+    /// Write a graphviz DOT of the graph
+    pub fn write_graphviz<'tcx, W>(&self, tcx: TyCtxt<'tcx>, w: &mut W) -> io::Result<()>
+    where
+        W: Write,
+    {
+        let kind = if self.is_subgraph { "subgraph" } else { "digraph" };
+        let cluster = if self.is_subgraph { "cluster_" } else { "" }; // Print border around graph
+        // FIXME(richkadel): If/when migrating the MIR graphviz to this generic implementation,
+        // prepend "Mir_" to the graphviz_safe_def_name(def_id)
+        writeln!(w, "{} {}{} {{", kind, cluster, self.graphviz_name)?;
+
+        // Global graph properties
+        let font = format!(r#"fontname="{}""#, tcx.sess.opts.unstable_opts.graphviz_font);
+        let mut graph_attrs = vec![&font[..]];
+        let mut content_attrs = vec![&font[..]];
+
+        let dark_mode = tcx.sess.opts.unstable_opts.graphviz_dark_mode;
+        if dark_mode {
+            graph_attrs.push(r#"bgcolor="black""#);
+            graph_attrs.push(r#"fontcolor="white""#);
+            content_attrs.push(r#"color="white""#);
+            content_attrs.push(r#"fontcolor="white""#);
+        }
+
+        writeln!(w, r#"    graph [{}];"#, graph_attrs.join(" "))?;
+        let content_attrs_str = content_attrs.join(" ");
+        writeln!(w, r#"    node [{}];"#, content_attrs_str)?;
+        writeln!(w, r#"    edge [{}];"#, content_attrs_str)?;
+
+        // Graph label
+        if let Some(graph_label) = &self.graph_label {
+            self.write_graph_label(graph_label, w)?;
+        }
+
+        // Nodes
+        for node in iterate::post_order_from(self.graph, self.graph.start_node()) {
+            self.write_node(node, dark_mode, w)?;
+        }
+
+        // Edges
+        for source in iterate::post_order_from(self.graph, self.graph.start_node()) {
+            self.write_edges(source, w)?;
+        }
+        writeln!(w, "}}")
+    }
+
+    /// Write a graphviz DOT node for the given node.
+    pub fn write_node<W>(&self, node: G::Node, dark_mode: bool, w: &mut W) -> io::Result<()>
+    where
+        W: Write,
+    {
+        // Start a new node with the label to follow, in one of DOT's pseudo-HTML tables.
+        write!(w, r#"    {} [shape="none", label=<"#, self.node(node))?;
+
+        write!(w, r#"<table border="0" cellborder="1" cellspacing="0">"#)?;
+
+        // FIXME(richkadel): If/when migrating the MIR graphviz to this generic implementation,
+        // we need generic way to know if node header should have a different color. For example,
+        // for MIR:
+        //
+        // let (blk, bgcolor) = if data.is_cleanup {
+        //     let color = if dark_mode { "royalblue" } else { "lightblue" };
+        //     (format!("{:?} (cleanup)", node), color)
+        // } else {
+        //     let color = if dark_mode { "dimgray" } else { "gray" };
+        //     (format!("{:?}", node), color)
+        // };
+        let color = if dark_mode { "dimgray" } else { "gray" };
+        let (blk, bgcolor) = (format!("{:?}", node), color);
+        write!(
+            w,
+            r#"<tr><td bgcolor="{bgcolor}" {attrs} colspan="{colspan}">{blk}</td></tr>"#,
+            attrs = r#"align="center""#,
+            colspan = 1,
+            blk = blk,
+            bgcolor = bgcolor
+        )?;
+
+        for section in (self.node_content_fn)(node) {
+            write!(
+                w,
+                r#"<tr><td align="left" balign="left">{}</td></tr>"#,
+                dot::escape_html(&section).replace('\n', "<br/>")
+            )?;
+        }
+
+        // Close the table
+        write!(w, "</table>")?;
+
+        // Close the node label and the node itself.
+        writeln!(w, ">];")
+    }
+
+    /// Write graphviz DOT edges with labels between the given node and all of its successors.
+    fn write_edges<W>(&self, source: G::Node, w: &mut W) -> io::Result<()>
+    where
+        W: Write,
+    {
+        let edge_labels = (self.edge_labels_fn)(source);
+        for (index, target) in self.graph.successors(source).enumerate() {
+            let src = self.node(source);
+            let trg = self.node(target);
+            let escaped_edge_label = if let Some(edge_label) = edge_labels.get(index) {
+                dot::escape_html(edge_label).replace('\n', r#"<br align="left"/>"#)
+            } else {
+                "".to_owned()
+            };
+            writeln!(w, r#"    {} -> {} [label=<{}>];"#, src, trg, escaped_edge_label)?;
+        }
+        Ok(())
+    }
+
+    /// Write the graphviz DOT label for the overall graph. This is essentially a block of text that
+    /// will appear below the graph.
+    fn write_graph_label<W>(&self, label: &str, w: &mut W) -> io::Result<()>
+    where
+        W: Write,
+    {
+        let lines = label.split('\n').map(|s| dot::escape_html(s)).collect::<Vec<_>>();
+        let escaped_label = lines.join(r#"<br align="left"/>"#);
+        writeln!(w, r#"    label=<<br/><br/>{}<br align="left"/><br/><br/><br/>>;"#, escaped_label)
+    }
+
+    fn node(&self, node: G::Node) -> String {
+        format!("{:?}__{}", node, self.graphviz_name)
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/graph_cyclic_cache.rs b/compiler/rustc_middle/src/mir/graph_cyclic_cache.rs
new file mode 100644
index 000000000..f97bf2883
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/graph_cyclic_cache.rs
@@ -0,0 +1,63 @@
+use rustc_data_structures::graph::{
+    self, DirectedGraph, WithNumNodes, WithStartNode, WithSuccessors,
+};
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::OnceCell;
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+
+/// Helper type to cache the result of `graph::is_cyclic`.
+#[derive(Clone, Debug)]
+pub(super) struct GraphIsCyclicCache {
+    cache: OnceCell<bool>,
+}
+
+impl GraphIsCyclicCache {
+    #[inline]
+    pub(super) fn new() -> Self {
+        GraphIsCyclicCache { cache: OnceCell::new() }
+    }
+
+    pub(super) fn is_cyclic<G>(&self, graph: &G) -> bool
+    where
+        G: ?Sized + DirectedGraph + WithStartNode + WithSuccessors + WithNumNodes,
+    {
+        *self.cache.get_or_init(|| graph::is_cyclic(graph))
+    }
+
+    /// Invalidates the cache.
+    #[inline]
+    pub(super) fn invalidate(&mut self) {
+        // Invalidating the cache requires mutating the MIR, which in turn requires a unique
+        // reference (`&mut`) to the `mir::Body`. Because of this, we can assume that all
+        // callers of `invalidate` have a unique reference to the MIR and thus to the
+        // cache. This means we never need to do synchronization when `invalidate` is called,
+        // we can simply reinitialize the `OnceCell`.
+        self.cache = OnceCell::new();
+    }
+}
+
+impl<S: Encoder> Encodable<S> for GraphIsCyclicCache {
+    #[inline]
+    fn encode(&self, s: &mut S) {
+        Encodable::encode(&(), s);
+    }
+}
+
+impl<D: Decoder> Decodable<D> for GraphIsCyclicCache {
+    #[inline]
+    fn decode(d: &mut D) -> Self {
+        let () = Decodable::decode(d);
+        Self::new()
+    }
+}
+
+impl<CTX> HashStable<CTX> for GraphIsCyclicCache {
+    #[inline]
+    fn hash_stable(&self, _: &mut CTX, _: &mut StableHasher) {
+        // do nothing
+    }
+}
+
+TrivialTypeTraversalAndLiftImpls! {
+    GraphIsCyclicCache,
+}
diff --git a/compiler/rustc_middle/src/mir/graphviz.rs b/compiler/rustc_middle/src/mir/graphviz.rs
new file mode 100644
index 000000000..5de56dad0
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/graphviz.rs
@@ -0,0 +1,134 @@
+use gsgdt::GraphvizSettings;
+use rustc_graphviz as dot;
+use rustc_hir::def_id::DefId;
+use rustc_middle::mir::*;
+use rustc_middle::ty::{self, TyCtxt};
+use std::fmt::Debug;
+use std::io::{self, Write};
+
+use super::generic_graph::mir_fn_to_generic_graph;
+use super::pretty::dump_mir_def_ids;
+
+/// Write a graphviz DOT graph of a list of MIRs.
+pub fn write_mir_graphviz<W>(tcx: TyCtxt<'_>, single: Option<DefId>, w: &mut W) -> io::Result<()>
+where
+    W: Write,
+{
+    let def_ids = dump_mir_def_ids(tcx, single);
+
+    let mirs =
+        def_ids
+            .iter()
+            .flat_map(|def_id| {
+                if tcx.is_const_fn_raw(*def_id) {
+                    vec![tcx.optimized_mir(*def_id), tcx.mir_for_ctfe(*def_id)]
+                } else {
+                    vec![tcx.instance_mir(ty::InstanceDef::Item(ty::WithOptConstParam::unknown(
+                        *def_id,
+                    )))]
+                }
+            })
+            .collect::<Vec<_>>();
+
+    let use_subgraphs = mirs.len() > 1;
+    if use_subgraphs {
+        writeln!(w, "digraph __crate__ {{")?;
+    }
+
+    for mir in mirs {
+        write_mir_fn_graphviz(tcx, mir, use_subgraphs, w)?;
+    }
+
+    if use_subgraphs {
+        writeln!(w, "}}")?;
+    }
+
+    Ok(())
+}
+
+/// Write a graphviz DOT graph of the MIR.
+pub fn write_mir_fn_graphviz<'tcx, W>(
+    tcx: TyCtxt<'tcx>,
+    body: &Body<'_>,
+    subgraph: bool,
+    w: &mut W,
+) -> io::Result<()>
+where
+    W: Write,
+{
+    // Global graph properties
+    let font = format!(r#"fontname="{}""#, tcx.sess.opts.unstable_opts.graphviz_font);
+    let mut graph_attrs = vec![&font[..]];
+    let mut content_attrs = vec![&font[..]];
+
+    let dark_mode = tcx.sess.opts.unstable_opts.graphviz_dark_mode;
+    if dark_mode {
+        graph_attrs.push(r#"bgcolor="black""#);
+        graph_attrs.push(r#"fontcolor="white""#);
+        content_attrs.push(r#"color="white""#);
+        content_attrs.push(r#"fontcolor="white""#);
+    }
+
+    // Graph label
+    let mut label = String::from("");
+    // FIXME: remove this unwrap
+    write_graph_label(tcx, body, &mut label).unwrap();
+    let g = mir_fn_to_generic_graph(tcx, body);
+    let settings = GraphvizSettings {
+        graph_attrs: Some(graph_attrs.join(" ")),
+        node_attrs: Some(content_attrs.join(" ")),
+        edge_attrs: Some(content_attrs.join(" ")),
+        graph_label: Some(label),
+    };
+    g.to_dot(w, &settings, subgraph)
+}
+
+/// Write the graphviz DOT label for the overall graph. This is essentially a block of text that
+/// will appear below the graph, showing the type of the `fn` this MIR represents and the types of
+/// all the variables and temporaries.
+fn write_graph_label<'tcx, W: std::fmt::Write>(
+    tcx: TyCtxt<'tcx>,
+    body: &Body<'_>,
+    w: &mut W,
+) -> std::fmt::Result {
+    let def_id = body.source.def_id();
+
+    write!(w, "fn {}(", dot::escape_html(&tcx.def_path_str(def_id)))?;
+
+    // fn argument types.
+    for (i, arg) in body.args_iter().enumerate() {
+        if i > 0 {
+            write!(w, ", ")?;
+        }
+        write!(w, "{:?}: {}", Place::from(arg), escape(&body.local_decls[arg].ty))?;
+    }
+
+    write!(w, ") -&gt; {}", escape(&body.return_ty()))?;
+    write!(w, r#"<br align="left"/>"#)?;
+
+    for local in body.vars_and_temps_iter() {
+        let decl = &body.local_decls[local];
+
+        write!(w, "let ")?;
+        if decl.mutability == Mutability::Mut {
+            write!(w, "mut ")?;
+        }
+
+        write!(w, r#"{:?}: {};<br align="left"/>"#, Place::from(local), escape(&decl.ty))?;
+    }
+
+    for var_debug_info in &body.var_debug_info {
+        write!(
+            w,
+            r#"debug {} =&gt; {};<br align="left"/>"#,
+            var_debug_info.name,
+            escape(&var_debug_info.value),
+        )?;
+    }
+
+    Ok(())
+}
+
+fn escape<T: Debug>(t: &T) -> String {
+    dot::escape_html(&format!("{:?}", t))
+}
diff --git a/compiler/rustc_middle/src/mir/interpret/allocation.rs b/compiler/rustc_middle/src/mir/interpret/allocation.rs
new file mode 100644
index 000000000..db7e0fb8a
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/interpret/allocation.rs
@@ -0,0 +1,1300 @@
+//! The virtual memory representation of the MIR interpreter.
+
+use std::borrow::Cow;
+use std::convert::{TryFrom, TryInto};
+use std::fmt;
+use std::hash;
+use std::iter;
+use std::ops::{Deref, Range};
+use std::ptr;
+
+use rustc_ast::Mutability;
+use rustc_data_structures::intern::Interned;
+use rustc_data_structures::sorted_map::SortedMap;
+use rustc_span::DUMMY_SP;
+use rustc_target::abi::{Align, HasDataLayout, Size};
+
+use super::{
+    read_target_uint, write_target_uint, AllocId, InterpError, InterpResult, Pointer, Provenance,
+    ResourceExhaustionInfo, Scalar, ScalarMaybeUninit, ScalarSizeMismatch, UndefinedBehaviorInfo,
+    UninitBytesAccess, UnsupportedOpInfo,
+};
+use crate::ty;
+
+/// This type represents an Allocation in the Miri/CTFE core engine.
+///
+/// Its public API is rather low-level, working directly with allocation offsets and a custom error
+/// type to account for the lack of an AllocId on this level. The Miri/CTFE core engine `memory`
+/// module provides higher-level access.
+// Note: for performance reasons when interning, some of the `Allocation` fields can be partially
+// hashed. (see the `Hash` impl below for more details), so the impl is not derived.
+#[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable)]
+#[derive(HashStable)]
+pub struct Allocation<Prov = AllocId, Extra = ()> {
+    /// The actual bytes of the allocation.
+    /// Note that the bytes of a pointer represent the offset of the pointer.
+    bytes: Box<[u8]>,
+    /// Maps from byte addresses to extra data for each pointer.
+    /// Only the first byte of a pointer is inserted into the map; i.e.,
+    /// every entry in this map applies to `pointer_size` consecutive bytes starting
+    /// at the given offset.
+    relocations: Relocations<Prov>,
+    /// Denotes which part of this allocation is initialized.
+    init_mask: InitMask,
+    /// The alignment of the allocation to detect unaligned reads.
+    /// (`Align` guarantees that this is a power of two.)
+    pub align: Align,
+    /// `true` if the allocation is mutable.
+    /// Also used by codegen to determine if a static should be put into mutable memory,
+    /// which happens for `static mut` and `static` with interior mutability.
+    pub mutability: Mutability,
+    /// Extra state for the machine.
+    pub extra: Extra,
+}
+
+/// This is the maximum size we will hash at a time, when interning an `Allocation` and its
+/// `InitMask`. Note, we hash that amount of bytes twice: at the start, and at the end of a buffer.
+/// Used when these two structures are large: we only partially hash the larger fields in that
+/// situation. See the comment at the top of their respective `Hash` impl for more details.
+const MAX_BYTES_TO_HASH: usize = 64;
+
+/// This is the maximum size (in bytes) for which a buffer will be fully hashed, when interning.
+/// Otherwise, it will be partially hashed in 2 slices, requiring at least 2 `MAX_BYTES_TO_HASH`
+/// bytes.
+const MAX_HASHED_BUFFER_LEN: usize = 2 * MAX_BYTES_TO_HASH;
+
+// Const allocations are only hashed for interning. However, they can be large, making the hashing
+// expensive especially since it uses `FxHash`: it's better suited to short keys, not potentially
+// big buffers like the actual bytes of allocation. We can partially hash some fields when they're
+// large.
+impl hash::Hash for Allocation {
+    fn hash<H: hash::Hasher>(&self, state: &mut H) {
+        // Partially hash the `bytes` buffer when it is large. To limit collisions with common
+        // prefixes and suffixes, we hash the length and some slices of the buffer.
+        let byte_count = self.bytes.len();
+        if byte_count > MAX_HASHED_BUFFER_LEN {
+            // Hash the buffer's length.
+            byte_count.hash(state);
+
+            // And its head and tail.
+            self.bytes[..MAX_BYTES_TO_HASH].hash(state);
+            self.bytes[byte_count - MAX_BYTES_TO_HASH..].hash(state);
+        } else {
+            self.bytes.hash(state);
+        }
+
+        // Hash the other fields as usual.
+        self.relocations.hash(state);
+        self.init_mask.hash(state);
+        self.align.hash(state);
+        self.mutability.hash(state);
+        self.extra.hash(state);
+    }
+}
+
+/// Interned types generally have an `Outer` type and an `Inner` type, where
+/// `Outer` is a newtype around `Interned<Inner>`, and all the operations are
+/// done on `Outer`, because all occurrences are interned. E.g. `Ty` is an
+/// outer type and `TyS` is its inner type.
+///
+/// Here things are different because only const allocations are interned. This
+/// means that both the inner type (`Allocation`) and the outer type
+/// (`ConstAllocation`) are used quite a bit.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, HashStable)]
+#[rustc_pass_by_value]
+pub struct ConstAllocation<'tcx, Prov = AllocId, Extra = ()>(
+    pub Interned<'tcx, Allocation<Prov, Extra>>,
+);
+
+impl<'tcx> fmt::Debug for ConstAllocation<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // This matches how `Allocation` is printed. We print it like this to
+        // avoid having to update expected output in a lot of tests.
+        write!(f, "{:?}", self.inner())
+    }
+}
+
+impl<'tcx, Prov, Extra> ConstAllocation<'tcx, Prov, Extra> {
+    pub fn inner(self) -> &'tcx Allocation<Prov, Extra> {
+        self.0.0
+    }
+}
+
+/// We have our own error type that does not know about the `AllocId`; that information
+/// is added when converting to `InterpError`.
+#[derive(Debug)]
+pub enum AllocError {
+    /// A scalar had the wrong size.
+    ScalarSizeMismatch(ScalarSizeMismatch),
+    /// Encountered a pointer where we needed raw bytes.
+    ReadPointerAsBytes,
+    /// Partially overwriting a pointer.
+    PartialPointerOverwrite(Size),
+    /// Using uninitialized data where it is not allowed.
+    InvalidUninitBytes(Option<UninitBytesAccess>),
+}
+pub type AllocResult<T = ()> = Result<T, AllocError>;
+
+impl From<ScalarSizeMismatch> for AllocError {
+    fn from(s: ScalarSizeMismatch) -> Self {
+        AllocError::ScalarSizeMismatch(s)
+    }
+}
+
+impl AllocError {
+    pub fn to_interp_error<'tcx>(self, alloc_id: AllocId) -> InterpError<'tcx> {
+        use AllocError::*;
+        match self {
+            ScalarSizeMismatch(s) => {
+                InterpError::UndefinedBehavior(UndefinedBehaviorInfo::ScalarSizeMismatch(s))
+            }
+            ReadPointerAsBytes => InterpError::Unsupported(UnsupportedOpInfo::ReadPointerAsBytes),
+            PartialPointerOverwrite(offset) => InterpError::Unsupported(
+                UnsupportedOpInfo::PartialPointerOverwrite(Pointer::new(alloc_id, offset)),
+            ),
+            InvalidUninitBytes(info) => InterpError::UndefinedBehavior(
+                UndefinedBehaviorInfo::InvalidUninitBytes(info.map(|b| (alloc_id, b))),
+            ),
+        }
+    }
+}
+
+/// The information that makes up a memory access: offset and size.
+#[derive(Copy, Clone)]
+pub struct AllocRange {
+    pub start: Size,
+    pub size: Size,
+}
+
+impl fmt::Debug for AllocRange {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "[{:#x}..{:#x}]", self.start.bytes(), self.end().bytes())
+    }
+}
+
+/// Free-starting constructor for less syntactic overhead.
+#[inline(always)]
+pub fn alloc_range(start: Size, size: Size) -> AllocRange {
+    AllocRange { start, size }
+}
+
+impl AllocRange {
+    #[inline]
+    pub fn from(r: Range<Size>) -> Self {
+        alloc_range(r.start, r.end - r.start) // `Size` subtraction (overflow-checked)
+    }
+
+    #[inline(always)]
+    pub fn end(self) -> Size {
+        self.start + self.size // This does overflow checking.
+    }
+
+    /// Returns the `subrange` within this range; panics if it is not a subrange.
+    #[inline]
+    pub fn subrange(self, subrange: AllocRange) -> AllocRange {
+        let sub_start = self.start + subrange.start;
+        let range = alloc_range(sub_start, subrange.size);
+        assert!(range.end() <= self.end(), "access outside the bounds for given AllocRange");
+        range
+    }
+}
+
+// The constructors are all without extra; the extra gets added by a machine hook later.
+impl<Prov> Allocation<Prov> {
+    /// Creates an allocation initialized by the given bytes
+    pub fn from_bytes<'a>(
+        slice: impl Into<Cow<'a, [u8]>>,
+        align: Align,
+        mutability: Mutability,
+    ) -> Self {
+        let bytes = Box::<[u8]>::from(slice.into());
+        let size = Size::from_bytes(bytes.len());
+        Self {
+            bytes,
+            relocations: Relocations::new(),
+            init_mask: InitMask::new(size, true),
+            align,
+            mutability,
+            extra: (),
+        }
+    }
+
+    pub fn from_bytes_byte_aligned_immutable<'a>(slice: impl Into<Cow<'a, [u8]>>) -> Self {
+        Allocation::from_bytes(slice, Align::ONE, Mutability::Not)
+    }
+
+    /// Try to create an Allocation of `size` bytes, failing if there is not enough memory
+    /// available to the compiler to do so.
+    ///
+    /// If `panic_on_fail` is true, this will never return `Err`.
+    pub fn uninit<'tcx>(size: Size, align: Align, panic_on_fail: bool) -> InterpResult<'tcx, Self> {
+        let bytes = Box::<[u8]>::try_new_zeroed_slice(size.bytes_usize()).map_err(|_| {
+            // This results in an error that can happen non-deterministically, since the memory
+            // available to the compiler can change between runs. Normally queries are always
+            // deterministic. However, we can be non-deterministic here because all uses of const
+            // evaluation (including ConstProp!) will make compilation fail (via hard error
+            // or ICE) upon encountering a `MemoryExhausted` error.
+            if panic_on_fail {
+                panic!("Allocation::uninit called with panic_on_fail had allocation failure")
+            }
+            ty::tls::with(|tcx| {
+                tcx.sess.delay_span_bug(DUMMY_SP, "exhausted memory during interpretation")
+            });
+            InterpError::ResourceExhaustion(ResourceExhaustionInfo::MemoryExhausted)
+        })?;
+        // SAFETY: the box was zero-allocated, which is a valid initial value for Box<[u8]>
+        let bytes = unsafe { bytes.assume_init() };
+        Ok(Allocation {
+            bytes,
+            relocations: Relocations::new(),
+            init_mask: InitMask::new(size, false),
+            align,
+            mutability: Mutability::Mut,
+            extra: (),
+        })
+    }
+}
+
+impl Allocation {
+    /// Adjust allocation from the ones in tcx to a custom Machine instance
+    /// with a different Provenance and Extra type.
+    pub fn adjust_from_tcx<Prov, Extra, Err>(
+        self,
+        cx: &impl HasDataLayout,
+        extra: Extra,
+        mut adjust_ptr: impl FnMut(Pointer<AllocId>) -> Result<Pointer<Prov>, Err>,
+    ) -> Result<Allocation<Prov, Extra>, Err> {
+        // Compute new pointer provenance, which also adjusts the bytes.
+        let mut bytes = self.bytes;
+        let mut new_relocations = Vec::with_capacity(self.relocations.0.len());
+        let ptr_size = cx.data_layout().pointer_size.bytes_usize();
+        let endian = cx.data_layout().endian;
+        for &(offset, alloc_id) in self.relocations.iter() {
+            let idx = offset.bytes_usize();
+            let ptr_bytes = &mut bytes[idx..idx + ptr_size];
+            let bits = read_target_uint(endian, ptr_bytes).unwrap();
+            let (ptr_prov, ptr_offset) =
+                adjust_ptr(Pointer::new(alloc_id, Size::from_bytes(bits)))?.into_parts();
+            write_target_uint(endian, ptr_bytes, ptr_offset.bytes().into()).unwrap();
+            new_relocations.push((offset, ptr_prov));
+        }
+        // Create allocation.
+        Ok(Allocation {
+            bytes,
+            relocations: Relocations::from_presorted(new_relocations),
+            init_mask: self.init_mask,
+            align: self.align,
+            mutability: self.mutability,
+            extra,
+        })
+    }
+}
+
+/// Raw accessors. Provide access to otherwise private bytes.
+impl<Prov, Extra> Allocation<Prov, Extra> {
+    pub fn len(&self) -> usize {
+        self.bytes.len()
+    }
+
+    pub fn size(&self) -> Size {
+        Size::from_bytes(self.len())
+    }
+
+    /// Looks at a slice which may describe uninitialized bytes or describe a relocation. This differs
+    /// from `get_bytes_with_uninit_and_ptr` in that it does no relocation checks (even on the
+    /// edges) at all.
+    /// This must not be used for reads affecting the interpreter execution.
+    pub fn inspect_with_uninit_and_ptr_outside_interpreter(&self, range: Range<usize>) -> &[u8] {
+        &self.bytes[range]
+    }
+
+    /// Returns the mask indicating which bytes are initialized.
+    pub fn init_mask(&self) -> &InitMask {
+        &self.init_mask
+    }
+
+    /// Returns the relocation list.
+    pub fn relocations(&self) -> &Relocations<Prov> {
+        &self.relocations
+    }
+}
+
+/// Byte accessors.
+impl<Prov: Provenance, Extra> Allocation<Prov, Extra> {
+    /// This is the entirely abstraction-violating way to just grab the raw bytes without
+    /// caring about relocations. It just deduplicates some code between `read_scalar`
+    /// and `get_bytes_internal`.
+    fn get_bytes_even_more_internal(&self, range: AllocRange) -> &[u8] {
+        &self.bytes[range.start.bytes_usize()..range.end().bytes_usize()]
+    }
+
+    /// The last argument controls whether we error out when there are uninitialized or pointer
+    /// bytes. However, we *always* error when there are relocations overlapping the edges of the
+    /// range.
+    ///
+    /// You should never call this, call `get_bytes` or `get_bytes_with_uninit_and_ptr` instead,
+    ///
+    /// This function also guarantees that the resulting pointer will remain stable
+    /// even when new allocations are pushed to the `HashMap`. `mem_copy_repeatedly` relies
+    /// on that.
+    ///
+    /// It is the caller's responsibility to check bounds and alignment beforehand.
+    fn get_bytes_internal(
+        &self,
+        cx: &impl HasDataLayout,
+        range: AllocRange,
+        check_init_and_ptr: bool,
+    ) -> AllocResult<&[u8]> {
+        if check_init_and_ptr {
+            self.check_init(range)?;
+            self.check_relocations(cx, range)?;
+        } else {
+            // We still don't want relocations on the *edges*.
+            self.check_relocation_edges(cx, range)?;
+        }
+
+        Ok(self.get_bytes_even_more_internal(range))
+    }
+
+    /// Checks that these bytes are initialized and not pointer bytes, and then return them
+    /// as a slice.
+    ///
+    /// It is the caller's responsibility to check bounds and alignment beforehand.
+    /// Most likely, you want to use the `PlaceTy` and `OperandTy`-based methods
+    /// on `InterpCx` instead.
+    #[inline]
+    pub fn get_bytes(&self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult<&[u8]> {
+        self.get_bytes_internal(cx, range, true)
+    }
+
+    /// It is the caller's responsibility to handle uninitialized and pointer bytes.
+    /// However, this still checks that there are no relocations on the *edges*.
+    ///
+    /// It is the caller's responsibility to check bounds and alignment beforehand.
+    #[inline]
+    pub fn get_bytes_with_uninit_and_ptr(
+        &self,
+        cx: &impl HasDataLayout,
+        range: AllocRange,
+    ) -> AllocResult<&[u8]> {
+        self.get_bytes_internal(cx, range, false)
+    }
+
+    /// Just calling this already marks everything as defined and removes relocations,
+    /// so be sure to actually put data there!
+    ///
+    /// It is the caller's responsibility to check bounds and alignment beforehand.
+    /// Most likely, you want to use the `PlaceTy` and `OperandTy`-based methods
+    /// on `InterpCx` instead.
+    pub fn get_bytes_mut(
+        &mut self,
+        cx: &impl HasDataLayout,
+        range: AllocRange,
+    ) -> AllocResult<&mut [u8]> {
+        self.mark_init(range, true);
+        self.clear_relocations(cx, range)?;
+
+        Ok(&mut self.bytes[range.start.bytes_usize()..range.end().bytes_usize()])
+    }
+
+    /// A raw pointer variant of `get_bytes_mut` that avoids invalidating existing aliases into this memory.
+    pub fn get_bytes_mut_ptr(
+        &mut self,
+        cx: &impl HasDataLayout,
+        range: AllocRange,
+    ) -> AllocResult<*mut [u8]> {
+        self.mark_init(range, true);
+        self.clear_relocations(cx, range)?;
+
+        assert!(range.end().bytes_usize() <= self.bytes.len()); // need to do our own bounds-check
+        let begin_ptr = self.bytes.as_mut_ptr().wrapping_add(range.start.bytes_usize());
+        let len = range.end().bytes_usize() - range.start.bytes_usize();
+        Ok(ptr::slice_from_raw_parts_mut(begin_ptr, len))
+    }
+}
+
+/// Reading and writing.
+impl<Prov: Provenance, Extra> Allocation<Prov, Extra> {
+    /// Validates that `ptr.offset` and `ptr.offset + size` do not point to the middle of a
+    /// relocation. If `allow_uninit`/`allow_ptr` is `false`, also enforces that the memory in the
+    /// given range contains no uninitialized bytes/relocations.
+    pub fn check_bytes(
+        &self,
+        cx: &impl HasDataLayout,
+        range: AllocRange,
+        allow_uninit: bool,
+        allow_ptr: bool,
+    ) -> AllocResult {
+        // Check bounds and relocations on the edges.
+        self.get_bytes_with_uninit_and_ptr(cx, range)?;
+        // Check uninit and ptr.
+        if !allow_uninit {
+            self.check_init(range)?;
+        }
+        if !allow_ptr {
+            self.check_relocations(cx, range)?;
+        }
+        Ok(())
+    }
+
+    /// Reads a *non-ZST* scalar.
+    ///
+    /// If `read_provenance` is `true`, this will also read provenance; otherwise (if the machine
+    /// supports that) provenance is entirely ignored.
+    ///
+    /// ZSTs can't be read because in order to obtain a `Pointer`, we need to check
+    /// for ZSTness anyway due to integer pointers being valid for ZSTs.
+    ///
+    /// It is the caller's responsibility to check bounds and alignment beforehand.
+    /// Most likely, you want to call `InterpCx::read_scalar` instead of this method.
+    pub fn read_scalar(
+        &self,
+        cx: &impl HasDataLayout,
+        range: AllocRange,
+        read_provenance: bool,
+    ) -> AllocResult<ScalarMaybeUninit<Prov>> {
+        if read_provenance {
+            assert_eq!(range.size, cx.data_layout().pointer_size);
+        }
+
+        // First and foremost, if anything is uninit, bail.
+        if self.is_init(range).is_err() {
+            // This inflates uninitialized bytes to the entire scalar, even if only a few
+            // bytes are uninitialized.
+            return Ok(ScalarMaybeUninit::Uninit);
+        }
+
+        // If we are doing a pointer read, and there is a relocation exactly where we
+        // are reading, then we can put data and relocation back together and return that.
+        if read_provenance && let Some(&prov) = self.relocations.get(&range.start) {
+            // We already checked init and relocations, so we can use this function.
+            let bytes = self.get_bytes_even_more_internal(range);
+            let bits = read_target_uint(cx.data_layout().endian, bytes).unwrap();
+            let ptr = Pointer::new(prov, Size::from_bytes(bits));
+            return Ok(ScalarMaybeUninit::from_pointer(ptr, cx));
+        }
+
+        // If we are *not* reading a pointer, and we can just ignore relocations,
+        // then do exactly that.
+        if !read_provenance && Prov::OFFSET_IS_ADDR {
+            // We just strip provenance.
+            let bytes = self.get_bytes_even_more_internal(range);
+            let bits = read_target_uint(cx.data_layout().endian, bytes).unwrap();
+            return Ok(ScalarMaybeUninit::Scalar(Scalar::from_uint(bits, range.size)));
+        }
+
+        // It's complicated. Better make sure there is no provenance anywhere.
+        // FIXME: If !OFFSET_IS_ADDR, this is the best we can do. But if OFFSET_IS_ADDR, then
+        // `read_pointer` is true and we ideally would distinguish the following two cases:
+        // - The entire `range` is covered by 2 relocations for the same provenance.
+        //   Then we should return a pointer with that provenance.
+        // - The range has inhomogeneous provenance. Then we should return just the
+        //   underlying bits.
+        let bytes = self.get_bytes(cx, range)?;
+        let bits = read_target_uint(cx.data_layout().endian, bytes).unwrap();
+        Ok(ScalarMaybeUninit::Scalar(Scalar::from_uint(bits, range.size)))
+    }
+
+    /// Writes a *non-ZST* scalar.
+    ///
+    /// ZSTs can't be read because in order to obtain a `Pointer`, we need to check
+    /// for ZSTness anyway due to integer pointers being valid for ZSTs.
+    ///
+    /// It is the caller's responsibility to check bounds and alignment beforehand.
+    /// Most likely, you want to call `InterpCx::write_scalar` instead of this method.
+    #[instrument(skip(self, cx), level = "debug")]
+    pub fn write_scalar(
+        &mut self,
+        cx: &impl HasDataLayout,
+        range: AllocRange,
+        val: ScalarMaybeUninit<Prov>,
+    ) -> AllocResult {
+        assert!(self.mutability == Mutability::Mut);
+
+        let val = match val {
+            ScalarMaybeUninit::Scalar(scalar) => scalar,
+            ScalarMaybeUninit::Uninit => {
+                return self.write_uninit(cx, range);
+            }
+        };
+
+        // `to_bits_or_ptr_internal` is the right method because we just want to store this data
+        // as-is into memory.
+        let (bytes, provenance) = match val.to_bits_or_ptr_internal(range.size)? {
+            Err(val) => {
+                let (provenance, offset) = val.into_parts();
+                (u128::from(offset.bytes()), Some(provenance))
+            }
+            Ok(data) => (data, None),
+        };
+
+        let endian = cx.data_layout().endian;
+        let dst = self.get_bytes_mut(cx, range)?;
+        write_target_uint(endian, dst, bytes).unwrap();
+
+        // See if we have to also write a relocation.
+        if let Some(provenance) = provenance {
+            self.relocations.0.insert(range.start, provenance);
+        }
+
+        Ok(())
+    }
+
+    /// Write "uninit" to the given memory range.
+    pub fn write_uninit(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult {
+        self.mark_init(range, false);
+        self.clear_relocations(cx, range)?;
+        return Ok(());
+    }
+}
+
+/// Relocations.
+impl<Prov: Copy, Extra> Allocation<Prov, Extra> {
+    /// Returns all relocations overlapping with the given pointer-offset pair.
+    fn get_relocations(&self, cx: &impl HasDataLayout, range: AllocRange) -> &[(Size, Prov)] {
+        // We have to go back `pointer_size - 1` bytes, as that one would still overlap with
+        // the beginning of this range.
+        let start = range.start.bytes().saturating_sub(cx.data_layout().pointer_size.bytes() - 1);
+        self.relocations.range(Size::from_bytes(start)..range.end())
+    }
+
+    /// Returns whether this allocation has relocations overlapping with the given range.
+    ///
+    /// Note: this function exists to allow `get_relocations` to be private, in order to somewhat
+    /// limit access to relocations outside of the `Allocation` abstraction.
+    ///
+    pub fn has_relocations(&self, cx: &impl HasDataLayout, range: AllocRange) -> bool {
+        !self.get_relocations(cx, range).is_empty()
+    }
+
+    /// Checks that there are no relocations overlapping with the given range.
+    #[inline(always)]
+    fn check_relocations(&self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult {
+        if self.has_relocations(cx, range) { Err(AllocError::ReadPointerAsBytes) } else { Ok(()) }
+    }
+
+    /// Removes all relocations inside the given range.
+    /// If there are relocations overlapping with the edges, they
+    /// are removed as well *and* the bytes they cover are marked as
+    /// uninitialized. This is a somewhat odd "spooky action at a distance",
+    /// but it allows strictly more code to run than if we would just error
+    /// immediately in that case.
+    fn clear_relocations(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult
+    where
+        Prov: Provenance,
+    {
+        // Find the start and end of the given range and its outermost relocations.
+        let (first, last) = {
+            // Find all relocations overlapping the given range.
+            let relocations = self.get_relocations(cx, range);
+            if relocations.is_empty() {
+                return Ok(());
+            }
+
+            (
+                relocations.first().unwrap().0,
+                relocations.last().unwrap().0 + cx.data_layout().pointer_size,
+            )
+        };
+        let start = range.start;
+        let end = range.end();
+
+        // We need to handle clearing the relocations from parts of a pointer.
+        // FIXME: Miri should preserve partial relocations; see
+        // https://github.com/rust-lang/miri/issues/2181.
+        if first < start {
+            if Prov::ERR_ON_PARTIAL_PTR_OVERWRITE {
+                return Err(AllocError::PartialPointerOverwrite(first));
+            }
+            warn!(
+                "Partial pointer overwrite! De-initializing memory at offsets {first:?}..{start:?}."
+            );
+            self.init_mask.set_range(first, start, false);
+        }
+        if last > end {
+            if Prov::ERR_ON_PARTIAL_PTR_OVERWRITE {
+                return Err(AllocError::PartialPointerOverwrite(
+                    last - cx.data_layout().pointer_size,
+                ));
+            }
+            warn!(
+                "Partial pointer overwrite! De-initializing memory at offsets {end:?}..{last:?}."
+            );
+            self.init_mask.set_range(end, last, false);
+        }
+
+        // Forget all the relocations.
+        // Since relocations do not overlap, we know that removing until `last` (exclusive) is fine,
+        // i.e., this will not remove any other relocations just after the ones we care about.
+        self.relocations.0.remove_range(first..last);
+
+        Ok(())
+    }
+
+    /// Errors if there are relocations overlapping with the edges of the
+    /// given memory range.
+    #[inline]
+    fn check_relocation_edges(&self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult {
+        self.check_relocations(cx, alloc_range(range.start, Size::ZERO))?;
+        self.check_relocations(cx, alloc_range(range.end(), Size::ZERO))?;
+        Ok(())
+    }
+}
+
+/// "Relocations" stores the provenance information of pointers stored in memory.
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
+pub struct Relocations<Prov = AllocId>(SortedMap<Size, Prov>);
+
+impl<Prov> Relocations<Prov> {
+    pub fn new() -> Self {
+        Relocations(SortedMap::new())
+    }
+
+    // The caller must guarantee that the given relocations are already sorted
+    // by address and contain no duplicates.
+    pub fn from_presorted(r: Vec<(Size, Prov)>) -> Self {
+        Relocations(SortedMap::from_presorted_elements(r))
+    }
+}
+
+impl<Prov> Deref for Relocations<Prov> {
+    type Target = SortedMap<Size, Prov>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+/// A partial, owned list of relocations to transfer into another allocation.
+///
+/// Offsets are already adjusted to the destination allocation.
+pub struct AllocationRelocations<Prov> {
+    dest_relocations: Vec<(Size, Prov)>,
+}
+
+impl<Prov: Copy, Extra> Allocation<Prov, Extra> {
+    pub fn prepare_relocation_copy(
+        &self,
+        cx: &impl HasDataLayout,
+        src: AllocRange,
+        dest: Size,
+        count: u64,
+    ) -> AllocationRelocations<Prov> {
+        let relocations = self.get_relocations(cx, src);
+        if relocations.is_empty() {
+            return AllocationRelocations { dest_relocations: Vec::new() };
+        }
+
+        let size = src.size;
+        let mut new_relocations = Vec::with_capacity(relocations.len() * (count as usize));
+
+        // If `count` is large, this is rather wasteful -- we are allocating a big array here, which
+        // is mostly filled with redundant information since it's just N copies of the same `Prov`s
+        // at slightly adjusted offsets. The reason we do this is so that in `mark_relocation_range`
+        // we can use `insert_presorted`. That wouldn't work with an `Iterator` that just produces
+        // the right sequence of relocations for all N copies.
+        for i in 0..count {
+            new_relocations.extend(relocations.iter().map(|&(offset, reloc)| {
+                // compute offset for current repetition
+                let dest_offset = dest + size * i; // `Size` operations
+                (
+                    // shift offsets from source allocation to destination allocation
+                    (offset + dest_offset) - src.start, // `Size` operations
+                    reloc,
+                )
+            }));
+        }
+
+        AllocationRelocations { dest_relocations: new_relocations }
+    }
+
+    /// Applies a relocation copy.
+    /// The affected range, as defined in the parameters to `prepare_relocation_copy` is expected
+    /// to be clear of relocations.
+    ///
+    /// This is dangerous to use as it can violate internal `Allocation` invariants!
+    /// It only exists to support an efficient implementation of `mem_copy_repeatedly`.
+    pub fn mark_relocation_range(&mut self, relocations: AllocationRelocations<Prov>) {
+        self.relocations.0.insert_presorted(relocations.dest_relocations);
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Uninitialized byte tracking
+////////////////////////////////////////////////////////////////////////////////
+
+type Block = u64;
+
+/// A bitmask where each bit refers to the byte with the same index. If the bit is `true`, the byte
+/// is initialized. If it is `false` the byte is uninitialized.
+// Note: for performance reasons when interning, some of the `InitMask` fields can be partially
+// hashed. (see the `Hash` impl below for more details), so the impl is not derived.
+#[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable)]
+#[derive(HashStable)]
+pub struct InitMask {
+    blocks: Vec<Block>,
+    len: Size,
+}
+
+// Const allocations are only hashed for interning. However, they can be large, making the hashing
+// expensive especially since it uses `FxHash`: it's better suited to short keys, not potentially
+// big buffers like the allocation's init mask. We can partially hash some fields when they're
+// large.
+impl hash::Hash for InitMask {
+    fn hash<H: hash::Hasher>(&self, state: &mut H) {
+        const MAX_BLOCKS_TO_HASH: usize = MAX_BYTES_TO_HASH / std::mem::size_of::<Block>();
+        const MAX_BLOCKS_LEN: usize = MAX_HASHED_BUFFER_LEN / std::mem::size_of::<Block>();
+
+        // Partially hash the `blocks` buffer when it is large. To limit collisions with common
+        // prefixes and suffixes, we hash the length and some slices of the buffer.
+        let block_count = self.blocks.len();
+        if block_count > MAX_BLOCKS_LEN {
+            // Hash the buffer's length.
+            block_count.hash(state);
+
+            // And its head and tail.
+            self.blocks[..MAX_BLOCKS_TO_HASH].hash(state);
+            self.blocks[block_count - MAX_BLOCKS_TO_HASH..].hash(state);
+        } else {
+            self.blocks.hash(state);
+        }
+
+        // Hash the other fields as usual.
+        self.len.hash(state);
+    }
+}
+
+impl InitMask {
+    pub const BLOCK_SIZE: u64 = 64;
+
+    #[inline]
+    fn bit_index(bits: Size) -> (usize, usize) {
+        // BLOCK_SIZE is the number of bits that can fit in a `Block`.
+        // Each bit in a `Block` represents the initialization state of one byte of an allocation,
+        // so we use `.bytes()` here.
+        let bits = bits.bytes();
+        let a = bits / InitMask::BLOCK_SIZE;
+        let b = bits % InitMask::BLOCK_SIZE;
+        (usize::try_from(a).unwrap(), usize::try_from(b).unwrap())
+    }
+
+    #[inline]
+    fn size_from_bit_index(block: impl TryInto<u64>, bit: impl TryInto<u64>) -> Size {
+        let block = block.try_into().ok().unwrap();
+        let bit = bit.try_into().ok().unwrap();
+        Size::from_bytes(block * InitMask::BLOCK_SIZE + bit)
+    }
+
+    pub fn new(size: Size, state: bool) -> Self {
+        let mut m = InitMask { blocks: vec![], len: Size::ZERO };
+        m.grow(size, state);
+        m
+    }
+
+    pub fn set_range(&mut self, start: Size, end: Size, new_state: bool) {
+        let len = self.len;
+        if end > len {
+            self.grow(end - len, new_state);
+        }
+        self.set_range_inbounds(start, end, new_state);
+    }
+
+    pub fn set_range_inbounds(&mut self, start: Size, end: Size, new_state: bool) {
+        let (blocka, bita) = Self::bit_index(start);
+        let (blockb, bitb) = Self::bit_index(end);
+        if blocka == blockb {
+            // First set all bits except the first `bita`,
+            // then unset the last `64 - bitb` bits.
+            let range = if bitb == 0 {
+                u64::MAX << bita
+            } else {
+                (u64::MAX << bita) & (u64::MAX >> (64 - bitb))
+            };
+            if new_state {
+                self.blocks[blocka] |= range;
+            } else {
+                self.blocks[blocka] &= !range;
+            }
+            return;
+        }
+        // across block boundaries
+        if new_state {
+            // Set `bita..64` to `1`.
+            self.blocks[blocka] |= u64::MAX << bita;
+            // Set `0..bitb` to `1`.
+            if bitb != 0 {
+                self.blocks[blockb] |= u64::MAX >> (64 - bitb);
+            }
+            // Fill in all the other blocks (much faster than one bit at a time).
+            for block in (blocka + 1)..blockb {
+                self.blocks[block] = u64::MAX;
+            }
+        } else {
+            // Set `bita..64` to `0`.
+            self.blocks[blocka] &= !(u64::MAX << bita);
+            // Set `0..bitb` to `0`.
+            if bitb != 0 {
+                self.blocks[blockb] &= !(u64::MAX >> (64 - bitb));
+            }
+            // Fill in all the other blocks (much faster than one bit at a time).
+            for block in (blocka + 1)..blockb {
+                self.blocks[block] = 0;
+            }
+        }
+    }
+
+    #[inline]
+    pub fn get(&self, i: Size) -> bool {
+        let (block, bit) = Self::bit_index(i);
+        (self.blocks[block] & (1 << bit)) != 0
+    }
+
+    #[inline]
+    pub fn set(&mut self, i: Size, new_state: bool) {
+        let (block, bit) = Self::bit_index(i);
+        self.set_bit(block, bit, new_state);
+    }
+
+    #[inline]
+    fn set_bit(&mut self, block: usize, bit: usize, new_state: bool) {
+        if new_state {
+            self.blocks[block] |= 1 << bit;
+        } else {
+            self.blocks[block] &= !(1 << bit);
+        }
+    }
+
+    pub fn grow(&mut self, amount: Size, new_state: bool) {
+        if amount.bytes() == 0 {
+            return;
+        }
+        let unused_trailing_bits =
+            u64::try_from(self.blocks.len()).unwrap() * Self::BLOCK_SIZE - self.len.bytes();
+        if amount.bytes() > unused_trailing_bits {
+            let additional_blocks = amount.bytes() / Self::BLOCK_SIZE + 1;
+            self.blocks.extend(
+                // FIXME(oli-obk): optimize this by repeating `new_state as Block`.
+                iter::repeat(0).take(usize::try_from(additional_blocks).unwrap()),
+            );
+        }
+        let start = self.len;
+        self.len += amount;
+        self.set_range_inbounds(start, start + amount, new_state); // `Size` operation
+    }
+
+    /// Returns the index of the first bit in `start..end` (end-exclusive) that is equal to is_init.
+    fn find_bit(&self, start: Size, end: Size, is_init: bool) -> Option<Size> {
+        /// A fast implementation of `find_bit`,
+        /// which skips over an entire block at a time if it's all 0s (resp. 1s),
+        /// and finds the first 1 (resp. 0) bit inside a block using `trailing_zeros` instead of a loop.
+        ///
+        /// Note that all examples below are written with 8 (instead of 64) bit blocks for simplicity,
+        /// and with the least significant bit (and lowest block) first:
+        /// ```text
+        ///        00000000|00000000
+        ///        ^      ^ ^      ^
+        /// index: 0      7 8      15
+        /// ```
+        /// Also, if not stated, assume that `is_init = true`, that is, we are searching for the first 1 bit.
+        fn find_bit_fast(
+            init_mask: &InitMask,
+            start: Size,
+            end: Size,
+            is_init: bool,
+        ) -> Option<Size> {
+            /// Search one block, returning the index of the first bit equal to `is_init`.
+            fn search_block(
+                bits: Block,
+                block: usize,
+                start_bit: usize,
+                is_init: bool,
+            ) -> Option<Size> {
+                // For the following examples, assume this function was called with:
+                //   bits = 0b00111011
+                //   start_bit = 3
+                //   is_init = false
+                // Note that, for the examples in this function, the most significant bit is written first,
+                // which is backwards compared to the comments in `find_bit`/`find_bit_fast`.
+
+                // Invert bits so we're always looking for the first set bit.
+                //        ! 0b00111011
+                //   bits = 0b11000100
+                let bits = if is_init { bits } else { !bits };
+                // Mask off unused start bits.
+                //          0b11000100
+                //        & 0b11111000
+                //   bits = 0b11000000
+                let bits = bits & (!0 << start_bit);
+                // Find set bit, if any.
+                //   bit = trailing_zeros(0b11000000)
+                //   bit = 6
+                if bits == 0 {
+                    None
+                } else {
+                    let bit = bits.trailing_zeros();
+                    Some(InitMask::size_from_bit_index(block, bit))
+                }
+            }
+
+            if start >= end {
+                return None;
+            }
+
+            // Convert `start` and `end` to block indexes and bit indexes within each block.
+            // We must convert `end` to an inclusive bound to handle block boundaries correctly.
+            //
+            // For example:
+            //
+            //   (a) 00000000|00000000    (b) 00000000|
+            //       ^~~~~~~~~~~^             ^~~~~~~~~^
+            //     start       end          start     end
+            //
+            // In both cases, the block index of `end` is 1.
+            // But we do want to search block 1 in (a), and we don't in (b).
+            //
+            // We subtract 1 from both end positions to make them inclusive:
+            //
+            //   (a) 00000000|00000000    (b) 00000000|
+            //       ^~~~~~~~~~^              ^~~~~~~^
+            //     start    end_inclusive   start end_inclusive
+            //
+            // For (a), the block index of `end_inclusive` is 1, and for (b), it's 0.
+            // This provides the desired behavior of searching blocks 0 and 1 for (a),
+            // and searching only block 0 for (b).
+            // There is no concern of overflows since we checked for `start >= end` above.
+            let (start_block, start_bit) = InitMask::bit_index(start);
+            let end_inclusive = Size::from_bytes(end.bytes() - 1);
+            let (end_block_inclusive, _) = InitMask::bit_index(end_inclusive);
+
+            // Handle first block: need to skip `start_bit` bits.
+            //
+            // We need to handle the first block separately,
+            // because there may be bits earlier in the block that should be ignored,
+            // such as the bit marked (1) in this example:
+            //
+            //       (1)
+            //       -|------
+            //   (c) 01000000|00000000|00000001
+            //          ^~~~~~~~~~~~~~~~~~^
+            //        start              end
+            if let Some(i) =
+                search_block(init_mask.blocks[start_block], start_block, start_bit, is_init)
+            {
+                // If the range is less than a block, we may find a matching bit after `end`.
+                //
+                // For example, we shouldn't successfully find bit (2), because it's after `end`:
+                //
+                //             (2)
+                //       -------|
+                //   (d) 00000001|00000000|00000001
+                //        ^~~~~^
+                //      start end
+                //
+                // An alternative would be to mask off end bits in the same way as we do for start bits,
+                // but performing this check afterwards is faster and simpler to implement.
+                if i < end {
+                    return Some(i);
+                } else {
+                    return None;
+                }
+            }
+
+            // Handle remaining blocks.
+            //
+            // We can skip over an entire block at once if it's all 0s (resp. 1s).
+            // The block marked (3) in this example is the first block that will be handled by this loop,
+            // and it will be skipped for that reason:
+            //
+            //                   (3)
+            //                --------
+            //   (e) 01000000|00000000|00000001
+            //          ^~~~~~~~~~~~~~~~~~^
+            //        start              end
+            if start_block < end_block_inclusive {
+                // This loop is written in a specific way for performance.
+                // Notably: `..end_block_inclusive + 1` is used for an inclusive range instead of `..=end_block_inclusive`,
+                // and `.zip(start_block + 1..)` is used to track the index instead of `.enumerate().skip().take()`,
+                // because both alternatives result in significantly worse codegen.
+                // `end_block_inclusive + 1` is guaranteed not to wrap, because `end_block_inclusive <= end / BLOCK_SIZE`,
+                // and `BLOCK_SIZE` (the number of bits per block) will always be at least 8 (1 byte).
+                for (&bits, block) in init_mask.blocks[start_block + 1..end_block_inclusive + 1]
+                    .iter()
+                    .zip(start_block + 1..)
+                {
+                    if let Some(i) = search_block(bits, block, 0, is_init) {
+                        // If this is the last block, we may find a matching bit after `end`.
+                        //
+                        // For example, we shouldn't successfully find bit (4), because it's after `end`:
+                        //
+                        //                               (4)
+                        //                         -------|
+                        //   (f) 00000001|00000000|00000001
+                        //          ^~~~~~~~~~~~~~~~~~^
+                        //        start              end
+                        //
+                        // As above with example (d), we could handle the end block separately and mask off end bits,
+                        // but unconditionally searching an entire block at once and performing this check afterwards
+                        // is faster and much simpler to implement.
+                        if i < end {
+                            return Some(i);
+                        } else {
+                            return None;
+                        }
+                    }
+                }
+            }
+
+            None
+        }
+
+        #[cfg_attr(not(debug_assertions), allow(dead_code))]
+        fn find_bit_slow(
+            init_mask: &InitMask,
+            start: Size,
+            end: Size,
+            is_init: bool,
+        ) -> Option<Size> {
+            (start..end).find(|&i| init_mask.get(i) == is_init)
+        }
+
+        let result = find_bit_fast(self, start, end, is_init);
+
+        debug_assert_eq!(
+            result,
+            find_bit_slow(self, start, end, is_init),
+            "optimized implementation of find_bit is wrong for start={:?} end={:?} is_init={} init_mask={:#?}",
+            start,
+            end,
+            is_init,
+            self
+        );
+
+        result
+    }
+}
+
+/// A contiguous chunk of initialized or uninitialized memory.
+pub enum InitChunk {
+    Init(Range<Size>),
+    Uninit(Range<Size>),
+}
+
+impl InitChunk {
+    #[inline]
+    pub fn is_init(&self) -> bool {
+        match self {
+            Self::Init(_) => true,
+            Self::Uninit(_) => false,
+        }
+    }
+
+    #[inline]
+    pub fn range(&self) -> Range<Size> {
+        match self {
+            Self::Init(r) => r.clone(),
+            Self::Uninit(r) => r.clone(),
+        }
+    }
+}
+
+impl InitMask {
+    /// Checks whether the range `start..end` (end-exclusive) is entirely initialized.
+    ///
+    /// Returns `Ok(())` if it's initialized. Otherwise returns a range of byte
+    /// indexes for the first contiguous span of the uninitialized access.
+    #[inline]
+    pub fn is_range_initialized(&self, start: Size, end: Size) -> Result<(), AllocRange> {
+        if end > self.len {
+            return Err(AllocRange::from(self.len..end));
+        }
+
+        let uninit_start = self.find_bit(start, end, false);
+
+        match uninit_start {
+            Some(uninit_start) => {
+                let uninit_end = self.find_bit(uninit_start, end, true).unwrap_or(end);
+                Err(AllocRange::from(uninit_start..uninit_end))
+            }
+            None => Ok(()),
+        }
+    }
+
+    /// Returns an iterator, yielding a range of byte indexes for each contiguous region
+    /// of initialized or uninitialized bytes inside the range `start..end` (end-exclusive).
+    ///
+    /// The iterator guarantees the following:
+    /// - Chunks are nonempty.
+    /// - Chunks are adjacent (each range's start is equal to the previous range's end).
+    /// - Chunks span exactly `start..end` (the first starts at `start`, the last ends at `end`).
+    /// - Chunks alternate between [`InitChunk::Init`] and [`InitChunk::Uninit`].
+    #[inline]
+    pub fn range_as_init_chunks(&self, start: Size, end: Size) -> InitChunkIter<'_> {
+        assert!(end <= self.len);
+
+        let is_init = if start < end {
+            self.get(start)
+        } else {
+            // `start..end` is empty: there are no chunks, so use some arbitrary value
+            false
+        };
+
+        InitChunkIter { init_mask: self, is_init, start, end }
+    }
+}
+
+/// Yields [`InitChunk`]s. See [`InitMask::range_as_init_chunks`].
+#[derive(Clone)]
+pub struct InitChunkIter<'a> {
+    init_mask: &'a InitMask,
+    /// Whether the next chunk we will return is initialized.
+    /// If there are no more chunks, contains some arbitrary value.
+    is_init: bool,
+    /// The current byte index into `init_mask`.
+    start: Size,
+    /// The end byte index into `init_mask`.
+    end: Size,
+}
+
+impl<'a> Iterator for InitChunkIter<'a> {
+    type Item = InitChunk;
+
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.start >= self.end {
+            return None;
+        }
+
+        let end_of_chunk =
+            self.init_mask.find_bit(self.start, self.end, !self.is_init).unwrap_or(self.end);
+        let range = self.start..end_of_chunk;
+
+        let ret =
+            Some(if self.is_init { InitChunk::Init(range) } else { InitChunk::Uninit(range) });
+
+        self.is_init = !self.is_init;
+        self.start = end_of_chunk;
+
+        ret
+    }
+}
+
+/// Uninitialized bytes.
+impl<Prov: Copy, Extra> Allocation<Prov, Extra> {
+    /// Checks whether the given range  is entirely initialized.
+    ///
+    /// Returns `Ok(())` if it's initialized. Otherwise returns the range of byte
+    /// indexes of the first contiguous uninitialized access.
+    fn is_init(&self, range: AllocRange) -> Result<(), AllocRange> {
+        self.init_mask.is_range_initialized(range.start, range.end()) // `Size` addition
+    }
+
+    /// Checks that a range of bytes is initialized. If not, returns the `InvalidUninitBytes`
+    /// error which will report the first range of bytes which is uninitialized.
+    fn check_init(&self, range: AllocRange) -> AllocResult {
+        self.is_init(range).map_err(|uninit_range| {
+            AllocError::InvalidUninitBytes(Some(UninitBytesAccess {
+                access: range,
+                uninit: uninit_range,
+            }))
+        })
+    }
+
+    fn mark_init(&mut self, range: AllocRange, is_init: bool) {
+        if range.size.bytes() == 0 {
+            return;
+        }
+        assert!(self.mutability == Mutability::Mut);
+        self.init_mask.set_range(range.start, range.end(), is_init);
+    }
+}
+
+/// Run-length encoding of the uninit mask.
+/// Used to copy parts of a mask multiple times to another allocation.
+pub struct InitMaskCompressed {
+    /// Whether the first range is initialized.
+    initial: bool,
+    /// The lengths of ranges that are run-length encoded.
+    /// The initialization state of the ranges alternate starting with `initial`.
+    ranges: smallvec::SmallVec<[u64; 1]>,
+}
+
+impl InitMaskCompressed {
+    pub fn no_bytes_init(&self) -> bool {
+        // The `ranges` are run-length encoded and of alternating initialization state.
+        // So if `ranges.len() > 1` then the second block is an initialized range.
+        !self.initial && self.ranges.len() == 1
+    }
+}
+
+/// Transferring the initialization mask to other allocations.
+impl<Prov, Extra> Allocation<Prov, Extra> {
+    /// Creates a run-length encoding of the initialization mask; panics if range is empty.
+    ///
+    /// This is essentially a more space-efficient version of
+    /// `InitMask::range_as_init_chunks(...).collect::<Vec<_>>()`.
+    pub fn compress_uninit_range(&self, range: AllocRange) -> InitMaskCompressed {
+        // Since we are copying `size` bytes from `src` to `dest + i * size` (`for i in 0..repeat`),
+        // a naive initialization mask copying algorithm would repeatedly have to read the initialization mask from
+        // the source and write it to the destination. Even if we optimized the memory accesses,
+        // we'd be doing all of this `repeat` times.
+        // Therefore we precompute a compressed version of the initialization mask of the source value and
+        // then write it back `repeat` times without computing any more information from the source.
+
+        // A precomputed cache for ranges of initialized / uninitialized bits
+        // 0000010010001110 will become
+        // `[5, 1, 2, 1, 3, 3, 1]`,
+        // where each element toggles the state.
+
+        let mut ranges = smallvec::SmallVec::<[u64; 1]>::new();
+
+        let mut chunks = self.init_mask.range_as_init_chunks(range.start, range.end()).peekable();
+
+        let initial = chunks.peek().expect("range should be nonempty").is_init();
+
+        // Here we rely on `range_as_init_chunks` to yield alternating init/uninit chunks.
+        for chunk in chunks {
+            let len = chunk.range().end.bytes() - chunk.range().start.bytes();
+            ranges.push(len);
+        }
+
+        InitMaskCompressed { ranges, initial }
+    }
+
+    /// Applies multiple instances of the run-length encoding to the initialization mask.
+    ///
+    /// This is dangerous to use as it can violate internal `Allocation` invariants!
+    /// It only exists to support an efficient implementation of `mem_copy_repeatedly`.
+    pub fn mark_compressed_init_range(
+        &mut self,
+        defined: &InitMaskCompressed,
+        range: AllocRange,
+        repeat: u64,
+    ) {
+        // An optimization where we can just overwrite an entire range of initialization
+        // bits if they are going to be uniformly `1` or `0`.
+        if defined.ranges.len() <= 1 {
+            self.init_mask.set_range_inbounds(
+                range.start,
+                range.start + range.size * repeat, // `Size` operations
+                defined.initial,
+            );
+            return;
+        }
+
+        for mut j in 0..repeat {
+            j *= range.size.bytes();
+            j += range.start.bytes();
+            let mut cur = defined.initial;
+            for range in &defined.ranges {
+                let old_j = j;
+                j += range;
+                self.init_mask.set_range_inbounds(
+                    Size::from_bytes(old_j),
+                    Size::from_bytes(j),
+                    cur,
+                );
+                cur = !cur;
+            }
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/interpret/error.rs b/compiler/rustc_middle/src/mir/interpret/error.rs
new file mode 100644
index 000000000..cecb55578
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/interpret/error.rs
@@ -0,0 +1,551 @@
+use super::{AllocId, AllocRange, ConstAlloc, Pointer, Scalar};
+
+use crate::mir::interpret::ConstValue;
+use crate::ty::{layout, query::TyCtxtAt, tls, Ty, ValTree};
+
+use rustc_data_structures::sync::Lock;
+use rustc_errors::{pluralize, struct_span_err, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_macros::HashStable;
+use rustc_session::CtfeBacktrace;
+use rustc_span::def_id::DefId;
+use rustc_target::abi::{call, Align, Size};
+use std::{any::Any, backtrace::Backtrace, fmt};
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+pub enum ErrorHandled {
+    /// Already reported an error for this evaluation, and the compilation is
+    /// *guaranteed* to fail. Warnings/lints *must not* produce `Reported`.
+    Reported(ErrorGuaranteed),
+    /// Already emitted a lint for this evaluation.
+    Linted,
+    /// Don't emit an error, the evaluation failed because the MIR was generic
+    /// and the substs didn't fully monomorphize it.
+    TooGeneric,
+}
+
+impl From<ErrorGuaranteed> for ErrorHandled {
+    fn from(err: ErrorGuaranteed) -> ErrorHandled {
+        ErrorHandled::Reported(err)
+    }
+}
+
+TrivialTypeTraversalAndLiftImpls! {
+    ErrorHandled,
+}
+
+pub type EvalToAllocationRawResult<'tcx> = Result<ConstAlloc<'tcx>, ErrorHandled>;
+pub type EvalToConstValueResult<'tcx> = Result<ConstValue<'tcx>, ErrorHandled>;
+pub type EvalToValTreeResult<'tcx> = Result<Option<ValTree<'tcx>>, ErrorHandled>;
+
+pub fn struct_error<'tcx>(
+    tcx: TyCtxtAt<'tcx>,
+    msg: &str,
+) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+    struct_span_err!(tcx.sess, tcx.span, E0080, "{}", msg)
+}
+
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(InterpErrorInfo<'_>, 8);
+
+/// Packages the kind of error we got from the const code interpreter
+/// up with a Rust-level backtrace of where the error occurred.
+/// These should always be constructed by calling `.into()` on
+/// an `InterpError`. In `rustc_mir::interpret`, we have `throw_err_*`
+/// macros for this.
+#[derive(Debug)]
+pub struct InterpErrorInfo<'tcx>(Box<InterpErrorInfoInner<'tcx>>);
+
+#[derive(Debug)]
+struct InterpErrorInfoInner<'tcx> {
+    kind: InterpError<'tcx>,
+    backtrace: Option<Box<Backtrace>>,
+}
+
+impl fmt::Display for InterpErrorInfo<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0.kind)
+    }
+}
+
+impl<'tcx> InterpErrorInfo<'tcx> {
+    pub fn print_backtrace(&self) {
+        if let Some(backtrace) = self.0.backtrace.as_ref() {
+            print_backtrace(backtrace);
+        }
+    }
+
+    pub fn into_kind(self) -> InterpError<'tcx> {
+        let InterpErrorInfo(box InterpErrorInfoInner { kind, .. }) = self;
+        kind
+    }
+
+    #[inline]
+    pub fn kind(&self) -> &InterpError<'tcx> {
+        &self.0.kind
+    }
+}
+
+fn print_backtrace(backtrace: &Backtrace) {
+    eprintln!("\n\nAn error occurred in miri:\n{}", backtrace);
+}
+
+impl From<ErrorHandled> for InterpErrorInfo<'_> {
+    fn from(err: ErrorHandled) -> Self {
+        match err {
+            ErrorHandled::Reported(ErrorGuaranteed { .. }) | ErrorHandled::Linted => {
+                err_inval!(ReferencedConstant)
+            }
+            ErrorHandled::TooGeneric => err_inval!(TooGeneric),
+        }
+        .into()
+    }
+}
+
+impl From<ErrorGuaranteed> for InterpErrorInfo<'_> {
+    fn from(err: ErrorGuaranteed) -> Self {
+        InterpError::InvalidProgram(InvalidProgramInfo::AlreadyReported(err)).into()
+    }
+}
+
+impl<'tcx> From<InterpError<'tcx>> for InterpErrorInfo<'tcx> {
+    fn from(kind: InterpError<'tcx>) -> Self {
+        let capture_backtrace = tls::with_opt(|tcx| {
+            if let Some(tcx) = tcx {
+                *Lock::borrow(&tcx.sess.ctfe_backtrace)
+            } else {
+                CtfeBacktrace::Disabled
+            }
+        });
+
+        let backtrace = match capture_backtrace {
+            CtfeBacktrace::Disabled => None,
+            CtfeBacktrace::Capture => Some(Box::new(Backtrace::force_capture())),
+            CtfeBacktrace::Immediate => {
+                // Print it now.
+                let backtrace = Backtrace::force_capture();
+                print_backtrace(&backtrace);
+                None
+            }
+        };
+
+        InterpErrorInfo(Box::new(InterpErrorInfoInner { kind, backtrace }))
+    }
+}
+
+/// Error information for when the program we executed turned out not to actually be a valid
+/// program. This cannot happen in stand-alone Miri, but it can happen during CTFE/ConstProp
+/// where we work on generic code or execution does not have all information available.
+pub enum InvalidProgramInfo<'tcx> {
+    /// Resolution can fail if we are in a too generic context.
+    TooGeneric,
+    /// Cannot compute this constant because it depends on another one
+    /// which already produced an error.
+    ReferencedConstant,
+    /// Abort in case errors are already reported.
+    AlreadyReported(ErrorGuaranteed),
+    /// An error occurred during layout computation.
+    Layout(layout::LayoutError<'tcx>),
+    /// An error occurred during FnAbi computation: the passed --target lacks FFI support
+    /// (which unfortunately typeck does not reject).
+    /// Not using `FnAbiError` as that contains a nested `LayoutError`.
+    FnAbiAdjustForForeignAbi(call::AdjustForForeignAbiError),
+    /// SizeOf of unsized type was requested.
+    SizeOfUnsizedType(Ty<'tcx>),
+}
+
+impl fmt::Display for InvalidProgramInfo<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use InvalidProgramInfo::*;
+        match self {
+            TooGeneric => write!(f, "encountered overly generic constant"),
+            ReferencedConstant => write!(f, "referenced constant has errors"),
+            AlreadyReported(ErrorGuaranteed { .. }) => {
+                write!(f, "encountered constants with type errors, stopping evaluation")
+            }
+            Layout(ref err) => write!(f, "{err}"),
+            FnAbiAdjustForForeignAbi(ref err) => write!(f, "{err}"),
+            SizeOfUnsizedType(ty) => write!(f, "size_of called on unsized type `{ty}`"),
+        }
+    }
+}
+
+/// Details of why a pointer had to be in-bounds.
+#[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
+pub enum CheckInAllocMsg {
+    /// We are dereferencing a pointer (i.e., creating a place).
+    DerefTest,
+    /// We are access memory.
+    MemoryAccessTest,
+    /// We are doing pointer arithmetic.
+    PointerArithmeticTest,
+    /// We are doing pointer offset_from.
+    OffsetFromTest,
+    /// None of the above -- generic/unspecific inbounds test.
+    InboundsTest,
+}
+
+impl fmt::Display for CheckInAllocMsg {
+    /// When this is printed as an error the context looks like this:
+    /// "{msg}{pointer} is a dangling pointer".
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "{}",
+            match *self {
+                CheckInAllocMsg::DerefTest => "dereferencing pointer failed: ",
+                CheckInAllocMsg::MemoryAccessTest => "memory access failed: ",
+                CheckInAllocMsg::PointerArithmeticTest => "out-of-bounds pointer arithmetic: ",
+                CheckInAllocMsg::OffsetFromTest => "out-of-bounds offset_from: ",
+                CheckInAllocMsg::InboundsTest => "out-of-bounds pointer use: ",
+            }
+        )
+    }
+}
+
+/// Details of an access to uninitialized bytes where it is not allowed.
+#[derive(Debug)]
+pub struct UninitBytesAccess {
+    /// Range of the original memory access.
+    pub access: AllocRange,
+    /// Range of the uninit memory that was encountered. (Might not be maximal.)
+    pub uninit: AllocRange,
+}
+
+/// Information about a size mismatch.
+#[derive(Debug)]
+pub struct ScalarSizeMismatch {
+    pub target_size: u64,
+    pub data_size: u64,
+}
+
+/// Error information for when the program caused Undefined Behavior.
+pub enum UndefinedBehaviorInfo {
+    /// Free-form case. Only for errors that are never caught!
+    Ub(String),
+    /// Unreachable code was executed.
+    Unreachable,
+    /// A slice/array index projection went out-of-bounds.
+    BoundsCheckFailed {
+        len: u64,
+        index: u64,
+    },
+    /// Something was divided by 0 (x / 0).
+    DivisionByZero,
+    /// Something was "remainded" by 0 (x % 0).
+    RemainderByZero,
+    /// Signed division overflowed (INT_MIN / -1).
+    DivisionOverflow,
+    /// Signed remainder overflowed (INT_MIN % -1).
+    RemainderOverflow,
+    /// Overflowing inbounds pointer arithmetic.
+    PointerArithOverflow,
+    /// Invalid metadata in a wide pointer (using `str` to avoid allocations).
+    InvalidMeta(&'static str),
+    /// Reading a C string that does not end within its allocation.
+    UnterminatedCString(Pointer),
+    /// Dereferencing a dangling pointer after it got freed.
+    PointerUseAfterFree(AllocId),
+    /// Used a pointer outside the bounds it is valid for.
+    /// (If `ptr_size > 0`, determines the size of the memory range that was expected to be in-bounds.)
+    PointerOutOfBounds {
+        alloc_id: AllocId,
+        alloc_size: Size,
+        ptr_offset: i64,
+        ptr_size: Size,
+        msg: CheckInAllocMsg,
+    },
+    /// Using an integer as a pointer in the wrong way.
+    DanglingIntPointer(u64, CheckInAllocMsg),
+    /// Used a pointer with bad alignment.
+    AlignmentCheckFailed {
+        required: Align,
+        has: Align,
+    },
+    /// Writing to read-only memory.
+    WriteToReadOnly(AllocId),
+    // Trying to access the data behind a function pointer.
+    DerefFunctionPointer(AllocId),
+    // Trying to access the data behind a vtable pointer.
+    DerefVTablePointer(AllocId),
+    /// The value validity check found a problem.
+    /// Should only be thrown by `validity.rs` and always point out which part of the value
+    /// is the problem.
+    ValidationFailure {
+        /// The "path" to the value in question, e.g. `.0[5].field` for a struct
+        /// field in the 6th element of an array that is the first element of a tuple.
+        path: Option<String>,
+        msg: String,
+    },
+    /// Using a non-boolean `u8` as bool.
+    InvalidBool(u8),
+    /// Using a non-character `u32` as character.
+    InvalidChar(u32),
+    /// The tag of an enum does not encode an actual discriminant.
+    InvalidTag(Scalar),
+    /// Using a pointer-not-to-a-function as function pointer.
+    InvalidFunctionPointer(Pointer),
+    /// Using a pointer-not-to-a-vtable as vtable pointer.
+    InvalidVTablePointer(Pointer),
+    /// Using a string that is not valid UTF-8,
+    InvalidStr(std::str::Utf8Error),
+    /// Using uninitialized data where it is not allowed.
+    InvalidUninitBytes(Option<(AllocId, UninitBytesAccess)>),
+    /// Working with a local that is not currently live.
+    DeadLocal,
+    /// Data size is not equal to target size.
+    ScalarSizeMismatch(ScalarSizeMismatch),
+    /// A discriminant of an uninhabited enum variant is written.
+    UninhabitedEnumVariantWritten,
+}
+
+impl fmt::Display for UndefinedBehaviorInfo {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use UndefinedBehaviorInfo::*;
+        match self {
+            Ub(msg) => write!(f, "{msg}"),
+            Unreachable => write!(f, "entering unreachable code"),
+            BoundsCheckFailed { ref len, ref index } => {
+                write!(f, "indexing out of bounds: the len is {len} but the index is {index}")
+            }
+            DivisionByZero => write!(f, "dividing by zero"),
+            RemainderByZero => write!(f, "calculating the remainder with a divisor of zero"),
+            DivisionOverflow => write!(f, "overflow in signed division (dividing MIN by -1)"),
+            RemainderOverflow => write!(f, "overflow in signed remainder (dividing MIN by -1)"),
+            PointerArithOverflow => write!(f, "overflowing in-bounds pointer arithmetic"),
+            InvalidMeta(msg) => write!(f, "invalid metadata in wide pointer: {msg}"),
+            UnterminatedCString(p) => write!(
+                f,
+                "reading a null-terminated string starting at {p:?} with no null found before end of allocation",
+            ),
+            PointerUseAfterFree(a) => {
+                write!(f, "pointer to {a:?} was dereferenced after this allocation got freed")
+            }
+            PointerOutOfBounds { alloc_id, alloc_size, ptr_offset, ptr_size: Size::ZERO, msg } => {
+                write!(
+                    f,
+                    "{msg}{alloc_id:?} has size {alloc_size}, so pointer at offset {ptr_offset} is out-of-bounds",
+                    alloc_size = alloc_size.bytes(),
+                )
+            }
+            PointerOutOfBounds { alloc_id, alloc_size, ptr_offset, ptr_size, msg } => write!(
+                f,
+                "{msg}{alloc_id:?} has size {alloc_size}, so pointer to {ptr_size} byte{ptr_size_p} starting at offset {ptr_offset} is out-of-bounds",
+                alloc_size = alloc_size.bytes(),
+                ptr_size = ptr_size.bytes(),
+                ptr_size_p = pluralize!(ptr_size.bytes()),
+            ),
+            DanglingIntPointer(i, msg) => {
+                write!(
+                    f,
+                    "{msg}{pointer} is a dangling pointer (it has no provenance)",
+                    pointer = Pointer::<Option<AllocId>>::from_addr(*i),
+                )
+            }
+            AlignmentCheckFailed { required, has } => write!(
+                f,
+                "accessing memory with alignment {has}, but alignment {required} is required",
+                has = has.bytes(),
+                required = required.bytes()
+            ),
+            WriteToReadOnly(a) => write!(f, "writing to {a:?} which is read-only"),
+            DerefFunctionPointer(a) => write!(f, "accessing {a:?} which contains a function"),
+            DerefVTablePointer(a) => write!(f, "accessing {a:?} which contains a vtable"),
+            ValidationFailure { path: None, msg } => {
+                write!(f, "constructing invalid value: {msg}")
+            }
+            ValidationFailure { path: Some(path), msg } => {
+                write!(f, "constructing invalid value at {path}: {msg}")
+            }
+            InvalidBool(b) => {
+                write!(f, "interpreting an invalid 8-bit value as a bool: 0x{b:02x}")
+            }
+            InvalidChar(c) => {
+                write!(f, "interpreting an invalid 32-bit value as a char: 0x{c:08x}")
+            }
+            InvalidTag(val) => write!(f, "enum value has invalid tag: {val:x}"),
+            InvalidFunctionPointer(p) => {
+                write!(f, "using {p:?} as function pointer but it does not point to a function")
+            }
+            InvalidVTablePointer(p) => {
+                write!(f, "using {p:?} as vtable pointer but it does not point to a vtable")
+            }
+            InvalidStr(err) => write!(f, "this string is not valid UTF-8: {err}"),
+            InvalidUninitBytes(Some((alloc, info))) => write!(
+                f,
+                "reading memory at {alloc:?}{access:?}, \
+                 but memory is uninitialized at {uninit:?}, \
+                 and this operation requires initialized memory",
+                access = info.access,
+                uninit = info.uninit,
+            ),
+            InvalidUninitBytes(None) => write!(
+                f,
+                "using uninitialized data, but this operation requires initialized memory"
+            ),
+            DeadLocal => write!(f, "accessing a dead local variable"),
+            ScalarSizeMismatch(self::ScalarSizeMismatch { target_size, data_size }) => write!(
+                f,
+                "scalar size mismatch: expected {target_size} bytes but got {data_size} bytes instead",
+            ),
+            UninhabitedEnumVariantWritten => {
+                write!(f, "writing discriminant of an uninhabited enum")
+            }
+        }
+    }
+}
+
+/// Error information for when the program did something that might (or might not) be correct
+/// to do according to the Rust spec, but due to limitations in the interpreter, the
+/// operation could not be carried out. These limitations can differ between CTFE and the
+/// Miri engine, e.g., CTFE does not support dereferencing pointers at integral addresses.
+pub enum UnsupportedOpInfo {
+    /// Free-form case. Only for errors that are never caught!
+    Unsupported(String),
+    /// Encountered a pointer where we needed raw bytes.
+    ReadPointerAsBytes,
+    /// Overwriting parts of a pointer; the resulting state cannot be represented in our
+    /// `Allocation` data structure. See <https://github.com/rust-lang/miri/issues/2181>.
+    PartialPointerOverwrite(Pointer<AllocId>),
+    //
+    // The variants below are only reachable from CTFE/const prop, miri will never emit them.
+    //
+    /// Accessing thread local statics
+    ThreadLocalStatic(DefId),
+    /// Accessing an unsupported extern static.
+    ReadExternStatic(DefId),
+}
+
+impl fmt::Display for UnsupportedOpInfo {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use UnsupportedOpInfo::*;
+        match self {
+            Unsupported(ref msg) => write!(f, "{msg}"),
+            ReadPointerAsBytes => write!(f, "unable to turn pointer into raw bytes"),
+            PartialPointerOverwrite(ptr) => {
+                write!(f, "unable to overwrite parts of a pointer in memory at {ptr:?}")
+            }
+            ThreadLocalStatic(did) => write!(f, "cannot access thread local static ({did:?})"),
+            ReadExternStatic(did) => write!(f, "cannot read from extern static ({did:?})"),
+        }
+    }
+}
+
+/// Error information for when the program exhausted the resources granted to it
+/// by the interpreter.
+pub enum ResourceExhaustionInfo {
+    /// The stack grew too big.
+    StackFrameLimitReached,
+    /// The program ran for too long.
+    ///
+    /// The exact limit is set by the `const_eval_limit` attribute.
+    StepLimitReached,
+    /// There is not enough memory to perform an allocation.
+    MemoryExhausted,
+}
+
+impl fmt::Display for ResourceExhaustionInfo {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use ResourceExhaustionInfo::*;
+        match self {
+            StackFrameLimitReached => {
+                write!(f, "reached the configured maximum number of stack frames")
+            }
+            StepLimitReached => {
+                write!(f, "exceeded interpreter step limit (see `#[const_eval_limit]`)")
+            }
+            MemoryExhausted => {
+                write!(f, "tried to allocate more memory than available to compiler")
+            }
+        }
+    }
+}
+
+/// A trait to work around not having trait object upcasting.
+pub trait AsAny: Any {
+    fn as_any(&self) -> &dyn Any;
+}
+impl<T: Any> AsAny for T {
+    #[inline(always)]
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+}
+
+/// A trait for machine-specific errors (or other "machine stop" conditions).
+pub trait MachineStopType: AsAny + fmt::Display + Send {
+    /// If `true`, emit a hard error instead of going through the `CONST_ERR` lint
+    fn is_hard_err(&self) -> bool {
+        false
+    }
+}
+
+impl dyn MachineStopType {
+    #[inline(always)]
+    pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
+        self.as_any().downcast_ref()
+    }
+}
+
+pub enum InterpError<'tcx> {
+    /// The program caused undefined behavior.
+    UndefinedBehavior(UndefinedBehaviorInfo),
+    /// The program did something the interpreter does not support (some of these *might* be UB
+    /// but the interpreter is not sure).
+    Unsupported(UnsupportedOpInfo),
+    /// The program was invalid (ill-typed, bad MIR, not sufficiently monomorphized, ...).
+    InvalidProgram(InvalidProgramInfo<'tcx>),
+    /// The program exhausted the interpreter's resources (stack/heap too big,
+    /// execution takes too long, ...).
+    ResourceExhaustion(ResourceExhaustionInfo),
+    /// Stop execution for a machine-controlled reason. This is never raised by
+    /// the core engine itself.
+    MachineStop(Box<dyn MachineStopType>),
+}
+
+pub type InterpResult<'tcx, T = ()> = Result<T, InterpErrorInfo<'tcx>>;
+
+impl fmt::Display for InterpError<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use InterpError::*;
+        match *self {
+            Unsupported(ref msg) => write!(f, "{msg}"),
+            InvalidProgram(ref msg) => write!(f, "{msg}"),
+            UndefinedBehavior(ref msg) => write!(f, "{msg}"),
+            ResourceExhaustion(ref msg) => write!(f, "{msg}"),
+            MachineStop(ref msg) => write!(f, "{msg}"),
+        }
+    }
+}
+
+// Forward `Debug` to `Display`, so it does not look awful.
+impl fmt::Debug for InterpError<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self, f)
+    }
+}
+
+impl InterpError<'_> {
+    /// Some errors do string formatting even if the error is never printed.
+    /// To avoid performance issues, there are places where we want to be sure to never raise these formatting errors,
+    /// so this method lets us detect them and `bug!` on unexpected errors.
+    pub fn formatted_string(&self) -> bool {
+        matches!(
+            self,
+            InterpError::Unsupported(UnsupportedOpInfo::Unsupported(_))
+                | InterpError::UndefinedBehavior(UndefinedBehaviorInfo::ValidationFailure { .. })
+                | InterpError::UndefinedBehavior(UndefinedBehaviorInfo::Ub(_))
+        )
+    }
+
+    /// Should this error be reported as a hard error, preventing compilation, or a soft error,
+    /// causing a deny-by-default lint?
+    pub fn is_hard_err(&self) -> bool {
+        use InterpError::*;
+        match *self {
+            MachineStop(ref err) => err.is_hard_err(),
+            UndefinedBehavior(_) => true,
+            ResourceExhaustion(ResourceExhaustionInfo::MemoryExhausted) => true,
+            _ => false,
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/interpret/mod.rs b/compiler/rustc_middle/src/mir/interpret/mod.rs
new file mode 100644
index 000000000..967f8ece1
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/interpret/mod.rs
@@ -0,0 +1,633 @@
+//! An interpreter for MIR used in CTFE and by miri.
+
+#[macro_export]
+macro_rules! err_unsup {
+    ($($tt:tt)*) => {
+        $crate::mir::interpret::InterpError::Unsupported(
+            $crate::mir::interpret::UnsupportedOpInfo::$($tt)*
+        )
+    };
+}
+
+#[macro_export]
+macro_rules! err_unsup_format {
+    ($($tt:tt)*) => { err_unsup!(Unsupported(format!($($tt)*))) };
+}
+
+#[macro_export]
+macro_rules! err_inval {
+    ($($tt:tt)*) => {
+        $crate::mir::interpret::InterpError::InvalidProgram(
+            $crate::mir::interpret::InvalidProgramInfo::$($tt)*
+        )
+    };
+}
+
+#[macro_export]
+macro_rules! err_ub {
+    ($($tt:tt)*) => {
+        $crate::mir::interpret::InterpError::UndefinedBehavior(
+            $crate::mir::interpret::UndefinedBehaviorInfo::$($tt)*
+        )
+    };
+}
+
+#[macro_export]
+macro_rules! err_ub_format {
+    ($($tt:tt)*) => { err_ub!(Ub(format!($($tt)*))) };
+}
+
+#[macro_export]
+macro_rules! err_exhaust {
+    ($($tt:tt)*) => {
+        $crate::mir::interpret::InterpError::ResourceExhaustion(
+            $crate::mir::interpret::ResourceExhaustionInfo::$($tt)*
+        )
+    };
+}
+
+#[macro_export]
+macro_rules! err_machine_stop {
+    ($($tt:tt)*) => {
+        $crate::mir::interpret::InterpError::MachineStop(Box::new($($tt)*))
+    };
+}
+
+// In the `throw_*` macros, avoid `return` to make them work with `try {}`.
+#[macro_export]
+macro_rules! throw_unsup {
+    ($($tt:tt)*) => { do yeet err_unsup!($($tt)*) };
+}
+
+#[macro_export]
+macro_rules! throw_unsup_format {
+    ($($tt:tt)*) => { throw_unsup!(Unsupported(format!($($tt)*))) };
+}
+
+#[macro_export]
+macro_rules! throw_inval {
+    ($($tt:tt)*) => { do yeet err_inval!($($tt)*) };
+}
+
+#[macro_export]
+macro_rules! throw_ub {
+    ($($tt:tt)*) => { do yeet err_ub!($($tt)*) };
+}
+
+#[macro_export]
+macro_rules! throw_ub_format {
+    ($($tt:tt)*) => { throw_ub!(Ub(format!($($tt)*))) };
+}
+
+#[macro_export]
+macro_rules! throw_exhaust {
+    ($($tt:tt)*) => { do yeet err_exhaust!($($tt)*) };
+}
+
+#[macro_export]
+macro_rules! throw_machine_stop {
+    ($($tt:tt)*) => { do yeet err_machine_stop!($($tt)*) };
+}
+
+mod allocation;
+mod error;
+mod pointer;
+mod queries;
+mod value;
+
+use std::convert::TryFrom;
+use std::fmt;
+use std::io;
+use std::io::{Read, Write};
+use std::num::{NonZeroU32, NonZeroU64};
+use std::sync::atomic::{AtomicU32, Ordering};
+
+use rustc_ast::LitKind;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::sync::{HashMapExt, Lock};
+use rustc_data_structures::tiny_list::TinyList;
+use rustc_hir::def_id::DefId;
+use rustc_macros::HashStable;
+use rustc_middle::ty::print::with_no_trimmed_paths;
+use rustc_serialize::{Decodable, Encodable};
+use rustc_target::abi::Endian;
+
+use crate::mir;
+use crate::ty::codec::{TyDecoder, TyEncoder};
+use crate::ty::subst::GenericArgKind;
+use crate::ty::{self, Instance, Ty, TyCtxt};
+
+pub use self::error::{
+    struct_error, CheckInAllocMsg, ErrorHandled, EvalToAllocationRawResult, EvalToConstValueResult,
+    EvalToValTreeResult, InterpError, InterpErrorInfo, InterpResult, InvalidProgramInfo,
+    MachineStopType, ResourceExhaustionInfo, ScalarSizeMismatch, UndefinedBehaviorInfo,
+    UninitBytesAccess, UnsupportedOpInfo,
+};
+
+pub use self::value::{get_slice_bytes, ConstAlloc, ConstValue, Scalar, ScalarMaybeUninit};
+
+pub use self::allocation::{
+    alloc_range, AllocRange, Allocation, ConstAllocation, InitChunk, InitChunkIter, InitMask,
+    Relocations,
+};
+
+pub use self::pointer::{Pointer, PointerArithmetic, Provenance};
+
+/// Uniquely identifies one of the following:
+/// - A constant
+/// - A static
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable, Lift)]
+pub struct GlobalId<'tcx> {
+    /// For a constant or static, the `Instance` of the item itself.
+    /// For a promoted global, the `Instance` of the function they belong to.
+    pub instance: ty::Instance<'tcx>,
+
+    /// The index for promoted globals within their function's `mir::Body`.
+    pub promoted: Option<mir::Promoted>,
+}
+
+impl<'tcx> GlobalId<'tcx> {
+    pub fn display(self, tcx: TyCtxt<'tcx>) -> String {
+        let instance_name = with_no_trimmed_paths!(tcx.def_path_str(self.instance.def.def_id()));
+        if let Some(promoted) = self.promoted {
+            format!("{}::{:?}", instance_name, promoted)
+        } else {
+            instance_name
+        }
+    }
+}
+
+/// Input argument for `tcx.lit_to_const`.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, HashStable)]
+pub struct LitToConstInput<'tcx> {
+    /// The absolute value of the resultant constant.
+    pub lit: &'tcx LitKind,
+    /// The type of the constant.
+    pub ty: Ty<'tcx>,
+    /// If the constant is negative.
+    pub neg: bool,
+}
+
+/// Error type for `tcx.lit_to_const`.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, HashStable)]
+pub enum LitToConstError {
+    /// The literal's inferred type did not match the expected `ty` in the input.
+    /// This is used for graceful error handling (`delay_span_bug`) in
+    /// type checking (`Const::from_anon_const`).
+    TypeError,
+    Reported,
+}
+
+#[derive(Copy, Clone, Eq, Hash, Ord, PartialEq, PartialOrd)]
+pub struct AllocId(pub NonZeroU64);
+
+// We want the `Debug` output to be readable as it is used by `derive(Debug)` for
+// all the Miri types.
+impl fmt::Debug for AllocId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if f.alternate() { write!(f, "a{}", self.0) } else { write!(f, "alloc{}", self.0) }
+    }
+}
+
+// No "Display" since AllocIds are not usually user-visible.
+
+#[derive(TyDecodable, TyEncodable)]
+enum AllocDiscriminant {
+    Alloc,
+    Fn,
+    VTable,
+    Static,
+}
+
+pub fn specialized_encode_alloc_id<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>>(
+    encoder: &mut E,
+    tcx: TyCtxt<'tcx>,
+    alloc_id: AllocId,
+) {
+    match tcx.global_alloc(alloc_id) {
+        GlobalAlloc::Memory(alloc) => {
+            trace!("encoding {:?} with {:#?}", alloc_id, alloc);
+            AllocDiscriminant::Alloc.encode(encoder);
+            alloc.encode(encoder);
+        }
+        GlobalAlloc::Function(fn_instance) => {
+            trace!("encoding {:?} with {:#?}", alloc_id, fn_instance);
+            AllocDiscriminant::Fn.encode(encoder);
+            fn_instance.encode(encoder);
+        }
+        GlobalAlloc::VTable(ty, poly_trait_ref) => {
+            trace!("encoding {:?} with {ty:#?}, {poly_trait_ref:#?}", alloc_id);
+            AllocDiscriminant::VTable.encode(encoder);
+            ty.encode(encoder);
+            poly_trait_ref.encode(encoder);
+        }
+        GlobalAlloc::Static(did) => {
+            assert!(!tcx.is_thread_local_static(did));
+            // References to statics doesn't need to know about their allocations,
+            // just about its `DefId`.
+            AllocDiscriminant::Static.encode(encoder);
+            did.encode(encoder);
+        }
+    }
+}
+
+// Used to avoid infinite recursion when decoding cyclic allocations.
+type DecodingSessionId = NonZeroU32;
+
+#[derive(Clone)]
+enum State {
+    Empty,
+    InProgressNonAlloc(TinyList<DecodingSessionId>),
+    InProgress(TinyList<DecodingSessionId>, AllocId),
+    Done(AllocId),
+}
+
+pub struct AllocDecodingState {
+    // For each `AllocId`, we keep track of which decoding state it's currently in.
+    decoding_state: Vec<Lock<State>>,
+    // The offsets of each allocation in the data stream.
+    data_offsets: Vec<u32>,
+}
+
+impl AllocDecodingState {
+    #[inline]
+    pub fn new_decoding_session(&self) -> AllocDecodingSession<'_> {
+        static DECODER_SESSION_ID: AtomicU32 = AtomicU32::new(0);
+        let counter = DECODER_SESSION_ID.fetch_add(1, Ordering::SeqCst);
+
+        // Make sure this is never zero.
+        let session_id = DecodingSessionId::new((counter & 0x7FFFFFFF) + 1).unwrap();
+
+        AllocDecodingSession { state: self, session_id }
+    }
+
+    pub fn new(data_offsets: Vec<u32>) -> Self {
+        let decoding_state = vec![Lock::new(State::Empty); data_offsets.len()];
+
+        Self { decoding_state, data_offsets }
+    }
+}
+
+#[derive(Copy, Clone)]
+pub struct AllocDecodingSession<'s> {
+    state: &'s AllocDecodingState,
+    session_id: DecodingSessionId,
+}
+
+impl<'s> AllocDecodingSession<'s> {
+    /// Decodes an `AllocId` in a thread-safe way.
+    pub fn decode_alloc_id<'tcx, D>(&self, decoder: &mut D) -> AllocId
+    where
+        D: TyDecoder<I = TyCtxt<'tcx>>,
+    {
+        // Read the index of the allocation.
+        let idx = usize::try_from(decoder.read_u32()).unwrap();
+        let pos = usize::try_from(self.state.data_offsets[idx]).unwrap();
+
+        // Decode the `AllocDiscriminant` now so that we know if we have to reserve an
+        // `AllocId`.
+        let (alloc_kind, pos) = decoder.with_position(pos, |decoder| {
+            let alloc_kind = AllocDiscriminant::decode(decoder);
+            (alloc_kind, decoder.position())
+        });
+
+        // Check the decoding state to see if it's already decoded or if we should
+        // decode it here.
+        let alloc_id = {
+            let mut entry = self.state.decoding_state[idx].lock();
+
+            match *entry {
+                State::Done(alloc_id) => {
+                    return alloc_id;
+                }
+                ref mut entry @ State::Empty => {
+                    // We are allowed to decode.
+                    match alloc_kind {
+                        AllocDiscriminant::Alloc => {
+                            // If this is an allocation, we need to reserve an
+                            // `AllocId` so we can decode cyclic graphs.
+                            let alloc_id = decoder.interner().reserve_alloc_id();
+                            *entry =
+                                State::InProgress(TinyList::new_single(self.session_id), alloc_id);
+                            Some(alloc_id)
+                        }
+                        AllocDiscriminant::Fn
+                        | AllocDiscriminant::Static
+                        | AllocDiscriminant::VTable => {
+                            // Fns and statics cannot be cyclic, and their `AllocId`
+                            // is determined later by interning.
+                            *entry =
+                                State::InProgressNonAlloc(TinyList::new_single(self.session_id));
+                            None
+                        }
+                    }
+                }
+                State::InProgressNonAlloc(ref mut sessions) => {
+                    if sessions.contains(&self.session_id) {
+                        bug!("this should be unreachable");
+                    } else {
+                        // Start decoding concurrently.
+                        sessions.insert(self.session_id);
+                        None
+                    }
+                }
+                State::InProgress(ref mut sessions, alloc_id) => {
+                    if sessions.contains(&self.session_id) {
+                        // Don't recurse.
+                        return alloc_id;
+                    } else {
+                        // Start decoding concurrently.
+                        sessions.insert(self.session_id);
+                        Some(alloc_id)
+                    }
+                }
+            }
+        };
+
+        // Now decode the actual data.
+        let alloc_id = decoder.with_position(pos, |decoder| {
+            match alloc_kind {
+                AllocDiscriminant::Alloc => {
+                    let alloc = <ConstAllocation<'tcx> as Decodable<_>>::decode(decoder);
+                    // We already have a reserved `AllocId`.
+                    let alloc_id = alloc_id.unwrap();
+                    trace!("decoded alloc {:?}: {:#?}", alloc_id, alloc);
+                    decoder.interner().set_alloc_id_same_memory(alloc_id, alloc);
+                    alloc_id
+                }
+                AllocDiscriminant::Fn => {
+                    assert!(alloc_id.is_none());
+                    trace!("creating fn alloc ID");
+                    let instance = ty::Instance::decode(decoder);
+                    trace!("decoded fn alloc instance: {:?}", instance);
+                    let alloc_id = decoder.interner().create_fn_alloc(instance);
+                    alloc_id
+                }
+                AllocDiscriminant::VTable => {
+                    assert!(alloc_id.is_none());
+                    trace!("creating vtable alloc ID");
+                    let ty = <Ty<'_> as Decodable<D>>::decode(decoder);
+                    let poly_trait_ref =
+                        <Option<ty::PolyExistentialTraitRef<'_>> as Decodable<D>>::decode(decoder);
+                    trace!("decoded vtable alloc instance: {ty:?}, {poly_trait_ref:?}");
+                    let alloc_id = decoder.interner().create_vtable_alloc(ty, poly_trait_ref);
+                    alloc_id
+                }
+                AllocDiscriminant::Static => {
+                    assert!(alloc_id.is_none());
+                    trace!("creating extern static alloc ID");
+                    let did = <DefId as Decodable<D>>::decode(decoder);
+                    trace!("decoded static def-ID: {:?}", did);
+                    let alloc_id = decoder.interner().create_static_alloc(did);
+                    alloc_id
+                }
+            }
+        });
+
+        self.state.decoding_state[idx].with_lock(|entry| {
+            *entry = State::Done(alloc_id);
+        });
+
+        alloc_id
+    }
+}
+
+/// An allocation in the global (tcx-managed) memory can be either a function pointer,
+/// a static, or a "real" allocation with some data in it.
+#[derive(Debug, Clone, Eq, PartialEq, Hash, TyDecodable, TyEncodable, HashStable)]
+pub enum GlobalAlloc<'tcx> {
+    /// The alloc ID is used as a function pointer.
+    Function(Instance<'tcx>),
+    /// This alloc ID points to a symbolic (not-reified) vtable.
+    VTable(Ty<'tcx>, Option<ty::PolyExistentialTraitRef<'tcx>>),
+    /// The alloc ID points to a "lazy" static variable that did not get computed (yet).
+    /// This is also used to break the cycle in recursive statics.
+    Static(DefId),
+    /// The alloc ID points to memory.
+    Memory(ConstAllocation<'tcx>),
+}
+
+impl<'tcx> GlobalAlloc<'tcx> {
+    /// Panics if the `GlobalAlloc` does not refer to an `GlobalAlloc::Memory`
+    #[track_caller]
+    #[inline]
+    pub fn unwrap_memory(&self) -> ConstAllocation<'tcx> {
+        match *self {
+            GlobalAlloc::Memory(mem) => mem,
+            _ => bug!("expected memory, got {:?}", self),
+        }
+    }
+
+    /// Panics if the `GlobalAlloc` is not `GlobalAlloc::Function`
+    #[track_caller]
+    #[inline]
+    pub fn unwrap_fn(&self) -> Instance<'tcx> {
+        match *self {
+            GlobalAlloc::Function(instance) => instance,
+            _ => bug!("expected function, got {:?}", self),
+        }
+    }
+
+    /// Panics if the `GlobalAlloc` is not `GlobalAlloc::VTable`
+    #[track_caller]
+    #[inline]
+    pub fn unwrap_vtable(&self) -> (Ty<'tcx>, Option<ty::PolyExistentialTraitRef<'tcx>>) {
+        match *self {
+            GlobalAlloc::VTable(ty, poly_trait_ref) => (ty, poly_trait_ref),
+            _ => bug!("expected vtable, got {:?}", self),
+        }
+    }
+}
+
+pub(crate) struct AllocMap<'tcx> {
+    /// Maps `AllocId`s to their corresponding allocations.
+    alloc_map: FxHashMap<AllocId, GlobalAlloc<'tcx>>,
+
+    /// Used to ensure that statics and functions only get one associated `AllocId`.
+    /// Should never contain a `GlobalAlloc::Memory`!
+    //
+    // FIXME: Should we just have two separate dedup maps for statics and functions each?
+    dedup: FxHashMap<GlobalAlloc<'tcx>, AllocId>,
+
+    /// The `AllocId` to assign to the next requested ID.
+    /// Always incremented; never gets smaller.
+    next_id: AllocId,
+}
+
+impl<'tcx> AllocMap<'tcx> {
+    pub(crate) fn new() -> Self {
+        AllocMap {
+            alloc_map: Default::default(),
+            dedup: Default::default(),
+            next_id: AllocId(NonZeroU64::new(1).unwrap()),
+        }
+    }
+    fn reserve(&mut self) -> AllocId {
+        let next = self.next_id;
+        self.next_id.0 = self.next_id.0.checked_add(1).expect(
+            "You overflowed a u64 by incrementing by 1... \
+             You've just earned yourself a free drink if we ever meet. \
+             Seriously, how did you do that?!",
+        );
+        next
+    }
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Obtains a new allocation ID that can be referenced but does not
+    /// yet have an allocation backing it.
+    ///
+    /// Make sure to call `set_alloc_id_memory` or `set_alloc_id_same_memory` before returning such
+    /// an `AllocId` from a query.
+    pub fn reserve_alloc_id(self) -> AllocId {
+        self.alloc_map.lock().reserve()
+    }
+
+    /// Reserves a new ID *if* this allocation has not been dedup-reserved before.
+    /// Should only be used for "symbolic" allocations (function pointers, vtables, statics), we
+    /// don't want to dedup IDs for "real" memory!
+    fn reserve_and_set_dedup(self, alloc: GlobalAlloc<'tcx>) -> AllocId {
+        let mut alloc_map = self.alloc_map.lock();
+        match alloc {
+            GlobalAlloc::Function(..) | GlobalAlloc::Static(..) | GlobalAlloc::VTable(..) => {}
+            GlobalAlloc::Memory(..) => bug!("Trying to dedup-reserve memory with real data!"),
+        }
+        if let Some(&alloc_id) = alloc_map.dedup.get(&alloc) {
+            return alloc_id;
+        }
+        let id = alloc_map.reserve();
+        debug!("creating alloc {alloc:?} with id {id:?}");
+        alloc_map.alloc_map.insert(id, alloc.clone());
+        alloc_map.dedup.insert(alloc, id);
+        id
+    }
+
+    /// Generates an `AllocId` for a static or return a cached one in case this function has been
+    /// called on the same static before.
+    pub fn create_static_alloc(self, static_id: DefId) -> AllocId {
+        self.reserve_and_set_dedup(GlobalAlloc::Static(static_id))
+    }
+
+    /// Generates an `AllocId` for a function.  Depending on the function type,
+    /// this might get deduplicated or assigned a new ID each time.
+    pub fn create_fn_alloc(self, instance: Instance<'tcx>) -> AllocId {
+        // Functions cannot be identified by pointers, as asm-equal functions can get deduplicated
+        // by the linker (we set the "unnamed_addr" attribute for LLVM) and functions can be
+        // duplicated across crates.
+        // We thus generate a new `AllocId` for every mention of a function. This means that
+        // `main as fn() == main as fn()` is false, while `let x = main as fn(); x == x` is true.
+        // However, formatting code relies on function identity (see #58320), so we only do
+        // this for generic functions.  Lifetime parameters are ignored.
+        let is_generic = instance
+            .substs
+            .into_iter()
+            .any(|kind| !matches!(kind.unpack(), GenericArgKind::Lifetime(_)));
+        if is_generic {
+            // Get a fresh ID.
+            let mut alloc_map = self.alloc_map.lock();
+            let id = alloc_map.reserve();
+            alloc_map.alloc_map.insert(id, GlobalAlloc::Function(instance));
+            id
+        } else {
+            // Deduplicate.
+            self.reserve_and_set_dedup(GlobalAlloc::Function(instance))
+        }
+    }
+
+    /// Generates an `AllocId` for a (symbolic, not-reified) vtable.  Will get deduplicated.
+    pub fn create_vtable_alloc(
+        self,
+        ty: Ty<'tcx>,
+        poly_trait_ref: Option<ty::PolyExistentialTraitRef<'tcx>>,
+    ) -> AllocId {
+        self.reserve_and_set_dedup(GlobalAlloc::VTable(ty, poly_trait_ref))
+    }
+
+    /// Interns the `Allocation` and return a new `AllocId`, even if there's already an identical
+    /// `Allocation` with a different `AllocId`.
+    /// Statics with identical content will still point to the same `Allocation`, i.e.,
+    /// their data will be deduplicated through `Allocation` interning -- but they
+    /// are different places in memory and as such need different IDs.
+    pub fn create_memory_alloc(self, mem: ConstAllocation<'tcx>) -> AllocId {
+        let id = self.reserve_alloc_id();
+        self.set_alloc_id_memory(id, mem);
+        id
+    }
+
+    /// Returns `None` in case the `AllocId` is dangling. An `InterpretCx` can still have a
+    /// local `Allocation` for that `AllocId`, but having such an `AllocId` in a constant is
+    /// illegal and will likely ICE.
+    /// This function exists to allow const eval to detect the difference between evaluation-
+    /// local dangling pointers and allocations in constants/statics.
+    #[inline]
+    pub fn try_get_global_alloc(self, id: AllocId) -> Option<GlobalAlloc<'tcx>> {
+        self.alloc_map.lock().alloc_map.get(&id).cloned()
+    }
+
+    #[inline]
+    #[track_caller]
+    /// Panics in case the `AllocId` is dangling. Since that is impossible for `AllocId`s in
+    /// constants (as all constants must pass interning and validation that check for dangling
+    /// ids), this function is frequently used throughout rustc, but should not be used within
+    /// the miri engine.
+    pub fn global_alloc(self, id: AllocId) -> GlobalAlloc<'tcx> {
+        match self.try_get_global_alloc(id) {
+            Some(alloc) => alloc,
+            None => bug!("could not find allocation for {id:?}"),
+        }
+    }
+
+    /// Freezes an `AllocId` created with `reserve` by pointing it at an `Allocation`. Trying to
+    /// call this function twice, even with the same `Allocation` will ICE the compiler.
+    pub fn set_alloc_id_memory(self, id: AllocId, mem: ConstAllocation<'tcx>) {
+        if let Some(old) = self.alloc_map.lock().alloc_map.insert(id, GlobalAlloc::Memory(mem)) {
+            bug!("tried to set allocation ID {id:?}, but it was already existing as {old:#?}");
+        }
+    }
+
+    /// Freezes an `AllocId` created with `reserve` by pointing it at an `Allocation`. May be called
+    /// twice for the same `(AllocId, Allocation)` pair.
+    fn set_alloc_id_same_memory(self, id: AllocId, mem: ConstAllocation<'tcx>) {
+        self.alloc_map.lock().alloc_map.insert_same(id, GlobalAlloc::Memory(mem));
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Methods to access integers in the target endianness
+////////////////////////////////////////////////////////////////////////////////
+
+#[inline]
+pub fn write_target_uint(
+    endianness: Endian,
+    mut target: &mut [u8],
+    data: u128,
+) -> Result<(), io::Error> {
+    // This u128 holds an "any-size uint" (since smaller uints can fits in it)
+    // So we do not write all bytes of the u128, just the "payload".
+    match endianness {
+        Endian::Little => target.write(&data.to_le_bytes())?,
+        Endian::Big => target.write(&data.to_be_bytes()[16 - target.len()..])?,
+    };
+    debug_assert!(target.len() == 0); // We should have filled the target buffer.
+    Ok(())
+}
+
+#[inline]
+pub fn read_target_uint(endianness: Endian, mut source: &[u8]) -> Result<u128, io::Error> {
+    // This u128 holds an "any-size uint" (since smaller uints can fits in it)
+    let mut buf = [0u8; std::mem::size_of::<u128>()];
+    // So we do not read exactly 16 bytes into the u128, just the "payload".
+    let uint = match endianness {
+        Endian::Little => {
+            source.read(&mut buf)?;
+            Ok(u128::from_le_bytes(buf))
+        }
+        Endian::Big => {
+            source.read(&mut buf[16 - source.len()..])?;
+            Ok(u128::from_be_bytes(buf))
+        }
+    };
+    debug_assert!(source.len() == 0); // We should have consumed the source buffer.
+    uint
+}
diff --git a/compiler/rustc_middle/src/mir/interpret/pointer.rs b/compiler/rustc_middle/src/mir/interpret/pointer.rs
new file mode 100644
index 000000000..384954cbb
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/interpret/pointer.rs
@@ -0,0 +1,307 @@
+use super::{AllocId, InterpResult};
+
+use rustc_macros::HashStable;
+use rustc_target::abi::{HasDataLayout, Size};
+
+use std::convert::{TryFrom, TryInto};
+use std::fmt;
+
+////////////////////////////////////////////////////////////////////////////////
+// Pointer arithmetic
+////////////////////////////////////////////////////////////////////////////////
+
+pub trait PointerArithmetic: HasDataLayout {
+    // These are not supposed to be overridden.
+
+    #[inline(always)]
+    fn pointer_size(&self) -> Size {
+        self.data_layout().pointer_size
+    }
+
+    #[inline(always)]
+    fn max_size_of_val(&self) -> Size {
+        Size::from_bytes(self.machine_isize_max())
+    }
+
+    #[inline]
+    fn machine_usize_max(&self) -> u64 {
+        self.pointer_size().unsigned_int_max().try_into().unwrap()
+    }
+
+    #[inline]
+    fn machine_isize_min(&self) -> i64 {
+        self.pointer_size().signed_int_min().try_into().unwrap()
+    }
+
+    #[inline]
+    fn machine_isize_max(&self) -> i64 {
+        self.pointer_size().signed_int_max().try_into().unwrap()
+    }
+
+    #[inline]
+    fn machine_usize_to_isize(&self, val: u64) -> i64 {
+        let val = val as i64;
+        // Now wrap-around into the machine_isize range.
+        if val > self.machine_isize_max() {
+            // This can only happen the the ptr size is < 64, so we know max_usize_plus_1 fits into
+            // i64.
+            debug_assert!(self.pointer_size().bits() < 64);
+            let max_usize_plus_1 = 1u128 << self.pointer_size().bits();
+            val - i64::try_from(max_usize_plus_1).unwrap()
+        } else {
+            val
+        }
+    }
+
+    /// Helper function: truncate given value-"overflowed flag" pair to pointer size and
+    /// update "overflowed flag" if there was an overflow.
+    /// This should be called by all the other methods before returning!
+    #[inline]
+    fn truncate_to_ptr(&self, (val, over): (u64, bool)) -> (u64, bool) {
+        let val = u128::from(val);
+        let max_ptr_plus_1 = 1u128 << self.pointer_size().bits();
+        (u64::try_from(val % max_ptr_plus_1).unwrap(), over || val >= max_ptr_plus_1)
+    }
+
+    #[inline]
+    fn overflowing_offset(&self, val: u64, i: u64) -> (u64, bool) {
+        // We do not need to check if i fits in a machine usize. If it doesn't,
+        // either the wrapping_add will wrap or res will not fit in a pointer.
+        let res = val.overflowing_add(i);
+        self.truncate_to_ptr(res)
+    }
+
+    #[inline]
+    fn overflowing_signed_offset(&self, val: u64, i: i64) -> (u64, bool) {
+        // We need to make sure that i fits in a machine isize.
+        let n = i.unsigned_abs();
+        if i >= 0 {
+            let (val, over) = self.overflowing_offset(val, n);
+            (val, over || i > self.machine_isize_max())
+        } else {
+            let res = val.overflowing_sub(n);
+            let (val, over) = self.truncate_to_ptr(res);
+            (val, over || i < self.machine_isize_min())
+        }
+    }
+
+    #[inline]
+    fn offset<'tcx>(&self, val: u64, i: u64) -> InterpResult<'tcx, u64> {
+        let (res, over) = self.overflowing_offset(val, i);
+        if over { throw_ub!(PointerArithOverflow) } else { Ok(res) }
+    }
+
+    #[inline]
+    fn signed_offset<'tcx>(&self, val: u64, i: i64) -> InterpResult<'tcx, u64> {
+        let (res, over) = self.overflowing_signed_offset(val, i);
+        if over { throw_ub!(PointerArithOverflow) } else { Ok(res) }
+    }
+}
+
+impl<T: HasDataLayout> PointerArithmetic for T {}
+
+/// This trait abstracts over the kind of provenance that is associated with a `Pointer`. It is
+/// mostly opaque; the `Machine` trait extends it with some more operations that also have access to
+/// some global state.
+/// We don't actually care about this `Debug` bound (we use `Provenance::fmt` to format the entire
+/// pointer), but `derive` adds some unnecessary bounds.
+pub trait Provenance: Copy + fmt::Debug {
+    /// Says whether the `offset` field of `Pointer`s with this provenance is the actual physical address.
+    /// If `true, ptr-to-int casts work by simply discarding the provenance.
+    /// If `false`, ptr-to-int casts are not supported. The offset *must* be relative in that case.
+    const OFFSET_IS_ADDR: bool;
+
+    /// We also use this trait to control whether to abort execution when a pointer is being partially overwritten
+    /// (this avoids a separate trait in `allocation.rs` just for this purpose).
+    const ERR_ON_PARTIAL_PTR_OVERWRITE: bool;
+
+    /// Determines how a pointer should be printed.
+    fn fmt(ptr: &Pointer<Self>, f: &mut fmt::Formatter<'_>) -> fmt::Result
+    where
+        Self: Sized;
+
+    /// If `OFFSET_IS_ADDR == false`, provenance must always be able to
+    /// identify the allocation this ptr points to (i.e., this must return `Some`).
+    /// Otherwise this function is best-effort (but must agree with `Machine::ptr_get_alloc`).
+    /// (Identifying the offset in that allocation, however, is harder -- use `Memory::ptr_get_alloc` for that.)
+    fn get_alloc_id(self) -> Option<AllocId>;
+}
+
+impl Provenance for AllocId {
+    // With the `AllocId` as provenance, the `offset` is interpreted *relative to the allocation*,
+    // so ptr-to-int casts are not possible (since we do not know the global physical offset).
+    const OFFSET_IS_ADDR: bool = false;
+
+    // For now, do not allow this, so that we keep our options open.
+    const ERR_ON_PARTIAL_PTR_OVERWRITE: bool = true;
+
+    fn fmt(ptr: &Pointer<Self>, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // Forward `alternate` flag to `alloc_id` printing.
+        if f.alternate() {
+            write!(f, "{:#?}", ptr.provenance)?;
+        } else {
+            write!(f, "{:?}", ptr.provenance)?;
+        }
+        // Print offset only if it is non-zero.
+        if ptr.offset.bytes() > 0 {
+            write!(f, "+{:#x}", ptr.offset.bytes())?;
+        }
+        Ok(())
+    }
+
+    fn get_alloc_id(self) -> Option<AllocId> {
+        Some(self)
+    }
+}
+
+/// Represents a pointer in the Miri engine.
+///
+/// Pointers are "tagged" with provenance information; typically the `AllocId` they belong to.
+#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, TyEncodable, TyDecodable, Hash)]
+#[derive(HashStable)]
+pub struct Pointer<Prov = AllocId> {
+    pub(super) offset: Size, // kept private to avoid accidental misinterpretation (meaning depends on `Prov` type)
+    pub provenance: Prov,
+}
+
+static_assert_size!(Pointer, 16);
+// `Option<Prov>` pointers are also passed around quite a bit
+// (but not stored in permanent machine state).
+static_assert_size!(Pointer<Option<AllocId>>, 16);
+
+// We want the `Debug` output to be readable as it is used by `derive(Debug)` for
+// all the Miri types.
+impl<Prov: Provenance> fmt::Debug for Pointer<Prov> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        Provenance::fmt(self, f)
+    }
+}
+
+impl<Prov: Provenance> fmt::Debug for Pointer<Option<Prov>> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.provenance {
+            Some(prov) => Provenance::fmt(&Pointer::new(prov, self.offset), f),
+            None => write!(f, "{:#x}[noalloc]", self.offset.bytes()),
+        }
+    }
+}
+
+impl<Prov: Provenance> fmt::Display for Pointer<Option<Prov>> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.provenance.is_none() && self.offset.bytes() == 0 {
+            write!(f, "null pointer")
+        } else {
+            fmt::Debug::fmt(self, f)
+        }
+    }
+}
+
+/// Produces a `Pointer` that points to the beginning of the `Allocation`.
+impl From<AllocId> for Pointer {
+    #[inline(always)]
+    fn from(alloc_id: AllocId) -> Self {
+        Pointer::new(alloc_id, Size::ZERO)
+    }
+}
+
+impl<Prov> From<Pointer<Prov>> for Pointer<Option<Prov>> {
+    #[inline(always)]
+    fn from(ptr: Pointer<Prov>) -> Self {
+        let (prov, offset) = ptr.into_parts();
+        Pointer::new(Some(prov), offset)
+    }
+}
+
+impl<Prov> Pointer<Option<Prov>> {
+    /// Convert this pointer that *might* have a provenance into a pointer that *definitely* has a
+    /// provenance, or an absolute address.
+    ///
+    /// This is rarely what you want; call `ptr_try_get_alloc_id` instead.
+    pub fn into_pointer_or_addr(self) -> Result<Pointer<Prov>, Size> {
+        match self.provenance {
+            Some(prov) => Ok(Pointer::new(prov, self.offset)),
+            None => Err(self.offset),
+        }
+    }
+
+    /// Returns the absolute address the pointer points to.
+    /// Only works if Prov::OFFSET_IS_ADDR is true!
+    pub fn addr(self) -> Size
+    where
+        Prov: Provenance,
+    {
+        assert!(Prov::OFFSET_IS_ADDR);
+        self.offset
+    }
+}
+
+impl<Prov> Pointer<Option<Prov>> {
+    #[inline(always)]
+    pub fn from_addr(addr: u64) -> Self {
+        Pointer { provenance: None, offset: Size::from_bytes(addr) }
+    }
+
+    #[inline(always)]
+    pub fn null() -> Self {
+        Pointer::from_addr(0)
+    }
+}
+
+impl<'tcx, Prov> Pointer<Prov> {
+    #[inline(always)]
+    pub fn new(provenance: Prov, offset: Size) -> Self {
+        Pointer { provenance, offset }
+    }
+
+    /// Obtain the constituents of this pointer. Not that the meaning of the offset depends on the type `Prov`!
+    /// This function must only be used in the implementation of `Machine::ptr_get_alloc`,
+    /// and when a `Pointer` is taken apart to be stored efficiently in an `Allocation`.
+    #[inline(always)]
+    pub fn into_parts(self) -> (Prov, Size) {
+        (self.provenance, self.offset)
+    }
+
+    pub fn map_provenance(self, f: impl FnOnce(Prov) -> Prov) -> Self {
+        Pointer { provenance: f(self.provenance), ..self }
+    }
+
+    #[inline]
+    pub fn offset(self, i: Size, cx: &impl HasDataLayout) -> InterpResult<'tcx, Self> {
+        Ok(Pointer {
+            offset: Size::from_bytes(cx.data_layout().offset(self.offset.bytes(), i.bytes())?),
+            ..self
+        })
+    }
+
+    #[inline]
+    pub fn overflowing_offset(self, i: Size, cx: &impl HasDataLayout) -> (Self, bool) {
+        let (res, over) = cx.data_layout().overflowing_offset(self.offset.bytes(), i.bytes());
+        let ptr = Pointer { offset: Size::from_bytes(res), ..self };
+        (ptr, over)
+    }
+
+    #[inline(always)]
+    pub fn wrapping_offset(self, i: Size, cx: &impl HasDataLayout) -> Self {
+        self.overflowing_offset(i, cx).0
+    }
+
+    #[inline]
+    pub fn signed_offset(self, i: i64, cx: &impl HasDataLayout) -> InterpResult<'tcx, Self> {
+        Ok(Pointer {
+            offset: Size::from_bytes(cx.data_layout().signed_offset(self.offset.bytes(), i)?),
+            ..self
+        })
+    }
+
+    #[inline]
+    pub fn overflowing_signed_offset(self, i: i64, cx: &impl HasDataLayout) -> (Self, bool) {
+        let (res, over) = cx.data_layout().overflowing_signed_offset(self.offset.bytes(), i);
+        let ptr = Pointer { offset: Size::from_bytes(res), ..self };
+        (ptr, over)
+    }
+
+    #[inline(always)]
+    pub fn wrapping_signed_offset(self, i: i64, cx: &impl HasDataLayout) -> Self {
+        self.overflowing_signed_offset(i, cx).0
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/interpret/queries.rs b/compiler/rustc_middle/src/mir/interpret/queries.rs
new file mode 100644
index 000000000..786927e2d
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/interpret/queries.rs
@@ -0,0 +1,217 @@
+use super::{ErrorHandled, EvalToConstValueResult, EvalToValTreeResult, GlobalId};
+
+use crate::mir;
+use crate::ty::subst::InternalSubsts;
+use crate::ty::visit::TypeVisitable;
+use crate::ty::{self, query::TyCtxtAt, query::TyCtxtEnsure, TyCtxt};
+use rustc_hir::def_id::DefId;
+use rustc_span::{Span, DUMMY_SP};
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Evaluates a constant without providing any substitutions. This is useful to evaluate consts
+    /// that can't take any generic arguments like statics, const items or enum discriminants. If a
+    /// generic parameter is used within the constant `ErrorHandled::ToGeneric` will be returned.
+    #[instrument(skip(self), level = "debug")]
+    pub fn const_eval_poly(self, def_id: DefId) -> EvalToConstValueResult<'tcx> {
+        // In some situations def_id will have substitutions within scope, but they aren't allowed
+        // to be used. So we can't use `Instance::mono`, instead we feed unresolved substitutions
+        // into `const_eval` which will return `ErrorHandled::ToGeneric` if any of them are
+        // encountered.
+        let substs = InternalSubsts::identity_for_item(self, def_id);
+        let instance = ty::Instance::new(def_id, substs);
+        let cid = GlobalId { instance, promoted: None };
+        let param_env = self.param_env(def_id).with_reveal_all_normalized(self);
+        self.const_eval_global_id(param_env, cid, None)
+    }
+    /// Resolves and evaluates a constant.
+    ///
+    /// The constant can be located on a trait like `<A as B>::C`, in which case the given
+    /// substitutions and environment are used to resolve the constant. Alternatively if the
+    /// constant has generic parameters in scope the substitutions are used to evaluate the value of
+    /// the constant. For example in `fn foo<T>() { let _ = [0; bar::<T>()]; }` the repeat count
+    /// constant `bar::<T>()` requires a substitution for `T`, if the substitution for `T` is still
+    /// too generic for the constant to be evaluated then `Err(ErrorHandled::TooGeneric)` is
+    /// returned.
+    #[instrument(level = "debug", skip(self))]
+    pub fn const_eval_resolve(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        ct: ty::Unevaluated<'tcx>,
+        span: Option<Span>,
+    ) -> EvalToConstValueResult<'tcx> {
+        // Cannot resolve `Unevaluated` constants that contain inference
+        // variables. We reject those here since `resolve_opt_const_arg`
+        // would fail otherwise.
+        //
+        // When trying to evaluate constants containing inference variables,
+        // use `Infcx::const_eval_resolve` instead.
+        if ct.substs.has_infer_types_or_consts() {
+            bug!("did not expect inference variables here");
+        }
+
+        match ty::Instance::resolve_opt_const_arg(self, param_env, ct.def, ct.substs) {
+            Ok(Some(instance)) => {
+                let cid = GlobalId { instance, promoted: ct.promoted };
+                self.const_eval_global_id(param_env, cid, span)
+            }
+            Ok(None) => Err(ErrorHandled::TooGeneric),
+            Err(error_reported) => Err(ErrorHandled::Reported(error_reported)),
+        }
+    }
+
+    #[instrument(level = "debug", skip(self))]
+    pub fn const_eval_resolve_for_typeck(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        ct: ty::Unevaluated<'tcx>,
+        span: Option<Span>,
+    ) -> EvalToValTreeResult<'tcx> {
+        // Cannot resolve `Unevaluated` constants that contain inference
+        // variables. We reject those here since `resolve_opt_const_arg`
+        // would fail otherwise.
+        //
+        // When trying to evaluate constants containing inference variables,
+        // use `Infcx::const_eval_resolve` instead.
+        if ct.substs.has_infer_types_or_consts() {
+            bug!("did not expect inference variables here");
+        }
+
+        match ty::Instance::resolve_opt_const_arg(self, param_env, ct.def, ct.substs) {
+            Ok(Some(instance)) => {
+                let cid = GlobalId { instance, promoted: ct.promoted };
+                self.const_eval_global_id_for_typeck(param_env, cid, span)
+            }
+            Ok(None) => Err(ErrorHandled::TooGeneric),
+            Err(error_reported) => Err(ErrorHandled::Reported(error_reported)),
+        }
+    }
+
+    pub fn const_eval_instance(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        instance: ty::Instance<'tcx>,
+        span: Option<Span>,
+    ) -> EvalToConstValueResult<'tcx> {
+        self.const_eval_global_id(param_env, GlobalId { instance, promoted: None }, span)
+    }
+
+    /// Evaluate a constant to a `ConstValue`.
+    #[instrument(skip(self), level = "debug")]
+    pub fn const_eval_global_id(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        cid: GlobalId<'tcx>,
+        span: Option<Span>,
+    ) -> EvalToConstValueResult<'tcx> {
+        let param_env = param_env.with_const();
+        // Const-eval shouldn't depend on lifetimes at all, so we can erase them, which should
+        // improve caching of queries.
+        let inputs = self.erase_regions(param_env.and(cid));
+        if let Some(span) = span {
+            self.at(span).eval_to_const_value_raw(inputs)
+        } else {
+            self.eval_to_const_value_raw(inputs)
+        }
+    }
+
+    /// Evaluate a constant to a type-level constant.
+    #[instrument(skip(self), level = "debug")]
+    pub fn const_eval_global_id_for_typeck(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        cid: GlobalId<'tcx>,
+        span: Option<Span>,
+    ) -> EvalToValTreeResult<'tcx> {
+        let param_env = param_env.with_const();
+        debug!(?param_env);
+        // Const-eval shouldn't depend on lifetimes at all, so we can erase them, which should
+        // improve caching of queries.
+        let inputs = self.erase_regions(param_env.and(cid));
+        debug!(?inputs);
+        if let Some(span) = span {
+            self.at(span).eval_to_valtree(inputs)
+        } else {
+            self.eval_to_valtree(inputs)
+        }
+    }
+
+    /// Evaluate a static's initializer, returning the allocation of the initializer's memory.
+    #[inline(always)]
+    pub fn eval_static_initializer(
+        self,
+        def_id: DefId,
+    ) -> Result<mir::ConstAllocation<'tcx>, ErrorHandled> {
+        self.at(DUMMY_SP).eval_static_initializer(def_id)
+    }
+}
+
+impl<'tcx> TyCtxtAt<'tcx> {
+    /// Evaluate a static's initializer, returning the allocation of the initializer's memory.
+    pub fn eval_static_initializer(
+        self,
+        def_id: DefId,
+    ) -> Result<mir::ConstAllocation<'tcx>, ErrorHandled> {
+        trace!("eval_static_initializer: Need to compute {:?}", def_id);
+        assert!(self.is_static(def_id));
+        let instance = ty::Instance::mono(*self, def_id);
+        let gid = GlobalId { instance, promoted: None };
+        self.eval_to_allocation(gid, ty::ParamEnv::reveal_all())
+    }
+
+    /// Evaluate anything constant-like, returning the allocation of the final memory.
+    fn eval_to_allocation(
+        self,
+        gid: GlobalId<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> Result<mir::ConstAllocation<'tcx>, ErrorHandled> {
+        let param_env = param_env.with_const();
+        trace!("eval_to_allocation: Need to compute {:?}", gid);
+        let raw_const = self.eval_to_allocation_raw(param_env.and(gid))?;
+        Ok(self.global_alloc(raw_const.alloc_id).unwrap_memory())
+    }
+}
+
+impl<'tcx> TyCtxtEnsure<'tcx> {
+    /// Evaluates a constant without providing any substitutions. This is useful to evaluate consts
+    /// that can't take any generic arguments like statics, const items or enum discriminants. If a
+    /// generic parameter is used within the constant `ErrorHandled::ToGeneric` will be returned.
+    #[instrument(skip(self), level = "debug")]
+    pub fn const_eval_poly(self, def_id: DefId) {
+        // In some situations def_id will have substitutions within scope, but they aren't allowed
+        // to be used. So we can't use `Instance::mono`, instead we feed unresolved substitutions
+        // into `const_eval` which will return `ErrorHandled::ToGeneric` if any of them are
+        // encountered.
+        let substs = InternalSubsts::identity_for_item(self.tcx, def_id);
+        let instance = ty::Instance::new(def_id, substs);
+        let cid = GlobalId { instance, promoted: None };
+        let param_env =
+            self.tcx.param_env(def_id).with_reveal_all_normalized(self.tcx).with_const();
+        // Const-eval shouldn't depend on lifetimes at all, so we can erase them, which should
+        // improve caching of queries.
+        let inputs = self.tcx.erase_regions(param_env.and(cid));
+        self.eval_to_const_value_raw(inputs)
+    }
+
+    /// Evaluate a static's initializer, returning the allocation of the initializer's memory.
+    pub fn eval_static_initializer(self, def_id: DefId) {
+        trace!("eval_static_initializer: Need to compute {:?}", def_id);
+        assert!(self.tcx.is_static(def_id));
+        let instance = ty::Instance::mono(self.tcx, def_id);
+        let gid = GlobalId { instance, promoted: None };
+        let param_env = ty::ParamEnv::reveal_all().with_const();
+        trace!("eval_to_allocation: Need to compute {:?}", gid);
+        self.eval_to_allocation_raw(param_env.and(gid))
+    }
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Destructure a mir constant ADT or array into its variant index and its field values.
+    /// Panics if the destructuring fails, use `try_destructure_mir_constant` for fallible version.
+    pub fn destructure_mir_constant(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        constant: mir::ConstantKind<'tcx>,
+    ) -> mir::DestructuredMirConstant<'tcx> {
+        self.try_destructure_mir_constant(param_env.and(constant)).unwrap()
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/interpret/value.rs b/compiler/rustc_middle/src/mir/interpret/value.rs
new file mode 100644
index 000000000..834c114ee
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/interpret/value.rs
@@ -0,0 +1,651 @@
+use std::convert::{TryFrom, TryInto};
+use std::fmt;
+
+use rustc_apfloat::{
+    ieee::{Double, Single},
+    Float,
+};
+use rustc_macros::HashStable;
+use rustc_target::abi::{HasDataLayout, Size};
+
+use crate::ty::{Lift, ParamEnv, ScalarInt, Ty, TyCtxt};
+
+use super::{
+    AllocId, AllocRange, ConstAllocation, InterpResult, Pointer, PointerArithmetic, Provenance,
+    ScalarSizeMismatch,
+};
+
+/// Represents the result of const evaluation via the `eval_to_allocation` query.
+#[derive(Copy, Clone, HashStable, TyEncodable, TyDecodable, Debug, Hash, Eq, PartialEq)]
+pub struct ConstAlloc<'tcx> {
+    // the value lives here, at offset 0, and that allocation definitely is an `AllocKind::Memory`
+    // (so you can use `AllocMap::unwrap_memory`).
+    pub alloc_id: AllocId,
+    pub ty: Ty<'tcx>,
+}
+
+/// Represents a constant value in Rust. `Scalar` and `Slice` are optimizations for
+/// array length computations, enum discriminants and the pattern matching logic.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable, Hash)]
+#[derive(HashStable)]
+pub enum ConstValue<'tcx> {
+    /// Used only for types with `layout::abi::Scalar` ABI.
+    ///
+    /// Not using the enum `Value` to encode that this must not be `Uninit`.
+    Scalar(Scalar),
+
+    /// Only used for ZSTs.
+    ZeroSized,
+
+    /// Used only for `&[u8]` and `&str`
+    Slice { data: ConstAllocation<'tcx>, start: usize, end: usize },
+
+    /// A value not represented/representable by `Scalar` or `Slice`
+    ByRef {
+        /// The backing memory of the value, may contain more memory than needed for just the value
+        /// in order to share `ConstAllocation`s between values
+        alloc: ConstAllocation<'tcx>,
+        /// Offset into `alloc`
+        offset: Size,
+    },
+}
+
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(ConstValue<'_>, 32);
+
+impl<'a, 'tcx> Lift<'tcx> for ConstValue<'a> {
+    type Lifted = ConstValue<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<ConstValue<'tcx>> {
+        Some(match self {
+            ConstValue::Scalar(s) => ConstValue::Scalar(s),
+            ConstValue::ZeroSized => ConstValue::ZeroSized,
+            ConstValue::Slice { data, start, end } => {
+                ConstValue::Slice { data: tcx.lift(data)?, start, end }
+            }
+            ConstValue::ByRef { alloc, offset } => {
+                ConstValue::ByRef { alloc: tcx.lift(alloc)?, offset }
+            }
+        })
+    }
+}
+
+impl<'tcx> ConstValue<'tcx> {
+    #[inline]
+    pub fn try_to_scalar(&self) -> Option<Scalar<AllocId>> {
+        match *self {
+            ConstValue::ByRef { .. } | ConstValue::Slice { .. } | ConstValue::ZeroSized => None,
+            ConstValue::Scalar(val) => Some(val),
+        }
+    }
+
+    pub fn try_to_scalar_int(&self) -> Option<ScalarInt> {
+        Some(self.try_to_scalar()?.assert_int())
+    }
+
+    pub fn try_to_bits(&self, size: Size) -> Option<u128> {
+        self.try_to_scalar_int()?.to_bits(size).ok()
+    }
+
+    pub fn try_to_bool(&self) -> Option<bool> {
+        self.try_to_scalar_int()?.try_into().ok()
+    }
+
+    pub fn try_to_machine_usize(&self, tcx: TyCtxt<'tcx>) -> Option<u64> {
+        self.try_to_scalar_int()?.try_to_machine_usize(tcx).ok()
+    }
+
+    pub fn try_to_bits_for_ty(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ParamEnv<'tcx>,
+        ty: Ty<'tcx>,
+    ) -> Option<u128> {
+        let size = tcx.layout_of(param_env.with_reveal_all_normalized(tcx).and(ty)).ok()?.size;
+        self.try_to_bits(size)
+    }
+
+    pub fn from_bool(b: bool) -> Self {
+        ConstValue::Scalar(Scalar::from_bool(b))
+    }
+
+    pub fn from_u64(i: u64) -> Self {
+        ConstValue::Scalar(Scalar::from_u64(i))
+    }
+
+    pub fn from_machine_usize(i: u64, cx: &impl HasDataLayout) -> Self {
+        ConstValue::Scalar(Scalar::from_machine_usize(i, cx))
+    }
+}
+
+/// A `Scalar` represents an immediate, primitive value existing outside of a
+/// `memory::Allocation`. It is in many ways like a small chunk of an `Allocation`, up to 16 bytes in
+/// size. Like a range of bytes in an `Allocation`, a `Scalar` can either represent the raw bytes
+/// of a simple value or a pointer into another `Allocation`
+///
+/// These variants would be private if there was a convenient way to achieve that in Rust.
+/// Do *not* match on a `Scalar`! Use the various `to_*` methods instead.
+#[derive(Clone, Copy, Eq, PartialEq, Ord, PartialOrd, TyEncodable, TyDecodable, Hash)]
+#[derive(HashStable)]
+pub enum Scalar<Prov = AllocId> {
+    /// The raw bytes of a simple value.
+    Int(ScalarInt),
+
+    /// A pointer into an `Allocation`. An `Allocation` in the `memory` module has a list of
+    /// relocations, but a `Scalar` is only large enough to contain one, so we just represent the
+    /// relocation and its associated offset together as a `Pointer` here.
+    ///
+    /// We also store the size of the pointer, such that a `Scalar` always knows how big it is.
+    /// The size is always the pointer size of the current target, but this is not information
+    /// that we always have readily available.
+    Ptr(Pointer<Prov>, u8),
+}
+
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(Scalar, 24);
+
+// We want the `Debug` output to be readable as it is used by `derive(Debug)` for
+// all the Miri types.
+impl<Prov: Provenance> fmt::Debug for Scalar<Prov> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Scalar::Ptr(ptr, _size) => write!(f, "{:?}", ptr),
+            Scalar::Int(int) => write!(f, "{:?}", int),
+        }
+    }
+}
+
+impl<Prov: Provenance> fmt::Display for Scalar<Prov> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Scalar::Ptr(ptr, _size) => write!(f, "pointer to {:?}", ptr),
+            Scalar::Int(int) => write!(f, "{}", int),
+        }
+    }
+}
+
+impl<Prov: Provenance> fmt::LowerHex for Scalar<Prov> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Scalar::Ptr(ptr, _size) => write!(f, "pointer to {:?}", ptr),
+            Scalar::Int(int) => write!(f, "{:#x}", int),
+        }
+    }
+}
+
+impl<Prov> From<Single> for Scalar<Prov> {
+    #[inline(always)]
+    fn from(f: Single) -> Self {
+        Scalar::from_f32(f)
+    }
+}
+
+impl<Prov> From<Double> for Scalar<Prov> {
+    #[inline(always)]
+    fn from(f: Double) -> Self {
+        Scalar::from_f64(f)
+    }
+}
+
+impl<Prov> From<ScalarInt> for Scalar<Prov> {
+    #[inline(always)]
+    fn from(ptr: ScalarInt) -> Self {
+        Scalar::Int(ptr)
+    }
+}
+
+impl<Prov> Scalar<Prov> {
+    #[inline(always)]
+    pub fn from_pointer(ptr: Pointer<Prov>, cx: &impl HasDataLayout) -> Self {
+        Scalar::Ptr(ptr, u8::try_from(cx.pointer_size().bytes()).unwrap())
+    }
+
+    /// Create a Scalar from a pointer with an `Option<_>` provenance (where `None` represents a
+    /// plain integer / "invalid" pointer).
+    pub fn from_maybe_pointer(ptr: Pointer<Option<Prov>>, cx: &impl HasDataLayout) -> Self {
+        match ptr.into_parts() {
+            (Some(prov), offset) => Scalar::from_pointer(Pointer::new(prov, offset), cx),
+            (None, offset) => {
+                Scalar::Int(ScalarInt::try_from_uint(offset.bytes(), cx.pointer_size()).unwrap())
+            }
+        }
+    }
+
+    #[inline]
+    pub fn null_ptr(cx: &impl HasDataLayout) -> Self {
+        Scalar::Int(ScalarInt::null(cx.pointer_size()))
+    }
+
+    #[inline]
+    pub fn from_bool(b: bool) -> Self {
+        Scalar::Int(b.into())
+    }
+
+    #[inline]
+    pub fn from_char(c: char) -> Self {
+        Scalar::Int(c.into())
+    }
+
+    #[inline]
+    pub fn try_from_uint(i: impl Into<u128>, size: Size) -> Option<Self> {
+        ScalarInt::try_from_uint(i, size).map(Scalar::Int)
+    }
+
+    #[inline]
+    pub fn from_uint(i: impl Into<u128>, size: Size) -> Self {
+        let i = i.into();
+        Self::try_from_uint(i, size)
+            .unwrap_or_else(|| bug!("Unsigned value {:#x} does not fit in {} bits", i, size.bits()))
+    }
+
+    #[inline]
+    pub fn from_u8(i: u8) -> Self {
+        Scalar::Int(i.into())
+    }
+
+    #[inline]
+    pub fn from_u16(i: u16) -> Self {
+        Scalar::Int(i.into())
+    }
+
+    #[inline]
+    pub fn from_u32(i: u32) -> Self {
+        Scalar::Int(i.into())
+    }
+
+    #[inline]
+    pub fn from_u64(i: u64) -> Self {
+        Scalar::Int(i.into())
+    }
+
+    #[inline]
+    pub fn from_machine_usize(i: u64, cx: &impl HasDataLayout) -> Self {
+        Self::from_uint(i, cx.data_layout().pointer_size)
+    }
+
+    #[inline]
+    pub fn try_from_int(i: impl Into<i128>, size: Size) -> Option<Self> {
+        ScalarInt::try_from_int(i, size).map(Scalar::Int)
+    }
+
+    #[inline]
+    pub fn from_int(i: impl Into<i128>, size: Size) -> Self {
+        let i = i.into();
+        Self::try_from_int(i, size)
+            .unwrap_or_else(|| bug!("Signed value {:#x} does not fit in {} bits", i, size.bits()))
+    }
+
+    #[inline]
+    pub fn from_i32(i: i32) -> Self {
+        Self::from_int(i, Size::from_bits(32))
+    }
+
+    #[inline]
+    pub fn from_i64(i: i64) -> Self {
+        Self::from_int(i, Size::from_bits(64))
+    }
+
+    #[inline]
+    pub fn from_machine_isize(i: i64, cx: &impl HasDataLayout) -> Self {
+        Self::from_int(i, cx.data_layout().pointer_size)
+    }
+
+    #[inline]
+    pub fn from_f32(f: Single) -> Self {
+        Scalar::Int(f.into())
+    }
+
+    #[inline]
+    pub fn from_f64(f: Double) -> Self {
+        Scalar::Int(f.into())
+    }
+
+    /// This is almost certainly not the method you want!  You should dispatch on the type
+    /// and use `to_{u8,u16,...}`/`scalar_to_ptr` to perform ptr-to-int / int-to-ptr casts as needed.
+    ///
+    /// This method only exists for the benefit of low-level operations that truly need to treat the
+    /// scalar in whatever form it is.
+    ///
+    /// This throws UB (instead of ICEing) on a size mismatch since size mismatches can arise in
+    /// Miri when someone declares a function that we shim (such as `malloc`) with a wrong type.
+    #[inline]
+    pub fn to_bits_or_ptr_internal(
+        self,
+        target_size: Size,
+    ) -> Result<Result<u128, Pointer<Prov>>, ScalarSizeMismatch> {
+        assert_ne!(target_size.bytes(), 0, "you should never look at the bits of a ZST");
+        Ok(match self {
+            Scalar::Int(int) => Ok(int.to_bits(target_size).map_err(|size| {
+                ScalarSizeMismatch { target_size: target_size.bytes(), data_size: size.bytes() }
+            })?),
+            Scalar::Ptr(ptr, sz) => {
+                if target_size.bytes() != u64::from(sz) {
+                    return Err(ScalarSizeMismatch {
+                        target_size: target_size.bytes(),
+                        data_size: sz.into(),
+                    });
+                }
+                Err(ptr)
+            }
+        })
+    }
+}
+
+impl<'tcx, Prov: Provenance> Scalar<Prov> {
+    pub fn to_pointer(self, cx: &impl HasDataLayout) -> InterpResult<'tcx, Pointer<Option<Prov>>> {
+        match self
+            .to_bits_or_ptr_internal(cx.pointer_size())
+            .map_err(|s| err_ub!(ScalarSizeMismatch(s)))?
+        {
+            Err(ptr) => Ok(ptr.into()),
+            Ok(bits) => {
+                let addr = u64::try_from(bits).unwrap();
+                Ok(Pointer::from_addr(addr))
+            }
+        }
+    }
+
+    /// Fundamental scalar-to-int (cast) operation. Many convenience wrappers exist below, that you
+    /// likely want to use instead.
+    ///
+    /// Will perform ptr-to-int casts if needed and possible.
+    /// If that fails, we know the offset is relative, so we return an "erased" Scalar
+    /// (which is useful for error messages but not much else).
+    #[inline]
+    pub fn try_to_int(self) -> Result<ScalarInt, Scalar<AllocId>> {
+        match self {
+            Scalar::Int(int) => Ok(int),
+            Scalar::Ptr(ptr, sz) => {
+                if Prov::OFFSET_IS_ADDR {
+                    Ok(ScalarInt::try_from_uint(ptr.offset.bytes(), Size::from_bytes(sz)).unwrap())
+                } else {
+                    // We know `offset` is relative, since `OFFSET_IS_ADDR == false`.
+                    let (prov, offset) = ptr.into_parts();
+                    // Because `OFFSET_IS_ADDR == false`, this unwrap can never fail.
+                    Err(Scalar::Ptr(Pointer::new(prov.get_alloc_id().unwrap(), offset), sz))
+                }
+            }
+        }
+    }
+
+    #[inline(always)]
+    pub fn assert_int(self) -> ScalarInt {
+        self.try_to_int().unwrap()
+    }
+
+    /// This throws UB (instead of ICEing) on a size mismatch since size mismatches can arise in
+    /// Miri when someone declares a function that we shim (such as `malloc`) with a wrong type.
+    #[inline]
+    pub fn to_bits(self, target_size: Size) -> InterpResult<'tcx, u128> {
+        assert_ne!(target_size.bytes(), 0, "you should never look at the bits of a ZST");
+        self.try_to_int().map_err(|_| err_unsup!(ReadPointerAsBytes))?.to_bits(target_size).map_err(
+            |size| {
+                err_ub!(ScalarSizeMismatch(ScalarSizeMismatch {
+                    target_size: target_size.bytes(),
+                    data_size: size.bytes(),
+                }))
+                .into()
+            },
+        )
+    }
+
+    #[inline(always)]
+    pub fn assert_bits(self, target_size: Size) -> u128 {
+        self.to_bits(target_size).unwrap()
+    }
+
+    pub fn to_bool(self) -> InterpResult<'tcx, bool> {
+        let val = self.to_u8()?;
+        match val {
+            0 => Ok(false),
+            1 => Ok(true),
+            _ => throw_ub!(InvalidBool(val)),
+        }
+    }
+
+    pub fn to_char(self) -> InterpResult<'tcx, char> {
+        let val = self.to_u32()?;
+        match std::char::from_u32(val) {
+            Some(c) => Ok(c),
+            None => throw_ub!(InvalidChar(val)),
+        }
+    }
+
+    /// Converts the scalar to produce an unsigned integer of the given size.
+    /// Fails if the scalar is a pointer.
+    #[inline]
+    pub fn to_uint(self, size: Size) -> InterpResult<'tcx, u128> {
+        self.to_bits(size)
+    }
+
+    /// Converts the scalar to produce a `u8`. Fails if the scalar is a pointer.
+    pub fn to_u8(self) -> InterpResult<'tcx, u8> {
+        self.to_uint(Size::from_bits(8)).map(|v| u8::try_from(v).unwrap())
+    }
+
+    /// Converts the scalar to produce a `u16`. Fails if the scalar is a pointer.
+    pub fn to_u16(self) -> InterpResult<'tcx, u16> {
+        self.to_uint(Size::from_bits(16)).map(|v| u16::try_from(v).unwrap())
+    }
+
+    /// Converts the scalar to produce a `u32`. Fails if the scalar is a pointer.
+    pub fn to_u32(self) -> InterpResult<'tcx, u32> {
+        self.to_uint(Size::from_bits(32)).map(|v| u32::try_from(v).unwrap())
+    }
+
+    /// Converts the scalar to produce a `u64`. Fails if the scalar is a pointer.
+    pub fn to_u64(self) -> InterpResult<'tcx, u64> {
+        self.to_uint(Size::from_bits(64)).map(|v| u64::try_from(v).unwrap())
+    }
+
+    /// Converts the scalar to produce a `u128`. Fails if the scalar is a pointer.
+    pub fn to_u128(self) -> InterpResult<'tcx, u128> {
+        self.to_uint(Size::from_bits(128))
+    }
+
+    /// Converts the scalar to produce a machine-pointer-sized unsigned integer.
+    /// Fails if the scalar is a pointer.
+    pub fn to_machine_usize(self, cx: &impl HasDataLayout) -> InterpResult<'tcx, u64> {
+        let b = self.to_uint(cx.data_layout().pointer_size)?;
+        Ok(u64::try_from(b).unwrap())
+    }
+
+    /// Converts the scalar to produce a signed integer of the given size.
+    /// Fails if the scalar is a pointer.
+    #[inline]
+    pub fn to_int(self, size: Size) -> InterpResult<'tcx, i128> {
+        let b = self.to_bits(size)?;
+        Ok(size.sign_extend(b) as i128)
+    }
+
+    /// Converts the scalar to produce an `i8`. Fails if the scalar is a pointer.
+    pub fn to_i8(self) -> InterpResult<'tcx, i8> {
+        self.to_int(Size::from_bits(8)).map(|v| i8::try_from(v).unwrap())
+    }
+
+    /// Converts the scalar to produce an `i16`. Fails if the scalar is a pointer.
+    pub fn to_i16(self) -> InterpResult<'tcx, i16> {
+        self.to_int(Size::from_bits(16)).map(|v| i16::try_from(v).unwrap())
+    }
+
+    /// Converts the scalar to produce an `i32`. Fails if the scalar is a pointer.
+    pub fn to_i32(self) -> InterpResult<'tcx, i32> {
+        self.to_int(Size::from_bits(32)).map(|v| i32::try_from(v).unwrap())
+    }
+
+    /// Converts the scalar to produce an `i64`. Fails if the scalar is a pointer.
+    pub fn to_i64(self) -> InterpResult<'tcx, i64> {
+        self.to_int(Size::from_bits(64)).map(|v| i64::try_from(v).unwrap())
+    }
+
+    /// Converts the scalar to produce an `i128`. Fails if the scalar is a pointer.
+    pub fn to_i128(self) -> InterpResult<'tcx, i128> {
+        self.to_int(Size::from_bits(128))
+    }
+
+    /// Converts the scalar to produce a machine-pointer-sized signed integer.
+    /// Fails if the scalar is a pointer.
+    pub fn to_machine_isize(self, cx: &impl HasDataLayout) -> InterpResult<'tcx, i64> {
+        let b = self.to_int(cx.data_layout().pointer_size)?;
+        Ok(i64::try_from(b).unwrap())
+    }
+
+    #[inline]
+    pub fn to_f32(self) -> InterpResult<'tcx, Single> {
+        // Going through `u32` to check size and truncation.
+        Ok(Single::from_bits(self.to_u32()?.into()))
+    }
+
+    #[inline]
+    pub fn to_f64(self) -> InterpResult<'tcx, Double> {
+        // Going through `u64` to check size and truncation.
+        Ok(Double::from_bits(self.to_u64()?.into()))
+    }
+}
+
+#[derive(Clone, Copy, Eq, PartialEq, TyEncodable, TyDecodable, HashStable, Hash)]
+pub enum ScalarMaybeUninit<Prov = AllocId> {
+    Scalar(Scalar<Prov>),
+    Uninit,
+}
+
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(ScalarMaybeUninit, 24);
+
+impl<Prov> From<Scalar<Prov>> for ScalarMaybeUninit<Prov> {
+    #[inline(always)]
+    fn from(s: Scalar<Prov>) -> Self {
+        ScalarMaybeUninit::Scalar(s)
+    }
+}
+
+// We want the `Debug` output to be readable as it is used by `derive(Debug)` for
+// all the Miri types.
+impl<Prov: Provenance> fmt::Debug for ScalarMaybeUninit<Prov> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            ScalarMaybeUninit::Uninit => write!(f, "<uninitialized>"),
+            ScalarMaybeUninit::Scalar(s) => write!(f, "{:?}", s),
+        }
+    }
+}
+
+impl<Prov: Provenance> fmt::LowerHex for ScalarMaybeUninit<Prov> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            ScalarMaybeUninit::Uninit => write!(f, "uninitialized bytes"),
+            ScalarMaybeUninit::Scalar(s) => write!(f, "{:x}", s),
+        }
+    }
+}
+
+impl<Prov> ScalarMaybeUninit<Prov> {
+    #[inline]
+    pub fn from_pointer(ptr: Pointer<Prov>, cx: &impl HasDataLayout) -> Self {
+        ScalarMaybeUninit::Scalar(Scalar::from_pointer(ptr, cx))
+    }
+
+    #[inline]
+    pub fn from_maybe_pointer(ptr: Pointer<Option<Prov>>, cx: &impl HasDataLayout) -> Self {
+        ScalarMaybeUninit::Scalar(Scalar::from_maybe_pointer(ptr, cx))
+    }
+
+    #[inline]
+    pub fn check_init<'tcx>(self) -> InterpResult<'tcx, Scalar<Prov>> {
+        match self {
+            ScalarMaybeUninit::Scalar(scalar) => Ok(scalar),
+            ScalarMaybeUninit::Uninit => throw_ub!(InvalidUninitBytes(None)),
+        }
+    }
+}
+
+impl<'tcx, Prov: Provenance> ScalarMaybeUninit<Prov> {
+    #[inline(always)]
+    pub fn to_pointer(self, cx: &impl HasDataLayout) -> InterpResult<'tcx, Pointer<Option<Prov>>> {
+        self.check_init()?.to_pointer(cx)
+    }
+
+    #[inline(always)]
+    pub fn to_bool(self) -> InterpResult<'tcx, bool> {
+        self.check_init()?.to_bool()
+    }
+
+    #[inline(always)]
+    pub fn to_char(self) -> InterpResult<'tcx, char> {
+        self.check_init()?.to_char()
+    }
+
+    #[inline(always)]
+    pub fn to_f32(self) -> InterpResult<'tcx, Single> {
+        self.check_init()?.to_f32()
+    }
+
+    #[inline(always)]
+    pub fn to_f64(self) -> InterpResult<'tcx, Double> {
+        self.check_init()?.to_f64()
+    }
+
+    #[inline(always)]
+    pub fn to_u8(self) -> InterpResult<'tcx, u8> {
+        self.check_init()?.to_u8()
+    }
+
+    #[inline(always)]
+    pub fn to_u16(self) -> InterpResult<'tcx, u16> {
+        self.check_init()?.to_u16()
+    }
+
+    #[inline(always)]
+    pub fn to_u32(self) -> InterpResult<'tcx, u32> {
+        self.check_init()?.to_u32()
+    }
+
+    #[inline(always)]
+    pub fn to_u64(self) -> InterpResult<'tcx, u64> {
+        self.check_init()?.to_u64()
+    }
+
+    #[inline(always)]
+    pub fn to_machine_usize(self, cx: &impl HasDataLayout) -> InterpResult<'tcx, u64> {
+        self.check_init()?.to_machine_usize(cx)
+    }
+
+    #[inline(always)]
+    pub fn to_i8(self) -> InterpResult<'tcx, i8> {
+        self.check_init()?.to_i8()
+    }
+
+    #[inline(always)]
+    pub fn to_i16(self) -> InterpResult<'tcx, i16> {
+        self.check_init()?.to_i16()
+    }
+
+    #[inline(always)]
+    pub fn to_i32(self) -> InterpResult<'tcx, i32> {
+        self.check_init()?.to_i32()
+    }
+
+    #[inline(always)]
+    pub fn to_i64(self) -> InterpResult<'tcx, i64> {
+        self.check_init()?.to_i64()
+    }
+
+    #[inline(always)]
+    pub fn to_machine_isize(self, cx: &impl HasDataLayout) -> InterpResult<'tcx, i64> {
+        self.check_init()?.to_machine_isize(cx)
+    }
+}
+
+/// Gets the bytes of a constant slice value.
+pub fn get_slice_bytes<'tcx>(cx: &impl HasDataLayout, val: ConstValue<'tcx>) -> &'tcx [u8] {
+    if let ConstValue::Slice { data, start, end } = val {
+        let len = end - start;
+        data.inner()
+            .get_bytes(
+                cx,
+                AllocRange { start: Size::from_bytes(start), size: Size::from_bytes(len) },
+            )
+            .unwrap_or_else(|err| bug!("const slice is invalid: {:?}", err))
+    } else {
+        bug!("expected const slice, but found another const value");
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
new file mode 100644
index 000000000..7ab71f900
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -0,0 +1,2900 @@
+//! MIR datatypes and passes. See the [rustc dev guide] for more info.
+//!
+//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/mir/index.html
+
+use crate::mir::interpret::{
+    AllocRange, ConstAllocation, ConstValue, GlobalAlloc, LitToConstInput, Scalar,
+};
+use crate::mir::visit::MirVisitable;
+use crate::ty::codec::{TyDecoder, TyEncoder};
+use crate::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeSuperFoldable};
+use crate::ty::print::{FmtPrinter, Printer};
+use crate::ty::subst::{GenericArg, InternalSubsts, Subst, SubstsRef};
+use crate::ty::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitor};
+use crate::ty::{self, List, Ty, TyCtxt};
+use crate::ty::{AdtDef, InstanceDef, ScalarInt, UserTypeAnnotationIndex};
+
+use rustc_data_structures::captures::Captures;
+use rustc_errors::ErrorGuaranteed;
+use rustc_hir::def::{CtorKind, Namespace};
+use rustc_hir::def_id::{DefId, LocalDefId, CRATE_DEF_ID};
+use rustc_hir::{self, GeneratorKind};
+use rustc_hir::{self as hir, HirId};
+use rustc_session::Session;
+use rustc_target::abi::{Size, VariantIdx};
+
+use polonius_engine::Atom;
+pub use rustc_ast::Mutability;
+use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::graph::dominators::Dominators;
+use rustc_index::bit_set::BitMatrix;
+use rustc_index::vec::{Idx, IndexVec};
+use rustc_serialize::{Decodable, Encodable};
+use rustc_span::symbol::Symbol;
+use rustc_span::{Span, DUMMY_SP};
+
+use either::Either;
+
+use std::borrow::Cow;
+use std::convert::TryInto;
+use std::fmt::{self, Debug, Display, Formatter, Write};
+use std::ops::{ControlFlow, Index, IndexMut};
+use std::{iter, mem};
+
+pub use self::query::*;
+pub use basic_blocks::BasicBlocks;
+
+mod basic_blocks;
+pub mod coverage;
+mod generic_graph;
+pub mod generic_graphviz;
+mod graph_cyclic_cache;
+pub mod graphviz;
+pub mod interpret;
+pub mod mono;
+pub mod patch;
+mod predecessors;
+pub mod pretty;
+mod query;
+pub mod spanview;
+mod syntax;
+pub use syntax::*;
+mod switch_sources;
+pub mod tcx;
+pub mod terminator;
+pub use terminator::*;
+
+pub mod traversal;
+mod type_foldable;
+mod type_visitable;
+pub mod visit;
+
+pub use self::generic_graph::graphviz_safe_def_name;
+pub use self::graphviz::write_mir_graphviz;
+pub use self::pretty::{
+    create_dump_file, display_allocation, dump_enabled, dump_mir, write_mir_pretty, PassWhere,
+};
+
+/// Types for locals
+pub type LocalDecls<'tcx> = IndexVec<Local, LocalDecl<'tcx>>;
+
+pub trait HasLocalDecls<'tcx> {
+    fn local_decls(&self) -> &LocalDecls<'tcx>;
+}
+
+impl<'tcx> HasLocalDecls<'tcx> for LocalDecls<'tcx> {
+    #[inline]
+    fn local_decls(&self) -> &LocalDecls<'tcx> {
+        self
+    }
+}
+
+impl<'tcx> HasLocalDecls<'tcx> for Body<'tcx> {
+    #[inline]
+    fn local_decls(&self) -> &LocalDecls<'tcx> {
+        &self.local_decls
+    }
+}
+
+/// A streamlined trait that you can implement to create a pass; the
+/// pass will be named after the type, and it will consist of a main
+/// loop that goes over each available MIR and applies `run_pass`.
+pub trait MirPass<'tcx> {
+    fn name(&self) -> Cow<'_, str> {
+        let name = std::any::type_name::<Self>();
+        if let Some(tail) = name.rfind(':') {
+            Cow::from(&name[tail + 1..])
+        } else {
+            Cow::from(name)
+        }
+    }
+
+    /// Returns `true` if this pass is enabled with the current combination of compiler flags.
+    fn is_enabled(&self, _sess: &Session) -> bool {
+        true
+    }
+
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>);
+
+    /// If this pass causes the MIR to enter a new phase, return that phase.
+    fn phase_change(&self) -> Option<MirPhase> {
+        None
+    }
+
+    fn is_mir_dump_enabled(&self) -> bool {
+        true
+    }
+}
+
+impl MirPhase {
+    /// Gets the index of the current MirPhase within the set of all `MirPhase`s.
+    pub fn phase_index(&self) -> usize {
+        *self as usize
+    }
+}
+
+/// Where a specific `mir::Body` comes from.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[derive(HashStable, TyEncodable, TyDecodable, TypeFoldable, TypeVisitable)]
+pub struct MirSource<'tcx> {
+    pub instance: InstanceDef<'tcx>,
+
+    /// If `Some`, this is a promoted rvalue within the parent function.
+    pub promoted: Option<Promoted>,
+}
+
+impl<'tcx> MirSource<'tcx> {
+    pub fn item(def_id: DefId) -> Self {
+        MirSource {
+            instance: InstanceDef::Item(ty::WithOptConstParam::unknown(def_id)),
+            promoted: None,
+        }
+    }
+
+    pub fn from_instance(instance: InstanceDef<'tcx>) -> Self {
+        MirSource { instance, promoted: None }
+    }
+
+    pub fn with_opt_param(self) -> ty::WithOptConstParam<DefId> {
+        self.instance.with_opt_param()
+    }
+
+    #[inline]
+    pub fn def_id(&self) -> DefId {
+        self.instance.def_id()
+    }
+}
+
+#[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable, TypeFoldable, TypeVisitable)]
+pub struct GeneratorInfo<'tcx> {
+    /// The yield type of the function, if it is a generator.
+    pub yield_ty: Option<Ty<'tcx>>,
+
+    /// Generator drop glue.
+    pub generator_drop: Option<Body<'tcx>>,
+
+    /// The layout of a generator. Produced by the state transformation.
+    pub generator_layout: Option<GeneratorLayout<'tcx>>,
+
+    /// If this is a generator then record the type of source expression that caused this generator
+    /// to be created.
+    pub generator_kind: GeneratorKind,
+}
+
+/// The lowered representation of a single function.
+#[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable, TypeFoldable, TypeVisitable)]
+pub struct Body<'tcx> {
+    /// A list of basic blocks. References to basic block use a newtyped index type [`BasicBlock`]
+    /// that indexes into this vector.
+    pub basic_blocks: BasicBlocks<'tcx>,
+
+    /// Records how far through the "desugaring and optimization" process this particular
+    /// MIR has traversed. This is particularly useful when inlining, since in that context
+    /// we instantiate the promoted constants and add them to our promoted vector -- but those
+    /// promoted items have already been optimized, whereas ours have not. This field allows
+    /// us to see the difference and forego optimization on the inlined promoted items.
+    pub phase: MirPhase,
+
+    pub source: MirSource<'tcx>,
+
+    /// A list of source scopes; these are referenced by statements
+    /// and used for debuginfo. Indexed by a `SourceScope`.
+    pub source_scopes: IndexVec<SourceScope, SourceScopeData<'tcx>>,
+
+    pub generator: Option<Box<GeneratorInfo<'tcx>>>,
+
+    /// Declarations of locals.
+    ///
+    /// The first local is the return value pointer, followed by `arg_count`
+    /// locals for the function arguments, followed by any user-declared
+    /// variables and temporaries.
+    pub local_decls: LocalDecls<'tcx>,
+
+    /// User type annotations.
+    pub user_type_annotations: ty::CanonicalUserTypeAnnotations<'tcx>,
+
+    /// The number of arguments this function takes.
+    ///
+    /// Starting at local 1, `arg_count` locals will be provided by the caller
+    /// and can be assumed to be initialized.
+    ///
+    /// If this MIR was built for a constant, this will be 0.
+    pub arg_count: usize,
+
+    /// Mark an argument local (which must be a tuple) as getting passed as
+    /// its individual components at the LLVM level.
+    ///
+    /// This is used for the "rust-call" ABI.
+    pub spread_arg: Option<Local>,
+
+    /// Debug information pertaining to user variables, including captures.
+    pub var_debug_info: Vec<VarDebugInfo<'tcx>>,
+
+    /// A span representing this MIR, for error reporting.
+    pub span: Span,
+
+    /// Constants that are required to evaluate successfully for this MIR to be well-formed.
+    /// We hold in this field all the constants we are not able to evaluate yet.
+    pub required_consts: Vec<Constant<'tcx>>,
+
+    /// Does this body use generic parameters. This is used for the `ConstEvaluatable` check.
+    ///
+    /// Note that this does not actually mean that this body is not computable right now.
+    /// The repeat count in the following example is polymorphic, but can still be evaluated
+    /// without knowing anything about the type parameter `T`.
+    ///
+    /// ```rust
+    /// fn test<T>() {
+    ///     let _ = [0; std::mem::size_of::<*mut T>()];
+    /// }
+    /// ```
+    ///
+    /// **WARNING**: Do not change this flags after the MIR was originally created, even if an optimization
+    /// removed the last mention of all generic params. We do not want to rely on optimizations and
+    /// potentially allow things like `[u8; std::mem::size_of::<T>() * 0]` due to this.
+    pub is_polymorphic: bool,
+
+    pub tainted_by_errors: Option<ErrorGuaranteed>,
+}
+
+impl<'tcx> Body<'tcx> {
+    pub fn new(
+        source: MirSource<'tcx>,
+        basic_blocks: IndexVec<BasicBlock, BasicBlockData<'tcx>>,
+        source_scopes: IndexVec<SourceScope, SourceScopeData<'tcx>>,
+        local_decls: LocalDecls<'tcx>,
+        user_type_annotations: ty::CanonicalUserTypeAnnotations<'tcx>,
+        arg_count: usize,
+        var_debug_info: Vec<VarDebugInfo<'tcx>>,
+        span: Span,
+        generator_kind: Option<GeneratorKind>,
+        tainted_by_errors: Option<ErrorGuaranteed>,
+    ) -> Self {
+        // We need `arg_count` locals, and one for the return place.
+        assert!(
+            local_decls.len() > arg_count,
+            "expected at least {} locals, got {}",
+            arg_count + 1,
+            local_decls.len()
+        );
+
+        let mut body = Body {
+            phase: MirPhase::Built,
+            source,
+            basic_blocks: BasicBlocks::new(basic_blocks),
+            source_scopes,
+            generator: generator_kind.map(|generator_kind| {
+                Box::new(GeneratorInfo {
+                    yield_ty: None,
+                    generator_drop: None,
+                    generator_layout: None,
+                    generator_kind,
+                })
+            }),
+            local_decls,
+            user_type_annotations,
+            arg_count,
+            spread_arg: None,
+            var_debug_info,
+            span,
+            required_consts: Vec::new(),
+            is_polymorphic: false,
+            tainted_by_errors,
+        };
+        body.is_polymorphic = body.has_param_types_or_consts();
+        body
+    }
+
+    /// Returns a partially initialized MIR body containing only a list of basic blocks.
+    ///
+    /// The returned MIR contains no `LocalDecl`s (even for the return place) or source scopes. It
+    /// is only useful for testing but cannot be `#[cfg(test)]` because it is used in a different
+    /// crate.
+    pub fn new_cfg_only(basic_blocks: IndexVec<BasicBlock, BasicBlockData<'tcx>>) -> Self {
+        let mut body = Body {
+            phase: MirPhase::Built,
+            source: MirSource::item(CRATE_DEF_ID.to_def_id()),
+            basic_blocks: BasicBlocks::new(basic_blocks),
+            source_scopes: IndexVec::new(),
+            generator: None,
+            local_decls: IndexVec::new(),
+            user_type_annotations: IndexVec::new(),
+            arg_count: 0,
+            spread_arg: None,
+            span: DUMMY_SP,
+            required_consts: Vec::new(),
+            var_debug_info: Vec::new(),
+            is_polymorphic: false,
+            tainted_by_errors: None,
+        };
+        body.is_polymorphic = body.has_param_types_or_consts();
+        body
+    }
+
+    #[inline]
+    pub fn basic_blocks(&self) -> &IndexVec<BasicBlock, BasicBlockData<'tcx>> {
+        &self.basic_blocks
+    }
+
+    #[inline]
+    pub fn basic_blocks_mut(&mut self) -> &mut IndexVec<BasicBlock, BasicBlockData<'tcx>> {
+        self.basic_blocks.as_mut()
+    }
+
+    #[inline]
+    pub fn local_kind(&self, local: Local) -> LocalKind {
+        let index = local.as_usize();
+        if index == 0 {
+            debug_assert!(
+                self.local_decls[local].mutability == Mutability::Mut,
+                "return place should be mutable"
+            );
+
+            LocalKind::ReturnPointer
+        } else if index < self.arg_count + 1 {
+            LocalKind::Arg
+        } else if self.local_decls[local].is_user_variable() {
+            LocalKind::Var
+        } else {
+            LocalKind::Temp
+        }
+    }
+
+    /// Returns an iterator over all user-declared mutable locals.
+    #[inline]
+    pub fn mut_vars_iter<'a>(&'a self) -> impl Iterator<Item = Local> + Captures<'tcx> + 'a {
+        (self.arg_count + 1..self.local_decls.len()).filter_map(move |index| {
+            let local = Local::new(index);
+            let decl = &self.local_decls[local];
+            if decl.is_user_variable() && decl.mutability == Mutability::Mut {
+                Some(local)
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Returns an iterator over all user-declared mutable arguments and locals.
+    #[inline]
+    pub fn mut_vars_and_args_iter<'a>(
+        &'a self,
+    ) -> impl Iterator<Item = Local> + Captures<'tcx> + 'a {
+        (1..self.local_decls.len()).filter_map(move |index| {
+            let local = Local::new(index);
+            let decl = &self.local_decls[local];
+            if (decl.is_user_variable() || index < self.arg_count + 1)
+                && decl.mutability == Mutability::Mut
+            {
+                Some(local)
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Returns an iterator over all function arguments.
+    #[inline]
+    pub fn args_iter(&self) -> impl Iterator<Item = Local> + ExactSizeIterator {
+        (1..self.arg_count + 1).map(Local::new)
+    }
+
+    /// Returns an iterator over all user-defined variables and compiler-generated temporaries (all
+    /// locals that are neither arguments nor the return place).
+    #[inline]
+    pub fn vars_and_temps_iter(
+        &self,
+    ) -> impl DoubleEndedIterator<Item = Local> + ExactSizeIterator {
+        (self.arg_count + 1..self.local_decls.len()).map(Local::new)
+    }
+
+    #[inline]
+    pub fn drain_vars_and_temps<'a>(&'a mut self) -> impl Iterator<Item = LocalDecl<'tcx>> + 'a {
+        self.local_decls.drain(self.arg_count + 1..)
+    }
+
+    /// Returns the source info associated with `location`.
+    pub fn source_info(&self, location: Location) -> &SourceInfo {
+        let block = &self[location.block];
+        let stmts = &block.statements;
+        let idx = location.statement_index;
+        if idx < stmts.len() {
+            &stmts[idx].source_info
+        } else {
+            assert_eq!(idx, stmts.len());
+            &block.terminator().source_info
+        }
+    }
+
+    /// Returns the return type; it always return first element from `local_decls` array.
+    #[inline]
+    pub fn return_ty(&self) -> Ty<'tcx> {
+        self.local_decls[RETURN_PLACE].ty
+    }
+
+    /// Returns the return type; it always return first element from `local_decls` array.
+    #[inline]
+    pub fn bound_return_ty(&self) -> ty::EarlyBinder<Ty<'tcx>> {
+        ty::EarlyBinder(self.local_decls[RETURN_PLACE].ty)
+    }
+
+    /// Gets the location of the terminator for the given block.
+    #[inline]
+    pub fn terminator_loc(&self, bb: BasicBlock) -> Location {
+        Location { block: bb, statement_index: self[bb].statements.len() }
+    }
+
+    pub fn stmt_at(&self, location: Location) -> Either<&Statement<'tcx>, &Terminator<'tcx>> {
+        let Location { block, statement_index } = location;
+        let block_data = &self.basic_blocks[block];
+        block_data
+            .statements
+            .get(statement_index)
+            .map(Either::Left)
+            .unwrap_or_else(|| Either::Right(block_data.terminator()))
+    }
+
+    #[inline]
+    pub fn yield_ty(&self) -> Option<Ty<'tcx>> {
+        self.generator.as_ref().and_then(|generator| generator.yield_ty)
+    }
+
+    #[inline]
+    pub fn generator_layout(&self) -> Option<&GeneratorLayout<'tcx>> {
+        self.generator.as_ref().and_then(|generator| generator.generator_layout.as_ref())
+    }
+
+    #[inline]
+    pub fn generator_drop(&self) -> Option<&Body<'tcx>> {
+        self.generator.as_ref().and_then(|generator| generator.generator_drop.as_ref())
+    }
+
+    #[inline]
+    pub fn generator_kind(&self) -> Option<GeneratorKind> {
+        self.generator.as_ref().map(|generator| generator.generator_kind)
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug, TyEncodable, TyDecodable, HashStable)]
+pub enum Safety {
+    Safe,
+    /// Unsafe because of compiler-generated unsafe code, like `await` desugaring
+    BuiltinUnsafe,
+    /// Unsafe because of an unsafe fn
+    FnUnsafe,
+    /// Unsafe because of an `unsafe` block
+    ExplicitUnsafe(hir::HirId),
+}
+
+impl<'tcx> Index<BasicBlock> for Body<'tcx> {
+    type Output = BasicBlockData<'tcx>;
+
+    #[inline]
+    fn index(&self, index: BasicBlock) -> &BasicBlockData<'tcx> {
+        &self.basic_blocks()[index]
+    }
+}
+
+impl<'tcx> IndexMut<BasicBlock> for Body<'tcx> {
+    #[inline]
+    fn index_mut(&mut self, index: BasicBlock) -> &mut BasicBlockData<'tcx> {
+        &mut self.basic_blocks.as_mut()[index]
+    }
+}
+
+#[derive(Copy, Clone, Debug, HashStable, TypeFoldable, TypeVisitable)]
+pub enum ClearCrossCrate<T> {
+    Clear,
+    Set(T),
+}
+
+impl<T> ClearCrossCrate<T> {
+    pub fn as_ref(&self) -> ClearCrossCrate<&T> {
+        match self {
+            ClearCrossCrate::Clear => ClearCrossCrate::Clear,
+            ClearCrossCrate::Set(v) => ClearCrossCrate::Set(v),
+        }
+    }
+
+    pub fn assert_crate_local(self) -> T {
+        match self {
+            ClearCrossCrate::Clear => bug!("unwrapping cross-crate data"),
+            ClearCrossCrate::Set(v) => v,
+        }
+    }
+}
+
+const TAG_CLEAR_CROSS_CRATE_CLEAR: u8 = 0;
+const TAG_CLEAR_CROSS_CRATE_SET: u8 = 1;
+
+impl<E: TyEncoder, T: Encodable<E>> Encodable<E> for ClearCrossCrate<T> {
+    #[inline]
+    fn encode(&self, e: &mut E) {
+        if E::CLEAR_CROSS_CRATE {
+            return;
+        }
+
+        match *self {
+            ClearCrossCrate::Clear => TAG_CLEAR_CROSS_CRATE_CLEAR.encode(e),
+            ClearCrossCrate::Set(ref val) => {
+                TAG_CLEAR_CROSS_CRATE_SET.encode(e);
+                val.encode(e);
+            }
+        }
+    }
+}
+impl<D: TyDecoder, T: Decodable<D>> Decodable<D> for ClearCrossCrate<T> {
+    #[inline]
+    fn decode(d: &mut D) -> ClearCrossCrate<T> {
+        if D::CLEAR_CROSS_CRATE {
+            return ClearCrossCrate::Clear;
+        }
+
+        let discr = u8::decode(d);
+
+        match discr {
+            TAG_CLEAR_CROSS_CRATE_CLEAR => ClearCrossCrate::Clear,
+            TAG_CLEAR_CROSS_CRATE_SET => {
+                let val = T::decode(d);
+                ClearCrossCrate::Set(val)
+            }
+            tag => panic!("Invalid tag for ClearCrossCrate: {:?}", tag),
+        }
+    }
+}
+
+/// Grouped information about the source code origin of a MIR entity.
+/// Intended to be inspected by diagnostics and debuginfo.
+/// Most passes can work with it as a whole, within a single function.
+// The unofficial Cranelift backend, at least as of #65828, needs `SourceInfo` to implement `Eq` and
+// `Hash`. Please ping @bjorn3 if removing them.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
+pub struct SourceInfo {
+    /// The source span for the AST pertaining to this MIR entity.
+    pub span: Span,
+
+    /// The source scope, keeping track of which bindings can be
+    /// seen by debuginfo, active lint levels, `unsafe {...}`, etc.
+    pub scope: SourceScope,
+}
+
+impl SourceInfo {
+    #[inline]
+    pub fn outermost(span: Span) -> Self {
+        SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE }
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Variables and temps
+
+rustc_index::newtype_index! {
+    pub struct Local {
+        derive [HashStable]
+        DEBUG_FORMAT = "_{}",
+        const RETURN_PLACE = 0,
+    }
+}
+
+impl Atom for Local {
+    fn index(self) -> usize {
+        Idx::index(self)
+    }
+}
+
+/// Classifies locals into categories. See `Body::local_kind`.
+#[derive(Clone, Copy, PartialEq, Eq, Debug, HashStable)]
+pub enum LocalKind {
+    /// User-declared variable binding.
+    Var,
+    /// Compiler-introduced temporary.
+    Temp,
+    /// Function argument.
+    Arg,
+    /// Location of function's return value.
+    ReturnPointer,
+}
+
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct VarBindingForm<'tcx> {
+    /// Is variable bound via `x`, `mut x`, `ref x`, or `ref mut x`?
+    pub binding_mode: ty::BindingMode,
+    /// If an explicit type was provided for this variable binding,
+    /// this holds the source Span of that type.
+    ///
+    /// NOTE: if you want to change this to a `HirId`, be wary that
+    /// doing so breaks incremental compilation (as of this writing),
+    /// while a `Span` does not cause our tests to fail.
+    pub opt_ty_info: Option<Span>,
+    /// Place of the RHS of the =, or the subject of the `match` where this
+    /// variable is initialized. None in the case of `let PATTERN;`.
+    /// Some((None, ..)) in the case of and `let [mut] x = ...` because
+    /// (a) the right-hand side isn't evaluated as a place expression.
+    /// (b) it gives a way to separate this case from the remaining cases
+    ///     for diagnostics.
+    pub opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
+    /// The span of the pattern in which this variable was bound.
+    pub pat_span: Span,
+}
+
+#[derive(Clone, Debug, TyEncodable, TyDecodable)]
+pub enum BindingForm<'tcx> {
+    /// This is a binding for a non-`self` binding, or a `self` that has an explicit type.
+    Var(VarBindingForm<'tcx>),
+    /// Binding for a `self`/`&self`/`&mut self` binding where the type is implicit.
+    ImplicitSelf(ImplicitSelfKind),
+    /// Reference used in a guard expression to ensure immutability.
+    RefForGuard,
+}
+
+/// Represents what type of implicit self a function has, if any.
+#[derive(Clone, Copy, PartialEq, Debug, TyEncodable, TyDecodable, HashStable)]
+pub enum ImplicitSelfKind {
+    /// Represents a `fn x(self);`.
+    Imm,
+    /// Represents a `fn x(mut self);`.
+    Mut,
+    /// Represents a `fn x(&self);`.
+    ImmRef,
+    /// Represents a `fn x(&mut self);`.
+    MutRef,
+    /// Represents when a function does not have a self argument or
+    /// when a function has a `self: X` argument.
+    None,
+}
+
+TrivialTypeTraversalAndLiftImpls! { BindingForm<'tcx>, }
+
+mod binding_form_impl {
+    use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+    use rustc_query_system::ich::StableHashingContext;
+
+    impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for super::BindingForm<'tcx> {
+        fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+            use super::BindingForm::*;
+            std::mem::discriminant(self).hash_stable(hcx, hasher);
+
+            match self {
+                Var(binding) => binding.hash_stable(hcx, hasher),
+                ImplicitSelf(kind) => kind.hash_stable(hcx, hasher),
+                RefForGuard => (),
+            }
+        }
+    }
+}
+
+/// `BlockTailInfo` is attached to the `LocalDecl` for temporaries
+/// created during evaluation of expressions in a block tail
+/// expression; that is, a block like `{ STMT_1; STMT_2; EXPR }`.
+///
+/// It is used to improve diagnostics when such temporaries are
+/// involved in borrow_check errors, e.g., explanations of where the
+/// temporaries come from, when their destructors are run, and/or how
+/// one might revise the code to satisfy the borrow checker's rules.
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct BlockTailInfo {
+    /// If `true`, then the value resulting from evaluating this tail
+    /// expression is ignored by the block's expression context.
+    ///
+    /// Examples include `{ ...; tail };` and `let _ = { ...; tail };`
+    /// but not e.g., `let _x = { ...; tail };`
+    pub tail_result_is_ignored: bool,
+
+    /// `Span` of the tail expression.
+    pub span: Span,
+}
+
+/// A MIR local.
+///
+/// This can be a binding declared by the user, a temporary inserted by the compiler, a function
+/// argument, or the return place.
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub struct LocalDecl<'tcx> {
+    /// Whether this is a mutable binding (i.e., `let x` or `let mut x`).
+    ///
+    /// Temporaries and the return place are always mutable.
+    pub mutability: Mutability,
+
+    // FIXME(matthewjasper) Don't store in this in `Body`
+    pub local_info: Option<Box<LocalInfo<'tcx>>>,
+
+    /// `true` if this is an internal local.
+    ///
+    /// These locals are not based on types in the source code and are only used
+    /// for a few desugarings at the moment.
+    ///
+    /// The generator transformation will sanity check the locals which are live
+    /// across a suspension point against the type components of the generator
+    /// which type checking knows are live across a suspension point. We need to
+    /// flag drop flags to avoid triggering this check as they are introduced
+    /// outside of type inference.
+    ///
+    /// This should be sound because the drop flags are fully algebraic, and
+    /// therefore don't affect the auto-trait or outlives properties of the
+    /// generator.
+    pub internal: bool,
+
+    /// If this local is a temporary and `is_block_tail` is `Some`,
+    /// then it is a temporary created for evaluation of some
+    /// subexpression of some block's tail expression (with no
+    /// intervening statement context).
+    // FIXME(matthewjasper) Don't store in this in `Body`
+    pub is_block_tail: Option<BlockTailInfo>,
+
+    /// The type of this local.
+    pub ty: Ty<'tcx>,
+
+    /// If the user manually ascribed a type to this variable,
+    /// e.g., via `let x: T`, then we carry that type here. The MIR
+    /// borrow checker needs this information since it can affect
+    /// region inference.
+    // FIXME(matthewjasper) Don't store in this in `Body`
+    pub user_ty: Option<Box<UserTypeProjections>>,
+
+    /// The *syntactic* (i.e., not visibility) source scope the local is defined
+    /// in. If the local was defined in a let-statement, this
+    /// is *within* the let-statement, rather than outside
+    /// of it.
+    ///
+    /// This is needed because the visibility source scope of locals within
+    /// a let-statement is weird.
+    ///
+    /// The reason is that we want the local to be *within* the let-statement
+    /// for lint purposes, but we want the local to be *after* the let-statement
+    /// for names-in-scope purposes.
+    ///
+    /// That's it, if we have a let-statement like the one in this
+    /// function:
+    ///
+    /// ```
+    /// fn foo(x: &str) {
+    ///     #[allow(unused_mut)]
+    ///     let mut x: u32 = { // <- one unused mut
+    ///         let mut y: u32 = x.parse().unwrap();
+    ///         y + 2
+    ///     };
+    ///     drop(x);
+    /// }
+    /// ```
+    ///
+    /// Then, from a lint point of view, the declaration of `x: u32`
+    /// (and `y: u32`) are within the `#[allow(unused_mut)]` scope - the
+    /// lint scopes are the same as the AST/HIR nesting.
+    ///
+    /// However, from a name lookup point of view, the scopes look more like
+    /// as if the let-statements were `match` expressions:
+    ///
+    /// ```
+    /// fn foo(x: &str) {
+    ///     match {
+    ///         match x.parse::<u32>().unwrap() {
+    ///             y => y + 2
+    ///         }
+    ///     } {
+    ///         x => drop(x)
+    ///     };
+    /// }
+    /// ```
+    ///
+    /// We care about the name-lookup scopes for debuginfo - if the
+    /// debuginfo instruction pointer is at the call to `x.parse()`, we
+    /// want `x` to refer to `x: &str`, but if it is at the call to
+    /// `drop(x)`, we want it to refer to `x: u32`.
+    ///
+    /// To allow both uses to work, we need to have more than a single scope
+    /// for a local. We have the `source_info.scope` represent the "syntactic"
+    /// lint scope (with a variable being under its let block) while the
+    /// `var_debug_info.source_info.scope` represents the "local variable"
+    /// scope (where the "rest" of a block is under all prior let-statements).
+    ///
+    /// The end result looks like this:
+    ///
+    /// ```text
+    /// ROOT SCOPE
+    ///  │{ argument x: &str }
+    ///  │
+    ///  │ │{ #[allow(unused_mut)] } // This is actually split into 2 scopes
+    ///  │ │                         // in practice because I'm lazy.
+    ///  │ │
+    ///  │ │← x.source_info.scope
+    ///  │ │← `x.parse().unwrap()`
+    ///  │ │
+    ///  │ │ │← y.source_info.scope
+    ///  │ │
+    ///  │ │ │{ let y: u32 }
+    ///  │ │ │
+    ///  │ │ │← y.var_debug_info.source_info.scope
+    ///  │ │ │← `y + 2`
+    ///  │
+    ///  │ │{ let x: u32 }
+    ///  │ │← x.var_debug_info.source_info.scope
+    ///  │ │← `drop(x)` // This accesses `x: u32`.
+    /// ```
+    pub source_info: SourceInfo,
+}
+
+// `LocalDecl` is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(LocalDecl<'_>, 56);
+
+/// Extra information about a some locals that's used for diagnostics and for
+/// classifying variables into local variables, statics, etc, which is needed e.g.
+/// for unsafety checking.
+///
+/// Not used for non-StaticRef temporaries, the return place, or anonymous
+/// function parameters.
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub enum LocalInfo<'tcx> {
+    /// A user-defined local variable or function parameter
+    ///
+    /// The `BindingForm` is solely used for local diagnostics when generating
+    /// warnings/errors when compiling the current crate, and therefore it need
+    /// not be visible across crates.
+    User(ClearCrossCrate<BindingForm<'tcx>>),
+    /// A temporary created that references the static with the given `DefId`.
+    StaticRef { def_id: DefId, is_thread_local: bool },
+    /// A temporary created that references the const with the given `DefId`
+    ConstRef { def_id: DefId },
+    /// A temporary created during the creation of an aggregate
+    /// (e.g. a temporary for `foo` in `MyStruct { my_field: foo }`)
+    AggregateTemp,
+    /// A temporary created during the pass `Derefer` to avoid it's retagging
+    DerefTemp,
+}
+
+impl<'tcx> LocalDecl<'tcx> {
+    /// Returns `true` only if local is a binding that can itself be
+    /// made mutable via the addition of the `mut` keyword, namely
+    /// something like the occurrences of `x` in:
+    /// - `fn foo(x: Type) { ... }`,
+    /// - `let x = ...`,
+    /// - or `match ... { C(x) => ... }`
+    pub fn can_be_made_mutable(&self) -> bool {
+        matches!(
+            self.local_info,
+            Some(box LocalInfo::User(ClearCrossCrate::Set(
+                BindingForm::Var(VarBindingForm {
+                    binding_mode: ty::BindingMode::BindByValue(_),
+                    opt_ty_info: _,
+                    opt_match_place: _,
+                    pat_span: _,
+                }) | BindingForm::ImplicitSelf(ImplicitSelfKind::Imm),
+            )))
+        )
+    }
+
+    /// Returns `true` if local is definitely not a `ref ident` or
+    /// `ref mut ident` binding. (Such bindings cannot be made into
+    /// mutable bindings, but the inverse does not necessarily hold).
+    pub fn is_nonref_binding(&self) -> bool {
+        matches!(
+            self.local_info,
+            Some(box LocalInfo::User(ClearCrossCrate::Set(
+                BindingForm::Var(VarBindingForm {
+                    binding_mode: ty::BindingMode::BindByValue(_),
+                    opt_ty_info: _,
+                    opt_match_place: _,
+                    pat_span: _,
+                }) | BindingForm::ImplicitSelf(_),
+            )))
+        )
+    }
+
+    /// Returns `true` if this variable is a named variable or function
+    /// parameter declared by the user.
+    #[inline]
+    pub fn is_user_variable(&self) -> bool {
+        matches!(self.local_info, Some(box LocalInfo::User(_)))
+    }
+
+    /// Returns `true` if this is a reference to a variable bound in a `match`
+    /// expression that is used to access said variable for the guard of the
+    /// match arm.
+    pub fn is_ref_for_guard(&self) -> bool {
+        matches!(
+            self.local_info,
+            Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::RefForGuard)))
+        )
+    }
+
+    /// Returns `Some` if this is a reference to a static item that is used to
+    /// access that static.
+    pub fn is_ref_to_static(&self) -> bool {
+        matches!(self.local_info, Some(box LocalInfo::StaticRef { .. }))
+    }
+
+    /// Returns `Some` if this is a reference to a thread-local static item that is used to
+    /// access that static.
+    pub fn is_ref_to_thread_local(&self) -> bool {
+        match self.local_info {
+            Some(box LocalInfo::StaticRef { is_thread_local, .. }) => is_thread_local,
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if this is a DerefTemp
+    pub fn is_deref_temp(&self) -> bool {
+        match self.local_info {
+            Some(box LocalInfo::DerefTemp) => return true,
+            _ => (),
+        }
+        return false;
+    }
+
+    /// Returns `true` is the local is from a compiler desugaring, e.g.,
+    /// `__next` from a `for` loop.
+    #[inline]
+    pub fn from_compiler_desugaring(&self) -> bool {
+        self.source_info.span.desugaring_kind().is_some()
+    }
+
+    /// Creates a new `LocalDecl` for a temporary: mutable, non-internal.
+    #[inline]
+    pub fn new(ty: Ty<'tcx>, span: Span) -> Self {
+        Self::with_source_info(ty, SourceInfo::outermost(span))
+    }
+
+    /// Like `LocalDecl::new`, but takes a `SourceInfo` instead of a `Span`.
+    #[inline]
+    pub fn with_source_info(ty: Ty<'tcx>, source_info: SourceInfo) -> Self {
+        LocalDecl {
+            mutability: Mutability::Mut,
+            local_info: None,
+            internal: false,
+            is_block_tail: None,
+            ty,
+            user_ty: None,
+            source_info,
+        }
+    }
+
+    /// Converts `self` into same `LocalDecl` except tagged as internal.
+    #[inline]
+    pub fn internal(mut self) -> Self {
+        self.internal = true;
+        self
+    }
+
+    /// Converts `self` into same `LocalDecl` except tagged as immutable.
+    #[inline]
+    pub fn immutable(mut self) -> Self {
+        self.mutability = Mutability::Not;
+        self
+    }
+
+    /// Converts `self` into same `LocalDecl` except tagged as internal temporary.
+    #[inline]
+    pub fn block_tail(mut self, info: BlockTailInfo) -> Self {
+        assert!(self.is_block_tail.is_none());
+        self.is_block_tail = Some(info);
+        self
+    }
+}
+
+#[derive(Clone, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub enum VarDebugInfoContents<'tcx> {
+    /// NOTE(eddyb) There's an unenforced invariant that this `Place` is
+    /// based on a `Local`, not a `Static`, and contains no indexing.
+    Place(Place<'tcx>),
+    Const(Constant<'tcx>),
+}
+
+impl<'tcx> Debug for VarDebugInfoContents<'tcx> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        match self {
+            VarDebugInfoContents::Const(c) => write!(fmt, "{}", c),
+            VarDebugInfoContents::Place(p) => write!(fmt, "{:?}", p),
+        }
+    }
+}
+
+/// Debug information pertaining to a user variable.
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub struct VarDebugInfo<'tcx> {
+    pub name: Symbol,
+
+    /// Source info of the user variable, including the scope
+    /// within which the variable is visible (to debuginfo)
+    /// (see `LocalDecl`'s `source_info` field for more details).
+    pub source_info: SourceInfo,
+
+    /// Where the data for this user variable is to be found.
+    pub value: VarDebugInfoContents<'tcx>,
+}
+
+///////////////////////////////////////////////////////////////////////////
+// BasicBlock
+
+rustc_index::newtype_index! {
+    /// A node in the MIR [control-flow graph][CFG].
+    ///
+    /// There are no branches (e.g., `if`s, function calls, etc.) within a basic block, which makes
+    /// it easier to do [data-flow analyses] and optimizations. Instead, branches are represented
+    /// as an edge in a graph between basic blocks.
+    ///
+    /// Basic blocks consist of a series of [statements][Statement], ending with a
+    /// [terminator][Terminator]. Basic blocks can have multiple predecessors and successors,
+    /// however there is a MIR pass ([`CriticalCallEdges`]) that removes *critical edges*, which
+    /// are edges that go from a multi-successor node to a multi-predecessor node. This pass is
+    /// needed because some analyses require that there are no critical edges in the CFG.
+    ///
+    /// Note that this type is just an index into [`Body.basic_blocks`](Body::basic_blocks);
+    /// the actual data that a basic block holds is in [`BasicBlockData`].
+    ///
+    /// Read more about basic blocks in the [rustc-dev-guide][guide-mir].
+    ///
+    /// [CFG]: https://rustc-dev-guide.rust-lang.org/appendix/background.html#cfg
+    /// [data-flow analyses]:
+    ///     https://rustc-dev-guide.rust-lang.org/appendix/background.html#what-is-a-dataflow-analysis
+    /// [`CriticalCallEdges`]: ../../rustc_const_eval/transform/add_call_guards/enum.AddCallGuards.html#variant.CriticalCallEdges
+    /// [guide-mir]: https://rustc-dev-guide.rust-lang.org/mir/
+    pub struct BasicBlock {
+        derive [HashStable]
+        DEBUG_FORMAT = "bb{}",
+        const START_BLOCK = 0,
+    }
+}
+
+impl BasicBlock {
+    pub fn start_location(self) -> Location {
+        Location { block: self, statement_index: 0 }
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// BasicBlockData
+
+/// Data for a basic block, including a list of its statements.
+///
+/// See [`BasicBlock`] for documentation on what basic blocks are at a high level.
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub struct BasicBlockData<'tcx> {
+    /// List of statements in this block.
+    pub statements: Vec<Statement<'tcx>>,
+
+    /// Terminator for this block.
+    ///
+    /// N.B., this should generally ONLY be `None` during construction.
+    /// Therefore, you should generally access it via the
+    /// `terminator()` or `terminator_mut()` methods. The only
+    /// exception is that certain passes, such as `simplify_cfg`, swap
+    /// out the terminator temporarily with `None` while they continue
+    /// to recurse over the set of basic blocks.
+    pub terminator: Option<Terminator<'tcx>>,
+
+    /// If true, this block lies on an unwind path. This is used
+    /// during codegen where distinct kinds of basic blocks may be
+    /// generated (particularly for MSVC cleanup). Unwind blocks must
+    /// only branch to other unwind blocks.
+    pub is_cleanup: bool,
+}
+
+impl<'tcx> BasicBlockData<'tcx> {
+    pub fn new(terminator: Option<Terminator<'tcx>>) -> BasicBlockData<'tcx> {
+        BasicBlockData { statements: vec![], terminator, is_cleanup: false }
+    }
+
+    /// Accessor for terminator.
+    ///
+    /// Terminator may not be None after construction of the basic block is complete. This accessor
+    /// provides a convenient way to reach the terminator.
+    #[inline]
+    pub fn terminator(&self) -> &Terminator<'tcx> {
+        self.terminator.as_ref().expect("invalid terminator state")
+    }
+
+    #[inline]
+    pub fn terminator_mut(&mut self) -> &mut Terminator<'tcx> {
+        self.terminator.as_mut().expect("invalid terminator state")
+    }
+
+    pub fn retain_statements<F>(&mut self, mut f: F)
+    where
+        F: FnMut(&mut Statement<'_>) -> bool,
+    {
+        for s in &mut self.statements {
+            if !f(s) {
+                s.make_nop();
+            }
+        }
+    }
+
+    pub fn expand_statements<F, I>(&mut self, mut f: F)
+    where
+        F: FnMut(&mut Statement<'tcx>) -> Option<I>,
+        I: iter::TrustedLen<Item = Statement<'tcx>>,
+    {
+        // Gather all the iterators we'll need to splice in, and their positions.
+        let mut splices: Vec<(usize, I)> = vec![];
+        let mut extra_stmts = 0;
+        for (i, s) in self.statements.iter_mut().enumerate() {
+            if let Some(mut new_stmts) = f(s) {
+                if let Some(first) = new_stmts.next() {
+                    // We can already store the first new statement.
+                    *s = first;
+
+                    // Save the other statements for optimized splicing.
+                    let remaining = new_stmts.size_hint().0;
+                    if remaining > 0 {
+                        splices.push((i + 1 + extra_stmts, new_stmts));
+                        extra_stmts += remaining;
+                    }
+                } else {
+                    s.make_nop();
+                }
+            }
+        }
+
+        // Splice in the new statements, from the end of the block.
+        // FIXME(eddyb) This could be more efficient with a "gap buffer"
+        // where a range of elements ("gap") is left uninitialized, with
+        // splicing adding new elements to the end of that gap and moving
+        // existing elements from before the gap to the end of the gap.
+        // For now, this is safe code, emulating a gap but initializing it.
+        let mut gap = self.statements.len()..self.statements.len() + extra_stmts;
+        self.statements.resize(
+            gap.end,
+            Statement { source_info: SourceInfo::outermost(DUMMY_SP), kind: StatementKind::Nop },
+        );
+        for (splice_start, new_stmts) in splices.into_iter().rev() {
+            let splice_end = splice_start + new_stmts.size_hint().0;
+            while gap.end > splice_end {
+                gap.start -= 1;
+                gap.end -= 1;
+                self.statements.swap(gap.start, gap.end);
+            }
+            self.statements.splice(splice_start..splice_end, new_stmts);
+            gap.end = splice_start;
+        }
+    }
+
+    pub fn visitable(&self, index: usize) -> &dyn MirVisitable<'tcx> {
+        if index < self.statements.len() { &self.statements[index] } else { &self.terminator }
+    }
+}
+
+impl<O> AssertKind<O> {
+    /// Getting a description does not require `O` to be printable, and does not
+    /// require allocation.
+    /// The caller is expected to handle `BoundsCheck` separately.
+    pub fn description(&self) -> &'static str {
+        use AssertKind::*;
+        match self {
+            Overflow(BinOp::Add, _, _) => "attempt to add with overflow",
+            Overflow(BinOp::Sub, _, _) => "attempt to subtract with overflow",
+            Overflow(BinOp::Mul, _, _) => "attempt to multiply with overflow",
+            Overflow(BinOp::Div, _, _) => "attempt to divide with overflow",
+            Overflow(BinOp::Rem, _, _) => "attempt to calculate the remainder with overflow",
+            OverflowNeg(_) => "attempt to negate with overflow",
+            Overflow(BinOp::Shr, _, _) => "attempt to shift right with overflow",
+            Overflow(BinOp::Shl, _, _) => "attempt to shift left with overflow",
+            Overflow(op, _, _) => bug!("{:?} cannot overflow", op),
+            DivisionByZero(_) => "attempt to divide by zero",
+            RemainderByZero(_) => "attempt to calculate the remainder with a divisor of zero",
+            ResumedAfterReturn(GeneratorKind::Gen) => "generator resumed after completion",
+            ResumedAfterReturn(GeneratorKind::Async(_)) => "`async fn` resumed after completion",
+            ResumedAfterPanic(GeneratorKind::Gen) => "generator resumed after panicking",
+            ResumedAfterPanic(GeneratorKind::Async(_)) => "`async fn` resumed after panicking",
+            BoundsCheck { .. } => bug!("Unexpected AssertKind"),
+        }
+    }
+
+    /// Format the message arguments for the `assert(cond, msg..)` terminator in MIR printing.
+    pub fn fmt_assert_args<W: Write>(&self, f: &mut W) -> fmt::Result
+    where
+        O: Debug,
+    {
+        use AssertKind::*;
+        match self {
+            BoundsCheck { ref len, ref index } => write!(
+                f,
+                "\"index out of bounds: the length is {{}} but the index is {{}}\", {:?}, {:?}",
+                len, index
+            ),
+
+            OverflowNeg(op) => {
+                write!(f, "\"attempt to negate `{{}}`, which would overflow\", {:?}", op)
+            }
+            DivisionByZero(op) => write!(f, "\"attempt to divide `{{}}` by zero\", {:?}", op),
+            RemainderByZero(op) => write!(
+                f,
+                "\"attempt to calculate the remainder of `{{}}` with a divisor of zero\", {:?}",
+                op
+            ),
+            Overflow(BinOp::Add, l, r) => write!(
+                f,
+                "\"attempt to compute `{{}} + {{}}`, which would overflow\", {:?}, {:?}",
+                l, r
+            ),
+            Overflow(BinOp::Sub, l, r) => write!(
+                f,
+                "\"attempt to compute `{{}} - {{}}`, which would overflow\", {:?}, {:?}",
+                l, r
+            ),
+            Overflow(BinOp::Mul, l, r) => write!(
+                f,
+                "\"attempt to compute `{{}} * {{}}`, which would overflow\", {:?}, {:?}",
+                l, r
+            ),
+            Overflow(BinOp::Div, l, r) => write!(
+                f,
+                "\"attempt to compute `{{}} / {{}}`, which would overflow\", {:?}, {:?}",
+                l, r
+            ),
+            Overflow(BinOp::Rem, l, r) => write!(
+                f,
+                "\"attempt to compute the remainder of `{{}} % {{}}`, which would overflow\", {:?}, {:?}",
+                l, r
+            ),
+            Overflow(BinOp::Shr, _, r) => {
+                write!(f, "\"attempt to shift right by `{{}}`, which would overflow\", {:?}", r)
+            }
+            Overflow(BinOp::Shl, _, r) => {
+                write!(f, "\"attempt to shift left by `{{}}`, which would overflow\", {:?}", r)
+            }
+            _ => write!(f, "\"{}\"", self.description()),
+        }
+    }
+}
+
+impl<O: fmt::Debug> fmt::Debug for AssertKind<O> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use AssertKind::*;
+        match self {
+            BoundsCheck { ref len, ref index } => write!(
+                f,
+                "index out of bounds: the length is {:?} but the index is {:?}",
+                len, index
+            ),
+            OverflowNeg(op) => write!(f, "attempt to negate `{:#?}`, which would overflow", op),
+            DivisionByZero(op) => write!(f, "attempt to divide `{:#?}` by zero", op),
+            RemainderByZero(op) => write!(
+                f,
+                "attempt to calculate the remainder of `{:#?}` with a divisor of zero",
+                op
+            ),
+            Overflow(BinOp::Add, l, r) => {
+                write!(f, "attempt to compute `{:#?} + {:#?}`, which would overflow", l, r)
+            }
+            Overflow(BinOp::Sub, l, r) => {
+                write!(f, "attempt to compute `{:#?} - {:#?}`, which would overflow", l, r)
+            }
+            Overflow(BinOp::Mul, l, r) => {
+                write!(f, "attempt to compute `{:#?} * {:#?}`, which would overflow", l, r)
+            }
+            Overflow(BinOp::Div, l, r) => {
+                write!(f, "attempt to compute `{:#?} / {:#?}`, which would overflow", l, r)
+            }
+            Overflow(BinOp::Rem, l, r) => write!(
+                f,
+                "attempt to compute the remainder of `{:#?} % {:#?}`, which would overflow",
+                l, r
+            ),
+            Overflow(BinOp::Shr, _, r) => {
+                write!(f, "attempt to shift right by `{:#?}`, which would overflow", r)
+            }
+            Overflow(BinOp::Shl, _, r) => {
+                write!(f, "attempt to shift left by `{:#?}`, which would overflow", r)
+            }
+            _ => write!(f, "{}", self.description()),
+        }
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Statements
+
+/// A statement in a basic block, including information about its source code.
+#[derive(Clone, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub struct Statement<'tcx> {
+    pub source_info: SourceInfo,
+    pub kind: StatementKind<'tcx>,
+}
+
+// `Statement` is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(Statement<'_>, 32);
+
+impl Statement<'_> {
+    /// Changes a statement to a nop. This is both faster than deleting instructions and avoids
+    /// invalidating statement indices in `Location`s.
+    pub fn make_nop(&mut self) {
+        self.kind = StatementKind::Nop
+    }
+
+    /// Changes a statement to a nop and returns the original statement.
+    #[must_use = "If you don't need the statement, use `make_nop` instead"]
+    pub fn replace_nop(&mut self) -> Self {
+        Statement {
+            source_info: self.source_info,
+            kind: mem::replace(&mut self.kind, StatementKind::Nop),
+        }
+    }
+}
+
+impl Debug for Statement<'_> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        use self::StatementKind::*;
+        match self.kind {
+            Assign(box (ref place, ref rv)) => write!(fmt, "{:?} = {:?}", place, rv),
+            FakeRead(box (ref cause, ref place)) => {
+                write!(fmt, "FakeRead({:?}, {:?})", cause, place)
+            }
+            Retag(ref kind, ref place) => write!(
+                fmt,
+                "Retag({}{:?})",
+                match kind {
+                    RetagKind::FnEntry => "[fn entry] ",
+                    RetagKind::TwoPhase => "[2phase] ",
+                    RetagKind::Raw => "[raw] ",
+                    RetagKind::Default => "",
+                },
+                place,
+            ),
+            StorageLive(ref place) => write!(fmt, "StorageLive({:?})", place),
+            StorageDead(ref place) => write!(fmt, "StorageDead({:?})", place),
+            SetDiscriminant { ref place, variant_index } => {
+                write!(fmt, "discriminant({:?}) = {:?}", place, variant_index)
+            }
+            Deinit(ref place) => write!(fmt, "Deinit({:?})", place),
+            AscribeUserType(box (ref place, ref c_ty), ref variance) => {
+                write!(fmt, "AscribeUserType({:?}, {:?}, {:?})", place, variance, c_ty)
+            }
+            Coverage(box self::Coverage { ref kind, code_region: Some(ref rgn) }) => {
+                write!(fmt, "Coverage::{:?} for {:?}", kind, rgn)
+            }
+            Coverage(box ref coverage) => write!(fmt, "Coverage::{:?}", coverage.kind),
+            CopyNonOverlapping(box crate::mir::CopyNonOverlapping {
+                ref src,
+                ref dst,
+                ref count,
+            }) => {
+                write!(fmt, "copy_nonoverlapping(src={:?}, dst={:?}, count={:?})", src, dst, count)
+            }
+            Nop => write!(fmt, "nop"),
+        }
+    }
+}
+
+impl<'tcx> StatementKind<'tcx> {
+    pub fn as_assign_mut(&mut self) -> Option<&mut (Place<'tcx>, Rvalue<'tcx>)> {
+        match self {
+            StatementKind::Assign(x) => Some(x),
+            _ => None,
+        }
+    }
+
+    pub fn as_assign(&self) -> Option<&(Place<'tcx>, Rvalue<'tcx>)> {
+        match self {
+            StatementKind::Assign(x) => Some(x),
+            _ => None,
+        }
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Places
+
+impl<V, T> ProjectionElem<V, T> {
+    /// Returns `true` if the target of this projection may refer to a different region of memory
+    /// than the base.
+    fn is_indirect(&self) -> bool {
+        match self {
+            Self::Deref => true,
+
+            Self::Field(_, _)
+            | Self::Index(_)
+            | Self::ConstantIndex { .. }
+            | Self::Subslice { .. }
+            | Self::Downcast(_, _) => false,
+        }
+    }
+
+    /// Returns `true` if this is a `Downcast` projection with the given `VariantIdx`.
+    pub fn is_downcast_to(&self, v: VariantIdx) -> bool {
+        matches!(*self, Self::Downcast(_, x) if x == v)
+    }
+
+    /// Returns `true` if this is a `Field` projection with the given index.
+    pub fn is_field_to(&self, f: Field) -> bool {
+        matches!(*self, Self::Field(x, _) if x == f)
+    }
+}
+
+/// Alias for projections as they appear in `UserTypeProjection`, where we
+/// need neither the `V` parameter for `Index` nor the `T` for `Field`.
+pub type ProjectionKind = ProjectionElem<(), ()>;
+
+rustc_index::newtype_index! {
+    /// A [newtype'd][wrapper] index type in the MIR [control-flow graph][CFG]
+    ///
+    /// A field (e.g., `f` in `_1.f`) is one variant of [`ProjectionElem`]. Conceptually,
+    /// rustc can identify that a field projection refers to either two different regions of memory
+    /// or the same one between the base and the 'projection element'.
+    /// Read more about projections in the [rustc-dev-guide][mir-datatypes]
+    ///
+    /// [wrapper]: https://rustc-dev-guide.rust-lang.org/appendix/glossary.html#newtype
+    /// [CFG]: https://rustc-dev-guide.rust-lang.org/appendix/background.html#cfg
+    /// [mir-datatypes]: https://rustc-dev-guide.rust-lang.org/mir/index.html#mir-data-types
+    pub struct Field {
+        derive [HashStable]
+        DEBUG_FORMAT = "field[{}]"
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub struct PlaceRef<'tcx> {
+    pub local: Local,
+    pub projection: &'tcx [PlaceElem<'tcx>],
+}
+
+// Once we stop implementing `Ord` for `DefId`,
+// this impl will be unnecessary. Until then, we'll
+// leave this impl in place to prevent re-adding a
+// dependnecy on the `Ord` impl for `DefId`
+impl<'tcx> !PartialOrd for PlaceRef<'tcx> {}
+
+impl<'tcx> Place<'tcx> {
+    // FIXME change this to a const fn by also making List::empty a const fn.
+    pub fn return_place() -> Place<'tcx> {
+        Place { local: RETURN_PLACE, projection: List::empty() }
+    }
+
+    /// Returns `true` if this `Place` contains a `Deref` projection.
+    ///
+    /// If `Place::is_indirect` returns false, the caller knows that the `Place` refers to the
+    /// same region of memory as its base.
+    pub fn is_indirect(&self) -> bool {
+        self.projection.iter().any(|elem| elem.is_indirect())
+    }
+
+    /// If MirPhase >= Derefered and if projection contains Deref,
+    /// It's guaranteed to be in the first place
+    pub fn has_deref(&self) -> bool {
+        // To make sure this is not accidently used in wrong mir phase
+        debug_assert!(!self.projection[1..].contains(&PlaceElem::Deref));
+        self.projection.first() == Some(&PlaceElem::Deref)
+    }
+
+    /// Finds the innermost `Local` from this `Place`, *if* it is either a local itself or
+    /// a single deref of a local.
+    #[inline(always)]
+    pub fn local_or_deref_local(&self) -> Option<Local> {
+        self.as_ref().local_or_deref_local()
+    }
+
+    /// If this place represents a local variable like `_X` with no
+    /// projections, return `Some(_X)`.
+    #[inline(always)]
+    pub fn as_local(&self) -> Option<Local> {
+        self.as_ref().as_local()
+    }
+
+    #[inline]
+    pub fn as_ref(&self) -> PlaceRef<'tcx> {
+        PlaceRef { local: self.local, projection: &self.projection }
+    }
+
+    /// Iterate over the projections in evaluation order, i.e., the first element is the base with
+    /// its projection and then subsequently more projections are added.
+    /// As a concrete example, given the place a.b.c, this would yield:
+    /// - (a, .b)
+    /// - (a.b, .c)
+    ///
+    /// Given a place without projections, the iterator is empty.
+    #[inline]
+    pub fn iter_projections(
+        self,
+    ) -> impl Iterator<Item = (PlaceRef<'tcx>, PlaceElem<'tcx>)> + DoubleEndedIterator {
+        self.as_ref().iter_projections()
+    }
+
+    /// Generates a new place by appending `more_projections` to the existing ones
+    /// and interning the result.
+    pub fn project_deeper(self, more_projections: &[PlaceElem<'tcx>], tcx: TyCtxt<'tcx>) -> Self {
+        if more_projections.is_empty() {
+            return self;
+        }
+
+        let mut v: Vec<PlaceElem<'tcx>>;
+
+        let new_projections = if self.projection.is_empty() {
+            more_projections
+        } else {
+            v = Vec::with_capacity(self.projection.len() + more_projections.len());
+            v.extend(self.projection);
+            v.extend(more_projections);
+            &v
+        };
+
+        Place { local: self.local, projection: tcx.intern_place_elems(new_projections) }
+    }
+}
+
+impl From<Local> for Place<'_> {
+    fn from(local: Local) -> Self {
+        Place { local, projection: List::empty() }
+    }
+}
+
+impl<'tcx> PlaceRef<'tcx> {
+    /// Finds the innermost `Local` from this `Place`, *if* it is either a local itself or
+    /// a single deref of a local.
+    pub fn local_or_deref_local(&self) -> Option<Local> {
+        match *self {
+            PlaceRef { local, projection: [] }
+            | PlaceRef { local, projection: [ProjectionElem::Deref] } => Some(local),
+            _ => None,
+        }
+    }
+
+    /// If MirPhase >= Derefered and if projection contains Deref,
+    /// It's guaranteed to be in the first place
+    pub fn has_deref(&self) -> bool {
+        self.projection.first() == Some(&PlaceElem::Deref)
+    }
+
+    /// If this place represents a local variable like `_X` with no
+    /// projections, return `Some(_X)`.
+    #[inline]
+    pub fn as_local(&self) -> Option<Local> {
+        match *self {
+            PlaceRef { local, projection: [] } => Some(local),
+            _ => None,
+        }
+    }
+
+    #[inline]
+    pub fn last_projection(&self) -> Option<(PlaceRef<'tcx>, PlaceElem<'tcx>)> {
+        if let &[ref proj_base @ .., elem] = self.projection {
+            Some((PlaceRef { local: self.local, projection: proj_base }, elem))
+        } else {
+            None
+        }
+    }
+
+    /// Iterate over the projections in evaluation order, i.e., the first element is the base with
+    /// its projection and then subsequently more projections are added.
+    /// As a concrete example, given the place a.b.c, this would yield:
+    /// - (a, .b)
+    /// - (a.b, .c)
+    ///
+    /// Given a place without projections, the iterator is empty.
+    #[inline]
+    pub fn iter_projections(
+        self,
+    ) -> impl Iterator<Item = (PlaceRef<'tcx>, PlaceElem<'tcx>)> + DoubleEndedIterator {
+        self.projection.iter().enumerate().map(move |(i, proj)| {
+            let base = PlaceRef { local: self.local, projection: &self.projection[..i] };
+            (base, *proj)
+        })
+    }
+}
+
+impl Debug for Place<'_> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        for elem in self.projection.iter().rev() {
+            match elem {
+                ProjectionElem::Downcast(_, _) | ProjectionElem::Field(_, _) => {
+                    write!(fmt, "(").unwrap();
+                }
+                ProjectionElem::Deref => {
+                    write!(fmt, "(*").unwrap();
+                }
+                ProjectionElem::Index(_)
+                | ProjectionElem::ConstantIndex { .. }
+                | ProjectionElem::Subslice { .. } => {}
+            }
+        }
+
+        write!(fmt, "{:?}", self.local)?;
+
+        for elem in self.projection.iter() {
+            match elem {
+                ProjectionElem::Downcast(Some(name), _index) => {
+                    write!(fmt, " as {})", name)?;
+                }
+                ProjectionElem::Downcast(None, index) => {
+                    write!(fmt, " as variant#{:?})", index)?;
+                }
+                ProjectionElem::Deref => {
+                    write!(fmt, ")")?;
+                }
+                ProjectionElem::Field(field, ty) => {
+                    write!(fmt, ".{:?}: {:?})", field.index(), ty)?;
+                }
+                ProjectionElem::Index(ref index) => {
+                    write!(fmt, "[{:?}]", index)?;
+                }
+                ProjectionElem::ConstantIndex { offset, min_length, from_end: false } => {
+                    write!(fmt, "[{:?} of {:?}]", offset, min_length)?;
+                }
+                ProjectionElem::ConstantIndex { offset, min_length, from_end: true } => {
+                    write!(fmt, "[-{:?} of {:?}]", offset, min_length)?;
+                }
+                ProjectionElem::Subslice { from, to, from_end: true } if to == 0 => {
+                    write!(fmt, "[{:?}:]", from)?;
+                }
+                ProjectionElem::Subslice { from, to, from_end: true } if from == 0 => {
+                    write!(fmt, "[:-{:?}]", to)?;
+                }
+                ProjectionElem::Subslice { from, to, from_end: true } => {
+                    write!(fmt, "[{:?}:-{:?}]", from, to)?;
+                }
+                ProjectionElem::Subslice { from, to, from_end: false } => {
+                    write!(fmt, "[{:?}..{:?}]", from, to)?;
+                }
+            }
+        }
+
+        Ok(())
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Scopes
+
+rustc_index::newtype_index! {
+    pub struct SourceScope {
+        derive [HashStable]
+        DEBUG_FORMAT = "scope[{}]",
+        const OUTERMOST_SOURCE_SCOPE = 0,
+    }
+}
+
+impl SourceScope {
+    /// Finds the original HirId this MIR item came from.
+    /// This is necessary after MIR optimizations, as otherwise we get a HirId
+    /// from the function that was inlined instead of the function call site.
+    pub fn lint_root<'tcx>(
+        self,
+        source_scopes: &IndexVec<SourceScope, SourceScopeData<'tcx>>,
+    ) -> Option<HirId> {
+        let mut data = &source_scopes[self];
+        // FIXME(oli-obk): we should be able to just walk the `inlined_parent_scope`, but it
+        // does not work as I thought it would. Needs more investigation and documentation.
+        while data.inlined.is_some() {
+            trace!(?data);
+            data = &source_scopes[data.parent_scope.unwrap()];
+        }
+        trace!(?data);
+        match &data.local_data {
+            ClearCrossCrate::Set(data) => Some(data.lint_root),
+            ClearCrossCrate::Clear => None,
+        }
+    }
+
+    /// The instance this source scope was inlined from, if any.
+    #[inline]
+    pub fn inlined_instance<'tcx>(
+        self,
+        source_scopes: &IndexVec<SourceScope, SourceScopeData<'tcx>>,
+    ) -> Option<ty::Instance<'tcx>> {
+        let scope_data = &source_scopes[self];
+        if let Some((inlined_instance, _)) = scope_data.inlined {
+            Some(inlined_instance)
+        } else if let Some(inlined_scope) = scope_data.inlined_parent_scope {
+            Some(source_scopes[inlined_scope].inlined.unwrap().0)
+        } else {
+            None
+        }
+    }
+}
+
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub struct SourceScopeData<'tcx> {
+    pub span: Span,
+    pub parent_scope: Option<SourceScope>,
+
+    /// Whether this scope is the root of a scope tree of another body,
+    /// inlined into this body by the MIR inliner.
+    /// `ty::Instance` is the callee, and the `Span` is the call site.
+    pub inlined: Option<(ty::Instance<'tcx>, Span)>,
+
+    /// Nearest (transitive) parent scope (if any) which is inlined.
+    /// This is an optimization over walking up `parent_scope`
+    /// until a scope with `inlined: Some(...)` is found.
+    pub inlined_parent_scope: Option<SourceScope>,
+
+    /// Crate-local information for this source scope, that can't (and
+    /// needn't) be tracked across crates.
+    pub local_data: ClearCrossCrate<SourceScopeLocalData>,
+}
+
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct SourceScopeLocalData {
+    /// An `HirId` with lint levels equivalent to this scope's lint levels.
+    pub lint_root: hir::HirId,
+    /// The unsafe block that contains this node.
+    pub safety: Safety,
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Operands
+
+impl<'tcx> Debug for Operand<'tcx> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        use self::Operand::*;
+        match *self {
+            Constant(ref a) => write!(fmt, "{:?}", a),
+            Copy(ref place) => write!(fmt, "{:?}", place),
+            Move(ref place) => write!(fmt, "move {:?}", place),
+        }
+    }
+}
+
+impl<'tcx> Operand<'tcx> {
+    /// Convenience helper to make a constant that refers to the fn
+    /// with given `DefId` and substs. Since this is used to synthesize
+    /// MIR, assumes `user_ty` is None.
+    pub fn function_handle(
+        tcx: TyCtxt<'tcx>,
+        def_id: DefId,
+        substs: SubstsRef<'tcx>,
+        span: Span,
+    ) -> Self {
+        let ty = tcx.bound_type_of(def_id).subst(tcx, substs);
+        Operand::Constant(Box::new(Constant {
+            span,
+            user_ty: None,
+            literal: ConstantKind::Val(ConstValue::ZeroSized, ty),
+        }))
+    }
+
+    pub fn is_move(&self) -> bool {
+        matches!(self, Operand::Move(..))
+    }
+
+    /// Convenience helper to make a literal-like constant from a given scalar value.
+    /// Since this is used to synthesize MIR, assumes `user_ty` is None.
+    pub fn const_from_scalar(
+        tcx: TyCtxt<'tcx>,
+        ty: Ty<'tcx>,
+        val: Scalar,
+        span: Span,
+    ) -> Operand<'tcx> {
+        debug_assert!({
+            let param_env_and_ty = ty::ParamEnv::empty().and(ty);
+            let type_size = tcx
+                .layout_of(param_env_and_ty)
+                .unwrap_or_else(|e| panic!("could not compute layout for {:?}: {:?}", ty, e))
+                .size;
+            let scalar_size = match val {
+                Scalar::Int(int) => int.size(),
+                _ => panic!("Invalid scalar type {:?}", val),
+            };
+            scalar_size == type_size
+        });
+        Operand::Constant(Box::new(Constant {
+            span,
+            user_ty: None,
+            literal: ConstantKind::Val(ConstValue::Scalar(val), ty),
+        }))
+    }
+
+    pub fn to_copy(&self) -> Self {
+        match *self {
+            Operand::Copy(_) | Operand::Constant(_) => self.clone(),
+            Operand::Move(place) => Operand::Copy(place),
+        }
+    }
+
+    /// Returns the `Place` that is the target of this `Operand`, or `None` if this `Operand` is a
+    /// constant.
+    pub fn place(&self) -> Option<Place<'tcx>> {
+        match self {
+            Operand::Copy(place) | Operand::Move(place) => Some(*place),
+            Operand::Constant(_) => None,
+        }
+    }
+
+    /// Returns the `Constant` that is the target of this `Operand`, or `None` if this `Operand` is a
+    /// place.
+    pub fn constant(&self) -> Option<&Constant<'tcx>> {
+        match self {
+            Operand::Constant(x) => Some(&**x),
+            Operand::Copy(_) | Operand::Move(_) => None,
+        }
+    }
+
+    /// Gets the `ty::FnDef` from an operand if it's a constant function item.
+    ///
+    /// While this is unlikely in general, it's the normal case of what you'll
+    /// find as the `func` in a [`TerminatorKind::Call`].
+    pub fn const_fn_def(&self) -> Option<(DefId, SubstsRef<'tcx>)> {
+        let const_ty = self.constant()?.literal.ty();
+        if let ty::FnDef(def_id, substs) = *const_ty.kind() { Some((def_id, substs)) } else { None }
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+/// Rvalues
+
+impl<'tcx> Rvalue<'tcx> {
+    /// Returns true if rvalue can be safely removed when the result is unused.
+    #[inline]
+    pub fn is_safe_to_remove(&self) -> bool {
+        match self {
+            // Pointer to int casts may be side-effects due to exposing the provenance.
+            // While the model is undecided, we should be conservative. See
+            // <https://www.ralfj.de/blog/2022/04/11/provenance-exposed.html>
+            Rvalue::Cast(CastKind::PointerExposeAddress, _, _) => false,
+
+            Rvalue::Use(_)
+            | Rvalue::CopyForDeref(_)
+            | Rvalue::Repeat(_, _)
+            | Rvalue::Ref(_, _, _)
+            | Rvalue::ThreadLocalRef(_)
+            | Rvalue::AddressOf(_, _)
+            | Rvalue::Len(_)
+            | Rvalue::Cast(
+                CastKind::Misc | CastKind::Pointer(_) | CastKind::PointerFromExposedAddress,
+                _,
+                _,
+            )
+            | Rvalue::BinaryOp(_, _)
+            | Rvalue::CheckedBinaryOp(_, _)
+            | Rvalue::NullaryOp(_, _)
+            | Rvalue::UnaryOp(_, _)
+            | Rvalue::Discriminant(_)
+            | Rvalue::Aggregate(_, _)
+            | Rvalue::ShallowInitBox(_, _) => true,
+        }
+    }
+}
+
+impl BorrowKind {
+    pub fn allows_two_phase_borrow(&self) -> bool {
+        match *self {
+            BorrowKind::Shared | BorrowKind::Shallow | BorrowKind::Unique => false,
+            BorrowKind::Mut { allow_two_phase_borrow } => allow_two_phase_borrow,
+        }
+    }
+
+    pub fn describe_mutability(&self) -> &str {
+        match *self {
+            BorrowKind::Shared | BorrowKind::Shallow | BorrowKind::Unique => "immutable",
+            BorrowKind::Mut { .. } => "mutable",
+        }
+    }
+}
+
+impl BinOp {
+    pub fn is_checkable(self) -> bool {
+        use self::BinOp::*;
+        matches!(self, Add | Sub | Mul | Shl | Shr)
+    }
+}
+
+impl<'tcx> Debug for Rvalue<'tcx> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        use self::Rvalue::*;
+
+        match *self {
+            Use(ref place) => write!(fmt, "{:?}", place),
+            Repeat(ref a, b) => {
+                write!(fmt, "[{:?}; ", a)?;
+                pretty_print_const(b, fmt, false)?;
+                write!(fmt, "]")
+            }
+            Len(ref a) => write!(fmt, "Len({:?})", a),
+            Cast(ref kind, ref place, ref ty) => {
+                write!(fmt, "{:?} as {:?} ({:?})", place, ty, kind)
+            }
+            BinaryOp(ref op, box (ref a, ref b)) => write!(fmt, "{:?}({:?}, {:?})", op, a, b),
+            CheckedBinaryOp(ref op, box (ref a, ref b)) => {
+                write!(fmt, "Checked{:?}({:?}, {:?})", op, a, b)
+            }
+            UnaryOp(ref op, ref a) => write!(fmt, "{:?}({:?})", op, a),
+            Discriminant(ref place) => write!(fmt, "discriminant({:?})", place),
+            NullaryOp(ref op, ref t) => write!(fmt, "{:?}({:?})", op, t),
+            ThreadLocalRef(did) => ty::tls::with(|tcx| {
+                let muta = tcx.static_mutability(did).unwrap().prefix_str();
+                write!(fmt, "&/*tls*/ {}{}", muta, tcx.def_path_str(did))
+            }),
+            Ref(region, borrow_kind, ref place) => {
+                let kind_str = match borrow_kind {
+                    BorrowKind::Shared => "",
+                    BorrowKind::Shallow => "shallow ",
+                    BorrowKind::Mut { .. } | BorrowKind::Unique => "mut ",
+                };
+
+                // When printing regions, add trailing space if necessary.
+                let print_region = ty::tls::with(|tcx| {
+                    tcx.sess.verbose() || tcx.sess.opts.unstable_opts.identify_regions
+                });
+                let region = if print_region {
+                    let mut region = region.to_string();
+                    if !region.is_empty() {
+                        region.push(' ');
+                    }
+                    region
+                } else {
+                    // Do not even print 'static
+                    String::new()
+                };
+                write!(fmt, "&{}{}{:?}", region, kind_str, place)
+            }
+
+            CopyForDeref(ref place) => write!(fmt, "deref_copy {:#?}", place),
+
+            AddressOf(mutability, ref place) => {
+                let kind_str = match mutability {
+                    Mutability::Mut => "mut",
+                    Mutability::Not => "const",
+                };
+
+                write!(fmt, "&raw {} {:?}", kind_str, place)
+            }
+
+            Aggregate(ref kind, ref places) => {
+                let fmt_tuple = |fmt: &mut Formatter<'_>, name: &str| {
+                    let mut tuple_fmt = fmt.debug_tuple(name);
+                    for place in places {
+                        tuple_fmt.field(place);
+                    }
+                    tuple_fmt.finish()
+                };
+
+                match **kind {
+                    AggregateKind::Array(_) => write!(fmt, "{:?}", places),
+
+                    AggregateKind::Tuple => {
+                        if places.is_empty() {
+                            write!(fmt, "()")
+                        } else {
+                            fmt_tuple(fmt, "")
+                        }
+                    }
+
+                    AggregateKind::Adt(adt_did, variant, substs, _user_ty, _) => {
+                        ty::tls::with(|tcx| {
+                            let variant_def = &tcx.adt_def(adt_did).variant(variant);
+                            let substs = tcx.lift(substs).expect("could not lift for printing");
+                            let name = FmtPrinter::new(tcx, Namespace::ValueNS)
+                                .print_def_path(variant_def.def_id, substs)?
+                                .into_buffer();
+
+                            match variant_def.ctor_kind {
+                                CtorKind::Const => fmt.write_str(&name),
+                                CtorKind::Fn => fmt_tuple(fmt, &name),
+                                CtorKind::Fictive => {
+                                    let mut struct_fmt = fmt.debug_struct(&name);
+                                    for (field, place) in iter::zip(&variant_def.fields, places) {
+                                        struct_fmt.field(field.name.as_str(), place);
+                                    }
+                                    struct_fmt.finish()
+                                }
+                            }
+                        })
+                    }
+
+                    AggregateKind::Closure(def_id, substs) => ty::tls::with(|tcx| {
+                        let name = if tcx.sess.opts.unstable_opts.span_free_formats {
+                            let substs = tcx.lift(substs).unwrap();
+                            format!(
+                                "[closure@{}]",
+                                tcx.def_path_str_with_substs(def_id.to_def_id(), substs),
+                            )
+                        } else {
+                            let span = tcx.def_span(def_id);
+                            format!(
+                                "[closure@{}]",
+                                tcx.sess.source_map().span_to_diagnostic_string(span)
+                            )
+                        };
+                        let mut struct_fmt = fmt.debug_struct(&name);
+
+                        // FIXME(project-rfc-2229#48): This should be a list of capture names/places
+                        if let Some(upvars) = tcx.upvars_mentioned(def_id) {
+                            for (&var_id, place) in iter::zip(upvars.keys(), places) {
+                                let var_name = tcx.hir().name(var_id);
+                                struct_fmt.field(var_name.as_str(), place);
+                            }
+                        }
+
+                        struct_fmt.finish()
+                    }),
+
+                    AggregateKind::Generator(def_id, _, _) => ty::tls::with(|tcx| {
+                        let name = format!("[generator@{:?}]", tcx.def_span(def_id));
+                        let mut struct_fmt = fmt.debug_struct(&name);
+
+                        // FIXME(project-rfc-2229#48): This should be a list of capture names/places
+                        if let Some(upvars) = tcx.upvars_mentioned(def_id) {
+                            for (&var_id, place) in iter::zip(upvars.keys(), places) {
+                                let var_name = tcx.hir().name(var_id);
+                                struct_fmt.field(var_name.as_str(), place);
+                            }
+                        }
+
+                        struct_fmt.finish()
+                    }),
+                }
+            }
+
+            ShallowInitBox(ref place, ref ty) => {
+                write!(fmt, "ShallowInitBox({:?}, {:?})", place, ty)
+            }
+        }
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+/// Constants
+///
+/// Two constants are equal if they are the same constant. Note that
+/// this does not necessarily mean that they are `==` in Rust. In
+/// particular, one must be wary of `NaN`!
+
+#[derive(Clone, Copy, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
+pub struct Constant<'tcx> {
+    pub span: Span,
+
+    /// Optional user-given type: for something like
+    /// `collect::<Vec<_>>`, this would be present and would
+    /// indicate that `Vec<_>` was explicitly specified.
+    ///
+    /// Needed for NLL to impose user-given type constraints.
+    pub user_ty: Option<UserTypeAnnotationIndex>,
+
+    pub literal: ConstantKind<'tcx>,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable, Debug)]
+#[derive(Lift)]
+pub enum ConstantKind<'tcx> {
+    /// This constant came from the type system
+    Ty(ty::Const<'tcx>),
+    /// This constant cannot go back into the type system, as it represents
+    /// something the type system cannot handle (e.g. pointers).
+    Val(interpret::ConstValue<'tcx>, Ty<'tcx>),
+}
+
+impl<'tcx> Constant<'tcx> {
+    pub fn check_static_ptr(&self, tcx: TyCtxt<'_>) -> Option<DefId> {
+        match self.literal.try_to_scalar() {
+            Some(Scalar::Ptr(ptr, _size)) => match tcx.global_alloc(ptr.provenance) {
+                GlobalAlloc::Static(def_id) => {
+                    assert!(!tcx.is_thread_local_static(def_id));
+                    Some(def_id)
+                }
+                _ => None,
+            },
+            _ => None,
+        }
+    }
+    #[inline]
+    pub fn ty(&self) -> Ty<'tcx> {
+        self.literal.ty()
+    }
+}
+
+impl<'tcx> ConstantKind<'tcx> {
+    /// Returns `None` if the constant is not trivially safe for use in the type system.
+    #[inline]
+    pub fn const_for_ty(&self) -> Option<ty::Const<'tcx>> {
+        match self {
+            ConstantKind::Ty(c) => Some(*c),
+            ConstantKind::Val(..) => None,
+        }
+    }
+
+    #[inline(always)]
+    pub fn ty(&self) -> Ty<'tcx> {
+        match self {
+            ConstantKind::Ty(c) => c.ty(),
+            ConstantKind::Val(_, ty) => *ty,
+        }
+    }
+
+    #[inline]
+    pub fn try_to_value(self, tcx: TyCtxt<'tcx>) -> Option<interpret::ConstValue<'tcx>> {
+        match self {
+            ConstantKind::Ty(c) => match c.kind() {
+                ty::ConstKind::Value(valtree) => Some(tcx.valtree_to_const_val((c.ty(), valtree))),
+                _ => None,
+            },
+            ConstantKind::Val(val, _) => Some(val),
+        }
+    }
+
+    #[inline]
+    pub fn try_to_scalar(self) -> Option<Scalar> {
+        match self {
+            ConstantKind::Ty(c) => match c.kind() {
+                ty::ConstKind::Value(valtree) => match valtree {
+                    ty::ValTree::Leaf(scalar_int) => Some(Scalar::Int(scalar_int)),
+                    ty::ValTree::Branch(_) => None,
+                },
+                _ => None,
+            },
+            ConstantKind::Val(val, _) => val.try_to_scalar(),
+        }
+    }
+
+    #[inline]
+    pub fn try_to_scalar_int(self) -> Option<ScalarInt> {
+        Some(self.try_to_scalar()?.assert_int())
+    }
+
+    #[inline]
+    pub fn try_to_bits(self, size: Size) -> Option<u128> {
+        self.try_to_scalar_int()?.to_bits(size).ok()
+    }
+
+    #[inline]
+    pub fn try_to_bool(self) -> Option<bool> {
+        self.try_to_scalar_int()?.try_into().ok()
+    }
+
+    #[inline]
+    pub fn eval(self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Self {
+        match self {
+            Self::Ty(c) => {
+                if let Some(val) = c.kind().try_eval_for_mir(tcx, param_env) {
+                    match val {
+                        Ok(val) => Self::Val(val, c.ty()),
+                        Err(_) => Self::Ty(tcx.const_error(self.ty())),
+                    }
+                } else {
+                    self
+                }
+            }
+            Self::Val(_, _) => self,
+        }
+    }
+
+    /// Panics if the value cannot be evaluated or doesn't contain a valid integer of the given type.
+    #[inline]
+    pub fn eval_bits(self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>, ty: Ty<'tcx>) -> u128 {
+        self.try_eval_bits(tcx, param_env, ty)
+            .unwrap_or_else(|| bug!("expected bits of {:#?}, got {:#?}", ty, self))
+    }
+
+    #[inline]
+    pub fn try_eval_bits(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        ty: Ty<'tcx>,
+    ) -> Option<u128> {
+        match self {
+            Self::Ty(ct) => ct.try_eval_bits(tcx, param_env, ty),
+            Self::Val(val, t) => {
+                assert_eq!(*t, ty);
+                let size =
+                    tcx.layout_of(param_env.with_reveal_all_normalized(tcx).and(ty)).ok()?.size;
+                val.try_to_bits(size)
+            }
+        }
+    }
+
+    #[inline]
+    pub fn try_eval_bool(&self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Option<bool> {
+        match self {
+            Self::Ty(ct) => ct.try_eval_bool(tcx, param_env),
+            Self::Val(val, _) => val.try_to_bool(),
+        }
+    }
+
+    #[inline]
+    pub fn try_eval_usize(&self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Option<u64> {
+        match self {
+            Self::Ty(ct) => ct.try_eval_usize(tcx, param_env),
+            Self::Val(val, _) => val.try_to_machine_usize(tcx),
+        }
+    }
+
+    #[inline]
+    pub fn from_value(val: ConstValue<'tcx>, ty: Ty<'tcx>) -> Self {
+        Self::Val(val, ty)
+    }
+
+    pub fn from_bits(
+        tcx: TyCtxt<'tcx>,
+        bits: u128,
+        param_env_ty: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
+    ) -> Self {
+        let size = tcx
+            .layout_of(param_env_ty)
+            .unwrap_or_else(|e| {
+                bug!("could not compute layout for {:?}: {:?}", param_env_ty.value, e)
+            })
+            .size;
+        let cv = ConstValue::Scalar(Scalar::from_uint(bits, size));
+
+        Self::Val(cv, param_env_ty.value)
+    }
+
+    #[inline]
+    pub fn from_bool(tcx: TyCtxt<'tcx>, v: bool) -> Self {
+        let cv = ConstValue::from_bool(v);
+        Self::Val(cv, tcx.types.bool)
+    }
+
+    #[inline]
+    pub fn zero_sized(ty: Ty<'tcx>) -> Self {
+        let cv = ConstValue::ZeroSized;
+        Self::Val(cv, ty)
+    }
+
+    pub fn from_usize(tcx: TyCtxt<'tcx>, n: u64) -> Self {
+        let ty = tcx.types.usize;
+        Self::from_bits(tcx, n as u128, ty::ParamEnv::empty().and(ty))
+    }
+
+    #[inline]
+    pub fn from_scalar(_tcx: TyCtxt<'tcx>, s: Scalar, ty: Ty<'tcx>) -> Self {
+        let val = ConstValue::Scalar(s);
+        Self::Val(val, ty)
+    }
+
+    /// Literals are converted to `ConstantKindVal`, const generic parameters are eagerly
+    /// converted to a constant, everything else becomes `Unevaluated`.
+    pub fn from_anon_const(
+        tcx: TyCtxt<'tcx>,
+        def_id: LocalDefId,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> Self {
+        Self::from_opt_const_arg_anon_const(tcx, ty::WithOptConstParam::unknown(def_id), param_env)
+    }
+
+    #[instrument(skip(tcx), level = "debug")]
+    pub fn from_inline_const(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Self {
+        let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+        let body_id = match tcx.hir().get(hir_id) {
+            hir::Node::AnonConst(ac) => ac.body,
+            _ => span_bug!(
+                tcx.def_span(def_id.to_def_id()),
+                "from_inline_const can only process anonymous constants"
+            ),
+        };
+        let expr = &tcx.hir().body(body_id).value;
+        let ty = tcx.typeck(def_id).node_type(hir_id);
+
+        let lit_input = match expr.kind {
+            hir::ExprKind::Lit(ref lit) => Some(LitToConstInput { lit: &lit.node, ty, neg: false }),
+            hir::ExprKind::Unary(hir::UnOp::Neg, ref expr) => match expr.kind {
+                hir::ExprKind::Lit(ref lit) => {
+                    Some(LitToConstInput { lit: &lit.node, ty, neg: true })
+                }
+                _ => None,
+            },
+            _ => None,
+        };
+        if let Some(lit_input) = lit_input {
+            // If an error occurred, ignore that it's a literal and leave reporting the error up to
+            // mir.
+            match tcx.at(expr.span).lit_to_mir_constant(lit_input) {
+                Ok(c) => return c,
+                Err(_) => {}
+            }
+        }
+
+        let typeck_root_def_id = tcx.typeck_root_def_id(def_id.to_def_id());
+        let parent_substs =
+            tcx.erase_regions(InternalSubsts::identity_for_item(tcx, typeck_root_def_id));
+        let substs =
+            ty::InlineConstSubsts::new(tcx, ty::InlineConstSubstsParts { parent_substs, ty })
+                .substs;
+        let uneval_const = tcx.mk_const(ty::ConstS {
+            kind: ty::ConstKind::Unevaluated(ty::Unevaluated {
+                def: ty::WithOptConstParam::unknown(def_id).to_global(),
+                substs,
+                promoted: None,
+            }),
+            ty,
+        });
+        debug!(?uneval_const);
+        debug_assert!(!uneval_const.has_free_regions());
+
+        Self::Ty(uneval_const)
+    }
+
+    #[instrument(skip(tcx), level = "debug")]
+    fn from_opt_const_arg_anon_const(
+        tcx: TyCtxt<'tcx>,
+        def: ty::WithOptConstParam<LocalDefId>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> Self {
+        let body_id = match tcx.hir().get_by_def_id(def.did) {
+            hir::Node::AnonConst(ac) => ac.body,
+            _ => span_bug!(
+                tcx.def_span(def.did.to_def_id()),
+                "from_anon_const can only process anonymous constants"
+            ),
+        };
+
+        let expr = &tcx.hir().body(body_id).value;
+        debug!(?expr);
+
+        // Unwrap a block, so that e.g. `{ P }` is recognised as a parameter. Const arguments
+        // currently have to be wrapped in curly brackets, so it's necessary to special-case.
+        let expr = match &expr.kind {
+            hir::ExprKind::Block(block, _) if block.stmts.is_empty() && block.expr.is_some() => {
+                block.expr.as_ref().unwrap()
+            }
+            _ => expr,
+        };
+        debug!("expr.kind: {:?}", expr.kind);
+
+        let ty = tcx.type_of(def.def_id_for_type_of());
+        debug!(?ty);
+
+        // FIXME(const_generics): We currently have to special case parameters because `min_const_generics`
+        // does not provide the parents generics to anonymous constants. We still allow generic const
+        // parameters by themselves however, e.g. `N`.  These constants would cause an ICE if we were to
+        // ever try to substitute the generic parameters in their bodies.
+        //
+        // While this doesn't happen as these constants are always used as `ty::ConstKind::Param`, it does
+        // cause issues if we were to remove that special-case and try to evaluate the constant instead.
+        use hir::{def::DefKind::ConstParam, def::Res, ExprKind, Path, QPath};
+        match expr.kind {
+            ExprKind::Path(QPath::Resolved(_, &Path { res: Res::Def(ConstParam, def_id), .. })) => {
+                // Find the name and index of the const parameter by indexing the generics of
+                // the parent item and construct a `ParamConst`.
+                let hir_id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
+                let item_id = tcx.hir().get_parent_node(hir_id);
+                let item_def_id = tcx.hir().local_def_id(item_id);
+                let generics = tcx.generics_of(item_def_id.to_def_id());
+                let index = generics.param_def_id_to_index[&def_id];
+                let name = tcx.hir().name(hir_id);
+                let ty_const = tcx.mk_const(ty::ConstS {
+                    kind: ty::ConstKind::Param(ty::ParamConst::new(index, name)),
+                    ty,
+                });
+                debug!(?ty_const);
+
+                return Self::Ty(ty_const);
+            }
+            _ => {}
+        }
+
+        let hir_id = tcx.hir().local_def_id_to_hir_id(def.did);
+        let parent_substs = if let Some(parent_hir_id) = tcx.hir().find_parent_node(hir_id) {
+            if let Some(parent_did) = tcx.hir().opt_local_def_id(parent_hir_id) {
+                InternalSubsts::identity_for_item(tcx, parent_did.to_def_id())
+            } else {
+                tcx.mk_substs(Vec::<GenericArg<'tcx>>::new().into_iter())
+            }
+        } else {
+            tcx.mk_substs(Vec::<GenericArg<'tcx>>::new().into_iter())
+        };
+        debug!(?parent_substs);
+
+        let did = def.did.to_def_id();
+        let child_substs = InternalSubsts::identity_for_item(tcx, did);
+        let substs = tcx.mk_substs(parent_substs.into_iter().chain(child_substs.into_iter()));
+        debug!(?substs);
+
+        let hir_id = tcx.hir().local_def_id_to_hir_id(def.did);
+        let span = tcx.hir().span(hir_id);
+        let uneval = ty::Unevaluated::new(def.to_global(), substs);
+        debug!(?span, ?param_env);
+
+        match tcx.const_eval_resolve(param_env, uneval, Some(span)) {
+            Ok(val) => {
+                debug!("evaluated const value: {:?}", val);
+                Self::Val(val, ty)
+            }
+            Err(_) => {
+                debug!("error encountered during evaluation");
+                // Error was handled in `const_eval_resolve`. Here we just create a
+                // new unevaluated const and error hard later in codegen
+                let ty_const = tcx.mk_const(ty::ConstS {
+                    kind: ty::ConstKind::Unevaluated(ty::Unevaluated {
+                        def: def.to_global(),
+                        substs: InternalSubsts::identity_for_item(tcx, def.did.to_def_id()),
+                        promoted: None,
+                    }),
+                    ty,
+                });
+                debug!(?ty_const);
+
+                Self::Ty(ty_const)
+            }
+        }
+    }
+
+    pub fn from_const(c: ty::Const<'tcx>, tcx: TyCtxt<'tcx>) -> Self {
+        match c.kind() {
+            ty::ConstKind::Value(valtree) => {
+                let const_val = tcx.valtree_to_const_val((c.ty(), valtree));
+                Self::Val(const_val, c.ty())
+            }
+            _ => Self::Ty(c),
+        }
+    }
+}
+
+/// A collection of projections into user types.
+///
+/// They are projections because a binding can occur a part of a
+/// parent pattern that has been ascribed a type.
+///
+/// Its a collection because there can be multiple type ascriptions on
+/// the path from the root of the pattern down to the binding itself.
+///
+/// An example:
+///
+/// ```ignore (illustrative)
+/// struct S<'a>((i32, &'a str), String);
+/// let S((_, w): (i32, &'static str), _): S = ...;
+/// //    ------  ^^^^^^^^^^^^^^^^^^^ (1)
+/// //  ---------------------------------  ^ (2)
+/// ```
+///
+/// The highlights labelled `(1)` show the subpattern `(_, w)` being
+/// ascribed the type `(i32, &'static str)`.
+///
+/// The highlights labelled `(2)` show the whole pattern being
+/// ascribed the type `S`.
+///
+/// In this example, when we descend to `w`, we will have built up the
+/// following two projected types:
+///
+///   * base: `S`,                   projection: `(base.0).1`
+///   * base: `(i32, &'static str)`, projection: `base.1`
+///
+/// The first will lead to the constraint `w: &'1 str` (for some
+/// inferred region `'1`). The second will lead to the constraint `w:
+/// &'static str`.
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub struct UserTypeProjections {
+    pub contents: Vec<(UserTypeProjection, Span)>,
+}
+
+impl<'tcx> UserTypeProjections {
+    pub fn none() -> Self {
+        UserTypeProjections { contents: vec![] }
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.contents.is_empty()
+    }
+
+    pub fn projections_and_spans(
+        &self,
+    ) -> impl Iterator<Item = &(UserTypeProjection, Span)> + ExactSizeIterator {
+        self.contents.iter()
+    }
+
+    pub fn projections(&self) -> impl Iterator<Item = &UserTypeProjection> + ExactSizeIterator {
+        self.contents.iter().map(|&(ref user_type, _span)| user_type)
+    }
+
+    pub fn push_projection(mut self, user_ty: &UserTypeProjection, span: Span) -> Self {
+        self.contents.push((user_ty.clone(), span));
+        self
+    }
+
+    fn map_projections(
+        mut self,
+        mut f: impl FnMut(UserTypeProjection) -> UserTypeProjection,
+    ) -> Self {
+        self.contents = self.contents.into_iter().map(|(proj, span)| (f(proj), span)).collect();
+        self
+    }
+
+    pub fn index(self) -> Self {
+        self.map_projections(|pat_ty_proj| pat_ty_proj.index())
+    }
+
+    pub fn subslice(self, from: u64, to: u64) -> Self {
+        self.map_projections(|pat_ty_proj| pat_ty_proj.subslice(from, to))
+    }
+
+    pub fn deref(self) -> Self {
+        self.map_projections(|pat_ty_proj| pat_ty_proj.deref())
+    }
+
+    pub fn leaf(self, field: Field) -> Self {
+        self.map_projections(|pat_ty_proj| pat_ty_proj.leaf(field))
+    }
+
+    pub fn variant(self, adt_def: AdtDef<'tcx>, variant_index: VariantIdx, field: Field) -> Self {
+        self.map_projections(|pat_ty_proj| pat_ty_proj.variant(adt_def, variant_index, field))
+    }
+}
+
+/// Encodes the effect of a user-supplied type annotation on the
+/// subcomponents of a pattern. The effect is determined by applying the
+/// given list of projections to some underlying base type. Often,
+/// the projection element list `projs` is empty, in which case this
+/// directly encodes a type in `base`. But in the case of complex patterns with
+/// subpatterns and bindings, we want to apply only a *part* of the type to a variable,
+/// in which case the `projs` vector is used.
+///
+/// Examples:
+///
+/// * `let x: T = ...` -- here, the `projs` vector is empty.
+///
+/// * `let (x, _): T = ...` -- here, the `projs` vector would contain
+///   `field[0]` (aka `.0`), indicating that the type of `s` is
+///   determined by finding the type of the `.0` field from `T`.
+#[derive(Clone, Debug, TyEncodable, TyDecodable, Hash, HashStable, PartialEq)]
+pub struct UserTypeProjection {
+    pub base: UserTypeAnnotationIndex,
+    pub projs: Vec<ProjectionKind>,
+}
+
+impl Copy for ProjectionKind {}
+
+impl UserTypeProjection {
+    pub(crate) fn index(mut self) -> Self {
+        self.projs.push(ProjectionElem::Index(()));
+        self
+    }
+
+    pub(crate) fn subslice(mut self, from: u64, to: u64) -> Self {
+        self.projs.push(ProjectionElem::Subslice { from, to, from_end: true });
+        self
+    }
+
+    pub(crate) fn deref(mut self) -> Self {
+        self.projs.push(ProjectionElem::Deref);
+        self
+    }
+
+    pub(crate) fn leaf(mut self, field: Field) -> Self {
+        self.projs.push(ProjectionElem::Field(field, ()));
+        self
+    }
+
+    pub(crate) fn variant(
+        mut self,
+        adt_def: AdtDef<'_>,
+        variant_index: VariantIdx,
+        field: Field,
+    ) -> Self {
+        self.projs.push(ProjectionElem::Downcast(
+            Some(adt_def.variant(variant_index).name),
+            variant_index,
+        ));
+        self.projs.push(ProjectionElem::Field(field, ()));
+        self
+    }
+}
+
+TrivialTypeTraversalAndLiftImpls! { ProjectionKind, }
+
+impl<'tcx> TypeFoldable<'tcx> for UserTypeProjection {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        Ok(UserTypeProjection {
+            base: self.base.try_fold_with(folder)?,
+            projs: self.projs.try_fold_with(folder)?,
+        })
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for UserTypeProjection {
+    fn visit_with<Vs: TypeVisitor<'tcx>>(&self, visitor: &mut Vs) -> ControlFlow<Vs::BreakTy> {
+        self.base.visit_with(visitor)
+        // Note: there's nothing in `self.proj` to visit.
+    }
+}
+
+rustc_index::newtype_index! {
+    pub struct Promoted {
+        derive [HashStable]
+        DEBUG_FORMAT = "promoted[{}]"
+    }
+}
+
+impl<'tcx> Debug for Constant<'tcx> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        write!(fmt, "{}", self)
+    }
+}
+
+impl<'tcx> Display for Constant<'tcx> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        match self.ty().kind() {
+            ty::FnDef(..) => {}
+            _ => write!(fmt, "const ")?,
+        }
+        Display::fmt(&self.literal, fmt)
+    }
+}
+
+impl<'tcx> Display for ConstantKind<'tcx> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        match *self {
+            ConstantKind::Ty(c) => pretty_print_const(c, fmt, true),
+            ConstantKind::Val(val, ty) => pretty_print_const_value(val, ty, fmt, true),
+        }
+    }
+}
+
+fn pretty_print_const<'tcx>(
+    c: ty::Const<'tcx>,
+    fmt: &mut Formatter<'_>,
+    print_types: bool,
+) -> fmt::Result {
+    use crate::ty::print::PrettyPrinter;
+    ty::tls::with(|tcx| {
+        let literal = tcx.lift(c).unwrap();
+        let mut cx = FmtPrinter::new(tcx, Namespace::ValueNS);
+        cx.print_alloc_ids = true;
+        let cx = cx.pretty_print_const(literal, print_types)?;
+        fmt.write_str(&cx.into_buffer())?;
+        Ok(())
+    })
+}
+
+fn pretty_print_byte_str(fmt: &mut Formatter<'_>, byte_str: &[u8]) -> fmt::Result {
+    fmt.write_str("b\"")?;
+    for &c in byte_str {
+        for e in std::ascii::escape_default(c) {
+            fmt.write_char(e as char)?;
+        }
+    }
+    fmt.write_str("\"")?;
+
+    Ok(())
+}
+
+fn comma_sep<'tcx>(fmt: &mut Formatter<'_>, elems: Vec<ConstantKind<'tcx>>) -> fmt::Result {
+    let mut first = true;
+    for elem in elems {
+        if !first {
+            fmt.write_str(", ")?;
+        }
+        fmt.write_str(&format!("{}", elem))?;
+        first = false;
+    }
+    Ok(())
+}
+
+// FIXME: Move that into `mir/pretty.rs`.
+fn pretty_print_const_value<'tcx>(
+    ct: ConstValue<'tcx>,
+    ty: Ty<'tcx>,
+    fmt: &mut Formatter<'_>,
+    print_ty: bool,
+) -> fmt::Result {
+    use crate::ty::print::PrettyPrinter;
+
+    ty::tls::with(|tcx| {
+        let ct = tcx.lift(ct).unwrap();
+        let ty = tcx.lift(ty).unwrap();
+
+        if tcx.sess.verbose() {
+            fmt.write_str(&format!("ConstValue({:?}: {})", ct, ty))?;
+            return Ok(());
+        }
+
+        let u8_type = tcx.types.u8;
+        match (ct, ty.kind()) {
+            // Byte/string slices, printed as (byte) string literals.
+            (ConstValue::Slice { data, start, end }, ty::Ref(_, inner, _)) => {
+                match inner.kind() {
+                    ty::Slice(t) => {
+                        if *t == u8_type {
+                            // The `inspect` here is okay since we checked the bounds, and there are
+                            // no relocations (we have an active slice reference here). We don't use
+                            // this result to affect interpreter execution.
+                            let byte_str = data
+                                .inner()
+                                .inspect_with_uninit_and_ptr_outside_interpreter(start..end);
+                            pretty_print_byte_str(fmt, byte_str)?;
+                            return Ok(());
+                        }
+                    }
+                    ty::Str => {
+                        // The `inspect` here is okay since we checked the bounds, and there are no
+                        // relocations (we have an active `str` reference here). We don't use this
+                        // result to affect interpreter execution.
+                        let slice = data
+                            .inner()
+                            .inspect_with_uninit_and_ptr_outside_interpreter(start..end);
+                        fmt.write_str(&format!("{:?}", String::from_utf8_lossy(slice)))?;
+                        return Ok(());
+                    }
+                    _ => {}
+                }
+            }
+            (ConstValue::ByRef { alloc, offset }, ty::Array(t, n)) if *t == u8_type => {
+                let n = n.kind().try_to_bits(tcx.data_layout.pointer_size).unwrap();
+                // cast is ok because we already checked for pointer size (32 or 64 bit) above
+                let range = AllocRange { start: offset, size: Size::from_bytes(n) };
+                let byte_str = alloc.inner().get_bytes(&tcx, range).unwrap();
+                fmt.write_str("*")?;
+                pretty_print_byte_str(fmt, byte_str)?;
+                return Ok(());
+            }
+            // Aggregates, printed as array/tuple/struct/variant construction syntax.
+            //
+            // NB: the `has_param_types_or_consts` check ensures that we can use
+            // the `destructure_const` query with an empty `ty::ParamEnv` without
+            // introducing ICEs (e.g. via `layout_of`) from missing bounds.
+            // E.g. `transmute([0usize; 2]): (u8, *mut T)` needs to know `T: Sized`
+            // to be able to destructure the tuple into `(0u8, *mut T)
+            //
+            // FIXME(eddyb) for `--emit=mir`/`-Z dump-mir`, we should provide the
+            // correct `ty::ParamEnv` to allow printing *all* constant values.
+            (_, ty::Array(..) | ty::Tuple(..) | ty::Adt(..)) if !ty.has_param_types_or_consts() => {
+                let ct = tcx.lift(ct).unwrap();
+                let ty = tcx.lift(ty).unwrap();
+                if let Some(contents) = tcx.try_destructure_mir_constant(
+                    ty::ParamEnv::reveal_all().and(ConstantKind::Val(ct, ty)),
+                ) {
+                    let fields = contents.fields.iter().copied().collect::<Vec<_>>();
+                    match *ty.kind() {
+                        ty::Array(..) => {
+                            fmt.write_str("[")?;
+                            comma_sep(fmt, fields)?;
+                            fmt.write_str("]")?;
+                        }
+                        ty::Tuple(..) => {
+                            fmt.write_str("(")?;
+                            comma_sep(fmt, fields)?;
+                            if contents.fields.len() == 1 {
+                                fmt.write_str(",")?;
+                            }
+                            fmt.write_str(")")?;
+                        }
+                        ty::Adt(def, _) if def.variants().is_empty() => {
+                            fmt.write_str(&format!("{{unreachable(): {}}}", ty))?;
+                        }
+                        ty::Adt(def, substs) => {
+                            let variant_idx = contents
+                                .variant
+                                .expect("destructed mir constant of adt without variant idx");
+                            let variant_def = &def.variant(variant_idx);
+                            let substs = tcx.lift(substs).unwrap();
+                            let mut cx = FmtPrinter::new(tcx, Namespace::ValueNS);
+                            cx.print_alloc_ids = true;
+                            let cx = cx.print_value_path(variant_def.def_id, substs)?;
+                            fmt.write_str(&cx.into_buffer())?;
+
+                            match variant_def.ctor_kind {
+                                CtorKind::Const => {}
+                                CtorKind::Fn => {
+                                    fmt.write_str("(")?;
+                                    comma_sep(fmt, fields)?;
+                                    fmt.write_str(")")?;
+                                }
+                                CtorKind::Fictive => {
+                                    fmt.write_str(" {{ ")?;
+                                    let mut first = true;
+                                    for (field_def, field) in iter::zip(&variant_def.fields, fields)
+                                    {
+                                        if !first {
+                                            fmt.write_str(", ")?;
+                                        }
+                                        fmt.write_str(&format!("{}: {}", field_def.name, field))?;
+                                        first = false;
+                                    }
+                                    fmt.write_str(" }}")?;
+                                }
+                            }
+                        }
+                        _ => unreachable!(),
+                    }
+                    return Ok(());
+                } else {
+                    // Fall back to debug pretty printing for invalid constants.
+                    fmt.write_str(&format!("{:?}", ct))?;
+                    if print_ty {
+                        fmt.write_str(&format!(": {}", ty))?;
+                    }
+                    return Ok(());
+                };
+            }
+            (ConstValue::Scalar(scalar), _) => {
+                let mut cx = FmtPrinter::new(tcx, Namespace::ValueNS);
+                cx.print_alloc_ids = true;
+                let ty = tcx.lift(ty).unwrap();
+                cx = cx.pretty_print_const_scalar(scalar, ty, print_ty)?;
+                fmt.write_str(&cx.into_buffer())?;
+                return Ok(());
+            }
+            (ConstValue::ZeroSized, ty::FnDef(d, s)) => {
+                let mut cx = FmtPrinter::new(tcx, Namespace::ValueNS);
+                cx.print_alloc_ids = true;
+                let cx = cx.print_value_path(*d, s)?;
+                fmt.write_str(&cx.into_buffer())?;
+                return Ok(());
+            }
+            // FIXME(oli-obk): also pretty print arrays and other aggregate constants by reading
+            // their fields instead of just dumping the memory.
+            _ => {}
+        }
+        // fallback
+        fmt.write_str(&format!("{:?}", ct))?;
+        if print_ty {
+            fmt.write_str(&format!(": {}", ty))?;
+        }
+        Ok(())
+    })
+}
+
+/// `Location` represents the position of the start of the statement; or, if
+/// `statement_index` equals the number of statements, then the start of the
+/// terminator.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Ord, PartialOrd, HashStable)]
+pub struct Location {
+    /// The block that the location is within.
+    pub block: BasicBlock,
+
+    pub statement_index: usize,
+}
+
+impl fmt::Debug for Location {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "{:?}[{}]", self.block, self.statement_index)
+    }
+}
+
+impl Location {
+    pub const START: Location = Location { block: START_BLOCK, statement_index: 0 };
+
+    /// Returns the location immediately after this one within the enclosing block.
+    ///
+    /// Note that if this location represents a terminator, then the
+    /// resulting location would be out of bounds and invalid.
+    pub fn successor_within_block(&self) -> Location {
+        Location { block: self.block, statement_index: self.statement_index + 1 }
+    }
+
+    /// Returns `true` if `other` is earlier in the control flow graph than `self`.
+    pub fn is_predecessor_of<'tcx>(&self, other: Location, body: &Body<'tcx>) -> bool {
+        // If we are in the same block as the other location and are an earlier statement
+        // then we are a predecessor of `other`.
+        if self.block == other.block && self.statement_index < other.statement_index {
+            return true;
+        }
+
+        let predecessors = body.basic_blocks.predecessors();
+
+        // If we're in another block, then we want to check that block is a predecessor of `other`.
+        let mut queue: Vec<BasicBlock> = predecessors[other.block].to_vec();
+        let mut visited = FxHashSet::default();
+
+        while let Some(block) = queue.pop() {
+            // If we haven't visited this block before, then make sure we visit its predecessors.
+            if visited.insert(block) {
+                queue.extend(predecessors[block].iter().cloned());
+            } else {
+                continue;
+            }
+
+            // If we found the block that `self` is in, then we are a predecessor of `other` (since
+            // we found that block by looking at the predecessors of `other`).
+            if self.block == block {
+                return true;
+            }
+        }
+
+        false
+    }
+
+    pub fn dominates(&self, other: Location, dominators: &Dominators<BasicBlock>) -> bool {
+        if self.block == other.block {
+            self.statement_index <= other.statement_index
+        } else {
+            dominators.is_dominated_by(other.block, self.block)
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/mono.rs b/compiler/rustc_middle/src/mir/mono.rs
new file mode 100644
index 000000000..21ae121e1
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/mono.rs
@@ -0,0 +1,527 @@
+use crate::dep_graph::{DepNode, WorkProduct, WorkProductId};
+use crate::ty::{subst::InternalSubsts, Instance, InstanceDef, SymbolName, TyCtxt};
+use rustc_attr::InlineAttr;
+use rustc_data_structures::base_n;
+use rustc_data_structures::fingerprint::Fingerprint;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_hir::def_id::{CrateNum, DefId, LOCAL_CRATE};
+use rustc_hir::ItemId;
+use rustc_index::vec::Idx;
+use rustc_query_system::ich::StableHashingContext;
+use rustc_session::config::OptLevel;
+use rustc_span::source_map::Span;
+use rustc_span::symbol::Symbol;
+use std::fmt;
+use std::hash::Hash;
+
+/// Describes how a monomorphization will be instantiated in object files.
+#[derive(PartialEq)]
+pub enum InstantiationMode {
+    /// There will be exactly one instance of the given MonoItem. It will have
+    /// external linkage so that it can be linked to from other codegen units.
+    GloballyShared {
+        /// In some compilation scenarios we may decide to take functions that
+        /// are typically `LocalCopy` and instead move them to `GloballyShared`
+        /// to avoid codegenning them a bunch of times. In this situation,
+        /// however, our local copy may conflict with other crates also
+        /// inlining the same function.
+        ///
+        /// This flag indicates that this situation is occurring, and informs
+        /// symbol name calculation that some extra mangling is needed to
+        /// avoid conflicts. Note that this may eventually go away entirely if
+        /// ThinLTO enables us to *always* have a globally shared instance of a
+        /// function within one crate's compilation.
+        may_conflict: bool,
+    },
+
+    /// Each codegen unit containing a reference to the given MonoItem will
+    /// have its own private copy of the function (with internal linkage).
+    LocalCopy,
+}
+
+#[derive(PartialEq, Eq, Clone, Copy, Debug, Hash, HashStable)]
+pub enum MonoItem<'tcx> {
+    Fn(Instance<'tcx>),
+    Static(DefId),
+    GlobalAsm(ItemId),
+}
+
+impl<'tcx> MonoItem<'tcx> {
+    /// Returns `true` if the mono item is user-defined (i.e. not compiler-generated, like shims).
+    pub fn is_user_defined(&self) -> bool {
+        match *self {
+            MonoItem::Fn(instance) => matches!(instance.def, InstanceDef::Item(..)),
+            MonoItem::Static(..) | MonoItem::GlobalAsm(..) => true,
+        }
+    }
+
+    pub fn size_estimate(&self, tcx: TyCtxt<'tcx>) -> usize {
+        match *self {
+            MonoItem::Fn(instance) => {
+                // Estimate the size of a function based on how many statements
+                // it contains.
+                tcx.instance_def_size_estimate(instance.def)
+            }
+            // Conservatively estimate the size of a static declaration
+            // or assembly to be 1.
+            MonoItem::Static(_) | MonoItem::GlobalAsm(_) => 1,
+        }
+    }
+
+    pub fn is_generic_fn(&self) -> bool {
+        match *self {
+            MonoItem::Fn(ref instance) => instance.substs.non_erasable_generics().next().is_some(),
+            MonoItem::Static(..) | MonoItem::GlobalAsm(..) => false,
+        }
+    }
+
+    pub fn symbol_name(&self, tcx: TyCtxt<'tcx>) -> SymbolName<'tcx> {
+        match *self {
+            MonoItem::Fn(instance) => tcx.symbol_name(instance),
+            MonoItem::Static(def_id) => tcx.symbol_name(Instance::mono(tcx, def_id)),
+            MonoItem::GlobalAsm(item_id) => {
+                SymbolName::new(tcx, &format!("global_asm_{:?}", item_id.def_id))
+            }
+        }
+    }
+
+    pub fn instantiation_mode(&self, tcx: TyCtxt<'tcx>) -> InstantiationMode {
+        let generate_cgu_internal_copies = tcx
+            .sess
+            .opts
+            .unstable_opts
+            .inline_in_all_cgus
+            .unwrap_or_else(|| tcx.sess.opts.optimize != OptLevel::No)
+            && !tcx.sess.link_dead_code();
+
+        match *self {
+            MonoItem::Fn(ref instance) => {
+                let entry_def_id = tcx.entry_fn(()).map(|(id, _)| id);
+                // If this function isn't inlined or otherwise has an extern
+                // indicator, then we'll be creating a globally shared version.
+                if tcx.codegen_fn_attrs(instance.def_id()).contains_extern_indicator()
+                    || !instance.def.generates_cgu_internal_copy(tcx)
+                    || Some(instance.def_id()) == entry_def_id
+                {
+                    return InstantiationMode::GloballyShared { may_conflict: false };
+                }
+
+                // At this point we don't have explicit linkage and we're an
+                // inlined function. If we're inlining into all CGUs then we'll
+                // be creating a local copy per CGU.
+                if generate_cgu_internal_copies {
+                    return InstantiationMode::LocalCopy;
+                }
+
+                // Finally, if this is `#[inline(always)]` we're sure to respect
+                // that with an inline copy per CGU, but otherwise we'll be
+                // creating one copy of this `#[inline]` function which may
+                // conflict with upstream crates as it could be an exported
+                // symbol.
+                match tcx.codegen_fn_attrs(instance.def_id()).inline {
+                    InlineAttr::Always => InstantiationMode::LocalCopy,
+                    _ => InstantiationMode::GloballyShared { may_conflict: true },
+                }
+            }
+            MonoItem::Static(..) | MonoItem::GlobalAsm(..) => {
+                InstantiationMode::GloballyShared { may_conflict: false }
+            }
+        }
+    }
+
+    pub fn explicit_linkage(&self, tcx: TyCtxt<'tcx>) -> Option<Linkage> {
+        let def_id = match *self {
+            MonoItem::Fn(ref instance) => instance.def_id(),
+            MonoItem::Static(def_id) => def_id,
+            MonoItem::GlobalAsm(..) => return None,
+        };
+
+        let codegen_fn_attrs = tcx.codegen_fn_attrs(def_id);
+        codegen_fn_attrs.linkage
+    }
+
+    /// Returns `true` if this instance is instantiable - whether it has no unsatisfied
+    /// predicates.
+    ///
+    /// In order to codegen an item, all of its predicates must hold, because
+    /// otherwise the item does not make sense. Type-checking ensures that
+    /// the predicates of every item that is *used by* a valid item *do*
+    /// hold, so we can rely on that.
+    ///
+    /// However, we codegen collector roots (reachable items) and functions
+    /// in vtables when they are seen, even if they are not used, and so they
+    /// might not be instantiable. For example, a programmer can define this
+    /// public function:
+    ///
+    ///     pub fn foo<'a>(s: &'a mut ()) where &'a mut (): Clone {
+    ///         <&mut () as Clone>::clone(&s);
+    ///     }
+    ///
+    /// That function can't be codegened, because the method `<&mut () as Clone>::clone`
+    /// does not exist. Luckily for us, that function can't ever be used,
+    /// because that would require for `&'a mut (): Clone` to hold, so we
+    /// can just not emit any code, or even a linker reference for it.
+    ///
+    /// Similarly, if a vtable method has such a signature, and therefore can't
+    /// be used, we can just not emit it and have a placeholder (a null pointer,
+    /// which will never be accessed) in its place.
+    pub fn is_instantiable(&self, tcx: TyCtxt<'tcx>) -> bool {
+        debug!("is_instantiable({:?})", self);
+        let (def_id, substs) = match *self {
+            MonoItem::Fn(ref instance) => (instance.def_id(), instance.substs),
+            MonoItem::Static(def_id) => (def_id, InternalSubsts::empty()),
+            // global asm never has predicates
+            MonoItem::GlobalAsm(..) => return true,
+        };
+
+        !tcx.subst_and_check_impossible_predicates((def_id, &substs))
+    }
+
+    pub fn local_span(&self, tcx: TyCtxt<'tcx>) -> Option<Span> {
+        match *self {
+            MonoItem::Fn(Instance { def, .. }) => def.def_id().as_local(),
+            MonoItem::Static(def_id) => def_id.as_local(),
+            MonoItem::GlobalAsm(item_id) => Some(item_id.def_id),
+        }
+        .map(|def_id| tcx.def_span(def_id))
+    }
+
+    // Only used by rustc_codegen_cranelift
+    pub fn codegen_dep_node(&self, tcx: TyCtxt<'tcx>) -> DepNode {
+        crate::dep_graph::make_compile_mono_item(tcx, self)
+    }
+
+    /// Returns the item's `CrateNum`
+    pub fn krate(&self) -> CrateNum {
+        match self {
+            MonoItem::Fn(ref instance) => instance.def_id().krate,
+            MonoItem::Static(def_id) => def_id.krate,
+            MonoItem::GlobalAsm(..) => LOCAL_CRATE,
+        }
+    }
+}
+
+impl<'tcx> fmt::Display for MonoItem<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            MonoItem::Fn(instance) => write!(f, "fn {}", instance),
+            MonoItem::Static(def_id) => {
+                write!(f, "static {}", Instance::new(def_id, InternalSubsts::empty()))
+            }
+            MonoItem::GlobalAsm(..) => write!(f, "global_asm"),
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct CodegenUnit<'tcx> {
+    /// A name for this CGU. Incremental compilation requires that
+    /// name be unique amongst **all** crates. Therefore, it should
+    /// contain something unique to this crate (e.g., a module path)
+    /// as well as the crate name and disambiguator.
+    name: Symbol,
+    items: FxHashMap<MonoItem<'tcx>, (Linkage, Visibility)>,
+    size_estimate: Option<usize>,
+    primary: bool,
+    /// True if this is CGU is used to hold code coverage information for dead code,
+    /// false otherwise.
+    is_code_coverage_dead_code_cgu: bool,
+}
+
+/// Specifies the linkage type for a `MonoItem`.
+///
+/// See <https://llvm.org/docs/LangRef.html#linkage-types> for more details about these variants.
+#[derive(Copy, Clone, PartialEq, Debug, TyEncodable, TyDecodable, HashStable)]
+pub enum Linkage {
+    External,
+    AvailableExternally,
+    LinkOnceAny,
+    LinkOnceODR,
+    WeakAny,
+    WeakODR,
+    Appending,
+    Internal,
+    Private,
+    ExternalWeak,
+    Common,
+}
+
+#[derive(Copy, Clone, PartialEq, Debug, HashStable)]
+pub enum Visibility {
+    Default,
+    Hidden,
+    Protected,
+}
+
+impl<'tcx> CodegenUnit<'tcx> {
+    #[inline]
+    pub fn new(name: Symbol) -> CodegenUnit<'tcx> {
+        CodegenUnit {
+            name,
+            items: Default::default(),
+            size_estimate: None,
+            primary: false,
+            is_code_coverage_dead_code_cgu: false,
+        }
+    }
+
+    pub fn name(&self) -> Symbol {
+        self.name
+    }
+
+    pub fn set_name(&mut self, name: Symbol) {
+        self.name = name;
+    }
+
+    pub fn is_primary(&self) -> bool {
+        self.primary
+    }
+
+    pub fn make_primary(&mut self) {
+        self.primary = true;
+    }
+
+    pub fn items(&self) -> &FxHashMap<MonoItem<'tcx>, (Linkage, Visibility)> {
+        &self.items
+    }
+
+    pub fn items_mut(&mut self) -> &mut FxHashMap<MonoItem<'tcx>, (Linkage, Visibility)> {
+        &mut self.items
+    }
+
+    pub fn is_code_coverage_dead_code_cgu(&self) -> bool {
+        self.is_code_coverage_dead_code_cgu
+    }
+
+    /// Marks this CGU as the one used to contain code coverage information for dead code.
+    pub fn make_code_coverage_dead_code_cgu(&mut self) {
+        self.is_code_coverage_dead_code_cgu = true;
+    }
+
+    pub fn mangle_name(human_readable_name: &str) -> String {
+        // We generate a 80 bit hash from the name. This should be enough to
+        // avoid collisions and is still reasonably short for filenames.
+        let mut hasher = StableHasher::new();
+        human_readable_name.hash(&mut hasher);
+        let hash: u128 = hasher.finish();
+        let hash = hash & ((1u128 << 80) - 1);
+        base_n::encode(hash, base_n::CASE_INSENSITIVE)
+    }
+
+    pub fn estimate_size(&mut self, tcx: TyCtxt<'tcx>) {
+        // Estimate the size of a codegen unit as (approximately) the number of MIR
+        // statements it corresponds to.
+        self.size_estimate = Some(self.items.keys().map(|mi| mi.size_estimate(tcx)).sum());
+    }
+
+    #[inline]
+    pub fn size_estimate(&self) -> usize {
+        // Should only be called if `estimate_size` has previously been called.
+        self.size_estimate.expect("estimate_size must be called before getting a size_estimate")
+    }
+
+    pub fn modify_size_estimate(&mut self, delta: usize) {
+        assert!(self.size_estimate.is_some());
+        if let Some(size_estimate) = self.size_estimate {
+            self.size_estimate = Some(size_estimate + delta);
+        }
+    }
+
+    pub fn contains_item(&self, item: &MonoItem<'tcx>) -> bool {
+        self.items().contains_key(item)
+    }
+
+    pub fn work_product_id(&self) -> WorkProductId {
+        WorkProductId::from_cgu_name(self.name().as_str())
+    }
+
+    pub fn previous_work_product(&self, tcx: TyCtxt<'_>) -> WorkProduct {
+        let work_product_id = self.work_product_id();
+        tcx.dep_graph
+            .previous_work_product(&work_product_id)
+            .unwrap_or_else(|| panic!("Could not find work-product for CGU `{}`", self.name()))
+    }
+
+    pub fn items_in_deterministic_order(
+        &self,
+        tcx: TyCtxt<'tcx>,
+    ) -> Vec<(MonoItem<'tcx>, (Linkage, Visibility))> {
+        // The codegen tests rely on items being process in the same order as
+        // they appear in the file, so for local items, we sort by node_id first
+        #[derive(PartialEq, Eq, PartialOrd, Ord)]
+        pub struct ItemSortKey<'tcx>(Option<usize>, SymbolName<'tcx>);
+
+        fn item_sort_key<'tcx>(tcx: TyCtxt<'tcx>, item: MonoItem<'tcx>) -> ItemSortKey<'tcx> {
+            ItemSortKey(
+                match item {
+                    MonoItem::Fn(ref instance) => {
+                        match instance.def {
+                            // We only want to take HirIds of user-defined
+                            // instances into account. The others don't matter for
+                            // the codegen tests and can even make item order
+                            // unstable.
+                            InstanceDef::Item(def) => def.did.as_local().map(Idx::index),
+                            InstanceDef::VTableShim(..)
+                            | InstanceDef::ReifyShim(..)
+                            | InstanceDef::Intrinsic(..)
+                            | InstanceDef::FnPtrShim(..)
+                            | InstanceDef::Virtual(..)
+                            | InstanceDef::ClosureOnceShim { .. }
+                            | InstanceDef::DropGlue(..)
+                            | InstanceDef::CloneShim(..) => None,
+                        }
+                    }
+                    MonoItem::Static(def_id) => def_id.as_local().map(Idx::index),
+                    MonoItem::GlobalAsm(item_id) => Some(item_id.def_id.index()),
+                },
+                item.symbol_name(tcx),
+            )
+        }
+
+        let mut items: Vec<_> = self.items().iter().map(|(&i, &l)| (i, l)).collect();
+        items.sort_by_cached_key(|&(i, _)| item_sort_key(tcx, i));
+        items
+    }
+
+    pub fn codegen_dep_node(&self, tcx: TyCtxt<'tcx>) -> DepNode {
+        crate::dep_graph::make_compile_codegen_unit(tcx, self.name())
+    }
+}
+
+impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for CodegenUnit<'tcx> {
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        let CodegenUnit {
+            ref items,
+            name,
+            // The size estimate is not relevant to the hash
+            size_estimate: _,
+            primary: _,
+            is_code_coverage_dead_code_cgu,
+        } = *self;
+
+        name.hash_stable(hcx, hasher);
+        is_code_coverage_dead_code_cgu.hash_stable(hcx, hasher);
+
+        let mut items: Vec<(Fingerprint, _)> = items
+            .iter()
+            .map(|(mono_item, &attrs)| {
+                let mut hasher = StableHasher::new();
+                mono_item.hash_stable(hcx, &mut hasher);
+                let mono_item_fingerprint = hasher.finish();
+                (mono_item_fingerprint, attrs)
+            })
+            .collect();
+
+        items.sort_unstable_by_key(|i| i.0);
+        items.hash_stable(hcx, hasher);
+    }
+}
+
+pub struct CodegenUnitNameBuilder<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    cache: FxHashMap<CrateNum, String>,
+}
+
+impl<'tcx> CodegenUnitNameBuilder<'tcx> {
+    pub fn new(tcx: TyCtxt<'tcx>) -> Self {
+        CodegenUnitNameBuilder { tcx, cache: Default::default() }
+    }
+
+    /// CGU names should fulfill the following requirements:
+    /// - They should be able to act as a file name on any kind of file system
+    /// - They should not collide with other CGU names, even for different versions
+    ///   of the same crate.
+    ///
+    /// Consequently, we don't use special characters except for '.' and '-' and we
+    /// prefix each name with the crate-name and crate-disambiguator.
+    ///
+    /// This function will build CGU names of the form:
+    ///
+    /// ```text
+    /// <crate-name>.<crate-disambiguator>[-in-<local-crate-id>](-<component>)*[.<special-suffix>]
+    /// <local-crate-id> = <local-crate-name>.<local-crate-disambiguator>
+    /// ```
+    ///
+    /// The '.' before `<special-suffix>` makes sure that names with a special
+    /// suffix can never collide with a name built out of regular Rust
+    /// identifiers (e.g., module paths).
+    pub fn build_cgu_name<I, C, S>(
+        &mut self,
+        cnum: CrateNum,
+        components: I,
+        special_suffix: Option<S>,
+    ) -> Symbol
+    where
+        I: IntoIterator<Item = C>,
+        C: fmt::Display,
+        S: fmt::Display,
+    {
+        let cgu_name = self.build_cgu_name_no_mangle(cnum, components, special_suffix);
+
+        if self.tcx.sess.opts.unstable_opts.human_readable_cgu_names {
+            cgu_name
+        } else {
+            Symbol::intern(&CodegenUnit::mangle_name(cgu_name.as_str()))
+        }
+    }
+
+    /// Same as `CodegenUnit::build_cgu_name()` but will never mangle the
+    /// resulting name.
+    pub fn build_cgu_name_no_mangle<I, C, S>(
+        &mut self,
+        cnum: CrateNum,
+        components: I,
+        special_suffix: Option<S>,
+    ) -> Symbol
+    where
+        I: IntoIterator<Item = C>,
+        C: fmt::Display,
+        S: fmt::Display,
+    {
+        use std::fmt::Write;
+
+        let mut cgu_name = String::with_capacity(64);
+
+        // Start out with the crate name and disambiguator
+        let tcx = self.tcx;
+        let crate_prefix = self.cache.entry(cnum).or_insert_with(|| {
+            // Whenever the cnum is not LOCAL_CRATE we also mix in the
+            // local crate's ID. Otherwise there can be collisions between CGUs
+            // instantiating stuff for upstream crates.
+            let local_crate_id = if cnum != LOCAL_CRATE {
+                let local_stable_crate_id = tcx.sess.local_stable_crate_id();
+                format!(
+                    "-in-{}.{:08x}",
+                    tcx.crate_name(LOCAL_CRATE),
+                    local_stable_crate_id.to_u64() as u32,
+                )
+            } else {
+                String::new()
+            };
+
+            let stable_crate_id = tcx.sess.local_stable_crate_id();
+            format!(
+                "{}.{:08x}{}",
+                tcx.crate_name(cnum),
+                stable_crate_id.to_u64() as u32,
+                local_crate_id,
+            )
+        });
+
+        write!(cgu_name, "{}", crate_prefix).unwrap();
+
+        // Add the components
+        for component in components {
+            write!(cgu_name, "-{}", component).unwrap();
+        }
+
+        if let Some(special_suffix) = special_suffix {
+            // We add a dot in here so it cannot clash with anything in a regular
+            // Rust identifier
+            write!(cgu_name, ".{}", special_suffix).unwrap();
+        }
+
+        Symbol::intern(&cgu_name)
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/patch.rs b/compiler/rustc_middle/src/mir/patch.rs
new file mode 100644
index 000000000..15496842d
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/patch.rs
@@ -0,0 +1,196 @@
+use rustc_index::vec::{Idx, IndexVec};
+use rustc_middle::mir::*;
+use rustc_middle::ty::Ty;
+use rustc_span::Span;
+
+/// This struct represents a patch to MIR, which can add
+/// new statements and basic blocks and patch over block
+/// terminators.
+pub struct MirPatch<'tcx> {
+    patch_map: IndexVec<BasicBlock, Option<TerminatorKind<'tcx>>>,
+    new_blocks: Vec<BasicBlockData<'tcx>>,
+    new_statements: Vec<(Location, StatementKind<'tcx>)>,
+    new_locals: Vec<LocalDecl<'tcx>>,
+    resume_block: Option<BasicBlock>,
+    body_span: Span,
+    next_local: usize,
+}
+
+impl<'tcx> MirPatch<'tcx> {
+    pub fn new(body: &Body<'tcx>) -> Self {
+        let mut result = MirPatch {
+            patch_map: IndexVec::from_elem(None, body.basic_blocks()),
+            new_blocks: vec![],
+            new_statements: vec![],
+            new_locals: vec![],
+            next_local: body.local_decls.len(),
+            resume_block: None,
+            body_span: body.span,
+        };
+
+        // Check if we already have a resume block
+        for (bb, block) in body.basic_blocks().iter_enumerated() {
+            if let TerminatorKind::Resume = block.terminator().kind && block.statements.is_empty() {
+                result.resume_block = Some(bb);
+                break;
+            }
+        }
+
+        result
+    }
+
+    pub fn resume_block(&mut self) -> BasicBlock {
+        if let Some(bb) = self.resume_block {
+            return bb;
+        }
+
+        let bb = self.new_block(BasicBlockData {
+            statements: vec![],
+            terminator: Some(Terminator {
+                source_info: SourceInfo::outermost(self.body_span),
+                kind: TerminatorKind::Resume,
+            }),
+            is_cleanup: true,
+        });
+        self.resume_block = Some(bb);
+        bb
+    }
+
+    pub fn is_patched(&self, bb: BasicBlock) -> bool {
+        self.patch_map[bb].is_some()
+    }
+
+    pub fn terminator_loc(&self, body: &Body<'tcx>, bb: BasicBlock) -> Location {
+        let offset = match bb.index().checked_sub(body.basic_blocks().len()) {
+            Some(index) => self.new_blocks[index].statements.len(),
+            None => body[bb].statements.len(),
+        };
+        Location { block: bb, statement_index: offset }
+    }
+
+    pub fn new_local_with_info(
+        &mut self,
+        ty: Ty<'tcx>,
+        span: Span,
+        local_info: Option<Box<LocalInfo<'tcx>>>,
+    ) -> Local {
+        let index = self.next_local;
+        self.next_local += 1;
+        let mut new_decl = LocalDecl::new(ty, span);
+        new_decl.local_info = local_info;
+        self.new_locals.push(new_decl);
+        Local::new(index as usize)
+    }
+
+    pub fn new_temp(&mut self, ty: Ty<'tcx>, span: Span) -> Local {
+        self.new_local_with_info(ty, span, None)
+    }
+
+    pub fn new_internal(&mut self, ty: Ty<'tcx>, span: Span) -> Local {
+        let index = self.next_local;
+        self.next_local += 1;
+        self.new_locals.push(LocalDecl::new(ty, span).internal());
+        Local::new(index as usize)
+    }
+
+    pub fn new_block(&mut self, data: BasicBlockData<'tcx>) -> BasicBlock {
+        let block = BasicBlock::new(self.patch_map.len());
+        debug!("MirPatch: new_block: {:?}: {:?}", block, data);
+        self.new_blocks.push(data);
+        self.patch_map.push(None);
+        block
+    }
+
+    pub fn patch_terminator(&mut self, block: BasicBlock, new: TerminatorKind<'tcx>) {
+        assert!(self.patch_map[block].is_none());
+        debug!("MirPatch: patch_terminator({:?}, {:?})", block, new);
+        self.patch_map[block] = Some(new);
+    }
+
+    pub fn add_statement(&mut self, loc: Location, stmt: StatementKind<'tcx>) {
+        debug!("MirPatch: add_statement({:?}, {:?})", loc, stmt);
+        self.new_statements.push((loc, stmt));
+    }
+
+    pub fn add_assign(&mut self, loc: Location, place: Place<'tcx>, rv: Rvalue<'tcx>) {
+        self.add_statement(loc, StatementKind::Assign(Box::new((place, rv))));
+    }
+
+    pub fn apply(self, body: &mut Body<'tcx>) {
+        debug!(
+            "MirPatch: {:?} new temps, starting from index {}: {:?}",
+            self.new_locals.len(),
+            body.local_decls.len(),
+            self.new_locals
+        );
+        debug!(
+            "MirPatch: {} new blocks, starting from index {}",
+            self.new_blocks.len(),
+            body.basic_blocks().len()
+        );
+        let bbs = if self.patch_map.is_empty() && self.new_blocks.is_empty() {
+            body.basic_blocks.as_mut_preserves_cfg()
+        } else {
+            body.basic_blocks.as_mut()
+        };
+        bbs.extend(self.new_blocks);
+        body.local_decls.extend(self.new_locals);
+        for (src, patch) in self.patch_map.into_iter_enumerated() {
+            if let Some(patch) = patch {
+                debug!("MirPatch: patching block {:?}", src);
+                bbs[src].terminator_mut().kind = patch;
+            }
+        }
+
+        let mut new_statements = self.new_statements;
+        new_statements.sort_by_key(|s| s.0);
+
+        let mut delta = 0;
+        let mut last_bb = START_BLOCK;
+        let mut stmts_and_targets: Vec<(Statement<'_>, BasicBlock)> = Vec::new();
+        for (mut loc, stmt) in new_statements {
+            if loc.block != last_bb {
+                delta = 0;
+                last_bb = loc.block;
+            }
+            debug!("MirPatch: adding statement {:?} at loc {:?}+{}", stmt, loc, delta);
+            loc.statement_index += delta;
+            let source_info = Self::source_info_for_index(&body[loc.block], loc);
+
+            // For mir-opt `Derefer` to work in all cases we need to
+            // get terminator's targets and apply the statement to all of them.
+            if loc.statement_index > body[loc.block].statements.len() {
+                let term = body[loc.block].terminator();
+                for i in term.successors() {
+                    stmts_and_targets.push((Statement { source_info, kind: stmt.clone() }, i));
+                }
+                delta += 1;
+                continue;
+            }
+
+            body[loc.block]
+                .statements
+                .insert(loc.statement_index, Statement { source_info, kind: stmt });
+            delta += 1;
+        }
+
+        for (stmt, target) in stmts_and_targets.into_iter().rev() {
+            body[target].statements.insert(0, stmt);
+        }
+    }
+
+    pub fn source_info_for_index(data: &BasicBlockData<'_>, loc: Location) -> SourceInfo {
+        match data.statements.get(loc.statement_index) {
+            Some(stmt) => stmt.source_info,
+            None => data.terminator().source_info,
+        }
+    }
+
+    pub fn source_info_for_location(&self, body: &Body<'tcx>, loc: Location) -> SourceInfo {
+        let data = match loc.block.index().checked_sub(body.basic_blocks().len()) {
+            Some(new) => &self.new_blocks[new],
+            None => &body[loc.block],
+        };
+        Self::source_info_for_index(data, loc)
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/predecessors.rs b/compiler/rustc_middle/src/mir/predecessors.rs
new file mode 100644
index 000000000..5f1fadaf3
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/predecessors.rs
@@ -0,0 +1,78 @@
+//! Lazily compute the reverse control-flow graph for the MIR.
+
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::OnceCell;
+use rustc_index::vec::IndexVec;
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+use smallvec::SmallVec;
+
+use crate::mir::{BasicBlock, BasicBlockData};
+
+// Typically 95%+ of basic blocks have 4 or fewer predecessors.
+pub type Predecessors = IndexVec<BasicBlock, SmallVec<[BasicBlock; 4]>>;
+
+#[derive(Clone, Debug)]
+pub(super) struct PredecessorCache {
+    cache: OnceCell<Predecessors>,
+}
+
+impl PredecessorCache {
+    #[inline]
+    pub(super) fn new() -> Self {
+        PredecessorCache { cache: OnceCell::new() }
+    }
+
+    /// Invalidates the predecessor cache.
+    #[inline]
+    pub(super) fn invalidate(&mut self) {
+        // Invalidating the predecessor cache requires mutating the MIR, which in turn requires a
+        // unique reference (`&mut`) to the `mir::Body`. Because of this, we can assume that all
+        // callers of `invalidate` have a unique reference to the MIR and thus to the predecessor
+        // cache. This means we never need to do synchronization when `invalidate` is called, we can
+        // simply reinitialize the `OnceCell`.
+        self.cache = OnceCell::new();
+    }
+
+    /// Returns the predecessor graph for this MIR.
+    #[inline]
+    pub(super) fn compute(
+        &self,
+        basic_blocks: &IndexVec<BasicBlock, BasicBlockData<'_>>,
+    ) -> &Predecessors {
+        self.cache.get_or_init(|| {
+            let mut preds = IndexVec::from_elem(SmallVec::new(), basic_blocks);
+            for (bb, data) in basic_blocks.iter_enumerated() {
+                if let Some(term) = &data.terminator {
+                    for succ in term.successors() {
+                        preds[succ].push(bb);
+                    }
+                }
+            }
+
+            preds
+        })
+    }
+}
+
+impl<S: Encoder> Encodable<S> for PredecessorCache {
+    #[inline]
+    fn encode(&self, _s: &mut S) {}
+}
+
+impl<D: Decoder> Decodable<D> for PredecessorCache {
+    #[inline]
+    fn decode(_: &mut D) -> Self {
+        Self::new()
+    }
+}
+
+impl<CTX> HashStable<CTX> for PredecessorCache {
+    #[inline]
+    fn hash_stable(&self, _: &mut CTX, _: &mut StableHasher) {
+        // do nothing
+    }
+}
+
+TrivialTypeTraversalAndLiftImpls! {
+    PredecessorCache,
+}
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
new file mode 100644
index 000000000..0ce41337b
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/pretty.rs
@@ -0,0 +1,1067 @@
+use std::collections::BTreeSet;
+use std::fmt::Display;
+use std::fmt::Write as _;
+use std::fs;
+use std::io::{self, Write};
+use std::path::{Path, PathBuf};
+
+use super::graphviz::write_mir_fn_graphviz;
+use super::spanview::write_mir_fn_spanview;
+use either::Either;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_hir::def_id::DefId;
+use rustc_index::vec::Idx;
+use rustc_middle::mir::interpret::{
+    read_target_uint, AllocId, Allocation, ConstAllocation, ConstValue, GlobalAlloc, Pointer,
+    Provenance,
+};
+use rustc_middle::mir::visit::Visitor;
+use rustc_middle::mir::MirSource;
+use rustc_middle::mir::*;
+use rustc_middle::ty::{self, TyCtxt};
+use rustc_target::abi::Size;
+
+const INDENT: &str = "    ";
+/// Alignment for lining up comments following MIR statements
+pub(crate) const ALIGN: usize = 40;
+
+/// An indication of where we are in the control flow graph. Used for printing
+/// extra information in `dump_mir`
+pub enum PassWhere {
+    /// We have not started dumping the control flow graph, but we are about to.
+    BeforeCFG,
+
+    /// We just finished dumping the control flow graph. This is right before EOF
+    AfterCFG,
+
+    /// We are about to start dumping the given basic block.
+    BeforeBlock(BasicBlock),
+
+    /// We are just about to dump the given statement or terminator.
+    BeforeLocation(Location),
+
+    /// We just dumped the given statement or terminator.
+    AfterLocation(Location),
+
+    /// We just dumped the terminator for a block but not the closing `}`.
+    AfterTerminator(BasicBlock),
+}
+
+/// If the session is properly configured, dumps a human-readable
+/// representation of the mir into:
+///
+/// ```text
+/// rustc.node<node_id>.<pass_num>.<pass_name>.<disambiguator>
+/// ```
+///
+/// Output from this function is controlled by passing `-Z dump-mir=<filter>`,
+/// where `<filter>` takes the following forms:
+///
+/// - `all` -- dump MIR for all fns, all passes, all everything
+/// - a filter defined by a set of substrings combined with `&` and `|`
+///   (`&` has higher precedence). At least one of the `|`-separated groups
+///   must match; an `|`-separated group matches if all of its `&`-separated
+///   substrings are matched.
+///
+/// Example:
+///
+/// - `nll` == match if `nll` appears in the name
+/// - `foo & nll` == match if `foo` and `nll` both appear in the name
+/// - `foo & nll | typeck` == match if `foo` and `nll` both appear in the name
+///   or `typeck` appears in the name.
+/// - `foo & nll | bar & typeck` == match if `foo` and `nll` both appear in the name
+///   or `typeck` and `bar` both appear in the name.
+#[inline]
+pub fn dump_mir<'tcx, F>(
+    tcx: TyCtxt<'tcx>,
+    pass_num: Option<&dyn Display>,
+    pass_name: &str,
+    disambiguator: &dyn Display,
+    body: &Body<'tcx>,
+    extra_data: F,
+) where
+    F: FnMut(PassWhere, &mut dyn Write) -> io::Result<()>,
+{
+    if !dump_enabled(tcx, pass_name, body.source.def_id()) {
+        return;
+    }
+
+    dump_matched_mir_node(tcx, pass_num, pass_name, disambiguator, body, extra_data);
+}
+
+pub fn dump_enabled<'tcx>(tcx: TyCtxt<'tcx>, pass_name: &str, def_id: DefId) -> bool {
+    let Some(ref filters) = tcx.sess.opts.unstable_opts.dump_mir else {
+        return false;
+    };
+    // see notes on #41697 below
+    let node_path = ty::print::with_forced_impl_filename_line!(tcx.def_path_str(def_id));
+    filters.split('|').any(|or_filter| {
+        or_filter.split('&').all(|and_filter| {
+            let and_filter_trimmed = and_filter.trim();
+            and_filter_trimmed == "all"
+                || pass_name.contains(and_filter_trimmed)
+                || node_path.contains(and_filter_trimmed)
+        })
+    })
+}
+
+// #41697 -- we use `with_forced_impl_filename_line()` because
+// `def_path_str()` would otherwise trigger `type_of`, and this can
+// run while we are already attempting to evaluate `type_of`.
+
+fn dump_matched_mir_node<'tcx, F>(
+    tcx: TyCtxt<'tcx>,
+    pass_num: Option<&dyn Display>,
+    pass_name: &str,
+    disambiguator: &dyn Display,
+    body: &Body<'tcx>,
+    mut extra_data: F,
+) where
+    F: FnMut(PassWhere, &mut dyn Write) -> io::Result<()>,
+{
+    let _: io::Result<()> = try {
+        let mut file =
+            create_dump_file(tcx, "mir", pass_num, pass_name, disambiguator, body.source)?;
+        // see notes on #41697 above
+        let def_path =
+            ty::print::with_forced_impl_filename_line!(tcx.def_path_str(body.source.def_id()));
+        write!(file, "// MIR for `{}", def_path)?;
+        match body.source.promoted {
+            None => write!(file, "`")?,
+            Some(promoted) => write!(file, "::{:?}`", promoted)?,
+        }
+        writeln!(file, " {} {}", disambiguator, pass_name)?;
+        if let Some(ref layout) = body.generator_layout() {
+            writeln!(file, "/* generator_layout = {:#?} */", layout)?;
+        }
+        writeln!(file)?;
+        extra_data(PassWhere::BeforeCFG, &mut file)?;
+        write_user_type_annotations(tcx, body, &mut file)?;
+        write_mir_fn(tcx, body, &mut extra_data, &mut file)?;
+        extra_data(PassWhere::AfterCFG, &mut file)?;
+    };
+
+    if tcx.sess.opts.unstable_opts.dump_mir_graphviz {
+        let _: io::Result<()> = try {
+            let mut file =
+                create_dump_file(tcx, "dot", pass_num, pass_name, disambiguator, body.source)?;
+            write_mir_fn_graphviz(tcx, body, false, &mut file)?;
+        };
+    }
+
+    if let Some(spanview) = tcx.sess.opts.unstable_opts.dump_mir_spanview {
+        let _: io::Result<()> = try {
+            let file_basename =
+                dump_file_basename(tcx, pass_num, pass_name, disambiguator, body.source);
+            let mut file = create_dump_file_with_basename(tcx, &file_basename, "html")?;
+            if body.source.def_id().is_local() {
+                write_mir_fn_spanview(tcx, body, spanview, &file_basename, &mut file)?;
+            }
+        };
+    }
+}
+
+/// Returns the file basename portion (without extension) of a filename path
+/// where we should dump a MIR representation output files.
+fn dump_file_basename<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    pass_num: Option<&dyn Display>,
+    pass_name: &str,
+    disambiguator: &dyn Display,
+    source: MirSource<'tcx>,
+) -> String {
+    let promotion_id = match source.promoted {
+        Some(id) => format!("-{:?}", id),
+        None => String::new(),
+    };
+
+    let pass_num = if tcx.sess.opts.unstable_opts.dump_mir_exclude_pass_number {
+        String::new()
+    } else {
+        match pass_num {
+            None => ".-------".to_string(),
+            Some(pass_num) => format!(".{}", pass_num),
+        }
+    };
+
+    let crate_name = tcx.crate_name(source.def_id().krate);
+    let item_name = tcx.def_path(source.def_id()).to_filename_friendly_no_crate();
+    // All drop shims have the same DefId, so we have to add the type
+    // to get unique file names.
+    let shim_disambiguator = match source.instance {
+        ty::InstanceDef::DropGlue(_, Some(ty)) => {
+            // Unfortunately, pretty-printed typed are not very filename-friendly.
+            // We dome some filtering.
+            let mut s = ".".to_owned();
+            s.extend(ty.to_string().chars().filter_map(|c| match c {
+                ' ' => None,
+                ':' | '<' | '>' => Some('_'),
+                c => Some(c),
+            }));
+            s
+        }
+        _ => String::new(),
+    };
+
+    format!(
+        "{}.{}{}{}{}.{}.{}",
+        crate_name, item_name, shim_disambiguator, promotion_id, pass_num, pass_name, disambiguator,
+    )
+}
+
+/// Returns the path to the filename where we should dump a given MIR.
+/// Also used by other bits of code (e.g., NLL inference) that dump
+/// graphviz data or other things.
+fn dump_path(tcx: TyCtxt<'_>, basename: &str, extension: &str) -> PathBuf {
+    let mut file_path = PathBuf::new();
+    file_path.push(Path::new(&tcx.sess.opts.unstable_opts.dump_mir_dir));
+
+    let file_name = format!("{}.{}", basename, extension,);
+
+    file_path.push(&file_name);
+
+    file_path
+}
+
+/// Attempts to open the MIR dump file with the given name and extension.
+fn create_dump_file_with_basename(
+    tcx: TyCtxt<'_>,
+    file_basename: &str,
+    extension: &str,
+) -> io::Result<io::BufWriter<fs::File>> {
+    let file_path = dump_path(tcx, file_basename, extension);
+    if let Some(parent) = file_path.parent() {
+        fs::create_dir_all(parent).map_err(|e| {
+            io::Error::new(
+                e.kind(),
+                format!("IO error creating MIR dump directory: {:?}; {}", parent, e),
+            )
+        })?;
+    }
+    Ok(io::BufWriter::new(fs::File::create(&file_path).map_err(|e| {
+        io::Error::new(e.kind(), format!("IO error creating MIR dump file: {:?}; {}", file_path, e))
+    })?))
+}
+
+/// Attempts to open a file where we should dump a given MIR or other
+/// bit of MIR-related data. Used by `mir-dump`, but also by other
+/// bits of code (e.g., NLL inference) that dump graphviz data or
+/// other things, and hence takes the extension as an argument.
+pub fn create_dump_file<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    extension: &str,
+    pass_num: Option<&dyn Display>,
+    pass_name: &str,
+    disambiguator: &dyn Display,
+    source: MirSource<'tcx>,
+) -> io::Result<io::BufWriter<fs::File>> {
+    create_dump_file_with_basename(
+        tcx,
+        &dump_file_basename(tcx, pass_num, pass_name, disambiguator, source),
+        extension,
+    )
+}
+
+/// Write out a human-readable textual representation for the given MIR.
+pub fn write_mir_pretty<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    single: Option<DefId>,
+    w: &mut dyn Write,
+) -> io::Result<()> {
+    writeln!(w, "// WARNING: This output format is intended for human consumers only")?;
+    writeln!(w, "// and is subject to change without notice. Knock yourself out.")?;
+
+    let mut first = true;
+    for def_id in dump_mir_def_ids(tcx, single) {
+        if first {
+            first = false;
+        } else {
+            // Put empty lines between all items
+            writeln!(w)?;
+        }
+
+        let render_body = |w: &mut dyn Write, body| -> io::Result<()> {
+            write_mir_fn(tcx, body, &mut |_, _| Ok(()), w)?;
+
+            for body in tcx.promoted_mir(def_id) {
+                writeln!(w)?;
+                write_mir_fn(tcx, body, &mut |_, _| Ok(()), w)?;
+            }
+            Ok(())
+        };
+
+        // For `const fn` we want to render both the optimized MIR and the MIR for ctfe.
+        if tcx.is_const_fn_raw(def_id) {
+            render_body(w, tcx.optimized_mir(def_id))?;
+            writeln!(w)?;
+            writeln!(w, "// MIR FOR CTFE")?;
+            // Do not use `render_body`, as that would render the promoteds again, but these
+            // are shared between mir_for_ctfe and optimized_mir
+            write_mir_fn(tcx, tcx.mir_for_ctfe(def_id), &mut |_, _| Ok(()), w)?;
+        } else {
+            let instance_mir =
+                tcx.instance_mir(ty::InstanceDef::Item(ty::WithOptConstParam::unknown(def_id)));
+            render_body(w, instance_mir)?;
+        }
+    }
+    Ok(())
+}
+
+/// Write out a human-readable textual representation for the given function.
+pub fn write_mir_fn<'tcx, F>(
+    tcx: TyCtxt<'tcx>,
+    body: &Body<'tcx>,
+    extra_data: &mut F,
+    w: &mut dyn Write,
+) -> io::Result<()>
+where
+    F: FnMut(PassWhere, &mut dyn Write) -> io::Result<()>,
+{
+    write_mir_intro(tcx, body, w)?;
+    for block in body.basic_blocks().indices() {
+        extra_data(PassWhere::BeforeBlock(block), w)?;
+        write_basic_block(tcx, block, body, extra_data, w)?;
+        if block.index() + 1 != body.basic_blocks().len() {
+            writeln!(w)?;
+        }
+    }
+
+    writeln!(w, "}}")?;
+
+    write_allocations(tcx, body, w)?;
+
+    Ok(())
+}
+
+/// Write out a human-readable textual representation for the given basic block.
+pub fn write_basic_block<'tcx, F>(
+    tcx: TyCtxt<'tcx>,
+    block: BasicBlock,
+    body: &Body<'tcx>,
+    extra_data: &mut F,
+    w: &mut dyn Write,
+) -> io::Result<()>
+where
+    F: FnMut(PassWhere, &mut dyn Write) -> io::Result<()>,
+{
+    let data = &body[block];
+
+    // Basic block label at the top.
+    let cleanup_text = if data.is_cleanup { " (cleanup)" } else { "" };
+    writeln!(w, "{}{:?}{}: {{", INDENT, block, cleanup_text)?;
+
+    // List of statements in the middle.
+    let mut current_location = Location { block, statement_index: 0 };
+    for statement in &data.statements {
+        extra_data(PassWhere::BeforeLocation(current_location), w)?;
+        let indented_body = format!("{0}{0}{1:?};", INDENT, statement);
+        writeln!(
+            w,
+            "{:A$} // {}{}",
+            indented_body,
+            if tcx.sess.verbose() { format!("{:?}: ", current_location) } else { String::new() },
+            comment(tcx, statement.source_info, body.span),
+            A = ALIGN,
+        )?;
+
+        write_extra(tcx, w, |visitor| {
+            visitor.visit_statement(statement, current_location);
+        })?;
+
+        extra_data(PassWhere::AfterLocation(current_location), w)?;
+
+        current_location.statement_index += 1;
+    }
+
+    // Terminator at the bottom.
+    extra_data(PassWhere::BeforeLocation(current_location), w)?;
+    let indented_terminator = format!("{0}{0}{1:?};", INDENT, data.terminator().kind);
+    writeln!(
+        w,
+        "{:A$} // {}{}",
+        indented_terminator,
+        if tcx.sess.verbose() { format!("{:?}: ", current_location) } else { String::new() },
+        comment(tcx, data.terminator().source_info, body.span),
+        A = ALIGN,
+    )?;
+
+    write_extra(tcx, w, |visitor| {
+        visitor.visit_terminator(data.terminator(), current_location);
+    })?;
+
+    extra_data(PassWhere::AfterLocation(current_location), w)?;
+    extra_data(PassWhere::AfterTerminator(block), w)?;
+
+    writeln!(w, "{}}}", INDENT)
+}
+
+/// After we print the main statement, we sometimes dump extra
+/// information. There's often a lot of little things "nuzzled up" in
+/// a statement.
+fn write_extra<'tcx, F>(tcx: TyCtxt<'tcx>, write: &mut dyn Write, mut visit_op: F) -> io::Result<()>
+where
+    F: FnMut(&mut ExtraComments<'tcx>),
+{
+    let mut extra_comments = ExtraComments { tcx, comments: vec![] };
+    visit_op(&mut extra_comments);
+    for comment in extra_comments.comments {
+        writeln!(write, "{:A$} // {}", "", comment, A = ALIGN)?;
+    }
+    Ok(())
+}
+
+struct ExtraComments<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    comments: Vec<String>,
+}
+
+impl<'tcx> ExtraComments<'tcx> {
+    fn push(&mut self, lines: &str) {
+        for line in lines.split('\n') {
+            self.comments.push(line.to_string());
+        }
+    }
+}
+
+fn use_verbose<'tcx>(ty: Ty<'tcx>, fn_def: bool) -> bool {
+    match *ty.kind() {
+        ty::Int(_) | ty::Uint(_) | ty::Bool | ty::Char | ty::Float(_) => false,
+        // Unit type
+        ty::Tuple(g_args) if g_args.is_empty() => false,
+        ty::Tuple(g_args) => g_args.iter().any(|g_arg| use_verbose(g_arg, fn_def)),
+        ty::Array(ty, _) => use_verbose(ty, fn_def),
+        ty::FnDef(..) => fn_def,
+        _ => true,
+    }
+}
+
+impl<'tcx> Visitor<'tcx> for ExtraComments<'tcx> {
+    fn visit_constant(&mut self, constant: &Constant<'tcx>, _location: Location) {
+        let Constant { span, user_ty, literal } = constant;
+        if use_verbose(literal.ty(), true) {
+            self.push("mir::Constant");
+            self.push(&format!(
+                "+ span: {}",
+                self.tcx.sess.source_map().span_to_embeddable_string(*span)
+            ));
+            if let Some(user_ty) = user_ty {
+                self.push(&format!("+ user_ty: {:?}", user_ty));
+            }
+
+            // FIXME: this is a poor version of `pretty_print_const_value`.
+            let fmt_val = |val: &ConstValue<'tcx>| match val {
+                ConstValue::ZeroSized => format!("<ZST>"),
+                ConstValue::Scalar(s) => format!("Scalar({:?})", s),
+                ConstValue::Slice { .. } => format!("Slice(..)"),
+                ConstValue::ByRef { .. } => format!("ByRef(..)"),
+            };
+
+            let fmt_valtree = |valtree: &ty::ValTree<'tcx>| match valtree {
+                ty::ValTree::Leaf(leaf) => format!("ValTree::Leaf({:?})", leaf),
+                ty::ValTree::Branch(_) => format!("ValTree::Branch(..)"),
+            };
+
+            let val = match literal {
+                ConstantKind::Ty(ct) => match ct.kind() {
+                    ty::ConstKind::Param(p) => format!("Param({})", p),
+                    ty::ConstKind::Unevaluated(uv) => format!(
+                        "Unevaluated({}, {:?}, {:?})",
+                        self.tcx.def_path_str(uv.def.did),
+                        uv.substs,
+                        uv.promoted,
+                    ),
+                    ty::ConstKind::Value(val) => format!("Value({})", fmt_valtree(&val)),
+                    ty::ConstKind::Error(_) => "Error".to_string(),
+                    // These variants shouldn't exist in the MIR.
+                    ty::ConstKind::Placeholder(_)
+                    | ty::ConstKind::Infer(_)
+                    | ty::ConstKind::Bound(..) => bug!("unexpected MIR constant: {:?}", literal),
+                },
+                // To keep the diffs small, we render this like we render `ty::Const::Value`.
+                //
+                // This changes once `ty::Const::Value` is represented using valtrees.
+                ConstantKind::Val(val, _) => format!("Value({})", fmt_val(&val)),
+            };
+
+            // This reflects what `Const` looked liked before `val` was renamed
+            // as `kind`. We print it like this to avoid having to update
+            // expected output in a lot of tests.
+            self.push(&format!("+ literal: Const {{ ty: {}, val: {} }}", literal.ty(), val));
+        }
+    }
+
+    fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
+        self.super_rvalue(rvalue, location);
+        if let Rvalue::Aggregate(kind, _) = rvalue {
+            match **kind {
+                AggregateKind::Closure(def_id, substs) => {
+                    self.push("closure");
+                    self.push(&format!("+ def_id: {:?}", def_id));
+                    self.push(&format!("+ substs: {:#?}", substs));
+                }
+
+                AggregateKind::Generator(def_id, substs, movability) => {
+                    self.push("generator");
+                    self.push(&format!("+ def_id: {:?}", def_id));
+                    self.push(&format!("+ substs: {:#?}", substs));
+                    self.push(&format!("+ movability: {:?}", movability));
+                }
+
+                AggregateKind::Adt(_, _, _, Some(user_ty), _) => {
+                    self.push("adt");
+                    self.push(&format!("+ user_ty: {:?}", user_ty));
+                }
+
+                _ => {}
+            }
+        }
+    }
+}
+
+fn comment(tcx: TyCtxt<'_>, SourceInfo { span, scope }: SourceInfo, function_span: Span) -> String {
+    let location = if tcx.sess.opts.unstable_opts.mir_pretty_relative_line_numbers {
+        tcx.sess.source_map().span_to_relative_line_string(span, function_span)
+    } else {
+        tcx.sess.source_map().span_to_embeddable_string(span)
+    };
+
+    format!("scope {} at {}", scope.index(), location,)
+}
+
+/// Prints local variables in a scope tree.
+fn write_scope_tree(
+    tcx: TyCtxt<'_>,
+    body: &Body<'_>,
+    scope_tree: &FxHashMap<SourceScope, Vec<SourceScope>>,
+    w: &mut dyn Write,
+    parent: SourceScope,
+    depth: usize,
+) -> io::Result<()> {
+    let indent = depth * INDENT.len();
+
+    // Local variable debuginfo.
+    for var_debug_info in &body.var_debug_info {
+        if var_debug_info.source_info.scope != parent {
+            // Not declared in this scope.
+            continue;
+        }
+
+        let indented_debug_info = format!(
+            "{0:1$}debug {2} => {3:?};",
+            INDENT, indent, var_debug_info.name, var_debug_info.value,
+        );
+
+        writeln!(
+            w,
+            "{0:1$} // in {2}",
+            indented_debug_info,
+            ALIGN,
+            comment(tcx, var_debug_info.source_info, body.span),
+        )?;
+    }
+
+    // Local variable types.
+    for (local, local_decl) in body.local_decls.iter_enumerated() {
+        if (1..body.arg_count + 1).contains(&local.index()) {
+            // Skip over argument locals, they're printed in the signature.
+            continue;
+        }
+
+        if local_decl.source_info.scope != parent {
+            // Not declared in this scope.
+            continue;
+        }
+
+        let mut_str = if local_decl.mutability == Mutability::Mut { "mut " } else { "" };
+
+        let mut indented_decl =
+            format!("{0:1$}let {2}{3:?}: {4:?}", INDENT, indent, mut_str, local, local_decl.ty);
+        if let Some(user_ty) = &local_decl.user_ty {
+            for user_ty in user_ty.projections() {
+                write!(indented_decl, " as {:?}", user_ty).unwrap();
+            }
+        }
+        indented_decl.push(';');
+
+        let local_name = if local == RETURN_PLACE { " return place" } else { "" };
+
+        writeln!(
+            w,
+            "{0:1$} //{2} in {3}",
+            indented_decl,
+            ALIGN,
+            local_name,
+            comment(tcx, local_decl.source_info, body.span),
+        )?;
+    }
+
+    let Some(children) = scope_tree.get(&parent) else {
+        return Ok(());
+    };
+
+    for &child in children {
+        let child_data = &body.source_scopes[child];
+        assert_eq!(child_data.parent_scope, Some(parent));
+
+        let (special, span) = if let Some((callee, callsite_span)) = child_data.inlined {
+            (
+                format!(
+                    " (inlined {}{})",
+                    if callee.def.requires_caller_location(tcx) { "#[track_caller] " } else { "" },
+                    callee
+                ),
+                Some(callsite_span),
+            )
+        } else {
+            (String::new(), None)
+        };
+
+        let indented_header = format!("{0:1$}scope {2}{3} {{", "", indent, child.index(), special);
+
+        if let Some(span) = span {
+            writeln!(
+                w,
+                "{0:1$} // at {2}",
+                indented_header,
+                ALIGN,
+                tcx.sess.source_map().span_to_embeddable_string(span),
+            )?;
+        } else {
+            writeln!(w, "{}", indented_header)?;
+        }
+
+        write_scope_tree(tcx, body, scope_tree, w, child, depth + 1)?;
+        writeln!(w, "{0:1$}}}", "", depth * INDENT.len())?;
+    }
+
+    Ok(())
+}
+
+/// Write out a human-readable textual representation of the MIR's `fn` type and the types of its
+/// local variables (both user-defined bindings and compiler temporaries).
+pub fn write_mir_intro<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    body: &Body<'_>,
+    w: &mut dyn Write,
+) -> io::Result<()> {
+    write_mir_sig(tcx, body, w)?;
+    writeln!(w, "{{")?;
+
+    // construct a scope tree and write it out
+    let mut scope_tree: FxHashMap<SourceScope, Vec<SourceScope>> = Default::default();
+    for (index, scope_data) in body.source_scopes.iter().enumerate() {
+        if let Some(parent) = scope_data.parent_scope {
+            scope_tree.entry(parent).or_default().push(SourceScope::new(index));
+        } else {
+            // Only the argument scope has no parent, because it's the root.
+            assert_eq!(index, OUTERMOST_SOURCE_SCOPE.index());
+        }
+    }
+
+    write_scope_tree(tcx, body, &scope_tree, w, OUTERMOST_SOURCE_SCOPE, 1)?;
+
+    // Add an empty line before the first block is printed.
+    writeln!(w)?;
+
+    Ok(())
+}
+
+/// Find all `AllocId`s mentioned (recursively) in the MIR body and print their corresponding
+/// allocations.
+pub fn write_allocations<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    body: &Body<'_>,
+    w: &mut dyn Write,
+) -> io::Result<()> {
+    fn alloc_ids_from_alloc(
+        alloc: ConstAllocation<'_>,
+    ) -> impl DoubleEndedIterator<Item = AllocId> + '_ {
+        alloc.inner().relocations().values().map(|id| *id)
+    }
+
+    fn alloc_ids_from_const_val(val: ConstValue<'_>) -> impl Iterator<Item = AllocId> + '_ {
+        match val {
+            ConstValue::Scalar(interpret::Scalar::Ptr(ptr, _)) => {
+                Either::Left(Either::Left(std::iter::once(ptr.provenance)))
+            }
+            ConstValue::Scalar(interpret::Scalar::Int { .. }) => {
+                Either::Left(Either::Right(std::iter::empty()))
+            }
+            ConstValue::ZeroSized => Either::Left(Either::Right(std::iter::empty())),
+            ConstValue::ByRef { alloc, .. } | ConstValue::Slice { data: alloc, .. } => {
+                Either::Right(alloc_ids_from_alloc(alloc))
+            }
+        }
+    }
+    struct CollectAllocIds(BTreeSet<AllocId>);
+
+    impl<'tcx> Visitor<'tcx> for CollectAllocIds {
+        fn visit_constant(&mut self, c: &Constant<'tcx>, loc: Location) {
+            match c.literal {
+                ConstantKind::Ty(c) => self.visit_const(c, loc),
+                ConstantKind::Val(val, _) => {
+                    self.0.extend(alloc_ids_from_const_val(val));
+                }
+            }
+        }
+    }
+
+    let mut visitor = CollectAllocIds(Default::default());
+    visitor.visit_body(body);
+
+    // `seen` contains all seen allocations, including the ones we have *not* printed yet.
+    // The protocol is to first `insert` into `seen`, and only if that returns `true`
+    // then push to `todo`.
+    let mut seen = visitor.0;
+    let mut todo: Vec<_> = seen.iter().copied().collect();
+    while let Some(id) = todo.pop() {
+        let mut write_allocation_track_relocs =
+            |w: &mut dyn Write, alloc: ConstAllocation<'tcx>| -> io::Result<()> {
+                // `.rev()` because we are popping them from the back of the `todo` vector.
+                for id in alloc_ids_from_alloc(alloc).rev() {
+                    if seen.insert(id) {
+                        todo.push(id);
+                    }
+                }
+                write!(w, "{}", display_allocation(tcx, alloc.inner()))
+            };
+        write!(w, "\n{id:?}")?;
+        match tcx.try_get_global_alloc(id) {
+            // This can't really happen unless there are bugs, but it doesn't cost us anything to
+            // gracefully handle it and allow buggy rustc to be debugged via allocation printing.
+            None => write!(w, " (deallocated)")?,
+            Some(GlobalAlloc::Function(inst)) => write!(w, " (fn: {inst})")?,
+            Some(GlobalAlloc::VTable(ty, Some(trait_ref))) => {
+                write!(w, " (vtable: impl {trait_ref} for {ty})")?
+            }
+            Some(GlobalAlloc::VTable(ty, None)) => {
+                write!(w, " (vtable: impl <auto trait> for {ty})")?
+            }
+            Some(GlobalAlloc::Static(did)) if !tcx.is_foreign_item(did) => {
+                match tcx.eval_static_initializer(did) {
+                    Ok(alloc) => {
+                        write!(w, " (static: {}, ", tcx.def_path_str(did))?;
+                        write_allocation_track_relocs(w, alloc)?;
+                    }
+                    Err(_) => write!(
+                        w,
+                        " (static: {}, error during initializer evaluation)",
+                        tcx.def_path_str(did)
+                    )?,
+                }
+            }
+            Some(GlobalAlloc::Static(did)) => {
+                write!(w, " (extern static: {})", tcx.def_path_str(did))?
+            }
+            Some(GlobalAlloc::Memory(alloc)) => {
+                write!(w, " (")?;
+                write_allocation_track_relocs(w, alloc)?
+            }
+        }
+        writeln!(w)?;
+    }
+    Ok(())
+}
+
+/// Dumps the size and metadata and content of an allocation to the given writer.
+/// The expectation is that the caller first prints other relevant metadata, so the exact
+/// format of this function is (*without* leading or trailing newline):
+///
+/// ```text
+/// size: {}, align: {}) {
+///     <bytes>
+/// }
+/// ```
+///
+/// The byte format is similar to how hex editors print bytes. Each line starts with the address of
+/// the start of the line, followed by all bytes in hex format (space separated).
+/// If the allocation is small enough to fit into a single line, no start address is given.
+/// After the hex dump, an ascii dump follows, replacing all unprintable characters (control
+/// characters or characters whose value is larger than 127) with a `.`
+/// This also prints relocations adequately.
+pub fn display_allocation<'a, 'tcx, Prov, Extra>(
+    tcx: TyCtxt<'tcx>,
+    alloc: &'a Allocation<Prov, Extra>,
+) -> RenderAllocation<'a, 'tcx, Prov, Extra> {
+    RenderAllocation { tcx, alloc }
+}
+
+#[doc(hidden)]
+pub struct RenderAllocation<'a, 'tcx, Prov, Extra> {
+    tcx: TyCtxt<'tcx>,
+    alloc: &'a Allocation<Prov, Extra>,
+}
+
+impl<'a, 'tcx, Prov: Provenance, Extra> std::fmt::Display
+    for RenderAllocation<'a, 'tcx, Prov, Extra>
+{
+    fn fmt(&self, w: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let RenderAllocation { tcx, alloc } = *self;
+        write!(w, "size: {}, align: {})", alloc.size().bytes(), alloc.align.bytes())?;
+        if alloc.size() == Size::ZERO {
+            // We are done.
+            return write!(w, " {{}}");
+        }
+        // Write allocation bytes.
+        writeln!(w, " {{")?;
+        write_allocation_bytes(tcx, alloc, w, "    ")?;
+        write!(w, "}}")?;
+        Ok(())
+    }
+}
+
+fn write_allocation_endline(w: &mut dyn std::fmt::Write, ascii: &str) -> std::fmt::Result {
+    for _ in 0..(BYTES_PER_LINE - ascii.chars().count()) {
+        write!(w, "   ")?;
+    }
+    writeln!(w, " │ {}", ascii)
+}
+
+/// Number of bytes to print per allocation hex dump line.
+const BYTES_PER_LINE: usize = 16;
+
+/// Prints the line start address and returns the new line start address.
+fn write_allocation_newline(
+    w: &mut dyn std::fmt::Write,
+    mut line_start: Size,
+    ascii: &str,
+    pos_width: usize,
+    prefix: &str,
+) -> Result<Size, std::fmt::Error> {
+    write_allocation_endline(w, ascii)?;
+    line_start += Size::from_bytes(BYTES_PER_LINE);
+    write!(w, "{}0x{:02$x} │ ", prefix, line_start.bytes(), pos_width)?;
+    Ok(line_start)
+}
+
+/// The `prefix` argument allows callers to add an arbitrary prefix before each line (even if there
+/// is only one line). Note that your prefix should contain a trailing space as the lines are
+/// printed directly after it.
+fn write_allocation_bytes<'tcx, Prov: Provenance, Extra>(
+    tcx: TyCtxt<'tcx>,
+    alloc: &Allocation<Prov, Extra>,
+    w: &mut dyn std::fmt::Write,
+    prefix: &str,
+) -> std::fmt::Result {
+    let num_lines = alloc.size().bytes_usize().saturating_sub(BYTES_PER_LINE);
+    // Number of chars needed to represent all line numbers.
+    let pos_width = hex_number_length(alloc.size().bytes());
+
+    if num_lines > 0 {
+        write!(w, "{}0x{:02$x} │ ", prefix, 0, pos_width)?;
+    } else {
+        write!(w, "{}", prefix)?;
+    }
+
+    let mut i = Size::ZERO;
+    let mut line_start = Size::ZERO;
+
+    let ptr_size = tcx.data_layout.pointer_size;
+
+    let mut ascii = String::new();
+
+    let oversized_ptr = |target: &mut String, width| {
+        if target.len() > width {
+            write!(target, " ({} ptr bytes)", ptr_size.bytes()).unwrap();
+        }
+    };
+
+    while i < alloc.size() {
+        // The line start already has a space. While we could remove that space from the line start
+        // printing and unconditionally print a space here, that would cause the single-line case
+        // to have a single space before it, which looks weird.
+        if i != line_start {
+            write!(w, " ")?;
+        }
+        if let Some(&prov) = alloc.relocations().get(&i) {
+            // Memory with a relocation must be defined
+            assert!(alloc.init_mask().is_range_initialized(i, i + ptr_size).is_ok());
+            let j = i.bytes_usize();
+            let offset = alloc
+                .inspect_with_uninit_and_ptr_outside_interpreter(j..j + ptr_size.bytes_usize());
+            let offset = read_target_uint(tcx.data_layout.endian, offset).unwrap();
+            let offset = Size::from_bytes(offset);
+            let relocation_width = |bytes| bytes * 3;
+            let ptr = Pointer::new(prov, offset);
+            let mut target = format!("{:?}", ptr);
+            if target.len() > relocation_width(ptr_size.bytes_usize() - 1) {
+                // This is too long, try to save some space.
+                target = format!("{:#?}", ptr);
+            }
+            if ((i - line_start) + ptr_size).bytes_usize() > BYTES_PER_LINE {
+                // This branch handles the situation where a relocation starts in the current line
+                // but ends in the next one.
+                let remainder = Size::from_bytes(BYTES_PER_LINE) - (i - line_start);
+                let overflow = ptr_size - remainder;
+                let remainder_width = relocation_width(remainder.bytes_usize()) - 2;
+                let overflow_width = relocation_width(overflow.bytes_usize() - 1) + 1;
+                ascii.push('╾');
+                for _ in 0..remainder.bytes() - 1 {
+                    ascii.push('─');
+                }
+                if overflow_width > remainder_width && overflow_width >= target.len() {
+                    // The case where the relocation fits into the part in the next line
+                    write!(w, "╾{0:─^1$}", "", remainder_width)?;
+                    line_start =
+                        write_allocation_newline(w, line_start, &ascii, pos_width, prefix)?;
+                    ascii.clear();
+                    write!(w, "{0:─^1$}╼", target, overflow_width)?;
+                } else {
+                    oversized_ptr(&mut target, remainder_width);
+                    write!(w, "╾{0:─^1$}", target, remainder_width)?;
+                    line_start =
+                        write_allocation_newline(w, line_start, &ascii, pos_width, prefix)?;
+                    write!(w, "{0:─^1$}╼", "", overflow_width)?;
+                    ascii.clear();
+                }
+                for _ in 0..overflow.bytes() - 1 {
+                    ascii.push('─');
+                }
+                ascii.push('╼');
+                i += ptr_size;
+                continue;
+            } else {
+                // This branch handles a relocation that starts and ends in the current line.
+                let relocation_width = relocation_width(ptr_size.bytes_usize() - 1);
+                oversized_ptr(&mut target, relocation_width);
+                ascii.push('╾');
+                write!(w, "╾{0:─^1$}╼", target, relocation_width)?;
+                for _ in 0..ptr_size.bytes() - 2 {
+                    ascii.push('─');
+                }
+                ascii.push('╼');
+                i += ptr_size;
+            }
+        } else if alloc.init_mask().is_range_initialized(i, i + Size::from_bytes(1)).is_ok() {
+            let j = i.bytes_usize();
+
+            // Checked definedness (and thus range) and relocations. This access also doesn't
+            // influence interpreter execution but is only for debugging.
+            let c = alloc.inspect_with_uninit_and_ptr_outside_interpreter(j..j + 1)[0];
+            write!(w, "{:02x}", c)?;
+            if c.is_ascii_control() || c >= 0x80 {
+                ascii.push('.');
+            } else {
+                ascii.push(char::from(c));
+            }
+            i += Size::from_bytes(1);
+        } else {
+            write!(w, "__")?;
+            ascii.push('░');
+            i += Size::from_bytes(1);
+        }
+        // Print a new line header if the next line still has some bytes to print.
+        if i == line_start + Size::from_bytes(BYTES_PER_LINE) && i != alloc.size() {
+            line_start = write_allocation_newline(w, line_start, &ascii, pos_width, prefix)?;
+            ascii.clear();
+        }
+    }
+    write_allocation_endline(w, &ascii)?;
+
+    Ok(())
+}
+
+fn write_mir_sig(tcx: TyCtxt<'_>, body: &Body<'_>, w: &mut dyn Write) -> io::Result<()> {
+    use rustc_hir::def::DefKind;
+
+    trace!("write_mir_sig: {:?}", body.source.instance);
+    let def_id = body.source.def_id();
+    let kind = tcx.def_kind(def_id);
+    let is_function = match kind {
+        DefKind::Fn | DefKind::AssocFn | DefKind::Ctor(..) => true,
+        _ => tcx.is_closure(def_id),
+    };
+    match (kind, body.source.promoted) {
+        (_, Some(i)) => write!(w, "{:?} in ", i)?,
+        (DefKind::Const | DefKind::AssocConst, _) => write!(w, "const ")?,
+        (DefKind::Static(hir::Mutability::Not), _) => write!(w, "static ")?,
+        (DefKind::Static(hir::Mutability::Mut), _) => write!(w, "static mut ")?,
+        (_, _) if is_function => write!(w, "fn ")?,
+        (DefKind::AnonConst | DefKind::InlineConst, _) => {} // things like anon const, not an item
+        _ => bug!("Unexpected def kind {:?}", kind),
+    }
+
+    ty::print::with_forced_impl_filename_line! {
+        // see notes on #41697 elsewhere
+        write!(w, "{}", tcx.def_path_str(def_id))?
+    }
+
+    if body.source.promoted.is_none() && is_function {
+        write!(w, "(")?;
+
+        // fn argument types.
+        for (i, arg) in body.args_iter().enumerate() {
+            if i != 0 {
+                write!(w, ", ")?;
+            }
+            write!(w, "{:?}: {}", Place::from(arg), body.local_decls[arg].ty)?;
+        }
+
+        write!(w, ") -> {}", body.return_ty())?;
+    } else {
+        assert_eq!(body.arg_count, 0);
+        write!(w, ": {} =", body.return_ty())?;
+    }
+
+    if let Some(yield_ty) = body.yield_ty() {
+        writeln!(w)?;
+        writeln!(w, "yields {}", yield_ty)?;
+    }
+
+    write!(w, " ")?;
+    // Next thing that gets printed is the opening {
+
+    Ok(())
+}
+
+fn write_user_type_annotations(
+    tcx: TyCtxt<'_>,
+    body: &Body<'_>,
+    w: &mut dyn Write,
+) -> io::Result<()> {
+    if !body.user_type_annotations.is_empty() {
+        writeln!(w, "| User Type Annotations")?;
+    }
+    for (index, annotation) in body.user_type_annotations.iter_enumerated() {
+        writeln!(
+            w,
+            "| {:?}: user_ty: {:?}, span: {}, inferred_ty: {:?}",
+            index.index(),
+            annotation.user_ty,
+            tcx.sess.source_map().span_to_embeddable_string(annotation.span),
+            annotation.inferred_ty,
+        )?;
+    }
+    if !body.user_type_annotations.is_empty() {
+        writeln!(w, "|")?;
+    }
+    Ok(())
+}
+
+pub fn dump_mir_def_ids(tcx: TyCtxt<'_>, single: Option<DefId>) -> Vec<DefId> {
+    if let Some(i) = single {
+        vec![i]
+    } else {
+        tcx.mir_keys(()).iter().map(|def_id| def_id.to_def_id()).collect()
+    }
+}
+
+/// Calc converted u64 decimal into hex and return it's length in chars
+///
+/// ```ignore (cannot-test-private-function)
+/// assert_eq!(1, hex_number_length(0));
+/// assert_eq!(1, hex_number_length(1));
+/// assert_eq!(2, hex_number_length(16));
+/// ```
+fn hex_number_length(x: u64) -> usize {
+    if x == 0 {
+        return 1;
+    }
+    let mut length = 0;
+    let mut x_left = x;
+    while x_left > 0 {
+        x_left /= 16;
+        length += 1;
+    }
+    length
+}
diff --git a/compiler/rustc_middle/src/mir/query.rs b/compiler/rustc_middle/src/mir/query.rs
new file mode 100644
index 000000000..dd9f8795f
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/query.rs
@@ -0,0 +1,476 @@
+//! Values computed by queries that use MIR.
+
+use crate::mir::{Body, ConstantKind, Promoted};
+use crate::ty::{self, OpaqueHiddenType, Ty, TyCtxt};
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::vec_map::VecMap;
+use rustc_errors::ErrorGuaranteed;
+use rustc_hir as hir;
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_index::bit_set::BitMatrix;
+use rustc_index::vec::IndexVec;
+use rustc_span::Span;
+use rustc_target::abi::VariantIdx;
+use smallvec::SmallVec;
+use std::cell::Cell;
+use std::fmt::{self, Debug};
+
+use super::{Field, SourceInfo};
+
+#[derive(Copy, Clone, PartialEq, TyEncodable, TyDecodable, HashStable, Debug)]
+pub enum UnsafetyViolationKind {
+    /// Unsafe operation outside `unsafe`.
+    General,
+    /// Unsafe operation in an `unsafe fn` but outside an `unsafe` block.
+    /// Has to be handled as a lint for backwards compatibility.
+    UnsafeFn,
+}
+
+#[derive(Copy, Clone, PartialEq, TyEncodable, TyDecodable, HashStable, Debug)]
+pub enum UnsafetyViolationDetails {
+    CallToUnsafeFunction,
+    UseOfInlineAssembly,
+    InitializingTypeWith,
+    CastOfPointerToInt,
+    UseOfMutableStatic,
+    UseOfExternStatic,
+    DerefOfRawPointer,
+    AccessToUnionField,
+    MutationOfLayoutConstrainedField,
+    BorrowOfLayoutConstrainedField,
+    CallToFunctionWith,
+}
+
+impl UnsafetyViolationDetails {
+    pub fn description_and_note(&self) -> (&'static str, &'static str) {
+        use UnsafetyViolationDetails::*;
+        match self {
+            CallToUnsafeFunction => (
+                "call to unsafe function",
+                "consult the function's documentation for information on how to avoid undefined \
+                 behavior",
+            ),
+            UseOfInlineAssembly => (
+                "use of inline assembly",
+                "inline assembly is entirely unchecked and can cause undefined behavior",
+            ),
+            InitializingTypeWith => (
+                "initializing type with `rustc_layout_scalar_valid_range` attr",
+                "initializing a layout restricted type's field with a value outside the valid \
+                 range is undefined behavior",
+            ),
+            CastOfPointerToInt => {
+                ("cast of pointer to int", "casting pointers to integers in constants")
+            }
+            UseOfMutableStatic => (
+                "use of mutable static",
+                "mutable statics can be mutated by multiple threads: aliasing violations or data \
+                 races will cause undefined behavior",
+            ),
+            UseOfExternStatic => (
+                "use of extern static",
+                "extern statics are not controlled by the Rust type system: invalid data, \
+                 aliasing violations or data races will cause undefined behavior",
+            ),
+            DerefOfRawPointer => (
+                "dereference of raw pointer",
+                "raw pointers may be null, dangling or unaligned; they can violate aliasing rules \
+                 and cause data races: all of these are undefined behavior",
+            ),
+            AccessToUnionField => (
+                "access to union field",
+                "the field may not be properly initialized: using uninitialized data will cause \
+                 undefined behavior",
+            ),
+            MutationOfLayoutConstrainedField => (
+                "mutation of layout constrained field",
+                "mutating layout constrained fields cannot statically be checked for valid values",
+            ),
+            BorrowOfLayoutConstrainedField => (
+                "borrow of layout constrained field with interior mutability",
+                "references to fields of layout constrained fields lose the constraints. Coupled \
+                 with interior mutability, the field can be changed to invalid values",
+            ),
+            CallToFunctionWith => (
+                "call to function with `#[target_feature]`",
+                "can only be called if the required target features are available",
+            ),
+        }
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, TyEncodable, TyDecodable, HashStable, Debug)]
+pub struct UnsafetyViolation {
+    pub source_info: SourceInfo,
+    pub lint_root: hir::HirId,
+    pub kind: UnsafetyViolationKind,
+    pub details: UnsafetyViolationDetails,
+}
+
+#[derive(Copy, Clone, PartialEq, TyEncodable, TyDecodable, HashStable, Debug)]
+pub enum UnusedUnsafe {
+    /// `unsafe` block contains no unsafe operations
+    /// > ``unnecessary `unsafe` block``
+    Unused,
+    /// `unsafe` block nested under another (used) `unsafe` block
+    /// > ``… because it's nested under this `unsafe` block``
+    InUnsafeBlock(hir::HirId),
+    /// `unsafe` block nested under `unsafe fn`
+    /// > ``… because it's nested under this `unsafe fn` ``
+    ///
+    /// the second HirId here indicates the first usage of the `unsafe` block,
+    /// which allows retrieval of the LintLevelSource for why that operation would
+    /// have been permitted without the block
+    InUnsafeFn(hir::HirId, hir::HirId),
+}
+
+#[derive(Copy, Clone, PartialEq, TyEncodable, TyDecodable, HashStable, Debug)]
+pub enum UsedUnsafeBlockData {
+    SomeDisallowedInUnsafeFn,
+    // the HirId here indicates the first usage of the `unsafe` block
+    // (i.e. the one that's first encountered in the MIR traversal of the unsafety check)
+    AllAllowedInUnsafeFn(hir::HirId),
+}
+
+#[derive(TyEncodable, TyDecodable, HashStable, Debug)]
+pub struct UnsafetyCheckResult {
+    /// Violations that are propagated *upwards* from this function.
+    pub violations: Vec<UnsafetyViolation>,
+
+    /// Used `unsafe` blocks in this function. This is used for the "unused_unsafe" lint.
+    ///
+    /// The keys are the used `unsafe` blocks, the UnusedUnsafeKind indicates whether
+    /// or not any of the usages happen at a place that doesn't allow `unsafe_op_in_unsafe_fn`.
+    pub used_unsafe_blocks: FxHashMap<hir::HirId, UsedUnsafeBlockData>,
+
+    /// This is `Some` iff the item is not a closure.
+    pub unused_unsafes: Option<Vec<(hir::HirId, UnusedUnsafe)>>,
+}
+
+rustc_index::newtype_index! {
+    pub struct GeneratorSavedLocal {
+        derive [HashStable]
+        DEBUG_FORMAT = "_{}",
+    }
+}
+
+/// The layout of generator state.
+#[derive(Clone, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub struct GeneratorLayout<'tcx> {
+    /// The type of every local stored inside the generator.
+    pub field_tys: IndexVec<GeneratorSavedLocal, Ty<'tcx>>,
+
+    /// Which of the above fields are in each variant. Note that one field may
+    /// be stored in multiple variants.
+    pub variant_fields: IndexVec<VariantIdx, IndexVec<Field, GeneratorSavedLocal>>,
+
+    /// The source that led to each variant being created (usually, a yield or
+    /// await).
+    pub variant_source_info: IndexVec<VariantIdx, SourceInfo>,
+
+    /// Which saved locals are storage-live at the same time. Locals that do not
+    /// have conflicts with each other are allowed to overlap in the computed
+    /// layout.
+    pub storage_conflicts: BitMatrix<GeneratorSavedLocal, GeneratorSavedLocal>,
+}
+
+impl Debug for GeneratorLayout<'_> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        /// Prints an iterator of (key, value) tuples as a map.
+        struct MapPrinter<'a, K, V>(Cell<Option<Box<dyn Iterator<Item = (K, V)> + 'a>>>);
+        impl<'a, K, V> MapPrinter<'a, K, V> {
+            fn new(iter: impl Iterator<Item = (K, V)> + 'a) -> Self {
+                Self(Cell::new(Some(Box::new(iter))))
+            }
+        }
+        impl<'a, K: Debug, V: Debug> Debug for MapPrinter<'a, K, V> {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt.debug_map().entries(self.0.take().unwrap()).finish()
+            }
+        }
+
+        /// Prints the generator variant name.
+        struct GenVariantPrinter(VariantIdx);
+        impl From<VariantIdx> for GenVariantPrinter {
+            fn from(idx: VariantIdx) -> Self {
+                GenVariantPrinter(idx)
+            }
+        }
+        impl Debug for GenVariantPrinter {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                let variant_name = ty::GeneratorSubsts::variant_name(self.0);
+                if fmt.alternate() {
+                    write!(fmt, "{:9}({:?})", variant_name, self.0)
+                } else {
+                    write!(fmt, "{}", variant_name)
+                }
+            }
+        }
+
+        /// Forces its contents to print in regular mode instead of alternate mode.
+        struct OneLinePrinter<T>(T);
+        impl<T: Debug> Debug for OneLinePrinter<T> {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                write!(fmt, "{:?}", self.0)
+            }
+        }
+
+        fmt.debug_struct("GeneratorLayout")
+            .field("field_tys", &MapPrinter::new(self.field_tys.iter_enumerated()))
+            .field(
+                "variant_fields",
+                &MapPrinter::new(
+                    self.variant_fields
+                        .iter_enumerated()
+                        .map(|(k, v)| (GenVariantPrinter(k), OneLinePrinter(v))),
+                ),
+            )
+            .field("storage_conflicts", &self.storage_conflicts)
+            .finish()
+    }
+}
+
+#[derive(Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct BorrowCheckResult<'tcx> {
+    /// All the opaque types that are restricted to concrete types
+    /// by this function. Unlike the value in `TypeckResults`, this has
+    /// unerased regions.
+    pub concrete_opaque_types: VecMap<LocalDefId, OpaqueHiddenType<'tcx>>,
+    pub closure_requirements: Option<ClosureRegionRequirements<'tcx>>,
+    pub used_mut_upvars: SmallVec<[Field; 8]>,
+    pub tainted_by_errors: Option<ErrorGuaranteed>,
+}
+
+/// The result of the `mir_const_qualif` query.
+///
+/// Each field (except `error_occurred`) corresponds to an implementer of the `Qualif` trait in
+/// `rustc_const_eval/src/transform/check_consts/qualifs.rs`. See that file for more information on each
+/// `Qualif`.
+#[derive(Clone, Copy, Debug, Default, TyEncodable, TyDecodable, HashStable)]
+pub struct ConstQualifs {
+    pub has_mut_interior: bool,
+    pub needs_drop: bool,
+    pub needs_non_const_drop: bool,
+    pub custom_eq: bool,
+    pub tainted_by_errors: Option<ErrorGuaranteed>,
+}
+
+/// After we borrow check a closure, we are left with various
+/// requirements that we have inferred between the free regions that
+/// appear in the closure's signature or on its field types. These
+/// requirements are then verified and proved by the closure's
+/// creating function. This struct encodes those requirements.
+///
+/// The requirements are listed as being between various `RegionVid`. The 0th
+/// region refers to `'static`; subsequent region vids refer to the free
+/// regions that appear in the closure (or generator's) type, in order of
+/// appearance. (This numbering is actually defined by the `UniversalRegions`
+/// struct in the NLL region checker. See for example
+/// `UniversalRegions::closure_mapping`.) Note the free regions in the
+/// closure's signature and captures are erased.
+///
+/// Example: If type check produces a closure with the closure substs:
+///
+/// ```text
+/// ClosureSubsts = [
+///     'a,                                         // From the parent.
+///     'b,
+///     i8,                                         // the "closure kind"
+///     for<'x> fn(&'<erased> &'x u32) -> &'x u32,  // the "closure signature"
+///     &'<erased> String,                          // some upvar
+/// ]
+/// ```
+///
+/// We would "renumber" each free region to a unique vid, as follows:
+///
+/// ```text
+/// ClosureSubsts = [
+///     '1,                                         // From the parent.
+///     '2,
+///     i8,                                         // the "closure kind"
+///     for<'x> fn(&'3 &'x u32) -> &'x u32,         // the "closure signature"
+///     &'4 String,                                 // some upvar
+/// ]
+/// ```
+///
+/// Now the code might impose a requirement like `'1: '2`. When an
+/// instance of the closure is created, the corresponding free regions
+/// can be extracted from its type and constrained to have the given
+/// outlives relationship.
+///
+/// In some cases, we have to record outlives requirements between types and
+/// regions as well. In that case, if those types include any regions, those
+/// regions are recorded using their external names (`ReStatic`,
+/// `ReEarlyBound`, `ReFree`). We use these because in a query response we
+/// cannot use `ReVar` (which is what we use internally within the rest of the
+/// NLL code).
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct ClosureRegionRequirements<'tcx> {
+    /// The number of external regions defined on the closure. In our
+    /// example above, it would be 3 -- one for `'static`, then `'1`
+    /// and `'2`. This is just used for a sanity check later on, to
+    /// make sure that the number of regions we see at the callsite
+    /// matches.
+    pub num_external_vids: usize,
+
+    /// Requirements between the various free regions defined in
+    /// indices.
+    pub outlives_requirements: Vec<ClosureOutlivesRequirement<'tcx>>,
+}
+
+/// Indicates an outlives-constraint between a type or between two
+/// free regions declared on the closure.
+#[derive(Copy, Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct ClosureOutlivesRequirement<'tcx> {
+    // This region or type ...
+    pub subject: ClosureOutlivesSubject<'tcx>,
+
+    // ... must outlive this one.
+    pub outlived_free_region: ty::RegionVid,
+
+    // If not, report an error here ...
+    pub blame_span: Span,
+
+    // ... due to this reason.
+    pub category: ConstraintCategory<'tcx>,
+}
+
+// Make sure this enum doesn't unintentionally grow
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+rustc_data_structures::static_assert_size!(ConstraintCategory<'_>, 16);
+
+/// Outlives-constraints can be categorized to determine whether and why they
+/// are interesting (for error reporting). Order of variants indicates sort
+/// order of the category, thereby influencing diagnostic output.
+///
+/// See also `rustc_const_eval::borrow_check::constraints`.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash)]
+#[derive(TyEncodable, TyDecodable, HashStable)]
+pub enum ConstraintCategory<'tcx> {
+    Return(ReturnConstraint),
+    Yield,
+    UseAsConst,
+    UseAsStatic,
+    TypeAnnotation,
+    Cast,
+
+    /// A constraint that came from checking the body of a closure.
+    ///
+    /// We try to get the category that the closure used when reporting this.
+    ClosureBounds,
+
+    /// Contains the function type if available.
+    CallArgument(Option<Ty<'tcx>>),
+    CopyBound,
+    SizedBound,
+    Assignment,
+    /// A constraint that came from a usage of a variable (e.g. in an ADT expression
+    /// like `Foo { field: my_val }`)
+    Usage,
+    OpaqueType,
+    ClosureUpvar(Field),
+
+    /// A constraint from a user-written predicate
+    /// with the provided span, written on the item
+    /// with the given `DefId`
+    Predicate(Span),
+
+    /// A "boring" constraint (caused by the given location) is one that
+    /// the user probably doesn't want to see described in diagnostics,
+    /// because it is kind of an artifact of the type system setup.
+    Boring,
+    // Boring and applicable everywhere.
+    BoringNoLocation,
+
+    /// A constraint that doesn't correspond to anything the user sees.
+    Internal,
+}
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash)]
+#[derive(TyEncodable, TyDecodable, HashStable)]
+pub enum ReturnConstraint {
+    Normal,
+    ClosureUpvar(Field),
+}
+
+/// The subject of a `ClosureOutlivesRequirement` -- that is, the thing
+/// that must outlive some region.
+#[derive(Copy, Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub enum ClosureOutlivesSubject<'tcx> {
+    /// Subject is a type, typically a type parameter, but could also
+    /// be a projection. Indicates a requirement like `T: 'a` being
+    /// passed to the caller, where the type here is `T`.
+    ///
+    /// The type here is guaranteed not to contain any free regions at
+    /// present.
+    Ty(Ty<'tcx>),
+
+    /// Subject is a free region from the closure. Indicates a requirement
+    /// like `'a: 'b` being passed to the caller; the region here is `'a`.
+    Region(ty::RegionVid),
+}
+
+/// The constituent parts of a type level constant of kind ADT or array.
+#[derive(Copy, Clone, Debug, HashStable)]
+pub struct DestructuredConst<'tcx> {
+    pub variant: Option<VariantIdx>,
+    pub fields: &'tcx [ty::Const<'tcx>],
+}
+
+/// The constituent parts of a mir constant of kind ADT or array.
+#[derive(Copy, Clone, Debug, HashStable)]
+pub struct DestructuredMirConstant<'tcx> {
+    pub variant: Option<VariantIdx>,
+    pub fields: &'tcx [ConstantKind<'tcx>],
+}
+
+/// Coverage information summarized from a MIR if instrumented for source code coverage (see
+/// compiler option `-Cinstrument-coverage`). This information is generated by the
+/// `InstrumentCoverage` MIR pass and can be retrieved via the `coverageinfo` query.
+#[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable)]
+pub struct CoverageInfo {
+    /// The total number of coverage region counters added to the MIR `Body`.
+    pub num_counters: u32,
+
+    /// The total number of coverage region counter expressions added to the MIR `Body`.
+    pub num_expressions: u32,
+}
+
+/// Shims which make dealing with `WithOptConstParam` easier.
+///
+/// For more information on why this is needed, consider looking
+/// at the docs for `WithOptConstParam` itself.
+impl<'tcx> TyCtxt<'tcx> {
+    #[inline]
+    pub fn mir_const_qualif_opt_const_arg(
+        self,
+        def: ty::WithOptConstParam<LocalDefId>,
+    ) -> ConstQualifs {
+        if let Some(param_did) = def.const_param_did {
+            self.mir_const_qualif_const_arg((def.did, param_did))
+        } else {
+            self.mir_const_qualif(def.did)
+        }
+    }
+
+    #[inline]
+    pub fn promoted_mir_opt_const_arg(
+        self,
+        def: ty::WithOptConstParam<DefId>,
+    ) -> &'tcx IndexVec<Promoted, Body<'tcx>> {
+        if let Some((did, param_did)) = def.as_const_arg() {
+            self.promoted_mir_of_const_arg((did, param_did))
+        } else {
+            self.promoted_mir(def.did)
+        }
+    }
+
+    #[inline]
+    pub fn mir_for_ctfe_opt_const_arg(self, def: ty::WithOptConstParam<DefId>) -> &'tcx Body<'tcx> {
+        if let Some((did, param_did)) = def.as_const_arg() {
+            self.mir_for_ctfe_of_const_arg((did, param_did))
+        } else {
+            self.mir_for_ctfe(def.did)
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/spanview.rs b/compiler/rustc_middle/src/mir/spanview.rs
new file mode 100644
index 000000000..4418b848e
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/spanview.rs
@@ -0,0 +1,691 @@
+use rustc_hir::def_id::DefId;
+use rustc_middle::hir;
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+use rustc_session::config::MirSpanview;
+use rustc_span::{BytePos, Pos, Span, SyntaxContext};
+
+use std::cmp;
+use std::io::{self, Write};
+
+pub const TOOLTIP_INDENT: &str = "    ";
+
+const CARET: char = '\u{2038}'; // Unicode `CARET`
+const ANNOTATION_LEFT_BRACKET: char = '\u{298a}'; // Unicode `Z NOTATION RIGHT BINDING BRACKET
+const ANNOTATION_RIGHT_BRACKET: char = '\u{2989}'; // Unicode `Z NOTATION LEFT BINDING BRACKET`
+const NEW_LINE_SPAN: &str = "</span>\n<span class=\"line\">";
+const HEADER: &str = r#"<!DOCTYPE html>
+<html>
+<head>"#;
+const START_BODY: &str = r#"</head>
+<body>"#;
+const FOOTER: &str = r#"</body>
+</html>"#;
+
+const STYLE_SECTION: &str = r#"<style>
+    .line {
+        counter-increment: line;
+    }
+    .line:before {
+        content: counter(line) ": ";
+        font-family: Menlo, Monaco, monospace;
+        font-style: italic;
+        width: 3.8em;
+        display: inline-block;
+        text-align: right;
+        filter: opacity(50%);
+        -webkit-user-select: none;
+    }
+    .code {
+        color: #dddddd;
+        background-color: #222222;
+        font-family: Menlo, Monaco, monospace;
+        line-height: 1.4em;
+        border-bottom: 2px solid #222222;
+        white-space: pre;
+        display: inline-block;
+    }
+    .odd {
+        background-color: #55bbff;
+        color: #223311;
+    }
+    .even {
+        background-color: #ee7756;
+        color: #551133;
+    }
+    .code {
+        --index: calc(var(--layer) - 1);
+        padding-top: calc(var(--index) * 0.15em);
+        filter:
+            hue-rotate(calc(var(--index) * 25deg))
+            saturate(calc(100% - (var(--index) * 2%)))
+            brightness(calc(100% - (var(--index) * 1.5%)));
+    }
+    .annotation {
+        color: #4444ff;
+        font-family: monospace;
+        font-style: italic;
+        display: none;
+        -webkit-user-select: none;
+    }
+    body:active .annotation {
+        /* requires holding mouse down anywhere on the page */
+        display: inline-block;
+    }
+    span:hover .annotation {
+        /* requires hover over a span ONLY on its first line */
+        display: inline-block;
+    }
+</style>"#;
+
+/// Metadata to highlight the span of a MIR BasicBlock, Statement, or Terminator.
+#[derive(Clone, Debug)]
+pub struct SpanViewable {
+    pub bb: BasicBlock,
+    pub span: Span,
+    pub id: String,
+    pub tooltip: String,
+}
+
+/// Write a spanview HTML+CSS file to analyze MIR element spans.
+pub fn write_mir_fn_spanview<'tcx, W>(
+    tcx: TyCtxt<'tcx>,
+    body: &Body<'tcx>,
+    spanview: MirSpanview,
+    title: &str,
+    w: &mut W,
+) -> io::Result<()>
+where
+    W: Write,
+{
+    let def_id = body.source.def_id();
+    let hir_body = hir_body(tcx, def_id);
+    if hir_body.is_none() {
+        return Ok(());
+    }
+    let body_span = hir_body.unwrap().value.span;
+    let mut span_viewables = Vec::new();
+    for (bb, data) in body.basic_blocks().iter_enumerated() {
+        match spanview {
+            MirSpanview::Statement => {
+                for (i, statement) in data.statements.iter().enumerate() {
+                    if let Some(span_viewable) =
+                        statement_span_viewable(tcx, body_span, bb, i, statement)
+                    {
+                        span_viewables.push(span_viewable);
+                    }
+                }
+                if let Some(span_viewable) = terminator_span_viewable(tcx, body_span, bb, data) {
+                    span_viewables.push(span_viewable);
+                }
+            }
+            MirSpanview::Terminator => {
+                if let Some(span_viewable) = terminator_span_viewable(tcx, body_span, bb, data) {
+                    span_viewables.push(span_viewable);
+                }
+            }
+            MirSpanview::Block => {
+                if let Some(span_viewable) = block_span_viewable(tcx, body_span, bb, data) {
+                    span_viewables.push(span_viewable);
+                }
+            }
+        }
+    }
+    write_document(tcx, fn_span(tcx, def_id), span_viewables, title, w)?;
+    Ok(())
+}
+
+/// Generate a spanview HTML+CSS document for the given local function `def_id`, and a pre-generated
+/// list `SpanViewable`s.
+pub fn write_document<'tcx, W>(
+    tcx: TyCtxt<'tcx>,
+    spanview_span: Span,
+    mut span_viewables: Vec<SpanViewable>,
+    title: &str,
+    w: &mut W,
+) -> io::Result<()>
+where
+    W: Write,
+{
+    let mut from_pos = spanview_span.lo();
+    let end_pos = spanview_span.hi();
+    let source_map = tcx.sess.source_map();
+    let start = source_map.lookup_char_pos(from_pos);
+    let indent_to_initial_start_col = " ".repeat(start.col.to_usize());
+    debug!(
+        "spanview_span={:?}; source is:\n{}{}",
+        spanview_span,
+        indent_to_initial_start_col,
+        source_map.span_to_snippet(spanview_span).expect("function should have printable source")
+    );
+    writeln!(w, "{}", HEADER)?;
+    writeln!(w, "<title>{}</title>", title)?;
+    writeln!(w, "{}", STYLE_SECTION)?;
+    writeln!(w, "{}", START_BODY)?;
+    write!(
+        w,
+        r#"<div class="code" style="counter-reset: line {}"><span class="line">{}"#,
+        start.line - 1,
+        indent_to_initial_start_col,
+    )?;
+    span_viewables.sort_unstable_by(|a, b| {
+        let a = a.span;
+        let b = b.span;
+        if a.lo() == b.lo() {
+            // Sort hi() in reverse order so shorter spans are attempted after longer spans.
+            // This should give shorter spans a higher "layer", so they are not covered by
+            // the longer spans.
+            b.hi().partial_cmp(&a.hi())
+        } else {
+            a.lo().partial_cmp(&b.lo())
+        }
+        .unwrap()
+    });
+    let mut ordered_viewables = &span_viewables[..];
+    const LOWEST_VIEWABLE_LAYER: usize = 1;
+    let mut alt = false;
+    while ordered_viewables.len() > 0 {
+        debug!(
+            "calling write_next_viewable with from_pos={}, end_pos={}, and viewables len={}",
+            from_pos.to_usize(),
+            end_pos.to_usize(),
+            ordered_viewables.len()
+        );
+        let curr_id = &ordered_viewables[0].id;
+        let (next_from_pos, next_ordered_viewables) = write_next_viewable_with_overlaps(
+            tcx,
+            from_pos,
+            end_pos,
+            ordered_viewables,
+            alt,
+            LOWEST_VIEWABLE_LAYER,
+            w,
+        )?;
+        debug!(
+            "DONE calling write_next_viewable, with new from_pos={}, \
+             and remaining viewables len={}",
+            next_from_pos.to_usize(),
+            next_ordered_viewables.len()
+        );
+        assert!(
+            from_pos != next_from_pos || ordered_viewables.len() != next_ordered_viewables.len(),
+            "write_next_viewable_with_overlaps() must make a state change"
+        );
+        from_pos = next_from_pos;
+        if next_ordered_viewables.len() != ordered_viewables.len() {
+            ordered_viewables = next_ordered_viewables;
+            if let Some(next_ordered_viewable) = ordered_viewables.first() {
+                if &next_ordered_viewable.id != curr_id {
+                    alt = !alt;
+                }
+            }
+        }
+    }
+    if from_pos < end_pos {
+        write_coverage_gap(tcx, from_pos, end_pos, w)?;
+    }
+    writeln!(w, r#"</span></div>"#)?;
+    writeln!(w, "{}", FOOTER)?;
+    Ok(())
+}
+
+/// Format a string showing the start line and column, and end line and column within a file.
+pub fn source_range_no_file<'tcx>(tcx: TyCtxt<'tcx>, span: Span) -> String {
+    let source_map = tcx.sess.source_map();
+    let start = source_map.lookup_char_pos(span.lo());
+    let end = source_map.lookup_char_pos(span.hi());
+    format!("{}:{}-{}:{}", start.line, start.col.to_usize() + 1, end.line, end.col.to_usize() + 1)
+}
+
+pub fn statement_kind_name(statement: &Statement<'_>) -> &'static str {
+    use StatementKind::*;
+    match statement.kind {
+        Assign(..) => "Assign",
+        FakeRead(..) => "FakeRead",
+        SetDiscriminant { .. } => "SetDiscriminant",
+        Deinit(..) => "Deinit",
+        StorageLive(..) => "StorageLive",
+        StorageDead(..) => "StorageDead",
+        Retag(..) => "Retag",
+        AscribeUserType(..) => "AscribeUserType",
+        Coverage(..) => "Coverage",
+        CopyNonOverlapping(..) => "CopyNonOverlapping",
+        Nop => "Nop",
+    }
+}
+
+pub fn terminator_kind_name(term: &Terminator<'_>) -> &'static str {
+    use TerminatorKind::*;
+    match term.kind {
+        Goto { .. } => "Goto",
+        SwitchInt { .. } => "SwitchInt",
+        Resume => "Resume",
+        Abort => "Abort",
+        Return => "Return",
+        Unreachable => "Unreachable",
+        Drop { .. } => "Drop",
+        DropAndReplace { .. } => "DropAndReplace",
+        Call { .. } => "Call",
+        Assert { .. } => "Assert",
+        Yield { .. } => "Yield",
+        GeneratorDrop => "GeneratorDrop",
+        FalseEdge { .. } => "FalseEdge",
+        FalseUnwind { .. } => "FalseUnwind",
+        InlineAsm { .. } => "InlineAsm",
+    }
+}
+
+fn statement_span_viewable<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    body_span: Span,
+    bb: BasicBlock,
+    i: usize,
+    statement: &Statement<'tcx>,
+) -> Option<SpanViewable> {
+    let span = statement.source_info.span;
+    if !body_span.contains(span) {
+        return None;
+    }
+    let id = format!("{}[{}]", bb.index(), i);
+    let tooltip = tooltip(tcx, &id, span, vec![statement.clone()], &None);
+    Some(SpanViewable { bb, span, id, tooltip })
+}
+
+fn terminator_span_viewable<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    body_span: Span,
+    bb: BasicBlock,
+    data: &BasicBlockData<'tcx>,
+) -> Option<SpanViewable> {
+    let term = data.terminator();
+    let span = term.source_info.span;
+    if !body_span.contains(span) {
+        return None;
+    }
+    let id = format!("{}:{}", bb.index(), terminator_kind_name(term));
+    let tooltip = tooltip(tcx, &id, span, vec![], &data.terminator);
+    Some(SpanViewable { bb, span, id, tooltip })
+}
+
+fn block_span_viewable<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    body_span: Span,
+    bb: BasicBlock,
+    data: &BasicBlockData<'tcx>,
+) -> Option<SpanViewable> {
+    let span = compute_block_span(data, body_span);
+    if !body_span.contains(span) {
+        return None;
+    }
+    let id = format!("{}", bb.index());
+    let tooltip = tooltip(tcx, &id, span, data.statements.clone(), &data.terminator);
+    Some(SpanViewable { bb, span, id, tooltip })
+}
+
+fn compute_block_span<'tcx>(data: &BasicBlockData<'tcx>, body_span: Span) -> Span {
+    let mut span = data.terminator().source_info.span;
+    for statement_span in data.statements.iter().map(|statement| statement.source_info.span) {
+        // Only combine Spans from the root context, and within the function's body_span.
+        if statement_span.ctxt() == SyntaxContext::root() && body_span.contains(statement_span) {
+            span = span.to(statement_span);
+        }
+    }
+    span
+}
+
+/// Recursively process each ordered span. Spans that overlap will have progressively varying
+/// styles, such as increased padding for each overlap. Non-overlapping adjacent spans will
+/// have alternating style choices, to help distinguish between them if, visually adjacent.
+/// The `layer` is incremented for each overlap, and the `alt` bool alternates between true
+/// and false, for each adjacent non-overlapping span. Source code between the spans (code
+/// that is not in any coverage region) has neutral styling.
+fn write_next_viewable_with_overlaps<'tcx, 'b, W>(
+    tcx: TyCtxt<'tcx>,
+    mut from_pos: BytePos,
+    mut to_pos: BytePos,
+    ordered_viewables: &'b [SpanViewable],
+    alt: bool,
+    layer: usize,
+    w: &mut W,
+) -> io::Result<(BytePos, &'b [SpanViewable])>
+where
+    W: Write,
+{
+    let debug_indent = "  ".repeat(layer);
+    let (viewable, mut remaining_viewables) =
+        ordered_viewables.split_first().expect("ordered_viewables should have some");
+
+    if from_pos < viewable.span.lo() {
+        debug!(
+            "{}advance from_pos to next SpanViewable (from from_pos={} to viewable.span.lo()={} \
+             of {:?}), with to_pos={}",
+            debug_indent,
+            from_pos.to_usize(),
+            viewable.span.lo().to_usize(),
+            viewable.span,
+            to_pos.to_usize()
+        );
+        let hi = cmp::min(viewable.span.lo(), to_pos);
+        write_coverage_gap(tcx, from_pos, hi, w)?;
+        from_pos = hi;
+        if from_pos < viewable.span.lo() {
+            debug!(
+                "{}EARLY RETURN: stopped before getting to next SpanViewable, at {}",
+                debug_indent,
+                from_pos.to_usize()
+            );
+            return Ok((from_pos, ordered_viewables));
+        }
+    }
+
+    if from_pos < viewable.span.hi() {
+        // Set to_pos to the end of this `viewable` to ensure the recursive calls stop writing
+        // with room to print the tail.
+        to_pos = cmp::min(viewable.span.hi(), to_pos);
+        debug!(
+            "{}update to_pos (if not closer) to viewable.span.hi()={}; to_pos is now {}",
+            debug_indent,
+            viewable.span.hi().to_usize(),
+            to_pos.to_usize()
+        );
+    }
+
+    let mut subalt = false;
+    while remaining_viewables.len() > 0 && remaining_viewables[0].span.overlaps(viewable.span) {
+        let overlapping_viewable = &remaining_viewables[0];
+        debug!("{}overlapping_viewable.span={:?}", debug_indent, overlapping_viewable.span);
+
+        let span =
+            trim_span(viewable.span, from_pos, cmp::min(overlapping_viewable.span.lo(), to_pos));
+        let mut some_html_snippet = if from_pos <= viewable.span.hi() || viewable.span.is_empty() {
+            // `viewable` is not yet fully rendered, so start writing the span, up to either the
+            // `to_pos` or the next `overlapping_viewable`, whichever comes first.
+            debug!(
+                "{}make html_snippet (may not write it if early exit) for partial span {:?} \
+                 of viewable.span {:?}",
+                debug_indent, span, viewable.span
+            );
+            from_pos = span.hi();
+            make_html_snippet(tcx, span, Some(&viewable))
+        } else {
+            None
+        };
+
+        // Defer writing the HTML snippet (until after early return checks) ONLY for empty spans.
+        // An empty Span with Some(html_snippet) is probably a tail marker. If there is an early
+        // exit, there should be another opportunity to write the tail marker.
+        if !span.is_empty() {
+            if let Some(ref html_snippet) = some_html_snippet {
+                debug!(
+                    "{}write html_snippet for that partial span of viewable.span {:?}",
+                    debug_indent, viewable.span
+                );
+                write_span(html_snippet, &viewable.tooltip, alt, layer, w)?;
+            }
+            some_html_snippet = None;
+        }
+
+        if from_pos < overlapping_viewable.span.lo() {
+            debug!(
+                "{}EARLY RETURN: from_pos={} has not yet reached the \
+                 overlapping_viewable.span {:?}",
+                debug_indent,
+                from_pos.to_usize(),
+                overlapping_viewable.span
+            );
+            // must have reached `to_pos` before reaching the start of the
+            // `overlapping_viewable.span`
+            return Ok((from_pos, ordered_viewables));
+        }
+
+        if from_pos == to_pos
+            && !(from_pos == overlapping_viewable.span.lo() && overlapping_viewable.span.is_empty())
+        {
+            debug!(
+                "{}EARLY RETURN: from_pos=to_pos={} and overlapping_viewable.span {:?} is not \
+                 empty, or not from_pos",
+                debug_indent,
+                to_pos.to_usize(),
+                overlapping_viewable.span
+            );
+            // `to_pos` must have occurred before the overlapping viewable. Return
+            // `ordered_viewables` so we can continue rendering the `viewable`, from after the
+            // `to_pos`.
+            return Ok((from_pos, ordered_viewables));
+        }
+
+        if let Some(ref html_snippet) = some_html_snippet {
+            debug!(
+                "{}write html_snippet for that partial span of viewable.span {:?}",
+                debug_indent, viewable.span
+            );
+            write_span(html_snippet, &viewable.tooltip, alt, layer, w)?;
+        }
+
+        debug!(
+            "{}recursively calling write_next_viewable with from_pos={}, to_pos={}, \
+             and viewables len={}",
+            debug_indent,
+            from_pos.to_usize(),
+            to_pos.to_usize(),
+            remaining_viewables.len()
+        );
+        // Write the overlaps (and the overlaps' overlaps, if any) up to `to_pos`.
+        let curr_id = &remaining_viewables[0].id;
+        let (next_from_pos, next_remaining_viewables) = write_next_viewable_with_overlaps(
+            tcx,
+            from_pos,
+            to_pos,
+            &remaining_viewables,
+            subalt,
+            layer + 1,
+            w,
+        )?;
+        debug!(
+            "{}DONE recursively calling write_next_viewable, with new from_pos={}, and remaining \
+             viewables len={}",
+            debug_indent,
+            next_from_pos.to_usize(),
+            next_remaining_viewables.len()
+        );
+        assert!(
+            from_pos != next_from_pos
+                || remaining_viewables.len() != next_remaining_viewables.len(),
+            "write_next_viewable_with_overlaps() must make a state change"
+        );
+        from_pos = next_from_pos;
+        if next_remaining_viewables.len() != remaining_viewables.len() {
+            remaining_viewables = next_remaining_viewables;
+            if let Some(next_ordered_viewable) = remaining_viewables.first() {
+                if &next_ordered_viewable.id != curr_id {
+                    subalt = !subalt;
+                }
+            }
+        }
+    }
+    if from_pos <= viewable.span.hi() {
+        let span = trim_span(viewable.span, from_pos, to_pos);
+        debug!(
+            "{}After overlaps, writing (end span?) {:?} of viewable.span {:?}",
+            debug_indent, span, viewable.span
+        );
+        if let Some(ref html_snippet) = make_html_snippet(tcx, span, Some(&viewable)) {
+            from_pos = span.hi();
+            write_span(html_snippet, &viewable.tooltip, alt, layer, w)?;
+        }
+    }
+    debug!("{}RETURN: No more overlap", debug_indent);
+    Ok((
+        from_pos,
+        if from_pos < viewable.span.hi() { ordered_viewables } else { remaining_viewables },
+    ))
+}
+
+#[inline(always)]
+fn write_coverage_gap<'tcx, W>(
+    tcx: TyCtxt<'tcx>,
+    lo: BytePos,
+    hi: BytePos,
+    w: &mut W,
+) -> io::Result<()>
+where
+    W: Write,
+{
+    let span = Span::with_root_ctxt(lo, hi);
+    if let Some(ref html_snippet) = make_html_snippet(tcx, span, None) {
+        write_span(html_snippet, "", false, 0, w)
+    } else {
+        Ok(())
+    }
+}
+
+fn write_span<W>(
+    html_snippet: &str,
+    tooltip: &str,
+    alt: bool,
+    layer: usize,
+    w: &mut W,
+) -> io::Result<()>
+where
+    W: Write,
+{
+    let maybe_alt_class = if layer > 0 {
+        if alt { " odd" } else { " even" }
+    } else {
+        ""
+    };
+    let maybe_title_attr = if !tooltip.is_empty() {
+        format!(" title=\"{}\"", escape_attr(tooltip))
+    } else {
+        "".to_owned()
+    };
+    if layer == 1 {
+        write!(w, "<span>")?;
+    }
+    for (i, line) in html_snippet.lines().enumerate() {
+        if i > 0 {
+            write!(w, "{}", NEW_LINE_SPAN)?;
+        }
+        write!(
+            w,
+            r#"<span class="code{}" style="--layer: {}"{}>{}</span>"#,
+            maybe_alt_class, layer, maybe_title_attr, line
+        )?;
+    }
+    // Check for and translate trailing newlines, because `str::lines()` ignores them
+    if html_snippet.ends_with('\n') {
+        write!(w, "{}", NEW_LINE_SPAN)?;
+    }
+    if layer == 1 {
+        write!(w, "</span>")?;
+    }
+    Ok(())
+}
+
+fn make_html_snippet<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    span: Span,
+    some_viewable: Option<&SpanViewable>,
+) -> Option<String> {
+    let source_map = tcx.sess.source_map();
+    let snippet = source_map
+        .span_to_snippet(span)
+        .unwrap_or_else(|err| bug!("span_to_snippet error for span {:?}: {:?}", span, err));
+    let html_snippet = if let Some(viewable) = some_viewable {
+        let is_head = span.lo() == viewable.span.lo();
+        let is_tail = span.hi() == viewable.span.hi();
+        let mut labeled_snippet = if is_head {
+            format!(r#"<span class="annotation">{}{}</span>"#, viewable.id, ANNOTATION_LEFT_BRACKET)
+        } else {
+            "".to_owned()
+        };
+        if span.is_empty() {
+            if is_head && is_tail {
+                labeled_snippet.push(CARET);
+            }
+        } else {
+            labeled_snippet.push_str(&escape_html(&snippet));
+        };
+        if is_tail {
+            labeled_snippet.push_str(&format!(
+                r#"<span class="annotation">{}{}</span>"#,
+                ANNOTATION_RIGHT_BRACKET, viewable.id
+            ));
+        }
+        labeled_snippet
+    } else {
+        escape_html(&snippet)
+    };
+    if html_snippet.is_empty() { None } else { Some(html_snippet) }
+}
+
+fn tooltip<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    spanview_id: &str,
+    span: Span,
+    statements: Vec<Statement<'tcx>>,
+    terminator: &Option<Terminator<'tcx>>,
+) -> String {
+    let source_map = tcx.sess.source_map();
+    let mut text = Vec::new();
+    text.push(format!("{}: {}:", spanview_id, &source_map.span_to_embeddable_string(span)));
+    for statement in statements {
+        let source_range = source_range_no_file(tcx, statement.source_info.span);
+        text.push(format!(
+            "\n{}{}: {}: {:?}",
+            TOOLTIP_INDENT,
+            source_range,
+            statement_kind_name(&statement),
+            statement
+        ));
+    }
+    if let Some(term) = terminator {
+        let source_range = source_range_no_file(tcx, term.source_info.span);
+        text.push(format!(
+            "\n{}{}: {}: {:?}",
+            TOOLTIP_INDENT,
+            source_range,
+            terminator_kind_name(term),
+            term.kind
+        ));
+    }
+    text.join("")
+}
+
+fn trim_span(span: Span, from_pos: BytePos, to_pos: BytePos) -> Span {
+    trim_span_hi(trim_span_lo(span, from_pos), to_pos)
+}
+
+fn trim_span_lo(span: Span, from_pos: BytePos) -> Span {
+    if from_pos <= span.lo() { span } else { span.with_lo(cmp::min(span.hi(), from_pos)) }
+}
+
+fn trim_span_hi(span: Span, to_pos: BytePos) -> Span {
+    if to_pos >= span.hi() { span } else { span.with_hi(cmp::max(span.lo(), to_pos)) }
+}
+
+fn fn_span<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> Span {
+    let fn_decl_span = tcx.def_span(def_id);
+    if let Some(body_span) = hir_body(tcx, def_id).map(|hir_body| hir_body.value.span) {
+        if fn_decl_span.eq_ctxt(body_span) { fn_decl_span.to(body_span) } else { body_span }
+    } else {
+        fn_decl_span
+    }
+}
+
+fn hir_body<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> Option<&'tcx rustc_hir::Body<'tcx>> {
+    let hir_node = tcx.hir().get_if_local(def_id).expect("expected DefId is local");
+    hir::map::associated_body(hir_node).map(|fn_body_id| tcx.hir().body(fn_body_id))
+}
+
+fn escape_html(s: &str) -> String {
+    s.replace('&', "&amp;").replace('<', "&lt;").replace('>', "&gt;")
+}
+
+fn escape_attr(s: &str) -> String {
+    s.replace('&', "&amp;")
+        .replace('\"', "&quot;")
+        .replace('\'', "&#39;")
+        .replace('<', "&lt;")
+        .replace('>', "&gt;")
+}
diff --git a/compiler/rustc_middle/src/mir/switch_sources.rs b/compiler/rustc_middle/src/mir/switch_sources.rs
new file mode 100644
index 000000000..b91c0c257
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/switch_sources.rs
@@ -0,0 +1,78 @@
+//! Lazily compute the inverse of each `SwitchInt`'s switch targets. Modeled after
+//! `Predecessors`/`PredecessorCache`.
+
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::OnceCell;
+use rustc_index::vec::IndexVec;
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+use smallvec::SmallVec;
+
+use crate::mir::{BasicBlock, BasicBlockData, Terminator, TerminatorKind};
+
+pub type SwitchSources = FxHashMap<(BasicBlock, BasicBlock), SmallVec<[Option<u128>; 1]>>;
+
+#[derive(Clone, Debug)]
+pub(super) struct SwitchSourceCache {
+    cache: OnceCell<SwitchSources>,
+}
+
+impl SwitchSourceCache {
+    #[inline]
+    pub(super) fn new() -> Self {
+        SwitchSourceCache { cache: OnceCell::new() }
+    }
+
+    /// Invalidates the switch source cache.
+    #[inline]
+    pub(super) fn invalidate(&mut self) {
+        self.cache = OnceCell::new();
+    }
+
+    /// Returns the switch sources for this MIR.
+    #[inline]
+    pub(super) fn compute(
+        &self,
+        basic_blocks: &IndexVec<BasicBlock, BasicBlockData<'_>>,
+    ) -> &SwitchSources {
+        self.cache.get_or_init(|| {
+            let mut switch_sources: SwitchSources = FxHashMap::default();
+            for (bb, data) in basic_blocks.iter_enumerated() {
+                if let Some(Terminator {
+                    kind: TerminatorKind::SwitchInt { targets, .. }, ..
+                }) = &data.terminator
+                {
+                    for (value, target) in targets.iter() {
+                        switch_sources.entry((target, bb)).or_default().push(Some(value));
+                    }
+                    switch_sources.entry((targets.otherwise(), bb)).or_default().push(None);
+                }
+            }
+
+            switch_sources
+        })
+    }
+}
+
+impl<S: Encoder> Encodable<S> for SwitchSourceCache {
+    #[inline]
+    fn encode(&self, _s: &mut S) {}
+}
+
+impl<D: Decoder> Decodable<D> for SwitchSourceCache {
+    #[inline]
+    fn decode(_: &mut D) -> Self {
+        Self::new()
+    }
+}
+
+impl<CTX> HashStable<CTX> for SwitchSourceCache {
+    #[inline]
+    fn hash_stable(&self, _: &mut CTX, _: &mut StableHasher) {
+        // do nothing
+    }
+}
+
+TrivialTypeTraversalAndLiftImpls! {
+    SwitchSourceCache,
+}
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
new file mode 100644
index 000000000..eb90169d0
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -0,0 +1,1168 @@
+//! This defines the syntax of MIR, i.e., the set of available MIR operations, and other definitions
+//! closely related to MIR semantics.
+//! This is in a dedicated file so that changes to this file can be reviewed more carefully.
+//! The intention is that this file only contains datatype declarations, no code.
+
+use super::{BasicBlock, Constant, Field, Local, SwitchTargets, UserTypeProjection};
+
+use crate::mir::coverage::{CodeRegion, CoverageKind};
+use crate::ty::adjustment::PointerCast;
+use crate::ty::subst::SubstsRef;
+use crate::ty::{self, List, Ty};
+use crate::ty::{Region, UserTypeAnnotationIndex};
+
+use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
+use rustc_hir::def_id::DefId;
+use rustc_hir::{self as hir};
+use rustc_hir::{self, GeneratorKind};
+use rustc_target::abi::VariantIdx;
+
+use rustc_ast::Mutability;
+use rustc_span::def_id::LocalDefId;
+use rustc_span::symbol::Symbol;
+use rustc_span::Span;
+use rustc_target::asm::InlineAsmRegOrRegClass;
+
+/// The various "big phases" that MIR goes through.
+///
+/// These phases all describe dialects of MIR. Since all MIR uses the same datastructures, the
+/// dialects forbid certain variants or values in certain phases. The sections below summarize the
+/// changes, but do not document them thoroughly. The full documentation is found in the appropriate
+/// documentation for the thing the change is affecting.
+///
+/// Warning: ordering of variants is significant.
+#[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(HashStable)]
+pub enum MirPhase {
+    /// The dialect of MIR used during all phases before `DropsLowered` is the same. This is also
+    /// the MIR that analysis such as borrowck uses.
+    ///
+    /// One important thing to remember about the behavior of this section of MIR is that drop terminators
+    /// (including drop and replace) are *conditional*. The elaborate drops pass will then replace each
+    /// instance of a drop terminator with a nop, an unconditional drop, or a drop conditioned on a drop
+    /// flag. Of course, this means that it is important that the drop elaboration can accurately recognize
+    /// when things are initialized and when things are de-initialized. That means any code running on this
+    /// version of MIR must be sure to produce output that drop elaboration can reason about. See the
+    /// section on the drop terminatorss for more details.
+    Built = 0,
+    // FIXME(oli-obk): it's unclear whether we still need this phase (and its corresponding query).
+    // We used to have this for pre-miri MIR based const eval.
+    Const = 1,
+    /// This phase checks the MIR for promotable elements and takes them out of the main MIR body
+    /// by creating a new MIR body per promoted element. After this phase (and thus the termination
+    /// of the `mir_promoted` query), these promoted elements are available in the `promoted_mir`
+    /// query.
+    ConstsPromoted = 2,
+    /// After this projections may only contain deref projections as the first element.
+    Derefered = 3,
+    /// Beginning with this phase, the following variants are disallowed:
+    /// * [`TerminatorKind::DropAndReplace`]
+    /// * [`TerminatorKind::FalseUnwind`]
+    /// * [`TerminatorKind::FalseEdge`]
+    /// * [`StatementKind::FakeRead`]
+    /// * [`StatementKind::AscribeUserType`]
+    /// * [`Rvalue::Ref`] with `BorrowKind::Shallow`
+    ///
+    /// And the following variant is allowed:
+    /// * [`StatementKind::Retag`]
+    ///
+    /// Furthermore, `Drop` now uses explicit drop flags visible in the MIR and reaching a `Drop`
+    /// terminator means that the auto-generated drop glue will be invoked. Also, `Copy` operands
+    /// are allowed for non-`Copy` types.
+    DropsLowered = 4,
+    /// Beginning with this phase, the following variant is disallowed:
+    /// * [`Rvalue::Aggregate`] for any `AggregateKind` except `Array`
+    ///
+    /// And the following variant is allowed:
+    /// * [`StatementKind::SetDiscriminant`]
+    Deaggregated = 5,
+    /// Before this phase, generators are in the "source code" form, featuring `yield` statements
+    /// and such. With this phase change, they are transformed into a proper state machine. Running
+    /// optimizations before this change can be potentially dangerous because the source code is to
+    /// some extent a "lie." In particular, `yield` terminators effectively make the value of all
+    /// locals visible to the caller. This means that dead store elimination before them, or code
+    /// motion across them, is not correct in general. This is also exasperated by type checking
+    /// having pre-computed a list of the types that it thinks are ok to be live across a yield
+    /// point - this is necessary to decide eg whether autotraits are implemented. Introducing new
+    /// types across a yield point will lead to ICEs becaues of this.
+    ///
+    /// Beginning with this phase, the following variants are disallowed:
+    /// * [`TerminatorKind::Yield`]
+    /// * [`TerminatorKind::GeneratorDrop`]
+    /// * [`ProjectionElem::Deref`] of `Box`
+    GeneratorsLowered = 6,
+    Optimized = 7,
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Borrow kinds
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, TyEncodable, TyDecodable)]
+#[derive(Hash, HashStable)]
+pub enum BorrowKind {
+    /// Data must be immutable and is aliasable.
+    Shared,
+
+    /// The immediately borrowed place must be immutable, but projections from
+    /// it don't need to be. For example, a shallow borrow of `a.b` doesn't
+    /// conflict with a mutable borrow of `a.b.c`.
+    ///
+    /// This is used when lowering matches: when matching on a place we want to
+    /// ensure that place have the same value from the start of the match until
+    /// an arm is selected. This prevents this code from compiling:
+    /// ```compile_fail,E0510
+    /// let mut x = &Some(0);
+    /// match *x {
+    ///     None => (),
+    ///     Some(_) if { x = &None; false } => (),
+    ///     Some(_) => (),
+    /// }
+    /// ```
+    /// This can't be a shared borrow because mutably borrowing (*x as Some).0
+    /// should not prevent `if let None = x { ... }`, for example, because the
+    /// mutating `(*x as Some).0` can't affect the discriminant of `x`.
+    /// We can also report errors with this kind of borrow differently.
+    Shallow,
+
+    /// Data must be immutable but not aliasable. This kind of borrow
+    /// cannot currently be expressed by the user and is used only in
+    /// implicit closure bindings. It is needed when the closure is
+    /// borrowing or mutating a mutable referent, e.g.:
+    /// ```
+    /// let mut z = 3;
+    /// let x: &mut isize = &mut z;
+    /// let y = || *x += 5;
+    /// ```
+    /// If we were to try to translate this closure into a more explicit
+    /// form, we'd encounter an error with the code as written:
+    /// ```compile_fail,E0594
+    /// struct Env<'a> { x: &'a &'a mut isize }
+    /// let mut z = 3;
+    /// let x: &mut isize = &mut z;
+    /// let y = (&mut Env { x: &x }, fn_ptr);  // Closure is pair of env and fn
+    /// fn fn_ptr(env: &mut Env) { **env.x += 5; }
+    /// ```
+    /// This is then illegal because you cannot mutate an `&mut` found
+    /// in an aliasable location. To solve, you'd have to translate with
+    /// an `&mut` borrow:
+    /// ```compile_fail,E0596
+    /// struct Env<'a> { x: &'a mut &'a mut isize }
+    /// let mut z = 3;
+    /// let x: &mut isize = &mut z;
+    /// let y = (&mut Env { x: &mut x }, fn_ptr); // changed from &x to &mut x
+    /// fn fn_ptr(env: &mut Env) { **env.x += 5; }
+    /// ```
+    /// Now the assignment to `**env.x` is legal, but creating a
+    /// mutable pointer to `x` is not because `x` is not mutable. We
+    /// could fix this by declaring `x` as `let mut x`. This is ok in
+    /// user code, if awkward, but extra weird for closures, since the
+    /// borrow is hidden.
+    ///
+    /// So we introduce a "unique imm" borrow -- the referent is
+    /// immutable, but not aliasable. This solves the problem. For
+    /// simplicity, we don't give users the way to express this
+    /// borrow, it's just used when translating closures.
+    Unique,
+
+    /// Data is mutable and not aliasable.
+    Mut {
+        /// `true` if this borrow arose from method-call auto-ref
+        /// (i.e., `adjustment::Adjust::Borrow`).
+        allow_two_phase_borrow: bool,
+    },
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Statements
+
+/// The various kinds of statements that can appear in MIR.
+///
+/// Not all of these are allowed at every [`MirPhase`]. Check the documentation there to see which
+/// ones you do not have to worry about. The MIR validator will generally enforce such restrictions,
+/// causing an ICE if they are violated.
+#[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub enum StatementKind<'tcx> {
+    /// Assign statements roughly correspond to an assignment in Rust proper (`x = ...`) except
+    /// without the possibility of dropping the previous value (that must be done separately, if at
+    /// all). The *exact* way this works is undecided. It probably does something like evaluating
+    /// the LHS to a place and the RHS to a value, and then storing the value to the place. Various
+    /// parts of this may do type specific things that are more complicated than simply copying
+    /// bytes.
+    ///
+    /// **Needs clarification**: The implication of the above idea would be that assignment implies
+    /// that the resulting value is initialized. I believe we could commit to this separately from
+    /// committing to whatever part of the memory model we would need to decide on to make the above
+    /// paragragh precise. Do we want to?
+    ///
+    /// Assignments in which the types of the place and rvalue differ are not well-formed.
+    ///
+    /// **Needs clarification**: Do we ever want to worry about non-free (in the body) lifetimes for
+    /// the typing requirement in post drop-elaboration MIR? I think probably not - I'm not sure we
+    /// could meaningfully require this anyway. How about free lifetimes? Is ignoring this
+    /// interesting for optimizations? Do we want to allow such optimizations?
+    ///
+    /// **Needs clarification**: We currently require that the LHS place not overlap with any place
+    /// read as part of computation of the RHS for some rvalues (generally those not producing
+    /// primitives). This requirement is under discussion in [#68364]. As a part of this discussion,
+    /// it is also unclear in what order the components are evaluated.
+    ///
+    /// [#68364]: https://github.com/rust-lang/rust/issues/68364
+    ///
+    /// See [`Rvalue`] documentation for details on each of those.
+    Assign(Box<(Place<'tcx>, Rvalue<'tcx>)>),
+
+    /// This represents all the reading that a pattern match may do (e.g., inspecting constants and
+    /// discriminant values), and the kind of pattern it comes from. This is in order to adapt
+    /// potential error messages to these specific patterns.
+    ///
+    /// Note that this also is emitted for regular `let` bindings to ensure that locals that are
+    /// never accessed still get some sanity checks for, e.g., `let x: ! = ..;`
+    ///
+    /// When executed at runtime this is a nop.
+    ///
+    /// Disallowed after drop elaboration.
+    FakeRead(Box<(FakeReadCause, Place<'tcx>)>),
+
+    /// Write the discriminant for a variant to the enum Place.
+    ///
+    /// This is permitted for both generators and ADTs. This does not necessarily write to the
+    /// entire place; instead, it writes to the minimum set of bytes as required by the layout for
+    /// the type.
+    SetDiscriminant { place: Box<Place<'tcx>>, variant_index: VariantIdx },
+
+    /// Deinitializes the place.
+    ///
+    /// This writes `uninit` bytes to the entire place.
+    Deinit(Box<Place<'tcx>>),
+
+    /// `StorageLive` and `StorageDead` statements mark the live range of a local.
+    ///
+    /// At any point during the execution of a function, each local is either allocated or
+    /// unallocated. Except as noted below, all locals except function parameters are initially
+    /// unallocated. `StorageLive` statements cause memory to be allocated for the local while
+    /// `StorageDead` statements cause the memory to be freed. Using a local in any way (not only
+    /// reading/writing from it) while it is unallocated is UB.
+    ///
+    /// Some locals have no `StorageLive` or `StorageDead` statements within the entire MIR body.
+    /// These locals are implicitly allocated for the full duration of the function. There is a
+    /// convenience method at `rustc_mir_dataflow::storage::always_storage_live_locals` for
+    /// computing these locals.
+    ///
+    /// If the local is already allocated, calling `StorageLive` again is UB. However, for an
+    /// unallocated local an additional `StorageDead` all is simply a nop.
+    StorageLive(Local),
+
+    /// See `StorageLive` above.
+    StorageDead(Local),
+
+    /// Retag references in the given place, ensuring they got fresh tags.
+    ///
+    /// This is part of the Stacked Borrows model. These statements are currently only interpreted
+    /// by miri and only generated when `-Z mir-emit-retag` is passed. See
+    /// <https://internals.rust-lang.org/t/stacked-borrows-an-aliasing-model-for-rust/8153/> for
+    /// more details.
+    ///
+    /// For code that is not specific to stacked borrows, you should consider retags to read
+    /// and modify the place in an opaque way.
+    Retag(RetagKind, Box<Place<'tcx>>),
+
+    /// Encodes a user's type ascription. These need to be preserved
+    /// intact so that NLL can respect them. For example:
+    /// ```ignore (illustrative)
+    /// let a: T = y;
+    /// ```
+    /// The effect of this annotation is to relate the type `T_y` of the place `y`
+    /// to the user-given type `T`. The effect depends on the specified variance:
+    ///
+    /// - `Covariant` -- requires that `T_y <: T`
+    /// - `Contravariant` -- requires that `T_y :> T`
+    /// - `Invariant` -- requires that `T_y == T`
+    /// - `Bivariant` -- no effect
+    ///
+    /// When executed at runtime this is a nop.
+    ///
+    /// Disallowed after drop elaboration.
+    AscribeUserType(Box<(Place<'tcx>, UserTypeProjection)>, ty::Variance),
+
+    /// Marks the start of a "coverage region", injected with '-Cinstrument-coverage'. A
+    /// `Coverage` statement carries metadata about the coverage region, used to inject a coverage
+    /// map into the binary. If `Coverage::kind` is a `Counter`, the statement also generates
+    /// executable code, to increment a counter variable at runtime, each time the code region is
+    /// executed.
+    Coverage(Box<Coverage>),
+
+    /// Denotes a call to the intrinsic function `copy_nonoverlapping`.
+    ///
+    /// First, all three operands are evaluated. `src` and `dest` must each be a reference, pointer,
+    /// or `Box` pointing to the same type `T`. `count` must evaluate to a `usize`. Then, `src` and
+    /// `dest` are dereferenced, and `count * size_of::<T>()` bytes beginning with the first byte of
+    /// the `src` place are copied to the continguous range of bytes beginning with the first byte
+    /// of `dest`.
+    ///
+    /// **Needs clarification**: In what order are operands computed and dereferenced? It should
+    /// probably match the order for assignment, but that is also undecided.
+    ///
+    /// **Needs clarification**: Is this typed or not, ie is there a typed load and store involved?
+    /// I vaguely remember Ralf saying somewhere that he thought it should not be.
+    CopyNonOverlapping(Box<CopyNonOverlapping<'tcx>>),
+
+    /// No-op. Useful for deleting instructions without affecting statement indices.
+    Nop,
+}
+
+/// Describes what kind of retag is to be performed.
+#[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, PartialEq, Eq, Hash, HashStable)]
+#[rustc_pass_by_value]
+pub enum RetagKind {
+    /// The initial retag when entering a function.
+    FnEntry,
+    /// Retag preparing for a two-phase borrow.
+    TwoPhase,
+    /// Retagging raw pointers.
+    Raw,
+    /// A "normal" retag.
+    Default,
+}
+
+/// The `FakeReadCause` describes the type of pattern why a FakeRead statement exists.
+#[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, Hash, HashStable, PartialEq)]
+pub enum FakeReadCause {
+    /// Inject a fake read of the borrowed input at the end of each guards
+    /// code.
+    ///
+    /// This should ensure that you cannot change the variant for an enum while
+    /// you are in the midst of matching on it.
+    ForMatchGuard,
+
+    /// `let x: !; match x {}` doesn't generate any read of x so we need to
+    /// generate a read of x to check that it is initialized and safe.
+    ///
+    /// If a closure pattern matches a Place starting with an Upvar, then we introduce a
+    /// FakeRead for that Place outside the closure, in such a case this option would be
+    /// Some(closure_def_id).
+    /// Otherwise, the value of the optional LocalDefId will be None.
+    //
+    // We can use LocaDefId here since fake read statements are removed
+    // before codegen in the `CleanupNonCodegenStatements` pass.
+    ForMatchedPlace(Option<LocalDefId>),
+
+    /// A fake read of the RefWithinGuard version of a bind-by-value variable
+    /// in a match guard to ensure that its value hasn't change by the time
+    /// we create the OutsideGuard version.
+    ForGuardBinding,
+
+    /// Officially, the semantics of
+    ///
+    /// `let pattern = <expr>;`
+    ///
+    /// is that `<expr>` is evaluated into a temporary and then this temporary is
+    /// into the pattern.
+    ///
+    /// However, if we see the simple pattern `let var = <expr>`, we optimize this to
+    /// evaluate `<expr>` directly into the variable `var`. This is mostly unobservable,
+    /// but in some cases it can affect the borrow checker, as in #53695.
+    /// Therefore, we insert a "fake read" here to ensure that we get
+    /// appropriate errors.
+    ///
+    /// If a closure pattern matches a Place starting with an Upvar, then we introduce a
+    /// FakeRead for that Place outside the closure, in such a case this option would be
+    /// Some(closure_def_id).
+    /// Otherwise, the value of the optional DefId will be None.
+    ForLet(Option<LocalDefId>),
+
+    /// If we have an index expression like
+    ///
+    /// (*x)[1][{ x = y; 4}]
+    ///
+    /// then the first bounds check is invalidated when we evaluate the second
+    /// index expression. Thus we create a fake borrow of `x` across the second
+    /// indexer, which will cause a borrow check error.
+    ForIndex,
+}
+
+#[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct Coverage {
+    pub kind: CoverageKind,
+    pub code_region: Option<CodeRegion>,
+}
+
+#[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct CopyNonOverlapping<'tcx> {
+    pub src: Operand<'tcx>,
+    pub dst: Operand<'tcx>,
+    /// Number of elements to copy from src to dest, not bytes.
+    pub count: Operand<'tcx>,
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Terminators
+
+/// The various kinds of terminators, representing ways of exiting from a basic block.
+///
+/// A note on unwinding: Panics may occur during the execution of some terminators. Depending on the
+/// `-C panic` flag, this may either cause the program to abort or the call stack to unwind. Such
+/// terminators have a `cleanup: Option<BasicBlock>` field on them. If stack unwinding occurs, then
+/// once the current function is reached, execution continues at the given basic block, if any. If
+/// `cleanup` is `None` then no cleanup is performed, and the stack continues unwinding. This is
+/// equivalent to the execution of a `Resume` terminator.
+///
+/// The basic block pointed to by a `cleanup` field must have its `cleanup` flag set. `cleanup`
+/// basic blocks have a couple restrictions:
+///  1. All `cleanup` fields in them must be `None`.
+///  2. `Return` terminators are not allowed in them. `Abort` and `Unwind` terminators are.
+///  3. All other basic blocks (in the current body) that are reachable from `cleanup` basic blocks
+///     must also be `cleanup`. This is a part of the type system and checked statically, so it is
+///     still an error to have such an edge in the CFG even if it's known that it won't be taken at
+///     runtime.
+#[derive(Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq)]
+pub enum TerminatorKind<'tcx> {
+    /// Block has one successor; we continue execution there.
+    Goto { target: BasicBlock },
+
+    /// Switches based on the computed value.
+    ///
+    /// First, evaluates the `discr` operand. The type of the operand must be a signed or unsigned
+    /// integer, char, or bool, and must match the given type. Then, if the list of switch targets
+    /// contains the computed value, continues execution at the associated basic block. Otherwise,
+    /// continues execution at the "otherwise" basic block.
+    ///
+    /// Target values may not appear more than once.
+    SwitchInt {
+        /// The discriminant value being tested.
+        discr: Operand<'tcx>,
+
+        /// The type of value being tested.
+        /// This is always the same as the type of `discr`.
+        /// FIXME: remove this redundant information. Currently, it is relied on by pretty-printing.
+        switch_ty: Ty<'tcx>,
+
+        targets: SwitchTargets,
+    },
+
+    /// Indicates that the landing pad is finished and that the process should continue unwinding.
+    ///
+    /// Like a return, this marks the end of this invocation of the function.
+    ///
+    /// Only permitted in cleanup blocks. `Resume` is not permitted with `-C unwind=abort` after
+    /// deaggregation runs.
+    Resume,
+
+    /// Indicates that the landing pad is finished and that the process should abort.
+    ///
+    /// Used to prevent unwinding for foreign items or with `-C unwind=abort`. Only permitted in
+    /// cleanup blocks.
+    Abort,
+
+    /// Returns from the function.
+    ///
+    /// Like function calls, the exact semantics of returns in Rust are unclear. Returning very
+    /// likely at least assigns the value currently in the return place (`_0`) to the place
+    /// specified in the associated `Call` terminator in the calling function, as if assigned via
+    /// `dest = move _0`. It might additionally do other things, like have side-effects in the
+    /// aliasing model.
+    ///
+    /// If the body is a generator body, this has slightly different semantics; it instead causes a
+    /// `GeneratorState::Returned(_0)` to be created (as if by an `Aggregate` rvalue) and assigned
+    /// to the return place.
+    Return,
+
+    /// Indicates a terminator that can never be reached.
+    ///
+    /// Executing this terminator is UB.
+    Unreachable,
+
+    /// The behavior of this statement differs significantly before and after drop elaboration.
+    /// After drop elaboration, `Drop` executes the drop glue for the specified place, after which
+    /// it continues execution/unwinds at the given basic blocks. It is possible that executing drop
+    /// glue is special - this would be part of Rust's memory model. (**FIXME**: due we have an
+    /// issue tracking if drop glue has any interesting semantics in addition to those of a function
+    /// call?)
+    ///
+    /// `Drop` before drop elaboration is a *conditional* execution of the drop glue. Specifically, the
+    /// `Drop` will be executed if...
+    ///
+    /// **Needs clarification**: End of that sentence. This in effect should document the exact
+    /// behavior of drop elaboration. The following sounds vaguely right, but I'm not quite sure:
+    ///
+    /// > The drop glue is executed if, among all statements executed within this `Body`, an assignment to
+    /// > the place or one of its "parents" occurred more recently than a move out of it. This does not
+    /// > consider indirect assignments.
+    Drop { place: Place<'tcx>, target: BasicBlock, unwind: Option<BasicBlock> },
+
+    /// Drops the place and assigns a new value to it.
+    ///
+    /// This first performs the exact same operation as the pre drop-elaboration `Drop` terminator;
+    /// it then additionally assigns the `value` to the `place` as if by an assignment statement.
+    /// This assignment occurs both in the unwind and the regular code paths. The semantics are best
+    /// explained by the elaboration:
+    ///
+    /// ```ignore (MIR)
+    /// BB0 {
+    ///   DropAndReplace(P <- V, goto BB1, unwind BB2)
+    /// }
+    /// ```
+    ///
+    /// becomes
+    ///
+    /// ```ignore (MIR)
+    /// BB0 {
+    ///   Drop(P, goto BB1, unwind BB2)
+    /// }
+    /// BB1 {
+    ///   // P is now uninitialized
+    ///   P <- V
+    /// }
+    /// BB2 {
+    ///   // P is now uninitialized -- its dtor panicked
+    ///   P <- V
+    /// }
+    /// ```
+    ///
+    /// Disallowed after drop elaboration.
+    DropAndReplace {
+        place: Place<'tcx>,
+        value: Operand<'tcx>,
+        target: BasicBlock,
+        unwind: Option<BasicBlock>,
+    },
+
+    /// Roughly speaking, evaluates the `func` operand and the arguments, and starts execution of
+    /// the referred to function. The operand types must match the argument types of the function.
+    /// The return place type must match the return type. The type of the `func` operand must be
+    /// callable, meaning either a function pointer, a function type, or a closure type.
+    ///
+    /// **Needs clarification**: The exact semantics of this. Current backends rely on `move`
+    /// operands not aliasing the return place. It is unclear how this is justified in MIR, see
+    /// [#71117].
+    ///
+    /// [#71117]: https://github.com/rust-lang/rust/issues/71117
+    Call {
+        /// The function that’s being called.
+        func: Operand<'tcx>,
+        /// Arguments the function is called with.
+        /// These are owned by the callee, which is free to modify them.
+        /// This allows the memory occupied by "by-value" arguments to be
+        /// reused across function calls without duplicating the contents.
+        args: Vec<Operand<'tcx>>,
+        /// Where the returned value will be written
+        destination: Place<'tcx>,
+        /// Where to go after this call returns. If none, the call necessarily diverges.
+        target: Option<BasicBlock>,
+        /// Cleanups to be done if the call unwinds.
+        cleanup: Option<BasicBlock>,
+        /// `true` if this is from a call in HIR rather than from an overloaded
+        /// operator. True for overloaded function call.
+        from_hir_call: bool,
+        /// This `Span` is the span of the function, without the dot and receiver
+        /// (e.g. `foo(a, b)` in `x.foo(a, b)`
+        fn_span: Span,
+    },
+
+    /// Evaluates the operand, which must have type `bool`. If it is not equal to `expected`,
+    /// initiates a panic. Initiating a panic corresponds to a `Call` terminator with some
+    /// unspecified constant as the function to call, all the operands stored in the `AssertMessage`
+    /// as parameters, and `None` for the destination. Keep in mind that the `cleanup` path is not
+    /// necessarily executed even in the case of a panic, for example in `-C panic=abort`. If the
+    /// assertion does not fail, execution continues at the specified basic block.
+    Assert {
+        cond: Operand<'tcx>,
+        expected: bool,
+        msg: AssertMessage<'tcx>,
+        target: BasicBlock,
+        cleanup: Option<BasicBlock>,
+    },
+
+    /// Marks a suspend point.
+    ///
+    /// Like `Return` terminators in generator bodies, this computes `value` and then a
+    /// `GeneratorState::Yielded(value)` as if by `Aggregate` rvalue. That value is then assigned to
+    /// the return place of the function calling this one, and execution continues in the calling
+    /// function. When next invoked with the same first argument, execution of this function
+    /// continues at the `resume` basic block, with the second argument written to the `resume_arg`
+    /// place. If the generator is dropped before then, the `drop` basic block is invoked.
+    ///
+    /// Not permitted in bodies that are not generator bodies, or after generator lowering.
+    ///
+    /// **Needs clarification**: What about the evaluation order of the `resume_arg` and `value`?
+    Yield {
+        /// The value to return.
+        value: Operand<'tcx>,
+        /// Where to resume to.
+        resume: BasicBlock,
+        /// The place to store the resume argument in.
+        resume_arg: Place<'tcx>,
+        /// Cleanup to be done if the generator is dropped at this suspend point.
+        drop: Option<BasicBlock>,
+    },
+
+    /// Indicates the end of dropping a generator.
+    ///
+    /// Semantically just a `return` (from the generators drop glue). Only permitted in the same situations
+    /// as `yield`.
+    ///
+    /// **Needs clarification**: Is that even correct? The generator drop code is always confusing
+    /// to me, because it's not even really in the current body.
+    ///
+    /// **Needs clarification**: Are there type system constraints on these terminators? Should
+    /// there be a "block type" like `cleanup` blocks for them?
+    GeneratorDrop,
+
+    /// A block where control flow only ever takes one real path, but borrowck needs to be more
+    /// conservative.
+    ///
+    /// At runtime this is semantically just a goto.
+    ///
+    /// Disallowed after drop elaboration.
+    FalseEdge {
+        /// The target normal control flow will take.
+        real_target: BasicBlock,
+        /// A block control flow could conceptually jump to, but won't in
+        /// practice.
+        imaginary_target: BasicBlock,
+    },
+
+    /// A terminator for blocks that only take one path in reality, but where we reserve the right
+    /// to unwind in borrowck, even if it won't happen in practice. This can arise in infinite loops
+    /// with no function calls for example.
+    ///
+    /// At runtime this is semantically just a goto.
+    ///
+    /// Disallowed after drop elaboration.
+    FalseUnwind {
+        /// The target normal control flow will take.
+        real_target: BasicBlock,
+        /// The imaginary cleanup block link. This particular path will never be taken
+        /// in practice, but in order to avoid fragility we want to always
+        /// consider it in borrowck. We don't want to accept programs which
+        /// pass borrowck only when `panic=abort` or some assertions are disabled
+        /// due to release vs. debug mode builds. This needs to be an `Option` because
+        /// of the `remove_noop_landing_pads` and `abort_unwinding_calls` passes.
+        unwind: Option<BasicBlock>,
+    },
+
+    /// Block ends with an inline assembly block. This is a terminator since
+    /// inline assembly is allowed to diverge.
+    InlineAsm {
+        /// The template for the inline assembly, with placeholders.
+        template: &'tcx [InlineAsmTemplatePiece],
+
+        /// The operands for the inline assembly, as `Operand`s or `Place`s.
+        operands: Vec<InlineAsmOperand<'tcx>>,
+
+        /// Miscellaneous options for the inline assembly.
+        options: InlineAsmOptions,
+
+        /// Source spans for each line of the inline assembly code. These are
+        /// used to map assembler errors back to the line in the source code.
+        line_spans: &'tcx [Span],
+
+        /// Destination block after the inline assembly returns, unless it is
+        /// diverging (InlineAsmOptions::NORETURN).
+        destination: Option<BasicBlock>,
+
+        /// Cleanup to be done if the inline assembly unwinds. This is present
+        /// if and only if InlineAsmOptions::MAY_UNWIND is set.
+        cleanup: Option<BasicBlock>,
+    },
+}
+
+/// Information about an assertion failure.
+#[derive(Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq, PartialOrd)]
+pub enum AssertKind<O> {
+    BoundsCheck { len: O, index: O },
+    Overflow(BinOp, O, O),
+    OverflowNeg(O),
+    DivisionByZero(O),
+    RemainderByZero(O),
+    ResumedAfterReturn(GeneratorKind),
+    ResumedAfterPanic(GeneratorKind),
+}
+
+#[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub enum InlineAsmOperand<'tcx> {
+    In {
+        reg: InlineAsmRegOrRegClass,
+        value: Operand<'tcx>,
+    },
+    Out {
+        reg: InlineAsmRegOrRegClass,
+        late: bool,
+        place: Option<Place<'tcx>>,
+    },
+    InOut {
+        reg: InlineAsmRegOrRegClass,
+        late: bool,
+        in_value: Operand<'tcx>,
+        out_place: Option<Place<'tcx>>,
+    },
+    Const {
+        value: Box<Constant<'tcx>>,
+    },
+    SymFn {
+        value: Box<Constant<'tcx>>,
+    },
+    SymStatic {
+        def_id: DefId,
+    },
+}
+
+/// Type for MIR `Assert` terminator error messages.
+pub type AssertMessage<'tcx> = AssertKind<Operand<'tcx>>;
+
+///////////////////////////////////////////////////////////////////////////
+// Places
+
+/// Places roughly correspond to a "location in memory." Places in MIR are the same mathematical
+/// object as places in Rust. This of course means that what exactly they are is undecided and part
+/// of the Rust memory model. However, they will likely contain at least the following pieces of
+/// information in some form:
+///
+///  1. The address in memory that the place refers to.
+///  2. The provenance with which the place is being accessed.
+///  3. The type of the place and an optional variant index. See [`PlaceTy`][super::tcx::PlaceTy].
+///  4. Optionally, some metadata. This exists if and only if the type of the place is not `Sized`.
+///
+/// We'll give a description below of how all pieces of the place except for the provenance are
+/// calculated. We cannot give a description of the provenance, because that is part of the
+/// undecided aliasing model - we only include it here at all to acknowledge its existence.
+///
+/// Each local naturally corresponds to the place `Place { local, projection: [] }`. This place has
+/// the address of the local's allocation and the type of the local.
+///
+/// **Needs clarification:** Unsized locals seem to present a bit of an issue. Their allocation
+/// can't actually be created on `StorageLive`, because it's unclear how big to make the allocation.
+/// Furthermore, MIR produces assignments to unsized locals, although that is not permitted under
+/// `#![feature(unsized_locals)]` in Rust. Besides just putting "unsized locals are special and
+/// different" in a bunch of places, I (JakobDegen) don't know how to incorporate this behavior into
+/// the current MIR semantics in a clean way - possibly this needs some design work first.
+///
+/// For places that are not locals, ie they have a non-empty list of projections, we define the
+/// values as a function of the parent place, that is the place with its last [`ProjectionElem`]
+/// stripped. The way this is computed of course depends on the kind of that last projection
+/// element:
+///
+///  - [`Downcast`](ProjectionElem::Downcast): This projection sets the place's variant index to the
+///    given one, and makes no other changes. A `Downcast` projection on a place with its variant
+///    index already set is not well-formed.
+///  - [`Field`](ProjectionElem::Field): `Field` projections take their parent place and create a
+///    place referring to one of the fields of the type. The resulting address is the parent
+///    address, plus the offset of the field. The type becomes the type of the field. If the parent
+///    was unsized and so had metadata associated with it, then the metadata is retained if the
+///    field is unsized and thrown out if it is sized.
+///
+///    These projections are only legal for tuples, ADTs, closures, and generators. If the ADT or
+///    generator has more than one variant, the parent place's variant index must be set, indicating
+///    which variant is being used. If it has just one variant, the variant index may or may not be
+///    included - the single possible variant is inferred if it is not included.
+///  - [`ConstantIndex`](ProjectionElem::ConstantIndex): Computes an offset in units of `T` into the
+///    place as described in the documentation for the `ProjectionElem`. The resulting address is
+///    the parent's address plus that offset, and the type is `T`. This is only legal if the parent
+///    place has type `[T;  N]` or `[T]` (*not* `&[T]`). Since such a `T` is always sized, any
+///    resulting metadata is thrown out.
+///  - [`Subslice`](ProjectionElem::Subslice): This projection calculates an offset and a new
+///    address in a similar manner as `ConstantIndex`. It is also only legal on `[T; N]` and `[T]`.
+///    However, this yields a `Place` of type `[T]`, and additionally sets the metadata to be the
+///    length of the subslice.
+///  - [`Index`](ProjectionElem::Index): Like `ConstantIndex`, only legal on `[T; N]` or `[T]`.
+///    However, `Index` additionally takes a local from which the value of the index is computed at
+///    runtime. Computing the value of the index involves interpreting the `Local` as a
+///    `Place { local, projection: [] }`, and then computing its value as if done via
+///    [`Operand::Copy`]. The array/slice is then indexed with the resulting value. The local must
+///    have type `usize`.
+///  - [`Deref`](ProjectionElem::Deref): Derefs are the last type of projection, and the most
+///    complicated. They are only legal on parent places that are references, pointers, or `Box`. A
+///    `Deref` projection begins by loading a value from the parent place, as if by
+///    [`Operand::Copy`]. It then dereferences the resulting pointer, creating a place of the
+///    pointee's type. The resulting address is the address that was stored in the pointer. If the
+///    pointee type is unsized, the pointer additionally stored the value of the metadata.
+///
+/// Computing a place may cause UB. One possibility is that the pointer used for a `Deref` may not
+/// be suitably aligned. Another possibility is that the place is not in bounds, meaning it does not
+/// point to an actual allocation.
+///
+/// However, if this is actually UB and when the UB kicks in is undecided. This is being discussed
+/// in [UCG#319]. The options include that every place must obey those rules, that only some places
+/// must obey them, or that places impose no rules of their own.
+///
+/// [UCG#319]: https://github.com/rust-lang/unsafe-code-guidelines/issues/319
+///
+/// Rust currently requires that every place obey those two rules. This is checked by MIRI and taken
+/// advantage of by codegen (via `gep inbounds`). That is possibly subject to change.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, TyEncodable, HashStable)]
+pub struct Place<'tcx> {
+    pub local: Local,
+
+    /// projection out of a place (access a field, deref a pointer, etc)
+    pub projection: &'tcx List<PlaceElem<'tcx>>,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(TyEncodable, TyDecodable, HashStable)]
+pub enum ProjectionElem<V, T> {
+    Deref,
+    Field(Field, T),
+    /// Index into a slice/array.
+    ///
+    /// Note that this does not also dereference, and so it does not exactly correspond to slice
+    /// indexing in Rust. In other words, in the below Rust code:
+    ///
+    /// ```rust
+    /// let x = &[1, 2, 3, 4];
+    /// let i = 2;
+    /// x[i];
+    /// ```
+    ///
+    /// The `x[i]` is turned into a `Deref` followed by an `Index`, not just an `Index`. The same
+    /// thing is true of the `ConstantIndex` and `Subslice` projections below.
+    Index(V),
+
+    /// These indices are generated by slice patterns. Easiest to explain
+    /// by example:
+    ///
+    /// ```ignore (illustrative)
+    /// [X, _, .._, _, _] => { offset: 0, min_length: 4, from_end: false },
+    /// [_, X, .._, _, _] => { offset: 1, min_length: 4, from_end: false },
+    /// [_, _, .._, X, _] => { offset: 2, min_length: 4, from_end: true },
+    /// [_, _, .._, _, X] => { offset: 1, min_length: 4, from_end: true },
+    /// ```
+    ConstantIndex {
+        /// index or -index (in Python terms), depending on from_end
+        offset: u64,
+        /// The thing being indexed must be at least this long. For arrays this
+        /// is always the exact length.
+        min_length: u64,
+        /// Counting backwards from end? This is always false when indexing an
+        /// array.
+        from_end: bool,
+    },
+
+    /// These indices are generated by slice patterns.
+    ///
+    /// If `from_end` is true `slice[from..slice.len() - to]`.
+    /// Otherwise `array[from..to]`.
+    Subslice {
+        from: u64,
+        to: u64,
+        /// Whether `to` counts from the start or end of the array/slice.
+        /// For `PlaceElem`s this is `true` if and only if the base is a slice.
+        /// For `ProjectionKind`, this can also be `true` for arrays.
+        from_end: bool,
+    },
+
+    /// "Downcast" to a variant of an enum or a generator.
+    ///
+    /// The included Symbol is the name of the variant, used for printing MIR.
+    Downcast(Option<Symbol>, VariantIdx),
+}
+
+/// Alias for projections as they appear in places, where the base is a place
+/// and the index is a local.
+pub type PlaceElem<'tcx> = ProjectionElem<Local, Ty<'tcx>>;
+
+///////////////////////////////////////////////////////////////////////////
+// Operands
+
+/// An operand in MIR represents a "value" in Rust, the definition of which is undecided and part of
+/// the memory model. One proposal for a definition of values can be found [on UCG][value-def].
+///
+/// [value-def]: https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/value-domain.md
+///
+/// The most common way to create values is via loading a place. Loading a place is an operation
+/// which reads the memory of the place and converts it to a value. This is a fundamentally *typed*
+/// operation. The nature of the value produced depends on the type of the conversion. Furthermore,
+/// there may be other effects: if the type has a validity constraint loading the place might be UB
+/// if the validity constraint is not met.
+///
+/// **Needs clarification:** Ralf proposes that loading a place not have side-effects.
+/// This is what is implemented in miri today. Are these the semantics we want for MIR? Is this
+/// something we can even decide without knowing more about Rust's memory model?
+///
+/// **Needs clarifiation:** Is loading a place that has its variant index set well-formed? Miri
+/// currently implements it, but it seems like this may be something to check against in the
+/// validator.
+#[derive(Clone, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
+pub enum Operand<'tcx> {
+    /// Creates a value by loading the given place.
+    ///
+    /// Before drop elaboration, the type of the place must be `Copy`. After drop elaboration there
+    /// is no such requirement.
+    Copy(Place<'tcx>),
+
+    /// Creates a value by performing loading the place, just like the `Copy` operand.
+    ///
+    /// This *may* additionally overwrite the place with `uninit` bytes, depending on how we decide
+    /// in [UCG#188]. You should not emit MIR that may attempt a subsequent second load of this
+    /// place without first re-initializing it.
+    ///
+    /// [UCG#188]: https://github.com/rust-lang/unsafe-code-guidelines/issues/188
+    Move(Place<'tcx>),
+
+    /// Constants are already semantically values, and remain unchanged.
+    Constant(Box<Constant<'tcx>>),
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Rvalues
+
+/// The various kinds of rvalues that can appear in MIR.
+///
+/// Not all of these are allowed at every [`MirPhase`] - when this is the case, it's stated below.
+///
+/// Computing any rvalue begins by evaluating the places and operands in some order (**Needs
+/// clarification**: Which order?). These are then used to produce a "value" - the same kind of
+/// value that an [`Operand`] produces.
+#[derive(Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq)]
+pub enum Rvalue<'tcx> {
+    /// Yields the operand unchanged
+    Use(Operand<'tcx>),
+
+    /// Creates an array where each element is the value of the operand.
+    ///
+    /// This is the cause of a bug in the case where the repetition count is zero because the value
+    /// is not dropped, see [#74836].
+    ///
+    /// Corresponds to source code like `[x; 32]`.
+    ///
+    /// [#74836]: https://github.com/rust-lang/rust/issues/74836
+    Repeat(Operand<'tcx>, ty::Const<'tcx>),
+
+    /// Creates a reference of the indicated kind to the place.
+    ///
+    /// There is not much to document here, because besides the obvious parts the semantics of this
+    /// are essentially entirely a part of the aliasing model. There are many UCG issues discussing
+    /// exactly what the behavior of this operation should be.
+    ///
+    /// `Shallow` borrows are disallowed after drop lowering.
+    Ref(Region<'tcx>, BorrowKind, Place<'tcx>),
+
+    /// Creates a pointer/reference to the given thread local.
+    ///
+    /// The yielded type is a `*mut T` if the static is mutable, otherwise if the static is extern a
+    /// `*const T`, and if neither of those apply a `&T`.
+    ///
+    /// **Note:** This is a runtime operation that actually executes code and is in this sense more
+    /// like a function call. Also, eliminating dead stores of this rvalue causes `fn main() {}` to
+    /// SIGILL for some reason that I (JakobDegen) never got a chance to look into.
+    ///
+    /// **Needs clarification**: Are there weird additional semantics here related to the runtime
+    /// nature of this operation?
+    ThreadLocalRef(DefId),
+
+    /// Creates a pointer with the indicated mutability to the place.
+    ///
+    /// This is generated by pointer casts like `&v as *const _` or raw address of expressions like
+    /// `&raw v` or `addr_of!(v)`.
+    ///
+    /// Like with references, the semantics of this operation are heavily dependent on the aliasing
+    /// model.
+    AddressOf(Mutability, Place<'tcx>),
+
+    /// Yields the length of the place, as a `usize`.
+    ///
+    /// If the type of the place is an array, this is the array length. For slices (`[T]`, not
+    /// `&[T]`) this accesses the place's metadata to determine the length. This rvalue is
+    /// ill-formed for places of other types.
+    Len(Place<'tcx>),
+
+    /// Performs essentially all of the casts that can be performed via `as`.
+    ///
+    /// This allows for casts from/to a variety of types.
+    ///
+    /// **FIXME**: Document exactly which `CastKind`s allow which types of casts. Figure out why
+    /// `ArrayToPointer` and `MutToConstPointer` are special.
+    Cast(CastKind, Operand<'tcx>, Ty<'tcx>),
+
+    /// * `Offset` has the same semantics as [`offset`](pointer::offset), except that the second
+    ///   parameter may be a `usize` as well.
+    /// * The comparison operations accept `bool`s, `char`s, signed or unsigned integers, floats,
+    ///   raw pointers, or function pointers and return a `bool`. The types of the operands must be
+    ///   matching, up to the usual caveat of the lifetimes in function pointers.
+    /// * Left and right shift operations accept signed or unsigned integers not necessarily of the
+    ///   same type and return a value of the same type as their LHS. Like in Rust, the RHS is
+    ///   truncated as needed.
+    /// * The `Bit*` operations accept signed integers, unsigned integers, or bools with matching
+    ///   types and return a value of that type.
+    /// * The remaining operations accept signed integers, unsigned integers, or floats with
+    ///   matching types and return a value of that type.
+    BinaryOp(BinOp, Box<(Operand<'tcx>, Operand<'tcx>)>),
+
+    /// Same as `BinaryOp`, but yields `(T, bool)` with a `bool` indicating an error condition.
+    ///
+    /// When overflow checking is disabled and we are generating run-time code, the error condition
+    /// is false. Otherwise, and always during CTFE, the error condition is determined as described
+    /// below.
+    ///
+    /// For addition, subtraction, and multiplication on integers the error condition is set when
+    /// the infinite precision result would be unequal to the actual result.
+    ///
+    /// For shift operations on integers the error condition is set when the value of right-hand
+    /// side is greater than or equal to the number of bits in the type of the left-hand side, or
+    /// when the value of right-hand side is negative.
+    ///
+    /// Other combinations of types and operators are unsupported.
+    CheckedBinaryOp(BinOp, Box<(Operand<'tcx>, Operand<'tcx>)>),
+
+    /// Computes a value as described by the operation.
+    NullaryOp(NullOp, Ty<'tcx>),
+
+    /// Exactly like `BinaryOp`, but less operands.
+    ///
+    /// Also does two's-complement arithmetic. Negation requires a signed integer or a float;
+    /// bitwise not requires a signed integer, unsigned integer, or bool. Both operation kinds
+    /// return a value with the same type as their operand.
+    UnaryOp(UnOp, Operand<'tcx>),
+
+    /// Computes the discriminant of the place, returning it as an integer of type
+    /// [`discriminant_ty`]. Returns zero for types without discriminant.
+    ///
+    /// The validity requirements for the underlying value are undecided for this rvalue, see
+    /// [#91095]. Note too that the value of the discriminant is not the same thing as the
+    /// variant index; use [`discriminant_for_variant`] to convert.
+    ///
+    /// [`discriminant_ty`]: crate::ty::Ty::discriminant_ty
+    /// [#91095]: https://github.com/rust-lang/rust/issues/91095
+    /// [`discriminant_for_variant`]: crate::ty::Ty::discriminant_for_variant
+    Discriminant(Place<'tcx>),
+
+    /// Creates an aggregate value, like a tuple or struct.
+    ///
+    /// This is needed because dataflow analysis needs to distinguish
+    /// `dest = Foo { x: ..., y: ... }` from `dest.x = ...; dest.y = ...;` in the case that `Foo`
+    /// has a destructor.
+    ///
+    /// Disallowed after deaggregation for all aggregate kinds except `Array` and `Generator`. After
+    /// generator lowering, `Generator` aggregate kinds are disallowed too.
+    Aggregate(Box<AggregateKind<'tcx>>, Vec<Operand<'tcx>>),
+
+    /// Transmutes a `*mut u8` into shallow-initialized `Box<T>`.
+    ///
+    /// This is different from a normal transmute because dataflow analysis will treat the box as
+    /// initialized but its content as uninitialized. Like other pointer casts, this in general
+    /// affects alias analysis.
+    ShallowInitBox(Operand<'tcx>, Ty<'tcx>),
+
+    /// A CopyForDeref is equivalent to a read from a place at the
+    /// codegen level, but is treated specially by drop elaboration. When such a read happens, it
+    /// is guaranteed (via nature of the mir_opt `Derefer` in rustc_mir_transform/src/deref_separator)
+    /// that the only use of the returned value is a deref operation, immediately
+    /// followed by one or more projections. Drop elaboration treats this rvalue as if the
+    /// read never happened and just projects further. This allows simplifying various MIR
+    /// optimizations and codegen backends that previously had to handle deref operations anywhere
+    /// in a place.
+    CopyForDeref(Place<'tcx>),
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
+pub enum CastKind {
+    /// An exposing pointer to address cast. A cast between a pointer and an integer type, or
+    /// between a function pointer and an integer type.
+    /// See the docs on `expose_addr` for more details.
+    PointerExposeAddress,
+    /// An address-to-pointer cast that picks up an exposed provenance.
+    /// See the docs on `from_exposed_addr` for more details.
+    PointerFromExposedAddress,
+    /// All sorts of pointer-to-pointer casts. Note that reference-to-raw-ptr casts are
+    /// translated into `&raw mut/const *r`, i.e., they are not actually casts.
+    Pointer(PointerCast),
+    /// Remaining unclassified casts.
+    Misc,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
+pub enum AggregateKind<'tcx> {
+    /// The type is of the element
+    Array(Ty<'tcx>),
+    Tuple,
+
+    /// The second field is the variant index. It's equal to 0 for struct
+    /// and union expressions. The fourth field is
+    /// active field number and is present only for union expressions
+    /// -- e.g., for a union expression `SomeUnion { c: .. }`, the
+    /// active field index would identity the field `c`
+    Adt(DefId, VariantIdx, SubstsRef<'tcx>, Option<UserTypeAnnotationIndex>, Option<usize>),
+
+    // Note: We can use LocalDefId since closures and generators a deaggregated
+    // before codegen.
+    Closure(LocalDefId, SubstsRef<'tcx>),
+    Generator(LocalDefId, SubstsRef<'tcx>, hir::Movability),
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
+pub enum NullOp {
+    /// Returns the size of a value of that type
+    SizeOf,
+    /// Returns the minimum alignment of a type
+    AlignOf,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
+pub enum UnOp {
+    /// The `!` operator for logical inversion
+    Not,
+    /// The `-` operator for negation
+    Neg,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
+pub enum BinOp {
+    /// The `+` operator (addition)
+    Add,
+    /// The `-` operator (subtraction)
+    Sub,
+    /// The `*` operator (multiplication)
+    Mul,
+    /// The `/` operator (division)
+    ///
+    /// Division by zero is UB, because the compiler should have inserted checks
+    /// prior to this.
+    Div,
+    /// The `%` operator (modulus)
+    ///
+    /// Using zero as the modulus (second operand) is UB, because the compiler
+    /// should have inserted checks prior to this.
+    Rem,
+    /// The `^` operator (bitwise xor)
+    BitXor,
+    /// The `&` operator (bitwise and)
+    BitAnd,
+    /// The `|` operator (bitwise or)
+    BitOr,
+    /// The `<<` operator (shift left)
+    ///
+    /// The offset is truncated to the size of the first operand before shifting.
+    Shl,
+    /// The `>>` operator (shift right)
+    ///
+    /// The offset is truncated to the size of the first operand before shifting.
+    Shr,
+    /// The `==` operator (equality)
+    Eq,
+    /// The `<` operator (less than)
+    Lt,
+    /// The `<=` operator (less than or equal to)
+    Le,
+    /// The `!=` operator (not equal to)
+    Ne,
+    /// The `>=` operator (greater than or equal to)
+    Ge,
+    /// The `>` operator (greater than)
+    Gt,
+    /// The `ptr.offset` operator
+    Offset,
+}
+
+// Some nodes are used a lot. Make sure they don't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+mod size_asserts {
+    use super::*;
+    // These are in alphabetical order, which is easy to maintain.
+    static_assert_size!(AggregateKind<'_>, 48);
+    static_assert_size!(Operand<'_>, 24);
+    static_assert_size!(Place<'_>, 16);
+    static_assert_size!(PlaceElem<'_>, 24);
+    static_assert_size!(Rvalue<'_>, 40);
+}
diff --git a/compiler/rustc_middle/src/mir/tcx.rs b/compiler/rustc_middle/src/mir/tcx.rs
new file mode 100644
index 000000000..405003156
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/tcx.rs
@@ -0,0 +1,307 @@
+/*!
+ * Methods for the various MIR types. These are intended for use after
+ * building is complete.
+ */
+
+use crate::mir::*;
+use crate::ty::subst::Subst;
+use crate::ty::{self, Ty, TyCtxt};
+use rustc_hir as hir;
+use rustc_target::abi::VariantIdx;
+
+#[derive(Copy, Clone, Debug, TypeFoldable, TypeVisitable)]
+pub struct PlaceTy<'tcx> {
+    pub ty: Ty<'tcx>,
+    /// Downcast to a particular variant of an enum or a generator, if included.
+    pub variant_index: Option<VariantIdx>,
+}
+
+// At least on 64 bit systems, `PlaceTy` should not be larger than two or three pointers.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(PlaceTy<'_>, 16);
+
+impl<'tcx> PlaceTy<'tcx> {
+    #[inline]
+    pub fn from_ty(ty: Ty<'tcx>) -> PlaceTy<'tcx> {
+        PlaceTy { ty, variant_index: None }
+    }
+
+    /// `place_ty.field_ty(tcx, f)` computes the type at a given field
+    /// of a record or enum-variant. (Most clients of `PlaceTy` can
+    /// instead just extract the relevant type directly from their
+    /// `PlaceElem`, but some instances of `ProjectionElem<V, T>` do
+    /// not carry a `Ty` for `T`.)
+    ///
+    /// Note that the resulting type has not been normalized.
+    pub fn field_ty(self, tcx: TyCtxt<'tcx>, f: Field) -> Ty<'tcx> {
+        let answer = match self.ty.kind() {
+            ty::Adt(adt_def, substs) => {
+                let variant_def = match self.variant_index {
+                    None => adt_def.non_enum_variant(),
+                    Some(variant_index) => {
+                        assert!(adt_def.is_enum());
+                        &adt_def.variant(variant_index)
+                    }
+                };
+                let field_def = &variant_def.fields[f.index()];
+                field_def.ty(tcx, substs)
+            }
+            ty::Tuple(tys) => tys[f.index()],
+            _ => bug!("extracting field of non-tuple non-adt: {:?}", self),
+        };
+        debug!("field_ty self: {:?} f: {:?} yields: {:?}", self, f, answer);
+        answer
+    }
+
+    /// Convenience wrapper around `projection_ty_core` for
+    /// `PlaceElem`, where we can just use the `Ty` that is already
+    /// stored inline on field projection elems.
+    pub fn projection_ty(self, tcx: TyCtxt<'tcx>, elem: PlaceElem<'tcx>) -> PlaceTy<'tcx> {
+        self.projection_ty_core(tcx, ty::ParamEnv::empty(), &elem, |_, _, ty| ty)
+    }
+
+    /// `place_ty.projection_ty_core(tcx, elem, |...| { ... })`
+    /// projects `place_ty` onto `elem`, returning the appropriate
+    /// `Ty` or downcast variant corresponding to that projection.
+    /// The `handle_field` callback must map a `Field` to its `Ty`,
+    /// (which should be trivial when `T` = `Ty`).
+    pub fn projection_ty_core<V, T>(
+        self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        elem: &ProjectionElem<V, T>,
+        mut handle_field: impl FnMut(&Self, Field, T) -> Ty<'tcx>,
+    ) -> PlaceTy<'tcx>
+    where
+        V: ::std::fmt::Debug,
+        T: ::std::fmt::Debug + Copy,
+    {
+        if self.variant_index.is_some() && !matches!(elem, ProjectionElem::Field(..)) {
+            bug!("cannot use non field projection on downcasted place")
+        }
+        let answer = match *elem {
+            ProjectionElem::Deref => {
+                let ty = self
+                    .ty
+                    .builtin_deref(true)
+                    .unwrap_or_else(|| {
+                        bug!("deref projection of non-dereferenceable ty {:?}", self)
+                    })
+                    .ty;
+                PlaceTy::from_ty(ty)
+            }
+            ProjectionElem::Index(_) | ProjectionElem::ConstantIndex { .. } => {
+                PlaceTy::from_ty(self.ty.builtin_index().unwrap())
+            }
+            ProjectionElem::Subslice { from, to, from_end } => {
+                PlaceTy::from_ty(match self.ty.kind() {
+                    ty::Slice(..) => self.ty,
+                    ty::Array(inner, _) if !from_end => tcx.mk_array(*inner, (to - from) as u64),
+                    ty::Array(inner, size) if from_end => {
+                        let size = size.eval_usize(tcx, param_env);
+                        let len = size - (from as u64) - (to as u64);
+                        tcx.mk_array(*inner, len)
+                    }
+                    _ => bug!("cannot subslice non-array type: `{:?}`", self),
+                })
+            }
+            ProjectionElem::Downcast(_name, index) => {
+                PlaceTy { ty: self.ty, variant_index: Some(index) }
+            }
+            ProjectionElem::Field(f, fty) => PlaceTy::from_ty(handle_field(&self, f, fty)),
+        };
+        debug!("projection_ty self: {:?} elem: {:?} yields: {:?}", self, elem, answer);
+        answer
+    }
+}
+
+impl<'tcx> Place<'tcx> {
+    pub fn ty_from<D>(
+        local: Local,
+        projection: &[PlaceElem<'tcx>],
+        local_decls: &D,
+        tcx: TyCtxt<'tcx>,
+    ) -> PlaceTy<'tcx>
+    where
+        D: HasLocalDecls<'tcx>,
+    {
+        projection
+            .iter()
+            .fold(PlaceTy::from_ty(local_decls.local_decls()[local].ty), |place_ty, &elem| {
+                place_ty.projection_ty(tcx, elem)
+            })
+    }
+
+    pub fn ty<D>(&self, local_decls: &D, tcx: TyCtxt<'tcx>) -> PlaceTy<'tcx>
+    where
+        D: HasLocalDecls<'tcx>,
+    {
+        Place::ty_from(self.local, &self.projection, local_decls, tcx)
+    }
+}
+
+impl<'tcx> PlaceRef<'tcx> {
+    pub fn ty<D>(&self, local_decls: &D, tcx: TyCtxt<'tcx>) -> PlaceTy<'tcx>
+    where
+        D: HasLocalDecls<'tcx>,
+    {
+        Place::ty_from(self.local, &self.projection, local_decls, tcx)
+    }
+}
+
+pub enum RvalueInitializationState {
+    Shallow,
+    Deep,
+}
+
+impl<'tcx> Rvalue<'tcx> {
+    pub fn ty<D>(&self, local_decls: &D, tcx: TyCtxt<'tcx>) -> Ty<'tcx>
+    where
+        D: HasLocalDecls<'tcx>,
+    {
+        match *self {
+            Rvalue::Use(ref operand) => operand.ty(local_decls, tcx),
+            Rvalue::Repeat(ref operand, count) => {
+                tcx.mk_ty(ty::Array(operand.ty(local_decls, tcx), count))
+            }
+            Rvalue::ThreadLocalRef(did) => {
+                let static_ty = tcx.type_of(did);
+                if tcx.is_mutable_static(did) {
+                    tcx.mk_mut_ptr(static_ty)
+                } else if tcx.is_foreign_item(did) {
+                    tcx.mk_imm_ptr(static_ty)
+                } else {
+                    // FIXME: These things don't *really* have 'static lifetime.
+                    tcx.mk_imm_ref(tcx.lifetimes.re_static, static_ty)
+                }
+            }
+            Rvalue::Ref(reg, bk, ref place) => {
+                let place_ty = place.ty(local_decls, tcx).ty;
+                tcx.mk_ref(reg, ty::TypeAndMut { ty: place_ty, mutbl: bk.to_mutbl_lossy() })
+            }
+            Rvalue::AddressOf(mutability, ref place) => {
+                let place_ty = place.ty(local_decls, tcx).ty;
+                tcx.mk_ptr(ty::TypeAndMut { ty: place_ty, mutbl: mutability })
+            }
+            Rvalue::Len(..) => tcx.types.usize,
+            Rvalue::Cast(.., ty) => ty,
+            Rvalue::BinaryOp(op, box (ref lhs, ref rhs)) => {
+                let lhs_ty = lhs.ty(local_decls, tcx);
+                let rhs_ty = rhs.ty(local_decls, tcx);
+                op.ty(tcx, lhs_ty, rhs_ty)
+            }
+            Rvalue::CheckedBinaryOp(op, box (ref lhs, ref rhs)) => {
+                let lhs_ty = lhs.ty(local_decls, tcx);
+                let rhs_ty = rhs.ty(local_decls, tcx);
+                let ty = op.ty(tcx, lhs_ty, rhs_ty);
+                tcx.intern_tup(&[ty, tcx.types.bool])
+            }
+            Rvalue::UnaryOp(UnOp::Not | UnOp::Neg, ref operand) => operand.ty(local_decls, tcx),
+            Rvalue::Discriminant(ref place) => place.ty(local_decls, tcx).ty.discriminant_ty(tcx),
+            Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf, _) => tcx.types.usize,
+            Rvalue::Aggregate(ref ak, ref ops) => match **ak {
+                AggregateKind::Array(ty) => tcx.mk_array(ty, ops.len() as u64),
+                AggregateKind::Tuple => tcx.mk_tup(ops.iter().map(|op| op.ty(local_decls, tcx))),
+                AggregateKind::Adt(did, _, substs, _, _) => {
+                    tcx.bound_type_of(did).subst(tcx, substs)
+                }
+                AggregateKind::Closure(did, substs) => tcx.mk_closure(did.to_def_id(), substs),
+                AggregateKind::Generator(did, substs, movability) => {
+                    tcx.mk_generator(did.to_def_id(), substs, movability)
+                }
+            },
+            Rvalue::ShallowInitBox(_, ty) => tcx.mk_box(ty),
+            Rvalue::CopyForDeref(ref place) => place.ty(local_decls, tcx).ty,
+        }
+    }
+
+    #[inline]
+    /// Returns `true` if this rvalue is deeply initialized (most rvalues) or
+    /// whether its only shallowly initialized (`Rvalue::Box`).
+    pub fn initialization_state(&self) -> RvalueInitializationState {
+        match *self {
+            Rvalue::ShallowInitBox(_, _) => RvalueInitializationState::Shallow,
+            _ => RvalueInitializationState::Deep,
+        }
+    }
+}
+
+impl<'tcx> Operand<'tcx> {
+    pub fn ty<D>(&self, local_decls: &D, tcx: TyCtxt<'tcx>) -> Ty<'tcx>
+    where
+        D: HasLocalDecls<'tcx>,
+    {
+        match self {
+            &Operand::Copy(ref l) | &Operand::Move(ref l) => l.ty(local_decls, tcx).ty,
+            &Operand::Constant(ref c) => c.literal.ty(),
+        }
+    }
+}
+
+impl<'tcx> BinOp {
+    pub fn ty(&self, tcx: TyCtxt<'tcx>, lhs_ty: Ty<'tcx>, rhs_ty: Ty<'tcx>) -> Ty<'tcx> {
+        // FIXME: handle SIMD correctly
+        match self {
+            &BinOp::Add
+            | &BinOp::Sub
+            | &BinOp::Mul
+            | &BinOp::Div
+            | &BinOp::Rem
+            | &BinOp::BitXor
+            | &BinOp::BitAnd
+            | &BinOp::BitOr => {
+                // these should be integers or floats of the same size.
+                assert_eq!(lhs_ty, rhs_ty);
+                lhs_ty
+            }
+            &BinOp::Shl | &BinOp::Shr | &BinOp::Offset => {
+                lhs_ty // lhs_ty can be != rhs_ty
+            }
+            &BinOp::Eq | &BinOp::Lt | &BinOp::Le | &BinOp::Ne | &BinOp::Ge | &BinOp::Gt => {
+                tcx.types.bool
+            }
+        }
+    }
+}
+
+impl BorrowKind {
+    pub fn to_mutbl_lossy(self) -> hir::Mutability {
+        match self {
+            BorrowKind::Mut { .. } => hir::Mutability::Mut,
+            BorrowKind::Shared => hir::Mutability::Not,
+
+            // We have no type corresponding to a unique imm borrow, so
+            // use `&mut`. It gives all the capabilities of a `&uniq`
+            // and hence is a safe "over approximation".
+            BorrowKind::Unique => hir::Mutability::Mut,
+
+            // We have no type corresponding to a shallow borrow, so use
+            // `&` as an approximation.
+            BorrowKind::Shallow => hir::Mutability::Not,
+        }
+    }
+}
+
+impl BinOp {
+    pub fn to_hir_binop(self) -> hir::BinOpKind {
+        match self {
+            BinOp::Add => hir::BinOpKind::Add,
+            BinOp::Sub => hir::BinOpKind::Sub,
+            BinOp::Mul => hir::BinOpKind::Mul,
+            BinOp::Div => hir::BinOpKind::Div,
+            BinOp::Rem => hir::BinOpKind::Rem,
+            BinOp::BitXor => hir::BinOpKind::BitXor,
+            BinOp::BitAnd => hir::BinOpKind::BitAnd,
+            BinOp::BitOr => hir::BinOpKind::BitOr,
+            BinOp::Shl => hir::BinOpKind::Shl,
+            BinOp::Shr => hir::BinOpKind::Shr,
+            BinOp::Eq => hir::BinOpKind::Eq,
+            BinOp::Ne => hir::BinOpKind::Ne,
+            BinOp::Lt => hir::BinOpKind::Lt,
+            BinOp::Gt => hir::BinOpKind::Gt,
+            BinOp::Le => hir::BinOpKind::Le,
+            BinOp::Ge => hir::BinOpKind::Ge,
+            BinOp::Offset => unreachable!(),
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/terminator.rs b/compiler/rustc_middle/src/mir/terminator.rs
new file mode 100644
index 000000000..9ccf5aea6
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/terminator.rs
@@ -0,0 +1,448 @@
+use crate::mir;
+use crate::mir::interpret::Scalar;
+use crate::ty::{self, Ty, TyCtxt};
+use smallvec::{smallvec, SmallVec};
+
+use super::{BasicBlock, InlineAsmOperand, Operand, SourceInfo, TerminatorKind};
+use rustc_ast::InlineAsmTemplatePiece;
+pub use rustc_ast::Mutability;
+use rustc_macros::HashStable;
+use std::borrow::Cow;
+use std::fmt::{self, Debug, Formatter, Write};
+use std::iter;
+use std::slice;
+
+pub use super::query::*;
+
+#[derive(Debug, Clone, TyEncodable, TyDecodable, Hash, HashStable, PartialEq, PartialOrd)]
+pub struct SwitchTargets {
+    /// Possible values. The locations to branch to in each case
+    /// are found in the corresponding indices from the `targets` vector.
+    values: SmallVec<[u128; 1]>,
+
+    /// Possible branch sites. The last element of this vector is used
+    /// for the otherwise branch, so targets.len() == values.len() + 1
+    /// should hold.
+    //
+    // This invariant is quite non-obvious and also could be improved.
+    // One way to make this invariant is to have something like this instead:
+    //
+    // branches: Vec<(ConstInt, BasicBlock)>,
+    // otherwise: Option<BasicBlock> // exhaustive if None
+    //
+    // However we’ve decided to keep this as-is until we figure a case
+    // where some other approach seems to be strictly better than other.
+    targets: SmallVec<[BasicBlock; 2]>,
+}
+
+impl SwitchTargets {
+    /// Creates switch targets from an iterator of values and target blocks.
+    ///
+    /// The iterator may be empty, in which case the `SwitchInt` instruction is equivalent to
+    /// `goto otherwise;`.
+    pub fn new(targets: impl Iterator<Item = (u128, BasicBlock)>, otherwise: BasicBlock) -> Self {
+        let (values, mut targets): (SmallVec<_>, SmallVec<_>) = targets.unzip();
+        targets.push(otherwise);
+        Self { values, targets }
+    }
+
+    /// Builds a switch targets definition that jumps to `then` if the tested value equals `value`,
+    /// and to `else_` if not.
+    pub fn static_if(value: u128, then: BasicBlock, else_: BasicBlock) -> Self {
+        Self { values: smallvec![value], targets: smallvec![then, else_] }
+    }
+
+    /// Returns the fallback target that is jumped to when none of the values match the operand.
+    pub fn otherwise(&self) -> BasicBlock {
+        *self.targets.last().unwrap()
+    }
+
+    /// Returns an iterator over the switch targets.
+    ///
+    /// The iterator will yield tuples containing the value and corresponding target to jump to, not
+    /// including the `otherwise` fallback target.
+    ///
+    /// Note that this may yield 0 elements. Only the `otherwise` branch is mandatory.
+    pub fn iter(&self) -> SwitchTargetsIter<'_> {
+        SwitchTargetsIter { inner: iter::zip(&self.values, &self.targets) }
+    }
+
+    /// Returns a slice with all possible jump targets (including the fallback target).
+    pub fn all_targets(&self) -> &[BasicBlock] {
+        &self.targets
+    }
+
+    pub fn all_targets_mut(&mut self) -> &mut [BasicBlock] {
+        &mut self.targets
+    }
+
+    /// Finds the `BasicBlock` to which this `SwitchInt` will branch given the
+    /// specific value.  This cannot fail, as it'll return the `otherwise`
+    /// branch if there's not a specific match for the value.
+    pub fn target_for_value(&self, value: u128) -> BasicBlock {
+        self.iter().find_map(|(v, t)| (v == value).then_some(t)).unwrap_or_else(|| self.otherwise())
+    }
+}
+
+pub struct SwitchTargetsIter<'a> {
+    inner: iter::Zip<slice::Iter<'a, u128>, slice::Iter<'a, BasicBlock>>,
+}
+
+impl<'a> Iterator for SwitchTargetsIter<'a> {
+    type Item = (u128, BasicBlock);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.inner.next().map(|(val, bb)| (*val, *bb))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+}
+
+impl<'a> ExactSizeIterator for SwitchTargetsIter<'a> {}
+
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct Terminator<'tcx> {
+    pub source_info: SourceInfo,
+    pub kind: TerminatorKind<'tcx>,
+}
+
+pub type Successors<'a> = impl Iterator<Item = BasicBlock> + 'a;
+pub type SuccessorsMut<'a> =
+    iter::Chain<std::option::IntoIter<&'a mut BasicBlock>, slice::IterMut<'a, BasicBlock>>;
+
+impl<'tcx> Terminator<'tcx> {
+    pub fn successors(&self) -> Successors<'_> {
+        self.kind.successors()
+    }
+
+    pub fn successors_mut(&mut self) -> SuccessorsMut<'_> {
+        self.kind.successors_mut()
+    }
+
+    pub fn unwind(&self) -> Option<&Option<BasicBlock>> {
+        self.kind.unwind()
+    }
+
+    pub fn unwind_mut(&mut self) -> Option<&mut Option<BasicBlock>> {
+        self.kind.unwind_mut()
+    }
+}
+
+impl<'tcx> TerminatorKind<'tcx> {
+    pub fn if_(
+        tcx: TyCtxt<'tcx>,
+        cond: Operand<'tcx>,
+        t: BasicBlock,
+        f: BasicBlock,
+    ) -> TerminatorKind<'tcx> {
+        TerminatorKind::SwitchInt {
+            discr: cond,
+            switch_ty: tcx.types.bool,
+            targets: SwitchTargets::static_if(0, f, t),
+        }
+    }
+
+    pub fn successors(&self) -> Successors<'_> {
+        use self::TerminatorKind::*;
+        match *self {
+            Resume
+            | Abort
+            | GeneratorDrop
+            | Return
+            | Unreachable
+            | Call { target: None, cleanup: None, .. }
+            | InlineAsm { destination: None, cleanup: None, .. } => {
+                None.into_iter().chain((&[]).into_iter().copied())
+            }
+            Goto { target: t }
+            | Call { target: None, cleanup: Some(t), .. }
+            | Call { target: Some(t), cleanup: None, .. }
+            | Yield { resume: t, drop: None, .. }
+            | DropAndReplace { target: t, unwind: None, .. }
+            | Drop { target: t, unwind: None, .. }
+            | Assert { target: t, cleanup: None, .. }
+            | FalseUnwind { real_target: t, unwind: None }
+            | InlineAsm { destination: Some(t), cleanup: None, .. }
+            | InlineAsm { destination: None, cleanup: Some(t), .. } => {
+                Some(t).into_iter().chain((&[]).into_iter().copied())
+            }
+            Call { target: Some(t), cleanup: Some(ref u), .. }
+            | Yield { resume: t, drop: Some(ref u), .. }
+            | DropAndReplace { target: t, unwind: Some(ref u), .. }
+            | Drop { target: t, unwind: Some(ref u), .. }
+            | Assert { target: t, cleanup: Some(ref u), .. }
+            | FalseUnwind { real_target: t, unwind: Some(ref u) }
+            | InlineAsm { destination: Some(t), cleanup: Some(ref u), .. } => {
+                Some(t).into_iter().chain(slice::from_ref(u).into_iter().copied())
+            }
+            SwitchInt { ref targets, .. } => {
+                None.into_iter().chain(targets.targets.iter().copied())
+            }
+            FalseEdge { real_target, ref imaginary_target } => Some(real_target)
+                .into_iter()
+                .chain(slice::from_ref(imaginary_target).into_iter().copied()),
+        }
+    }
+
+    pub fn successors_mut(&mut self) -> SuccessorsMut<'_> {
+        use self::TerminatorKind::*;
+        match *self {
+            Resume
+            | Abort
+            | GeneratorDrop
+            | Return
+            | Unreachable
+            | Call { target: None, cleanup: None, .. }
+            | InlineAsm { destination: None, cleanup: None, .. } => None.into_iter().chain(&mut []),
+            Goto { target: ref mut t }
+            | Call { target: None, cleanup: Some(ref mut t), .. }
+            | Call { target: Some(ref mut t), cleanup: None, .. }
+            | Yield { resume: ref mut t, drop: None, .. }
+            | DropAndReplace { target: ref mut t, unwind: None, .. }
+            | Drop { target: ref mut t, unwind: None, .. }
+            | Assert { target: ref mut t, cleanup: None, .. }
+            | FalseUnwind { real_target: ref mut t, unwind: None }
+            | InlineAsm { destination: Some(ref mut t), cleanup: None, .. }
+            | InlineAsm { destination: None, cleanup: Some(ref mut t), .. } => {
+                Some(t).into_iter().chain(&mut [])
+            }
+            Call { target: Some(ref mut t), cleanup: Some(ref mut u), .. }
+            | Yield { resume: ref mut t, drop: Some(ref mut u), .. }
+            | DropAndReplace { target: ref mut t, unwind: Some(ref mut u), .. }
+            | Drop { target: ref mut t, unwind: Some(ref mut u), .. }
+            | Assert { target: ref mut t, cleanup: Some(ref mut u), .. }
+            | FalseUnwind { real_target: ref mut t, unwind: Some(ref mut u) }
+            | InlineAsm { destination: Some(ref mut t), cleanup: Some(ref mut u), .. } => {
+                Some(t).into_iter().chain(slice::from_mut(u))
+            }
+            SwitchInt { ref mut targets, .. } => None.into_iter().chain(&mut targets.targets),
+            FalseEdge { ref mut real_target, ref mut imaginary_target } => {
+                Some(real_target).into_iter().chain(slice::from_mut(imaginary_target))
+            }
+        }
+    }
+
+    pub fn unwind(&self) -> Option<&Option<BasicBlock>> {
+        match *self {
+            TerminatorKind::Goto { .. }
+            | TerminatorKind::Resume
+            | TerminatorKind::Abort
+            | TerminatorKind::Return
+            | TerminatorKind::Unreachable
+            | TerminatorKind::GeneratorDrop
+            | TerminatorKind::Yield { .. }
+            | TerminatorKind::SwitchInt { .. }
+            | TerminatorKind::FalseEdge { .. } => None,
+            TerminatorKind::Call { cleanup: ref unwind, .. }
+            | TerminatorKind::Assert { cleanup: ref unwind, .. }
+            | TerminatorKind::DropAndReplace { ref unwind, .. }
+            | TerminatorKind::Drop { ref unwind, .. }
+            | TerminatorKind::FalseUnwind { ref unwind, .. }
+            | TerminatorKind::InlineAsm { cleanup: ref unwind, .. } => Some(unwind),
+        }
+    }
+
+    pub fn unwind_mut(&mut self) -> Option<&mut Option<BasicBlock>> {
+        match *self {
+            TerminatorKind::Goto { .. }
+            | TerminatorKind::Resume
+            | TerminatorKind::Abort
+            | TerminatorKind::Return
+            | TerminatorKind::Unreachable
+            | TerminatorKind::GeneratorDrop
+            | TerminatorKind::Yield { .. }
+            | TerminatorKind::SwitchInt { .. }
+            | TerminatorKind::FalseEdge { .. } => None,
+            TerminatorKind::Call { cleanup: ref mut unwind, .. }
+            | TerminatorKind::Assert { cleanup: ref mut unwind, .. }
+            | TerminatorKind::DropAndReplace { ref mut unwind, .. }
+            | TerminatorKind::Drop { ref mut unwind, .. }
+            | TerminatorKind::FalseUnwind { ref mut unwind, .. }
+            | TerminatorKind::InlineAsm { cleanup: ref mut unwind, .. } => Some(unwind),
+        }
+    }
+
+    pub fn as_switch(&self) -> Option<(&Operand<'tcx>, Ty<'tcx>, &SwitchTargets)> {
+        match self {
+            TerminatorKind::SwitchInt { discr, switch_ty, targets } => {
+                Some((discr, *switch_ty, targets))
+            }
+            _ => None,
+        }
+    }
+
+    pub fn as_goto(&self) -> Option<BasicBlock> {
+        match self {
+            TerminatorKind::Goto { target } => Some(*target),
+            _ => None,
+        }
+    }
+}
+
+impl<'tcx> Debug for TerminatorKind<'tcx> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        self.fmt_head(fmt)?;
+        let successor_count = self.successors().count();
+        let labels = self.fmt_successor_labels();
+        assert_eq!(successor_count, labels.len());
+
+        match successor_count {
+            0 => Ok(()),
+
+            1 => write!(fmt, " -> {:?}", self.successors().next().unwrap()),
+
+            _ => {
+                write!(fmt, " -> [")?;
+                for (i, target) in self.successors().enumerate() {
+                    if i > 0 {
+                        write!(fmt, ", ")?;
+                    }
+                    write!(fmt, "{}: {:?}", labels[i], target)?;
+                }
+                write!(fmt, "]")
+            }
+        }
+    }
+}
+
+impl<'tcx> TerminatorKind<'tcx> {
+    /// Writes the "head" part of the terminator; that is, its name and the data it uses to pick the
+    /// successor basic block, if any. The only information not included is the list of possible
+    /// successors, which may be rendered differently between the text and the graphviz format.
+    pub fn fmt_head<W: Write>(&self, fmt: &mut W) -> fmt::Result {
+        use self::TerminatorKind::*;
+        match self {
+            Goto { .. } => write!(fmt, "goto"),
+            SwitchInt { discr, .. } => write!(fmt, "switchInt({:?})", discr),
+            Return => write!(fmt, "return"),
+            GeneratorDrop => write!(fmt, "generator_drop"),
+            Resume => write!(fmt, "resume"),
+            Abort => write!(fmt, "abort"),
+            Yield { value, resume_arg, .. } => write!(fmt, "{:?} = yield({:?})", resume_arg, value),
+            Unreachable => write!(fmt, "unreachable"),
+            Drop { place, .. } => write!(fmt, "drop({:?})", place),
+            DropAndReplace { place, value, .. } => {
+                write!(fmt, "replace({:?} <- {:?})", place, value)
+            }
+            Call { func, args, destination, .. } => {
+                write!(fmt, "{:?} = ", destination)?;
+                write!(fmt, "{:?}(", func)?;
+                for (index, arg) in args.iter().enumerate() {
+                    if index > 0 {
+                        write!(fmt, ", ")?;
+                    }
+                    write!(fmt, "{:?}", arg)?;
+                }
+                write!(fmt, ")")
+            }
+            Assert { cond, expected, msg, .. } => {
+                write!(fmt, "assert(")?;
+                if !expected {
+                    write!(fmt, "!")?;
+                }
+                write!(fmt, "{:?}, ", cond)?;
+                msg.fmt_assert_args(fmt)?;
+                write!(fmt, ")")
+            }
+            FalseEdge { .. } => write!(fmt, "falseEdge"),
+            FalseUnwind { .. } => write!(fmt, "falseUnwind"),
+            InlineAsm { template, ref operands, options, .. } => {
+                write!(fmt, "asm!(\"{}\"", InlineAsmTemplatePiece::to_string(template))?;
+                for op in operands {
+                    write!(fmt, ", ")?;
+                    let print_late = |&late| if late { "late" } else { "" };
+                    match op {
+                        InlineAsmOperand::In { reg, value } => {
+                            write!(fmt, "in({}) {:?}", reg, value)?;
+                        }
+                        InlineAsmOperand::Out { reg, late, place: Some(place) } => {
+                            write!(fmt, "{}out({}) {:?}", print_late(late), reg, place)?;
+                        }
+                        InlineAsmOperand::Out { reg, late, place: None } => {
+                            write!(fmt, "{}out({}) _", print_late(late), reg)?;
+                        }
+                        InlineAsmOperand::InOut {
+                            reg,
+                            late,
+                            in_value,
+                            out_place: Some(out_place),
+                        } => {
+                            write!(
+                                fmt,
+                                "in{}out({}) {:?} => {:?}",
+                                print_late(late),
+                                reg,
+                                in_value,
+                                out_place
+                            )?;
+                        }
+                        InlineAsmOperand::InOut { reg, late, in_value, out_place: None } => {
+                            write!(fmt, "in{}out({}) {:?} => _", print_late(late), reg, in_value)?;
+                        }
+                        InlineAsmOperand::Const { value } => {
+                            write!(fmt, "const {:?}", value)?;
+                        }
+                        InlineAsmOperand::SymFn { value } => {
+                            write!(fmt, "sym_fn {:?}", value)?;
+                        }
+                        InlineAsmOperand::SymStatic { def_id } => {
+                            write!(fmt, "sym_static {:?}", def_id)?;
+                        }
+                    }
+                }
+                write!(fmt, ", options({:?}))", options)
+            }
+        }
+    }
+
+    /// Returns the list of labels for the edges to the successor basic blocks.
+    pub fn fmt_successor_labels(&self) -> Vec<Cow<'static, str>> {
+        use self::TerminatorKind::*;
+        match *self {
+            Return | Resume | Abort | Unreachable | GeneratorDrop => vec![],
+            Goto { .. } => vec!["".into()],
+            SwitchInt { ref targets, switch_ty, .. } => ty::tls::with(|tcx| {
+                let param_env = ty::ParamEnv::empty();
+                let switch_ty = tcx.lift(switch_ty).unwrap();
+                let size = tcx.layout_of(param_env.and(switch_ty)).unwrap().size;
+                targets
+                    .values
+                    .iter()
+                    .map(|&u| {
+                        mir::ConstantKind::from_scalar(tcx, Scalar::from_uint(u, size), switch_ty)
+                            .to_string()
+                            .into()
+                    })
+                    .chain(iter::once("otherwise".into()))
+                    .collect()
+            }),
+            Call { target: Some(_), cleanup: Some(_), .. } => {
+                vec!["return".into(), "unwind".into()]
+            }
+            Call { target: Some(_), cleanup: None, .. } => vec!["return".into()],
+            Call { target: None, cleanup: Some(_), .. } => vec!["unwind".into()],
+            Call { target: None, cleanup: None, .. } => vec![],
+            Yield { drop: Some(_), .. } => vec!["resume".into(), "drop".into()],
+            Yield { drop: None, .. } => vec!["resume".into()],
+            DropAndReplace { unwind: None, .. } | Drop { unwind: None, .. } => {
+                vec!["return".into()]
+            }
+            DropAndReplace { unwind: Some(_), .. } | Drop { unwind: Some(_), .. } => {
+                vec!["return".into(), "unwind".into()]
+            }
+            Assert { cleanup: None, .. } => vec!["".into()],
+            Assert { .. } => vec!["success".into(), "unwind".into()],
+            FalseEdge { .. } => vec!["real".into(), "imaginary".into()],
+            FalseUnwind { unwind: Some(_), .. } => vec!["real".into(), "cleanup".into()],
+            FalseUnwind { unwind: None, .. } => vec!["real".into()],
+            InlineAsm { destination: Some(_), cleanup: Some(_), .. } => {
+                vec!["return".into(), "unwind".into()]
+            }
+            InlineAsm { destination: Some(_), cleanup: None, .. } => vec!["return".into()],
+            InlineAsm { destination: None, cleanup: Some(_), .. } => vec!["unwind".into()],
+            InlineAsm { destination: None, cleanup: None, .. } => vec![],
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/traversal.rs b/compiler/rustc_middle/src/mir/traversal.rs
new file mode 100644
index 000000000..627dc32f3
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/traversal.rs
@@ -0,0 +1,388 @@
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::OnceCell;
+use rustc_index::bit_set::BitSet;
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+
+use super::*;
+
+/// Preorder traversal of a graph.
+///
+/// Preorder traversal is when each node is visited after at least one of its predecessors. If you
+/// are familiar with some basic graph theory, then this performs a depth first search and returns
+/// nodes in order of discovery time.
+///
+/// ```text
+///
+///         A
+///        / \
+///       /   \
+///      B     C
+///       \   /
+///        \ /
+///         D
+/// ```
+///
+/// A preorder traversal of this graph is either `A B D C` or `A C D B`
+#[derive(Clone)]
+pub struct Preorder<'a, 'tcx> {
+    body: &'a Body<'tcx>,
+    visited: BitSet<BasicBlock>,
+    worklist: Vec<BasicBlock>,
+    root_is_start_block: bool,
+}
+
+impl<'a, 'tcx> Preorder<'a, 'tcx> {
+    pub fn new(body: &'a Body<'tcx>, root: BasicBlock) -> Preorder<'a, 'tcx> {
+        let worklist = vec![root];
+
+        Preorder {
+            body,
+            visited: BitSet::new_empty(body.basic_blocks().len()),
+            worklist,
+            root_is_start_block: root == START_BLOCK,
+        }
+    }
+}
+
+pub fn preorder<'a, 'tcx>(body: &'a Body<'tcx>) -> Preorder<'a, 'tcx> {
+    Preorder::new(body, START_BLOCK)
+}
+
+impl<'a, 'tcx> Iterator for Preorder<'a, 'tcx> {
+    type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
+
+    fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
+        while let Some(idx) = self.worklist.pop() {
+            if !self.visited.insert(idx) {
+                continue;
+            }
+
+            let data = &self.body[idx];
+
+            if let Some(ref term) = data.terminator {
+                self.worklist.extend(term.successors());
+            }
+
+            return Some((idx, data));
+        }
+
+        None
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // All the blocks, minus the number of blocks we've visited.
+        let upper = self.body.basic_blocks().len() - self.visited.count();
+
+        let lower = if self.root_is_start_block {
+            // We will visit all remaining blocks exactly once.
+            upper
+        } else {
+            self.worklist.len()
+        };
+
+        (lower, Some(upper))
+    }
+}
+
+/// Postorder traversal of a graph.
+///
+/// Postorder traversal is when each node is visited after all of its successors, except when the
+/// successor is only reachable by a back-edge. If you are familiar with some basic graph theory,
+/// then this performs a depth first search and returns nodes in order of completion time.
+///
+///
+/// ```text
+///
+///         A
+///        / \
+///       /   \
+///      B     C
+///       \   /
+///        \ /
+///         D
+/// ```
+///
+/// A Postorder traversal of this graph is `D B C A` or `D C B A`
+pub struct Postorder<'a, 'tcx> {
+    basic_blocks: &'a IndexVec<BasicBlock, BasicBlockData<'tcx>>,
+    visited: BitSet<BasicBlock>,
+    visit_stack: Vec<(BasicBlock, Successors<'a>)>,
+    root_is_start_block: bool,
+}
+
+impl<'a, 'tcx> Postorder<'a, 'tcx> {
+    pub fn new(
+        basic_blocks: &'a IndexVec<BasicBlock, BasicBlockData<'tcx>>,
+        root: BasicBlock,
+    ) -> Postorder<'a, 'tcx> {
+        let mut po = Postorder {
+            basic_blocks,
+            visited: BitSet::new_empty(basic_blocks.len()),
+            visit_stack: Vec::new(),
+            root_is_start_block: root == START_BLOCK,
+        };
+
+        let data = &po.basic_blocks[root];
+
+        if let Some(ref term) = data.terminator {
+            po.visited.insert(root);
+            po.visit_stack.push((root, term.successors()));
+            po.traverse_successor();
+        }
+
+        po
+    }
+
+    fn traverse_successor(&mut self) {
+        // This is quite a complex loop due to 1. the borrow checker not liking it much
+        // and 2. what exactly is going on is not clear
+        //
+        // It does the actual traversal of the graph, while the `next` method on the iterator
+        // just pops off of the stack. `visit_stack` is a stack containing pairs of nodes and
+        // iterators over the successors of those nodes. Each iteration attempts to get the next
+        // node from the top of the stack, then pushes that node and an iterator over the
+        // successors to the top of the stack. This loop only grows `visit_stack`, stopping when
+        // we reach a child that has no children that we haven't already visited.
+        //
+        // For a graph that looks like this:
+        //
+        //         A
+        //        / \
+        //       /   \
+        //      B     C
+        //      |     |
+        //      |     |
+        //      D     |
+        //       \   /
+        //        \ /
+        //         E
+        //
+        // The state of the stack starts out with just the root node (`A` in this case);
+        //     [(A, [B, C])]
+        //
+        // When the first call to `traverse_successor` happens, the following happens:
+        //
+        //     [(B, [D]),  // `B` taken from the successors of `A`, pushed to the
+        //                 // top of the stack along with the successors of `B`
+        //      (A, [C])]
+        //
+        //     [(D, [E]),  // `D` taken from successors of `B`, pushed to stack
+        //      (B, []),
+        //      (A, [C])]
+        //
+        //     [(E, []),   // `E` taken from successors of `D`, pushed to stack
+        //      (D, []),
+        //      (B, []),
+        //      (A, [C])]
+        //
+        // Now that the top of the stack has no successors we can traverse, each item will
+        // be popped off during iteration until we get back to `A`. This yields [E, D, B].
+        //
+        // When we yield `B` and call `traverse_successor`, we push `C` to the stack, but
+        // since we've already visited `E`, that child isn't added to the stack. The last
+        // two iterations yield `C` and finally `A` for a final traversal of [E, D, B, C, A]
+        loop {
+            let bb = if let Some(&mut (_, ref mut iter)) = self.visit_stack.last_mut() {
+                if let Some(bb) = iter.next() {
+                    bb
+                } else {
+                    break;
+                }
+            } else {
+                break;
+            };
+
+            if self.visited.insert(bb) {
+                if let Some(term) = &self.basic_blocks[bb].terminator {
+                    self.visit_stack.push((bb, term.successors()));
+                }
+            }
+        }
+    }
+}
+
+pub fn postorder<'a, 'tcx>(body: &'a Body<'tcx>) -> Postorder<'a, 'tcx> {
+    Postorder::new(&body.basic_blocks, START_BLOCK)
+}
+
+impl<'a, 'tcx> Iterator for Postorder<'a, 'tcx> {
+    type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
+
+    fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
+        let next = self.visit_stack.pop();
+        if next.is_some() {
+            self.traverse_successor();
+        }
+
+        next.map(|(bb, _)| (bb, &self.basic_blocks[bb]))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // All the blocks, minus the number of blocks we've visited.
+        let upper = self.basic_blocks.len() - self.visited.count();
+
+        let lower = if self.root_is_start_block {
+            // We will visit all remaining blocks exactly once.
+            upper
+        } else {
+            self.visit_stack.len()
+        };
+
+        (lower, Some(upper))
+    }
+}
+
+/// Reverse postorder traversal of a graph
+///
+/// Reverse postorder is the reverse order of a postorder traversal.
+/// This is different to a preorder traversal and represents a natural
+/// linearization of control-flow.
+///
+/// ```text
+///
+///         A
+///        / \
+///       /   \
+///      B     C
+///       \   /
+///        \ /
+///         D
+/// ```
+///
+/// A reverse postorder traversal of this graph is either `A B C D` or `A C B D`
+/// Note that for a graph containing no loops (i.e., A DAG), this is equivalent to
+/// a topological sort.
+///
+/// Construction of a `ReversePostorder` traversal requires doing a full
+/// postorder traversal of the graph, therefore this traversal should be
+/// constructed as few times as possible. Use the `reset` method to be able
+/// to re-use the traversal
+#[derive(Clone)]
+pub struct ReversePostorder<'a, 'tcx> {
+    body: &'a Body<'tcx>,
+    blocks: Vec<BasicBlock>,
+    idx: usize,
+}
+
+impl<'a, 'tcx> ReversePostorder<'a, 'tcx> {
+    pub fn new(body: &'a Body<'tcx>, root: BasicBlock) -> ReversePostorder<'a, 'tcx> {
+        let blocks: Vec<_> = Postorder::new(&body.basic_blocks, root).map(|(bb, _)| bb).collect();
+        let len = blocks.len();
+        ReversePostorder { body, blocks, idx: len }
+    }
+}
+
+impl<'a, 'tcx> Iterator for ReversePostorder<'a, 'tcx> {
+    type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
+
+    fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
+        if self.idx == 0 {
+            return None;
+        }
+        self.idx -= 1;
+
+        self.blocks.get(self.idx).map(|&bb| (bb, &self.body[bb]))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.idx, Some(self.idx))
+    }
+}
+
+impl<'a, 'tcx> ExactSizeIterator for ReversePostorder<'a, 'tcx> {}
+
+/// Returns an iterator over all basic blocks reachable from the `START_BLOCK` in no particular
+/// order.
+///
+/// This is clearer than writing `preorder` in cases where the order doesn't matter.
+pub fn reachable<'a, 'tcx>(
+    body: &'a Body<'tcx>,
+) -> impl 'a + Iterator<Item = (BasicBlock, &'a BasicBlockData<'tcx>)> {
+    preorder(body)
+}
+
+/// Returns a `BitSet` containing all basic blocks reachable from the `START_BLOCK`.
+pub fn reachable_as_bitset<'tcx>(body: &Body<'tcx>) -> BitSet<BasicBlock> {
+    let mut iter = preorder(body);
+    (&mut iter).for_each(drop);
+    iter.visited
+}
+
+#[derive(Clone)]
+pub struct ReversePostorderIter<'a, 'tcx> {
+    body: &'a Body<'tcx>,
+    blocks: &'a [BasicBlock],
+    idx: usize,
+}
+
+impl<'a, 'tcx> Iterator for ReversePostorderIter<'a, 'tcx> {
+    type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
+
+    fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
+        if self.idx == 0 {
+            return None;
+        }
+        self.idx -= 1;
+
+        self.blocks.get(self.idx).map(|&bb| (bb, &self.body[bb]))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.idx, Some(self.idx))
+    }
+}
+
+impl<'a, 'tcx> ExactSizeIterator for ReversePostorderIter<'a, 'tcx> {}
+
+pub fn reverse_postorder<'a, 'tcx>(body: &'a Body<'tcx>) -> ReversePostorderIter<'a, 'tcx> {
+    let blocks = body.basic_blocks.postorder();
+    let len = blocks.len();
+    ReversePostorderIter { body, blocks, idx: len }
+}
+
+#[derive(Clone, Debug)]
+pub(super) struct PostorderCache {
+    cache: OnceCell<Vec<BasicBlock>>,
+}
+
+impl PostorderCache {
+    #[inline]
+    pub(super) fn new() -> Self {
+        PostorderCache { cache: OnceCell::new() }
+    }
+
+    /// Invalidates the postorder cache.
+    #[inline]
+    pub(super) fn invalidate(&mut self) {
+        self.cache = OnceCell::new();
+    }
+
+    /// Returns the `&[BasicBlocks]` represents the postorder graph for this MIR.
+    #[inline]
+    pub(super) fn compute(&self, body: &IndexVec<BasicBlock, BasicBlockData<'_>>) -> &[BasicBlock] {
+        self.cache.get_or_init(|| Postorder::new(body, START_BLOCK).map(|(bb, _)| bb).collect())
+    }
+}
+
+impl<S: Encoder> Encodable<S> for PostorderCache {
+    #[inline]
+    fn encode(&self, _s: &mut S) {}
+}
+
+impl<D: Decoder> Decodable<D> for PostorderCache {
+    #[inline]
+    fn decode(_: &mut D) -> Self {
+        Self::new()
+    }
+}
+
+impl<CTX> HashStable<CTX> for PostorderCache {
+    #[inline]
+    fn hash_stable(&self, _: &mut CTX, _: &mut StableHasher) {
+        // do nothing
+    }
+}
+
+TrivialTypeTraversalAndLiftImpls! {
+    PostorderCache,
+}
diff --git a/compiler/rustc_middle/src/mir/type_foldable.rs b/compiler/rustc_middle/src/mir/type_foldable.rs
new file mode 100644
index 000000000..82a6b0c50
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/type_foldable.rs
@@ -0,0 +1,240 @@
+//! `TypeFoldable` implementations for MIR types
+
+use super::*;
+use crate::ty;
+use rustc_data_structures::functor::IdFunctor;
+
+TrivialTypeTraversalAndLiftImpls! {
+    BlockTailInfo,
+    MirPhase,
+    SourceInfo,
+    FakeReadCause,
+    RetagKind,
+    SourceScope,
+    SourceScopeLocalData,
+    UserTypeAnnotationIndex,
+}
+
+impl<'tcx> TypeFoldable<'tcx> for Terminator<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        use crate::mir::TerminatorKind::*;
+
+        let kind = match self.kind {
+            Goto { target } => Goto { target },
+            SwitchInt { discr, switch_ty, targets } => SwitchInt {
+                discr: discr.try_fold_with(folder)?,
+                switch_ty: switch_ty.try_fold_with(folder)?,
+                targets,
+            },
+            Drop { place, target, unwind } => {
+                Drop { place: place.try_fold_with(folder)?, target, unwind }
+            }
+            DropAndReplace { place, value, target, unwind } => DropAndReplace {
+                place: place.try_fold_with(folder)?,
+                value: value.try_fold_with(folder)?,
+                target,
+                unwind,
+            },
+            Yield { value, resume, resume_arg, drop } => Yield {
+                value: value.try_fold_with(folder)?,
+                resume,
+                resume_arg: resume_arg.try_fold_with(folder)?,
+                drop,
+            },
+            Call { func, args, destination, target, cleanup, from_hir_call, fn_span } => Call {
+                func: func.try_fold_with(folder)?,
+                args: args.try_fold_with(folder)?,
+                destination: destination.try_fold_with(folder)?,
+                target,
+                cleanup,
+                from_hir_call,
+                fn_span,
+            },
+            Assert { cond, expected, msg, target, cleanup } => {
+                use AssertKind::*;
+                let msg = match msg {
+                    BoundsCheck { len, index } => BoundsCheck {
+                        len: len.try_fold_with(folder)?,
+                        index: index.try_fold_with(folder)?,
+                    },
+                    Overflow(op, l, r) => {
+                        Overflow(op, l.try_fold_with(folder)?, r.try_fold_with(folder)?)
+                    }
+                    OverflowNeg(op) => OverflowNeg(op.try_fold_with(folder)?),
+                    DivisionByZero(op) => DivisionByZero(op.try_fold_with(folder)?),
+                    RemainderByZero(op) => RemainderByZero(op.try_fold_with(folder)?),
+                    ResumedAfterReturn(_) | ResumedAfterPanic(_) => msg,
+                };
+                Assert { cond: cond.try_fold_with(folder)?, expected, msg, target, cleanup }
+            }
+            GeneratorDrop => GeneratorDrop,
+            Resume => Resume,
+            Abort => Abort,
+            Return => Return,
+            Unreachable => Unreachable,
+            FalseEdge { real_target, imaginary_target } => {
+                FalseEdge { real_target, imaginary_target }
+            }
+            FalseUnwind { real_target, unwind } => FalseUnwind { real_target, unwind },
+            InlineAsm { template, operands, options, line_spans, destination, cleanup } => {
+                InlineAsm {
+                    template,
+                    operands: operands.try_fold_with(folder)?,
+                    options,
+                    line_spans,
+                    destination,
+                    cleanup,
+                }
+            }
+        };
+        Ok(Terminator { source_info: self.source_info, kind })
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for GeneratorKind {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
+        Ok(self)
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for Place<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        Ok(Place {
+            local: self.local.try_fold_with(folder)?,
+            projection: self.projection.try_fold_with(folder)?,
+        })
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<PlaceElem<'tcx>> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        ty::util::fold_list(self, folder, |tcx, v| tcx.intern_place_elems(v))
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for Rvalue<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        use crate::mir::Rvalue::*;
+        Ok(match self {
+            Use(op) => Use(op.try_fold_with(folder)?),
+            Repeat(op, len) => Repeat(op.try_fold_with(folder)?, len.try_fold_with(folder)?),
+            ThreadLocalRef(did) => ThreadLocalRef(did.try_fold_with(folder)?),
+            Ref(region, bk, place) => {
+                Ref(region.try_fold_with(folder)?, bk, place.try_fold_with(folder)?)
+            }
+            CopyForDeref(place) => CopyForDeref(place.try_fold_with(folder)?),
+            AddressOf(mutability, place) => AddressOf(mutability, place.try_fold_with(folder)?),
+            Len(place) => Len(place.try_fold_with(folder)?),
+            Cast(kind, op, ty) => Cast(kind, op.try_fold_with(folder)?, ty.try_fold_with(folder)?),
+            BinaryOp(op, box (rhs, lhs)) => {
+                BinaryOp(op, Box::new((rhs.try_fold_with(folder)?, lhs.try_fold_with(folder)?)))
+            }
+            CheckedBinaryOp(op, box (rhs, lhs)) => CheckedBinaryOp(
+                op,
+                Box::new((rhs.try_fold_with(folder)?, lhs.try_fold_with(folder)?)),
+            ),
+            UnaryOp(op, val) => UnaryOp(op, val.try_fold_with(folder)?),
+            Discriminant(place) => Discriminant(place.try_fold_with(folder)?),
+            NullaryOp(op, ty) => NullaryOp(op, ty.try_fold_with(folder)?),
+            Aggregate(kind, fields) => {
+                let kind = kind.try_map_id(|kind| {
+                    Ok(match kind {
+                        AggregateKind::Array(ty) => AggregateKind::Array(ty.try_fold_with(folder)?),
+                        AggregateKind::Tuple => AggregateKind::Tuple,
+                        AggregateKind::Adt(def, v, substs, user_ty, n) => AggregateKind::Adt(
+                            def,
+                            v,
+                            substs.try_fold_with(folder)?,
+                            user_ty.try_fold_with(folder)?,
+                            n,
+                        ),
+                        AggregateKind::Closure(id, substs) => {
+                            AggregateKind::Closure(id, substs.try_fold_with(folder)?)
+                        }
+                        AggregateKind::Generator(id, substs, movablity) => {
+                            AggregateKind::Generator(id, substs.try_fold_with(folder)?, movablity)
+                        }
+                    })
+                })?;
+                Aggregate(kind, fields.try_fold_with(folder)?)
+            }
+            ShallowInitBox(op, ty) => {
+                ShallowInitBox(op.try_fold_with(folder)?, ty.try_fold_with(folder)?)
+            }
+        })
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for Operand<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        Ok(match self {
+            Operand::Copy(place) => Operand::Copy(place.try_fold_with(folder)?),
+            Operand::Move(place) => Operand::Move(place.try_fold_with(folder)?),
+            Operand::Constant(c) => Operand::Constant(c.try_fold_with(folder)?),
+        })
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for PlaceElem<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        use crate::mir::ProjectionElem::*;
+
+        Ok(match self {
+            Deref => Deref,
+            Field(f, ty) => Field(f, ty.try_fold_with(folder)?),
+            Index(v) => Index(v.try_fold_with(folder)?),
+            Downcast(symbol, variantidx) => Downcast(symbol, variantidx),
+            ConstantIndex { offset, min_length, from_end } => {
+                ConstantIndex { offset, min_length, from_end }
+            }
+            Subslice { from, to, from_end } => Subslice { from, to, from_end },
+        })
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for Field {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
+        Ok(self)
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for GeneratorSavedLocal {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
+        Ok(self)
+    }
+}
+
+impl<'tcx, R: Idx, C: Idx> TypeFoldable<'tcx> for BitMatrix<R, C> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
+        Ok(self)
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for Constant<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        Ok(Constant {
+            span: self.span,
+            user_ty: self.user_ty.try_fold_with(folder)?,
+            literal: self.literal.try_fold_with(folder)?,
+        })
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for ConstantKind<'tcx> {
+    #[inline(always)]
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        folder.try_fold_mir_const(self)
+    }
+}
+
+impl<'tcx> TypeSuperFoldable<'tcx> for ConstantKind<'tcx> {
+    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        match self {
+            ConstantKind::Ty(c) => Ok(ConstantKind::Ty(c.try_fold_with(folder)?)),
+            ConstantKind::Val(v, t) => Ok(ConstantKind::Val(v, t.try_fold_with(folder)?)),
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/type_visitable.rs b/compiler/rustc_middle/src/mir/type_visitable.rs
new file mode 100644
index 000000000..6a0801cb0
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/type_visitable.rs
@@ -0,0 +1,190 @@
+//! `TypeVisitable` implementations for MIR types
+
+use super::*;
+use crate::ty;
+
+impl<'tcx> TypeVisitable<'tcx> for Terminator<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        use crate::mir::TerminatorKind::*;
+
+        match self.kind {
+            SwitchInt { ref discr, switch_ty, .. } => {
+                discr.visit_with(visitor)?;
+                switch_ty.visit_with(visitor)
+            }
+            Drop { ref place, .. } => place.visit_with(visitor),
+            DropAndReplace { ref place, ref value, .. } => {
+                place.visit_with(visitor)?;
+                value.visit_with(visitor)
+            }
+            Yield { ref value, .. } => value.visit_with(visitor),
+            Call { ref func, ref args, ref destination, .. } => {
+                destination.visit_with(visitor)?;
+                func.visit_with(visitor)?;
+                args.visit_with(visitor)
+            }
+            Assert { ref cond, ref msg, .. } => {
+                cond.visit_with(visitor)?;
+                use AssertKind::*;
+                match msg {
+                    BoundsCheck { ref len, ref index } => {
+                        len.visit_with(visitor)?;
+                        index.visit_with(visitor)
+                    }
+                    Overflow(_, l, r) => {
+                        l.visit_with(visitor)?;
+                        r.visit_with(visitor)
+                    }
+                    OverflowNeg(op) | DivisionByZero(op) | RemainderByZero(op) => {
+                        op.visit_with(visitor)
+                    }
+                    ResumedAfterReturn(_) | ResumedAfterPanic(_) => ControlFlow::CONTINUE,
+                }
+            }
+            InlineAsm { ref operands, .. } => operands.visit_with(visitor),
+            Goto { .. }
+            | Resume
+            | Abort
+            | Return
+            | GeneratorDrop
+            | Unreachable
+            | FalseEdge { .. }
+            | FalseUnwind { .. } => ControlFlow::CONTINUE,
+        }
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for GeneratorKind {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
+        ControlFlow::CONTINUE
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for Place<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.local.visit_with(visitor)?;
+        self.projection.visit_with(visitor)
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for &'tcx ty::List<PlaceElem<'tcx>> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.iter().try_for_each(|t| t.visit_with(visitor))
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for Rvalue<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        use crate::mir::Rvalue::*;
+        match *self {
+            Use(ref op) => op.visit_with(visitor),
+            CopyForDeref(ref place) => {
+                let op = &Operand::Copy(*place);
+                op.visit_with(visitor)
+            }
+            Repeat(ref op, _) => op.visit_with(visitor),
+            ThreadLocalRef(did) => did.visit_with(visitor),
+            Ref(region, _, ref place) => {
+                region.visit_with(visitor)?;
+                place.visit_with(visitor)
+            }
+            AddressOf(_, ref place) => place.visit_with(visitor),
+            Len(ref place) => place.visit_with(visitor),
+            Cast(_, ref op, ty) => {
+                op.visit_with(visitor)?;
+                ty.visit_with(visitor)
+            }
+            BinaryOp(_, box (ref rhs, ref lhs)) | CheckedBinaryOp(_, box (ref rhs, ref lhs)) => {
+                rhs.visit_with(visitor)?;
+                lhs.visit_with(visitor)
+            }
+            UnaryOp(_, ref val) => val.visit_with(visitor),
+            Discriminant(ref place) => place.visit_with(visitor),
+            NullaryOp(_, ty) => ty.visit_with(visitor),
+            Aggregate(ref kind, ref fields) => {
+                match **kind {
+                    AggregateKind::Array(ty) => {
+                        ty.visit_with(visitor)?;
+                    }
+                    AggregateKind::Tuple => {}
+                    AggregateKind::Adt(_, _, substs, user_ty, _) => {
+                        substs.visit_with(visitor)?;
+                        user_ty.visit_with(visitor)?;
+                    }
+                    AggregateKind::Closure(_, substs) => {
+                        substs.visit_with(visitor)?;
+                    }
+                    AggregateKind::Generator(_, substs, _) => {
+                        substs.visit_with(visitor)?;
+                    }
+                }
+                fields.visit_with(visitor)
+            }
+            ShallowInitBox(ref op, ty) => {
+                op.visit_with(visitor)?;
+                ty.visit_with(visitor)
+            }
+        }
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for Operand<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        match *self {
+            Operand::Copy(ref place) | Operand::Move(ref place) => place.visit_with(visitor),
+            Operand::Constant(ref c) => c.visit_with(visitor),
+        }
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for PlaceElem<'tcx> {
+    fn visit_with<Vs: TypeVisitor<'tcx>>(&self, visitor: &mut Vs) -> ControlFlow<Vs::BreakTy> {
+        use crate::mir::ProjectionElem::*;
+
+        match self {
+            Field(_, ty) => ty.visit_with(visitor),
+            Index(v) => v.visit_with(visitor),
+            _ => ControlFlow::CONTINUE,
+        }
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for Field {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
+        ControlFlow::CONTINUE
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for GeneratorSavedLocal {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
+        ControlFlow::CONTINUE
+    }
+}
+
+impl<'tcx, R: Idx, C: Idx> TypeVisitable<'tcx> for BitMatrix<R, C> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
+        ControlFlow::CONTINUE
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for Constant<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.literal.visit_with(visitor)?;
+        self.user_ty.visit_with(visitor)
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for ConstantKind<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        visitor.visit_mir_const(*self)
+    }
+}
+
+impl<'tcx> TypeSuperVisitable<'tcx> for ConstantKind<'tcx> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        match *self {
+            ConstantKind::Ty(c) => c.visit_with(visitor),
+            ConstantKind::Val(_, t) => t.visit_with(visitor),
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/mir/visit.rs b/compiler/rustc_middle/src/mir/visit.rs
new file mode 100644
index 000000000..891608764
--- /dev/null
+++ b/compiler/rustc_middle/src/mir/visit.rs
@@ -0,0 +1,1330 @@
+//! # The MIR Visitor
+//!
+//! ## Overview
+//!
+//! There are two visitors, one for immutable and one for mutable references,
+//! but both are generated by the following macro. The code is written according
+//! to the following conventions:
+//!
+//! - introduce a `visit_foo` and a `super_foo` method for every MIR type
+//! - `visit_foo`, by default, calls `super_foo`
+//! - `super_foo`, by default, destructures the `foo` and calls `visit_foo`
+//!
+//! This allows you as a user to override `visit_foo` for types are
+//! interested in, and invoke (within that method) call
+//! `self.super_foo` to get the default behavior. Just as in an OO
+//! language, you should never call `super` methods ordinarily except
+//! in that circumstance.
+//!
+//! For the most part, we do not destructure things external to the
+//! MIR, e.g., types, spans, etc, but simply visit them and stop. This
+//! avoids duplication with other visitors like `TypeFoldable`.
+//!
+//! ## Updating
+//!
+//! The code is written in a very deliberate style intended to minimize
+//! the chance of things being overlooked. You'll notice that we always
+//! use pattern matching to reference fields and we ensure that all
+//! matches are exhaustive.
+//!
+//! For example, the `super_basic_block_data` method begins like this:
+//!
+//! ```ignore (pseudo-rust)
+//! fn super_basic_block_data(
+//!     &mut self,
+//!     block: BasicBlock,
+//!     data: & $($mutability)? BasicBlockData<'tcx>
+//! ) {
+//!     let BasicBlockData {
+//!         statements,
+//!         terminator,
+//!         is_cleanup: _
+//!     } = *data;
+//!
+//!     for statement in statements {
+//!         self.visit_statement(block, statement);
+//!     }
+//!
+//!     ...
+//! }
+//! ```
+//!
+//! Here we used `let BasicBlockData { <fields> } = *data` deliberately,
+//! rather than writing `data.statements` in the body. This is because if one
+//! adds a new field to `BasicBlockData`, one will be forced to revise this code,
+//! and hence one will (hopefully) invoke the correct visit methods (if any).
+//!
+//! For this to work, ALL MATCHES MUST BE EXHAUSTIVE IN FIELDS AND VARIANTS.
+//! That means you never write `..` to skip over fields, nor do you write `_`
+//! to skip over variants in a `match`.
+//!
+//! The only place that `_` is acceptable is to match a field (or
+//! variant argument) that does not require visiting, as in
+//! `is_cleanup` above.
+
+use crate::mir::*;
+use crate::ty::subst::SubstsRef;
+use crate::ty::{CanonicalUserTypeAnnotation, Ty};
+use rustc_span::Span;
+
+macro_rules! make_mir_visitor {
+    ($visitor_trait_name:ident, $($mutability:ident)?) => {
+        pub trait $visitor_trait_name<'tcx> {
+            // Override these, and call `self.super_xxx` to revert back to the
+            // default behavior.
+
+            fn visit_body(
+                &mut self,
+                body: &$($mutability)? Body<'tcx>,
+            ) {
+                self.super_body(body);
+            }
+
+            fn visit_basic_block_data(
+                &mut self,
+                block: BasicBlock,
+                data: & $($mutability)? BasicBlockData<'tcx>,
+            ) {
+                self.super_basic_block_data(block, data);
+            }
+
+            fn visit_source_scope_data(
+                &mut self,
+                scope_data: & $($mutability)? SourceScopeData<'tcx>,
+            ) {
+                self.super_source_scope_data(scope_data);
+            }
+
+            fn visit_statement(
+                &mut self,
+                statement: & $($mutability)? Statement<'tcx>,
+                location: Location,
+            ) {
+                self.super_statement(statement, location);
+            }
+
+            fn visit_assign(
+                &mut self,
+                place: & $($mutability)? Place<'tcx>,
+                rvalue: & $($mutability)? Rvalue<'tcx>,
+                location: Location,
+            ) {
+                self.super_assign(place, rvalue, location);
+            }
+
+            fn visit_terminator(
+                &mut self,
+                terminator: & $($mutability)? Terminator<'tcx>,
+                location: Location,
+            ) {
+                self.super_terminator(terminator, location);
+            }
+
+            fn visit_assert_message(
+                &mut self,
+                msg: & $($mutability)? AssertMessage<'tcx>,
+                location: Location,
+            ) {
+                self.super_assert_message(msg, location);
+            }
+
+            fn visit_rvalue(
+                &mut self,
+                rvalue: & $($mutability)? Rvalue<'tcx>,
+                location: Location,
+            ) {
+                self.super_rvalue(rvalue, location);
+            }
+
+            fn visit_operand(
+                &mut self,
+                operand: & $($mutability)? Operand<'tcx>,
+                location: Location,
+            ) {
+                self.super_operand(operand, location);
+            }
+
+            fn visit_ascribe_user_ty(
+                &mut self,
+                place: & $($mutability)? Place<'tcx>,
+                variance: $(& $mutability)? ty::Variance,
+                user_ty: & $($mutability)? UserTypeProjection,
+                location: Location,
+            ) {
+                self.super_ascribe_user_ty(place, variance, user_ty, location);
+            }
+
+            fn visit_coverage(
+                &mut self,
+                coverage: & $($mutability)? Coverage,
+                location: Location,
+            ) {
+                self.super_coverage(coverage, location);
+            }
+
+            fn visit_retag(
+                &mut self,
+                kind: $(& $mutability)? RetagKind,
+                place: & $($mutability)? Place<'tcx>,
+                location: Location,
+            ) {
+                self.super_retag(kind, place, location);
+            }
+
+            fn visit_place(
+                &mut self,
+                place: & $($mutability)? Place<'tcx>,
+                context: PlaceContext,
+                location: Location,
+            ) {
+                self.super_place(place, context, location);
+            }
+
+            visit_place_fns!($($mutability)?);
+
+            fn visit_constant(
+                &mut self,
+                constant: & $($mutability)? Constant<'tcx>,
+                location: Location,
+            ) {
+                self.super_constant(constant, location);
+            }
+
+            fn visit_span(
+                &mut self,
+                span: $(& $mutability)? Span,
+            ) {
+                self.super_span(span);
+            }
+
+            fn visit_source_info(
+                &mut self,
+                source_info: & $($mutability)? SourceInfo,
+            ) {
+                self.super_source_info(source_info);
+            }
+
+            fn visit_ty(
+                &mut self,
+                ty: $(& $mutability)? Ty<'tcx>,
+                _: TyContext,
+            ) {
+                self.super_ty(ty);
+            }
+
+            fn visit_user_type_projection(
+                &mut self,
+                ty: & $($mutability)? UserTypeProjection,
+            ) {
+                self.super_user_type_projection(ty);
+            }
+
+            fn visit_user_type_annotation(
+                &mut self,
+                index: UserTypeAnnotationIndex,
+                ty: & $($mutability)? CanonicalUserTypeAnnotation<'tcx>,
+            ) {
+                self.super_user_type_annotation(index, ty);
+            }
+
+            fn visit_region(
+                &mut self,
+                region: $(& $mutability)? ty::Region<'tcx>,
+                _: Location,
+            ) {
+                self.super_region(region);
+            }
+
+            fn visit_const(
+                &mut self,
+                constant: $(& $mutability)? ty::Const<'tcx>,
+                _: Location,
+            ) {
+                self.super_const(constant);
+            }
+
+            fn visit_substs(
+                &mut self,
+                substs: & $($mutability)? SubstsRef<'tcx>,
+                _: Location,
+            ) {
+                self.super_substs(substs);
+            }
+
+            fn visit_local_decl(
+                &mut self,
+                local: Local,
+                local_decl: & $($mutability)? LocalDecl<'tcx>,
+            ) {
+                self.super_local_decl(local, local_decl);
+            }
+
+            fn visit_var_debug_info(
+                &mut self,
+                var_debug_info: & $($mutability)* VarDebugInfo<'tcx>,
+            ) {
+                self.super_var_debug_info(var_debug_info);
+            }
+
+            fn visit_local(
+                &mut self,
+                _local: $(& $mutability)? Local,
+                _context: PlaceContext,
+                _location: Location,
+            ) {}
+
+            fn visit_source_scope(
+                &mut self,
+                scope: $(& $mutability)? SourceScope,
+            ) {
+                self.super_source_scope(scope);
+            }
+
+            // The `super_xxx` methods comprise the default behavior and are
+            // not meant to be overridden.
+
+            fn super_body(
+                &mut self,
+                body: &$($mutability)? Body<'tcx>,
+            ) {
+                let span = body.span;
+                if let Some(gen) = &$($mutability)? body.generator {
+                    if let Some(yield_ty) = $(& $mutability)? gen.yield_ty {
+                        self.visit_ty(
+                            yield_ty,
+                            TyContext::YieldTy(SourceInfo::outermost(span))
+                        );
+                    }
+                }
+
+                // for best performance, we want to use an iterator rather
+                // than a for-loop, to avoid calling `body::Body::invalidate` for
+                // each basic block.
+                #[allow(unused_macro_rules)]
+                macro_rules! basic_blocks {
+                    (mut) => (body.basic_blocks_mut().iter_enumerated_mut());
+                    () => (body.basic_blocks().iter_enumerated());
+                }
+                for (bb, data) in basic_blocks!($($mutability)?) {
+                    self.visit_basic_block_data(bb, data);
+                }
+
+                for scope in &$($mutability)? body.source_scopes {
+                    self.visit_source_scope_data(scope);
+                }
+
+                self.visit_ty(
+                    $(& $mutability)? body.return_ty(),
+                    TyContext::ReturnTy(SourceInfo::outermost(body.span))
+                );
+
+                for local in body.local_decls.indices() {
+                    self.visit_local_decl(local, & $($mutability)? body.local_decls[local]);
+                }
+
+                #[allow(unused_macro_rules)]
+                macro_rules! type_annotations {
+                    (mut) => (body.user_type_annotations.iter_enumerated_mut());
+                    () => (body.user_type_annotations.iter_enumerated());
+                }
+
+                for (index, annotation) in type_annotations!($($mutability)?) {
+                    self.visit_user_type_annotation(
+                        index, annotation
+                    );
+                }
+
+                for var_debug_info in &$($mutability)? body.var_debug_info {
+                    self.visit_var_debug_info(var_debug_info);
+                }
+
+                self.visit_span($(& $mutability)? body.span);
+
+                for const_ in &$($mutability)? body.required_consts {
+                    let location = START_BLOCK.start_location();
+                    self.visit_constant(const_, location);
+                }
+            }
+
+            fn super_basic_block_data(&mut self,
+                                      block: BasicBlock,
+                                      data: & $($mutability)? BasicBlockData<'tcx>) {
+                let BasicBlockData {
+                    statements,
+                    terminator,
+                    is_cleanup: _
+                } = data;
+
+                let mut index = 0;
+                for statement in statements {
+                    let location = Location { block, statement_index: index };
+                    self.visit_statement(statement, location);
+                    index += 1;
+                }
+
+                if let Some(terminator) = terminator {
+                    let location = Location { block, statement_index: index };
+                    self.visit_terminator(terminator, location);
+                }
+            }
+
+            fn super_source_scope_data(
+                &mut self,
+                scope_data: & $($mutability)? SourceScopeData<'tcx>,
+            ) {
+                let SourceScopeData {
+                    span,
+                    parent_scope,
+                    inlined,
+                    inlined_parent_scope,
+                    local_data: _,
+                } = scope_data;
+
+                self.visit_span($(& $mutability)? *span);
+                if let Some(parent_scope) = parent_scope {
+                    self.visit_source_scope($(& $mutability)? *parent_scope);
+                }
+                if let Some((callee, callsite_span)) = inlined {
+                    let location = START_BLOCK.start_location();
+
+                    self.visit_span($(& $mutability)? *callsite_span);
+
+                    let ty::Instance { def: callee_def, substs: callee_substs } = callee;
+                    match callee_def {
+                        ty::InstanceDef::Item(_def_id) => {}
+
+                        ty::InstanceDef::Intrinsic(_def_id) |
+                        ty::InstanceDef::VTableShim(_def_id) |
+                        ty::InstanceDef::ReifyShim(_def_id) |
+                        ty::InstanceDef::Virtual(_def_id, _) |
+                        ty::InstanceDef::ClosureOnceShim { call_once: _def_id, track_caller: _ } |
+                        ty::InstanceDef::DropGlue(_def_id, None) => {}
+
+                        ty::InstanceDef::FnPtrShim(_def_id, ty) |
+                        ty::InstanceDef::DropGlue(_def_id, Some(ty)) |
+                        ty::InstanceDef::CloneShim(_def_id, ty) => {
+                            // FIXME(eddyb) use a better `TyContext` here.
+                            self.visit_ty($(& $mutability)? *ty, TyContext::Location(location));
+                        }
+                    }
+                    self.visit_substs(callee_substs, location);
+                }
+                if let Some(inlined_parent_scope) = inlined_parent_scope {
+                    self.visit_source_scope($(& $mutability)? *inlined_parent_scope);
+                }
+            }
+
+            fn super_statement(&mut self,
+                               statement: & $($mutability)? Statement<'tcx>,
+                               location: Location) {
+                let Statement {
+                    source_info,
+                    kind,
+                } = statement;
+
+                self.visit_source_info(source_info);
+                match kind {
+                    StatementKind::Assign(
+                        box (place, rvalue)
+                    ) => {
+                        self.visit_assign(place, rvalue, location);
+                    }
+                    StatementKind::FakeRead(box (_, place)) => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect),
+                            location
+                        );
+                    }
+                    StatementKind::SetDiscriminant { place, .. } => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::MutatingUse(MutatingUseContext::SetDiscriminant),
+                            location
+                        );
+                    }
+                    StatementKind::Deinit(place) => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::MutatingUse(MutatingUseContext::Deinit),
+                            location
+                        )
+                    }
+                    StatementKind::StorageLive(local) => {
+                        self.visit_local(
+                            $(& $mutability)? *local,
+                            PlaceContext::NonUse(NonUseContext::StorageLive),
+                            location
+                        );
+                    }
+                    StatementKind::StorageDead(local) => {
+                        self.visit_local(
+                            $(& $mutability)? *local,
+                            PlaceContext::NonUse(NonUseContext::StorageDead),
+                            location
+                        );
+                    }
+                    StatementKind::Retag(kind, place) => {
+                        self.visit_retag($(& $mutability)? *kind, place, location);
+                    }
+                    StatementKind::AscribeUserType(
+                        box (place, user_ty),
+                        variance
+                    ) => {
+                        self.visit_ascribe_user_ty(place, $(& $mutability)? *variance, user_ty, location);
+                    }
+                    StatementKind::Coverage(coverage) => {
+                        self.visit_coverage(
+                            coverage,
+                            location
+                        )
+                    }
+                    StatementKind::CopyNonOverlapping(box crate::mir::CopyNonOverlapping{
+                        src,
+                        dst,
+                        count,
+                    }) => {
+                      self.visit_operand(src, location);
+                      self.visit_operand(dst, location);
+                      self.visit_operand(count, location)
+                    }
+                    StatementKind::Nop => {}
+                }
+            }
+
+            fn super_assign(&mut self,
+                            place: &$($mutability)? Place<'tcx>,
+                            rvalue: &$($mutability)? Rvalue<'tcx>,
+                            location: Location) {
+                self.visit_place(
+                    place,
+                    PlaceContext::MutatingUse(MutatingUseContext::Store),
+                    location
+                );
+                self.visit_rvalue(rvalue, location);
+            }
+
+            fn super_terminator(&mut self,
+                                terminator: &$($mutability)? Terminator<'tcx>,
+                                location: Location) {
+                let Terminator { source_info, kind } = terminator;
+
+                self.visit_source_info(source_info);
+                match kind {
+                    TerminatorKind::Goto { .. } |
+                    TerminatorKind::Resume |
+                    TerminatorKind::Abort |
+                    TerminatorKind::GeneratorDrop |
+                    TerminatorKind::Unreachable |
+                    TerminatorKind::FalseEdge { .. } |
+                    TerminatorKind::FalseUnwind { .. } => {}
+
+                    TerminatorKind::Return => {
+                        // `return` logically moves from the return place `_0`. Note that the place
+                        // cannot be changed by any visitor, though.
+                        let $($mutability)? local = RETURN_PLACE;
+                        self.visit_local(
+                            $(& $mutability)? local,
+                            PlaceContext::NonMutatingUse(NonMutatingUseContext::Move),
+                            location,
+                        );
+
+                        assert_eq!(
+                            local,
+                            RETURN_PLACE,
+                            "`MutVisitor` tried to mutate return place of `return` terminator"
+                        );
+                    }
+
+                    TerminatorKind::SwitchInt {
+                        discr,
+                        switch_ty,
+                        targets: _
+                    } => {
+                        self.visit_operand(discr, location);
+                        self.visit_ty($(& $mutability)? *switch_ty, TyContext::Location(location));
+                    }
+
+                    TerminatorKind::Drop {
+                        place,
+                        target: _,
+                        unwind: _,
+                    } => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::MutatingUse(MutatingUseContext::Drop),
+                            location
+                        );
+                    }
+
+                    TerminatorKind::DropAndReplace {
+                        place,
+                        value,
+                        target: _,
+                        unwind: _,
+                    } => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::MutatingUse(MutatingUseContext::Drop),
+                            location
+                        );
+                        self.visit_operand(value, location);
+                    }
+
+                    TerminatorKind::Call {
+                        func,
+                        args,
+                        destination,
+                        target: _,
+                        cleanup: _,
+                        from_hir_call: _,
+                        fn_span: _
+                    } => {
+                        self.visit_operand(func, location);
+                        for arg in args {
+                            self.visit_operand(arg, location);
+                        }
+                        self.visit_place(
+                            destination,
+                            PlaceContext::MutatingUse(MutatingUseContext::Call),
+                            location
+                        );
+                    }
+
+                    TerminatorKind::Assert {
+                        cond,
+                        expected: _,
+                        msg,
+                        target: _,
+                        cleanup: _,
+                    } => {
+                        self.visit_operand(cond, location);
+                        self.visit_assert_message(msg, location);
+                    }
+
+                    TerminatorKind::Yield {
+                        value,
+                        resume: _,
+                        resume_arg,
+                        drop: _,
+                    } => {
+                        self.visit_operand(value, location);
+                        self.visit_place(
+                            resume_arg,
+                            PlaceContext::MutatingUse(MutatingUseContext::Yield),
+                            location,
+                        );
+                    }
+
+                    TerminatorKind::InlineAsm {
+                        template: _,
+                        operands,
+                        options: _,
+                        line_spans: _,
+                        destination: _,
+                        cleanup: _,
+                    } => {
+                        for op in operands {
+                            match op {
+                                InlineAsmOperand::In { value, .. } => {
+                                    self.visit_operand(value, location);
+                                }
+                                InlineAsmOperand::Out { place: Some(place), .. } => {
+                                    self.visit_place(
+                                        place,
+                                        PlaceContext::MutatingUse(MutatingUseContext::AsmOutput),
+                                        location,
+                                    );
+                                }
+                                InlineAsmOperand::InOut { in_value, out_place, .. } => {
+                                    self.visit_operand(in_value, location);
+                                    if let Some(out_place) = out_place {
+                                        self.visit_place(
+                                            out_place,
+                                            PlaceContext::MutatingUse(MutatingUseContext::AsmOutput),
+                                            location,
+                                        );
+                                    }
+                                }
+                                InlineAsmOperand::Const { value }
+                                | InlineAsmOperand::SymFn { value } => {
+                                    self.visit_constant(value, location);
+                                }
+                                InlineAsmOperand::Out { place: None, .. }
+                                | InlineAsmOperand::SymStatic { def_id: _ } => {}
+                            }
+                        }
+                    }
+                }
+            }
+
+            fn super_assert_message(&mut self,
+                                    msg: & $($mutability)? AssertMessage<'tcx>,
+                                    location: Location) {
+                use crate::mir::AssertKind::*;
+                match msg {
+                    BoundsCheck { len, index } => {
+                        self.visit_operand(len, location);
+                        self.visit_operand(index, location);
+                    }
+                    Overflow(_, l, r) => {
+                        self.visit_operand(l, location);
+                        self.visit_operand(r, location);
+                    }
+                    OverflowNeg(op) | DivisionByZero(op) | RemainderByZero(op) => {
+                        self.visit_operand(op, location);
+                    }
+                    ResumedAfterReturn(_) | ResumedAfterPanic(_) => {
+                        // Nothing to visit
+                    }
+                }
+            }
+
+            fn super_rvalue(&mut self,
+                            rvalue: & $($mutability)? Rvalue<'tcx>,
+                            location: Location) {
+                match rvalue {
+                    Rvalue::Use(operand) => {
+                        self.visit_operand(operand, location);
+                    }
+
+                    Rvalue::Repeat(value, _) => {
+                        self.visit_operand(value, location);
+                    }
+
+                    Rvalue::ThreadLocalRef(_) => {}
+
+                    Rvalue::Ref(r, bk, path) => {
+                        self.visit_region($(& $mutability)? *r, location);
+                        let ctx = match bk {
+                            BorrowKind::Shared => PlaceContext::NonMutatingUse(
+                                NonMutatingUseContext::SharedBorrow
+                            ),
+                            BorrowKind::Shallow => PlaceContext::NonMutatingUse(
+                                NonMutatingUseContext::ShallowBorrow
+                            ),
+                            BorrowKind::Unique => PlaceContext::NonMutatingUse(
+                                NonMutatingUseContext::UniqueBorrow
+                            ),
+                            BorrowKind::Mut { .. } =>
+                                PlaceContext::MutatingUse(MutatingUseContext::Borrow),
+                        };
+                        self.visit_place(path, ctx, location);
+                    }
+                    Rvalue::CopyForDeref(place) => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect),
+                            location
+                        );
+                    }
+
+                    Rvalue::AddressOf(m, path) => {
+                        let ctx = match m {
+                            Mutability::Mut => PlaceContext::MutatingUse(
+                                MutatingUseContext::AddressOf
+                            ),
+                            Mutability::Not => PlaceContext::NonMutatingUse(
+                                NonMutatingUseContext::AddressOf
+                            ),
+                        };
+                        self.visit_place(path, ctx, location);
+                    }
+
+                    Rvalue::Len(path) => {
+                        self.visit_place(
+                            path,
+                            PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect),
+                            location
+                        );
+                    }
+
+                    Rvalue::Cast(_cast_kind, operand, ty) => {
+                        self.visit_operand(operand, location);
+                        self.visit_ty($(& $mutability)? *ty, TyContext::Location(location));
+                    }
+
+                    Rvalue::BinaryOp(_bin_op, box(lhs, rhs))
+                    | Rvalue::CheckedBinaryOp(_bin_op, box(lhs, rhs)) => {
+                        self.visit_operand(lhs, location);
+                        self.visit_operand(rhs, location);
+                    }
+
+                    Rvalue::UnaryOp(_un_op, op) => {
+                        self.visit_operand(op, location);
+                    }
+
+                    Rvalue::Discriminant(place) => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect),
+                            location
+                        );
+                    }
+
+                    Rvalue::NullaryOp(_op, ty) => {
+                        self.visit_ty($(& $mutability)? *ty, TyContext::Location(location));
+                    }
+
+                    Rvalue::Aggregate(kind, operands) => {
+                        let kind = &$($mutability)? **kind;
+                        match kind {
+                            AggregateKind::Array(ty) => {
+                                self.visit_ty($(& $mutability)? *ty, TyContext::Location(location));
+                            }
+                            AggregateKind::Tuple => {
+                            }
+                            AggregateKind::Adt(
+                                _adt_def,
+                                _variant_index,
+                                substs,
+                                _user_substs,
+                                _active_field_index
+                            ) => {
+                                self.visit_substs(substs, location);
+                            }
+                            AggregateKind::Closure(
+                                _,
+                                closure_substs
+                            ) => {
+                                self.visit_substs(closure_substs, location);
+                            }
+                            AggregateKind::Generator(
+                                _,
+                                generator_substs,
+                                _movability,
+                            ) => {
+                                self.visit_substs(generator_substs, location);
+                            }
+                        }
+
+                        for operand in operands {
+                            self.visit_operand(operand, location);
+                        }
+                    }
+
+                    Rvalue::ShallowInitBox(operand, ty) => {
+                        self.visit_operand(operand, location);
+                        self.visit_ty($(& $mutability)? *ty, TyContext::Location(location));
+                    }
+                }
+            }
+
+            fn super_operand(&mut self,
+                             operand: & $($mutability)? Operand<'tcx>,
+                             location: Location) {
+                match operand {
+                    Operand::Copy(place) => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
+                            location
+                        );
+                    }
+                    Operand::Move(place) => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::NonMutatingUse(NonMutatingUseContext::Move),
+                            location
+                        );
+                    }
+                    Operand::Constant(constant) => {
+                        self.visit_constant(constant, location);
+                    }
+                }
+            }
+
+            fn super_ascribe_user_ty(&mut self,
+                                     place: & $($mutability)? Place<'tcx>,
+                                     _variance: $(& $mutability)? ty::Variance,
+                                     user_ty: & $($mutability)? UserTypeProjection,
+                                     location: Location) {
+                self.visit_place(
+                    place,
+                    PlaceContext::NonUse(NonUseContext::AscribeUserTy),
+                    location
+                );
+                self.visit_user_type_projection(user_ty);
+            }
+
+            fn super_coverage(&mut self,
+                              _coverage: & $($mutability)? Coverage,
+                              _location: Location) {
+            }
+
+            fn super_retag(&mut self,
+                           _kind: $(& $mutability)? RetagKind,
+                           place: & $($mutability)? Place<'tcx>,
+                           location: Location) {
+                self.visit_place(
+                    place,
+                    PlaceContext::MutatingUse(MutatingUseContext::Retag),
+                    location,
+                );
+            }
+
+            fn super_local_decl(&mut self,
+                                local: Local,
+                                local_decl: & $($mutability)? LocalDecl<'tcx>) {
+                let LocalDecl {
+                    mutability: _,
+                    ty,
+                    user_ty,
+                    source_info,
+                    internal: _,
+                    local_info: _,
+                    is_block_tail: _,
+                } = local_decl;
+
+                self.visit_ty($(& $mutability)? *ty, TyContext::LocalDecl {
+                    local,
+                    source_info: *source_info,
+                });
+                if let Some(user_ty) = user_ty {
+                    for (user_ty, _) in & $($mutability)? user_ty.contents {
+                        self.visit_user_type_projection(user_ty);
+                    }
+                }
+                self.visit_source_info(source_info);
+            }
+
+            fn super_var_debug_info(
+                &mut self,
+                var_debug_info: & $($mutability)? VarDebugInfo<'tcx>
+            ) {
+                let VarDebugInfo {
+                    name: _,
+                    source_info,
+                    value,
+                } = var_debug_info;
+
+                self.visit_source_info(source_info);
+                let location = START_BLOCK.start_location();
+                match value {
+                    VarDebugInfoContents::Const(c) => self.visit_constant(c, location),
+                    VarDebugInfoContents::Place(place) =>
+                        self.visit_place(
+                            place,
+                            PlaceContext::NonUse(NonUseContext::VarDebugInfo),
+                            location
+                        ),
+                }
+            }
+
+            fn super_source_scope(
+                &mut self,
+                _scope: $(& $mutability)? SourceScope
+            ) {}
+
+            fn super_constant(
+                &mut self,
+                constant: & $($mutability)? Constant<'tcx>,
+                location: Location
+            ) {
+                let Constant {
+                    span,
+                    user_ty,
+                    literal,
+                } = constant;
+
+                self.visit_span($(& $mutability)? *span);
+                drop(user_ty); // no visit method for this
+                match literal {
+                    ConstantKind::Ty(ct) => self.visit_const($(& $mutability)? *ct, location),
+                    ConstantKind::Val(_, ty) => self.visit_ty($(& $mutability)? *ty, TyContext::Location(location)),
+                }
+            }
+
+            fn super_span(&mut self, _span: $(& $mutability)? Span) {
+            }
+
+            fn super_source_info(&mut self, source_info: & $($mutability)? SourceInfo) {
+                let SourceInfo {
+                    span,
+                    scope,
+                } = source_info;
+
+                self.visit_span($(& $mutability)? *span);
+                self.visit_source_scope($(& $mutability)? *scope);
+            }
+
+            fn super_user_type_projection(
+                &mut self,
+                _ty: & $($mutability)? UserTypeProjection,
+            ) {
+            }
+
+            fn super_user_type_annotation(
+                &mut self,
+                _index: UserTypeAnnotationIndex,
+                ty: & $($mutability)? CanonicalUserTypeAnnotation<'tcx>,
+            ) {
+                self.visit_span($(& $mutability)? ty.span);
+                self.visit_ty($(& $mutability)? ty.inferred_ty, TyContext::UserTy(ty.span));
+            }
+
+            fn super_ty(&mut self, _ty: $(& $mutability)? Ty<'tcx>) {
+            }
+
+            fn super_region(&mut self, _region: $(& $mutability)? ty::Region<'tcx>) {
+            }
+
+            fn super_const(&mut self, _const: $(& $mutability)? ty::Const<'tcx>) {
+            }
+
+            fn super_substs(&mut self, _substs: & $($mutability)? SubstsRef<'tcx>) {
+            }
+
+            // Convenience methods
+
+            fn visit_location(
+                &mut self,
+                body: &$($mutability)? Body<'tcx>,
+                location: Location
+            ) {
+                #[allow(unused_macro_rules)]
+                macro_rules! basic_blocks {
+                    (mut) => (body.basic_blocks_mut());
+                    () => (body.basic_blocks());
+                }
+                let basic_block = & $($mutability)? basic_blocks!($($mutability)?)[location.block];
+                if basic_block.statements.len() == location.statement_index {
+                    if let Some(ref $($mutability)? terminator) = basic_block.terminator {
+                        self.visit_terminator(terminator, location)
+                    }
+                } else {
+                    let statement = & $($mutability)?
+                        basic_block.statements[location.statement_index];
+                    self.visit_statement(statement, location)
+                }
+            }
+        }
+    }
+}
+
+macro_rules! visit_place_fns {
+    (mut) => {
+        fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
+
+        fn super_place(
+            &mut self,
+            place: &mut Place<'tcx>,
+            context: PlaceContext,
+            location: Location,
+        ) {
+            self.visit_local(&mut place.local, context, location);
+
+            if let Some(new_projection) = self.process_projection(&place.projection, location) {
+                place.projection = self.tcx().intern_place_elems(&new_projection);
+            }
+        }
+
+        fn process_projection<'a>(
+            &mut self,
+            projection: &'a [PlaceElem<'tcx>],
+            location: Location,
+        ) -> Option<Vec<PlaceElem<'tcx>>> {
+            let mut projection = Cow::Borrowed(projection);
+
+            for i in 0..projection.len() {
+                if let Some(&elem) = projection.get(i) {
+                    if let Some(elem) = self.process_projection_elem(elem, location) {
+                        // This converts the borrowed projection into `Cow::Owned(_)` and returns a
+                        // clone of the projection so we can mutate and reintern later.
+                        let vec = projection.to_mut();
+                        vec[i] = elem;
+                    }
+                }
+            }
+
+            match projection {
+                Cow::Borrowed(_) => None,
+                Cow::Owned(vec) => Some(vec),
+            }
+        }
+
+        fn process_projection_elem(
+            &mut self,
+            elem: PlaceElem<'tcx>,
+            location: Location,
+        ) -> Option<PlaceElem<'tcx>> {
+            match elem {
+                PlaceElem::Index(local) => {
+                    let mut new_local = local;
+                    self.visit_local(
+                        &mut new_local,
+                        PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
+                        location,
+                    );
+
+                    if new_local == local { None } else { Some(PlaceElem::Index(new_local)) }
+                }
+                PlaceElem::Field(field, ty) => {
+                    let mut new_ty = ty;
+                    self.visit_ty(&mut new_ty, TyContext::Location(location));
+                    if ty != new_ty { Some(PlaceElem::Field(field, new_ty)) } else { None }
+                }
+                PlaceElem::Deref
+                | PlaceElem::ConstantIndex { .. }
+                | PlaceElem::Subslice { .. }
+                | PlaceElem::Downcast(..) => None,
+            }
+        }
+    };
+
+    () => {
+        fn visit_projection(
+            &mut self,
+            place_ref: PlaceRef<'tcx>,
+            context: PlaceContext,
+            location: Location,
+        ) {
+            self.super_projection(place_ref, context, location);
+        }
+
+        fn visit_projection_elem(
+            &mut self,
+            local: Local,
+            proj_base: &[PlaceElem<'tcx>],
+            elem: PlaceElem<'tcx>,
+            context: PlaceContext,
+            location: Location,
+        ) {
+            self.super_projection_elem(local, proj_base, elem, context, location);
+        }
+
+        fn super_place(&mut self, place: &Place<'tcx>, context: PlaceContext, location: Location) {
+            let mut context = context;
+
+            if !place.projection.is_empty() {
+                if context.is_use() {
+                    // ^ Only change the context if it is a real use, not a "use" in debuginfo.
+                    context = if context.is_mutating_use() {
+                        PlaceContext::MutatingUse(MutatingUseContext::Projection)
+                    } else {
+                        PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection)
+                    };
+                }
+            }
+
+            self.visit_local(place.local, context, location);
+
+            self.visit_projection(place.as_ref(), context, location);
+        }
+
+        fn super_projection(
+            &mut self,
+            place_ref: PlaceRef<'tcx>,
+            context: PlaceContext,
+            location: Location,
+        ) {
+            for (base, elem) in place_ref.iter_projections().rev() {
+                let base_proj = base.projection;
+                self.visit_projection_elem(place_ref.local, base_proj, elem, context, location);
+            }
+        }
+
+        fn super_projection_elem(
+            &mut self,
+            _local: Local,
+            _proj_base: &[PlaceElem<'tcx>],
+            elem: PlaceElem<'tcx>,
+            _context: PlaceContext,
+            location: Location,
+        ) {
+            match elem {
+                ProjectionElem::Field(_field, ty) => {
+                    self.visit_ty(ty, TyContext::Location(location));
+                }
+                ProjectionElem::Index(local) => {
+                    self.visit_local(
+                        local,
+                        PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
+                        location,
+                    );
+                }
+                ProjectionElem::Deref
+                | ProjectionElem::Subslice { from: _, to: _, from_end: _ }
+                | ProjectionElem::ConstantIndex { offset: _, min_length: _, from_end: _ }
+                | ProjectionElem::Downcast(_, _) => {}
+            }
+        }
+    };
+}
+
+make_mir_visitor!(Visitor,);
+make_mir_visitor!(MutVisitor, mut);
+
+pub trait MirVisitable<'tcx> {
+    fn apply(&self, location: Location, visitor: &mut dyn Visitor<'tcx>);
+}
+
+impl<'tcx> MirVisitable<'tcx> for Statement<'tcx> {
+    fn apply(&self, location: Location, visitor: &mut dyn Visitor<'tcx>) {
+        visitor.visit_statement(self, location)
+    }
+}
+
+impl<'tcx> MirVisitable<'tcx> for Terminator<'tcx> {
+    fn apply(&self, location: Location, visitor: &mut dyn Visitor<'tcx>) {
+        visitor.visit_terminator(self, location)
+    }
+}
+
+impl<'tcx> MirVisitable<'tcx> for Option<Terminator<'tcx>> {
+    fn apply(&self, location: Location, visitor: &mut dyn Visitor<'tcx>) {
+        visitor.visit_terminator(self.as_ref().unwrap(), location)
+    }
+}
+
+/// Extra information passed to `visit_ty` and friends to give context
+/// about where the type etc appears.
+#[derive(Debug)]
+pub enum TyContext {
+    LocalDecl {
+        /// The index of the local variable we are visiting.
+        local: Local,
+
+        /// The source location where this local variable was declared.
+        source_info: SourceInfo,
+    },
+
+    /// The inferred type of a user type annotation.
+    UserTy(Span),
+
+    /// The return type of the function.
+    ReturnTy(SourceInfo),
+
+    YieldTy(SourceInfo),
+
+    /// A type found at some location.
+    Location(Location),
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum NonMutatingUseContext {
+    /// Being inspected in some way, like loading a len.
+    Inspect,
+    /// Consumed as part of an operand.
+    Copy,
+    /// Consumed as part of an operand.
+    Move,
+    /// Shared borrow.
+    SharedBorrow,
+    /// Shallow borrow.
+    ShallowBorrow,
+    /// Unique borrow.
+    UniqueBorrow,
+    /// AddressOf for *const pointer.
+    AddressOf,
+    /// Used as base for another place, e.g., `x` in `x.y`. Will not mutate the place.
+    /// For example, the projection `x.y` is not marked as a mutation in these cases:
+    /// ```ignore (illustrative)
+    /// z = x.y;
+    /// f(&x.y);
+    /// ```
+    Projection,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum MutatingUseContext {
+    /// Appears as LHS of an assignment.
+    Store,
+    /// Appears on `SetDiscriminant`
+    SetDiscriminant,
+    /// Appears on `Deinit`
+    Deinit,
+    /// Output operand of an inline assembly block.
+    AsmOutput,
+    /// Destination of a call.
+    Call,
+    /// Destination of a yield.
+    Yield,
+    /// Being dropped.
+    Drop,
+    /// Mutable borrow.
+    Borrow,
+    /// AddressOf for *mut pointer.
+    AddressOf,
+    /// Used as base for another place, e.g., `x` in `x.y`. Could potentially mutate the place.
+    /// For example, the projection `x.y` is marked as a mutation in these cases:
+    /// ```ignore (illustrative)
+    /// x.y = ...;
+    /// f(&mut x.y);
+    /// ```
+    Projection,
+    /// Retagging, a "Stacked Borrows" shadow state operation
+    Retag,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum NonUseContext {
+    /// Starting a storage live range.
+    StorageLive,
+    /// Ending a storage live range.
+    StorageDead,
+    /// User type annotation assertions for NLL.
+    AscribeUserTy,
+    /// The data of a user variable, for debug info.
+    VarDebugInfo,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum PlaceContext {
+    NonMutatingUse(NonMutatingUseContext),
+    MutatingUse(MutatingUseContext),
+    NonUse(NonUseContext),
+}
+
+impl PlaceContext {
+    /// Returns `true` if this place context represents a drop.
+    #[inline]
+    pub fn is_drop(&self) -> bool {
+        matches!(self, PlaceContext::MutatingUse(MutatingUseContext::Drop))
+    }
+
+    /// Returns `true` if this place context represents a borrow.
+    pub fn is_borrow(&self) -> bool {
+        matches!(
+            self,
+            PlaceContext::NonMutatingUse(
+                NonMutatingUseContext::SharedBorrow
+                    | NonMutatingUseContext::ShallowBorrow
+                    | NonMutatingUseContext::UniqueBorrow
+            ) | PlaceContext::MutatingUse(MutatingUseContext::Borrow)
+        )
+    }
+
+    /// Returns `true` if this place context represents a storage live or storage dead marker.
+    #[inline]
+    pub fn is_storage_marker(&self) -> bool {
+        matches!(
+            self,
+            PlaceContext::NonUse(NonUseContext::StorageLive | NonUseContext::StorageDead)
+        )
+    }
+
+    /// Returns `true` if this place context represents a use that potentially changes the value.
+    #[inline]
+    pub fn is_mutating_use(&self) -> bool {
+        matches!(self, PlaceContext::MutatingUse(..))
+    }
+
+    /// Returns `true` if this place context represents a use.
+    #[inline]
+    pub fn is_use(&self) -> bool {
+        !matches!(self, PlaceContext::NonUse(..))
+    }
+
+    /// Returns `true` if this place context represents an assignment statement.
+    pub fn is_place_assignment(&self) -> bool {
+        matches!(
+            self,
+            PlaceContext::MutatingUse(
+                MutatingUseContext::Store
+                    | MutatingUseContext::Call
+                    | MutatingUseContext::AsmOutput,
+            )
+        )
+    }
+}
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
new file mode 100644
index 000000000..d8483e7e4
--- /dev/null
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -0,0 +1,2060 @@
+//! Defines the various compiler queries.
+//!
+//! For more information on the query system, see
+//! ["Queries: demand-driven compilation"](https://rustc-dev-guide.rust-lang.org/query.html).
+//! This chapter includes instructions for adding new queries.
+
+// Each of these queries corresponds to a function pointer field in the
+// `Providers` struct for requesting a value of that type, and a method
+// on `tcx: TyCtxt` (and `tcx.at(span)`) for doing that request in a way
+// which memoizes and does dep-graph tracking, wrapping around the actual
+// `Providers` that the driver creates (using several `rustc_*` crates).
+//
+// The result type of each query must implement `Clone`, and additionally
+// `ty::query::values::Value`, which produces an appropriate placeholder
+// (error) value if the query resulted in a query cycle.
+// Queries marked with `fatal_cycle` do not need the latter implementation,
+// as they will raise an fatal error on query cycles instead.
+rustc_queries! {
+    query trigger_delay_span_bug(key: DefId) -> () {
+        desc { "trigger a delay span bug" }
+    }
+
+    query resolutions(_: ()) -> &'tcx ty::ResolverOutputs {
+        eval_always
+        no_hash
+        desc { "get the resolver outputs" }
+    }
+
+    query resolver_for_lowering(_: ()) -> &'tcx Steal<ty::ResolverAstLowering> {
+        eval_always
+        no_hash
+        desc { "get the resolver for lowering" }
+    }
+
+    /// Return the span for a definition.
+    /// Contrary to `def_span` below, this query returns the full absolute span of the definition.
+    /// This span is meant for dep-tracking rather than diagnostics. It should not be used outside
+    /// of rustc_middle::hir::source_map.
+    query source_span(key: LocalDefId) -> Span {
+        desc { "get the source span" }
+    }
+
+    /// Represents crate as a whole (as distinct from the top-level crate module).
+    /// If you call `hir_crate` (e.g., indirectly by calling `tcx.hir().krate()`),
+    /// we will have to assume that any change means that you need to be recompiled.
+    /// This is because the `hir_crate` query gives you access to all other items.
+    /// To avoid this fate, do not call `tcx.hir().krate()`; instead,
+    /// prefer wrappers like `tcx.visit_all_items_in_krate()`.
+    query hir_crate(key: ()) -> Crate<'tcx> {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        desc { "get the crate HIR" }
+    }
+
+    /// All items in the crate.
+    query hir_crate_items(_: ()) -> rustc_middle::hir::ModuleItems {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        desc { "get HIR crate items" }
+    }
+
+    /// The items in a module.
+    ///
+    /// This can be conveniently accessed by `tcx.hir().visit_item_likes_in_module`.
+    /// Avoid calling this query directly.
+    query hir_module_items(key: LocalDefId) -> rustc_middle::hir::ModuleItems {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { |tcx| "HIR module items in `{}`", tcx.def_path_str(key.to_def_id()) }
+        cache_on_disk_if { true }
+    }
+
+    /// Gives access to the HIR node for the HIR owner `key`.
+    ///
+    /// This can be conveniently accessed by methods on `tcx.hir()`.
+    /// Avoid calling this query directly.
+    query hir_owner(key: LocalDefId) -> Option<crate::hir::Owner<'tcx>> {
+        desc { |tcx| "HIR owner of `{}`", tcx.def_path_str(key.to_def_id()) }
+    }
+
+    /// Gives access to the HIR ID for the given `LocalDefId` owner `key`.
+    ///
+    /// This can be conveniently accessed by methods on `tcx.hir()`.
+    /// Avoid calling this query directly.
+    query local_def_id_to_hir_id(key: LocalDefId) -> hir::HirId {
+        desc { |tcx| "HIR ID of `{}`", tcx.def_path_str(key.to_def_id()) }
+    }
+
+    /// Gives access to the HIR node's parent for the HIR owner `key`.
+    ///
+    /// This can be conveniently accessed by methods on `tcx.hir()`.
+    /// Avoid calling this query directly.
+    query hir_owner_parent(key: LocalDefId) -> hir::HirId {
+        desc { |tcx| "HIR parent of `{}`", tcx.def_path_str(key.to_def_id()) }
+    }
+
+    /// Gives access to the HIR nodes and bodies inside the HIR owner `key`.
+    ///
+    /// This can be conveniently accessed by methods on `tcx.hir()`.
+    /// Avoid calling this query directly.
+    query hir_owner_nodes(key: LocalDefId) -> hir::MaybeOwner<&'tcx hir::OwnerNodes<'tcx>> {
+        desc { |tcx| "HIR owner items in `{}`", tcx.def_path_str(key.to_def_id()) }
+    }
+
+    /// Gives access to the HIR attributes inside the HIR owner `key`.
+    ///
+    /// This can be conveniently accessed by methods on `tcx.hir()`.
+    /// Avoid calling this query directly.
+    query hir_attrs(key: LocalDefId) -> &'tcx hir::AttributeMap<'tcx> {
+        desc { |tcx| "HIR owner attributes in `{}`", tcx.def_path_str(key.to_def_id()) }
+    }
+
+    /// Computes the `DefId` of the corresponding const parameter in case the `key` is a
+    /// const argument and returns `None` otherwise.
+    ///
+    /// ```ignore (incomplete)
+    /// let a = foo::<7>();
+    /// //            ^ Calling `opt_const_param_of` for this argument,
+    ///
+    /// fn foo<const N: usize>()
+    /// //           ^ returns this `DefId`.
+    ///
+    /// fn bar() {
+    /// // ^ While calling `opt_const_param_of` for other bodies returns `None`.
+    /// }
+    /// ```
+    // It looks like caching this query on disk actually slightly
+    // worsened performance in #74376.
+    //
+    // Once const generics are more prevalently used, we might want to
+    // consider only caching calls returning `Some`.
+    query opt_const_param_of(key: LocalDefId) -> Option<DefId> {
+        desc { |tcx| "computing the optional const parameter of `{}`", tcx.def_path_str(key.to_def_id()) }
+    }
+
+    /// Given the def_id of a const-generic parameter, computes the associated default const
+    /// parameter. e.g. `fn example<const N: usize=3>` called on `N` would return `3`.
+    query const_param_default(param: DefId) -> ty::Const<'tcx> {
+        desc { |tcx| "compute const default for a given parameter `{}`", tcx.def_path_str(param)  }
+        cache_on_disk_if { param.is_local() }
+        separate_provide_extern
+    }
+
+    /// Returns the [`Ty`][rustc_middle::ty::Ty] of the given [`DefId`]. If the [`DefId`] points
+    /// to an alias, it will "skip" this alias to return the aliased type.
+    ///
+    /// [`DefId`]: rustc_hir::def_id::DefId
+    query type_of(key: DefId) -> Ty<'tcx> {
+        desc { |tcx|
+            "{action} `{path}`",
+            action = {
+                use rustc_hir::def::DefKind;
+                match tcx.def_kind(key) {
+                    DefKind::TyAlias => "expanding type alias",
+                    DefKind::TraitAlias => "expanding trait alias",
+                    _ => "computing type of",
+                }
+            },
+            path = tcx.def_path_str(key),
+        }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    query analysis(key: ()) -> Result<(), ErrorGuaranteed> {
+        eval_always
+        desc { "running analysis passes on this crate" }
+    }
+
+    /// This query checks the fulfillment of collected lint expectations.
+    /// All lint emitting queries have to be done before this is executed
+    /// to ensure that all expectations can be fulfilled.
+    ///
+    /// This is an extra query to enable other drivers (like rustdoc) to
+    /// only execute a small subset of the `analysis` query, while allowing
+    /// lints to be expected. In rustc, this query will be executed as part of
+    /// the `analysis` query and doesn't have to be called a second time.
+    ///
+    /// Tools can additionally pass in a tool filter. That will restrict the
+    /// expectations to only trigger for lints starting with the listed tool
+    /// name. This is useful for cases were not all linting code from rustc
+    /// was called. With the default `None` all registered lints will also
+    /// be checked for expectation fulfillment.
+    query check_expectations(key: Option<Symbol>) -> () {
+        eval_always
+        desc { "checking lint expectations (RFC 2383)" }
+    }
+
+    /// Maps from the `DefId` of an item (trait/struct/enum/fn) to its
+    /// associated generics.
+    query generics_of(key: DefId) -> ty::Generics {
+        desc { |tcx| "computing generics of `{}`", tcx.def_path_str(key) }
+        storage(ArenaCacheSelector<'tcx>)
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    /// Maps from the `DefId` of an item (trait/struct/enum/fn) to the
+    /// predicates (where-clauses) that must be proven true in order
+    /// to reference it. This is almost always the "predicates query"
+    /// that you want.
+    ///
+    /// `predicates_of` builds on `predicates_defined_on` -- in fact,
+    /// it is almost always the same as that query, except for the
+    /// case of traits. For traits, `predicates_of` contains
+    /// an additional `Self: Trait<...>` predicate that users don't
+    /// actually write. This reflects the fact that to invoke the
+    /// trait (e.g., via `Default::default`) you must supply types
+    /// that actually implement the trait. (However, this extra
+    /// predicate gets in the way of some checks, which are intended
+    /// to operate over only the actual where-clauses written by the
+    /// user.)
+    query predicates_of(key: DefId) -> ty::GenericPredicates<'tcx> {
+        desc { |tcx| "computing predicates of `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+    }
+
+    /// Returns the list of bounds that can be used for
+    /// `SelectionCandidate::ProjectionCandidate(_)` and
+    /// `ProjectionTyCandidate::TraitDef`.
+    /// Specifically this is the bounds written on the trait's type
+    /// definition, or those after the `impl` keyword
+    ///
+    /// ```ignore (incomplete)
+    /// type X: Bound + 'lt
+    /// //      ^^^^^^^^^^^
+    /// impl Debug + Display
+    /// //   ^^^^^^^^^^^^^^^
+    /// ```
+    ///
+    /// `key` is the `DefId` of the associated type or opaque type.
+    ///
+    /// Bounds from the parent (e.g. with nested impl trait) are not included.
+    query explicit_item_bounds(key: DefId) -> &'tcx [(ty::Predicate<'tcx>, Span)] {
+        desc { |tcx| "finding item bounds for `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    /// Elaborated version of the predicates from `explicit_item_bounds`.
+    ///
+    /// For example:
+    ///
+    /// ```
+    /// trait MyTrait {
+    ///     type MyAType: Eq + ?Sized;
+    /// }
+    /// ```
+    ///
+    /// `explicit_item_bounds` returns `[<Self as MyTrait>::MyAType: Eq]`,
+    /// and `item_bounds` returns
+    /// ```text
+    /// [
+    ///     <Self as Trait>::MyAType: Eq,
+    ///     <Self as Trait>::MyAType: PartialEq<<Self as Trait>::MyAType>
+    /// ]
+    /// ```
+    ///
+    /// Bounds from the parent (e.g. with nested impl trait) are not included.
+    query item_bounds(key: DefId) -> &'tcx ty::List<ty::Predicate<'tcx>> {
+        desc { |tcx| "elaborating item bounds for `{}`", tcx.def_path_str(key) }
+    }
+
+    query native_libraries(_: CrateNum) -> Vec<NativeLib> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "looking up the native libraries of a linked crate" }
+        separate_provide_extern
+    }
+
+    query lint_levels(_: ()) -> LintLevelMap {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        desc { "computing the lint levels for items in this crate" }
+    }
+
+    query parent_module_from_def_id(key: LocalDefId) -> LocalDefId {
+        eval_always
+        desc { |tcx| "parent module of `{}`", tcx.def_path_str(key.to_def_id()) }
+    }
+
+    query expn_that_defined(key: DefId) -> rustc_span::ExpnId {
+        desc { |tcx| "expansion that defined `{}`", tcx.def_path_str(key) }
+        separate_provide_extern
+    }
+
+    query is_panic_runtime(_: CrateNum) -> bool {
+        fatal_cycle
+        desc { "checking if the crate is_panic_runtime" }
+        separate_provide_extern
+    }
+
+    /// Fetch the THIR for a given body. If typeck for that body failed, returns an empty `Thir`.
+    query thir_body(key: ty::WithOptConstParam<LocalDefId>)
+        -> Result<(&'tcx Steal<thir::Thir<'tcx>>, thir::ExprId), ErrorGuaranteed>
+    {
+        // Perf tests revealed that hashing THIR is inefficient (see #85729).
+        no_hash
+        desc { |tcx| "building THIR for `{}`", tcx.def_path_str(key.did.to_def_id()) }
+    }
+
+    /// Create a THIR tree for debugging.
+    query thir_tree(key: ty::WithOptConstParam<LocalDefId>) -> String {
+        no_hash
+        storage(ArenaCacheSelector<'tcx>)
+        desc { |tcx| "constructing THIR tree for `{}`", tcx.def_path_str(key.did.to_def_id()) }
+    }
+
+    /// Set of all the `DefId`s in this crate that have MIR associated with
+    /// them. This includes all the body owners, but also things like struct
+    /// constructors.
+    query mir_keys(_: ()) -> rustc_data_structures::fx::FxIndexSet<LocalDefId> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "getting a list of all mir_keys" }
+    }
+
+    /// Maps DefId's that have an associated `mir::Body` to the result
+    /// of the MIR const-checking pass. This is the set of qualifs in
+    /// the final value of a `const`.
+    query mir_const_qualif(key: DefId) -> mir::ConstQualifs {
+        desc { |tcx| "const checking `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+    query mir_const_qualif_const_arg(
+        key: (LocalDefId, DefId)
+    ) -> mir::ConstQualifs {
+        desc {
+            |tcx| "const checking the const argument `{}`",
+            tcx.def_path_str(key.0.to_def_id())
+        }
+    }
+
+    /// Fetch the MIR for a given `DefId` right after it's built - this includes
+    /// unreachable code.
+    query mir_built(key: ty::WithOptConstParam<LocalDefId>) -> &'tcx Steal<mir::Body<'tcx>> {
+        desc { |tcx| "building MIR for `{}`", tcx.def_path_str(key.did.to_def_id()) }
+    }
+
+    /// Fetch the MIR for a given `DefId` up till the point where it is
+    /// ready for const qualification.
+    ///
+    /// See the README for the `mir` module for details.
+    query mir_const(key: ty::WithOptConstParam<LocalDefId>) -> &'tcx Steal<mir::Body<'tcx>> {
+        desc {
+            |tcx| "processing MIR for {}`{}`",
+            if key.const_param_did.is_some() { "the const argument " } else { "" },
+            tcx.def_path_str(key.did.to_def_id()),
+        }
+        no_hash
+    }
+
+    /// Try to build an abstract representation of the given constant.
+    query thir_abstract_const(
+        key: DefId
+    ) -> Result<Option<&'tcx [ty::abstract_const::Node<'tcx>]>, ErrorGuaranteed> {
+        desc {
+            |tcx| "building an abstract representation for {}", tcx.def_path_str(key),
+        }
+        separate_provide_extern
+    }
+    /// Try to build an abstract representation of the given constant.
+    query thir_abstract_const_of_const_arg(
+        key: (LocalDefId, DefId)
+    ) -> Result<Option<&'tcx [ty::abstract_const::Node<'tcx>]>, ErrorGuaranteed> {
+        desc {
+            |tcx|
+            "building an abstract representation for the const argument {}",
+            tcx.def_path_str(key.0.to_def_id()),
+        }
+    }
+
+    query try_unify_abstract_consts(key:
+        ty::ParamEnvAnd<'tcx, (ty::Unevaluated<'tcx, ()>, ty::Unevaluated<'tcx, ()>
+    )>) -> bool {
+        desc {
+            |tcx| "trying to unify the generic constants {} and {}",
+            tcx.def_path_str(key.value.0.def.did), tcx.def_path_str(key.value.1.def.did)
+        }
+    }
+
+    query mir_drops_elaborated_and_const_checked(
+        key: ty::WithOptConstParam<LocalDefId>
+    ) -> &'tcx Steal<mir::Body<'tcx>> {
+        no_hash
+        desc { |tcx| "elaborating drops for `{}`", tcx.def_path_str(key.did.to_def_id()) }
+    }
+
+    query mir_for_ctfe(
+        key: DefId
+    ) -> &'tcx mir::Body<'tcx> {
+        desc { |tcx| "caching mir of `{}` for CTFE", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    query mir_for_ctfe_of_const_arg(key: (LocalDefId, DefId)) -> &'tcx mir::Body<'tcx> {
+        desc {
+            |tcx| "MIR for CTFE of the const argument `{}`",
+            tcx.def_path_str(key.0.to_def_id())
+        }
+    }
+
+    query mir_promoted(key: ty::WithOptConstParam<LocalDefId>) ->
+        (
+            &'tcx Steal<mir::Body<'tcx>>,
+            &'tcx Steal<IndexVec<mir::Promoted, mir::Body<'tcx>>>
+        ) {
+        no_hash
+        desc {
+            |tcx| "processing {}`{}`",
+            if key.const_param_did.is_some() { "the const argument " } else { "" },
+            tcx.def_path_str(key.did.to_def_id()),
+        }
+    }
+
+    query symbols_for_closure_captures(
+        key: (LocalDefId, LocalDefId)
+    ) -> Vec<rustc_span::Symbol> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc {
+            |tcx| "symbols for captures of closure `{}` in `{}`",
+            tcx.def_path_str(key.1.to_def_id()),
+            tcx.def_path_str(key.0.to_def_id())
+        }
+    }
+
+    /// MIR after our optimization passes have run. This is MIR that is ready
+    /// for codegen. This is also the only query that can fetch non-local MIR, at present.
+    query optimized_mir(key: DefId) -> &'tcx mir::Body<'tcx> {
+        desc { |tcx| "optimizing MIR for `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    /// Returns coverage summary info for a function, after executing the `InstrumentCoverage`
+    /// MIR pass (assuming the -Cinstrument-coverage option is enabled).
+    query coverageinfo(key: ty::InstanceDef<'tcx>) -> mir::CoverageInfo {
+        desc { |tcx| "retrieving coverage info from MIR for `{}`", tcx.def_path_str(key.def_id()) }
+        storage(ArenaCacheSelector<'tcx>)
+    }
+
+    /// Returns the `CodeRegions` for a function that has instrumented coverage, in case the
+    /// function was optimized out before codegen, and before being added to the Coverage Map.
+    query covered_code_regions(key: DefId) -> Vec<&'tcx mir::coverage::CodeRegion> {
+        desc {
+            |tcx| "retrieving the covered `CodeRegion`s, if instrumented, for `{}`",
+            tcx.def_path_str(key)
+        }
+        storage(ArenaCacheSelector<'tcx>)
+        cache_on_disk_if { key.is_local() }
+    }
+
+    /// The `DefId` is the `DefId` of the containing MIR body. Promoteds do not have their own
+    /// `DefId`. This function returns all promoteds in the specified body. The body references
+    /// promoteds by the `DefId` and the `mir::Promoted` index. This is necessary, because
+    /// after inlining a body may refer to promoteds from other bodies. In that case you still
+    /// need to use the `DefId` of the original body.
+    query promoted_mir(key: DefId) -> &'tcx IndexVec<mir::Promoted, mir::Body<'tcx>> {
+        desc { |tcx| "optimizing promoted MIR for `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+    query promoted_mir_of_const_arg(
+        key: (LocalDefId, DefId)
+    ) -> &'tcx IndexVec<mir::Promoted, mir::Body<'tcx>> {
+        desc {
+            |tcx| "optimizing promoted MIR for the const argument `{}`",
+            tcx.def_path_str(key.0.to_def_id()),
+        }
+    }
+
+    /// Erases regions from `ty` to yield a new type.
+    /// Normally you would just use `tcx.erase_regions(value)`,
+    /// however, which uses this query as a kind of cache.
+    query erase_regions_ty(ty: Ty<'tcx>) -> Ty<'tcx> {
+        // This query is not expected to have input -- as a result, it
+        // is not a good candidates for "replay" because it is essentially a
+        // pure function of its input (and hence the expectation is that
+        // no caller would be green **apart** from just these
+        // queries). Making it anonymous avoids hashing the result, which
+        // may save a bit of time.
+        anon
+        desc { "erasing regions from `{:?}`", ty }
+    }
+
+    query wasm_import_module_map(_: CrateNum) -> FxHashMap<DefId, String> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "wasm import module map" }
+    }
+
+    /// Maps from the `DefId` of an item (trait/struct/enum/fn) to the
+    /// predicates (where-clauses) directly defined on it. This is
+    /// equal to the `explicit_predicates_of` predicates plus the
+    /// `inferred_outlives_of` predicates.
+    query predicates_defined_on(key: DefId) -> ty::GenericPredicates<'tcx> {
+        desc { |tcx| "computing predicates of `{}`", tcx.def_path_str(key) }
+    }
+
+    /// Returns everything that looks like a predicate written explicitly
+    /// by the user on a trait item.
+    ///
+    /// Traits are unusual, because predicates on associated types are
+    /// converted into bounds on that type for backwards compatibility:
+    ///
+    /// trait X where Self::U: Copy { type U; }
+    ///
+    /// becomes
+    ///
+    /// trait X { type U: Copy; }
+    ///
+    /// `explicit_predicates_of` and `explicit_item_bounds` will then take
+    /// the appropriate subsets of the predicates here.
+    query trait_explicit_predicates_and_bounds(key: LocalDefId) -> ty::GenericPredicates<'tcx> {
+        desc { |tcx| "computing explicit predicates of trait `{}`", tcx.def_path_str(key.to_def_id()) }
+    }
+
+    /// Returns the predicates written explicitly by the user.
+    query explicit_predicates_of(key: DefId) -> ty::GenericPredicates<'tcx> {
+        desc { |tcx| "computing explicit predicates of `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    /// Returns the inferred outlives predicates (e.g., for `struct
+    /// Foo<'a, T> { x: &'a T }`, this would return `T: 'a`).
+    query inferred_outlives_of(key: DefId) -> &'tcx [(ty::Predicate<'tcx>, Span)] {
+        desc { |tcx| "computing inferred outlives predicates of `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    /// Maps from the `DefId` of a trait to the list of
+    /// super-predicates. This is a subset of the full list of
+    /// predicates. We store these in a separate map because we must
+    /// evaluate them even during type conversion, often before the
+    /// full predicates are available (note that supertraits have
+    /// additional acyclicity requirements).
+    query super_predicates_of(key: DefId) -> ty::GenericPredicates<'tcx> {
+        desc { |tcx| "computing the super predicates of `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    /// The `Option<Ident>` is the name of an associated type. If it is `None`, then this query
+    /// returns the full set of predicates. If `Some<Ident>`, then the query returns only the
+    /// subset of super-predicates that reference traits that define the given associated type.
+    /// This is used to avoid cycles in resolving types like `T::Item`.
+    query super_predicates_that_define_assoc_type(key: (DefId, Option<rustc_span::symbol::Ident>)) -> ty::GenericPredicates<'tcx> {
+        desc { |tcx| "computing the super traits of `{}`{}",
+            tcx.def_path_str(key.0),
+            if let Some(assoc_name) = key.1 { format!(" with associated type name `{}`", assoc_name) } else { "".to_string() },
+        }
+    }
+
+    /// To avoid cycles within the predicates of a single item we compute
+    /// per-type-parameter predicates for resolving `T::AssocTy`.
+    query type_param_predicates(key: (DefId, LocalDefId, rustc_span::symbol::Ident)) -> ty::GenericPredicates<'tcx> {
+        desc { |tcx| "computing the bounds for type parameter `{}`", tcx.hir().ty_param_name(key.1) }
+    }
+
+    query trait_def(key: DefId) -> ty::TraitDef {
+        desc { |tcx| "computing trait definition for `{}`", tcx.def_path_str(key) }
+        storage(ArenaCacheSelector<'tcx>)
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+    query adt_def(key: DefId) -> ty::AdtDef<'tcx> {
+        desc { |tcx| "computing ADT definition for `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+    query adt_destructor(key: DefId) -> Option<ty::Destructor> {
+        desc { |tcx| "computing `Drop` impl for `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    // The cycle error here should be reported as an error by `check_representable`.
+    // We consider the type as Sized in the meanwhile to avoid
+    // further errors (done in impl Value for AdtSizedConstraint).
+    // Use `cycle_delay_bug` to delay the cycle error here to be emitted later
+    // in case we accidentally otherwise don't emit an error.
+    query adt_sized_constraint(
+        key: DefId
+    ) -> AdtSizedConstraint<'tcx> {
+        desc { |tcx| "computing `Sized` constraints for `{}`", tcx.def_path_str(key) }
+        cycle_delay_bug
+    }
+
+    query adt_dtorck_constraint(
+        key: DefId
+    ) -> Result<&'tcx DropckConstraint<'tcx>, NoSolution> {
+        desc { |tcx| "computing drop-check constraints for `{}`", tcx.def_path_str(key) }
+    }
+
+    /// Returns `true` if this is a const fn, use the `is_const_fn` to know whether your crate
+    /// actually sees it as const fn (e.g., the const-fn-ness might be unstable and you might
+    /// not have the feature gate active).
+    ///
+    /// **Do not call this function manually.** It is only meant to cache the base data for the
+    /// `is_const_fn` function. Consider using `is_const_fn` or `is_const_fn_raw` instead.
+    query constness(key: DefId) -> hir::Constness {
+        desc { |tcx| "checking if item is const: `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    query asyncness(key: DefId) -> hir::IsAsync {
+        desc { |tcx| "checking if the function is async: `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    /// Returns `true` if calls to the function may be promoted.
+    ///
+    /// This is either because the function is e.g., a tuple-struct or tuple-variant
+    /// constructor, or because it has the `#[rustc_promotable]` attribute. The attribute should
+    /// be removed in the future in favour of some form of check which figures out whether the
+    /// function does not inspect the bits of any of its arguments (so is essentially just a
+    /// constructor function).
+    query is_promotable_const_fn(key: DefId) -> bool {
+        desc { |tcx| "checking if item is promotable: `{}`", tcx.def_path_str(key) }
+    }
+
+    /// Returns `true` if this is a foreign item (i.e., linked via `extern { ... }`).
+    query is_foreign_item(key: DefId) -> bool {
+        desc { |tcx| "checking if `{}` is a foreign item", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    /// Returns `Some(generator_kind)` if the node pointed to by `def_id` is a generator.
+    query generator_kind(def_id: DefId) -> Option<hir::GeneratorKind> {
+        desc { |tcx| "looking up generator kind of `{}`", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+
+    /// Gets a map with the variance of every item; use `item_variance` instead.
+    query crate_variances(_: ()) -> ty::CrateVariancesMap<'tcx> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "computing the variances for items in this crate" }
+    }
+
+    /// Maps from the `DefId` of a type or region parameter to its (inferred) variance.
+    query variances_of(def_id: DefId) -> &'tcx [ty::Variance] {
+        desc { |tcx| "computing the variances of `{}`", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+
+    /// Maps from thee `DefId` of a type to its (inferred) outlives.
+    query inferred_outlives_crate(_: ()) -> ty::CratePredicatesMap<'tcx> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "computing the inferred outlives predicates for items in this crate" }
+    }
+
+    /// Maps from an impl/trait `DefId` to a list of the `DefId`s of its items.
+    query associated_item_def_ids(key: DefId) -> &'tcx [DefId] {
+        desc { |tcx| "collecting associated items of `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    /// Maps from a trait item to the trait item "descriptor".
+    query associated_item(key: DefId) -> ty::AssocItem {
+        desc { |tcx| "computing associated item data for `{}`", tcx.def_path_str(key) }
+        storage(ArenaCacheSelector<'tcx>)
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    /// Collects the associated items defined on a trait or impl.
+    query associated_items(key: DefId) -> ty::AssocItems<'tcx> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { |tcx| "collecting associated items of {}", tcx.def_path_str(key) }
+    }
+
+    /// Maps from associated items on a trait to the corresponding associated
+    /// item on the impl specified by `impl_id`.
+    ///
+    /// For example, with the following code
+    ///
+    /// ```
+    /// struct Type {}
+    ///                         // DefId
+    /// trait Trait {           // trait_id
+    ///     fn f();             // trait_f
+    ///     fn g() {}           // trait_g
+    /// }
+    ///
+    /// impl Trait for Type {   // impl_id
+    ///     fn f() {}           // impl_f
+    ///     fn g() {}           // impl_g
+    /// }
+    /// ```
+    ///
+    /// The map returned for `tcx.impl_item_implementor_ids(impl_id)` would be
+    ///`{ trait_f: impl_f, trait_g: impl_g }`
+    query impl_item_implementor_ids(impl_id: DefId) -> FxHashMap<DefId, DefId> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { |tcx| "comparing impl items against trait for {}", tcx.def_path_str(impl_id) }
+    }
+
+    /// Given an `impl_id`, return the trait it implements.
+    /// Return `None` if this is an inherent impl.
+    query impl_trait_ref(impl_id: DefId) -> Option<ty::TraitRef<'tcx>> {
+        desc { |tcx| "computing trait implemented by `{}`", tcx.def_path_str(impl_id) }
+        cache_on_disk_if { impl_id.is_local() }
+        separate_provide_extern
+    }
+    query impl_polarity(impl_id: DefId) -> ty::ImplPolarity {
+        desc { |tcx| "computing implementation polarity of `{}`", tcx.def_path_str(impl_id) }
+        cache_on_disk_if { impl_id.is_local() }
+        separate_provide_extern
+    }
+
+    query issue33140_self_ty(key: DefId) -> Option<ty::Ty<'tcx>> {
+        desc { |tcx| "computing Self type wrt issue #33140 `{}`", tcx.def_path_str(key) }
+    }
+
+    /// Maps a `DefId` of a type to a list of its inherent impls.
+    /// Contains implementations of methods that are inherent to a type.
+    /// Methods in these implementations don't need to be exported.
+    query inherent_impls(key: DefId) -> &'tcx [DefId] {
+        desc { |tcx| "collecting inherent impls for `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    query incoherent_impls(key: SimplifiedType) -> &'tcx [DefId] {
+        desc { |tcx| "collecting all inherent impls for `{:?}`", key }
+    }
+
+    /// The result of unsafety-checking this `LocalDefId`.
+    query unsafety_check_result(key: LocalDefId) -> &'tcx mir::UnsafetyCheckResult {
+        desc { |tcx| "unsafety-checking `{}`", tcx.def_path_str(key.to_def_id()) }
+        cache_on_disk_if { true }
+    }
+    query unsafety_check_result_for_const_arg(key: (LocalDefId, DefId)) -> &'tcx mir::UnsafetyCheckResult {
+        desc {
+            |tcx| "unsafety-checking the const argument `{}`",
+            tcx.def_path_str(key.0.to_def_id())
+        }
+    }
+
+    /// Unsafety-check this `LocalDefId` with THIR unsafeck. This should be
+    /// used with `-Zthir-unsafeck`.
+    query thir_check_unsafety(key: LocalDefId) {
+        desc { |tcx| "unsafety-checking `{}`", tcx.def_path_str(key.to_def_id()) }
+        cache_on_disk_if { true }
+    }
+    query thir_check_unsafety_for_const_arg(key: (LocalDefId, DefId)) {
+        desc {
+            |tcx| "unsafety-checking the const argument `{}`",
+            tcx.def_path_str(key.0.to_def_id())
+        }
+    }
+
+    /// HACK: when evaluated, this reports an "unsafe derive on repr(packed)" error.
+    ///
+    /// Unsafety checking is executed for each method separately, but we only want
+    /// to emit this error once per derive. As there are some impls with multiple
+    /// methods, we use a query for deduplication.
+    query unsafe_derive_on_repr_packed(key: LocalDefId) -> () {
+        desc { |tcx| "processing `{}`", tcx.def_path_str(key.to_def_id()) }
+    }
+
+    /// Computes the signature of the function.
+    query fn_sig(key: DefId) -> ty::PolyFnSig<'tcx> {
+        desc { |tcx| "computing function signature of `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    /// Performs lint checking for the module.
+    query lint_mod(key: LocalDefId) -> () {
+        desc { |tcx| "linting {}", describe_as_module(key, tcx) }
+    }
+
+    /// Checks the attributes in the module.
+    query check_mod_attrs(key: LocalDefId) -> () {
+        desc { |tcx| "checking attributes in {}", describe_as_module(key, tcx) }
+    }
+
+    /// Checks for uses of unstable APIs in the module.
+    query check_mod_unstable_api_usage(key: LocalDefId) -> () {
+        desc { |tcx| "checking for unstable API usage in {}", describe_as_module(key, tcx) }
+    }
+
+    /// Checks the const bodies in the module for illegal operations (e.g. `if` or `loop`).
+    query check_mod_const_bodies(key: LocalDefId) -> () {
+        desc { |tcx| "checking consts in {}", describe_as_module(key, tcx) }
+    }
+
+    /// Checks the loops in the module.
+    query check_mod_loops(key: LocalDefId) -> () {
+        desc { |tcx| "checking loops in {}", describe_as_module(key, tcx) }
+    }
+
+    query check_mod_naked_functions(key: LocalDefId) -> () {
+        desc { |tcx| "checking naked functions in {}", describe_as_module(key, tcx) }
+    }
+
+    query check_mod_item_types(key: LocalDefId) -> () {
+        desc { |tcx| "checking item types in {}", describe_as_module(key, tcx) }
+    }
+
+    query check_mod_privacy(key: LocalDefId) -> () {
+        desc { |tcx| "checking privacy in {}", describe_as_module(key, tcx) }
+    }
+
+    query check_mod_liveness(key: LocalDefId) -> () {
+        desc { |tcx| "checking liveness of variables in {}", describe_as_module(key, tcx) }
+    }
+
+    /// Return the live symbols in the crate for dead code check.
+    ///
+    /// The second return value maps from ADTs to ignored derived traits (e.g. Debug and Clone) and
+    /// their respective impl (i.e., part of the derive macro)
+    query live_symbols_and_ignored_derived_traits(_: ()) -> (
+        FxHashSet<LocalDefId>,
+        FxHashMap<LocalDefId, Vec<(DefId, DefId)>>
+    ) {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "find live symbols in crate" }
+    }
+
+    query check_mod_deathness(key: LocalDefId) -> () {
+        desc { |tcx| "checking deathness of variables in {}", describe_as_module(key, tcx) }
+    }
+
+    query check_mod_impl_wf(key: LocalDefId) -> () {
+        desc { |tcx| "checking that impls are well-formed in {}", describe_as_module(key, tcx) }
+    }
+
+    query check_mod_type_wf(key: LocalDefId) -> () {
+        desc { |tcx| "checking that types are well-formed in {}", describe_as_module(key, tcx) }
+    }
+
+    query collect_mod_item_types(key: LocalDefId) -> () {
+        desc { |tcx| "collecting item types in {}", describe_as_module(key, tcx) }
+    }
+
+    /// Caches `CoerceUnsized` kinds for impls on custom types.
+    query coerce_unsized_info(key: DefId) -> ty::adjustment::CoerceUnsizedInfo {
+        desc { |tcx| "computing CoerceUnsized info for `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    query typeck_item_bodies(_: ()) -> () {
+        desc { "type-checking all item bodies" }
+    }
+
+    query typeck(key: LocalDefId) -> &'tcx ty::TypeckResults<'tcx> {
+        desc { |tcx| "type-checking `{}`", tcx.def_path_str(key.to_def_id()) }
+        cache_on_disk_if { true }
+    }
+    query typeck_const_arg(
+        key: (LocalDefId, DefId)
+    ) -> &'tcx ty::TypeckResults<'tcx> {
+        desc {
+            |tcx| "type-checking the const argument `{}`",
+            tcx.def_path_str(key.0.to_def_id()),
+        }
+    }
+    query diagnostic_only_typeck(key: LocalDefId) -> &'tcx ty::TypeckResults<'tcx> {
+        desc { |tcx| "type-checking `{}`", tcx.def_path_str(key.to_def_id()) }
+        cache_on_disk_if { true }
+        load_cached(tcx, id) {
+            let typeck_results: Option<ty::TypeckResults<'tcx>> = tcx
+                .on_disk_cache().as_ref()
+                .and_then(|c| c.try_load_query_result(*tcx, id));
+
+            typeck_results.map(|x| &*tcx.arena.alloc(x))
+        }
+    }
+
+    query used_trait_imports(key: LocalDefId) -> &'tcx FxHashSet<LocalDefId> {
+        desc { |tcx| "used_trait_imports `{}`", tcx.def_path_str(key.to_def_id()) }
+        cache_on_disk_if { true }
+    }
+
+    query has_typeck_results(def_id: DefId) -> bool {
+        desc { |tcx| "checking whether `{}` has a body", tcx.def_path_str(def_id) }
+    }
+
+    query coherent_trait(def_id: DefId) -> () {
+        desc { |tcx| "coherence checking all impls of trait `{}`", tcx.def_path_str(def_id) }
+    }
+
+    /// Borrow-checks the function body. If this is a closure, returns
+    /// additional requirements that the closure's creator must verify.
+    query mir_borrowck(key: LocalDefId) -> &'tcx mir::BorrowCheckResult<'tcx> {
+        desc { |tcx| "borrow-checking `{}`", tcx.def_path_str(key.to_def_id()) }
+        cache_on_disk_if(tcx) { tcx.is_typeck_child(key.to_def_id()) }
+    }
+    query mir_borrowck_const_arg(key: (LocalDefId, DefId)) -> &'tcx mir::BorrowCheckResult<'tcx> {
+        desc {
+            |tcx| "borrow-checking the const argument`{}`",
+            tcx.def_path_str(key.0.to_def_id())
+        }
+    }
+
+    /// Gets a complete map from all types to their inherent impls.
+    /// Not meant to be used directly outside of coherence.
+    query crate_inherent_impls(k: ()) -> CrateInherentImpls {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "all inherent impls defined in crate" }
+    }
+
+    /// Checks all types in the crate for overlap in their inherent impls. Reports errors.
+    /// Not meant to be used directly outside of coherence.
+    query crate_inherent_impls_overlap_check(_: ()) -> () {
+        desc { "check for overlap between inherent impls defined in this crate" }
+    }
+
+    /// Checks whether all impls in the crate pass the overlap check, returning
+    /// which impls fail it. If all impls are correct, the returned slice is empty.
+    query orphan_check_impl(key: LocalDefId) -> Result<(), ErrorGuaranteed> {
+        desc { |tcx|
+            "checking whether impl `{}` follows the orphan rules",
+            tcx.def_path_str(key.to_def_id()),
+        }
+    }
+
+    /// Check whether the function has any recursion that could cause the inliner to trigger
+    /// a cycle. Returns the call stack causing the cycle. The call stack does not contain the
+    /// current function, just all intermediate functions.
+    query mir_callgraph_reachable(key: (ty::Instance<'tcx>, LocalDefId)) -> bool {
+        fatal_cycle
+        desc { |tcx|
+            "computing if `{}` (transitively) calls `{}`",
+            key.0,
+            tcx.def_path_str(key.1.to_def_id()),
+        }
+    }
+
+    /// Obtain all the calls into other local functions
+    query mir_inliner_callees(key: ty::InstanceDef<'tcx>) -> &'tcx [(DefId, SubstsRef<'tcx>)] {
+        fatal_cycle
+        desc { |tcx|
+            "computing all local function calls in `{}`",
+            tcx.def_path_str(key.def_id()),
+        }
+    }
+
+    /// Evaluates a constant and returns the computed allocation.
+    ///
+    /// **Do not use this** directly, use the `tcx.eval_static_initializer` wrapper.
+    query eval_to_allocation_raw(key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>)
+        -> EvalToAllocationRawResult<'tcx> {
+        desc { |tcx|
+            "const-evaluating + checking `{}`",
+            key.value.display(tcx)
+        }
+        cache_on_disk_if { true }
+    }
+
+    /// Evaluates const items or anonymous constants
+    /// (such as enum variant explicit discriminants or array lengths)
+    /// into a representation suitable for the type system and const generics.
+    ///
+    /// **Do not use this** directly, use one of the following wrappers: `tcx.const_eval_poly`,
+    /// `tcx.const_eval_resolve`, `tcx.const_eval_instance`, or `tcx.const_eval_global_id`.
+    query eval_to_const_value_raw(key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>)
+        -> EvalToConstValueResult<'tcx> {
+        desc { |tcx|
+            "simplifying constant for the type system `{}`",
+            key.value.display(tcx)
+        }
+        cache_on_disk_if { true }
+    }
+
+    /// Evaluate a constant and convert it to a type level constant or
+    /// return `None` if that is not possible.
+    query eval_to_valtree(
+        key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>
+    ) -> EvalToValTreeResult<'tcx> {
+        desc { "evaluating type-level constant" }
+    }
+
+    /// Converts a type level constant value into `ConstValue`
+    query valtree_to_const_val(key: (Ty<'tcx>, ty::ValTree<'tcx>)) -> ConstValue<'tcx> {
+        desc { "converting type-level constant value to mir constant value"}
+    }
+
+    /// Destructures array, ADT or tuple constants into the constants
+    /// of their fields.
+    query destructure_const(key: ty::Const<'tcx>) -> ty::DestructuredConst<'tcx> {
+        desc { "destructuring type level constant"}
+    }
+
+    /// Tries to destructure an `mir::ConstantKind` ADT or array into its variant index
+    /// and its field values.
+    query try_destructure_mir_constant(key: ty::ParamEnvAnd<'tcx, mir::ConstantKind<'tcx>>) -> Option<mir::DestructuredMirConstant<'tcx>> {
+        desc { "destructuring mir constant"}
+        remap_env_constness
+    }
+
+    /// Dereference a constant reference or raw pointer and turn the result into a constant
+    /// again.
+    query deref_mir_constant(
+        key: ty::ParamEnvAnd<'tcx, mir::ConstantKind<'tcx>>
+    ) -> mir::ConstantKind<'tcx> {
+        desc { "dereferencing mir constant" }
+        remap_env_constness
+    }
+
+    query const_caller_location(key: (rustc_span::Symbol, u32, u32)) -> ConstValue<'tcx> {
+        desc { "get a &core::panic::Location referring to a span" }
+    }
+
+    // FIXME get rid of this with valtrees
+    query lit_to_const(
+        key: LitToConstInput<'tcx>
+    ) -> Result<ty::Const<'tcx>, LitToConstError> {
+        desc { "converting literal to const" }
+    }
+
+    query lit_to_mir_constant(key: LitToConstInput<'tcx>) -> Result<mir::ConstantKind<'tcx>, LitToConstError> {
+        desc { "converting literal to mir constant" }
+    }
+
+    query check_match(key: DefId) {
+        desc { |tcx| "match-checking `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+    }
+
+    /// Performs part of the privacy check and computes "access levels".
+    query privacy_access_levels(_: ()) -> &'tcx AccessLevels {
+        eval_always
+        desc { "privacy access levels" }
+    }
+    query check_private_in_public(_: ()) -> () {
+        eval_always
+        desc { "checking for private elements in public interfaces" }
+    }
+
+    query reachable_set(_: ()) -> FxHashSet<LocalDefId> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "reachability" }
+    }
+
+    /// Per-body `region::ScopeTree`. The `DefId` should be the owner `DefId` for the body;
+    /// in the case of closures, this will be redirected to the enclosing function.
+    query region_scope_tree(def_id: DefId) -> &'tcx crate::middle::region::ScopeTree {
+        desc { |tcx| "computing drop scopes for `{}`", tcx.def_path_str(def_id) }
+    }
+
+    /// Generates a MIR body for the shim.
+    query mir_shims(key: ty::InstanceDef<'tcx>) -> mir::Body<'tcx> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { |tcx| "generating MIR shim for `{}`", tcx.def_path_str(key.def_id()) }
+    }
+
+    /// The `symbol_name` query provides the symbol name for calling a
+    /// given instance from the local crate. In particular, it will also
+    /// look up the correct symbol name of instances from upstream crates.
+    query symbol_name(key: ty::Instance<'tcx>) -> ty::SymbolName<'tcx> {
+        desc { "computing the symbol for `{}`", key }
+        cache_on_disk_if { true }
+    }
+
+    query opt_def_kind(def_id: DefId) -> Option<DefKind> {
+        desc { |tcx| "looking up definition kind of `{}`", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+
+    /// Gets the span for the definition.
+    query def_span(def_id: DefId) -> Span {
+        desc { |tcx| "looking up span for `{}`", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+
+    /// Gets the span for the identifier of the definition.
+    query def_ident_span(def_id: DefId) -> Option<Span> {
+        desc { |tcx| "looking up span for `{}`'s identifier", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+
+    query lookup_stability(def_id: DefId) -> Option<attr::Stability> {
+        desc { |tcx| "looking up stability of `{}`", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+
+    query lookup_const_stability(def_id: DefId) -> Option<attr::ConstStability> {
+        desc { |tcx| "looking up const stability of `{}`", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+
+    query should_inherit_track_caller(def_id: DefId) -> bool {
+        desc { |tcx| "computing should_inherit_track_caller of `{}`", tcx.def_path_str(def_id) }
+    }
+
+    query lookup_deprecation_entry(def_id: DefId) -> Option<DeprecationEntry> {
+        desc { |tcx| "checking whether `{}` is deprecated", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+
+    /// Determines whether an item is annotated with `doc(hidden)`.
+    query is_doc_hidden(def_id: DefId) -> bool {
+        desc { |tcx| "checking whether `{}` is `doc(hidden)`", tcx.def_path_str(def_id) }
+    }
+
+    /// Returns the attributes on the item at `def_id`.
+    ///
+    /// Do not use this directly, use `tcx.get_attrs` instead.
+    query item_attrs(def_id: DefId) -> &'tcx [ast::Attribute] {
+        desc { |tcx| "collecting attributes of `{}`", tcx.def_path_str(def_id) }
+        separate_provide_extern
+    }
+
+    query codegen_fn_attrs(def_id: DefId) -> CodegenFnAttrs {
+        desc { |tcx| "computing codegen attributes of `{}`", tcx.def_path_str(def_id) }
+        storage(ArenaCacheSelector<'tcx>)
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+
+    query asm_target_features(def_id: DefId) -> &'tcx FxHashSet<Symbol> {
+        desc { |tcx| "computing target features for inline asm of `{}`", tcx.def_path_str(def_id) }
+    }
+
+    query fn_arg_names(def_id: DefId) -> &'tcx [rustc_span::symbol::Ident] {
+        desc { |tcx| "looking up function parameter names for `{}`", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+    /// Gets the rendered value of the specified constant or associated constant.
+    /// Used by rustdoc.
+    query rendered_const(def_id: DefId) -> String {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { |tcx| "rendering constant intializer of `{}`", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+    query impl_parent(def_id: DefId) -> Option<DefId> {
+        desc { |tcx| "computing specialization parent impl of `{}`", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+
+    query is_ctfe_mir_available(key: DefId) -> bool {
+        desc { |tcx| "checking if item has ctfe mir available: `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+    query is_mir_available(key: DefId) -> bool {
+        desc { |tcx| "checking if item has mir available: `{}`", tcx.def_path_str(key) }
+        cache_on_disk_if { key.is_local() }
+        separate_provide_extern
+    }
+
+    query own_existential_vtable_entries(
+        key: ty::PolyExistentialTraitRef<'tcx>
+    ) -> &'tcx [DefId] {
+        desc { |tcx| "finding all existential vtable entries for trait {}", tcx.def_path_str(key.def_id()) }
+    }
+
+    query vtable_entries(key: ty::PolyTraitRef<'tcx>)
+                        -> &'tcx [ty::VtblEntry<'tcx>] {
+        desc { |tcx| "finding all vtable entries for trait {}", tcx.def_path_str(key.def_id()) }
+    }
+
+    query vtable_trait_upcasting_coercion_new_vptr_slot(key: (ty::Ty<'tcx>, ty::Ty<'tcx>)) -> Option<usize> {
+        desc { |tcx| "finding the slot within vtable for trait object {} vtable ptr during trait upcasting coercion from {} vtable",
+            key.1, key.0 }
+    }
+
+    query vtable_allocation(key: (Ty<'tcx>, Option<ty::PolyExistentialTraitRef<'tcx>>)) -> mir::interpret::AllocId {
+        desc { |tcx| "vtable const allocation for <{} as {}>",
+            key.0,
+            key.1.map(|trait_ref| format!("{}", trait_ref)).unwrap_or("_".to_owned())
+        }
+    }
+
+    query codegen_fulfill_obligation(
+        key: (ty::ParamEnv<'tcx>, ty::PolyTraitRef<'tcx>)
+    ) -> Result<&'tcx ImplSource<'tcx, ()>, traits::CodegenObligationError> {
+        cache_on_disk_if { true }
+        desc { |tcx|
+            "checking if `{}` fulfills its obligations",
+            tcx.def_path_str(key.1.def_id())
+        }
+    }
+
+    /// Return all `impl` blocks in the current crate.
+    query all_local_trait_impls(_: ()) -> &'tcx rustc_data_structures::fx::FxIndexMap<DefId, Vec<LocalDefId>> {
+        desc { "local trait impls" }
+    }
+
+    /// Given a trait `trait_id`, return all known `impl` blocks.
+    query trait_impls_of(trait_id: DefId) -> ty::trait_def::TraitImpls {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { |tcx| "trait impls of `{}`", tcx.def_path_str(trait_id) }
+    }
+
+    query specialization_graph_of(trait_id: DefId) -> specialization_graph::Graph {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { |tcx| "building specialization graph of trait `{}`", tcx.def_path_str(trait_id) }
+        cache_on_disk_if { true }
+    }
+    query object_safety_violations(trait_id: DefId) -> &'tcx [traits::ObjectSafetyViolation] {
+        desc { |tcx| "determine object safety of trait `{}`", tcx.def_path_str(trait_id) }
+    }
+
+    /// Gets the ParameterEnvironment for a given item; this environment
+    /// will be in "user-facing" mode, meaning that it is suitable for
+    /// type-checking etc, and it does not normalize specializable
+    /// associated types. This is almost always what you want,
+    /// unless you are doing MIR optimizations, in which case you
+    /// might want to use `reveal_all()` method to change modes.
+    query param_env(def_id: DefId) -> ty::ParamEnv<'tcx> {
+        desc { |tcx| "computing normalized predicates of `{}`", tcx.def_path_str(def_id) }
+    }
+
+    /// Like `param_env`, but returns the `ParamEnv` in `Reveal::All` mode.
+    /// Prefer this over `tcx.param_env(def_id).with_reveal_all_normalized(tcx)`,
+    /// as this method is more efficient.
+    query param_env_reveal_all_normalized(def_id: DefId) -> ty::ParamEnv<'tcx> {
+        desc { |tcx| "computing revealed normalized predicates of `{}`", tcx.def_path_str(def_id) }
+    }
+
+    /// Trait selection queries. These are best used by invoking `ty.is_copy_modulo_regions()`,
+    /// `ty.is_copy()`, etc, since that will prune the environment where possible.
+    query is_copy_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
+        desc { "computing whether `{}` is `Copy`", env.value }
+        remap_env_constness
+    }
+    /// Query backing `Ty::is_sized`.
+    query is_sized_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
+        desc { "computing whether `{}` is `Sized`", env.value }
+        remap_env_constness
+    }
+    /// Query backing `Ty::is_freeze`.
+    query is_freeze_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
+        desc { "computing whether `{}` is freeze", env.value }
+        remap_env_constness
+    }
+    /// Query backing `Ty::is_unpin`.
+    query is_unpin_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
+        desc { "computing whether `{}` is `Unpin`", env.value }
+        remap_env_constness
+    }
+    /// Query backing `Ty::needs_drop`.
+    query needs_drop_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
+        desc { "computing whether `{}` needs drop", env.value }
+        remap_env_constness
+    }
+    /// Query backing `Ty::has_significant_drop_raw`.
+    query has_significant_drop_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
+        desc { "computing whether `{}` has a significant drop", env.value }
+        remap_env_constness
+    }
+
+    /// Query backing `Ty::is_structural_eq_shallow`.
+    ///
+    /// This is only correct for ADTs. Call `is_structural_eq_shallow` to handle all types
+    /// correctly.
+    query has_structural_eq_impls(ty: Ty<'tcx>) -> bool {
+        desc {
+            "computing whether `{:?}` implements `PartialStructuralEq` and `StructuralEq`",
+            ty
+        }
+    }
+
+    /// A list of types where the ADT requires drop if and only if any of
+    /// those types require drop. If the ADT is known to always need drop
+    /// then `Err(AlwaysRequiresDrop)` is returned.
+    query adt_drop_tys(def_id: DefId) -> Result<&'tcx ty::List<Ty<'tcx>>, AlwaysRequiresDrop> {
+        desc { |tcx| "computing when `{}` needs drop", tcx.def_path_str(def_id) }
+        cache_on_disk_if { true }
+    }
+
+    /// A list of types where the ADT requires drop if and only if any of those types
+    /// has significant drop. A type marked with the attribute `rustc_insignificant_dtor`
+    /// is considered to not be significant. A drop is significant if it is implemented
+    /// by the user or does anything that will have any observable behavior (other than
+    /// freeing up memory). If the ADT is known to have a significant destructor then
+    /// `Err(AlwaysRequiresDrop)` is returned.
+    query adt_significant_drop_tys(def_id: DefId) -> Result<&'tcx ty::List<Ty<'tcx>>, AlwaysRequiresDrop> {
+        desc { |tcx| "computing when `{}` has a significant destructor", tcx.def_path_str(def_id) }
+        cache_on_disk_if { false }
+    }
+
+    /// Computes the layout of a type. Note that this implicitly
+    /// executes in "reveal all" mode, and will normalize the input type.
+    query layout_of(
+        key: ty::ParamEnvAnd<'tcx, Ty<'tcx>>
+    ) -> Result<ty::layout::TyAndLayout<'tcx>, ty::layout::LayoutError<'tcx>> {
+        desc { "computing layout of `{}`", key.value }
+        remap_env_constness
+    }
+
+    /// Compute a `FnAbi` suitable for indirect calls, i.e. to `fn` pointers.
+    ///
+    /// NB: this doesn't handle virtual calls - those should use `fn_abi_of_instance`
+    /// instead, where the instance is an `InstanceDef::Virtual`.
+    query fn_abi_of_fn_ptr(
+        key: ty::ParamEnvAnd<'tcx, (ty::PolyFnSig<'tcx>, &'tcx ty::List<Ty<'tcx>>)>
+    ) -> Result<&'tcx abi::call::FnAbi<'tcx, Ty<'tcx>>, ty::layout::FnAbiError<'tcx>> {
+        desc { "computing call ABI of `{}` function pointers", key.value.0 }
+        remap_env_constness
+    }
+
+    /// Compute a `FnAbi` suitable for declaring/defining an `fn` instance, and for
+    /// direct calls to an `fn`.
+    ///
+    /// NB: that includes virtual calls, which are represented by "direct calls"
+    /// to an `InstanceDef::Virtual` instance (of `<dyn Trait as Trait>::fn`).
+    query fn_abi_of_instance(
+        key: ty::ParamEnvAnd<'tcx, (ty::Instance<'tcx>, &'tcx ty::List<Ty<'tcx>>)>
+    ) -> Result<&'tcx abi::call::FnAbi<'tcx, Ty<'tcx>>, ty::layout::FnAbiError<'tcx>> {
+        desc { "computing call ABI of `{}`", key.value.0 }
+        remap_env_constness
+    }
+
+    query dylib_dependency_formats(_: CrateNum)
+                                    -> &'tcx [(CrateNum, LinkagePreference)] {
+        desc { "dylib dependency formats of crate" }
+        separate_provide_extern
+    }
+
+    query dependency_formats(_: ()) -> Lrc<crate::middle::dependency_format::Dependencies> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "get the linkage format of all dependencies" }
+    }
+
+    query is_compiler_builtins(_: CrateNum) -> bool {
+        fatal_cycle
+        desc { "checking if the crate is_compiler_builtins" }
+        separate_provide_extern
+    }
+    query has_global_allocator(_: CrateNum) -> bool {
+        // This query depends on untracked global state in CStore
+        eval_always
+        fatal_cycle
+        desc { "checking if the crate has_global_allocator" }
+        separate_provide_extern
+    }
+    query has_panic_handler(_: CrateNum) -> bool {
+        fatal_cycle
+        desc { "checking if the crate has_panic_handler" }
+        separate_provide_extern
+    }
+    query is_profiler_runtime(_: CrateNum) -> bool {
+        fatal_cycle
+        desc { "query a crate is `#![profiler_runtime]`" }
+        separate_provide_extern
+    }
+    query has_ffi_unwind_calls(key: LocalDefId) -> bool {
+        desc { |tcx| "check if `{}` contains FFI-unwind calls", tcx.def_path_str(key.to_def_id()) }
+        cache_on_disk_if { true }
+    }
+    query required_panic_strategy(_: CrateNum) -> Option<PanicStrategy> {
+        fatal_cycle
+        desc { "query a crate's required panic strategy" }
+        separate_provide_extern
+    }
+    query panic_in_drop_strategy(_: CrateNum) -> PanicStrategy {
+        fatal_cycle
+        desc { "query a crate's configured panic-in-drop strategy" }
+        separate_provide_extern
+    }
+    query is_no_builtins(_: CrateNum) -> bool {
+        fatal_cycle
+        desc { "test whether a crate has `#![no_builtins]`" }
+        separate_provide_extern
+    }
+    query symbol_mangling_version(_: CrateNum) -> SymbolManglingVersion {
+        fatal_cycle
+        desc { "query a crate's symbol mangling version" }
+        separate_provide_extern
+    }
+
+    query extern_crate(def_id: DefId) -> Option<&'tcx ExternCrate> {
+        eval_always
+        desc { "getting crate's ExternCrateData" }
+        separate_provide_extern
+    }
+
+    query specializes(_: (DefId, DefId)) -> bool {
+        desc { "computing whether impls specialize one another" }
+    }
+    query in_scope_traits_map(_: LocalDefId)
+        -> Option<&'tcx FxHashMap<ItemLocalId, Box<[TraitCandidate]>>> {
+        desc { "traits in scope at a block" }
+    }
+
+    query module_reexports(def_id: LocalDefId) -> Option<&'tcx [ModChild]> {
+        desc { |tcx| "looking up reexports of module `{}`", tcx.def_path_str(def_id.to_def_id()) }
+    }
+
+    query impl_defaultness(def_id: DefId) -> hir::Defaultness {
+        desc { |tcx| "looking up whether `{}` is a default impl", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+
+    query check_well_formed(key: LocalDefId) -> () {
+        desc { |tcx| "checking that `{}` is well-formed", tcx.def_path_str(key.to_def_id()) }
+    }
+
+    // The `DefId`s of all non-generic functions and statics in the given crate
+    // that can be reached from outside the crate.
+    //
+    // We expect this items to be available for being linked to.
+    //
+    // This query can also be called for `LOCAL_CRATE`. In this case it will
+    // compute which items will be reachable to other crates, taking into account
+    // the kind of crate that is currently compiled. Crates with only a
+    // C interface have fewer reachable things.
+    //
+    // Does not include external symbols that don't have a corresponding DefId,
+    // like the compiler-generated `main` function and so on.
+    query reachable_non_generics(_: CrateNum)
+        -> DefIdMap<SymbolExportInfo> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "looking up the exported symbols of a crate" }
+        separate_provide_extern
+    }
+    query is_reachable_non_generic(def_id: DefId) -> bool {
+        desc { |tcx| "checking whether `{}` is an exported symbol", tcx.def_path_str(def_id) }
+        cache_on_disk_if { def_id.is_local() }
+        separate_provide_extern
+    }
+    query is_unreachable_local_definition(def_id: LocalDefId) -> bool {
+        desc { |tcx|
+            "checking whether `{}` is reachable from outside the crate",
+            tcx.def_path_str(def_id.to_def_id()),
+        }
+    }
+
+    /// The entire set of monomorphizations the local crate can safely link
+    /// to because they are exported from upstream crates. Do not depend on
+    /// this directly, as its value changes anytime a monomorphization gets
+    /// added or removed in any upstream crate. Instead use the narrower
+    /// `upstream_monomorphizations_for`, `upstream_drop_glue_for`, or, even
+    /// better, `Instance::upstream_monomorphization()`.
+    query upstream_monomorphizations(_: ()) -> DefIdMap<FxHashMap<SubstsRef<'tcx>, CrateNum>> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "collecting available upstream monomorphizations" }
+    }
+
+    /// Returns the set of upstream monomorphizations available for the
+    /// generic function identified by the given `def_id`. The query makes
+    /// sure to make a stable selection if the same monomorphization is
+    /// available in multiple upstream crates.
+    ///
+    /// You likely want to call `Instance::upstream_monomorphization()`
+    /// instead of invoking this query directly.
+    query upstream_monomorphizations_for(def_id: DefId)
+        -> Option<&'tcx FxHashMap<SubstsRef<'tcx>, CrateNum>>
+    {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { |tcx|
+            "collecting available upstream monomorphizations for `{}`",
+            tcx.def_path_str(def_id),
+        }
+        separate_provide_extern
+    }
+
+    /// Returns the upstream crate that exports drop-glue for the given
+    /// type (`substs` is expected to be a single-item list containing the
+    /// type one wants drop-glue for).
+    ///
+    /// This is a subset of `upstream_monomorphizations_for` in order to
+    /// increase dep-tracking granularity. Otherwise adding or removing any
+    /// type with drop-glue in any upstream crate would invalidate all
+    /// functions calling drop-glue of an upstream type.
+    ///
+    /// You likely want to call `Instance::upstream_monomorphization()`
+    /// instead of invoking this query directly.
+    ///
+    /// NOTE: This query could easily be extended to also support other
+    ///       common functions that have are large set of monomorphizations
+    ///       (like `Clone::clone` for example).
+    query upstream_drop_glue_for(substs: SubstsRef<'tcx>) -> Option<CrateNum> {
+        desc { "available upstream drop-glue for `{:?}`", substs }
+    }
+
+    query foreign_modules(_: CrateNum) -> FxHashMap<DefId, ForeignModule> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "looking up the foreign modules of a linked crate" }
+        separate_provide_extern
+    }
+
+    /// Identifies the entry-point (e.g., the `main` function) for a given
+    /// crate, returning `None` if there is no entry point (such as for library crates).
+    query entry_fn(_: ()) -> Option<(DefId, EntryFnType)> {
+        desc { "looking up the entry function of a crate" }
+    }
+    query proc_macro_decls_static(_: ()) -> Option<LocalDefId> {
+        desc { "looking up the derive registrar for a crate" }
+    }
+    // The macro which defines `rustc_metadata::provide_extern` depends on this query's name.
+    // Changing the name should cause a compiler error, but in case that changes, be aware.
+    query crate_hash(_: CrateNum) -> Svh {
+        eval_always
+        desc { "looking up the hash a crate" }
+        separate_provide_extern
+    }
+    query crate_host_hash(_: CrateNum) -> Option<Svh> {
+        eval_always
+        desc { "looking up the hash of a host version of a crate" }
+        separate_provide_extern
+    }
+    query extra_filename(_: CrateNum) -> String {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        desc { "looking up the extra filename for a crate" }
+        separate_provide_extern
+    }
+    query crate_extern_paths(_: CrateNum) -> Vec<PathBuf> {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        desc { "looking up the paths for extern crates" }
+        separate_provide_extern
+    }
+
+    /// Given a crate and a trait, look up all impls of that trait in the crate.
+    /// Return `(impl_id, self_ty)`.
+    query implementations_of_trait(_: (CrateNum, DefId)) -> &'tcx [(DefId, Option<SimplifiedType>)] {
+        desc { "looking up implementations of a trait in a crate" }
+        separate_provide_extern
+    }
+
+    /// Collects all incoherent impls for the given crate and type.
+    ///
+    /// Do not call this directly, but instead use the `incoherent_impls` query.
+    /// This query is only used to get the data necessary for that query.
+    query crate_incoherent_impls(key: (CrateNum, SimplifiedType)) -> &'tcx [DefId] {
+        desc { |tcx| "collecting all impls for a type in a crate" }
+        separate_provide_extern
+    }
+
+    query is_dllimport_foreign_item(def_id: DefId) -> bool {
+        desc { |tcx| "is_dllimport_foreign_item({})", tcx.def_path_str(def_id) }
+    }
+    query is_statically_included_foreign_item(def_id: DefId) -> bool {
+        desc { |tcx| "is_statically_included_foreign_item({})", tcx.def_path_str(def_id) }
+    }
+    query native_library_kind(def_id: DefId)
+        -> Option<NativeLibKind> {
+        desc { |tcx| "native_library_kind({})", tcx.def_path_str(def_id) }
+    }
+
+    /// Does lifetime resolution, but does not descend into trait items. This
+    /// should only be used for resolving lifetimes of on trait definitions,
+    /// and is used to avoid cycles. Importantly, `resolve_lifetimes` still visits
+    /// the same lifetimes and is responsible for diagnostics.
+    /// See `rustc_resolve::late::lifetimes for details.
+    query resolve_lifetimes_trait_definition(_: LocalDefId) -> ResolveLifetimes {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "resolving lifetimes for a trait definition" }
+    }
+    /// Does lifetime resolution on items. Importantly, we can't resolve
+    /// lifetimes directly on things like trait methods, because of trait params.
+    /// See `rustc_resolve::late::lifetimes for details.
+    query resolve_lifetimes(_: LocalDefId) -> ResolveLifetimes {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "resolving lifetimes" }
+    }
+    query named_region_map(_: LocalDefId) ->
+        Option<&'tcx FxHashMap<ItemLocalId, Region>> {
+        desc { "looking up a named region" }
+    }
+    query is_late_bound_map(_: LocalDefId) -> Option<&'tcx FxIndexSet<LocalDefId>> {
+        desc { "testing if a region is late bound" }
+    }
+    /// For a given item (like a struct), gets the default lifetimes to be used
+    /// for each parameter if a trait object were to be passed for that parameter.
+    /// For example, for `struct Foo<'a, T, U>`, this would be `['static, 'static]`.
+    /// For `struct Foo<'a, T: 'a, U>`, this would instead be `['a, 'static]`.
+    query object_lifetime_defaults(_: LocalDefId) -> Option<&'tcx [ObjectLifetimeDefault]> {
+        desc { "looking up lifetime defaults for a region on an item" }
+    }
+    query late_bound_vars_map(_: LocalDefId)
+        -> Option<&'tcx FxHashMap<ItemLocalId, Vec<ty::BoundVariableKind>>> {
+        desc { "looking up late bound vars" }
+    }
+
+    query visibility(def_id: DefId) -> ty::Visibility {
+        desc { |tcx| "computing visibility of `{}`", tcx.def_path_str(def_id) }
+        separate_provide_extern
+    }
+
+    /// Computes the set of modules from which this type is visibly uninhabited.
+    /// To check whether a type is uninhabited at all (not just from a given module), you could
+    /// check whether the forest is empty.
+    query type_uninhabited_from(
+        key: ty::ParamEnvAnd<'tcx, Ty<'tcx>>
+    ) -> ty::inhabitedness::DefIdForest<'tcx> {
+        desc { "computing the inhabitedness of `{:?}`", key }
+        remap_env_constness
+    }
+
+    query dep_kind(_: CrateNum) -> CrateDepKind {
+        eval_always
+        desc { "fetching what a dependency looks like" }
+        separate_provide_extern
+    }
+
+    /// Gets the name of the crate.
+    query crate_name(_: CrateNum) -> Symbol {
+        eval_always
+        desc { "fetching what a crate is named" }
+        separate_provide_extern
+    }
+    query module_children(def_id: DefId) -> &'tcx [ModChild] {
+        desc { |tcx| "collecting child items of module `{}`", tcx.def_path_str(def_id) }
+        separate_provide_extern
+    }
+    query extern_mod_stmt_cnum(def_id: LocalDefId) -> Option<CrateNum> {
+        desc { |tcx| "computing crate imported by `{}`", tcx.def_path_str(def_id.to_def_id()) }
+    }
+
+    query lib_features(_: ()) -> LibFeatures {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "calculating the lib features map" }
+    }
+    query defined_lib_features(_: CrateNum) -> &'tcx [(Symbol, Option<Symbol>)] {
+        desc { "calculating the lib features defined in a crate" }
+        separate_provide_extern
+    }
+    query stability_implications(_: CrateNum) -> FxHashMap<Symbol, Symbol> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "calculating the implications between `#[unstable]` features defined in a crate" }
+        separate_provide_extern
+    }
+    /// Whether the function is an intrinsic
+    query is_intrinsic(def_id: DefId) -> bool {
+        desc { |tcx| "is_intrinsic({})", tcx.def_path_str(def_id) }
+        separate_provide_extern
+    }
+    /// Returns the lang items defined in another crate by loading it from metadata.
+    query get_lang_items(_: ()) -> LanguageItems {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        desc { "calculating the lang items map" }
+    }
+
+    /// Returns all diagnostic items defined in all crates.
+    query all_diagnostic_items(_: ()) -> rustc_hir::diagnostic_items::DiagnosticItems {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        desc { "calculating the diagnostic items map" }
+    }
+
+    /// Returns the lang items defined in another crate by loading it from metadata.
+    query defined_lang_items(_: CrateNum) -> &'tcx [(DefId, usize)] {
+        desc { "calculating the lang items defined in a crate" }
+        separate_provide_extern
+    }
+
+    /// Returns the diagnostic items defined in a crate.
+    query diagnostic_items(_: CrateNum) -> rustc_hir::diagnostic_items::DiagnosticItems {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "calculating the diagnostic items map in a crate" }
+        separate_provide_extern
+    }
+
+    query missing_lang_items(_: CrateNum) -> &'tcx [LangItem] {
+        desc { "calculating the missing lang items in a crate" }
+        separate_provide_extern
+    }
+    query visible_parent_map(_: ()) -> DefIdMap<DefId> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "calculating the visible parent map" }
+    }
+    query trimmed_def_paths(_: ()) -> FxHashMap<DefId, Symbol> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "calculating trimmed def paths" }
+    }
+    query missing_extern_crate_item(_: CrateNum) -> bool {
+        eval_always
+        desc { "seeing if we're missing an `extern crate` item for this crate" }
+        separate_provide_extern
+    }
+    query used_crate_source(_: CrateNum) -> Lrc<CrateSource> {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        desc { "looking at the source for a crate" }
+        separate_provide_extern
+    }
+    /// Returns the debugger visualizers defined for this crate.
+    query debugger_visualizers(_: CrateNum) -> Vec<rustc_span::DebuggerVisualizerFile> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "looking up the debugger visualizers for this crate" }
+        separate_provide_extern
+    }
+    query postorder_cnums(_: ()) -> &'tcx [CrateNum] {
+        eval_always
+        desc { "generating a postorder list of CrateNums" }
+    }
+    /// Returns whether or not the crate with CrateNum 'cnum'
+    /// is marked as a private dependency
+    query is_private_dep(c: CrateNum) -> bool {
+        eval_always
+        desc { "check whether crate {} is a private dependency", c }
+        separate_provide_extern
+    }
+    query allocator_kind(_: ()) -> Option<AllocatorKind> {
+        eval_always
+        desc { "allocator kind for the current crate" }
+    }
+
+    query upvars_mentioned(def_id: DefId) -> Option<&'tcx FxIndexMap<hir::HirId, hir::Upvar>> {
+        desc { |tcx| "collecting upvars mentioned in `{}`", tcx.def_path_str(def_id) }
+    }
+    query maybe_unused_trait_imports(_: ()) -> &'tcx FxIndexSet<LocalDefId> {
+        desc { "fetching potentially unused trait imports" }
+    }
+    query maybe_unused_extern_crates(_: ()) -> &'tcx [(LocalDefId, Span)] {
+        desc { "looking up all possibly unused extern crates" }
+    }
+    query names_imported_by_glob_use(def_id: LocalDefId) -> &'tcx FxHashSet<Symbol> {
+        desc { |tcx| "names_imported_by_glob_use for `{}`", tcx.def_path_str(def_id.to_def_id()) }
+    }
+
+    query stability_index(_: ()) -> stability::Index {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        desc { "calculating the stability index for the local crate" }
+    }
+    query crates(_: ()) -> &'tcx [CrateNum] {
+        eval_always
+        desc { "fetching all foreign CrateNum instances" }
+    }
+
+    /// A list of all traits in a crate, used by rustdoc and error reporting.
+    /// NOTE: Not named just `traits` due to a naming conflict.
+    query traits_in_crate(_: CrateNum) -> &'tcx [DefId] {
+        desc { "fetching all traits in a crate" }
+        separate_provide_extern
+    }
+
+    /// The list of symbols exported from the given crate.
+    ///
+    /// - All names contained in `exported_symbols(cnum)` are guaranteed to
+    ///   correspond to a publicly visible symbol in `cnum` machine code.
+    /// - The `exported_symbols` sets of different crates do not intersect.
+    query exported_symbols(cnum: CrateNum) -> &'tcx [(ExportedSymbol<'tcx>, SymbolExportInfo)] {
+        desc { "exported_symbols" }
+        cache_on_disk_if { *cnum == LOCAL_CRATE }
+        separate_provide_extern
+    }
+
+    query collect_and_partition_mono_items(_: ()) -> (&'tcx DefIdSet, &'tcx [CodegenUnit<'tcx>]) {
+        eval_always
+        desc { "collect_and_partition_mono_items" }
+    }
+    query is_codegened_item(def_id: DefId) -> bool {
+        desc { |tcx| "determining whether `{}` needs codegen", tcx.def_path_str(def_id) }
+    }
+
+    /// All items participating in code generation together with items inlined into them.
+    query codegened_and_inlined_items(_: ()) -> &'tcx DefIdSet {
+        eval_always
+       desc { "codegened_and_inlined_items" }
+    }
+
+    query codegen_unit(_: Symbol) -> &'tcx CodegenUnit<'tcx> {
+        desc { "codegen_unit" }
+    }
+    query unused_generic_params(key: ty::InstanceDef<'tcx>) -> FiniteBitSet<u32> {
+        cache_on_disk_if { key.def_id().is_local() }
+        desc {
+            |tcx| "determining which generic parameters are unused by `{}`",
+                tcx.def_path_str(key.def_id())
+        }
+        separate_provide_extern
+    }
+    query backend_optimization_level(_: ()) -> OptLevel {
+        desc { "optimization level used by backend" }
+    }
+
+    /// Return the filenames where output artefacts shall be stored.
+    ///
+    /// This query returns an `&Arc` because codegen backends need the value even after the `TyCtxt`
+    /// has been destroyed.
+    query output_filenames(_: ()) -> &'tcx Arc<OutputFilenames> {
+        eval_always
+        desc { "output_filenames" }
+    }
+
+    /// Do not call this query directly: invoke `normalize` instead.
+    query normalize_projection_ty(
+        goal: CanonicalProjectionGoal<'tcx>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, NormalizationResult<'tcx>>>,
+        NoSolution,
+    > {
+        desc { "normalizing `{:?}`", goal }
+        remap_env_constness
+    }
+
+    /// Do not call this query directly: invoke `try_normalize_erasing_regions` instead.
+    query try_normalize_generic_arg_after_erasing_regions(
+        goal: ParamEnvAnd<'tcx, GenericArg<'tcx>>
+    ) -> Result<GenericArg<'tcx>, NoSolution> {
+        desc { "normalizing `{}`", goal.value }
+        remap_env_constness
+    }
+
+    /// Do not call this query directly: invoke `try_normalize_erasing_regions` instead.
+    query try_normalize_mir_const_after_erasing_regions(
+        goal: ParamEnvAnd<'tcx, mir::ConstantKind<'tcx>>
+    ) -> Result<mir::ConstantKind<'tcx>, NoSolution> {
+        desc { "normalizing `{}`", goal.value }
+        remap_env_constness
+    }
+
+    query implied_outlives_bounds(
+        goal: CanonicalTyGoal<'tcx>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, Vec<OutlivesBound<'tcx>>>>,
+        NoSolution,
+    > {
+        desc { "computing implied outlives bounds for `{:?}`", goal }
+        remap_env_constness
+    }
+
+    /// Do not call this query directly:
+    /// invoke `DropckOutlives::new(dropped_ty)).fully_perform(typeck.infcx)` instead.
+    query dropck_outlives(
+        goal: CanonicalTyGoal<'tcx>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, DropckOutlivesResult<'tcx>>>,
+        NoSolution,
+    > {
+        desc { "computing dropck types for `{:?}`", goal }
+        remap_env_constness
+    }
+
+    /// Do not call this query directly: invoke `infcx.predicate_may_hold()` or
+    /// `infcx.predicate_must_hold()` instead.
+    query evaluate_obligation(
+        goal: CanonicalPredicateGoal<'tcx>
+    ) -> Result<traits::EvaluationResult, traits::OverflowError> {
+        desc { "evaluating trait selection obligation `{}`", goal.value.value }
+    }
+
+    query evaluate_goal(
+        goal: traits::CanonicalChalkEnvironmentAndGoal<'tcx>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, ()>>,
+        NoSolution
+    > {
+        desc { "evaluating trait selection obligation `{}`", goal.value }
+    }
+
+    /// Do not call this query directly: part of the `Eq` type-op
+    query type_op_ascribe_user_type(
+        goal: CanonicalTypeOpAscribeUserTypeGoal<'tcx>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, ()>>,
+        NoSolution,
+    > {
+        desc { "evaluating `type_op_ascribe_user_type` `{:?}`", goal }
+        remap_env_constness
+    }
+
+    /// Do not call this query directly: part of the `Eq` type-op
+    query type_op_eq(
+        goal: CanonicalTypeOpEqGoal<'tcx>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, ()>>,
+        NoSolution,
+    > {
+        desc { "evaluating `type_op_eq` `{:?}`", goal }
+        remap_env_constness
+    }
+
+    /// Do not call this query directly: part of the `Subtype` type-op
+    query type_op_subtype(
+        goal: CanonicalTypeOpSubtypeGoal<'tcx>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, ()>>,
+        NoSolution,
+    > {
+        desc { "evaluating `type_op_subtype` `{:?}`", goal }
+        remap_env_constness
+    }
+
+    /// Do not call this query directly: part of the `ProvePredicate` type-op
+    query type_op_prove_predicate(
+        goal: CanonicalTypeOpProvePredicateGoal<'tcx>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, ()>>,
+        NoSolution,
+    > {
+        desc { "evaluating `type_op_prove_predicate` `{:?}`", goal }
+    }
+
+    /// Do not call this query directly: part of the `Normalize` type-op
+    query type_op_normalize_ty(
+        goal: CanonicalTypeOpNormalizeGoal<'tcx, Ty<'tcx>>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, Ty<'tcx>>>,
+        NoSolution,
+    > {
+        desc { "normalizing `{:?}`", goal }
+        remap_env_constness
+    }
+
+    /// Do not call this query directly: part of the `Normalize` type-op
+    query type_op_normalize_predicate(
+        goal: CanonicalTypeOpNormalizeGoal<'tcx, ty::Predicate<'tcx>>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, ty::Predicate<'tcx>>>,
+        NoSolution,
+    > {
+        desc { "normalizing `{:?}`", goal }
+        remap_env_constness
+    }
+
+    /// Do not call this query directly: part of the `Normalize` type-op
+    query type_op_normalize_poly_fn_sig(
+        goal: CanonicalTypeOpNormalizeGoal<'tcx, ty::PolyFnSig<'tcx>>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, ty::PolyFnSig<'tcx>>>,
+        NoSolution,
+    > {
+        desc { "normalizing `{:?}`", goal }
+        remap_env_constness
+    }
+
+    /// Do not call this query directly: part of the `Normalize` type-op
+    query type_op_normalize_fn_sig(
+        goal: CanonicalTypeOpNormalizeGoal<'tcx, ty::FnSig<'tcx>>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, ty::FnSig<'tcx>>>,
+        NoSolution,
+    > {
+        desc { "normalizing `{:?}`", goal }
+        remap_env_constness
+    }
+
+    query subst_and_check_impossible_predicates(key: (DefId, SubstsRef<'tcx>)) -> bool {
+        desc { |tcx|
+            "impossible substituted predicates:`{}`",
+            tcx.def_path_str(key.0)
+        }
+    }
+
+    query method_autoderef_steps(
+        goal: CanonicalTyGoal<'tcx>
+    ) -> MethodAutoderefStepsResult<'tcx> {
+        desc { "computing autoderef types for `{:?}`", goal }
+        remap_env_constness
+    }
+
+    query supported_target_features(_: CrateNum) -> FxHashMap<String, Option<Symbol>> {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        desc { "looking up supported target features" }
+    }
+
+    /// Get an estimate of the size of an InstanceDef based on its MIR for CGU partitioning.
+    query instance_def_size_estimate(def: ty::InstanceDef<'tcx>)
+        -> usize {
+        desc { |tcx| "estimating size for `{}`", tcx.def_path_str(def.def_id()) }
+    }
+
+    query features_query(_: ()) -> &'tcx rustc_feature::Features {
+        eval_always
+        desc { "looking up enabled feature gates" }
+    }
+
+    /// Attempt to resolve the given `DefId` to an `Instance`, for the
+    /// given generics args (`SubstsRef`), returning one of:
+    ///  * `Ok(Some(instance))` on success
+    ///  * `Ok(None)` when the `SubstsRef` are still too generic,
+    ///    and therefore don't allow finding the final `Instance`
+    ///  * `Err(ErrorGuaranteed)` when the `Instance` resolution process
+    ///    couldn't complete due to errors elsewhere - this is distinct
+    ///    from `Ok(None)` to avoid misleading diagnostics when an error
+    ///    has already been/will be emitted, for the original cause
+    query resolve_instance(
+        key: ty::ParamEnvAnd<'tcx, (DefId, SubstsRef<'tcx>)>
+    ) -> Result<Option<ty::Instance<'tcx>>, ErrorGuaranteed> {
+        desc { "resolving instance `{}`", ty::Instance::new(key.value.0, key.value.1) }
+        remap_env_constness
+    }
+
+    query resolve_instance_of_const_arg(
+        key: ty::ParamEnvAnd<'tcx, (LocalDefId, DefId, SubstsRef<'tcx>)>
+    ) -> Result<Option<ty::Instance<'tcx>>, ErrorGuaranteed> {
+        desc {
+            "resolving instance of the const argument `{}`",
+            ty::Instance::new(key.value.0.to_def_id(), key.value.2),
+        }
+        remap_env_constness
+    }
+
+    query normalize_opaque_types(key: &'tcx ty::List<ty::Predicate<'tcx>>) -> &'tcx ty::List<ty::Predicate<'tcx>> {
+        desc { "normalizing opaque types in {:?}", key }
+    }
+
+    /// Checks whether a type is definitely uninhabited. This is
+    /// conservative: for some types that are uninhabited we return `false`,
+    /// but we only return `true` for types that are definitely uninhabited.
+    /// `ty.conservative_is_privately_uninhabited` implies that any value of type `ty`
+    /// will be `Abi::Uninhabited`. (Note that uninhabited types may have nonzero
+    /// size, to account for partial initialisation. See #49298 for details.)
+    query conservative_is_privately_uninhabited(key: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
+        desc { "conservatively checking if {:?} is privately uninhabited", key }
+        remap_env_constness
+    }
+
+    query limits(key: ()) -> Limits {
+        desc { "looking up limits" }
+    }
+
+    /// Performs an HIR-based well-formed check on the item with the given `HirId`. If
+    /// we get an `Unimplemented` error that matches the provided `Predicate`, return
+    /// the cause of the newly created obligation.
+    ///
+    /// This is only used by error-reporting code to get a better cause (in particular, a better
+    /// span) for an *existing* error. Therefore, it is best-effort, and may never handle
+    /// all of the cases that the normal `ty::Ty`-based wfcheck does. This is fine,
+    /// because the `ty::Ty`-based wfcheck is always run.
+    query diagnostic_hir_wf_check(key: (ty::Predicate<'tcx>, traits::WellFormedLoc)) -> Option<traits::ObligationCause<'tcx>> {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        no_hash
+        desc { "performing HIR wf-checking for predicate {:?} at item {:?}", key.0, key.1 }
+    }
+
+
+    /// The list of backend features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,
+    /// `--target` and similar).
+    query global_backend_features(_: ()) -> Vec<String> {
+        storage(ArenaCacheSelector<'tcx>)
+        eval_always
+        desc { "computing the backend features for CLI flags" }
+    }
+
+    query generator_diagnostic_data(key: DefId) -> Option<GeneratorDiagnosticData<'tcx>> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { |tcx| "looking up generator diagnostic data of `{}`", tcx.def_path_str(key) }
+        separate_provide_extern
+    }
+
+    query permits_uninit_init(key: TyAndLayout<'tcx>) -> bool {
+        desc { "checking to see if {:?} permits being left uninit", key.ty }
+    }
+
+    query permits_zero_init(key: TyAndLayout<'tcx>) -> bool {
+        desc { "checking to see if {:?} permits being left zeroed", key.ty }
+    }
+}
diff --git a/compiler/rustc_middle/src/tests.rs b/compiler/rustc_middle/src/tests.rs
new file mode 100644
index 000000000..757e0bd3b
--- /dev/null
+++ b/compiler/rustc_middle/src/tests.rs
@@ -0,0 +1,13 @@
+use super::*;
+
+// FIXME(#27438): right now the unit tests of librustc_middle don't refer to any actual
+//                functions generated in librustc_data_structures (all
+//                references are through generic functions), but statics are
+//                referenced from time to time. Due to this bug we won't
+//                actually correctly link in the statics unless we also
+//                reference a function, so be sure to reference a dummy
+//                function.
+#[test]
+fn noop() {
+    rustc_data_structures::__noop_fix_for_27438();
+}
diff --git a/compiler/rustc_middle/src/thir.rs b/compiler/rustc_middle/src/thir.rs
new file mode 100644
index 000000000..b856af1d8
--- /dev/null
+++ b/compiler/rustc_middle/src/thir.rs
@@ -0,0 +1,821 @@
+//! THIR datatypes and definitions. See the [rustc dev guide] for more info.
+//!
+//! If you compare the THIR [`ExprKind`] to [`hir::ExprKind`], you will see it is
+//! a good bit simpler. In fact, a number of the more straight-forward
+//! MIR simplifications are already done in the lowering to THIR. For
+//! example, method calls and overloaded operators are absent: they are
+//! expected to be converted into [`ExprKind::Call`] instances.
+//!
+//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/thir.html
+
+use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
+use rustc_hir as hir;
+use rustc_hir::def::CtorKind;
+use rustc_hir::def_id::DefId;
+use rustc_hir::RangeEnd;
+use rustc_index::newtype_index;
+use rustc_index::vec::IndexVec;
+use rustc_middle::infer::canonical::Canonical;
+use rustc_middle::middle::region;
+use rustc_middle::mir::interpret::AllocId;
+use rustc_middle::mir::{self, BinOp, BorrowKind, FakeReadCause, Field, Mutability, UnOp};
+use rustc_middle::ty::adjustment::PointerCast;
+use rustc_middle::ty::subst::SubstsRef;
+use rustc_middle::ty::CanonicalUserTypeAnnotation;
+use rustc_middle::ty::{self, AdtDef, Ty, UpvarSubsts, UserType};
+use rustc_span::{Span, Symbol, DUMMY_SP};
+use rustc_target::abi::VariantIdx;
+use rustc_target::asm::InlineAsmRegOrRegClass;
+
+use rustc_span::def_id::LocalDefId;
+use std::fmt;
+use std::ops::Index;
+
+pub mod visit;
+
+newtype_index! {
+    /// An index to an [`Arm`] stored in [`Thir::arms`]
+    #[derive(HashStable)]
+    pub struct ArmId {
+        DEBUG_FORMAT = "a{}"
+    }
+}
+
+newtype_index! {
+    /// An index to an [`Expr`] stored in [`Thir::exprs`]
+    #[derive(HashStable)]
+    pub struct ExprId {
+        DEBUG_FORMAT = "e{}"
+    }
+}
+
+newtype_index! {
+    #[derive(HashStable)]
+    /// An index to a [`Stmt`] stored in [`Thir::stmts`]
+    pub struct StmtId {
+        DEBUG_FORMAT = "s{}"
+    }
+}
+
+macro_rules! thir_with_elements {
+    ($($name:ident: $id:ty => $value:ty,)*) => {
+        /// A container for a THIR body.
+        ///
+        /// This can be indexed directly by any THIR index (e.g. [`ExprId`]).
+        #[derive(Debug, HashStable, Clone)]
+        pub struct Thir<'tcx> {
+            $(
+                pub $name: IndexVec<$id, $value>,
+            )*
+        }
+
+        impl<'tcx> Thir<'tcx> {
+            pub fn new() -> Thir<'tcx> {
+                Thir {
+                    $(
+                        $name: IndexVec::new(),
+                    )*
+                }
+            }
+        }
+
+        $(
+            impl<'tcx> Index<$id> for Thir<'tcx> {
+                type Output = $value;
+                fn index(&self, index: $id) -> &Self::Output {
+                    &self.$name[index]
+                }
+            }
+        )*
+    }
+}
+
+thir_with_elements! {
+    arms: ArmId => Arm<'tcx>,
+    exprs: ExprId => Expr<'tcx>,
+    stmts: StmtId => Stmt<'tcx>,
+}
+
+#[derive(Copy, Clone, Debug, HashStable)]
+pub enum LintLevel {
+    Inherited,
+    Explicit(hir::HirId),
+}
+
+#[derive(Clone, Debug, HashStable)]
+pub struct Block {
+    /// Whether the block itself has a label. Used by `label: {}`
+    /// and `try` blocks.
+    ///
+    /// This does *not* include labels on loops, e.g. `'label: loop {}`.
+    pub targeted_by_break: bool,
+    pub region_scope: region::Scope,
+    pub opt_destruction_scope: Option<region::Scope>,
+    /// The span of the block, including the opening braces,
+    /// the label, and the `unsafe` keyword, if present.
+    pub span: Span,
+    /// The statements in the blocK.
+    pub stmts: Box<[StmtId]>,
+    /// The trailing expression of the block, if any.
+    pub expr: Option<ExprId>,
+    pub safety_mode: BlockSafety,
+}
+
+#[derive(Clone, Debug, HashStable)]
+pub struct Adt<'tcx> {
+    /// The ADT we're constructing.
+    pub adt_def: AdtDef<'tcx>,
+    /// The variant of the ADT.
+    pub variant_index: VariantIdx,
+    pub substs: SubstsRef<'tcx>,
+
+    /// Optional user-given substs: for something like `let x =
+    /// Bar::<T> { ... }`.
+    pub user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+
+    pub fields: Box<[FieldExpr]>,
+    /// The base, e.g. `Foo {x: 1, .. base}`.
+    pub base: Option<FruInfo<'tcx>>,
+}
+
+#[derive(Copy, Clone, Debug, HashStable)]
+pub enum BlockSafety {
+    Safe,
+    /// A compiler-generated unsafe block
+    BuiltinUnsafe,
+    /// An `unsafe` block. The `HirId` is the ID of the block.
+    ExplicitUnsafe(hir::HirId),
+}
+
+#[derive(Clone, Debug, HashStable)]
+pub struct Stmt<'tcx> {
+    pub kind: StmtKind<'tcx>,
+    pub opt_destruction_scope: Option<region::Scope>,
+}
+
+#[derive(Clone, Debug, HashStable)]
+pub enum StmtKind<'tcx> {
+    /// An expression with a trailing semicolon.
+    Expr {
+        /// The scope for this statement; may be used as lifetime of temporaries.
+        scope: region::Scope,
+
+        /// The expression being evaluated in this statement.
+        expr: ExprId,
+    },
+
+    /// A `let` binding.
+    Let {
+        /// The scope for variables bound in this `let`; it covers this and
+        /// all the remaining statements in the block.
+        remainder_scope: region::Scope,
+
+        /// The scope for the initialization itself; might be used as
+        /// lifetime of temporaries.
+        init_scope: region::Scope,
+
+        /// `let <PAT> = ...`
+        ///
+        /// If a type annotation is included, it is added as an ascription pattern.
+        pattern: Pat<'tcx>,
+
+        /// `let pat: ty = <INIT>`
+        initializer: Option<ExprId>,
+
+        /// `let pat: ty = <INIT> else { <ELSE> }
+        else_block: Option<Block>,
+
+        /// The lint level for this `let` statement.
+        lint_level: LintLevel,
+    },
+}
+
+#[derive(Clone, Debug, Copy, PartialEq, Eq, Hash, HashStable, TyEncodable, TyDecodable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct LocalVarId(pub hir::HirId);
+
+/// A THIR expression.
+#[derive(Clone, Debug, HashStable)]
+pub struct Expr<'tcx> {
+    /// The type of this expression
+    pub ty: Ty<'tcx>,
+
+    /// The lifetime of this expression if it should be spilled into a
+    /// temporary; should be `None` only if in a constant context
+    pub temp_lifetime: Option<region::Scope>,
+
+    /// span of the expression in the source
+    pub span: Span,
+
+    /// kind of expression
+    pub kind: ExprKind<'tcx>,
+}
+
+#[derive(Clone, Debug, HashStable)]
+pub enum ExprKind<'tcx> {
+    /// `Scope`s are used to explicitly mark destruction scopes,
+    /// and to track the `HirId` of the expressions within the scope.
+    Scope {
+        region_scope: region::Scope,
+        lint_level: LintLevel,
+        value: ExprId,
+    },
+    /// A `box <value>` expression.
+    Box {
+        value: ExprId,
+    },
+    /// An `if` expression.
+    If {
+        if_then_scope: region::Scope,
+        cond: ExprId,
+        then: ExprId,
+        else_opt: Option<ExprId>,
+    },
+    /// A function call. Method calls and overloaded operators are converted to plain function calls.
+    Call {
+        /// The type of the function. This is often a [`FnDef`] or a [`FnPtr`].
+        ///
+        /// [`FnDef`]: ty::TyKind::FnDef
+        /// [`FnPtr`]: ty::TyKind::FnPtr
+        ty: Ty<'tcx>,
+        /// The function itself.
+        fun: ExprId,
+        /// The arguments passed to the function.
+        ///
+        /// Note: in some cases (like calling a closure), the function call `f(...args)` gets
+        /// rewritten as a call to a function trait method (e.g. `FnOnce::call_once(f, (...args))`).
+        args: Box<[ExprId]>,
+        /// Whether this is from an overloaded operator rather than a
+        /// function call from HIR. `true` for overloaded function call.
+        from_hir_call: bool,
+        /// The span of the function, without the dot and receiver
+        /// (e.g. `foo(a, b)` in `x.foo(a, b)`).
+        fn_span: Span,
+    },
+    /// A *non-overloaded* dereference.
+    Deref {
+        arg: ExprId,
+    },
+    /// A *non-overloaded* binary operation.
+    Binary {
+        op: BinOp,
+        lhs: ExprId,
+        rhs: ExprId,
+    },
+    /// A logical operation. This is distinct from `BinaryOp` because
+    /// the operands need to be lazily evaluated.
+    LogicalOp {
+        op: LogicalOp,
+        lhs: ExprId,
+        rhs: ExprId,
+    },
+    /// A *non-overloaded* unary operation. Note that here the deref (`*`)
+    /// operator is represented by `ExprKind::Deref`.
+    Unary {
+        op: UnOp,
+        arg: ExprId,
+    },
+    /// A cast: `<source> as <type>`. The type we cast to is the type of
+    /// the parent expression.
+    Cast {
+        source: ExprId,
+    },
+    Use {
+        source: ExprId,
+    }, // Use a lexpr to get a vexpr.
+    /// A coercion from `!` to any type.
+    NeverToAny {
+        source: ExprId,
+    },
+    /// A pointer cast. More information can be found in [`PointerCast`].
+    Pointer {
+        cast: PointerCast,
+        source: ExprId,
+    },
+    /// A `loop` expression.
+    Loop {
+        body: ExprId,
+    },
+    Let {
+        expr: ExprId,
+        pat: Pat<'tcx>,
+    },
+    /// A `match` expression.
+    Match {
+        scrutinee: ExprId,
+        arms: Box<[ArmId]>,
+    },
+    /// A block.
+    Block {
+        body: Block,
+    },
+    /// An assignment: `lhs = rhs`.
+    Assign {
+        lhs: ExprId,
+        rhs: ExprId,
+    },
+    /// A *non-overloaded* operation assignment, e.g. `lhs += rhs`.
+    AssignOp {
+        op: BinOp,
+        lhs: ExprId,
+        rhs: ExprId,
+    },
+    /// Access to a field of a struct, a tuple, an union, or an enum.
+    Field {
+        lhs: ExprId,
+        /// Variant containing the field.
+        variant_index: VariantIdx,
+        /// This can be a named (`.foo`) or unnamed (`.0`) field.
+        name: Field,
+    },
+    /// A *non-overloaded* indexing operation.
+    Index {
+        lhs: ExprId,
+        index: ExprId,
+    },
+    /// A local variable.
+    VarRef {
+        id: LocalVarId,
+    },
+    /// Used to represent upvars mentioned in a closure/generator
+    UpvarRef {
+        /// DefId of the closure/generator
+        closure_def_id: DefId,
+
+        /// HirId of the root variable
+        var_hir_id: LocalVarId,
+    },
+    /// A borrow, e.g. `&arg`.
+    Borrow {
+        borrow_kind: BorrowKind,
+        arg: ExprId,
+    },
+    /// A `&raw [const|mut] $place_expr` raw borrow resulting in type `*[const|mut] T`.
+    AddressOf {
+        mutability: hir::Mutability,
+        arg: ExprId,
+    },
+    /// A `break` expression.
+    Break {
+        label: region::Scope,
+        value: Option<ExprId>,
+    },
+    /// A `continue` expression.
+    Continue {
+        label: region::Scope,
+    },
+    /// A `return` expression.
+    Return {
+        value: Option<ExprId>,
+    },
+    /// An inline `const` block, e.g. `const {}`.
+    ConstBlock {
+        did: DefId,
+        substs: SubstsRef<'tcx>,
+    },
+    /// An array literal constructed from one repeated element, e.g. `[1; 5]`.
+    Repeat {
+        value: ExprId,
+        count: ty::Const<'tcx>,
+    },
+    /// An array, e.g. `[a, b, c, d]`.
+    Array {
+        fields: Box<[ExprId]>,
+    },
+    /// A tuple, e.g. `(a, b, c, d)`.
+    Tuple {
+        fields: Box<[ExprId]>,
+    },
+    /// An ADT constructor, e.g. `Foo {x: 1, y: 2}`.
+    Adt(Box<Adt<'tcx>>),
+    /// A type ascription on a place.
+    PlaceTypeAscription {
+        source: ExprId,
+        /// Type that the user gave to this expression
+        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+    },
+    /// A type ascription on a value, e.g. `42: i32`.
+    ValueTypeAscription {
+        source: ExprId,
+        /// Type that the user gave to this expression
+        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+    },
+    /// A closure definition.
+    Closure {
+        closure_id: LocalDefId,
+        substs: UpvarSubsts<'tcx>,
+        upvars: Box<[ExprId]>,
+        movability: Option<hir::Movability>,
+        fake_reads: Vec<(ExprId, FakeReadCause, hir::HirId)>,
+    },
+    /// A literal.
+    Literal {
+        lit: &'tcx hir::Lit,
+        neg: bool,
+    },
+    /// For literals that don't correspond to anything in the HIR
+    NonHirLiteral {
+        lit: ty::ScalarInt,
+        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+    },
+    /// A literal of a ZST type.
+    ZstLiteral {
+        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+    },
+    /// Associated constants and named constants
+    NamedConst {
+        def_id: DefId,
+        substs: SubstsRef<'tcx>,
+        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+    },
+    ConstParam {
+        param: ty::ParamConst,
+        def_id: DefId,
+    },
+    // FIXME improve docs for `StaticRef` by distinguishing it from `NamedConst`
+    /// A literal containing the address of a `static`.
+    ///
+    /// This is only distinguished from `Literal` so that we can register some
+    /// info for diagnostics.
+    StaticRef {
+        alloc_id: AllocId,
+        ty: Ty<'tcx>,
+        def_id: DefId,
+    },
+    /// Inline assembly, i.e. `asm!()`.
+    InlineAsm {
+        template: &'tcx [InlineAsmTemplatePiece],
+        operands: Box<[InlineAsmOperand<'tcx>]>,
+        options: InlineAsmOptions,
+        line_spans: &'tcx [Span],
+    },
+    /// An expression taking a reference to a thread local.
+    ThreadLocalRef(DefId),
+    /// A `yield` expression.
+    Yield {
+        value: ExprId,
+    },
+}
+
+/// Represents the association of a field identifier and an expression.
+///
+/// This is used in struct constructors.
+#[derive(Clone, Debug, HashStable)]
+pub struct FieldExpr {
+    pub name: Field,
+    pub expr: ExprId,
+}
+
+#[derive(Clone, Debug, HashStable)]
+pub struct FruInfo<'tcx> {
+    pub base: ExprId,
+    pub field_types: Box<[Ty<'tcx>]>,
+}
+
+/// A `match` arm.
+#[derive(Clone, Debug, HashStable)]
+pub struct Arm<'tcx> {
+    pub pattern: Pat<'tcx>,
+    pub guard: Option<Guard<'tcx>>,
+    pub body: ExprId,
+    pub lint_level: LintLevel,
+    pub scope: region::Scope,
+    pub span: Span,
+}
+
+/// A `match` guard.
+#[derive(Clone, Debug, HashStable)]
+pub enum Guard<'tcx> {
+    If(ExprId),
+    IfLet(Pat<'tcx>, ExprId),
+}
+
+#[derive(Copy, Clone, Debug, HashStable)]
+pub enum LogicalOp {
+    /// The `&&` operator.
+    And,
+    /// The `||` operator.
+    Or,
+}
+
+#[derive(Clone, Debug, HashStable)]
+pub enum InlineAsmOperand<'tcx> {
+    In {
+        reg: InlineAsmRegOrRegClass,
+        expr: ExprId,
+    },
+    Out {
+        reg: InlineAsmRegOrRegClass,
+        late: bool,
+        expr: Option<ExprId>,
+    },
+    InOut {
+        reg: InlineAsmRegOrRegClass,
+        late: bool,
+        expr: ExprId,
+    },
+    SplitInOut {
+        reg: InlineAsmRegOrRegClass,
+        late: bool,
+        in_expr: ExprId,
+        out_expr: Option<ExprId>,
+    },
+    Const {
+        value: mir::ConstantKind<'tcx>,
+        span: Span,
+    },
+    SymFn {
+        value: mir::ConstantKind<'tcx>,
+        span: Span,
+    },
+    SymStatic {
+        def_id: DefId,
+    },
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, HashStable)]
+pub enum BindingMode {
+    ByValue,
+    ByRef(BorrowKind),
+}
+
+#[derive(Clone, Debug, HashStable)]
+pub struct FieldPat<'tcx> {
+    pub field: Field,
+    pub pattern: Pat<'tcx>,
+}
+
+#[derive(Clone, Debug, HashStable)]
+pub struct Pat<'tcx> {
+    pub ty: Ty<'tcx>,
+    pub span: Span,
+    pub kind: Box<PatKind<'tcx>>,
+}
+
+impl<'tcx> Pat<'tcx> {
+    pub fn wildcard_from_ty(ty: Ty<'tcx>) -> Self {
+        Pat { ty, span: DUMMY_SP, kind: Box::new(PatKind::Wild) }
+    }
+}
+
+#[derive(Clone, Debug, HashStable)]
+pub struct Ascription<'tcx> {
+    pub annotation: CanonicalUserTypeAnnotation<'tcx>,
+    /// Variance to use when relating the `user_ty` to the **type of the value being
+    /// matched**. Typically, this is `Variance::Covariant`, since the value being matched must
+    /// have a type that is some subtype of the ascribed type.
+    ///
+    /// Note that this variance does not apply for any bindings within subpatterns. The type
+    /// assigned to those bindings must be exactly equal to the `user_ty` given here.
+    ///
+    /// The only place where this field is not `Covariant` is when matching constants, where
+    /// we currently use `Contravariant` -- this is because the constant type just needs to
+    /// be "comparable" to the type of the input value. So, for example:
+    ///
+    /// ```text
+    /// match x { "foo" => .. }
+    /// ```
+    ///
+    /// requires that `&'static str <: T_x`, where `T_x` is the type of `x`. Really, we should
+    /// probably be checking for a `PartialEq` impl instead, but this preserves the behavior
+    /// of the old type-check for now. See #57280 for details.
+    pub variance: ty::Variance,
+}
+
+#[derive(Clone, Debug, HashStable)]
+pub enum PatKind<'tcx> {
+    /// A wildcard pattern: `_`.
+    Wild,
+
+    AscribeUserType {
+        ascription: Ascription<'tcx>,
+        subpattern: Pat<'tcx>,
+    },
+
+    /// `x`, `ref x`, `x @ P`, etc.
+    Binding {
+        mutability: Mutability,
+        name: Symbol,
+        mode: BindingMode,
+        var: LocalVarId,
+        ty: Ty<'tcx>,
+        subpattern: Option<Pat<'tcx>>,
+        /// Is this the leftmost occurrence of the binding, i.e., is `var` the
+        /// `HirId` of this pattern?
+        is_primary: bool,
+    },
+
+    /// `Foo(...)` or `Foo{...}` or `Foo`, where `Foo` is a variant name from an ADT with
+    /// multiple variants.
+    Variant {
+        adt_def: AdtDef<'tcx>,
+        substs: SubstsRef<'tcx>,
+        variant_index: VariantIdx,
+        subpatterns: Vec<FieldPat<'tcx>>,
+    },
+
+    /// `(...)`, `Foo(...)`, `Foo{...}`, or `Foo`, where `Foo` is a variant name from an ADT with
+    /// a single variant.
+    Leaf {
+        subpatterns: Vec<FieldPat<'tcx>>,
+    },
+
+    /// `box P`, `&P`, `&mut P`, etc.
+    Deref {
+        subpattern: Pat<'tcx>,
+    },
+
+    /// One of the following:
+    /// * `&str`, which will be handled as a string pattern and thus exhaustiveness
+    ///   checking will detect if you use the same string twice in different patterns.
+    /// * integer, bool, char or float, which will be handled by exhaustiveness to cover exactly
+    ///   its own value, similar to `&str`, but these values are much simpler.
+    /// * Opaque constants, that must not be matched structurally. So anything that does not derive
+    ///   `PartialEq` and `Eq`.
+    Constant {
+        value: mir::ConstantKind<'tcx>,
+    },
+
+    Range(PatRange<'tcx>),
+
+    /// Matches against a slice, checking the length and extracting elements.
+    /// irrefutable when there is a slice pattern and both `prefix` and `suffix` are empty.
+    /// e.g., `&[ref xs @ ..]`.
+    Slice {
+        prefix: Vec<Pat<'tcx>>,
+        slice: Option<Pat<'tcx>>,
+        suffix: Vec<Pat<'tcx>>,
+    },
+
+    /// Fixed match against an array; irrefutable.
+    Array {
+        prefix: Vec<Pat<'tcx>>,
+        slice: Option<Pat<'tcx>>,
+        suffix: Vec<Pat<'tcx>>,
+    },
+
+    /// An or-pattern, e.g. `p | q`.
+    /// Invariant: `pats.len() >= 2`.
+    Or {
+        pats: Vec<Pat<'tcx>>,
+    },
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, HashStable)]
+pub struct PatRange<'tcx> {
+    pub lo: mir::ConstantKind<'tcx>,
+    pub hi: mir::ConstantKind<'tcx>,
+    pub end: RangeEnd,
+}
+
+impl<'tcx> fmt::Display for Pat<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // Printing lists is a chore.
+        let mut first = true;
+        let mut start_or_continue = |s| {
+            if first {
+                first = false;
+                ""
+            } else {
+                s
+            }
+        };
+        let mut start_or_comma = || start_or_continue(", ");
+
+        match *self.kind {
+            PatKind::Wild => write!(f, "_"),
+            PatKind::AscribeUserType { ref subpattern, .. } => write!(f, "{}: _", subpattern),
+            PatKind::Binding { mutability, name, mode, ref subpattern, .. } => {
+                let is_mut = match mode {
+                    BindingMode::ByValue => mutability == Mutability::Mut,
+                    BindingMode::ByRef(bk) => {
+                        write!(f, "ref ")?;
+                        matches!(bk, BorrowKind::Mut { .. })
+                    }
+                };
+                if is_mut {
+                    write!(f, "mut ")?;
+                }
+                write!(f, "{}", name)?;
+                if let Some(ref subpattern) = *subpattern {
+                    write!(f, " @ {}", subpattern)?;
+                }
+                Ok(())
+            }
+            PatKind::Variant { ref subpatterns, .. } | PatKind::Leaf { ref subpatterns } => {
+                let variant = match *self.kind {
+                    PatKind::Variant { adt_def, variant_index, .. } => {
+                        Some(adt_def.variant(variant_index))
+                    }
+                    _ => self.ty.ty_adt_def().and_then(|adt| {
+                        if !adt.is_enum() { Some(adt.non_enum_variant()) } else { None }
+                    }),
+                };
+
+                if let Some(variant) = variant {
+                    write!(f, "{}", variant.name)?;
+
+                    // Only for Adt we can have `S {...}`,
+                    // which we handle separately here.
+                    if variant.ctor_kind == CtorKind::Fictive {
+                        write!(f, " {{ ")?;
+
+                        let mut printed = 0;
+                        for p in subpatterns {
+                            if let PatKind::Wild = *p.pattern.kind {
+                                continue;
+                            }
+                            let name = variant.fields[p.field.index()].name;
+                            write!(f, "{}{}: {}", start_or_comma(), name, p.pattern)?;
+                            printed += 1;
+                        }
+
+                        if printed < variant.fields.len() {
+                            write!(f, "{}..", start_or_comma())?;
+                        }
+
+                        return write!(f, " }}");
+                    }
+                }
+
+                let num_fields = variant.map_or(subpatterns.len(), |v| v.fields.len());
+                if num_fields != 0 || variant.is_none() {
+                    write!(f, "(")?;
+                    for i in 0..num_fields {
+                        write!(f, "{}", start_or_comma())?;
+
+                        // Common case: the field is where we expect it.
+                        if let Some(p) = subpatterns.get(i) {
+                            if p.field.index() == i {
+                                write!(f, "{}", p.pattern)?;
+                                continue;
+                            }
+                        }
+
+                        // Otherwise, we have to go looking for it.
+                        if let Some(p) = subpatterns.iter().find(|p| p.field.index() == i) {
+                            write!(f, "{}", p.pattern)?;
+                        } else {
+                            write!(f, "_")?;
+                        }
+                    }
+                    write!(f, ")")?;
+                }
+
+                Ok(())
+            }
+            PatKind::Deref { ref subpattern } => {
+                match self.ty.kind() {
+                    ty::Adt(def, _) if def.is_box() => write!(f, "box ")?,
+                    ty::Ref(_, _, mutbl) => {
+                        write!(f, "&{}", mutbl.prefix_str())?;
+                    }
+                    _ => bug!("{} is a bad Deref pattern type", self.ty),
+                }
+                write!(f, "{}", subpattern)
+            }
+            PatKind::Constant { value } => write!(f, "{}", value),
+            PatKind::Range(PatRange { lo, hi, end }) => {
+                write!(f, "{}", lo)?;
+                write!(f, "{}", end)?;
+                write!(f, "{}", hi)
+            }
+            PatKind::Slice { ref prefix, ref slice, ref suffix }
+            | PatKind::Array { ref prefix, ref slice, ref suffix } => {
+                write!(f, "[")?;
+                for p in prefix {
+                    write!(f, "{}{}", start_or_comma(), p)?;
+                }
+                if let Some(ref slice) = *slice {
+                    write!(f, "{}", start_or_comma())?;
+                    match *slice.kind {
+                        PatKind::Wild => {}
+                        _ => write!(f, "{}", slice)?,
+                    }
+                    write!(f, "..")?;
+                }
+                for p in suffix {
+                    write!(f, "{}{}", start_or_comma(), p)?;
+                }
+                write!(f, "]")
+            }
+            PatKind::Or { ref pats } => {
+                for pat in pats {
+                    write!(f, "{}{}", start_or_continue(" | "), pat)?;
+                }
+                Ok(())
+            }
+        }
+    }
+}
+
+// Some nodes are used a lot. Make sure they don't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+mod size_asserts {
+    use super::*;
+    // These are in alphabetical order, which is easy to maintain.
+    rustc_data_structures::static_assert_size!(Block, 56);
+    rustc_data_structures::static_assert_size!(Expr<'_>, 104);
+    rustc_data_structures::static_assert_size!(Pat<'_>, 24);
+    rustc_data_structures::static_assert_size!(Stmt<'_>, 120);
+}
diff --git a/compiler/rustc_middle/src/thir/visit.rs b/compiler/rustc_middle/src/thir/visit.rs
new file mode 100644
index 000000000..97249fdd1
--- /dev/null
+++ b/compiler/rustc_middle/src/thir/visit.rs
@@ -0,0 +1,244 @@
+use super::{
+    Arm, Block, Expr, ExprKind, Guard, InlineAsmOperand, Pat, PatKind, Stmt, StmtKind, Thir,
+};
+
+pub trait Visitor<'a, 'tcx: 'a>: Sized {
+    fn thir(&self) -> &'a Thir<'tcx>;
+
+    fn visit_expr(&mut self, expr: &Expr<'tcx>) {
+        walk_expr(self, expr);
+    }
+
+    fn visit_stmt(&mut self, stmt: &Stmt<'tcx>) {
+        walk_stmt(self, stmt);
+    }
+
+    fn visit_block(&mut self, block: &Block) {
+        walk_block(self, block);
+    }
+
+    fn visit_arm(&mut self, arm: &Arm<'tcx>) {
+        walk_arm(self, arm);
+    }
+
+    fn visit_pat(&mut self, pat: &Pat<'tcx>) {
+        walk_pat(self, pat);
+    }
+
+    // Note: We don't have visitors for `ty::Const` and `mir::ConstantKind`
+    // (even though these types occur in THIR) for consistency and to reduce confusion,
+    // since the lazy creation of constants during thir construction causes most
+    // 'constants' to not be of type `ty::Const` or `mir::ConstantKind` at that
+    // stage (they are mostly still identified by `DefId` or `hir::Lit`, see
+    // the variants `Literal`, `NonHirLiteral` and `NamedConst` in `thir::ExprKind`).
+    // You have to manually visit `ty::Const` and `mir::ConstantKind` through the
+    // other `visit*` functions.
+}
+
+pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Expr<'tcx>) {
+    use ExprKind::*;
+    match expr.kind {
+        Scope { value, region_scope: _, lint_level: _ } => {
+            visitor.visit_expr(&visitor.thir()[value])
+        }
+        Box { value } => visitor.visit_expr(&visitor.thir()[value]),
+        If { cond, then, else_opt, if_then_scope: _ } => {
+            visitor.visit_expr(&visitor.thir()[cond]);
+            visitor.visit_expr(&visitor.thir()[then]);
+            if let Some(else_expr) = else_opt {
+                visitor.visit_expr(&visitor.thir()[else_expr]);
+            }
+        }
+        Call { fun, ref args, ty: _, from_hir_call: _, fn_span: _ } => {
+            visitor.visit_expr(&visitor.thir()[fun]);
+            for &arg in &**args {
+                visitor.visit_expr(&visitor.thir()[arg]);
+            }
+        }
+        Deref { arg } => visitor.visit_expr(&visitor.thir()[arg]),
+        Binary { lhs, rhs, op: _ } | LogicalOp { lhs, rhs, op: _ } => {
+            visitor.visit_expr(&visitor.thir()[lhs]);
+            visitor.visit_expr(&visitor.thir()[rhs]);
+        }
+        Unary { arg, op: _ } => visitor.visit_expr(&visitor.thir()[arg]),
+        Cast { source } => visitor.visit_expr(&visitor.thir()[source]),
+        Use { source } => visitor.visit_expr(&visitor.thir()[source]),
+        NeverToAny { source } => visitor.visit_expr(&visitor.thir()[source]),
+        Pointer { source, cast: _ } => visitor.visit_expr(&visitor.thir()[source]),
+        Let { expr, .. } => {
+            visitor.visit_expr(&visitor.thir()[expr]);
+        }
+        Loop { body } => visitor.visit_expr(&visitor.thir()[body]),
+        Match { scrutinee, ref arms } => {
+            visitor.visit_expr(&visitor.thir()[scrutinee]);
+            for &arm in &**arms {
+                visitor.visit_arm(&visitor.thir()[arm]);
+            }
+        }
+        Block { ref body } => visitor.visit_block(body),
+        Assign { lhs, rhs } | AssignOp { lhs, rhs, op: _ } => {
+            visitor.visit_expr(&visitor.thir()[lhs]);
+            visitor.visit_expr(&visitor.thir()[rhs]);
+        }
+        Field { lhs, variant_index: _, name: _ } => visitor.visit_expr(&visitor.thir()[lhs]),
+        Index { lhs, index } => {
+            visitor.visit_expr(&visitor.thir()[lhs]);
+            visitor.visit_expr(&visitor.thir()[index]);
+        }
+        VarRef { id: _ } | UpvarRef { closure_def_id: _, var_hir_id: _ } => {}
+        Borrow { arg, borrow_kind: _ } => visitor.visit_expr(&visitor.thir()[arg]),
+        AddressOf { arg, mutability: _ } => visitor.visit_expr(&visitor.thir()[arg]),
+        Break { value, label: _ } => {
+            if let Some(value) = value {
+                visitor.visit_expr(&visitor.thir()[value])
+            }
+        }
+        Continue { label: _ } => {}
+        Return { value } => {
+            if let Some(value) = value {
+                visitor.visit_expr(&visitor.thir()[value])
+            }
+        }
+        ConstBlock { did: _, substs: _ } => {}
+        Repeat { value, count: _ } => {
+            visitor.visit_expr(&visitor.thir()[value]);
+        }
+        Array { ref fields } | Tuple { ref fields } => {
+            for &field in &**fields {
+                visitor.visit_expr(&visitor.thir()[field]);
+            }
+        }
+        Adt(box crate::thir::Adt {
+            ref fields,
+            ref base,
+            adt_def: _,
+            variant_index: _,
+            substs: _,
+            user_ty: _,
+        }) => {
+            for field in &**fields {
+                visitor.visit_expr(&visitor.thir()[field.expr]);
+            }
+            if let Some(base) = base {
+                visitor.visit_expr(&visitor.thir()[base.base]);
+            }
+        }
+        PlaceTypeAscription { source, user_ty: _ } | ValueTypeAscription { source, user_ty: _ } => {
+            visitor.visit_expr(&visitor.thir()[source])
+        }
+        Closure { closure_id: _, substs: _, upvars: _, movability: _, fake_reads: _ } => {}
+        Literal { lit: _, neg: _ } => {}
+        NonHirLiteral { lit: _, user_ty: _ } => {}
+        ZstLiteral { user_ty: _ } => {}
+        NamedConst { def_id: _, substs: _, user_ty: _ } => {}
+        ConstParam { param: _, def_id: _ } => {}
+        StaticRef { alloc_id: _, ty: _, def_id: _ } => {}
+        InlineAsm { ref operands, template: _, options: _, line_spans: _ } => {
+            for op in &**operands {
+                use InlineAsmOperand::*;
+                match op {
+                    In { expr, reg: _ }
+                    | Out { expr: Some(expr), reg: _, late: _ }
+                    | InOut { expr, reg: _, late: _ } => visitor.visit_expr(&visitor.thir()[*expr]),
+                    SplitInOut { in_expr, out_expr, reg: _, late: _ } => {
+                        visitor.visit_expr(&visitor.thir()[*in_expr]);
+                        if let Some(out_expr) = out_expr {
+                            visitor.visit_expr(&visitor.thir()[*out_expr]);
+                        }
+                    }
+                    Out { expr: None, reg: _, late: _ }
+                    | Const { value: _, span: _ }
+                    | SymFn { value: _, span: _ }
+                    | SymStatic { def_id: _ } => {}
+                }
+            }
+        }
+        ThreadLocalRef(_) => {}
+        Yield { value } => visitor.visit_expr(&visitor.thir()[value]),
+    }
+}
+
+pub fn walk_stmt<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, stmt: &Stmt<'tcx>) {
+    match &stmt.kind {
+        StmtKind::Expr { expr, scope: _ } => visitor.visit_expr(&visitor.thir()[*expr]),
+        StmtKind::Let {
+            initializer,
+            remainder_scope: _,
+            init_scope: _,
+            ref pattern,
+            lint_level: _,
+            else_block,
+        } => {
+            if let Some(init) = initializer {
+                visitor.visit_expr(&visitor.thir()[*init]);
+            }
+            visitor.visit_pat(pattern);
+            if let Some(block) = else_block {
+                visitor.visit_block(block)
+            }
+        }
+    }
+}
+
+pub fn walk_block<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, block: &Block) {
+    for &stmt in &*block.stmts {
+        visitor.visit_stmt(&visitor.thir()[stmt]);
+    }
+    if let Some(expr) = block.expr {
+        visitor.visit_expr(&visitor.thir()[expr]);
+    }
+}
+
+pub fn walk_arm<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, arm: &Arm<'tcx>) {
+    match arm.guard {
+        Some(Guard::If(expr)) => visitor.visit_expr(&visitor.thir()[expr]),
+        Some(Guard::IfLet(ref pat, expr)) => {
+            visitor.visit_pat(pat);
+            visitor.visit_expr(&visitor.thir()[expr]);
+        }
+        None => {}
+    }
+    visitor.visit_pat(&arm.pattern);
+    visitor.visit_expr(&visitor.thir()[arm.body]);
+}
+
+pub fn walk_pat<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, pat: &Pat<'tcx>) {
+    use PatKind::*;
+    match pat.kind.as_ref() {
+        AscribeUserType { subpattern, ascription: _ }
+        | Deref { subpattern }
+        | Binding {
+            subpattern: Some(subpattern),
+            mutability: _,
+            mode: _,
+            var: _,
+            ty: _,
+            is_primary: _,
+            name: _,
+        } => visitor.visit_pat(&subpattern),
+        Binding { .. } | Wild => {}
+        Variant { subpatterns, adt_def: _, substs: _, variant_index: _ } | Leaf { subpatterns } => {
+            for subpattern in subpatterns {
+                visitor.visit_pat(&subpattern.pattern);
+            }
+        }
+        Constant { value: _ } => {}
+        Range(_) => {}
+        Slice { prefix, slice, suffix } | Array { prefix, slice, suffix } => {
+            for subpattern in prefix {
+                visitor.visit_pat(&subpattern);
+            }
+            if let Some(pat) = slice {
+                visitor.visit_pat(pat);
+            }
+            for subpattern in suffix {
+                visitor.visit_pat(&subpattern);
+            }
+        }
+        Or { pats } => {
+            for pat in pats {
+                visitor.visit_pat(&pat);
+            }
+        }
+    };
+}
diff --git a/compiler/rustc_middle/src/traits/chalk.rs b/compiler/rustc_middle/src/traits/chalk.rs
new file mode 100644
index 000000000..6d4af8bea
--- /dev/null
+++ b/compiler/rustc_middle/src/traits/chalk.rs
@@ -0,0 +1,403 @@
+//! Types required for Chalk-related queries
+//!
+//! The primary purpose of this file is defining an implementation for the
+//! `chalk_ir::interner::Interner` trait. The primary purpose of this trait, as
+//! its name suggest, is to provide an abstraction boundary for creating
+//! interned Chalk types.
+
+use rustc_middle::ty::{self, AdtDef, TyCtxt};
+
+use rustc_hir::def_id::DefId;
+use rustc_target::spec::abi::Abi;
+
+use std::cmp::Ordering;
+use std::fmt;
+use std::hash::{Hash, Hasher};
+
+#[derive(Copy, Clone)]
+pub struct RustInterner<'tcx> {
+    pub tcx: TyCtxt<'tcx>,
+}
+
+/// We don't ever actually need this. It's only required for derives.
+impl<'tcx> Hash for RustInterner<'tcx> {
+    fn hash<H: Hasher>(&self, _state: &mut H) {}
+}
+
+/// We don't ever actually need this. It's only required for derives.
+impl<'tcx> Ord for RustInterner<'tcx> {
+    fn cmp(&self, _other: &Self) -> Ordering {
+        Ordering::Equal
+    }
+}
+
+/// We don't ever actually need this. It's only required for derives.
+impl<'tcx> PartialOrd for RustInterner<'tcx> {
+    fn partial_cmp(&self, _other: &Self) -> Option<Ordering> {
+        None
+    }
+}
+
+/// We don't ever actually need this. It's only required for derives.
+impl<'tcx> PartialEq for RustInterner<'tcx> {
+    fn eq(&self, _other: &Self) -> bool {
+        false
+    }
+}
+
+/// We don't ever actually need this. It's only required for derives.
+impl<'tcx> Eq for RustInterner<'tcx> {}
+
+impl fmt::Debug for RustInterner<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "RustInterner")
+    }
+}
+
+// Right now, there is no interning at all. I was running into problems with
+// adding interning in `ty/context.rs` for Chalk types with
+// `parallel-compiler = true`. -jackh726
+impl<'tcx> chalk_ir::interner::Interner for RustInterner<'tcx> {
+    type InternedType = Box<chalk_ir::TyData<Self>>;
+    type InternedLifetime = Box<chalk_ir::LifetimeData<Self>>;
+    type InternedConst = Box<chalk_ir::ConstData<Self>>;
+    type InternedConcreteConst = ty::ValTree<'tcx>;
+    type InternedGenericArg = Box<chalk_ir::GenericArgData<Self>>;
+    type InternedGoal = Box<chalk_ir::GoalData<Self>>;
+    type InternedGoals = Vec<chalk_ir::Goal<Self>>;
+    type InternedSubstitution = Vec<chalk_ir::GenericArg<Self>>;
+    type InternedProgramClause = Box<chalk_ir::ProgramClauseData<Self>>;
+    type InternedProgramClauses = Vec<chalk_ir::ProgramClause<Self>>;
+    type InternedQuantifiedWhereClauses = Vec<chalk_ir::QuantifiedWhereClause<Self>>;
+    type InternedVariableKinds = Vec<chalk_ir::VariableKind<Self>>;
+    type InternedCanonicalVarKinds = Vec<chalk_ir::CanonicalVarKind<Self>>;
+    type InternedVariances = Vec<chalk_ir::Variance>;
+    type InternedConstraints = Vec<chalk_ir::InEnvironment<chalk_ir::Constraint<Self>>>;
+    type DefId = DefId;
+    type InternedAdtId = AdtDef<'tcx>;
+    type Identifier = ();
+    type FnAbi = Abi;
+
+    fn debug_program_clause_implication(
+        pci: &chalk_ir::ProgramClauseImplication<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        let mut write = || {
+            write!(fmt, "{:?}", pci.consequence)?;
+
+            let conditions = pci.conditions.interned();
+            let constraints = pci.constraints.interned();
+
+            let conds = conditions.len();
+            let consts = constraints.len();
+            if conds == 0 && consts == 0 {
+                return Ok(());
+            }
+
+            write!(fmt, " :- ")?;
+
+            if conds != 0 {
+                for cond in &conditions[..conds - 1] {
+                    write!(fmt, "{:?}, ", cond)?;
+                }
+                write!(fmt, "{:?}", conditions[conds - 1])?;
+            }
+
+            if conds != 0 && consts != 0 {
+                write!(fmt, " ; ")?;
+            }
+
+            if consts != 0 {
+                for constraint in &constraints[..consts - 1] {
+                    write!(fmt, "{:?}, ", constraint)?;
+                }
+                write!(fmt, "{:?}", constraints[consts - 1])?;
+            }
+
+            Ok(())
+        };
+        Some(write())
+    }
+
+    fn debug_substitution(
+        substitution: &chalk_ir::Substitution<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        Some(write!(fmt, "{:?}", substitution.interned()))
+    }
+
+    fn debug_separator_trait_ref(
+        separator_trait_ref: &chalk_ir::SeparatorTraitRef<'_, Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        let substitution = &separator_trait_ref.trait_ref.substitution;
+        let parameters = substitution.interned();
+        Some(write!(
+            fmt,
+            "{:?}{}{:?}{:?}",
+            parameters[0],
+            separator_trait_ref.separator,
+            separator_trait_ref.trait_ref.trait_id,
+            chalk_ir::debug::Angle(&parameters[1..])
+        ))
+    }
+
+    fn debug_quantified_where_clauses(
+        clauses: &chalk_ir::QuantifiedWhereClauses<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        Some(write!(fmt, "{:?}", clauses.interned()))
+    }
+
+    fn debug_ty(ty: &chalk_ir::Ty<Self>, fmt: &mut fmt::Formatter<'_>) -> Option<fmt::Result> {
+        match &ty.interned().kind {
+            chalk_ir::TyKind::Ref(chalk_ir::Mutability::Not, lifetime, ty) => {
+                Some(write!(fmt, "(&{:?} {:?})", lifetime, ty))
+            }
+            chalk_ir::TyKind::Ref(chalk_ir::Mutability::Mut, lifetime, ty) => {
+                Some(write!(fmt, "(&{:?} mut {:?})", lifetime, ty))
+            }
+            chalk_ir::TyKind::Array(ty, len) => Some(write!(fmt, "[{:?}; {:?}]", ty, len)),
+            chalk_ir::TyKind::Slice(ty) => Some(write!(fmt, "[{:?}]", ty)),
+            chalk_ir::TyKind::Tuple(len, substs) => Some((|| {
+                write!(fmt, "(")?;
+                for (idx, substitution) in substs.interned().iter().enumerate() {
+                    if idx == *len && *len != 1 {
+                        // Don't add a trailing comma if the tuple has more than one element
+                        write!(fmt, "{:?}", substitution)?;
+                    } else {
+                        write!(fmt, "{:?},", substitution)?;
+                    }
+                }
+                write!(fmt, ")")
+            })()),
+            _ => None,
+        }
+    }
+
+    fn debug_alias(
+        alias_ty: &chalk_ir::AliasTy<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        match alias_ty {
+            chalk_ir::AliasTy::Projection(projection_ty) => {
+                Self::debug_projection_ty(projection_ty, fmt)
+            }
+            chalk_ir::AliasTy::Opaque(opaque_ty) => Self::debug_opaque_ty(opaque_ty, fmt),
+        }
+    }
+
+    fn debug_projection_ty(
+        projection_ty: &chalk_ir::ProjectionTy<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        Some(write!(
+            fmt,
+            "projection: {:?} {:?}",
+            projection_ty.associated_ty_id, projection_ty.substitution,
+        ))
+    }
+
+    fn debug_opaque_ty(
+        opaque_ty: &chalk_ir::OpaqueTy<Self>,
+        fmt: &mut fmt::Formatter<'_>,
+    ) -> Option<fmt::Result> {
+        Some(write!(fmt, "{:?}", opaque_ty.opaque_ty_id))
+    }
+
+    fn intern_ty(self, ty: chalk_ir::TyKind<Self>) -> Self::InternedType {
+        let flags = ty.compute_flags(self);
+        Box::new(chalk_ir::TyData { kind: ty, flags: flags })
+    }
+
+    fn ty_data<'a>(self, ty: &'a Self::InternedType) -> &'a chalk_ir::TyData<Self> {
+        ty
+    }
+
+    fn intern_lifetime(self, lifetime: chalk_ir::LifetimeData<Self>) -> Self::InternedLifetime {
+        Box::new(lifetime)
+    }
+
+    fn lifetime_data<'a>(
+        self,
+        lifetime: &'a Self::InternedLifetime,
+    ) -> &'a chalk_ir::LifetimeData<Self> {
+        &lifetime
+    }
+
+    fn intern_const(self, constant: chalk_ir::ConstData<Self>) -> Self::InternedConst {
+        Box::new(constant)
+    }
+
+    fn const_data<'a>(self, constant: &'a Self::InternedConst) -> &'a chalk_ir::ConstData<Self> {
+        &constant
+    }
+
+    fn const_eq(
+        self,
+        _ty: &Self::InternedType,
+        c1: &Self::InternedConcreteConst,
+        c2: &Self::InternedConcreteConst,
+    ) -> bool {
+        c1 == c2
+    }
+
+    fn intern_generic_arg(self, data: chalk_ir::GenericArgData<Self>) -> Self::InternedGenericArg {
+        Box::new(data)
+    }
+
+    fn generic_arg_data<'a>(
+        self,
+        data: &'a Self::InternedGenericArg,
+    ) -> &'a chalk_ir::GenericArgData<Self> {
+        &data
+    }
+
+    fn intern_goal(self, goal: chalk_ir::GoalData<Self>) -> Self::InternedGoal {
+        Box::new(goal)
+    }
+
+    fn goal_data<'a>(self, goal: &'a Self::InternedGoal) -> &'a chalk_ir::GoalData<Self> {
+        &goal
+    }
+
+    fn intern_goals<E>(
+        self,
+        data: impl IntoIterator<Item = Result<chalk_ir::Goal<Self>, E>>,
+    ) -> Result<Self::InternedGoals, E> {
+        data.into_iter().collect::<Result<Vec<_>, _>>()
+    }
+
+    fn goals_data<'a>(self, goals: &'a Self::InternedGoals) -> &'a [chalk_ir::Goal<Self>] {
+        goals
+    }
+
+    fn intern_substitution<E>(
+        self,
+        data: impl IntoIterator<Item = Result<chalk_ir::GenericArg<Self>, E>>,
+    ) -> Result<Self::InternedSubstitution, E> {
+        data.into_iter().collect::<Result<Vec<_>, _>>()
+    }
+
+    fn substitution_data<'a>(
+        self,
+        substitution: &'a Self::InternedSubstitution,
+    ) -> &'a [chalk_ir::GenericArg<Self>] {
+        substitution
+    }
+
+    fn intern_program_clause(
+        self,
+        data: chalk_ir::ProgramClauseData<Self>,
+    ) -> Self::InternedProgramClause {
+        Box::new(data)
+    }
+
+    fn program_clause_data<'a>(
+        self,
+        clause: &'a Self::InternedProgramClause,
+    ) -> &'a chalk_ir::ProgramClauseData<Self> {
+        &clause
+    }
+
+    fn intern_program_clauses<E>(
+        self,
+        data: impl IntoIterator<Item = Result<chalk_ir::ProgramClause<Self>, E>>,
+    ) -> Result<Self::InternedProgramClauses, E> {
+        data.into_iter().collect::<Result<Vec<_>, _>>()
+    }
+
+    fn program_clauses_data<'a>(
+        self,
+        clauses: &'a Self::InternedProgramClauses,
+    ) -> &'a [chalk_ir::ProgramClause<Self>] {
+        clauses
+    }
+
+    fn intern_quantified_where_clauses<E>(
+        self,
+        data: impl IntoIterator<Item = Result<chalk_ir::QuantifiedWhereClause<Self>, E>>,
+    ) -> Result<Self::InternedQuantifiedWhereClauses, E> {
+        data.into_iter().collect::<Result<Vec<_>, _>>()
+    }
+
+    fn quantified_where_clauses_data<'a>(
+        self,
+        clauses: &'a Self::InternedQuantifiedWhereClauses,
+    ) -> &'a [chalk_ir::QuantifiedWhereClause<Self>] {
+        clauses
+    }
+
+    fn intern_generic_arg_kinds<E>(
+        self,
+        data: impl IntoIterator<Item = Result<chalk_ir::VariableKind<Self>, E>>,
+    ) -> Result<Self::InternedVariableKinds, E> {
+        data.into_iter().collect::<Result<Vec<_>, _>>()
+    }
+
+    fn variable_kinds_data<'a>(
+        self,
+        parameter_kinds: &'a Self::InternedVariableKinds,
+    ) -> &'a [chalk_ir::VariableKind<Self>] {
+        parameter_kinds
+    }
+
+    fn intern_canonical_var_kinds<E>(
+        self,
+        data: impl IntoIterator<Item = Result<chalk_ir::CanonicalVarKind<Self>, E>>,
+    ) -> Result<Self::InternedCanonicalVarKinds, E> {
+        data.into_iter().collect::<Result<Vec<_>, _>>()
+    }
+
+    fn canonical_var_kinds_data<'a>(
+        self,
+        canonical_var_kinds: &'a Self::InternedCanonicalVarKinds,
+    ) -> &'a [chalk_ir::CanonicalVarKind<Self>] {
+        canonical_var_kinds
+    }
+
+    fn intern_constraints<E>(
+        self,
+        data: impl IntoIterator<Item = Result<chalk_ir::InEnvironment<chalk_ir::Constraint<Self>>, E>>,
+    ) -> Result<Self::InternedConstraints, E> {
+        data.into_iter().collect::<Result<Vec<_>, _>>()
+    }
+
+    fn constraints_data<'a>(
+        self,
+        constraints: &'a Self::InternedConstraints,
+    ) -> &'a [chalk_ir::InEnvironment<chalk_ir::Constraint<Self>>] {
+        constraints
+    }
+
+    fn intern_variances<E>(
+        self,
+        data: impl IntoIterator<Item = Result<chalk_ir::Variance, E>>,
+    ) -> Result<Self::InternedVariances, E> {
+        data.into_iter().collect::<Result<Vec<_>, _>>()
+    }
+
+    fn variances_data<'a>(
+        self,
+        variances: &'a Self::InternedVariances,
+    ) -> &'a [chalk_ir::Variance] {
+        variances
+    }
+}
+
+impl<'tcx> chalk_ir::interner::HasInterner for RustInterner<'tcx> {
+    type Interner = Self;
+}
+
+/// A chalk environment and goal.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, HashStable, TypeFoldable, TypeVisitable)]
+pub struct ChalkEnvironmentAndGoal<'tcx> {
+    pub environment: &'tcx ty::List<ty::Predicate<'tcx>>,
+    pub goal: ty::Predicate<'tcx>,
+}
+
+impl<'tcx> fmt::Display for ChalkEnvironmentAndGoal<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "environment: {:?}, goal: {}", self.environment, self.goal)
+    }
+}
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
new file mode 100644
index 000000000..72b848c3e
--- /dev/null
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -0,0 +1,1026 @@
+//! Trait Resolution. See the [rustc dev guide] for more information on how this works.
+//!
+//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html
+
+mod chalk;
+pub mod query;
+pub mod select;
+pub mod specialization_graph;
+mod structural_impls;
+pub mod util;
+
+use crate::infer::canonical::Canonical;
+use crate::ty::abstract_const::NotConstEvaluatable;
+use crate::ty::subst::SubstsRef;
+use crate::ty::{self, AdtKind, Predicate, Ty, TyCtxt};
+
+use rustc_data_structures::sync::Lrc;
+use rustc_errors::{Applicability, Diagnostic};
+use rustc_hir as hir;
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_span::symbol::Symbol;
+use rustc_span::{Span, DUMMY_SP};
+use smallvec::SmallVec;
+
+use std::borrow::Cow;
+use std::hash::{Hash, Hasher};
+
+pub use self::select::{EvaluationCache, EvaluationResult, OverflowError, SelectionCache};
+
+pub type CanonicalChalkEnvironmentAndGoal<'tcx> = Canonical<'tcx, ChalkEnvironmentAndGoal<'tcx>>;
+
+pub use self::ObligationCauseCode::*;
+
+pub use self::chalk::{ChalkEnvironmentAndGoal, RustInterner as ChalkRustInterner};
+
+/// Depending on the stage of compilation, we want projection to be
+/// more or less conservative.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, HashStable)]
+pub enum Reveal {
+    /// At type-checking time, we refuse to project any associated
+    /// type that is marked `default`. Non-`default` ("final") types
+    /// are always projected. This is necessary in general for
+    /// soundness of specialization. However, we *could* allow
+    /// projections in fully-monomorphic cases. We choose not to,
+    /// because we prefer for `default type` to force the type
+    /// definition to be treated abstractly by any consumers of the
+    /// impl. Concretely, that means that the following example will
+    /// fail to compile:
+    ///
+    /// ```compile_fail,E0308
+    /// #![feature(specialization)]
+    /// trait Assoc {
+    ///     type Output;
+    /// }
+    ///
+    /// impl<T> Assoc for T {
+    ///     default type Output = bool;
+    /// }
+    ///
+    /// fn main() {
+    ///     let x: <() as Assoc>::Output = true;
+    /// }
+    /// ```
+    ///
+    /// We also do not reveal the hidden type of opaque types during
+    /// type-checking.
+    UserFacing,
+
+    /// At codegen time, all monomorphic projections will succeed.
+    /// Also, `impl Trait` is normalized to the concrete type,
+    /// which has to be already collected by type-checking.
+    ///
+    /// NOTE: as `impl Trait`'s concrete type should *never*
+    /// be observable directly by the user, `Reveal::All`
+    /// should not be used by checks which may expose
+    /// type equality or type contents to the user.
+    /// There are some exceptions, e.g., around auto traits and
+    /// transmute-checking, which expose some details, but
+    /// not the whole concrete type of the `impl Trait`.
+    All,
+}
+
+/// The reason why we incurred this obligation; used for error reporting.
+///
+/// Non-misc `ObligationCauseCode`s are stored on the heap. This gives the
+/// best trade-off between keeping the type small (which makes copies cheaper)
+/// while not doing too many heap allocations.
+///
+/// We do not want to intern this as there are a lot of obligation causes which
+/// only live for a short period of time.
+#[derive(Clone, Debug, PartialEq, Eq, Lift)]
+pub struct ObligationCause<'tcx> {
+    pub span: Span,
+
+    /// The ID of the fn body that triggered this obligation. This is
+    /// used for region obligations to determine the precise
+    /// environment in which the region obligation should be evaluated
+    /// (in particular, closures can add new assumptions). See the
+    /// field `region_obligations` of the `FulfillmentContext` for more
+    /// information.
+    pub body_id: hir::HirId,
+
+    code: InternedObligationCauseCode<'tcx>,
+}
+
+// This custom hash function speeds up hashing for `Obligation` deduplication
+// greatly by skipping the `code` field, which can be large and complex. That
+// shouldn't affect hash quality much since there are several other fields in
+// `Obligation` which should be unique enough, especially the predicate itself
+// which is hashed as an interned pointer. See #90996.
+impl Hash for ObligationCause<'_> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.body_id.hash(state);
+        self.span.hash(state);
+    }
+}
+
+impl<'tcx> ObligationCause<'tcx> {
+    #[inline]
+    pub fn new(
+        span: Span,
+        body_id: hir::HirId,
+        code: ObligationCauseCode<'tcx>,
+    ) -> ObligationCause<'tcx> {
+        ObligationCause { span, body_id, code: code.into() }
+    }
+
+    pub fn misc(span: Span, body_id: hir::HirId) -> ObligationCause<'tcx> {
+        ObligationCause::new(span, body_id, MiscObligation)
+    }
+
+    #[inline(always)]
+    pub fn dummy() -> ObligationCause<'tcx> {
+        ObligationCause::dummy_with_span(DUMMY_SP)
+    }
+
+    #[inline(always)]
+    pub fn dummy_with_span(span: Span) -> ObligationCause<'tcx> {
+        ObligationCause { span, body_id: hir::CRATE_HIR_ID, code: Default::default() }
+    }
+
+    pub fn span(&self) -> Span {
+        match *self.code() {
+            ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
+                arm_span,
+                ..
+            }) => arm_span,
+            _ => self.span,
+        }
+    }
+
+    #[inline]
+    pub fn code(&self) -> &ObligationCauseCode<'tcx> {
+        &self.code
+    }
+
+    pub fn map_code(
+        &mut self,
+        f: impl FnOnce(InternedObligationCauseCode<'tcx>) -> ObligationCauseCode<'tcx>,
+    ) {
+        self.code = f(std::mem::take(&mut self.code)).into();
+    }
+
+    pub fn derived_cause(
+        mut self,
+        parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
+        variant: impl FnOnce(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
+    ) -> ObligationCause<'tcx> {
+        /*!
+         * Creates a cause for obligations that are derived from
+         * `obligation` by a recursive search (e.g., for a builtin
+         * bound, or eventually a `auto trait Foo`). If `obligation`
+         * is itself a derived obligation, this is just a clone, but
+         * otherwise we create a "derived obligation" cause so as to
+         * keep track of the original root obligation for error
+         * reporting.
+         */
+
+        // NOTE(flaper87): As of now, it keeps track of the whole error
+        // chain. Ideally, we should have a way to configure this either
+        // by using -Z verbose or just a CLI argument.
+        self.code =
+            variant(DerivedObligationCause { parent_trait_pred, parent_code: self.code }).into();
+        self
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
+pub struct UnifyReceiverContext<'tcx> {
+    pub assoc_item: ty::AssocItem,
+    pub param_env: ty::ParamEnv<'tcx>,
+    pub substs: SubstsRef<'tcx>,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash, Lift, Default)]
+pub struct InternedObligationCauseCode<'tcx> {
+    /// `None` for `ObligationCauseCode::MiscObligation` (a common case, occurs ~60% of
+    /// the time). `Some` otherwise.
+    code: Option<Lrc<ObligationCauseCode<'tcx>>>,
+}
+
+impl<'tcx> ObligationCauseCode<'tcx> {
+    #[inline(always)]
+    fn into(self) -> InternedObligationCauseCode<'tcx> {
+        InternedObligationCauseCode {
+            code: if let ObligationCauseCode::MiscObligation = self {
+                None
+            } else {
+                Some(Lrc::new(self))
+            },
+        }
+    }
+}
+
+impl<'tcx> std::ops::Deref for InternedObligationCauseCode<'tcx> {
+    type Target = ObligationCauseCode<'tcx>;
+
+    fn deref(&self) -> &Self::Target {
+        self.code.as_deref().unwrap_or(&ObligationCauseCode::MiscObligation)
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
+pub enum ObligationCauseCode<'tcx> {
+    /// Not well classified or should be obvious from the span.
+    MiscObligation,
+
+    /// A slice or array is WF only if `T: Sized`.
+    SliceOrArrayElem,
+
+    /// A tuple is WF only if its middle elements are `Sized`.
+    TupleElem,
+
+    /// This is the trait reference from the given projection.
+    ProjectionWf(ty::ProjectionTy<'tcx>),
+
+    /// In an impl of trait `X` for type `Y`, type `Y` must
+    /// also implement all supertraits of `X`.
+    ItemObligation(DefId),
+
+    /// Like `ItemObligation`, but with extra detail on the source of the obligation.
+    BindingObligation(DefId, Span),
+
+    /// A type like `&'a T` is WF only if `T: 'a`.
+    ReferenceOutlivesReferent(Ty<'tcx>),
+
+    /// A type like `Box<Foo<'a> + 'b>` is WF only if `'b: 'a`.
+    ObjectTypeBound(Ty<'tcx>, ty::Region<'tcx>),
+
+    /// Obligation incurred due to an object cast.
+    ObjectCastObligation(/* Concrete type */ Ty<'tcx>, /* Object type */ Ty<'tcx>),
+
+    /// Obligation incurred due to a coercion.
+    Coercion {
+        source: Ty<'tcx>,
+        target: Ty<'tcx>,
+    },
+
+    /// Various cases where expressions must be `Sized` / `Copy` / etc.
+    /// `L = X` implies that `L` is `Sized`.
+    AssignmentLhsSized,
+    /// `(x1, .., xn)` must be `Sized`.
+    TupleInitializerSized,
+    /// `S { ... }` must be `Sized`.
+    StructInitializerSized,
+    /// Type of each variable must be `Sized`.
+    VariableType(hir::HirId),
+    /// Argument type must be `Sized`.
+    SizedArgumentType(Option<Span>),
+    /// Return type must be `Sized`.
+    SizedReturnType,
+    /// Yield type must be `Sized`.
+    SizedYieldType,
+    /// Box expression result type must be `Sized`.
+    SizedBoxType,
+    /// Inline asm operand type must be `Sized`.
+    InlineAsmSized,
+    /// `[expr; N]` requires `type_of(expr): Copy`.
+    RepeatElementCopy {
+        /// If element is a `const fn` we display a help message suggesting to move the
+        /// function call to a new `const` item while saying that `T` doesn't implement `Copy`.
+        is_const_fn: bool,
+    },
+
+    /// Types of fields (other than the last, except for packed structs) in a struct must be sized.
+    FieldSized {
+        adt_kind: AdtKind,
+        span: Span,
+        last: bool,
+    },
+
+    /// Constant expressions must be sized.
+    ConstSized,
+
+    /// `static` items must have `Sync` type.
+    SharedStatic,
+
+    BuiltinDerivedObligation(DerivedObligationCause<'tcx>),
+
+    ImplDerivedObligation(Box<ImplDerivedObligationCause<'tcx>>),
+
+    DerivedObligation(DerivedObligationCause<'tcx>),
+
+    FunctionArgumentObligation {
+        /// The node of the relevant argument in the function call.
+        arg_hir_id: hir::HirId,
+        /// The node of the function call.
+        call_hir_id: hir::HirId,
+        /// The obligation introduced by this argument.
+        parent_code: InternedObligationCauseCode<'tcx>,
+    },
+
+    /// Error derived when matching traits/impls; see ObligationCause for more details
+    CompareImplItemObligation {
+        impl_item_def_id: LocalDefId,
+        trait_item_def_id: DefId,
+        kind: ty::AssocKind,
+    },
+
+    /// Checking that the bounds of a trait's associated type hold for a given impl
+    CheckAssociatedTypeBounds {
+        impl_item_def_id: LocalDefId,
+        trait_item_def_id: DefId,
+    },
+
+    /// Checking that this expression can be assigned to its target.
+    ExprAssignable,
+
+    /// Computing common supertype in the arms of a match expression
+    MatchExpressionArm(Box<MatchExpressionArmCause<'tcx>>),
+
+    /// Type error arising from type checking a pattern against an expected type.
+    Pattern {
+        /// The span of the scrutinee or type expression which caused the `root_ty` type.
+        span: Option<Span>,
+        /// The root expected type induced by a scrutinee or type expression.
+        root_ty: Ty<'tcx>,
+        /// Whether the `Span` came from an expression or a type expression.
+        origin_expr: bool,
+    },
+
+    /// Constants in patterns must have `Structural` type.
+    ConstPatternStructural,
+
+    /// Computing common supertype in an if expression
+    IfExpression(Box<IfExpressionCause<'tcx>>),
+
+    /// Computing common supertype of an if expression with no else counter-part
+    IfExpressionWithNoElse,
+
+    /// `main` has wrong type
+    MainFunctionType,
+
+    /// `start` has wrong type
+    StartFunctionType,
+
+    /// Intrinsic has wrong type
+    IntrinsicType,
+
+    /// A let else block does not diverge
+    LetElse,
+
+    /// Method receiver
+    MethodReceiver,
+
+    UnifyReceiver(Box<UnifyReceiverContext<'tcx>>),
+
+    /// `return` with no expression
+    ReturnNoExpression,
+
+    /// `return` with an expression
+    ReturnValue(hir::HirId),
+
+    /// Return type of this function
+    ReturnType,
+
+    /// Opaque return type of this function
+    OpaqueReturnType(Option<(Ty<'tcx>, Span)>),
+
+    /// Block implicit return
+    BlockTailExpression(hir::HirId),
+
+    /// #[feature(trivial_bounds)] is not enabled
+    TrivialBound,
+
+    /// If `X` is the concrete type of an opaque type `impl Y`, then `X` must implement `Y`
+    OpaqueType,
+
+    AwaitableExpr(Option<hir::HirId>),
+
+    ForLoopIterator,
+
+    QuestionMark,
+
+    /// Well-formed checking. If a `WellFormedLoc` is provided,
+    /// then it will be used to perform HIR-based wf checking
+    /// after an error occurs, in order to generate a more precise error span.
+    /// This is purely for diagnostic purposes - it is always
+    /// correct to use `MiscObligation` instead, or to specify
+    /// `WellFormed(None)`
+    WellFormed(Option<WellFormedLoc>),
+
+    /// From `match_impl`. The cause for us having to match an impl, and the DefId we are matching against.
+    MatchImpl(ObligationCause<'tcx>, DefId),
+
+    BinOp {
+        rhs_span: Option<Span>,
+        is_lit: bool,
+        output_pred: Option<Predicate<'tcx>>,
+    },
+}
+
+/// The 'location' at which we try to perform HIR-based wf checking.
+/// This information is used to obtain an `hir::Ty`, which
+/// we can walk in order to obtain precise spans for any
+/// 'nested' types (e.g. `Foo` in `Option<Foo>`).
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable)]
+pub enum WellFormedLoc {
+    /// Use the type of the provided definition.
+    Ty(LocalDefId),
+    /// Use the type of the parameter of the provided function.
+    /// We cannot use `hir::Param`, since the function may
+    /// not have a body (e.g. a trait method definition)
+    Param {
+        /// The function to lookup the parameter in
+        function: LocalDefId,
+        /// The index of the parameter to use.
+        /// Parameters are indexed from 0, with the return type
+        /// being the last 'parameter'
+        param_idx: u16,
+    },
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
+pub struct ImplDerivedObligationCause<'tcx> {
+    pub derived: DerivedObligationCause<'tcx>,
+    pub impl_def_id: DefId,
+    pub span: Span,
+}
+
+impl<'tcx> ObligationCauseCode<'tcx> {
+    // Return the base obligation, ignoring derived obligations.
+    pub fn peel_derives(&self) -> &Self {
+        let mut base_cause = self;
+        while let Some((parent_code, _)) = base_cause.parent() {
+            base_cause = parent_code;
+        }
+        base_cause
+    }
+
+    pub fn parent(&self) -> Option<(&Self, Option<ty::PolyTraitPredicate<'tcx>>)> {
+        match self {
+            FunctionArgumentObligation { parent_code, .. } => Some((parent_code, None)),
+            BuiltinDerivedObligation(derived)
+            | DerivedObligation(derived)
+            | ImplDerivedObligation(box ImplDerivedObligationCause { derived, .. }) => {
+                Some((&derived.parent_code, Some(derived.parent_trait_pred)))
+            }
+            _ => None,
+        }
+    }
+}
+
+// `ObligationCauseCode` is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(ObligationCauseCode<'_>, 48);
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub enum StatementAsExpression {
+    CorrectType,
+    NeedsBoxing,
+}
+
+impl<'tcx> ty::Lift<'tcx> for StatementAsExpression {
+    type Lifted = StatementAsExpression;
+    fn lift_to_tcx(self, _tcx: TyCtxt<'tcx>) -> Option<StatementAsExpression> {
+        Some(self)
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
+pub struct MatchExpressionArmCause<'tcx> {
+    pub arm_block_id: Option<hir::HirId>,
+    pub arm_ty: Ty<'tcx>,
+    pub arm_span: Span,
+    pub prior_arm_block_id: Option<hir::HirId>,
+    pub prior_arm_ty: Ty<'tcx>,
+    pub prior_arm_span: Span,
+    pub scrut_span: Span,
+    pub source: hir::MatchSource,
+    pub prior_arms: Vec<Span>,
+    pub scrut_hir_id: hir::HirId,
+    pub opt_suggest_box_span: Option<Span>,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[derive(Lift, TypeFoldable, TypeVisitable)]
+pub struct IfExpressionCause<'tcx> {
+    pub then_id: hir::HirId,
+    pub else_id: hir::HirId,
+    pub then_ty: Ty<'tcx>,
+    pub else_ty: Ty<'tcx>,
+    pub outer_span: Option<Span>,
+    pub opt_suggest_box_span: Option<Span>,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
+pub struct DerivedObligationCause<'tcx> {
+    /// The trait predicate of the parent obligation that led to the
+    /// current obligation. Note that only trait obligations lead to
+    /// derived obligations, so we just store the trait predicate here
+    /// directly.
+    pub parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
+
+    /// The parent trait had this cause.
+    pub parent_code: InternedObligationCauseCode<'tcx>,
+}
+
+#[derive(Clone, Debug, TypeFoldable, TypeVisitable, Lift)]
+pub enum SelectionError<'tcx> {
+    /// The trait is not implemented.
+    Unimplemented,
+    /// After a closure impl has selected, its "outputs" were evaluated
+    /// (which for closures includes the "input" type params) and they
+    /// didn't resolve. See `confirm_poly_trait_refs` for more.
+    OutputTypeParameterMismatch(
+        ty::PolyTraitRef<'tcx>,
+        ty::PolyTraitRef<'tcx>,
+        ty::error::TypeError<'tcx>,
+    ),
+    /// The trait pointed by `DefId` is not object safe.
+    TraitNotObjectSafe(DefId),
+    /// A given constant couldn't be evaluated.
+    NotConstEvaluatable(NotConstEvaluatable),
+    /// Exceeded the recursion depth during type projection.
+    Overflow(OverflowError),
+    /// Signaling that an error has already been emitted, to avoid
+    /// multiple errors being shown.
+    ErrorReporting,
+    /// Multiple applicable `impl`s where found. The `DefId`s correspond to
+    /// all the `impl`s' Items.
+    Ambiguous(Vec<DefId>),
+}
+
+/// When performing resolution, it is typically the case that there
+/// can be one of three outcomes:
+///
+/// - `Ok(Some(r))`: success occurred with result `r`
+/// - `Ok(None)`: could not definitely determine anything, usually due
+///   to inconclusive type inference.
+/// - `Err(e)`: error `e` occurred
+pub type SelectionResult<'tcx, T> = Result<Option<T>, SelectionError<'tcx>>;
+
+/// Given the successful resolution of an obligation, the `ImplSource`
+/// indicates where the impl comes from.
+///
+/// For example, the obligation may be satisfied by a specific impl (case A),
+/// or it may be relative to some bound that is in scope (case B).
+///
+/// ```ignore (illustrative)
+/// impl<T:Clone> Clone<T> for Option<T> { ... } // Impl_1
+/// impl<T:Clone> Clone<T> for Box<T> { ... }    // Impl_2
+/// impl Clone for i32 { ... }                   // Impl_3
+///
+/// fn foo<T: Clone>(concrete: Option<Box<i32>>, param: T, mixed: Option<T>) {
+///     // Case A: ImplSource points at a specific impl. Only possible when
+///     // type is concretely known. If the impl itself has bounded
+///     // type parameters, ImplSource will carry resolutions for those as well:
+///     concrete.clone(); // ImplSource(Impl_1, [ImplSource(Impl_2, [ImplSource(Impl_3)])])
+///
+///     // Case A: ImplSource points at a specific impl. Only possible when
+///     // type is concretely known. If the impl itself has bounded
+///     // type parameters, ImplSource will carry resolutions for those as well:
+///     concrete.clone(); // ImplSource(Impl_1, [ImplSource(Impl_2, [ImplSource(Impl_3)])])
+///
+///     // Case B: ImplSource must be provided by caller. This applies when
+///     // type is a type parameter.
+///     param.clone();    // ImplSource::Param
+///
+///     // Case C: A mix of cases A and B.
+///     mixed.clone();    // ImplSource(Impl_1, [ImplSource::Param])
+/// }
+/// ```
+///
+/// ### The type parameter `N`
+///
+/// See explanation on `ImplSourceUserDefinedData`.
+#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub enum ImplSource<'tcx, N> {
+    /// ImplSource identifying a particular impl.
+    UserDefined(ImplSourceUserDefinedData<'tcx, N>),
+
+    /// ImplSource for auto trait implementations.
+    /// This carries the information and nested obligations with regards
+    /// to an auto implementation for a trait `Trait`. The nested obligations
+    /// ensure the trait implementation holds for all the constituent types.
+    AutoImpl(ImplSourceAutoImplData<N>),
+
+    /// Successful resolution to an obligation provided by the caller
+    /// for some type parameter. The `Vec<N>` represents the
+    /// obligations incurred from normalizing the where-clause (if
+    /// any).
+    Param(Vec<N>, ty::BoundConstness),
+
+    /// Virtual calls through an object.
+    Object(ImplSourceObjectData<'tcx, N>),
+
+    /// Successful resolution for a builtin trait.
+    Builtin(ImplSourceBuiltinData<N>),
+
+    /// ImplSource for trait upcasting coercion
+    TraitUpcasting(ImplSourceTraitUpcastingData<'tcx, N>),
+
+    /// ImplSource automatically generated for a closure. The `DefId` is the ID
+    /// of the closure expression. This is an `ImplSource::UserDefined` in spirit, but the
+    /// impl is generated by the compiler and does not appear in the source.
+    Closure(ImplSourceClosureData<'tcx, N>),
+
+    /// Same as above, but for a function pointer type with the given signature.
+    FnPointer(ImplSourceFnPointerData<'tcx, N>),
+
+    /// ImplSource for a builtin `DeterminantKind` trait implementation.
+    DiscriminantKind(ImplSourceDiscriminantKindData),
+
+    /// ImplSource for a builtin `Pointee` trait implementation.
+    Pointee(ImplSourcePointeeData),
+
+    /// ImplSource automatically generated for a generator.
+    Generator(ImplSourceGeneratorData<'tcx, N>),
+
+    /// ImplSource for a trait alias.
+    TraitAlias(ImplSourceTraitAliasData<'tcx, N>),
+
+    /// ImplSource for a `const Drop` implementation.
+    ConstDestruct(ImplSourceConstDestructData<N>),
+}
+
+impl<'tcx, N> ImplSource<'tcx, N> {
+    pub fn nested_obligations(self) -> Vec<N> {
+        match self {
+            ImplSource::UserDefined(i) => i.nested,
+            ImplSource::Param(n, _) => n,
+            ImplSource::Builtin(i) => i.nested,
+            ImplSource::AutoImpl(d) => d.nested,
+            ImplSource::Closure(c) => c.nested,
+            ImplSource::Generator(c) => c.nested,
+            ImplSource::Object(d) => d.nested,
+            ImplSource::FnPointer(d) => d.nested,
+            ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
+            | ImplSource::Pointee(ImplSourcePointeeData) => Vec::new(),
+            ImplSource::TraitAlias(d) => d.nested,
+            ImplSource::TraitUpcasting(d) => d.nested,
+            ImplSource::ConstDestruct(i) => i.nested,
+        }
+    }
+
+    pub fn borrow_nested_obligations(&self) -> &[N] {
+        match &self {
+            ImplSource::UserDefined(i) => &i.nested[..],
+            ImplSource::Param(n, _) => &n,
+            ImplSource::Builtin(i) => &i.nested,
+            ImplSource::AutoImpl(d) => &d.nested,
+            ImplSource::Closure(c) => &c.nested,
+            ImplSource::Generator(c) => &c.nested,
+            ImplSource::Object(d) => &d.nested,
+            ImplSource::FnPointer(d) => &d.nested,
+            ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
+            | ImplSource::Pointee(ImplSourcePointeeData) => &[],
+            ImplSource::TraitAlias(d) => &d.nested,
+            ImplSource::TraitUpcasting(d) => &d.nested,
+            ImplSource::ConstDestruct(i) => &i.nested,
+        }
+    }
+
+    pub fn map<M, F>(self, f: F) -> ImplSource<'tcx, M>
+    where
+        F: FnMut(N) -> M,
+    {
+        match self {
+            ImplSource::UserDefined(i) => ImplSource::UserDefined(ImplSourceUserDefinedData {
+                impl_def_id: i.impl_def_id,
+                substs: i.substs,
+                nested: i.nested.into_iter().map(f).collect(),
+            }),
+            ImplSource::Param(n, ct) => ImplSource::Param(n.into_iter().map(f).collect(), ct),
+            ImplSource::Builtin(i) => ImplSource::Builtin(ImplSourceBuiltinData {
+                nested: i.nested.into_iter().map(f).collect(),
+            }),
+            ImplSource::Object(o) => ImplSource::Object(ImplSourceObjectData {
+                upcast_trait_ref: o.upcast_trait_ref,
+                vtable_base: o.vtable_base,
+                nested: o.nested.into_iter().map(f).collect(),
+            }),
+            ImplSource::AutoImpl(d) => ImplSource::AutoImpl(ImplSourceAutoImplData {
+                trait_def_id: d.trait_def_id,
+                nested: d.nested.into_iter().map(f).collect(),
+            }),
+            ImplSource::Closure(c) => ImplSource::Closure(ImplSourceClosureData {
+                closure_def_id: c.closure_def_id,
+                substs: c.substs,
+                nested: c.nested.into_iter().map(f).collect(),
+            }),
+            ImplSource::Generator(c) => ImplSource::Generator(ImplSourceGeneratorData {
+                generator_def_id: c.generator_def_id,
+                substs: c.substs,
+                nested: c.nested.into_iter().map(f).collect(),
+            }),
+            ImplSource::FnPointer(p) => ImplSource::FnPointer(ImplSourceFnPointerData {
+                fn_ty: p.fn_ty,
+                nested: p.nested.into_iter().map(f).collect(),
+            }),
+            ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData) => {
+                ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
+            }
+            ImplSource::Pointee(ImplSourcePointeeData) => {
+                ImplSource::Pointee(ImplSourcePointeeData)
+            }
+            ImplSource::TraitAlias(d) => ImplSource::TraitAlias(ImplSourceTraitAliasData {
+                alias_def_id: d.alias_def_id,
+                substs: d.substs,
+                nested: d.nested.into_iter().map(f).collect(),
+            }),
+            ImplSource::TraitUpcasting(d) => {
+                ImplSource::TraitUpcasting(ImplSourceTraitUpcastingData {
+                    upcast_trait_ref: d.upcast_trait_ref,
+                    vtable_vptr_slot: d.vtable_vptr_slot,
+                    nested: d.nested.into_iter().map(f).collect(),
+                })
+            }
+            ImplSource::ConstDestruct(i) => {
+                ImplSource::ConstDestruct(ImplSourceConstDestructData {
+                    nested: i.nested.into_iter().map(f).collect(),
+                })
+            }
+        }
+    }
+}
+
+/// Identifies a particular impl in the source, along with a set of
+/// substitutions from the impl's type/lifetime parameters. The
+/// `nested` vector corresponds to the nested obligations attached to
+/// the impl's type parameters.
+///
+/// The type parameter `N` indicates the type used for "nested
+/// obligations" that are required by the impl. During type-check, this
+/// is `Obligation`, as one might expect. During codegen, however, this
+/// is `()`, because codegen only requires a shallow resolution of an
+/// impl, and nested obligations are satisfied later.
+#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct ImplSourceUserDefinedData<'tcx, N> {
+    pub impl_def_id: DefId,
+    pub substs: SubstsRef<'tcx>,
+    pub nested: Vec<N>,
+}
+
+#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct ImplSourceGeneratorData<'tcx, N> {
+    pub generator_def_id: DefId,
+    pub substs: SubstsRef<'tcx>,
+    /// Nested obligations. This can be non-empty if the generator
+    /// signature contains associated types.
+    pub nested: Vec<N>,
+}
+
+#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct ImplSourceClosureData<'tcx, N> {
+    pub closure_def_id: DefId,
+    pub substs: SubstsRef<'tcx>,
+    /// Nested obligations. This can be non-empty if the closure
+    /// signature contains associated types.
+    pub nested: Vec<N>,
+}
+
+#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct ImplSourceAutoImplData<N> {
+    pub trait_def_id: DefId,
+    pub nested: Vec<N>,
+}
+
+#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct ImplSourceTraitUpcastingData<'tcx, N> {
+    /// `Foo` upcast to the obligation trait. This will be some supertrait of `Foo`.
+    pub upcast_trait_ref: ty::PolyTraitRef<'tcx>,
+
+    /// The vtable is formed by concatenating together the method lists of
+    /// the base object trait and all supertraits, pointers to supertrait vtable will
+    /// be provided when necessary; this is the position of `upcast_trait_ref`'s vtable
+    /// within that vtable.
+    pub vtable_vptr_slot: Option<usize>,
+
+    pub nested: Vec<N>,
+}
+
+#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct ImplSourceBuiltinData<N> {
+    pub nested: Vec<N>,
+}
+
+#[derive(PartialEq, Eq, Clone, TyEncodable, TyDecodable, HashStable, Lift)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct ImplSourceObjectData<'tcx, N> {
+    /// `Foo` upcast to the obligation trait. This will be some supertrait of `Foo`.
+    pub upcast_trait_ref: ty::PolyTraitRef<'tcx>,
+
+    /// The vtable is formed by concatenating together the method lists of
+    /// the base object trait and all supertraits, pointers to supertrait vtable will
+    /// be provided when necessary; this is the start of `upcast_trait_ref`'s methods
+    /// in that vtable.
+    pub vtable_base: usize,
+
+    pub nested: Vec<N>,
+}
+
+#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct ImplSourceFnPointerData<'tcx, N> {
+    pub fn_ty: Ty<'tcx>,
+    pub nested: Vec<N>,
+}
+
+// FIXME(@lcnr): This should be  refactored and merged with other builtin vtables.
+#[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
+pub struct ImplSourceDiscriminantKindData;
+
+#[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
+pub struct ImplSourcePointeeData;
+
+#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct ImplSourceConstDestructData<N> {
+    pub nested: Vec<N>,
+}
+
+#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct ImplSourceTraitAliasData<'tcx, N> {
+    pub alias_def_id: DefId,
+    pub substs: SubstsRef<'tcx>,
+    pub nested: Vec<N>,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash, HashStable, PartialOrd, Ord)]
+pub enum ObjectSafetyViolation {
+    /// `Self: Sized` declared on the trait.
+    SizedSelf(SmallVec<[Span; 1]>),
+
+    /// Supertrait reference references `Self` an in illegal location
+    /// (e.g., `trait Foo : Bar<Self>`).
+    SupertraitSelf(SmallVec<[Span; 1]>),
+
+    /// Method has something illegal.
+    Method(Symbol, MethodViolationCode, Span),
+
+    /// Associated const.
+    AssocConst(Symbol, Span),
+
+    /// GAT
+    GAT(Symbol, Span),
+}
+
+impl ObjectSafetyViolation {
+    pub fn error_msg(&self) -> Cow<'static, str> {
+        match self {
+            ObjectSafetyViolation::SizedSelf(_) => "it requires `Self: Sized`".into(),
+            ObjectSafetyViolation::SupertraitSelf(ref spans) => {
+                if spans.iter().any(|sp| *sp != DUMMY_SP) {
+                    "it uses `Self` as a type parameter".into()
+                } else {
+                    "it cannot use `Self` as a type parameter in a supertrait or `where`-clause"
+                        .into()
+                }
+            }
+            ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod(_), _) => {
+                format!("associated function `{}` has no `self` parameter", name).into()
+            }
+            ObjectSafetyViolation::Method(
+                name,
+                MethodViolationCode::ReferencesSelfInput(_),
+                DUMMY_SP,
+            ) => format!("method `{}` references the `Self` type in its parameters", name).into(),
+            ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfInput(_), _) => {
+                format!("method `{}` references the `Self` type in this parameter", name).into()
+            }
+            ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfOutput, _) => {
+                format!("method `{}` references the `Self` type in its return type", name).into()
+            }
+            ObjectSafetyViolation::Method(
+                name,
+                MethodViolationCode::WhereClauseReferencesSelf,
+                _,
+            ) => {
+                format!("method `{}` references the `Self` type in its `where` clause", name).into()
+            }
+            ObjectSafetyViolation::Method(name, MethodViolationCode::Generic, _) => {
+                format!("method `{}` has generic type parameters", name).into()
+            }
+            ObjectSafetyViolation::Method(
+                name,
+                MethodViolationCode::UndispatchableReceiver(_),
+                _,
+            ) => format!("method `{}`'s `self` parameter cannot be dispatched on", name).into(),
+            ObjectSafetyViolation::AssocConst(name, DUMMY_SP) => {
+                format!("it contains associated `const` `{}`", name).into()
+            }
+            ObjectSafetyViolation::AssocConst(..) => "it contains this associated `const`".into(),
+            ObjectSafetyViolation::GAT(name, _) => {
+                format!("it contains the generic associated type `{}`", name).into()
+            }
+        }
+    }
+
+    pub fn solution(&self, err: &mut Diagnostic) {
+        match self {
+            ObjectSafetyViolation::SizedSelf(_) | ObjectSafetyViolation::SupertraitSelf(_) => {}
+            ObjectSafetyViolation::Method(
+                name,
+                MethodViolationCode::StaticMethod(Some((add_self_sugg, make_sized_sugg))),
+                _,
+            ) => {
+                err.span_suggestion(
+                    add_self_sugg.1,
+                    format!(
+                        "consider turning `{}` into a method by giving it a `&self` argument",
+                        name
+                    ),
+                    add_self_sugg.0.to_string(),
+                    Applicability::MaybeIncorrect,
+                );
+                err.span_suggestion(
+                    make_sized_sugg.1,
+                    format!(
+                        "alternatively, consider constraining `{}` so it does not apply to \
+                             trait objects",
+                        name
+                    ),
+                    make_sized_sugg.0.to_string(),
+                    Applicability::MaybeIncorrect,
+                );
+            }
+            ObjectSafetyViolation::Method(
+                name,
+                MethodViolationCode::UndispatchableReceiver(Some(span)),
+                _,
+            ) => {
+                err.span_suggestion(
+                    *span,
+                    &format!(
+                        "consider changing method `{}`'s `self` parameter to be `&self`",
+                        name
+                    ),
+                    "&Self",
+                    Applicability::MachineApplicable,
+                );
+            }
+            ObjectSafetyViolation::AssocConst(name, _)
+            | ObjectSafetyViolation::GAT(name, _)
+            | ObjectSafetyViolation::Method(name, ..) => {
+                err.help(&format!("consider moving `{}` to another trait", name));
+            }
+        }
+    }
+
+    pub fn spans(&self) -> SmallVec<[Span; 1]> {
+        // When `span` comes from a separate crate, it'll be `DUMMY_SP`. Treat it as `None` so
+        // diagnostics use a `note` instead of a `span_label`.
+        match self {
+            ObjectSafetyViolation::SupertraitSelf(spans)
+            | ObjectSafetyViolation::SizedSelf(spans) => spans.clone(),
+            ObjectSafetyViolation::AssocConst(_, span)
+            | ObjectSafetyViolation::GAT(_, span)
+            | ObjectSafetyViolation::Method(_, _, span)
+                if *span != DUMMY_SP =>
+            {
+                smallvec![*span]
+            }
+            _ => smallvec![],
+        }
+    }
+}
+
+/// Reasons a method might not be object-safe.
+#[derive(Clone, Debug, PartialEq, Eq, Hash, HashStable, PartialOrd, Ord)]
+pub enum MethodViolationCode {
+    /// e.g., `fn foo()`
+    StaticMethod(Option<(/* add &self */ (String, Span), /* add Self: Sized */ (String, Span))>),
+
+    /// e.g., `fn foo(&self, x: Self)`
+    ReferencesSelfInput(Option<Span>),
+
+    /// e.g., `fn foo(&self) -> Self`
+    ReferencesSelfOutput,
+
+    /// e.g., `fn foo(&self) where Self: Clone`
+    WhereClauseReferencesSelf,
+
+    /// e.g., `fn foo<A>()`
+    Generic,
+
+    /// the method's receiver (`self` argument) can't be dispatched on
+    UndispatchableReceiver(Option<Span>),
+}
+
+/// These are the error cases for `codegen_fulfill_obligation`.
+#[derive(Copy, Clone, Debug, Hash, HashStable, Encodable, Decodable)]
+pub enum CodegenObligationError {
+    /// Ambiguity can happen when monomorphizing during trans
+    /// expands to some humongous type that never occurred
+    /// statically -- this humongous type can then overflow,
+    /// leading to an ambiguous result. So report this as an
+    /// overflow bug, since I believe this is the only case
+    /// where ambiguity can result.
+    Ambiguity,
+    /// This can trigger when we probe for the source of a `'static` lifetime requirement
+    /// on a trait object: `impl Foo for dyn Trait {}` has an implicit `'static` bound.
+    /// This can also trigger when we have a global bound that is not actually satisfied,
+    /// but was included during typeck due to the trivial_bounds feature.
+    Unimplemented,
+    FulfillmentError,
+}
diff --git a/compiler/rustc_middle/src/traits/query.rs b/compiler/rustc_middle/src/traits/query.rs
new file mode 100644
index 000000000..1f9b474ad
--- /dev/null
+++ b/compiler/rustc_middle/src/traits/query.rs
@@ -0,0 +1,230 @@
+//! Experimental types for the trait query interface. The methods
+//! defined in this module are all based on **canonicalization**,
+//! which makes a canonical query by replacing unbound inference
+//! variables and regions, so that results can be reused more broadly.
+//! The providers for the queries defined here can be found in
+//! `rustc_traits`.
+
+use crate::infer::canonical::{Canonical, QueryResponse};
+use crate::ty::error::TypeError;
+use crate::ty::subst::GenericArg;
+use crate::ty::{self, Ty, TyCtxt};
+use rustc_errors::struct_span_err;
+use rustc_span::source_map::Span;
+use std::iter::FromIterator;
+
+pub mod type_op {
+    use crate::ty::fold::TypeFoldable;
+    use crate::ty::subst::UserSubsts;
+    use crate::ty::{Predicate, Ty};
+    use rustc_hir::def_id::DefId;
+    use std::fmt;
+
+    #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq, HashStable, Lift)]
+    #[derive(TypeFoldable, TypeVisitable)]
+    pub struct AscribeUserType<'tcx> {
+        pub mir_ty: Ty<'tcx>,
+        pub def_id: DefId,
+        pub user_substs: UserSubsts<'tcx>,
+    }
+
+    impl<'tcx> AscribeUserType<'tcx> {
+        pub fn new(mir_ty: Ty<'tcx>, def_id: DefId, user_substs: UserSubsts<'tcx>) -> Self {
+            Self { mir_ty, def_id, user_substs }
+        }
+    }
+
+    #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq, HashStable, Lift)]
+    #[derive(TypeFoldable, TypeVisitable)]
+    pub struct Eq<'tcx> {
+        pub a: Ty<'tcx>,
+        pub b: Ty<'tcx>,
+    }
+
+    #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq, HashStable, Lift)]
+    #[derive(TypeFoldable, TypeVisitable)]
+    pub struct Subtype<'tcx> {
+        pub sub: Ty<'tcx>,
+        pub sup: Ty<'tcx>,
+    }
+
+    #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq, HashStable, Lift)]
+    #[derive(TypeFoldable, TypeVisitable)]
+    pub struct ProvePredicate<'tcx> {
+        pub predicate: Predicate<'tcx>,
+    }
+
+    impl<'tcx> ProvePredicate<'tcx> {
+        pub fn new(predicate: Predicate<'tcx>) -> Self {
+            ProvePredicate { predicate }
+        }
+    }
+
+    #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq, HashStable, Lift)]
+    #[derive(TypeFoldable, TypeVisitable)]
+    pub struct Normalize<T> {
+        pub value: T,
+    }
+
+    impl<'tcx, T> Normalize<T>
+    where
+        T: fmt::Debug + TypeFoldable<'tcx>,
+    {
+        pub fn new(value: T) -> Self {
+            Self { value }
+        }
+    }
+}
+
+pub type CanonicalProjectionGoal<'tcx> =
+    Canonical<'tcx, ty::ParamEnvAnd<'tcx, ty::ProjectionTy<'tcx>>>;
+
+pub type CanonicalTyGoal<'tcx> = Canonical<'tcx, ty::ParamEnvAnd<'tcx, Ty<'tcx>>>;
+
+pub type CanonicalPredicateGoal<'tcx> = Canonical<'tcx, ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>>;
+
+pub type CanonicalTypeOpAscribeUserTypeGoal<'tcx> =
+    Canonical<'tcx, ty::ParamEnvAnd<'tcx, type_op::AscribeUserType<'tcx>>>;
+
+pub type CanonicalTypeOpEqGoal<'tcx> = Canonical<'tcx, ty::ParamEnvAnd<'tcx, type_op::Eq<'tcx>>>;
+
+pub type CanonicalTypeOpSubtypeGoal<'tcx> =
+    Canonical<'tcx, ty::ParamEnvAnd<'tcx, type_op::Subtype<'tcx>>>;
+
+pub type CanonicalTypeOpProvePredicateGoal<'tcx> =
+    Canonical<'tcx, ty::ParamEnvAnd<'tcx, type_op::ProvePredicate<'tcx>>>;
+
+pub type CanonicalTypeOpNormalizeGoal<'tcx, T> =
+    Canonical<'tcx, ty::ParamEnvAnd<'tcx, type_op::Normalize<T>>>;
+
+#[derive(Copy, Clone, Debug, HashStable)]
+pub struct NoSolution;
+
+pub type Fallible<T> = Result<T, NoSolution>;
+
+impl<'tcx> From<TypeError<'tcx>> for NoSolution {
+    fn from(_: TypeError<'tcx>) -> NoSolution {
+        NoSolution
+    }
+}
+
+#[derive(Clone, Debug, Default, HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub struct DropckOutlivesResult<'tcx> {
+    pub kinds: Vec<GenericArg<'tcx>>,
+    pub overflows: Vec<Ty<'tcx>>,
+}
+
+impl<'tcx> DropckOutlivesResult<'tcx> {
+    pub fn report_overflows(&self, tcx: TyCtxt<'tcx>, span: Span, ty: Ty<'tcx>) {
+        if let Some(overflow_ty) = self.overflows.get(0) {
+            let mut err = struct_span_err!(
+                tcx.sess,
+                span,
+                E0320,
+                "overflow while adding drop-check rules for {}",
+                ty,
+            );
+            err.note(&format!("overflowed on {}", overflow_ty));
+            err.emit();
+        }
+    }
+
+    pub fn into_kinds_reporting_overflows(
+        self,
+        tcx: TyCtxt<'tcx>,
+        span: Span,
+        ty: Ty<'tcx>,
+    ) -> Vec<GenericArg<'tcx>> {
+        self.report_overflows(tcx, span, ty);
+        let DropckOutlivesResult { kinds, overflows: _ } = self;
+        kinds
+    }
+}
+
+/// A set of constraints that need to be satisfied in order for
+/// a type to be valid for destruction.
+#[derive(Clone, Debug, HashStable)]
+pub struct DropckConstraint<'tcx> {
+    /// Types that are required to be alive in order for this
+    /// type to be valid for destruction.
+    pub outlives: Vec<ty::subst::GenericArg<'tcx>>,
+
+    /// Types that could not be resolved: projections and params.
+    pub dtorck_types: Vec<Ty<'tcx>>,
+
+    /// If, during the computation of the dtorck constraint, we
+    /// overflow, that gets recorded here. The caller is expected to
+    /// report an error.
+    pub overflows: Vec<Ty<'tcx>>,
+}
+
+impl<'tcx> DropckConstraint<'tcx> {
+    pub fn empty() -> DropckConstraint<'tcx> {
+        DropckConstraint { outlives: vec![], dtorck_types: vec![], overflows: vec![] }
+    }
+}
+
+impl<'tcx> FromIterator<DropckConstraint<'tcx>> for DropckConstraint<'tcx> {
+    fn from_iter<I: IntoIterator<Item = DropckConstraint<'tcx>>>(iter: I) -> Self {
+        let mut result = Self::empty();
+
+        for DropckConstraint { outlives, dtorck_types, overflows } in iter {
+            result.outlives.extend(outlives);
+            result.dtorck_types.extend(dtorck_types);
+            result.overflows.extend(overflows);
+        }
+
+        result
+    }
+}
+
+#[derive(Debug, HashStable)]
+pub struct CandidateStep<'tcx> {
+    pub self_ty: Canonical<'tcx, QueryResponse<'tcx, Ty<'tcx>>>,
+    pub autoderefs: usize,
+    /// `true` if the type results from a dereference of a raw pointer.
+    /// when assembling candidates, we include these steps, but not when
+    /// picking methods. This so that if we have `foo: *const Foo` and `Foo` has methods
+    /// `fn by_raw_ptr(self: *const Self)` and `fn by_ref(&self)`, then
+    /// `foo.by_raw_ptr()` will work and `foo.by_ref()` won't.
+    pub from_unsafe_deref: bool,
+    pub unsize: bool,
+}
+
+#[derive(Copy, Clone, Debug, HashStable)]
+pub struct MethodAutoderefStepsResult<'tcx> {
+    /// The valid autoderef steps that could be find.
+    pub steps: &'tcx [CandidateStep<'tcx>],
+    /// If Some(T), a type autoderef reported an error on.
+    pub opt_bad_ty: Option<&'tcx MethodAutoderefBadTy<'tcx>>,
+    /// If `true`, `steps` has been truncated due to reaching the
+    /// recursion limit.
+    pub reached_recursion_limit: bool,
+}
+
+#[derive(Debug, HashStable)]
+pub struct MethodAutoderefBadTy<'tcx> {
+    pub reached_raw_pointer: bool,
+    pub ty: Canonical<'tcx, QueryResponse<'tcx, Ty<'tcx>>>,
+}
+
+/// Result from the `normalize_projection_ty` query.
+#[derive(Clone, Debug, HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub struct NormalizationResult<'tcx> {
+    /// Result of normalization.
+    pub normalized_ty: Ty<'tcx>,
+}
+
+/// Outlives bounds are relationships between generic parameters,
+/// whether they both be regions (`'a: 'b`) or whether types are
+/// involved (`T: 'a`). These relationships can be extracted from the
+/// full set of predicates we understand or also from types (in which
+/// case they are called implied bounds). They are fed to the
+/// `OutlivesEnv` which in turn is supplied to the region checker and
+/// other parts of the inference system.
+#[derive(Clone, Debug, TypeFoldable, TypeVisitable, Lift, HashStable)]
+pub enum OutlivesBound<'tcx> {
+    RegionSubRegion(ty::Region<'tcx>, ty::Region<'tcx>),
+    RegionSubParam(ty::Region<'tcx>, ty::ParamTy),
+    RegionSubProjection(ty::Region<'tcx>, ty::ProjectionTy<'tcx>),
+}
diff --git a/compiler/rustc_middle/src/traits/select.rs b/compiler/rustc_middle/src/traits/select.rs
new file mode 100644
index 000000000..e836ba47e
--- /dev/null
+++ b/compiler/rustc_middle/src/traits/select.rs
@@ -0,0 +1,312 @@
+//! Candidate selection. See the [rustc dev guide] for more information on how this works.
+//!
+//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html#selection
+
+use self::EvaluationResult::*;
+
+use super::{SelectionError, SelectionResult};
+use rustc_errors::ErrorGuaranteed;
+
+use crate::ty;
+
+use rustc_hir::def_id::DefId;
+use rustc_query_system::cache::Cache;
+
+pub type SelectionCache<'tcx> = Cache<
+    // This cache does not use `ParamEnvAnd` in its keys because `ParamEnv::and` can replace
+    // caller bounds with an empty list if the `TraitPredicate` looks global, which may happen
+    // after erasing lifetimes from the predicate.
+    (ty::ParamEnv<'tcx>, ty::TraitPredicate<'tcx>),
+    SelectionResult<'tcx, SelectionCandidate<'tcx>>,
+>;
+
+pub type EvaluationCache<'tcx> = Cache<
+    // See above: this cache does not use `ParamEnvAnd` in its keys due to sometimes incorrectly
+    // caching with the wrong `ParamEnv`.
+    (ty::ParamEnv<'tcx>, ty::PolyTraitPredicate<'tcx>),
+    EvaluationResult,
+>;
+
+/// The selection process begins by considering all impls, where
+/// clauses, and so forth that might resolve an obligation. Sometimes
+/// we'll be able to say definitively that (e.g.) an impl does not
+/// apply to the obligation: perhaps it is defined for `usize` but the
+/// obligation is for `i32`. In that case, we drop the impl out of the
+/// list. But the other cases are considered *candidates*.
+///
+/// For selection to succeed, there must be exactly one matching
+/// candidate. If the obligation is fully known, this is guaranteed
+/// by coherence. However, if the obligation contains type parameters
+/// or variables, there may be multiple such impls.
+///
+/// It is not a real problem if multiple matching impls exist because
+/// of type variables - it just means the obligation isn't sufficiently
+/// elaborated. In that case we report an ambiguity, and the caller can
+/// try again after more type information has been gathered or report a
+/// "type annotations needed" error.
+///
+/// However, with type parameters, this can be a real problem - type
+/// parameters don't unify with regular types, but they *can* unify
+/// with variables from blanket impls, and (unless we know its bounds
+/// will always be satisfied) picking the blanket impl will be wrong
+/// for at least *some* substitutions. To make this concrete, if we have
+///
+/// ```rust, ignore
+/// trait AsDebug { type Out: fmt::Debug; fn debug(self) -> Self::Out; }
+/// impl<T: fmt::Debug> AsDebug for T {
+///     type Out = T;
+///     fn debug(self) -> fmt::Debug { self }
+/// }
+/// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
+/// ```
+///
+/// we can't just use the impl to resolve the `<T as AsDebug>` obligation
+/// -- a type from another crate (that doesn't implement `fmt::Debug`) could
+/// implement `AsDebug`.
+///
+/// Because where-clauses match the type exactly, multiple clauses can
+/// only match if there are unresolved variables, and we can mostly just
+/// report this ambiguity in that case. This is still a problem - we can't
+/// *do anything* with ambiguities that involve only regions. This is issue
+/// #21974.
+///
+/// If a single where-clause matches and there are no inference
+/// variables left, then it definitely matches and we can just select
+/// it.
+///
+/// In fact, we even select the where-clause when the obligation contains
+/// inference variables. The can lead to inference making "leaps of logic",
+/// for example in this situation:
+///
+/// ```rust, ignore
+/// pub trait Foo<T> { fn foo(&self) -> T; }
+/// impl<T> Foo<()> for T { fn foo(&self) { } }
+/// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
+///
+/// pub fn foo<T>(t: T) where T: Foo<bool> {
+///     println!("{:?}", <T as Foo<_>>::foo(&t));
+/// }
+/// fn main() { foo(false); }
+/// ```
+///
+/// Here the obligation `<T as Foo<$0>>` can be matched by both the blanket
+/// impl and the where-clause. We select the where-clause and unify `$0=bool`,
+/// so the program prints "false". However, if the where-clause is omitted,
+/// the blanket impl is selected, we unify `$0=()`, and the program prints
+/// "()".
+///
+/// Exactly the same issues apply to projection and object candidates, except
+/// that we can have both a projection candidate and a where-clause candidate
+/// for the same obligation. In that case either would do (except that
+/// different "leaps of logic" would occur if inference variables are
+/// present), and we just pick the where-clause. This is, for example,
+/// required for associated types to work in default impls, as the bounds
+/// are visible both as projection bounds and as where-clauses from the
+/// parameter environment.
+#[derive(PartialEq, Eq, Debug, Clone, TypeFoldable, TypeVisitable)]
+pub enum SelectionCandidate<'tcx> {
+    BuiltinCandidate {
+        /// `false` if there are no *further* obligations.
+        has_nested: bool,
+    },
+
+    /// Implementation of transmutability trait.
+    TransmutabilityCandidate,
+
+    ParamCandidate(ty::PolyTraitPredicate<'tcx>),
+    ImplCandidate(DefId),
+    AutoImplCandidate(DefId),
+
+    /// This is a trait matching with a projected type as `Self`, and we found
+    /// an applicable bound in the trait definition. The `usize` is an index
+    /// into the list returned by `tcx.item_bounds`.
+    ProjectionCandidate(usize),
+
+    /// Implementation of a `Fn`-family trait by one of the anonymous types
+    /// generated for an `||` expression.
+    ClosureCandidate,
+
+    /// Implementation of a `Generator` trait by one of the anonymous types
+    /// generated for a generator.
+    GeneratorCandidate,
+
+    /// Implementation of a `Fn`-family trait by one of the anonymous
+    /// types generated for a fn pointer type (e.g., `fn(int) -> int`)
+    FnPointerCandidate {
+        is_const: bool,
+    },
+
+    /// Builtin implementation of `DiscriminantKind`.
+    DiscriminantKindCandidate,
+
+    /// Builtin implementation of `Pointee`.
+    PointeeCandidate,
+
+    TraitAliasCandidate(DefId),
+
+    /// Matching `dyn Trait` with a supertrait of `Trait`. The index is the
+    /// position in the iterator returned by
+    /// `rustc_infer::traits::util::supertraits`.
+    ObjectCandidate(usize),
+
+    /// Perform trait upcasting coercion of `dyn Trait` to a supertrait of `Trait`.
+    /// The index is the position in the iterator returned by
+    /// `rustc_infer::traits::util::supertraits`.
+    TraitUpcastingUnsizeCandidate(usize),
+
+    BuiltinObjectCandidate,
+
+    BuiltinUnsizeCandidate,
+
+    /// Implementation of `const Destruct`, optionally from a custom `impl const Drop`.
+    ConstDestructCandidate(Option<DefId>),
+}
+
+/// The result of trait evaluation. The order is important
+/// here as the evaluation of a list is the maximum of the
+/// evaluations.
+///
+/// The evaluation results are ordered:
+///     - `EvaluatedToOk` implies `EvaluatedToOkModuloRegions`
+///       implies `EvaluatedToAmbig` implies `EvaluatedToUnknown`
+///     - `EvaluatedToErr` implies `EvaluatedToRecur`
+///     - the "union" of evaluation results is equal to their maximum -
+///     all the "potential success" candidates can potentially succeed,
+///     so they are noops when unioned with a definite error, and within
+///     the categories it's easy to see that the unions are correct.
+#[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq, HashStable)]
+pub enum EvaluationResult {
+    /// Evaluation successful.
+    EvaluatedToOk,
+    /// Evaluation successful, but there were unevaluated region obligations.
+    EvaluatedToOkModuloRegions,
+    /// Evaluation successful, but need to rerun because opaque types got
+    /// hidden types assigned without it being known whether the opaque types
+    /// are within their defining scope
+    EvaluatedToOkModuloOpaqueTypes,
+    /// Evaluation is known to be ambiguous -- it *might* hold for some
+    /// assignment of inference variables, but it might not.
+    ///
+    /// While this has the same meaning as `EvaluatedToUnknown` -- we can't
+    /// know whether this obligation holds or not -- it is the result we
+    /// would get with an empty stack, and therefore is cacheable.
+    EvaluatedToAmbig,
+    /// Evaluation failed because of recursion involving inference
+    /// variables. We are somewhat imprecise there, so we don't actually
+    /// know the real result.
+    ///
+    /// This can't be trivially cached for the same reason as `EvaluatedToRecur`.
+    EvaluatedToUnknown,
+    /// Evaluation failed because we encountered an obligation we are already
+    /// trying to prove on this branch.
+    ///
+    /// We know this branch can't be a part of a minimal proof-tree for
+    /// the "root" of our cycle, because then we could cut out the recursion
+    /// and maintain a valid proof tree. However, this does not mean
+    /// that all the obligations on this branch do not hold -- it's possible
+    /// that we entered this branch "speculatively", and that there
+    /// might be some other way to prove this obligation that does not
+    /// go through this cycle -- so we can't cache this as a failure.
+    ///
+    /// For example, suppose we have this:
+    ///
+    /// ```rust,ignore (pseudo-Rust)
+    /// pub trait Trait { fn xyz(); }
+    /// // This impl is "useless", but we can still have
+    /// // an `impl Trait for SomeUnsizedType` somewhere.
+    /// impl<T: Trait + Sized> Trait for T { fn xyz() {} }
+    ///
+    /// pub fn foo<T: Trait + ?Sized>() {
+    ///     <T as Trait>::xyz();
+    /// }
+    /// ```
+    ///
+    /// When checking `foo`, we have to prove `T: Trait`. This basically
+    /// translates into this:
+    ///
+    /// ```plain,ignore
+    /// (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait
+    /// ```
+    ///
+    /// When we try to prove it, we first go the first option, which
+    /// recurses. This shows us that the impl is "useless" -- it won't
+    /// tell us that `T: Trait` unless it already implemented `Trait`
+    /// by some other means. However, that does not prevent `T: Trait`
+    /// does not hold, because of the bound (which can indeed be satisfied
+    /// by `SomeUnsizedType` from another crate).
+    //
+    // FIXME: when an `EvaluatedToRecur` goes past its parent root, we
+    // ought to convert it to an `EvaluatedToErr`, because we know
+    // there definitely isn't a proof tree for that obligation. Not
+    // doing so is still sound -- there isn't any proof tree, so the
+    // branch still can't be a part of a minimal one -- but does not re-enable caching.
+    EvaluatedToRecur,
+    /// Evaluation failed.
+    EvaluatedToErr,
+}
+
+impl EvaluationResult {
+    /// Returns `true` if this evaluation result is known to apply, even
+    /// considering outlives constraints.
+    pub fn must_apply_considering_regions(self) -> bool {
+        self == EvaluatedToOk
+    }
+
+    /// Returns `true` if this evaluation result is known to apply, ignoring
+    /// outlives constraints.
+    pub fn must_apply_modulo_regions(self) -> bool {
+        self <= EvaluatedToOkModuloRegions
+    }
+
+    pub fn may_apply(self) -> bool {
+        match self {
+            EvaluatedToOkModuloOpaqueTypes
+            | EvaluatedToOk
+            | EvaluatedToOkModuloRegions
+            | EvaluatedToAmbig
+            | EvaluatedToUnknown => true,
+
+            EvaluatedToErr | EvaluatedToRecur => false,
+        }
+    }
+
+    pub fn is_stack_dependent(self) -> bool {
+        match self {
+            EvaluatedToUnknown | EvaluatedToRecur => true,
+
+            EvaluatedToOkModuloOpaqueTypes
+            | EvaluatedToOk
+            | EvaluatedToOkModuloRegions
+            | EvaluatedToAmbig
+            | EvaluatedToErr => false,
+        }
+    }
+}
+
+/// Indicates that trait evaluation caused overflow and in which pass.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable)]
+pub enum OverflowError {
+    Error(ErrorGuaranteed),
+    Canonical,
+    ErrorReporting,
+}
+
+impl From<ErrorGuaranteed> for OverflowError {
+    fn from(e: ErrorGuaranteed) -> OverflowError {
+        OverflowError::Error(e)
+    }
+}
+
+TrivialTypeTraversalAndLiftImpls! {
+    OverflowError,
+}
+
+impl<'tcx> From<OverflowError> for SelectionError<'tcx> {
+    fn from(overflow_error: OverflowError) -> SelectionError<'tcx> {
+        match overflow_error {
+            OverflowError::Error(e) => SelectionError::Overflow(OverflowError::Error(e)),
+            OverflowError::Canonical => SelectionError::Overflow(OverflowError::Canonical),
+            OverflowError::ErrorReporting => SelectionError::ErrorReporting,
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/traits/specialization_graph.rs b/compiler/rustc_middle/src/traits/specialization_graph.rs
new file mode 100644
index 000000000..2465f8e25
--- /dev/null
+++ b/compiler/rustc_middle/src/traits/specialization_graph.rs
@@ -0,0 +1,261 @@
+use crate::ty::fast_reject::SimplifiedType;
+use crate::ty::visit::TypeVisitable;
+use crate::ty::{self, TyCtxt};
+use rustc_data_structures::fx::FxIndexMap;
+use rustc_errors::ErrorGuaranteed;
+use rustc_hir::def_id::{DefId, DefIdMap};
+use rustc_span::symbol::sym;
+
+/// A per-trait graph of impls in specialization order. At the moment, this
+/// graph forms a tree rooted with the trait itself, with all other nodes
+/// representing impls, and parent-child relationships representing
+/// specializations.
+///
+/// The graph provides two key services:
+///
+/// - Construction. This implicitly checks for overlapping impls (i.e., impls
+///   that overlap but where neither specializes the other -- an artifact of the
+///   simple "chain" rule.
+///
+/// - Parent extraction. In particular, the graph can give you the *immediate*
+///   parents of a given specializing impl, which is needed for extracting
+///   default items amongst other things. In the simple "chain" rule, every impl
+///   has at most one parent.
+#[derive(TyEncodable, TyDecodable, HashStable, Debug)]
+pub struct Graph {
+    /// All impls have a parent; the "root" impls have as their parent the `def_id`
+    /// of the trait.
+    pub parent: DefIdMap<DefId>,
+
+    /// The "root" impls are found by looking up the trait's def_id.
+    pub children: DefIdMap<Children>,
+
+    /// Whether an error was emitted while constructing the graph.
+    pub has_errored: Option<ErrorGuaranteed>,
+}
+
+impl Graph {
+    pub fn new() -> Graph {
+        Graph { parent: Default::default(), children: Default::default(), has_errored: None }
+    }
+
+    /// The parent of a given impl, which is the `DefId` of the trait when the
+    /// impl is a "specialization root".
+    pub fn parent(&self, child: DefId) -> DefId {
+        *self.parent.get(&child).unwrap_or_else(|| panic!("Failed to get parent for {:?}", child))
+    }
+}
+
+/// What kind of overlap check are we doing -- this exists just for testing and feature-gating
+/// purposes.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, HashStable, Debug, TyEncodable, TyDecodable)]
+pub enum OverlapMode {
+    /// The 1.0 rules (either types fail to unify, or where clauses are not implemented for crate-local types)
+    Stable,
+    /// Feature-gated test: Stable, *or* there is an explicit negative impl that rules out one of the where-clauses.
+    WithNegative,
+    /// Just check for negative impls, not for "where clause not implemented": used for testing.
+    Strict,
+}
+
+impl OverlapMode {
+    pub fn get<'tcx>(tcx: TyCtxt<'tcx>, trait_id: DefId) -> OverlapMode {
+        let with_negative_coherence = tcx.features().with_negative_coherence;
+        let strict_coherence = tcx.has_attr(trait_id, sym::rustc_strict_coherence);
+
+        if with_negative_coherence {
+            if strict_coherence { OverlapMode::Strict } else { OverlapMode::WithNegative }
+        } else if strict_coherence {
+            bug!("To use strict_coherence you need to set with_negative_coherence feature flag");
+        } else {
+            OverlapMode::Stable
+        }
+    }
+
+    pub fn use_negative_impl(&self) -> bool {
+        *self == OverlapMode::Strict || *self == OverlapMode::WithNegative
+    }
+
+    pub fn use_implicit_negative(&self) -> bool {
+        *self == OverlapMode::Stable || *self == OverlapMode::WithNegative
+    }
+}
+
+/// Children of a given impl, grouped into blanket/non-blanket varieties as is
+/// done in `TraitDef`.
+#[derive(Default, TyEncodable, TyDecodable, Debug, HashStable)]
+pub struct Children {
+    // Impls of a trait (or specializations of a given impl). To allow for
+    // quicker lookup, the impls are indexed by a simplified version of their
+    // `Self` type: impls with a simplifiable `Self` are stored in
+    // `non_blanket_impls` keyed by it, while all other impls are stored in
+    // `blanket_impls`.
+    //
+    // A similar division is used within `TraitDef`, but the lists there collect
+    // together *all* the impls for a trait, and are populated prior to building
+    // the specialization graph.
+    /// Impls of the trait.
+    pub non_blanket_impls: FxIndexMap<SimplifiedType, Vec<DefId>>,
+
+    /// Blanket impls associated with the trait.
+    pub blanket_impls: Vec<DefId>,
+}
+
+/// A node in the specialization graph is either an impl or a trait
+/// definition; either can serve as a source of item definitions.
+/// There is always exactly one trait definition node: the root.
+#[derive(Debug, Copy, Clone)]
+pub enum Node {
+    Impl(DefId),
+    Trait(DefId),
+}
+
+impl Node {
+    pub fn is_from_trait(&self) -> bool {
+        matches!(self, Node::Trait(..))
+    }
+
+    /// Trys to find the associated item that implements `trait_item_def_id`
+    /// defined in this node.
+    ///
+    /// If this returns `None`, the item can potentially still be found in
+    /// parents of this node.
+    pub fn item<'tcx>(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        trait_item_def_id: DefId,
+    ) -> Option<&'tcx ty::AssocItem> {
+        match *self {
+            Node::Trait(_) => Some(tcx.associated_item(trait_item_def_id)),
+            Node::Impl(impl_def_id) => {
+                let id = tcx.impl_item_implementor_ids(impl_def_id).get(&trait_item_def_id)?;
+                Some(tcx.associated_item(*id))
+            }
+        }
+    }
+
+    pub fn def_id(&self) -> DefId {
+        match *self {
+            Node::Impl(did) => did,
+            Node::Trait(did) => did,
+        }
+    }
+}
+
+#[derive(Copy, Clone)]
+pub struct Ancestors<'tcx> {
+    trait_def_id: DefId,
+    specialization_graph: &'tcx Graph,
+    current_source: Option<Node>,
+}
+
+impl Iterator for Ancestors<'_> {
+    type Item = Node;
+    fn next(&mut self) -> Option<Node> {
+        let cur = self.current_source.take();
+        if let Some(Node::Impl(cur_impl)) = cur {
+            let parent = self.specialization_graph.parent(cur_impl);
+
+            self.current_source = if parent == self.trait_def_id {
+                Some(Node::Trait(parent))
+            } else {
+                Some(Node::Impl(parent))
+            };
+        }
+        cur
+    }
+}
+
+/// Information about the most specialized definition of an associated item.
+pub struct LeafDef {
+    /// The associated item described by this `LeafDef`.
+    pub item: ty::AssocItem,
+
+    /// The node in the specialization graph containing the definition of `item`.
+    pub defining_node: Node,
+
+    /// The "top-most" (ie. least specialized) specialization graph node that finalized the
+    /// definition of `item`.
+    ///
+    /// Example:
+    ///
+    /// ```
+    /// #![feature(specialization)]
+    /// trait Tr {
+    ///     fn assoc(&self);
+    /// }
+    ///
+    /// impl<T> Tr for T {
+    ///     default fn assoc(&self) {}
+    /// }
+    ///
+    /// impl Tr for u8 {}
+    /// ```
+    ///
+    /// If we start the leaf definition search at `impl Tr for u8`, that impl will be the
+    /// `finalizing_node`, while `defining_node` will be the generic impl.
+    ///
+    /// If the leaf definition search is started at the generic impl, `finalizing_node` will be
+    /// `None`, since the most specialized impl we found still allows overriding the method
+    /// (doesn't finalize it).
+    pub finalizing_node: Option<Node>,
+}
+
+impl LeafDef {
+    /// Returns whether this definition is known to not be further specializable.
+    pub fn is_final(&self) -> bool {
+        self.finalizing_node.is_some()
+    }
+}
+
+impl<'tcx> Ancestors<'tcx> {
+    /// Finds the bottom-most (ie. most specialized) definition of an associated
+    /// item.
+    pub fn leaf_def(mut self, tcx: TyCtxt<'tcx>, trait_item_def_id: DefId) -> Option<LeafDef> {
+        let mut finalizing_node = None;
+
+        self.find_map(|node| {
+            if let Some(item) = node.item(tcx, trait_item_def_id) {
+                if finalizing_node.is_none() {
+                    let is_specializable = item.defaultness(tcx).is_default()
+                        || tcx.impl_defaultness(node.def_id()).is_default();
+
+                    if !is_specializable {
+                        finalizing_node = Some(node);
+                    }
+                }
+
+                Some(LeafDef { item: *item, defining_node: node, finalizing_node })
+            } else {
+                // Item not mentioned. This "finalizes" any defaulted item provided by an ancestor.
+                finalizing_node = Some(node);
+                None
+            }
+        })
+    }
+}
+
+/// Walk up the specialization ancestors of a given impl, starting with that
+/// impl itself.
+///
+/// Returns `Err` if an error was reported while building the specialization
+/// graph.
+pub fn ancestors<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    trait_def_id: DefId,
+    start_from_impl: DefId,
+) -> Result<Ancestors<'tcx>, ErrorGuaranteed> {
+    let specialization_graph = tcx.specialization_graph_of(trait_def_id);
+
+    if let Some(reported) = specialization_graph.has_errored {
+        Err(reported)
+    } else if let Some(reported) = tcx.type_of(start_from_impl).error_reported() {
+        Err(reported)
+    } else {
+        Ok(Ancestors {
+            trait_def_id,
+            specialization_graph,
+            current_source: Some(Node::Impl(start_from_impl)),
+        })
+    }
+}
diff --git a/compiler/rustc_middle/src/traits/structural_impls.rs b/compiler/rustc_middle/src/traits/structural_impls.rs
new file mode 100644
index 000000000..7fbd57ac7
--- /dev/null
+++ b/compiler/rustc_middle/src/traits/structural_impls.rs
@@ -0,0 +1,135 @@
+use crate::traits;
+
+use std::fmt;
+
+// Structural impls for the structs in `traits`.
+
+impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSource<'tcx, N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            super::ImplSource::UserDefined(ref v) => write!(f, "{:?}", v),
+
+            super::ImplSource::AutoImpl(ref t) => write!(f, "{:?}", t),
+
+            super::ImplSource::Closure(ref d) => write!(f, "{:?}", d),
+
+            super::ImplSource::Generator(ref d) => write!(f, "{:?}", d),
+
+            super::ImplSource::FnPointer(ref d) => write!(f, "({:?})", d),
+
+            super::ImplSource::DiscriminantKind(ref d) => write!(f, "{:?}", d),
+
+            super::ImplSource::Pointee(ref d) => write!(f, "{:?}", d),
+
+            super::ImplSource::Object(ref d) => write!(f, "{:?}", d),
+
+            super::ImplSource::Param(ref n, ct) => {
+                write!(f, "ImplSourceParamData({:?}, {:?})", n, ct)
+            }
+
+            super::ImplSource::Builtin(ref d) => write!(f, "{:?}", d),
+
+            super::ImplSource::TraitAlias(ref d) => write!(f, "{:?}", d),
+
+            super::ImplSource::TraitUpcasting(ref d) => write!(f, "{:?}", d),
+
+            super::ImplSource::ConstDestruct(ref d) => write!(f, "{:?}", d),
+        }
+    }
+}
+
+impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSourceUserDefinedData<'tcx, N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "ImplSourceUserDefinedData(impl_def_id={:?}, substs={:?}, nested={:?})",
+            self.impl_def_id, self.substs, self.nested
+        )
+    }
+}
+
+impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSourceGeneratorData<'tcx, N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "ImplSourceGeneratorData(generator_def_id={:?}, substs={:?}, nested={:?})",
+            self.generator_def_id, self.substs, self.nested
+        )
+    }
+}
+
+impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSourceClosureData<'tcx, N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "ImplSourceClosureData(closure_def_id={:?}, substs={:?}, nested={:?})",
+            self.closure_def_id, self.substs, self.nested
+        )
+    }
+}
+
+impl<N: fmt::Debug> fmt::Debug for traits::ImplSourceBuiltinData<N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "ImplSourceBuiltinData(nested={:?})", self.nested)
+    }
+}
+
+impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSourceTraitUpcastingData<'tcx, N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "ImplSourceTraitUpcastingData(upcast={:?}, vtable_vptr_slot={:?}, nested={:?})",
+            self.upcast_trait_ref, self.vtable_vptr_slot, self.nested
+        )
+    }
+}
+
+impl<N: fmt::Debug> fmt::Debug for traits::ImplSourceAutoImplData<N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "ImplSourceAutoImplData(trait_def_id={:?}, nested={:?})",
+            self.trait_def_id, self.nested
+        )
+    }
+}
+
+impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSourceObjectData<'tcx, N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "ImplSourceObjectData(upcast={:?}, vtable_base={}, nested={:?})",
+            self.upcast_trait_ref, self.vtable_base, self.nested
+        )
+    }
+}
+
+impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSourceFnPointerData<'tcx, N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "ImplSourceFnPointerData(fn_ty={:?}, nested={:?})", self.fn_ty, self.nested)
+    }
+}
+
+impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSourceTraitAliasData<'tcx, N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "ImplSourceTraitAliasData(alias_def_id={:?}, substs={:?}, nested={:?})",
+            self.alias_def_id, self.substs, self.nested
+        )
+    }
+}
+
+impl<N: fmt::Debug> fmt::Debug for traits::ImplSourceConstDestructData<N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "ImplSourceConstDestructData(nested={:?})", self.nested)
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Lift implementations
+
+TrivialTypeTraversalAndLiftImpls! {
+    super::ImplSourceDiscriminantKindData,
+    super::ImplSourcePointeeData,
+}
diff --git a/compiler/rustc_middle/src/traits/util.rs b/compiler/rustc_middle/src/traits/util.rs
new file mode 100644
index 000000000..d54b8c599
--- /dev/null
+++ b/compiler/rustc_middle/src/traits/util.rs
@@ -0,0 +1,49 @@
+use rustc_data_structures::fx::FxHashSet;
+
+use crate::ty::{PolyTraitRef, TyCtxt};
+
+/// Given a PolyTraitRef, get the PolyTraitRefs of the trait's (transitive) supertraits.
+///
+/// A simplified version of the same function at `rustc_infer::traits::util::supertraits`.
+pub fn supertraits<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    trait_ref: PolyTraitRef<'tcx>,
+) -> impl Iterator<Item = PolyTraitRef<'tcx>> {
+    Elaborator { tcx, visited: FxHashSet::from_iter([trait_ref]), stack: vec![trait_ref] }
+}
+
+struct Elaborator<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    visited: FxHashSet<PolyTraitRef<'tcx>>,
+    stack: Vec<PolyTraitRef<'tcx>>,
+}
+
+impl<'tcx> Elaborator<'tcx> {
+    fn elaborate(&mut self, trait_ref: PolyTraitRef<'tcx>) {
+        let supertrait_refs = self
+            .tcx
+            .super_predicates_of(trait_ref.def_id())
+            .predicates
+            .into_iter()
+            .flat_map(|(pred, _)| {
+                pred.subst_supertrait(self.tcx, &trait_ref).to_opt_poly_trait_pred()
+            })
+            .map(|t| t.map_bound(|pred| pred.trait_ref))
+            .filter(|supertrait_ref| self.visited.insert(*supertrait_ref));
+
+        self.stack.extend(supertrait_refs);
+    }
+}
+
+impl<'tcx> Iterator for Elaborator<'tcx> {
+    type Item = PolyTraitRef<'tcx>;
+
+    fn next(&mut self) -> Option<PolyTraitRef<'tcx>> {
+        if let Some(trait_ref) = self.stack.pop() {
+            self.elaborate(trait_ref);
+            Some(trait_ref)
+        } else {
+            None
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/_match.rs b/compiler/rustc_middle/src/ty/_match.rs
new file mode 100644
index 000000000..e6aab30a1
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/_match.rs
@@ -0,0 +1,124 @@
+use crate::ty::error::TypeError;
+use crate::ty::relate::{self, Relate, RelateResult, TypeRelation};
+use crate::ty::{self, InferConst, Ty, TyCtxt};
+
+/// A type "A" *matches* "B" if the fresh types in B could be
+/// substituted with values so as to make it equal to A. Matching is
+/// intended to be used only on freshened types, and it basically
+/// indicates if the non-freshened versions of A and B could have been
+/// unified.
+///
+/// It is only an approximation. If it yields false, unification would
+/// definitely fail, but a true result doesn't mean unification would
+/// succeed. This is because we don't track the "side-constraints" on
+/// type variables, nor do we track if the same freshened type appears
+/// more than once. To some extent these approximations could be
+/// fixed, given effort.
+///
+/// Like subtyping, matching is really a binary relation, so the only
+/// important thing about the result is Ok/Err. Also, matching never
+/// affects any type variables or unification state.
+pub struct Match<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+}
+
+impl<'tcx> Match<'tcx> {
+    pub fn new(tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Match<'tcx> {
+        Match { tcx, param_env }
+    }
+}
+
+impl<'tcx> TypeRelation<'tcx> for Match<'tcx> {
+    fn tag(&self) -> &'static str {
+        "Match"
+    }
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.param_env
+    }
+    fn a_is_expected(&self) -> bool {
+        true
+    } // irrelevant
+
+    fn relate_with_variance<T: Relate<'tcx>>(
+        &mut self,
+        _: ty::Variance,
+        _: ty::VarianceDiagInfo<'tcx>,
+        a: T,
+        b: T,
+    ) -> RelateResult<'tcx, T> {
+        self.relate(a, b)
+    }
+
+    #[instrument(skip(self), level = "debug")]
+    fn regions(
+        &mut self,
+        a: ty::Region<'tcx>,
+        b: ty::Region<'tcx>,
+    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+        Ok(a)
+    }
+
+    #[instrument(skip(self), level = "debug")]
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        if a == b {
+            return Ok(a);
+        }
+
+        match (a.kind(), b.kind()) {
+            (
+                _,
+                &ty::Infer(ty::FreshTy(_))
+                | &ty::Infer(ty::FreshIntTy(_))
+                | &ty::Infer(ty::FreshFloatTy(_)),
+            ) => Ok(a),
+
+            (&ty::Infer(_), _) | (_, &ty::Infer(_)) => {
+                Err(TypeError::Sorts(relate::expected_found(self, a, b)))
+            }
+
+            (&ty::Error(_), _) | (_, &ty::Error(_)) => Ok(self.tcx().ty_error()),
+
+            _ => relate::super_relate_tys(self, a, b),
+        }
+    }
+
+    fn consts(
+        &mut self,
+        a: ty::Const<'tcx>,
+        b: ty::Const<'tcx>,
+    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
+        debug!("{}.consts({:?}, {:?})", self.tag(), a, b);
+        if a == b {
+            return Ok(a);
+        }
+
+        match (a.kind(), b.kind()) {
+            (_, ty::ConstKind::Infer(InferConst::Fresh(_))) => {
+                return Ok(a);
+            }
+
+            (ty::ConstKind::Infer(_), _) | (_, ty::ConstKind::Infer(_)) => {
+                return Err(TypeError::ConstMismatch(relate::expected_found(self, a, b)));
+            }
+
+            _ => {}
+        }
+
+        relate::super_relate_consts(self, a, b)
+    }
+
+    fn binders<T>(
+        &mut self,
+        a: ty::Binder<'tcx, T>,
+        b: ty::Binder<'tcx, T>,
+    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
+    where
+        T: Relate<'tcx>,
+    {
+        Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/abstract_const.rs b/compiler/rustc_middle/src/ty/abstract_const.rs
new file mode 100644
index 000000000..bed809930
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/abstract_const.rs
@@ -0,0 +1,194 @@
+//! A subset of a mir body used for const evaluatability checking.
+use crate::mir;
+use crate::ty::visit::TypeVisitable;
+use crate::ty::{self, subst::Subst, DelaySpanBugEmitted, EarlyBinder, SubstsRef, Ty, TyCtxt};
+use rustc_errors::ErrorGuaranteed;
+use rustc_hir::def_id::DefId;
+use std::cmp;
+use std::ops::ControlFlow;
+
+rustc_index::newtype_index! {
+    /// An index into an `AbstractConst`.
+    pub struct NodeId {
+        derive [HashStable]
+        DEBUG_FORMAT = "n{}",
+    }
+}
+
+/// A tree representing an anonymous constant.
+///
+/// This is only able to represent a subset of `MIR`,
+/// and should not leak any information about desugarings.
+#[derive(Debug, Clone, Copy)]
+pub struct AbstractConst<'tcx> {
+    // FIXME: Consider adding something like `IndexSlice`
+    // and use this here.
+    inner: &'tcx [Node<'tcx>],
+    substs: SubstsRef<'tcx>,
+}
+
+impl<'tcx> AbstractConst<'tcx> {
+    pub fn new(
+        tcx: TyCtxt<'tcx>,
+        uv: ty::Unevaluated<'tcx, ()>,
+    ) -> Result<Option<AbstractConst<'tcx>>, ErrorGuaranteed> {
+        let inner = tcx.thir_abstract_const_opt_const_arg(uv.def)?;
+        debug!("AbstractConst::new({:?}) = {:?}", uv, inner);
+        Ok(inner.map(|inner| AbstractConst { inner, substs: tcx.erase_regions(uv.substs) }))
+    }
+
+    pub fn from_const(
+        tcx: TyCtxt<'tcx>,
+        ct: ty::Const<'tcx>,
+    ) -> Result<Option<AbstractConst<'tcx>>, ErrorGuaranteed> {
+        match ct.kind() {
+            ty::ConstKind::Unevaluated(uv) => AbstractConst::new(tcx, uv.shrink()),
+            ty::ConstKind::Error(DelaySpanBugEmitted { reported, .. }) => Err(reported),
+            _ => Ok(None),
+        }
+    }
+
+    #[inline]
+    pub fn subtree(self, node: NodeId) -> AbstractConst<'tcx> {
+        AbstractConst { inner: &self.inner[..=node.index()], substs: self.substs }
+    }
+
+    #[inline]
+    pub fn root(self, tcx: TyCtxt<'tcx>) -> Node<'tcx> {
+        let node = self.inner.last().copied().unwrap();
+        match node {
+            Node::Leaf(leaf) => Node::Leaf(EarlyBinder(leaf).subst(tcx, self.substs)),
+            Node::Cast(kind, operand, ty) => {
+                Node::Cast(kind, operand, EarlyBinder(ty).subst(tcx, self.substs))
+            }
+            // Don't perform substitution on the following as they can't directly contain generic params
+            Node::Binop(_, _, _) | Node::UnaryOp(_, _) | Node::FunctionCall(_, _) => node,
+        }
+    }
+
+    pub fn unify_failure_kind(self, tcx: TyCtxt<'tcx>) -> FailureKind {
+        let mut failure_kind = FailureKind::Concrete;
+        walk_abstract_const::<!, _>(tcx, self, |node| {
+            match node.root(tcx) {
+                Node::Leaf(leaf) => {
+                    if leaf.has_infer_types_or_consts() {
+                        failure_kind = FailureKind::MentionsInfer;
+                    } else if leaf.has_param_types_or_consts() {
+                        failure_kind = cmp::min(failure_kind, FailureKind::MentionsParam);
+                    }
+                }
+                Node::Cast(_, _, ty) => {
+                    if ty.has_infer_types_or_consts() {
+                        failure_kind = FailureKind::MentionsInfer;
+                    } else if ty.has_param_types_or_consts() {
+                        failure_kind = cmp::min(failure_kind, FailureKind::MentionsParam);
+                    }
+                }
+                Node::Binop(_, _, _) | Node::UnaryOp(_, _) | Node::FunctionCall(_, _) => {}
+            }
+            ControlFlow::CONTINUE
+        });
+        failure_kind
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+pub enum CastKind {
+    /// thir::ExprKind::As
+    As,
+    /// thir::ExprKind::Use
+    Use,
+}
+
+/// A node of an `AbstractConst`.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+pub enum Node<'tcx> {
+    Leaf(ty::Const<'tcx>),
+    Binop(mir::BinOp, NodeId, NodeId),
+    UnaryOp(mir::UnOp, NodeId),
+    FunctionCall(NodeId, &'tcx [NodeId]),
+    Cast(CastKind, NodeId, Ty<'tcx>),
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+pub enum NotConstEvaluatable {
+    Error(ErrorGuaranteed),
+    MentionsInfer,
+    MentionsParam,
+}
+
+impl From<ErrorGuaranteed> for NotConstEvaluatable {
+    fn from(e: ErrorGuaranteed) -> NotConstEvaluatable {
+        NotConstEvaluatable::Error(e)
+    }
+}
+
+TrivialTypeTraversalAndLiftImpls! {
+    NotConstEvaluatable,
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    #[inline]
+    pub fn thir_abstract_const_opt_const_arg(
+        self,
+        def: ty::WithOptConstParam<DefId>,
+    ) -> Result<Option<&'tcx [Node<'tcx>]>, ErrorGuaranteed> {
+        if let Some((did, param_did)) = def.as_const_arg() {
+            self.thir_abstract_const_of_const_arg((did, param_did))
+        } else {
+            self.thir_abstract_const(def.did)
+        }
+    }
+}
+
+#[instrument(skip(tcx, f), level = "debug")]
+pub fn walk_abstract_const<'tcx, R, F>(
+    tcx: TyCtxt<'tcx>,
+    ct: AbstractConst<'tcx>,
+    mut f: F,
+) -> ControlFlow<R>
+where
+    F: FnMut(AbstractConst<'tcx>) -> ControlFlow<R>,
+{
+    #[instrument(skip(tcx, f), level = "debug")]
+    fn recurse<'tcx, R>(
+        tcx: TyCtxt<'tcx>,
+        ct: AbstractConst<'tcx>,
+        f: &mut dyn FnMut(AbstractConst<'tcx>) -> ControlFlow<R>,
+    ) -> ControlFlow<R> {
+        f(ct)?;
+        let root = ct.root(tcx);
+        debug!(?root);
+        match root {
+            Node::Leaf(_) => ControlFlow::CONTINUE,
+            Node::Binop(_, l, r) => {
+                recurse(tcx, ct.subtree(l), f)?;
+                recurse(tcx, ct.subtree(r), f)
+            }
+            Node::UnaryOp(_, v) => recurse(tcx, ct.subtree(v), f),
+            Node::FunctionCall(func, args) => {
+                recurse(tcx, ct.subtree(func), f)?;
+                args.iter().try_for_each(|&arg| recurse(tcx, ct.subtree(arg), f))
+            }
+            Node::Cast(_, operand, _) => recurse(tcx, ct.subtree(operand), f),
+        }
+    }
+
+    recurse(tcx, ct, &mut f)
+}
+
+// We were unable to unify the abstract constant with
+// a constant found in the caller bounds, there are
+// now three possible cases here.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
+pub enum FailureKind {
+    /// The abstract const still references an inference
+    /// variable, in this case we return `TooGeneric`.
+    MentionsInfer,
+    /// The abstract const references a generic parameter,
+    /// this means that we emit an error here.
+    MentionsParam,
+    /// The substs are concrete enough that we can simply
+    /// try and evaluate the given constant.
+    Concrete,
+}
diff --git a/compiler/rustc_middle/src/ty/adjustment.rs b/compiler/rustc_middle/src/ty/adjustment.rs
new file mode 100644
index 000000000..d36cf2fe3
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/adjustment.rs
@@ -0,0 +1,198 @@
+use crate::ty::subst::SubstsRef;
+use crate::ty::{self, Ty, TyCtxt};
+use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use rustc_hir::lang_items::LangItem;
+use rustc_macros::HashStable;
+use rustc_span::Span;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
+pub enum PointerCast {
+    /// Go from a fn-item type to a fn-pointer type.
+    ReifyFnPointer,
+
+    /// Go from a safe fn pointer to an unsafe fn pointer.
+    UnsafeFnPointer,
+
+    /// Go from a non-capturing closure to an fn pointer or an unsafe fn pointer.
+    /// It cannot convert a closure that requires unsafe.
+    ClosureFnPointer(hir::Unsafety),
+
+    /// Go from a mut raw pointer to a const raw pointer.
+    MutToConstPointer,
+
+    /// Go from `*const [T; N]` to `*const T`
+    ArrayToPointer,
+
+    /// Unsize a pointer/reference value, e.g., `&[T; n]` to
+    /// `&[T]`. Note that the source could be a thin or fat pointer.
+    /// This will do things like convert thin pointers to fat
+    /// pointers, or convert structs containing thin pointers to
+    /// structs containing fat pointers, or convert between fat
+    /// pointers. We don't store the details of how the transform is
+    /// done (in fact, we don't know that, because it might depend on
+    /// the precise type parameters). We just store the target
+    /// type. Codegen backends and miri figure out what has to be done
+    /// based on the precise source/target type at hand.
+    Unsize,
+}
+
+/// Represents coercing a value to a different type of value.
+///
+/// We transform values by following a number of `Adjust` steps in order.
+/// See the documentation on variants of `Adjust` for more details.
+///
+/// Here are some common scenarios:
+///
+/// 1. The simplest cases are where a pointer is not adjusted fat vs thin.
+///    Here the pointer will be dereferenced N times (where a dereference can
+///    happen to raw or borrowed pointers or any smart pointer which implements
+///    `Deref`, including `Box<_>`). The types of dereferences is given by
+///    `autoderefs`. It can then be auto-referenced zero or one times, indicated
+///    by `autoref`, to either a raw or borrowed pointer. In these cases unsize is
+///    `false`.
+///
+/// 2. A thin-to-fat coercion involves unsizing the underlying data. We start
+///    with a thin pointer, deref a number of times, unsize the underlying data,
+///    then autoref. The 'unsize' phase may change a fixed length array to a
+///    dynamically sized one, a concrete object to a trait object, or statically
+///    sized struct to a dynamically sized one. E.g., `&[i32; 4]` -> `&[i32]` is
+///    represented by:
+///
+///    ```ignore (illustrative)
+///    Deref(None) -> [i32; 4],
+///    Borrow(AutoBorrow::Ref) -> &[i32; 4],
+///    Unsize -> &[i32],
+///    ```
+///
+///    Note that for a struct, the 'deep' unsizing of the struct is not recorded.
+///    E.g., `struct Foo<T> { x: T }` we can coerce `&Foo<[i32; 4]>` to `&Foo<[i32]>`
+///    The autoderef and -ref are the same as in the above example, but the type
+///    stored in `unsize` is `Foo<[i32]>`, we don't store any further detail about
+///    the underlying conversions from `[i32; 4]` to `[i32]`.
+///
+/// 3. Coercing a `Box<T>` to `Box<dyn Trait>` is an interesting special case. In
+///    that case, we have the pointer we need coming in, so there are no
+///    autoderefs, and no autoref. Instead we just do the `Unsize` transformation.
+///    At some point, of course, `Box` should move out of the compiler, in which
+///    case this is analogous to transforming a struct. E.g., `Box<[i32; 4]>` ->
+///    `Box<[i32]>` is an `Adjust::Unsize` with the target `Box<[i32]>`.
+#[derive(Clone, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub struct Adjustment<'tcx> {
+    pub kind: Adjust<'tcx>,
+    pub target: Ty<'tcx>,
+}
+
+impl<'tcx> Adjustment<'tcx> {
+    pub fn is_region_borrow(&self) -> bool {
+        matches!(self.kind, Adjust::Borrow(AutoBorrow::Ref(..)))
+    }
+}
+
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub enum Adjust<'tcx> {
+    /// Go from ! to any type.
+    NeverToAny,
+
+    /// Dereference once, producing a place.
+    Deref(Option<OverloadedDeref<'tcx>>),
+
+    /// Take the address and produce either a `&` or `*` pointer.
+    Borrow(AutoBorrow<'tcx>),
+
+    Pointer(PointerCast),
+}
+
+/// An overloaded autoderef step, representing a `Deref(Mut)::deref(_mut)`
+/// call, with the signature `&'a T -> &'a U` or `&'a mut T -> &'a mut U`.
+/// The target type is `U` in both cases, with the region and mutability
+/// being those shared by both the receiver and the returned reference.
+#[derive(Copy, Clone, PartialEq, Debug, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct OverloadedDeref<'tcx> {
+    pub region: ty::Region<'tcx>,
+    pub mutbl: hir::Mutability,
+    /// The `Span` associated with the field access or method call
+    /// that triggered this overloaded deref.
+    pub span: Span,
+}
+
+impl<'tcx> OverloadedDeref<'tcx> {
+    pub fn method_call(&self, tcx: TyCtxt<'tcx>, source: Ty<'tcx>) -> (DefId, SubstsRef<'tcx>) {
+        let trait_def_id = match self.mutbl {
+            hir::Mutability::Not => tcx.require_lang_item(LangItem::Deref, None),
+            hir::Mutability::Mut => tcx.require_lang_item(LangItem::DerefMut, None),
+        };
+        let method_def_id = tcx
+            .associated_items(trait_def_id)
+            .in_definition_order()
+            .find(|m| m.kind == ty::AssocKind::Fn)
+            .unwrap()
+            .def_id;
+        (method_def_id, tcx.mk_substs_trait(source, &[]))
+    }
+}
+
+/// At least for initial deployment, we want to limit two-phase borrows to
+/// only a few specific cases. Right now, those are mostly "things that desugar"
+/// into method calls:
+/// - using `x.some_method()` syntax, where some_method takes `&mut self`,
+/// - using `Foo::some_method(&mut x, ...)` syntax,
+/// - binary assignment operators (`+=`, `-=`, `*=`, etc.).
+/// Anything else should be rejected until generalized two-phase borrow support
+/// is implemented. Right now, dataflow can't handle the general case where there
+/// is more than one use of a mutable borrow, and we don't want to accept too much
+/// new code via two-phase borrows, so we try to limit where we create two-phase
+/// capable mutable borrows.
+/// See #49434 for tracking.
+#[derive(Copy, Clone, PartialEq, Debug, TyEncodable, TyDecodable, HashStable)]
+pub enum AllowTwoPhase {
+    Yes,
+    No,
+}
+
+#[derive(Copy, Clone, PartialEq, Debug, TyEncodable, TyDecodable, HashStable)]
+pub enum AutoBorrowMutability {
+    Mut { allow_two_phase_borrow: AllowTwoPhase },
+    Not,
+}
+
+impl From<AutoBorrowMutability> for hir::Mutability {
+    fn from(m: AutoBorrowMutability) -> Self {
+        match m {
+            AutoBorrowMutability::Mut { .. } => hir::Mutability::Mut,
+            AutoBorrowMutability::Not => hir::Mutability::Not,
+        }
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Debug, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub enum AutoBorrow<'tcx> {
+    /// Converts from T to &T.
+    Ref(ty::Region<'tcx>, AutoBorrowMutability),
+
+    /// Converts from T to *T.
+    RawPtr(hir::Mutability),
+}
+
+/// Information for `CoerceUnsized` impls, storing information we
+/// have computed about the coercion.
+///
+/// This struct can be obtained via the `coerce_impl_info` query.
+/// Demanding this struct also has the side-effect of reporting errors
+/// for inappropriate impls.
+#[derive(Clone, Copy, TyEncodable, TyDecodable, Debug, HashStable)]
+pub struct CoerceUnsizedInfo {
+    /// If this is a "custom coerce" impl, then what kind of custom
+    /// coercion is it? This applies to impls of `CoerceUnsized` for
+    /// structs, primarily, where we store a bit of info about which
+    /// fields need to be coerced.
+    pub custom_kind: Option<CustomCoerceUnsized>,
+}
+
+#[derive(Clone, Copy, TyEncodable, TyDecodable, Debug, HashStable)]
+pub enum CustomCoerceUnsized {
+    /// Records the index of the field being coerced.
+    Struct(usize),
+}
diff --git a/compiler/rustc_middle/src/ty/adt.rs b/compiler/rustc_middle/src/ty/adt.rs
new file mode 100644
index 000000000..2e596b275
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/adt.rs
@@ -0,0 +1,569 @@
+use crate::mir::interpret::ErrorHandled;
+use crate::ty;
+use crate::ty::util::{Discr, IntTypeExt};
+use rustc_data_structures::captures::Captures;
+use rustc_data_structures::fingerprint::Fingerprint;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::intern::Interned;
+use rustc_data_structures::stable_hasher::HashingControls;
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_hir as hir;
+use rustc_hir::def::{CtorKind, DefKind, Res};
+use rustc_hir::def_id::DefId;
+use rustc_index::vec::{Idx, IndexVec};
+use rustc_query_system::ich::StableHashingContext;
+use rustc_session::DataTypeKind;
+use rustc_span::symbol::sym;
+use rustc_target::abi::VariantIdx;
+
+use std::cell::RefCell;
+use std::cmp::Ordering;
+use std::hash::{Hash, Hasher};
+use std::ops::Range;
+use std::str;
+
+use super::{
+    Destructor, FieldDef, GenericPredicates, ReprOptions, Ty, TyCtxt, VariantDef, VariantDiscr,
+};
+
+#[derive(Copy, Clone, HashStable, Debug)]
+pub struct AdtSizedConstraint<'tcx>(pub &'tcx [Ty<'tcx>]);
+
+bitflags! {
+    #[derive(HashStable, TyEncodable, TyDecodable)]
+    pub struct AdtFlags: u32 {
+        const NO_ADT_FLAGS        = 0;
+        /// Indicates whether the ADT is an enum.
+        const IS_ENUM             = 1 << 0;
+        /// Indicates whether the ADT is a union.
+        const IS_UNION            = 1 << 1;
+        /// Indicates whether the ADT is a struct.
+        const IS_STRUCT           = 1 << 2;
+        /// Indicates whether the ADT is a struct and has a constructor.
+        const HAS_CTOR            = 1 << 3;
+        /// Indicates whether the type is `PhantomData`.
+        const IS_PHANTOM_DATA     = 1 << 4;
+        /// Indicates whether the type has a `#[fundamental]` attribute.
+        const IS_FUNDAMENTAL      = 1 << 5;
+        /// Indicates whether the type is `Box`.
+        const IS_BOX              = 1 << 6;
+        /// Indicates whether the type is `ManuallyDrop`.
+        const IS_MANUALLY_DROP    = 1 << 7;
+        /// Indicates whether the variant list of this ADT is `#[non_exhaustive]`.
+        /// (i.e., this flag is never set unless this ADT is an enum).
+        const IS_VARIANT_LIST_NON_EXHAUSTIVE = 1 << 8;
+        /// Indicates whether the type is `UnsafeCell`.
+        const IS_UNSAFE_CELL              = 1 << 9;
+    }
+}
+
+/// The definition of a user-defined type, e.g., a `struct`, `enum`, or `union`.
+///
+/// These are all interned (by `alloc_adt_def`) into the global arena.
+///
+/// The initialism *ADT* stands for an [*algebraic data type (ADT)*][adt].
+/// This is slightly wrong because `union`s are not ADTs.
+/// Moreover, Rust only allows recursive data types through indirection.
+///
+/// [adt]: https://en.wikipedia.org/wiki/Algebraic_data_type
+///
+/// # Recursive types
+///
+/// It may seem impossible to represent recursive types using [`Ty`],
+/// since [`TyKind::Adt`] includes [`AdtDef`], which includes its fields,
+/// creating a cycle. However, `AdtDef` does not actually include the *types*
+/// of its fields; it includes just their [`DefId`]s.
+///
+/// [`TyKind::Adt`]: ty::TyKind::Adt
+///
+/// For example, the following type:
+///
+/// ```
+/// struct S { x: Box<S> }
+/// ```
+///
+/// is essentially represented with [`Ty`] as the following pseudocode:
+///
+/// ```ignore (illustrative)
+/// struct S { x }
+/// ```
+///
+/// where `x` here represents the `DefId` of `S.x`. Then, the `DefId`
+/// can be used with [`TyCtxt::type_of()`] to get the type of the field.
+#[derive(TyEncodable, TyDecodable)]
+pub struct AdtDefData {
+    /// The `DefId` of the struct, enum or union item.
+    pub did: DefId,
+    /// Variants of the ADT. If this is a struct or union, then there will be a single variant.
+    variants: IndexVec<VariantIdx, VariantDef>,
+    /// Flags of the ADT (e.g., is this a struct? is this non-exhaustive?).
+    flags: AdtFlags,
+    /// Repr options provided by the user.
+    repr: ReprOptions,
+}
+
+impl PartialOrd for AdtDefData {
+    fn partial_cmp(&self, other: &AdtDefData) -> Option<Ordering> {
+        Some(self.cmp(&other))
+    }
+}
+
+/// There should be only one AdtDef for each `did`, therefore
+/// it is fine to implement `Ord` only based on `did`.
+impl Ord for AdtDefData {
+    fn cmp(&self, other: &AdtDefData) -> Ordering {
+        self.did.cmp(&other.did)
+    }
+}
+
+/// There should be only one AdtDef for each `did`, therefore
+/// it is fine to implement `PartialEq` only based on `did`.
+impl PartialEq for AdtDefData {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        self.did == other.did
+    }
+}
+
+impl Eq for AdtDefData {}
+
+/// There should be only one AdtDef for each `did`, therefore
+/// it is fine to implement `Hash` only based on `did`.
+impl Hash for AdtDefData {
+    #[inline]
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        self.did.hash(s)
+    }
+}
+
+impl<'a> HashStable<StableHashingContext<'a>> for AdtDefData {
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        thread_local! {
+            static CACHE: RefCell<FxHashMap<(usize, HashingControls), Fingerprint>> = Default::default();
+        }
+
+        let hash: Fingerprint = CACHE.with(|cache| {
+            let addr = self as *const AdtDefData as usize;
+            let hashing_controls = hcx.hashing_controls();
+            *cache.borrow_mut().entry((addr, hashing_controls)).or_insert_with(|| {
+                let ty::AdtDefData { did, ref variants, ref flags, ref repr } = *self;
+
+                let mut hasher = StableHasher::new();
+                did.hash_stable(hcx, &mut hasher);
+                variants.hash_stable(hcx, &mut hasher);
+                flags.hash_stable(hcx, &mut hasher);
+                repr.hash_stable(hcx, &mut hasher);
+
+                hasher.finish()
+            })
+        });
+
+        hash.hash_stable(hcx, hasher);
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Ord, PartialOrd, HashStable)]
+#[rustc_pass_by_value]
+pub struct AdtDef<'tcx>(pub Interned<'tcx, AdtDefData>);
+
+impl<'tcx> AdtDef<'tcx> {
+    #[inline]
+    pub fn did(self) -> DefId {
+        self.0.0.did
+    }
+
+    #[inline]
+    pub fn variants(self) -> &'tcx IndexVec<VariantIdx, VariantDef> {
+        &self.0.0.variants
+    }
+
+    #[inline]
+    pub fn variant(self, idx: VariantIdx) -> &'tcx VariantDef {
+        &self.0.0.variants[idx]
+    }
+
+    #[inline]
+    pub fn flags(self) -> AdtFlags {
+        self.0.0.flags
+    }
+
+    #[inline]
+    pub fn repr(self) -> ReprOptions {
+        self.0.0.repr
+    }
+}
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, TyEncodable, TyDecodable)]
+pub enum AdtKind {
+    Struct,
+    Union,
+    Enum,
+}
+
+impl Into<DataTypeKind> for AdtKind {
+    fn into(self) -> DataTypeKind {
+        match self {
+            AdtKind::Struct => DataTypeKind::Struct,
+            AdtKind::Union => DataTypeKind::Union,
+            AdtKind::Enum => DataTypeKind::Enum,
+        }
+    }
+}
+
+impl AdtDefData {
+    /// Creates a new `AdtDefData`.
+    pub(super) fn new(
+        tcx: TyCtxt<'_>,
+        did: DefId,
+        kind: AdtKind,
+        variants: IndexVec<VariantIdx, VariantDef>,
+        repr: ReprOptions,
+    ) -> Self {
+        debug!("AdtDef::new({:?}, {:?}, {:?}, {:?})", did, kind, variants, repr);
+        let mut flags = AdtFlags::NO_ADT_FLAGS;
+
+        if kind == AdtKind::Enum && tcx.has_attr(did, sym::non_exhaustive) {
+            debug!("found non-exhaustive variant list for {:?}", did);
+            flags = flags | AdtFlags::IS_VARIANT_LIST_NON_EXHAUSTIVE;
+        }
+
+        flags |= match kind {
+            AdtKind::Enum => AdtFlags::IS_ENUM,
+            AdtKind::Union => AdtFlags::IS_UNION,
+            AdtKind::Struct => AdtFlags::IS_STRUCT,
+        };
+
+        if kind == AdtKind::Struct && variants[VariantIdx::new(0)].ctor_def_id.is_some() {
+            flags |= AdtFlags::HAS_CTOR;
+        }
+
+        if tcx.has_attr(did, sym::fundamental) {
+            flags |= AdtFlags::IS_FUNDAMENTAL;
+        }
+        if Some(did) == tcx.lang_items().phantom_data() {
+            flags |= AdtFlags::IS_PHANTOM_DATA;
+        }
+        if Some(did) == tcx.lang_items().owned_box() {
+            flags |= AdtFlags::IS_BOX;
+        }
+        if Some(did) == tcx.lang_items().manually_drop() {
+            flags |= AdtFlags::IS_MANUALLY_DROP;
+        }
+        if Some(did) == tcx.lang_items().unsafe_cell_type() {
+            flags |= AdtFlags::IS_UNSAFE_CELL;
+        }
+
+        AdtDefData { did, variants, flags, repr }
+    }
+}
+
+impl<'tcx> AdtDef<'tcx> {
+    /// Returns `true` if this is a struct.
+    #[inline]
+    pub fn is_struct(self) -> bool {
+        self.flags().contains(AdtFlags::IS_STRUCT)
+    }
+
+    /// Returns `true` if this is a union.
+    #[inline]
+    pub fn is_union(self) -> bool {
+        self.flags().contains(AdtFlags::IS_UNION)
+    }
+
+    /// Returns `true` if this is an enum.
+    #[inline]
+    pub fn is_enum(self) -> bool {
+        self.flags().contains(AdtFlags::IS_ENUM)
+    }
+
+    /// Returns `true` if the variant list of this ADT is `#[non_exhaustive]`.
+    #[inline]
+    pub fn is_variant_list_non_exhaustive(self) -> bool {
+        self.flags().contains(AdtFlags::IS_VARIANT_LIST_NON_EXHAUSTIVE)
+    }
+
+    /// Returns the kind of the ADT.
+    #[inline]
+    pub fn adt_kind(self) -> AdtKind {
+        if self.is_enum() {
+            AdtKind::Enum
+        } else if self.is_union() {
+            AdtKind::Union
+        } else {
+            AdtKind::Struct
+        }
+    }
+
+    /// Returns a description of this abstract data type.
+    pub fn descr(self) -> &'static str {
+        match self.adt_kind() {
+            AdtKind::Struct => "struct",
+            AdtKind::Union => "union",
+            AdtKind::Enum => "enum",
+        }
+    }
+
+    /// Returns a description of a variant of this abstract data type.
+    #[inline]
+    pub fn variant_descr(self) -> &'static str {
+        match self.adt_kind() {
+            AdtKind::Struct => "struct",
+            AdtKind::Union => "union",
+            AdtKind::Enum => "variant",
+        }
+    }
+
+    /// If this function returns `true`, it implies that `is_struct` must return `true`.
+    #[inline]
+    pub fn has_ctor(self) -> bool {
+        self.flags().contains(AdtFlags::HAS_CTOR)
+    }
+
+    /// Returns `true` if this type is `#[fundamental]` for the purposes
+    /// of coherence checking.
+    #[inline]
+    pub fn is_fundamental(self) -> bool {
+        self.flags().contains(AdtFlags::IS_FUNDAMENTAL)
+    }
+
+    /// Returns `true` if this is `PhantomData<T>`.
+    #[inline]
+    pub fn is_phantom_data(self) -> bool {
+        self.flags().contains(AdtFlags::IS_PHANTOM_DATA)
+    }
+
+    /// Returns `true` if this is Box<T>.
+    #[inline]
+    pub fn is_box(self) -> bool {
+        self.flags().contains(AdtFlags::IS_BOX)
+    }
+
+    /// Returns `true` if this is UnsafeCell<T>.
+    #[inline]
+    pub fn is_unsafe_cell(self) -> bool {
+        self.flags().contains(AdtFlags::IS_UNSAFE_CELL)
+    }
+
+    /// Returns `true` if this is `ManuallyDrop<T>`.
+    #[inline]
+    pub fn is_manually_drop(self) -> bool {
+        self.flags().contains(AdtFlags::IS_MANUALLY_DROP)
+    }
+
+    /// Returns `true` if this type has a destructor.
+    pub fn has_dtor(self, tcx: TyCtxt<'tcx>) -> bool {
+        self.destructor(tcx).is_some()
+    }
+
+    pub fn has_non_const_dtor(self, tcx: TyCtxt<'tcx>) -> bool {
+        matches!(self.destructor(tcx), Some(Destructor { constness: hir::Constness::NotConst, .. }))
+    }
+
+    /// Asserts this is a struct or union and returns its unique variant.
+    pub fn non_enum_variant(self) -> &'tcx VariantDef {
+        assert!(self.is_struct() || self.is_union());
+        &self.variant(VariantIdx::new(0))
+    }
+
+    #[inline]
+    pub fn predicates(self, tcx: TyCtxt<'tcx>) -> GenericPredicates<'tcx> {
+        tcx.predicates_of(self.did())
+    }
+
+    /// Returns an iterator over all fields contained
+    /// by this ADT.
+    #[inline]
+    pub fn all_fields(self) -> impl Iterator<Item = &'tcx FieldDef> + Clone {
+        self.variants().iter().flat_map(|v| v.fields.iter())
+    }
+
+    /// Whether the ADT lacks fields. Note that this includes uninhabited enums,
+    /// e.g., `enum Void {}` is considered payload free as well.
+    pub fn is_payloadfree(self) -> bool {
+        // Treat the ADT as not payload-free if arbitrary_enum_discriminant is used (#88621).
+        // This would disallow the following kind of enum from being casted into integer.
+        // ```
+        // enum Enum {
+        //    Foo() = 1,
+        //    Bar{} = 2,
+        //    Baz = 3,
+        // }
+        // ```
+        if self
+            .variants()
+            .iter()
+            .any(|v| matches!(v.discr, VariantDiscr::Explicit(_)) && v.ctor_kind != CtorKind::Const)
+        {
+            return false;
+        }
+        self.variants().iter().all(|v| v.fields.is_empty())
+    }
+
+    /// Return a `VariantDef` given a variant id.
+    pub fn variant_with_id(self, vid: DefId) -> &'tcx VariantDef {
+        self.variants().iter().find(|v| v.def_id == vid).expect("variant_with_id: unknown variant")
+    }
+
+    /// Return a `VariantDef` given a constructor id.
+    pub fn variant_with_ctor_id(self, cid: DefId) -> &'tcx VariantDef {
+        self.variants()
+            .iter()
+            .find(|v| v.ctor_def_id == Some(cid))
+            .expect("variant_with_ctor_id: unknown variant")
+    }
+
+    /// Return the index of `VariantDef` given a variant id.
+    pub fn variant_index_with_id(self, vid: DefId) -> VariantIdx {
+        self.variants()
+            .iter_enumerated()
+            .find(|(_, v)| v.def_id == vid)
+            .expect("variant_index_with_id: unknown variant")
+            .0
+    }
+
+    /// Return the index of `VariantDef` given a constructor id.
+    pub fn variant_index_with_ctor_id(self, cid: DefId) -> VariantIdx {
+        self.variants()
+            .iter_enumerated()
+            .find(|(_, v)| v.ctor_def_id == Some(cid))
+            .expect("variant_index_with_ctor_id: unknown variant")
+            .0
+    }
+
+    pub fn variant_of_res(self, res: Res) -> &'tcx VariantDef {
+        match res {
+            Res::Def(DefKind::Variant, vid) => self.variant_with_id(vid),
+            Res::Def(DefKind::Ctor(..), cid) => self.variant_with_ctor_id(cid),
+            Res::Def(DefKind::Struct, _)
+            | Res::Def(DefKind::Union, _)
+            | Res::Def(DefKind::TyAlias, _)
+            | Res::Def(DefKind::AssocTy, _)
+            | Res::SelfTy { .. }
+            | Res::SelfCtor(..) => self.non_enum_variant(),
+            _ => bug!("unexpected res {:?} in variant_of_res", res),
+        }
+    }
+
+    #[inline]
+    pub fn eval_explicit_discr(self, tcx: TyCtxt<'tcx>, expr_did: DefId) -> Option<Discr<'tcx>> {
+        assert!(self.is_enum());
+        let param_env = tcx.param_env(expr_did);
+        let repr_type = self.repr().discr_type();
+        match tcx.const_eval_poly(expr_did) {
+            Ok(val) => {
+                let ty = repr_type.to_ty(tcx);
+                if let Some(b) = val.try_to_bits_for_ty(tcx, param_env, ty) {
+                    trace!("discriminants: {} ({:?})", b, repr_type);
+                    Some(Discr { val: b, ty })
+                } else {
+                    info!("invalid enum discriminant: {:#?}", val);
+                    crate::mir::interpret::struct_error(
+                        tcx.at(tcx.def_span(expr_did)),
+                        "constant evaluation of enum discriminant resulted in non-integer",
+                    )
+                    .emit();
+                    None
+                }
+            }
+            Err(err) => {
+                let msg = match err {
+                    ErrorHandled::Reported(_) | ErrorHandled::Linted => {
+                        "enum discriminant evaluation failed"
+                    }
+                    ErrorHandled::TooGeneric => "enum discriminant depends on generics",
+                };
+                tcx.sess.delay_span_bug(tcx.def_span(expr_did), msg);
+                None
+            }
+        }
+    }
+
+    #[inline]
+    pub fn discriminants(
+        self,
+        tcx: TyCtxt<'tcx>,
+    ) -> impl Iterator<Item = (VariantIdx, Discr<'tcx>)> + Captures<'tcx> {
+        assert!(self.is_enum());
+        let repr_type = self.repr().discr_type();
+        let initial = repr_type.initial_discriminant(tcx);
+        let mut prev_discr = None::<Discr<'tcx>>;
+        self.variants().iter_enumerated().map(move |(i, v)| {
+            let mut discr = prev_discr.map_or(initial, |d| d.wrap_incr(tcx));
+            if let VariantDiscr::Explicit(expr_did) = v.discr {
+                if let Some(new_discr) = self.eval_explicit_discr(tcx, expr_did) {
+                    discr = new_discr;
+                }
+            }
+            prev_discr = Some(discr);
+
+            (i, discr)
+        })
+    }
+
+    #[inline]
+    pub fn variant_range(self) -> Range<VariantIdx> {
+        VariantIdx::new(0)..VariantIdx::new(self.variants().len())
+    }
+
+    /// Computes the discriminant value used by a specific variant.
+    /// Unlike `discriminants`, this is (amortized) constant-time,
+    /// only doing at most one query for evaluating an explicit
+    /// discriminant (the last one before the requested variant),
+    /// assuming there are no constant-evaluation errors there.
+    #[inline]
+    pub fn discriminant_for_variant(
+        self,
+        tcx: TyCtxt<'tcx>,
+        variant_index: VariantIdx,
+    ) -> Discr<'tcx> {
+        assert!(self.is_enum());
+        let (val, offset) = self.discriminant_def_for_variant(variant_index);
+        let explicit_value = val
+            .and_then(|expr_did| self.eval_explicit_discr(tcx, expr_did))
+            .unwrap_or_else(|| self.repr().discr_type().initial_discriminant(tcx));
+        explicit_value.checked_add(tcx, offset as u128).0
+    }
+
+    /// Yields a `DefId` for the discriminant and an offset to add to it
+    /// Alternatively, if there is no explicit discriminant, returns the
+    /// inferred discriminant directly.
+    pub fn discriminant_def_for_variant(self, variant_index: VariantIdx) -> (Option<DefId>, u32) {
+        assert!(!self.variants().is_empty());
+        let mut explicit_index = variant_index.as_u32();
+        let expr_did;
+        loop {
+            match self.variant(VariantIdx::from_u32(explicit_index)).discr {
+                ty::VariantDiscr::Relative(0) => {
+                    expr_did = None;
+                    break;
+                }
+                ty::VariantDiscr::Relative(distance) => {
+                    explicit_index -= distance;
+                }
+                ty::VariantDiscr::Explicit(did) => {
+                    expr_did = Some(did);
+                    break;
+                }
+            }
+        }
+        (expr_did, variant_index.as_u32() - explicit_index)
+    }
+
+    pub fn destructor(self, tcx: TyCtxt<'tcx>) -> Option<Destructor> {
+        tcx.adt_destructor(self.did())
+    }
+
+    /// Returns a list of types such that `Self: Sized` if and only
+    /// if that type is `Sized`, or `TyErr` if this type is recursive.
+    ///
+    /// Oddly enough, checking that the sized-constraint is `Sized` is
+    /// actually more expressive than checking all members:
+    /// the `Sized` trait is inductive, so an associated type that references
+    /// `Self` would prevent its containing ADT from being `Sized`.
+    ///
+    /// Due to normalization being eager, this applies even if
+    /// the associated type is behind a pointer (e.g., issue #31299).
+    pub fn sized_constraint(self, tcx: TyCtxt<'tcx>) -> ty::EarlyBinder<&'tcx [Ty<'tcx>]> {
+        ty::EarlyBinder(tcx.adt_sized_constraint(self.did()).0)
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/assoc.rs b/compiler/rustc_middle/src/ty/assoc.rs
new file mode 100644
index 000000000..c97156ac1
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/assoc.rs
@@ -0,0 +1,195 @@
+pub use self::AssocItemContainer::*;
+
+use crate::ty::{self, DefIdTree};
+use rustc_data_structures::sorted_map::SortedIndexMultiMap;
+use rustc_hir as hir;
+use rustc_hir::def::{DefKind, Namespace};
+use rustc_hir::def_id::DefId;
+use rustc_span::symbol::{Ident, Symbol};
+
+use super::{TyCtxt, Visibility};
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug, HashStable, Hash, Encodable, Decodable)]
+pub enum AssocItemContainer {
+    TraitContainer,
+    ImplContainer,
+}
+
+/// Information about an associated item
+#[derive(Copy, Clone, Debug, PartialEq, HashStable, Eq, Hash, Encodable, Decodable)]
+pub struct AssocItem {
+    pub def_id: DefId,
+    pub name: Symbol,
+    pub kind: AssocKind,
+    pub container: AssocItemContainer,
+
+    /// If this is an item in an impl of a trait then this is the `DefId` of
+    /// the associated item on the trait that this implements.
+    pub trait_item_def_id: Option<DefId>,
+
+    /// Whether this is a method with an explicit self
+    /// as its first parameter, allowing method calls.
+    pub fn_has_self_parameter: bool,
+}
+
+impl AssocItem {
+    pub fn ident(&self, tcx: TyCtxt<'_>) -> Ident {
+        Ident::new(self.name, tcx.def_ident_span(self.def_id).unwrap())
+    }
+
+    pub fn defaultness(&self, tcx: TyCtxt<'_>) -> hir::Defaultness {
+        tcx.impl_defaultness(self.def_id)
+    }
+
+    #[inline]
+    pub fn visibility(&self, tcx: TyCtxt<'_>) -> Visibility {
+        tcx.visibility(self.def_id)
+    }
+
+    #[inline]
+    pub fn container_id(&self, tcx: TyCtxt<'_>) -> DefId {
+        tcx.parent(self.def_id)
+    }
+
+    #[inline]
+    pub fn trait_container(&self, tcx: TyCtxt<'_>) -> Option<DefId> {
+        match self.container {
+            AssocItemContainer::ImplContainer => None,
+            AssocItemContainer::TraitContainer => Some(tcx.parent(self.def_id)),
+        }
+    }
+
+    #[inline]
+    pub fn impl_container(&self, tcx: TyCtxt<'_>) -> Option<DefId> {
+        match self.container {
+            AssocItemContainer::ImplContainer => Some(tcx.parent(self.def_id)),
+            AssocItemContainer::TraitContainer => None,
+        }
+    }
+
+    pub fn signature(&self, tcx: TyCtxt<'_>) -> String {
+        match self.kind {
+            ty::AssocKind::Fn => {
+                // We skip the binder here because the binder would deanonymize all
+                // late-bound regions, and we don't want method signatures to show up
+                // `as for<'r> fn(&'r MyType)`.  Pretty-printing handles late-bound
+                // regions just fine, showing `fn(&MyType)`.
+                tcx.fn_sig(self.def_id).skip_binder().to_string()
+            }
+            ty::AssocKind::Type => format!("type {};", self.name),
+            ty::AssocKind::Const => {
+                format!("const {}: {:?};", self.name, tcx.type_of(self.def_id))
+            }
+        }
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Debug, HashStable, Eq, Hash, Encodable, Decodable)]
+pub enum AssocKind {
+    Const,
+    Fn,
+    Type,
+}
+
+impl AssocKind {
+    pub fn namespace(&self) -> Namespace {
+        match *self {
+            ty::AssocKind::Type => Namespace::TypeNS,
+            ty::AssocKind::Const | ty::AssocKind::Fn => Namespace::ValueNS,
+        }
+    }
+
+    pub fn as_def_kind(&self) -> DefKind {
+        match self {
+            AssocKind::Const => DefKind::AssocConst,
+            AssocKind::Fn => DefKind::AssocFn,
+            AssocKind::Type => DefKind::AssocTy,
+        }
+    }
+}
+
+impl std::fmt::Display for AssocKind {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            AssocKind::Fn => write!(f, "method"),
+            AssocKind::Const => write!(f, "associated const"),
+            AssocKind::Type => write!(f, "associated type"),
+        }
+    }
+}
+
+/// A list of `ty::AssocItem`s in definition order that allows for efficient lookup by name.
+///
+/// When doing lookup by name, we try to postpone hygienic comparison for as long as possible since
+/// it is relatively expensive. Instead, items are indexed by `Symbol` and hygienic comparison is
+/// done only on items with the same name.
+#[derive(Debug, Clone, PartialEq, HashStable)]
+pub struct AssocItems<'tcx> {
+    pub(super) items: SortedIndexMultiMap<u32, Symbol, &'tcx ty::AssocItem>,
+}
+
+impl<'tcx> AssocItems<'tcx> {
+    /// Constructs an `AssociatedItems` map from a series of `ty::AssocItem`s in definition order.
+    pub fn new(items_in_def_order: impl IntoIterator<Item = &'tcx ty::AssocItem>) -> Self {
+        let items = items_in_def_order.into_iter().map(|item| (item.name, item)).collect();
+        AssocItems { items }
+    }
+
+    /// Returns a slice of associated items in the order they were defined.
+    ///
+    /// New code should avoid relying on definition order. If you need a particular associated item
+    /// for a known trait, make that trait a lang item instead of indexing this array.
+    pub fn in_definition_order(&self) -> impl '_ + Iterator<Item = &ty::AssocItem> {
+        self.items.iter().map(|(_, v)| *v)
+    }
+
+    pub fn len(&self) -> usize {
+        self.items.len()
+    }
+
+    /// Returns an iterator over all associated items with the given name, ignoring hygiene.
+    pub fn filter_by_name_unhygienic(
+        &self,
+        name: Symbol,
+    ) -> impl '_ + Iterator<Item = &ty::AssocItem> {
+        self.items.get_by_key(name).copied()
+    }
+
+    /// Returns the associated item with the given name and `AssocKind`, if one exists.
+    pub fn find_by_name_and_kind(
+        &self,
+        tcx: TyCtxt<'_>,
+        ident: Ident,
+        kind: AssocKind,
+        parent_def_id: DefId,
+    ) -> Option<&ty::AssocItem> {
+        self.filter_by_name_unhygienic(ident.name)
+            .filter(|item| item.kind == kind)
+            .find(|item| tcx.hygienic_eq(ident, item.ident(tcx), parent_def_id))
+    }
+
+    /// Returns the associated item with the given name and any of `AssocKind`, if one exists.
+    pub fn find_by_name_and_kinds(
+        &self,
+        tcx: TyCtxt<'_>,
+        ident: Ident,
+        // Sorted in order of what kinds to look at
+        kinds: &[AssocKind],
+        parent_def_id: DefId,
+    ) -> Option<&ty::AssocItem> {
+        kinds.iter().find_map(|kind| self.find_by_name_and_kind(tcx, ident, *kind, parent_def_id))
+    }
+
+    /// Returns the associated item with the given name in the given `Namespace`, if one exists.
+    pub fn find_by_name_and_namespace(
+        &self,
+        tcx: TyCtxt<'_>,
+        ident: Ident,
+        ns: Namespace,
+        parent_def_id: DefId,
+    ) -> Option<&ty::AssocItem> {
+        self.filter_by_name_unhygienic(ident.name)
+            .filter(|item| item.kind.namespace() == ns)
+            .find(|item| tcx.hygienic_eq(ident, item.ident(tcx), parent_def_id))
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/binding.rs b/compiler/rustc_middle/src/ty/binding.rs
new file mode 100644
index 000000000..3d65429f2
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/binding.rs
@@ -0,0 +1,22 @@
+use rustc_hir::BindingAnnotation;
+use rustc_hir::BindingAnnotation::*;
+use rustc_hir::Mutability;
+
+#[derive(Clone, PartialEq, TyEncodable, TyDecodable, Debug, Copy, HashStable)]
+pub enum BindingMode {
+    BindByReference(Mutability),
+    BindByValue(Mutability),
+}
+
+TrivialTypeTraversalAndLiftImpls! { BindingMode, }
+
+impl BindingMode {
+    pub fn convert(ba: BindingAnnotation) -> BindingMode {
+        match ba {
+            Unannotated => BindingMode::BindByValue(Mutability::Not),
+            Mutable => BindingMode::BindByValue(Mutability::Mut),
+            Ref => BindingMode::BindByReference(Mutability::Not),
+            RefMut => BindingMode::BindByReference(Mutability::Mut),
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/cast.rs b/compiler/rustc_middle/src/ty/cast.rs
new file mode 100644
index 000000000..c4b743dd4
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/cast.rs
@@ -0,0 +1,73 @@
+// Helpers for handling cast expressions, used in both
+// typeck and codegen.
+
+use crate::ty::{self, Ty};
+
+use rustc_macros::HashStable;
+
+/// Types that are represented as ints.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum IntTy {
+    U(ty::UintTy),
+    I,
+    CEnum,
+    Bool,
+    Char,
+}
+
+impl IntTy {
+    pub fn is_signed(self) -> bool {
+        matches!(self, Self::I)
+    }
+}
+
+// Valid types for the result of a non-coercion cast
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum CastTy<'tcx> {
+    /// Various types that are represented as ints and handled mostly
+    /// in the same way, merged for easier matching.
+    Int(IntTy),
+    /// Floating-point types.
+    Float,
+    /// Function pointers.
+    FnPtr,
+    /// Raw pointers.
+    Ptr(ty::TypeAndMut<'tcx>),
+}
+
+/// Cast Kind. See [RFC 401](https://rust-lang.github.io/rfcs/0401-coercions.html)
+/// (or librustc_typeck/check/cast.rs).
+#[derive(Copy, Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub enum CastKind {
+    CoercionCast,
+    PtrPtrCast,
+    PtrAddrCast,
+    AddrPtrCast,
+    NumericCast,
+    EnumCast,
+    PrimIntCast,
+    U8CharCast,
+    ArrayPtrCast,
+    FnPtrPtrCast,
+    FnPtrAddrCast,
+}
+
+impl<'tcx> CastTy<'tcx> {
+    /// Returns `Some` for integral/pointer casts.
+    /// Casts like unsizing casts will return `None`.
+    pub fn from_ty(t: Ty<'tcx>) -> Option<CastTy<'tcx>> {
+        match *t.kind() {
+            ty::Bool => Some(CastTy::Int(IntTy::Bool)),
+            ty::Char => Some(CastTy::Int(IntTy::Char)),
+            ty::Int(_) => Some(CastTy::Int(IntTy::I)),
+            ty::Infer(ty::InferTy::IntVar(_)) => Some(CastTy::Int(IntTy::I)),
+            ty::Infer(ty::InferTy::FloatVar(_)) => Some(CastTy::Float),
+            ty::Uint(u) => Some(CastTy::Int(IntTy::U(u))),
+            ty::Float(_) => Some(CastTy::Float),
+            ty::Adt(d, _) if d.is_enum() && d.is_payloadfree() => Some(CastTy::Int(IntTy::CEnum)),
+            ty::RawPtr(mt) => Some(CastTy::Ptr(mt)),
+            ty::FnPtr(..) => Some(CastTy::FnPtr),
+            _ => None,
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/closure.rs b/compiler/rustc_middle/src/ty/closure.rs
new file mode 100644
index 000000000..0d6c26a58
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/closure.rs
@@ -0,0 +1,454 @@
+use crate::hir::place::{
+    Place as HirPlace, PlaceBase as HirPlaceBase, ProjectionKind as HirProjectionKind,
+};
+use crate::{mir, ty};
+
+use std::fmt::Write;
+
+use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
+use rustc_hir as hir;
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_span::{Span, Symbol};
+
+use super::{Ty, TyCtxt};
+
+use self::BorrowKind::*;
+
+// Captures are represented using fields inside a structure.
+// This represents accessing self in the closure structure
+pub const CAPTURE_STRUCT_LOCAL: mir::Local = mir::Local::from_u32(1);
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct UpvarPath {
+    pub hir_id: hir::HirId,
+}
+
+/// Upvars do not get their own `NodeId`. Instead, we use the pair of
+/// the original var ID (that is, the root variable that is referenced
+/// by the upvar) and the ID of the closure expression.
+#[derive(Clone, Copy, PartialEq, Eq, Hash, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct UpvarId {
+    pub var_path: UpvarPath,
+    pub closure_expr_id: LocalDefId,
+}
+
+impl UpvarId {
+    pub fn new(var_hir_id: hir::HirId, closure_def_id: LocalDefId) -> UpvarId {
+        UpvarId { var_path: UpvarPath { hir_id: var_hir_id }, closure_expr_id: closure_def_id }
+    }
+}
+
+/// Information describing the capture of an upvar. This is computed
+/// during `typeck`, specifically by `regionck`.
+#[derive(PartialEq, Clone, Debug, Copy, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub enum UpvarCapture {
+    /// Upvar is captured by value. This is always true when the
+    /// closure is labeled `move`, but can also be true in other cases
+    /// depending on inference.
+    ByValue,
+
+    /// Upvar is captured by reference.
+    ByRef(BorrowKind),
+}
+
+pub type UpvarListMap = FxHashMap<DefId, FxIndexMap<hir::HirId, UpvarId>>;
+pub type UpvarCaptureMap = FxHashMap<UpvarId, UpvarCapture>;
+
+/// Given the closure DefId this map provides a map of root variables to minimum
+/// set of `CapturedPlace`s that need to be tracked to support all captures of that closure.
+pub type MinCaptureInformationMap<'tcx> = FxHashMap<LocalDefId, RootVariableMinCaptureList<'tcx>>;
+
+/// Part of `MinCaptureInformationMap`; Maps a root variable to the list of `CapturedPlace`.
+/// Used to track the minimum set of `Place`s that need to be captured to support all
+/// Places captured by the closure starting at a given root variable.
+///
+/// This provides a convenient and quick way of checking if a variable being used within
+/// a closure is a capture of a local variable.
+pub type RootVariableMinCaptureList<'tcx> = FxIndexMap<hir::HirId, MinCaptureList<'tcx>>;
+
+/// Part of `MinCaptureInformationMap`; List of `CapturePlace`s.
+pub type MinCaptureList<'tcx> = Vec<CapturedPlace<'tcx>>;
+
+/// Represents the various closure traits in the language. This
+/// will determine the type of the environment (`self`, in the
+/// desugaring) argument that the closure expects.
+///
+/// You can get the environment type of a closure using
+/// `tcx.closure_env_ty()`.
+#[derive(Clone, Copy, PartialOrd, Ord, PartialEq, Eq, Hash, Debug, TyEncodable, TyDecodable)]
+#[derive(HashStable)]
+pub enum ClosureKind {
+    // Warning: Ordering is significant here! The ordering is chosen
+    // because the trait Fn is a subtrait of FnMut and so in turn, and
+    // hence we order it so that Fn < FnMut < FnOnce.
+    Fn,
+    FnMut,
+    FnOnce,
+}
+
+impl<'tcx> ClosureKind {
+    // This is the initial value used when doing upvar inference.
+    pub const LATTICE_BOTTOM: ClosureKind = ClosureKind::Fn;
+
+    /// Returns `true` if a type that impls this closure kind
+    /// must also implement `other`.
+    pub fn extends(self, other: ty::ClosureKind) -> bool {
+        matches!(
+            (self, other),
+            (ClosureKind::Fn, ClosureKind::Fn)
+                | (ClosureKind::Fn, ClosureKind::FnMut)
+                | (ClosureKind::Fn, ClosureKind::FnOnce)
+                | (ClosureKind::FnMut, ClosureKind::FnMut)
+                | (ClosureKind::FnMut, ClosureKind::FnOnce)
+                | (ClosureKind::FnOnce, ClosureKind::FnOnce)
+        )
+    }
+
+    /// Returns the representative scalar type for this closure kind.
+    /// See `Ty::to_opt_closure_kind` for more details.
+    pub fn to_ty(self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        match self {
+            ClosureKind::Fn => tcx.types.i8,
+            ClosureKind::FnMut => tcx.types.i16,
+            ClosureKind::FnOnce => tcx.types.i32,
+        }
+    }
+
+    pub fn from_def_id(tcx: TyCtxt<'_>, def_id: DefId) -> Option<ClosureKind> {
+        if Some(def_id) == tcx.lang_items().fn_once_trait() {
+            Some(ClosureKind::FnOnce)
+        } else if Some(def_id) == tcx.lang_items().fn_mut_trait() {
+            Some(ClosureKind::FnMut)
+        } else if Some(def_id) == tcx.lang_items().fn_trait() {
+            Some(ClosureKind::Fn)
+        } else {
+            None
+        }
+    }
+
+    pub fn to_def_id(&self, tcx: TyCtxt<'_>) -> DefId {
+        match self {
+            ClosureKind::Fn => tcx.lang_items().fn_once_trait().unwrap(),
+            ClosureKind::FnMut => tcx.lang_items().fn_mut_trait().unwrap(),
+            ClosureKind::FnOnce => tcx.lang_items().fn_trait().unwrap(),
+        }
+    }
+}
+
+/// A composite describing a `Place` that is captured by a closure.
+#[derive(PartialEq, Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct CapturedPlace<'tcx> {
+    /// The `Place` that is captured.
+    pub place: HirPlace<'tcx>,
+
+    /// `CaptureKind` and expression(s) that resulted in such capture of `place`.
+    pub info: CaptureInfo,
+
+    /// Represents if `place` can be mutated or not.
+    pub mutability: hir::Mutability,
+
+    /// Region of the resulting reference if the upvar is captured by ref.
+    pub region: Option<ty::Region<'tcx>>,
+}
+
+impl<'tcx> CapturedPlace<'tcx> {
+    pub fn to_string(&self, tcx: TyCtxt<'tcx>) -> String {
+        place_to_string_for_capture(tcx, &self.place)
+    }
+
+    /// Returns a symbol of the captured upvar, which looks like `name__field1__field2`.
+    fn to_symbol(&self, tcx: TyCtxt<'tcx>) -> Symbol {
+        let hir_id = match self.place.base {
+            HirPlaceBase::Upvar(upvar_id) => upvar_id.var_path.hir_id,
+            base => bug!("Expected an upvar, found {:?}", base),
+        };
+        let mut symbol = tcx.hir().name(hir_id).as_str().to_string();
+
+        let mut ty = self.place.base_ty;
+        for proj in self.place.projections.iter() {
+            match proj.kind {
+                HirProjectionKind::Field(idx, variant) => match ty.kind() {
+                    ty::Tuple(_) => write!(&mut symbol, "__{}", idx).unwrap(),
+                    ty::Adt(def, ..) => {
+                        write!(
+                            &mut symbol,
+                            "__{}",
+                            def.variant(variant).fields[idx as usize].name.as_str(),
+                        )
+                        .unwrap();
+                    }
+                    ty => {
+                        span_bug!(
+                            self.get_capture_kind_span(tcx),
+                            "Unexpected type {:?} for `Field` projection",
+                            ty
+                        )
+                    }
+                },
+
+                // Ignore derefs for now, as they are likely caused by
+                // autoderefs that don't appear in the original code.
+                HirProjectionKind::Deref => {}
+                proj => bug!("Unexpected projection {:?} in captured place", proj),
+            }
+            ty = proj.ty;
+        }
+
+        Symbol::intern(&symbol)
+    }
+
+    /// Returns the hir-id of the root variable for the captured place.
+    /// e.g., if `a.b.c` was captured, would return the hir-id for `a`.
+    pub fn get_root_variable(&self) -> hir::HirId {
+        match self.place.base {
+            HirPlaceBase::Upvar(upvar_id) => upvar_id.var_path.hir_id,
+            base => bug!("Expected upvar, found={:?}", base),
+        }
+    }
+
+    /// Returns the `LocalDefId` of the closure that captured this Place
+    pub fn get_closure_local_def_id(&self) -> LocalDefId {
+        match self.place.base {
+            HirPlaceBase::Upvar(upvar_id) => upvar_id.closure_expr_id,
+            base => bug!("expected upvar, found={:?}", base),
+        }
+    }
+
+    /// Return span pointing to use that resulted in selecting the captured path
+    pub fn get_path_span(&self, tcx: TyCtxt<'tcx>) -> Span {
+        if let Some(path_expr_id) = self.info.path_expr_id {
+            tcx.hir().span(path_expr_id)
+        } else if let Some(capture_kind_expr_id) = self.info.capture_kind_expr_id {
+            tcx.hir().span(capture_kind_expr_id)
+        } else {
+            // Fallback on upvars mentioned if neither path or capture expr id is captured
+
+            // Safe to unwrap since we know this place is captured by the closure, therefore the closure must have upvars.
+            tcx.upvars_mentioned(self.get_closure_local_def_id()).unwrap()
+                [&self.get_root_variable()]
+                .span
+        }
+    }
+
+    /// Return span pointing to use that resulted in selecting the current capture kind
+    pub fn get_capture_kind_span(&self, tcx: TyCtxt<'tcx>) -> Span {
+        if let Some(capture_kind_expr_id) = self.info.capture_kind_expr_id {
+            tcx.hir().span(capture_kind_expr_id)
+        } else if let Some(path_expr_id) = self.info.path_expr_id {
+            tcx.hir().span(path_expr_id)
+        } else {
+            // Fallback on upvars mentioned if neither path or capture expr id is captured
+
+            // Safe to unwrap since we know this place is captured by the closure, therefore the closure must have upvars.
+            tcx.upvars_mentioned(self.get_closure_local_def_id()).unwrap()
+                [&self.get_root_variable()]
+                .span
+        }
+    }
+}
+
+fn symbols_for_closure_captures<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    def_id: (LocalDefId, LocalDefId),
+) -> Vec<Symbol> {
+    let typeck_results = tcx.typeck(def_id.0);
+    let captures = typeck_results.closure_min_captures_flattened(def_id.1);
+    captures.into_iter().map(|captured_place| captured_place.to_symbol(tcx)).collect()
+}
+
+/// Return true if the `proj_possible_ancestor` represents an ancestor path
+/// to `proj_capture` or `proj_possible_ancestor` is same as `proj_capture`,
+/// assuming they both start off of the same root variable.
+///
+/// **Note:** It's the caller's responsibility to ensure that both lists of projections
+///           start off of the same root variable.
+///
+/// Eg: 1. `foo.x` which is represented using `projections=[Field(x)]` is an ancestor of
+///        `foo.x.y` which is represented using `projections=[Field(x), Field(y)]`.
+///        Note both `foo.x` and `foo.x.y` start off of the same root variable `foo`.
+///     2. Since we only look at the projections here function will return `bar.x` as an a valid
+///        ancestor of `foo.x.y`. It's the caller's responsibility to ensure that both projections
+///        list are being applied to the same root variable.
+pub fn is_ancestor_or_same_capture(
+    proj_possible_ancestor: &[HirProjectionKind],
+    proj_capture: &[HirProjectionKind],
+) -> bool {
+    // We want to make sure `is_ancestor_or_same_capture("x.0.0", "x.0")` to return false.
+    // Therefore we can't just check if all projections are same in the zipped iterator below.
+    if proj_possible_ancestor.len() > proj_capture.len() {
+        return false;
+    }
+
+    proj_possible_ancestor.iter().zip(proj_capture).all(|(a, b)| a == b)
+}
+
+/// Part of `MinCaptureInformationMap`; describes the capture kind (&, &mut, move)
+/// for a particular capture as well as identifying the part of the source code
+/// that triggered this capture to occur.
+#[derive(PartialEq, Clone, Debug, Copy, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct CaptureInfo {
+    /// Expr Id pointing to use that resulted in selecting the current capture kind
+    ///
+    /// Eg:
+    /// ```rust,no_run
+    /// let mut t = (0,1);
+    ///
+    /// let c = || {
+    ///     println!("{t:?}"); // L1
+    ///     t.1 = 4; // L2
+    /// };
+    /// ```
+    /// `capture_kind_expr_id` will point to the use on L2 and `path_expr_id` will point to the
+    /// use on L1.
+    ///
+    /// If the user doesn't enable feature `capture_disjoint_fields` (RFC 2229) then, it is
+    /// possible that we don't see the use of a particular place resulting in capture_kind_expr_id being
+    /// None. In such case we fallback on uvpars_mentioned for span.
+    ///
+    /// Eg:
+    /// ```rust,no_run
+    /// let x = 5;
+    ///
+    /// let c = || {
+    ///     let _ = x;
+    /// };
+    /// ```
+    ///
+    /// In this example, if `capture_disjoint_fields` is **not** set, then x will be captured,
+    /// but we won't see it being used during capture analysis, since it's essentially a discard.
+    pub capture_kind_expr_id: Option<hir::HirId>,
+    /// Expr Id pointing to use that resulted the corresponding place being captured
+    ///
+    /// See `capture_kind_expr_id` for example.
+    ///
+    pub path_expr_id: Option<hir::HirId>,
+
+    /// Capture mode that was selected
+    pub capture_kind: UpvarCapture,
+}
+
+pub fn place_to_string_for_capture<'tcx>(tcx: TyCtxt<'tcx>, place: &HirPlace<'tcx>) -> String {
+    let mut curr_string: String = match place.base {
+        HirPlaceBase::Upvar(upvar_id) => tcx.hir().name(upvar_id.var_path.hir_id).to_string(),
+        _ => bug!("Capture_information should only contain upvars"),
+    };
+
+    for (i, proj) in place.projections.iter().enumerate() {
+        match proj.kind {
+            HirProjectionKind::Deref => {
+                curr_string = format!("*{}", curr_string);
+            }
+            HirProjectionKind::Field(idx, variant) => match place.ty_before_projection(i).kind() {
+                ty::Adt(def, ..) => {
+                    curr_string = format!(
+                        "{}.{}",
+                        curr_string,
+                        def.variant(variant).fields[idx as usize].name.as_str()
+                    );
+                }
+                ty::Tuple(_) => {
+                    curr_string = format!("{}.{}", curr_string, idx);
+                }
+                _ => {
+                    bug!(
+                        "Field projection applied to a type other than Adt or Tuple: {:?}.",
+                        place.ty_before_projection(i).kind()
+                    )
+                }
+            },
+            proj => bug!("{:?} unexpected because it isn't captured", proj),
+        }
+    }
+
+    curr_string
+}
+
+#[derive(Clone, PartialEq, Debug, TyEncodable, TyDecodable, Copy, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub enum BorrowKind {
+    /// Data must be immutable and is aliasable.
+    ImmBorrow,
+
+    /// Data must be immutable but not aliasable. This kind of borrow
+    /// cannot currently be expressed by the user and is used only in
+    /// implicit closure bindings. It is needed when the closure
+    /// is borrowing or mutating a mutable referent, e.g.:
+    ///
+    /// ```
+    /// let mut z = 3;
+    /// let x: &mut isize = &mut z;
+    /// let y = || *x += 5;
+    /// ```
+    ///
+    /// If we were to try to translate this closure into a more explicit
+    /// form, we'd encounter an error with the code as written:
+    ///
+    /// ```compile_fail,E0594
+    /// struct Env<'a> { x: &'a &'a mut isize }
+    /// let mut z = 3;
+    /// let x: &mut isize = &mut z;
+    /// let y = (&mut Env { x: &x }, fn_ptr);  // Closure is pair of env and fn
+    /// fn fn_ptr(env: &mut Env) { **env.x += 5; }
+    /// ```
+    ///
+    /// This is then illegal because you cannot mutate a `&mut` found
+    /// in an aliasable location. To solve, you'd have to translate with
+    /// an `&mut` borrow:
+    ///
+    /// ```compile_fail,E0596
+    /// struct Env<'a> { x: &'a mut &'a mut isize }
+    /// let mut z = 3;
+    /// let x: &mut isize = &mut z;
+    /// let y = (&mut Env { x: &mut x }, fn_ptr); // changed from &x to &mut x
+    /// fn fn_ptr(env: &mut Env) { **env.x += 5; }
+    /// ```
+    ///
+    /// Now the assignment to `**env.x` is legal, but creating a
+    /// mutable pointer to `x` is not because `x` is not mutable. We
+    /// could fix this by declaring `x` as `let mut x`. This is ok in
+    /// user code, if awkward, but extra weird for closures, since the
+    /// borrow is hidden.
+    ///
+    /// So we introduce a "unique imm" borrow -- the referent is
+    /// immutable, but not aliasable. This solves the problem. For
+    /// simplicity, we don't give users the way to express this
+    /// borrow, it's just used when translating closures.
+    UniqueImmBorrow,
+
+    /// Data is mutable and not aliasable.
+    MutBorrow,
+}
+
+impl BorrowKind {
+    pub fn from_mutbl(m: hir::Mutability) -> BorrowKind {
+        match m {
+            hir::Mutability::Mut => MutBorrow,
+            hir::Mutability::Not => ImmBorrow,
+        }
+    }
+
+    /// Returns a mutability `m` such that an `&m T` pointer could be used to obtain this borrow
+    /// kind. Because borrow kinds are richer than mutabilities, we sometimes have to pick a
+    /// mutability that is stronger than necessary so that it at least *would permit* the borrow in
+    /// question.
+    pub fn to_mutbl_lossy(self) -> hir::Mutability {
+        match self {
+            MutBorrow => hir::Mutability::Mut,
+            ImmBorrow => hir::Mutability::Not,
+
+            // We have no type corresponding to a unique imm borrow, so
+            // use `&mut`. It gives all the capabilities of a `&uniq`
+            // and hence is a safe "over approximation".
+            UniqueImmBorrow => hir::Mutability::Mut,
+        }
+    }
+}
+
+pub fn provide(providers: &mut ty::query::Providers) {
+    *providers = ty::query::Providers { symbols_for_closure_captures, ..*providers }
+}
diff --git a/compiler/rustc_middle/src/ty/codec.rs b/compiler/rustc_middle/src/ty/codec.rs
new file mode 100644
index 000000000..51137c526
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/codec.rs
@@ -0,0 +1,527 @@
+//! This module contains some shared code for encoding and decoding various
+//! things from the `ty` module, and in particular implements support for
+//! "shorthands" which allow to have pointers back into the already encoded
+//! stream instead of re-encoding the same thing twice.
+//!
+//! The functionality in here is shared between persisting to crate metadata and
+//! persisting to incr. comp. caches.
+
+use crate::arena::ArenaAllocatable;
+use crate::infer::canonical::{CanonicalVarInfo, CanonicalVarInfos};
+use crate::mir::{
+    self,
+    interpret::{AllocId, ConstAllocation},
+};
+use crate::traits;
+use crate::ty::subst::SubstsRef;
+use crate::ty::{self, AdtDef, Ty};
+use rustc_data_structures::fx::FxHashMap;
+use rustc_middle::ty::TyCtxt;
+use rustc_serialize::{Decodable, Encodable};
+use rustc_span::Span;
+pub use rustc_type_ir::{TyDecoder, TyEncoder};
+use std::hash::Hash;
+use std::intrinsics;
+use std::marker::DiscriminantKind;
+
+/// The shorthand encoding uses an enum's variant index `usize`
+/// and is offset by this value so it never matches a real variant.
+/// This offset is also chosen so that the first byte is never < 0x80.
+pub const SHORTHAND_OFFSET: usize = 0x80;
+
+pub trait EncodableWithShorthand<E: TyEncoder>: Copy + Eq + Hash {
+    type Variant: Encodable<E>;
+    fn variant(&self) -> &Self::Variant;
+}
+
+#[allow(rustc::usage_of_ty_tykind)]
+impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> EncodableWithShorthand<E> for Ty<'tcx> {
+    type Variant = ty::TyKind<'tcx>;
+
+    #[inline]
+    fn variant(&self) -> &Self::Variant {
+        self.kind()
+    }
+}
+
+impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> EncodableWithShorthand<E> for ty::PredicateKind<'tcx> {
+    type Variant = ty::PredicateKind<'tcx>;
+
+    #[inline]
+    fn variant(&self) -> &Self::Variant {
+        self
+    }
+}
+
+/// Trait for decoding to a reference.
+///
+/// This is a separate trait from `Decodable` so that we can implement it for
+/// upstream types, such as `FxHashSet`.
+///
+/// The `TyDecodable` derive macro will use this trait for fields that are
+/// references (and don't use a type alias to hide that).
+///
+/// `Decodable` can still be implemented in cases where `Decodable` is required
+/// by a trait bound.
+pub trait RefDecodable<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> {
+    fn decode(d: &mut D) -> &'tcx Self;
+}
+
+/// Encode the given value or a previously cached shorthand.
+pub fn encode_with_shorthand<'tcx, E, T, M>(encoder: &mut E, value: &T, cache: M)
+where
+    E: TyEncoder<I = TyCtxt<'tcx>>,
+    M: for<'b> Fn(&'b mut E) -> &'b mut FxHashMap<T, usize>,
+    T: EncodableWithShorthand<E>,
+    // The discriminant and shorthand must have the same size.
+    T::Variant: DiscriminantKind<Discriminant = isize>,
+{
+    let existing_shorthand = cache(encoder).get(value).copied();
+    if let Some(shorthand) = existing_shorthand {
+        encoder.emit_usize(shorthand);
+        return;
+    }
+
+    let variant = value.variant();
+
+    let start = encoder.position();
+    variant.encode(encoder);
+    let len = encoder.position() - start;
+
+    // The shorthand encoding uses the same usize as the
+    // discriminant, with an offset so they can't conflict.
+    let discriminant = intrinsics::discriminant_value(variant);
+    assert!(SHORTHAND_OFFSET > discriminant as usize);
+
+    let shorthand = start + SHORTHAND_OFFSET;
+
+    // Get the number of bits that leb128 could fit
+    // in the same space as the fully encoded type.
+    let leb128_bits = len * 7;
+
+    // Check that the shorthand is a not longer than the
+    // full encoding itself, i.e., it's an obvious win.
+    if leb128_bits >= 64 || (shorthand as u64) < (1 << leb128_bits) {
+        cache(encoder).insert(*value, shorthand);
+    }
+}
+
+impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> Encodable<E> for Ty<'tcx> {
+    fn encode(&self, e: &mut E) {
+        encode_with_shorthand(e, self, TyEncoder::type_shorthands);
+    }
+}
+
+impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> Encodable<E>
+    for ty::Binder<'tcx, ty::PredicateKind<'tcx>>
+{
+    fn encode(&self, e: &mut E) {
+        self.bound_vars().encode(e);
+        encode_with_shorthand(e, &self.skip_binder(), TyEncoder::predicate_shorthands);
+    }
+}
+
+impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> Encodable<E> for ty::Predicate<'tcx> {
+    fn encode(&self, e: &mut E) {
+        self.kind().encode(e);
+    }
+}
+
+impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> Encodable<E> for ty::Region<'tcx> {
+    fn encode(&self, e: &mut E) {
+        self.kind().encode(e);
+    }
+}
+
+impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> Encodable<E> for ty::Const<'tcx> {
+    fn encode(&self, e: &mut E) {
+        self.0.0.encode(e);
+    }
+}
+
+impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> Encodable<E> for ConstAllocation<'tcx> {
+    fn encode(&self, e: &mut E) {
+        self.inner().encode(e)
+    }
+}
+
+impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> Encodable<E> for AdtDef<'tcx> {
+    fn encode(&self, e: &mut E) {
+        self.0.0.encode(e)
+    }
+}
+
+impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> Encodable<E> for AllocId {
+    fn encode(&self, e: &mut E) {
+        e.encode_alloc_id(self)
+    }
+}
+
+#[inline]
+fn decode_arena_allocable<
+    'tcx,
+    D: TyDecoder<I = TyCtxt<'tcx>>,
+    T: ArenaAllocatable<'tcx> + Decodable<D>,
+>(
+    decoder: &mut D,
+) -> &'tcx T
+where
+    D: TyDecoder,
+{
+    decoder.interner().arena.alloc(Decodable::decode(decoder))
+}
+
+#[inline]
+fn decode_arena_allocable_slice<
+    'tcx,
+    D: TyDecoder<I = TyCtxt<'tcx>>,
+    T: ArenaAllocatable<'tcx> + Decodable<D>,
+>(
+    decoder: &mut D,
+) -> &'tcx [T]
+where
+    D: TyDecoder,
+{
+    decoder.interner().arena.alloc_from_iter(<Vec<T> as Decodable<D>>::decode(decoder))
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for Ty<'tcx> {
+    #[allow(rustc::usage_of_ty_tykind)]
+    fn decode(decoder: &mut D) -> Ty<'tcx> {
+        // Handle shorthands first, if we have a usize > 0x80.
+        if decoder.positioned_at_shorthand() {
+            let pos = decoder.read_usize();
+            assert!(pos >= SHORTHAND_OFFSET);
+            let shorthand = pos - SHORTHAND_OFFSET;
+
+            decoder.cached_ty_for_shorthand(shorthand, |decoder| {
+                decoder.with_position(shorthand, Ty::decode)
+            })
+        } else {
+            let tcx = decoder.interner();
+            tcx.mk_ty(rustc_type_ir::TyKind::decode(decoder))
+        }
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D>
+    for ty::Binder<'tcx, ty::PredicateKind<'tcx>>
+{
+    fn decode(decoder: &mut D) -> ty::Binder<'tcx, ty::PredicateKind<'tcx>> {
+        let bound_vars = Decodable::decode(decoder);
+        // Handle shorthands first, if we have a usize > 0x80.
+        ty::Binder::bind_with_vars(
+            if decoder.positioned_at_shorthand() {
+                let pos = decoder.read_usize();
+                assert!(pos >= SHORTHAND_OFFSET);
+                let shorthand = pos - SHORTHAND_OFFSET;
+
+                decoder.with_position(shorthand, ty::PredicateKind::decode)
+            } else {
+                ty::PredicateKind::decode(decoder)
+            },
+            bound_vars,
+        )
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for ty::Predicate<'tcx> {
+    fn decode(decoder: &mut D) -> ty::Predicate<'tcx> {
+        let predicate_kind = Decodable::decode(decoder);
+        decoder.interner().mk_predicate(predicate_kind)
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for SubstsRef<'tcx> {
+    fn decode(decoder: &mut D) -> Self {
+        let len = decoder.read_usize();
+        let tcx = decoder.interner();
+        tcx.mk_substs(
+            (0..len).map::<ty::subst::GenericArg<'tcx>, _>(|_| Decodable::decode(decoder)),
+        )
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for mir::Place<'tcx> {
+    fn decode(decoder: &mut D) -> Self {
+        let local: mir::Local = Decodable::decode(decoder);
+        let len = decoder.read_usize();
+        let projection = decoder.interner().mk_place_elems(
+            (0..len).map::<mir::PlaceElem<'tcx>, _>(|_| Decodable::decode(decoder)),
+        );
+        mir::Place { local, projection }
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for ty::Region<'tcx> {
+    fn decode(decoder: &mut D) -> Self {
+        decoder.interner().mk_region(Decodable::decode(decoder))
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for CanonicalVarInfos<'tcx> {
+    fn decode(decoder: &mut D) -> Self {
+        let len = decoder.read_usize();
+        let interned: Vec<CanonicalVarInfo<'tcx>> =
+            (0..len).map(|_| Decodable::decode(decoder)).collect();
+        decoder.interner().intern_canonical_var_infos(interned.as_slice())
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for AllocId {
+    fn decode(decoder: &mut D) -> Self {
+        decoder.decode_alloc_id()
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for ty::SymbolName<'tcx> {
+    fn decode(decoder: &mut D) -> Self {
+        ty::SymbolName::new(decoder.interner(), &decoder.read_str())
+    }
+}
+
+macro_rules! impl_decodable_via_ref {
+    ($($t:ty),+) => {
+        $(impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for $t {
+            fn decode(decoder: &mut D) -> Self {
+                RefDecodable::decode(decoder)
+            }
+        })*
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for ty::List<Ty<'tcx>> {
+    fn decode(decoder: &mut D) -> &'tcx Self {
+        let len = decoder.read_usize();
+        decoder.interner().mk_type_list((0..len).map::<Ty<'tcx>, _>(|_| Decodable::decode(decoder)))
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D>
+    for ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>
+{
+    fn decode(decoder: &mut D) -> &'tcx Self {
+        let len = decoder.read_usize();
+        decoder.interner().mk_poly_existential_predicates(
+            (0..len).map::<ty::Binder<'tcx, _>, _>(|_| Decodable::decode(decoder)),
+        )
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for ty::Const<'tcx> {
+    fn decode(decoder: &mut D) -> Self {
+        decoder.interner().mk_const(Decodable::decode(decoder))
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for [ty::ValTree<'tcx>] {
+    fn decode(decoder: &mut D) -> &'tcx Self {
+        decoder.interner().arena.alloc_from_iter(
+            (0..decoder.read_usize()).map(|_| Decodable::decode(decoder)).collect::<Vec<_>>(),
+        )
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for ConstAllocation<'tcx> {
+    fn decode(decoder: &mut D) -> Self {
+        decoder.interner().intern_const_alloc(Decodable::decode(decoder))
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for AdtDef<'tcx> {
+    fn decode(decoder: &mut D) -> Self {
+        decoder.interner().intern_adt_def(Decodable::decode(decoder))
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D>
+    for [(ty::Predicate<'tcx>, Span)]
+{
+    fn decode(decoder: &mut D) -> &'tcx Self {
+        decoder.interner().arena.alloc_from_iter(
+            (0..decoder.read_usize()).map(|_| Decodable::decode(decoder)).collect::<Vec<_>>(),
+        )
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D>
+    for [ty::abstract_const::Node<'tcx>]
+{
+    fn decode(decoder: &mut D) -> &'tcx Self {
+        decoder.interner().arena.alloc_from_iter(
+            (0..decoder.read_usize()).map(|_| Decodable::decode(decoder)).collect::<Vec<_>>(),
+        )
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D>
+    for [ty::abstract_const::NodeId]
+{
+    fn decode(decoder: &mut D) -> &'tcx Self {
+        decoder.interner().arena.alloc_from_iter(
+            (0..decoder.read_usize()).map(|_| Decodable::decode(decoder)).collect::<Vec<_>>(),
+        )
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D>
+    for ty::List<ty::BoundVariableKind>
+{
+    fn decode(decoder: &mut D) -> &'tcx Self {
+        let len = decoder.read_usize();
+        decoder.interner().mk_bound_variable_kinds(
+            (0..len).map::<ty::BoundVariableKind, _>(|_| Decodable::decode(decoder)),
+        )
+    }
+}
+
+impl_decodable_via_ref! {
+    &'tcx ty::TypeckResults<'tcx>,
+    &'tcx ty::List<Ty<'tcx>>,
+    &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
+    &'tcx traits::ImplSource<'tcx, ()>,
+    &'tcx mir::Body<'tcx>,
+    &'tcx mir::UnsafetyCheckResult,
+    &'tcx mir::BorrowCheckResult<'tcx>,
+    &'tcx mir::coverage::CodeRegion,
+    &'tcx ty::List<ty::BoundVariableKind>
+}
+
+#[macro_export]
+macro_rules! __impl_decoder_methods {
+    ($($name:ident -> $ty:ty;)*) => {
+        $(
+            #[inline]
+            fn $name(&mut self) -> $ty {
+                self.opaque.$name()
+            }
+        )*
+    }
+}
+
+macro_rules! impl_arena_allocatable_decoder {
+    ([]$args:tt) => {};
+    ([decode $(, $attrs:ident)*]
+     [$name:ident: $ty:ty]) => {
+        impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for $ty {
+            #[inline]
+            fn decode(decoder: &mut D) -> &'tcx Self {
+                decode_arena_allocable(decoder)
+            }
+        }
+
+        impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for [$ty] {
+            #[inline]
+            fn decode(decoder: &mut D) -> &'tcx Self {
+                decode_arena_allocable_slice(decoder)
+            }
+        }
+    };
+}
+
+macro_rules! impl_arena_allocatable_decoders {
+    ([$($a:tt $name:ident: $ty:ty,)*]) => {
+        $(
+            impl_arena_allocatable_decoder!($a [$name: $ty]);
+        )*
+    }
+}
+
+rustc_hir::arena_types!(impl_arena_allocatable_decoders);
+arena_types!(impl_arena_allocatable_decoders);
+
+macro_rules! impl_arena_copy_decoder {
+    (<$tcx:tt> $($ty:ty,)*) => {
+        $(impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for $ty {
+            #[inline]
+            fn decode(decoder: &mut D) -> &'tcx Self {
+                decoder.interner().arena.alloc(Decodable::decode(decoder))
+            }
+        }
+
+        impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for [$ty] {
+            #[inline]
+            fn decode(decoder: &mut D) -> &'tcx Self {
+                decoder.interner().arena.alloc_from_iter(<Vec<_> as Decodable<D>>::decode(decoder))
+            }
+        })*
+    };
+}
+
+impl_arena_copy_decoder! {<'tcx>
+    Span,
+    rustc_span::symbol::Ident,
+    ty::Variance,
+    rustc_span::def_id::DefId,
+    rustc_span::def_id::LocalDefId,
+    (rustc_middle::middle::exported_symbols::ExportedSymbol<'tcx>, rustc_middle::middle::exported_symbols::SymbolExportInfo),
+}
+
+#[macro_export]
+macro_rules! implement_ty_decoder {
+    ($DecoderName:ident <$($typaram:tt),*>) => {
+        mod __ty_decoder_impl {
+            use std::borrow::Cow;
+            use rustc_serialize::Decoder;
+
+            use super::$DecoderName;
+
+            impl<$($typaram ),*> Decoder for $DecoderName<$($typaram),*> {
+                $crate::__impl_decoder_methods! {
+                    read_u128 -> u128;
+                    read_u64 -> u64;
+                    read_u32 -> u32;
+                    read_u16 -> u16;
+                    read_u8 -> u8;
+                    read_usize -> usize;
+
+                    read_i128 -> i128;
+                    read_i64 -> i64;
+                    read_i32 -> i32;
+                    read_i16 -> i16;
+                    read_i8 -> i8;
+                    read_isize -> isize;
+
+                    read_bool -> bool;
+                    read_f64 -> f64;
+                    read_f32 -> f32;
+                    read_char -> char;
+                    read_str -> &str;
+                }
+
+                #[inline]
+                fn read_raw_bytes(&mut self, len: usize) -> &[u8] {
+                    self.opaque.read_raw_bytes(len)
+                }
+            }
+        }
+    }
+}
+
+macro_rules! impl_binder_encode_decode {
+    ($($t:ty),+ $(,)?) => {
+        $(
+            impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> Encodable<E> for ty::Binder<'tcx, $t> {
+                fn encode(&self, e: &mut E) {
+                    self.bound_vars().encode(e);
+                    self.as_ref().skip_binder().encode(e);
+                }
+            }
+            impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for ty::Binder<'tcx, $t> {
+                fn decode(decoder: &mut D) -> Self {
+                    let bound_vars = Decodable::decode(decoder);
+                    ty::Binder::bind_with_vars(Decodable::decode(decoder), bound_vars)
+                }
+            }
+        )*
+    }
+}
+
+impl_binder_encode_decode! {
+    &'tcx ty::List<Ty<'tcx>>,
+    ty::FnSig<'tcx>,
+    ty::ExistentialPredicate<'tcx>,
+    ty::TraitRef<'tcx>,
+    Vec<ty::GeneratorInteriorTypeCause<'tcx>>,
+    ty::ExistentialTraitRef<'tcx>,
+}
diff --git a/compiler/rustc_middle/src/ty/consts.rs b/compiler/rustc_middle/src/ty/consts.rs
new file mode 100644
index 000000000..f8792edc0
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/consts.rs
@@ -0,0 +1,326 @@
+use crate::mir::interpret::LitToConstInput;
+use crate::mir::ConstantKind;
+use crate::ty::{
+    self, InlineConstSubsts, InlineConstSubstsParts, InternalSubsts, ParamEnv, ParamEnvAnd, Ty,
+    TyCtxt, TypeVisitable,
+};
+use rustc_data_structures::intern::Interned;
+use rustc_errors::ErrorGuaranteed;
+use rustc_hir as hir;
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_macros::HashStable;
+use std::fmt;
+
+mod int;
+mod kind;
+mod valtree;
+
+pub use int::*;
+pub use kind::*;
+pub use valtree::*;
+
+/// Use this rather than `ConstS`, whenever possible.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, HashStable)]
+#[rustc_pass_by_value]
+pub struct Const<'tcx>(pub Interned<'tcx, ConstS<'tcx>>);
+
+impl<'tcx> fmt::Debug for Const<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // This reflects what `Const` looked liked before `Interned` was
+        // introduced. We print it like this to avoid having to update expected
+        // output in a lot of tests.
+        write!(f, "Const {{ ty: {:?}, kind: {:?} }}", self.ty(), self.kind())
+    }
+}
+
+/// Typed constant value.
+#[derive(PartialEq, Eq, PartialOrd, Ord, Hash, HashStable, TyEncodable, TyDecodable)]
+pub struct ConstS<'tcx> {
+    pub ty: Ty<'tcx>,
+    pub kind: ConstKind<'tcx>,
+}
+
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(ConstS<'_>, 48);
+
+impl<'tcx> Const<'tcx> {
+    #[inline]
+    pub fn ty(self) -> Ty<'tcx> {
+        self.0.ty
+    }
+
+    #[inline]
+    pub fn kind(self) -> ConstKind<'tcx> {
+        self.0.kind
+    }
+
+    /// Literals and const generic parameters are eagerly converted to a constant, everything else
+    /// becomes `Unevaluated`.
+    pub fn from_anon_const(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Self {
+        Self::from_opt_const_arg_anon_const(tcx, ty::WithOptConstParam::unknown(def_id))
+    }
+
+    #[instrument(skip(tcx), level = "debug")]
+    pub fn from_opt_const_arg_anon_const(
+        tcx: TyCtxt<'tcx>,
+        def: ty::WithOptConstParam<LocalDefId>,
+    ) -> Self {
+        debug!("Const::from_anon_const(def={:?})", def);
+
+        let body_id = match tcx.hir().get_by_def_id(def.did) {
+            hir::Node::AnonConst(ac) => ac.body,
+            _ => span_bug!(
+                tcx.def_span(def.did.to_def_id()),
+                "from_anon_const can only process anonymous constants"
+            ),
+        };
+
+        let expr = &tcx.hir().body(body_id).value;
+        debug!(?expr);
+
+        let ty = tcx.type_of(def.def_id_for_type_of());
+
+        match Self::try_eval_lit_or_param(tcx, ty, expr) {
+            Some(v) => v,
+            None => tcx.mk_const(ty::ConstS {
+                kind: ty::ConstKind::Unevaluated(ty::Unevaluated {
+                    def: def.to_global(),
+                    substs: InternalSubsts::identity_for_item(tcx, def.did.to_def_id()),
+                    promoted: None,
+                }),
+                ty,
+            }),
+        }
+    }
+
+    #[instrument(skip(tcx), level = "debug")]
+    fn try_eval_lit_or_param(
+        tcx: TyCtxt<'tcx>,
+        ty: Ty<'tcx>,
+        expr: &'tcx hir::Expr<'tcx>,
+    ) -> Option<Self> {
+        // Unwrap a block, so that e.g. `{ P }` is recognised as a parameter. Const arguments
+        // currently have to be wrapped in curly brackets, so it's necessary to special-case.
+        let expr = match &expr.kind {
+            hir::ExprKind::Block(block, _) if block.stmts.is_empty() && block.expr.is_some() => {
+                block.expr.as_ref().unwrap()
+            }
+            _ => expr,
+        };
+
+        let lit_input = match expr.kind {
+            hir::ExprKind::Lit(ref lit) => Some(LitToConstInput { lit: &lit.node, ty, neg: false }),
+            hir::ExprKind::Unary(hir::UnOp::Neg, ref expr) => match expr.kind {
+                hir::ExprKind::Lit(ref lit) => {
+                    Some(LitToConstInput { lit: &lit.node, ty, neg: true })
+                }
+                _ => None,
+            },
+            _ => None,
+        };
+
+        if let Some(lit_input) = lit_input {
+            // If an error occurred, ignore that it's a literal and leave reporting the error up to
+            // mir.
+            match tcx.at(expr.span).lit_to_const(lit_input) {
+                Ok(c) => return Some(c),
+                Err(e) => {
+                    tcx.sess.delay_span_bug(
+                        expr.span,
+                        &format!("Const::from_anon_const: couldn't lit_to_const {:?}", e),
+                    );
+                }
+            }
+        }
+
+        use hir::{def::DefKind::ConstParam, def::Res, ExprKind, Path, QPath};
+        match expr.kind {
+            ExprKind::Path(QPath::Resolved(_, &Path { res: Res::Def(ConstParam, def_id), .. })) => {
+                // Find the name and index of the const parameter by indexing the generics of
+                // the parent item and construct a `ParamConst`.
+                let hir_id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
+                let item_id = tcx.hir().get_parent_node(hir_id);
+                let item_def_id = tcx.hir().local_def_id(item_id);
+                let generics = tcx.generics_of(item_def_id.to_def_id());
+                let index = generics.param_def_id_to_index[&def_id];
+                let name = tcx.hir().name(hir_id);
+                Some(tcx.mk_const(ty::ConstS {
+                    kind: ty::ConstKind::Param(ty::ParamConst::new(index, name)),
+                    ty,
+                }))
+            }
+            _ => None,
+        }
+    }
+
+    pub fn from_inline_const(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Self {
+        debug!("Const::from_inline_const(def_id={:?})", def_id);
+
+        let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+
+        let body_id = match tcx.hir().get(hir_id) {
+            hir::Node::AnonConst(ac) => ac.body,
+            _ => span_bug!(
+                tcx.def_span(def_id.to_def_id()),
+                "from_inline_const can only process anonymous constants"
+            ),
+        };
+
+        let expr = &tcx.hir().body(body_id).value;
+
+        let ty = tcx.typeck(def_id).node_type(hir_id);
+
+        let ret = match Self::try_eval_lit_or_param(tcx, ty, expr) {
+            Some(v) => v,
+            None => {
+                let typeck_root_def_id = tcx.typeck_root_def_id(def_id.to_def_id());
+                let parent_substs =
+                    tcx.erase_regions(InternalSubsts::identity_for_item(tcx, typeck_root_def_id));
+                let substs =
+                    InlineConstSubsts::new(tcx, InlineConstSubstsParts { parent_substs, ty })
+                        .substs;
+                tcx.mk_const(ty::ConstS {
+                    kind: ty::ConstKind::Unevaluated(ty::Unevaluated {
+                        def: ty::WithOptConstParam::unknown(def_id).to_global(),
+                        substs,
+                        promoted: None,
+                    }),
+                    ty,
+                })
+            }
+        };
+        debug_assert!(!ret.has_free_regions());
+        ret
+    }
+
+    /// Interns the given value as a constant.
+    #[inline]
+    pub fn from_value(tcx: TyCtxt<'tcx>, val: ty::ValTree<'tcx>, ty: Ty<'tcx>) -> Self {
+        tcx.mk_const(ConstS { kind: ConstKind::Value(val), ty })
+    }
+
+    /// Panics if self.kind != ty::ConstKind::Value
+    pub fn to_valtree(self) -> ty::ValTree<'tcx> {
+        match self.kind() {
+            ty::ConstKind::Value(valtree) => valtree,
+            _ => bug!("expected ConstKind::Value, got {:?}", self.kind()),
+        }
+    }
+
+    pub fn from_scalar_int(tcx: TyCtxt<'tcx>, i: ScalarInt, ty: Ty<'tcx>) -> Self {
+        let valtree = ty::ValTree::from_scalar_int(i);
+        Self::from_value(tcx, valtree, ty)
+    }
+
+    #[inline]
+    /// Creates a constant with the given integer value and interns it.
+    pub fn from_bits(tcx: TyCtxt<'tcx>, bits: u128, ty: ParamEnvAnd<'tcx, Ty<'tcx>>) -> Self {
+        let size = tcx
+            .layout_of(ty)
+            .unwrap_or_else(|e| panic!("could not compute layout for {:?}: {:?}", ty, e))
+            .size;
+        Self::from_scalar_int(tcx, ScalarInt::try_from_uint(bits, size).unwrap(), ty.value)
+    }
+
+    #[inline]
+    /// Creates an interned zst constant.
+    pub fn zero_sized(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Self {
+        let valtree = ty::ValTree::zst();
+        Self::from_value(tcx, valtree, ty)
+    }
+
+    #[inline]
+    /// Creates an interned bool constant.
+    pub fn from_bool(tcx: TyCtxt<'tcx>, v: bool) -> Self {
+        Self::from_bits(tcx, v as u128, ParamEnv::empty().and(tcx.types.bool))
+    }
+
+    #[inline]
+    /// Creates an interned usize constant.
+    pub fn from_usize(tcx: TyCtxt<'tcx>, n: u64) -> Self {
+        Self::from_bits(tcx, n as u128, ParamEnv::empty().and(tcx.types.usize))
+    }
+
+    #[inline]
+    /// Attempts to evaluate the given constant to bits. Can fail to evaluate in the presence of
+    /// generics (or erroneous code) or if the value can't be represented as bits (e.g. because it
+    /// contains const generic parameters or pointers).
+    pub fn try_eval_bits(
+        self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ParamEnv<'tcx>,
+        ty: Ty<'tcx>,
+    ) -> Option<u128> {
+        assert_eq!(self.ty(), ty);
+        let size = tcx.layout_of(param_env.with_reveal_all_normalized(tcx).and(ty)).ok()?.size;
+        // if `ty` does not depend on generic parameters, use an empty param_env
+        self.kind().eval(tcx, param_env).try_to_bits(size)
+    }
+
+    #[inline]
+    pub fn try_eval_bool(self, tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>) -> Option<bool> {
+        self.kind().eval(tcx, param_env).try_to_bool()
+    }
+
+    #[inline]
+    pub fn try_eval_usize(self, tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>) -> Option<u64> {
+        self.kind().eval(tcx, param_env).try_to_machine_usize(tcx)
+    }
+
+    #[inline]
+    /// Tries to evaluate the constant if it is `Unevaluated`. If that doesn't succeed, return the
+    /// unevaluated constant.
+    pub fn eval(self, tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>) -> Const<'tcx> {
+        if let Some(val) = self.kind().try_eval_for_typeck(tcx, param_env) {
+            match val {
+                Ok(val) => Const::from_value(tcx, val, self.ty()),
+                Err(ErrorGuaranteed { .. }) => tcx.const_error(self.ty()),
+            }
+        } else {
+            // Either the constant isn't evaluatable or ValTree creation failed.
+            self
+        }
+    }
+
+    #[inline]
+    /// Tries to evaluate the constant if it is `Unevaluated` and creates a ConstValue if the
+    /// evaluation succeeds. If it doesn't succeed, returns the unevaluated constant.
+    pub fn eval_for_mir(self, tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>) -> ConstantKind<'tcx> {
+        if let Some(val) = self.kind().try_eval_for_mir(tcx, param_env) {
+            match val {
+                Ok(const_val) => ConstantKind::from_value(const_val, self.ty()),
+                Err(ErrorGuaranteed { .. }) => ConstantKind::Ty(tcx.const_error(self.ty())),
+            }
+        } else {
+            ConstantKind::Ty(self)
+        }
+    }
+
+    #[inline]
+    /// Panics if the value cannot be evaluated or doesn't contain a valid integer of the given type.
+    pub fn eval_bits(self, tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>, ty: Ty<'tcx>) -> u128 {
+        self.try_eval_bits(tcx, param_env, ty)
+            .unwrap_or_else(|| bug!("expected bits of {:#?}, got {:#?}", ty, self))
+    }
+
+    #[inline]
+    /// Panics if the value cannot be evaluated or doesn't contain a valid `usize`.
+    pub fn eval_usize(self, tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>) -> u64 {
+        self.try_eval_usize(tcx, param_env)
+            .unwrap_or_else(|| bug!("expected usize, got {:#?}", self))
+    }
+}
+
+pub fn const_param_default<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> Const<'tcx> {
+    let default_def_id = match tcx.hir().get_by_def_id(def_id.expect_local()) {
+        hir::Node::GenericParam(hir::GenericParam {
+            kind: hir::GenericParamKind::Const { ty: _, default: Some(ac) },
+            ..
+        }) => tcx.hir().local_def_id(ac.hir_id),
+        _ => span_bug!(
+            tcx.def_span(def_id),
+            "`const_param_default` expected a generic parameter with a constant"
+        ),
+    };
+    Const::from_anon_const(tcx, default_def_id)
+}
diff --git a/compiler/rustc_middle/src/ty/consts/int.rs b/compiler/rustc_middle/src/ty/consts/int.rs
new file mode 100644
index 000000000..7436f0f6f
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/consts/int.rs
@@ -0,0 +1,483 @@
+use rustc_apfloat::ieee::{Double, Single};
+use rustc_apfloat::Float;
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+use rustc_target::abi::Size;
+use std::convert::{TryFrom, TryInto};
+use std::fmt;
+use std::num::NonZeroU8;
+
+use crate::ty::TyCtxt;
+
+#[derive(Copy, Clone)]
+/// A type for representing any integer. Only used for printing.
+pub struct ConstInt {
+    /// The "untyped" variant of `ConstInt`.
+    int: ScalarInt,
+    /// Whether the value is of a signed integer type.
+    signed: bool,
+    /// Whether the value is a `usize` or `isize` type.
+    is_ptr_sized_integral: bool,
+}
+
+impl ConstInt {
+    pub fn new(int: ScalarInt, signed: bool, is_ptr_sized_integral: bool) -> Self {
+        Self { int, signed, is_ptr_sized_integral }
+    }
+}
+
+impl std::fmt::Debug for ConstInt {
+    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let Self { int, signed, is_ptr_sized_integral } = *self;
+        let size = int.size().bytes();
+        let raw = int.data;
+        if signed {
+            let bit_size = size * 8;
+            let min = 1u128 << (bit_size - 1);
+            let max = min - 1;
+            if raw == min {
+                match (size, is_ptr_sized_integral) {
+                    (_, true) => write!(fmt, "isize::MIN"),
+                    (1, _) => write!(fmt, "i8::MIN"),
+                    (2, _) => write!(fmt, "i16::MIN"),
+                    (4, _) => write!(fmt, "i32::MIN"),
+                    (8, _) => write!(fmt, "i64::MIN"),
+                    (16, _) => write!(fmt, "i128::MIN"),
+                    _ => bug!("ConstInt 0x{:x} with size = {} and signed = {}", raw, size, signed),
+                }
+            } else if raw == max {
+                match (size, is_ptr_sized_integral) {
+                    (_, true) => write!(fmt, "isize::MAX"),
+                    (1, _) => write!(fmt, "i8::MAX"),
+                    (2, _) => write!(fmt, "i16::MAX"),
+                    (4, _) => write!(fmt, "i32::MAX"),
+                    (8, _) => write!(fmt, "i64::MAX"),
+                    (16, _) => write!(fmt, "i128::MAX"),
+                    _ => bug!("ConstInt 0x{:x} with size = {} and signed = {}", raw, size, signed),
+                }
+            } else {
+                match size {
+                    1 => write!(fmt, "{}", raw as i8)?,
+                    2 => write!(fmt, "{}", raw as i16)?,
+                    4 => write!(fmt, "{}", raw as i32)?,
+                    8 => write!(fmt, "{}", raw as i64)?,
+                    16 => write!(fmt, "{}", raw as i128)?,
+                    _ => bug!("ConstInt 0x{:x} with size = {} and signed = {}", raw, size, signed),
+                }
+                if fmt.alternate() {
+                    match (size, is_ptr_sized_integral) {
+                        (_, true) => write!(fmt, "_isize")?,
+                        (1, _) => write!(fmt, "_i8")?,
+                        (2, _) => write!(fmt, "_i16")?,
+                        (4, _) => write!(fmt, "_i32")?,
+                        (8, _) => write!(fmt, "_i64")?,
+                        (16, _) => write!(fmt, "_i128")?,
+                        _ => bug!(),
+                    }
+                }
+                Ok(())
+            }
+        } else {
+            let max = Size::from_bytes(size).truncate(u128::MAX);
+            if raw == max {
+                match (size, is_ptr_sized_integral) {
+                    (_, true) => write!(fmt, "usize::MAX"),
+                    (1, _) => write!(fmt, "u8::MAX"),
+                    (2, _) => write!(fmt, "u16::MAX"),
+                    (4, _) => write!(fmt, "u32::MAX"),
+                    (8, _) => write!(fmt, "u64::MAX"),
+                    (16, _) => write!(fmt, "u128::MAX"),
+                    _ => bug!("ConstInt 0x{:x} with size = {} and signed = {}", raw, size, signed),
+                }
+            } else {
+                match size {
+                    1 => write!(fmt, "{}", raw as u8)?,
+                    2 => write!(fmt, "{}", raw as u16)?,
+                    4 => write!(fmt, "{}", raw as u32)?,
+                    8 => write!(fmt, "{}", raw as u64)?,
+                    16 => write!(fmt, "{}", raw as u128)?,
+                    _ => bug!("ConstInt 0x{:x} with size = {} and signed = {}", raw, size, signed),
+                }
+                if fmt.alternate() {
+                    match (size, is_ptr_sized_integral) {
+                        (_, true) => write!(fmt, "_usize")?,
+                        (1, _) => write!(fmt, "_u8")?,
+                        (2, _) => write!(fmt, "_u16")?,
+                        (4, _) => write!(fmt, "_u32")?,
+                        (8, _) => write!(fmt, "_u64")?,
+                        (16, _) => write!(fmt, "_u128")?,
+                        _ => bug!(),
+                    }
+                }
+                Ok(())
+            }
+        }
+    }
+}
+
+/// The raw bytes of a simple value.
+///
+/// This is a packed struct in order to allow this type to be optimally embedded in enums
+/// (like Scalar).
+#[derive(Clone, Copy, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[repr(packed)]
+pub struct ScalarInt {
+    /// The first `size` bytes of `data` are the value.
+    /// Do not try to read less or more bytes than that. The remaining bytes must be 0.
+    data: u128,
+    size: NonZeroU8,
+}
+
+// Cannot derive these, as the derives take references to the fields, and we
+// can't take references to fields of packed structs.
+impl<CTX> crate::ty::HashStable<CTX> for ScalarInt {
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut crate::ty::StableHasher) {
+        // Using a block `{self.data}` here to force a copy instead of using `self.data`
+        // directly, because `hash_stable` takes `&self` and would thus borrow `self.data`.
+        // Since `Self` is a packed struct, that would create a possibly unaligned reference,
+        // which is UB.
+        { self.data }.hash_stable(hcx, hasher);
+        self.size.get().hash_stable(hcx, hasher);
+    }
+}
+
+impl<S: Encoder> Encodable<S> for ScalarInt {
+    fn encode(&self, s: &mut S) {
+        s.emit_u128(self.data);
+        s.emit_u8(self.size.get());
+    }
+}
+
+impl<D: Decoder> Decodable<D> for ScalarInt {
+    fn decode(d: &mut D) -> ScalarInt {
+        ScalarInt { data: d.read_u128(), size: NonZeroU8::new(d.read_u8()).unwrap() }
+    }
+}
+
+impl ScalarInt {
+    pub const TRUE: ScalarInt = ScalarInt { data: 1_u128, size: NonZeroU8::new(1).unwrap() };
+
+    pub const FALSE: ScalarInt = ScalarInt { data: 0_u128, size: NonZeroU8::new(1).unwrap() };
+
+    #[inline]
+    pub fn size(self) -> Size {
+        Size::from_bytes(self.size.get())
+    }
+
+    /// Make sure the `data` fits in `size`.
+    /// This is guaranteed by all constructors here, but having had this check saved us from
+    /// bugs many times in the past, so keeping it around is definitely worth it.
+    #[inline(always)]
+    fn check_data(self) {
+        // Using a block `{self.data}` here to force a copy instead of using `self.data`
+        // directly, because `debug_assert_eq` takes references to its arguments and formatting
+        // arguments and would thus borrow `self.data`. Since `Self`
+        // is a packed struct, that would create a possibly unaligned reference, which
+        // is UB.
+        debug_assert_eq!(
+            self.size().truncate(self.data),
+            { self.data },
+            "Scalar value {:#x} exceeds size of {} bytes",
+            { self.data },
+            self.size
+        );
+    }
+
+    #[inline]
+    pub fn null(size: Size) -> Self {
+        Self { data: 0, size: NonZeroU8::new(size.bytes() as u8).unwrap() }
+    }
+
+    #[inline]
+    pub fn is_null(self) -> bool {
+        self.data == 0
+    }
+
+    #[inline]
+    pub fn try_from_uint(i: impl Into<u128>, size: Size) -> Option<Self> {
+        let data = i.into();
+        if size.truncate(data) == data {
+            Some(Self { data, size: NonZeroU8::new(size.bytes() as u8).unwrap() })
+        } else {
+            None
+        }
+    }
+
+    #[inline]
+    pub fn try_from_int(i: impl Into<i128>, size: Size) -> Option<Self> {
+        let i = i.into();
+        // `into` performed sign extension, we have to truncate
+        let truncated = size.truncate(i as u128);
+        if size.sign_extend(truncated) as i128 == i {
+            Some(Self { data: truncated, size: NonZeroU8::new(size.bytes() as u8).unwrap() })
+        } else {
+            None
+        }
+    }
+
+    #[inline]
+    pub fn assert_bits(self, target_size: Size) -> u128 {
+        self.to_bits(target_size).unwrap_or_else(|size| {
+            bug!("expected int of size {}, but got size {}", target_size.bytes(), size.bytes())
+        })
+    }
+
+    #[inline]
+    pub fn to_bits(self, target_size: Size) -> Result<u128, Size> {
+        assert_ne!(target_size.bytes(), 0, "you should never look at the bits of a ZST");
+        if target_size.bytes() == u64::from(self.size.get()) {
+            self.check_data();
+            Ok(self.data)
+        } else {
+            Err(self.size())
+        }
+    }
+
+    #[inline]
+    pub fn try_to_machine_usize<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Result<u64, Size> {
+        Ok(self.to_bits(tcx.data_layout.pointer_size)? as u64)
+    }
+
+    /// Tries to convert the `ScalarInt` to an unsigned integer of the given size.
+    /// Fails if the size of the `ScalarInt` is unequal to `size` and returns the
+    /// `ScalarInt`s size in that case.
+    #[inline]
+    pub fn try_to_uint(self, size: Size) -> Result<u128, Size> {
+        self.to_bits(size)
+    }
+
+    // Tries to convert the `ScalarInt` to `u8`. Fails if the `size` of the `ScalarInt`
+    // in not equal to `Size { raw: 1 }` and returns the `size` value of the `ScalarInt` in
+    // that case.
+    #[inline]
+    pub fn try_to_u8(self) -> Result<u8, Size> {
+        self.to_bits(Size::from_bits(8)).map(|v| u8::try_from(v).unwrap())
+    }
+
+    /// Tries to convert the `ScalarInt` to `u16`. Fails if the size of the `ScalarInt`
+    /// in not equal to `Size { raw: 2 }` and returns the `size` value of the `ScalarInt` in
+    /// that case.
+    #[inline]
+    pub fn try_to_u16(self) -> Result<u16, Size> {
+        self.to_bits(Size::from_bits(16)).map(|v| u16::try_from(v).unwrap())
+    }
+
+    /// Tries to convert the `ScalarInt` to `u32`. Fails if the `size` of the `ScalarInt`
+    /// in not equal to `Size { raw: 4 }` and returns the `size` value of the `ScalarInt` in
+    /// that case.
+    #[inline]
+    pub fn try_to_u32(self) -> Result<u32, Size> {
+        self.to_bits(Size::from_bits(32)).map(|v| u32::try_from(v).unwrap())
+    }
+
+    /// Tries to convert the `ScalarInt` to `u64`. Fails if the `size` of the `ScalarInt`
+    /// in not equal to `Size { raw: 8 }` and returns the `size` value of the `ScalarInt` in
+    /// that case.
+    #[inline]
+    pub fn try_to_u64(self) -> Result<u64, Size> {
+        self.to_bits(Size::from_bits(64)).map(|v| u64::try_from(v).unwrap())
+    }
+
+    /// Tries to convert the `ScalarInt` to `u128`. Fails if the `size` of the `ScalarInt`
+    /// in not equal to `Size { raw: 16 }` and returns the `size` value of the `ScalarInt` in
+    /// that case.
+    #[inline]
+    pub fn try_to_u128(self) -> Result<u128, Size> {
+        self.to_bits(Size::from_bits(128))
+    }
+
+    /// Tries to convert the `ScalarInt` to a signed integer of the given size.
+    /// Fails if the size of the `ScalarInt` is unequal to `size` and returns the
+    /// `ScalarInt`s size in that case.
+    #[inline]
+    pub fn try_to_int(self, size: Size) -> Result<i128, Size> {
+        let b = self.to_bits(size)?;
+        Ok(size.sign_extend(b) as i128)
+    }
+
+    /// Tries to convert the `ScalarInt` to i8.
+    /// Fails if the size of the `ScalarInt` is unequal to `Size { raw: 1 }`
+    /// and returns the `ScalarInt`s size in that case.
+    pub fn try_to_i8(self) -> Result<i8, Size> {
+        self.try_to_int(Size::from_bits(8)).map(|v| i8::try_from(v).unwrap())
+    }
+
+    /// Tries to convert the `ScalarInt` to i16.
+    /// Fails if the size of the `ScalarInt` is unequal to `Size { raw: 2 }`
+    /// and returns the `ScalarInt`s size in that case.
+    pub fn try_to_i16(self) -> Result<i16, Size> {
+        self.try_to_int(Size::from_bits(16)).map(|v| i16::try_from(v).unwrap())
+    }
+
+    /// Tries to convert the `ScalarInt` to i32.
+    /// Fails if the size of the `ScalarInt` is unequal to `Size { raw: 4 }`
+    /// and returns the `ScalarInt`s size in that case.
+    pub fn try_to_i32(self) -> Result<i32, Size> {
+        self.try_to_int(Size::from_bits(32)).map(|v| i32::try_from(v).unwrap())
+    }
+
+    /// Tries to convert the `ScalarInt` to i64.
+    /// Fails if the size of the `ScalarInt` is unequal to `Size { raw: 8 }`
+    /// and returns the `ScalarInt`s size in that case.
+    pub fn try_to_i64(self) -> Result<i64, Size> {
+        self.try_to_int(Size::from_bits(64)).map(|v| i64::try_from(v).unwrap())
+    }
+
+    /// Tries to convert the `ScalarInt` to i128.
+    /// Fails if the size of the `ScalarInt` is unequal to `Size { raw: 16 }`
+    /// and returns the `ScalarInt`s size in that case.
+    pub fn try_to_i128(self) -> Result<i128, Size> {
+        self.try_to_int(Size::from_bits(128)).map(|v| i128::try_from(v).unwrap())
+    }
+}
+
+macro_rules! from {
+    ($($ty:ty),*) => {
+        $(
+            impl From<$ty> for ScalarInt {
+                #[inline]
+                fn from(u: $ty) -> Self {
+                    Self {
+                        data: u128::from(u),
+                        size: NonZeroU8::new(std::mem::size_of::<$ty>() as u8).unwrap(),
+                    }
+                }
+            }
+        )*
+    }
+}
+
+macro_rules! try_from {
+    ($($ty:ty),*) => {
+        $(
+            impl TryFrom<ScalarInt> for $ty {
+                type Error = Size;
+                #[inline]
+                fn try_from(int: ScalarInt) -> Result<Self, Size> {
+                    // The `unwrap` cannot fail because to_bits (if it succeeds)
+                    // is guaranteed to return a value that fits into the size.
+                    int.to_bits(Size::from_bytes(std::mem::size_of::<$ty>()))
+                       .map(|u| u.try_into().unwrap())
+                }
+            }
+        )*
+    }
+}
+
+from!(u8, u16, u32, u64, u128, bool);
+try_from!(u8, u16, u32, u64, u128);
+
+impl TryFrom<ScalarInt> for bool {
+    type Error = Size;
+    #[inline]
+    fn try_from(int: ScalarInt) -> Result<Self, Size> {
+        int.to_bits(Size::from_bytes(1)).and_then(|u| match u {
+            0 => Ok(false),
+            1 => Ok(true),
+            _ => Err(Size::from_bytes(1)),
+        })
+    }
+}
+
+impl From<char> for ScalarInt {
+    #[inline]
+    fn from(c: char) -> Self {
+        Self { data: c as u128, size: NonZeroU8::new(std::mem::size_of::<char>() as u8).unwrap() }
+    }
+}
+
+/// Error returned when a conversion from ScalarInt to char fails.
+#[derive(Debug)]
+pub struct CharTryFromScalarInt;
+
+impl TryFrom<ScalarInt> for char {
+    type Error = CharTryFromScalarInt;
+
+    #[inline]
+    fn try_from(int: ScalarInt) -> Result<Self, Self::Error> {
+        let Ok(bits) = int.to_bits(Size::from_bytes(std::mem::size_of::<char>())) else  {
+            return Err(CharTryFromScalarInt);
+        };
+        match char::from_u32(bits.try_into().unwrap()) {
+            Some(c) => Ok(c),
+            None => Err(CharTryFromScalarInt),
+        }
+    }
+}
+
+impl From<Single> for ScalarInt {
+    #[inline]
+    fn from(f: Single) -> Self {
+        // We trust apfloat to give us properly truncated data.
+        Self { data: f.to_bits(), size: NonZeroU8::new((Single::BITS / 8) as u8).unwrap() }
+    }
+}
+
+impl TryFrom<ScalarInt> for Single {
+    type Error = Size;
+    #[inline]
+    fn try_from(int: ScalarInt) -> Result<Self, Size> {
+        int.to_bits(Size::from_bytes(4)).map(Self::from_bits)
+    }
+}
+
+impl From<Double> for ScalarInt {
+    #[inline]
+    fn from(f: Double) -> Self {
+        // We trust apfloat to give us properly truncated data.
+        Self { data: f.to_bits(), size: NonZeroU8::new((Double::BITS / 8) as u8).unwrap() }
+    }
+}
+
+impl TryFrom<ScalarInt> for Double {
+    type Error = Size;
+    #[inline]
+    fn try_from(int: ScalarInt) -> Result<Self, Size> {
+        int.to_bits(Size::from_bytes(8)).map(Self::from_bits)
+    }
+}
+
+impl fmt::Debug for ScalarInt {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // Dispatch to LowerHex below.
+        write!(f, "0x{:x}", self)
+    }
+}
+
+impl fmt::LowerHex for ScalarInt {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.check_data();
+        if f.alternate() {
+            // Like regular ints, alternate flag adds leading `0x`.
+            write!(f, "0x")?;
+        }
+        // Format as hex number wide enough to fit any value of the given `size`.
+        // So data=20, size=1 will be "0x14", but with size=4 it'll be "0x00000014".
+        // Using a block `{self.data}` here to force a copy instead of using `self.data`
+        // directly, because `write!` takes references to its formatting arguments and
+        // would thus borrow `self.data`. Since `Self`
+        // is a packed struct, that would create a possibly unaligned reference, which
+        // is UB.
+        write!(f, "{:01$x}", { self.data }, self.size.get() as usize * 2)
+    }
+}
+
+impl fmt::UpperHex for ScalarInt {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.check_data();
+        // Format as hex number wide enough to fit any value of the given `size`.
+        // So data=20, size=1 will be "0x14", but with size=4 it'll be "0x00000014".
+        // Using a block `{self.data}` here to force a copy instead of using `self.data`
+        // directly, because `write!` takes references to its formatting arguments and
+        // would thus borrow `self.data`. Since `Self`
+        // is a packed struct, that would create a possibly unaligned reference, which
+        // is UB.
+        write!(f, "{:01$X}", { self.data }, self.size.get() as usize * 2)
+    }
+}
+
+impl fmt::Display for ScalarInt {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.check_data();
+        write!(f, "{}", { self.data })
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/consts/kind.rs b/compiler/rustc_middle/src/ty/consts/kind.rs
new file mode 100644
index 000000000..cb0137d2e
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/consts/kind.rs
@@ -0,0 +1,239 @@
+use std::convert::TryInto;
+
+use crate::mir::interpret::{AllocId, ConstValue, Scalar};
+use crate::mir::Promoted;
+use crate::ty::subst::{InternalSubsts, SubstsRef};
+use crate::ty::ParamEnv;
+use crate::ty::{self, TyCtxt, TypeVisitable};
+use rustc_errors::ErrorGuaranteed;
+use rustc_hir::def_id::DefId;
+use rustc_macros::HashStable;
+use rustc_target::abi::Size;
+
+use super::ScalarInt;
+/// An unevaluated, potentially generic, constant.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable, Lift)]
+#[derive(Hash, HashStable)]
+pub struct Unevaluated<'tcx, P = Option<Promoted>> {
+    pub def: ty::WithOptConstParam<DefId>,
+    pub substs: SubstsRef<'tcx>,
+    pub promoted: P,
+}
+
+impl<'tcx> Unevaluated<'tcx> {
+    #[inline]
+    pub fn shrink(self) -> Unevaluated<'tcx, ()> {
+        debug_assert_eq!(self.promoted, None);
+        Unevaluated { def: self.def, substs: self.substs, promoted: () }
+    }
+}
+
+impl<'tcx> Unevaluated<'tcx, ()> {
+    #[inline]
+    pub fn expand(self) -> Unevaluated<'tcx> {
+        Unevaluated { def: self.def, substs: self.substs, promoted: None }
+    }
+}
+
+impl<'tcx, P: Default> Unevaluated<'tcx, P> {
+    #[inline]
+    pub fn new(def: ty::WithOptConstParam<DefId>, substs: SubstsRef<'tcx>) -> Unevaluated<'tcx, P> {
+        Unevaluated { def, substs, promoted: Default::default() }
+    }
+}
+
+/// Represents a constant in Rust.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable)]
+#[derive(Hash, HashStable)]
+pub enum ConstKind<'tcx> {
+    /// A const generic parameter.
+    Param(ty::ParamConst),
+
+    /// Infer the value of the const.
+    Infer(InferConst<'tcx>),
+
+    /// Bound const variable, used only when preparing a trait query.
+    Bound(ty::DebruijnIndex, ty::BoundVar),
+
+    /// A placeholder const - universally quantified higher-ranked const.
+    Placeholder(ty::PlaceholderConst<'tcx>),
+
+    /// Used in the HIR by using `Unevaluated` everywhere and later normalizing to one of the other
+    /// variants when the code is monomorphic enough for that.
+    Unevaluated(Unevaluated<'tcx>),
+
+    /// Used to hold computed value.
+    Value(ty::ValTree<'tcx>),
+
+    /// A placeholder for a const which could not be computed; this is
+    /// propagated to avoid useless error messages.
+    Error(ty::DelaySpanBugEmitted),
+}
+
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(ConstKind<'_>, 40);
+
+impl<'tcx> ConstKind<'tcx> {
+    #[inline]
+    pub fn try_to_value(self) -> Option<ty::ValTree<'tcx>> {
+        if let ConstKind::Value(val) = self { Some(val) } else { None }
+    }
+
+    #[inline]
+    pub fn try_to_scalar(self) -> Option<Scalar<AllocId>> {
+        self.try_to_value()?.try_to_scalar()
+    }
+
+    #[inline]
+    pub fn try_to_scalar_int(self) -> Option<ScalarInt> {
+        self.try_to_value()?.try_to_scalar_int()
+    }
+
+    #[inline]
+    pub fn try_to_bits(self, size: Size) -> Option<u128> {
+        self.try_to_scalar_int()?.to_bits(size).ok()
+    }
+
+    #[inline]
+    pub fn try_to_bool(self) -> Option<bool> {
+        self.try_to_scalar_int()?.try_into().ok()
+    }
+
+    #[inline]
+    pub fn try_to_machine_usize(self, tcx: TyCtxt<'tcx>) -> Option<u64> {
+        self.try_to_value()?.try_to_machine_usize(tcx)
+    }
+}
+
+/// An inference variable for a const, for use in const generics.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable, Hash)]
+#[derive(HashStable)]
+pub enum InferConst<'tcx> {
+    /// Infer the value of the const.
+    Var(ty::ConstVid<'tcx>),
+    /// A fresh const variable. See `infer::freshen` for more details.
+    Fresh(u32),
+}
+
+enum EvalMode {
+    Typeck,
+    Mir,
+}
+
+enum EvalResult<'tcx> {
+    ValTree(ty::ValTree<'tcx>),
+    ConstVal(ConstValue<'tcx>),
+}
+
+impl<'tcx> ConstKind<'tcx> {
+    #[inline]
+    /// Tries to evaluate the constant if it is `Unevaluated`. If that doesn't succeed, return the
+    /// unevaluated constant.
+    pub fn eval(self, tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>) -> Self {
+        self.try_eval_for_typeck(tcx, param_env).and_then(Result::ok).map_or(self, ConstKind::Value)
+    }
+
+    #[inline]
+    /// Tries to evaluate the constant if it is `Unevaluated`. If that isn't possible or necessary
+    /// return `None`.
+    // FIXME(@lcnr): Completely rework the evaluation/normalization system for `ty::Const` once valtrees are merged.
+    pub fn try_eval_for_mir(
+        self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ParamEnv<'tcx>,
+    ) -> Option<Result<ConstValue<'tcx>, ErrorGuaranteed>> {
+        match self.try_eval_inner(tcx, param_env, EvalMode::Mir) {
+            Some(Ok(EvalResult::ValTree(_))) => unreachable!(),
+            Some(Ok(EvalResult::ConstVal(v))) => Some(Ok(v)),
+            Some(Err(e)) => Some(Err(e)),
+            None => None,
+        }
+    }
+
+    #[inline]
+    /// Tries to evaluate the constant if it is `Unevaluated`. If that isn't possible or necessary
+    /// return `None`.
+    // FIXME(@lcnr): Completely rework the evaluation/normalization system for `ty::Const` once valtrees are merged.
+    pub fn try_eval_for_typeck(
+        self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ParamEnv<'tcx>,
+    ) -> Option<Result<ty::ValTree<'tcx>, ErrorGuaranteed>> {
+        match self.try_eval_inner(tcx, param_env, EvalMode::Typeck) {
+            Some(Ok(EvalResult::ValTree(v))) => Some(Ok(v)),
+            Some(Ok(EvalResult::ConstVal(_))) => unreachable!(),
+            Some(Err(e)) => Some(Err(e)),
+            None => None,
+        }
+    }
+
+    #[inline]
+    fn try_eval_inner(
+        self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ParamEnv<'tcx>,
+        eval_mode: EvalMode,
+    ) -> Option<Result<EvalResult<'tcx>, ErrorGuaranteed>> {
+        if let ConstKind::Unevaluated(unevaluated) = self {
+            use crate::mir::interpret::ErrorHandled;
+
+            // HACK(eddyb) this erases lifetimes even though `const_eval_resolve`
+            // also does later, but we want to do it before checking for
+            // inference variables.
+            // Note that we erase regions *before* calling `with_reveal_all_normalized`,
+            // so that we don't try to invoke this query with
+            // any region variables.
+            let param_env_and = tcx
+                .erase_regions(param_env)
+                .with_reveal_all_normalized(tcx)
+                .and(tcx.erase_regions(unevaluated));
+
+            // HACK(eddyb) when the query key would contain inference variables,
+            // attempt using identity substs and `ParamEnv` instead, that will succeed
+            // when the expression doesn't depend on any parameters.
+            // FIXME(eddyb, skinny121) pass `InferCtxt` into here when it's available, so that
+            // we can call `infcx.const_eval_resolve` which handles inference variables.
+            let param_env_and = if param_env_and.needs_infer() {
+                tcx.param_env(unevaluated.def.did).and(ty::Unevaluated {
+                    def: unevaluated.def,
+                    substs: InternalSubsts::identity_for_item(tcx, unevaluated.def.did),
+                    promoted: unevaluated.promoted,
+                })
+            } else {
+                param_env_and
+            };
+
+            // FIXME(eddyb) maybe the `const_eval_*` methods should take
+            // `ty::ParamEnvAnd` instead of having them separate.
+            let (param_env, unevaluated) = param_env_and.into_parts();
+            // try to resolve e.g. associated constants to their definition on an impl, and then
+            // evaluate the const.
+            match eval_mode {
+                EvalMode::Typeck => {
+                    match tcx.const_eval_resolve_for_typeck(param_env, unevaluated, None) {
+                        // NOTE(eddyb) `val` contains no lifetimes/types/consts,
+                        // and we use the original type, so nothing from `substs`
+                        // (which may be identity substs, see above),
+                        // can leak through `val` into the const we return.
+                        Ok(val) => Some(Ok(EvalResult::ValTree(val?))),
+                        Err(ErrorHandled::TooGeneric | ErrorHandled::Linted) => None,
+                        Err(ErrorHandled::Reported(e)) => Some(Err(e)),
+                    }
+                }
+                EvalMode::Mir => {
+                    match tcx.const_eval_resolve(param_env, unevaluated, None) {
+                        // NOTE(eddyb) `val` contains no lifetimes/types/consts,
+                        // and we use the original type, so nothing from `substs`
+                        // (which may be identity substs, see above),
+                        // can leak through `val` into the const we return.
+                        Ok(val) => Some(Ok(EvalResult::ConstVal(val))),
+                        Err(ErrorHandled::TooGeneric | ErrorHandled::Linted) => None,
+                        Err(ErrorHandled::Reported(e)) => Some(Err(e)),
+                    }
+                }
+            }
+        } else {
+            None
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/consts/valtree.rs b/compiler/rustc_middle/src/ty/consts/valtree.rs
new file mode 100644
index 000000000..93707bb18
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/consts/valtree.rs
@@ -0,0 +1,104 @@
+use super::ScalarInt;
+use crate::mir::interpret::{AllocId, Scalar};
+use crate::ty::{self, Ty, TyCtxt};
+use rustc_macros::{HashStable, TyDecodable, TyEncodable};
+
+#[derive(Copy, Clone, Debug, Hash, TyEncodable, TyDecodable, Eq, PartialEq, Ord, PartialOrd)]
+#[derive(HashStable)]
+/// This datastructure is used to represent the value of constants used in the type system.
+///
+/// We explicitly choose a different datastructure from the way values are processed within
+/// CTFE, as in the type system equal values (according to their `PartialEq`) must also have
+/// equal representation (`==` on the rustc data structure, e.g. `ValTree`) and vice versa.
+/// Since CTFE uses `AllocId` to represent pointers, it often happens that two different
+/// `AllocId`s point to equal values. So we may end up with different representations for
+/// two constants whose value is `&42`. Furthermore any kind of struct that has padding will
+/// have arbitrary values within that padding, even if the values of the struct are the same.
+///
+/// `ValTree` does not have this problem with representation, as it only contains integers or
+/// lists of (nested) `ValTree`.
+pub enum ValTree<'tcx> {
+    /// ZSTs, integers, `bool`, `char` are represented as scalars.
+    /// See the `ScalarInt` documentation for how `ScalarInt` guarantees that equal values
+    /// of these types have the same representation.
+    Leaf(ScalarInt),
+
+    //SliceOrStr(ValSlice<'tcx>),
+    // dont use SliceOrStr for now
+    /// The fields of any kind of aggregate. Structs, tuples and arrays are represented by
+    /// listing their fields' values in order.
+    /// Enums are represented by storing their discriminant as a field, followed by all
+    /// the fields of the variant.
+    Branch(&'tcx [ValTree<'tcx>]),
+}
+
+impl<'tcx> ValTree<'tcx> {
+    pub fn zst() -> Self {
+        Self::Branch(&[])
+    }
+
+    #[inline]
+    pub fn unwrap_leaf(self) -> ScalarInt {
+        match self {
+            Self::Leaf(s) => s,
+            _ => bug!("expected leaf, got {:?}", self),
+        }
+    }
+
+    #[inline]
+    pub fn unwrap_branch(self) -> &'tcx [Self] {
+        match self {
+            Self::Branch(branch) => branch,
+            _ => bug!("expected branch, got {:?}", self),
+        }
+    }
+
+    pub fn from_raw_bytes<'a>(tcx: TyCtxt<'tcx>, bytes: &'a [u8]) -> Self {
+        let branches = bytes.iter().map(|b| Self::Leaf(ScalarInt::from(*b)));
+        let interned = tcx.arena.alloc_from_iter(branches);
+
+        Self::Branch(interned)
+    }
+
+    pub fn from_scalar_int(i: ScalarInt) -> Self {
+        Self::Leaf(i)
+    }
+
+    pub fn try_to_scalar(self) -> Option<Scalar<AllocId>> {
+        self.try_to_scalar_int().map(Scalar::Int)
+    }
+
+    pub fn try_to_scalar_int(self) -> Option<ScalarInt> {
+        match self {
+            Self::Leaf(s) => Some(s),
+            Self::Branch(_) => None,
+        }
+    }
+
+    pub fn try_to_machine_usize(self, tcx: TyCtxt<'tcx>) -> Option<u64> {
+        self.try_to_scalar_int().map(|s| s.try_to_machine_usize(tcx).ok()).flatten()
+    }
+
+    /// Get the values inside the ValTree as a slice of bytes. This only works for
+    /// constants with types &str, &[u8], or [u8; _].
+    pub fn try_to_raw_bytes(self, tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Option<&'tcx [u8]> {
+        match ty.kind() {
+            ty::Ref(_, inner_ty, _) => match inner_ty.kind() {
+                // `&str` can be interpreted as raw bytes
+                ty::Str => {}
+                // `&[u8]` can be interpreted as raw bytes
+                ty::Slice(slice_ty) if *slice_ty == tcx.types.u8 => {}
+                // other `&_` can't be interpreted as raw bytes
+                _ => return None,
+            },
+            // `[u8; N]` can be interpreted as raw bytes
+            ty::Array(array_ty, _) if *array_ty == tcx.types.u8 => {}
+            // Otherwise, type cannot be interpreted as raw bytes
+            _ => return None,
+        }
+
+        Some(tcx.arena.alloc_from_iter(
+            self.unwrap_branch().into_iter().map(|v| v.unwrap_leaf().try_to_u8().unwrap()),
+        ))
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
new file mode 100644
index 000000000..0a0f45ce1
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -0,0 +1,3018 @@
+//! Type context book-keeping.
+
+use crate::arena::Arena;
+use crate::dep_graph::{DepGraph, DepKind, DepKindStruct};
+use crate::hir::place::Place as HirPlace;
+use crate::infer::canonical::{Canonical, CanonicalVarInfo, CanonicalVarInfos};
+use crate::lint::{struct_lint_level, LintLevelSource};
+use crate::middle::codegen_fn_attrs::CodegenFnAttrs;
+use crate::middle::resolve_lifetime;
+use crate::middle::stability;
+use crate::mir::interpret::{self, Allocation, ConstAllocation};
+use crate::mir::{
+    Body, BorrowCheckResult, Field, Local, Place, PlaceElem, ProjectionKind, Promoted,
+};
+use crate::thir::Thir;
+use crate::traits;
+use crate::ty::query::{self, TyCtxtAt};
+use crate::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, Subst, SubstsRef, UserSubsts};
+use crate::ty::{
+    self, AdtDef, AdtDefData, AdtKind, Binder, BindingMode, BoundVar, CanonicalPolyFnSig,
+    ClosureSizeProfileData, Const, ConstS, ConstVid, DefIdTree, ExistentialPredicate, FloatTy,
+    FloatVar, FloatVid, GenericParamDefKind, InferConst, InferTy, IntTy, IntVar, IntVid, List,
+    ParamConst, ParamTy, PolyFnSig, Predicate, PredicateKind, PredicateS, ProjectionTy, Region,
+    RegionKind, ReprOptions, TraitObjectVisitor, Ty, TyKind, TyS, TyVar, TyVid, TypeAndMut, UintTy,
+};
+use rustc_ast as ast;
+use rustc_data_structures::fingerprint::Fingerprint;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::intern::{Interned, WithStableHash};
+use rustc_data_structures::memmap::Mmap;
+use rustc_data_structures::profiling::SelfProfilerRef;
+use rustc_data_structures::sharded::{IntoPointer, ShardedHashMap};
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::steal::Steal;
+use rustc_data_structures::sync::{self, Lock, Lrc, ReadGuard, RwLock, WorkerLocal};
+use rustc_data_structures::vec_map::VecMap;
+use rustc_errors::{DecorateLint, ErrorGuaranteed, LintDiagnosticBuilder, MultiSpan};
+use rustc_hir as hir;
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, LocalDefId, LOCAL_CRATE};
+use rustc_hir::definitions::Definitions;
+use rustc_hir::intravisit::Visitor;
+use rustc_hir::lang_items::LangItem;
+use rustc_hir::{
+    Constness, ExprKind, HirId, ImplItemKind, ItemKind, ItemLocalId, ItemLocalMap, ItemLocalSet,
+    Node, TraitCandidate, TraitItemKind,
+};
+use rustc_index::vec::{Idx, IndexVec};
+use rustc_macros::HashStable;
+use rustc_middle::mir::FakeReadCause;
+use rustc_query_system::ich::StableHashingContext;
+use rustc_serialize::opaque::{FileEncodeResult, FileEncoder};
+use rustc_session::config::{CrateType, OutputFilenames};
+use rustc_session::cstore::CrateStoreDyn;
+use rustc_session::lint::{Level, Lint};
+use rustc_session::Limit;
+use rustc_session::Session;
+use rustc_span::def_id::{DefPathHash, StableCrateId};
+use rustc_span::source_map::SourceMap;
+use rustc_span::symbol::{kw, sym, Ident, Symbol};
+use rustc_span::{Span, DUMMY_SP};
+use rustc_target::abi::{Layout, LayoutS, TargetDataLayout, VariantIdx};
+use rustc_target::spec::abi;
+use rustc_type_ir::sty::TyKind::*;
+use rustc_type_ir::{InternAs, InternIteratorElement, Interner, TypeFlags};
+
+use std::any::Any;
+use std::borrow::Borrow;
+use std::cmp::Ordering;
+use std::collections::hash_map::{self, Entry};
+use std::fmt;
+use std::hash::{Hash, Hasher};
+use std::iter;
+use std::mem;
+use std::ops::{Bound, Deref};
+use std::sync::Arc;
+
+use super::{ImplPolarity, RvalueScopes};
+
+pub trait OnDiskCache<'tcx>: rustc_data_structures::sync::Sync {
+    /// Creates a new `OnDiskCache` instance from the serialized data in `data`.
+    fn new(sess: &'tcx Session, data: Mmap, start_pos: usize) -> Self
+    where
+        Self: Sized;
+
+    fn new_empty(source_map: &'tcx SourceMap) -> Self
+    where
+        Self: Sized;
+
+    fn drop_serialized_data(&self, tcx: TyCtxt<'tcx>);
+
+    fn serialize(&self, tcx: TyCtxt<'tcx>, encoder: FileEncoder) -> FileEncodeResult;
+}
+
+#[allow(rustc::usage_of_ty_tykind)]
+impl<'tcx> Interner for TyCtxt<'tcx> {
+    type AdtDef = ty::AdtDef<'tcx>;
+    type SubstsRef = ty::SubstsRef<'tcx>;
+    type DefId = DefId;
+    type Ty = Ty<'tcx>;
+    type Const = ty::Const<'tcx>;
+    type Region = Region<'tcx>;
+    type TypeAndMut = TypeAndMut<'tcx>;
+    type Mutability = hir::Mutability;
+    type Movability = hir::Movability;
+    type PolyFnSig = PolyFnSig<'tcx>;
+    type ListBinderExistentialPredicate = &'tcx List<Binder<'tcx, ExistentialPredicate<'tcx>>>;
+    type BinderListTy = Binder<'tcx, &'tcx List<Ty<'tcx>>>;
+    type ListTy = &'tcx List<Ty<'tcx>>;
+    type ProjectionTy = ty::ProjectionTy<'tcx>;
+    type ParamTy = ParamTy;
+    type BoundTy = ty::BoundTy;
+    type PlaceholderType = ty::PlaceholderType;
+    type InferTy = InferTy;
+    type DelaySpanBugEmitted = DelaySpanBugEmitted;
+    type PredicateKind = ty::PredicateKind<'tcx>;
+    type AllocId = crate::mir::interpret::AllocId;
+
+    type EarlyBoundRegion = ty::EarlyBoundRegion;
+    type BoundRegion = ty::BoundRegion;
+    type FreeRegion = ty::FreeRegion;
+    type RegionVid = ty::RegionVid;
+    type PlaceholderRegion = ty::PlaceholderRegion;
+}
+
+/// A type that is not publicly constructable. This prevents people from making [`TyKind::Error`]s
+/// except through the error-reporting functions on a [`tcx`][TyCtxt].
+#[derive(Copy, Clone, Debug, Eq, Hash, PartialEq, PartialOrd, Ord)]
+#[derive(TyEncodable, TyDecodable, HashStable)]
+pub struct DelaySpanBugEmitted {
+    pub reported: ErrorGuaranteed,
+    _priv: (),
+}
+
+type InternedSet<'tcx, T> = ShardedHashMap<InternedInSet<'tcx, T>, ()>;
+
+pub struct CtxtInterners<'tcx> {
+    /// The arena that types, regions, etc. are allocated from.
+    arena: &'tcx WorkerLocal<Arena<'tcx>>,
+
+    // Specifically use a speedy hash algorithm for these hash sets, since
+    // they're accessed quite often.
+    type_: InternedSet<'tcx, WithStableHash<TyS<'tcx>>>,
+    substs: InternedSet<'tcx, InternalSubsts<'tcx>>,
+    canonical_var_infos: InternedSet<'tcx, List<CanonicalVarInfo<'tcx>>>,
+    region: InternedSet<'tcx, RegionKind<'tcx>>,
+    poly_existential_predicates:
+        InternedSet<'tcx, List<ty::Binder<'tcx, ExistentialPredicate<'tcx>>>>,
+    predicate: InternedSet<'tcx, PredicateS<'tcx>>,
+    predicates: InternedSet<'tcx, List<Predicate<'tcx>>>,
+    projs: InternedSet<'tcx, List<ProjectionKind>>,
+    place_elems: InternedSet<'tcx, List<PlaceElem<'tcx>>>,
+    const_: InternedSet<'tcx, ConstS<'tcx>>,
+    const_allocation: InternedSet<'tcx, Allocation>,
+    bound_variable_kinds: InternedSet<'tcx, List<ty::BoundVariableKind>>,
+    layout: InternedSet<'tcx, LayoutS<'tcx>>,
+    adt_def: InternedSet<'tcx, AdtDefData>,
+}
+
+impl<'tcx> CtxtInterners<'tcx> {
+    fn new(arena: &'tcx WorkerLocal<Arena<'tcx>>) -> CtxtInterners<'tcx> {
+        CtxtInterners {
+            arena,
+            type_: Default::default(),
+            substs: Default::default(),
+            region: Default::default(),
+            poly_existential_predicates: Default::default(),
+            canonical_var_infos: Default::default(),
+            predicate: Default::default(),
+            predicates: Default::default(),
+            projs: Default::default(),
+            place_elems: Default::default(),
+            const_: Default::default(),
+            const_allocation: Default::default(),
+            bound_variable_kinds: Default::default(),
+            layout: Default::default(),
+            adt_def: Default::default(),
+        }
+    }
+
+    /// Interns a type.
+    #[allow(rustc::usage_of_ty_tykind)]
+    #[inline(never)]
+    fn intern_ty(
+        &self,
+        kind: TyKind<'tcx>,
+        sess: &Session,
+        definitions: &rustc_hir::definitions::Definitions,
+        cstore: &CrateStoreDyn,
+        source_span: &IndexVec<LocalDefId, Span>,
+    ) -> Ty<'tcx> {
+        Ty(Interned::new_unchecked(
+            self.type_
+                .intern(kind, |kind| {
+                    let flags = super::flags::FlagComputation::for_kind(&kind);
+
+                    // It's impossible to hash inference regions (and will ICE), so we don't need to try to cache them.
+                    // Without incremental, we rarely stable-hash types, so let's not do it proactively.
+                    let stable_hash = if flags.flags.intersects(TypeFlags::HAS_RE_INFER)
+                        || sess.opts.incremental.is_none()
+                    {
+                        Fingerprint::ZERO
+                    } else {
+                        let mut hasher = StableHasher::new();
+                        let mut hcx = StableHashingContext::ignore_spans(
+                            sess,
+                            definitions,
+                            cstore,
+                            source_span,
+                        );
+                        kind.hash_stable(&mut hcx, &mut hasher);
+                        hasher.finish()
+                    };
+
+                    let ty_struct = TyS {
+                        kind,
+                        flags: flags.flags,
+                        outer_exclusive_binder: flags.outer_exclusive_binder,
+                    };
+
+                    InternedInSet(
+                        self.arena.alloc(WithStableHash { internee: ty_struct, stable_hash }),
+                    )
+                })
+                .0,
+        ))
+    }
+
+    #[inline(never)]
+    fn intern_predicate(&self, kind: Binder<'tcx, PredicateKind<'tcx>>) -> Predicate<'tcx> {
+        Predicate(Interned::new_unchecked(
+            self.predicate
+                .intern(kind, |kind| {
+                    let flags = super::flags::FlagComputation::for_predicate(kind);
+
+                    let predicate_struct = PredicateS {
+                        kind,
+                        flags: flags.flags,
+                        outer_exclusive_binder: flags.outer_exclusive_binder,
+                    };
+
+                    InternedInSet(self.arena.alloc(predicate_struct))
+                })
+                .0,
+        ))
+    }
+}
+
+pub struct CommonTypes<'tcx> {
+    pub unit: Ty<'tcx>,
+    pub bool: Ty<'tcx>,
+    pub char: Ty<'tcx>,
+    pub isize: Ty<'tcx>,
+    pub i8: Ty<'tcx>,
+    pub i16: Ty<'tcx>,
+    pub i32: Ty<'tcx>,
+    pub i64: Ty<'tcx>,
+    pub i128: Ty<'tcx>,
+    pub usize: Ty<'tcx>,
+    pub u8: Ty<'tcx>,
+    pub u16: Ty<'tcx>,
+    pub u32: Ty<'tcx>,
+    pub u64: Ty<'tcx>,
+    pub u128: Ty<'tcx>,
+    pub f32: Ty<'tcx>,
+    pub f64: Ty<'tcx>,
+    pub str_: Ty<'tcx>,
+    pub never: Ty<'tcx>,
+    pub self_param: Ty<'tcx>,
+
+    /// Dummy type used for the `Self` of a `TraitRef` created for converting
+    /// a trait object, and which gets removed in `ExistentialTraitRef`.
+    /// This type must not appear anywhere in other converted types.
+    pub trait_object_dummy_self: Ty<'tcx>,
+}
+
+pub struct CommonLifetimes<'tcx> {
+    /// `ReEmpty` in the root universe.
+    pub re_root_empty: Region<'tcx>,
+
+    /// `ReStatic`
+    pub re_static: Region<'tcx>,
+
+    /// Erased region, used outside of type inference.
+    pub re_erased: Region<'tcx>,
+}
+
+pub struct CommonConsts<'tcx> {
+    pub unit: Const<'tcx>,
+}
+
+pub struct LocalTableInContext<'a, V> {
+    hir_owner: LocalDefId,
+    data: &'a ItemLocalMap<V>,
+}
+
+/// Validate that the given HirId (respectively its `local_id` part) can be
+/// safely used as a key in the maps of a TypeckResults. For that to be
+/// the case, the HirId must have the same `owner` as all the other IDs in
+/// this table (signified by `hir_owner`). Otherwise the HirId
+/// would be in a different frame of reference and using its `local_id`
+/// would result in lookup errors, or worse, in silently wrong data being
+/// stored/returned.
+#[inline]
+fn validate_hir_id_for_typeck_results(hir_owner: LocalDefId, hir_id: hir::HirId) {
+    if hir_id.owner != hir_owner {
+        invalid_hir_id_for_typeck_results(hir_owner, hir_id);
+    }
+}
+
+#[cold]
+#[inline(never)]
+fn invalid_hir_id_for_typeck_results(hir_owner: LocalDefId, hir_id: hir::HirId) {
+    ty::tls::with(|tcx| {
+        bug!(
+            "node {} with HirId::owner {:?} cannot be placed in TypeckResults with hir_owner {:?}",
+            tcx.hir().node_to_string(hir_id),
+            hir_id.owner,
+            hir_owner
+        )
+    });
+}
+
+impl<'a, V> LocalTableInContext<'a, V> {
+    pub fn contains_key(&self, id: hir::HirId) -> bool {
+        validate_hir_id_for_typeck_results(self.hir_owner, id);
+        self.data.contains_key(&id.local_id)
+    }
+
+    pub fn get(&self, id: hir::HirId) -> Option<&V> {
+        validate_hir_id_for_typeck_results(self.hir_owner, id);
+        self.data.get(&id.local_id)
+    }
+
+    pub fn iter(&self) -> hash_map::Iter<'_, hir::ItemLocalId, V> {
+        self.data.iter()
+    }
+}
+
+impl<'a, V> ::std::ops::Index<hir::HirId> for LocalTableInContext<'a, V> {
+    type Output = V;
+
+    fn index(&self, key: hir::HirId) -> &V {
+        self.get(key).expect("LocalTableInContext: key not found")
+    }
+}
+
+pub struct LocalTableInContextMut<'a, V> {
+    hir_owner: LocalDefId,
+    data: &'a mut ItemLocalMap<V>,
+}
+
+impl<'a, V> LocalTableInContextMut<'a, V> {
+    pub fn get_mut(&mut self, id: hir::HirId) -> Option<&mut V> {
+        validate_hir_id_for_typeck_results(self.hir_owner, id);
+        self.data.get_mut(&id.local_id)
+    }
+
+    pub fn entry(&mut self, id: hir::HirId) -> Entry<'_, hir::ItemLocalId, V> {
+        validate_hir_id_for_typeck_results(self.hir_owner, id);
+        self.data.entry(id.local_id)
+    }
+
+    pub fn insert(&mut self, id: hir::HirId, val: V) -> Option<V> {
+        validate_hir_id_for_typeck_results(self.hir_owner, id);
+        self.data.insert(id.local_id, val)
+    }
+
+    pub fn remove(&mut self, id: hir::HirId) -> Option<V> {
+        validate_hir_id_for_typeck_results(self.hir_owner, id);
+        self.data.remove(&id.local_id)
+    }
+}
+
+/// Whenever a value may be live across a generator yield, the type of that value winds up in the
+/// `GeneratorInteriorTypeCause` struct. This struct adds additional information about such
+/// captured types that can be useful for diagnostics. In particular, it stores the span that
+/// caused a given type to be recorded, along with the scope that enclosed the value (which can
+/// be used to find the await that the value is live across).
+///
+/// For example:
+///
+/// ```ignore (pseudo-Rust)
+/// async move {
+///     let x: T = expr;
+///     foo.await
+///     ...
+/// }
+/// ```
+///
+/// Here, we would store the type `T`, the span of the value `x`, the "scope-span" for
+/// the scope that contains `x`, the expr `T` evaluated from, and the span of `foo.await`.
+#[derive(TyEncodable, TyDecodable, Clone, Debug, Eq, Hash, PartialEq, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct GeneratorInteriorTypeCause<'tcx> {
+    /// Type of the captured binding.
+    pub ty: Ty<'tcx>,
+    /// Span of the binding that was captured.
+    pub span: Span,
+    /// Span of the scope of the captured binding.
+    pub scope_span: Option<Span>,
+    /// Span of `.await` or `yield` expression.
+    pub yield_span: Span,
+    /// Expr which the type evaluated from.
+    pub expr: Option<hir::HirId>,
+}
+
+// This type holds diagnostic information on generators and async functions across crate boundaries
+// and is used to provide better error messages
+#[derive(TyEncodable, TyDecodable, Clone, Debug, HashStable)]
+pub struct GeneratorDiagnosticData<'tcx> {
+    pub generator_interior_types: ty::Binder<'tcx, Vec<GeneratorInteriorTypeCause<'tcx>>>,
+    pub hir_owner: DefId,
+    pub nodes_types: ItemLocalMap<Ty<'tcx>>,
+    pub adjustments: ItemLocalMap<Vec<ty::adjustment::Adjustment<'tcx>>>,
+}
+
+#[derive(TyEncodable, TyDecodable, Debug, HashStable)]
+pub struct TypeckResults<'tcx> {
+    /// The `HirId::owner` all `ItemLocalId`s in this table are relative to.
+    pub hir_owner: LocalDefId,
+
+    /// Resolved definitions for `<T>::X` associated paths and
+    /// method calls, including those of overloaded operators.
+    type_dependent_defs: ItemLocalMap<Result<(DefKind, DefId), ErrorGuaranteed>>,
+
+    /// Resolved field indices for field accesses in expressions (`S { field }`, `obj.field`)
+    /// or patterns (`S { field }`). The index is often useful by itself, but to learn more
+    /// about the field you also need definition of the variant to which the field
+    /// belongs, but it may not exist if it's a tuple field (`tuple.0`).
+    field_indices: ItemLocalMap<usize>,
+
+    /// Stores the types for various nodes in the AST. Note that this table
+    /// is not guaranteed to be populated outside inference. See
+    /// typeck::check::fn_ctxt for details.
+    node_types: ItemLocalMap<Ty<'tcx>>,
+
+    /// Stores the type parameters which were substituted to obtain the type
+    /// of this node. This only applies to nodes that refer to entities
+    /// parameterized by type parameters, such as generic fns, types, or
+    /// other items.
+    node_substs: ItemLocalMap<SubstsRef<'tcx>>,
+
+    /// This will either store the canonicalized types provided by the user
+    /// or the substitutions that the user explicitly gave (if any) attached
+    /// to `id`. These will not include any inferred values. The canonical form
+    /// is used to capture things like `_` or other unspecified values.
+    ///
+    /// For example, if the user wrote `foo.collect::<Vec<_>>()`, then the
+    /// canonical substitutions would include only `for<X> { Vec<X> }`.
+    ///
+    /// See also `AscribeUserType` statement in MIR.
+    user_provided_types: ItemLocalMap<CanonicalUserType<'tcx>>,
+
+    /// Stores the canonicalized types provided by the user. See also
+    /// `AscribeUserType` statement in MIR.
+    pub user_provided_sigs: DefIdMap<CanonicalPolyFnSig<'tcx>>,
+
+    adjustments: ItemLocalMap<Vec<ty::adjustment::Adjustment<'tcx>>>,
+
+    /// Stores the actual binding mode for all instances of hir::BindingAnnotation.
+    pat_binding_modes: ItemLocalMap<BindingMode>,
+
+    /// Stores the types which were implicitly dereferenced in pattern binding modes
+    /// for later usage in THIR lowering. For example,
+    ///
+    /// ```
+    /// match &&Some(5i32) {
+    ///     Some(n) => {},
+    ///     _ => {},
+    /// }
+    /// ```
+    /// leads to a `vec![&&Option<i32>, &Option<i32>]`. Empty vectors are not stored.
+    ///
+    /// See:
+    /// <https://github.com/rust-lang/rfcs/blob/master/text/2005-match-ergonomics.md#definitions>
+    pat_adjustments: ItemLocalMap<Vec<Ty<'tcx>>>,
+
+    /// Records the reasons that we picked the kind of each closure;
+    /// not all closures are present in the map.
+    closure_kind_origins: ItemLocalMap<(Span, HirPlace<'tcx>)>,
+
+    /// For each fn, records the "liberated" types of its arguments
+    /// and return type. Liberated means that all bound regions
+    /// (including late-bound regions) are replaced with free
+    /// equivalents. This table is not used in codegen (since regions
+    /// are erased there) and hence is not serialized to metadata.
+    ///
+    /// This table also contains the "revealed" values for any `impl Trait`
+    /// that appear in the signature and whose values are being inferred
+    /// by this function.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # use std::fmt::Debug;
+    /// fn foo(x: &u32) -> impl Debug { *x }
+    /// ```
+    ///
+    /// The function signature here would be:
+    ///
+    /// ```ignore (illustrative)
+    /// for<'a> fn(&'a u32) -> Foo
+    /// ```
+    ///
+    /// where `Foo` is an opaque type created for this function.
+    ///
+    ///
+    /// The *liberated* form of this would be
+    ///
+    /// ```ignore (illustrative)
+    /// fn(&'a u32) -> u32
+    /// ```
+    ///
+    /// Note that `'a` is not bound (it would be an `ReFree`) and
+    /// that the `Foo` opaque type is replaced by its hidden type.
+    liberated_fn_sigs: ItemLocalMap<ty::FnSig<'tcx>>,
+
+    /// For each FRU expression, record the normalized types of the fields
+    /// of the struct - this is needed because it is non-trivial to
+    /// normalize while preserving regions. This table is used only in
+    /// MIR construction and hence is not serialized to metadata.
+    fru_field_types: ItemLocalMap<Vec<Ty<'tcx>>>,
+
+    /// For every coercion cast we add the HIR node ID of the cast
+    /// expression to this set.
+    coercion_casts: ItemLocalSet,
+
+    /// Set of trait imports actually used in the method resolution.
+    /// This is used for warning unused imports. During type
+    /// checking, this `Lrc` should not be cloned: it must have a ref-count
+    /// of 1 so that we can insert things into the set mutably.
+    pub used_trait_imports: Lrc<FxHashSet<LocalDefId>>,
+
+    /// If any errors occurred while type-checking this body,
+    /// this field will be set to `Some(ErrorGuaranteed)`.
+    pub tainted_by_errors: Option<ErrorGuaranteed>,
+
+    /// All the opaque types that have hidden types set
+    /// by this function. For return-position-impl-trait we also store the
+    /// type here, so that mir-borrowck can figure out hidden types,
+    /// even if they are only set in dead code (which doesn't show up in MIR).
+    /// For type-alias-impl-trait, this map is only used to prevent query cycles,
+    /// so the hidden types are all `None`.
+    pub concrete_opaque_types: VecMap<LocalDefId, Option<Ty<'tcx>>>,
+
+    /// Tracks the minimum captures required for a closure;
+    /// see `MinCaptureInformationMap` for more details.
+    pub closure_min_captures: ty::MinCaptureInformationMap<'tcx>,
+
+    /// Tracks the fake reads required for a closure and the reason for the fake read.
+    /// When performing pattern matching for closures, there are times we don't end up
+    /// reading places that are mentioned in a closure (because of _ patterns). However,
+    /// to ensure the places are initialized, we introduce fake reads.
+    /// Consider these two examples:
+    /// ``` (discriminant matching with only wildcard arm)
+    /// let x: u8;
+    /// let c = || match x { _ => () };
+    /// ```
+    /// In this example, we don't need to actually read/borrow `x` in `c`, and so we don't
+    /// want to capture it. However, we do still want an error here, because `x` should have
+    /// to be initialized at the point where c is created. Therefore, we add a "fake read"
+    /// instead.
+    /// ``` (destructured assignments)
+    /// let c = || {
+    ///     let (t1, t2) = t;
+    /// }
+    /// ```
+    /// In the second example, we capture the disjoint fields of `t` (`t.0` & `t.1`), but
+    /// we never capture `t`. This becomes an issue when we build MIR as we require
+    /// information on `t` in order to create place `t.0` and `t.1`. We can solve this
+    /// issue by fake reading `t`.
+    pub closure_fake_reads: FxHashMap<LocalDefId, Vec<(HirPlace<'tcx>, FakeReadCause, hir::HirId)>>,
+
+    /// Tracks the rvalue scoping rules which defines finer scoping for rvalue expressions
+    /// by applying extended parameter rules.
+    /// Details may be find in `rustc_typeck::check::rvalue_scopes`.
+    pub rvalue_scopes: RvalueScopes,
+
+    /// Stores the type, expression, span and optional scope span of all types
+    /// that are live across the yield of this generator (if a generator).
+    pub generator_interior_types: ty::Binder<'tcx, Vec<GeneratorInteriorTypeCause<'tcx>>>,
+
+    /// We sometimes treat byte string literals (which are of type `&[u8; N]`)
+    /// as `&[u8]`, depending on the pattern  in which they are used.
+    /// This hashset records all instances where we behave
+    /// like this to allow `const_to_pat` to reliably handle this situation.
+    pub treat_byte_string_as_slice: ItemLocalSet,
+
+    /// Contains the data for evaluating the effect of feature `capture_disjoint_fields`
+    /// on closure size.
+    pub closure_size_eval: FxHashMap<LocalDefId, ClosureSizeProfileData<'tcx>>,
+}
+
+impl<'tcx> TypeckResults<'tcx> {
+    pub fn new(hir_owner: LocalDefId) -> TypeckResults<'tcx> {
+        TypeckResults {
+            hir_owner,
+            type_dependent_defs: Default::default(),
+            field_indices: Default::default(),
+            user_provided_types: Default::default(),
+            user_provided_sigs: Default::default(),
+            node_types: Default::default(),
+            node_substs: Default::default(),
+            adjustments: Default::default(),
+            pat_binding_modes: Default::default(),
+            pat_adjustments: Default::default(),
+            closure_kind_origins: Default::default(),
+            liberated_fn_sigs: Default::default(),
+            fru_field_types: Default::default(),
+            coercion_casts: Default::default(),
+            used_trait_imports: Lrc::new(Default::default()),
+            tainted_by_errors: None,
+            concrete_opaque_types: Default::default(),
+            closure_min_captures: Default::default(),
+            closure_fake_reads: Default::default(),
+            rvalue_scopes: Default::default(),
+            generator_interior_types: ty::Binder::dummy(Default::default()),
+            treat_byte_string_as_slice: Default::default(),
+            closure_size_eval: Default::default(),
+        }
+    }
+
+    /// Returns the final resolution of a `QPath` in an `Expr` or `Pat` node.
+    pub fn qpath_res(&self, qpath: &hir::QPath<'_>, id: hir::HirId) -> Res {
+        match *qpath {
+            hir::QPath::Resolved(_, ref path) => path.res,
+            hir::QPath::TypeRelative(..) | hir::QPath::LangItem(..) => self
+                .type_dependent_def(id)
+                .map_or(Res::Err, |(kind, def_id)| Res::Def(kind, def_id)),
+        }
+    }
+
+    pub fn type_dependent_defs(
+        &self,
+    ) -> LocalTableInContext<'_, Result<(DefKind, DefId), ErrorGuaranteed>> {
+        LocalTableInContext { hir_owner: self.hir_owner, data: &self.type_dependent_defs }
+    }
+
+    pub fn type_dependent_def(&self, id: HirId) -> Option<(DefKind, DefId)> {
+        validate_hir_id_for_typeck_results(self.hir_owner, id);
+        self.type_dependent_defs.get(&id.local_id).cloned().and_then(|r| r.ok())
+    }
+
+    pub fn type_dependent_def_id(&self, id: HirId) -> Option<DefId> {
+        self.type_dependent_def(id).map(|(_, def_id)| def_id)
+    }
+
+    pub fn type_dependent_defs_mut(
+        &mut self,
+    ) -> LocalTableInContextMut<'_, Result<(DefKind, DefId), ErrorGuaranteed>> {
+        LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.type_dependent_defs }
+    }
+
+    pub fn field_indices(&self) -> LocalTableInContext<'_, usize> {
+        LocalTableInContext { hir_owner: self.hir_owner, data: &self.field_indices }
+    }
+
+    pub fn field_indices_mut(&mut self) -> LocalTableInContextMut<'_, usize> {
+        LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.field_indices }
+    }
+
+    pub fn user_provided_types(&self) -> LocalTableInContext<'_, CanonicalUserType<'tcx>> {
+        LocalTableInContext { hir_owner: self.hir_owner, data: &self.user_provided_types }
+    }
+
+    pub fn user_provided_types_mut(
+        &mut self,
+    ) -> LocalTableInContextMut<'_, CanonicalUserType<'tcx>> {
+        LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.user_provided_types }
+    }
+
+    pub fn node_types(&self) -> LocalTableInContext<'_, Ty<'tcx>> {
+        LocalTableInContext { hir_owner: self.hir_owner, data: &self.node_types }
+    }
+
+    pub fn node_types_mut(&mut self) -> LocalTableInContextMut<'_, Ty<'tcx>> {
+        LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.node_types }
+    }
+
+    pub fn get_generator_diagnostic_data(&self) -> GeneratorDiagnosticData<'tcx> {
+        let generator_interior_type = self.generator_interior_types.map_bound_ref(|vec| {
+            vec.iter()
+                .map(|item| {
+                    GeneratorInteriorTypeCause {
+                        ty: item.ty,
+                        span: item.span,
+                        scope_span: item.scope_span,
+                        yield_span: item.yield_span,
+                        expr: None, //FIXME: Passing expression over crate boundaries is impossible at the moment
+                    }
+                })
+                .collect::<Vec<_>>()
+        });
+        GeneratorDiagnosticData {
+            generator_interior_types: generator_interior_type,
+            hir_owner: self.hir_owner.to_def_id(),
+            nodes_types: self.node_types.clone(),
+            adjustments: self.adjustments.clone(),
+        }
+    }
+
+    pub fn node_type(&self, id: hir::HirId) -> Ty<'tcx> {
+        self.node_type_opt(id).unwrap_or_else(|| {
+            bug!("node_type: no type for node `{}`", tls::with(|tcx| tcx.hir().node_to_string(id)))
+        })
+    }
+
+    pub fn node_type_opt(&self, id: hir::HirId) -> Option<Ty<'tcx>> {
+        validate_hir_id_for_typeck_results(self.hir_owner, id);
+        self.node_types.get(&id.local_id).cloned()
+    }
+
+    pub fn node_substs_mut(&mut self) -> LocalTableInContextMut<'_, SubstsRef<'tcx>> {
+        LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.node_substs }
+    }
+
+    pub fn node_substs(&self, id: hir::HirId) -> SubstsRef<'tcx> {
+        validate_hir_id_for_typeck_results(self.hir_owner, id);
+        self.node_substs.get(&id.local_id).cloned().unwrap_or_else(|| InternalSubsts::empty())
+    }
+
+    pub fn node_substs_opt(&self, id: hir::HirId) -> Option<SubstsRef<'tcx>> {
+        validate_hir_id_for_typeck_results(self.hir_owner, id);
+        self.node_substs.get(&id.local_id).cloned()
+    }
+
+    // Returns the type of a pattern as a monotype. Like @expr_ty, this function
+    // doesn't provide type parameter substitutions.
+    pub fn pat_ty(&self, pat: &hir::Pat<'_>) -> Ty<'tcx> {
+        self.node_type(pat.hir_id)
+    }
+
+    // Returns the type of an expression as a monotype.
+    //
+    // NB (1): This is the PRE-ADJUSTMENT TYPE for the expression.  That is, in
+    // some cases, we insert `Adjustment` annotations such as auto-deref or
+    // auto-ref.  The type returned by this function does not consider such
+    // adjustments.  See `expr_ty_adjusted()` instead.
+    //
+    // NB (2): This type doesn't provide type parameter substitutions; e.g., if you
+    // ask for the type of "id" in "id(3)", it will return "fn(&isize) -> isize"
+    // instead of "fn(ty) -> T with T = isize".
+    pub fn expr_ty(&self, expr: &hir::Expr<'_>) -> Ty<'tcx> {
+        self.node_type(expr.hir_id)
+    }
+
+    pub fn expr_ty_opt(&self, expr: &hir::Expr<'_>) -> Option<Ty<'tcx>> {
+        self.node_type_opt(expr.hir_id)
+    }
+
+    pub fn adjustments(&self) -> LocalTableInContext<'_, Vec<ty::adjustment::Adjustment<'tcx>>> {
+        LocalTableInContext { hir_owner: self.hir_owner, data: &self.adjustments }
+    }
+
+    pub fn adjustments_mut(
+        &mut self,
+    ) -> LocalTableInContextMut<'_, Vec<ty::adjustment::Adjustment<'tcx>>> {
+        LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.adjustments }
+    }
+
+    pub fn expr_adjustments(&self, expr: &hir::Expr<'_>) -> &[ty::adjustment::Adjustment<'tcx>] {
+        validate_hir_id_for_typeck_results(self.hir_owner, expr.hir_id);
+        self.adjustments.get(&expr.hir_id.local_id).map_or(&[], |a| &a[..])
+    }
+
+    /// Returns the type of `expr`, considering any `Adjustment`
+    /// entry recorded for that expression.
+    pub fn expr_ty_adjusted(&self, expr: &hir::Expr<'_>) -> Ty<'tcx> {
+        self.expr_adjustments(expr).last().map_or_else(|| self.expr_ty(expr), |adj| adj.target)
+    }
+
+    pub fn expr_ty_adjusted_opt(&self, expr: &hir::Expr<'_>) -> Option<Ty<'tcx>> {
+        self.expr_adjustments(expr).last().map(|adj| adj.target).or_else(|| self.expr_ty_opt(expr))
+    }
+
+    pub fn is_method_call(&self, expr: &hir::Expr<'_>) -> bool {
+        // Only paths and method calls/overloaded operators have
+        // entries in type_dependent_defs, ignore the former here.
+        if let hir::ExprKind::Path(_) = expr.kind {
+            return false;
+        }
+
+        matches!(self.type_dependent_defs().get(expr.hir_id), Some(Ok((DefKind::AssocFn, _))))
+    }
+
+    pub fn extract_binding_mode(&self, s: &Session, id: HirId, sp: Span) -> Option<BindingMode> {
+        self.pat_binding_modes().get(id).copied().or_else(|| {
+            s.delay_span_bug(sp, "missing binding mode");
+            None
+        })
+    }
+
+    pub fn pat_binding_modes(&self) -> LocalTableInContext<'_, BindingMode> {
+        LocalTableInContext { hir_owner: self.hir_owner, data: &self.pat_binding_modes }
+    }
+
+    pub fn pat_binding_modes_mut(&mut self) -> LocalTableInContextMut<'_, BindingMode> {
+        LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.pat_binding_modes }
+    }
+
+    pub fn pat_adjustments(&self) -> LocalTableInContext<'_, Vec<Ty<'tcx>>> {
+        LocalTableInContext { hir_owner: self.hir_owner, data: &self.pat_adjustments }
+    }
+
+    pub fn pat_adjustments_mut(&mut self) -> LocalTableInContextMut<'_, Vec<Ty<'tcx>>> {
+        LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.pat_adjustments }
+    }
+
+    /// For a given closure, returns the iterator of `ty::CapturedPlace`s that are captured
+    /// by the closure.
+    pub fn closure_min_captures_flattened(
+        &self,
+        closure_def_id: LocalDefId,
+    ) -> impl Iterator<Item = &ty::CapturedPlace<'tcx>> {
+        self.closure_min_captures
+            .get(&closure_def_id)
+            .map(|closure_min_captures| closure_min_captures.values().flat_map(|v| v.iter()))
+            .into_iter()
+            .flatten()
+    }
+
+    pub fn closure_kind_origins(&self) -> LocalTableInContext<'_, (Span, HirPlace<'tcx>)> {
+        LocalTableInContext { hir_owner: self.hir_owner, data: &self.closure_kind_origins }
+    }
+
+    pub fn closure_kind_origins_mut(
+        &mut self,
+    ) -> LocalTableInContextMut<'_, (Span, HirPlace<'tcx>)> {
+        LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.closure_kind_origins }
+    }
+
+    pub fn liberated_fn_sigs(&self) -> LocalTableInContext<'_, ty::FnSig<'tcx>> {
+        LocalTableInContext { hir_owner: self.hir_owner, data: &self.liberated_fn_sigs }
+    }
+
+    pub fn liberated_fn_sigs_mut(&mut self) -> LocalTableInContextMut<'_, ty::FnSig<'tcx>> {
+        LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.liberated_fn_sigs }
+    }
+
+    pub fn fru_field_types(&self) -> LocalTableInContext<'_, Vec<Ty<'tcx>>> {
+        LocalTableInContext { hir_owner: self.hir_owner, data: &self.fru_field_types }
+    }
+
+    pub fn fru_field_types_mut(&mut self) -> LocalTableInContextMut<'_, Vec<Ty<'tcx>>> {
+        LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.fru_field_types }
+    }
+
+    pub fn is_coercion_cast(&self, hir_id: hir::HirId) -> bool {
+        validate_hir_id_for_typeck_results(self.hir_owner, hir_id);
+        self.coercion_casts.contains(&hir_id.local_id)
+    }
+
+    pub fn set_coercion_cast(&mut self, id: ItemLocalId) {
+        self.coercion_casts.insert(id);
+    }
+
+    pub fn coercion_casts(&self) -> &ItemLocalSet {
+        &self.coercion_casts
+    }
+}
+
+rustc_index::newtype_index! {
+    pub struct UserTypeAnnotationIndex {
+        derive [HashStable]
+        DEBUG_FORMAT = "UserType({})",
+        const START_INDEX = 0,
+    }
+}
+
+/// Mapping of type annotation indices to canonical user type annotations.
+pub type CanonicalUserTypeAnnotations<'tcx> =
+    IndexVec<UserTypeAnnotationIndex, CanonicalUserTypeAnnotation<'tcx>>;
+
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub struct CanonicalUserTypeAnnotation<'tcx> {
+    pub user_ty: CanonicalUserType<'tcx>,
+    pub span: Span,
+    pub inferred_ty: Ty<'tcx>,
+}
+
+/// Canonicalized user type annotation.
+pub type CanonicalUserType<'tcx> = Canonical<'tcx, UserType<'tcx>>;
+
+impl<'tcx> CanonicalUserType<'tcx> {
+    /// Returns `true` if this represents a substitution of the form `[?0, ?1, ?2]`,
+    /// i.e., each thing is mapped to a canonical variable with the same index.
+    pub fn is_identity(&self) -> bool {
+        match self.value {
+            UserType::Ty(_) => false,
+            UserType::TypeOf(_, user_substs) => {
+                if user_substs.user_self_ty.is_some() {
+                    return false;
+                }
+
+                iter::zip(user_substs.substs, BoundVar::new(0)..).all(|(kind, cvar)| {
+                    match kind.unpack() {
+                        GenericArgKind::Type(ty) => match ty.kind() {
+                            ty::Bound(debruijn, b) => {
+                                // We only allow a `ty::INNERMOST` index in substitutions.
+                                assert_eq!(*debruijn, ty::INNERMOST);
+                                cvar == b.var
+                            }
+                            _ => false,
+                        },
+
+                        GenericArgKind::Lifetime(r) => match *r {
+                            ty::ReLateBound(debruijn, br) => {
+                                // We only allow a `ty::INNERMOST` index in substitutions.
+                                assert_eq!(debruijn, ty::INNERMOST);
+                                cvar == br.var
+                            }
+                            _ => false,
+                        },
+
+                        GenericArgKind::Const(ct) => match ct.kind() {
+                            ty::ConstKind::Bound(debruijn, b) => {
+                                // We only allow a `ty::INNERMOST` index in substitutions.
+                                assert_eq!(debruijn, ty::INNERMOST);
+                                cvar == b
+                            }
+                            _ => false,
+                        },
+                    }
+                })
+            }
+        }
+    }
+}
+
+/// A user-given type annotation attached to a constant. These arise
+/// from constants that are named via paths, like `Foo::<A>::new` and
+/// so forth.
+#[derive(Copy, Clone, Debug, PartialEq, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub enum UserType<'tcx> {
+    Ty(Ty<'tcx>),
+
+    /// The canonical type is the result of `type_of(def_id)` with the
+    /// given substitutions applied.
+    TypeOf(DefId, UserSubsts<'tcx>),
+}
+
+impl<'tcx> CommonTypes<'tcx> {
+    fn new(
+        interners: &CtxtInterners<'tcx>,
+        sess: &Session,
+        definitions: &rustc_hir::definitions::Definitions,
+        cstore: &CrateStoreDyn,
+        source_span: &IndexVec<LocalDefId, Span>,
+    ) -> CommonTypes<'tcx> {
+        let mk = |ty| interners.intern_ty(ty, sess, definitions, cstore, source_span);
+
+        CommonTypes {
+            unit: mk(Tuple(List::empty())),
+            bool: mk(Bool),
+            char: mk(Char),
+            never: mk(Never),
+            isize: mk(Int(ty::IntTy::Isize)),
+            i8: mk(Int(ty::IntTy::I8)),
+            i16: mk(Int(ty::IntTy::I16)),
+            i32: mk(Int(ty::IntTy::I32)),
+            i64: mk(Int(ty::IntTy::I64)),
+            i128: mk(Int(ty::IntTy::I128)),
+            usize: mk(Uint(ty::UintTy::Usize)),
+            u8: mk(Uint(ty::UintTy::U8)),
+            u16: mk(Uint(ty::UintTy::U16)),
+            u32: mk(Uint(ty::UintTy::U32)),
+            u64: mk(Uint(ty::UintTy::U64)),
+            u128: mk(Uint(ty::UintTy::U128)),
+            f32: mk(Float(ty::FloatTy::F32)),
+            f64: mk(Float(ty::FloatTy::F64)),
+            str_: mk(Str),
+            self_param: mk(ty::Param(ty::ParamTy { index: 0, name: kw::SelfUpper })),
+
+            trait_object_dummy_self: mk(Infer(ty::FreshTy(0))),
+        }
+    }
+}
+
+impl<'tcx> CommonLifetimes<'tcx> {
+    fn new(interners: &CtxtInterners<'tcx>) -> CommonLifetimes<'tcx> {
+        let mk = |r| {
+            Region(Interned::new_unchecked(
+                interners.region.intern(r, |r| InternedInSet(interners.arena.alloc(r))).0,
+            ))
+        };
+
+        CommonLifetimes {
+            re_root_empty: mk(ty::ReEmpty(ty::UniverseIndex::ROOT)),
+            re_static: mk(ty::ReStatic),
+            re_erased: mk(ty::ReErased),
+        }
+    }
+}
+
+impl<'tcx> CommonConsts<'tcx> {
+    fn new(interners: &CtxtInterners<'tcx>, types: &CommonTypes<'tcx>) -> CommonConsts<'tcx> {
+        let mk_const = |c| {
+            Const(Interned::new_unchecked(
+                interners.const_.intern(c, |c| InternedInSet(interners.arena.alloc(c))).0,
+            ))
+        };
+
+        CommonConsts {
+            unit: mk_const(ty::ConstS {
+                kind: ty::ConstKind::Value(ty::ValTree::zst()),
+                ty: types.unit,
+            }),
+        }
+    }
+}
+
+// This struct contains information regarding the `ReFree(FreeRegion)` corresponding to a lifetime
+// conflict.
+#[derive(Debug)]
+pub struct FreeRegionInfo {
+    // `LocalDefId` corresponding to FreeRegion
+    pub def_id: LocalDefId,
+    // the bound region corresponding to FreeRegion
+    pub boundregion: ty::BoundRegionKind,
+    // checks if bound region is in Impl Item
+    pub is_impl_item: bool,
+}
+
+/// The central data structure of the compiler. It stores references
+/// to the various **arenas** and also houses the results of the
+/// various **compiler queries** that have been performed. See the
+/// [rustc dev guide] for more details.
+///
+/// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/ty.html
+#[derive(Copy, Clone)]
+#[rustc_diagnostic_item = "TyCtxt"]
+#[rustc_pass_by_value]
+pub struct TyCtxt<'tcx> {
+    gcx: &'tcx GlobalCtxt<'tcx>,
+}
+
+impl<'tcx> Deref for TyCtxt<'tcx> {
+    type Target = &'tcx GlobalCtxt<'tcx>;
+    #[inline(always)]
+    fn deref(&self) -> &Self::Target {
+        &self.gcx
+    }
+}
+
+pub struct GlobalCtxt<'tcx> {
+    pub arena: &'tcx WorkerLocal<Arena<'tcx>>,
+    pub hir_arena: &'tcx WorkerLocal<hir::Arena<'tcx>>,
+
+    interners: CtxtInterners<'tcx>,
+
+    pub sess: &'tcx Session,
+
+    /// This only ever stores a `LintStore` but we don't want a dependency on that type here.
+    ///
+    /// FIXME(Centril): consider `dyn LintStoreMarker` once
+    /// we can upcast to `Any` for some additional type safety.
+    pub lint_store: Lrc<dyn Any + sync::Sync + sync::Send>,
+
+    pub dep_graph: DepGraph,
+
+    pub prof: SelfProfilerRef,
+
+    /// Common types, pre-interned for your convenience.
+    pub types: CommonTypes<'tcx>,
+
+    /// Common lifetimes, pre-interned for your convenience.
+    pub lifetimes: CommonLifetimes<'tcx>,
+
+    /// Common consts, pre-interned for your convenience.
+    pub consts: CommonConsts<'tcx>,
+
+    definitions: RwLock<Definitions>,
+    cstore: Box<CrateStoreDyn>,
+
+    /// Output of the resolver.
+    pub(crate) untracked_resolutions: ty::ResolverOutputs,
+    untracked_resolver_for_lowering: Steal<ty::ResolverAstLowering>,
+    /// The entire crate as AST. This field serves as the input for the hir_crate query,
+    /// which lowers it from AST to HIR. It must not be read or used by anything else.
+    pub untracked_crate: Steal<Lrc<ast::Crate>>,
+
+    /// This provides access to the incremental compilation on-disk cache for query results.
+    /// Do not access this directly. It is only meant to be used by
+    /// `DepGraph::try_mark_green()` and the query infrastructure.
+    /// This is `None` if we are not incremental compilation mode
+    pub on_disk_cache: Option<&'tcx dyn OnDiskCache<'tcx>>,
+
+    pub queries: &'tcx dyn query::QueryEngine<'tcx>,
+    pub query_caches: query::QueryCaches<'tcx>,
+    query_kinds: &'tcx [DepKindStruct],
+
+    // Internal caches for metadata decoding. No need to track deps on this.
+    pub ty_rcache: Lock<FxHashMap<ty::CReaderCacheKey, Ty<'tcx>>>,
+    pub pred_rcache: Lock<FxHashMap<ty::CReaderCacheKey, Predicate<'tcx>>>,
+
+    /// Caches the results of trait selection. This cache is used
+    /// for things that do not have to do with the parameters in scope.
+    pub selection_cache: traits::SelectionCache<'tcx>,
+
+    /// Caches the results of trait evaluation. This cache is used
+    /// for things that do not have to do with the parameters in scope.
+    /// Merge this with `selection_cache`?
+    pub evaluation_cache: traits::EvaluationCache<'tcx>,
+
+    /// The definite name of the current crate after taking into account
+    /// attributes, commandline parameters, etc.
+    crate_name: Symbol,
+
+    /// Data layout specification for the current target.
+    pub data_layout: TargetDataLayout,
+
+    /// Stores memory for globals (statics/consts).
+    pub(crate) alloc_map: Lock<interpret::AllocMap<'tcx>>,
+
+    output_filenames: Arc<OutputFilenames>,
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Expects a body and returns its codegen attributes.
+    ///
+    /// Unlike `codegen_fn_attrs`, this returns `CodegenFnAttrs::EMPTY` for
+    /// constants.
+    pub fn body_codegen_attrs(self, def_id: DefId) -> &'tcx CodegenFnAttrs {
+        let def_kind = self.def_kind(def_id);
+        if def_kind.has_codegen_attrs() {
+            self.codegen_fn_attrs(def_id)
+        } else if matches!(
+            def_kind,
+            DefKind::AnonConst | DefKind::AssocConst | DefKind::Const | DefKind::InlineConst
+        ) {
+            CodegenFnAttrs::EMPTY
+        } else {
+            bug!(
+                "body_codegen_fn_attrs called on unexpected definition: {:?} {:?}",
+                def_id,
+                def_kind
+            )
+        }
+    }
+
+    pub fn typeck_opt_const_arg(
+        self,
+        def: ty::WithOptConstParam<LocalDefId>,
+    ) -> &'tcx TypeckResults<'tcx> {
+        if let Some(param_did) = def.const_param_did {
+            self.typeck_const_arg((def.did, param_did))
+        } else {
+            self.typeck(def.did)
+        }
+    }
+
+    pub fn mir_borrowck_opt_const_arg(
+        self,
+        def: ty::WithOptConstParam<LocalDefId>,
+    ) -> &'tcx BorrowCheckResult<'tcx> {
+        if let Some(param_did) = def.const_param_did {
+            self.mir_borrowck_const_arg((def.did, param_did))
+        } else {
+            self.mir_borrowck(def.did)
+        }
+    }
+
+    pub fn alloc_steal_thir(self, thir: Thir<'tcx>) -> &'tcx Steal<Thir<'tcx>> {
+        self.arena.alloc(Steal::new(thir))
+    }
+
+    pub fn alloc_steal_mir(self, mir: Body<'tcx>) -> &'tcx Steal<Body<'tcx>> {
+        self.arena.alloc(Steal::new(mir))
+    }
+
+    pub fn alloc_steal_promoted(
+        self,
+        promoted: IndexVec<Promoted, Body<'tcx>>,
+    ) -> &'tcx Steal<IndexVec<Promoted, Body<'tcx>>> {
+        self.arena.alloc(Steal::new(promoted))
+    }
+
+    pub fn alloc_adt_def(
+        self,
+        did: DefId,
+        kind: AdtKind,
+        variants: IndexVec<VariantIdx, ty::VariantDef>,
+        repr: ReprOptions,
+    ) -> ty::AdtDef<'tcx> {
+        self.intern_adt_def(ty::AdtDefData::new(self, did, kind, variants, repr))
+    }
+
+    /// Allocates a read-only byte or string literal for `mir::interpret`.
+    pub fn allocate_bytes(self, bytes: &[u8]) -> interpret::AllocId {
+        // Create an allocation that just contains these bytes.
+        let alloc = interpret::Allocation::from_bytes_byte_aligned_immutable(bytes);
+        let alloc = self.intern_const_alloc(alloc);
+        self.create_memory_alloc(alloc)
+    }
+
+    /// Returns a range of the start/end indices specified with the
+    /// `rustc_layout_scalar_valid_range` attribute.
+    // FIXME(eddyb) this is an awkward spot for this method, maybe move it?
+    pub fn layout_scalar_valid_range(self, def_id: DefId) -> (Bound<u128>, Bound<u128>) {
+        let get = |name| {
+            let Some(attr) = self.get_attr(def_id, name) else {
+                return Bound::Unbounded;
+            };
+            debug!("layout_scalar_valid_range: attr={:?}", attr);
+            if let Some(
+                &[
+                    ast::NestedMetaItem::Literal(ast::Lit {
+                        kind: ast::LitKind::Int(a, _), ..
+                    }),
+                ],
+            ) = attr.meta_item_list().as_deref()
+            {
+                Bound::Included(a)
+            } else {
+                self.sess
+                    .delay_span_bug(attr.span, "invalid rustc_layout_scalar_valid_range attribute");
+                Bound::Unbounded
+            }
+        };
+        (
+            get(sym::rustc_layout_scalar_valid_range_start),
+            get(sym::rustc_layout_scalar_valid_range_end),
+        )
+    }
+
+    pub fn lift<T: Lift<'tcx>>(self, value: T) -> Option<T::Lifted> {
+        value.lift_to_tcx(self)
+    }
+
+    /// Creates a type context and call the closure with a `TyCtxt` reference
+    /// to the context. The closure enforces that the type context and any interned
+    /// value (types, substs, etc.) can only be used while `ty::tls` has a valid
+    /// reference to the context, to allow formatting values that need it.
+    pub fn create_global_ctxt(
+        s: &'tcx Session,
+        lint_store: Lrc<dyn Any + sync::Send + sync::Sync>,
+        arena: &'tcx WorkerLocal<Arena<'tcx>>,
+        hir_arena: &'tcx WorkerLocal<hir::Arena<'tcx>>,
+        definitions: Definitions,
+        cstore: Box<CrateStoreDyn>,
+        untracked_resolutions: ty::ResolverOutputs,
+        untracked_resolver_for_lowering: ty::ResolverAstLowering,
+        krate: Lrc<ast::Crate>,
+        dep_graph: DepGraph,
+        on_disk_cache: Option<&'tcx dyn OnDiskCache<'tcx>>,
+        queries: &'tcx dyn query::QueryEngine<'tcx>,
+        query_kinds: &'tcx [DepKindStruct],
+        crate_name: &str,
+        output_filenames: OutputFilenames,
+    ) -> GlobalCtxt<'tcx> {
+        let data_layout = TargetDataLayout::parse(&s.target).unwrap_or_else(|err| {
+            s.fatal(&err);
+        });
+        let interners = CtxtInterners::new(arena);
+        let common_types = CommonTypes::new(
+            &interners,
+            s,
+            &definitions,
+            &*cstore,
+            // This is only used to create a stable hashing context.
+            &untracked_resolutions.source_span,
+        );
+        let common_lifetimes = CommonLifetimes::new(&interners);
+        let common_consts = CommonConsts::new(&interners, &common_types);
+
+        GlobalCtxt {
+            sess: s,
+            lint_store,
+            arena,
+            hir_arena,
+            interners,
+            dep_graph,
+            definitions: RwLock::new(definitions),
+            cstore,
+            prof: s.prof.clone(),
+            types: common_types,
+            lifetimes: common_lifetimes,
+            consts: common_consts,
+            untracked_resolutions,
+            untracked_resolver_for_lowering: Steal::new(untracked_resolver_for_lowering),
+            untracked_crate: Steal::new(krate),
+            on_disk_cache,
+            queries,
+            query_caches: query::QueryCaches::default(),
+            query_kinds,
+            ty_rcache: Default::default(),
+            pred_rcache: Default::default(),
+            selection_cache: Default::default(),
+            evaluation_cache: Default::default(),
+            crate_name: Symbol::intern(crate_name),
+            data_layout,
+            alloc_map: Lock::new(interpret::AllocMap::new()),
+            output_filenames: Arc::new(output_filenames),
+        }
+    }
+
+    pub(crate) fn query_kind(self, k: DepKind) -> &'tcx DepKindStruct {
+        &self.query_kinds[k as usize]
+    }
+
+    /// Constructs a `TyKind::Error` type and registers a `delay_span_bug` to ensure it gets used.
+    #[track_caller]
+    pub fn ty_error(self) -> Ty<'tcx> {
+        self.ty_error_with_message(DUMMY_SP, "TyKind::Error constructed but no error reported")
+    }
+
+    /// Constructs a `TyKind::Error` type and registers a `delay_span_bug` with the given `msg` to
+    /// ensure it gets used.
+    #[track_caller]
+    pub fn ty_error_with_message<S: Into<MultiSpan>>(self, span: S, msg: &str) -> Ty<'tcx> {
+        let reported = self.sess.delay_span_bug(span, msg);
+        self.mk_ty(Error(DelaySpanBugEmitted { reported, _priv: () }))
+    }
+
+    /// Like [TyCtxt::ty_error] but for constants.
+    #[track_caller]
+    pub fn const_error(self, ty: Ty<'tcx>) -> Const<'tcx> {
+        self.const_error_with_message(
+            ty,
+            DUMMY_SP,
+            "ty::ConstKind::Error constructed but no error reported",
+        )
+    }
+
+    /// Like [TyCtxt::ty_error_with_message] but for constants.
+    #[track_caller]
+    pub fn const_error_with_message<S: Into<MultiSpan>>(
+        self,
+        ty: Ty<'tcx>,
+        span: S,
+        msg: &str,
+    ) -> Const<'tcx> {
+        let reported = self.sess.delay_span_bug(span, msg);
+        self.mk_const(ty::ConstS {
+            kind: ty::ConstKind::Error(DelaySpanBugEmitted { reported, _priv: () }),
+            ty,
+        })
+    }
+
+    pub fn consider_optimizing<T: Fn() -> String>(self, msg: T) -> bool {
+        let cname = self.crate_name(LOCAL_CRATE);
+        self.sess.consider_optimizing(cname.as_str(), msg)
+    }
+
+    /// Obtain all lang items of this crate and all dependencies (recursively)
+    pub fn lang_items(self) -> &'tcx rustc_hir::lang_items::LanguageItems {
+        self.get_lang_items(())
+    }
+
+    /// Obtain the given diagnostic item's `DefId`. Use `is_diagnostic_item` if you just want to
+    /// compare against another `DefId`, since `is_diagnostic_item` is cheaper.
+    pub fn get_diagnostic_item(self, name: Symbol) -> Option<DefId> {
+        self.all_diagnostic_items(()).name_to_id.get(&name).copied()
+    }
+
+    /// Obtain the diagnostic item's name
+    pub fn get_diagnostic_name(self, id: DefId) -> Option<Symbol> {
+        self.diagnostic_items(id.krate).id_to_name.get(&id).copied()
+    }
+
+    /// Check whether the diagnostic item with the given `name` has the given `DefId`.
+    pub fn is_diagnostic_item(self, name: Symbol, did: DefId) -> bool {
+        self.diagnostic_items(did.krate).name_to_id.get(&name) == Some(&did)
+    }
+
+    pub fn stability(self) -> &'tcx stability::Index {
+        self.stability_index(())
+    }
+
+    pub fn features(self) -> &'tcx rustc_feature::Features {
+        self.features_query(())
+    }
+
+    pub fn def_key(self, id: DefId) -> rustc_hir::definitions::DefKey {
+        // Accessing the DefKey is ok, since it is part of DefPathHash.
+        if let Some(id) = id.as_local() {
+            self.definitions_untracked().def_key(id)
+        } else {
+            self.cstore.def_key(id)
+        }
+    }
+
+    /// Converts a `DefId` into its fully expanded `DefPath` (every
+    /// `DefId` is really just an interned `DefPath`).
+    ///
+    /// Note that if `id` is not local to this crate, the result will
+    ///  be a non-local `DefPath`.
+    pub fn def_path(self, id: DefId) -> rustc_hir::definitions::DefPath {
+        // Accessing the DefPath is ok, since it is part of DefPathHash.
+        if let Some(id) = id.as_local() {
+            self.definitions_untracked().def_path(id)
+        } else {
+            self.cstore.def_path(id)
+        }
+    }
+
+    #[inline]
+    pub fn def_path_hash(self, def_id: DefId) -> rustc_hir::definitions::DefPathHash {
+        // Accessing the DefPathHash is ok, it is incr. comp. stable.
+        if let Some(def_id) = def_id.as_local() {
+            self.definitions_untracked().def_path_hash(def_id)
+        } else {
+            self.cstore.def_path_hash(def_id)
+        }
+    }
+
+    #[inline]
+    pub fn stable_crate_id(self, crate_num: CrateNum) -> StableCrateId {
+        if crate_num == LOCAL_CRATE {
+            self.sess.local_stable_crate_id()
+        } else {
+            self.cstore.stable_crate_id(crate_num)
+        }
+    }
+
+    /// Maps a StableCrateId to the corresponding CrateNum. This method assumes
+    /// that the crate in question has already been loaded by the CrateStore.
+    #[inline]
+    pub fn stable_crate_id_to_crate_num(self, stable_crate_id: StableCrateId) -> CrateNum {
+        if stable_crate_id == self.sess.local_stable_crate_id() {
+            LOCAL_CRATE
+        } else {
+            self.cstore.stable_crate_id_to_crate_num(stable_crate_id)
+        }
+    }
+
+    /// Converts a `DefPathHash` to its corresponding `DefId` in the current compilation
+    /// session, if it still exists. This is used during incremental compilation to
+    /// turn a deserialized `DefPathHash` into its current `DefId`.
+    pub fn def_path_hash_to_def_id(self, hash: DefPathHash, err: &mut dyn FnMut() -> !) -> DefId {
+        debug!("def_path_hash_to_def_id({:?})", hash);
+
+        let stable_crate_id = hash.stable_crate_id();
+
+        // If this is a DefPathHash from the local crate, we can look up the
+        // DefId in the tcx's `Definitions`.
+        if stable_crate_id == self.sess.local_stable_crate_id() {
+            self.definitions.read().local_def_path_hash_to_def_id(hash, err).to_def_id()
+        } else {
+            // If this is a DefPathHash from an upstream crate, let the CrateStore map
+            // it to a DefId.
+            let cnum = self.cstore.stable_crate_id_to_crate_num(stable_crate_id);
+            self.cstore.def_path_hash_to_def_id(cnum, hash)
+        }
+    }
+
+    pub fn def_path_debug_str(self, def_id: DefId) -> String {
+        // We are explicitly not going through queries here in order to get
+        // crate name and stable crate id since this code is called from debug!()
+        // statements within the query system and we'd run into endless
+        // recursion otherwise.
+        let (crate_name, stable_crate_id) = if def_id.is_local() {
+            (self.crate_name, self.sess.local_stable_crate_id())
+        } else {
+            let cstore = &self.cstore;
+            (cstore.crate_name(def_id.krate), cstore.stable_crate_id(def_id.krate))
+        };
+
+        format!(
+            "{}[{:04x}]{}",
+            crate_name,
+            // Don't print the whole stable crate id. That's just
+            // annoying in debug output.
+            stable_crate_id.to_u64() >> 8 * 6,
+            self.def_path(def_id).to_string_no_crate_verbose()
+        )
+    }
+
+    /// Create a new definition within the incr. comp. engine.
+    pub fn create_def(self, parent: LocalDefId, data: hir::definitions::DefPathData) -> LocalDefId {
+        // This function modifies `self.definitions` using a side-effect.
+        // We need to ensure that these side effects are re-run by the incr. comp. engine.
+        // Depending on the forever-red node will tell the graph that the calling query
+        // needs to be re-evaluated.
+        use rustc_query_system::dep_graph::DepNodeIndex;
+        self.dep_graph.read_index(DepNodeIndex::FOREVER_RED_NODE);
+
+        // The following call has the side effect of modifying the tables inside `definitions`.
+        // These very tables are relied on by the incr. comp. engine to decode DepNodes and to
+        // decode the on-disk cache.
+        //
+        // Any LocalDefId which is used within queries, either as key or result, either:
+        // - has been created before the construction of the TyCtxt;
+        // - has been created by this call to `create_def`.
+        // As a consequence, this LocalDefId is always re-created before it is needed by the incr.
+        // comp. engine itself.
+        //
+        // This call also writes to the value of `source_span` and `expn_that_defined` queries.
+        // This is fine because:
+        // - those queries are `eval_always` so we won't miss their result changing;
+        // - this write will have happened before these queries are called.
+        self.definitions.write().create_def(parent, data)
+    }
+
+    pub fn iter_local_def_id(self) -> impl Iterator<Item = LocalDefId> + 'tcx {
+        // Create a dependency to the crate to be sure we re-execute this when the amount of
+        // definitions change.
+        self.ensure().hir_crate(());
+        // Leak a read lock once we start iterating on definitions, to prevent adding new onces
+        // while iterating.  If some query needs to add definitions, it should be `ensure`d above.
+        let definitions = self.definitions.leak();
+        definitions.iter_local_def_id()
+    }
+
+    pub fn def_path_table(self) -> &'tcx rustc_hir::definitions::DefPathTable {
+        // Create a dependency to the crate to be sure we reexcute this when the amount of
+        // definitions change.
+        self.ensure().hir_crate(());
+        // Leak a read lock once we start iterating on definitions, to prevent adding new onces
+        // while iterating.  If some query needs to add definitions, it should be `ensure`d above.
+        let definitions = self.definitions.leak();
+        definitions.def_path_table()
+    }
+
+    pub fn def_path_hash_to_def_index_map(
+        self,
+    ) -> &'tcx rustc_hir::def_path_hash_map::DefPathHashMap {
+        // Create a dependency to the crate to be sure we reexcute this when the amount of
+        // definitions change.
+        self.ensure().hir_crate(());
+        // Leak a read lock once we start iterating on definitions, to prevent adding new onces
+        // while iterating.  If some query needs to add definitions, it should be `ensure`d above.
+        let definitions = self.definitions.leak();
+        definitions.def_path_hash_to_def_index_map()
+    }
+
+    /// Note that this is *untracked* and should only be used within the query
+    /// system if the result is otherwise tracked through queries
+    pub fn cstore_untracked(self) -> &'tcx CrateStoreDyn {
+        &*self.cstore
+    }
+
+    /// Note that this is *untracked* and should only be used within the query
+    /// system if the result is otherwise tracked through queries
+    #[inline]
+    pub fn definitions_untracked(self) -> ReadGuard<'tcx, Definitions> {
+        self.definitions.read()
+    }
+
+    /// Note that this is *untracked* and should only be used within the query
+    /// system if the result is otherwise tracked through queries
+    #[inline]
+    pub fn source_span_untracked(self, def_id: LocalDefId) -> Span {
+        self.untracked_resolutions.source_span.get(def_id).copied().unwrap_or(DUMMY_SP)
+    }
+
+    #[inline(always)]
+    pub fn with_stable_hashing_context<R>(
+        self,
+        f: impl FnOnce(StableHashingContext<'_>) -> R,
+    ) -> R {
+        let definitions = self.definitions_untracked();
+        let hcx = StableHashingContext::new(
+            self.sess,
+            &*definitions,
+            &*self.cstore,
+            &self.untracked_resolutions.source_span,
+        );
+        f(hcx)
+    }
+
+    pub fn serialize_query_result_cache(self, encoder: FileEncoder) -> FileEncodeResult {
+        self.on_disk_cache.as_ref().map_or(Ok(0), |c| c.serialize(self, encoder))
+    }
+
+    /// If `true`, we should use lazy normalization for constants, otherwise
+    /// we still evaluate them eagerly.
+    #[inline]
+    pub fn lazy_normalization(self) -> bool {
+        let features = self.features();
+        // Note: We only use lazy normalization for generic const expressions.
+        features.generic_const_exprs
+    }
+
+    #[inline]
+    pub fn local_crate_exports_generics(self) -> bool {
+        debug_assert!(self.sess.opts.share_generics());
+
+        self.sess.crate_types().iter().any(|crate_type| {
+            match crate_type {
+                CrateType::Executable
+                | CrateType::Staticlib
+                | CrateType::ProcMacro
+                | CrateType::Cdylib => false,
+
+                // FIXME rust-lang/rust#64319, rust-lang/rust#64872:
+                // We want to block export of generics from dylibs,
+                // but we must fix rust-lang/rust#65890 before we can
+                // do that robustly.
+                CrateType::Dylib => true,
+
+                CrateType::Rlib => true,
+            }
+        })
+    }
+
+    // Returns the `DefId` and the `BoundRegionKind` corresponding to the given region.
+    pub fn is_suitable_region(self, region: Region<'tcx>) -> Option<FreeRegionInfo> {
+        let (suitable_region_binding_scope, bound_region) = match *region {
+            ty::ReFree(ref free_region) => {
+                (free_region.scope.expect_local(), free_region.bound_region)
+            }
+            ty::ReEarlyBound(ref ebr) => (
+                self.local_parent(ebr.def_id.expect_local()),
+                ty::BoundRegionKind::BrNamed(ebr.def_id, ebr.name),
+            ),
+            _ => return None, // not a free region
+        };
+
+        let is_impl_item = match self.hir().find_by_def_id(suitable_region_binding_scope) {
+            Some(Node::Item(..) | Node::TraitItem(..)) => false,
+            Some(Node::ImplItem(..)) => {
+                self.is_bound_region_in_impl_item(suitable_region_binding_scope)
+            }
+            _ => return None,
+        };
+
+        Some(FreeRegionInfo {
+            def_id: suitable_region_binding_scope,
+            boundregion: bound_region,
+            is_impl_item,
+        })
+    }
+
+    /// Given a `DefId` for an `fn`, return all the `dyn` and `impl` traits in its return type.
+    pub fn return_type_impl_or_dyn_traits(
+        self,
+        scope_def_id: LocalDefId,
+    ) -> Vec<&'tcx hir::Ty<'tcx>> {
+        let hir_id = self.hir().local_def_id_to_hir_id(scope_def_id);
+        let Some(hir::FnDecl { output: hir::FnRetTy::Return(hir_output), .. }) = self.hir().fn_decl_by_hir_id(hir_id) else {
+            return vec![];
+        };
+
+        let mut v = TraitObjectVisitor(vec![], self.hir());
+        v.visit_ty(hir_output);
+        v.0
+    }
+
+    pub fn return_type_impl_trait(self, scope_def_id: LocalDefId) -> Option<(Ty<'tcx>, Span)> {
+        // `type_of()` will fail on these (#55796, #86483), so only allow `fn`s or closures.
+        match self.hir().get_by_def_id(scope_def_id) {
+            Node::Item(&hir::Item { kind: ItemKind::Fn(..), .. }) => {}
+            Node::TraitItem(&hir::TraitItem { kind: TraitItemKind::Fn(..), .. }) => {}
+            Node::ImplItem(&hir::ImplItem { kind: ImplItemKind::Fn(..), .. }) => {}
+            Node::Expr(&hir::Expr { kind: ExprKind::Closure { .. }, .. }) => {}
+            _ => return None,
+        }
+
+        let ret_ty = self.type_of(scope_def_id);
+        match ret_ty.kind() {
+            ty::FnDef(_, _) => {
+                let sig = ret_ty.fn_sig(self);
+                let output = self.erase_late_bound_regions(sig.output());
+                if output.is_impl_trait() {
+                    let hir_id = self.hir().local_def_id_to_hir_id(scope_def_id);
+                    let fn_decl = self.hir().fn_decl_by_hir_id(hir_id).unwrap();
+                    Some((output, fn_decl.output.span()))
+                } else {
+                    None
+                }
+            }
+            _ => None,
+        }
+    }
+
+    // Checks if the bound region is in Impl Item.
+    pub fn is_bound_region_in_impl_item(self, suitable_region_binding_scope: LocalDefId) -> bool {
+        let container_id = self.parent(suitable_region_binding_scope.to_def_id());
+        if self.impl_trait_ref(container_id).is_some() {
+            // For now, we do not try to target impls of traits. This is
+            // because this message is going to suggest that the user
+            // change the fn signature, but they may not be free to do so,
+            // since the signature must match the trait.
+            //
+            // FIXME(#42706) -- in some cases, we could do better here.
+            return true;
+        }
+        false
+    }
+
+    /// Determines whether identifiers in the assembly have strict naming rules.
+    /// Currently, only NVPTX* targets need it.
+    pub fn has_strict_asm_symbol_naming(self) -> bool {
+        self.sess.target.arch.contains("nvptx")
+    }
+
+    /// Returns `&'static core::panic::Location<'static>`.
+    pub fn caller_location_ty(self) -> Ty<'tcx> {
+        self.mk_imm_ref(
+            self.lifetimes.re_static,
+            self.bound_type_of(self.require_lang_item(LangItem::PanicLocation, None))
+                .subst(self, self.mk_substs([self.lifetimes.re_static.into()].iter())),
+        )
+    }
+
+    /// Returns a displayable description and article for the given `def_id` (e.g. `("a", "struct")`).
+    pub fn article_and_description(self, def_id: DefId) -> (&'static str, &'static str) {
+        match self.def_kind(def_id) {
+            DefKind::Generator => match self.generator_kind(def_id).unwrap() {
+                rustc_hir::GeneratorKind::Async(..) => ("an", "async closure"),
+                rustc_hir::GeneratorKind::Gen => ("a", "generator"),
+            },
+            def_kind => (def_kind.article(), def_kind.descr(def_id)),
+        }
+    }
+
+    pub fn type_length_limit(self) -> Limit {
+        self.limits(()).type_length_limit
+    }
+
+    pub fn recursion_limit(self) -> Limit {
+        self.limits(()).recursion_limit
+    }
+
+    pub fn move_size_limit(self) -> Limit {
+        self.limits(()).move_size_limit
+    }
+
+    pub fn const_eval_limit(self) -> Limit {
+        self.limits(()).const_eval_limit
+    }
+
+    pub fn all_traits(self) -> impl Iterator<Item = DefId> + 'tcx {
+        iter::once(LOCAL_CRATE)
+            .chain(self.crates(()).iter().copied())
+            .flat_map(move |cnum| self.traits_in_crate(cnum).iter().copied())
+    }
+}
+
+/// A trait implemented for all `X<'a>` types that can be safely and
+/// efficiently converted to `X<'tcx>` as long as they are part of the
+/// provided `TyCtxt<'tcx>`.
+/// This can be done, for example, for `Ty<'tcx>` or `SubstsRef<'tcx>`
+/// by looking them up in their respective interners.
+///
+/// However, this is still not the best implementation as it does
+/// need to compare the components, even for interned values.
+/// It would be more efficient if `TypedArena` provided a way to
+/// determine whether the address is in the allocated range.
+///
+/// `None` is returned if the value or one of the components is not part
+/// of the provided context.
+/// For `Ty`, `None` can be returned if either the type interner doesn't
+/// contain the `TyKind` key or if the address of the interned
+/// pointer differs. The latter case is possible if a primitive type,
+/// e.g., `()` or `u8`, was interned in a different context.
+pub trait Lift<'tcx>: fmt::Debug {
+    type Lifted: fmt::Debug + 'tcx;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted>;
+}
+
+macro_rules! nop_lift {
+    ($set:ident; $ty:ty => $lifted:ty) => {
+        impl<'a, 'tcx> Lift<'tcx> for $ty {
+            type Lifted = $lifted;
+            fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+                if tcx.interners.$set.contains_pointer_to(&InternedInSet(&*self.0.0)) {
+                    // SAFETY: `self` is interned and therefore valid
+                    // for the entire lifetime of the `TyCtxt`.
+                    Some(unsafe { mem::transmute(self) })
+                } else {
+                    None
+                }
+            }
+        }
+    };
+}
+
+// Can't use the macros as we have reuse the `substs` here.
+//
+// See `intern_type_list` for more info.
+impl<'a, 'tcx> Lift<'tcx> for &'a List<Ty<'a>> {
+    type Lifted = &'tcx List<Ty<'tcx>>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        if self.is_empty() {
+            return Some(List::empty());
+        }
+        if tcx.interners.substs.contains_pointer_to(&InternedInSet(self.as_substs())) {
+            // SAFETY: `self` is interned and therefore valid
+            // for the entire lifetime of the `TyCtxt`.
+            Some(unsafe { mem::transmute::<&'a List<Ty<'a>>, &'tcx List<Ty<'tcx>>>(self) })
+        } else {
+            None
+        }
+    }
+}
+
+macro_rules! nop_list_lift {
+    ($set:ident; $ty:ty => $lifted:ty) => {
+        impl<'a, 'tcx> Lift<'tcx> for &'a List<$ty> {
+            type Lifted = &'tcx List<$lifted>;
+            fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+                if self.is_empty() {
+                    return Some(List::empty());
+                }
+                if tcx.interners.$set.contains_pointer_to(&InternedInSet(self)) {
+                    Some(unsafe { mem::transmute(self) })
+                } else {
+                    None
+                }
+            }
+        }
+    };
+}
+
+nop_lift! {type_; Ty<'a> => Ty<'tcx>}
+nop_lift! {region; Region<'a> => Region<'tcx>}
+nop_lift! {const_; Const<'a> => Const<'tcx>}
+nop_lift! {const_allocation; ConstAllocation<'a> => ConstAllocation<'tcx>}
+nop_lift! {predicate; Predicate<'a> => Predicate<'tcx>}
+
+nop_list_lift! {poly_existential_predicates; ty::Binder<'a, ExistentialPredicate<'a>> => ty::Binder<'tcx, ExistentialPredicate<'tcx>>}
+nop_list_lift! {predicates; Predicate<'a> => Predicate<'tcx>}
+nop_list_lift! {canonical_var_infos; CanonicalVarInfo<'a> => CanonicalVarInfo<'tcx>}
+nop_list_lift! {projs; ProjectionKind => ProjectionKind}
+nop_list_lift! {bound_variable_kinds; ty::BoundVariableKind => ty::BoundVariableKind}
+
+// This is the impl for `&'a InternalSubsts<'a>`.
+nop_list_lift! {substs; GenericArg<'a> => GenericArg<'tcx>}
+
+CloneLiftImpls! { for<'tcx> { Constness, traits::WellFormedLoc, } }
+
+pub mod tls {
+    use super::{ptr_eq, GlobalCtxt, TyCtxt};
+
+    use crate::dep_graph::TaskDepsRef;
+    use crate::ty::query;
+    use rustc_data_structures::sync::{self, Lock};
+    use rustc_data_structures::thin_vec::ThinVec;
+    use rustc_errors::Diagnostic;
+    use std::mem;
+
+    #[cfg(not(parallel_compiler))]
+    use std::cell::Cell;
+
+    #[cfg(parallel_compiler)]
+    use rustc_rayon_core as rayon_core;
+
+    /// This is the implicit state of rustc. It contains the current
+    /// `TyCtxt` and query. It is updated when creating a local interner or
+    /// executing a new query. Whenever there's a `TyCtxt` value available
+    /// you should also have access to an `ImplicitCtxt` through the functions
+    /// in this module.
+    #[derive(Clone)]
+    pub struct ImplicitCtxt<'a, 'tcx> {
+        /// The current `TyCtxt`.
+        pub tcx: TyCtxt<'tcx>,
+
+        /// The current query job, if any. This is updated by `JobOwner::start` in
+        /// `ty::query::plumbing` when executing a query.
+        pub query: Option<query::QueryJobId>,
+
+        /// Where to store diagnostics for the current query job, if any.
+        /// This is updated by `JobOwner::start` in `ty::query::plumbing` when executing a query.
+        pub diagnostics: Option<&'a Lock<ThinVec<Diagnostic>>>,
+
+        /// Used to prevent layout from recursing too deeply.
+        pub layout_depth: usize,
+
+        /// The current dep graph task. This is used to add dependencies to queries
+        /// when executing them.
+        pub task_deps: TaskDepsRef<'a>,
+    }
+
+    impl<'a, 'tcx> ImplicitCtxt<'a, 'tcx> {
+        pub fn new(gcx: &'tcx GlobalCtxt<'tcx>) -> Self {
+            let tcx = TyCtxt { gcx };
+            ImplicitCtxt {
+                tcx,
+                query: None,
+                diagnostics: None,
+                layout_depth: 0,
+                task_deps: TaskDepsRef::Ignore,
+            }
+        }
+    }
+
+    /// Sets Rayon's thread-local variable, which is preserved for Rayon jobs
+    /// to `value` during the call to `f`. It is restored to its previous value after.
+    /// This is used to set the pointer to the new `ImplicitCtxt`.
+    #[cfg(parallel_compiler)]
+    #[inline]
+    fn set_tlv<F: FnOnce() -> R, R>(value: usize, f: F) -> R {
+        rayon_core::tlv::with(value, f)
+    }
+
+    /// Gets Rayon's thread-local variable, which is preserved for Rayon jobs.
+    /// This is used to get the pointer to the current `ImplicitCtxt`.
+    #[cfg(parallel_compiler)]
+    #[inline]
+    pub fn get_tlv() -> usize {
+        rayon_core::tlv::get()
+    }
+
+    #[cfg(not(parallel_compiler))]
+    thread_local! {
+        /// A thread local variable that stores a pointer to the current `ImplicitCtxt`.
+        static TLV: Cell<usize> = const { Cell::new(0) };
+    }
+
+    /// Sets TLV to `value` during the call to `f`.
+    /// It is restored to its previous value after.
+    /// This is used to set the pointer to the new `ImplicitCtxt`.
+    #[cfg(not(parallel_compiler))]
+    #[inline]
+    fn set_tlv<F: FnOnce() -> R, R>(value: usize, f: F) -> R {
+        let old = get_tlv();
+        let _reset = rustc_data_structures::OnDrop(move || TLV.with(|tlv| tlv.set(old)));
+        TLV.with(|tlv| tlv.set(value));
+        f()
+    }
+
+    /// Gets the pointer to the current `ImplicitCtxt`.
+    #[cfg(not(parallel_compiler))]
+    #[inline]
+    fn get_tlv() -> usize {
+        TLV.with(|tlv| tlv.get())
+    }
+
+    /// Sets `context` as the new current `ImplicitCtxt` for the duration of the function `f`.
+    #[inline]
+    pub fn enter_context<'a, 'tcx, F, R>(context: &ImplicitCtxt<'a, 'tcx>, f: F) -> R
+    where
+        F: FnOnce(&ImplicitCtxt<'a, 'tcx>) -> R,
+    {
+        set_tlv(context as *const _ as usize, || f(&context))
+    }
+
+    /// Allows access to the current `ImplicitCtxt` in a closure if one is available.
+    #[inline]
+    pub fn with_context_opt<F, R>(f: F) -> R
+    where
+        F: for<'a, 'tcx> FnOnce(Option<&ImplicitCtxt<'a, 'tcx>>) -> R,
+    {
+        let context = get_tlv();
+        if context == 0 {
+            f(None)
+        } else {
+            // We could get an `ImplicitCtxt` pointer from another thread.
+            // Ensure that `ImplicitCtxt` is `Sync`.
+            sync::assert_sync::<ImplicitCtxt<'_, '_>>();
+
+            unsafe { f(Some(&*(context as *const ImplicitCtxt<'_, '_>))) }
+        }
+    }
+
+    /// Allows access to the current `ImplicitCtxt`.
+    /// Panics if there is no `ImplicitCtxt` available.
+    #[inline]
+    pub fn with_context<F, R>(f: F) -> R
+    where
+        F: for<'a, 'tcx> FnOnce(&ImplicitCtxt<'a, 'tcx>) -> R,
+    {
+        with_context_opt(|opt_context| f(opt_context.expect("no ImplicitCtxt stored in tls")))
+    }
+
+    /// Allows access to the current `ImplicitCtxt` whose tcx field is the same as the tcx argument
+    /// passed in. This means the closure is given an `ImplicitCtxt` with the same `'tcx` lifetime
+    /// as the `TyCtxt` passed in.
+    /// This will panic if you pass it a `TyCtxt` which is different from the current
+    /// `ImplicitCtxt`'s `tcx` field.
+    #[inline]
+    pub fn with_related_context<'tcx, F, R>(tcx: TyCtxt<'tcx>, f: F) -> R
+    where
+        F: FnOnce(&ImplicitCtxt<'_, 'tcx>) -> R,
+    {
+        with_context(|context| unsafe {
+            assert!(ptr_eq(context.tcx.gcx, tcx.gcx));
+            let context: &ImplicitCtxt<'_, '_> = mem::transmute(context);
+            f(context)
+        })
+    }
+
+    /// Allows access to the `TyCtxt` in the current `ImplicitCtxt`.
+    /// Panics if there is no `ImplicitCtxt` available.
+    #[inline]
+    pub fn with<F, R>(f: F) -> R
+    where
+        F: for<'tcx> FnOnce(TyCtxt<'tcx>) -> R,
+    {
+        with_context(|context| f(context.tcx))
+    }
+
+    /// Allows access to the `TyCtxt` in the current `ImplicitCtxt`.
+    /// The closure is passed None if there is no `ImplicitCtxt` available.
+    #[inline]
+    pub fn with_opt<F, R>(f: F) -> R
+    where
+        F: for<'tcx> FnOnce(Option<TyCtxt<'tcx>>) -> R,
+    {
+        with_context_opt(|opt_context| f(opt_context.map(|context| context.tcx)))
+    }
+}
+
+macro_rules! sty_debug_print {
+    ($fmt: expr, $ctxt: expr, $($variant: ident),*) => {{
+        // Curious inner module to allow variant names to be used as
+        // variable names.
+        #[allow(non_snake_case)]
+        mod inner {
+            use crate::ty::{self, TyCtxt};
+            use crate::ty::context::InternedInSet;
+
+            #[derive(Copy, Clone)]
+            struct DebugStat {
+                total: usize,
+                lt_infer: usize,
+                ty_infer: usize,
+                ct_infer: usize,
+                all_infer: usize,
+            }
+
+            pub fn go(fmt: &mut std::fmt::Formatter<'_>, tcx: TyCtxt<'_>) -> std::fmt::Result {
+                let mut total = DebugStat {
+                    total: 0,
+                    lt_infer: 0,
+                    ty_infer: 0,
+                    ct_infer: 0,
+                    all_infer: 0,
+                };
+                $(let mut $variant = total;)*
+
+                let shards = tcx.interners.type_.lock_shards();
+                let types = shards.iter().flat_map(|shard| shard.keys());
+                for &InternedInSet(t) in types {
+                    let variant = match t.kind {
+                        ty::Bool | ty::Char | ty::Int(..) | ty::Uint(..) |
+                            ty::Float(..) | ty::Str | ty::Never => continue,
+                        ty::Error(_) => /* unimportant */ continue,
+                        $(ty::$variant(..) => &mut $variant,)*
+                    };
+                    let lt = t.flags.intersects(ty::TypeFlags::HAS_RE_INFER);
+                    let ty = t.flags.intersects(ty::TypeFlags::HAS_TY_INFER);
+                    let ct = t.flags.intersects(ty::TypeFlags::HAS_CT_INFER);
+
+                    variant.total += 1;
+                    total.total += 1;
+                    if lt { total.lt_infer += 1; variant.lt_infer += 1 }
+                    if ty { total.ty_infer += 1; variant.ty_infer += 1 }
+                    if ct { total.ct_infer += 1; variant.ct_infer += 1 }
+                    if lt && ty && ct { total.all_infer += 1; variant.all_infer += 1 }
+                }
+                writeln!(fmt, "Ty interner             total           ty lt ct all")?;
+                $(writeln!(fmt, "    {:18}: {uses:6} {usespc:4.1}%, \
+                            {ty:4.1}% {lt:5.1}% {ct:4.1}% {all:4.1}%",
+                    stringify!($variant),
+                    uses = $variant.total,
+                    usespc = $variant.total as f64 * 100.0 / total.total as f64,
+                    ty = $variant.ty_infer as f64 * 100.0  / total.total as f64,
+                    lt = $variant.lt_infer as f64 * 100.0  / total.total as f64,
+                    ct = $variant.ct_infer as f64 * 100.0  / total.total as f64,
+                    all = $variant.all_infer as f64 * 100.0  / total.total as f64)?;
+                )*
+                writeln!(fmt, "                  total {uses:6}        \
+                          {ty:4.1}% {lt:5.1}% {ct:4.1}% {all:4.1}%",
+                    uses = total.total,
+                    ty = total.ty_infer as f64 * 100.0  / total.total as f64,
+                    lt = total.lt_infer as f64 * 100.0  / total.total as f64,
+                    ct = total.ct_infer as f64 * 100.0  / total.total as f64,
+                    all = total.all_infer as f64 * 100.0  / total.total as f64)
+            }
+        }
+
+        inner::go($fmt, $ctxt)
+    }}
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    pub fn debug_stats(self) -> impl std::fmt::Debug + 'tcx {
+        struct DebugStats<'tcx>(TyCtxt<'tcx>);
+
+        impl<'tcx> std::fmt::Debug for DebugStats<'tcx> {
+            fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+                sty_debug_print!(
+                    fmt,
+                    self.0,
+                    Adt,
+                    Array,
+                    Slice,
+                    RawPtr,
+                    Ref,
+                    FnDef,
+                    FnPtr,
+                    Placeholder,
+                    Generator,
+                    GeneratorWitness,
+                    Dynamic,
+                    Closure,
+                    Tuple,
+                    Bound,
+                    Param,
+                    Infer,
+                    Projection,
+                    Opaque,
+                    Foreign
+                )?;
+
+                writeln!(fmt, "InternalSubsts interner: #{}", self.0.interners.substs.len())?;
+                writeln!(fmt, "Region interner: #{}", self.0.interners.region.len())?;
+                writeln!(
+                    fmt,
+                    "Const Allocation interner: #{}",
+                    self.0.interners.const_allocation.len()
+                )?;
+                writeln!(fmt, "Layout interner: #{}", self.0.interners.layout.len())?;
+
+                Ok(())
+            }
+        }
+
+        DebugStats(self)
+    }
+}
+
+// This type holds a `T` in the interner. The `T` is stored in the arena and
+// this type just holds a pointer to it, but it still effectively owns it. It
+// impls `Borrow` so that it can be looked up using the original
+// (non-arena-memory-owning) types.
+struct InternedInSet<'tcx, T: ?Sized>(&'tcx T);
+
+impl<'tcx, T: 'tcx + ?Sized> Clone for InternedInSet<'tcx, T> {
+    fn clone(&self) -> Self {
+        InternedInSet(self.0)
+    }
+}
+
+impl<'tcx, T: 'tcx + ?Sized> Copy for InternedInSet<'tcx, T> {}
+
+impl<'tcx, T: 'tcx + ?Sized> IntoPointer for InternedInSet<'tcx, T> {
+    fn into_pointer(&self) -> *const () {
+        self.0 as *const _ as *const ()
+    }
+}
+
+#[allow(rustc::usage_of_ty_tykind)]
+impl<'tcx> Borrow<TyKind<'tcx>> for InternedInSet<'tcx, WithStableHash<TyS<'tcx>>> {
+    fn borrow<'a>(&'a self) -> &'a TyKind<'tcx> {
+        &self.0.kind
+    }
+}
+
+impl<'tcx> PartialEq for InternedInSet<'tcx, WithStableHash<TyS<'tcx>>> {
+    fn eq(&self, other: &InternedInSet<'tcx, WithStableHash<TyS<'tcx>>>) -> bool {
+        // The `Borrow` trait requires that `x.borrow() == y.borrow()` equals
+        // `x == y`.
+        self.0.kind == other.0.kind
+    }
+}
+
+impl<'tcx> Eq for InternedInSet<'tcx, WithStableHash<TyS<'tcx>>> {}
+
+impl<'tcx> Hash for InternedInSet<'tcx, WithStableHash<TyS<'tcx>>> {
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        // The `Borrow` trait requires that `x.borrow().hash(s) == x.hash(s)`.
+        self.0.kind.hash(s)
+    }
+}
+
+impl<'tcx> Borrow<Binder<'tcx, PredicateKind<'tcx>>> for InternedInSet<'tcx, PredicateS<'tcx>> {
+    fn borrow<'a>(&'a self) -> &'a Binder<'tcx, PredicateKind<'tcx>> {
+        &self.0.kind
+    }
+}
+
+impl<'tcx> PartialEq for InternedInSet<'tcx, PredicateS<'tcx>> {
+    fn eq(&self, other: &InternedInSet<'tcx, PredicateS<'tcx>>) -> bool {
+        // The `Borrow` trait requires that `x.borrow() == y.borrow()` equals
+        // `x == y`.
+        self.0.kind == other.0.kind
+    }
+}
+
+impl<'tcx> Eq for InternedInSet<'tcx, PredicateS<'tcx>> {}
+
+impl<'tcx> Hash for InternedInSet<'tcx, PredicateS<'tcx>> {
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        // The `Borrow` trait requires that `x.borrow().hash(s) == x.hash(s)`.
+        self.0.kind.hash(s)
+    }
+}
+
+impl<'tcx, T> Borrow<[T]> for InternedInSet<'tcx, List<T>> {
+    fn borrow<'a>(&'a self) -> &'a [T] {
+        &self.0[..]
+    }
+}
+
+impl<'tcx, T: PartialEq> PartialEq for InternedInSet<'tcx, List<T>> {
+    fn eq(&self, other: &InternedInSet<'tcx, List<T>>) -> bool {
+        // The `Borrow` trait requires that `x.borrow() == y.borrow()` equals
+        // `x == y`.
+        self.0[..] == other.0[..]
+    }
+}
+
+impl<'tcx, T: Eq> Eq for InternedInSet<'tcx, List<T>> {}
+
+impl<'tcx, T: Hash> Hash for InternedInSet<'tcx, List<T>> {
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        // The `Borrow` trait requires that `x.borrow().hash(s) == x.hash(s)`.
+        self.0[..].hash(s)
+    }
+}
+
+macro_rules! direct_interners {
+    ($($name:ident: $method:ident($ty:ty): $ret_ctor:ident -> $ret_ty:ty,)+) => {
+        $(impl<'tcx> Borrow<$ty> for InternedInSet<'tcx, $ty> {
+            fn borrow<'a>(&'a self) -> &'a $ty {
+                &self.0
+            }
+        }
+
+        impl<'tcx> PartialEq for InternedInSet<'tcx, $ty> {
+            fn eq(&self, other: &Self) -> bool {
+                // The `Borrow` trait requires that `x.borrow() == y.borrow()`
+                // equals `x == y`.
+                self.0 == other.0
+            }
+        }
+
+        impl<'tcx> Eq for InternedInSet<'tcx, $ty> {}
+
+        impl<'tcx> Hash for InternedInSet<'tcx, $ty> {
+            fn hash<H: Hasher>(&self, s: &mut H) {
+                // The `Borrow` trait requires that `x.borrow().hash(s) ==
+                // x.hash(s)`.
+                self.0.hash(s)
+            }
+        }
+
+        impl<'tcx> TyCtxt<'tcx> {
+            pub fn $method(self, v: $ty) -> $ret_ty {
+                $ret_ctor(Interned::new_unchecked(self.interners.$name.intern(v, |v| {
+                    InternedInSet(self.interners.arena.alloc(v))
+                }).0))
+            }
+        })+
+    }
+}
+
+direct_interners! {
+    region: mk_region(RegionKind<'tcx>): Region -> Region<'tcx>,
+    const_: mk_const(ConstS<'tcx>): Const -> Const<'tcx>,
+    const_allocation: intern_const_alloc(Allocation): ConstAllocation -> ConstAllocation<'tcx>,
+    layout: intern_layout(LayoutS<'tcx>): Layout -> Layout<'tcx>,
+    adt_def: intern_adt_def(AdtDefData): AdtDef -> AdtDef<'tcx>,
+}
+
+macro_rules! slice_interners {
+    ($($field:ident: $method:ident($ty:ty)),+ $(,)?) => (
+        impl<'tcx> TyCtxt<'tcx> {
+            $(pub fn $method(self, v: &[$ty]) -> &'tcx List<$ty> {
+                self.interners.$field.intern_ref(v, || {
+                    InternedInSet(List::from_arena(&*self.arena, v))
+                }).0
+            })+
+        }
+    );
+}
+
+slice_interners!(
+    substs: _intern_substs(GenericArg<'tcx>),
+    canonical_var_infos: _intern_canonical_var_infos(CanonicalVarInfo<'tcx>),
+    poly_existential_predicates:
+        _intern_poly_existential_predicates(ty::Binder<'tcx, ExistentialPredicate<'tcx>>),
+    predicates: _intern_predicates(Predicate<'tcx>),
+    projs: _intern_projs(ProjectionKind),
+    place_elems: _intern_place_elems(PlaceElem<'tcx>),
+    bound_variable_kinds: _intern_bound_variable_kinds(ty::BoundVariableKind),
+);
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Given a `fn` type, returns an equivalent `unsafe fn` type;
+    /// that is, a `fn` type that is equivalent in every way for being
+    /// unsafe.
+    pub fn safe_to_unsafe_fn_ty(self, sig: PolyFnSig<'tcx>) -> Ty<'tcx> {
+        assert_eq!(sig.unsafety(), hir::Unsafety::Normal);
+        self.mk_fn_ptr(sig.map_bound(|sig| ty::FnSig { unsafety: hir::Unsafety::Unsafe, ..sig }))
+    }
+
+    /// Given the def_id of a Trait `trait_def_id` and the name of an associated item `assoc_name`
+    /// returns true if the `trait_def_id` defines an associated item of name `assoc_name`.
+    pub fn trait_may_define_assoc_type(self, trait_def_id: DefId, assoc_name: Ident) -> bool {
+        self.super_traits_of(trait_def_id).any(|trait_did| {
+            self.associated_items(trait_did)
+                .find_by_name_and_kind(self, assoc_name, ty::AssocKind::Type, trait_did)
+                .is_some()
+        })
+    }
+
+    /// Given a `ty`, return whether it's an `impl Future<...>`.
+    pub fn ty_is_opaque_future(self, ty: Ty<'_>) -> bool {
+        let ty::Opaque(def_id, _) = ty.kind() else { return false };
+        let future_trait = self.lang_items().future_trait().unwrap();
+
+        self.explicit_item_bounds(def_id).iter().any(|(predicate, _)| {
+            let ty::PredicateKind::Trait(trait_predicate) = predicate.kind().skip_binder() else {
+                return false;
+            };
+            trait_predicate.trait_ref.def_id == future_trait
+                && trait_predicate.polarity == ImplPolarity::Positive
+        })
+    }
+
+    /// Computes the def-ids of the transitive supertraits of `trait_def_id`. This (intentionally)
+    /// does not compute the full elaborated super-predicates but just the set of def-ids. It is used
+    /// to identify which traits may define a given associated type to help avoid cycle errors.
+    /// Returns a `DefId` iterator.
+    fn super_traits_of(self, trait_def_id: DefId) -> impl Iterator<Item = DefId> + 'tcx {
+        let mut set = FxHashSet::default();
+        let mut stack = vec![trait_def_id];
+
+        set.insert(trait_def_id);
+
+        iter::from_fn(move || -> Option<DefId> {
+            let trait_did = stack.pop()?;
+            let generic_predicates = self.super_predicates_of(trait_did);
+
+            for (predicate, _) in generic_predicates.predicates {
+                if let ty::PredicateKind::Trait(data) = predicate.kind().skip_binder() {
+                    if set.insert(data.def_id()) {
+                        stack.push(data.def_id());
+                    }
+                }
+            }
+
+            Some(trait_did)
+        })
+    }
+
+    /// Given a closure signature, returns an equivalent fn signature. Detuples
+    /// and so forth -- so e.g., if we have a sig with `Fn<(u32, i32)>` then
+    /// you would get a `fn(u32, i32)`.
+    /// `unsafety` determines the unsafety of the fn signature. If you pass
+    /// `hir::Unsafety::Unsafe` in the previous example, then you would get
+    /// an `unsafe fn (u32, i32)`.
+    /// It cannot convert a closure that requires unsafe.
+    pub fn signature_unclosure(
+        self,
+        sig: PolyFnSig<'tcx>,
+        unsafety: hir::Unsafety,
+    ) -> PolyFnSig<'tcx> {
+        sig.map_bound(|s| {
+            let params_iter = match s.inputs()[0].kind() {
+                ty::Tuple(params) => params.into_iter(),
+                _ => bug!(),
+            };
+            self.mk_fn_sig(params_iter, s.output(), s.c_variadic, unsafety, abi::Abi::Rust)
+        })
+    }
+
+    /// Same a `self.mk_region(kind)`, but avoids accessing the interners if
+    /// `*r == kind`.
+    #[inline]
+    pub fn reuse_or_mk_region(self, r: Region<'tcx>, kind: RegionKind<'tcx>) -> Region<'tcx> {
+        if *r == kind { r } else { self.mk_region(kind) }
+    }
+
+    #[allow(rustc::usage_of_ty_tykind)]
+    #[inline]
+    pub fn mk_ty(self, st: TyKind<'tcx>) -> Ty<'tcx> {
+        self.interners.intern_ty(
+            st,
+            self.sess,
+            &self.definitions.read(),
+            &*self.cstore,
+            // This is only used to create a stable hashing context.
+            &self.untracked_resolutions.source_span,
+        )
+    }
+
+    #[inline]
+    pub fn mk_predicate(self, binder: Binder<'tcx, PredicateKind<'tcx>>) -> Predicate<'tcx> {
+        self.interners.intern_predicate(binder)
+    }
+
+    #[inline]
+    pub fn reuse_or_mk_predicate(
+        self,
+        pred: Predicate<'tcx>,
+        binder: Binder<'tcx, PredicateKind<'tcx>>,
+    ) -> Predicate<'tcx> {
+        if pred.kind() != binder { self.mk_predicate(binder) } else { pred }
+    }
+
+    pub fn mk_mach_int(self, tm: IntTy) -> Ty<'tcx> {
+        match tm {
+            IntTy::Isize => self.types.isize,
+            IntTy::I8 => self.types.i8,
+            IntTy::I16 => self.types.i16,
+            IntTy::I32 => self.types.i32,
+            IntTy::I64 => self.types.i64,
+            IntTy::I128 => self.types.i128,
+        }
+    }
+
+    pub fn mk_mach_uint(self, tm: UintTy) -> Ty<'tcx> {
+        match tm {
+            UintTy::Usize => self.types.usize,
+            UintTy::U8 => self.types.u8,
+            UintTy::U16 => self.types.u16,
+            UintTy::U32 => self.types.u32,
+            UintTy::U64 => self.types.u64,
+            UintTy::U128 => self.types.u128,
+        }
+    }
+
+    pub fn mk_mach_float(self, tm: FloatTy) -> Ty<'tcx> {
+        match tm {
+            FloatTy::F32 => self.types.f32,
+            FloatTy::F64 => self.types.f64,
+        }
+    }
+
+    #[inline]
+    pub fn mk_static_str(self) -> Ty<'tcx> {
+        self.mk_imm_ref(self.lifetimes.re_static, self.types.str_)
+    }
+
+    #[inline]
+    pub fn mk_adt(self, def: AdtDef<'tcx>, substs: SubstsRef<'tcx>) -> Ty<'tcx> {
+        // Take a copy of substs so that we own the vectors inside.
+        self.mk_ty(Adt(def, substs))
+    }
+
+    #[inline]
+    pub fn mk_foreign(self, def_id: DefId) -> Ty<'tcx> {
+        self.mk_ty(Foreign(def_id))
+    }
+
+    fn mk_generic_adt(self, wrapper_def_id: DefId, ty_param: Ty<'tcx>) -> Ty<'tcx> {
+        let adt_def = self.adt_def(wrapper_def_id);
+        let substs =
+            InternalSubsts::for_item(self, wrapper_def_id, |param, substs| match param.kind {
+                GenericParamDefKind::Lifetime | GenericParamDefKind::Const { .. } => bug!(),
+                GenericParamDefKind::Type { has_default, .. } => {
+                    if param.index == 0 {
+                        ty_param.into()
+                    } else {
+                        assert!(has_default);
+                        self.bound_type_of(param.def_id).subst(self, substs).into()
+                    }
+                }
+            });
+        self.mk_ty(Adt(adt_def, substs))
+    }
+
+    #[inline]
+    pub fn mk_box(self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        let def_id = self.require_lang_item(LangItem::OwnedBox, None);
+        self.mk_generic_adt(def_id, ty)
+    }
+
+    #[inline]
+    pub fn mk_lang_item(self, ty: Ty<'tcx>, item: LangItem) -> Option<Ty<'tcx>> {
+        let def_id = self.lang_items().require(item).ok()?;
+        Some(self.mk_generic_adt(def_id, ty))
+    }
+
+    #[inline]
+    pub fn mk_diagnostic_item(self, ty: Ty<'tcx>, name: Symbol) -> Option<Ty<'tcx>> {
+        let def_id = self.get_diagnostic_item(name)?;
+        Some(self.mk_generic_adt(def_id, ty))
+    }
+
+    #[inline]
+    pub fn mk_maybe_uninit(self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        let def_id = self.require_lang_item(LangItem::MaybeUninit, None);
+        self.mk_generic_adt(def_id, ty)
+    }
+
+    #[inline]
+    pub fn mk_ptr(self, tm: TypeAndMut<'tcx>) -> Ty<'tcx> {
+        self.mk_ty(RawPtr(tm))
+    }
+
+    #[inline]
+    pub fn mk_ref(self, r: Region<'tcx>, tm: TypeAndMut<'tcx>) -> Ty<'tcx> {
+        self.mk_ty(Ref(r, tm.ty, tm.mutbl))
+    }
+
+    #[inline]
+    pub fn mk_mut_ref(self, r: Region<'tcx>, ty: Ty<'tcx>) -> Ty<'tcx> {
+        self.mk_ref(r, TypeAndMut { ty, mutbl: hir::Mutability::Mut })
+    }
+
+    #[inline]
+    pub fn mk_imm_ref(self, r: Region<'tcx>, ty: Ty<'tcx>) -> Ty<'tcx> {
+        self.mk_ref(r, TypeAndMut { ty, mutbl: hir::Mutability::Not })
+    }
+
+    #[inline]
+    pub fn mk_mut_ptr(self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        self.mk_ptr(TypeAndMut { ty, mutbl: hir::Mutability::Mut })
+    }
+
+    #[inline]
+    pub fn mk_imm_ptr(self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        self.mk_ptr(TypeAndMut { ty, mutbl: hir::Mutability::Not })
+    }
+
+    #[inline]
+    pub fn mk_array(self, ty: Ty<'tcx>, n: u64) -> Ty<'tcx> {
+        self.mk_ty(Array(ty, ty::Const::from_usize(self, n)))
+    }
+
+    #[inline]
+    pub fn mk_slice(self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        self.mk_ty(Slice(ty))
+    }
+
+    #[inline]
+    pub fn intern_tup(self, ts: &[Ty<'tcx>]) -> Ty<'tcx> {
+        self.mk_ty(Tuple(self.intern_type_list(&ts)))
+    }
+
+    pub fn mk_tup<I: InternAs<[Ty<'tcx>], Ty<'tcx>>>(self, iter: I) -> I::Output {
+        iter.intern_with(|ts| self.mk_ty(Tuple(self.intern_type_list(&ts))))
+    }
+
+    #[inline]
+    pub fn mk_unit(self) -> Ty<'tcx> {
+        self.types.unit
+    }
+
+    #[inline]
+    pub fn mk_diverging_default(self) -> Ty<'tcx> {
+        if self.features().never_type_fallback { self.types.never } else { self.types.unit }
+    }
+
+    #[inline]
+    pub fn mk_fn_def(self, def_id: DefId, substs: SubstsRef<'tcx>) -> Ty<'tcx> {
+        self.mk_ty(FnDef(def_id, substs))
+    }
+
+    #[inline]
+    pub fn mk_fn_ptr(self, fty: PolyFnSig<'tcx>) -> Ty<'tcx> {
+        self.mk_ty(FnPtr(fty))
+    }
+
+    #[inline]
+    pub fn mk_dynamic(
+        self,
+        obj: &'tcx List<ty::Binder<'tcx, ExistentialPredicate<'tcx>>>,
+        reg: ty::Region<'tcx>,
+    ) -> Ty<'tcx> {
+        self.mk_ty(Dynamic(obj, reg))
+    }
+
+    #[inline]
+    pub fn mk_projection(self, item_def_id: DefId, substs: SubstsRef<'tcx>) -> Ty<'tcx> {
+        self.mk_ty(Projection(ProjectionTy { item_def_id, substs }))
+    }
+
+    #[inline]
+    pub fn mk_closure(self, closure_id: DefId, closure_substs: SubstsRef<'tcx>) -> Ty<'tcx> {
+        self.mk_ty(Closure(closure_id, closure_substs))
+    }
+
+    #[inline]
+    pub fn mk_generator(
+        self,
+        id: DefId,
+        generator_substs: SubstsRef<'tcx>,
+        movability: hir::Movability,
+    ) -> Ty<'tcx> {
+        self.mk_ty(Generator(id, generator_substs, movability))
+    }
+
+    #[inline]
+    pub fn mk_generator_witness(self, types: ty::Binder<'tcx, &'tcx List<Ty<'tcx>>>) -> Ty<'tcx> {
+        self.mk_ty(GeneratorWitness(types))
+    }
+
+    #[inline]
+    pub fn mk_ty_var(self, v: TyVid) -> Ty<'tcx> {
+        self.mk_ty_infer(TyVar(v))
+    }
+
+    #[inline]
+    pub fn mk_const_var(self, v: ConstVid<'tcx>, ty: Ty<'tcx>) -> Const<'tcx> {
+        self.mk_const(ty::ConstS { kind: ty::ConstKind::Infer(InferConst::Var(v)), ty })
+    }
+
+    #[inline]
+    pub fn mk_int_var(self, v: IntVid) -> Ty<'tcx> {
+        self.mk_ty_infer(IntVar(v))
+    }
+
+    #[inline]
+    pub fn mk_float_var(self, v: FloatVid) -> Ty<'tcx> {
+        self.mk_ty_infer(FloatVar(v))
+    }
+
+    #[inline]
+    pub fn mk_ty_infer(self, it: InferTy) -> Ty<'tcx> {
+        self.mk_ty(Infer(it))
+    }
+
+    #[inline]
+    pub fn mk_const_infer(self, ic: InferConst<'tcx>, ty: Ty<'tcx>) -> ty::Const<'tcx> {
+        self.mk_const(ty::ConstS { kind: ty::ConstKind::Infer(ic), ty })
+    }
+
+    #[inline]
+    pub fn mk_ty_param(self, index: u32, name: Symbol) -> Ty<'tcx> {
+        self.mk_ty(Param(ParamTy { index, name }))
+    }
+
+    #[inline]
+    pub fn mk_const_param(self, index: u32, name: Symbol, ty: Ty<'tcx>) -> Const<'tcx> {
+        self.mk_const(ty::ConstS { kind: ty::ConstKind::Param(ParamConst { index, name }), ty })
+    }
+
+    pub fn mk_param_from_def(self, param: &ty::GenericParamDef) -> GenericArg<'tcx> {
+        match param.kind {
+            GenericParamDefKind::Lifetime => {
+                self.mk_region(ty::ReEarlyBound(param.to_early_bound_region_data())).into()
+            }
+            GenericParamDefKind::Type { .. } => self.mk_ty_param(param.index, param.name).into(),
+            GenericParamDefKind::Const { .. } => {
+                self.mk_const_param(param.index, param.name, self.type_of(param.def_id)).into()
+            }
+        }
+    }
+
+    #[inline]
+    pub fn mk_opaque(self, def_id: DefId, substs: SubstsRef<'tcx>) -> Ty<'tcx> {
+        self.mk_ty(Opaque(def_id, substs))
+    }
+
+    pub fn mk_place_field(self, place: Place<'tcx>, f: Field, ty: Ty<'tcx>) -> Place<'tcx> {
+        self.mk_place_elem(place, PlaceElem::Field(f, ty))
+    }
+
+    pub fn mk_place_deref(self, place: Place<'tcx>) -> Place<'tcx> {
+        self.mk_place_elem(place, PlaceElem::Deref)
+    }
+
+    pub fn mk_place_downcast(
+        self,
+        place: Place<'tcx>,
+        adt_def: AdtDef<'tcx>,
+        variant_index: VariantIdx,
+    ) -> Place<'tcx> {
+        self.mk_place_elem(
+            place,
+            PlaceElem::Downcast(Some(adt_def.variant(variant_index).name), variant_index),
+        )
+    }
+
+    pub fn mk_place_downcast_unnamed(
+        self,
+        place: Place<'tcx>,
+        variant_index: VariantIdx,
+    ) -> Place<'tcx> {
+        self.mk_place_elem(place, PlaceElem::Downcast(None, variant_index))
+    }
+
+    pub fn mk_place_index(self, place: Place<'tcx>, index: Local) -> Place<'tcx> {
+        self.mk_place_elem(place, PlaceElem::Index(index))
+    }
+
+    /// This method copies `Place`'s projection, add an element and reintern it. Should not be used
+    /// to build a full `Place` it's just a convenient way to grab a projection and modify it in
+    /// flight.
+    pub fn mk_place_elem(self, place: Place<'tcx>, elem: PlaceElem<'tcx>) -> Place<'tcx> {
+        let mut projection = place.projection.to_vec();
+        projection.push(elem);
+
+        Place { local: place.local, projection: self.intern_place_elems(&projection) }
+    }
+
+    pub fn intern_poly_existential_predicates(
+        self,
+        eps: &[ty::Binder<'tcx, ExistentialPredicate<'tcx>>],
+    ) -> &'tcx List<ty::Binder<'tcx, ExistentialPredicate<'tcx>>> {
+        assert!(!eps.is_empty());
+        assert!(
+            eps.array_windows()
+                .all(|[a, b]| a.skip_binder().stable_cmp(self, &b.skip_binder())
+                    != Ordering::Greater)
+        );
+        self._intern_poly_existential_predicates(eps)
+    }
+
+    pub fn intern_predicates(self, preds: &[Predicate<'tcx>]) -> &'tcx List<Predicate<'tcx>> {
+        // FIXME consider asking the input slice to be sorted to avoid
+        // re-interning permutations, in which case that would be asserted
+        // here.
+        if preds.is_empty() {
+            // The macro-generated method below asserts we don't intern an empty slice.
+            List::empty()
+        } else {
+            self._intern_predicates(preds)
+        }
+    }
+
+    pub fn intern_type_list(self, ts: &[Ty<'tcx>]) -> &'tcx List<Ty<'tcx>> {
+        if ts.is_empty() {
+            List::empty()
+        } else {
+            // Actually intern type lists as lists of `GenericArg`s.
+            //
+            // Transmuting from `Ty<'tcx>` to `GenericArg<'tcx>` is sound
+            // as explained in ty_slice_as_generic_arg`. With this,
+            // we guarantee that even when transmuting between `List<Ty<'tcx>>`
+            // and `List<GenericArg<'tcx>>`, the uniqueness requirement for
+            // lists is upheld.
+            let substs = self._intern_substs(ty::subst::ty_slice_as_generic_args(ts));
+            substs.try_as_type_list().unwrap()
+        }
+    }
+
+    pub fn intern_substs(self, ts: &[GenericArg<'tcx>]) -> &'tcx List<GenericArg<'tcx>> {
+        if ts.is_empty() { List::empty() } else { self._intern_substs(ts) }
+    }
+
+    pub fn intern_projs(self, ps: &[ProjectionKind]) -> &'tcx List<ProjectionKind> {
+        if ps.is_empty() { List::empty() } else { self._intern_projs(ps) }
+    }
+
+    pub fn intern_place_elems(self, ts: &[PlaceElem<'tcx>]) -> &'tcx List<PlaceElem<'tcx>> {
+        if ts.is_empty() { List::empty() } else { self._intern_place_elems(ts) }
+    }
+
+    pub fn intern_canonical_var_infos(
+        self,
+        ts: &[CanonicalVarInfo<'tcx>],
+    ) -> CanonicalVarInfos<'tcx> {
+        if ts.is_empty() { List::empty() } else { self._intern_canonical_var_infos(ts) }
+    }
+
+    pub fn intern_bound_variable_kinds(
+        self,
+        ts: &[ty::BoundVariableKind],
+    ) -> &'tcx List<ty::BoundVariableKind> {
+        if ts.is_empty() { List::empty() } else { self._intern_bound_variable_kinds(ts) }
+    }
+
+    pub fn mk_fn_sig<I>(
+        self,
+        inputs: I,
+        output: I::Item,
+        c_variadic: bool,
+        unsafety: hir::Unsafety,
+        abi: abi::Abi,
+    ) -> <I::Item as InternIteratorElement<Ty<'tcx>, ty::FnSig<'tcx>>>::Output
+    where
+        I: Iterator<Item: InternIteratorElement<Ty<'tcx>, ty::FnSig<'tcx>>>,
+    {
+        inputs.chain(iter::once(output)).intern_with(|xs| ty::FnSig {
+            inputs_and_output: self.intern_type_list(xs),
+            c_variadic,
+            unsafety,
+            abi,
+        })
+    }
+
+    pub fn mk_poly_existential_predicates<
+        I: InternAs<
+            [ty::Binder<'tcx, ExistentialPredicate<'tcx>>],
+            &'tcx List<ty::Binder<'tcx, ExistentialPredicate<'tcx>>>,
+        >,
+    >(
+        self,
+        iter: I,
+    ) -> I::Output {
+        iter.intern_with(|xs| self.intern_poly_existential_predicates(xs))
+    }
+
+    pub fn mk_predicates<I: InternAs<[Predicate<'tcx>], &'tcx List<Predicate<'tcx>>>>(
+        self,
+        iter: I,
+    ) -> I::Output {
+        iter.intern_with(|xs| self.intern_predicates(xs))
+    }
+
+    pub fn mk_type_list<I: InternAs<[Ty<'tcx>], &'tcx List<Ty<'tcx>>>>(self, iter: I) -> I::Output {
+        iter.intern_with(|xs| self.intern_type_list(xs))
+    }
+
+    pub fn mk_substs<I: InternAs<[GenericArg<'tcx>], &'tcx List<GenericArg<'tcx>>>>(
+        self,
+        iter: I,
+    ) -> I::Output {
+        iter.intern_with(|xs| self.intern_substs(xs))
+    }
+
+    pub fn mk_place_elems<I: InternAs<[PlaceElem<'tcx>], &'tcx List<PlaceElem<'tcx>>>>(
+        self,
+        iter: I,
+    ) -> I::Output {
+        iter.intern_with(|xs| self.intern_place_elems(xs))
+    }
+
+    pub fn mk_substs_trait(self, self_ty: Ty<'tcx>, rest: &[GenericArg<'tcx>]) -> SubstsRef<'tcx> {
+        self.mk_substs(iter::once(self_ty.into()).chain(rest.iter().cloned()))
+    }
+
+    pub fn mk_bound_variable_kinds<
+        I: InternAs<[ty::BoundVariableKind], &'tcx List<ty::BoundVariableKind>>,
+    >(
+        self,
+        iter: I,
+    ) -> I::Output {
+        iter.intern_with(|xs| self.intern_bound_variable_kinds(xs))
+    }
+
+    /// Walks upwards from `id` to find a node which might change lint levels with attributes.
+    /// It stops at `bound` and just returns it if reached.
+    pub fn maybe_lint_level_root_bounded(self, mut id: HirId, bound: HirId) -> HirId {
+        let hir = self.hir();
+        loop {
+            if id == bound {
+                return bound;
+            }
+
+            if hir.attrs(id).iter().any(|attr| Level::from_attr(attr).is_some()) {
+                return id;
+            }
+            let next = hir.get_parent_node(id);
+            if next == id {
+                bug!("lint traversal reached the root of the crate");
+            }
+            id = next;
+        }
+    }
+
+    pub fn lint_level_at_node(
+        self,
+        lint: &'static Lint,
+        mut id: hir::HirId,
+    ) -> (Level, LintLevelSource) {
+        let sets = self.lint_levels(());
+        loop {
+            if let Some(pair) = sets.level_and_source(lint, id, self.sess) {
+                return pair;
+            }
+            let next = self.hir().get_parent_node(id);
+            if next == id {
+                bug!("lint traversal reached the root of the crate");
+            }
+            id = next;
+        }
+    }
+
+    /// Emit a lint at `span` from a lint struct (some type that implements `DecorateLint`,
+    /// typically generated by `#[derive(LintDiagnostic)]`).
+    pub fn emit_spanned_lint(
+        self,
+        lint: &'static Lint,
+        hir_id: HirId,
+        span: impl Into<MultiSpan>,
+        decorator: impl for<'a> DecorateLint<'a, ()>,
+    ) {
+        self.struct_span_lint_hir(lint, hir_id, span, |diag| decorator.decorate_lint(diag))
+    }
+
+    pub fn struct_span_lint_hir(
+        self,
+        lint: &'static Lint,
+        hir_id: HirId,
+        span: impl Into<MultiSpan>,
+        decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a, ()>),
+    ) {
+        let (level, src) = self.lint_level_at_node(lint, hir_id);
+        struct_lint_level(self.sess, lint, level, src, Some(span.into()), decorate);
+    }
+
+    /// Emit a lint from a lint struct (some type that implements `DecorateLint`, typically
+    /// generated by `#[derive(LintDiagnostic)]`).
+    pub fn emit_lint(
+        self,
+        lint: &'static Lint,
+        id: HirId,
+        decorator: impl for<'a> DecorateLint<'a, ()>,
+    ) {
+        self.struct_lint_node(lint, id, |diag| decorator.decorate_lint(diag))
+    }
+
+    pub fn struct_lint_node(
+        self,
+        lint: &'static Lint,
+        id: HirId,
+        decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a, ()>),
+    ) {
+        let (level, src) = self.lint_level_at_node(lint, id);
+        struct_lint_level(self.sess, lint, level, src, None, decorate);
+    }
+
+    pub fn in_scope_traits(self, id: HirId) -> Option<&'tcx [TraitCandidate]> {
+        let map = self.in_scope_traits_map(id.owner)?;
+        let candidates = map.get(&id.local_id)?;
+        Some(&*candidates)
+    }
+
+    pub fn named_region(self, id: HirId) -> Option<resolve_lifetime::Region> {
+        debug!(?id, "named_region");
+        self.named_region_map(id.owner).and_then(|map| map.get(&id.local_id).cloned())
+    }
+
+    pub fn is_late_bound(self, id: HirId) -> bool {
+        self.is_late_bound_map(id.owner).map_or(false, |set| {
+            let def_id = self.hir().local_def_id(id);
+            set.contains(&def_id)
+        })
+    }
+
+    pub fn late_bound_vars(self, id: HirId) -> &'tcx List<ty::BoundVariableKind> {
+        self.mk_bound_variable_kinds(
+            self.late_bound_vars_map(id.owner)
+                .and_then(|map| map.get(&id.local_id).cloned())
+                .unwrap_or_else(|| {
+                    bug!("No bound vars found for {:?} ({:?})", self.hir().node_to_string(id), id)
+                })
+                .iter(),
+        )
+    }
+
+    /// Whether the `def_id` counts as const fn in the current crate, considering all active
+    /// feature gates
+    pub fn is_const_fn(self, def_id: DefId) -> bool {
+        if self.is_const_fn_raw(def_id) {
+            match self.lookup_const_stability(def_id) {
+                Some(stability) if stability.is_const_unstable() => {
+                    // has a `rustc_const_unstable` attribute, check whether the user enabled the
+                    // corresponding feature gate.
+                    self.features()
+                        .declared_lib_features
+                        .iter()
+                        .any(|&(sym, _)| sym == stability.feature)
+                }
+                // functions without const stability are either stable user written
+                // const fn or the user is using feature gates and we thus don't
+                // care what they do
+                _ => true,
+            }
+        } else {
+            false
+        }
+    }
+
+    /// Whether the trait impl is marked const. This does not consider stability or feature gates.
+    pub fn is_const_trait_impl_raw(self, def_id: DefId) -> bool {
+        let Some(local_def_id) = def_id.as_local() else { return false };
+        let hir_id = self.local_def_id_to_hir_id(local_def_id);
+        let node = self.hir().get(hir_id);
+
+        matches!(
+            node,
+            hir::Node::Item(hir::Item {
+                kind: hir::ItemKind::Impl(hir::Impl { constness: hir::Constness::Const, .. }),
+                ..
+            })
+        )
+    }
+}
+
+impl<'tcx> TyCtxtAt<'tcx> {
+    /// Constructs a `TyKind::Error` type and registers a `delay_span_bug` to ensure it gets used.
+    #[track_caller]
+    pub fn ty_error(self) -> Ty<'tcx> {
+        self.tcx.ty_error_with_message(self.span, "TyKind::Error constructed but no error reported")
+    }
+
+    /// Constructs a `TyKind::Error` type and registers a `delay_span_bug` with the given `msg to
+    /// ensure it gets used.
+    #[track_caller]
+    pub fn ty_error_with_message(self, msg: &str) -> Ty<'tcx> {
+        self.tcx.ty_error_with_message(self.span, msg)
+    }
+}
+
+// We are comparing types with different invariant lifetimes, so `ptr::eq`
+// won't work for us.
+fn ptr_eq<T, U>(t: *const T, u: *const U) -> bool {
+    t as *const () == u as *const ()
+}
+
+pub fn provide(providers: &mut ty::query::Providers) {
+    providers.resolutions = |tcx, ()| &tcx.untracked_resolutions;
+    providers.resolver_for_lowering = |tcx, ()| &tcx.untracked_resolver_for_lowering;
+    providers.module_reexports =
+        |tcx, id| tcx.resolutions(()).reexport_map.get(&id).map(|v| &v[..]);
+    providers.crate_name = |tcx, id| {
+        assert_eq!(id, LOCAL_CRATE);
+        tcx.crate_name
+    };
+    providers.maybe_unused_trait_imports =
+        |tcx, ()| &tcx.resolutions(()).maybe_unused_trait_imports;
+    providers.maybe_unused_extern_crates =
+        |tcx, ()| &tcx.resolutions(()).maybe_unused_extern_crates[..];
+    providers.names_imported_by_glob_use = |tcx, id| {
+        tcx.arena.alloc(tcx.resolutions(()).glob_map.get(&id).cloned().unwrap_or_default())
+    };
+
+    providers.extern_mod_stmt_cnum =
+        |tcx, id| tcx.resolutions(()).extern_crate_map.get(&id).cloned();
+    providers.output_filenames = |tcx, ()| &tcx.output_filenames;
+    providers.features_query = |tcx, ()| tcx.sess.features_untracked();
+    providers.is_panic_runtime = |tcx, cnum| {
+        assert_eq!(cnum, LOCAL_CRATE);
+        tcx.sess.contains_name(tcx.hir().krate_attrs(), sym::panic_runtime)
+    };
+    providers.is_compiler_builtins = |tcx, cnum| {
+        assert_eq!(cnum, LOCAL_CRATE);
+        tcx.sess.contains_name(tcx.hir().krate_attrs(), sym::compiler_builtins)
+    };
+    providers.has_panic_handler = |tcx, cnum| {
+        assert_eq!(cnum, LOCAL_CRATE);
+        // We want to check if the panic handler was defined in this crate
+        tcx.lang_items().panic_impl().map_or(false, |did| did.is_local())
+    };
+}
diff --git a/compiler/rustc_middle/src/ty/diagnostics.rs b/compiler/rustc_middle/src/ty/diagnostics.rs
new file mode 100644
index 000000000..dd2f43210
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/diagnostics.rs
@@ -0,0 +1,501 @@
+//! Diagnostics related methods for `Ty`.
+
+use std::ops::ControlFlow;
+
+use crate::ty::{
+    visit::TypeVisitable, Const, ConstKind, DefIdTree, ExistentialPredicate, InferConst, InferTy,
+    PolyTraitPredicate, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor,
+};
+
+use rustc_data_structures::fx::FxHashMap;
+use rustc_errors::{Applicability, Diagnostic, DiagnosticArgValue, IntoDiagnosticArg};
+use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use rustc_hir::WherePredicate;
+use rustc_span::Span;
+use rustc_type_ir::sty::TyKind::*;
+
+impl<'tcx> IntoDiagnosticArg for Ty<'tcx> {
+    fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static> {
+        format!("{}", self).into_diagnostic_arg()
+    }
+}
+
+impl<'tcx> Ty<'tcx> {
+    /// Similar to `Ty::is_primitive`, but also considers inferred numeric values to be primitive.
+    pub fn is_primitive_ty(self) -> bool {
+        matches!(
+            self.kind(),
+            Bool | Char
+                | Str
+                | Int(_)
+                | Uint(_)
+                | Float(_)
+                | Infer(
+                    InferTy::IntVar(_)
+                        | InferTy::FloatVar(_)
+                        | InferTy::FreshIntTy(_)
+                        | InferTy::FreshFloatTy(_)
+                )
+        )
+    }
+
+    /// Whether the type is succinctly representable as a type instead of just referred to with a
+    /// description in error messages. This is used in the main error message.
+    pub fn is_simple_ty(self) -> bool {
+        match self.kind() {
+            Bool
+            | Char
+            | Str
+            | Int(_)
+            | Uint(_)
+            | Float(_)
+            | Infer(
+                InferTy::IntVar(_)
+                | InferTy::FloatVar(_)
+                | InferTy::FreshIntTy(_)
+                | InferTy::FreshFloatTy(_),
+            ) => true,
+            Ref(_, x, _) | Array(x, _) | Slice(x) => x.peel_refs().is_simple_ty(),
+            Tuple(tys) if tys.is_empty() => true,
+            _ => false,
+        }
+    }
+
+    /// Whether the type is succinctly representable as a type instead of just referred to with a
+    /// description in error messages. This is used in the primary span label. Beyond what
+    /// `is_simple_ty` includes, it also accepts ADTs with no type arguments and references to
+    /// ADTs with no type arguments.
+    pub fn is_simple_text(self) -> bool {
+        match self.kind() {
+            Adt(_, substs) => substs.non_erasable_generics().next().is_none(),
+            Ref(_, ty, _) => ty.is_simple_text(),
+            _ => self.is_simple_ty(),
+        }
+    }
+}
+
+pub trait IsSuggestable<'tcx> {
+    /// Whether this makes sense to suggest in a diagnostic.
+    ///
+    /// We filter out certain types and constants since they don't provide
+    /// meaningful rendered suggestions when pretty-printed. We leave some
+    /// nonsense, such as region vars, since those render as `'_` and are
+    /// usually okay to reinterpret as elided lifetimes.
+    ///
+    /// Only if `infer_suggestable` is true, we consider type and const
+    /// inference variables to be suggestable.
+    fn is_suggestable(self, tcx: TyCtxt<'tcx>, infer_suggestable: bool) -> bool;
+}
+
+impl<'tcx, T> IsSuggestable<'tcx> for T
+where
+    T: TypeVisitable<'tcx>,
+{
+    fn is_suggestable(self, tcx: TyCtxt<'tcx>, infer_suggestable: bool) -> bool {
+        self.visit_with(&mut IsSuggestableVisitor { tcx, infer_suggestable }).is_continue()
+    }
+}
+
+pub fn suggest_arbitrary_trait_bound<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    generics: &hir::Generics<'_>,
+    err: &mut Diagnostic,
+    trait_pred: PolyTraitPredicate<'tcx>,
+) -> bool {
+    if !trait_pred.is_suggestable(tcx, false) {
+        return false;
+    }
+
+    let param_name = trait_pred.skip_binder().self_ty().to_string();
+    let constraint = trait_pred.print_modifiers_and_trait_path().to_string();
+    let param = generics.params.iter().find(|p| p.name.ident().as_str() == param_name);
+
+    // Skip, there is a param named Self
+    if param.is_some() && param_name == "Self" {
+        return false;
+    }
+
+    // Suggest a where clause bound for a non-type parameter.
+    err.span_suggestion_verbose(
+        generics.tail_span_for_predicate_suggestion(),
+        &format!(
+            "consider {} `where` clause, but there might be an alternative better way to express \
+             this requirement",
+            if generics.where_clause_span.is_empty() { "introducing a" } else { "extending the" },
+        ),
+        format!("{} {}: {}", generics.add_where_or_trailing_comma(), param_name, constraint),
+        Applicability::MaybeIncorrect,
+    );
+    true
+}
+
+#[derive(Debug)]
+enum SuggestChangingConstraintsMessage<'a> {
+    RestrictBoundFurther,
+    RestrictType { ty: &'a str },
+    RestrictTypeFurther { ty: &'a str },
+    RemovingQSized,
+}
+
+fn suggest_removing_unsized_bound(
+    tcx: TyCtxt<'_>,
+    generics: &hir::Generics<'_>,
+    suggestions: &mut Vec<(Span, String, SuggestChangingConstraintsMessage<'_>)>,
+    param: &hir::GenericParam<'_>,
+    def_id: Option<DefId>,
+) {
+    // See if there's a `?Sized` bound that can be removed to suggest that.
+    // First look at the `where` clause because we can have `where T: ?Sized`,
+    // then look at params.
+    let param_def_id = tcx.hir().local_def_id(param.hir_id);
+    for (where_pos, predicate) in generics.predicates.iter().enumerate() {
+        let WherePredicate::BoundPredicate(predicate) = predicate else {
+            continue;
+        };
+        if !predicate.is_param_bound(param_def_id.to_def_id()) {
+            continue;
+        };
+
+        for (pos, bound) in predicate.bounds.iter().enumerate() {
+            let    hir::GenericBound::Trait(poly, hir::TraitBoundModifier::Maybe) = bound else {
+                continue;
+            };
+            if poly.trait_ref.trait_def_id() != def_id {
+                continue;
+            }
+            let sp = generics.span_for_bound_removal(where_pos, pos);
+            suggestions.push((
+                sp,
+                String::new(),
+                SuggestChangingConstraintsMessage::RemovingQSized,
+            ));
+        }
+    }
+}
+
+/// Suggest restricting a type param with a new bound.
+pub fn suggest_constraining_type_param(
+    tcx: TyCtxt<'_>,
+    generics: &hir::Generics<'_>,
+    err: &mut Diagnostic,
+    param_name: &str,
+    constraint: &str,
+    def_id: Option<DefId>,
+) -> bool {
+    suggest_constraining_type_params(
+        tcx,
+        generics,
+        err,
+        [(param_name, constraint, def_id)].into_iter(),
+    )
+}
+
+/// Suggest restricting a type param with a new bound.
+pub fn suggest_constraining_type_params<'a>(
+    tcx: TyCtxt<'_>,
+    generics: &hir::Generics<'_>,
+    err: &mut Diagnostic,
+    param_names_and_constraints: impl Iterator<Item = (&'a str, &'a str, Option<DefId>)>,
+) -> bool {
+    let mut grouped = FxHashMap::default();
+    param_names_and_constraints.for_each(|(param_name, constraint, def_id)| {
+        grouped.entry(param_name).or_insert(Vec::new()).push((constraint, def_id))
+    });
+
+    let mut applicability = Applicability::MachineApplicable;
+    let mut suggestions = Vec::new();
+
+    for (param_name, mut constraints) in grouped {
+        let param = generics.params.iter().find(|p| p.name.ident().as_str() == param_name);
+        let Some(param) = param else { return false };
+
+        {
+            let mut sized_constraints =
+                constraints.drain_filter(|(_, def_id)| *def_id == tcx.lang_items().sized_trait());
+            if let Some((constraint, def_id)) = sized_constraints.next() {
+                applicability = Applicability::MaybeIncorrect;
+
+                err.span_label(
+                    param.span,
+                    &format!("this type parameter needs to be `{}`", constraint),
+                );
+                suggest_removing_unsized_bound(tcx, generics, &mut suggestions, param, def_id);
+            }
+        }
+
+        if constraints.is_empty() {
+            continue;
+        }
+
+        let mut constraint = constraints.iter().map(|&(c, _)| c).collect::<Vec<_>>();
+        constraint.sort();
+        constraint.dedup();
+        let constraint = constraint.join(" + ");
+        let mut suggest_restrict = |span, bound_list_non_empty| {
+            suggestions.push((
+                span,
+                if bound_list_non_empty {
+                    format!(" + {}", constraint)
+                } else {
+                    format!(" {}", constraint)
+                },
+                SuggestChangingConstraintsMessage::RestrictBoundFurther,
+            ))
+        };
+
+        // When the type parameter has been provided bounds
+        //
+        //    Message:
+        //      fn foo<T>(t: T) where T: Foo { ... }
+        //                            ^^^^^^
+        //                            |
+        //                            help: consider further restricting this bound with `+ Bar`
+        //
+        //    Suggestion:
+        //      fn foo<T>(t: T) where T: Foo { ... }
+        //                                  ^
+        //                                  |
+        //                                  replace with: ` + Bar`
+        //
+        // Or, if user has provided some bounds, suggest restricting them:
+        //
+        //   fn foo<T: Foo>(t: T) { ... }
+        //             ---
+        //             |
+        //             help: consider further restricting this bound with `+ Bar`
+        //
+        // Suggestion for tools in this case is:
+        //
+        //   fn foo<T: Foo>(t: T) { ... }
+        //          --
+        //          |
+        //          replace with: `T: Bar +`
+        let param_def_id = tcx.hir().local_def_id(param.hir_id);
+        if let Some(span) = generics.bounds_span_for_suggestions(param_def_id) {
+            suggest_restrict(span, true);
+            continue;
+        }
+
+        if generics.has_where_clause_predicates {
+            // This part is a bit tricky, because using the `where` clause user can
+            // provide zero, one or many bounds for the same type parameter, so we
+            // have following cases to consider:
+            //
+            // When the type parameter has been provided zero bounds
+            //
+            //    Message:
+            //      fn foo<X, Y>(x: X, y: Y) where Y: Foo { ... }
+            //             - help: consider restricting this type parameter with `where X: Bar`
+            //
+            //    Suggestion:
+            //      fn foo<X, Y>(x: X, y: Y) where Y: Foo { ... }
+            //                                           - insert: `, X: Bar`
+            suggestions.push((
+                generics.tail_span_for_predicate_suggestion(),
+                constraints
+                    .iter()
+                    .map(|&(constraint, _)| format!(", {}: {}", param_name, constraint))
+                    .collect::<String>(),
+                SuggestChangingConstraintsMessage::RestrictTypeFurther { ty: param_name },
+            ));
+            continue;
+        }
+
+        // Additionally, there may be no `where` clause but the generic parameter has a default:
+        //
+        //    Message:
+        //      trait Foo<T=()> {... }
+        //                - help: consider further restricting this type parameter with `where T: Zar`
+        //
+        //    Suggestion:
+        //      trait Foo<T=()> {... }
+        //                     - insert: `where T: Zar`
+        if matches!(param.kind, hir::GenericParamKind::Type { default: Some(_), .. }) {
+            // Suggest a bound, but there is no existing `where` clause *and* the type param has a
+            // default (`<T=Foo>`), so we suggest adding `where T: Bar`.
+            suggestions.push((
+                generics.tail_span_for_predicate_suggestion(),
+                format!(" where {}: {}", param_name, constraint),
+                SuggestChangingConstraintsMessage::RestrictTypeFurther { ty: param_name },
+            ));
+            continue;
+        }
+
+        // If user has provided a colon, don't suggest adding another:
+        //
+        //   fn foo<T:>(t: T) { ... }
+        //            - insert: consider restricting this type parameter with `T: Foo`
+        if let Some(colon_span) = param.colon_span {
+            suggestions.push((
+                colon_span.shrink_to_hi(),
+                format!(" {}", constraint),
+                SuggestChangingConstraintsMessage::RestrictType { ty: param_name },
+            ));
+            continue;
+        }
+
+        // If user hasn't provided any bounds, suggest adding a new one:
+        //
+        //   fn foo<T>(t: T) { ... }
+        //          - help: consider restricting this type parameter with `T: Foo`
+        suggestions.push((
+            param.span.shrink_to_hi(),
+            format!(": {}", constraint),
+            SuggestChangingConstraintsMessage::RestrictType { ty: param_name },
+        ));
+    }
+
+    if suggestions.len() == 1 {
+        let (span, suggestion, msg) = suggestions.pop().unwrap();
+
+        let s;
+        let msg = match msg {
+            SuggestChangingConstraintsMessage::RestrictBoundFurther => {
+                "consider further restricting this bound"
+            }
+            SuggestChangingConstraintsMessage::RestrictType { ty } => {
+                s = format!("consider restricting type parameter `{}`", ty);
+                &s
+            }
+            SuggestChangingConstraintsMessage::RestrictTypeFurther { ty } => {
+                s = format!("consider further restricting type parameter `{}`", ty);
+                &s
+            }
+            SuggestChangingConstraintsMessage::RemovingQSized => {
+                "consider removing the `?Sized` bound to make the type parameter `Sized`"
+            }
+        };
+
+        err.span_suggestion_verbose(span, msg, suggestion, applicability);
+    } else if suggestions.len() > 1 {
+        err.multipart_suggestion_verbose(
+            "consider restricting type parameters",
+            suggestions.into_iter().map(|(span, suggestion, _)| (span, suggestion)).collect(),
+            applicability,
+        );
+    }
+
+    true
+}
+
+/// Collect al types that have an implicit `'static` obligation that we could suggest `'_` for.
+pub struct TraitObjectVisitor<'tcx>(pub Vec<&'tcx hir::Ty<'tcx>>, pub crate::hir::map::Map<'tcx>);
+
+impl<'v> hir::intravisit::Visitor<'v> for TraitObjectVisitor<'v> {
+    fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) {
+        match ty.kind {
+            hir::TyKind::TraitObject(
+                _,
+                hir::Lifetime {
+                    name:
+                        hir::LifetimeName::ImplicitObjectLifetimeDefault | hir::LifetimeName::Static,
+                    ..
+                },
+                _,
+            ) => {
+                self.0.push(ty);
+            }
+            hir::TyKind::OpaqueDef(item_id, _) => {
+                self.0.push(ty);
+                let item = self.1.item(item_id);
+                hir::intravisit::walk_item(self, item);
+            }
+            _ => {}
+        }
+        hir::intravisit::walk_ty(self, ty);
+    }
+}
+
+/// Collect al types that have an implicit `'static` obligation that we could suggest `'_` for.
+pub struct StaticLifetimeVisitor<'tcx>(pub Vec<Span>, pub crate::hir::map::Map<'tcx>);
+
+impl<'v> hir::intravisit::Visitor<'v> for StaticLifetimeVisitor<'v> {
+    fn visit_lifetime(&mut self, lt: &'v hir::Lifetime) {
+        if let hir::LifetimeName::ImplicitObjectLifetimeDefault | hir::LifetimeName::Static =
+            lt.name
+        {
+            self.0.push(lt.span);
+        }
+    }
+}
+
+pub struct IsSuggestableVisitor<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    infer_suggestable: bool,
+}
+
+impl<'tcx> TypeVisitor<'tcx> for IsSuggestableVisitor<'tcx> {
+    type BreakTy = ();
+
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        match t.kind() {
+            Infer(InferTy::TyVar(_)) if self.infer_suggestable => {}
+
+            FnDef(..)
+            | Closure(..)
+            | Infer(..)
+            | Generator(..)
+            | GeneratorWitness(..)
+            | Bound(_, _)
+            | Placeholder(_)
+            | Error(_) => {
+                return ControlFlow::Break(());
+            }
+
+            Opaque(did, _) => {
+                let parent = self.tcx.parent(*did);
+                if let hir::def::DefKind::TyAlias | hir::def::DefKind::AssocTy = self.tcx.def_kind(parent)
+                    && let Opaque(parent_did, _) = self.tcx.type_of(parent).kind()
+                    && parent_did == did
+                {
+                    // Okay
+                } else {
+                    return ControlFlow::Break(());
+                }
+            }
+
+            Dynamic(dty, _) => {
+                for pred in *dty {
+                    match pred.skip_binder() {
+                        ExistentialPredicate::Trait(_) | ExistentialPredicate::Projection(_) => {
+                            // Okay
+                        }
+                        _ => return ControlFlow::Break(()),
+                    }
+                }
+            }
+
+            Param(param) => {
+                // FIXME: It would be nice to make this not use string manipulation,
+                // but it's pretty hard to do this, since `ty::ParamTy` is missing
+                // sufficient info to determine if it is synthetic, and we don't
+                // always have a convenient way of getting `ty::Generics` at the call
+                // sites we invoke `IsSuggestable::is_suggestable`.
+                if param.name.as_str().starts_with("impl ") {
+                    return ControlFlow::Break(());
+                }
+            }
+
+            _ => {}
+        }
+
+        t.super_visit_with(self)
+    }
+
+    fn visit_const(&mut self, c: Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+        match c.kind() {
+            ConstKind::Infer(InferConst::Var(_)) if self.infer_suggestable => {}
+
+            ConstKind::Infer(..)
+            | ConstKind::Bound(..)
+            | ConstKind::Placeholder(..)
+            | ConstKind::Error(..) => {
+                return ControlFlow::Break(());
+            }
+            _ => {}
+        }
+
+        c.super_visit_with(self)
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/erase_regions.rs b/compiler/rustc_middle/src/ty/erase_regions.rs
new file mode 100644
index 000000000..3226950e7
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/erase_regions.rs
@@ -0,0 +1,74 @@
+use crate::mir;
+use crate::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
+use crate::ty::visit::TypeVisitable;
+use crate::ty::{self, Ty, TyCtxt, TypeFlags};
+
+pub(super) fn provide(providers: &mut ty::query::Providers) {
+    *providers = ty::query::Providers { erase_regions_ty, ..*providers };
+}
+
+fn erase_regions_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Ty<'tcx> {
+    // N.B., use `super_fold_with` here. If we used `fold_with`, it
+    // could invoke the `erase_regions_ty` query recursively.
+    ty.super_fold_with(&mut RegionEraserVisitor { tcx })
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Returns an equivalent value with all free regions removed (note
+    /// that late-bound regions remain, because they are important for
+    /// subtyping, but they are anonymized and normalized as well)..
+    pub fn erase_regions<T>(self, value: T) -> T
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        // If there's nothing to erase avoid performing the query at all
+        if !value.has_type_flags(TypeFlags::HAS_RE_LATE_BOUND | TypeFlags::HAS_FREE_REGIONS) {
+            return value;
+        }
+        debug!("erase_regions({:?})", value);
+        let value1 = value.fold_with(&mut RegionEraserVisitor { tcx: self });
+        debug!("erase_regions = {:?}", value1);
+        value1
+    }
+}
+
+struct RegionEraserVisitor<'tcx> {
+    tcx: TyCtxt<'tcx>,
+}
+
+impl<'tcx> TypeFolder<'tcx> for RegionEraserVisitor<'tcx> {
+    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        if ty.needs_infer() { ty.super_fold_with(self) } else { self.tcx.erase_regions_ty(ty) }
+    }
+
+    fn fold_binder<T>(&mut self, t: ty::Binder<'tcx, T>) -> ty::Binder<'tcx, T>
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        let u = self.tcx.anonymize_bound_vars(t);
+        u.super_fold_with(self)
+    }
+
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        // because late-bound regions affect subtyping, we can't
+        // erase the bound/free distinction, but we can replace
+        // all free regions with 'erased.
+        //
+        // Note that we *CAN* replace early-bound regions -- the
+        // type system never "sees" those, they get substituted
+        // away. In codegen, they will always be erased to 'erased
+        // whenever a substitution occurs.
+        match *r {
+            ty::ReLateBound(..) => r,
+            _ => self.tcx.lifetimes.re_erased,
+        }
+    }
+
+    fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
+        c.super_fold_with(self)
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/error.rs b/compiler/rustc_middle/src/ty/error.rs
new file mode 100644
index 000000000..4b0bc3c11
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/error.rs
@@ -0,0 +1,965 @@
+use crate::traits::{ObligationCause, ObligationCauseCode};
+use crate::ty::diagnostics::suggest_constraining_type_param;
+use crate::ty::print::{FmtPrinter, Printer};
+use crate::ty::{self, BoundRegionKind, Region, Ty, TyCtxt};
+use rustc_errors::Applicability::{MachineApplicable, MaybeIncorrect};
+use rustc_errors::{pluralize, Diagnostic, MultiSpan};
+use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use rustc_span::symbol::{sym, Symbol};
+use rustc_span::{BytePos, Span};
+use rustc_target::spec::abi;
+
+use std::borrow::Cow;
+use std::fmt;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, TypeFoldable, TypeVisitable)]
+pub struct ExpectedFound<T> {
+    pub expected: T,
+    pub found: T,
+}
+
+impl<T> ExpectedFound<T> {
+    pub fn new(a_is_expected: bool, a: T, b: T) -> Self {
+        if a_is_expected {
+            ExpectedFound { expected: a, found: b }
+        } else {
+            ExpectedFound { expected: b, found: a }
+        }
+    }
+}
+
+// Data structures used in type unification
+#[derive(Clone, Debug, TypeFoldable, TypeVisitable)]
+pub enum TypeError<'tcx> {
+    Mismatch,
+    ConstnessMismatch(ExpectedFound<ty::BoundConstness>),
+    PolarityMismatch(ExpectedFound<ty::ImplPolarity>),
+    UnsafetyMismatch(ExpectedFound<hir::Unsafety>),
+    AbiMismatch(ExpectedFound<abi::Abi>),
+    Mutability,
+    ArgumentMutability(usize),
+    TupleSize(ExpectedFound<usize>),
+    FixedArraySize(ExpectedFound<u64>),
+    ArgCount,
+    FieldMisMatch(Symbol, Symbol),
+
+    RegionsDoesNotOutlive(Region<'tcx>, Region<'tcx>),
+    RegionsInsufficientlyPolymorphic(BoundRegionKind, Region<'tcx>),
+    RegionsOverlyPolymorphic(BoundRegionKind, Region<'tcx>),
+    RegionsPlaceholderMismatch,
+
+    Sorts(ExpectedFound<Ty<'tcx>>),
+    ArgumentSorts(ExpectedFound<Ty<'tcx>>, usize),
+    IntMismatch(ExpectedFound<ty::IntVarValue>),
+    FloatMismatch(ExpectedFound<ty::FloatTy>),
+    Traits(ExpectedFound<DefId>),
+    VariadicMismatch(ExpectedFound<bool>),
+
+    /// Instantiating a type variable with the given type would have
+    /// created a cycle (because it appears somewhere within that
+    /// type).
+    CyclicTy(Ty<'tcx>),
+    CyclicConst(ty::Const<'tcx>),
+    ProjectionMismatched(ExpectedFound<DefId>),
+    ExistentialMismatch(
+        ExpectedFound<&'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>>,
+    ),
+    ObjectUnsafeCoercion(DefId),
+    ConstMismatch(ExpectedFound<ty::Const<'tcx>>),
+
+    IntrinsicCast,
+    /// Safe `#[target_feature]` functions are not assignable to safe function pointers.
+    TargetFeatureCast(DefId),
+}
+
+/// Explains the source of a type err in a short, human readable way. This is meant to be placed
+/// in parentheses after some larger message. You should also invoke `note_and_explain_type_err()`
+/// afterwards to present additional details, particularly when it comes to lifetime-related
+/// errors.
+impl<'tcx> fmt::Display for TypeError<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use self::TypeError::*;
+        fn report_maybe_different(
+            f: &mut fmt::Formatter<'_>,
+            expected: &str,
+            found: &str,
+        ) -> fmt::Result {
+            // A naive approach to making sure that we're not reporting silly errors such as:
+            // (expected closure, found closure).
+            if expected == found {
+                write!(f, "expected {}, found a different {}", expected, found)
+            } else {
+                write!(f, "expected {}, found {}", expected, found)
+            }
+        }
+
+        let br_string = |br: ty::BoundRegionKind| match br {
+            ty::BrNamed(_, name) => format!(" {}", name),
+            _ => String::new(),
+        };
+
+        match *self {
+            CyclicTy(_) => write!(f, "cyclic type of infinite size"),
+            CyclicConst(_) => write!(f, "encountered a self-referencing constant"),
+            Mismatch => write!(f, "types differ"),
+            ConstnessMismatch(values) => {
+                write!(f, "expected {} bound, found {} bound", values.expected, values.found)
+            }
+            PolarityMismatch(values) => {
+                write!(f, "expected {} polarity, found {} polarity", values.expected, values.found)
+            }
+            UnsafetyMismatch(values) => {
+                write!(f, "expected {} fn, found {} fn", values.expected, values.found)
+            }
+            AbiMismatch(values) => {
+                write!(f, "expected {} fn, found {} fn", values.expected, values.found)
+            }
+            ArgumentMutability(_) | Mutability => write!(f, "types differ in mutability"),
+            TupleSize(values) => write!(
+                f,
+                "expected a tuple with {} element{}, found one with {} element{}",
+                values.expected,
+                pluralize!(values.expected),
+                values.found,
+                pluralize!(values.found)
+            ),
+            FixedArraySize(values) => write!(
+                f,
+                "expected an array with a fixed size of {} element{}, found one with {} element{}",
+                values.expected,
+                pluralize!(values.expected),
+                values.found,
+                pluralize!(values.found)
+            ),
+            ArgCount => write!(f, "incorrect number of function parameters"),
+            FieldMisMatch(adt, field) => write!(f, "field type mismatch: {}.{}", adt, field),
+            RegionsDoesNotOutlive(..) => write!(f, "lifetime mismatch"),
+            // Actually naming the region here is a bit confusing because context is lacking
+            RegionsInsufficientlyPolymorphic(..) => {
+                write!(f, "one type is more general than the other")
+            }
+            RegionsOverlyPolymorphic(br, _) => write!(
+                f,
+                "expected concrete lifetime, found bound lifetime parameter{}",
+                br_string(br)
+            ),
+            RegionsPlaceholderMismatch => write!(f, "one type is more general than the other"),
+            ArgumentSorts(values, _) | Sorts(values) => ty::tls::with(|tcx| {
+                report_maybe_different(
+                    f,
+                    &values.expected.sort_string(tcx),
+                    &values.found.sort_string(tcx),
+                )
+            }),
+            Traits(values) => ty::tls::with(|tcx| {
+                report_maybe_different(
+                    f,
+                    &format!("trait `{}`", tcx.def_path_str(values.expected)),
+                    &format!("trait `{}`", tcx.def_path_str(values.found)),
+                )
+            }),
+            IntMismatch(ref values) => {
+                let expected = match values.expected {
+                    ty::IntVarValue::IntType(ty) => ty.name_str(),
+                    ty::IntVarValue::UintType(ty) => ty.name_str(),
+                };
+                let found = match values.found {
+                    ty::IntVarValue::IntType(ty) => ty.name_str(),
+                    ty::IntVarValue::UintType(ty) => ty.name_str(),
+                };
+                write!(f, "expected `{}`, found `{}`", expected, found)
+            }
+            FloatMismatch(ref values) => {
+                write!(
+                    f,
+                    "expected `{}`, found `{}`",
+                    values.expected.name_str(),
+                    values.found.name_str()
+                )
+            }
+            VariadicMismatch(ref values) => write!(
+                f,
+                "expected {} fn, found {} function",
+                if values.expected { "variadic" } else { "non-variadic" },
+                if values.found { "variadic" } else { "non-variadic" }
+            ),
+            ProjectionMismatched(ref values) => ty::tls::with(|tcx| {
+                write!(
+                    f,
+                    "expected {}, found {}",
+                    tcx.def_path_str(values.expected),
+                    tcx.def_path_str(values.found)
+                )
+            }),
+            ExistentialMismatch(ref values) => report_maybe_different(
+                f,
+                &format!("trait `{}`", values.expected),
+                &format!("trait `{}`", values.found),
+            ),
+            ConstMismatch(ref values) => {
+                write!(f, "expected `{}`, found `{}`", values.expected, values.found)
+            }
+            IntrinsicCast => write!(f, "cannot coerce intrinsics to function pointers"),
+            TargetFeatureCast(_) => write!(
+                f,
+                "cannot coerce functions with `#[target_feature]` to safe function pointers"
+            ),
+            ObjectUnsafeCoercion(_) => write!(f, "coercion to object-unsafe trait object"),
+        }
+    }
+}
+
+impl<'tcx> TypeError<'tcx> {
+    pub fn must_include_note(&self) -> bool {
+        use self::TypeError::*;
+        match self {
+            CyclicTy(_) | CyclicConst(_) | UnsafetyMismatch(_) | ConstnessMismatch(_)
+            | PolarityMismatch(_) | Mismatch | AbiMismatch(_) | FixedArraySize(_)
+            | ArgumentSorts(..) | Sorts(_) | IntMismatch(_) | FloatMismatch(_)
+            | VariadicMismatch(_) | TargetFeatureCast(_) => false,
+
+            Mutability
+            | ArgumentMutability(_)
+            | TupleSize(_)
+            | ArgCount
+            | FieldMisMatch(..)
+            | RegionsDoesNotOutlive(..)
+            | RegionsInsufficientlyPolymorphic(..)
+            | RegionsOverlyPolymorphic(..)
+            | RegionsPlaceholderMismatch
+            | Traits(_)
+            | ProjectionMismatched(_)
+            | ExistentialMismatch(_)
+            | ConstMismatch(_)
+            | IntrinsicCast
+            | ObjectUnsafeCoercion(_) => true,
+        }
+    }
+}
+
+impl<'tcx> Ty<'tcx> {
+    pub fn sort_string(self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
+        match *self.kind() {
+            ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Str | ty::Never => {
+                format!("`{}`", self).into()
+            }
+            ty::Tuple(ref tys) if tys.is_empty() => format!("`{}`", self).into(),
+
+            ty::Adt(def, _) => format!("{} `{}`", def.descr(), tcx.def_path_str(def.did())).into(),
+            ty::Foreign(def_id) => format!("extern type `{}`", tcx.def_path_str(def_id)).into(),
+            ty::Array(t, n) => {
+                if t.is_simple_ty() {
+                    return format!("array `{}`", self).into();
+                }
+
+                let n = tcx.lift(n).unwrap();
+                if let ty::ConstKind::Value(v) = n.kind() {
+                    if let Some(n) = v.try_to_machine_usize(tcx) {
+                        return format!("array of {} element{}", n, pluralize!(n)).into();
+                    }
+                }
+                "array".into()
+            }
+            ty::Slice(ty) if ty.is_simple_ty() => format!("slice `{}`", self).into(),
+            ty::Slice(_) => "slice".into(),
+            ty::RawPtr(_) => "*-ptr".into(),
+            ty::Ref(_, ty, mutbl) => {
+                let tymut = ty::TypeAndMut { ty, mutbl };
+                let tymut_string = tymut.to_string();
+                if tymut_string != "_"
+                    && (ty.is_simple_text() || tymut_string.len() < "mutable reference".len())
+                {
+                    format!("`&{}`", tymut_string).into()
+                } else {
+                    // Unknown type name, it's long or has type arguments
+                    match mutbl {
+                        hir::Mutability::Mut => "mutable reference",
+                        _ => "reference",
+                    }
+                    .into()
+                }
+            }
+            ty::FnDef(..) => "fn item".into(),
+            ty::FnPtr(_) => "fn pointer".into(),
+            ty::Dynamic(ref inner, ..) if let Some(principal) = inner.principal() => {
+                format!("trait object `dyn {}`", tcx.def_path_str(principal.def_id())).into()
+            }
+            ty::Dynamic(..) => "trait object".into(),
+            ty::Closure(..) => "closure".into(),
+            ty::Generator(def_id, ..) => tcx.generator_kind(def_id).unwrap().descr().into(),
+            ty::GeneratorWitness(..) => "generator witness".into(),
+            ty::Tuple(..) => "tuple".into(),
+            ty::Infer(ty::TyVar(_)) => "inferred type".into(),
+            ty::Infer(ty::IntVar(_)) => "integer".into(),
+            ty::Infer(ty::FloatVar(_)) => "floating-point number".into(),
+            ty::Placeholder(..) => "placeholder type".into(),
+            ty::Bound(..) => "bound type".into(),
+            ty::Infer(ty::FreshTy(_)) => "fresh type".into(),
+            ty::Infer(ty::FreshIntTy(_)) => "fresh integral type".into(),
+            ty::Infer(ty::FreshFloatTy(_)) => "fresh floating-point type".into(),
+            ty::Projection(_) => "associated type".into(),
+            ty::Param(p) => format!("type parameter `{}`", p).into(),
+            ty::Opaque(..) => "opaque type".into(),
+            ty::Error(_) => "type error".into(),
+        }
+    }
+
+    pub fn prefix_string(self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
+        match *self.kind() {
+            ty::Infer(_)
+            | ty::Error(_)
+            | ty::Bool
+            | ty::Char
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Str
+            | ty::Never => "type".into(),
+            ty::Tuple(ref tys) if tys.is_empty() => "unit type".into(),
+            ty::Adt(def, _) => def.descr().into(),
+            ty::Foreign(_) => "extern type".into(),
+            ty::Array(..) => "array".into(),
+            ty::Slice(_) => "slice".into(),
+            ty::RawPtr(_) => "raw pointer".into(),
+            ty::Ref(.., mutbl) => match mutbl {
+                hir::Mutability::Mut => "mutable reference",
+                _ => "reference",
+            }
+            .into(),
+            ty::FnDef(..) => "fn item".into(),
+            ty::FnPtr(_) => "fn pointer".into(),
+            ty::Dynamic(..) => "trait object".into(),
+            ty::Closure(..) => "closure".into(),
+            ty::Generator(def_id, ..) => tcx.generator_kind(def_id).unwrap().descr().into(),
+            ty::GeneratorWitness(..) => "generator witness".into(),
+            ty::Tuple(..) => "tuple".into(),
+            ty::Placeholder(..) => "higher-ranked type".into(),
+            ty::Bound(..) => "bound type variable".into(),
+            ty::Projection(_) => "associated type".into(),
+            ty::Param(_) => "type parameter".into(),
+            ty::Opaque(..) => "opaque type".into(),
+        }
+    }
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    pub fn note_and_explain_type_err(
+        self,
+        diag: &mut Diagnostic,
+        err: &TypeError<'tcx>,
+        cause: &ObligationCause<'tcx>,
+        sp: Span,
+        body_owner_def_id: DefId,
+    ) {
+        use self::TypeError::*;
+        debug!("note_and_explain_type_err err={:?} cause={:?}", err, cause);
+        match err {
+            ArgumentSorts(values, _) | Sorts(values) => {
+                match (values.expected.kind(), values.found.kind()) {
+                    (ty::Closure(..), ty::Closure(..)) => {
+                        diag.note("no two closures, even if identical, have the same type");
+                        diag.help("consider boxing your closure and/or using it as a trait object");
+                    }
+                    (ty::Opaque(..), ty::Opaque(..)) => {
+                        // Issue #63167
+                        diag.note("distinct uses of `impl Trait` result in different opaque types");
+                    }
+                    (ty::Float(_), ty::Infer(ty::IntVar(_)))
+                        if let Ok(
+                            // Issue #53280
+                            snippet,
+                        ) = self.sess.source_map().span_to_snippet(sp) =>
+                    {
+                        if snippet.chars().all(|c| c.is_digit(10) || c == '-' || c == '_') {
+                            diag.span_suggestion(
+                                sp,
+                                "use a float literal",
+                                format!("{}.0", snippet),
+                                MachineApplicable,
+                            );
+                        }
+                    }
+                    (ty::Param(expected), ty::Param(found)) => {
+                        let generics = self.generics_of(body_owner_def_id);
+                        let e_span = self.def_span(generics.type_param(expected, self).def_id);
+                        if !sp.contains(e_span) {
+                            diag.span_label(e_span, "expected type parameter");
+                        }
+                        let f_span = self.def_span(generics.type_param(found, self).def_id);
+                        if !sp.contains(f_span) {
+                            diag.span_label(f_span, "found type parameter");
+                        }
+                        diag.note(
+                            "a type parameter was expected, but a different one was found; \
+                             you might be missing a type parameter or trait bound",
+                        );
+                        diag.note(
+                            "for more information, visit \
+                             https://doc.rust-lang.org/book/ch10-02-traits.html\
+                             #traits-as-parameters",
+                        );
+                    }
+                    (ty::Projection(_), ty::Projection(_)) => {
+                        diag.note("an associated type was expected, but a different one was found");
+                    }
+                    (ty::Param(p), ty::Projection(proj)) | (ty::Projection(proj), ty::Param(p)) => {
+                        let generics = self.generics_of(body_owner_def_id);
+                        let p_span = self.def_span(generics.type_param(p, self).def_id);
+                        if !sp.contains(p_span) {
+                            diag.span_label(p_span, "this type parameter");
+                        }
+                        let hir = self.hir();
+                        let mut note = true;
+                        if let Some(generics) = generics
+                            .type_param(p, self)
+                            .def_id
+                            .as_local()
+                            .map(|id| hir.local_def_id_to_hir_id(id))
+                            .and_then(|id| self.hir().find(self.hir().get_parent_node(id)))
+                            .as_ref()
+                            .and_then(|node| node.generics())
+                        {
+                            // Synthesize the associated type restriction `Add<Output = Expected>`.
+                            // FIXME: extract this logic for use in other diagnostics.
+                            let (trait_ref, assoc_substs) = proj.trait_ref_and_own_substs(self);
+                            let path =
+                                self.def_path_str_with_substs(trait_ref.def_id, trait_ref.substs);
+                            let item_name = self.item_name(proj.item_def_id);
+                            let item_args = self.format_generic_args(assoc_substs);
+
+                            let path = if path.ends_with('>') {
+                                format!(
+                                    "{}, {}{} = {}>",
+                                    &path[..path.len() - 1],
+                                    item_name,
+                                    item_args,
+                                    p
+                                )
+                            } else {
+                                format!("{}<{}{} = {}>", path, item_name, item_args, p)
+                            };
+                            note = !suggest_constraining_type_param(
+                                self,
+                                generics,
+                                diag,
+                                &format!("{}", proj.self_ty()),
+                                &path,
+                                None,
+                            );
+                        }
+                        if note {
+                            diag.note("you might be missing a type parameter or trait bound");
+                        }
+                    }
+                    (ty::Param(p), ty::Dynamic(..) | ty::Opaque(..))
+                    | (ty::Dynamic(..) | ty::Opaque(..), ty::Param(p)) => {
+                        let generics = self.generics_of(body_owner_def_id);
+                        let p_span = self.def_span(generics.type_param(p, self).def_id);
+                        if !sp.contains(p_span) {
+                            diag.span_label(p_span, "this type parameter");
+                        }
+                        diag.help("type parameters must be constrained to match other types");
+                        if self.sess.teach(&diag.get_code().unwrap()) {
+                            diag.help(
+                                "given a type parameter `T` and a method `foo`:
+```
+trait Trait<T> { fn foo(&self) -> T; }
+```
+the only ways to implement method `foo` are:
+- constrain `T` with an explicit type:
+```
+impl Trait<String> for X {
+    fn foo(&self) -> String { String::new() }
+}
+```
+- add a trait bound to `T` and call a method on that trait that returns `Self`:
+```
+impl<T: std::default::Default> Trait<T> for X {
+    fn foo(&self) -> T { <T as std::default::Default>::default() }
+}
+```
+- change `foo` to return an argument of type `T`:
+```
+impl<T> Trait<T> for X {
+    fn foo(&self, x: T) -> T { x }
+}
+```",
+                            );
+                        }
+                        diag.note(
+                            "for more information, visit \
+                             https://doc.rust-lang.org/book/ch10-02-traits.html\
+                             #traits-as-parameters",
+                        );
+                    }
+                    (ty::Param(p), ty::Closure(..) | ty::Generator(..)) => {
+                        let generics = self.generics_of(body_owner_def_id);
+                        let p_span = self.def_span(generics.type_param(p, self).def_id);
+                        if !sp.contains(p_span) {
+                            diag.span_label(p_span, "this type parameter");
+                        }
+                        diag.help(&format!(
+                            "every closure has a distinct type and so could not always match the \
+                             caller-chosen type of parameter `{}`",
+                            p
+                        ));
+                    }
+                    (ty::Param(p), _) | (_, ty::Param(p)) => {
+                        let generics = self.generics_of(body_owner_def_id);
+                        let p_span = self.def_span(generics.type_param(p, self).def_id);
+                        if !sp.contains(p_span) {
+                            diag.span_label(p_span, "this type parameter");
+                        }
+                    }
+                    (ty::Projection(proj_ty), _) => {
+                        self.expected_projection(
+                            diag,
+                            proj_ty,
+                            values,
+                            body_owner_def_id,
+                            cause.code(),
+                        );
+                    }
+                    (_, ty::Projection(proj_ty)) => {
+                        let msg = format!(
+                            "consider constraining the associated type `{}` to `{}`",
+                            values.found, values.expected,
+                        );
+                        if !(self.suggest_constraining_opaque_associated_type(
+                            diag,
+                            &msg,
+                            proj_ty,
+                            values.expected,
+                        ) || self.suggest_constraint(
+                            diag,
+                            &msg,
+                            body_owner_def_id,
+                            proj_ty,
+                            values.expected,
+                        )) {
+                            diag.help(&msg);
+                            diag.note(
+                                "for more information, visit \
+                                https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
+                            );
+                        }
+                    }
+                    _ => {}
+                }
+                debug!(
+                    "note_and_explain_type_err expected={:?} ({:?}) found={:?} ({:?})",
+                    values.expected,
+                    values.expected.kind(),
+                    values.found,
+                    values.found.kind(),
+                );
+            }
+            CyclicTy(ty) => {
+                // Watch out for various cases of cyclic types and try to explain.
+                if ty.is_closure() || ty.is_generator() {
+                    diag.note(
+                        "closures cannot capture themselves or take themselves as argument;\n\
+                         this error may be the result of a recent compiler bug-fix,\n\
+                         see issue #46062 <https://github.com/rust-lang/rust/issues/46062>\n\
+                         for more information",
+                    );
+                }
+            }
+            TargetFeatureCast(def_id) => {
+                let target_spans =
+                    self.get_attrs(*def_id, sym::target_feature).map(|attr| attr.span);
+                diag.note(
+                    "functions with `#[target_feature]` can only be coerced to `unsafe` function pointers"
+                );
+                diag.span_labels(target_spans, "`#[target_feature]` added here");
+            }
+            _ => {}
+        }
+    }
+
+    fn suggest_constraint(
+        self,
+        diag: &mut Diagnostic,
+        msg: &str,
+        body_owner_def_id: DefId,
+        proj_ty: &ty::ProjectionTy<'tcx>,
+        ty: Ty<'tcx>,
+    ) -> bool {
+        let assoc = self.associated_item(proj_ty.item_def_id);
+        let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
+        if let Some(item) = self.hir().get_if_local(body_owner_def_id) {
+            if let Some(hir_generics) = item.generics() {
+                // Get the `DefId` for the type parameter corresponding to `A` in `<A as T>::Foo`.
+                // This will also work for `impl Trait`.
+                let def_id = if let ty::Param(param_ty) = proj_ty.self_ty().kind() {
+                    let generics = self.generics_of(body_owner_def_id);
+                    generics.type_param(param_ty, self).def_id
+                } else {
+                    return false;
+                };
+                let Some(def_id) = def_id.as_local() else {
+                    return false;
+                };
+
+                // First look in the `where` clause, as this might be
+                // `fn foo<T>(x: T) where T: Trait`.
+                for pred in hir_generics.bounds_for_param(def_id) {
+                    if self.constrain_generic_bound_associated_type_structured_suggestion(
+                        diag,
+                        &trait_ref,
+                        pred.bounds,
+                        &assoc,
+                        assoc_substs,
+                        ty,
+                        msg,
+                        false,
+                    ) {
+                        return true;
+                    }
+                }
+            }
+        }
+        false
+    }
+
+    /// An associated type was expected and a different type was found.
+    ///
+    /// We perform a few different checks to see what we can suggest:
+    ///
+    ///  - In the current item, look for associated functions that return the expected type and
+    ///    suggest calling them. (Not a structured suggestion.)
+    ///  - If any of the item's generic bounds can be constrained, we suggest constraining the
+    ///    associated type to the found type.
+    ///  - If the associated type has a default type and was expected inside of a `trait`, we
+    ///    mention that this is disallowed.
+    ///  - If all other things fail, and the error is not because of a mismatch between the `trait`
+    ///    and the `impl`, we provide a generic `help` to constrain the assoc type or call an assoc
+    ///    fn that returns the type.
+    fn expected_projection(
+        self,
+        diag: &mut Diagnostic,
+        proj_ty: &ty::ProjectionTy<'tcx>,
+        values: &ExpectedFound<Ty<'tcx>>,
+        body_owner_def_id: DefId,
+        cause_code: &ObligationCauseCode<'_>,
+    ) {
+        let msg = format!(
+            "consider constraining the associated type `{}` to `{}`",
+            values.expected, values.found
+        );
+        let body_owner = self.hir().get_if_local(body_owner_def_id);
+        let current_method_ident = body_owner.and_then(|n| n.ident()).map(|i| i.name);
+
+        // We don't want to suggest calling an assoc fn in a scope where that isn't feasible.
+        let callable_scope = matches!(
+            body_owner,
+            Some(
+                hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })
+                    | hir::Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Fn(..), .. })
+                    | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }),
+            )
+        );
+        let impl_comparison =
+            matches!(cause_code, ObligationCauseCode::CompareImplItemObligation { .. });
+        let assoc = self.associated_item(proj_ty.item_def_id);
+        if !callable_scope || impl_comparison {
+            // We do not want to suggest calling functions when the reason of the
+            // type error is a comparison of an `impl` with its `trait` or when the
+            // scope is outside of a `Body`.
+        } else {
+            // If we find a suitable associated function that returns the expected type, we don't
+            // want the more general suggestion later in this method about "consider constraining
+            // the associated type or calling a method that returns the associated type".
+            let point_at_assoc_fn = self.point_at_methods_that_satisfy_associated_type(
+                diag,
+                assoc.container_id(self),
+                current_method_ident,
+                proj_ty.item_def_id,
+                values.expected,
+            );
+            // Possibly suggest constraining the associated type to conform to the
+            // found type.
+            if self.suggest_constraint(diag, &msg, body_owner_def_id, proj_ty, values.found)
+                || point_at_assoc_fn
+            {
+                return;
+            }
+        }
+
+        self.suggest_constraining_opaque_associated_type(diag, &msg, proj_ty, values.found);
+
+        if self.point_at_associated_type(diag, body_owner_def_id, values.found) {
+            return;
+        }
+
+        if !impl_comparison {
+            // Generic suggestion when we can't be more specific.
+            if callable_scope {
+                diag.help(&format!(
+                    "{} or calling a method that returns `{}`",
+                    msg, values.expected
+                ));
+            } else {
+                diag.help(&msg);
+            }
+            diag.note(
+                "for more information, visit \
+                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
+            );
+        }
+        if self.sess.teach(&diag.get_code().unwrap()) {
+            diag.help(
+                "given an associated type `T` and a method `foo`:
+```
+trait Trait {
+type T;
+fn foo(&self) -> Self::T;
+}
+```
+the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
+```
+impl Trait for X {
+type T = String;
+fn foo(&self) -> Self::T { String::new() }
+}
+```",
+            );
+        }
+    }
+
+    /// When the expected `impl Trait` is not defined in the current item, it will come from
+    /// a return type. This can occur when dealing with `TryStream` (#71035).
+    fn suggest_constraining_opaque_associated_type(
+        self,
+        diag: &mut Diagnostic,
+        msg: &str,
+        proj_ty: &ty::ProjectionTy<'tcx>,
+        ty: Ty<'tcx>,
+    ) -> bool {
+        let assoc = self.associated_item(proj_ty.item_def_id);
+        if let ty::Opaque(def_id, _) = *proj_ty.self_ty().kind() {
+            let opaque_local_def_id = def_id.as_local();
+            let opaque_hir_ty = if let Some(opaque_local_def_id) = opaque_local_def_id {
+                match &self.hir().expect_item(opaque_local_def_id).kind {
+                    hir::ItemKind::OpaqueTy(opaque_hir_ty) => opaque_hir_ty,
+                    _ => bug!("The HirId comes from a `ty::Opaque`"),
+                }
+            } else {
+                return false;
+            };
+
+            let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
+
+            self.constrain_generic_bound_associated_type_structured_suggestion(
+                diag,
+                &trait_ref,
+                opaque_hir_ty.bounds,
+                assoc,
+                assoc_substs,
+                ty,
+                msg,
+                true,
+            )
+        } else {
+            false
+        }
+    }
+
+    fn point_at_methods_that_satisfy_associated_type(
+        self,
+        diag: &mut Diagnostic,
+        assoc_container_id: DefId,
+        current_method_ident: Option<Symbol>,
+        proj_ty_item_def_id: DefId,
+        expected: Ty<'tcx>,
+    ) -> bool {
+        let items = self.associated_items(assoc_container_id);
+        // Find all the methods in the trait that could be called to construct the
+        // expected associated type.
+        // FIXME: consider suggesting the use of associated `const`s.
+        let methods: Vec<(Span, String)> = items
+            .items
+            .iter()
+            .filter(|(name, item)| {
+                ty::AssocKind::Fn == item.kind && Some(**name) != current_method_ident
+            })
+            .filter_map(|(_, item)| {
+                let method = self.fn_sig(item.def_id);
+                match *method.output().skip_binder().kind() {
+                    ty::Projection(ty::ProjectionTy { item_def_id, .. })
+                        if item_def_id == proj_ty_item_def_id =>
+                    {
+                        Some((
+                            self.def_span(item.def_id),
+                            format!("consider calling `{}`", self.def_path_str(item.def_id)),
+                        ))
+                    }
+                    _ => None,
+                }
+            })
+            .collect();
+        if !methods.is_empty() {
+            // Use a single `help:` to show all the methods in the trait that can
+            // be used to construct the expected associated type.
+            let mut span: MultiSpan =
+                methods.iter().map(|(sp, _)| *sp).collect::<Vec<Span>>().into();
+            let msg = format!(
+                "{some} method{s} {are} available that return{r} `{ty}`",
+                some = if methods.len() == 1 { "a" } else { "some" },
+                s = pluralize!(methods.len()),
+                are = pluralize!("is", methods.len()),
+                r = if methods.len() == 1 { "s" } else { "" },
+                ty = expected
+            );
+            for (sp, label) in methods.into_iter() {
+                span.push_span_label(sp, label);
+            }
+            diag.span_help(span, &msg);
+            return true;
+        }
+        false
+    }
+
+    fn point_at_associated_type(
+        self,
+        diag: &mut Diagnostic,
+        body_owner_def_id: DefId,
+        found: Ty<'tcx>,
+    ) -> bool {
+        let Some(hir_id) = body_owner_def_id.as_local() else {
+            return false;
+        };
+        let hir_id = self.hir().local_def_id_to_hir_id(hir_id);
+        // When `body_owner` is an `impl` or `trait` item, look in its associated types for
+        // `expected` and point at it.
+        let parent_id = self.hir().get_parent_item(hir_id);
+        let item = self.hir().find_by_def_id(parent_id);
+        debug!("expected_projection parent item {:?}", item);
+        match item {
+            Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Trait(.., items), .. })) => {
+                // FIXME: account for `#![feature(specialization)]`
+                for item in &items[..] {
+                    match item.kind {
+                        hir::AssocItemKind::Type => {
+                            // FIXME: account for returning some type in a trait fn impl that has
+                            // an assoc type as a return type (#72076).
+                            if let hir::Defaultness::Default { has_value: true } =
+                                self.impl_defaultness(item.id.def_id)
+                            {
+                                if self.type_of(item.id.def_id) == found {
+                                    diag.span_label(
+                                        item.span,
+                                        "associated type defaults can't be assumed inside the \
+                                            trait defining them",
+                                    );
+                                    return true;
+                                }
+                            }
+                        }
+                        _ => {}
+                    }
+                }
+            }
+            Some(hir::Node::Item(hir::Item {
+                kind: hir::ItemKind::Impl(hir::Impl { items, .. }),
+                ..
+            })) => {
+                for item in &items[..] {
+                    if let hir::AssocItemKind::Type = item.kind {
+                        if self.type_of(item.id.def_id) == found {
+                            diag.span_label(item.span, "expected this associated type");
+                            return true;
+                        }
+                    }
+                }
+            }
+            _ => {}
+        }
+        false
+    }
+
+    /// Given a slice of `hir::GenericBound`s, if any of them corresponds to the `trait_ref`
+    /// requirement, provide a structured suggestion to constrain it to a given type `ty`.
+    ///
+    /// `is_bound_surely_present` indicates whether we know the bound we're looking for is
+    /// inside `bounds`. If that's the case then we can consider `bounds` containing only one
+    /// trait bound as the one we're looking for. This can help in cases where the associated
+    /// type is defined on a supertrait of the one present in the bounds.
+    fn constrain_generic_bound_associated_type_structured_suggestion(
+        self,
+        diag: &mut Diagnostic,
+        trait_ref: &ty::TraitRef<'tcx>,
+        bounds: hir::GenericBounds<'_>,
+        assoc: &ty::AssocItem,
+        assoc_substs: &[ty::GenericArg<'tcx>],
+        ty: Ty<'tcx>,
+        msg: &str,
+        is_bound_surely_present: bool,
+    ) -> bool {
+        // FIXME: we would want to call `resolve_vars_if_possible` on `ty` before suggesting.
+
+        let trait_bounds = bounds.iter().filter_map(|bound| match bound {
+            hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => Some(ptr),
+            _ => None,
+        });
+
+        let matching_trait_bounds = trait_bounds
+            .clone()
+            .filter(|ptr| ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id))
+            .collect::<Vec<_>>();
+
+        let span = match &matching_trait_bounds[..] {
+            &[ptr] => ptr.span,
+            &[] if is_bound_surely_present => match &trait_bounds.collect::<Vec<_>>()[..] {
+                &[ptr] => ptr.span,
+                _ => return false,
+            },
+            _ => return false,
+        };
+
+        self.constrain_associated_type_structured_suggestion(
+            diag,
+            span,
+            assoc,
+            assoc_substs,
+            ty,
+            msg,
+        )
+    }
+
+    /// Given a span corresponding to a bound, provide a structured suggestion to set an
+    /// associated type to a given type `ty`.
+    fn constrain_associated_type_structured_suggestion(
+        self,
+        diag: &mut Diagnostic,
+        span: Span,
+        assoc: &ty::AssocItem,
+        assoc_substs: &[ty::GenericArg<'tcx>],
+        ty: Ty<'tcx>,
+        msg: &str,
+    ) -> bool {
+        if let Ok(has_params) =
+            self.sess.source_map().span_to_snippet(span).map(|snippet| snippet.ends_with('>'))
+        {
+            let (span, sugg) = if has_params {
+                let pos = span.hi() - BytePos(1);
+                let span = Span::new(pos, pos, span.ctxt(), span.parent());
+                (span, format!(", {} = {}", assoc.ident(self), ty))
+            } else {
+                let item_args = self.format_generic_args(assoc_substs);
+                (span.shrink_to_hi(), format!("<{}{} = {}>", assoc.ident(self), item_args, ty))
+            };
+            diag.span_suggestion_verbose(span, msg, sugg, MaybeIncorrect);
+            return true;
+        }
+        false
+    }
+
+    fn format_generic_args(self, args: &[ty::GenericArg<'tcx>]) -> String {
+        FmtPrinter::new(self, hir::def::Namespace::TypeNS)
+            .path_generic_args(Ok, args)
+            .expect("could not write to `String`.")
+            .into_buffer()
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/fast_reject.rs b/compiler/rustc_middle/src/ty/fast_reject.rs
new file mode 100644
index 000000000..8d019a3ba
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/fast_reject.rs
@@ -0,0 +1,405 @@
+use crate::mir::Mutability;
+use crate::ty::subst::GenericArgKind;
+use crate::ty::{self, Ty, TyCtxt, TypeVisitable};
+use rustc_hir::def_id::DefId;
+use std::fmt::Debug;
+use std::hash::Hash;
+use std::iter;
+
+use self::SimplifiedTypeGen::*;
+
+pub type SimplifiedType = SimplifiedTypeGen<DefId>;
+
+/// See `simplify_type`
+///
+/// Note that we keep this type generic over the type of identifier it uses
+/// because we sometimes need to use SimplifiedTypeGen values as stable sorting
+/// keys (in which case we use a DefPathHash as id-type) but in the general case
+/// the non-stable but fast to construct DefId-version is the better choice.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, TyEncodable, TyDecodable, HashStable)]
+pub enum SimplifiedTypeGen<D>
+where
+    D: Copy + Debug + Eq,
+{
+    BoolSimplifiedType,
+    CharSimplifiedType,
+    IntSimplifiedType(ty::IntTy),
+    UintSimplifiedType(ty::UintTy),
+    FloatSimplifiedType(ty::FloatTy),
+    AdtSimplifiedType(D),
+    ForeignSimplifiedType(D),
+    StrSimplifiedType,
+    ArraySimplifiedType,
+    SliceSimplifiedType,
+    RefSimplifiedType(Mutability),
+    PtrSimplifiedType(Mutability),
+    NeverSimplifiedType,
+    TupleSimplifiedType(usize),
+    /// A trait object, all of whose components are markers
+    /// (e.g., `dyn Send + Sync`).
+    MarkerTraitObjectSimplifiedType,
+    TraitSimplifiedType(D),
+    ClosureSimplifiedType(D),
+    GeneratorSimplifiedType(D),
+    GeneratorWitnessSimplifiedType(usize),
+    OpaqueSimplifiedType(D),
+    FunctionSimplifiedType(usize),
+    PlaceholderSimplifiedType,
+}
+
+/// Generic parameters are pretty much just bound variables, e.g.
+/// the type of `fn foo<'a, T>(x: &'a T) -> u32 { ... }` can be thought of as
+/// `for<'a, T> fn(&'a T) -> u32`.
+///
+/// Typecheck of `foo` has to succeed for all possible generic arguments, so
+/// during typeck, we have to treat its generic parameters as if they
+/// were placeholders.
+///
+/// But when calling `foo` we only have to provide a specific generic argument.
+/// In that case the generic parameters are instantiated with inference variables.
+/// As we use `simplify_type` before that instantiation happens, we just treat
+/// generic parameters as if they were inference variables in that case.
+#[derive(PartialEq, Eq, Debug, Clone, Copy)]
+pub enum TreatParams {
+    /// Treat parameters as placeholders in the given environment.
+    ///
+    /// Note that this also causes us to treat projections as if they were
+    /// placeholders. This is only correct if the given projection cannot
+    /// be normalized in the current context. Even if normalization fails,
+    /// it may still succeed later if the projection contains any inference
+    /// variables.
+    AsPlaceholder,
+    AsInfer,
+}
+
+/// Tries to simplify a type by only returning the outermost injective¹ layer, if one exists.
+///
+/// **This function should only be used if you need to store or retrieve the type from some
+/// hashmap. If you want to quickly decide whether two types may unify, use the [DeepRejectCtxt]
+/// instead.**
+///
+/// The idea is to get something simple that we can use to quickly decide if two types could unify,
+/// for example during method lookup. If this function returns `Some(x)` it can only unify with
+/// types for which this method returns either `Some(x)` as well or `None`.
+///
+/// A special case here are parameters and projections, which are only injective
+/// if they are treated as placeholders.
+///
+/// For example when storing impls based on their simplified self type, we treat
+/// generic parameters as if they were inference variables. We must not simplify them here,
+/// as they can unify with any other type.
+///
+/// With projections we have to be even more careful, as treating them as placeholders
+/// is only correct if they are fully normalized.
+///
+/// ¹ meaning that if the outermost layers are different, then the whole types are also different.
+pub fn simplify_type<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    ty: Ty<'tcx>,
+    treat_params: TreatParams,
+) -> Option<SimplifiedType> {
+    match *ty.kind() {
+        ty::Bool => Some(BoolSimplifiedType),
+        ty::Char => Some(CharSimplifiedType),
+        ty::Int(int_type) => Some(IntSimplifiedType(int_type)),
+        ty::Uint(uint_type) => Some(UintSimplifiedType(uint_type)),
+        ty::Float(float_type) => Some(FloatSimplifiedType(float_type)),
+        ty::Adt(def, _) => Some(AdtSimplifiedType(def.did())),
+        ty::Str => Some(StrSimplifiedType),
+        ty::Array(..) => Some(ArraySimplifiedType),
+        ty::Slice(..) => Some(SliceSimplifiedType),
+        ty::RawPtr(ptr) => Some(PtrSimplifiedType(ptr.mutbl)),
+        ty::Dynamic(trait_info, ..) => match trait_info.principal_def_id() {
+            Some(principal_def_id) if !tcx.trait_is_auto(principal_def_id) => {
+                Some(TraitSimplifiedType(principal_def_id))
+            }
+            _ => Some(MarkerTraitObjectSimplifiedType),
+        },
+        ty::Ref(_, _, mutbl) => Some(RefSimplifiedType(mutbl)),
+        ty::FnDef(def_id, _) | ty::Closure(def_id, _) => Some(ClosureSimplifiedType(def_id)),
+        ty::Generator(def_id, _, _) => Some(GeneratorSimplifiedType(def_id)),
+        ty::GeneratorWitness(tys) => Some(GeneratorWitnessSimplifiedType(tys.skip_binder().len())),
+        ty::Never => Some(NeverSimplifiedType),
+        ty::Tuple(tys) => Some(TupleSimplifiedType(tys.len())),
+        ty::FnPtr(f) => Some(FunctionSimplifiedType(f.skip_binder().inputs().len())),
+        ty::Placeholder(..) => Some(PlaceholderSimplifiedType),
+        ty::Param(_) => match treat_params {
+            TreatParams::AsPlaceholder => Some(PlaceholderSimplifiedType),
+            TreatParams::AsInfer => None,
+        },
+        ty::Projection(_) => match treat_params {
+            // When treating `ty::Param` as a placeholder, projections also
+            // don't unify with anything else as long as they are fully normalized.
+            //
+            // We will have to be careful with lazy normalization here.
+            TreatParams::AsPlaceholder if !ty.has_infer_types_or_consts() => {
+                debug!("treating `{}` as a placeholder", ty);
+                Some(PlaceholderSimplifiedType)
+            }
+            TreatParams::AsPlaceholder | TreatParams::AsInfer => None,
+        },
+        ty::Opaque(def_id, _) => Some(OpaqueSimplifiedType(def_id)),
+        ty::Foreign(def_id) => Some(ForeignSimplifiedType(def_id)),
+        ty::Bound(..) | ty::Infer(_) | ty::Error(_) => None,
+    }
+}
+
+impl<D: Copy + Debug + Eq> SimplifiedTypeGen<D> {
+    pub fn def(self) -> Option<D> {
+        match self {
+            AdtSimplifiedType(d)
+            | ForeignSimplifiedType(d)
+            | TraitSimplifiedType(d)
+            | ClosureSimplifiedType(d)
+            | GeneratorSimplifiedType(d)
+            | OpaqueSimplifiedType(d) => Some(d),
+            _ => None,
+        }
+    }
+
+    pub fn map_def<U, F>(self, map: F) -> SimplifiedTypeGen<U>
+    where
+        F: Fn(D) -> U,
+        U: Copy + Debug + Eq,
+    {
+        match self {
+            BoolSimplifiedType => BoolSimplifiedType,
+            CharSimplifiedType => CharSimplifiedType,
+            IntSimplifiedType(t) => IntSimplifiedType(t),
+            UintSimplifiedType(t) => UintSimplifiedType(t),
+            FloatSimplifiedType(t) => FloatSimplifiedType(t),
+            AdtSimplifiedType(d) => AdtSimplifiedType(map(d)),
+            ForeignSimplifiedType(d) => ForeignSimplifiedType(map(d)),
+            StrSimplifiedType => StrSimplifiedType,
+            ArraySimplifiedType => ArraySimplifiedType,
+            SliceSimplifiedType => SliceSimplifiedType,
+            RefSimplifiedType(m) => RefSimplifiedType(m),
+            PtrSimplifiedType(m) => PtrSimplifiedType(m),
+            NeverSimplifiedType => NeverSimplifiedType,
+            MarkerTraitObjectSimplifiedType => MarkerTraitObjectSimplifiedType,
+            TupleSimplifiedType(n) => TupleSimplifiedType(n),
+            TraitSimplifiedType(d) => TraitSimplifiedType(map(d)),
+            ClosureSimplifiedType(d) => ClosureSimplifiedType(map(d)),
+            GeneratorSimplifiedType(d) => GeneratorSimplifiedType(map(d)),
+            GeneratorWitnessSimplifiedType(n) => GeneratorWitnessSimplifiedType(n),
+            OpaqueSimplifiedType(d) => OpaqueSimplifiedType(map(d)),
+            FunctionSimplifiedType(n) => FunctionSimplifiedType(n),
+            PlaceholderSimplifiedType => PlaceholderSimplifiedType,
+        }
+    }
+}
+
+/// Given generic arguments from an obligation and an impl,
+/// could these two be unified after replacing parameters in the
+/// the impl with inference variables.
+///
+/// For obligations, parameters won't be replaced by inference
+/// variables and only unify with themselves. We treat them
+/// the same way we treat placeholders.
+///
+/// We also use this function during coherence. For coherence the
+/// impls only have to overlap for some value, so we treat parameters
+/// on both sides like inference variables. This behavior is toggled
+/// using the `treat_obligation_params` field.
+#[derive(Debug, Clone, Copy)]
+pub struct DeepRejectCtxt {
+    pub treat_obligation_params: TreatParams,
+}
+
+impl DeepRejectCtxt {
+    pub fn generic_args_may_unify<'tcx>(
+        self,
+        obligation_arg: ty::GenericArg<'tcx>,
+        impl_arg: ty::GenericArg<'tcx>,
+    ) -> bool {
+        match (obligation_arg.unpack(), impl_arg.unpack()) {
+            // We don't fast reject based on regions for now.
+            (GenericArgKind::Lifetime(_), GenericArgKind::Lifetime(_)) => true,
+            (GenericArgKind::Type(obl), GenericArgKind::Type(imp)) => {
+                self.types_may_unify(obl, imp)
+            }
+            (GenericArgKind::Const(obl), GenericArgKind::Const(imp)) => {
+                self.consts_may_unify(obl, imp)
+            }
+            _ => bug!("kind mismatch: {obligation_arg} {impl_arg}"),
+        }
+    }
+
+    pub fn types_may_unify<'tcx>(self, obligation_ty: Ty<'tcx>, impl_ty: Ty<'tcx>) -> bool {
+        match impl_ty.kind() {
+            // Start by checking whether the type in the impl may unify with
+            // pretty much everything. Just return `true` in that case.
+            ty::Param(_) | ty::Projection(_) | ty::Error(_) => return true,
+            // These types only unify with inference variables or their own
+            // variant.
+            ty::Bool
+            | ty::Char
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Adt(..)
+            | ty::Str
+            | ty::Array(..)
+            | ty::Slice(..)
+            | ty::RawPtr(..)
+            | ty::Dynamic(..)
+            | ty::Ref(..)
+            | ty::Never
+            | ty::Tuple(..)
+            | ty::FnPtr(..)
+            | ty::Foreign(..)
+            | ty::Opaque(..) => {}
+            ty::FnDef(..)
+            | ty::Closure(..)
+            | ty::Generator(..)
+            | ty::GeneratorWitness(..)
+            | ty::Placeholder(..)
+            | ty::Bound(..)
+            | ty::Infer(_) => bug!("unexpected impl_ty: {impl_ty}"),
+        }
+
+        let k = impl_ty.kind();
+        match *obligation_ty.kind() {
+            // Purely rigid types, use structural equivalence.
+            ty::Bool
+            | ty::Char
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Str
+            | ty::Never
+            | ty::Foreign(_) => obligation_ty == impl_ty,
+            ty::Ref(_, obl_ty, obl_mutbl) => match k {
+                &ty::Ref(_, impl_ty, impl_mutbl) => {
+                    obl_mutbl == impl_mutbl && self.types_may_unify(obl_ty, impl_ty)
+                }
+                _ => false,
+            },
+            ty::Adt(obl_def, obl_substs) => match k {
+                &ty::Adt(impl_def, impl_substs) => {
+                    obl_def == impl_def
+                        && iter::zip(obl_substs, impl_substs)
+                            .all(|(obl, imp)| self.generic_args_may_unify(obl, imp))
+                }
+                _ => false,
+            },
+            ty::Slice(obl_ty) => {
+                matches!(k, &ty::Slice(impl_ty) if self.types_may_unify(obl_ty, impl_ty))
+            }
+            ty::Array(obl_ty, obl_len) => match k {
+                &ty::Array(impl_ty, impl_len) => {
+                    self.types_may_unify(obl_ty, impl_ty)
+                        && self.consts_may_unify(obl_len, impl_len)
+                }
+                _ => false,
+            },
+            ty::Tuple(obl) => match k {
+                &ty::Tuple(imp) => {
+                    obl.len() == imp.len()
+                        && iter::zip(obl, imp).all(|(obl, imp)| self.types_may_unify(obl, imp))
+                }
+                _ => false,
+            },
+            ty::RawPtr(obl) => match k {
+                ty::RawPtr(imp) => obl.mutbl == imp.mutbl && self.types_may_unify(obl.ty, imp.ty),
+                _ => false,
+            },
+            ty::Dynamic(obl_preds, ..) => {
+                // Ideally we would walk the existential predicates here or at least
+                // compare their length. But considering that the relevant `Relate` impl
+                // actually sorts and deduplicates these, that doesn't work.
+                matches!(k, ty::Dynamic(impl_preds, ..) if
+                    obl_preds.principal_def_id() == impl_preds.principal_def_id()
+                )
+            }
+            ty::FnPtr(obl_sig) => match k {
+                ty::FnPtr(impl_sig) => {
+                    let ty::FnSig { inputs_and_output, c_variadic, unsafety, abi } =
+                        obl_sig.skip_binder();
+                    let impl_sig = impl_sig.skip_binder();
+
+                    abi == impl_sig.abi
+                        && c_variadic == impl_sig.c_variadic
+                        && unsafety == impl_sig.unsafety
+                        && inputs_and_output.len() == impl_sig.inputs_and_output.len()
+                        && iter::zip(inputs_and_output, impl_sig.inputs_and_output)
+                            .all(|(obl, imp)| self.types_may_unify(obl, imp))
+                }
+                _ => false,
+            },
+
+            // Opaque types in impls should be forbidden, but that doesn't
+            // stop compilation. So this match arm should never return true
+            // if compilation succeeds.
+            ty::Opaque(..) => matches!(k, ty::Opaque(..)),
+
+            // Impls cannot contain these types as these cannot be named directly.
+            ty::FnDef(..) | ty::Closure(..) | ty::Generator(..) => false,
+
+            ty::Placeholder(..) => false,
+
+            // Depending on the value of `treat_obligation_params`, we either
+            // treat generic parameters like placeholders or like inference variables.
+            ty::Param(_) => match self.treat_obligation_params {
+                TreatParams::AsPlaceholder => false,
+                TreatParams::AsInfer => true,
+            },
+
+            ty::Infer(_) => true,
+
+            // As we're walking the whole type, it may encounter projections
+            // inside of binders and what not, so we're just going to assume that
+            // projections can unify with other stuff.
+            //
+            // Looking forward to lazy normalization this is the safer strategy anyways.
+            ty::Projection(_) => true,
+
+            ty::Error(_) => true,
+
+            ty::GeneratorWitness(..) | ty::Bound(..) => {
+                bug!("unexpected obligation type: {:?}", obligation_ty)
+            }
+        }
+    }
+
+    pub fn consts_may_unify(self, obligation_ct: ty::Const<'_>, impl_ct: ty::Const<'_>) -> bool {
+        match impl_ct.kind() {
+            ty::ConstKind::Param(_) | ty::ConstKind::Unevaluated(_) | ty::ConstKind::Error(_) => {
+                return true;
+            }
+            ty::ConstKind::Value(_) => {}
+            ty::ConstKind::Infer(_) | ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(_) => {
+                bug!("unexpected impl arg: {:?}", impl_ct)
+            }
+        }
+
+        let k = impl_ct.kind();
+        match obligation_ct.kind() {
+            ty::ConstKind::Param(_) => match self.treat_obligation_params {
+                TreatParams::AsPlaceholder => false,
+                TreatParams::AsInfer => true,
+            },
+
+            // As we don't necessarily eagerly evaluate constants,
+            // they might unify with any value.
+            ty::ConstKind::Unevaluated(_) | ty::ConstKind::Error(_) => true,
+            ty::ConstKind::Value(obl) => match k {
+                ty::ConstKind::Value(imp) => {
+                    // FIXME(valtrees): Once we have valtrees, we can just
+                    // compare them directly here.
+                    match (obl.try_to_scalar_int(), imp.try_to_scalar_int()) {
+                        (Some(obl), Some(imp)) => obl == imp,
+                        _ => true,
+                    }
+                }
+                _ => true,
+            },
+
+            ty::ConstKind::Infer(_) => true,
+
+            ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(_) => {
+                bug!("unexpected obl const: {:?}", obligation_ct)
+            }
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/flags.rs b/compiler/rustc_middle/src/ty/flags.rs
new file mode 100644
index 000000000..ea6bb8a7a
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/flags.rs
@@ -0,0 +1,342 @@
+use crate::ty::subst::{GenericArg, GenericArgKind};
+use crate::ty::{self, InferConst, Term, Ty, TypeFlags};
+use std::slice;
+
+#[derive(Debug)]
+pub struct FlagComputation {
+    pub flags: TypeFlags,
+
+    // see `Ty::outer_exclusive_binder` for details
+    pub outer_exclusive_binder: ty::DebruijnIndex,
+}
+
+impl FlagComputation {
+    fn new() -> FlagComputation {
+        FlagComputation { flags: TypeFlags::empty(), outer_exclusive_binder: ty::INNERMOST }
+    }
+
+    #[allow(rustc::usage_of_ty_tykind)]
+    pub fn for_kind(kind: &ty::TyKind<'_>) -> FlagComputation {
+        let mut result = FlagComputation::new();
+        result.add_kind(kind);
+        result
+    }
+
+    pub fn for_predicate<'tcx>(binder: ty::Binder<'tcx, ty::PredicateKind<'_>>) -> FlagComputation {
+        let mut result = FlagComputation::new();
+        result.add_predicate(binder);
+        result
+    }
+
+    pub fn for_const(c: ty::Const<'_>) -> TypeFlags {
+        let mut result = FlagComputation::new();
+        result.add_const(c);
+        result.flags
+    }
+
+    pub fn for_unevaluated_const(uv: ty::Unevaluated<'_>) -> TypeFlags {
+        let mut result = FlagComputation::new();
+        result.add_unevaluated_const(uv);
+        result.flags
+    }
+
+    fn add_flags(&mut self, flags: TypeFlags) {
+        self.flags = self.flags | flags;
+    }
+
+    /// indicates that `self` refers to something at binding level `binder`
+    fn add_bound_var(&mut self, binder: ty::DebruijnIndex) {
+        let exclusive_binder = binder.shifted_in(1);
+        self.add_exclusive_binder(exclusive_binder);
+    }
+
+    /// indicates that `self` refers to something *inside* binding
+    /// level `binder` -- not bound by `binder`, but bound by the next
+    /// binder internal to it
+    fn add_exclusive_binder(&mut self, exclusive_binder: ty::DebruijnIndex) {
+        self.outer_exclusive_binder = self.outer_exclusive_binder.max(exclusive_binder);
+    }
+
+    /// Adds the flags/depth from a set of types that appear within the current type, but within a
+    /// region binder.
+    fn bound_computation<T, F>(&mut self, value: ty::Binder<'_, T>, f: F)
+    where
+        F: FnOnce(&mut Self, T),
+    {
+        let mut computation = FlagComputation::new();
+
+        if !value.bound_vars().is_empty() {
+            computation.flags = computation.flags | TypeFlags::HAS_RE_LATE_BOUND;
+        }
+
+        f(&mut computation, value.skip_binder());
+
+        self.add_flags(computation.flags);
+
+        // The types that contributed to `computation` occurred within
+        // a region binder, so subtract one from the region depth
+        // within when adding the depth to `self`.
+        let outer_exclusive_binder = computation.outer_exclusive_binder;
+        if outer_exclusive_binder > ty::INNERMOST {
+            self.add_exclusive_binder(outer_exclusive_binder.shifted_out(1));
+        } // otherwise, this binder captures nothing
+    }
+
+    #[allow(rustc::usage_of_ty_tykind)]
+    fn add_kind(&mut self, kind: &ty::TyKind<'_>) {
+        match kind {
+            &ty::Bool
+            | &ty::Char
+            | &ty::Int(_)
+            | &ty::Float(_)
+            | &ty::Uint(_)
+            | &ty::Never
+            | &ty::Str
+            | &ty::Foreign(..) => {}
+
+            &ty::Error(_) => self.add_flags(TypeFlags::HAS_ERROR),
+
+            &ty::Param(_) => {
+                self.add_flags(TypeFlags::HAS_TY_PARAM);
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
+            }
+
+            &ty::Generator(_, ref substs, _) => {
+                let substs = substs.as_generator();
+                let should_remove_further_specializable =
+                    !self.flags.contains(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
+                self.add_substs(substs.parent_substs());
+                if should_remove_further_specializable {
+                    self.flags -= TypeFlags::STILL_FURTHER_SPECIALIZABLE;
+                }
+
+                self.add_ty(substs.resume_ty());
+                self.add_ty(substs.return_ty());
+                self.add_ty(substs.witness());
+                self.add_ty(substs.yield_ty());
+                self.add_ty(substs.tupled_upvars_ty());
+            }
+
+            &ty::GeneratorWitness(ts) => {
+                self.bound_computation(ts, |flags, ts| flags.add_tys(ts));
+            }
+
+            &ty::Closure(_, substs) => {
+                let substs = substs.as_closure();
+                let should_remove_further_specializable =
+                    !self.flags.contains(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
+                self.add_substs(substs.parent_substs());
+                if should_remove_further_specializable {
+                    self.flags -= TypeFlags::STILL_FURTHER_SPECIALIZABLE;
+                }
+
+                self.add_ty(substs.sig_as_fn_ptr_ty());
+                self.add_ty(substs.kind_ty());
+                self.add_ty(substs.tupled_upvars_ty());
+            }
+
+            &ty::Bound(debruijn, _) => {
+                self.add_bound_var(debruijn);
+            }
+
+            &ty::Placeholder(..) => {
+                self.add_flags(TypeFlags::HAS_TY_PLACEHOLDER);
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
+            }
+
+            &ty::Infer(infer) => {
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
+                match infer {
+                    ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => {
+                        self.add_flags(TypeFlags::HAS_TY_FRESH)
+                    }
+
+                    ty::TyVar(_) | ty::IntVar(_) | ty::FloatVar(_) => {
+                        self.add_flags(TypeFlags::HAS_TY_INFER)
+                    }
+                }
+            }
+
+            &ty::Adt(_, substs) => {
+                self.add_substs(substs);
+            }
+
+            &ty::Projection(data) => {
+                self.add_flags(TypeFlags::HAS_TY_PROJECTION);
+                self.add_projection_ty(data);
+            }
+
+            &ty::Opaque(_, substs) => {
+                self.add_flags(TypeFlags::HAS_TY_OPAQUE);
+                self.add_substs(substs);
+            }
+
+            &ty::Dynamic(obj, r) => {
+                for predicate in obj.iter() {
+                    self.bound_computation(predicate, |computation, predicate| match predicate {
+                        ty::ExistentialPredicate::Trait(tr) => computation.add_substs(tr.substs),
+                        ty::ExistentialPredicate::Projection(p) => {
+                            computation.add_existential_projection(&p);
+                        }
+                        ty::ExistentialPredicate::AutoTrait(_) => {}
+                    });
+                }
+
+                self.add_region(r);
+            }
+
+            &ty::Array(tt, len) => {
+                self.add_ty(tt);
+                self.add_const(len);
+            }
+
+            &ty::Slice(tt) => self.add_ty(tt),
+
+            &ty::RawPtr(ref m) => {
+                self.add_ty(m.ty);
+            }
+
+            &ty::Ref(r, ty, _) => {
+                self.add_region(r);
+                self.add_ty(ty);
+            }
+
+            &ty::Tuple(types) => {
+                self.add_tys(types);
+            }
+
+            &ty::FnDef(_, substs) => {
+                self.add_substs(substs);
+            }
+
+            &ty::FnPtr(fn_sig) => self.bound_computation(fn_sig, |computation, fn_sig| {
+                computation.add_tys(fn_sig.inputs());
+                computation.add_ty(fn_sig.output());
+            }),
+        }
+    }
+
+    fn add_predicate(&mut self, binder: ty::Binder<'_, ty::PredicateKind<'_>>) {
+        self.bound_computation(binder, |computation, atom| computation.add_predicate_atom(atom));
+    }
+
+    fn add_predicate_atom(&mut self, atom: ty::PredicateKind<'_>) {
+        match atom {
+            ty::PredicateKind::Trait(trait_pred) => {
+                self.add_substs(trait_pred.trait_ref.substs);
+            }
+            ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(a, b)) => {
+                self.add_region(a);
+                self.add_region(b);
+            }
+            ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty, region)) => {
+                self.add_ty(ty);
+                self.add_region(region);
+            }
+            ty::PredicateKind::Subtype(ty::SubtypePredicate { a_is_expected: _, a, b }) => {
+                self.add_ty(a);
+                self.add_ty(b);
+            }
+            ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
+                self.add_ty(a);
+                self.add_ty(b);
+            }
+            ty::PredicateKind::Projection(ty::ProjectionPredicate { projection_ty, term }) => {
+                self.add_projection_ty(projection_ty);
+                match term {
+                    Term::Ty(ty) => self.add_ty(ty),
+                    Term::Const(c) => self.add_const(c),
+                }
+            }
+            ty::PredicateKind::WellFormed(arg) => {
+                self.add_substs(slice::from_ref(&arg));
+            }
+            ty::PredicateKind::ObjectSafe(_def_id) => {}
+            ty::PredicateKind::ClosureKind(_def_id, substs, _kind) => {
+                self.add_substs(substs);
+            }
+            ty::PredicateKind::ConstEvaluatable(uv) => {
+                self.add_unevaluated_const(uv);
+            }
+            ty::PredicateKind::ConstEquate(expected, found) => {
+                self.add_const(expected);
+                self.add_const(found);
+            }
+            ty::PredicateKind::TypeWellFormedFromEnv(ty) => {
+                self.add_ty(ty);
+            }
+        }
+    }
+
+    fn add_ty(&mut self, ty: Ty<'_>) {
+        self.add_flags(ty.flags());
+        self.add_exclusive_binder(ty.outer_exclusive_binder());
+    }
+
+    fn add_tys(&mut self, tys: &[Ty<'_>]) {
+        for &ty in tys {
+            self.add_ty(ty);
+        }
+    }
+
+    fn add_region(&mut self, r: ty::Region<'_>) {
+        self.add_flags(r.type_flags());
+        if let ty::ReLateBound(debruijn, _) = *r {
+            self.add_bound_var(debruijn);
+        }
+    }
+
+    fn add_const(&mut self, c: ty::Const<'_>) {
+        self.add_ty(c.ty());
+        match c.kind() {
+            ty::ConstKind::Unevaluated(unevaluated) => self.add_unevaluated_const(unevaluated),
+            ty::ConstKind::Infer(infer) => {
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
+                match infer {
+                    InferConst::Fresh(_) => self.add_flags(TypeFlags::HAS_CT_FRESH),
+                    InferConst::Var(_) => self.add_flags(TypeFlags::HAS_CT_INFER),
+                }
+            }
+            ty::ConstKind::Bound(debruijn, _) => {
+                self.add_bound_var(debruijn);
+            }
+            ty::ConstKind::Param(_) => {
+                self.add_flags(TypeFlags::HAS_CT_PARAM);
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
+            }
+            ty::ConstKind::Placeholder(_) => {
+                self.add_flags(TypeFlags::HAS_CT_PLACEHOLDER);
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
+            }
+            ty::ConstKind::Value(_) => {}
+            ty::ConstKind::Error(_) => self.add_flags(TypeFlags::HAS_ERROR),
+        }
+    }
+
+    fn add_unevaluated_const<P>(&mut self, ct: ty::Unevaluated<'_, P>) {
+        self.add_substs(ct.substs);
+        self.add_flags(TypeFlags::HAS_CT_PROJECTION);
+    }
+
+    fn add_existential_projection(&mut self, projection: &ty::ExistentialProjection<'_>) {
+        self.add_substs(projection.substs);
+        match projection.term {
+            ty::Term::Ty(ty) => self.add_ty(ty),
+            ty::Term::Const(ct) => self.add_const(ct),
+        }
+    }
+
+    fn add_projection_ty(&mut self, projection_ty: ty::ProjectionTy<'_>) {
+        self.add_substs(projection_ty.substs);
+    }
+
+    fn add_substs(&mut self, substs: &[GenericArg<'_>]) {
+        for kind in substs {
+            match kind.unpack() {
+                GenericArgKind::Type(ty) => self.add_ty(ty),
+                GenericArgKind::Lifetime(lt) => self.add_region(lt),
+                GenericArgKind::Const(ct) => self.add_const(ct),
+            }
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/fold.rs b/compiler/rustc_middle/src/ty/fold.rs
new file mode 100644
index 000000000..5e96e278b
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/fold.rs
@@ -0,0 +1,797 @@
+//! A folding traversal mechanism for complex data structures that contain type
+//! information.
+//!
+//! This is a modifying traversal. It consumes the data structure, producing a
+//! (possibly) modified version of it. Both fallible and infallible versions are
+//! available. The name is potentially confusing, because this traversal is more
+//! like `Iterator::map` than `Iterator::fold`.
+//!
+//! This traversal has limited flexibility. Only a small number of "types of
+//! interest" within the complex data structures can receive custom
+//! modification. These are the ones containing the most important type-related
+//! information, such as `Ty`, `Predicate`, `Region`, and `Const`.
+//!
+//! There are three groups of traits involved in each traversal.
+//! - `TypeFoldable`. This is implemented once for many types, including:
+//!   - Types of interest, for which the the methods delegate to the
+//!     folder.
+//!   - All other types, including generic containers like `Vec` and `Option`.
+//!     It defines a "skeleton" of how they should be folded.
+//! - `TypeSuperFoldable`. This is implemented only for each type of interest,
+//!   and defines the folding "skeleton" for these types.
+//! - `TypeFolder`/`FallibleTypeFolder. One of these is implemented for each
+//!   folder. This defines how types of interest are folded.
+//!
+//! This means each fold is a mixture of (a) generic folding operations, and (b)
+//! custom fold operations that are specific to the folder.
+//! - The `TypeFoldable` impls handle most of the traversal, and call into
+//!   `TypeFolder`/`FallibleTypeFolder` when they encounter a type of interest.
+//! - A `TypeFolder`/`FallibleTypeFolder` may call into another `TypeFoldable`
+//!   impl, because some of the types of interest are recursive and can contain
+//!   other types of interest.
+//! - A `TypeFolder`/`FallibleTypeFolder` may also call into a `TypeSuperFoldable`
+//!   impl, because each folder might provide custom handling only for some types
+//!   of interest, or only for some variants of each type of interest, and then
+//!   use default traversal for the remaining cases.
+//!
+//! For example, if you have `struct S(Ty, U)` where `S: TypeFoldable` and `U:
+//! TypeFoldable`, and an instance `s = S(ty, u)`, it would be folded like so:
+//! ```text
+//! s.fold_with(folder) calls
+//! - ty.fold_with(folder) calls
+//!   - folder.fold_ty(ty) may call
+//!     - ty.super_fold_with(folder)
+//! - u.fold_with(folder)
+//! ```
+use crate::mir;
+use crate::ty::{self, Binder, BoundTy, Ty, TyCtxt, TypeVisitable};
+use rustc_data_structures::fx::FxIndexMap;
+use rustc_hir::def_id::DefId;
+
+use std::collections::BTreeMap;
+
+/// This trait is implemented for every type that can be folded,
+/// providing the skeleton of the traversal.
+///
+/// To implement this conveniently, use the derive macro located in
+/// `rustc_macros`.
+pub trait TypeFoldable<'tcx>: TypeVisitable<'tcx> {
+    /// The entry point for folding. To fold a value `t` with a folder `f`
+    /// call: `t.try_fold_with(f)`.
+    ///
+    /// For most types, this just traverses the value, calling `try_fold_with`
+    /// on each field/element.
+    ///
+    /// For types of interest (such as `Ty`), the implementation of method
+    /// calls a folder method specifically for that type (such as
+    /// `F::try_fold_ty`). This is where control transfers from `TypeFoldable`
+    /// to `TypeFolder`.
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error>;
+
+    /// A convenient alternative to `try_fold_with` for use with infallible
+    /// folders. Do not override this method, to ensure coherence with
+    /// `try_fold_with`.
+    fn fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self {
+        self.try_fold_with(folder).into_ok()
+    }
+}
+
+// This trait is implemented for types of interest.
+pub trait TypeSuperFoldable<'tcx>: TypeFoldable<'tcx> {
+    /// Provides a default fold for a type of interest. This should only be
+    /// called within `TypeFolder` methods, when a non-custom traversal is
+    /// desired for the value of the type of interest passed to that method.
+    /// For example, in `MyFolder::try_fold_ty(ty)`, it is valid to call
+    /// `ty.try_super_fold_with(self)`, but any other folding should be done
+    /// with `xyz.try_fold_with(self)`.
+    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self, F::Error>;
+
+    /// A convenient alternative to `try_super_fold_with` for use with
+    /// infallible folders. Do not override this method, to ensure coherence
+    /// with `try_super_fold_with`.
+    fn super_fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self {
+        self.try_super_fold_with(folder).into_ok()
+    }
+}
+
+/// This trait is implemented for every infallible folding traversal. There is
+/// a fold method defined for every type of interest. Each such method has a
+/// default that does an "identity" fold. Implementations of these methods
+/// often fall back to a `super_fold_with` method if the primary argument
+/// doesn't satisfy a particular condition.
+///
+/// A blanket implementation of [`FallibleTypeFolder`] will defer to
+/// the infallible methods of this trait to ensure that the two APIs
+/// are coherent.
+pub trait TypeFolder<'tcx>: FallibleTypeFolder<'tcx, Error = !> {
+    fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
+
+    fn fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Binder<'tcx, T>
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        t.super_fold_with(self)
+    }
+
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+        t.super_fold_with(self)
+    }
+
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        r.super_fold_with(self)
+    }
+
+    fn fold_const(&mut self, c: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        c.super_fold_with(self)
+    }
+
+    fn fold_unevaluated(&mut self, uv: ty::Unevaluated<'tcx>) -> ty::Unevaluated<'tcx> {
+        uv.super_fold_with(self)
+    }
+
+    fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
+        p.super_fold_with(self)
+    }
+
+    fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
+        bug!("most type folders should not be folding MIR datastructures: {:?}", c)
+    }
+}
+
+/// This trait is implemented for every folding traversal. There is a fold
+/// method defined for every type of interest. Each such method has a default
+/// that does an "identity" fold.
+///
+/// A blanket implementation of this trait (that defers to the relevant
+/// method of [`TypeFolder`]) is provided for all infallible folders in
+/// order to ensure the two APIs are coherent.
+pub trait FallibleTypeFolder<'tcx>: Sized {
+    type Error;
+
+    fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
+
+    fn try_fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Result<Binder<'tcx, T>, Self::Error>
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        t.try_super_fold_with(self)
+    }
+
+    fn try_fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
+        t.try_super_fold_with(self)
+    }
+
+    fn try_fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, Self::Error> {
+        r.try_super_fold_with(self)
+    }
+
+    fn try_fold_const(&mut self, c: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, Self::Error> {
+        c.try_super_fold_with(self)
+    }
+
+    fn try_fold_unevaluated(
+        &mut self,
+        c: ty::Unevaluated<'tcx>,
+    ) -> Result<ty::Unevaluated<'tcx>, Self::Error> {
+        c.try_super_fold_with(self)
+    }
+
+    fn try_fold_predicate(
+        &mut self,
+        p: ty::Predicate<'tcx>,
+    ) -> Result<ty::Predicate<'tcx>, Self::Error> {
+        p.try_super_fold_with(self)
+    }
+
+    fn try_fold_mir_const(
+        &mut self,
+        c: mir::ConstantKind<'tcx>,
+    ) -> Result<mir::ConstantKind<'tcx>, Self::Error> {
+        bug!("most type folders should not be folding MIR datastructures: {:?}", c)
+    }
+}
+
+// This blanket implementation of the fallible trait for infallible folders
+// delegates to infallible methods to ensure coherence.
+impl<'tcx, F> FallibleTypeFolder<'tcx> for F
+where
+    F: TypeFolder<'tcx>,
+{
+    type Error = !;
+
+    fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
+        TypeFolder::tcx(self)
+    }
+
+    fn try_fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Result<Binder<'tcx, T>, !>
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        Ok(self.fold_binder(t))
+    }
+
+    fn try_fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, !> {
+        Ok(self.fold_ty(t))
+    }
+
+    fn try_fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, !> {
+        Ok(self.fold_region(r))
+    }
+
+    fn try_fold_const(&mut self, c: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, !> {
+        Ok(self.fold_const(c))
+    }
+
+    fn try_fold_unevaluated(
+        &mut self,
+        c: ty::Unevaluated<'tcx>,
+    ) -> Result<ty::Unevaluated<'tcx>, !> {
+        Ok(self.fold_unevaluated(c))
+    }
+
+    fn try_fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> Result<ty::Predicate<'tcx>, !> {
+        Ok(self.fold_predicate(p))
+    }
+
+    fn try_fold_mir_const(
+        &mut self,
+        c: mir::ConstantKind<'tcx>,
+    ) -> Result<mir::ConstantKind<'tcx>, !> {
+        Ok(self.fold_mir_const(c))
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Some sample folders
+
+pub struct BottomUpFolder<'tcx, F, G, H>
+where
+    F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
+    G: FnMut(ty::Region<'tcx>) -> ty::Region<'tcx>,
+    H: FnMut(ty::Const<'tcx>) -> ty::Const<'tcx>,
+{
+    pub tcx: TyCtxt<'tcx>,
+    pub ty_op: F,
+    pub lt_op: G,
+    pub ct_op: H,
+}
+
+impl<'tcx, F, G, H> TypeFolder<'tcx> for BottomUpFolder<'tcx, F, G, H>
+where
+    F: FnMut(Ty<'tcx>) -> Ty<'tcx>,
+    G: FnMut(ty::Region<'tcx>) -> ty::Region<'tcx>,
+    H: FnMut(ty::Const<'tcx>) -> ty::Const<'tcx>,
+{
+    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        let t = ty.super_fold_with(self);
+        (self.ty_op)(t)
+    }
+
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        let r = r.super_fold_with(self);
+        (self.lt_op)(r)
+    }
+
+    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        let ct = ct.super_fold_with(self);
+        (self.ct_op)(ct)
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Region folder
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Folds the escaping and free regions in `value` using `f`, and
+    /// sets `skipped_regions` to true if any late-bound region was found
+    /// and skipped.
+    pub fn fold_regions<T>(
+        self,
+        value: T,
+        mut f: impl FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
+    ) -> T
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        value.fold_with(&mut RegionFolder::new(self, &mut f))
+    }
+}
+
+/// Folds over the substructure of a type, visiting its component
+/// types and all regions that occur *free* within it.
+///
+/// That is, `Ty` can contain function or method types that bind
+/// regions at the call site (`ReLateBound`), and occurrences of
+/// regions (aka "lifetimes") that are bound within a type are not
+/// visited by this folder; only regions that occur free will be
+/// visited by `fld_r`.
+
+pub struct RegionFolder<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+
+    /// Stores the index of a binder *just outside* the stuff we have
+    /// visited.  So this begins as INNERMOST; when we pass through a
+    /// binder, it is incremented (via `shift_in`).
+    current_index: ty::DebruijnIndex,
+
+    /// Callback invokes for each free region. The `DebruijnIndex`
+    /// points to the binder *just outside* the ones we have passed
+    /// through.
+    fold_region_fn:
+        &'a mut (dyn FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx> + 'a),
+}
+
+impl<'a, 'tcx> RegionFolder<'a, 'tcx> {
+    #[inline]
+    pub fn new(
+        tcx: TyCtxt<'tcx>,
+        fold_region_fn: &'a mut dyn FnMut(ty::Region<'tcx>, ty::DebruijnIndex) -> ty::Region<'tcx>,
+    ) -> RegionFolder<'a, 'tcx> {
+        RegionFolder { tcx, current_index: ty::INNERMOST, fold_region_fn }
+    }
+}
+
+impl<'a, 'tcx> TypeFolder<'tcx> for RegionFolder<'a, 'tcx> {
+    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_binder<T: TypeFoldable<'tcx>>(
+        &mut self,
+        t: ty::Binder<'tcx, T>,
+    ) -> ty::Binder<'tcx, T> {
+        self.current_index.shift_in(1);
+        let t = t.super_fold_with(self);
+        self.current_index.shift_out(1);
+        t
+    }
+
+    #[instrument(skip(self), level = "debug")]
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        match *r {
+            ty::ReLateBound(debruijn, _) if debruijn < self.current_index => {
+                debug!(?self.current_index, "skipped bound region");
+                r
+            }
+            _ => {
+                debug!(?self.current_index, "folding free region");
+                (self.fold_region_fn)(r, self.current_index)
+            }
+        }
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Bound vars replacer
+
+pub trait BoundVarReplacerDelegate<'tcx> {
+    fn replace_region(&mut self, br: ty::BoundRegion) -> ty::Region<'tcx>;
+    fn replace_ty(&mut self, bt: ty::BoundTy) -> Ty<'tcx>;
+    fn replace_const(&mut self, bv: ty::BoundVar, ty: Ty<'tcx>) -> ty::Const<'tcx>;
+}
+
+pub struct FnMutDelegate<R, T, C> {
+    pub regions: R,
+    pub types: T,
+    pub consts: C,
+}
+impl<'tcx, R, T, C> BoundVarReplacerDelegate<'tcx> for FnMutDelegate<R, T, C>
+where
+    R: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
+    T: FnMut(ty::BoundTy) -> Ty<'tcx>,
+    C: FnMut(ty::BoundVar, Ty<'tcx>) -> ty::Const<'tcx>,
+{
+    fn replace_region(&mut self, br: ty::BoundRegion) -> ty::Region<'tcx> {
+        (self.regions)(br)
+    }
+    fn replace_ty(&mut self, bt: ty::BoundTy) -> Ty<'tcx> {
+        (self.types)(bt)
+    }
+    fn replace_const(&mut self, bv: ty::BoundVar, ty: Ty<'tcx>) -> ty::Const<'tcx> {
+        (self.consts)(bv, ty)
+    }
+}
+
+/// Replaces the escaping bound vars (late bound regions or bound types) in a type.
+struct BoundVarReplacer<'tcx, D> {
+    tcx: TyCtxt<'tcx>,
+
+    /// As with `RegionFolder`, represents the index of a binder *just outside*
+    /// the ones we have visited.
+    current_index: ty::DebruijnIndex,
+
+    delegate: D,
+}
+
+impl<'tcx, D: BoundVarReplacerDelegate<'tcx>> BoundVarReplacer<'tcx, D> {
+    fn new(tcx: TyCtxt<'tcx>, delegate: D) -> Self {
+        BoundVarReplacer { tcx, current_index: ty::INNERMOST, delegate }
+    }
+}
+
+impl<'tcx, D> TypeFolder<'tcx> for BoundVarReplacer<'tcx, D>
+where
+    D: BoundVarReplacerDelegate<'tcx>,
+{
+    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_binder<T: TypeFoldable<'tcx>>(
+        &mut self,
+        t: ty::Binder<'tcx, T>,
+    ) -> ty::Binder<'tcx, T> {
+        self.current_index.shift_in(1);
+        let t = t.super_fold_with(self);
+        self.current_index.shift_out(1);
+        t
+    }
+
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+        match *t.kind() {
+            ty::Bound(debruijn, bound_ty) if debruijn == self.current_index => {
+                let ty = self.delegate.replace_ty(bound_ty);
+                ty::fold::shift_vars(self.tcx, ty, self.current_index.as_u32())
+            }
+            _ if t.has_vars_bound_at_or_above(self.current_index) => t.super_fold_with(self),
+            _ => t,
+        }
+    }
+
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        match *r {
+            ty::ReLateBound(debruijn, br) if debruijn == self.current_index => {
+                let region = self.delegate.replace_region(br);
+                if let ty::ReLateBound(debruijn1, br) = *region {
+                    // If the callback returns a late-bound region,
+                    // that region should always use the INNERMOST
+                    // debruijn index. Then we adjust it to the
+                    // correct depth.
+                    assert_eq!(debruijn1, ty::INNERMOST);
+                    self.tcx.reuse_or_mk_region(region, ty::ReLateBound(debruijn, br))
+                } else {
+                    region
+                }
+            }
+            _ => r,
+        }
+    }
+
+    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        match ct.kind() {
+            ty::ConstKind::Bound(debruijn, bound_const) if debruijn == self.current_index => {
+                let ct = self.delegate.replace_const(bound_const, ct.ty());
+                ty::fold::shift_vars(self.tcx, ct, self.current_index.as_u32())
+            }
+            _ => ct.super_fold_with(self),
+        }
+    }
+
+    fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
+        if p.has_vars_bound_at_or_above(self.current_index) { p.super_fold_with(self) } else { p }
+    }
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Replaces all regions bound by the given `Binder` with the
+    /// results returned by the closure; the closure is expected to
+    /// return a free region (relative to this binder), and hence the
+    /// binder is removed in the return type. The closure is invoked
+    /// once for each unique `BoundRegionKind`; multiple references to the
+    /// same `BoundRegionKind` will reuse the previous result. A map is
+    /// returned at the end with each bound region and the free region
+    /// that replaced it.
+    ///
+    /// # Panics
+    ///
+    /// This method only replaces late bound regions. Any types or
+    /// constants bound by `value` will cause an ICE.
+    pub fn replace_late_bound_regions<T, F>(
+        self,
+        value: Binder<'tcx, T>,
+        mut fld_r: F,
+    ) -> (T, BTreeMap<ty::BoundRegion, ty::Region<'tcx>>)
+    where
+        F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
+        T: TypeFoldable<'tcx>,
+    {
+        let mut region_map = BTreeMap::new();
+        let real_fld_r = |br: ty::BoundRegion| *region_map.entry(br).or_insert_with(|| fld_r(br));
+        let value = self.replace_late_bound_regions_uncached(value, real_fld_r);
+        (value, region_map)
+    }
+
+    pub fn replace_late_bound_regions_uncached<T, F>(
+        self,
+        value: Binder<'tcx, T>,
+        replace_regions: F,
+    ) -> T
+    where
+        F: FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
+        T: TypeFoldable<'tcx>,
+    {
+        let value = value.skip_binder();
+        if !value.has_escaping_bound_vars() {
+            value
+        } else {
+            let delegate = FnMutDelegate {
+                regions: replace_regions,
+                types: |b| bug!("unexpected bound ty in binder: {b:?}"),
+                consts: |b, ty| bug!("unexpected bound ct in binder: {b:?} {ty}"),
+            };
+            let mut replacer = BoundVarReplacer::new(self, delegate);
+            value.fold_with(&mut replacer)
+        }
+    }
+
+    /// Replaces all escaping bound vars. The `fld_r` closure replaces escaping
+    /// bound regions; the `fld_t` closure replaces escaping bound types and the `fld_c`
+    /// closure replaces escaping bound consts.
+    pub fn replace_escaping_bound_vars_uncached<T: TypeFoldable<'tcx>>(
+        self,
+        value: T,
+        delegate: impl BoundVarReplacerDelegate<'tcx>,
+    ) -> T {
+        if !value.has_escaping_bound_vars() {
+            value
+        } else {
+            let mut replacer = BoundVarReplacer::new(self, delegate);
+            value.fold_with(&mut replacer)
+        }
+    }
+
+    /// Replaces all types or regions bound by the given `Binder`. The `fld_r`
+    /// closure replaces bound regions, the `fld_t` closure replaces bound
+    /// types, and `fld_c` replaces bound constants.
+    pub fn replace_bound_vars_uncached<T: TypeFoldable<'tcx>>(
+        self,
+        value: Binder<'tcx, T>,
+        delegate: impl BoundVarReplacerDelegate<'tcx>,
+    ) -> T {
+        self.replace_escaping_bound_vars_uncached(value.skip_binder(), delegate)
+    }
+
+    /// Replaces any late-bound regions bound in `value` with
+    /// free variants attached to `all_outlive_scope`.
+    pub fn liberate_late_bound_regions<T>(
+        self,
+        all_outlive_scope: DefId,
+        value: ty::Binder<'tcx, T>,
+    ) -> T
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        self.replace_late_bound_regions_uncached(value, |br| {
+            self.mk_region(ty::ReFree(ty::FreeRegion {
+                scope: all_outlive_scope,
+                bound_region: br.kind,
+            }))
+        })
+    }
+
+    pub fn shift_bound_var_indices<T>(self, bound_vars: usize, value: T) -> T
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        let shift_bv = |bv: ty::BoundVar| ty::BoundVar::from_usize(bv.as_usize() + bound_vars);
+        self.replace_escaping_bound_vars_uncached(
+            value,
+            FnMutDelegate {
+                regions: |r: ty::BoundRegion| {
+                    self.mk_region(ty::ReLateBound(
+                        ty::INNERMOST,
+                        ty::BoundRegion { var: shift_bv(r.var), kind: r.kind },
+                    ))
+                },
+                types: |t: ty::BoundTy| {
+                    self.mk_ty(ty::Bound(
+                        ty::INNERMOST,
+                        ty::BoundTy { var: shift_bv(t.var), kind: t.kind },
+                    ))
+                },
+                consts: |c, ty: Ty<'tcx>| {
+                    self.mk_const(ty::ConstS {
+                        kind: ty::ConstKind::Bound(ty::INNERMOST, shift_bv(c)),
+                        ty,
+                    })
+                },
+            },
+        )
+    }
+
+    /// Replaces any late-bound regions bound in `value` with `'erased`. Useful in codegen but also
+    /// method lookup and a few other places where precise region relationships are not required.
+    pub fn erase_late_bound_regions<T>(self, value: Binder<'tcx, T>) -> T
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        self.replace_late_bound_regions(value, |_| self.lifetimes.re_erased).0
+    }
+
+    /// Rewrite any late-bound regions so that they are anonymous. Region numbers are
+    /// assigned starting at 0 and increasing monotonically in the order traversed
+    /// by the fold operation.
+    ///
+    /// The chief purpose of this function is to canonicalize regions so that two
+    /// `FnSig`s or `TraitRef`s which are equivalent up to region naming will become
+    /// structurally identical. For example, `for<'a, 'b> fn(&'a isize, &'b isize)` and
+    /// `for<'a, 'b> fn(&'b isize, &'a isize)` will become identical after anonymization.
+    pub fn anonymize_late_bound_regions<T>(self, sig: Binder<'tcx, T>) -> Binder<'tcx, T>
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        let mut counter = 0;
+        let inner = self
+            .replace_late_bound_regions(sig, |_| {
+                let br = ty::BoundRegion {
+                    var: ty::BoundVar::from_u32(counter),
+                    kind: ty::BrAnon(counter),
+                };
+                let r = self.mk_region(ty::ReLateBound(ty::INNERMOST, br));
+                counter += 1;
+                r
+            })
+            .0;
+        let bound_vars = self.mk_bound_variable_kinds(
+            (0..counter).map(|i| ty::BoundVariableKind::Region(ty::BrAnon(i))),
+        );
+        Binder::bind_with_vars(inner, bound_vars)
+    }
+
+    /// Anonymize all bound variables in `value`, this is mostly used to improve caching.
+    pub fn anonymize_bound_vars<T>(self, value: Binder<'tcx, T>) -> Binder<'tcx, T>
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        struct Anonymize<'a, 'tcx> {
+            tcx: TyCtxt<'tcx>,
+            map: &'a mut FxIndexMap<ty::BoundVar, ty::BoundVariableKind>,
+        }
+        impl<'tcx> BoundVarReplacerDelegate<'tcx> for Anonymize<'_, 'tcx> {
+            fn replace_region(&mut self, br: ty::BoundRegion) -> ty::Region<'tcx> {
+                let entry = self.map.entry(br.var);
+                let index = entry.index();
+                let var = ty::BoundVar::from_usize(index);
+                let kind = entry
+                    .or_insert_with(|| ty::BoundVariableKind::Region(ty::BrAnon(index as u32)))
+                    .expect_region();
+                let br = ty::BoundRegion { var, kind };
+                self.tcx.mk_region(ty::ReLateBound(ty::INNERMOST, br))
+            }
+            fn replace_ty(&mut self, bt: ty::BoundTy) -> Ty<'tcx> {
+                let entry = self.map.entry(bt.var);
+                let index = entry.index();
+                let var = ty::BoundVar::from_usize(index);
+                let kind = entry
+                    .or_insert_with(|| ty::BoundVariableKind::Ty(ty::BoundTyKind::Anon))
+                    .expect_ty();
+                self.tcx.mk_ty(ty::Bound(ty::INNERMOST, BoundTy { var, kind }))
+            }
+            fn replace_const(&mut self, bv: ty::BoundVar, ty: Ty<'tcx>) -> ty::Const<'tcx> {
+                let entry = self.map.entry(bv);
+                let index = entry.index();
+                let var = ty::BoundVar::from_usize(index);
+                let () = entry.or_insert_with(|| ty::BoundVariableKind::Const).expect_const();
+                self.tcx.mk_const(ty::ConstS { ty, kind: ty::ConstKind::Bound(ty::INNERMOST, var) })
+            }
+        }
+
+        let mut map = Default::default();
+        let delegate = Anonymize { tcx: self, map: &mut map };
+        let inner = self.replace_escaping_bound_vars_uncached(value.skip_binder(), delegate);
+        let bound_vars = self.mk_bound_variable_kinds(map.into_values());
+        Binder::bind_with_vars(inner, bound_vars)
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Shifter
+//
+// Shifts the De Bruijn indices on all escaping bound vars by a
+// fixed amount. Useful in substitution or when otherwise introducing
+// a binding level that is not intended to capture the existing bound
+// vars. See comment on `shift_vars_through_binders` method in
+// `subst.rs` for more details.
+
+struct Shifter<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    current_index: ty::DebruijnIndex,
+    amount: u32,
+}
+
+impl<'tcx> Shifter<'tcx> {
+    pub fn new(tcx: TyCtxt<'tcx>, amount: u32) -> Self {
+        Shifter { tcx, current_index: ty::INNERMOST, amount }
+    }
+}
+
+impl<'tcx> TypeFolder<'tcx> for Shifter<'tcx> {
+    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_binder<T: TypeFoldable<'tcx>>(
+        &mut self,
+        t: ty::Binder<'tcx, T>,
+    ) -> ty::Binder<'tcx, T> {
+        self.current_index.shift_in(1);
+        let t = t.super_fold_with(self);
+        self.current_index.shift_out(1);
+        t
+    }
+
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        match *r {
+            ty::ReLateBound(debruijn, br) => {
+                if self.amount == 0 || debruijn < self.current_index {
+                    r
+                } else {
+                    let debruijn = debruijn.shifted_in(self.amount);
+                    let shifted = ty::ReLateBound(debruijn, br);
+                    self.tcx.mk_region(shifted)
+                }
+            }
+            _ => r,
+        }
+    }
+
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        match *ty.kind() {
+            ty::Bound(debruijn, bound_ty) => {
+                if self.amount == 0 || debruijn < self.current_index {
+                    ty
+                } else {
+                    let debruijn = debruijn.shifted_in(self.amount);
+                    self.tcx.mk_ty(ty::Bound(debruijn, bound_ty))
+                }
+            }
+
+            _ => ty.super_fold_with(self),
+        }
+    }
+
+    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        if let ty::ConstKind::Bound(debruijn, bound_ct) = ct.kind() {
+            if self.amount == 0 || debruijn < self.current_index {
+                ct
+            } else {
+                let debruijn = debruijn.shifted_in(self.amount);
+                self.tcx.mk_const(ty::ConstS {
+                    kind: ty::ConstKind::Bound(debruijn, bound_ct),
+                    ty: ct.ty(),
+                })
+            }
+        } else {
+            ct.super_fold_with(self)
+        }
+    }
+}
+
+pub fn shift_region<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    region: ty::Region<'tcx>,
+    amount: u32,
+) -> ty::Region<'tcx> {
+    match *region {
+        ty::ReLateBound(debruijn, br) if amount > 0 => {
+            tcx.mk_region(ty::ReLateBound(debruijn.shifted_in(amount), br))
+        }
+        _ => region,
+    }
+}
+
+pub fn shift_vars<'tcx, T>(tcx: TyCtxt<'tcx>, value: T, amount: u32) -> T
+where
+    T: TypeFoldable<'tcx>,
+{
+    debug!("shift_vars(value={:?}, amount={})", value, amount);
+
+    value.fold_with(&mut Shifter::new(tcx, amount))
+}
diff --git a/compiler/rustc_middle/src/ty/generics.rs b/compiler/rustc_middle/src/ty/generics.rs
new file mode 100644
index 000000000..add2df258
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/generics.rs
@@ -0,0 +1,349 @@
+use crate::middle::resolve_lifetime::ObjectLifetimeDefault;
+use crate::ty;
+use crate::ty::subst::{Subst, SubstsRef};
+use crate::ty::EarlyBinder;
+use rustc_ast as ast;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_hir::def_id::DefId;
+use rustc_span::symbol::Symbol;
+use rustc_span::Span;
+
+use super::{EarlyBoundRegion, InstantiatedPredicates, ParamConst, ParamTy, Predicate, TyCtxt};
+
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub enum GenericParamDefKind {
+    Lifetime,
+    Type { has_default: bool, object_lifetime_default: ObjectLifetimeDefault, synthetic: bool },
+    Const { has_default: bool },
+}
+
+impl GenericParamDefKind {
+    pub fn descr(&self) -> &'static str {
+        match self {
+            GenericParamDefKind::Lifetime => "lifetime",
+            GenericParamDefKind::Type { .. } => "type",
+            GenericParamDefKind::Const { .. } => "constant",
+        }
+    }
+    pub fn to_ord(&self) -> ast::ParamKindOrd {
+        match self {
+            GenericParamDefKind::Lifetime => ast::ParamKindOrd::Lifetime,
+            GenericParamDefKind::Type { .. } => ast::ParamKindOrd::Type,
+            GenericParamDefKind::Const { .. } => ast::ParamKindOrd::Const,
+        }
+    }
+
+    pub fn is_ty_or_const(&self) -> bool {
+        match self {
+            GenericParamDefKind::Lifetime => false,
+            GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => true,
+        }
+    }
+
+    pub fn is_synthetic(&self) -> bool {
+        match self {
+            GenericParamDefKind::Type { synthetic, .. } => *synthetic,
+            _ => false,
+        }
+    }
+}
+
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct GenericParamDef {
+    pub name: Symbol,
+    pub def_id: DefId,
+    pub index: u32,
+
+    /// `pure_wrt_drop`, set by the (unsafe) `#[may_dangle]` attribute
+    /// on generic parameter `'a`/`T`, asserts data behind the parameter
+    /// `'a`/`T` won't be accessed during the parent type's `Drop` impl.
+    pub pure_wrt_drop: bool,
+
+    pub kind: GenericParamDefKind,
+}
+
+impl GenericParamDef {
+    pub fn to_early_bound_region_data(&self) -> ty::EarlyBoundRegion {
+        if let GenericParamDefKind::Lifetime = self.kind {
+            ty::EarlyBoundRegion { def_id: self.def_id, index: self.index, name: self.name }
+        } else {
+            bug!("cannot convert a non-lifetime parameter def to an early bound region")
+        }
+    }
+
+    pub fn has_default(&self) -> bool {
+        match self.kind {
+            GenericParamDefKind::Type { has_default, .. }
+            | GenericParamDefKind::Const { has_default } => has_default,
+            GenericParamDefKind::Lifetime => false,
+        }
+    }
+
+    pub fn default_value<'tcx>(
+        &self,
+        tcx: TyCtxt<'tcx>,
+    ) -> Option<EarlyBinder<ty::GenericArg<'tcx>>> {
+        match self.kind {
+            GenericParamDefKind::Type { has_default, .. } if has_default => {
+                Some(tcx.bound_type_of(self.def_id).map_bound(|t| t.into()))
+            }
+            GenericParamDefKind::Const { has_default } if has_default => {
+                Some(tcx.bound_const_param_default(self.def_id).map_bound(|c| c.into()))
+            }
+            _ => None,
+        }
+    }
+}
+
+#[derive(Default)]
+pub struct GenericParamCount {
+    pub lifetimes: usize,
+    pub types: usize,
+    pub consts: usize,
+}
+
+/// Information about the formal type/lifetime parameters associated
+/// with an item or method. Analogous to `hir::Generics`.
+///
+/// The ordering of parameters is the same as in `Subst` (excluding child generics):
+/// `Self` (optionally), `Lifetime` params..., `Type` params...
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct Generics {
+    pub parent: Option<DefId>,
+    pub parent_count: usize,
+    pub params: Vec<GenericParamDef>,
+
+    /// Reverse map to the `index` field of each `GenericParamDef`.
+    #[stable_hasher(ignore)]
+    pub param_def_id_to_index: FxHashMap<DefId, u32>,
+
+    pub has_self: bool,
+    pub has_late_bound_regions: Option<Span>,
+}
+
+impl<'tcx> Generics {
+    #[inline]
+    pub fn count(&self) -> usize {
+        self.parent_count + self.params.len()
+    }
+
+    pub fn own_counts(&self) -> GenericParamCount {
+        // We could cache this as a property of `GenericParamCount`, but
+        // the aim is to refactor this away entirely eventually and the
+        // presence of this method will be a constant reminder.
+        let mut own_counts = GenericParamCount::default();
+
+        for param in &self.params {
+            match param.kind {
+                GenericParamDefKind::Lifetime => own_counts.lifetimes += 1,
+                GenericParamDefKind::Type { .. } => own_counts.types += 1,
+                GenericParamDefKind::Const { .. } => own_counts.consts += 1,
+            }
+        }
+
+        own_counts
+    }
+
+    pub fn own_defaults(&self) -> GenericParamCount {
+        let mut own_defaults = GenericParamCount::default();
+
+        for param in &self.params {
+            match param.kind {
+                GenericParamDefKind::Lifetime => (),
+                GenericParamDefKind::Type { has_default, .. } => {
+                    own_defaults.types += has_default as usize;
+                }
+                GenericParamDefKind::Const { has_default } => {
+                    own_defaults.consts += has_default as usize;
+                }
+            }
+        }
+
+        own_defaults
+    }
+
+    pub fn requires_monomorphization(&self, tcx: TyCtxt<'tcx>) -> bool {
+        if self.own_requires_monomorphization() {
+            return true;
+        }
+
+        if let Some(parent_def_id) = self.parent {
+            let parent = tcx.generics_of(parent_def_id);
+            parent.requires_monomorphization(tcx)
+        } else {
+            false
+        }
+    }
+
+    pub fn own_requires_monomorphization(&self) -> bool {
+        for param in &self.params {
+            match param.kind {
+                GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
+                    return true;
+                }
+                GenericParamDefKind::Lifetime => {}
+            }
+        }
+        false
+    }
+
+    /// Returns the `GenericParamDef` with the given index.
+    pub fn param_at(&'tcx self, param_index: usize, tcx: TyCtxt<'tcx>) -> &'tcx GenericParamDef {
+        if let Some(index) = param_index.checked_sub(self.parent_count) {
+            &self.params[index]
+        } else {
+            tcx.generics_of(self.parent.expect("parent_count > 0 but no parent?"))
+                .param_at(param_index, tcx)
+        }
+    }
+
+    /// Returns the `GenericParamDef` associated with this `EarlyBoundRegion`.
+    pub fn region_param(
+        &'tcx self,
+        param: &EarlyBoundRegion,
+        tcx: TyCtxt<'tcx>,
+    ) -> &'tcx GenericParamDef {
+        let param = self.param_at(param.index as usize, tcx);
+        match param.kind {
+            GenericParamDefKind::Lifetime => param,
+            _ => bug!("expected lifetime parameter, but found another generic parameter"),
+        }
+    }
+
+    /// Returns the `GenericParamDef` associated with this `ParamTy`.
+    pub fn type_param(&'tcx self, param: &ParamTy, tcx: TyCtxt<'tcx>) -> &'tcx GenericParamDef {
+        let param = self.param_at(param.index as usize, tcx);
+        match param.kind {
+            GenericParamDefKind::Type { .. } => param,
+            _ => bug!("expected type parameter, but found another generic parameter"),
+        }
+    }
+
+    /// Returns the `GenericParamDef` associated with this `ParamConst`.
+    pub fn const_param(&'tcx self, param: &ParamConst, tcx: TyCtxt<'tcx>) -> &GenericParamDef {
+        let param = self.param_at(param.index as usize, tcx);
+        match param.kind {
+            GenericParamDefKind::Const { .. } => param,
+            _ => bug!("expected const parameter, but found another generic parameter"),
+        }
+    }
+
+    /// Returns `true` if `params` has `impl Trait`.
+    pub fn has_impl_trait(&'tcx self) -> bool {
+        self.params.iter().any(|param| {
+            matches!(param.kind, ty::GenericParamDefKind::Type { synthetic: true, .. })
+        })
+    }
+
+    /// Returns the substs corresponding to the generic parameters
+    /// of this item, excluding `Self`.
+    ///
+    /// **This should only be used for diagnostics purposes.**
+    pub fn own_substs_no_defaults(
+        &'tcx self,
+        tcx: TyCtxt<'tcx>,
+        substs: &'tcx [ty::GenericArg<'tcx>],
+    ) -> &'tcx [ty::GenericArg<'tcx>] {
+        let mut own_params = self.parent_count..self.count();
+        if self.has_self && self.parent.is_none() {
+            own_params.start = 1;
+        }
+
+        // Filter the default arguments.
+        //
+        // This currently uses structural equality instead
+        // of semantic equivalance. While not ideal, that's
+        // good enough for now as this should only be used
+        // for diagnostics anyways.
+        own_params.end -= self
+            .params
+            .iter()
+            .rev()
+            .take_while(|param| {
+                param.default_value(tcx).map_or(false, |default| {
+                    default.subst(tcx, substs) == substs[param.index as usize]
+                })
+            })
+            .count();
+
+        &substs[own_params]
+    }
+
+    /// Returns the substs corresponding to the generic parameters of this item, excluding `Self`.
+    ///
+    /// **This should only be used for diagnostics purposes.**
+    pub fn own_substs(
+        &'tcx self,
+        substs: &'tcx [ty::GenericArg<'tcx>],
+    ) -> &'tcx [ty::GenericArg<'tcx>] {
+        let own = &substs[self.parent_count..][..self.params.len()];
+        if self.has_self && self.parent.is_none() { &own[1..] } else { &own }
+    }
+}
+
+/// Bounds on generics.
+#[derive(Copy, Clone, Default, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct GenericPredicates<'tcx> {
+    pub parent: Option<DefId>,
+    pub predicates: &'tcx [(Predicate<'tcx>, Span)],
+}
+
+impl<'tcx> GenericPredicates<'tcx> {
+    pub fn instantiate(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        substs: SubstsRef<'tcx>,
+    ) -> InstantiatedPredicates<'tcx> {
+        let mut instantiated = InstantiatedPredicates::empty();
+        self.instantiate_into(tcx, &mut instantiated, substs);
+        instantiated
+    }
+
+    pub fn instantiate_own(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        substs: SubstsRef<'tcx>,
+    ) -> InstantiatedPredicates<'tcx> {
+        InstantiatedPredicates {
+            predicates: self
+                .predicates
+                .iter()
+                .map(|(p, _)| EarlyBinder(*p).subst(tcx, substs))
+                .collect(),
+            spans: self.predicates.iter().map(|(_, sp)| *sp).collect(),
+        }
+    }
+
+    fn instantiate_into(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        instantiated: &mut InstantiatedPredicates<'tcx>,
+        substs: SubstsRef<'tcx>,
+    ) {
+        if let Some(def_id) = self.parent {
+            tcx.predicates_of(def_id).instantiate_into(tcx, instantiated, substs);
+        }
+        instantiated
+            .predicates
+            .extend(self.predicates.iter().map(|(p, _)| EarlyBinder(*p).subst(tcx, substs)));
+        instantiated.spans.extend(self.predicates.iter().map(|(_, sp)| *sp));
+    }
+
+    pub fn instantiate_identity(&self, tcx: TyCtxt<'tcx>) -> InstantiatedPredicates<'tcx> {
+        let mut instantiated = InstantiatedPredicates::empty();
+        self.instantiate_identity_into(tcx, &mut instantiated);
+        instantiated
+    }
+
+    fn instantiate_identity_into(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        instantiated: &mut InstantiatedPredicates<'tcx>,
+    ) {
+        if let Some(def_id) = self.parent {
+            tcx.predicates_of(def_id).instantiate_identity_into(tcx, instantiated);
+        }
+        instantiated.predicates.extend(self.predicates.iter().map(|(p, _)| p));
+        instantiated.spans.extend(self.predicates.iter().map(|(_, s)| s));
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/impls_ty.rs b/compiler/rustc_middle/src/ty/impls_ty.rs
new file mode 100644
index 000000000..cd00b26b8
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/impls_ty.rs
@@ -0,0 +1,135 @@
+//! This module contains `HashStable` implementations for various data types
+//! from `rustc_middle::ty` in no particular order.
+
+use crate::middle::region;
+use crate::mir;
+use crate::ty;
+use crate::ty::fast_reject::SimplifiedType;
+use rustc_data_structures::fingerprint::Fingerprint;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::stable_hasher::HashingControls;
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher, ToStableHashKey};
+use rustc_query_system::ich::StableHashingContext;
+use std::cell::RefCell;
+
+impl<'a, 'tcx, T> HashStable<StableHashingContext<'a>> for &'tcx ty::List<T>
+where
+    T: HashStable<StableHashingContext<'a>>,
+{
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        thread_local! {
+            static CACHE: RefCell<FxHashMap<(usize, usize, HashingControls), Fingerprint>> =
+                RefCell::new(Default::default());
+        }
+
+        let hash = CACHE.with(|cache| {
+            let key = (self.as_ptr() as usize, self.len(), hcx.hashing_controls());
+            if let Some(&hash) = cache.borrow().get(&key) {
+                return hash;
+            }
+
+            let mut hasher = StableHasher::new();
+            (&self[..]).hash_stable(hcx, &mut hasher);
+
+            let hash: Fingerprint = hasher.finish();
+            cache.borrow_mut().insert(key, hash);
+            hash
+        });
+
+        hash.hash_stable(hcx, hasher);
+    }
+}
+
+impl<'a, 'tcx, T> ToStableHashKey<StableHashingContext<'a>> for &'tcx ty::List<T>
+where
+    T: HashStable<StableHashingContext<'a>>,
+{
+    type KeyType = Fingerprint;
+
+    #[inline]
+    fn to_stable_hash_key(&self, hcx: &StableHashingContext<'a>) -> Fingerprint {
+        let mut hasher = StableHasher::new();
+        let mut hcx: StableHashingContext<'a> = hcx.clone();
+        self.hash_stable(&mut hcx, &mut hasher);
+        hasher.finish()
+    }
+}
+
+impl<'a> ToStableHashKey<StableHashingContext<'a>> for SimplifiedType {
+    type KeyType = Fingerprint;
+
+    #[inline]
+    fn to_stable_hash_key(&self, hcx: &StableHashingContext<'a>) -> Fingerprint {
+        let mut hasher = StableHasher::new();
+        let mut hcx: StableHashingContext<'a> = hcx.clone();
+        self.hash_stable(&mut hcx, &mut hasher);
+        hasher.finish()
+    }
+}
+
+impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for ty::subst::GenericArg<'tcx> {
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        self.unpack().hash_stable(hcx, hasher);
+    }
+}
+
+impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for ty::subst::GenericArgKind<'tcx> {
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        match self {
+            // WARNING: We dedup cache the `HashStable` results for `List`
+            // while ignoring types and freely transmute
+            // between `List<Ty<'tcx>>` and `List<GenericArg<'tcx>>`.
+            // See `fn intern_type_list` for more details.
+            //
+            // We therefore hash types without adding a hash for their discriminant.
+            //
+            // In order to make it very unlikely for the sequence of bytes being hashed for
+            // a `GenericArgKind::Type` to be the same as the sequence of bytes being
+            // hashed for one of the other variants, we hash some very high number instead
+            // of their actual discriminant since `TyKind` should never start with anything
+            // that high.
+            ty::subst::GenericArgKind::Type(ty) => ty.hash_stable(hcx, hasher),
+            ty::subst::GenericArgKind::Const(ct) => {
+                0xF3u8.hash_stable(hcx, hasher);
+                ct.hash_stable(hcx, hasher);
+            }
+            ty::subst::GenericArgKind::Lifetime(lt) => {
+                0xF5u8.hash_stable(hcx, hasher);
+                lt.hash_stable(hcx, hasher);
+            }
+        }
+    }
+}
+
+// AllocIds get resolved to whatever they point to (to be stable)
+impl<'a> HashStable<StableHashingContext<'a>> for mir::interpret::AllocId {
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        ty::tls::with_opt(|tcx| {
+            trace!("hashing {:?}", *self);
+            let tcx = tcx.expect("can't hash AllocIds during hir lowering");
+            tcx.try_get_global_alloc(*self).hash_stable(hcx, hasher);
+        });
+    }
+}
+
+// `Relocations` with default type parameters is a sorted map.
+impl<'a, Prov> HashStable<StableHashingContext<'a>> for mir::interpret::Relocations<Prov>
+where
+    Prov: HashStable<StableHashingContext<'a>>,
+{
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        self.len().hash_stable(hcx, hasher);
+        for reloc in self.iter() {
+            reloc.hash_stable(hcx, hasher);
+        }
+    }
+}
+
+impl<'a> ToStableHashKey<StableHashingContext<'a>> for region::Scope {
+    type KeyType = region::Scope;
+
+    #[inline]
+    fn to_stable_hash_key(&self, _: &StableHashingContext<'a>) -> region::Scope {
+        *self
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/inhabitedness/def_id_forest.rs b/compiler/rustc_middle/src/ty/inhabitedness/def_id_forest.rs
new file mode 100644
index 000000000..c4ad698ba
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/inhabitedness/def_id_forest.rs
@@ -0,0 +1,145 @@
+use crate::ty::context::TyCtxt;
+use crate::ty::{DefId, DefIdTree};
+use rustc_span::def_id::CRATE_DEF_ID;
+use smallvec::SmallVec;
+use std::mem;
+
+use DefIdForest::*;
+
+/// Represents a forest of `DefId`s closed under the ancestor relation. That is,
+/// if a `DefId` representing a module is contained in the forest then all
+/// `DefId`s defined in that module or submodules are also implicitly contained
+/// in the forest.
+///
+/// This is used to represent a set of modules in which a type is visibly
+/// uninhabited.
+///
+/// We store the minimal set of `DefId`s required to represent the whole set. If A and B are
+/// `DefId`s in the `DefIdForest`, and A is a parent of B, then only A will be stored. When this is
+/// used with `type_uninhabited_from`, there will very rarely be more than one `DefId` stored.
+#[derive(Copy, Clone, HashStable, Debug)]
+pub enum DefIdForest<'a> {
+    Empty,
+    Single(DefId),
+    /// This variant is very rare.
+    /// Invariant: >1 elements
+    Multiple(&'a [DefId]),
+}
+
+/// Tests whether a slice of roots contains a given DefId.
+#[inline]
+fn slice_contains<'tcx>(tcx: TyCtxt<'tcx>, slice: &[DefId], id: DefId) -> bool {
+    slice.iter().any(|root_id| tcx.is_descendant_of(id, *root_id))
+}
+
+impl<'tcx> DefIdForest<'tcx> {
+    /// Creates an empty forest.
+    pub fn empty() -> DefIdForest<'tcx> {
+        DefIdForest::Empty
+    }
+
+    /// Creates a forest consisting of a single tree representing the entire
+    /// crate.
+    #[inline]
+    pub fn full() -> DefIdForest<'tcx> {
+        DefIdForest::from_id(CRATE_DEF_ID.to_def_id())
+    }
+
+    /// Creates a forest containing a `DefId` and all its descendants.
+    pub fn from_id(id: DefId) -> DefIdForest<'tcx> {
+        DefIdForest::Single(id)
+    }
+
+    fn as_slice(&self) -> &[DefId] {
+        match self {
+            Empty => &[],
+            Single(id) => std::slice::from_ref(id),
+            Multiple(root_ids) => root_ids,
+        }
+    }
+
+    // Only allocates in the rare `Multiple` case.
+    fn from_vec(tcx: TyCtxt<'tcx>, root_ids: SmallVec<[DefId; 1]>) -> DefIdForest<'tcx> {
+        match &root_ids[..] {
+            [] => Empty,
+            [id] => Single(*id),
+            _ => DefIdForest::Multiple(tcx.arena.alloc_from_iter(root_ids)),
+        }
+    }
+
+    /// Tests whether the forest is empty.
+    pub fn is_empty(&self) -> bool {
+        match self {
+            Empty => true,
+            Single(..) | Multiple(..) => false,
+        }
+    }
+
+    /// Iterate over the set of roots.
+    fn iter(&self) -> impl Iterator<Item = DefId> + '_ {
+        self.as_slice().iter().copied()
+    }
+
+    /// Tests whether the forest contains a given DefId.
+    pub fn contains(&self, tcx: TyCtxt<'tcx>, id: DefId) -> bool {
+        slice_contains(tcx, self.as_slice(), id)
+    }
+
+    /// Calculate the intersection of a collection of forests.
+    pub fn intersection<I>(tcx: TyCtxt<'tcx>, iter: I) -> DefIdForest<'tcx>
+    where
+        I: IntoIterator<Item = DefIdForest<'tcx>>,
+    {
+        let mut iter = iter.into_iter();
+        let mut ret: SmallVec<[_; 1]> = if let Some(first) = iter.next() {
+            SmallVec::from_slice(first.as_slice())
+        } else {
+            return DefIdForest::full();
+        };
+
+        let mut next_ret: SmallVec<[_; 1]> = SmallVec::new();
+        for next_forest in iter {
+            // No need to continue if the intersection is already empty.
+            if ret.is_empty() || next_forest.is_empty() {
+                return DefIdForest::empty();
+            }
+
+            // We keep the elements in `ret` that are also in `next_forest`.
+            next_ret.extend(ret.iter().copied().filter(|&id| next_forest.contains(tcx, id)));
+            // We keep the elements in `next_forest` that are also in `ret`.
+            next_ret.extend(next_forest.iter().filter(|&id| slice_contains(tcx, &ret, id)));
+
+            mem::swap(&mut next_ret, &mut ret);
+            next_ret.clear();
+        }
+        DefIdForest::from_vec(tcx, ret)
+    }
+
+    /// Calculate the union of a collection of forests.
+    pub fn union<I>(tcx: TyCtxt<'tcx>, iter: I) -> DefIdForest<'tcx>
+    where
+        I: IntoIterator<Item = DefIdForest<'tcx>>,
+    {
+        let mut ret: SmallVec<[_; 1]> = SmallVec::new();
+        let mut next_ret: SmallVec<[_; 1]> = SmallVec::new();
+        for next_forest in iter {
+            // Union with the empty set is a no-op.
+            if next_forest.is_empty() {
+                continue;
+            }
+
+            // We add everything in `ret` that is not in `next_forest`.
+            next_ret.extend(ret.iter().copied().filter(|&id| !next_forest.contains(tcx, id)));
+            // We add everything in `next_forest` that we haven't added yet.
+            for id in next_forest.iter() {
+                if !slice_contains(tcx, &next_ret, id) {
+                    next_ret.push(id);
+                }
+            }
+
+            mem::swap(&mut next_ret, &mut ret);
+            next_ret.clear();
+        }
+        DefIdForest::from_vec(tcx, ret)
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/inhabitedness/mod.rs b/compiler/rustc_middle/src/ty/inhabitedness/mod.rs
new file mode 100644
index 000000000..3d22f5a04
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/inhabitedness/mod.rs
@@ -0,0 +1,234 @@
+pub use self::def_id_forest::DefIdForest;
+
+use crate::ty;
+use crate::ty::context::TyCtxt;
+use crate::ty::{AdtDef, FieldDef, Ty, VariantDef};
+use crate::ty::{AdtKind, Visibility};
+use crate::ty::{DefId, SubstsRef};
+
+use rustc_type_ir::sty::TyKind::*;
+
+mod def_id_forest;
+
+// The methods in this module calculate `DefIdForest`s of modules in which an
+// `AdtDef`/`VariantDef`/`FieldDef` is visibly uninhabited.
+//
+// # Example
+// ```rust
+// enum Void {}
+// mod a {
+//     pub mod b {
+//         pub struct SecretlyUninhabited {
+//             _priv: !,
+//         }
+//     }
+// }
+//
+// mod c {
+//     pub struct AlsoSecretlyUninhabited {
+//         _priv: Void,
+//     }
+//     mod d {
+//     }
+// }
+//
+// struct Foo {
+//     x: a::b::SecretlyUninhabited,
+//     y: c::AlsoSecretlyUninhabited,
+// }
+// ```
+// In this code, the type `Foo` will only be visibly uninhabited inside the
+// modules `b`, `c` and `d`. Calling `uninhabited_from` on `Foo` or its `AdtDef` will
+// return the forest of modules {`b`, `c`->`d`} (represented in a `DefIdForest` by the
+// set {`b`, `c`}).
+//
+// We need this information for pattern-matching on `Foo` or types that contain
+// `Foo`.
+//
+// # Example
+// ```rust
+// let foo_result: Result<T, Foo> = ... ;
+// let Ok(t) = foo_result;
+// ```
+// This code should only compile in modules where the uninhabitedness of `Foo` is
+// visible.
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Checks whether a type is visibly uninhabited from a particular module.
+    ///
+    /// # Example
+    /// ```
+    /// #![feature(never_type)]
+    /// # fn main() {}
+    /// enum Void {}
+    /// mod a {
+    ///     pub mod b {
+    ///         pub struct SecretlyUninhabited {
+    ///             _priv: !,
+    ///         }
+    ///     }
+    /// }
+    ///
+    /// mod c {
+    ///     use super::Void;
+    ///     pub struct AlsoSecretlyUninhabited {
+    ///         _priv: Void,
+    ///     }
+    ///     mod d {
+    ///     }
+    /// }
+    ///
+    /// struct Foo {
+    ///     x: a::b::SecretlyUninhabited,
+    ///     y: c::AlsoSecretlyUninhabited,
+    /// }
+    /// ```
+    /// In this code, the type `Foo` will only be visibly uninhabited inside the
+    /// modules b, c and d. This effects pattern-matching on `Foo` or types that
+    /// contain `Foo`.
+    ///
+    /// # Example
+    /// ```ignore (illustrative)
+    /// let foo_result: Result<T, Foo> = ... ;
+    /// let Ok(t) = foo_result;
+    /// ```
+    /// This code should only compile in modules where the uninhabitedness of Foo is
+    /// visible.
+    pub fn is_ty_uninhabited_from(
+        self,
+        module: DefId,
+        ty: Ty<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> bool {
+        // To check whether this type is uninhabited at all (not just from the
+        // given node), you could check whether the forest is empty.
+        // ```
+        // forest.is_empty()
+        // ```
+        ty.uninhabited_from(self, param_env).contains(self, module)
+    }
+}
+
+impl<'tcx> AdtDef<'tcx> {
+    /// Calculates the forest of `DefId`s from which this ADT is visibly uninhabited.
+    fn uninhabited_from(
+        self,
+        tcx: TyCtxt<'tcx>,
+        substs: SubstsRef<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> DefIdForest<'tcx> {
+        // Non-exhaustive ADTs from other crates are always considered inhabited.
+        if self.is_variant_list_non_exhaustive() && !self.did().is_local() {
+            DefIdForest::empty()
+        } else {
+            DefIdForest::intersection(
+                tcx,
+                self.variants()
+                    .iter()
+                    .map(|v| v.uninhabited_from(tcx, substs, self.adt_kind(), param_env)),
+            )
+        }
+    }
+}
+
+impl<'tcx> VariantDef {
+    /// Calculates the forest of `DefId`s from which this variant is visibly uninhabited.
+    pub fn uninhabited_from(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        substs: SubstsRef<'tcx>,
+        adt_kind: AdtKind,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> DefIdForest<'tcx> {
+        let is_enum = match adt_kind {
+            // For now, `union`s are never considered uninhabited.
+            // The precise semantics of inhabitedness with respect to unions is currently undecided.
+            AdtKind::Union => return DefIdForest::empty(),
+            AdtKind::Enum => true,
+            AdtKind::Struct => false,
+        };
+        // Non-exhaustive variants from other crates are always considered inhabited.
+        if self.is_field_list_non_exhaustive() && !self.def_id.is_local() {
+            DefIdForest::empty()
+        } else {
+            DefIdForest::union(
+                tcx,
+                self.fields.iter().map(|f| f.uninhabited_from(tcx, substs, is_enum, param_env)),
+            )
+        }
+    }
+}
+
+impl<'tcx> FieldDef {
+    /// Calculates the forest of `DefId`s from which this field is visibly uninhabited.
+    fn uninhabited_from(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        substs: SubstsRef<'tcx>,
+        is_enum: bool,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> DefIdForest<'tcx> {
+        let data_uninhabitedness = move || self.ty(tcx, substs).uninhabited_from(tcx, param_env);
+        // FIXME(canndrew): Currently enum fields are (incorrectly) stored with
+        // `Visibility::Invisible` so we need to override `self.vis` if we're
+        // dealing with an enum.
+        if is_enum {
+            data_uninhabitedness()
+        } else {
+            match self.vis {
+                Visibility::Invisible => DefIdForest::empty(),
+                Visibility::Restricted(from) => {
+                    let forest = DefIdForest::from_id(from);
+                    let iter = Some(forest).into_iter().chain(Some(data_uninhabitedness()));
+                    DefIdForest::intersection(tcx, iter)
+                }
+                Visibility::Public => data_uninhabitedness(),
+            }
+        }
+    }
+}
+
+impl<'tcx> Ty<'tcx> {
+    /// Calculates the forest of `DefId`s from which this type is visibly uninhabited.
+    fn uninhabited_from(
+        self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> DefIdForest<'tcx> {
+        tcx.type_uninhabited_from(param_env.and(self))
+    }
+}
+
+// Query provider for `type_uninhabited_from`.
+pub(crate) fn type_uninhabited_from<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    key: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
+) -> DefIdForest<'tcx> {
+    let ty = key.value;
+    let param_env = key.param_env;
+    match *ty.kind() {
+        Adt(def, substs) => def.uninhabited_from(tcx, substs, param_env),
+
+        Never => DefIdForest::full(),
+
+        Tuple(ref tys) => {
+            DefIdForest::union(tcx, tys.iter().map(|ty| ty.uninhabited_from(tcx, param_env)))
+        }
+
+        Array(ty, len) => match len.try_eval_usize(tcx, param_env) {
+            Some(0) | None => DefIdForest::empty(),
+            // If the array is definitely non-empty, it's uninhabited if
+            // the type of its elements is uninhabited.
+            Some(1..) => ty.uninhabited_from(tcx, param_env),
+        },
+
+        // References to uninitialised memory are valid for any type, including
+        // uninhabited types, in unsafe code, so we treat all references as
+        // inhabited.
+        // The precise semantics of inhabitedness with respect to references is currently
+        // undecided.
+        Ref(..) => DefIdForest::empty(),
+
+        _ => DefIdForest::empty(),
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/instance.rs b/compiler/rustc_middle/src/ty/instance.rs
new file mode 100644
index 000000000..53218225d
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/instance.rs
@@ -0,0 +1,746 @@
+use crate::middle::codegen_fn_attrs::CodegenFnAttrFlags;
+use crate::ty::print::{FmtPrinter, Printer};
+use crate::ty::subst::{InternalSubsts, Subst};
+use crate::ty::{
+    self, EarlyBinder, SubstsRef, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitable,
+};
+use rustc_errors::ErrorGuaranteed;
+use rustc_hir::def::Namespace;
+use rustc_hir::def_id::{CrateNum, DefId};
+use rustc_hir::lang_items::LangItem;
+use rustc_macros::HashStable;
+use rustc_middle::ty::normalize_erasing_regions::NormalizationError;
+use rustc_span::Symbol;
+
+use std::fmt;
+
+/// A monomorphized `InstanceDef`.
+///
+/// Monomorphization happens on-the-fly and no monomorphized MIR is ever created. Instead, this type
+/// simply couples a potentially generic `InstanceDef` with some substs, and codegen and const eval
+/// will do all required substitution as they run.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, TyEncodable, TyDecodable)]
+#[derive(HashStable, Lift)]
+pub struct Instance<'tcx> {
+    pub def: InstanceDef<'tcx>,
+    pub substs: SubstsRef<'tcx>,
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
+#[derive(TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
+pub enum InstanceDef<'tcx> {
+    /// A user-defined callable item.
+    ///
+    /// This includes:
+    /// - `fn` items
+    /// - closures
+    /// - generators
+    Item(ty::WithOptConstParam<DefId>),
+
+    /// An intrinsic `fn` item (with `"rust-intrinsic"` or `"platform-intrinsic"` ABI).
+    ///
+    /// Alongside `Virtual`, this is the only `InstanceDef` that does not have its own callable MIR.
+    /// Instead, codegen and const eval "magically" evaluate calls to intrinsics purely in the
+    /// caller.
+    Intrinsic(DefId),
+
+    /// `<T as Trait>::method` where `method` receives unsizeable `self: Self` (part of the
+    /// `unsized_locals` feature).
+    ///
+    /// The generated shim will take `Self` via `*mut Self` - conceptually this is `&owned Self` -
+    /// and dereference the argument to call the original function.
+    VTableShim(DefId),
+
+    /// `fn()` pointer where the function itself cannot be turned into a pointer.
+    ///
+    /// One example is `<dyn Trait as Trait>::fn`, where the shim contains
+    /// a virtual call, which codegen supports only via a direct call to the
+    /// `<dyn Trait as Trait>::fn` instance (an `InstanceDef::Virtual`).
+    ///
+    /// Another example is functions annotated with `#[track_caller]`, which
+    /// must have their implicit caller location argument populated for a call.
+    /// Because this is a required part of the function's ABI but can't be tracked
+    /// as a property of the function pointer, we use a single "caller location"
+    /// (the definition of the function itself).
+    ReifyShim(DefId),
+
+    /// `<fn() as FnTrait>::call_*` (generated `FnTrait` implementation for `fn()` pointers).
+    ///
+    /// `DefId` is `FnTrait::call_*`.
+    FnPtrShim(DefId, Ty<'tcx>),
+
+    /// Dynamic dispatch to `<dyn Trait as Trait>::fn`.
+    ///
+    /// This `InstanceDef` does not have callable MIR. Calls to `Virtual` instances must be
+    /// codegen'd as virtual calls through the vtable.
+    ///
+    /// If this is reified to a `fn` pointer, a `ReifyShim` is used (see `ReifyShim` above for more
+    /// details on that).
+    Virtual(DefId, usize),
+
+    /// `<[FnMut closure] as FnOnce>::call_once`.
+    ///
+    /// The `DefId` is the ID of the `call_once` method in `FnOnce`.
+    ClosureOnceShim { call_once: DefId, track_caller: bool },
+
+    /// `core::ptr::drop_in_place::<T>`.
+    ///
+    /// The `DefId` is for `core::ptr::drop_in_place`.
+    /// The `Option<Ty<'tcx>>` is either `Some(T)`, or `None` for empty drop
+    /// glue.
+    DropGlue(DefId, Option<Ty<'tcx>>),
+
+    /// Compiler-generated `<T as Clone>::clone` implementation.
+    ///
+    /// For all types that automatically implement `Copy`, a trivial `Clone` impl is provided too.
+    /// Additionally, arrays, tuples, and closures get a `Clone` shim even if they aren't `Copy`.
+    ///
+    /// The `DefId` is for `Clone::clone`, the `Ty` is the type `T` with the builtin `Clone` impl.
+    CloneShim(DefId, Ty<'tcx>),
+}
+
+impl<'tcx> Instance<'tcx> {
+    /// Returns the `Ty` corresponding to this `Instance`, with generic substitutions applied and
+    /// lifetimes erased, allowing a `ParamEnv` to be specified for use during normalization.
+    pub fn ty(&self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Ty<'tcx> {
+        let ty = tcx.type_of(self.def.def_id());
+        tcx.subst_and_normalize_erasing_regions(self.substs, param_env, ty)
+    }
+
+    /// Finds a crate that contains a monomorphization of this instance that
+    /// can be linked to from the local crate. A return value of `None` means
+    /// no upstream crate provides such an exported monomorphization.
+    ///
+    /// This method already takes into account the global `-Zshare-generics`
+    /// setting, always returning `None` if `share-generics` is off.
+    pub fn upstream_monomorphization(&self, tcx: TyCtxt<'tcx>) -> Option<CrateNum> {
+        // If we are not in share generics mode, we don't link to upstream
+        // monomorphizations but always instantiate our own internal versions
+        // instead.
+        if !tcx.sess.opts.share_generics() {
+            return None;
+        }
+
+        // If this is an item that is defined in the local crate, no upstream
+        // crate can know about it/provide a monomorphization.
+        if self.def_id().is_local() {
+            return None;
+        }
+
+        // If this a non-generic instance, it cannot be a shared monomorphization.
+        self.substs.non_erasable_generics().next()?;
+
+        match self.def {
+            InstanceDef::Item(def) => tcx
+                .upstream_monomorphizations_for(def.did)
+                .and_then(|monos| monos.get(&self.substs).cloned()),
+            InstanceDef::DropGlue(_, Some(_)) => tcx.upstream_drop_glue_for(self.substs),
+            _ => None,
+        }
+    }
+}
+
+impl<'tcx> InstanceDef<'tcx> {
+    #[inline]
+    pub fn def_id(self) -> DefId {
+        match self {
+            InstanceDef::Item(def) => def.did,
+            InstanceDef::VTableShim(def_id)
+            | InstanceDef::ReifyShim(def_id)
+            | InstanceDef::FnPtrShim(def_id, _)
+            | InstanceDef::Virtual(def_id, _)
+            | InstanceDef::Intrinsic(def_id)
+            | InstanceDef::ClosureOnceShim { call_once: def_id, track_caller: _ }
+            | InstanceDef::DropGlue(def_id, _)
+            | InstanceDef::CloneShim(def_id, _) => def_id,
+        }
+    }
+
+    /// Returns the `DefId` of instances which might not require codegen locally.
+    pub fn def_id_if_not_guaranteed_local_codegen(self) -> Option<DefId> {
+        match self {
+            ty::InstanceDef::Item(def) => Some(def.did),
+            ty::InstanceDef::DropGlue(def_id, Some(_)) => Some(def_id),
+            InstanceDef::VTableShim(..)
+            | InstanceDef::ReifyShim(..)
+            | InstanceDef::FnPtrShim(..)
+            | InstanceDef::Virtual(..)
+            | InstanceDef::Intrinsic(..)
+            | InstanceDef::ClosureOnceShim { .. }
+            | InstanceDef::DropGlue(..)
+            | InstanceDef::CloneShim(..) => None,
+        }
+    }
+
+    #[inline]
+    pub fn with_opt_param(self) -> ty::WithOptConstParam<DefId> {
+        match self {
+            InstanceDef::Item(def) => def,
+            InstanceDef::VTableShim(def_id)
+            | InstanceDef::ReifyShim(def_id)
+            | InstanceDef::FnPtrShim(def_id, _)
+            | InstanceDef::Virtual(def_id, _)
+            | InstanceDef::Intrinsic(def_id)
+            | InstanceDef::ClosureOnceShim { call_once: def_id, track_caller: _ }
+            | InstanceDef::DropGlue(def_id, _)
+            | InstanceDef::CloneShim(def_id, _) => ty::WithOptConstParam::unknown(def_id),
+        }
+    }
+
+    #[inline]
+    pub fn get_attrs(&self, tcx: TyCtxt<'tcx>, attr: Symbol) -> ty::Attributes<'tcx> {
+        tcx.get_attrs(self.def_id(), attr)
+    }
+
+    /// Returns `true` if the LLVM version of this instance is unconditionally
+    /// marked with `inline`. This implies that a copy of this instance is
+    /// generated in every codegen unit.
+    /// Note that this is only a hint. See the documentation for
+    /// `generates_cgu_internal_copy` for more information.
+    pub fn requires_inline(&self, tcx: TyCtxt<'tcx>) -> bool {
+        use rustc_hir::definitions::DefPathData;
+        let def_id = match *self {
+            ty::InstanceDef::Item(def) => def.did,
+            ty::InstanceDef::DropGlue(_, Some(_)) => return false,
+            _ => return true,
+        };
+        matches!(
+            tcx.def_key(def_id).disambiguated_data.data,
+            DefPathData::Ctor | DefPathData::ClosureExpr
+        )
+    }
+
+    /// Returns `true` if the machine code for this instance is instantiated in
+    /// each codegen unit that references it.
+    /// Note that this is only a hint! The compiler can globally decide to *not*
+    /// do this in order to speed up compilation. CGU-internal copies are
+    /// only exist to enable inlining. If inlining is not performed (e.g. at
+    /// `-Copt-level=0`) then the time for generating them is wasted and it's
+    /// better to create a single copy with external linkage.
+    pub fn generates_cgu_internal_copy(&self, tcx: TyCtxt<'tcx>) -> bool {
+        if self.requires_inline(tcx) {
+            return true;
+        }
+        if let ty::InstanceDef::DropGlue(.., Some(ty)) = *self {
+            // Drop glue generally wants to be instantiated at every codegen
+            // unit, but without an #[inline] hint. We should make this
+            // available to normal end-users.
+            if tcx.sess.opts.incremental.is_none() {
+                return true;
+            }
+            // When compiling with incremental, we can generate a *lot* of
+            // codegen units. Including drop glue into all of them has a
+            // considerable compile time cost.
+            //
+            // We include enums without destructors to allow, say, optimizing
+            // drops of `Option::None` before LTO. We also respect the intent of
+            // `#[inline]` on `Drop::drop` implementations.
+            return ty.ty_adt_def().map_or(true, |adt_def| {
+                adt_def.destructor(tcx).map_or_else(
+                    || adt_def.is_enum(),
+                    |dtor| tcx.codegen_fn_attrs(dtor.did).requests_inline(),
+                )
+            });
+        }
+        tcx.codegen_fn_attrs(self.def_id()).requests_inline()
+    }
+
+    pub fn requires_caller_location(&self, tcx: TyCtxt<'_>) -> bool {
+        match *self {
+            InstanceDef::Item(ty::WithOptConstParam { did: def_id, .. })
+            | InstanceDef::Virtual(def_id, _) => {
+                tcx.body_codegen_attrs(def_id).flags.contains(CodegenFnAttrFlags::TRACK_CALLER)
+            }
+            InstanceDef::ClosureOnceShim { call_once: _, track_caller } => track_caller,
+            _ => false,
+        }
+    }
+
+    /// Returns `true` when the MIR body associated with this instance should be monomorphized
+    /// by its users (e.g. codegen or miri) by substituting the `substs` from `Instance` (see
+    /// `Instance::substs_for_mir_body`).
+    ///
+    /// Otherwise, returns `false` only for some kinds of shims where the construction of the MIR
+    /// body should perform necessary substitutions.
+    pub fn has_polymorphic_mir_body(&self) -> bool {
+        match *self {
+            InstanceDef::CloneShim(..)
+            | InstanceDef::FnPtrShim(..)
+            | InstanceDef::DropGlue(_, Some(_)) => false,
+            InstanceDef::ClosureOnceShim { .. }
+            | InstanceDef::DropGlue(..)
+            | InstanceDef::Item(_)
+            | InstanceDef::Intrinsic(..)
+            | InstanceDef::ReifyShim(..)
+            | InstanceDef::Virtual(..)
+            | InstanceDef::VTableShim(..) => true,
+        }
+    }
+}
+
+impl<'tcx> fmt::Display for Instance<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        ty::tls::with(|tcx| {
+            let substs = tcx.lift(self.substs).expect("could not lift for printing");
+            let s = FmtPrinter::new(tcx, Namespace::ValueNS)
+                .print_def_path(self.def_id(), substs)?
+                .into_buffer();
+            f.write_str(&s)
+        })?;
+
+        match self.def {
+            InstanceDef::Item(_) => Ok(()),
+            InstanceDef::VTableShim(_) => write!(f, " - shim(vtable)"),
+            InstanceDef::ReifyShim(_) => write!(f, " - shim(reify)"),
+            InstanceDef::Intrinsic(_) => write!(f, " - intrinsic"),
+            InstanceDef::Virtual(_, num) => write!(f, " - virtual#{}", num),
+            InstanceDef::FnPtrShim(_, ty) => write!(f, " - shim({})", ty),
+            InstanceDef::ClosureOnceShim { .. } => write!(f, " - shim"),
+            InstanceDef::DropGlue(_, None) => write!(f, " - shim(None)"),
+            InstanceDef::DropGlue(_, Some(ty)) => write!(f, " - shim(Some({}))", ty),
+            InstanceDef::CloneShim(_, ty) => write!(f, " - shim({})", ty),
+        }
+    }
+}
+
+impl<'tcx> Instance<'tcx> {
+    pub fn new(def_id: DefId, substs: SubstsRef<'tcx>) -> Instance<'tcx> {
+        assert!(
+            !substs.has_escaping_bound_vars(),
+            "substs of instance {:?} not normalized for codegen: {:?}",
+            def_id,
+            substs
+        );
+        Instance { def: InstanceDef::Item(ty::WithOptConstParam::unknown(def_id)), substs }
+    }
+
+    pub fn mono(tcx: TyCtxt<'tcx>, def_id: DefId) -> Instance<'tcx> {
+        let substs = InternalSubsts::for_item(tcx, def_id, |param, _| match param.kind {
+            ty::GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
+            ty::GenericParamDefKind::Type { .. } => {
+                bug!("Instance::mono: {:?} has type parameters", def_id)
+            }
+            ty::GenericParamDefKind::Const { .. } => {
+                bug!("Instance::mono: {:?} has const parameters", def_id)
+            }
+        });
+
+        Instance::new(def_id, substs)
+    }
+
+    #[inline]
+    pub fn def_id(&self) -> DefId {
+        self.def.def_id()
+    }
+
+    /// Resolves a `(def_id, substs)` pair to an (optional) instance -- most commonly,
+    /// this is used to find the precise code that will run for a trait method invocation,
+    /// if known.
+    ///
+    /// Returns `Ok(None)` if we cannot resolve `Instance` to a specific instance.
+    /// For example, in a context like this,
+    ///
+    /// ```ignore (illustrative)
+    /// fn foo<T: Debug>(t: T) { ... }
+    /// ```
+    ///
+    /// trying to resolve `Debug::fmt` applied to `T` will yield `Ok(None)`, because we do not
+    /// know what code ought to run. (Note that this setting is also affected by the
+    /// `RevealMode` in the parameter environment.)
+    ///
+    /// Presuming that coherence and type-check have succeeded, if this method is invoked
+    /// in a monomorphic context (i.e., like during codegen), then it is guaranteed to return
+    /// `Ok(Some(instance))`.
+    ///
+    /// Returns `Err(ErrorGuaranteed)` when the `Instance` resolution process
+    /// couldn't complete due to errors elsewhere - this is distinct
+    /// from `Ok(None)` to avoid misleading diagnostics when an error
+    /// has already been/will be emitted, for the original cause
+    pub fn resolve(
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        def_id: DefId,
+        substs: SubstsRef<'tcx>,
+    ) -> Result<Option<Instance<'tcx>>, ErrorGuaranteed> {
+        Instance::resolve_opt_const_arg(
+            tcx,
+            param_env,
+            ty::WithOptConstParam::unknown(def_id),
+            substs,
+        )
+    }
+
+    // This should be kept up to date with `resolve`.
+    pub fn resolve_opt_const_arg(
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        def: ty::WithOptConstParam<DefId>,
+        substs: SubstsRef<'tcx>,
+    ) -> Result<Option<Instance<'tcx>>, ErrorGuaranteed> {
+        // All regions in the result of this query are erased, so it's
+        // fine to erase all of the input regions.
+
+        // HACK(eddyb) erase regions in `substs` first, so that `param_env.and(...)`
+        // below is more likely to ignore the bounds in scope (e.g. if the only
+        // generic parameters mentioned by `substs` were lifetime ones).
+        let substs = tcx.erase_regions(substs);
+
+        // FIXME(eddyb) should this always use `param_env.with_reveal_all()`?
+        if let Some((did, param_did)) = def.as_const_arg() {
+            tcx.resolve_instance_of_const_arg(
+                tcx.erase_regions(param_env.and((did, param_did, substs))),
+            )
+        } else {
+            tcx.resolve_instance(tcx.erase_regions(param_env.and((def.did, substs))))
+        }
+    }
+
+    pub fn resolve_for_fn_ptr(
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        def_id: DefId,
+        substs: SubstsRef<'tcx>,
+    ) -> Option<Instance<'tcx>> {
+        debug!("resolve(def_id={:?}, substs={:?})", def_id, substs);
+        // Use either `resolve_closure` or `resolve_for_vtable`
+        assert!(!tcx.is_closure(def_id), "Called `resolve_for_fn_ptr` on closure: {:?}", def_id);
+        Instance::resolve(tcx, param_env, def_id, substs).ok().flatten().map(|mut resolved| {
+            match resolved.def {
+                InstanceDef::Item(def) if resolved.def.requires_caller_location(tcx) => {
+                    debug!(" => fn pointer created for function with #[track_caller]");
+                    resolved.def = InstanceDef::ReifyShim(def.did);
+                }
+                InstanceDef::Virtual(def_id, _) => {
+                    debug!(" => fn pointer created for virtual call");
+                    resolved.def = InstanceDef::ReifyShim(def_id);
+                }
+                _ => {}
+            }
+
+            resolved
+        })
+    }
+
+    pub fn resolve_for_vtable(
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        def_id: DefId,
+        substs: SubstsRef<'tcx>,
+    ) -> Option<Instance<'tcx>> {
+        debug!("resolve_for_vtable(def_id={:?}, substs={:?})", def_id, substs);
+        let fn_sig = tcx.fn_sig(def_id);
+        let is_vtable_shim = !fn_sig.inputs().skip_binder().is_empty()
+            && fn_sig.input(0).skip_binder().is_param(0)
+            && tcx.generics_of(def_id).has_self;
+        if is_vtable_shim {
+            debug!(" => associated item with unsizeable self: Self");
+            Some(Instance { def: InstanceDef::VTableShim(def_id), substs })
+        } else {
+            Instance::resolve(tcx, param_env, def_id, substs).ok().flatten().map(|mut resolved| {
+                match resolved.def {
+                    InstanceDef::Item(def) => {
+                        // We need to generate a shim when we cannot guarantee that
+                        // the caller of a trait object method will be aware of
+                        // `#[track_caller]` - this ensures that the caller
+                        // and callee ABI will always match.
+                        //
+                        // The shim is generated when all of these conditions are met:
+                        //
+                        // 1) The underlying method expects a caller location parameter
+                        // in the ABI
+                        if resolved.def.requires_caller_location(tcx)
+                            // 2) The caller location parameter comes from having `#[track_caller]`
+                            // on the implementation, and *not* on the trait method.
+                            && !tcx.should_inherit_track_caller(def.did)
+                            // If the method implementation comes from the trait definition itself
+                            // (e.g. `trait Foo { #[track_caller] my_fn() { /* impl */ } }`),
+                            // then we don't need to generate a shim. This check is needed because
+                            // `should_inherit_track_caller` returns `false` if our method
+                            // implementation comes from the trait block, and not an impl block
+                            && !matches!(
+                                tcx.opt_associated_item(def.did),
+                                Some(ty::AssocItem {
+                                    container: ty::AssocItemContainer::TraitContainer,
+                                    ..
+                                })
+                            )
+                        {
+                            if tcx.is_closure(def.did) {
+                                debug!(" => vtable fn pointer created for closure with #[track_caller]: {:?} for method {:?} {:?}",
+                                       def.did, def_id, substs);
+
+                                // Create a shim for the `FnOnce/FnMut/Fn` method we are calling
+                                // - unlike functions, invoking a closure always goes through a
+                                // trait.
+                                resolved = Instance { def: InstanceDef::ReifyShim(def_id), substs };
+                            } else {
+                                debug!(
+                                    " => vtable fn pointer created for function with #[track_caller]: {:?}", def.did
+                                );
+                                resolved.def = InstanceDef::ReifyShim(def.did);
+                            }
+                        }
+                    }
+                    InstanceDef::Virtual(def_id, _) => {
+                        debug!(" => vtable fn pointer created for virtual call");
+                        resolved.def = InstanceDef::ReifyShim(def_id);
+                    }
+                    _ => {}
+                }
+
+                resolved
+            })
+        }
+    }
+
+    pub fn resolve_closure(
+        tcx: TyCtxt<'tcx>,
+        def_id: DefId,
+        substs: ty::SubstsRef<'tcx>,
+        requested_kind: ty::ClosureKind,
+    ) -> Option<Instance<'tcx>> {
+        let actual_kind = substs.as_closure().kind();
+
+        match needs_fn_once_adapter_shim(actual_kind, requested_kind) {
+            Ok(true) => Instance::fn_once_adapter_instance(tcx, def_id, substs),
+            _ => Some(Instance::new(def_id, substs)),
+        }
+    }
+
+    pub fn resolve_drop_in_place(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> ty::Instance<'tcx> {
+        let def_id = tcx.require_lang_item(LangItem::DropInPlace, None);
+        let substs = tcx.intern_substs(&[ty.into()]);
+        Instance::resolve(tcx, ty::ParamEnv::reveal_all(), def_id, substs).unwrap().unwrap()
+    }
+
+    pub fn fn_once_adapter_instance(
+        tcx: TyCtxt<'tcx>,
+        closure_did: DefId,
+        substs: ty::SubstsRef<'tcx>,
+    ) -> Option<Instance<'tcx>> {
+        debug!("fn_once_adapter_shim({:?}, {:?})", closure_did, substs);
+        let fn_once = tcx.require_lang_item(LangItem::FnOnce, None);
+        let call_once = tcx
+            .associated_items(fn_once)
+            .in_definition_order()
+            .find(|it| it.kind == ty::AssocKind::Fn)
+            .unwrap()
+            .def_id;
+        let track_caller =
+            tcx.codegen_fn_attrs(closure_did).flags.contains(CodegenFnAttrFlags::TRACK_CALLER);
+        let def = ty::InstanceDef::ClosureOnceShim { call_once, track_caller };
+
+        let self_ty = tcx.mk_closure(closure_did, substs);
+
+        let sig = substs.as_closure().sig();
+        let sig =
+            tcx.try_normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), sig).ok()?;
+        assert_eq!(sig.inputs().len(), 1);
+        let substs = tcx.mk_substs_trait(self_ty, &[sig.inputs()[0].into()]);
+
+        debug!("fn_once_adapter_shim: self_ty={:?} sig={:?}", self_ty, sig);
+        Some(Instance { def, substs })
+    }
+
+    /// Depending on the kind of `InstanceDef`, the MIR body associated with an
+    /// instance is expressed in terms of the generic parameters of `self.def_id()`, and in other
+    /// cases the MIR body is expressed in terms of the types found in the substitution array.
+    /// In the former case, we want to substitute those generic types and replace them with the
+    /// values from the substs when monomorphizing the function body. But in the latter case, we
+    /// don't want to do that substitution, since it has already been done effectively.
+    ///
+    /// This function returns `Some(substs)` in the former case and `None` otherwise -- i.e., if
+    /// this function returns `None`, then the MIR body does not require substitution during
+    /// codegen.
+    fn substs_for_mir_body(&self) -> Option<SubstsRef<'tcx>> {
+        if self.def.has_polymorphic_mir_body() { Some(self.substs) } else { None }
+    }
+
+    pub fn subst_mir<T>(&self, tcx: TyCtxt<'tcx>, v: &T) -> T
+    where
+        T: TypeFoldable<'tcx> + Copy,
+    {
+        if let Some(substs) = self.substs_for_mir_body() {
+            EarlyBinder(*v).subst(tcx, substs)
+        } else {
+            *v
+        }
+    }
+
+    #[inline(always)]
+    pub fn subst_mir_and_normalize_erasing_regions<T>(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        v: T,
+    ) -> T
+    where
+        T: TypeFoldable<'tcx> + Clone,
+    {
+        if let Some(substs) = self.substs_for_mir_body() {
+            tcx.subst_and_normalize_erasing_regions(substs, param_env, v)
+        } else {
+            tcx.normalize_erasing_regions(param_env, v)
+        }
+    }
+
+    #[inline(always)]
+    pub fn try_subst_mir_and_normalize_erasing_regions<T>(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        v: T,
+    ) -> Result<T, NormalizationError<'tcx>>
+    where
+        T: TypeFoldable<'tcx> + Clone,
+    {
+        if let Some(substs) = self.substs_for_mir_body() {
+            tcx.try_subst_and_normalize_erasing_regions(substs, param_env, v)
+        } else {
+            tcx.try_normalize_erasing_regions(param_env, v)
+        }
+    }
+
+    /// Returns a new `Instance` where generic parameters in `instance.substs` are replaced by
+    /// identity parameters if they are determined to be unused in `instance.def`.
+    pub fn polymorphize(self, tcx: TyCtxt<'tcx>) -> Self {
+        debug!("polymorphize: running polymorphization analysis");
+        if !tcx.sess.opts.unstable_opts.polymorphize {
+            return self;
+        }
+
+        let polymorphized_substs = polymorphize(tcx, self.def, self.substs);
+        debug!("polymorphize: self={:?} polymorphized_substs={:?}", self, polymorphized_substs);
+        Self { def: self.def, substs: polymorphized_substs }
+    }
+}
+
+fn polymorphize<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    instance: ty::InstanceDef<'tcx>,
+    substs: SubstsRef<'tcx>,
+) -> SubstsRef<'tcx> {
+    debug!("polymorphize({:?}, {:?})", instance, substs);
+    let unused = tcx.unused_generic_params(instance);
+    debug!("polymorphize: unused={:?}", unused);
+
+    // If this is a closure or generator then we need to handle the case where another closure
+    // from the function is captured as an upvar and hasn't been polymorphized. In this case,
+    // the unpolymorphized upvar closure would result in a polymorphized closure producing
+    // multiple mono items (and eventually symbol clashes).
+    let def_id = instance.def_id();
+    let upvars_ty = if tcx.is_closure(def_id) {
+        Some(substs.as_closure().tupled_upvars_ty())
+    } else if tcx.type_of(def_id).is_generator() {
+        Some(substs.as_generator().tupled_upvars_ty())
+    } else {
+        None
+    };
+    let has_upvars = upvars_ty.map_or(false, |ty| !ty.tuple_fields().is_empty());
+    debug!("polymorphize: upvars_ty={:?} has_upvars={:?}", upvars_ty, has_upvars);
+
+    struct PolymorphizationFolder<'tcx> {
+        tcx: TyCtxt<'tcx>,
+    }
+
+    impl<'tcx> ty::TypeFolder<'tcx> for PolymorphizationFolder<'tcx> {
+        fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
+            self.tcx
+        }
+
+        fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+            debug!("fold_ty: ty={:?}", ty);
+            match *ty.kind() {
+                ty::Closure(def_id, substs) => {
+                    let polymorphized_substs = polymorphize(
+                        self.tcx,
+                        ty::InstanceDef::Item(ty::WithOptConstParam::unknown(def_id)),
+                        substs,
+                    );
+                    if substs == polymorphized_substs {
+                        ty
+                    } else {
+                        self.tcx.mk_closure(def_id, polymorphized_substs)
+                    }
+                }
+                ty::Generator(def_id, substs, movability) => {
+                    let polymorphized_substs = polymorphize(
+                        self.tcx,
+                        ty::InstanceDef::Item(ty::WithOptConstParam::unknown(def_id)),
+                        substs,
+                    );
+                    if substs == polymorphized_substs {
+                        ty
+                    } else {
+                        self.tcx.mk_generator(def_id, polymorphized_substs, movability)
+                    }
+                }
+                _ => ty.super_fold_with(self),
+            }
+        }
+    }
+
+    InternalSubsts::for_item(tcx, def_id, |param, _| {
+        let is_unused = unused.contains(param.index).unwrap_or(false);
+        debug!("polymorphize: param={:?} is_unused={:?}", param, is_unused);
+        match param.kind {
+            // Upvar case: If parameter is a type parameter..
+            ty::GenericParamDefKind::Type { .. } if
+                // ..and has upvars..
+                has_upvars &&
+                // ..and this param has the same type as the tupled upvars..
+                upvars_ty == Some(substs[param.index as usize].expect_ty()) => {
+                    // ..then double-check that polymorphization marked it used..
+                    debug_assert!(!is_unused);
+                    // ..and polymorphize any closures/generators captured as upvars.
+                    let upvars_ty = upvars_ty.unwrap();
+                    let polymorphized_upvars_ty = upvars_ty.fold_with(
+                        &mut PolymorphizationFolder { tcx });
+                    debug!("polymorphize: polymorphized_upvars_ty={:?}", polymorphized_upvars_ty);
+                    ty::GenericArg::from(polymorphized_upvars_ty)
+                },
+
+            // Simple case: If parameter is a const or type parameter..
+            ty::GenericParamDefKind::Const { .. } | ty::GenericParamDefKind::Type { .. } if
+                // ..and is within range and unused..
+                unused.contains(param.index).unwrap_or(false) =>
+                    // ..then use the identity for this parameter.
+                    tcx.mk_param_from_def(param),
+
+            // Otherwise, use the parameter as before.
+            _ => substs[param.index as usize],
+        }
+    })
+}
+
+fn needs_fn_once_adapter_shim(
+    actual_closure_kind: ty::ClosureKind,
+    trait_closure_kind: ty::ClosureKind,
+) -> Result<bool, ()> {
+    match (actual_closure_kind, trait_closure_kind) {
+        (ty::ClosureKind::Fn, ty::ClosureKind::Fn)
+        | (ty::ClosureKind::FnMut, ty::ClosureKind::FnMut)
+        | (ty::ClosureKind::FnOnce, ty::ClosureKind::FnOnce) => {
+            // No adapter needed.
+            Ok(false)
+        }
+        (ty::ClosureKind::Fn, ty::ClosureKind::FnMut) => {
+            // The closure fn `llfn` is a `fn(&self, ...)`.  We want a
+            // `fn(&mut self, ...)`. In fact, at codegen time, these are
+            // basically the same thing, so we can just return llfn.
+            Ok(false)
+        }
+        (ty::ClosureKind::Fn | ty::ClosureKind::FnMut, ty::ClosureKind::FnOnce) => {
+            // The closure fn `llfn` is a `fn(&self, ...)` or `fn(&mut
+            // self, ...)`.  We want a `fn(self, ...)`. We can produce
+            // this by doing something like:
+            //
+            //     fn call_once(self, ...) { call_mut(&self, ...) }
+            //     fn call_once(mut self, ...) { call_mut(&mut self, ...) }
+            //
+            // These are both the same at codegen time.
+            Ok(true)
+        }
+        (ty::ClosureKind::FnMut | ty::ClosureKind::FnOnce, _) => Err(()),
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/layout.rs b/compiler/rustc_middle/src/ty/layout.rs
new file mode 100644
index 000000000..ad78d24e9
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/layout.rs
@@ -0,0 +1,3504 @@
+use crate::middle::codegen_fn_attrs::CodegenFnAttrFlags;
+use crate::mir::{GeneratorLayout, GeneratorSavedLocal};
+use crate::ty::normalize_erasing_regions::NormalizationError;
+use crate::ty::subst::Subst;
+use crate::ty::{self, subst::SubstsRef, EarlyBinder, ReprOptions, Ty, TyCtxt, TypeVisitable};
+use rustc_ast as ast;
+use rustc_attr as attr;
+use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use rustc_hir::lang_items::LangItem;
+use rustc_index::bit_set::BitSet;
+use rustc_index::vec::{Idx, IndexVec};
+use rustc_session::{config::OptLevel, DataTypeKind, FieldInfo, SizeKind, VariantInfo};
+use rustc_span::symbol::Symbol;
+use rustc_span::{Span, DUMMY_SP};
+use rustc_target::abi::call::{
+    ArgAbi, ArgAttribute, ArgAttributes, ArgExtension, Conv, FnAbi, PassMode, Reg, RegKind,
+};
+use rustc_target::abi::*;
+use rustc_target::spec::{abi::Abi as SpecAbi, HasTargetSpec, PanicStrategy, Target};
+
+use std::cmp;
+use std::fmt;
+use std::iter;
+use std::num::NonZeroUsize;
+use std::ops::Bound;
+
+use rand::{seq::SliceRandom, SeedableRng};
+use rand_xoshiro::Xoshiro128StarStar;
+
+pub fn provide(providers: &mut ty::query::Providers) {
+    *providers =
+        ty::query::Providers { layout_of, fn_abi_of_fn_ptr, fn_abi_of_instance, ..*providers };
+}
+
+pub trait IntegerExt {
+    fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>, signed: bool) -> Ty<'tcx>;
+    fn from_attr<C: HasDataLayout>(cx: &C, ity: attr::IntType) -> Integer;
+    fn from_int_ty<C: HasDataLayout>(cx: &C, ity: ty::IntTy) -> Integer;
+    fn from_uint_ty<C: HasDataLayout>(cx: &C, uty: ty::UintTy) -> Integer;
+    fn repr_discr<'tcx>(
+        tcx: TyCtxt<'tcx>,
+        ty: Ty<'tcx>,
+        repr: &ReprOptions,
+        min: i128,
+        max: i128,
+    ) -> (Integer, bool);
+}
+
+impl IntegerExt for Integer {
+    #[inline]
+    fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>, signed: bool) -> Ty<'tcx> {
+        match (*self, signed) {
+            (I8, false) => tcx.types.u8,
+            (I16, false) => tcx.types.u16,
+            (I32, false) => tcx.types.u32,
+            (I64, false) => tcx.types.u64,
+            (I128, false) => tcx.types.u128,
+            (I8, true) => tcx.types.i8,
+            (I16, true) => tcx.types.i16,
+            (I32, true) => tcx.types.i32,
+            (I64, true) => tcx.types.i64,
+            (I128, true) => tcx.types.i128,
+        }
+    }
+
+    /// Gets the Integer type from an attr::IntType.
+    fn from_attr<C: HasDataLayout>(cx: &C, ity: attr::IntType) -> Integer {
+        let dl = cx.data_layout();
+
+        match ity {
+            attr::SignedInt(ast::IntTy::I8) | attr::UnsignedInt(ast::UintTy::U8) => I8,
+            attr::SignedInt(ast::IntTy::I16) | attr::UnsignedInt(ast::UintTy::U16) => I16,
+            attr::SignedInt(ast::IntTy::I32) | attr::UnsignedInt(ast::UintTy::U32) => I32,
+            attr::SignedInt(ast::IntTy::I64) | attr::UnsignedInt(ast::UintTy::U64) => I64,
+            attr::SignedInt(ast::IntTy::I128) | attr::UnsignedInt(ast::UintTy::U128) => I128,
+            attr::SignedInt(ast::IntTy::Isize) | attr::UnsignedInt(ast::UintTy::Usize) => {
+                dl.ptr_sized_integer()
+            }
+        }
+    }
+
+    fn from_int_ty<C: HasDataLayout>(cx: &C, ity: ty::IntTy) -> Integer {
+        match ity {
+            ty::IntTy::I8 => I8,
+            ty::IntTy::I16 => I16,
+            ty::IntTy::I32 => I32,
+            ty::IntTy::I64 => I64,
+            ty::IntTy::I128 => I128,
+            ty::IntTy::Isize => cx.data_layout().ptr_sized_integer(),
+        }
+    }
+    fn from_uint_ty<C: HasDataLayout>(cx: &C, ity: ty::UintTy) -> Integer {
+        match ity {
+            ty::UintTy::U8 => I8,
+            ty::UintTy::U16 => I16,
+            ty::UintTy::U32 => I32,
+            ty::UintTy::U64 => I64,
+            ty::UintTy::U128 => I128,
+            ty::UintTy::Usize => cx.data_layout().ptr_sized_integer(),
+        }
+    }
+
+    /// Finds the appropriate Integer type and signedness for the given
+    /// signed discriminant range and `#[repr]` attribute.
+    /// N.B.: `u128` values above `i128::MAX` will be treated as signed, but
+    /// that shouldn't affect anything, other than maybe debuginfo.
+    fn repr_discr<'tcx>(
+        tcx: TyCtxt<'tcx>,
+        ty: Ty<'tcx>,
+        repr: &ReprOptions,
+        min: i128,
+        max: i128,
+    ) -> (Integer, bool) {
+        // Theoretically, negative values could be larger in unsigned representation
+        // than the unsigned representation of the signed minimum. However, if there
+        // are any negative values, the only valid unsigned representation is u128
+        // which can fit all i128 values, so the result remains unaffected.
+        let unsigned_fit = Integer::fit_unsigned(cmp::max(min as u128, max as u128));
+        let signed_fit = cmp::max(Integer::fit_signed(min), Integer::fit_signed(max));
+
+        if let Some(ity) = repr.int {
+            let discr = Integer::from_attr(&tcx, ity);
+            let fit = if ity.is_signed() { signed_fit } else { unsigned_fit };
+            if discr < fit {
+                bug!(
+                    "Integer::repr_discr: `#[repr]` hint too small for \
+                      discriminant range of enum `{}",
+                    ty
+                )
+            }
+            return (discr, ity.is_signed());
+        }
+
+        let at_least = if repr.c() {
+            // This is usually I32, however it can be different on some platforms,
+            // notably hexagon and arm-none/thumb-none
+            tcx.data_layout().c_enum_min_size
+        } else {
+            // repr(Rust) enums try to be as small as possible
+            I8
+        };
+
+        // If there are no negative values, we can use the unsigned fit.
+        if min >= 0 {
+            (cmp::max(unsigned_fit, at_least), false)
+        } else {
+            (cmp::max(signed_fit, at_least), true)
+        }
+    }
+}
+
+pub trait PrimitiveExt {
+    fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
+    fn to_int_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
+}
+
+impl PrimitiveExt for Primitive {
+    #[inline]
+    fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        match *self {
+            Int(i, signed) => i.to_ty(tcx, signed),
+            F32 => tcx.types.f32,
+            F64 => tcx.types.f64,
+            Pointer => tcx.mk_mut_ptr(tcx.mk_unit()),
+        }
+    }
+
+    /// Return an *integer* type matching this primitive.
+    /// Useful in particular when dealing with enum discriminants.
+    #[inline]
+    fn to_int_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        match *self {
+            Int(i, signed) => i.to_ty(tcx, signed),
+            Pointer => tcx.types.usize,
+            F32 | F64 => bug!("floats do not have an int type"),
+        }
+    }
+}
+
+/// The first half of a fat pointer.
+///
+/// - For a trait object, this is the address of the box.
+/// - For a slice, this is the base address.
+pub const FAT_PTR_ADDR: usize = 0;
+
+/// The second half of a fat pointer.
+///
+/// - For a trait object, this is the address of the vtable.
+/// - For a slice, this is the length.
+pub const FAT_PTR_EXTRA: usize = 1;
+
+/// The maximum supported number of lanes in a SIMD vector.
+///
+/// This value is selected based on backend support:
+/// * LLVM does not appear to have a vector width limit.
+/// * Cranelift stores the base-2 log of the lane count in a 4 bit integer.
+pub const MAX_SIMD_LANES: u64 = 1 << 0xF;
+
+#[derive(Copy, Clone, Debug, HashStable, TyEncodable, TyDecodable)]
+pub enum LayoutError<'tcx> {
+    Unknown(Ty<'tcx>),
+    SizeOverflow(Ty<'tcx>),
+    NormalizationFailure(Ty<'tcx>, NormalizationError<'tcx>),
+}
+
+impl<'tcx> fmt::Display for LayoutError<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            LayoutError::Unknown(ty) => write!(f, "the type `{}` has an unknown layout", ty),
+            LayoutError::SizeOverflow(ty) => {
+                write!(f, "values of the type `{}` are too big for the current architecture", ty)
+            }
+            LayoutError::NormalizationFailure(t, e) => write!(
+                f,
+                "unable to determine layout for `{}` because `{}` cannot be normalized",
+                t,
+                e.get_type_for_failure()
+            ),
+        }
+    }
+}
+
+/// Enforce some basic invariants on layouts.
+fn sanity_check_layout<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    layout: &TyAndLayout<'tcx>,
+) {
+    // Type-level uninhabitedness should always imply ABI uninhabitedness.
+    if tcx.conservative_is_privately_uninhabited(param_env.and(layout.ty)) {
+        assert!(layout.abi.is_uninhabited());
+    }
+
+    if layout.size.bytes() % layout.align.abi.bytes() != 0 {
+        bug!("size is not a multiple of align, in the following layout:\n{layout:#?}");
+    }
+
+    if cfg!(debug_assertions) {
+        fn check_layout_abi<'tcx>(tcx: TyCtxt<'tcx>, layout: Layout<'tcx>) {
+            match layout.abi() {
+                Abi::Scalar(scalar) => {
+                    // No padding in scalars.
+                    assert_eq!(
+                        layout.align().abi,
+                        scalar.align(&tcx).abi,
+                        "alignment mismatch between ABI and layout in {layout:#?}"
+                    );
+                    assert_eq!(
+                        layout.size(),
+                        scalar.size(&tcx),
+                        "size mismatch between ABI and layout in {layout:#?}"
+                    );
+                }
+                Abi::Vector { count, element } => {
+                    // No padding in vectors. Alignment can be strengthened, though.
+                    assert!(
+                        layout.align().abi >= element.align(&tcx).abi,
+                        "alignment mismatch between ABI and layout in {layout:#?}"
+                    );
+                    let size = element.size(&tcx) * count;
+                    assert_eq!(
+                        layout.size(),
+                        size.align_to(tcx.data_layout().vector_align(size).abi),
+                        "size mismatch between ABI and layout in {layout:#?}"
+                    );
+                }
+                Abi::ScalarPair(scalar1, scalar2) => {
+                    // Sanity-check scalar pairs. These are a bit more flexible and support
+                    // padding, but we can at least ensure both fields actually fit into the layout
+                    // and the alignment requirement has not been weakened.
+                    let align1 = scalar1.align(&tcx).abi;
+                    let align2 = scalar2.align(&tcx).abi;
+                    assert!(
+                        layout.align().abi >= cmp::max(align1, align2),
+                        "alignment mismatch between ABI and layout in {layout:#?}",
+                    );
+                    let field2_offset = scalar1.size(&tcx).align_to(align2);
+                    assert!(
+                        layout.size() >= field2_offset + scalar2.size(&tcx),
+                        "size mismatch between ABI and layout in {layout:#?}"
+                    );
+                }
+                Abi::Uninhabited | Abi::Aggregate { .. } => {} // Nothing to check.
+            }
+        }
+
+        check_layout_abi(tcx, layout.layout);
+
+        if let Variants::Multiple { variants, .. } = &layout.variants {
+            for variant in variants {
+                check_layout_abi(tcx, *variant);
+                // No nested "multiple".
+                assert!(matches!(variant.variants(), Variants::Single { .. }));
+                // Skip empty variants.
+                if variant.size() == Size::ZERO
+                    || variant.fields().count() == 0
+                    || variant.abi().is_uninhabited()
+                {
+                    // These are never actually accessed anyway, so we can skip them. (Note that
+                    // sometimes, variants with fields have size 0, and sometimes, variants without
+                    // fields have non-0 size.)
+                    continue;
+                }
+                // Variants should have the same or a smaller size as the full thing.
+                if variant.size() > layout.size {
+                    bug!(
+                        "Type with size {} bytes has variant with size {} bytes: {layout:#?}",
+                        layout.size.bytes(),
+                        variant.size().bytes(),
+                    )
+                }
+                // The top-level ABI and the ABI of the variants should be coherent.
+                let abi_coherent = match (layout.abi, variant.abi()) {
+                    (Abi::Scalar(..), Abi::Scalar(..)) => true,
+                    (Abi::ScalarPair(..), Abi::ScalarPair(..)) => true,
+                    (Abi::Uninhabited, _) => true,
+                    (Abi::Aggregate { .. }, _) => true,
+                    _ => false,
+                };
+                if !abi_coherent {
+                    bug!(
+                        "Variant ABI is incompatible with top-level ABI:\nvariant={:#?}\nTop-level: {layout:#?}",
+                        variant
+                    );
+                }
+            }
+        }
+    }
+}
+
+#[instrument(skip(tcx, query), level = "debug")]
+fn layout_of<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
+) -> Result<TyAndLayout<'tcx>, LayoutError<'tcx>> {
+    ty::tls::with_related_context(tcx, move |icx| {
+        let (param_env, ty) = query.into_parts();
+        debug!(?ty);
+
+        if !tcx.recursion_limit().value_within_limit(icx.layout_depth) {
+            tcx.sess.fatal(&format!("overflow representing the type `{}`", ty));
+        }
+
+        // Update the ImplicitCtxt to increase the layout_depth
+        let icx = ty::tls::ImplicitCtxt { layout_depth: icx.layout_depth + 1, ..icx.clone() };
+
+        ty::tls::enter_context(&icx, |_| {
+            let param_env = param_env.with_reveal_all_normalized(tcx);
+            let unnormalized_ty = ty;
+
+            // FIXME: We might want to have two different versions of `layout_of`:
+            // One that can be called after typecheck has completed and can use
+            // `normalize_erasing_regions` here and another one that can be called
+            // before typecheck has completed and uses `try_normalize_erasing_regions`.
+            let ty = match tcx.try_normalize_erasing_regions(param_env, ty) {
+                Ok(t) => t,
+                Err(normalization_error) => {
+                    return Err(LayoutError::NormalizationFailure(ty, normalization_error));
+                }
+            };
+
+            if ty != unnormalized_ty {
+                // Ensure this layout is also cached for the normalized type.
+                return tcx.layout_of(param_env.and(ty));
+            }
+
+            let cx = LayoutCx { tcx, param_env };
+
+            let layout = cx.layout_of_uncached(ty)?;
+            let layout = TyAndLayout { ty, layout };
+
+            cx.record_layout_for_printing(layout);
+
+            sanity_check_layout(tcx, param_env, &layout);
+
+            Ok(layout)
+        })
+    })
+}
+
+pub struct LayoutCx<'tcx, C> {
+    pub tcx: C,
+    pub param_env: ty::ParamEnv<'tcx>,
+}
+
+#[derive(Copy, Clone, Debug)]
+enum StructKind {
+    /// A tuple, closure, or univariant which cannot be coerced to unsized.
+    AlwaysSized,
+    /// A univariant, the last field of which may be coerced to unsized.
+    MaybeUnsized,
+    /// A univariant, but with a prefix of an arbitrary size & alignment (e.g., enum tag).
+    Prefixed(Size, Align),
+}
+
+// Invert a bijective mapping, i.e. `invert(map)[y] = x` if `map[x] = y`.
+// This is used to go between `memory_index` (source field order to memory order)
+// and `inverse_memory_index` (memory order to source field order).
+// See also `FieldsShape::Arbitrary::memory_index` for more details.
+// FIXME(eddyb) build a better abstraction for permutations, if possible.
+fn invert_mapping(map: &[u32]) -> Vec<u32> {
+    let mut inverse = vec![0; map.len()];
+    for i in 0..map.len() {
+        inverse[map[i] as usize] = i as u32;
+    }
+    inverse
+}
+
+impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
+    fn scalar_pair(&self, a: Scalar, b: Scalar) -> LayoutS<'tcx> {
+        let dl = self.data_layout();
+        let b_align = b.align(dl);
+        let align = a.align(dl).max(b_align).max(dl.aggregate_align);
+        let b_offset = a.size(dl).align_to(b_align.abi);
+        let size = (b_offset + b.size(dl)).align_to(align.abi);
+
+        // HACK(nox): We iter on `b` and then `a` because `max_by_key`
+        // returns the last maximum.
+        let largest_niche = Niche::from_scalar(dl, b_offset, b)
+            .into_iter()
+            .chain(Niche::from_scalar(dl, Size::ZERO, a))
+            .max_by_key(|niche| niche.available(dl));
+
+        LayoutS {
+            variants: Variants::Single { index: VariantIdx::new(0) },
+            fields: FieldsShape::Arbitrary {
+                offsets: vec![Size::ZERO, b_offset],
+                memory_index: vec![0, 1],
+            },
+            abi: Abi::ScalarPair(a, b),
+            largest_niche,
+            align,
+            size,
+        }
+    }
+
+    fn univariant_uninterned(
+        &self,
+        ty: Ty<'tcx>,
+        fields: &[TyAndLayout<'_>],
+        repr: &ReprOptions,
+        kind: StructKind,
+    ) -> Result<LayoutS<'tcx>, LayoutError<'tcx>> {
+        let dl = self.data_layout();
+        let pack = repr.pack;
+        if pack.is_some() && repr.align.is_some() {
+            self.tcx.sess.delay_span_bug(DUMMY_SP, "struct cannot be packed and aligned");
+            return Err(LayoutError::Unknown(ty));
+        }
+
+        let mut align = if pack.is_some() { dl.i8_align } else { dl.aggregate_align };
+
+        let mut inverse_memory_index: Vec<u32> = (0..fields.len() as u32).collect();
+
+        let optimize = !repr.inhibit_struct_field_reordering_opt();
+        if optimize {
+            let end =
+                if let StructKind::MaybeUnsized = kind { fields.len() - 1 } else { fields.len() };
+            let optimizing = &mut inverse_memory_index[..end];
+            let field_align = |f: &TyAndLayout<'_>| {
+                if let Some(pack) = pack { f.align.abi.min(pack) } else { f.align.abi }
+            };
+
+            // If `-Z randomize-layout` was enabled for the type definition we can shuffle
+            // the field ordering to try and catch some code making assumptions about layouts
+            // we don't guarantee
+            if repr.can_randomize_type_layout() {
+                // `ReprOptions.layout_seed` is a deterministic seed that we can use to
+                // randomize field ordering with
+                let mut rng = Xoshiro128StarStar::seed_from_u64(repr.field_shuffle_seed);
+
+                // Shuffle the ordering of the fields
+                optimizing.shuffle(&mut rng);
+
+            // Otherwise we just leave things alone and actually optimize the type's fields
+            } else {
+                match kind {
+                    StructKind::AlwaysSized | StructKind::MaybeUnsized => {
+                        optimizing.sort_by_key(|&x| {
+                            // Place ZSTs first to avoid "interesting offsets",
+                            // especially with only one or two non-ZST fields.
+                            let f = &fields[x as usize];
+                            (!f.is_zst(), cmp::Reverse(field_align(f)))
+                        });
+                    }
+
+                    StructKind::Prefixed(..) => {
+                        // Sort in ascending alignment so that the layout stays optimal
+                        // regardless of the prefix
+                        optimizing.sort_by_key(|&x| field_align(&fields[x as usize]));
+                    }
+                }
+
+                // FIXME(Kixiron): We can always shuffle fields within a given alignment class
+                //                 regardless of the status of `-Z randomize-layout`
+            }
+        }
+
+        // inverse_memory_index holds field indices by increasing memory offset.
+        // That is, if field 5 has offset 0, the first element of inverse_memory_index is 5.
+        // We now write field offsets to the corresponding offset slot;
+        // field 5 with offset 0 puts 0 in offsets[5].
+        // At the bottom of this function, we invert `inverse_memory_index` to
+        // produce `memory_index` (see `invert_mapping`).
+
+        let mut sized = true;
+        let mut offsets = vec![Size::ZERO; fields.len()];
+        let mut offset = Size::ZERO;
+        let mut largest_niche = None;
+        let mut largest_niche_available = 0;
+
+        if let StructKind::Prefixed(prefix_size, prefix_align) = kind {
+            let prefix_align =
+                if let Some(pack) = pack { prefix_align.min(pack) } else { prefix_align };
+            align = align.max(AbiAndPrefAlign::new(prefix_align));
+            offset = prefix_size.align_to(prefix_align);
+        }
+
+        for &i in &inverse_memory_index {
+            let field = fields[i as usize];
+            if !sized {
+                self.tcx.sess.delay_span_bug(
+                    DUMMY_SP,
+                    &format!(
+                        "univariant: field #{} of `{}` comes after unsized field",
+                        offsets.len(),
+                        ty
+                    ),
+                );
+            }
+
+            if field.is_unsized() {
+                sized = false;
+            }
+
+            // Invariant: offset < dl.obj_size_bound() <= 1<<61
+            let field_align = if let Some(pack) = pack {
+                field.align.min(AbiAndPrefAlign::new(pack))
+            } else {
+                field.align
+            };
+            offset = offset.align_to(field_align.abi);
+            align = align.max(field_align);
+
+            debug!("univariant offset: {:?} field: {:#?}", offset, field);
+            offsets[i as usize] = offset;
+
+            if let Some(mut niche) = field.largest_niche {
+                let available = niche.available(dl);
+                if available > largest_niche_available {
+                    largest_niche_available = available;
+                    niche.offset += offset;
+                    largest_niche = Some(niche);
+                }
+            }
+
+            offset = offset.checked_add(field.size, dl).ok_or(LayoutError::SizeOverflow(ty))?;
+        }
+
+        if let Some(repr_align) = repr.align {
+            align = align.max(AbiAndPrefAlign::new(repr_align));
+        }
+
+        debug!("univariant min_size: {:?}", offset);
+        let min_size = offset;
+
+        // As stated above, inverse_memory_index holds field indices by increasing offset.
+        // This makes it an already-sorted view of the offsets vec.
+        // To invert it, consider:
+        // If field 5 has offset 0, offsets[0] is 5, and memory_index[5] should be 0.
+        // Field 5 would be the first element, so memory_index is i:
+        // Note: if we didn't optimize, it's already right.
+
+        let memory_index =
+            if optimize { invert_mapping(&inverse_memory_index) } else { inverse_memory_index };
+
+        let size = min_size.align_to(align.abi);
+        let mut abi = Abi::Aggregate { sized };
+
+        // Unpack newtype ABIs and find scalar pairs.
+        if sized && size.bytes() > 0 {
+            // All other fields must be ZSTs.
+            let mut non_zst_fields = fields.iter().enumerate().filter(|&(_, f)| !f.is_zst());
+
+            match (non_zst_fields.next(), non_zst_fields.next(), non_zst_fields.next()) {
+                // We have exactly one non-ZST field.
+                (Some((i, field)), None, None) => {
+                    // Field fills the struct and it has a scalar or scalar pair ABI.
+                    if offsets[i].bytes() == 0 && align.abi == field.align.abi && size == field.size
+                    {
+                        match field.abi {
+                            // For plain scalars, or vectors of them, we can't unpack
+                            // newtypes for `#[repr(C)]`, as that affects C ABIs.
+                            Abi::Scalar(_) | Abi::Vector { .. } if optimize => {
+                                abi = field.abi;
+                            }
+                            // But scalar pairs are Rust-specific and get
+                            // treated as aggregates by C ABIs anyway.
+                            Abi::ScalarPair(..) => {
+                                abi = field.abi;
+                            }
+                            _ => {}
+                        }
+                    }
+                }
+
+                // Two non-ZST fields, and they're both scalars.
+                (Some((i, a)), Some((j, b)), None) => {
+                    match (a.abi, b.abi) {
+                        (Abi::Scalar(a), Abi::Scalar(b)) => {
+                            // Order by the memory placement, not source order.
+                            let ((i, a), (j, b)) = if offsets[i] < offsets[j] {
+                                ((i, a), (j, b))
+                            } else {
+                                ((j, b), (i, a))
+                            };
+                            let pair = self.scalar_pair(a, b);
+                            let pair_offsets = match pair.fields {
+                                FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
+                                    assert_eq!(memory_index, &[0, 1]);
+                                    offsets
+                                }
+                                _ => bug!(),
+                            };
+                            if offsets[i] == pair_offsets[0]
+                                && offsets[j] == pair_offsets[1]
+                                && align == pair.align
+                                && size == pair.size
+                            {
+                                // We can use `ScalarPair` only when it matches our
+                                // already computed layout (including `#[repr(C)]`).
+                                abi = pair.abi;
+                            }
+                        }
+                        _ => {}
+                    }
+                }
+
+                _ => {}
+            }
+        }
+
+        if fields.iter().any(|f| f.abi.is_uninhabited()) {
+            abi = Abi::Uninhabited;
+        }
+
+        Ok(LayoutS {
+            variants: Variants::Single { index: VariantIdx::new(0) },
+            fields: FieldsShape::Arbitrary { offsets, memory_index },
+            abi,
+            largest_niche,
+            align,
+            size,
+        })
+    }
+
+    fn layout_of_uncached(&self, ty: Ty<'tcx>) -> Result<Layout<'tcx>, LayoutError<'tcx>> {
+        let tcx = self.tcx;
+        let param_env = self.param_env;
+        let dl = self.data_layout();
+        let scalar_unit = |value: Primitive| {
+            let size = value.size(dl);
+            assert!(size.bits() <= 128);
+            Scalar::Initialized { value, valid_range: WrappingRange::full(size) }
+        };
+        let scalar =
+            |value: Primitive| tcx.intern_layout(LayoutS::scalar(self, scalar_unit(value)));
+
+        let univariant = |fields: &[TyAndLayout<'_>], repr: &ReprOptions, kind| {
+            Ok(tcx.intern_layout(self.univariant_uninterned(ty, fields, repr, kind)?))
+        };
+        debug_assert!(!ty.has_infer_types_or_consts());
+
+        Ok(match *ty.kind() {
+            // Basic scalars.
+            ty::Bool => tcx.intern_layout(LayoutS::scalar(
+                self,
+                Scalar::Initialized {
+                    value: Int(I8, false),
+                    valid_range: WrappingRange { start: 0, end: 1 },
+                },
+            )),
+            ty::Char => tcx.intern_layout(LayoutS::scalar(
+                self,
+                Scalar::Initialized {
+                    value: Int(I32, false),
+                    valid_range: WrappingRange { start: 0, end: 0x10FFFF },
+                },
+            )),
+            ty::Int(ity) => scalar(Int(Integer::from_int_ty(dl, ity), true)),
+            ty::Uint(ity) => scalar(Int(Integer::from_uint_ty(dl, ity), false)),
+            ty::Float(fty) => scalar(match fty {
+                ty::FloatTy::F32 => F32,
+                ty::FloatTy::F64 => F64,
+            }),
+            ty::FnPtr(_) => {
+                let mut ptr = scalar_unit(Pointer);
+                ptr.valid_range_mut().start = 1;
+                tcx.intern_layout(LayoutS::scalar(self, ptr))
+            }
+
+            // The never type.
+            ty::Never => tcx.intern_layout(LayoutS {
+                variants: Variants::Single { index: VariantIdx::new(0) },
+                fields: FieldsShape::Primitive,
+                abi: Abi::Uninhabited,
+                largest_niche: None,
+                align: dl.i8_align,
+                size: Size::ZERO,
+            }),
+
+            // Potentially-wide pointers.
+            ty::Ref(_, pointee, _) | ty::RawPtr(ty::TypeAndMut { ty: pointee, .. }) => {
+                let mut data_ptr = scalar_unit(Pointer);
+                if !ty.is_unsafe_ptr() {
+                    data_ptr.valid_range_mut().start = 1;
+                }
+
+                let pointee = tcx.normalize_erasing_regions(param_env, pointee);
+                if pointee.is_sized(tcx.at(DUMMY_SP), param_env) {
+                    return Ok(tcx.intern_layout(LayoutS::scalar(self, data_ptr)));
+                }
+
+                let unsized_part = tcx.struct_tail_erasing_lifetimes(pointee, param_env);
+                let metadata = match unsized_part.kind() {
+                    ty::Foreign(..) => {
+                        return Ok(tcx.intern_layout(LayoutS::scalar(self, data_ptr)));
+                    }
+                    ty::Slice(_) | ty::Str => scalar_unit(Int(dl.ptr_sized_integer(), false)),
+                    ty::Dynamic(..) => {
+                        let mut vtable = scalar_unit(Pointer);
+                        vtable.valid_range_mut().start = 1;
+                        vtable
+                    }
+                    _ => return Err(LayoutError::Unknown(unsized_part)),
+                };
+
+                // Effectively a (ptr, meta) tuple.
+                tcx.intern_layout(self.scalar_pair(data_ptr, metadata))
+            }
+
+            // Arrays and slices.
+            ty::Array(element, mut count) => {
+                if count.has_projections() {
+                    count = tcx.normalize_erasing_regions(param_env, count);
+                    if count.has_projections() {
+                        return Err(LayoutError::Unknown(ty));
+                    }
+                }
+
+                let count = count.try_eval_usize(tcx, param_env).ok_or(LayoutError::Unknown(ty))?;
+                let element = self.layout_of(element)?;
+                let size =
+                    element.size.checked_mul(count, dl).ok_or(LayoutError::SizeOverflow(ty))?;
+
+                let abi =
+                    if count != 0 && tcx.conservative_is_privately_uninhabited(param_env.and(ty)) {
+                        Abi::Uninhabited
+                    } else {
+                        Abi::Aggregate { sized: true }
+                    };
+
+                let largest_niche = if count != 0 { element.largest_niche } else { None };
+
+                tcx.intern_layout(LayoutS {
+                    variants: Variants::Single { index: VariantIdx::new(0) },
+                    fields: FieldsShape::Array { stride: element.size, count },
+                    abi,
+                    largest_niche,
+                    align: element.align,
+                    size,
+                })
+            }
+            ty::Slice(element) => {
+                let element = self.layout_of(element)?;
+                tcx.intern_layout(LayoutS {
+                    variants: Variants::Single { index: VariantIdx::new(0) },
+                    fields: FieldsShape::Array { stride: element.size, count: 0 },
+                    abi: Abi::Aggregate { sized: false },
+                    largest_niche: None,
+                    align: element.align,
+                    size: Size::ZERO,
+                })
+            }
+            ty::Str => tcx.intern_layout(LayoutS {
+                variants: Variants::Single { index: VariantIdx::new(0) },
+                fields: FieldsShape::Array { stride: Size::from_bytes(1), count: 0 },
+                abi: Abi::Aggregate { sized: false },
+                largest_niche: None,
+                align: dl.i8_align,
+                size: Size::ZERO,
+            }),
+
+            // Odd unit types.
+            ty::FnDef(..) => univariant(&[], &ReprOptions::default(), StructKind::AlwaysSized)?,
+            ty::Dynamic(..) | ty::Foreign(..) => {
+                let mut unit = self.univariant_uninterned(
+                    ty,
+                    &[],
+                    &ReprOptions::default(),
+                    StructKind::AlwaysSized,
+                )?;
+                match unit.abi {
+                    Abi::Aggregate { ref mut sized } => *sized = false,
+                    _ => bug!(),
+                }
+                tcx.intern_layout(unit)
+            }
+
+            ty::Generator(def_id, substs, _) => self.generator_layout(ty, def_id, substs)?,
+
+            ty::Closure(_, ref substs) => {
+                let tys = substs.as_closure().upvar_tys();
+                univariant(
+                    &tys.map(|ty| self.layout_of(ty)).collect::<Result<Vec<_>, _>>()?,
+                    &ReprOptions::default(),
+                    StructKind::AlwaysSized,
+                )?
+            }
+
+            ty::Tuple(tys) => {
+                let kind =
+                    if tys.len() == 0 { StructKind::AlwaysSized } else { StructKind::MaybeUnsized };
+
+                univariant(
+                    &tys.iter().map(|k| self.layout_of(k)).collect::<Result<Vec<_>, _>>()?,
+                    &ReprOptions::default(),
+                    kind,
+                )?
+            }
+
+            // SIMD vector types.
+            ty::Adt(def, substs) if def.repr().simd() => {
+                if !def.is_struct() {
+                    // Should have yielded E0517 by now.
+                    tcx.sess.delay_span_bug(
+                        DUMMY_SP,
+                        "#[repr(simd)] was applied to an ADT that is not a struct",
+                    );
+                    return Err(LayoutError::Unknown(ty));
+                }
+
+                // Supported SIMD vectors are homogeneous ADTs with at least one field:
+                //
+                // * #[repr(simd)] struct S(T, T, T, T);
+                // * #[repr(simd)] struct S { x: T, y: T, z: T, w: T }
+                // * #[repr(simd)] struct S([T; 4])
+                //
+                // where T is a primitive scalar (integer/float/pointer).
+
+                // SIMD vectors with zero fields are not supported.
+                // (should be caught by typeck)
+                if def.non_enum_variant().fields.is_empty() {
+                    tcx.sess.fatal(&format!("monomorphising SIMD type `{}` of zero length", ty));
+                }
+
+                // Type of the first ADT field:
+                let f0_ty = def.non_enum_variant().fields[0].ty(tcx, substs);
+
+                // Heterogeneous SIMD vectors are not supported:
+                // (should be caught by typeck)
+                for fi in &def.non_enum_variant().fields {
+                    if fi.ty(tcx, substs) != f0_ty {
+                        tcx.sess.fatal(&format!("monomorphising heterogeneous SIMD type `{}`", ty));
+                    }
+                }
+
+                // The element type and number of elements of the SIMD vector
+                // are obtained from:
+                //
+                // * the element type and length of the single array field, if
+                // the first field is of array type, or
+                //
+                // * the homogenous field type and the number of fields.
+                let (e_ty, e_len, is_array) = if let ty::Array(e_ty, _) = f0_ty.kind() {
+                    // First ADT field is an array:
+
+                    // SIMD vectors with multiple array fields are not supported:
+                    // (should be caught by typeck)
+                    if def.non_enum_variant().fields.len() != 1 {
+                        tcx.sess.fatal(&format!(
+                            "monomorphising SIMD type `{}` with more than one array field",
+                            ty
+                        ));
+                    }
+
+                    // Extract the number of elements from the layout of the array field:
+                    let FieldsShape::Array { count, .. } = self.layout_of(f0_ty)?.layout.fields() else {
+                        return Err(LayoutError::Unknown(ty));
+                    };
+
+                    (*e_ty, *count, true)
+                } else {
+                    // First ADT field is not an array:
+                    (f0_ty, def.non_enum_variant().fields.len() as _, false)
+                };
+
+                // SIMD vectors of zero length are not supported.
+                // Additionally, lengths are capped at 2^16 as a fixed maximum backends must
+                // support.
+                //
+                // Can't be caught in typeck if the array length is generic.
+                if e_len == 0 {
+                    tcx.sess.fatal(&format!("monomorphising SIMD type `{}` of zero length", ty));
+                } else if e_len > MAX_SIMD_LANES {
+                    tcx.sess.fatal(&format!(
+                        "monomorphising SIMD type `{}` of length greater than {}",
+                        ty, MAX_SIMD_LANES,
+                    ));
+                }
+
+                // Compute the ABI of the element type:
+                let e_ly = self.layout_of(e_ty)?;
+                let Abi::Scalar(e_abi) = e_ly.abi else {
+                    // This error isn't caught in typeck, e.g., if
+                    // the element type of the vector is generic.
+                    tcx.sess.fatal(&format!(
+                        "monomorphising SIMD type `{}` with a non-primitive-scalar \
+                        (integer/float/pointer) element type `{}`",
+                        ty, e_ty
+                    ))
+                };
+
+                // Compute the size and alignment of the vector:
+                let size = e_ly.size.checked_mul(e_len, dl).ok_or(LayoutError::SizeOverflow(ty))?;
+                let align = dl.vector_align(size);
+                let size = size.align_to(align.abi);
+
+                // Compute the placement of the vector fields:
+                let fields = if is_array {
+                    FieldsShape::Arbitrary { offsets: vec![Size::ZERO], memory_index: vec![0] }
+                } else {
+                    FieldsShape::Array { stride: e_ly.size, count: e_len }
+                };
+
+                tcx.intern_layout(LayoutS {
+                    variants: Variants::Single { index: VariantIdx::new(0) },
+                    fields,
+                    abi: Abi::Vector { element: e_abi, count: e_len },
+                    largest_niche: e_ly.largest_niche,
+                    size,
+                    align,
+                })
+            }
+
+            // ADTs.
+            ty::Adt(def, substs) => {
+                // Cache the field layouts.
+                let variants = def
+                    .variants()
+                    .iter()
+                    .map(|v| {
+                        v.fields
+                            .iter()
+                            .map(|field| self.layout_of(field.ty(tcx, substs)))
+                            .collect::<Result<Vec<_>, _>>()
+                    })
+                    .collect::<Result<IndexVec<VariantIdx, _>, _>>()?;
+
+                if def.is_union() {
+                    if def.repr().pack.is_some() && def.repr().align.is_some() {
+                        self.tcx.sess.delay_span_bug(
+                            tcx.def_span(def.did()),
+                            "union cannot be packed and aligned",
+                        );
+                        return Err(LayoutError::Unknown(ty));
+                    }
+
+                    let mut align =
+                        if def.repr().pack.is_some() { dl.i8_align } else { dl.aggregate_align };
+
+                    if let Some(repr_align) = def.repr().align {
+                        align = align.max(AbiAndPrefAlign::new(repr_align));
+                    }
+
+                    let optimize = !def.repr().inhibit_union_abi_opt();
+                    let mut size = Size::ZERO;
+                    let mut abi = Abi::Aggregate { sized: true };
+                    let index = VariantIdx::new(0);
+                    for field in &variants[index] {
+                        assert!(!field.is_unsized());
+                        align = align.max(field.align);
+
+                        // If all non-ZST fields have the same ABI, forward this ABI
+                        if optimize && !field.is_zst() {
+                            // Discard valid range information and allow undef
+                            let field_abi = match field.abi {
+                                Abi::Scalar(x) => Abi::Scalar(x.to_union()),
+                                Abi::ScalarPair(x, y) => {
+                                    Abi::ScalarPair(x.to_union(), y.to_union())
+                                }
+                                Abi::Vector { element: x, count } => {
+                                    Abi::Vector { element: x.to_union(), count }
+                                }
+                                Abi::Uninhabited | Abi::Aggregate { .. } => {
+                                    Abi::Aggregate { sized: true }
+                                }
+                            };
+
+                            if size == Size::ZERO {
+                                // first non ZST: initialize 'abi'
+                                abi = field_abi;
+                            } else if abi != field_abi {
+                                // different fields have different ABI: reset to Aggregate
+                                abi = Abi::Aggregate { sized: true };
+                            }
+                        }
+
+                        size = cmp::max(size, field.size);
+                    }
+
+                    if let Some(pack) = def.repr().pack {
+                        align = align.min(AbiAndPrefAlign::new(pack));
+                    }
+
+                    return Ok(tcx.intern_layout(LayoutS {
+                        variants: Variants::Single { index },
+                        fields: FieldsShape::Union(
+                            NonZeroUsize::new(variants[index].len())
+                                .ok_or(LayoutError::Unknown(ty))?,
+                        ),
+                        abi,
+                        largest_niche: None,
+                        align,
+                        size: size.align_to(align.abi),
+                    }));
+                }
+
+                // A variant is absent if it's uninhabited and only has ZST fields.
+                // Present uninhabited variants only require space for their fields,
+                // but *not* an encoding of the discriminant (e.g., a tag value).
+                // See issue #49298 for more details on the need to leave space
+                // for non-ZST uninhabited data (mostly partial initialization).
+                let absent = |fields: &[TyAndLayout<'_>]| {
+                    let uninhabited = fields.iter().any(|f| f.abi.is_uninhabited());
+                    let is_zst = fields.iter().all(|f| f.is_zst());
+                    uninhabited && is_zst
+                };
+                let (present_first, present_second) = {
+                    let mut present_variants = variants
+                        .iter_enumerated()
+                        .filter_map(|(i, v)| if absent(v) { None } else { Some(i) });
+                    (present_variants.next(), present_variants.next())
+                };
+                let present_first = match present_first {
+                    Some(present_first) => present_first,
+                    // Uninhabited because it has no variants, or only absent ones.
+                    None if def.is_enum() => {
+                        return Ok(tcx.layout_of(param_env.and(tcx.types.never))?.layout);
+                    }
+                    // If it's a struct, still compute a layout so that we can still compute the
+                    // field offsets.
+                    None => VariantIdx::new(0),
+                };
+
+                let is_struct = !def.is_enum() ||
+                    // Only one variant is present.
+                    (present_second.is_none() &&
+                    // Representation optimizations are allowed.
+                    !def.repr().inhibit_enum_layout_opt());
+                if is_struct {
+                    // Struct, or univariant enum equivalent to a struct.
+                    // (Typechecking will reject discriminant-sizing attrs.)
+
+                    let v = present_first;
+                    let kind = if def.is_enum() || variants[v].is_empty() {
+                        StructKind::AlwaysSized
+                    } else {
+                        let param_env = tcx.param_env(def.did());
+                        let last_field = def.variant(v).fields.last().unwrap();
+                        let always_sized =
+                            tcx.type_of(last_field.did).is_sized(tcx.at(DUMMY_SP), param_env);
+                        if !always_sized {
+                            StructKind::MaybeUnsized
+                        } else {
+                            StructKind::AlwaysSized
+                        }
+                    };
+
+                    let mut st = self.univariant_uninterned(ty, &variants[v], &def.repr(), kind)?;
+                    st.variants = Variants::Single { index: v };
+
+                    if def.is_unsafe_cell() {
+                        let hide_niches = |scalar: &mut _| match scalar {
+                            Scalar::Initialized { value, valid_range } => {
+                                *valid_range = WrappingRange::full(value.size(dl))
+                            }
+                            // Already doesn't have any niches
+                            Scalar::Union { .. } => {}
+                        };
+                        match &mut st.abi {
+                            Abi::Uninhabited => {}
+                            Abi::Scalar(scalar) => hide_niches(scalar),
+                            Abi::ScalarPair(a, b) => {
+                                hide_niches(a);
+                                hide_niches(b);
+                            }
+                            Abi::Vector { element, count: _ } => hide_niches(element),
+                            Abi::Aggregate { sized: _ } => {}
+                        }
+                        st.largest_niche = None;
+                        return Ok(tcx.intern_layout(st));
+                    }
+
+                    let (start, end) = self.tcx.layout_scalar_valid_range(def.did());
+                    match st.abi {
+                        Abi::Scalar(ref mut scalar) | Abi::ScalarPair(ref mut scalar, _) => {
+                            // the asserts ensure that we are not using the
+                            // `#[rustc_layout_scalar_valid_range(n)]`
+                            // attribute to widen the range of anything as that would probably
+                            // result in UB somewhere
+                            // FIXME(eddyb) the asserts are probably not needed,
+                            // as larger validity ranges would result in missed
+                            // optimizations, *not* wrongly assuming the inner
+                            // value is valid. e.g. unions enlarge validity ranges,
+                            // because the values may be uninitialized.
+                            if let Bound::Included(start) = start {
+                                // FIXME(eddyb) this might be incorrect - it doesn't
+                                // account for wrap-around (end < start) ranges.
+                                let valid_range = scalar.valid_range_mut();
+                                assert!(valid_range.start <= start);
+                                valid_range.start = start;
+                            }
+                            if let Bound::Included(end) = end {
+                                // FIXME(eddyb) this might be incorrect - it doesn't
+                                // account for wrap-around (end < start) ranges.
+                                let valid_range = scalar.valid_range_mut();
+                                assert!(valid_range.end >= end);
+                                valid_range.end = end;
+                            }
+
+                            // Update `largest_niche` if we have introduced a larger niche.
+                            let niche = Niche::from_scalar(dl, Size::ZERO, *scalar);
+                            if let Some(niche) = niche {
+                                match st.largest_niche {
+                                    Some(largest_niche) => {
+                                        // Replace the existing niche even if they're equal,
+                                        // because this one is at a lower offset.
+                                        if largest_niche.available(dl) <= niche.available(dl) {
+                                            st.largest_niche = Some(niche);
+                                        }
+                                    }
+                                    None => st.largest_niche = Some(niche),
+                                }
+                            }
+                        }
+                        _ => assert!(
+                            start == Bound::Unbounded && end == Bound::Unbounded,
+                            "nonscalar layout for layout_scalar_valid_range type {:?}: {:#?}",
+                            def,
+                            st,
+                        ),
+                    }
+
+                    return Ok(tcx.intern_layout(st));
+                }
+
+                // At this point, we have handled all unions and
+                // structs. (We have also handled univariant enums
+                // that allow representation optimization.)
+                assert!(def.is_enum());
+
+                // The current code for niche-filling relies on variant indices
+                // instead of actual discriminants, so dataful enums with
+                // explicit discriminants (RFC #2363) would misbehave.
+                let no_explicit_discriminants = def
+                    .variants()
+                    .iter_enumerated()
+                    .all(|(i, v)| v.discr == ty::VariantDiscr::Relative(i.as_u32()));
+
+                let mut niche_filling_layout = None;
+
+                // Niche-filling enum optimization.
+                if !def.repr().inhibit_enum_layout_opt() && no_explicit_discriminants {
+                    let mut dataful_variant = None;
+                    let mut niche_variants = VariantIdx::MAX..=VariantIdx::new(0);
+
+                    // Find one non-ZST variant.
+                    'variants: for (v, fields) in variants.iter_enumerated() {
+                        if absent(fields) {
+                            continue 'variants;
+                        }
+                        for f in fields {
+                            if !f.is_zst() {
+                                if dataful_variant.is_none() {
+                                    dataful_variant = Some(v);
+                                    continue 'variants;
+                                } else {
+                                    dataful_variant = None;
+                                    break 'variants;
+                                }
+                            }
+                        }
+                        niche_variants = *niche_variants.start().min(&v)..=v;
+                    }
+
+                    if niche_variants.start() > niche_variants.end() {
+                        dataful_variant = None;
+                    }
+
+                    if let Some(i) = dataful_variant {
+                        let count = (niche_variants.end().as_u32()
+                            - niche_variants.start().as_u32()
+                            + 1) as u128;
+
+                        // Find the field with the largest niche
+                        let niche_candidate = variants[i]
+                            .iter()
+                            .enumerate()
+                            .filter_map(|(j, field)| Some((j, field.largest_niche?)))
+                            .max_by_key(|(_, niche)| niche.available(dl));
+
+                        if let Some((field_index, niche, (niche_start, niche_scalar))) =
+                            niche_candidate.and_then(|(field_index, niche)| {
+                                Some((field_index, niche, niche.reserve(self, count)?))
+                            })
+                        {
+                            let mut align = dl.aggregate_align;
+                            let st = variants
+                                .iter_enumerated()
+                                .map(|(j, v)| {
+                                    let mut st = self.univariant_uninterned(
+                                        ty,
+                                        v,
+                                        &def.repr(),
+                                        StructKind::AlwaysSized,
+                                    )?;
+                                    st.variants = Variants::Single { index: j };
+
+                                    align = align.max(st.align);
+
+                                    Ok(tcx.intern_layout(st))
+                                })
+                                .collect::<Result<IndexVec<VariantIdx, _>, _>>()?;
+
+                            let offset = st[i].fields().offset(field_index) + niche.offset;
+
+                            // Align the total size to the largest alignment.
+                            let size = st[i].size().align_to(align.abi);
+
+                            let abi = if st.iter().all(|v| v.abi().is_uninhabited()) {
+                                Abi::Uninhabited
+                            } else if align == st[i].align() && size == st[i].size() {
+                                // When the total alignment and size match, we can use the
+                                // same ABI as the scalar variant with the reserved niche.
+                                match st[i].abi() {
+                                    Abi::Scalar(_) => Abi::Scalar(niche_scalar),
+                                    Abi::ScalarPair(first, second) => {
+                                        // Only the niche is guaranteed to be initialised,
+                                        // so use union layout for the other primitive.
+                                        if offset.bytes() == 0 {
+                                            Abi::ScalarPair(niche_scalar, second.to_union())
+                                        } else {
+                                            Abi::ScalarPair(first.to_union(), niche_scalar)
+                                        }
+                                    }
+                                    _ => Abi::Aggregate { sized: true },
+                                }
+                            } else {
+                                Abi::Aggregate { sized: true }
+                            };
+
+                            let largest_niche = Niche::from_scalar(dl, offset, niche_scalar);
+
+                            niche_filling_layout = Some(LayoutS {
+                                variants: Variants::Multiple {
+                                    tag: niche_scalar,
+                                    tag_encoding: TagEncoding::Niche {
+                                        dataful_variant: i,
+                                        niche_variants,
+                                        niche_start,
+                                    },
+                                    tag_field: 0,
+                                    variants: st,
+                                },
+                                fields: FieldsShape::Arbitrary {
+                                    offsets: vec![offset],
+                                    memory_index: vec![0],
+                                },
+                                abi,
+                                largest_niche,
+                                size,
+                                align,
+                            });
+                        }
+                    }
+                }
+
+                let (mut min, mut max) = (i128::MAX, i128::MIN);
+                let discr_type = def.repr().discr_type();
+                let bits = Integer::from_attr(self, discr_type).size().bits();
+                for (i, discr) in def.discriminants(tcx) {
+                    if variants[i].iter().any(|f| f.abi.is_uninhabited()) {
+                        continue;
+                    }
+                    let mut x = discr.val as i128;
+                    if discr_type.is_signed() {
+                        // sign extend the raw representation to be an i128
+                        x = (x << (128 - bits)) >> (128 - bits);
+                    }
+                    if x < min {
+                        min = x;
+                    }
+                    if x > max {
+                        max = x;
+                    }
+                }
+                // We might have no inhabited variants, so pretend there's at least one.
+                if (min, max) == (i128::MAX, i128::MIN) {
+                    min = 0;
+                    max = 0;
+                }
+                assert!(min <= max, "discriminant range is {}...{}", min, max);
+                let (min_ity, signed) = Integer::repr_discr(tcx, ty, &def.repr(), min, max);
+
+                let mut align = dl.aggregate_align;
+                let mut size = Size::ZERO;
+
+                // We're interested in the smallest alignment, so start large.
+                let mut start_align = Align::from_bytes(256).unwrap();
+                assert_eq!(Integer::for_align(dl, start_align), None);
+
+                // repr(C) on an enum tells us to make a (tag, union) layout,
+                // so we need to grow the prefix alignment to be at least
+                // the alignment of the union. (This value is used both for
+                // determining the alignment of the overall enum, and the
+                // determining the alignment of the payload after the tag.)
+                let mut prefix_align = min_ity.align(dl).abi;
+                if def.repr().c() {
+                    for fields in &variants {
+                        for field in fields {
+                            prefix_align = prefix_align.max(field.align.abi);
+                        }
+                    }
+                }
+
+                // Create the set of structs that represent each variant.
+                let mut layout_variants = variants
+                    .iter_enumerated()
+                    .map(|(i, field_layouts)| {
+                        let mut st = self.univariant_uninterned(
+                            ty,
+                            &field_layouts,
+                            &def.repr(),
+                            StructKind::Prefixed(min_ity.size(), prefix_align),
+                        )?;
+                        st.variants = Variants::Single { index: i };
+                        // Find the first field we can't move later
+                        // to make room for a larger discriminant.
+                        for field in
+                            st.fields.index_by_increasing_offset().map(|j| field_layouts[j])
+                        {
+                            if !field.is_zst() || field.align.abi.bytes() != 1 {
+                                start_align = start_align.min(field.align.abi);
+                                break;
+                            }
+                        }
+                        size = cmp::max(size, st.size);
+                        align = align.max(st.align);
+                        Ok(st)
+                    })
+                    .collect::<Result<IndexVec<VariantIdx, _>, _>>()?;
+
+                // Align the maximum variant size to the largest alignment.
+                size = size.align_to(align.abi);
+
+                if size.bytes() >= dl.obj_size_bound() {
+                    return Err(LayoutError::SizeOverflow(ty));
+                }
+
+                let typeck_ity = Integer::from_attr(dl, def.repr().discr_type());
+                if typeck_ity < min_ity {
+                    // It is a bug if Layout decided on a greater discriminant size than typeck for
+                    // some reason at this point (based on values discriminant can take on). Mostly
+                    // because this discriminant will be loaded, and then stored into variable of
+                    // type calculated by typeck. Consider such case (a bug): typeck decided on
+                    // byte-sized discriminant, but layout thinks we need a 16-bit to store all
+                    // discriminant values. That would be a bug, because then, in codegen, in order
+                    // to store this 16-bit discriminant into 8-bit sized temporary some of the
+                    // space necessary to represent would have to be discarded (or layout is wrong
+                    // on thinking it needs 16 bits)
+                    bug!(
+                        "layout decided on a larger discriminant type ({:?}) than typeck ({:?})",
+                        min_ity,
+                        typeck_ity
+                    );
+                    // However, it is fine to make discr type however large (as an optimisation)
+                    // after this point – we’ll just truncate the value we load in codegen.
+                }
+
+                // Check to see if we should use a different type for the
+                // discriminant. We can safely use a type with the same size
+                // as the alignment of the first field of each variant.
+                // We increase the size of the discriminant to avoid LLVM copying
+                // padding when it doesn't need to. This normally causes unaligned
+                // load/stores and excessive memcpy/memset operations. By using a
+                // bigger integer size, LLVM can be sure about its contents and
+                // won't be so conservative.
+
+                // Use the initial field alignment
+                let mut ity = if def.repr().c() || def.repr().int.is_some() {
+                    min_ity
+                } else {
+                    Integer::for_align(dl, start_align).unwrap_or(min_ity)
+                };
+
+                // If the alignment is not larger than the chosen discriminant size,
+                // don't use the alignment as the final size.
+                if ity <= min_ity {
+                    ity = min_ity;
+                } else {
+                    // Patch up the variants' first few fields.
+                    let old_ity_size = min_ity.size();
+                    let new_ity_size = ity.size();
+                    for variant in &mut layout_variants {
+                        match variant.fields {
+                            FieldsShape::Arbitrary { ref mut offsets, .. } => {
+                                for i in offsets {
+                                    if *i <= old_ity_size {
+                                        assert_eq!(*i, old_ity_size);
+                                        *i = new_ity_size;
+                                    }
+                                }
+                                // We might be making the struct larger.
+                                if variant.size <= old_ity_size {
+                                    variant.size = new_ity_size;
+                                }
+                            }
+                            _ => bug!(),
+                        }
+                    }
+                }
+
+                let tag_mask = ity.size().unsigned_int_max();
+                let tag = Scalar::Initialized {
+                    value: Int(ity, signed),
+                    valid_range: WrappingRange {
+                        start: (min as u128 & tag_mask),
+                        end: (max as u128 & tag_mask),
+                    },
+                };
+                let mut abi = Abi::Aggregate { sized: true };
+
+                if layout_variants.iter().all(|v| v.abi.is_uninhabited()) {
+                    abi = Abi::Uninhabited;
+                } else if tag.size(dl) == size {
+                    // Make sure we only use scalar layout when the enum is entirely its
+                    // own tag (i.e. it has no padding nor any non-ZST variant fields).
+                    abi = Abi::Scalar(tag);
+                } else {
+                    // Try to use a ScalarPair for all tagged enums.
+                    let mut common_prim = None;
+                    let mut common_prim_initialized_in_all_variants = true;
+                    for (field_layouts, layout_variant) in iter::zip(&variants, &layout_variants) {
+                        let FieldsShape::Arbitrary { ref offsets, .. } = layout_variant.fields else {
+                            bug!();
+                        };
+                        let mut fields =
+                            iter::zip(field_layouts, offsets).filter(|p| !p.0.is_zst());
+                        let (field, offset) = match (fields.next(), fields.next()) {
+                            (None, None) => {
+                                common_prim_initialized_in_all_variants = false;
+                                continue;
+                            }
+                            (Some(pair), None) => pair,
+                            _ => {
+                                common_prim = None;
+                                break;
+                            }
+                        };
+                        let prim = match field.abi {
+                            Abi::Scalar(scalar) => {
+                                common_prim_initialized_in_all_variants &=
+                                    matches!(scalar, Scalar::Initialized { .. });
+                                scalar.primitive()
+                            }
+                            _ => {
+                                common_prim = None;
+                                break;
+                            }
+                        };
+                        if let Some(pair) = common_prim {
+                            // This is pretty conservative. We could go fancier
+                            // by conflating things like i32 and u32, or even
+                            // realising that (u8, u8) could just cohabit with
+                            // u16 or even u32.
+                            if pair != (prim, offset) {
+                                common_prim = None;
+                                break;
+                            }
+                        } else {
+                            common_prim = Some((prim, offset));
+                        }
+                    }
+                    if let Some((prim, offset)) = common_prim {
+                        let prim_scalar = if common_prim_initialized_in_all_variants {
+                            scalar_unit(prim)
+                        } else {
+                            // Common prim might be uninit.
+                            Scalar::Union { value: prim }
+                        };
+                        let pair = self.scalar_pair(tag, prim_scalar);
+                        let pair_offsets = match pair.fields {
+                            FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
+                                assert_eq!(memory_index, &[0, 1]);
+                                offsets
+                            }
+                            _ => bug!(),
+                        };
+                        if pair_offsets[0] == Size::ZERO
+                            && pair_offsets[1] == *offset
+                            && align == pair.align
+                            && size == pair.size
+                        {
+                            // We can use `ScalarPair` only when it matches our
+                            // already computed layout (including `#[repr(C)]`).
+                            abi = pair.abi;
+                        }
+                    }
+                }
+
+                // If we pick a "clever" (by-value) ABI, we might have to adjust the ABI of the
+                // variants to ensure they are consistent. This is because a downcast is
+                // semantically a NOP, and thus should not affect layout.
+                if matches!(abi, Abi::Scalar(..) | Abi::ScalarPair(..)) {
+                    for variant in &mut layout_variants {
+                        // We only do this for variants with fields; the others are not accessed anyway.
+                        // Also do not overwrite any already existing "clever" ABIs.
+                        if variant.fields.count() > 0
+                            && matches!(variant.abi, Abi::Aggregate { .. })
+                        {
+                            variant.abi = abi;
+                            // Also need to bump up the size and alignment, so that the entire value fits in here.
+                            variant.size = cmp::max(variant.size, size);
+                            variant.align.abi = cmp::max(variant.align.abi, align.abi);
+                        }
+                    }
+                }
+
+                let largest_niche = Niche::from_scalar(dl, Size::ZERO, tag);
+
+                let layout_variants =
+                    layout_variants.into_iter().map(|v| tcx.intern_layout(v)).collect();
+
+                let tagged_layout = LayoutS {
+                    variants: Variants::Multiple {
+                        tag,
+                        tag_encoding: TagEncoding::Direct,
+                        tag_field: 0,
+                        variants: layout_variants,
+                    },
+                    fields: FieldsShape::Arbitrary {
+                        offsets: vec![Size::ZERO],
+                        memory_index: vec![0],
+                    },
+                    largest_niche,
+                    abi,
+                    align,
+                    size,
+                };
+
+                let best_layout = match (tagged_layout, niche_filling_layout) {
+                    (tagged_layout, Some(niche_filling_layout)) => {
+                        // Pick the smaller layout; otherwise,
+                        // pick the layout with the larger niche; otherwise,
+                        // pick tagged as it has simpler codegen.
+                        cmp::min_by_key(tagged_layout, niche_filling_layout, |layout| {
+                            let niche_size = layout.largest_niche.map_or(0, |n| n.available(dl));
+                            (layout.size, cmp::Reverse(niche_size))
+                        })
+                    }
+                    (tagged_layout, None) => tagged_layout,
+                };
+
+                tcx.intern_layout(best_layout)
+            }
+
+            // Types with no meaningful known layout.
+            ty::Projection(_) | ty::Opaque(..) => {
+                // NOTE(eddyb) `layout_of` query should've normalized these away,
+                // if that was possible, so there's no reason to try again here.
+                return Err(LayoutError::Unknown(ty));
+            }
+
+            ty::Placeholder(..) | ty::GeneratorWitness(..) | ty::Infer(_) => {
+                bug!("Layout::compute: unexpected type `{}`", ty)
+            }
+
+            ty::Bound(..) | ty::Param(_) | ty::Error(_) => {
+                return Err(LayoutError::Unknown(ty));
+            }
+        })
+    }
+}
+
+/// Overlap eligibility and variant assignment for each GeneratorSavedLocal.
+#[derive(Clone, Debug, PartialEq)]
+enum SavedLocalEligibility {
+    Unassigned,
+    Assigned(VariantIdx),
+    // FIXME: Use newtype_index so we aren't wasting bytes
+    Ineligible(Option<u32>),
+}
+
+// When laying out generators, we divide our saved local fields into two
+// categories: overlap-eligible and overlap-ineligible.
+//
+// Those fields which are ineligible for overlap go in a "prefix" at the
+// beginning of the layout, and always have space reserved for them.
+//
+// Overlap-eligible fields are only assigned to one variant, so we lay
+// those fields out for each variant and put them right after the
+// prefix.
+//
+// Finally, in the layout details, we point to the fields from the
+// variants they are assigned to. It is possible for some fields to be
+// included in multiple variants. No field ever "moves around" in the
+// layout; its offset is always the same.
+//
+// Also included in the layout are the upvars and the discriminant.
+// These are included as fields on the "outer" layout; they are not part
+// of any variant.
+impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
+    /// Compute the eligibility and assignment of each local.
+    fn generator_saved_local_eligibility(
+        &self,
+        info: &GeneratorLayout<'tcx>,
+    ) -> (BitSet<GeneratorSavedLocal>, IndexVec<GeneratorSavedLocal, SavedLocalEligibility>) {
+        use SavedLocalEligibility::*;
+
+        let mut assignments: IndexVec<GeneratorSavedLocal, SavedLocalEligibility> =
+            IndexVec::from_elem_n(Unassigned, info.field_tys.len());
+
+        // The saved locals not eligible for overlap. These will get
+        // "promoted" to the prefix of our generator.
+        let mut ineligible_locals = BitSet::new_empty(info.field_tys.len());
+
+        // Figure out which of our saved locals are fields in only
+        // one variant. The rest are deemed ineligible for overlap.
+        for (variant_index, fields) in info.variant_fields.iter_enumerated() {
+            for local in fields {
+                match assignments[*local] {
+                    Unassigned => {
+                        assignments[*local] = Assigned(variant_index);
+                    }
+                    Assigned(idx) => {
+                        // We've already seen this local at another suspension
+                        // point, so it is no longer a candidate.
+                        trace!(
+                            "removing local {:?} in >1 variant ({:?}, {:?})",
+                            local,
+                            variant_index,
+                            idx
+                        );
+                        ineligible_locals.insert(*local);
+                        assignments[*local] = Ineligible(None);
+                    }
+                    Ineligible(_) => {}
+                }
+            }
+        }
+
+        // Next, check every pair of eligible locals to see if they
+        // conflict.
+        for local_a in info.storage_conflicts.rows() {
+            let conflicts_a = info.storage_conflicts.count(local_a);
+            if ineligible_locals.contains(local_a) {
+                continue;
+            }
+
+            for local_b in info.storage_conflicts.iter(local_a) {
+                // local_a and local_b are storage live at the same time, therefore they
+                // cannot overlap in the generator layout. The only way to guarantee
+                // this is if they are in the same variant, or one is ineligible
+                // (which means it is stored in every variant).
+                if ineligible_locals.contains(local_b)
+                    || assignments[local_a] == assignments[local_b]
+                {
+                    continue;
+                }
+
+                // If they conflict, we will choose one to make ineligible.
+                // This is not always optimal; it's just a greedy heuristic that
+                // seems to produce good results most of the time.
+                let conflicts_b = info.storage_conflicts.count(local_b);
+                let (remove, other) =
+                    if conflicts_a > conflicts_b { (local_a, local_b) } else { (local_b, local_a) };
+                ineligible_locals.insert(remove);
+                assignments[remove] = Ineligible(None);
+                trace!("removing local {:?} due to conflict with {:?}", remove, other);
+            }
+        }
+
+        // Count the number of variants in use. If only one of them, then it is
+        // impossible to overlap any locals in our layout. In this case it's
+        // always better to make the remaining locals ineligible, so we can
+        // lay them out with the other locals in the prefix and eliminate
+        // unnecessary padding bytes.
+        {
+            let mut used_variants = BitSet::new_empty(info.variant_fields.len());
+            for assignment in &assignments {
+                if let Assigned(idx) = assignment {
+                    used_variants.insert(*idx);
+                }
+            }
+            if used_variants.count() < 2 {
+                for assignment in assignments.iter_mut() {
+                    *assignment = Ineligible(None);
+                }
+                ineligible_locals.insert_all();
+            }
+        }
+
+        // Write down the order of our locals that will be promoted to the prefix.
+        {
+            for (idx, local) in ineligible_locals.iter().enumerate() {
+                assignments[local] = Ineligible(Some(idx as u32));
+            }
+        }
+        debug!("generator saved local assignments: {:?}", assignments);
+
+        (ineligible_locals, assignments)
+    }
+
+    /// Compute the full generator layout.
+    fn generator_layout(
+        &self,
+        ty: Ty<'tcx>,
+        def_id: hir::def_id::DefId,
+        substs: SubstsRef<'tcx>,
+    ) -> Result<Layout<'tcx>, LayoutError<'tcx>> {
+        use SavedLocalEligibility::*;
+        let tcx = self.tcx;
+        let subst_field = |ty: Ty<'tcx>| EarlyBinder(ty).subst(tcx, substs);
+
+        let Some(info) = tcx.generator_layout(def_id) else {
+            return Err(LayoutError::Unknown(ty));
+        };
+        let (ineligible_locals, assignments) = self.generator_saved_local_eligibility(&info);
+
+        // Build a prefix layout, including "promoting" all ineligible
+        // locals as part of the prefix. We compute the layout of all of
+        // these fields at once to get optimal packing.
+        let tag_index = substs.as_generator().prefix_tys().count();
+
+        // `info.variant_fields` already accounts for the reserved variants, so no need to add them.
+        let max_discr = (info.variant_fields.len() - 1) as u128;
+        let discr_int = Integer::fit_unsigned(max_discr);
+        let discr_int_ty = discr_int.to_ty(tcx, false);
+        let tag = Scalar::Initialized {
+            value: Primitive::Int(discr_int, false),
+            valid_range: WrappingRange { start: 0, end: max_discr },
+        };
+        let tag_layout = self.tcx.intern_layout(LayoutS::scalar(self, tag));
+        let tag_layout = TyAndLayout { ty: discr_int_ty, layout: tag_layout };
+
+        let promoted_layouts = ineligible_locals
+            .iter()
+            .map(|local| subst_field(info.field_tys[local]))
+            .map(|ty| tcx.mk_maybe_uninit(ty))
+            .map(|ty| self.layout_of(ty));
+        let prefix_layouts = substs
+            .as_generator()
+            .prefix_tys()
+            .map(|ty| self.layout_of(ty))
+            .chain(iter::once(Ok(tag_layout)))
+            .chain(promoted_layouts)
+            .collect::<Result<Vec<_>, _>>()?;
+        let prefix = self.univariant_uninterned(
+            ty,
+            &prefix_layouts,
+            &ReprOptions::default(),
+            StructKind::AlwaysSized,
+        )?;
+
+        let (prefix_size, prefix_align) = (prefix.size, prefix.align);
+
+        // Split the prefix layout into the "outer" fields (upvars and
+        // discriminant) and the "promoted" fields. Promoted fields will
+        // get included in each variant that requested them in
+        // GeneratorLayout.
+        debug!("prefix = {:#?}", prefix);
+        let (outer_fields, promoted_offsets, promoted_memory_index) = match prefix.fields {
+            FieldsShape::Arbitrary { mut offsets, memory_index } => {
+                let mut inverse_memory_index = invert_mapping(&memory_index);
+
+                // "a" (`0..b_start`) and "b" (`b_start..`) correspond to
+                // "outer" and "promoted" fields respectively.
+                let b_start = (tag_index + 1) as u32;
+                let offsets_b = offsets.split_off(b_start as usize);
+                let offsets_a = offsets;
+
+                // Disentangle the "a" and "b" components of `inverse_memory_index`
+                // by preserving the order but keeping only one disjoint "half" each.
+                // FIXME(eddyb) build a better abstraction for permutations, if possible.
+                let inverse_memory_index_b: Vec<_> =
+                    inverse_memory_index.iter().filter_map(|&i| i.checked_sub(b_start)).collect();
+                inverse_memory_index.retain(|&i| i < b_start);
+                let inverse_memory_index_a = inverse_memory_index;
+
+                // Since `inverse_memory_index_{a,b}` each only refer to their
+                // respective fields, they can be safely inverted
+                let memory_index_a = invert_mapping(&inverse_memory_index_a);
+                let memory_index_b = invert_mapping(&inverse_memory_index_b);
+
+                let outer_fields =
+                    FieldsShape::Arbitrary { offsets: offsets_a, memory_index: memory_index_a };
+                (outer_fields, offsets_b, memory_index_b)
+            }
+            _ => bug!(),
+        };
+
+        let mut size = prefix.size;
+        let mut align = prefix.align;
+        let variants = info
+            .variant_fields
+            .iter_enumerated()
+            .map(|(index, variant_fields)| {
+                // Only include overlap-eligible fields when we compute our variant layout.
+                let variant_only_tys = variant_fields
+                    .iter()
+                    .filter(|local| match assignments[**local] {
+                        Unassigned => bug!(),
+                        Assigned(v) if v == index => true,
+                        Assigned(_) => bug!("assignment does not match variant"),
+                        Ineligible(_) => false,
+                    })
+                    .map(|local| subst_field(info.field_tys[*local]));
+
+                let mut variant = self.univariant_uninterned(
+                    ty,
+                    &variant_only_tys
+                        .map(|ty| self.layout_of(ty))
+                        .collect::<Result<Vec<_>, _>>()?,
+                    &ReprOptions::default(),
+                    StructKind::Prefixed(prefix_size, prefix_align.abi),
+                )?;
+                variant.variants = Variants::Single { index };
+
+                let FieldsShape::Arbitrary { offsets, memory_index } = variant.fields else {
+                    bug!();
+                };
+
+                // Now, stitch the promoted and variant-only fields back together in
+                // the order they are mentioned by our GeneratorLayout.
+                // Because we only use some subset (that can differ between variants)
+                // of the promoted fields, we can't just pick those elements of the
+                // `promoted_memory_index` (as we'd end up with gaps).
+                // So instead, we build an "inverse memory_index", as if all of the
+                // promoted fields were being used, but leave the elements not in the
+                // subset as `INVALID_FIELD_IDX`, which we can filter out later to
+                // obtain a valid (bijective) mapping.
+                const INVALID_FIELD_IDX: u32 = !0;
+                let mut combined_inverse_memory_index =
+                    vec![INVALID_FIELD_IDX; promoted_memory_index.len() + memory_index.len()];
+                let mut offsets_and_memory_index = iter::zip(offsets, memory_index);
+                let combined_offsets = variant_fields
+                    .iter()
+                    .enumerate()
+                    .map(|(i, local)| {
+                        let (offset, memory_index) = match assignments[*local] {
+                            Unassigned => bug!(),
+                            Assigned(_) => {
+                                let (offset, memory_index) =
+                                    offsets_and_memory_index.next().unwrap();
+                                (offset, promoted_memory_index.len() as u32 + memory_index)
+                            }
+                            Ineligible(field_idx) => {
+                                let field_idx = field_idx.unwrap() as usize;
+                                (promoted_offsets[field_idx], promoted_memory_index[field_idx])
+                            }
+                        };
+                        combined_inverse_memory_index[memory_index as usize] = i as u32;
+                        offset
+                    })
+                    .collect();
+
+                // Remove the unused slots and invert the mapping to obtain the
+                // combined `memory_index` (also see previous comment).
+                combined_inverse_memory_index.retain(|&i| i != INVALID_FIELD_IDX);
+                let combined_memory_index = invert_mapping(&combined_inverse_memory_index);
+
+                variant.fields = FieldsShape::Arbitrary {
+                    offsets: combined_offsets,
+                    memory_index: combined_memory_index,
+                };
+
+                size = size.max(variant.size);
+                align = align.max(variant.align);
+                Ok(tcx.intern_layout(variant))
+            })
+            .collect::<Result<IndexVec<VariantIdx, _>, _>>()?;
+
+        size = size.align_to(align.abi);
+
+        let abi =
+            if prefix.abi.is_uninhabited() || variants.iter().all(|v| v.abi().is_uninhabited()) {
+                Abi::Uninhabited
+            } else {
+                Abi::Aggregate { sized: true }
+            };
+
+        let layout = tcx.intern_layout(LayoutS {
+            variants: Variants::Multiple {
+                tag,
+                tag_encoding: TagEncoding::Direct,
+                tag_field: tag_index,
+                variants,
+            },
+            fields: outer_fields,
+            abi,
+            largest_niche: prefix.largest_niche,
+            size,
+            align,
+        });
+        debug!("generator layout ({:?}): {:#?}", ty, layout);
+        Ok(layout)
+    }
+
+    /// This is invoked by the `layout_of` query to record the final
+    /// layout of each type.
+    #[inline(always)]
+    fn record_layout_for_printing(&self, layout: TyAndLayout<'tcx>) {
+        // If we are running with `-Zprint-type-sizes`, maybe record layouts
+        // for dumping later.
+        if self.tcx.sess.opts.unstable_opts.print_type_sizes {
+            self.record_layout_for_printing_outlined(layout)
+        }
+    }
+
+    fn record_layout_for_printing_outlined(&self, layout: TyAndLayout<'tcx>) {
+        // Ignore layouts that are done with non-empty environments or
+        // non-monomorphic layouts, as the user only wants to see the stuff
+        // resulting from the final codegen session.
+        if layout.ty.has_param_types_or_consts() || !self.param_env.caller_bounds().is_empty() {
+            return;
+        }
+
+        // (delay format until we actually need it)
+        let record = |kind, packed, opt_discr_size, variants| {
+            let type_desc = format!("{:?}", layout.ty);
+            self.tcx.sess.code_stats.record_type_size(
+                kind,
+                type_desc,
+                layout.align.abi,
+                layout.size,
+                packed,
+                opt_discr_size,
+                variants,
+            );
+        };
+
+        let adt_def = match *layout.ty.kind() {
+            ty::Adt(ref adt_def, _) => {
+                debug!("print-type-size t: `{:?}` process adt", layout.ty);
+                adt_def
+            }
+
+            ty::Closure(..) => {
+                debug!("print-type-size t: `{:?}` record closure", layout.ty);
+                record(DataTypeKind::Closure, false, None, vec![]);
+                return;
+            }
+
+            _ => {
+                debug!("print-type-size t: `{:?}` skip non-nominal", layout.ty);
+                return;
+            }
+        };
+
+        let adt_kind = adt_def.adt_kind();
+        let adt_packed = adt_def.repr().pack.is_some();
+
+        let build_variant_info = |n: Option<Symbol>, flds: &[Symbol], layout: TyAndLayout<'tcx>| {
+            let mut min_size = Size::ZERO;
+            let field_info: Vec<_> = flds
+                .iter()
+                .enumerate()
+                .map(|(i, &name)| {
+                    let field_layout = layout.field(self, i);
+                    let offset = layout.fields.offset(i);
+                    let field_end = offset + field_layout.size;
+                    if min_size < field_end {
+                        min_size = field_end;
+                    }
+                    FieldInfo {
+                        name,
+                        offset: offset.bytes(),
+                        size: field_layout.size.bytes(),
+                        align: field_layout.align.abi.bytes(),
+                    }
+                })
+                .collect();
+
+            VariantInfo {
+                name: n,
+                kind: if layout.is_unsized() { SizeKind::Min } else { SizeKind::Exact },
+                align: layout.align.abi.bytes(),
+                size: if min_size.bytes() == 0 { layout.size.bytes() } else { min_size.bytes() },
+                fields: field_info,
+            }
+        };
+
+        match layout.variants {
+            Variants::Single { index } => {
+                if !adt_def.variants().is_empty() && layout.fields != FieldsShape::Primitive {
+                    debug!(
+                        "print-type-size `{:#?}` variant {}",
+                        layout,
+                        adt_def.variant(index).name
+                    );
+                    let variant_def = &adt_def.variant(index);
+                    let fields: Vec<_> = variant_def.fields.iter().map(|f| f.name).collect();
+                    record(
+                        adt_kind.into(),
+                        adt_packed,
+                        None,
+                        vec![build_variant_info(Some(variant_def.name), &fields, layout)],
+                    );
+                } else {
+                    // (This case arises for *empty* enums; so give it
+                    // zero variants.)
+                    record(adt_kind.into(), adt_packed, None, vec![]);
+                }
+            }
+
+            Variants::Multiple { tag, ref tag_encoding, .. } => {
+                debug!(
+                    "print-type-size `{:#?}` adt general variants def {}",
+                    layout.ty,
+                    adt_def.variants().len()
+                );
+                let variant_infos: Vec<_> = adt_def
+                    .variants()
+                    .iter_enumerated()
+                    .map(|(i, variant_def)| {
+                        let fields: Vec<_> = variant_def.fields.iter().map(|f| f.name).collect();
+                        build_variant_info(
+                            Some(variant_def.name),
+                            &fields,
+                            layout.for_variant(self, i),
+                        )
+                    })
+                    .collect();
+                record(
+                    adt_kind.into(),
+                    adt_packed,
+                    match tag_encoding {
+                        TagEncoding::Direct => Some(tag.size(self)),
+                        _ => None,
+                    },
+                    variant_infos,
+                );
+            }
+        }
+    }
+}
+
+/// Type size "skeleton", i.e., the only information determining a type's size.
+/// While this is conservative, (aside from constant sizes, only pointers,
+/// newtypes thereof and null pointer optimized enums are allowed), it is
+/// enough to statically check common use cases of transmute.
+#[derive(Copy, Clone, Debug)]
+pub enum SizeSkeleton<'tcx> {
+    /// Any statically computable Layout.
+    Known(Size),
+
+    /// A potentially-fat pointer.
+    Pointer {
+        /// If true, this pointer is never null.
+        non_zero: bool,
+        /// The type which determines the unsized metadata, if any,
+        /// of this pointer. Either a type parameter or a projection
+        /// depending on one, with regions erased.
+        tail: Ty<'tcx>,
+    },
+}
+
+impl<'tcx> SizeSkeleton<'tcx> {
+    pub fn compute(
+        ty: Ty<'tcx>,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> Result<SizeSkeleton<'tcx>, LayoutError<'tcx>> {
+        debug_assert!(!ty.has_infer_types_or_consts());
+
+        // First try computing a static layout.
+        let err = match tcx.layout_of(param_env.and(ty)) {
+            Ok(layout) => {
+                return Ok(SizeSkeleton::Known(layout.size));
+            }
+            Err(err) => err,
+        };
+
+        match *ty.kind() {
+            ty::Ref(_, pointee, _) | ty::RawPtr(ty::TypeAndMut { ty: pointee, .. }) => {
+                let non_zero = !ty.is_unsafe_ptr();
+                let tail = tcx.struct_tail_erasing_lifetimes(pointee, param_env);
+                match tail.kind() {
+                    ty::Param(_) | ty::Projection(_) => {
+                        debug_assert!(tail.has_param_types_or_consts());
+                        Ok(SizeSkeleton::Pointer { non_zero, tail: tcx.erase_regions(tail) })
+                    }
+                    _ => bug!(
+                        "SizeSkeleton::compute({}): layout errored ({}), yet \
+                              tail `{}` is not a type parameter or a projection",
+                        ty,
+                        err,
+                        tail
+                    ),
+                }
+            }
+
+            ty::Adt(def, substs) => {
+                // Only newtypes and enums w/ nullable pointer optimization.
+                if def.is_union() || def.variants().is_empty() || def.variants().len() > 2 {
+                    return Err(err);
+                }
+
+                // Get a zero-sized variant or a pointer newtype.
+                let zero_or_ptr_variant = |i| {
+                    let i = VariantIdx::new(i);
+                    let fields =
+                        def.variant(i).fields.iter().map(|field| {
+                            SizeSkeleton::compute(field.ty(tcx, substs), tcx, param_env)
+                        });
+                    let mut ptr = None;
+                    for field in fields {
+                        let field = field?;
+                        match field {
+                            SizeSkeleton::Known(size) => {
+                                if size.bytes() > 0 {
+                                    return Err(err);
+                                }
+                            }
+                            SizeSkeleton::Pointer { .. } => {
+                                if ptr.is_some() {
+                                    return Err(err);
+                                }
+                                ptr = Some(field);
+                            }
+                        }
+                    }
+                    Ok(ptr)
+                };
+
+                let v0 = zero_or_ptr_variant(0)?;
+                // Newtype.
+                if def.variants().len() == 1 {
+                    if let Some(SizeSkeleton::Pointer { non_zero, tail }) = v0 {
+                        return Ok(SizeSkeleton::Pointer {
+                            non_zero: non_zero
+                                || match tcx.layout_scalar_valid_range(def.did()) {
+                                    (Bound::Included(start), Bound::Unbounded) => start > 0,
+                                    (Bound::Included(start), Bound::Included(end)) => {
+                                        0 < start && start < end
+                                    }
+                                    _ => false,
+                                },
+                            tail,
+                        });
+                    } else {
+                        return Err(err);
+                    }
+                }
+
+                let v1 = zero_or_ptr_variant(1)?;
+                // Nullable pointer enum optimization.
+                match (v0, v1) {
+                    (Some(SizeSkeleton::Pointer { non_zero: true, tail }), None)
+                    | (None, Some(SizeSkeleton::Pointer { non_zero: true, tail })) => {
+                        Ok(SizeSkeleton::Pointer { non_zero: false, tail })
+                    }
+                    _ => Err(err),
+                }
+            }
+
+            ty::Projection(_) | ty::Opaque(..) => {
+                let normalized = tcx.normalize_erasing_regions(param_env, ty);
+                if ty == normalized {
+                    Err(err)
+                } else {
+                    SizeSkeleton::compute(normalized, tcx, param_env)
+                }
+            }
+
+            _ => Err(err),
+        }
+    }
+
+    pub fn same_size(self, other: SizeSkeleton<'tcx>) -> bool {
+        match (self, other) {
+            (SizeSkeleton::Known(a), SizeSkeleton::Known(b)) => a == b,
+            (SizeSkeleton::Pointer { tail: a, .. }, SizeSkeleton::Pointer { tail: b, .. }) => {
+                a == b
+            }
+            _ => false,
+        }
+    }
+}
+
+pub trait HasTyCtxt<'tcx>: HasDataLayout {
+    fn tcx(&self) -> TyCtxt<'tcx>;
+}
+
+pub trait HasParamEnv<'tcx> {
+    fn param_env(&self) -> ty::ParamEnv<'tcx>;
+}
+
+impl<'tcx> HasDataLayout for TyCtxt<'tcx> {
+    #[inline]
+    fn data_layout(&self) -> &TargetDataLayout {
+        &self.data_layout
+    }
+}
+
+impl<'tcx> HasTargetSpec for TyCtxt<'tcx> {
+    fn target_spec(&self) -> &Target {
+        &self.sess.target
+    }
+}
+
+impl<'tcx> HasTyCtxt<'tcx> for TyCtxt<'tcx> {
+    #[inline]
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        *self
+    }
+}
+
+impl<'tcx> HasDataLayout for ty::query::TyCtxtAt<'tcx> {
+    #[inline]
+    fn data_layout(&self) -> &TargetDataLayout {
+        &self.data_layout
+    }
+}
+
+impl<'tcx> HasTargetSpec for ty::query::TyCtxtAt<'tcx> {
+    fn target_spec(&self) -> &Target {
+        &self.sess.target
+    }
+}
+
+impl<'tcx> HasTyCtxt<'tcx> for ty::query::TyCtxtAt<'tcx> {
+    #[inline]
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        **self
+    }
+}
+
+impl<'tcx, C> HasParamEnv<'tcx> for LayoutCx<'tcx, C> {
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.param_env
+    }
+}
+
+impl<'tcx, T: HasDataLayout> HasDataLayout for LayoutCx<'tcx, T> {
+    fn data_layout(&self) -> &TargetDataLayout {
+        self.tcx.data_layout()
+    }
+}
+
+impl<'tcx, T: HasTargetSpec> HasTargetSpec for LayoutCx<'tcx, T> {
+    fn target_spec(&self) -> &Target {
+        self.tcx.target_spec()
+    }
+}
+
+impl<'tcx, T: HasTyCtxt<'tcx>> HasTyCtxt<'tcx> for LayoutCx<'tcx, T> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx.tcx()
+    }
+}
+
+pub trait MaybeResult<T> {
+    type Error;
+
+    fn from(x: Result<T, Self::Error>) -> Self;
+    fn to_result(self) -> Result<T, Self::Error>;
+}
+
+impl<T> MaybeResult<T> for T {
+    type Error = !;
+
+    fn from(Ok(x): Result<T, Self::Error>) -> Self {
+        x
+    }
+    fn to_result(self) -> Result<T, Self::Error> {
+        Ok(self)
+    }
+}
+
+impl<T, E> MaybeResult<T> for Result<T, E> {
+    type Error = E;
+
+    fn from(x: Result<T, Self::Error>) -> Self {
+        x
+    }
+    fn to_result(self) -> Result<T, Self::Error> {
+        self
+    }
+}
+
+pub type TyAndLayout<'tcx> = rustc_target::abi::TyAndLayout<'tcx, Ty<'tcx>>;
+
+/// Trait for contexts that want to be able to compute layouts of types.
+/// This automatically gives access to `LayoutOf`, through a blanket `impl`.
+pub trait LayoutOfHelpers<'tcx>: HasDataLayout + HasTyCtxt<'tcx> + HasParamEnv<'tcx> {
+    /// The `TyAndLayout`-wrapping type (or `TyAndLayout` itself), which will be
+    /// returned from `layout_of` (see also `handle_layout_err`).
+    type LayoutOfResult: MaybeResult<TyAndLayout<'tcx>>;
+
+    /// `Span` to use for `tcx.at(span)`, from `layout_of`.
+    // FIXME(eddyb) perhaps make this mandatory to get contexts to track it better?
+    #[inline]
+    fn layout_tcx_at_span(&self) -> Span {
+        DUMMY_SP
+    }
+
+    /// Helper used for `layout_of`, to adapt `tcx.layout_of(...)` into a
+    /// `Self::LayoutOfResult` (which does not need to be a `Result<...>`).
+    ///
+    /// Most `impl`s, which propagate `LayoutError`s, should simply return `err`,
+    /// but this hook allows e.g. codegen to return only `TyAndLayout` from its
+    /// `cx.layout_of(...)`, without any `Result<...>` around it to deal with
+    /// (and any `LayoutError`s are turned into fatal errors or ICEs).
+    fn handle_layout_err(
+        &self,
+        err: LayoutError<'tcx>,
+        span: Span,
+        ty: Ty<'tcx>,
+    ) -> <Self::LayoutOfResult as MaybeResult<TyAndLayout<'tcx>>>::Error;
+}
+
+/// Blanket extension trait for contexts that can compute layouts of types.
+pub trait LayoutOf<'tcx>: LayoutOfHelpers<'tcx> {
+    /// Computes the layout of a type. Note that this implicitly
+    /// executes in "reveal all" mode, and will normalize the input type.
+    #[inline]
+    fn layout_of(&self, ty: Ty<'tcx>) -> Self::LayoutOfResult {
+        self.spanned_layout_of(ty, DUMMY_SP)
+    }
+
+    /// Computes the layout of a type, at `span`. Note that this implicitly
+    /// executes in "reveal all" mode, and will normalize the input type.
+    // FIXME(eddyb) avoid passing information like this, and instead add more
+    // `TyCtxt::at`-like APIs to be able to do e.g. `cx.at(span).layout_of(ty)`.
+    #[inline]
+    fn spanned_layout_of(&self, ty: Ty<'tcx>, span: Span) -> Self::LayoutOfResult {
+        let span = if !span.is_dummy() { span } else { self.layout_tcx_at_span() };
+        let tcx = self.tcx().at(span);
+
+        MaybeResult::from(
+            tcx.layout_of(self.param_env().and(ty))
+                .map_err(|err| self.handle_layout_err(err, span, ty)),
+        )
+    }
+}
+
+impl<'tcx, C: LayoutOfHelpers<'tcx>> LayoutOf<'tcx> for C {}
+
+impl<'tcx> LayoutOfHelpers<'tcx> for LayoutCx<'tcx, TyCtxt<'tcx>> {
+    type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
+
+    #[inline]
+    fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
+        err
+    }
+}
+
+impl<'tcx> LayoutOfHelpers<'tcx> for LayoutCx<'tcx, ty::query::TyCtxtAt<'tcx>> {
+    type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
+
+    #[inline]
+    fn layout_tcx_at_span(&self) -> Span {
+        self.tcx.span
+    }
+
+    #[inline]
+    fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
+        err
+    }
+}
+
+impl<'tcx, C> TyAbiInterface<'tcx, C> for Ty<'tcx>
+where
+    C: HasTyCtxt<'tcx> + HasParamEnv<'tcx>,
+{
+    fn ty_and_layout_for_variant(
+        this: TyAndLayout<'tcx>,
+        cx: &C,
+        variant_index: VariantIdx,
+    ) -> TyAndLayout<'tcx> {
+        let layout = match this.variants {
+            Variants::Single { index }
+                // If all variants but one are uninhabited, the variant layout is the enum layout.
+                if index == variant_index &&
+                // Don't confuse variants of uninhabited enums with the enum itself.
+                // For more details see https://github.com/rust-lang/rust/issues/69763.
+                this.fields != FieldsShape::Primitive =>
+            {
+                this.layout
+            }
+
+            Variants::Single { index } => {
+                let tcx = cx.tcx();
+                let param_env = cx.param_env();
+
+                // Deny calling for_variant more than once for non-Single enums.
+                if let Ok(original_layout) = tcx.layout_of(param_env.and(this.ty)) {
+                    assert_eq!(original_layout.variants, Variants::Single { index });
+                }
+
+                let fields = match this.ty.kind() {
+                    ty::Adt(def, _) if def.variants().is_empty() =>
+                        bug!("for_variant called on zero-variant enum"),
+                    ty::Adt(def, _) => def.variant(variant_index).fields.len(),
+                    _ => bug!(),
+                };
+                tcx.intern_layout(LayoutS {
+                    variants: Variants::Single { index: variant_index },
+                    fields: match NonZeroUsize::new(fields) {
+                        Some(fields) => FieldsShape::Union(fields),
+                        None => FieldsShape::Arbitrary { offsets: vec![], memory_index: vec![] },
+                    },
+                    abi: Abi::Uninhabited,
+                    largest_niche: None,
+                    align: tcx.data_layout.i8_align,
+                    size: Size::ZERO,
+                })
+            }
+
+            Variants::Multiple { ref variants, .. } => variants[variant_index],
+        };
+
+        assert_eq!(*layout.variants(), Variants::Single { index: variant_index });
+
+        TyAndLayout { ty: this.ty, layout }
+    }
+
+    fn ty_and_layout_field(this: TyAndLayout<'tcx>, cx: &C, i: usize) -> TyAndLayout<'tcx> {
+        enum TyMaybeWithLayout<'tcx> {
+            Ty(Ty<'tcx>),
+            TyAndLayout(TyAndLayout<'tcx>),
+        }
+
+        fn field_ty_or_layout<'tcx>(
+            this: TyAndLayout<'tcx>,
+            cx: &(impl HasTyCtxt<'tcx> + HasParamEnv<'tcx>),
+            i: usize,
+        ) -> TyMaybeWithLayout<'tcx> {
+            let tcx = cx.tcx();
+            let tag_layout = |tag: Scalar| -> TyAndLayout<'tcx> {
+                TyAndLayout {
+                    layout: tcx.intern_layout(LayoutS::scalar(cx, tag)),
+                    ty: tag.primitive().to_ty(tcx),
+                }
+            };
+
+            match *this.ty.kind() {
+                ty::Bool
+                | ty::Char
+                | ty::Int(_)
+                | ty::Uint(_)
+                | ty::Float(_)
+                | ty::FnPtr(_)
+                | ty::Never
+                | ty::FnDef(..)
+                | ty::GeneratorWitness(..)
+                | ty::Foreign(..)
+                | ty::Dynamic(..) => bug!("TyAndLayout::field({:?}): not applicable", this),
+
+                // Potentially-fat pointers.
+                ty::Ref(_, pointee, _) | ty::RawPtr(ty::TypeAndMut { ty: pointee, .. }) => {
+                    assert!(i < this.fields.count());
+
+                    // Reuse the fat `*T` type as its own thin pointer data field.
+                    // This provides information about, e.g., DST struct pointees
+                    // (which may have no non-DST form), and will work as long
+                    // as the `Abi` or `FieldsShape` is checked by users.
+                    if i == 0 {
+                        let nil = tcx.mk_unit();
+                        let unit_ptr_ty = if this.ty.is_unsafe_ptr() {
+                            tcx.mk_mut_ptr(nil)
+                        } else {
+                            tcx.mk_mut_ref(tcx.lifetimes.re_static, nil)
+                        };
+
+                        // NOTE(eddyb) using an empty `ParamEnv`, and `unwrap`-ing
+                        // the `Result` should always work because the type is
+                        // always either `*mut ()` or `&'static mut ()`.
+                        return TyMaybeWithLayout::TyAndLayout(TyAndLayout {
+                            ty: this.ty,
+                            ..tcx.layout_of(ty::ParamEnv::reveal_all().and(unit_ptr_ty)).unwrap()
+                        });
+                    }
+
+                    match tcx.struct_tail_erasing_lifetimes(pointee, cx.param_env()).kind() {
+                        ty::Slice(_) | ty::Str => TyMaybeWithLayout::Ty(tcx.types.usize),
+                        ty::Dynamic(_, _) => {
+                            TyMaybeWithLayout::Ty(tcx.mk_imm_ref(
+                                tcx.lifetimes.re_static,
+                                tcx.mk_array(tcx.types.usize, 3),
+                            ))
+                            /* FIXME: use actual fn pointers
+                            Warning: naively computing the number of entries in the
+                            vtable by counting the methods on the trait + methods on
+                            all parent traits does not work, because some methods can
+                            be not object safe and thus excluded from the vtable.
+                            Increase this counter if you tried to implement this but
+                            failed to do it without duplicating a lot of code from
+                            other places in the compiler: 2
+                            tcx.mk_tup(&[
+                                tcx.mk_array(tcx.types.usize, 3),
+                                tcx.mk_array(Option<fn()>),
+                            ])
+                            */
+                        }
+                        _ => bug!("TyAndLayout::field({:?}): not applicable", this),
+                    }
+                }
+
+                // Arrays and slices.
+                ty::Array(element, _) | ty::Slice(element) => TyMaybeWithLayout::Ty(element),
+                ty::Str => TyMaybeWithLayout::Ty(tcx.types.u8),
+
+                // Tuples, generators and closures.
+                ty::Closure(_, ref substs) => field_ty_or_layout(
+                    TyAndLayout { ty: substs.as_closure().tupled_upvars_ty(), ..this },
+                    cx,
+                    i,
+                ),
+
+                ty::Generator(def_id, ref substs, _) => match this.variants {
+                    Variants::Single { index } => TyMaybeWithLayout::Ty(
+                        substs
+                            .as_generator()
+                            .state_tys(def_id, tcx)
+                            .nth(index.as_usize())
+                            .unwrap()
+                            .nth(i)
+                            .unwrap(),
+                    ),
+                    Variants::Multiple { tag, tag_field, .. } => {
+                        if i == tag_field {
+                            return TyMaybeWithLayout::TyAndLayout(tag_layout(tag));
+                        }
+                        TyMaybeWithLayout::Ty(substs.as_generator().prefix_tys().nth(i).unwrap())
+                    }
+                },
+
+                ty::Tuple(tys) => TyMaybeWithLayout::Ty(tys[i]),
+
+                // ADTs.
+                ty::Adt(def, substs) => {
+                    match this.variants {
+                        Variants::Single { index } => {
+                            TyMaybeWithLayout::Ty(def.variant(index).fields[i].ty(tcx, substs))
+                        }
+
+                        // Discriminant field for enums (where applicable).
+                        Variants::Multiple { tag, .. } => {
+                            assert_eq!(i, 0);
+                            return TyMaybeWithLayout::TyAndLayout(tag_layout(tag));
+                        }
+                    }
+                }
+
+                ty::Projection(_)
+                | ty::Bound(..)
+                | ty::Placeholder(..)
+                | ty::Opaque(..)
+                | ty::Param(_)
+                | ty::Infer(_)
+                | ty::Error(_) => bug!("TyAndLayout::field: unexpected type `{}`", this.ty),
+            }
+        }
+
+        match field_ty_or_layout(this, cx, i) {
+            TyMaybeWithLayout::Ty(field_ty) => {
+                cx.tcx().layout_of(cx.param_env().and(field_ty)).unwrap_or_else(|e| {
+                    bug!(
+                        "failed to get layout for `{}`: {},\n\
+                         despite it being a field (#{}) of an existing layout: {:#?}",
+                        field_ty,
+                        e,
+                        i,
+                        this
+                    )
+                })
+            }
+            TyMaybeWithLayout::TyAndLayout(field_layout) => field_layout,
+        }
+    }
+
+    fn ty_and_layout_pointee_info_at(
+        this: TyAndLayout<'tcx>,
+        cx: &C,
+        offset: Size,
+    ) -> Option<PointeeInfo> {
+        let tcx = cx.tcx();
+        let param_env = cx.param_env();
+
+        let addr_space_of_ty = |ty: Ty<'tcx>| {
+            if ty.is_fn() { cx.data_layout().instruction_address_space } else { AddressSpace::DATA }
+        };
+
+        let pointee_info = match *this.ty.kind() {
+            ty::RawPtr(mt) if offset.bytes() == 0 => {
+                tcx.layout_of(param_env.and(mt.ty)).ok().map(|layout| PointeeInfo {
+                    size: layout.size,
+                    align: layout.align.abi,
+                    safe: None,
+                    address_space: addr_space_of_ty(mt.ty),
+                })
+            }
+            ty::FnPtr(fn_sig) if offset.bytes() == 0 => {
+                tcx.layout_of(param_env.and(tcx.mk_fn_ptr(fn_sig))).ok().map(|layout| PointeeInfo {
+                    size: layout.size,
+                    align: layout.align.abi,
+                    safe: None,
+                    address_space: cx.data_layout().instruction_address_space,
+                })
+            }
+            ty::Ref(_, ty, mt) if offset.bytes() == 0 => {
+                let address_space = addr_space_of_ty(ty);
+                let kind = if tcx.sess.opts.optimize == OptLevel::No {
+                    // Use conservative pointer kind if not optimizing. This saves us the
+                    // Freeze/Unpin queries, and can save time in the codegen backend (noalias
+                    // attributes in LLVM have compile-time cost even in unoptimized builds).
+                    PointerKind::SharedMutable
+                } else {
+                    match mt {
+                        hir::Mutability::Not => {
+                            if ty.is_freeze(tcx.at(DUMMY_SP), cx.param_env()) {
+                                PointerKind::Frozen
+                            } else {
+                                PointerKind::SharedMutable
+                            }
+                        }
+                        hir::Mutability::Mut => {
+                            // References to self-referential structures should not be considered
+                            // noalias, as another pointer to the structure can be obtained, that
+                            // is not based-on the original reference. We consider all !Unpin
+                            // types to be potentially self-referential here.
+                            if ty.is_unpin(tcx.at(DUMMY_SP), cx.param_env()) {
+                                PointerKind::UniqueBorrowed
+                            } else {
+                                PointerKind::UniqueBorrowedPinned
+                            }
+                        }
+                    }
+                };
+
+                tcx.layout_of(param_env.and(ty)).ok().map(|layout| PointeeInfo {
+                    size: layout.size,
+                    align: layout.align.abi,
+                    safe: Some(kind),
+                    address_space,
+                })
+            }
+
+            _ => {
+                let mut data_variant = match this.variants {
+                    // Within the discriminant field, only the niche itself is
+                    // always initialized, so we only check for a pointer at its
+                    // offset.
+                    //
+                    // If the niche is a pointer, it's either valid (according
+                    // to its type), or null (which the niche field's scalar
+                    // validity range encodes).  This allows using
+                    // `dereferenceable_or_null` for e.g., `Option<&T>`, and
+                    // this will continue to work as long as we don't start
+                    // using more niches than just null (e.g., the first page of
+                    // the address space, or unaligned pointers).
+                    Variants::Multiple {
+                        tag_encoding: TagEncoding::Niche { dataful_variant, .. },
+                        tag_field,
+                        ..
+                    } if this.fields.offset(tag_field) == offset => {
+                        Some(this.for_variant(cx, dataful_variant))
+                    }
+                    _ => Some(this),
+                };
+
+                if let Some(variant) = data_variant {
+                    // We're not interested in any unions.
+                    if let FieldsShape::Union(_) = variant.fields {
+                        data_variant = None;
+                    }
+                }
+
+                let mut result = None;
+
+                if let Some(variant) = data_variant {
+                    let ptr_end = offset + Pointer.size(cx);
+                    for i in 0..variant.fields.count() {
+                        let field_start = variant.fields.offset(i);
+                        if field_start <= offset {
+                            let field = variant.field(cx, i);
+                            result = field.to_result().ok().and_then(|field| {
+                                if ptr_end <= field_start + field.size {
+                                    // We found the right field, look inside it.
+                                    let field_info =
+                                        field.pointee_info_at(cx, offset - field_start);
+                                    field_info
+                                } else {
+                                    None
+                                }
+                            });
+                            if result.is_some() {
+                                break;
+                            }
+                        }
+                    }
+                }
+
+                // FIXME(eddyb) This should be for `ptr::Unique<T>`, not `Box<T>`.
+                if let Some(ref mut pointee) = result {
+                    if let ty::Adt(def, _) = this.ty.kind() {
+                        if def.is_box() && offset.bytes() == 0 {
+                            pointee.safe = Some(PointerKind::UniqueOwned);
+                        }
+                    }
+                }
+
+                result
+            }
+        };
+
+        debug!(
+            "pointee_info_at (offset={:?}, type kind: {:?}) => {:?}",
+            offset,
+            this.ty.kind(),
+            pointee_info
+        );
+
+        pointee_info
+    }
+
+    fn is_adt(this: TyAndLayout<'tcx>) -> bool {
+        matches!(this.ty.kind(), ty::Adt(..))
+    }
+
+    fn is_never(this: TyAndLayout<'tcx>) -> bool {
+        this.ty.kind() == &ty::Never
+    }
+
+    fn is_tuple(this: TyAndLayout<'tcx>) -> bool {
+        matches!(this.ty.kind(), ty::Tuple(..))
+    }
+
+    fn is_unit(this: TyAndLayout<'tcx>) -> bool {
+        matches!(this.ty.kind(), ty::Tuple(list) if list.len() == 0)
+    }
+}
+
+impl<'tcx> ty::Instance<'tcx> {
+    // NOTE(eddyb) this is private to avoid using it from outside of
+    // `fn_abi_of_instance` - any other uses are either too high-level
+    // for `Instance` (e.g. typeck would use `Ty::fn_sig` instead),
+    // or should go through `FnAbi` instead, to avoid losing any
+    // adjustments `fn_abi_of_instance` might be performing.
+    fn fn_sig_for_fn_abi(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> ty::PolyFnSig<'tcx> {
+        let ty = self.ty(tcx, param_env);
+        match *ty.kind() {
+            ty::FnDef(..) => {
+                // HACK(davidtwco,eddyb): This is a workaround for polymorphization considering
+                // parameters unused if they show up in the signature, but not in the `mir::Body`
+                // (i.e. due to being inside a projection that got normalized, see
+                // `src/test/ui/polymorphization/normalized_sig_types.rs`), and codegen not keeping
+                // track of a polymorphization `ParamEnv` to allow normalizing later.
+                let mut sig = match *ty.kind() {
+                    ty::FnDef(def_id, substs) => tcx
+                        .normalize_erasing_regions(tcx.param_env(def_id), tcx.bound_fn_sig(def_id))
+                        .subst(tcx, substs),
+                    _ => unreachable!(),
+                };
+
+                if let ty::InstanceDef::VTableShim(..) = self.def {
+                    // Modify `fn(self, ...)` to `fn(self: *mut Self, ...)`.
+                    sig = sig.map_bound(|mut sig| {
+                        let mut inputs_and_output = sig.inputs_and_output.to_vec();
+                        inputs_and_output[0] = tcx.mk_mut_ptr(inputs_and_output[0]);
+                        sig.inputs_and_output = tcx.intern_type_list(&inputs_and_output);
+                        sig
+                    });
+                }
+                sig
+            }
+            ty::Closure(def_id, substs) => {
+                let sig = substs.as_closure().sig();
+
+                let bound_vars = tcx.mk_bound_variable_kinds(
+                    sig.bound_vars()
+                        .iter()
+                        .chain(iter::once(ty::BoundVariableKind::Region(ty::BrEnv))),
+                );
+                let br = ty::BoundRegion {
+                    var: ty::BoundVar::from_usize(bound_vars.len() - 1),
+                    kind: ty::BoundRegionKind::BrEnv,
+                };
+                let env_region = ty::ReLateBound(ty::INNERMOST, br);
+                let env_ty = tcx.closure_env_ty(def_id, substs, env_region).unwrap();
+
+                let sig = sig.skip_binder();
+                ty::Binder::bind_with_vars(
+                    tcx.mk_fn_sig(
+                        iter::once(env_ty).chain(sig.inputs().iter().cloned()),
+                        sig.output(),
+                        sig.c_variadic,
+                        sig.unsafety,
+                        sig.abi,
+                    ),
+                    bound_vars,
+                )
+            }
+            ty::Generator(_, substs, _) => {
+                let sig = substs.as_generator().poly_sig();
+
+                let bound_vars = tcx.mk_bound_variable_kinds(
+                    sig.bound_vars()
+                        .iter()
+                        .chain(iter::once(ty::BoundVariableKind::Region(ty::BrEnv))),
+                );
+                let br = ty::BoundRegion {
+                    var: ty::BoundVar::from_usize(bound_vars.len() - 1),
+                    kind: ty::BoundRegionKind::BrEnv,
+                };
+                let env_region = ty::ReLateBound(ty::INNERMOST, br);
+                let env_ty = tcx.mk_mut_ref(tcx.mk_region(env_region), ty);
+
+                let pin_did = tcx.require_lang_item(LangItem::Pin, None);
+                let pin_adt_ref = tcx.adt_def(pin_did);
+                let pin_substs = tcx.intern_substs(&[env_ty.into()]);
+                let env_ty = tcx.mk_adt(pin_adt_ref, pin_substs);
+
+                let sig = sig.skip_binder();
+                let state_did = tcx.require_lang_item(LangItem::GeneratorState, None);
+                let state_adt_ref = tcx.adt_def(state_did);
+                let state_substs = tcx.intern_substs(&[sig.yield_ty.into(), sig.return_ty.into()]);
+                let ret_ty = tcx.mk_adt(state_adt_ref, state_substs);
+                ty::Binder::bind_with_vars(
+                    tcx.mk_fn_sig(
+                        [env_ty, sig.resume_ty].iter(),
+                        &ret_ty,
+                        false,
+                        hir::Unsafety::Normal,
+                        rustc_target::spec::abi::Abi::Rust,
+                    ),
+                    bound_vars,
+                )
+            }
+            _ => bug!("unexpected type {:?} in Instance::fn_sig", ty),
+        }
+    }
+}
+
+/// Calculates whether a function's ABI can unwind or not.
+///
+/// This takes two primary parameters:
+///
+/// * `codegen_fn_attr_flags` - these are flags calculated as part of the
+///   codegen attrs for a defined function. For function pointers this set of
+///   flags is the empty set. This is only applicable for Rust-defined
+///   functions, and generally isn't needed except for small optimizations where
+///   we try to say a function which otherwise might look like it could unwind
+///   doesn't actually unwind (such as for intrinsics and such).
+///
+/// * `abi` - this is the ABI that the function is defined with. This is the
+///   primary factor for determining whether a function can unwind or not.
+///
+/// Note that in this case unwinding is not necessarily panicking in Rust. Rust
+/// panics are implemented with unwinds on most platform (when
+/// `-Cpanic=unwind`), but this also accounts for `-Cpanic=abort` build modes.
+/// Notably unwinding is disallowed for more non-Rust ABIs unless it's
+/// specifically in the name (e.g. `"C-unwind"`). Unwinding within each ABI is
+/// defined for each ABI individually, but it always corresponds to some form of
+/// stack-based unwinding (the exact mechanism of which varies
+/// platform-by-platform).
+///
+/// Rust functions are classified whether or not they can unwind based on the
+/// active "panic strategy". In other words Rust functions are considered to
+/// unwind in `-Cpanic=unwind` mode and cannot unwind in `-Cpanic=abort` mode.
+/// Note that Rust supports intermingling panic=abort and panic=unwind code, but
+/// only if the final panic mode is panic=abort. In this scenario any code
+/// previously compiled assuming that a function can unwind is still correct, it
+/// just never happens to actually unwind at runtime.
+///
+/// This function's answer to whether or not a function can unwind is quite
+/// impactful throughout the compiler. This affects things like:
+///
+/// * Calling a function which can't unwind means codegen simply ignores any
+///   associated unwinding cleanup.
+/// * Calling a function which can unwind from a function which can't unwind
+///   causes the `abort_unwinding_calls` MIR pass to insert a landing pad that
+///   aborts the process.
+/// * This affects whether functions have the LLVM `nounwind` attribute, which
+///   affects various optimizations and codegen.
+///
+/// FIXME: this is actually buggy with respect to Rust functions. Rust functions
+/// compiled with `-Cpanic=unwind` and referenced from another crate compiled
+/// with `-Cpanic=abort` will look like they can't unwind when in fact they
+/// might (from a foreign exception or similar).
+#[inline]
+pub fn fn_can_unwind<'tcx>(tcx: TyCtxt<'tcx>, fn_def_id: Option<DefId>, abi: SpecAbi) -> bool {
+    if let Some(did) = fn_def_id {
+        // Special attribute for functions which can't unwind.
+        if tcx.codegen_fn_attrs(did).flags.contains(CodegenFnAttrFlags::NEVER_UNWIND) {
+            return false;
+        }
+
+        // With `-C panic=abort`, all non-FFI functions are required to not unwind.
+        //
+        // Note that this is true regardless ABI specified on the function -- a `extern "C-unwind"`
+        // function defined in Rust is also required to abort.
+        if tcx.sess.panic_strategy() == PanicStrategy::Abort && !tcx.is_foreign_item(did) {
+            return false;
+        }
+
+        // With -Z panic-in-drop=abort, drop_in_place never unwinds.
+        //
+        // This is not part of `codegen_fn_attrs` as it can differ between crates
+        // and therefore cannot be computed in core.
+        if tcx.sess.opts.unstable_opts.panic_in_drop == PanicStrategy::Abort {
+            if Some(did) == tcx.lang_items().drop_in_place_fn() {
+                return false;
+            }
+        }
+    }
+
+    // Otherwise if this isn't special then unwinding is generally determined by
+    // the ABI of the itself. ABIs like `C` have variants which also
+    // specifically allow unwinding (`C-unwind`), but not all platform-specific
+    // ABIs have such an option. Otherwise the only other thing here is Rust
+    // itself, and those ABIs are determined by the panic strategy configured
+    // for this compilation.
+    //
+    // Unfortunately at this time there's also another caveat. Rust [RFC
+    // 2945][rfc] has been accepted and is in the process of being implemented
+    // and stabilized. In this interim state we need to deal with historical
+    // rustc behavior as well as plan for future rustc behavior.
+    //
+    // Historically functions declared with `extern "C"` were marked at the
+    // codegen layer as `nounwind`. This happened regardless of `panic=unwind`
+    // or not. This is UB for functions in `panic=unwind` mode that then
+    // actually panic and unwind. Note that this behavior is true for both
+    // externally declared functions as well as Rust-defined function.
+    //
+    // To fix this UB rustc would like to change in the future to catch unwinds
+    // from function calls that may unwind within a Rust-defined `extern "C"`
+    // function and forcibly abort the process, thereby respecting the
+    // `nounwind` attribute emitted for `extern "C"`. This behavior change isn't
+    // ready to roll out, so determining whether or not the `C` family of ABIs
+    // unwinds is conditional not only on their definition but also whether the
+    // `#![feature(c_unwind)]` feature gate is active.
+    //
+    // Note that this means that unlike historical compilers rustc now, by
+    // default, unconditionally thinks that the `C` ABI may unwind. This will
+    // prevent some optimization opportunities, however, so we try to scope this
+    // change and only assume that `C` unwinds with `panic=unwind` (as opposed
+    // to `panic=abort`).
+    //
+    // Eventually the check against `c_unwind` here will ideally get removed and
+    // this'll be a little cleaner as it'll be a straightforward check of the
+    // ABI.
+    //
+    // [rfc]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md
+    use SpecAbi::*;
+    match abi {
+        C { unwind }
+        | System { unwind }
+        | Cdecl { unwind }
+        | Stdcall { unwind }
+        | Fastcall { unwind }
+        | Vectorcall { unwind }
+        | Thiscall { unwind }
+        | Aapcs { unwind }
+        | Win64 { unwind }
+        | SysV64 { unwind } => {
+            unwind
+                || (!tcx.features().c_unwind && tcx.sess.panic_strategy() == PanicStrategy::Unwind)
+        }
+        PtxKernel
+        | Msp430Interrupt
+        | X86Interrupt
+        | AmdGpuKernel
+        | EfiApi
+        | AvrInterrupt
+        | AvrNonBlockingInterrupt
+        | CCmseNonSecureCall
+        | Wasm
+        | RustIntrinsic
+        | PlatformIntrinsic
+        | Unadjusted => false,
+        Rust | RustCall | RustCold => tcx.sess.panic_strategy() == PanicStrategy::Unwind,
+    }
+}
+
+#[inline]
+pub fn conv_from_spec_abi(tcx: TyCtxt<'_>, abi: SpecAbi) -> Conv {
+    use rustc_target::spec::abi::Abi::*;
+    match tcx.sess.target.adjust_abi(abi) {
+        RustIntrinsic | PlatformIntrinsic | Rust | RustCall => Conv::Rust,
+        RustCold => Conv::RustCold,
+
+        // It's the ABI's job to select this, not ours.
+        System { .. } => bug!("system abi should be selected elsewhere"),
+        EfiApi => bug!("eficall abi should be selected elsewhere"),
+
+        Stdcall { .. } => Conv::X86Stdcall,
+        Fastcall { .. } => Conv::X86Fastcall,
+        Vectorcall { .. } => Conv::X86VectorCall,
+        Thiscall { .. } => Conv::X86ThisCall,
+        C { .. } => Conv::C,
+        Unadjusted => Conv::C,
+        Win64 { .. } => Conv::X86_64Win64,
+        SysV64 { .. } => Conv::X86_64SysV,
+        Aapcs { .. } => Conv::ArmAapcs,
+        CCmseNonSecureCall => Conv::CCmseNonSecureCall,
+        PtxKernel => Conv::PtxKernel,
+        Msp430Interrupt => Conv::Msp430Intr,
+        X86Interrupt => Conv::X86Intr,
+        AmdGpuKernel => Conv::AmdGpuKernel,
+        AvrInterrupt => Conv::AvrInterrupt,
+        AvrNonBlockingInterrupt => Conv::AvrNonBlockingInterrupt,
+        Wasm => Conv::C,
+
+        // These API constants ought to be more specific...
+        Cdecl { .. } => Conv::C,
+    }
+}
+
+/// Error produced by attempting to compute or adjust a `FnAbi`.
+#[derive(Copy, Clone, Debug, HashStable)]
+pub enum FnAbiError<'tcx> {
+    /// Error produced by a `layout_of` call, while computing `FnAbi` initially.
+    Layout(LayoutError<'tcx>),
+
+    /// Error produced by attempting to adjust a `FnAbi`, for a "foreign" ABI.
+    AdjustForForeignAbi(call::AdjustForForeignAbiError),
+}
+
+impl<'tcx> From<LayoutError<'tcx>> for FnAbiError<'tcx> {
+    fn from(err: LayoutError<'tcx>) -> Self {
+        Self::Layout(err)
+    }
+}
+
+impl From<call::AdjustForForeignAbiError> for FnAbiError<'_> {
+    fn from(err: call::AdjustForForeignAbiError) -> Self {
+        Self::AdjustForForeignAbi(err)
+    }
+}
+
+impl<'tcx> fmt::Display for FnAbiError<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::Layout(err) => err.fmt(f),
+            Self::AdjustForForeignAbi(err) => err.fmt(f),
+        }
+    }
+}
+
+// FIXME(eddyb) maybe use something like this for an unified `fn_abi_of`, not
+// just for error handling.
+#[derive(Debug)]
+pub enum FnAbiRequest<'tcx> {
+    OfFnPtr { sig: ty::PolyFnSig<'tcx>, extra_args: &'tcx ty::List<Ty<'tcx>> },
+    OfInstance { instance: ty::Instance<'tcx>, extra_args: &'tcx ty::List<Ty<'tcx>> },
+}
+
+/// Trait for contexts that want to be able to compute `FnAbi`s.
+/// This automatically gives access to `FnAbiOf`, through a blanket `impl`.
+pub trait FnAbiOfHelpers<'tcx>: LayoutOfHelpers<'tcx> {
+    /// The `&FnAbi`-wrapping type (or `&FnAbi` itself), which will be
+    /// returned from `fn_abi_of_*` (see also `handle_fn_abi_err`).
+    type FnAbiOfResult: MaybeResult<&'tcx FnAbi<'tcx, Ty<'tcx>>>;
+
+    /// Helper used for `fn_abi_of_*`, to adapt `tcx.fn_abi_of_*(...)` into a
+    /// `Self::FnAbiOfResult` (which does not need to be a `Result<...>`).
+    ///
+    /// Most `impl`s, which propagate `FnAbiError`s, should simply return `err`,
+    /// but this hook allows e.g. codegen to return only `&FnAbi` from its
+    /// `cx.fn_abi_of_*(...)`, without any `Result<...>` around it to deal with
+    /// (and any `FnAbiError`s are turned into fatal errors or ICEs).
+    fn handle_fn_abi_err(
+        &self,
+        err: FnAbiError<'tcx>,
+        span: Span,
+        fn_abi_request: FnAbiRequest<'tcx>,
+    ) -> <Self::FnAbiOfResult as MaybeResult<&'tcx FnAbi<'tcx, Ty<'tcx>>>>::Error;
+}
+
+/// Blanket extension trait for contexts that can compute `FnAbi`s.
+pub trait FnAbiOf<'tcx>: FnAbiOfHelpers<'tcx> {
+    /// Compute a `FnAbi` suitable for indirect calls, i.e. to `fn` pointers.
+    ///
+    /// NB: this doesn't handle virtual calls - those should use `fn_abi_of_instance`
+    /// instead, where the instance is an `InstanceDef::Virtual`.
+    #[inline]
+    fn fn_abi_of_fn_ptr(
+        &self,
+        sig: ty::PolyFnSig<'tcx>,
+        extra_args: &'tcx ty::List<Ty<'tcx>>,
+    ) -> Self::FnAbiOfResult {
+        // FIXME(eddyb) get a better `span` here.
+        let span = self.layout_tcx_at_span();
+        let tcx = self.tcx().at(span);
+
+        MaybeResult::from(tcx.fn_abi_of_fn_ptr(self.param_env().and((sig, extra_args))).map_err(
+            |err| self.handle_fn_abi_err(err, span, FnAbiRequest::OfFnPtr { sig, extra_args }),
+        ))
+    }
+
+    /// Compute a `FnAbi` suitable for declaring/defining an `fn` instance, and for
+    /// direct calls to an `fn`.
+    ///
+    /// NB: that includes virtual calls, which are represented by "direct calls"
+    /// to an `InstanceDef::Virtual` instance (of `<dyn Trait as Trait>::fn`).
+    #[inline]
+    fn fn_abi_of_instance(
+        &self,
+        instance: ty::Instance<'tcx>,
+        extra_args: &'tcx ty::List<Ty<'tcx>>,
+    ) -> Self::FnAbiOfResult {
+        // FIXME(eddyb) get a better `span` here.
+        let span = self.layout_tcx_at_span();
+        let tcx = self.tcx().at(span);
+
+        MaybeResult::from(
+            tcx.fn_abi_of_instance(self.param_env().and((instance, extra_args))).map_err(|err| {
+                // HACK(eddyb) at least for definitions of/calls to `Instance`s,
+                // we can get some kind of span even if one wasn't provided.
+                // However, we don't do this early in order to avoid calling
+                // `def_span` unconditionally (which may have a perf penalty).
+                let span = if !span.is_dummy() { span } else { tcx.def_span(instance.def_id()) };
+                self.handle_fn_abi_err(err, span, FnAbiRequest::OfInstance { instance, extra_args })
+            }),
+        )
+    }
+}
+
+impl<'tcx, C: FnAbiOfHelpers<'tcx>> FnAbiOf<'tcx> for C {}
+
+fn fn_abi_of_fn_ptr<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    query: ty::ParamEnvAnd<'tcx, (ty::PolyFnSig<'tcx>, &'tcx ty::List<Ty<'tcx>>)>,
+) -> Result<&'tcx FnAbi<'tcx, Ty<'tcx>>, FnAbiError<'tcx>> {
+    let (param_env, (sig, extra_args)) = query.into_parts();
+
+    LayoutCx { tcx, param_env }.fn_abi_new_uncached(sig, extra_args, None, None, false)
+}
+
+fn fn_abi_of_instance<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    query: ty::ParamEnvAnd<'tcx, (ty::Instance<'tcx>, &'tcx ty::List<Ty<'tcx>>)>,
+) -> Result<&'tcx FnAbi<'tcx, Ty<'tcx>>, FnAbiError<'tcx>> {
+    let (param_env, (instance, extra_args)) = query.into_parts();
+
+    let sig = instance.fn_sig_for_fn_abi(tcx, param_env);
+
+    let caller_location = if instance.def.requires_caller_location(tcx) {
+        Some(tcx.caller_location_ty())
+    } else {
+        None
+    };
+
+    LayoutCx { tcx, param_env }.fn_abi_new_uncached(
+        sig,
+        extra_args,
+        caller_location,
+        Some(instance.def_id()),
+        matches!(instance.def, ty::InstanceDef::Virtual(..)),
+    )
+}
+
+impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
+    // FIXME(eddyb) perhaps group the signature/type-containing (or all of them?)
+    // arguments of this method, into a separate `struct`.
+    fn fn_abi_new_uncached(
+        &self,
+        sig: ty::PolyFnSig<'tcx>,
+        extra_args: &[Ty<'tcx>],
+        caller_location: Option<Ty<'tcx>>,
+        fn_def_id: Option<DefId>,
+        // FIXME(eddyb) replace this with something typed, like an `enum`.
+        force_thin_self_ptr: bool,
+    ) -> Result<&'tcx FnAbi<'tcx, Ty<'tcx>>, FnAbiError<'tcx>> {
+        debug!("fn_abi_new_uncached({:?}, {:?})", sig, extra_args);
+
+        let sig = self.tcx.normalize_erasing_late_bound_regions(self.param_env, sig);
+
+        let conv = conv_from_spec_abi(self.tcx(), sig.abi);
+
+        let mut inputs = sig.inputs();
+        let extra_args = if sig.abi == RustCall {
+            assert!(!sig.c_variadic && extra_args.is_empty());
+
+            if let Some(input) = sig.inputs().last() {
+                if let ty::Tuple(tupled_arguments) = input.kind() {
+                    inputs = &sig.inputs()[0..sig.inputs().len() - 1];
+                    tupled_arguments
+                } else {
+                    bug!(
+                        "argument to function with \"rust-call\" ABI \
+                            is not a tuple"
+                    );
+                }
+            } else {
+                bug!(
+                    "argument to function with \"rust-call\" ABI \
+                        is not a tuple"
+                );
+            }
+        } else {
+            assert!(sig.c_variadic || extra_args.is_empty());
+            extra_args
+        };
+
+        let target = &self.tcx.sess.target;
+        let target_env_gnu_like = matches!(&target.env[..], "gnu" | "musl" | "uclibc");
+        let win_x64_gnu = target.os == "windows" && target.arch == "x86_64" && target.env == "gnu";
+        let linux_s390x_gnu_like =
+            target.os == "linux" && target.arch == "s390x" && target_env_gnu_like;
+        let linux_sparc64_gnu_like =
+            target.os == "linux" && target.arch == "sparc64" && target_env_gnu_like;
+        let linux_powerpc_gnu_like =
+            target.os == "linux" && target.arch == "powerpc" && target_env_gnu_like;
+        use SpecAbi::*;
+        let rust_abi = matches!(sig.abi, RustIntrinsic | PlatformIntrinsic | Rust | RustCall);
+
+        // Handle safe Rust thin and fat pointers.
+        let adjust_for_rust_scalar = |attrs: &mut ArgAttributes,
+                                      scalar: Scalar,
+                                      layout: TyAndLayout<'tcx>,
+                                      offset: Size,
+                                      is_return: bool| {
+            // Booleans are always a noundef i1 that needs to be zero-extended.
+            if scalar.is_bool() {
+                attrs.ext(ArgExtension::Zext);
+                attrs.set(ArgAttribute::NoUndef);
+                return;
+            }
+
+            // Scalars which have invalid values cannot be undef.
+            if !scalar.is_always_valid(self) {
+                attrs.set(ArgAttribute::NoUndef);
+            }
+
+            // Only pointer types handled below.
+            let Scalar::Initialized { value: Pointer, valid_range} = scalar else { return };
+
+            if !valid_range.contains(0) {
+                attrs.set(ArgAttribute::NonNull);
+            }
+
+            if let Some(pointee) = layout.pointee_info_at(self, offset) {
+                if let Some(kind) = pointee.safe {
+                    attrs.pointee_align = Some(pointee.align);
+
+                    // `Box` (`UniqueBorrowed`) are not necessarily dereferenceable
+                    // for the entire duration of the function as they can be deallocated
+                    // at any time. Same for shared mutable references. If LLVM had a
+                    // way to say "dereferenceable on entry" we could use it here.
+                    attrs.pointee_size = match kind {
+                        PointerKind::UniqueBorrowed
+                        | PointerKind::UniqueBorrowedPinned
+                        | PointerKind::Frozen => pointee.size,
+                        PointerKind::SharedMutable | PointerKind::UniqueOwned => Size::ZERO,
+                    };
+
+                    // `Box`, `&T`, and `&mut T` cannot be undef.
+                    // Note that this only applies to the value of the pointer itself;
+                    // this attribute doesn't make it UB for the pointed-to data to be undef.
+                    attrs.set(ArgAttribute::NoUndef);
+
+                    // The aliasing rules for `Box<T>` are still not decided, but currently we emit
+                    // `noalias` for it. This can be turned off using an unstable flag.
+                    // See https://github.com/rust-lang/unsafe-code-guidelines/issues/326
+                    let noalias_for_box =
+                        self.tcx().sess.opts.unstable_opts.box_noalias.unwrap_or(true);
+
+                    // `&mut` pointer parameters never alias other parameters,
+                    // or mutable global data
+                    //
+                    // `&T` where `T` contains no `UnsafeCell<U>` is immutable,
+                    // and can be marked as both `readonly` and `noalias`, as
+                    // LLVM's definition of `noalias` is based solely on memory
+                    // dependencies rather than pointer equality
+                    //
+                    // Due to past miscompiles in LLVM, we apply a separate NoAliasMutRef attribute
+                    // for UniqueBorrowed arguments, so that the codegen backend can decide whether
+                    // or not to actually emit the attribute. It can also be controlled with the
+                    // `-Zmutable-noalias` debugging option.
+                    let no_alias = match kind {
+                        PointerKind::SharedMutable
+                        | PointerKind::UniqueBorrowed
+                        | PointerKind::UniqueBorrowedPinned => false,
+                        PointerKind::UniqueOwned => noalias_for_box,
+                        PointerKind::Frozen => !is_return,
+                    };
+                    if no_alias {
+                        attrs.set(ArgAttribute::NoAlias);
+                    }
+
+                    if kind == PointerKind::Frozen && !is_return {
+                        attrs.set(ArgAttribute::ReadOnly);
+                    }
+
+                    if kind == PointerKind::UniqueBorrowed && !is_return {
+                        attrs.set(ArgAttribute::NoAliasMutRef);
+                    }
+                }
+            }
+        };
+
+        let arg_of = |ty: Ty<'tcx>, arg_idx: Option<usize>| -> Result<_, FnAbiError<'tcx>> {
+            let is_return = arg_idx.is_none();
+
+            let layout = self.layout_of(ty)?;
+            let layout = if force_thin_self_ptr && arg_idx == Some(0) {
+                // Don't pass the vtable, it's not an argument of the virtual fn.
+                // Instead, pass just the data pointer, but give it the type `*const/mut dyn Trait`
+                // or `&/&mut dyn Trait` because this is special-cased elsewhere in codegen
+                make_thin_self_ptr(self, layout)
+            } else {
+                layout
+            };
+
+            let mut arg = ArgAbi::new(self, layout, |layout, scalar, offset| {
+                let mut attrs = ArgAttributes::new();
+                adjust_for_rust_scalar(&mut attrs, scalar, *layout, offset, is_return);
+                attrs
+            });
+
+            if arg.layout.is_zst() {
+                // For some forsaken reason, x86_64-pc-windows-gnu
+                // doesn't ignore zero-sized struct arguments.
+                // The same is true for {s390x,sparc64,powerpc}-unknown-linux-{gnu,musl,uclibc}.
+                if is_return
+                    || rust_abi
+                    || (!win_x64_gnu
+                        && !linux_s390x_gnu_like
+                        && !linux_sparc64_gnu_like
+                        && !linux_powerpc_gnu_like)
+                {
+                    arg.mode = PassMode::Ignore;
+                }
+            }
+
+            Ok(arg)
+        };
+
+        let mut fn_abi = FnAbi {
+            ret: arg_of(sig.output(), None)?,
+            args: inputs
+                .iter()
+                .copied()
+                .chain(extra_args.iter().copied())
+                .chain(caller_location)
+                .enumerate()
+                .map(|(i, ty)| arg_of(ty, Some(i)))
+                .collect::<Result<_, _>>()?,
+            c_variadic: sig.c_variadic,
+            fixed_count: inputs.len(),
+            conv,
+            can_unwind: fn_can_unwind(self.tcx(), fn_def_id, sig.abi),
+        };
+        self.fn_abi_adjust_for_abi(&mut fn_abi, sig.abi)?;
+        debug!("fn_abi_new_uncached = {:?}", fn_abi);
+        Ok(self.tcx.arena.alloc(fn_abi))
+    }
+
+    fn fn_abi_adjust_for_abi(
+        &self,
+        fn_abi: &mut FnAbi<'tcx, Ty<'tcx>>,
+        abi: SpecAbi,
+    ) -> Result<(), FnAbiError<'tcx>> {
+        if abi == SpecAbi::Unadjusted {
+            return Ok(());
+        }
+
+        if abi == SpecAbi::Rust
+            || abi == SpecAbi::RustCall
+            || abi == SpecAbi::RustIntrinsic
+            || abi == SpecAbi::PlatformIntrinsic
+        {
+            let fixup = |arg: &mut ArgAbi<'tcx, Ty<'tcx>>| {
+                if arg.is_ignore() {
+                    return;
+                }
+
+                match arg.layout.abi {
+                    Abi::Aggregate { .. } => {}
+
+                    // This is a fun case! The gist of what this is doing is
+                    // that we want callers and callees to always agree on the
+                    // ABI of how they pass SIMD arguments. If we were to *not*
+                    // make these arguments indirect then they'd be immediates
+                    // in LLVM, which means that they'd used whatever the
+                    // appropriate ABI is for the callee and the caller. That
+                    // means, for example, if the caller doesn't have AVX
+                    // enabled but the callee does, then passing an AVX argument
+                    // across this boundary would cause corrupt data to show up.
+                    //
+                    // This problem is fixed by unconditionally passing SIMD
+                    // arguments through memory between callers and callees
+                    // which should get them all to agree on ABI regardless of
+                    // target feature sets. Some more information about this
+                    // issue can be found in #44367.
+                    //
+                    // Note that the platform intrinsic ABI is exempt here as
+                    // that's how we connect up to LLVM and it's unstable
+                    // anyway, we control all calls to it in libstd.
+                    Abi::Vector { .. }
+                        if abi != SpecAbi::PlatformIntrinsic
+                            && self.tcx.sess.target.simd_types_indirect =>
+                    {
+                        arg.make_indirect();
+                        return;
+                    }
+
+                    _ => return,
+                }
+
+                let size = arg.layout.size;
+                if arg.layout.is_unsized() || size > Pointer.size(self) {
+                    arg.make_indirect();
+                } else {
+                    // We want to pass small aggregates as immediates, but using
+                    // a LLVM aggregate type for this leads to bad optimizations,
+                    // so we pick an appropriately sized integer type instead.
+                    arg.cast_to(Reg { kind: RegKind::Integer, size });
+                }
+            };
+            fixup(&mut fn_abi.ret);
+            for arg in &mut fn_abi.args {
+                fixup(arg);
+            }
+        } else {
+            fn_abi.adjust_for_foreign_abi(self, abi)?;
+        }
+
+        Ok(())
+    }
+}
+
+fn make_thin_self_ptr<'tcx>(
+    cx: &(impl HasTyCtxt<'tcx> + HasParamEnv<'tcx>),
+    layout: TyAndLayout<'tcx>,
+) -> TyAndLayout<'tcx> {
+    let tcx = cx.tcx();
+    let fat_pointer_ty = if layout.is_unsized() {
+        // unsized `self` is passed as a pointer to `self`
+        // FIXME (mikeyhew) change this to use &own if it is ever added to the language
+        tcx.mk_mut_ptr(layout.ty)
+    } else {
+        match layout.abi {
+            Abi::ScalarPair(..) => (),
+            _ => bug!("receiver type has unsupported layout: {:?}", layout),
+        }
+
+        // In the case of Rc<Self>, we need to explicitly pass a *mut RcBox<Self>
+        // with a Scalar (not ScalarPair) ABI. This is a hack that is understood
+        // elsewhere in the compiler as a method on a `dyn Trait`.
+        // To get the type `*mut RcBox<Self>`, we just keep unwrapping newtypes until we
+        // get a built-in pointer type
+        let mut fat_pointer_layout = layout;
+        'descend_newtypes: while !fat_pointer_layout.ty.is_unsafe_ptr()
+            && !fat_pointer_layout.ty.is_region_ptr()
+        {
+            for i in 0..fat_pointer_layout.fields.count() {
+                let field_layout = fat_pointer_layout.field(cx, i);
+
+                if !field_layout.is_zst() {
+                    fat_pointer_layout = field_layout;
+                    continue 'descend_newtypes;
+                }
+            }
+
+            bug!("receiver has no non-zero-sized fields {:?}", fat_pointer_layout);
+        }
+
+        fat_pointer_layout.ty
+    };
+
+    // we now have a type like `*mut RcBox<dyn Trait>`
+    // change its layout to that of `*mut ()`, a thin pointer, but keep the same type
+    // this is understood as a special case elsewhere in the compiler
+    let unit_ptr_ty = tcx.mk_mut_ptr(tcx.mk_unit());
+
+    TyAndLayout {
+        ty: fat_pointer_ty,
+
+        // NOTE(eddyb) using an empty `ParamEnv`, and `unwrap`-ing the `Result`
+        // should always work because the type is always `*mut ()`.
+        ..tcx.layout_of(ty::ParamEnv::reveal_all().and(unit_ptr_ty)).unwrap()
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/list.rs b/compiler/rustc_middle/src/ty/list.rs
new file mode 100644
index 000000000..db3b5cfd1
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/list.rs
@@ -0,0 +1,215 @@
+use crate::arena::Arena;
+use rustc_serialize::{Encodable, Encoder};
+use std::alloc::Layout;
+use std::cmp::Ordering;
+use std::fmt;
+use std::hash::{Hash, Hasher};
+use std::iter;
+use std::mem;
+use std::ops::Deref;
+use std::ptr;
+use std::slice;
+
+/// `List<T>` is a bit like `&[T]`, but with some critical differences.
+/// - IMPORTANT: Every `List<T>` is *required* to have unique contents. The
+///   type's correctness relies on this, *but it does not enforce it*.
+///   Therefore, any code that creates a `List<T>` must ensure uniqueness
+///   itself. In practice this is achieved by interning.
+/// - The length is stored within the `List<T>`, so `&List<Ty>` is a thin
+///   pointer.
+/// - Because of this, you cannot get a `List<T>` that is a sub-list of another
+///   `List<T>`. You can get a sub-slice `&[T]`, however.
+/// - `List<T>` can be used with `CopyTaggedPtr`, which is useful within
+///   structs whose size must be minimized.
+/// - Because of the uniqueness assumption, we can use the address of a
+///   `List<T>` for faster equality comparisons and hashing.
+/// - `T` must be `Copy`. This lets `List<T>` be stored in a dropless arena and
+///   iterators return a `T` rather than a `&T`.
+/// - `T` must not be zero-sized.
+#[repr(C)]
+pub struct List<T> {
+    len: usize,
+
+    /// Although this claims to be a zero-length array, in practice `len`
+    /// elements are actually present.
+    data: [T; 0],
+
+    opaque: OpaqueListContents,
+}
+
+extern "C" {
+    /// A dummy type used to force `List` to be unsized while not requiring
+    /// references to it be wide pointers.
+    type OpaqueListContents;
+}
+
+impl<T> List<T> {
+    /// Returns a reference to the (unique, static) empty list.
+    #[inline(always)]
+    pub fn empty<'a>() -> &'a List<T> {
+        #[repr(align(64))]
+        struct MaxAlign;
+
+        assert!(mem::align_of::<T>() <= mem::align_of::<MaxAlign>());
+
+        #[repr(C)]
+        struct InOrder<T, U>(T, U);
+
+        // The empty slice is static and contains a single `0` usize (for the
+        // length) that is 64-byte aligned, thus featuring the necessary
+        // trailing padding for elements with up to 64-byte alignment.
+        static EMPTY_SLICE: InOrder<usize, MaxAlign> = InOrder(0, MaxAlign);
+        unsafe { &*(&EMPTY_SLICE as *const _ as *const List<T>) }
+    }
+
+    pub fn len(&self) -> usize {
+        self.len
+    }
+}
+
+impl<T: Copy> List<T> {
+    /// Allocates a list from `arena` and copies the contents of `slice` into it.
+    ///
+    /// WARNING: the contents *must be unique*, such that no list with these
+    /// contents has been previously created. If not, operations such as `eq`
+    /// and `hash` might give incorrect results.
+    ///
+    /// Panics if `T` is `Drop`, or `T` is zero-sized, or the slice is empty
+    /// (because the empty list exists statically, and is available via
+    /// `empty()`).
+    #[inline]
+    pub(super) fn from_arena<'tcx>(arena: &'tcx Arena<'tcx>, slice: &[T]) -> &'tcx List<T> {
+        assert!(!mem::needs_drop::<T>());
+        assert!(mem::size_of::<T>() != 0);
+        assert!(!slice.is_empty());
+
+        let (layout, _offset) =
+            Layout::new::<usize>().extend(Layout::for_value::<[T]>(slice)).unwrap();
+        let mem = arena.dropless.alloc_raw(layout) as *mut List<T>;
+        unsafe {
+            // Write the length
+            ptr::addr_of_mut!((*mem).len).write(slice.len());
+
+            // Write the elements
+            ptr::addr_of_mut!((*mem).data)
+                .cast::<T>()
+                .copy_from_nonoverlapping(slice.as_ptr(), slice.len());
+
+            &*mem
+        }
+    }
+
+    // If this method didn't exist, we would use `slice.iter` due to
+    // deref coercion.
+    //
+    // This would be weird, as `self.into_iter` iterates over `T` directly.
+    #[inline(always)]
+    pub fn iter(&self) -> <&'_ List<T> as IntoIterator>::IntoIter {
+        self.into_iter()
+    }
+}
+
+impl<T: fmt::Debug> fmt::Debug for List<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+impl<S: Encoder, T: Encodable<S>> Encodable<S> for List<T> {
+    #[inline]
+    fn encode(&self, s: &mut S) {
+        (**self).encode(s);
+    }
+}
+
+impl<T: PartialEq> PartialEq for List<T> {
+    #[inline]
+    fn eq(&self, other: &List<T>) -> bool {
+        // Pointer equality implies list equality (due to the unique contents
+        // assumption).
+        ptr::eq(self, other)
+    }
+}
+
+impl<T: Eq> Eq for List<T> {}
+
+impl<T> Ord for List<T>
+where
+    T: Ord,
+{
+    fn cmp(&self, other: &List<T>) -> Ordering {
+        // Pointer equality implies list equality (due to the unique contents
+        // assumption), but the contents must be compared otherwise.
+        if self == other { Ordering::Equal } else { <[T] as Ord>::cmp(&**self, &**other) }
+    }
+}
+
+impl<T> PartialOrd for List<T>
+where
+    T: PartialOrd,
+{
+    fn partial_cmp(&self, other: &List<T>) -> Option<Ordering> {
+        // Pointer equality implies list equality (due to the unique contents
+        // assumption), but the contents must be compared otherwise.
+        if self == other {
+            Some(Ordering::Equal)
+        } else {
+            <[T] as PartialOrd>::partial_cmp(&**self, &**other)
+        }
+    }
+}
+
+impl<T> Hash for List<T> {
+    #[inline]
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        // Pointer hashing is sufficient (due to the unique contents
+        // assumption).
+        (self as *const List<T>).hash(s)
+    }
+}
+
+impl<T> Deref for List<T> {
+    type Target = [T];
+    #[inline(always)]
+    fn deref(&self) -> &[T] {
+        self.as_ref()
+    }
+}
+
+impl<T> AsRef<[T]> for List<T> {
+    #[inline(always)]
+    fn as_ref(&self) -> &[T] {
+        unsafe { slice::from_raw_parts(self.data.as_ptr(), self.len) }
+    }
+}
+
+impl<'a, T: Copy> IntoIterator for &'a List<T> {
+    type Item = T;
+    type IntoIter = iter::Copied<<&'a [T] as IntoIterator>::IntoIter>;
+    #[inline(always)]
+    fn into_iter(self) -> Self::IntoIter {
+        self[..].iter().copied()
+    }
+}
+
+unsafe impl<T: Sync> Sync for List<T> {}
+
+unsafe impl<'a, T: 'a> rustc_data_structures::tagged_ptr::Pointer for &'a List<T> {
+    const BITS: usize = std::mem::align_of::<usize>().trailing_zeros() as usize;
+
+    #[inline]
+    fn into_usize(self) -> usize {
+        self as *const List<T> as usize
+    }
+
+    #[inline]
+    unsafe fn from_usize(ptr: usize) -> &'a List<T> {
+        &*(ptr as *const List<T>)
+    }
+
+    unsafe fn with_ref<R, F: FnOnce(&Self) -> R>(ptr: usize, f: F) -> R {
+        // `Self` is `&'a List<T>` which impls `Copy`, so this is fine.
+        let ptr = Self::from_usize(ptr);
+        f(&ptr)
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
new file mode 100644
index 000000000..02da02568
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -0,0 +1,2518 @@
+//! Defines how the compiler represents types internally.
+//!
+//! Two important entities in this module are:
+//!
+//! - [`rustc_middle::ty::Ty`], used to represent the semantics of a type.
+//! - [`rustc_middle::ty::TyCtxt`], the central data structure in the compiler.
+//!
+//! For more information, see ["The `ty` module: representing types"] in the rustc-dev-guide.
+//!
+//! ["The `ty` module: representing types"]: https://rustc-dev-guide.rust-lang.org/ty.html
+
+pub use self::fold::{FallibleTypeFolder, TypeFoldable, TypeFolder, TypeSuperFoldable};
+pub use self::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitor};
+pub use self::AssocItemContainer::*;
+pub use self::BorrowKind::*;
+pub use self::IntVarValue::*;
+pub use self::Variance::*;
+use crate::metadata::ModChild;
+use crate::middle::privacy::AccessLevels;
+use crate::mir::{Body, GeneratorLayout};
+use crate::traits::{self, Reveal};
+use crate::ty;
+use crate::ty::fast_reject::SimplifiedType;
+use crate::ty::util::Discr;
+pub use adt::*;
+pub use assoc::*;
+pub use generics::*;
+use rustc_ast as ast;
+use rustc_ast::node_id::NodeMap;
+use rustc_attr as attr;
+use rustc_data_structures::fingerprint::Fingerprint;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, FxIndexSet};
+use rustc_data_structures::intern::{Interned, WithStableHash};
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::tagged_ptr::CopyTaggedPtr;
+use rustc_hir as hir;
+use rustc_hir::def::{CtorKind, CtorOf, DefKind, LifetimeRes, Res};
+use rustc_hir::def_id::{CrateNum, DefId, LocalDefId, LocalDefIdMap};
+use rustc_hir::Node;
+use rustc_index::vec::IndexVec;
+use rustc_macros::HashStable;
+use rustc_query_system::ich::StableHashingContext;
+use rustc_span::hygiene::MacroKind;
+use rustc_span::symbol::{kw, sym, Ident, Symbol};
+use rustc_span::{ExpnId, Span};
+use rustc_target::abi::{Align, VariantIdx};
+pub use subst::*;
+pub use vtable::*;
+
+use std::fmt::Debug;
+use std::hash::{Hash, Hasher};
+use std::ops::ControlFlow;
+use std::{fmt, str};
+
+pub use crate::ty::diagnostics::*;
+pub use rustc_type_ir::InferTy::*;
+pub use rustc_type_ir::RegionKind::*;
+pub use rustc_type_ir::TyKind::*;
+pub use rustc_type_ir::*;
+
+pub use self::binding::BindingMode;
+pub use self::binding::BindingMode::*;
+pub use self::closure::{
+    is_ancestor_or_same_capture, place_to_string_for_capture, BorrowKind, CaptureInfo,
+    CapturedPlace, ClosureKind, MinCaptureInformationMap, MinCaptureList,
+    RootVariableMinCaptureList, UpvarCapture, UpvarCaptureMap, UpvarId, UpvarListMap, UpvarPath,
+    CAPTURE_STRUCT_LOCAL,
+};
+pub use self::consts::{
+    Const, ConstInt, ConstKind, ConstS, InferConst, ScalarInt, Unevaluated, ValTree,
+};
+pub use self::context::{
+    tls, CanonicalUserType, CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations,
+    CtxtInterners, DelaySpanBugEmitted, FreeRegionInfo, GeneratorDiagnosticData,
+    GeneratorInteriorTypeCause, GlobalCtxt, Lift, OnDiskCache, TyCtxt, TypeckResults, UserType,
+    UserTypeAnnotationIndex,
+};
+pub use self::instance::{Instance, InstanceDef};
+pub use self::list::List;
+pub use self::parameterized::ParameterizedOverTcx;
+pub use self::rvalue_scopes::RvalueScopes;
+pub use self::sty::BoundRegionKind::*;
+pub use self::sty::{
+    Article, Binder, BoundRegion, BoundRegionKind, BoundTy, BoundTyKind, BoundVar,
+    BoundVariableKind, CanonicalPolyFnSig, ClosureSubsts, ClosureSubstsParts, ConstVid,
+    EarlyBinder, EarlyBoundRegion, ExistentialPredicate, ExistentialProjection,
+    ExistentialTraitRef, FnSig, FreeRegion, GenSig, GeneratorSubsts, GeneratorSubstsParts,
+    InlineConstSubsts, InlineConstSubstsParts, ParamConst, ParamTy, PolyExistentialProjection,
+    PolyExistentialTraitRef, PolyFnSig, PolyGenSig, PolyTraitRef, ProjectionTy, Region, RegionKind,
+    RegionVid, TraitRef, TyKind, TypeAndMut, UpvarSubsts, VarianceDiagInfo,
+};
+pub use self::trait_def::TraitDef;
+
+pub mod _match;
+pub mod abstract_const;
+pub mod adjustment;
+pub mod binding;
+pub mod cast;
+pub mod codec;
+pub mod error;
+pub mod fast_reject;
+pub mod flags;
+pub mod fold;
+pub mod inhabitedness;
+pub mod layout;
+pub mod normalize_erasing_regions;
+pub mod print;
+pub mod query;
+pub mod relate;
+pub mod subst;
+pub mod trait_def;
+pub mod util;
+pub mod visit;
+pub mod vtable;
+pub mod walk;
+
+mod adt;
+mod assoc;
+mod closure;
+mod consts;
+mod context;
+mod diagnostics;
+mod erase_regions;
+mod generics;
+mod impls_ty;
+mod instance;
+mod list;
+mod parameterized;
+mod rvalue_scopes;
+mod structural_impls;
+mod sty;
+
+// Data types
+
+pub type RegisteredTools = FxHashSet<Ident>;
+
+#[derive(Debug)]
+pub struct ResolverOutputs {
+    pub visibilities: FxHashMap<LocalDefId, Visibility>,
+    /// This field is used to decide whether we should make `PRIVATE_IN_PUBLIC` a hard error.
+    pub has_pub_restricted: bool,
+    /// Item with a given `LocalDefId` was defined during macro expansion with ID `ExpnId`.
+    pub expn_that_defined: FxHashMap<LocalDefId, ExpnId>,
+    /// Reference span for definitions.
+    pub source_span: IndexVec<LocalDefId, Span>,
+    pub access_levels: AccessLevels,
+    pub extern_crate_map: FxHashMap<LocalDefId, CrateNum>,
+    pub maybe_unused_trait_imports: FxIndexSet<LocalDefId>,
+    pub maybe_unused_extern_crates: Vec<(LocalDefId, Span)>,
+    pub reexport_map: FxHashMap<LocalDefId, Vec<ModChild>>,
+    pub glob_map: FxHashMap<LocalDefId, FxHashSet<Symbol>>,
+    /// Extern prelude entries. The value is `true` if the entry was introduced
+    /// via `extern crate` item and not `--extern` option or compiler built-in.
+    pub extern_prelude: FxHashMap<Symbol, bool>,
+    pub main_def: Option<MainDefinition>,
+    pub trait_impls: FxIndexMap<DefId, Vec<LocalDefId>>,
+    /// A list of proc macro LocalDefIds, written out in the order in which
+    /// they are declared in the static array generated by proc_macro_harness.
+    pub proc_macros: Vec<LocalDefId>,
+    /// Mapping from ident span to path span for paths that don't exist as written, but that
+    /// exist under `std`. For example, wrote `str::from_utf8` instead of `std::str::from_utf8`.
+    pub confused_type_with_std_module: FxHashMap<Span, Span>,
+    pub registered_tools: RegisteredTools,
+}
+
+/// Resolutions that should only be used for lowering.
+/// This struct is meant to be consumed by lowering.
+#[derive(Debug)]
+pub struct ResolverAstLowering {
+    pub legacy_const_generic_args: FxHashMap<DefId, Option<Vec<usize>>>,
+
+    /// Resolutions for nodes that have a single resolution.
+    pub partial_res_map: NodeMap<hir::def::PartialRes>,
+    /// Resolutions for import nodes, which have multiple resolutions in different namespaces.
+    pub import_res_map: NodeMap<hir::def::PerNS<Option<Res<ast::NodeId>>>>,
+    /// Resolutions for labels (node IDs of their corresponding blocks or loops).
+    pub label_res_map: NodeMap<ast::NodeId>,
+    /// Resolutions for lifetimes.
+    pub lifetimes_res_map: NodeMap<LifetimeRes>,
+    /// Mapping from generics `def_id`s to TAIT generics `def_id`s.
+    /// For each captured lifetime (e.g., 'a), we create a new lifetime parameter that is a generic
+    /// defined on the TAIT, so we have type Foo<'a1> = ... and we establish a mapping in this
+    /// field from the original parameter 'a to the new parameter 'a1.
+    pub generics_def_id_map: Vec<FxHashMap<LocalDefId, LocalDefId>>,
+    /// Lifetime parameters that lowering will have to introduce.
+    pub extra_lifetime_params_map: NodeMap<Vec<(Ident, ast::NodeId, LifetimeRes)>>,
+
+    pub next_node_id: ast::NodeId,
+
+    pub node_id_to_def_id: FxHashMap<ast::NodeId, LocalDefId>,
+    pub def_id_to_node_id: IndexVec<LocalDefId, ast::NodeId>,
+
+    pub trait_map: NodeMap<Vec<hir::TraitCandidate>>,
+    /// A small map keeping true kinds of built-in macros that appear to be fn-like on
+    /// the surface (`macro` items in libcore), but are actually attributes or derives.
+    pub builtin_macro_kinds: FxHashMap<LocalDefId, MacroKind>,
+}
+
+#[derive(Clone, Copy, Debug)]
+pub struct MainDefinition {
+    pub res: Res<ast::NodeId>,
+    pub is_import: bool,
+    pub span: Span,
+}
+
+impl MainDefinition {
+    pub fn opt_fn_def_id(self) -> Option<DefId> {
+        if let Res::Def(DefKind::Fn, def_id) = self.res { Some(def_id) } else { None }
+    }
+}
+
+/// The "header" of an impl is everything outside the body: a Self type, a trait
+/// ref (in the case of a trait impl), and a set of predicates (from the
+/// bounds / where-clauses).
+#[derive(Clone, Debug, TypeFoldable, TypeVisitable)]
+pub struct ImplHeader<'tcx> {
+    pub impl_def_id: DefId,
+    pub self_ty: Ty<'tcx>,
+    pub trait_ref: Option<TraitRef<'tcx>>,
+    pub predicates: Vec<Predicate<'tcx>>,
+}
+
+#[derive(Copy, Clone, Debug, TypeFoldable, TypeVisitable)]
+pub enum ImplSubject<'tcx> {
+    Trait(TraitRef<'tcx>),
+    Inherent(Ty<'tcx>),
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash, TyEncodable, TyDecodable, HashStable, Debug)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub enum ImplPolarity {
+    /// `impl Trait for Type`
+    Positive,
+    /// `impl !Trait for Type`
+    Negative,
+    /// `#[rustc_reservation_impl] impl Trait for Type`
+    ///
+    /// This is a "stability hack", not a real Rust feature.
+    /// See #64631 for details.
+    Reservation,
+}
+
+impl ImplPolarity {
+    /// Flips polarity by turning `Positive` into `Negative` and `Negative` into `Positive`.
+    pub fn flip(&self) -> Option<ImplPolarity> {
+        match self {
+            ImplPolarity::Positive => Some(ImplPolarity::Negative),
+            ImplPolarity::Negative => Some(ImplPolarity::Positive),
+            ImplPolarity::Reservation => None,
+        }
+    }
+}
+
+impl fmt::Display for ImplPolarity {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::Positive => f.write_str("positive"),
+            Self::Negative => f.write_str("negative"),
+            Self::Reservation => f.write_str("reservation"),
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Copy, Hash, Encodable, Decodable, HashStable)]
+pub enum Visibility {
+    /// Visible everywhere (including in other crates).
+    Public,
+    /// Visible only in the given crate-local module.
+    Restricted(DefId),
+    /// Not visible anywhere in the local crate. This is the visibility of private external items.
+    Invisible,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable, TyEncodable, TyDecodable)]
+pub enum BoundConstness {
+    /// `T: Trait`
+    NotConst,
+    /// `T: ~const Trait`
+    ///
+    /// Requires resolving to const only when we are in a const context.
+    ConstIfConst,
+}
+
+impl BoundConstness {
+    /// Reduce `self` and `constness` to two possible combined states instead of four.
+    pub fn and(&mut self, constness: hir::Constness) -> hir::Constness {
+        match (constness, self) {
+            (hir::Constness::Const, BoundConstness::ConstIfConst) => hir::Constness::Const,
+            (_, this) => {
+                *this = BoundConstness::NotConst;
+                hir::Constness::NotConst
+            }
+        }
+    }
+}
+
+impl fmt::Display for BoundConstness {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::NotConst => f.write_str("normal"),
+            Self::ConstIfConst => f.write_str("`~const`"),
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Copy, Hash, TyEncodable, TyDecodable, HashStable)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct ClosureSizeProfileData<'tcx> {
+    /// Tuple containing the types of closure captures before the feature `capture_disjoint_fields`
+    pub before_feature_tys: Ty<'tcx>,
+    /// Tuple containing the types of closure captures after the feature `capture_disjoint_fields`
+    pub after_feature_tys: Ty<'tcx>,
+}
+
+pub trait DefIdTree: Copy {
+    fn opt_parent(self, id: DefId) -> Option<DefId>;
+
+    #[inline]
+    #[track_caller]
+    fn parent(self, id: DefId) -> DefId {
+        match self.opt_parent(id) {
+            Some(id) => id,
+            // not `unwrap_or_else` to avoid breaking caller tracking
+            None => bug!("{id:?} doesn't have a parent"),
+        }
+    }
+
+    #[inline]
+    #[track_caller]
+    fn opt_local_parent(self, id: LocalDefId) -> Option<LocalDefId> {
+        self.opt_parent(id.to_def_id()).map(DefId::expect_local)
+    }
+
+    #[inline]
+    #[track_caller]
+    fn local_parent(self, id: LocalDefId) -> LocalDefId {
+        self.parent(id.to_def_id()).expect_local()
+    }
+
+    fn is_descendant_of(self, mut descendant: DefId, ancestor: DefId) -> bool {
+        if descendant.krate != ancestor.krate {
+            return false;
+        }
+
+        while descendant != ancestor {
+            match self.opt_parent(descendant) {
+                Some(parent) => descendant = parent,
+                None => return false,
+            }
+        }
+        true
+    }
+}
+
+impl<'tcx> DefIdTree for TyCtxt<'tcx> {
+    #[inline]
+    fn opt_parent(self, id: DefId) -> Option<DefId> {
+        self.def_key(id).parent.map(|index| DefId { index, ..id })
+    }
+}
+
+impl Visibility {
+    /// Returns `true` if an item with this visibility is accessible from the given block.
+    pub fn is_accessible_from<T: DefIdTree>(self, module: DefId, tree: T) -> bool {
+        let restriction = match self {
+            // Public items are visible everywhere.
+            Visibility::Public => return true,
+            // Private items from other crates are visible nowhere.
+            Visibility::Invisible => return false,
+            // Restricted items are visible in an arbitrary local module.
+            Visibility::Restricted(other) if other.krate != module.krate => return false,
+            Visibility::Restricted(module) => module,
+        };
+
+        tree.is_descendant_of(module, restriction)
+    }
+
+    /// Returns `true` if this visibility is at least as accessible as the given visibility
+    pub fn is_at_least<T: DefIdTree>(self, vis: Visibility, tree: T) -> bool {
+        let vis_restriction = match vis {
+            Visibility::Public => return self == Visibility::Public,
+            Visibility::Invisible => return true,
+            Visibility::Restricted(module) => module,
+        };
+
+        self.is_accessible_from(vis_restriction, tree)
+    }
+
+    // Returns `true` if this item is visible anywhere in the local crate.
+    pub fn is_visible_locally(self) -> bool {
+        match self {
+            Visibility::Public => true,
+            Visibility::Restricted(def_id) => def_id.is_local(),
+            Visibility::Invisible => false,
+        }
+    }
+
+    pub fn is_public(self) -> bool {
+        matches!(self, Visibility::Public)
+    }
+}
+
+/// The crate variances map is computed during typeck and contains the
+/// variance of every item in the local crate. You should not use it
+/// directly, because to do so will make your pass dependent on the
+/// HIR of every item in the local crate. Instead, use
+/// `tcx.variances_of()` to get the variance for a *particular*
+/// item.
+#[derive(HashStable, Debug)]
+pub struct CrateVariancesMap<'tcx> {
+    /// For each item with generics, maps to a vector of the variance
+    /// of its generics. If an item has no generics, it will have no
+    /// entry.
+    pub variances: FxHashMap<DefId, &'tcx [ty::Variance]>,
+}
+
+// Contains information needed to resolve types and (in the future) look up
+// the types of AST nodes.
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+pub struct CReaderCacheKey {
+    pub cnum: Option<CrateNum>,
+    pub pos: usize,
+}
+
+/// Represents a type.
+///
+/// IMPORTANT:
+/// - This is a very "dumb" struct (with no derives and no `impls`).
+/// - Values of this type are always interned and thus unique, and are stored
+///   as an `Interned<TyS>`.
+/// - `Ty` (which contains a reference to a `Interned<TyS>`) or `Interned<TyS>`
+///   should be used everywhere instead of `TyS`. In particular, `Ty` has most
+///   of the relevant methods.
+#[derive(PartialEq, Eq, PartialOrd, Ord)]
+#[allow(rustc::usage_of_ty_tykind)]
+pub(crate) struct TyS<'tcx> {
+    /// This field shouldn't be used directly and may be removed in the future.
+    /// Use `Ty::kind()` instead.
+    kind: TyKind<'tcx>,
+
+    /// This field provides fast access to information that is also contained
+    /// in `kind`.
+    ///
+    /// This field shouldn't be used directly and may be removed in the future.
+    /// Use `Ty::flags()` instead.
+    flags: TypeFlags,
+
+    /// This field provides fast access to information that is also contained
+    /// in `kind`.
+    ///
+    /// This is a kind of confusing thing: it stores the smallest
+    /// binder such that
+    ///
+    /// (a) the binder itself captures nothing but
+    /// (b) all the late-bound things within the type are captured
+    ///     by some sub-binder.
+    ///
+    /// So, for a type without any late-bound things, like `u32`, this
+    /// will be *innermost*, because that is the innermost binder that
+    /// captures nothing. But for a type `&'D u32`, where `'D` is a
+    /// late-bound region with De Bruijn index `D`, this would be `D + 1`
+    /// -- the binder itself does not capture `D`, but `D` is captured
+    /// by an inner binder.
+    ///
+    /// We call this concept an "exclusive" binder `D` because all
+    /// De Bruijn indices within the type are contained within `0..D`
+    /// (exclusive).
+    outer_exclusive_binder: ty::DebruijnIndex,
+}
+
+// `TyS` is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(TyS<'_>, 40);
+
+// We are actually storing a stable hash cache next to the type, so let's
+// also check the full size
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(WithStableHash<TyS<'_>>, 56);
+
+/// Use this rather than `TyS`, whenever possible.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, HashStable)]
+#[rustc_diagnostic_item = "Ty"]
+#[rustc_pass_by_value]
+pub struct Ty<'tcx>(Interned<'tcx, WithStableHash<TyS<'tcx>>>);
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// A "bool" type used in rustc_mir_transform unit tests when we
+    /// have not spun up a TyCtxt.
+    pub const BOOL_TY_FOR_UNIT_TESTING: Ty<'tcx> = Ty(Interned::new_unchecked(&WithStableHash {
+        internee: TyS {
+            kind: ty::Bool,
+            flags: TypeFlags::empty(),
+            outer_exclusive_binder: DebruijnIndex::from_usize(0),
+        },
+        stable_hash: Fingerprint::ZERO,
+    }));
+}
+
+impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for TyS<'tcx> {
+    #[inline]
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        let TyS {
+            kind,
+
+            // The other fields just provide fast access to information that is
+            // also contained in `kind`, so no need to hash them.
+            flags: _,
+
+            outer_exclusive_binder: _,
+        } = self;
+
+        kind.hash_stable(hcx, hasher)
+    }
+}
+
+impl ty::EarlyBoundRegion {
+    /// Does this early bound region have a name? Early bound regions normally
+    /// always have names except when using anonymous lifetimes (`'_`).
+    pub fn has_name(&self) -> bool {
+        self.name != kw::UnderscoreLifetime
+    }
+}
+
+/// Represents a predicate.
+///
+/// See comments on `TyS`, which apply here too (albeit for
+/// `PredicateS`/`Predicate` rather than `TyS`/`Ty`).
+#[derive(Debug)]
+pub(crate) struct PredicateS<'tcx> {
+    kind: Binder<'tcx, PredicateKind<'tcx>>,
+    flags: TypeFlags,
+    /// See the comment for the corresponding field of [TyS].
+    outer_exclusive_binder: ty::DebruijnIndex,
+}
+
+// This type is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(PredicateS<'_>, 56);
+
+/// Use this rather than `PredicateS`, whenever possible.
+#[derive(Clone, Copy, PartialEq, Eq, Hash)]
+#[rustc_pass_by_value]
+pub struct Predicate<'tcx>(Interned<'tcx, PredicateS<'tcx>>);
+
+impl<'tcx> Predicate<'tcx> {
+    /// Gets the inner `Binder<'tcx, PredicateKind<'tcx>>`.
+    #[inline]
+    pub fn kind(self) -> Binder<'tcx, PredicateKind<'tcx>> {
+        self.0.kind
+    }
+
+    #[inline(always)]
+    pub fn flags(self) -> TypeFlags {
+        self.0.flags
+    }
+
+    #[inline(always)]
+    pub fn outer_exclusive_binder(self) -> DebruijnIndex {
+        self.0.outer_exclusive_binder
+    }
+
+    /// Flips the polarity of a Predicate.
+    ///
+    /// Given `T: Trait` predicate it returns `T: !Trait` and given `T: !Trait` returns `T: Trait`.
+    pub fn flip_polarity(self, tcx: TyCtxt<'tcx>) -> Option<Predicate<'tcx>> {
+        let kind = self
+            .kind()
+            .map_bound(|kind| match kind {
+                PredicateKind::Trait(TraitPredicate { trait_ref, constness, polarity }) => {
+                    Some(PredicateKind::Trait(TraitPredicate {
+                        trait_ref,
+                        constness,
+                        polarity: polarity.flip()?,
+                    }))
+                }
+
+                _ => None,
+            })
+            .transpose()?;
+
+        Some(tcx.mk_predicate(kind))
+    }
+
+    pub fn without_const(mut self, tcx: TyCtxt<'tcx>) -> Self {
+        if let PredicateKind::Trait(TraitPredicate { trait_ref, constness, polarity }) = self.kind().skip_binder()
+            && constness != BoundConstness::NotConst
+        {
+            self = tcx.mk_predicate(self.kind().rebind(PredicateKind::Trait(TraitPredicate {
+                trait_ref,
+                constness: BoundConstness::NotConst,
+                polarity,
+            })));
+        }
+        self
+    }
+}
+
+impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for Predicate<'tcx> {
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        let PredicateS {
+            ref kind,
+
+            // The other fields just provide fast access to information that is
+            // also contained in `kind`, so no need to hash them.
+            flags: _,
+            outer_exclusive_binder: _,
+        } = self.0.0;
+
+        kind.hash_stable(hcx, hasher);
+    }
+}
+
+impl rustc_errors::IntoDiagnosticArg for Predicate<'_> {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        rustc_errors::DiagnosticArgValue::Str(std::borrow::Cow::Owned(self.to_string()))
+    }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub enum PredicateKind<'tcx> {
+    /// Corresponds to `where Foo: Bar<A, B, C>`. `Foo` here would be
+    /// the `Self` type of the trait reference and `A`, `B`, and `C`
+    /// would be the type parameters.
+    Trait(TraitPredicate<'tcx>),
+
+    /// `where 'a: 'b`
+    RegionOutlives(RegionOutlivesPredicate<'tcx>),
+
+    /// `where T: 'a`
+    TypeOutlives(TypeOutlivesPredicate<'tcx>),
+
+    /// `where <T as TraitRef>::Name == X`, approximately.
+    /// See the `ProjectionPredicate` struct for details.
+    Projection(ProjectionPredicate<'tcx>),
+
+    /// No syntax: `T` well-formed.
+    WellFormed(GenericArg<'tcx>),
+
+    /// Trait must be object-safe.
+    ObjectSafe(DefId),
+
+    /// No direct syntax. May be thought of as `where T: FnFoo<...>`
+    /// for some substitutions `...` and `T` being a closure type.
+    /// Satisfied (or refuted) once we know the closure's kind.
+    ClosureKind(DefId, SubstsRef<'tcx>, ClosureKind),
+
+    /// `T1 <: T2`
+    ///
+    /// This obligation is created most often when we have two
+    /// unresolved type variables and hence don't have enough
+    /// information to process the subtyping obligation yet.
+    Subtype(SubtypePredicate<'tcx>),
+
+    /// `T1` coerced to `T2`
+    ///
+    /// Like a subtyping obligation, this is created most often
+    /// when we have two unresolved type variables and hence
+    /// don't have enough information to process the coercion
+    /// obligation yet. At the moment, we actually process coercions
+    /// very much like subtyping and don't handle the full coercion
+    /// logic.
+    Coerce(CoercePredicate<'tcx>),
+
+    /// Constant initializer must evaluate successfully.
+    ConstEvaluatable(ty::Unevaluated<'tcx, ()>),
+
+    /// Constants must be equal. The first component is the const that is expected.
+    ConstEquate(Const<'tcx>, Const<'tcx>),
+
+    /// Represents a type found in the environment that we can use for implied bounds.
+    ///
+    /// Only used for Chalk.
+    TypeWellFormedFromEnv(Ty<'tcx>),
+}
+
+/// The crate outlives map is computed during typeck and contains the
+/// outlives of every item in the local crate. You should not use it
+/// directly, because to do so will make your pass dependent on the
+/// HIR of every item in the local crate. Instead, use
+/// `tcx.inferred_outlives_of()` to get the outlives for a *particular*
+/// item.
+#[derive(HashStable, Debug)]
+pub struct CratePredicatesMap<'tcx> {
+    /// For each struct with outlive bounds, maps to a vector of the
+    /// predicate of its outlive bounds. If an item has no outlives
+    /// bounds, it will have no entry.
+    pub predicates: FxHashMap<DefId, &'tcx [(Predicate<'tcx>, Span)]>,
+}
+
+impl<'tcx> Predicate<'tcx> {
+    /// Performs a substitution suitable for going from a
+    /// poly-trait-ref to supertraits that must hold if that
+    /// poly-trait-ref holds. This is slightly different from a normal
+    /// substitution in terms of what happens with bound regions. See
+    /// lengthy comment below for details.
+    pub fn subst_supertrait(
+        self,
+        tcx: TyCtxt<'tcx>,
+        trait_ref: &ty::PolyTraitRef<'tcx>,
+    ) -> Predicate<'tcx> {
+        // The interaction between HRTB and supertraits is not entirely
+        // obvious. Let me walk you (and myself) through an example.
+        //
+        // Let's start with an easy case. Consider two traits:
+        //
+        //     trait Foo<'a>: Bar<'a,'a> { }
+        //     trait Bar<'b,'c> { }
+        //
+        // Now, if we have a trait reference `for<'x> T: Foo<'x>`, then
+        // we can deduce that `for<'x> T: Bar<'x,'x>`. Basically, if we
+        // knew that `Foo<'x>` (for any 'x) then we also know that
+        // `Bar<'x,'x>` (for any 'x). This more-or-less falls out from
+        // normal substitution.
+        //
+        // In terms of why this is sound, the idea is that whenever there
+        // is an impl of `T:Foo<'a>`, it must show that `T:Bar<'a,'a>`
+        // holds.  So if there is an impl of `T:Foo<'a>` that applies to
+        // all `'a`, then we must know that `T:Bar<'a,'a>` holds for all
+        // `'a`.
+        //
+        // Another example to be careful of is this:
+        //
+        //     trait Foo1<'a>: for<'b> Bar1<'a,'b> { }
+        //     trait Bar1<'b,'c> { }
+        //
+        // Here, if we have `for<'x> T: Foo1<'x>`, then what do we know?
+        // The answer is that we know `for<'x,'b> T: Bar1<'x,'b>`. The
+        // reason is similar to the previous example: any impl of
+        // `T:Foo1<'x>` must show that `for<'b> T: Bar1<'x, 'b>`.  So
+        // basically we would want to collapse the bound lifetimes from
+        // the input (`trait_ref`) and the supertraits.
+        //
+        // To achieve this in practice is fairly straightforward. Let's
+        // consider the more complicated scenario:
+        //
+        // - We start out with `for<'x> T: Foo1<'x>`. In this case, `'x`
+        //   has a De Bruijn index of 1. We want to produce `for<'x,'b> T: Bar1<'x,'b>`,
+        //   where both `'x` and `'b` would have a DB index of 1.
+        //   The substitution from the input trait-ref is therefore going to be
+        //   `'a => 'x` (where `'x` has a DB index of 1).
+        // - The supertrait-ref is `for<'b> Bar1<'a,'b>`, where `'a` is an
+        //   early-bound parameter and `'b' is a late-bound parameter with a
+        //   DB index of 1.
+        // - If we replace `'a` with `'x` from the input, it too will have
+        //   a DB index of 1, and thus we'll have `for<'x,'b> Bar1<'x,'b>`
+        //   just as we wanted.
+        //
+        // There is only one catch. If we just apply the substitution `'a
+        // => 'x` to `for<'b> Bar1<'a,'b>`, the substitution code will
+        // adjust the DB index because we substituting into a binder (it
+        // tries to be so smart...) resulting in `for<'x> for<'b>
+        // Bar1<'x,'b>` (we have no syntax for this, so use your
+        // imagination). Basically the 'x will have DB index of 2 and 'b
+        // will have DB index of 1. Not quite what we want. So we apply
+        // the substitution to the *contents* of the trait reference,
+        // rather than the trait reference itself (put another way, the
+        // substitution code expects equal binding levels in the values
+        // from the substitution and the value being substituted into, and
+        // this trick achieves that).
+
+        // Working through the second example:
+        // trait_ref: for<'x> T: Foo1<'^0.0>; substs: [T, '^0.0]
+        // predicate: for<'b> Self: Bar1<'a, '^0.0>; substs: [Self, 'a, '^0.0]
+        // We want to end up with:
+        //     for<'x, 'b> T: Bar1<'^0.0, '^0.1>
+        // To do this:
+        // 1) We must shift all bound vars in predicate by the length
+        //    of trait ref's bound vars. So, we would end up with predicate like
+        //    Self: Bar1<'a, '^0.1>
+        // 2) We can then apply the trait substs to this, ending up with
+        //    T: Bar1<'^0.0, '^0.1>
+        // 3) Finally, to create the final bound vars, we concatenate the bound
+        //    vars of the trait ref with those of the predicate:
+        //    ['x, 'b]
+        let bound_pred = self.kind();
+        let pred_bound_vars = bound_pred.bound_vars();
+        let trait_bound_vars = trait_ref.bound_vars();
+        // 1) Self: Bar1<'a, '^0.0> -> Self: Bar1<'a, '^0.1>
+        let shifted_pred =
+            tcx.shift_bound_var_indices(trait_bound_vars.len(), bound_pred.skip_binder());
+        // 2) Self: Bar1<'a, '^0.1> -> T: Bar1<'^0.0, '^0.1>
+        let new = EarlyBinder(shifted_pred).subst(tcx, trait_ref.skip_binder().substs);
+        // 3) ['x] + ['b] -> ['x, 'b]
+        let bound_vars =
+            tcx.mk_bound_variable_kinds(trait_bound_vars.iter().chain(pred_bound_vars));
+        tcx.reuse_or_mk_predicate(self, ty::Binder::bind_with_vars(new, bound_vars))
+    }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub struct TraitPredicate<'tcx> {
+    pub trait_ref: TraitRef<'tcx>,
+
+    pub constness: BoundConstness,
+
+    /// If polarity is Positive: we are proving that the trait is implemented.
+    ///
+    /// If polarity is Negative: we are proving that a negative impl of this trait
+    /// exists. (Note that coherence also checks whether negative impls of supertraits
+    /// exist via a series of predicates.)
+    ///
+    /// If polarity is Reserved: that's a bug.
+    pub polarity: ImplPolarity,
+}
+
+pub type PolyTraitPredicate<'tcx> = ty::Binder<'tcx, TraitPredicate<'tcx>>;
+
+impl<'tcx> TraitPredicate<'tcx> {
+    pub fn remap_constness(&mut self, param_env: &mut ParamEnv<'tcx>) {
+        *param_env = param_env.with_constness(self.constness.and(param_env.constness()))
+    }
+
+    /// Remap the constness of this predicate before emitting it for diagnostics.
+    pub fn remap_constness_diag(&mut self, param_env: ParamEnv<'tcx>) {
+        // this is different to `remap_constness` that callees want to print this predicate
+        // in case of selection errors. `T: ~const Drop` bounds cannot end up here when the
+        // param_env is not const because it is always satisfied in non-const contexts.
+        if let hir::Constness::NotConst = param_env.constness() {
+            self.constness = ty::BoundConstness::NotConst;
+        }
+    }
+
+    pub fn def_id(self) -> DefId {
+        self.trait_ref.def_id
+    }
+
+    pub fn self_ty(self) -> Ty<'tcx> {
+        self.trait_ref.self_ty()
+    }
+
+    #[inline]
+    pub fn is_const_if_const(self) -> bool {
+        self.constness == BoundConstness::ConstIfConst
+    }
+
+    pub fn is_constness_satisfied_by(self, constness: hir::Constness) -> bool {
+        match (self.constness, constness) {
+            (BoundConstness::NotConst, _)
+            | (BoundConstness::ConstIfConst, hir::Constness::Const) => true,
+            (BoundConstness::ConstIfConst, hir::Constness::NotConst) => false,
+        }
+    }
+}
+
+impl<'tcx> PolyTraitPredicate<'tcx> {
+    pub fn def_id(self) -> DefId {
+        // Ok to skip binder since trait `DefId` does not care about regions.
+        self.skip_binder().def_id()
+    }
+
+    pub fn self_ty(self) -> ty::Binder<'tcx, Ty<'tcx>> {
+        self.map_bound(|trait_ref| trait_ref.self_ty())
+    }
+
+    /// Remap the constness of this predicate before emitting it for diagnostics.
+    pub fn remap_constness_diag(&mut self, param_env: ParamEnv<'tcx>) {
+        *self = self.map_bound(|mut p| {
+            p.remap_constness_diag(param_env);
+            p
+        });
+    }
+
+    #[inline]
+    pub fn is_const_if_const(self) -> bool {
+        self.skip_binder().is_const_if_const()
+    }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub struct OutlivesPredicate<A, B>(pub A, pub B); // `A: B`
+pub type RegionOutlivesPredicate<'tcx> = OutlivesPredicate<ty::Region<'tcx>, ty::Region<'tcx>>;
+pub type TypeOutlivesPredicate<'tcx> = OutlivesPredicate<Ty<'tcx>, ty::Region<'tcx>>;
+pub type PolyRegionOutlivesPredicate<'tcx> = ty::Binder<'tcx, RegionOutlivesPredicate<'tcx>>;
+pub type PolyTypeOutlivesPredicate<'tcx> = ty::Binder<'tcx, TypeOutlivesPredicate<'tcx>>;
+
+/// Encodes that `a` must be a subtype of `b`. The `a_is_expected` flag indicates
+/// whether the `a` type is the type that we should label as "expected" when
+/// presenting user diagnostics.
+#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub struct SubtypePredicate<'tcx> {
+    pub a_is_expected: bool,
+    pub a: Ty<'tcx>,
+    pub b: Ty<'tcx>,
+}
+pub type PolySubtypePredicate<'tcx> = ty::Binder<'tcx, SubtypePredicate<'tcx>>;
+
+/// Encodes that we have to coerce *from* the `a` type to the `b` type.
+#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub struct CoercePredicate<'tcx> {
+    pub a: Ty<'tcx>,
+    pub b: Ty<'tcx>,
+}
+pub type PolyCoercePredicate<'tcx> = ty::Binder<'tcx, CoercePredicate<'tcx>>;
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub enum Term<'tcx> {
+    Ty(Ty<'tcx>),
+    Const(Const<'tcx>),
+}
+
+impl<'tcx> From<Ty<'tcx>> for Term<'tcx> {
+    fn from(ty: Ty<'tcx>) -> Self {
+        Term::Ty(ty)
+    }
+}
+
+impl<'tcx> From<Const<'tcx>> for Term<'tcx> {
+    fn from(c: Const<'tcx>) -> Self {
+        Term::Const(c)
+    }
+}
+
+impl<'tcx> Term<'tcx> {
+    pub fn ty(&self) -> Option<Ty<'tcx>> {
+        if let Term::Ty(ty) = self { Some(*ty) } else { None }
+    }
+
+    pub fn ct(&self) -> Option<Const<'tcx>> {
+        if let Term::Const(c) = self { Some(*c) } else { None }
+    }
+
+    pub fn into_arg(self) -> GenericArg<'tcx> {
+        match self {
+            Term::Ty(ty) => ty.into(),
+            Term::Const(c) => c.into(),
+        }
+    }
+}
+
+/// This kind of predicate has no *direct* correspondent in the
+/// syntax, but it roughly corresponds to the syntactic forms:
+///
+/// 1. `T: TraitRef<..., Item = Type>`
+/// 2. `<T as TraitRef<...>>::Item == Type` (NYI)
+///
+/// In particular, form #1 is "desugared" to the combination of a
+/// normal trait predicate (`T: TraitRef<...>`) and one of these
+/// predicates. Form #2 is a broader form in that it also permits
+/// equality between arbitrary types. Processing an instance of
+/// Form #2 eventually yields one of these `ProjectionPredicate`
+/// instances to normalize the LHS.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub struct ProjectionPredicate<'tcx> {
+    pub projection_ty: ProjectionTy<'tcx>,
+    pub term: Term<'tcx>,
+}
+
+pub type PolyProjectionPredicate<'tcx> = Binder<'tcx, ProjectionPredicate<'tcx>>;
+
+impl<'tcx> PolyProjectionPredicate<'tcx> {
+    /// Returns the `DefId` of the trait of the associated item being projected.
+    #[inline]
+    pub fn trait_def_id(&self, tcx: TyCtxt<'tcx>) -> DefId {
+        self.skip_binder().projection_ty.trait_def_id(tcx)
+    }
+
+    /// Get the [PolyTraitRef] required for this projection to be well formed.
+    /// Note that for generic associated types the predicates of the associated
+    /// type also need to be checked.
+    #[inline]
+    pub fn required_poly_trait_ref(&self, tcx: TyCtxt<'tcx>) -> PolyTraitRef<'tcx> {
+        // Note: unlike with `TraitRef::to_poly_trait_ref()`,
+        // `self.0.trait_ref` is permitted to have escaping regions.
+        // This is because here `self` has a `Binder` and so does our
+        // return value, so we are preserving the number of binding
+        // levels.
+        self.map_bound(|predicate| predicate.projection_ty.trait_ref(tcx))
+    }
+
+    pub fn term(&self) -> Binder<'tcx, Term<'tcx>> {
+        self.map_bound(|predicate| predicate.term)
+    }
+
+    /// The `DefId` of the `TraitItem` for the associated type.
+    ///
+    /// Note that this is not the `DefId` of the `TraitRef` containing this
+    /// associated type, which is in `tcx.associated_item(projection_def_id()).container`.
+    pub fn projection_def_id(&self) -> DefId {
+        // Ok to skip binder since trait `DefId` does not care about regions.
+        self.skip_binder().projection_ty.item_def_id
+    }
+}
+
+pub trait ToPolyTraitRef<'tcx> {
+    fn to_poly_trait_ref(&self) -> PolyTraitRef<'tcx>;
+}
+
+impl<'tcx> ToPolyTraitRef<'tcx> for PolyTraitPredicate<'tcx> {
+    fn to_poly_trait_ref(&self) -> PolyTraitRef<'tcx> {
+        self.map_bound_ref(|trait_pred| trait_pred.trait_ref)
+    }
+}
+
+pub trait ToPredicate<'tcx> {
+    fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx>;
+}
+
+impl<'tcx> ToPredicate<'tcx> for Binder<'tcx, PredicateKind<'tcx>> {
+    #[inline(always)]
+    fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {
+        tcx.mk_predicate(self)
+    }
+}
+
+impl<'tcx> ToPredicate<'tcx> for PolyTraitPredicate<'tcx> {
+    fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {
+        self.map_bound(PredicateKind::Trait).to_predicate(tcx)
+    }
+}
+
+impl<'tcx> ToPredicate<'tcx> for PolyRegionOutlivesPredicate<'tcx> {
+    fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {
+        self.map_bound(PredicateKind::RegionOutlives).to_predicate(tcx)
+    }
+}
+
+impl<'tcx> ToPredicate<'tcx> for PolyTypeOutlivesPredicate<'tcx> {
+    fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {
+        self.map_bound(PredicateKind::TypeOutlives).to_predicate(tcx)
+    }
+}
+
+impl<'tcx> ToPredicate<'tcx> for PolyProjectionPredicate<'tcx> {
+    fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {
+        self.map_bound(PredicateKind::Projection).to_predicate(tcx)
+    }
+}
+
+impl<'tcx> Predicate<'tcx> {
+    pub fn to_opt_poly_trait_pred(self) -> Option<PolyTraitPredicate<'tcx>> {
+        let predicate = self.kind();
+        match predicate.skip_binder() {
+            PredicateKind::Trait(t) => Some(predicate.rebind(t)),
+            PredicateKind::Projection(..)
+            | PredicateKind::Subtype(..)
+            | PredicateKind::Coerce(..)
+            | PredicateKind::RegionOutlives(..)
+            | PredicateKind::WellFormed(..)
+            | PredicateKind::ObjectSafe(..)
+            | PredicateKind::ClosureKind(..)
+            | PredicateKind::TypeOutlives(..)
+            | PredicateKind::ConstEvaluatable(..)
+            | PredicateKind::ConstEquate(..)
+            | PredicateKind::TypeWellFormedFromEnv(..) => None,
+        }
+    }
+
+    pub fn to_opt_poly_projection_pred(self) -> Option<PolyProjectionPredicate<'tcx>> {
+        let predicate = self.kind();
+        match predicate.skip_binder() {
+            PredicateKind::Projection(t) => Some(predicate.rebind(t)),
+            PredicateKind::Trait(..)
+            | PredicateKind::Subtype(..)
+            | PredicateKind::Coerce(..)
+            | PredicateKind::RegionOutlives(..)
+            | PredicateKind::WellFormed(..)
+            | PredicateKind::ObjectSafe(..)
+            | PredicateKind::ClosureKind(..)
+            | PredicateKind::TypeOutlives(..)
+            | PredicateKind::ConstEvaluatable(..)
+            | PredicateKind::ConstEquate(..)
+            | PredicateKind::TypeWellFormedFromEnv(..) => None,
+        }
+    }
+
+    pub fn to_opt_type_outlives(self) -> Option<PolyTypeOutlivesPredicate<'tcx>> {
+        let predicate = self.kind();
+        match predicate.skip_binder() {
+            PredicateKind::TypeOutlives(data) => Some(predicate.rebind(data)),
+            PredicateKind::Trait(..)
+            | PredicateKind::Projection(..)
+            | PredicateKind::Subtype(..)
+            | PredicateKind::Coerce(..)
+            | PredicateKind::RegionOutlives(..)
+            | PredicateKind::WellFormed(..)
+            | PredicateKind::ObjectSafe(..)
+            | PredicateKind::ClosureKind(..)
+            | PredicateKind::ConstEvaluatable(..)
+            | PredicateKind::ConstEquate(..)
+            | PredicateKind::TypeWellFormedFromEnv(..) => None,
+        }
+    }
+}
+
+/// Represents the bounds declared on a particular set of type
+/// parameters. Should eventually be generalized into a flag list of
+/// where-clauses. You can obtain an `InstantiatedPredicates` list from a
+/// `GenericPredicates` by using the `instantiate` method. Note that this method
+/// reflects an important semantic invariant of `InstantiatedPredicates`: while
+/// the `GenericPredicates` are expressed in terms of the bound type
+/// parameters of the impl/trait/whatever, an `InstantiatedPredicates` instance
+/// represented a set of bounds for some particular instantiation,
+/// meaning that the generic parameters have been substituted with
+/// their values.
+///
+/// Example:
+/// ```ignore (illustrative)
+/// struct Foo<T, U: Bar<T>> { ... }
+/// ```
+/// Here, the `GenericPredicates` for `Foo` would contain a list of bounds like
+/// `[[], [U:Bar<T>]]`. Now if there were some particular reference
+/// like `Foo<isize,usize>`, then the `InstantiatedPredicates` would be `[[],
+/// [usize:Bar<isize>]]`.
+#[derive(Clone, Debug, TypeFoldable, TypeVisitable)]
+pub struct InstantiatedPredicates<'tcx> {
+    pub predicates: Vec<Predicate<'tcx>>,
+    pub spans: Vec<Span>,
+}
+
+impl<'tcx> InstantiatedPredicates<'tcx> {
+    pub fn empty() -> InstantiatedPredicates<'tcx> {
+        InstantiatedPredicates { predicates: vec![], spans: vec![] }
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.predicates.is_empty()
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, TyEncodable, TyDecodable, Lift)]
+#[derive(TypeFoldable, TypeVisitable)]
+pub struct OpaqueTypeKey<'tcx> {
+    pub def_id: LocalDefId,
+    pub substs: SubstsRef<'tcx>,
+}
+
+#[derive(Copy, Clone, Debug, TypeFoldable, TypeVisitable, HashStable, TyEncodable, TyDecodable)]
+pub struct OpaqueHiddenType<'tcx> {
+    /// The span of this particular definition of the opaque type. So
+    /// for example:
+    ///
+    /// ```ignore (incomplete snippet)
+    /// type Foo = impl Baz;
+    /// fn bar() -> Foo {
+    /// //          ^^^ This is the span we are looking for!
+    /// }
+    /// ```
+    ///
+    /// In cases where the fn returns `(impl Trait, impl Trait)` or
+    /// other such combinations, the result is currently
+    /// over-approximated, but better than nothing.
+    pub span: Span,
+
+    /// The type variable that represents the value of the opaque type
+    /// that we require. In other words, after we compile this function,
+    /// we will be created a constraint like:
+    /// ```ignore (pseudo-rust)
+    /// Foo<'a, T> = ?C
+    /// ```
+    /// where `?C` is the value of this type variable. =) It may
+    /// naturally refer to the type and lifetime parameters in scope
+    /// in this function, though ultimately it should only reference
+    /// those that are arguments to `Foo` in the constraint above. (In
+    /// other words, `?C` should not include `'b`, even though it's a
+    /// lifetime parameter on `foo`.)
+    pub ty: Ty<'tcx>,
+}
+
+impl<'tcx> OpaqueHiddenType<'tcx> {
+    pub fn report_mismatch(&self, other: &Self, tcx: TyCtxt<'tcx>) {
+        // Found different concrete types for the opaque type.
+        let mut err = tcx.sess.struct_span_err(
+            other.span,
+            "concrete type differs from previous defining opaque type use",
+        );
+        err.span_label(other.span, format!("expected `{}`, got `{}`", self.ty, other.ty));
+        if self.span == other.span {
+            err.span_label(
+                self.span,
+                "this expression supplies two conflicting concrete types for the same opaque type",
+            );
+        } else {
+            err.span_note(self.span, "previous use here");
+        }
+        err.emit();
+    }
+}
+
+/// The "placeholder index" fully defines a placeholder region, type, or const. Placeholders are
+/// identified by both a universe, as well as a name residing within that universe. Distinct bound
+/// regions/types/consts within the same universe simply have an unknown relationship to one
+/// another.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, PartialOrd, Ord)]
+#[derive(HashStable, TyEncodable, TyDecodable)]
+pub struct Placeholder<T> {
+    pub universe: UniverseIndex,
+    pub name: T,
+}
+
+pub type PlaceholderRegion = Placeholder<BoundRegionKind>;
+
+pub type PlaceholderType = Placeholder<BoundVar>;
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable)]
+#[derive(TyEncodable, TyDecodable, PartialOrd, Ord)]
+pub struct BoundConst<'tcx> {
+    pub var: BoundVar,
+    pub ty: Ty<'tcx>,
+}
+
+pub type PlaceholderConst<'tcx> = Placeholder<BoundVar>;
+
+/// A `DefId` which, in case it is a const argument, is potentially bundled with
+/// the `DefId` of the generic parameter it instantiates.
+///
+/// This is used to avoid calls to `type_of` for const arguments during typeck
+/// which cause cycle errors.
+///
+/// ```rust
+/// struct A;
+/// impl A {
+///     fn foo<const N: usize>(&self) -> [u8; N] { [0; N] }
+///     //           ^ const parameter
+/// }
+/// struct B;
+/// impl B {
+///     fn foo<const M: u8>(&self) -> usize { 42 }
+///     //           ^ const parameter
+/// }
+///
+/// fn main() {
+///     let a = A;
+///     let _b = a.foo::<{ 3 + 7 }>();
+///     //               ^^^^^^^^^ const argument
+/// }
+/// ```
+///
+/// Let's look at the call `a.foo::<{ 3 + 7 }>()` here. We do not know
+/// which `foo` is used until we know the type of `a`.
+///
+/// We only know the type of `a` once we are inside of `typeck(main)`.
+/// We also end up normalizing the type of `_b` during `typeck(main)` which
+/// requires us to evaluate the const argument.
+///
+/// To evaluate that const argument we need to know its type,
+/// which we would get using `type_of(const_arg)`. This requires us to
+/// resolve `foo` as it can be either `usize` or `u8` in this example.
+/// However, resolving `foo` once again requires `typeck(main)` to get the type of `a`,
+/// which results in a cycle.
+///
+/// In short we must not call `type_of(const_arg)` during `typeck(main)`.
+///
+/// When first creating the `ty::Const` of the const argument inside of `typeck` we have
+/// already resolved `foo` so we know which const parameter this argument instantiates.
+/// This means that we also know the expected result of `type_of(const_arg)` even if we
+/// aren't allowed to call that query: it is equal to `type_of(const_param)` which is
+/// trivial to compute.
+///
+/// If we now want to use that constant in a place which potentially needs its type
+/// we also pass the type of its `const_param`. This is the point of `WithOptConstParam`,
+/// except that instead of a `Ty` we bundle the `DefId` of the const parameter.
+/// Meaning that we need to use `type_of(const_param_did)` if `const_param_did` is `Some`
+/// to get the type of `did`.
+#[derive(Copy, Clone, Debug, TypeFoldable, TypeVisitable, Lift, TyEncodable, TyDecodable)]
+#[derive(PartialEq, Eq, PartialOrd, Ord)]
+#[derive(Hash, HashStable)]
+pub struct WithOptConstParam<T> {
+    pub did: T,
+    /// The `DefId` of the corresponding generic parameter in case `did` is
+    /// a const argument.
+    ///
+    /// Note that even if `did` is a const argument, this may still be `None`.
+    /// All queries taking `WithOptConstParam` start by calling `tcx.opt_const_param_of(def.did)`
+    /// to potentially update `param_did` in the case it is `None`.
+    pub const_param_did: Option<DefId>,
+}
+
+impl<T> WithOptConstParam<T> {
+    /// Creates a new `WithOptConstParam` setting `const_param_did` to `None`.
+    #[inline(always)]
+    pub fn unknown(did: T) -> WithOptConstParam<T> {
+        WithOptConstParam { did, const_param_did: None }
+    }
+}
+
+impl WithOptConstParam<LocalDefId> {
+    /// Returns `Some((did, param_did))` if `def_id` is a const argument,
+    /// `None` otherwise.
+    #[inline(always)]
+    pub fn try_lookup(did: LocalDefId, tcx: TyCtxt<'_>) -> Option<(LocalDefId, DefId)> {
+        tcx.opt_const_param_of(did).map(|param_did| (did, param_did))
+    }
+
+    /// In case `self` is unknown but `self.did` is a const argument, this returns
+    /// a `WithOptConstParam` with the correct `const_param_did`.
+    #[inline(always)]
+    pub fn try_upgrade(self, tcx: TyCtxt<'_>) -> Option<WithOptConstParam<LocalDefId>> {
+        if self.const_param_did.is_none() {
+            if let const_param_did @ Some(_) = tcx.opt_const_param_of(self.did) {
+                return Some(WithOptConstParam { did: self.did, const_param_did });
+            }
+        }
+
+        None
+    }
+
+    pub fn to_global(self) -> WithOptConstParam<DefId> {
+        WithOptConstParam { did: self.did.to_def_id(), const_param_did: self.const_param_did }
+    }
+
+    pub fn def_id_for_type_of(self) -> DefId {
+        if let Some(did) = self.const_param_did { did } else { self.did.to_def_id() }
+    }
+}
+
+impl WithOptConstParam<DefId> {
+    pub fn as_local(self) -> Option<WithOptConstParam<LocalDefId>> {
+        self.did
+            .as_local()
+            .map(|did| WithOptConstParam { did, const_param_did: self.const_param_did })
+    }
+
+    pub fn as_const_arg(self) -> Option<(LocalDefId, DefId)> {
+        if let Some(param_did) = self.const_param_did {
+            if let Some(did) = self.did.as_local() {
+                return Some((did, param_did));
+            }
+        }
+
+        None
+    }
+
+    pub fn is_local(self) -> bool {
+        self.did.is_local()
+    }
+
+    pub fn def_id_for_type_of(self) -> DefId {
+        self.const_param_did.unwrap_or(self.did)
+    }
+}
+
+/// When type checking, we use the `ParamEnv` to track
+/// details about the set of where-clauses that are in scope at this
+/// particular point.
+#[derive(Copy, Clone, Hash, PartialEq, Eq)]
+pub struct ParamEnv<'tcx> {
+    /// This packs both caller bounds and the reveal enum into one pointer.
+    ///
+    /// Caller bounds are `Obligation`s that the caller must satisfy. This is
+    /// basically the set of bounds on the in-scope type parameters, translated
+    /// into `Obligation`s, and elaborated and normalized.
+    ///
+    /// Use the `caller_bounds()` method to access.
+    ///
+    /// Typically, this is `Reveal::UserFacing`, but during codegen we
+    /// want `Reveal::All`.
+    ///
+    /// Note: This is packed, use the reveal() method to access it.
+    packed: CopyTaggedPtr<&'tcx List<Predicate<'tcx>>, ParamTag, true>,
+}
+
+#[derive(Copy, Clone)]
+struct ParamTag {
+    reveal: traits::Reveal,
+    constness: hir::Constness,
+}
+
+unsafe impl rustc_data_structures::tagged_ptr::Tag for ParamTag {
+    const BITS: usize = 2;
+    #[inline]
+    fn into_usize(self) -> usize {
+        match self {
+            Self { reveal: traits::Reveal::UserFacing, constness: hir::Constness::NotConst } => 0,
+            Self { reveal: traits::Reveal::All, constness: hir::Constness::NotConst } => 1,
+            Self { reveal: traits::Reveal::UserFacing, constness: hir::Constness::Const } => 2,
+            Self { reveal: traits::Reveal::All, constness: hir::Constness::Const } => 3,
+        }
+    }
+    #[inline]
+    unsafe fn from_usize(ptr: usize) -> Self {
+        match ptr {
+            0 => Self { reveal: traits::Reveal::UserFacing, constness: hir::Constness::NotConst },
+            1 => Self { reveal: traits::Reveal::All, constness: hir::Constness::NotConst },
+            2 => Self { reveal: traits::Reveal::UserFacing, constness: hir::Constness::Const },
+            3 => Self { reveal: traits::Reveal::All, constness: hir::Constness::Const },
+            _ => std::hint::unreachable_unchecked(),
+        }
+    }
+}
+
+impl<'tcx> fmt::Debug for ParamEnv<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("ParamEnv")
+            .field("caller_bounds", &self.caller_bounds())
+            .field("reveal", &self.reveal())
+            .field("constness", &self.constness())
+            .finish()
+    }
+}
+
+impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for ParamEnv<'tcx> {
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        self.caller_bounds().hash_stable(hcx, hasher);
+        self.reveal().hash_stable(hcx, hasher);
+        self.constness().hash_stable(hcx, hasher);
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for ParamEnv<'tcx> {
+    fn try_fold_with<F: ty::fold::FallibleTypeFolder<'tcx>>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        Ok(ParamEnv::new(
+            self.caller_bounds().try_fold_with(folder)?,
+            self.reveal().try_fold_with(folder)?,
+            self.constness().try_fold_with(folder)?,
+        ))
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for ParamEnv<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.caller_bounds().visit_with(visitor)?;
+        self.reveal().visit_with(visitor)?;
+        self.constness().visit_with(visitor)
+    }
+}
+
+impl<'tcx> ParamEnv<'tcx> {
+    /// Construct a trait environment suitable for contexts where
+    /// there are no where-clauses in scope. Hidden types (like `impl
+    /// Trait`) are left hidden, so this is suitable for ordinary
+    /// type-checking.
+    #[inline]
+    pub fn empty() -> Self {
+        Self::new(List::empty(), Reveal::UserFacing, hir::Constness::NotConst)
+    }
+
+    #[inline]
+    pub fn caller_bounds(self) -> &'tcx List<Predicate<'tcx>> {
+        self.packed.pointer()
+    }
+
+    #[inline]
+    pub fn reveal(self) -> traits::Reveal {
+        self.packed.tag().reveal
+    }
+
+    #[inline]
+    pub fn constness(self) -> hir::Constness {
+        self.packed.tag().constness
+    }
+
+    #[inline]
+    pub fn is_const(self) -> bool {
+        self.packed.tag().constness == hir::Constness::Const
+    }
+
+    /// Construct a trait environment with no where-clauses in scope
+    /// where the values of all `impl Trait` and other hidden types
+    /// are revealed. This is suitable for monomorphized, post-typeck
+    /// environments like codegen or doing optimizations.
+    ///
+    /// N.B., if you want to have predicates in scope, use `ParamEnv::new`,
+    /// or invoke `param_env.with_reveal_all()`.
+    #[inline]
+    pub fn reveal_all() -> Self {
+        Self::new(List::empty(), Reveal::All, hir::Constness::NotConst)
+    }
+
+    /// Construct a trait environment with the given set of predicates.
+    #[inline]
+    pub fn new(
+        caller_bounds: &'tcx List<Predicate<'tcx>>,
+        reveal: Reveal,
+        constness: hir::Constness,
+    ) -> Self {
+        ty::ParamEnv { packed: CopyTaggedPtr::new(caller_bounds, ParamTag { reveal, constness }) }
+    }
+
+    pub fn with_user_facing(mut self) -> Self {
+        self.packed.set_tag(ParamTag { reveal: Reveal::UserFacing, ..self.packed.tag() });
+        self
+    }
+
+    #[inline]
+    pub fn with_constness(mut self, constness: hir::Constness) -> Self {
+        self.packed.set_tag(ParamTag { constness, ..self.packed.tag() });
+        self
+    }
+
+    #[inline]
+    pub fn with_const(mut self) -> Self {
+        self.packed.set_tag(ParamTag { constness: hir::Constness::Const, ..self.packed.tag() });
+        self
+    }
+
+    #[inline]
+    pub fn without_const(mut self) -> Self {
+        self.packed.set_tag(ParamTag { constness: hir::Constness::NotConst, ..self.packed.tag() });
+        self
+    }
+
+    #[inline]
+    pub fn remap_constness_with(&mut self, mut constness: ty::BoundConstness) {
+        *self = self.with_constness(constness.and(self.constness()))
+    }
+
+    /// Returns a new parameter environment with the same clauses, but
+    /// which "reveals" the true results of projections in all cases
+    /// (even for associated types that are specializable). This is
+    /// the desired behavior during codegen and certain other special
+    /// contexts; normally though we want to use `Reveal::UserFacing`,
+    /// which is the default.
+    /// All opaque types in the caller_bounds of the `ParamEnv`
+    /// will be normalized to their underlying types.
+    /// See PR #65989 and issue #65918 for more details
+    pub fn with_reveal_all_normalized(self, tcx: TyCtxt<'tcx>) -> Self {
+        if self.packed.tag().reveal == traits::Reveal::All {
+            return self;
+        }
+
+        ParamEnv::new(
+            tcx.normalize_opaque_types(self.caller_bounds()),
+            Reveal::All,
+            self.constness(),
+        )
+    }
+
+    /// Returns this same environment but with no caller bounds.
+    #[inline]
+    pub fn without_caller_bounds(self) -> Self {
+        Self::new(List::empty(), self.reveal(), self.constness())
+    }
+
+    /// Creates a suitable environment in which to perform trait
+    /// queries on the given value. When type-checking, this is simply
+    /// the pair of the environment plus value. But when reveal is set to
+    /// All, then if `value` does not reference any type parameters, we will
+    /// pair it with the empty environment. This improves caching and is generally
+    /// invisible.
+    ///
+    /// N.B., we preserve the environment when type-checking because it
+    /// is possible for the user to have wacky where-clauses like
+    /// `where Box<u32>: Copy`, which are clearly never
+    /// satisfiable. We generally want to behave as if they were true,
+    /// although the surrounding function is never reachable.
+    pub fn and<T: TypeVisitable<'tcx>>(self, value: T) -> ParamEnvAnd<'tcx, T> {
+        match self.reveal() {
+            Reveal::UserFacing => ParamEnvAnd { param_env: self, value },
+
+            Reveal::All => {
+                if value.is_global() {
+                    ParamEnvAnd { param_env: self.without_caller_bounds(), value }
+                } else {
+                    ParamEnvAnd { param_env: self, value }
+                }
+            }
+        }
+    }
+}
+
+// FIXME(ecstaticmorse): Audit all occurrences of `without_const().to_predicate(tcx)` to ensure that
+// the constness of trait bounds is being propagated correctly.
+impl<'tcx> PolyTraitRef<'tcx> {
+    #[inline]
+    pub fn with_constness(self, constness: BoundConstness) -> PolyTraitPredicate<'tcx> {
+        self.map_bound(|trait_ref| ty::TraitPredicate {
+            trait_ref,
+            constness,
+            polarity: ty::ImplPolarity::Positive,
+        })
+    }
+
+    #[inline]
+    pub fn without_const(self) -> PolyTraitPredicate<'tcx> {
+        self.with_constness(BoundConstness::NotConst)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable)]
+#[derive(HashStable)]
+pub struct ParamEnvAnd<'tcx, T> {
+    pub param_env: ParamEnv<'tcx>,
+    pub value: T,
+}
+
+impl<'tcx, T> ParamEnvAnd<'tcx, T> {
+    pub fn into_parts(self) -> (ParamEnv<'tcx>, T) {
+        (self.param_env, self.value)
+    }
+
+    #[inline]
+    pub fn without_const(mut self) -> Self {
+        self.param_env = self.param_env.without_const();
+        self
+    }
+}
+
+#[derive(Copy, Clone, Debug, HashStable, Encodable, Decodable)]
+pub struct Destructor {
+    /// The `DefId` of the destructor method
+    pub did: DefId,
+    /// The constness of the destructor method
+    pub constness: hir::Constness,
+}
+
+bitflags! {
+    #[derive(HashStable, TyEncodable, TyDecodable)]
+    pub struct VariantFlags: u32 {
+        const NO_VARIANT_FLAGS        = 0;
+        /// Indicates whether the field list of this variant is `#[non_exhaustive]`.
+        const IS_FIELD_LIST_NON_EXHAUSTIVE = 1 << 0;
+        /// Indicates whether this variant was obtained as part of recovering from
+        /// a syntactic error. May be incomplete or bogus.
+        const IS_RECOVERED = 1 << 1;
+    }
+}
+
+/// Definition of a variant -- a struct's fields or an enum variant.
+#[derive(Debug, HashStable, TyEncodable, TyDecodable)]
+pub struct VariantDef {
+    /// `DefId` that identifies the variant itself.
+    /// If this variant belongs to a struct or union, then this is a copy of its `DefId`.
+    pub def_id: DefId,
+    /// `DefId` that identifies the variant's constructor.
+    /// If this variant is a struct variant, then this is `None`.
+    pub ctor_def_id: Option<DefId>,
+    /// Variant or struct name.
+    pub name: Symbol,
+    /// Discriminant of this variant.
+    pub discr: VariantDiscr,
+    /// Fields of this variant.
+    pub fields: Vec<FieldDef>,
+    /// Type of constructor of variant.
+    pub ctor_kind: CtorKind,
+    /// Flags of the variant (e.g. is field list non-exhaustive)?
+    flags: VariantFlags,
+}
+
+impl VariantDef {
+    /// Creates a new `VariantDef`.
+    ///
+    /// `variant_did` is the `DefId` that identifies the enum variant (if this `VariantDef`
+    /// represents an enum variant).
+    ///
+    /// `ctor_did` is the `DefId` that identifies the constructor of unit or
+    /// tuple-variants/structs. If this is a `struct`-variant then this should be `None`.
+    ///
+    /// `parent_did` is the `DefId` of the `AdtDef` representing the enum or struct that
+    /// owns this variant. It is used for checking if a struct has `#[non_exhaustive]` w/out having
+    /// to go through the redirect of checking the ctor's attributes - but compiling a small crate
+    /// requires loading the `AdtDef`s for all the structs in the universe (e.g., coherence for any
+    /// built-in trait), and we do not want to load attributes twice.
+    ///
+    /// If someone speeds up attribute loading to not be a performance concern, they can
+    /// remove this hack and use the constructor `DefId` everywhere.
+    pub fn new(
+        name: Symbol,
+        variant_did: Option<DefId>,
+        ctor_def_id: Option<DefId>,
+        discr: VariantDiscr,
+        fields: Vec<FieldDef>,
+        ctor_kind: CtorKind,
+        adt_kind: AdtKind,
+        parent_did: DefId,
+        recovered: bool,
+        is_field_list_non_exhaustive: bool,
+    ) -> Self {
+        debug!(
+            "VariantDef::new(name = {:?}, variant_did = {:?}, ctor_def_id = {:?}, discr = {:?},
+             fields = {:?}, ctor_kind = {:?}, adt_kind = {:?}, parent_did = {:?})",
+            name, variant_did, ctor_def_id, discr, fields, ctor_kind, adt_kind, parent_did,
+        );
+
+        let mut flags = VariantFlags::NO_VARIANT_FLAGS;
+        if is_field_list_non_exhaustive {
+            flags |= VariantFlags::IS_FIELD_LIST_NON_EXHAUSTIVE;
+        }
+
+        if recovered {
+            flags |= VariantFlags::IS_RECOVERED;
+        }
+
+        VariantDef {
+            def_id: variant_did.unwrap_or(parent_did),
+            ctor_def_id,
+            name,
+            discr,
+            fields,
+            ctor_kind,
+            flags,
+        }
+    }
+
+    /// Is this field list non-exhaustive?
+    #[inline]
+    pub fn is_field_list_non_exhaustive(&self) -> bool {
+        self.flags.intersects(VariantFlags::IS_FIELD_LIST_NON_EXHAUSTIVE)
+    }
+
+    /// Was this variant obtained as part of recovering from a syntactic error?
+    #[inline]
+    pub fn is_recovered(&self) -> bool {
+        self.flags.intersects(VariantFlags::IS_RECOVERED)
+    }
+
+    /// Computes the `Ident` of this variant by looking up the `Span`
+    pub fn ident(&self, tcx: TyCtxt<'_>) -> Ident {
+        Ident::new(self.name, tcx.def_ident_span(self.def_id).unwrap())
+    }
+}
+
+impl PartialEq for VariantDef {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        // There should be only one `VariantDef` for each `def_id`, therefore
+        // it is fine to implement `PartialEq` only based on `def_id`.
+        //
+        // Below, we exhaustively destructure `self` and `other` so that if the
+        // definition of `VariantDef` changes, a compile-error will be produced,
+        // reminding us to revisit this assumption.
+
+        let Self {
+            def_id: lhs_def_id,
+            ctor_def_id: _,
+            name: _,
+            discr: _,
+            fields: _,
+            ctor_kind: _,
+            flags: _,
+        } = &self;
+
+        let Self {
+            def_id: rhs_def_id,
+            ctor_def_id: _,
+            name: _,
+            discr: _,
+            fields: _,
+            ctor_kind: _,
+            flags: _,
+        } = other;
+
+        lhs_def_id == rhs_def_id
+    }
+}
+
+impl Eq for VariantDef {}
+
+impl Hash for VariantDef {
+    #[inline]
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        // There should be only one `VariantDef` for each `def_id`, therefore
+        // it is fine to implement `Hash` only based on `def_id`.
+        //
+        // Below, we exhaustively destructure `self` so that if the definition
+        // of `VariantDef` changes, a compile-error will be produced, reminding
+        // us to revisit this assumption.
+
+        let Self { def_id, ctor_def_id: _, name: _, discr: _, fields: _, ctor_kind: _, flags: _ } =
+            &self;
+
+        def_id.hash(s)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
+pub enum VariantDiscr {
+    /// Explicit value for this variant, i.e., `X = 123`.
+    /// The `DefId` corresponds to the embedded constant.
+    Explicit(DefId),
+
+    /// The previous variant's discriminant plus one.
+    /// For efficiency reasons, the distance from the
+    /// last `Explicit` discriminant is being stored,
+    /// or `0` for the first variant, if it has none.
+    Relative(u32),
+}
+
+#[derive(Debug, HashStable, TyEncodable, TyDecodable)]
+pub struct FieldDef {
+    pub did: DefId,
+    pub name: Symbol,
+    pub vis: Visibility,
+}
+
+impl PartialEq for FieldDef {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        // There should be only one `FieldDef` for each `did`, therefore it is
+        // fine to implement `PartialEq` only based on `did`.
+        //
+        // Below, we exhaustively destructure `self` so that if the definition
+        // of `FieldDef` changes, a compile-error will be produced, reminding
+        // us to revisit this assumption.
+
+        let Self { did: lhs_did, name: _, vis: _ } = &self;
+
+        let Self { did: rhs_did, name: _, vis: _ } = other;
+
+        lhs_did == rhs_did
+    }
+}
+
+impl Eq for FieldDef {}
+
+impl Hash for FieldDef {
+    #[inline]
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        // There should be only one `FieldDef` for each `did`, therefore it is
+        // fine to implement `Hash` only based on `did`.
+        //
+        // Below, we exhaustively destructure `self` so that if the definition
+        // of `FieldDef` changes, a compile-error will be produced, reminding
+        // us to revisit this assumption.
+
+        let Self { did, name: _, vis: _ } = &self;
+
+        did.hash(s)
+    }
+}
+
+bitflags! {
+    #[derive(TyEncodable, TyDecodable, Default, HashStable)]
+    pub struct ReprFlags: u8 {
+        const IS_C               = 1 << 0;
+        const IS_SIMD            = 1 << 1;
+        const IS_TRANSPARENT     = 1 << 2;
+        // Internal only for now. If true, don't reorder fields.
+        const IS_LINEAR          = 1 << 3;
+        // If true, the type's layout can be randomized using
+        // the seed stored in `ReprOptions.layout_seed`
+        const RANDOMIZE_LAYOUT   = 1 << 4;
+        // Any of these flags being set prevent field reordering optimisation.
+        const IS_UNOPTIMISABLE   = ReprFlags::IS_C.bits
+                                 | ReprFlags::IS_SIMD.bits
+                                 | ReprFlags::IS_LINEAR.bits;
+    }
+}
+
+/// Represents the repr options provided by the user,
+#[derive(Copy, Clone, Debug, Eq, PartialEq, TyEncodable, TyDecodable, Default, HashStable)]
+pub struct ReprOptions {
+    pub int: Option<attr::IntType>,
+    pub align: Option<Align>,
+    pub pack: Option<Align>,
+    pub flags: ReprFlags,
+    /// The seed to be used for randomizing a type's layout
+    ///
+    /// Note: This could technically be a `[u8; 16]` (a `u128`) which would
+    /// be the "most accurate" hash as it'd encompass the item and crate
+    /// hash without loss, but it does pay the price of being larger.
+    /// Everything's a tradeoff, a `u64` seed should be sufficient for our
+    /// purposes (primarily `-Z randomize-layout`)
+    pub field_shuffle_seed: u64,
+}
+
+impl ReprOptions {
+    pub fn new(tcx: TyCtxt<'_>, did: DefId) -> ReprOptions {
+        let mut flags = ReprFlags::empty();
+        let mut size = None;
+        let mut max_align: Option<Align> = None;
+        let mut min_pack: Option<Align> = None;
+
+        // Generate a deterministically-derived seed from the item's path hash
+        // to allow for cross-crate compilation to actually work
+        let mut field_shuffle_seed = tcx.def_path_hash(did).0.to_smaller_hash();
+
+        // If the user defined a custom seed for layout randomization, xor the item's
+        // path hash with the user defined seed, this will allowing determinism while
+        // still allowing users to further randomize layout generation for e.g. fuzzing
+        if let Some(user_seed) = tcx.sess.opts.unstable_opts.layout_seed {
+            field_shuffle_seed ^= user_seed;
+        }
+
+        for attr in tcx.get_attrs(did, sym::repr) {
+            for r in attr::parse_repr_attr(&tcx.sess, attr) {
+                flags.insert(match r {
+                    attr::ReprC => ReprFlags::IS_C,
+                    attr::ReprPacked(pack) => {
+                        let pack = Align::from_bytes(pack as u64).unwrap();
+                        min_pack = Some(if let Some(min_pack) = min_pack {
+                            min_pack.min(pack)
+                        } else {
+                            pack
+                        });
+                        ReprFlags::empty()
+                    }
+                    attr::ReprTransparent => ReprFlags::IS_TRANSPARENT,
+                    attr::ReprSimd => ReprFlags::IS_SIMD,
+                    attr::ReprInt(i) => {
+                        size = Some(i);
+                        ReprFlags::empty()
+                    }
+                    attr::ReprAlign(align) => {
+                        max_align = max_align.max(Some(Align::from_bytes(align as u64).unwrap()));
+                        ReprFlags::empty()
+                    }
+                });
+            }
+        }
+
+        // If `-Z randomize-layout` was enabled for the type definition then we can
+        // consider performing layout randomization
+        if tcx.sess.opts.unstable_opts.randomize_layout {
+            flags.insert(ReprFlags::RANDOMIZE_LAYOUT);
+        }
+
+        // This is here instead of layout because the choice must make it into metadata.
+        if !tcx.consider_optimizing(|| format!("Reorder fields of {:?}", tcx.def_path_str(did))) {
+            flags.insert(ReprFlags::IS_LINEAR);
+        }
+
+        Self { int: size, align: max_align, pack: min_pack, flags, field_shuffle_seed }
+    }
+
+    #[inline]
+    pub fn simd(&self) -> bool {
+        self.flags.contains(ReprFlags::IS_SIMD)
+    }
+
+    #[inline]
+    pub fn c(&self) -> bool {
+        self.flags.contains(ReprFlags::IS_C)
+    }
+
+    #[inline]
+    pub fn packed(&self) -> bool {
+        self.pack.is_some()
+    }
+
+    #[inline]
+    pub fn transparent(&self) -> bool {
+        self.flags.contains(ReprFlags::IS_TRANSPARENT)
+    }
+
+    #[inline]
+    pub fn linear(&self) -> bool {
+        self.flags.contains(ReprFlags::IS_LINEAR)
+    }
+
+    /// Returns the discriminant type, given these `repr` options.
+    /// This must only be called on enums!
+    pub fn discr_type(&self) -> attr::IntType {
+        self.int.unwrap_or(attr::SignedInt(ast::IntTy::Isize))
+    }
+
+    /// Returns `true` if this `#[repr()]` should inhabit "smart enum
+    /// layout" optimizations, such as representing `Foo<&T>` as a
+    /// single pointer.
+    pub fn inhibit_enum_layout_opt(&self) -> bool {
+        self.c() || self.int.is_some()
+    }
+
+    /// Returns `true` if this `#[repr()]` should inhibit struct field reordering
+    /// optimizations, such as with `repr(C)`, `repr(packed(1))`, or `repr(<int>)`.
+    pub fn inhibit_struct_field_reordering_opt(&self) -> bool {
+        if let Some(pack) = self.pack {
+            if pack.bytes() == 1 {
+                return true;
+            }
+        }
+
+        self.flags.intersects(ReprFlags::IS_UNOPTIMISABLE) || self.int.is_some()
+    }
+
+    /// Returns `true` if this type is valid for reordering and `-Z randomize-layout`
+    /// was enabled for its declaration crate
+    pub fn can_randomize_type_layout(&self) -> bool {
+        !self.inhibit_struct_field_reordering_opt()
+            && self.flags.contains(ReprFlags::RANDOMIZE_LAYOUT)
+    }
+
+    /// Returns `true` if this `#[repr()]` should inhibit union ABI optimisations.
+    pub fn inhibit_union_abi_opt(&self) -> bool {
+        self.c()
+    }
+}
+
+impl<'tcx> FieldDef {
+    /// Returns the type of this field. The resulting type is not normalized. The `subst` is
+    /// typically obtained via the second field of [`TyKind::Adt`].
+    pub fn ty(&self, tcx: TyCtxt<'tcx>, subst: SubstsRef<'tcx>) -> Ty<'tcx> {
+        tcx.bound_type_of(self.did).subst(tcx, subst)
+    }
+
+    /// Computes the `Ident` of this variant by looking up the `Span`
+    pub fn ident(&self, tcx: TyCtxt<'_>) -> Ident {
+        Ident::new(self.name, tcx.def_ident_span(self.did).unwrap())
+    }
+}
+
+pub type Attributes<'tcx> = impl Iterator<Item = &'tcx ast::Attribute>;
+#[derive(Debug, PartialEq, Eq)]
+pub enum ImplOverlapKind {
+    /// These impls are always allowed to overlap.
+    Permitted {
+        /// Whether or not the impl is permitted due to the trait being a `#[marker]` trait
+        marker: bool,
+    },
+    /// These impls are allowed to overlap, but that raises
+    /// an issue #33140 future-compatibility warning.
+    ///
+    /// Some background: in Rust 1.0, the trait-object types `Send + Sync` (today's
+    /// `dyn Send + Sync`) and `Sync + Send` (now `dyn Sync + Send`) were different.
+    ///
+    /// The widely-used version 0.1.0 of the crate `traitobject` had accidentally relied
+    /// that difference, making what reduces to the following set of impls:
+    ///
+    /// ```compile_fail,(E0119)
+    /// trait Trait {}
+    /// impl Trait for dyn Send + Sync {}
+    /// impl Trait for dyn Sync + Send {}
+    /// ```
+    ///
+    /// Obviously, once we made these types be identical, that code causes a coherence
+    /// error and a fairly big headache for us. However, luckily for us, the trait
+    /// `Trait` used in this case is basically a marker trait, and therefore having
+    /// overlapping impls for it is sound.
+    ///
+    /// To handle this, we basically regard the trait as a marker trait, with an additional
+    /// future-compatibility warning. To avoid accidentally "stabilizing" this feature,
+    /// it has the following restrictions:
+    ///
+    /// 1. The trait must indeed be a marker-like trait (i.e., no items), and must be
+    /// positive impls.
+    /// 2. The trait-ref of both impls must be equal.
+    /// 3. The trait-ref of both impls must be a trait object type consisting only of
+    /// marker traits.
+    /// 4. Neither of the impls can have any where-clauses.
+    ///
+    /// Once `traitobject` 0.1.0 is no longer an active concern, this hack can be removed.
+    Issue33140,
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    pub fn typeck_body(self, body: hir::BodyId) -> &'tcx TypeckResults<'tcx> {
+        self.typeck(self.hir().body_owner_def_id(body))
+    }
+
+    pub fn provided_trait_methods(self, id: DefId) -> impl 'tcx + Iterator<Item = &'tcx AssocItem> {
+        self.associated_items(id)
+            .in_definition_order()
+            .filter(move |item| item.kind == AssocKind::Fn && item.defaultness(self).has_value())
+    }
+
+    /// Look up the name of a definition across crates. This does not look at HIR.
+    pub fn opt_item_name(self, def_id: DefId) -> Option<Symbol> {
+        if let Some(cnum) = def_id.as_crate_root() {
+            Some(self.crate_name(cnum))
+        } else {
+            let def_key = self.def_key(def_id);
+            match def_key.disambiguated_data.data {
+                // The name of a constructor is that of its parent.
+                rustc_hir::definitions::DefPathData::Ctor => self
+                    .opt_item_name(DefId { krate: def_id.krate, index: def_key.parent.unwrap() }),
+                // The name of opaque types only exists in HIR.
+                rustc_hir::definitions::DefPathData::ImplTrait
+                    if let Some(def_id) = def_id.as_local() =>
+                    self.hir().opt_name(self.hir().local_def_id_to_hir_id(def_id)),
+                _ => def_key.get_opt_name(),
+            }
+        }
+    }
+
+    /// Look up the name of a definition across crates. This does not look at HIR.
+    ///
+    /// This method will ICE if the corresponding item does not have a name.  In these cases, use
+    /// [`opt_item_name`] instead.
+    ///
+    /// [`opt_item_name`]: Self::opt_item_name
+    pub fn item_name(self, id: DefId) -> Symbol {
+        self.opt_item_name(id).unwrap_or_else(|| {
+            bug!("item_name: no name for {:?}", self.def_path(id));
+        })
+    }
+
+    /// Look up the name and span of a definition.
+    ///
+    /// See [`item_name`][Self::item_name] for more information.
+    pub fn opt_item_ident(self, def_id: DefId) -> Option<Ident> {
+        let def = self.opt_item_name(def_id)?;
+        let span = def_id
+            .as_local()
+            .and_then(|id| self.def_ident_span(id))
+            .unwrap_or(rustc_span::DUMMY_SP);
+        Some(Ident::new(def, span))
+    }
+
+    pub fn opt_associated_item(self, def_id: DefId) -> Option<&'tcx AssocItem> {
+        if let DefKind::AssocConst | DefKind::AssocFn | DefKind::AssocTy = self.def_kind(def_id) {
+            Some(self.associated_item(def_id))
+        } else {
+            None
+        }
+    }
+
+    pub fn field_index(self, hir_id: hir::HirId, typeck_results: &TypeckResults<'_>) -> usize {
+        typeck_results.field_indices().get(hir_id).cloned().expect("no index for a field")
+    }
+
+    pub fn find_field_index(self, ident: Ident, variant: &VariantDef) -> Option<usize> {
+        variant
+            .fields
+            .iter()
+            .position(|field| self.hygienic_eq(ident, field.ident(self), variant.def_id))
+    }
+
+    /// Returns `true` if the impls are the same polarity and the trait either
+    /// has no items or is annotated `#[marker]` and prevents item overrides.
+    pub fn impls_are_allowed_to_overlap(
+        self,
+        def_id1: DefId,
+        def_id2: DefId,
+    ) -> Option<ImplOverlapKind> {
+        // If either trait impl references an error, they're allowed to overlap,
+        // as one of them essentially doesn't exist.
+        if self.impl_trait_ref(def_id1).map_or(false, |tr| tr.references_error())
+            || self.impl_trait_ref(def_id2).map_or(false, |tr| tr.references_error())
+        {
+            return Some(ImplOverlapKind::Permitted { marker: false });
+        }
+
+        match (self.impl_polarity(def_id1), self.impl_polarity(def_id2)) {
+            (ImplPolarity::Reservation, _) | (_, ImplPolarity::Reservation) => {
+                // `#[rustc_reservation_impl]` impls don't overlap with anything
+                debug!(
+                    "impls_are_allowed_to_overlap({:?}, {:?}) = Some(Permitted) (reservations)",
+                    def_id1, def_id2
+                );
+                return Some(ImplOverlapKind::Permitted { marker: false });
+            }
+            (ImplPolarity::Positive, ImplPolarity::Negative)
+            | (ImplPolarity::Negative, ImplPolarity::Positive) => {
+                // `impl AutoTrait for Type` + `impl !AutoTrait for Type`
+                debug!(
+                    "impls_are_allowed_to_overlap({:?}, {:?}) - None (differing polarities)",
+                    def_id1, def_id2
+                );
+                return None;
+            }
+            (ImplPolarity::Positive, ImplPolarity::Positive)
+            | (ImplPolarity::Negative, ImplPolarity::Negative) => {}
+        };
+
+        let is_marker_overlap = {
+            let is_marker_impl = |def_id: DefId| -> bool {
+                let trait_ref = self.impl_trait_ref(def_id);
+                trait_ref.map_or(false, |tr| self.trait_def(tr.def_id).is_marker)
+            };
+            is_marker_impl(def_id1) && is_marker_impl(def_id2)
+        };
+
+        if is_marker_overlap {
+            debug!(
+                "impls_are_allowed_to_overlap({:?}, {:?}) = Some(Permitted) (marker overlap)",
+                def_id1, def_id2
+            );
+            Some(ImplOverlapKind::Permitted { marker: true })
+        } else {
+            if let Some(self_ty1) = self.issue33140_self_ty(def_id1) {
+                if let Some(self_ty2) = self.issue33140_self_ty(def_id2) {
+                    if self_ty1 == self_ty2 {
+                        debug!(
+                            "impls_are_allowed_to_overlap({:?}, {:?}) - issue #33140 HACK",
+                            def_id1, def_id2
+                        );
+                        return Some(ImplOverlapKind::Issue33140);
+                    } else {
+                        debug!(
+                            "impls_are_allowed_to_overlap({:?}, {:?}) - found {:?} != {:?}",
+                            def_id1, def_id2, self_ty1, self_ty2
+                        );
+                    }
+                }
+            }
+
+            debug!("impls_are_allowed_to_overlap({:?}, {:?}) = None", def_id1, def_id2);
+            None
+        }
+    }
+
+    /// Returns `ty::VariantDef` if `res` refers to a struct,
+    /// or variant or their constructors, panics otherwise.
+    pub fn expect_variant_res(self, res: Res) -> &'tcx VariantDef {
+        match res {
+            Res::Def(DefKind::Variant, did) => {
+                let enum_did = self.parent(did);
+                self.adt_def(enum_did).variant_with_id(did)
+            }
+            Res::Def(DefKind::Struct | DefKind::Union, did) => self.adt_def(did).non_enum_variant(),
+            Res::Def(DefKind::Ctor(CtorOf::Variant, ..), variant_ctor_did) => {
+                let variant_did = self.parent(variant_ctor_did);
+                let enum_did = self.parent(variant_did);
+                self.adt_def(enum_did).variant_with_ctor_id(variant_ctor_did)
+            }
+            Res::Def(DefKind::Ctor(CtorOf::Struct, ..), ctor_did) => {
+                let struct_did = self.parent(ctor_did);
+                self.adt_def(struct_did).non_enum_variant()
+            }
+            _ => bug!("expect_variant_res used with unexpected res {:?}", res),
+        }
+    }
+
+    /// Returns the possibly-auto-generated MIR of a `(DefId, Subst)` pair.
+    #[instrument(skip(self), level = "debug")]
+    pub fn instance_mir(self, instance: ty::InstanceDef<'tcx>) -> &'tcx Body<'tcx> {
+        match instance {
+            ty::InstanceDef::Item(def) => {
+                debug!("calling def_kind on def: {:?}", def);
+                let def_kind = self.def_kind(def.did);
+                debug!("returned from def_kind: {:?}", def_kind);
+                match def_kind {
+                    DefKind::Const
+                    | DefKind::Static(..)
+                    | DefKind::AssocConst
+                    | DefKind::Ctor(..)
+                    | DefKind::AnonConst
+                    | DefKind::InlineConst => self.mir_for_ctfe_opt_const_arg(def),
+                    // If the caller wants `mir_for_ctfe` of a function they should not be using
+                    // `instance_mir`, so we'll assume const fn also wants the optimized version.
+                    _ => {
+                        assert_eq!(def.const_param_did, None);
+                        self.optimized_mir(def.did)
+                    }
+                }
+            }
+            ty::InstanceDef::VTableShim(..)
+            | ty::InstanceDef::ReifyShim(..)
+            | ty::InstanceDef::Intrinsic(..)
+            | ty::InstanceDef::FnPtrShim(..)
+            | ty::InstanceDef::Virtual(..)
+            | ty::InstanceDef::ClosureOnceShim { .. }
+            | ty::InstanceDef::DropGlue(..)
+            | ty::InstanceDef::CloneShim(..) => self.mir_shims(instance),
+        }
+    }
+
+    // FIXME(@lcnr): Remove this function.
+    pub fn get_attrs_unchecked(self, did: DefId) -> &'tcx [ast::Attribute] {
+        if let Some(did) = did.as_local() {
+            self.hir().attrs(self.hir().local_def_id_to_hir_id(did))
+        } else {
+            self.item_attrs(did)
+        }
+    }
+
+    /// Gets all attributes with the given name.
+    pub fn get_attrs(self, did: DefId, attr: Symbol) -> ty::Attributes<'tcx> {
+        let filter_fn = move |a: &&ast::Attribute| a.has_name(attr);
+        if let Some(did) = did.as_local() {
+            self.hir().attrs(self.hir().local_def_id_to_hir_id(did)).iter().filter(filter_fn)
+        } else if cfg!(debug_assertions) && rustc_feature::is_builtin_only_local(attr) {
+            bug!("tried to access the `only_local` attribute `{}` from an extern crate", attr);
+        } else {
+            self.item_attrs(did).iter().filter(filter_fn)
+        }
+    }
+
+    pub fn get_attr(self, did: DefId, attr: Symbol) -> Option<&'tcx ast::Attribute> {
+        self.get_attrs(did, attr).next()
+    }
+
+    /// Determines whether an item is annotated with an attribute.
+    pub fn has_attr(self, did: DefId, attr: Symbol) -> bool {
+        if cfg!(debug_assertions) && !did.is_local() && rustc_feature::is_builtin_only_local(attr) {
+            bug!("tried to access the `only_local` attribute `{}` from an extern crate", attr);
+        } else {
+            self.get_attrs(did, attr).next().is_some()
+        }
+    }
+
+    /// Returns `true` if this is an `auto trait`.
+    pub fn trait_is_auto(self, trait_def_id: DefId) -> bool {
+        self.trait_def(trait_def_id).has_auto_impl
+    }
+
+    /// Returns layout of a generator. Layout might be unavailable if the
+    /// generator is tainted by errors.
+    pub fn generator_layout(self, def_id: DefId) -> Option<&'tcx GeneratorLayout<'tcx>> {
+        self.optimized_mir(def_id).generator_layout()
+    }
+
+    /// Given the `DefId` of an impl, returns the `DefId` of the trait it implements.
+    /// If it implements no trait, returns `None`.
+    pub fn trait_id_of_impl(self, def_id: DefId) -> Option<DefId> {
+        self.impl_trait_ref(def_id).map(|tr| tr.def_id)
+    }
+
+    /// If the given `DefId` describes an item belonging to a trait,
+    /// returns the `DefId` of the trait that the trait item belongs to;
+    /// otherwise, returns `None`.
+    pub fn trait_of_item(self, def_id: DefId) -> Option<DefId> {
+        if let DefKind::AssocConst | DefKind::AssocFn | DefKind::AssocTy = self.def_kind(def_id) {
+            let parent = self.parent(def_id);
+            if let DefKind::Trait | DefKind::TraitAlias = self.def_kind(parent) {
+                return Some(parent);
+            }
+        }
+        None
+    }
+
+    /// If the given `DefId` describes a method belonging to an impl, returns the
+    /// `DefId` of the impl that the method belongs to; otherwise, returns `None`.
+    pub fn impl_of_method(self, def_id: DefId) -> Option<DefId> {
+        if let DefKind::AssocConst | DefKind::AssocFn | DefKind::AssocTy = self.def_kind(def_id) {
+            let parent = self.parent(def_id);
+            if let DefKind::Impl = self.def_kind(parent) {
+                return Some(parent);
+            }
+        }
+        None
+    }
+
+    /// If the given `DefId` belongs to a trait that was automatically derived, returns `true`.
+    pub fn is_builtin_derive(self, def_id: DefId) -> bool {
+        self.has_attr(def_id, sym::automatically_derived)
+    }
+
+    /// Looks up the span of `impl_did` if the impl is local; otherwise returns `Err`
+    /// with the name of the crate containing the impl.
+    pub fn span_of_impl(self, impl_did: DefId) -> Result<Span, Symbol> {
+        if let Some(impl_did) = impl_did.as_local() {
+            Ok(self.def_span(impl_did))
+        } else {
+            Err(self.crate_name(impl_did.krate))
+        }
+    }
+
+    /// Hygienically compares a use-site name (`use_name`) for a field or an associated item with
+    /// its supposed definition name (`def_name`). The method also needs `DefId` of the supposed
+    /// definition's parent/scope to perform comparison.
+    pub fn hygienic_eq(self, use_name: Ident, def_name: Ident, def_parent_def_id: DefId) -> bool {
+        // We could use `Ident::eq` here, but we deliberately don't. The name
+        // comparison fails frequently, and we want to avoid the expensive
+        // `normalize_to_macros_2_0()` calls required for the span comparison whenever possible.
+        use_name.name == def_name.name
+            && use_name
+                .span
+                .ctxt()
+                .hygienic_eq(def_name.span.ctxt(), self.expn_that_defined(def_parent_def_id))
+    }
+
+    pub fn adjust_ident(self, mut ident: Ident, scope: DefId) -> Ident {
+        ident.span.normalize_to_macros_2_0_and_adjust(self.expn_that_defined(scope));
+        ident
+    }
+
+    pub fn adjust_ident_and_get_scope(
+        self,
+        mut ident: Ident,
+        scope: DefId,
+        block: hir::HirId,
+    ) -> (Ident, DefId) {
+        let scope = ident
+            .span
+            .normalize_to_macros_2_0_and_adjust(self.expn_that_defined(scope))
+            .and_then(|actual_expansion| actual_expansion.expn_data().parent_module)
+            .unwrap_or_else(|| self.parent_module(block).to_def_id());
+        (ident, scope)
+    }
+
+    pub fn is_object_safe(self, key: DefId) -> bool {
+        self.object_safety_violations(key).is_empty()
+    }
+
+    #[inline]
+    pub fn is_const_fn_raw(self, def_id: DefId) -> bool {
+        matches!(self.def_kind(def_id), DefKind::Fn | DefKind::AssocFn | DefKind::Ctor(..))
+            && self.constness(def_id) == hir::Constness::Const
+    }
+
+    #[inline]
+    pub fn is_const_default_method(self, def_id: DefId) -> bool {
+        matches!(self.trait_of_item(def_id), Some(trait_id) if self.has_attr(trait_id, sym::const_trait))
+    }
+}
+
+/// Yields the parent function's `LocalDefId` if `def_id` is an `impl Trait` definition.
+pub fn is_impl_trait_defn(tcx: TyCtxt<'_>, def_id: DefId) -> Option<LocalDefId> {
+    let def_id = def_id.as_local()?;
+    if let Node::Item(item) = tcx.hir().get_by_def_id(def_id) {
+        if let hir::ItemKind::OpaqueTy(ref opaque_ty) = item.kind {
+            return match opaque_ty.origin {
+                hir::OpaqueTyOrigin::FnReturn(parent) | hir::OpaqueTyOrigin::AsyncFn(parent) => {
+                    Some(parent)
+                }
+                hir::OpaqueTyOrigin::TyAlias => None,
+            };
+        }
+    }
+    None
+}
+
+pub fn int_ty(ity: ast::IntTy) -> IntTy {
+    match ity {
+        ast::IntTy::Isize => IntTy::Isize,
+        ast::IntTy::I8 => IntTy::I8,
+        ast::IntTy::I16 => IntTy::I16,
+        ast::IntTy::I32 => IntTy::I32,
+        ast::IntTy::I64 => IntTy::I64,
+        ast::IntTy::I128 => IntTy::I128,
+    }
+}
+
+pub fn uint_ty(uty: ast::UintTy) -> UintTy {
+    match uty {
+        ast::UintTy::Usize => UintTy::Usize,
+        ast::UintTy::U8 => UintTy::U8,
+        ast::UintTy::U16 => UintTy::U16,
+        ast::UintTy::U32 => UintTy::U32,
+        ast::UintTy::U64 => UintTy::U64,
+        ast::UintTy::U128 => UintTy::U128,
+    }
+}
+
+pub fn float_ty(fty: ast::FloatTy) -> FloatTy {
+    match fty {
+        ast::FloatTy::F32 => FloatTy::F32,
+        ast::FloatTy::F64 => FloatTy::F64,
+    }
+}
+
+pub fn ast_int_ty(ity: IntTy) -> ast::IntTy {
+    match ity {
+        IntTy::Isize => ast::IntTy::Isize,
+        IntTy::I8 => ast::IntTy::I8,
+        IntTy::I16 => ast::IntTy::I16,
+        IntTy::I32 => ast::IntTy::I32,
+        IntTy::I64 => ast::IntTy::I64,
+        IntTy::I128 => ast::IntTy::I128,
+    }
+}
+
+pub fn ast_uint_ty(uty: UintTy) -> ast::UintTy {
+    match uty {
+        UintTy::Usize => ast::UintTy::Usize,
+        UintTy::U8 => ast::UintTy::U8,
+        UintTy::U16 => ast::UintTy::U16,
+        UintTy::U32 => ast::UintTy::U32,
+        UintTy::U64 => ast::UintTy::U64,
+        UintTy::U128 => ast::UintTy::U128,
+    }
+}
+
+pub fn provide(providers: &mut ty::query::Providers) {
+    closure::provide(providers);
+    context::provide(providers);
+    erase_regions::provide(providers);
+    layout::provide(providers);
+    util::provide(providers);
+    print::provide(providers);
+    super::util::bug::provide(providers);
+    super::middle::provide(providers);
+    *providers = ty::query::Providers {
+        trait_impls_of: trait_def::trait_impls_of_provider,
+        incoherent_impls: trait_def::incoherent_impls_provider,
+        type_uninhabited_from: inhabitedness::type_uninhabited_from,
+        const_param_default: consts::const_param_default,
+        vtable_allocation: vtable::vtable_allocation_provider,
+        ..*providers
+    };
+}
+
+/// A map for the local crate mapping each type to a vector of its
+/// inherent impls. This is not meant to be used outside of coherence;
+/// rather, you should request the vector for a specific type via
+/// `tcx.inherent_impls(def_id)` so as to minimize your dependencies
+/// (constructing this map requires touching the entire crate).
+#[derive(Clone, Debug, Default, HashStable)]
+pub struct CrateInherentImpls {
+    pub inherent_impls: LocalDefIdMap<Vec<DefId>>,
+    pub incoherent_impls: FxHashMap<SimplifiedType, Vec<LocalDefId>>,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, TyEncodable, HashStable)]
+pub struct SymbolName<'tcx> {
+    /// `&str` gives a consistent ordering, which ensures reproducible builds.
+    pub name: &'tcx str,
+}
+
+impl<'tcx> SymbolName<'tcx> {
+    pub fn new(tcx: TyCtxt<'tcx>, name: &str) -> SymbolName<'tcx> {
+        SymbolName {
+            name: unsafe { str::from_utf8_unchecked(tcx.arena.alloc_slice(name.as_bytes())) },
+        }
+    }
+}
+
+impl<'tcx> fmt::Display for SymbolName<'tcx> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(&self.name, fmt)
+    }
+}
+
+impl<'tcx> fmt::Debug for SymbolName<'tcx> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(&self.name, fmt)
+    }
+}
+
+#[derive(Debug, Default, Copy, Clone)]
+pub struct FoundRelationships {
+    /// This is true if we identified that this Ty (`?T`) is found in a `?T: Foo`
+    /// obligation, where:
+    ///
+    ///  * `Foo` is not `Sized`
+    ///  * `(): Foo` may be satisfied
+    pub self_in_trait: bool,
+    /// This is true if we identified that this Ty (`?T`) is found in a `<_ as
+    /// _>::AssocType = ?T`
+    pub output: bool,
+}
+
+/// The constituent parts of a type level constant of kind ADT or array.
+#[derive(Copy, Clone, Debug, HashStable)]
+pub struct DestructuredConst<'tcx> {
+    pub variant: Option<VariantIdx>,
+    pub fields: &'tcx [ty::Const<'tcx>],
+}
diff --git a/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs b/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs
new file mode 100644
index 000000000..9d8a81165
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs
@@ -0,0 +1,283 @@
+//! Methods for normalizing when you don't care about regions (and
+//! aren't doing type inference). If either of those things don't
+//! apply to you, use `infcx.normalize(...)`.
+//!
+//! The methods in this file use a `TypeFolder` to recursively process
+//! contents, invoking the underlying
+//! `normalize_generic_arg_after_erasing_regions` query for each type
+//! or constant found within. (This underlying query is what is cached.)
+
+use crate::mir;
+use crate::traits::query::NoSolution;
+use crate::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeFolder};
+use crate::ty::subst::{Subst, SubstsRef};
+use crate::ty::{self, EarlyBinder, Ty, TyCtxt};
+
+#[derive(Debug, Copy, Clone, HashStable, TyEncodable, TyDecodable)]
+pub enum NormalizationError<'tcx> {
+    Type(Ty<'tcx>),
+    Const(ty::Const<'tcx>),
+    ConstantKind(mir::ConstantKind<'tcx>),
+}
+
+impl<'tcx> NormalizationError<'tcx> {
+    pub fn get_type_for_failure(&self) -> String {
+        match self {
+            NormalizationError::Type(t) => format!("{}", t),
+            NormalizationError::Const(c) => format!("{}", c),
+            NormalizationError::ConstantKind(ck) => format!("{}", ck),
+        }
+    }
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Erase the regions in `value` and then fully normalize all the
+    /// types found within. The result will also have regions erased.
+    ///
+    /// This should only be used outside of type inference. For example,
+    /// it assumes that normalization will succeed.
+    pub fn normalize_erasing_regions<T>(self, param_env: ty::ParamEnv<'tcx>, value: T) -> T
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        debug!(
+            "normalize_erasing_regions::<{}>(value={:?}, param_env={:?})",
+            std::any::type_name::<T>(),
+            value,
+            param_env,
+        );
+
+        // Erase first before we do the real query -- this keeps the
+        // cache from being too polluted.
+        let value = self.erase_regions(value);
+        debug!(?value);
+
+        if !value.has_projections() {
+            value
+        } else {
+            value.fold_with(&mut NormalizeAfterErasingRegionsFolder { tcx: self, param_env })
+        }
+    }
+
+    /// Tries to erase the regions in `value` and then fully normalize all the
+    /// types found within. The result will also have regions erased.
+    ///
+    /// Contrary to `normalize_erasing_regions` this function does not assume that normalization
+    /// succeeds.
+    pub fn try_normalize_erasing_regions<T>(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        value: T,
+    ) -> Result<T, NormalizationError<'tcx>>
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        debug!(
+            "try_normalize_erasing_regions::<{}>(value={:?}, param_env={:?})",
+            std::any::type_name::<T>(),
+            value,
+            param_env,
+        );
+
+        // Erase first before we do the real query -- this keeps the
+        // cache from being too polluted.
+        let value = self.erase_regions(value);
+        debug!(?value);
+
+        if !value.has_projections() {
+            Ok(value)
+        } else {
+            let mut folder = TryNormalizeAfterErasingRegionsFolder::new(self, param_env);
+            value.try_fold_with(&mut folder)
+        }
+    }
+
+    /// If you have a `Binder<'tcx, T>`, you can do this to strip out the
+    /// late-bound regions and then normalize the result, yielding up
+    /// a `T` (with regions erased). This is appropriate when the
+    /// binder is being instantiated at the call site.
+    ///
+    /// N.B., currently, higher-ranked type bounds inhibit
+    /// normalization. Therefore, each time we erase them in
+    /// codegen, we need to normalize the contents.
+    pub fn normalize_erasing_late_bound_regions<T>(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        value: ty::Binder<'tcx, T>,
+    ) -> T
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        let value = self.erase_late_bound_regions(value);
+        self.normalize_erasing_regions(param_env, value)
+    }
+
+    /// If you have a `Binder<'tcx, T>`, you can do this to strip out the
+    /// late-bound regions and then normalize the result, yielding up
+    /// a `T` (with regions erased). This is appropriate when the
+    /// binder is being instantiated at the call site.
+    ///
+    /// N.B., currently, higher-ranked type bounds inhibit
+    /// normalization. Therefore, each time we erase them in
+    /// codegen, we need to normalize the contents.
+    pub fn try_normalize_erasing_late_bound_regions<T>(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        value: ty::Binder<'tcx, T>,
+    ) -> Result<T, NormalizationError<'tcx>>
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        let value = self.erase_late_bound_regions(value);
+        self.try_normalize_erasing_regions(param_env, value)
+    }
+
+    /// Monomorphizes a type from the AST by first applying the
+    /// in-scope substitutions and then normalizing any associated
+    /// types.
+    /// Panics if normalization fails. In case normalization might fail
+    /// use `try_subst_and_normalize_erasing_regions` instead.
+    pub fn subst_and_normalize_erasing_regions<T>(
+        self,
+        param_substs: SubstsRef<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        value: T,
+    ) -> T
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        debug!(
+            "subst_and_normalize_erasing_regions(\
+             param_substs={:?}, \
+             value={:?}, \
+             param_env={:?})",
+            param_substs, value, param_env,
+        );
+        let substituted = EarlyBinder(value).subst(self, param_substs);
+        self.normalize_erasing_regions(param_env, substituted)
+    }
+
+    /// Monomorphizes a type from the AST by first applying the
+    /// in-scope substitutions and then trying to normalize any associated
+    /// types. Contrary to `subst_and_normalize_erasing_regions` this does
+    /// not assume that normalization succeeds.
+    pub fn try_subst_and_normalize_erasing_regions<T>(
+        self,
+        param_substs: SubstsRef<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        value: T,
+    ) -> Result<T, NormalizationError<'tcx>>
+    where
+        T: TypeFoldable<'tcx>,
+    {
+        debug!(
+            "subst_and_normalize_erasing_regions(\
+             param_substs={:?}, \
+             value={:?}, \
+             param_env={:?})",
+            param_substs, value, param_env,
+        );
+        let substituted = EarlyBinder(value).subst(self, param_substs);
+        self.try_normalize_erasing_regions(param_env, substituted)
+    }
+}
+
+struct NormalizeAfterErasingRegionsFolder<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+}
+
+impl<'tcx> NormalizeAfterErasingRegionsFolder<'tcx> {
+    #[instrument(skip(self), level = "debug")]
+    fn normalize_generic_arg_after_erasing_regions(
+        &self,
+        arg: ty::GenericArg<'tcx>,
+    ) -> ty::GenericArg<'tcx> {
+        let arg = self.param_env.and(arg);
+        debug!(?arg);
+
+        self.tcx.try_normalize_generic_arg_after_erasing_regions(arg).unwrap_or_else(|_| bug!(
+                "Failed to normalize {:?}, maybe try to call `try_normalize_erasing_regions` instead",
+                arg.value
+            ))
+    }
+}
+
+impl<'tcx> TypeFolder<'tcx> for NormalizeAfterErasingRegionsFolder<'tcx> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        self.normalize_generic_arg_after_erasing_regions(ty.into()).expect_ty()
+    }
+
+    fn fold_const(&mut self, c: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        self.normalize_generic_arg_after_erasing_regions(c.into()).expect_const()
+    }
+
+    #[inline]
+    fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
+        // FIXME: This *probably* needs canonicalization too!
+        let arg = self.param_env.and(c);
+        self.tcx
+            .try_normalize_mir_const_after_erasing_regions(arg)
+            .unwrap_or_else(|_| bug!("failed to normalize {:?}", c))
+    }
+}
+
+struct TryNormalizeAfterErasingRegionsFolder<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+}
+
+impl<'tcx> TryNormalizeAfterErasingRegionsFolder<'tcx> {
+    fn new(tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Self {
+        TryNormalizeAfterErasingRegionsFolder { tcx, param_env }
+    }
+
+    #[instrument(skip(self), level = "debug")]
+    fn try_normalize_generic_arg_after_erasing_regions(
+        &self,
+        arg: ty::GenericArg<'tcx>,
+    ) -> Result<ty::GenericArg<'tcx>, NoSolution> {
+        let arg = self.param_env.and(arg);
+        debug!(?arg);
+
+        self.tcx.try_normalize_generic_arg_after_erasing_regions(arg)
+    }
+}
+
+impl<'tcx> FallibleTypeFolder<'tcx> for TryNormalizeAfterErasingRegionsFolder<'tcx> {
+    type Error = NormalizationError<'tcx>;
+
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn try_fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
+        match self.try_normalize_generic_arg_after_erasing_regions(ty.into()) {
+            Ok(t) => Ok(t.expect_ty()),
+            Err(_) => Err(NormalizationError::Type(ty)),
+        }
+    }
+
+    fn try_fold_const(&mut self, c: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, Self::Error> {
+        match self.try_normalize_generic_arg_after_erasing_regions(c.into()) {
+            Ok(t) => Ok(t.expect_const()),
+            Err(_) => Err(NormalizationError::Const(c)),
+        }
+    }
+
+    fn try_fold_mir_const(
+        &mut self,
+        c: mir::ConstantKind<'tcx>,
+    ) -> Result<mir::ConstantKind<'tcx>, Self::Error> {
+        // FIXME: This *probably* needs canonicalization too!
+        let arg = self.param_env.and(c);
+        match self.tcx.try_normalize_mir_const_after_erasing_regions(arg) {
+            Ok(c) => Ok(c),
+            Err(_) => Err(NormalizationError::ConstantKind(c)),
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/parameterized.rs b/compiler/rustc_middle/src/ty/parameterized.rs
new file mode 100644
index 000000000..e189ee2fc
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/parameterized.rs
@@ -0,0 +1,119 @@
+use rustc_hir::def_id::DefId;
+use rustc_index::vec::{Idx, IndexVec};
+
+use crate::middle::exported_symbols::ExportedSymbol;
+use crate::mir::Body;
+use crate::ty::abstract_const::Node;
+use crate::ty::{
+    self, Const, FnSig, GeneratorDiagnosticData, GenericPredicates, Predicate, TraitRef, Ty,
+};
+
+pub trait ParameterizedOverTcx: 'static {
+    #[allow(unused_lifetimes)]
+    type Value<'tcx>;
+}
+
+impl<T: ParameterizedOverTcx> ParameterizedOverTcx for &'static [T] {
+    type Value<'tcx> = &'tcx [T::Value<'tcx>];
+}
+
+impl<T: ParameterizedOverTcx> ParameterizedOverTcx for Option<T> {
+    type Value<'tcx> = Option<T::Value<'tcx>>;
+}
+
+impl<A: ParameterizedOverTcx, B: ParameterizedOverTcx> ParameterizedOverTcx for (A, B) {
+    type Value<'tcx> = (A::Value<'tcx>, B::Value<'tcx>);
+}
+
+impl<I: Idx + 'static, T: ParameterizedOverTcx> ParameterizedOverTcx for IndexVec<I, T> {
+    type Value<'tcx> = IndexVec<I, T::Value<'tcx>>;
+}
+
+impl<T: ParameterizedOverTcx> ParameterizedOverTcx for ty::Binder<'static, T> {
+    type Value<'tcx> = ty::Binder<'tcx, T::Value<'tcx>>;
+}
+
+#[macro_export]
+macro_rules! trivially_parameterized_over_tcx {
+    ($($ty:ty),+ $(,)?) => {
+        $(
+            impl $crate::ty::ParameterizedOverTcx for $ty {
+                #[allow(unused_lifetimes)]
+                type Value<'tcx> = $ty;
+            }
+        )*
+    }
+}
+
+trivially_parameterized_over_tcx! {
+    usize,
+    (),
+    u32,
+    std::string::String,
+    crate::metadata::ModChild,
+    crate::middle::codegen_fn_attrs::CodegenFnAttrs,
+    crate::middle::exported_symbols::SymbolExportInfo,
+    crate::mir::ConstQualifs,
+    ty::Generics,
+    ty::ImplPolarity,
+    ty::ReprOptions,
+    ty::TraitDef,
+    ty::Visibility,
+    ty::adjustment::CoerceUnsizedInfo,
+    ty::fast_reject::SimplifiedTypeGen<DefId>,
+    rustc_ast::Attribute,
+    rustc_ast::MacArgs,
+    rustc_attr::ConstStability,
+    rustc_attr::Deprecation,
+    rustc_attr::Stability,
+    rustc_hir::Constness,
+    rustc_hir::Defaultness,
+    rustc_hir::GeneratorKind,
+    rustc_hir::IsAsync,
+    rustc_hir::LangItem,
+    rustc_hir::def::DefKind,
+    rustc_hir::def_id::DefIndex,
+    rustc_hir::definitions::DefKey,
+    rustc_index::bit_set::FiniteBitSet<u32>,
+    rustc_session::cstore::ForeignModule,
+    rustc_session::cstore::LinkagePreference,
+    rustc_session::cstore::NativeLib,
+    rustc_span::DebuggerVisualizerFile,
+    rustc_span::ExpnData,
+    rustc_span::ExpnHash,
+    rustc_span::ExpnId,
+    rustc_span::SourceFile,
+    rustc_span::Span,
+    rustc_span::Symbol,
+    rustc_span::def_id::DefPathHash,
+    rustc_span::hygiene::SyntaxContextData,
+    rustc_span::symbol::Ident,
+    rustc_type_ir::Variance,
+}
+
+// HACK(compiler-errors): This macro rule can only take an ident,
+// not a path, due to parsing ambiguity reasons. That means we gotta
+// import all of these types above.
+#[macro_export]
+macro_rules! parameterized_over_tcx {
+    ($($ident:ident),+ $(,)?) => {
+        $(
+            impl $crate::ty::ParameterizedOverTcx for $ident<'static> {
+                type Value<'tcx> = $ident<'tcx>;
+            }
+        )*
+    }
+}
+
+parameterized_over_tcx! {
+    Ty,
+    FnSig,
+    GenericPredicates,
+    TraitRef,
+    Const,
+    Predicate,
+    GeneratorDiagnosticData,
+    Body,
+    Node,
+    ExportedSymbol,
+}
diff --git a/compiler/rustc_middle/src/ty/print/mod.rs b/compiler/rustc_middle/src/ty/print/mod.rs
new file mode 100644
index 000000000..d57cf8f01
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/print/mod.rs
@@ -0,0 +1,327 @@
+use crate::ty::subst::{GenericArg, Subst};
+use crate::ty::{self, DefIdTree, Ty, TyCtxt};
+
+use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::sso::SsoHashSet;
+use rustc_hir::def_id::{CrateNum, DefId};
+use rustc_hir::definitions::{DefPathData, DisambiguatedDefPathData};
+
+// `pretty` is a separate module only for organization.
+mod pretty;
+pub use self::pretty::*;
+
+// FIXME(eddyb) false positive, the lifetime parameters are used with `P:  Printer<...>`.
+#[allow(unused_lifetimes)]
+pub trait Print<'tcx, P> {
+    type Output;
+    type Error;
+
+    fn print(&self, cx: P) -> Result<Self::Output, Self::Error>;
+}
+
+/// Interface for outputting user-facing "type-system entities"
+/// (paths, types, lifetimes, constants, etc.) as a side-effect
+/// (e.g. formatting, like `PrettyPrinter` implementors do) or by
+/// constructing some alternative representation (e.g. an AST),
+/// which the associated types allow passing through the methods.
+///
+/// For pretty-printing/formatting in particular, see `PrettyPrinter`.
+//
+// FIXME(eddyb) find a better name; this is more general than "printing".
+pub trait Printer<'tcx>: Sized {
+    type Error;
+
+    type Path;
+    type Region;
+    type Type;
+    type DynExistential;
+    type Const;
+
+    fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
+
+    fn print_def_path(
+        self,
+        def_id: DefId,
+        substs: &'tcx [GenericArg<'tcx>],
+    ) -> Result<Self::Path, Self::Error> {
+        self.default_print_def_path(def_id, substs)
+    }
+
+    fn print_impl_path(
+        self,
+        impl_def_id: DefId,
+        substs: &'tcx [GenericArg<'tcx>],
+        self_ty: Ty<'tcx>,
+        trait_ref: Option<ty::TraitRef<'tcx>>,
+    ) -> Result<Self::Path, Self::Error> {
+        self.default_print_impl_path(impl_def_id, substs, self_ty, trait_ref)
+    }
+
+    fn print_region(self, region: ty::Region<'tcx>) -> Result<Self::Region, Self::Error>;
+
+    fn print_type(self, ty: Ty<'tcx>) -> Result<Self::Type, Self::Error>;
+
+    fn print_dyn_existential(
+        self,
+        predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
+    ) -> Result<Self::DynExistential, Self::Error>;
+
+    fn print_const(self, ct: ty::Const<'tcx>) -> Result<Self::Const, Self::Error>;
+
+    fn path_crate(self, cnum: CrateNum) -> Result<Self::Path, Self::Error>;
+
+    fn path_qualified(
+        self,
+        self_ty: Ty<'tcx>,
+        trait_ref: Option<ty::TraitRef<'tcx>>,
+    ) -> Result<Self::Path, Self::Error>;
+
+    fn path_append_impl(
+        self,
+        print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
+        disambiguated_data: &DisambiguatedDefPathData,
+        self_ty: Ty<'tcx>,
+        trait_ref: Option<ty::TraitRef<'tcx>>,
+    ) -> Result<Self::Path, Self::Error>;
+
+    fn path_append(
+        self,
+        print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
+        disambiguated_data: &DisambiguatedDefPathData,
+    ) -> Result<Self::Path, Self::Error>;
+
+    fn path_generic_args(
+        self,
+        print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
+        args: &[GenericArg<'tcx>],
+    ) -> Result<Self::Path, Self::Error>;
+
+    // Defaults (should not be overridden):
+
+    #[instrument(skip(self), level = "debug")]
+    fn default_print_def_path(
+        self,
+        def_id: DefId,
+        substs: &'tcx [GenericArg<'tcx>],
+    ) -> Result<Self::Path, Self::Error> {
+        let key = self.tcx().def_key(def_id);
+        debug!(?key);
+
+        match key.disambiguated_data.data {
+            DefPathData::CrateRoot => {
+                assert!(key.parent.is_none());
+                self.path_crate(def_id.krate)
+            }
+
+            DefPathData::Impl => {
+                let generics = self.tcx().generics_of(def_id);
+                let self_ty = self.tcx().bound_type_of(def_id);
+                let impl_trait_ref = self.tcx().bound_impl_trait_ref(def_id);
+                let (self_ty, impl_trait_ref) = if substs.len() >= generics.count() {
+                    (
+                        self_ty.subst(self.tcx(), substs),
+                        impl_trait_ref.map(|i| i.subst(self.tcx(), substs)),
+                    )
+                } else {
+                    (self_ty.0, impl_trait_ref.map(|i| i.0))
+                };
+                self.print_impl_path(def_id, substs, self_ty, impl_trait_ref)
+            }
+
+            _ => {
+                let parent_def_id = DefId { index: key.parent.unwrap(), ..def_id };
+
+                let mut parent_substs = substs;
+                let mut trait_qualify_parent = false;
+                if !substs.is_empty() {
+                    let generics = self.tcx().generics_of(def_id);
+                    parent_substs = &substs[..generics.parent_count.min(substs.len())];
+
+                    match key.disambiguated_data.data {
+                        // Closures' own generics are only captures, don't print them.
+                        DefPathData::ClosureExpr => {}
+                        // This covers both `DefKind::AnonConst` and `DefKind::InlineConst`.
+                        // Anon consts doesn't have their own generics, and inline consts' own
+                        // generics are their inferred types, so don't print them.
+                        DefPathData::AnonConst => {}
+
+                        // If we have any generic arguments to print, we do that
+                        // on top of the same path, but without its own generics.
+                        _ => {
+                            if !generics.params.is_empty() && substs.len() >= generics.count() {
+                                let args = generics.own_substs_no_defaults(self.tcx(), substs);
+                                return self.path_generic_args(
+                                    |cx| cx.print_def_path(def_id, parent_substs),
+                                    args,
+                                );
+                            }
+                        }
+                    }
+
+                    // FIXME(eddyb) try to move this into the parent's printing
+                    // logic, instead of doing it when printing the child.
+                    trait_qualify_parent = generics.has_self
+                        && generics.parent == Some(parent_def_id)
+                        && parent_substs.len() == generics.parent_count
+                        && self.tcx().generics_of(parent_def_id).parent_count == 0;
+                }
+
+                self.path_append(
+                    |cx: Self| {
+                        if trait_qualify_parent {
+                            let trait_ref = ty::TraitRef::new(
+                                parent_def_id,
+                                cx.tcx().intern_substs(parent_substs),
+                            );
+                            cx.path_qualified(trait_ref.self_ty(), Some(trait_ref))
+                        } else {
+                            cx.print_def_path(parent_def_id, parent_substs)
+                        }
+                    },
+                    &key.disambiguated_data,
+                )
+            }
+        }
+    }
+
+    fn default_print_impl_path(
+        self,
+        impl_def_id: DefId,
+        _substs: &'tcx [GenericArg<'tcx>],
+        self_ty: Ty<'tcx>,
+        impl_trait_ref: Option<ty::TraitRef<'tcx>>,
+    ) -> Result<Self::Path, Self::Error> {
+        debug!(
+            "default_print_impl_path: impl_def_id={:?}, self_ty={}, impl_trait_ref={:?}",
+            impl_def_id, self_ty, impl_trait_ref
+        );
+
+        let key = self.tcx().def_key(impl_def_id);
+        let parent_def_id = DefId { index: key.parent.unwrap(), ..impl_def_id };
+
+        // Decide whether to print the parent path for the impl.
+        // Logically, since impls are global, it's never needed, but
+        // users may find it useful. Currently, we omit the parent if
+        // the impl is either in the same module as the self-type or
+        // as the trait.
+        let in_self_mod = match characteristic_def_id_of_type(self_ty) {
+            None => false,
+            Some(ty_def_id) => self.tcx().parent(ty_def_id) == parent_def_id,
+        };
+        let in_trait_mod = match impl_trait_ref {
+            None => false,
+            Some(trait_ref) => self.tcx().parent(trait_ref.def_id) == parent_def_id,
+        };
+
+        if !in_self_mod && !in_trait_mod {
+            // If the impl is not co-located with either self-type or
+            // trait-type, then fallback to a format that identifies
+            // the module more clearly.
+            self.path_append_impl(
+                |cx| cx.print_def_path(parent_def_id, &[]),
+                &key.disambiguated_data,
+                self_ty,
+                impl_trait_ref,
+            )
+        } else {
+            // Otherwise, try to give a good form that would be valid language
+            // syntax. Preferably using associated item notation.
+            self.path_qualified(self_ty, impl_trait_ref)
+        }
+    }
+}
+
+/// As a heuristic, when we see an impl, if we see that the
+/// 'self type' is a type defined in the same module as the impl,
+/// we can omit including the path to the impl itself. This
+/// function tries to find a "characteristic `DefId`" for a
+/// type. It's just a heuristic so it makes some questionable
+/// decisions and we may want to adjust it later.
+///
+/// Visited set is needed to avoid full iteration over
+/// deeply nested tuples that have no DefId.
+fn characteristic_def_id_of_type_cached<'a>(
+    ty: Ty<'a>,
+    visited: &mut SsoHashSet<Ty<'a>>,
+) -> Option<DefId> {
+    match *ty.kind() {
+        ty::Adt(adt_def, _) => Some(adt_def.did()),
+
+        ty::Dynamic(data, ..) => data.principal_def_id(),
+
+        ty::Array(subty, _) | ty::Slice(subty) => {
+            characteristic_def_id_of_type_cached(subty, visited)
+        }
+
+        ty::RawPtr(mt) => characteristic_def_id_of_type_cached(mt.ty, visited),
+
+        ty::Ref(_, ty, _) => characteristic_def_id_of_type_cached(ty, visited),
+
+        ty::Tuple(ref tys) => tys.iter().find_map(|ty| {
+            if visited.insert(ty) {
+                return characteristic_def_id_of_type_cached(ty, visited);
+            }
+            return None;
+        }),
+
+        ty::FnDef(def_id, _)
+        | ty::Closure(def_id, _)
+        | ty::Generator(def_id, _, _)
+        | ty::Foreign(def_id) => Some(def_id),
+
+        ty::Bool
+        | ty::Char
+        | ty::Int(_)
+        | ty::Uint(_)
+        | ty::Str
+        | ty::FnPtr(_)
+        | ty::Projection(_)
+        | ty::Placeholder(..)
+        | ty::Param(_)
+        | ty::Opaque(..)
+        | ty::Infer(_)
+        | ty::Bound(..)
+        | ty::Error(_)
+        | ty::GeneratorWitness(..)
+        | ty::Never
+        | ty::Float(_) => None,
+    }
+}
+pub fn characteristic_def_id_of_type(ty: Ty<'_>) -> Option<DefId> {
+    characteristic_def_id_of_type_cached(ty, &mut SsoHashSet::new())
+}
+
+impl<'tcx, P: Printer<'tcx>> Print<'tcx, P> for ty::Region<'tcx> {
+    type Output = P::Region;
+    type Error = P::Error;
+    fn print(&self, cx: P) -> Result<Self::Output, Self::Error> {
+        cx.print_region(*self)
+    }
+}
+
+impl<'tcx, P: Printer<'tcx>> Print<'tcx, P> for Ty<'tcx> {
+    type Output = P::Type;
+    type Error = P::Error;
+
+    fn print(&self, cx: P) -> Result<Self::Output, Self::Error> {
+        cx.print_type(*self)
+    }
+}
+
+impl<'tcx, P: Printer<'tcx>> Print<'tcx, P>
+    for &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>
+{
+    type Output = P::DynExistential;
+    type Error = P::Error;
+    fn print(&self, cx: P) -> Result<Self::Output, Self::Error> {
+        cx.print_dyn_existential(self)
+    }
+}
+
+impl<'tcx, P: Printer<'tcx>> Print<'tcx, P> for ty::Const<'tcx> {
+    type Output = P::Const;
+    type Error = P::Error;
+    fn print(&self, cx: P) -> Result<Self::Output, Self::Error> {
+        cx.print_const(*self)
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
new file mode 100644
index 000000000..7f2e81a71
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -0,0 +1,2789 @@
+use crate::mir::interpret::{AllocRange, GlobalAlloc, Pointer, Provenance, Scalar};
+use crate::ty::subst::{GenericArg, GenericArgKind, Subst};
+use crate::ty::{
+    self, ConstInt, DefIdTree, ParamConst, ScalarInt, Term, Ty, TyCtxt, TypeFoldable,
+    TypeSuperFoldable, TypeSuperVisitable, TypeVisitable,
+};
+use rustc_apfloat::ieee::{Double, Single};
+use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
+use rustc_data_structures::sso::SsoHashSet;
+use rustc_hir as hir;
+use rustc_hir::def::{self, CtorKind, DefKind, Namespace};
+use rustc_hir::def_id::{DefId, DefIdSet, CRATE_DEF_ID, LOCAL_CRATE};
+use rustc_hir::definitions::{DefPathData, DefPathDataName, DisambiguatedDefPathData};
+use rustc_session::config::TrimmedDefPaths;
+use rustc_session::cstore::{ExternCrate, ExternCrateSource};
+use rustc_span::symbol::{kw, Ident, Symbol};
+use rustc_target::abi::Size;
+use rustc_target::spec::abi::Abi;
+
+use std::cell::Cell;
+use std::char;
+use std::collections::BTreeMap;
+use std::convert::TryFrom;
+use std::fmt::{self, Write as _};
+use std::iter;
+use std::ops::{ControlFlow, Deref, DerefMut};
+
+// `pretty` is a separate module only for organization.
+use super::*;
+
+macro_rules! p {
+    (@$lit:literal) => {
+        write!(scoped_cx!(), $lit)?
+    };
+    (@write($($data:expr),+)) => {
+        write!(scoped_cx!(), $($data),+)?
+    };
+    (@print($x:expr)) => {
+        scoped_cx!() = $x.print(scoped_cx!())?
+    };
+    (@$method:ident($($arg:expr),*)) => {
+        scoped_cx!() = scoped_cx!().$method($($arg),*)?
+    };
+    ($($elem:tt $(($($args:tt)*))?),+) => {{
+        $(p!(@ $elem $(($($args)*))?);)+
+    }};
+}
+macro_rules! define_scoped_cx {
+    ($cx:ident) => {
+        #[allow(unused_macros)]
+        macro_rules! scoped_cx {
+            () => {
+                $cx
+            };
+        }
+    };
+}
+
+thread_local! {
+    static FORCE_IMPL_FILENAME_LINE: Cell<bool> = const { Cell::new(false) };
+    static SHOULD_PREFIX_WITH_CRATE: Cell<bool> = const { Cell::new(false) };
+    static NO_TRIMMED_PATH: Cell<bool> = const { Cell::new(false) };
+    static NO_QUERIES: Cell<bool> = const { Cell::new(false) };
+    static NO_VISIBLE_PATH: Cell<bool> = const { Cell::new(false) };
+}
+
+macro_rules! define_helper {
+    ($($(#[$a:meta])* fn $name:ident($helper:ident, $tl:ident);)+) => {
+        $(
+            #[must_use]
+            pub struct $helper(bool);
+
+            impl $helper {
+                pub fn new() -> $helper {
+                    $helper($tl.with(|c| c.replace(true)))
+                }
+            }
+
+            $(#[$a])*
+            pub macro $name($e:expr) {
+                {
+                    let _guard = $helper::new();
+                    $e
+                }
+            }
+
+            impl Drop for $helper {
+                fn drop(&mut self) {
+                    $tl.with(|c| c.set(self.0))
+                }
+            }
+        )+
+    }
+}
+
+define_helper!(
+    /// Avoids running any queries during any prints that occur
+    /// during the closure. This may alter the appearance of some
+    /// types (e.g. forcing verbose printing for opaque types).
+    /// This method is used during some queries (e.g. `explicit_item_bounds`
+    /// for opaque types), to ensure that any debug printing that
+    /// occurs during the query computation does not end up recursively
+    /// calling the same query.
+    fn with_no_queries(NoQueriesGuard, NO_QUERIES);
+    /// Force us to name impls with just the filename/line number. We
+    /// normally try to use types. But at some points, notably while printing
+    /// cycle errors, this can result in extra or suboptimal error output,
+    /// so this variable disables that check.
+    fn with_forced_impl_filename_line(ForcedImplGuard, FORCE_IMPL_FILENAME_LINE);
+    /// Adds the `crate::` prefix to paths where appropriate.
+    fn with_crate_prefix(CratePrefixGuard, SHOULD_PREFIX_WITH_CRATE);
+    /// Prevent path trimming if it is turned on. Path trimming affects `Display` impl
+    /// of various rustc types, for example `std::vec::Vec` would be trimmed to `Vec`,
+    /// if no other `Vec` is found.
+    fn with_no_trimmed_paths(NoTrimmedGuard, NO_TRIMMED_PATH);
+    /// Prevent selection of visible paths. `Display` impl of DefId will prefer
+    /// visible (public) reexports of types as paths.
+    fn with_no_visible_paths(NoVisibleGuard, NO_VISIBLE_PATH);
+);
+
+/// The "region highlights" are used to control region printing during
+/// specific error messages. When a "region highlight" is enabled, it
+/// gives an alternate way to print specific regions. For now, we
+/// always print those regions using a number, so something like "`'0`".
+///
+/// Regions not selected by the region highlight mode are presently
+/// unaffected.
+#[derive(Copy, Clone)]
+pub struct RegionHighlightMode<'tcx> {
+    tcx: TyCtxt<'tcx>,
+
+    /// If enabled, when we see the selected region, use "`'N`"
+    /// instead of the ordinary behavior.
+    highlight_regions: [Option<(ty::Region<'tcx>, usize)>; 3],
+
+    /// If enabled, when printing a "free region" that originated from
+    /// the given `ty::BoundRegionKind`, print it as "`'1`". Free regions that would ordinarily
+    /// have names print as normal.
+    ///
+    /// This is used when you have a signature like `fn foo(x: &u32,
+    /// y: &'a u32)` and we want to give a name to the region of the
+    /// reference `x`.
+    highlight_bound_region: Option<(ty::BoundRegionKind, usize)>,
+}
+
+impl<'tcx> RegionHighlightMode<'tcx> {
+    pub fn new(tcx: TyCtxt<'tcx>) -> Self {
+        Self {
+            tcx,
+            highlight_regions: Default::default(),
+            highlight_bound_region: Default::default(),
+        }
+    }
+
+    /// If `region` and `number` are both `Some`, invokes
+    /// `highlighting_region`.
+    pub fn maybe_highlighting_region(
+        &mut self,
+        region: Option<ty::Region<'tcx>>,
+        number: Option<usize>,
+    ) {
+        if let Some(k) = region {
+            if let Some(n) = number {
+                self.highlighting_region(k, n);
+            }
+        }
+    }
+
+    /// Highlights the region inference variable `vid` as `'N`.
+    pub fn highlighting_region(&mut self, region: ty::Region<'tcx>, number: usize) {
+        let num_slots = self.highlight_regions.len();
+        let first_avail_slot =
+            self.highlight_regions.iter_mut().find(|s| s.is_none()).unwrap_or_else(|| {
+                bug!("can only highlight {} placeholders at a time", num_slots,)
+            });
+        *first_avail_slot = Some((region, number));
+    }
+
+    /// Convenience wrapper for `highlighting_region`.
+    pub fn highlighting_region_vid(&mut self, vid: ty::RegionVid, number: usize) {
+        self.highlighting_region(self.tcx.mk_region(ty::ReVar(vid)), number)
+    }
+
+    /// Returns `Some(n)` with the number to use for the given region, if any.
+    fn region_highlighted(&self, region: ty::Region<'tcx>) -> Option<usize> {
+        self.highlight_regions.iter().find_map(|h| match h {
+            Some((r, n)) if *r == region => Some(*n),
+            _ => None,
+        })
+    }
+
+    /// Highlight the given bound region.
+    /// We can only highlight one bound region at a time. See
+    /// the field `highlight_bound_region` for more detailed notes.
+    pub fn highlighting_bound_region(&mut self, br: ty::BoundRegionKind, number: usize) {
+        assert!(self.highlight_bound_region.is_none());
+        self.highlight_bound_region = Some((br, number));
+    }
+}
+
+/// Trait for printers that pretty-print using `fmt::Write` to the printer.
+pub trait PrettyPrinter<'tcx>:
+    Printer<
+        'tcx,
+        Error = fmt::Error,
+        Path = Self,
+        Region = Self,
+        Type = Self,
+        DynExistential = Self,
+        Const = Self,
+    > + fmt::Write
+{
+    /// Like `print_def_path` but for value paths.
+    fn print_value_path(
+        self,
+        def_id: DefId,
+        substs: &'tcx [GenericArg<'tcx>],
+    ) -> Result<Self::Path, Self::Error> {
+        self.print_def_path(def_id, substs)
+    }
+
+    fn in_binder<T>(self, value: &ty::Binder<'tcx, T>) -> Result<Self, Self::Error>
+    where
+        T: Print<'tcx, Self, Output = Self, Error = Self::Error> + TypeFoldable<'tcx>,
+    {
+        value.as_ref().skip_binder().print(self)
+    }
+
+    fn wrap_binder<T, F: FnOnce(&T, Self) -> Result<Self, fmt::Error>>(
+        self,
+        value: &ty::Binder<'tcx, T>,
+        f: F,
+    ) -> Result<Self, Self::Error>
+    where
+        T: Print<'tcx, Self, Output = Self, Error = Self::Error> + TypeFoldable<'tcx>,
+    {
+        f(value.as_ref().skip_binder(), self)
+    }
+
+    /// Prints comma-separated elements.
+    fn comma_sep<T>(mut self, mut elems: impl Iterator<Item = T>) -> Result<Self, Self::Error>
+    where
+        T: Print<'tcx, Self, Output = Self, Error = Self::Error>,
+    {
+        if let Some(first) = elems.next() {
+            self = first.print(self)?;
+            for elem in elems {
+                self.write_str(", ")?;
+                self = elem.print(self)?;
+            }
+        }
+        Ok(self)
+    }
+
+    /// Prints `{f: t}` or `{f as t}` depending on the `cast` argument
+    fn typed_value(
+        mut self,
+        f: impl FnOnce(Self) -> Result<Self, Self::Error>,
+        t: impl FnOnce(Self) -> Result<Self, Self::Error>,
+        conversion: &str,
+    ) -> Result<Self::Const, Self::Error> {
+        self.write_str("{")?;
+        self = f(self)?;
+        self.write_str(conversion)?;
+        self = t(self)?;
+        self.write_str("}")?;
+        Ok(self)
+    }
+
+    /// Prints `<...>` around what `f` prints.
+    fn generic_delimiters(
+        self,
+        f: impl FnOnce(Self) -> Result<Self, Self::Error>,
+    ) -> Result<Self, Self::Error>;
+
+    /// Returns `true` if the region should be printed in
+    /// optional positions, e.g., `&'a T` or `dyn Tr + 'b`.
+    /// This is typically the case for all non-`'_` regions.
+    fn should_print_region(&self, region: ty::Region<'tcx>) -> bool;
+
+    // Defaults (should not be overridden):
+
+    /// If possible, this returns a global path resolving to `def_id` that is visible
+    /// from at least one local module, and returns `true`. If the crate defining `def_id` is
+    /// declared with an `extern crate`, the path is guaranteed to use the `extern crate`.
+    fn try_print_visible_def_path(self, def_id: DefId) -> Result<(Self, bool), Self::Error> {
+        if NO_VISIBLE_PATH.with(|flag| flag.get()) {
+            return Ok((self, false));
+        }
+
+        let mut callers = Vec::new();
+        self.try_print_visible_def_path_recur(def_id, &mut callers)
+    }
+
+    /// Try to see if this path can be trimmed to a unique symbol name.
+    fn try_print_trimmed_def_path(
+        mut self,
+        def_id: DefId,
+    ) -> Result<(Self::Path, bool), Self::Error> {
+        if !self.tcx().sess.opts.unstable_opts.trim_diagnostic_paths
+            || matches!(self.tcx().sess.opts.trimmed_def_paths, TrimmedDefPaths::Never)
+            || NO_TRIMMED_PATH.with(|flag| flag.get())
+            || SHOULD_PREFIX_WITH_CRATE.with(|flag| flag.get())
+        {
+            return Ok((self, false));
+        }
+
+        match self.tcx().trimmed_def_paths(()).get(&def_id) {
+            None => Ok((self, false)),
+            Some(symbol) => {
+                self.write_str(symbol.as_str())?;
+                Ok((self, true))
+            }
+        }
+    }
+
+    /// Does the work of `try_print_visible_def_path`, building the
+    /// full definition path recursively before attempting to
+    /// post-process it into the valid and visible version that
+    /// accounts for re-exports.
+    ///
+    /// This method should only be called by itself or
+    /// `try_print_visible_def_path`.
+    ///
+    /// `callers` is a chain of visible_parent's leading to `def_id`,
+    /// to support cycle detection during recursion.
+    ///
+    /// This method returns false if we can't print the visible path, so
+    /// `print_def_path` can fall back on the item's real definition path.
+    fn try_print_visible_def_path_recur(
+        mut self,
+        def_id: DefId,
+        callers: &mut Vec<DefId>,
+    ) -> Result<(Self, bool), Self::Error> {
+        define_scoped_cx!(self);
+
+        debug!("try_print_visible_def_path: def_id={:?}", def_id);
+
+        // If `def_id` is a direct or injected extern crate, return the
+        // path to the crate followed by the path to the item within the crate.
+        if let Some(cnum) = def_id.as_crate_root() {
+            if cnum == LOCAL_CRATE {
+                return Ok((self.path_crate(cnum)?, true));
+            }
+
+            // In local mode, when we encounter a crate other than
+            // LOCAL_CRATE, execution proceeds in one of two ways:
+            //
+            // 1. For a direct dependency, where user added an
+            //    `extern crate` manually, we put the `extern
+            //    crate` as the parent. So you wind up with
+            //    something relative to the current crate.
+            // 2. For an extern inferred from a path or an indirect crate,
+            //    where there is no explicit `extern crate`, we just prepend
+            //    the crate name.
+            match self.tcx().extern_crate(def_id) {
+                Some(&ExternCrate { src, dependency_of, span, .. }) => match (src, dependency_of) {
+                    (ExternCrateSource::Extern(def_id), LOCAL_CRATE) => {
+                        // NOTE(eddyb) the only reason `span` might be dummy,
+                        // that we're aware of, is that it's the `std`/`core`
+                        // `extern crate` injected by default.
+                        // FIXME(eddyb) find something better to key this on,
+                        // or avoid ending up with `ExternCrateSource::Extern`,
+                        // for the injected `std`/`core`.
+                        if span.is_dummy() {
+                            return Ok((self.path_crate(cnum)?, true));
+                        }
+
+                        // Disable `try_print_trimmed_def_path` behavior within
+                        // the `print_def_path` call, to avoid infinite recursion
+                        // in cases where the `extern crate foo` has non-trivial
+                        // parents, e.g. it's nested in `impl foo::Trait for Bar`
+                        // (see also issues #55779 and #87932).
+                        self = with_no_visible_paths!(self.print_def_path(def_id, &[])?);
+
+                        return Ok((self, true));
+                    }
+                    (ExternCrateSource::Path, LOCAL_CRATE) => {
+                        return Ok((self.path_crate(cnum)?, true));
+                    }
+                    _ => {}
+                },
+                None => {
+                    return Ok((self.path_crate(cnum)?, true));
+                }
+            }
+        }
+
+        if def_id.is_local() {
+            return Ok((self, false));
+        }
+
+        let visible_parent_map = self.tcx().visible_parent_map(());
+
+        let mut cur_def_key = self.tcx().def_key(def_id);
+        debug!("try_print_visible_def_path: cur_def_key={:?}", cur_def_key);
+
+        // For a constructor, we want the name of its parent rather than <unnamed>.
+        if let DefPathData::Ctor = cur_def_key.disambiguated_data.data {
+            let parent = DefId {
+                krate: def_id.krate,
+                index: cur_def_key
+                    .parent
+                    .expect("`DefPathData::Ctor` / `VariantData` missing a parent"),
+            };
+
+            cur_def_key = self.tcx().def_key(parent);
+        }
+
+        let Some(visible_parent) = visible_parent_map.get(&def_id).cloned() else {
+            return Ok((self, false));
+        };
+
+        let actual_parent = self.tcx().opt_parent(def_id);
+        debug!(
+            "try_print_visible_def_path: visible_parent={:?} actual_parent={:?}",
+            visible_parent, actual_parent,
+        );
+
+        let mut data = cur_def_key.disambiguated_data.data;
+        debug!(
+            "try_print_visible_def_path: data={:?} visible_parent={:?} actual_parent={:?}",
+            data, visible_parent, actual_parent,
+        );
+
+        match data {
+            // In order to output a path that could actually be imported (valid and visible),
+            // we need to handle re-exports correctly.
+            //
+            // For example, take `std::os::unix::process::CommandExt`, this trait is actually
+            // defined at `std::sys::unix::ext::process::CommandExt` (at time of writing).
+            //
+            // `std::os::unix` reexports the contents of `std::sys::unix::ext`. `std::sys` is
+            // private so the "true" path to `CommandExt` isn't accessible.
+            //
+            // In this case, the `visible_parent_map` will look something like this:
+            //
+            // (child) -> (parent)
+            // `std::sys::unix::ext::process::CommandExt` -> `std::sys::unix::ext::process`
+            // `std::sys::unix::ext::process` -> `std::sys::unix::ext`
+            // `std::sys::unix::ext` -> `std::os`
+            //
+            // This is correct, as the visible parent of `std::sys::unix::ext` is in fact
+            // `std::os`.
+            //
+            // When printing the path to `CommandExt` and looking at the `cur_def_key` that
+            // corresponds to `std::sys::unix::ext`, we would normally print `ext` and then go
+            // to the parent - resulting in a mangled path like
+            // `std::os::ext::process::CommandExt`.
+            //
+            // Instead, we must detect that there was a re-export and instead print `unix`
+            // (which is the name `std::sys::unix::ext` was re-exported as in `std::os`). To
+            // do this, we compare the parent of `std::sys::unix::ext` (`std::sys::unix`) with
+            // the visible parent (`std::os`). If these do not match, then we iterate over
+            // the children of the visible parent (as was done when computing
+            // `visible_parent_map`), looking for the specific child we currently have and then
+            // have access to the re-exported name.
+            DefPathData::TypeNs(ref mut name) if Some(visible_parent) != actual_parent => {
+                // Item might be re-exported several times, but filter for the one
+                // that's public and whose identifier isn't `_`.
+                let reexport = self
+                    .tcx()
+                    .module_children(visible_parent)
+                    .iter()
+                    .filter(|child| child.res.opt_def_id() == Some(def_id))
+                    .find(|child| child.vis.is_public() && child.ident.name != kw::Underscore)
+                    .map(|child| child.ident.name);
+
+                if let Some(new_name) = reexport {
+                    *name = new_name;
+                } else {
+                    // There is no name that is public and isn't `_`, so bail.
+                    return Ok((self, false));
+                }
+            }
+            // Re-exported `extern crate` (#43189).
+            DefPathData::CrateRoot => {
+                data = DefPathData::TypeNs(self.tcx().crate_name(def_id.krate));
+            }
+            _ => {}
+        }
+        debug!("try_print_visible_def_path: data={:?}", data);
+
+        if callers.contains(&visible_parent) {
+            return Ok((self, false));
+        }
+        callers.push(visible_parent);
+        // HACK(eddyb) this bypasses `path_append`'s prefix printing to avoid
+        // knowing ahead of time whether the entire path will succeed or not.
+        // To support printers that do not implement `PrettyPrinter`, a `Vec` or
+        // linked list on the stack would need to be built, before any printing.
+        match self.try_print_visible_def_path_recur(visible_parent, callers)? {
+            (cx, false) => return Ok((cx, false)),
+            (cx, true) => self = cx,
+        }
+        callers.pop();
+
+        Ok((self.path_append(Ok, &DisambiguatedDefPathData { data, disambiguator: 0 })?, true))
+    }
+
+    fn pretty_path_qualified(
+        self,
+        self_ty: Ty<'tcx>,
+        trait_ref: Option<ty::TraitRef<'tcx>>,
+    ) -> Result<Self::Path, Self::Error> {
+        if trait_ref.is_none() {
+            // Inherent impls. Try to print `Foo::bar` for an inherent
+            // impl on `Foo`, but fallback to `<Foo>::bar` if self-type is
+            // anything other than a simple path.
+            match self_ty.kind() {
+                ty::Adt(..)
+                | ty::Foreign(_)
+                | ty::Bool
+                | ty::Char
+                | ty::Str
+                | ty::Int(_)
+                | ty::Uint(_)
+                | ty::Float(_) => {
+                    return self_ty.print(self);
+                }
+
+                _ => {}
+            }
+        }
+
+        self.generic_delimiters(|mut cx| {
+            define_scoped_cx!(cx);
+
+            p!(print(self_ty));
+            if let Some(trait_ref) = trait_ref {
+                p!(" as ", print(trait_ref.print_only_trait_path()));
+            }
+            Ok(cx)
+        })
+    }
+
+    fn pretty_path_append_impl(
+        mut self,
+        print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
+        self_ty: Ty<'tcx>,
+        trait_ref: Option<ty::TraitRef<'tcx>>,
+    ) -> Result<Self::Path, Self::Error> {
+        self = print_prefix(self)?;
+
+        self.generic_delimiters(|mut cx| {
+            define_scoped_cx!(cx);
+
+            p!("impl ");
+            if let Some(trait_ref) = trait_ref {
+                p!(print(trait_ref.print_only_trait_path()), " for ");
+            }
+            p!(print(self_ty));
+
+            Ok(cx)
+        })
+    }
+
+    fn pretty_print_type(mut self, ty: Ty<'tcx>) -> Result<Self::Type, Self::Error> {
+        define_scoped_cx!(self);
+
+        match *ty.kind() {
+            ty::Bool => p!("bool"),
+            ty::Char => p!("char"),
+            ty::Int(t) => p!(write("{}", t.name_str())),
+            ty::Uint(t) => p!(write("{}", t.name_str())),
+            ty::Float(t) => p!(write("{}", t.name_str())),
+            ty::RawPtr(ref tm) => {
+                p!(write(
+                    "*{} ",
+                    match tm.mutbl {
+                        hir::Mutability::Mut => "mut",
+                        hir::Mutability::Not => "const",
+                    }
+                ));
+                p!(print(tm.ty))
+            }
+            ty::Ref(r, ty, mutbl) => {
+                p!("&");
+                if self.should_print_region(r) {
+                    p!(print(r), " ");
+                }
+                p!(print(ty::TypeAndMut { ty, mutbl }))
+            }
+            ty::Never => p!("!"),
+            ty::Tuple(ref tys) => {
+                p!("(", comma_sep(tys.iter()));
+                if tys.len() == 1 {
+                    p!(",");
+                }
+                p!(")")
+            }
+            ty::FnDef(def_id, substs) => {
+                let sig = self.tcx().bound_fn_sig(def_id).subst(self.tcx(), substs);
+                p!(print(sig), " {{", print_value_path(def_id, substs), "}}");
+            }
+            ty::FnPtr(ref bare_fn) => p!(print(bare_fn)),
+            ty::Infer(infer_ty) => {
+                let verbose = self.tcx().sess.verbose();
+                if let ty::TyVar(ty_vid) = infer_ty {
+                    if let Some(name) = self.ty_infer_name(ty_vid) {
+                        p!(write("{}", name))
+                    } else {
+                        if verbose {
+                            p!(write("{:?}", infer_ty))
+                        } else {
+                            p!(write("{}", infer_ty))
+                        }
+                    }
+                } else {
+                    if verbose { p!(write("{:?}", infer_ty)) } else { p!(write("{}", infer_ty)) }
+                }
+            }
+            ty::Error(_) => p!("[type error]"),
+            ty::Param(ref param_ty) => p!(print(param_ty)),
+            ty::Bound(debruijn, bound_ty) => match bound_ty.kind {
+                ty::BoundTyKind::Anon => self.pretty_print_bound_var(debruijn, bound_ty.var)?,
+                ty::BoundTyKind::Param(p) => p!(write("{}", p)),
+            },
+            ty::Adt(def, substs) => {
+                p!(print_def_path(def.did(), substs));
+            }
+            ty::Dynamic(data, r) => {
+                let print_r = self.should_print_region(r);
+                if print_r {
+                    p!("(");
+                }
+                p!("dyn ", print(data));
+                if print_r {
+                    p!(" + ", print(r), ")");
+                }
+            }
+            ty::Foreign(def_id) => {
+                p!(print_def_path(def_id, &[]));
+            }
+            ty::Projection(ref data) => p!(print(data)),
+            ty::Placeholder(placeholder) => p!(write("Placeholder({:?})", placeholder)),
+            ty::Opaque(def_id, substs) => {
+                // FIXME(eddyb) print this with `print_def_path`.
+                // We use verbose printing in 'NO_QUERIES' mode, to
+                // avoid needing to call `predicates_of`. This should
+                // only affect certain debug messages (e.g. messages printed
+                // from `rustc_middle::ty` during the computation of `tcx.predicates_of`),
+                // and should have no effect on any compiler output.
+                if self.tcx().sess.verbose() || NO_QUERIES.with(|q| q.get()) {
+                    p!(write("Opaque({:?}, {:?})", def_id, substs));
+                    return Ok(self);
+                }
+
+                let parent = self.tcx().parent(def_id);
+                match self.tcx().def_kind(parent) {
+                    DefKind::TyAlias | DefKind::AssocTy => {
+                        if let ty::Opaque(d, _) = *self.tcx().type_of(parent).kind() {
+                            if d == def_id {
+                                // If the type alias directly starts with the `impl` of the
+                                // opaque type we're printing, then skip the `::{opaque#1}`.
+                                p!(print_def_path(parent, substs));
+                                return Ok(self);
+                            }
+                        }
+                        // Complex opaque type, e.g. `type Foo = (i32, impl Debug);`
+                        p!(print_def_path(def_id, substs));
+                        return Ok(self);
+                    }
+                    _ => return self.pretty_print_opaque_impl_type(def_id, substs),
+                }
+            }
+            ty::Str => p!("str"),
+            ty::Generator(did, substs, movability) => {
+                p!(write("["));
+                match movability {
+                    hir::Movability::Movable => {}
+                    hir::Movability::Static => p!("static "),
+                }
+
+                if !self.tcx().sess.verbose() {
+                    p!("generator");
+                    // FIXME(eddyb) should use `def_span`.
+                    if let Some(did) = did.as_local() {
+                        let span = self.tcx().def_span(did);
+                        p!(write(
+                            "@{}",
+                            // This may end up in stderr diagnostics but it may also be emitted
+                            // into MIR. Hence we use the remapped path if available
+                            self.tcx().sess.source_map().span_to_embeddable_string(span)
+                        ));
+                    } else {
+                        p!(write("@"), print_def_path(did, substs));
+                    }
+                } else {
+                    p!(print_def_path(did, substs));
+                    p!(" upvar_tys=(");
+                    if !substs.as_generator().is_valid() {
+                        p!("unavailable");
+                    } else {
+                        self = self.comma_sep(substs.as_generator().upvar_tys())?;
+                    }
+                    p!(")");
+
+                    if substs.as_generator().is_valid() {
+                        p!(" ", print(substs.as_generator().witness()));
+                    }
+                }
+
+                p!("]")
+            }
+            ty::GeneratorWitness(types) => {
+                p!(in_binder(&types));
+            }
+            ty::Closure(did, substs) => {
+                p!(write("["));
+                if !self.tcx().sess.verbose() {
+                    p!(write("closure"));
+                    // FIXME(eddyb) should use `def_span`.
+                    if let Some(did) = did.as_local() {
+                        if self.tcx().sess.opts.unstable_opts.span_free_formats {
+                            p!("@", print_def_path(did.to_def_id(), substs));
+                        } else {
+                            let span = self.tcx().def_span(did);
+                            p!(write(
+                                "@{}",
+                                // This may end up in stderr diagnostics but it may also be emitted
+                                // into MIR. Hence we use the remapped path if available
+                                self.tcx().sess.source_map().span_to_embeddable_string(span)
+                            ));
+                        }
+                    } else {
+                        p!(write("@"), print_def_path(did, substs));
+                    }
+                } else {
+                    p!(print_def_path(did, substs));
+                    if !substs.as_closure().is_valid() {
+                        p!(" closure_substs=(unavailable)");
+                        p!(write(" substs={:?}", substs));
+                    } else {
+                        p!(" closure_kind_ty=", print(substs.as_closure().kind_ty()));
+                        p!(
+                            " closure_sig_as_fn_ptr_ty=",
+                            print(substs.as_closure().sig_as_fn_ptr_ty())
+                        );
+                        p!(" upvar_tys=(");
+                        self = self.comma_sep(substs.as_closure().upvar_tys())?;
+                        p!(")");
+                    }
+                }
+                p!("]");
+            }
+            ty::Array(ty, sz) => {
+                p!("[", print(ty), "; ");
+                if self.tcx().sess.verbose() {
+                    p!(write("{:?}", sz));
+                } else if let ty::ConstKind::Unevaluated(..) = sz.kind() {
+                    // Do not try to evaluate unevaluated constants. If we are const evaluating an
+                    // array length anon const, rustc will (with debug assertions) print the
+                    // constant's path. Which will end up here again.
+                    p!("_");
+                } else if let Some(n) = sz.kind().try_to_bits(self.tcx().data_layout.pointer_size) {
+                    p!(write("{}", n));
+                } else if let ty::ConstKind::Param(param) = sz.kind() {
+                    p!(print(param));
+                } else {
+                    p!("_");
+                }
+                p!("]")
+            }
+            ty::Slice(ty) => p!("[", print(ty), "]"),
+        }
+
+        Ok(self)
+    }
+
+    fn pretty_print_opaque_impl_type(
+        mut self,
+        def_id: DefId,
+        substs: &'tcx ty::List<ty::GenericArg<'tcx>>,
+    ) -> Result<Self::Type, Self::Error> {
+        let tcx = self.tcx();
+
+        // Grab the "TraitA + TraitB" from `impl TraitA + TraitB`,
+        // by looking up the projections associated with the def_id.
+        let bounds = tcx.bound_explicit_item_bounds(def_id);
+
+        let mut traits = FxIndexMap::default();
+        let mut fn_traits = FxIndexMap::default();
+        let mut is_sized = false;
+
+        for predicate in bounds.transpose_iter().map(|e| e.map_bound(|(p, _)| *p)) {
+            let predicate = predicate.subst(tcx, substs);
+            let bound_predicate = predicate.kind();
+
+            match bound_predicate.skip_binder() {
+                ty::PredicateKind::Trait(pred) => {
+                    let trait_ref = bound_predicate.rebind(pred.trait_ref);
+
+                    // Don't print + Sized, but rather + ?Sized if absent.
+                    if Some(trait_ref.def_id()) == tcx.lang_items().sized_trait() {
+                        is_sized = true;
+                        continue;
+                    }
+
+                    self.insert_trait_and_projection(trait_ref, None, &mut traits, &mut fn_traits);
+                }
+                ty::PredicateKind::Projection(pred) => {
+                    let proj_ref = bound_predicate.rebind(pred);
+                    let trait_ref = proj_ref.required_poly_trait_ref(tcx);
+
+                    // Projection type entry -- the def-id for naming, and the ty.
+                    let proj_ty = (proj_ref.projection_def_id(), proj_ref.term());
+
+                    self.insert_trait_and_projection(
+                        trait_ref,
+                        Some(proj_ty),
+                        &mut traits,
+                        &mut fn_traits,
+                    );
+                }
+                _ => {}
+            }
+        }
+
+        write!(self, "impl ")?;
+
+        let mut first = true;
+        // Insert parenthesis around (Fn(A, B) -> C) if the opaque ty has more than one other trait
+        let paren_needed = fn_traits.len() > 1 || traits.len() > 0 || !is_sized;
+
+        for (fn_once_trait_ref, entry) in fn_traits {
+            write!(self, "{}", if first { "" } else { " + " })?;
+            write!(self, "{}", if paren_needed { "(" } else { "" })?;
+
+            self = self.wrap_binder(&fn_once_trait_ref, |trait_ref, mut cx| {
+                define_scoped_cx!(cx);
+                // Get the (single) generic ty (the args) of this FnOnce trait ref.
+                let generics = tcx.generics_of(trait_ref.def_id);
+                let args = generics.own_substs_no_defaults(tcx, trait_ref.substs);
+
+                match (entry.return_ty, args[0].expect_ty()) {
+                    // We can only print `impl Fn() -> ()` if we have a tuple of args and we recorded
+                    // a return type.
+                    (Some(return_ty), arg_tys) if matches!(arg_tys.kind(), ty::Tuple(_)) => {
+                        let name = if entry.fn_trait_ref.is_some() {
+                            "Fn"
+                        } else if entry.fn_mut_trait_ref.is_some() {
+                            "FnMut"
+                        } else {
+                            "FnOnce"
+                        };
+
+                        p!(write("{}(", name));
+
+                        for (idx, ty) in arg_tys.tuple_fields().iter().enumerate() {
+                            if idx > 0 {
+                                p!(", ");
+                            }
+                            p!(print(ty));
+                        }
+
+                        p!(")");
+                        if let Term::Ty(ty) = return_ty.skip_binder() {
+                            if !ty.is_unit() {
+                                p!(" -> ", print(return_ty));
+                            }
+                        }
+                        p!(write("{}", if paren_needed { ")" } else { "" }));
+
+                        first = false;
+                    }
+                    // If we got here, we can't print as a `impl Fn(A, B) -> C`. Just record the
+                    // trait_refs we collected in the OpaqueFnEntry as normal trait refs.
+                    _ => {
+                        if entry.has_fn_once {
+                            traits.entry(fn_once_trait_ref).or_default().extend(
+                                // Group the return ty with its def id, if we had one.
+                                entry
+                                    .return_ty
+                                    .map(|ty| (tcx.lang_items().fn_once_output().unwrap(), ty)),
+                            );
+                        }
+                        if let Some(trait_ref) = entry.fn_mut_trait_ref {
+                            traits.entry(trait_ref).or_default();
+                        }
+                        if let Some(trait_ref) = entry.fn_trait_ref {
+                            traits.entry(trait_ref).or_default();
+                        }
+                    }
+                }
+
+                Ok(cx)
+            })?;
+        }
+
+        // Print the rest of the trait types (that aren't Fn* family of traits)
+        for (trait_ref, assoc_items) in traits {
+            write!(self, "{}", if first { "" } else { " + " })?;
+
+            self = self.wrap_binder(&trait_ref, |trait_ref, mut cx| {
+                define_scoped_cx!(cx);
+                p!(print(trait_ref.print_only_trait_name()));
+
+                let generics = tcx.generics_of(trait_ref.def_id);
+                let args = generics.own_substs_no_defaults(tcx, trait_ref.substs);
+
+                if !args.is_empty() || !assoc_items.is_empty() {
+                    let mut first = true;
+
+                    for ty in args {
+                        if first {
+                            p!("<");
+                            first = false;
+                        } else {
+                            p!(", ");
+                        }
+                        p!(print(ty));
+                    }
+
+                    for (assoc_item_def_id, term) in assoc_items {
+                        // Skip printing `<[generator@] as Generator<_>>::Return` from async blocks,
+                        // unless we can find out what generator return type it comes from.
+                        let term = if let Some(ty) = term.skip_binder().ty()
+                            && let ty::Projection(ty::ProjectionTy { item_def_id, substs }) = ty.kind()
+                            && Some(*item_def_id) == tcx.lang_items().generator_return()
+                        {
+                            if let ty::Generator(_, substs, _) = substs.type_at(0).kind() {
+                                let return_ty = substs.as_generator().return_ty();
+                                if !return_ty.is_ty_infer() {
+                                    return_ty.into()
+                                } else {
+                                    continue;
+                                }
+                            } else {
+                                continue;
+                            }
+                        } else {
+                            term.skip_binder()
+                        };
+
+                        if first {
+                            p!("<");
+                            first = false;
+                        } else {
+                            p!(", ");
+                        }
+
+                        p!(write("{} = ", tcx.associated_item(assoc_item_def_id).name));
+
+                        match term {
+                            Term::Ty(ty) => {
+                                p!(print(ty))
+                            }
+                            Term::Const(c) => {
+                                p!(print(c));
+                            }
+                        };
+                    }
+
+                    if !first {
+                        p!(">");
+                    }
+                }
+
+                first = false;
+                Ok(cx)
+            })?;
+        }
+
+        if !is_sized {
+            write!(self, "{}?Sized", if first { "" } else { " + " })?;
+        } else if first {
+            write!(self, "Sized")?;
+        }
+
+        Ok(self)
+    }
+
+    /// Insert the trait ref and optionally a projection type associated with it into either the
+    /// traits map or fn_traits map, depending on if the trait is in the Fn* family of traits.
+    fn insert_trait_and_projection(
+        &mut self,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+        proj_ty: Option<(DefId, ty::Binder<'tcx, Term<'tcx>>)>,
+        traits: &mut FxIndexMap<
+            ty::PolyTraitRef<'tcx>,
+            FxIndexMap<DefId, ty::Binder<'tcx, Term<'tcx>>>,
+        >,
+        fn_traits: &mut FxIndexMap<ty::PolyTraitRef<'tcx>, OpaqueFnEntry<'tcx>>,
+    ) {
+        let trait_def_id = trait_ref.def_id();
+
+        // If our trait_ref is FnOnce or any of its children, project it onto the parent FnOnce
+        // super-trait ref and record it there.
+        if let Some(fn_once_trait) = self.tcx().lang_items().fn_once_trait() {
+            // If we have a FnOnce, then insert it into
+            if trait_def_id == fn_once_trait {
+                let entry = fn_traits.entry(trait_ref).or_default();
+                // Optionally insert the return_ty as well.
+                if let Some((_, ty)) = proj_ty {
+                    entry.return_ty = Some(ty);
+                }
+                entry.has_fn_once = true;
+                return;
+            } else if Some(trait_def_id) == self.tcx().lang_items().fn_mut_trait() {
+                let super_trait_ref = crate::traits::util::supertraits(self.tcx(), trait_ref)
+                    .find(|super_trait_ref| super_trait_ref.def_id() == fn_once_trait)
+                    .unwrap();
+
+                fn_traits.entry(super_trait_ref).or_default().fn_mut_trait_ref = Some(trait_ref);
+                return;
+            } else if Some(trait_def_id) == self.tcx().lang_items().fn_trait() {
+                let super_trait_ref = crate::traits::util::supertraits(self.tcx(), trait_ref)
+                    .find(|super_trait_ref| super_trait_ref.def_id() == fn_once_trait)
+                    .unwrap();
+
+                fn_traits.entry(super_trait_ref).or_default().fn_trait_ref = Some(trait_ref);
+                return;
+            }
+        }
+
+        // Otherwise, just group our traits and projection types.
+        traits.entry(trait_ref).or_default().extend(proj_ty);
+    }
+
+    fn pretty_print_bound_var(
+        &mut self,
+        debruijn: ty::DebruijnIndex,
+        var: ty::BoundVar,
+    ) -> Result<(), Self::Error> {
+        if debruijn == ty::INNERMOST {
+            write!(self, "^{}", var.index())
+        } else {
+            write!(self, "^{}_{}", debruijn.index(), var.index())
+        }
+    }
+
+    fn ty_infer_name(&self, _: ty::TyVid) -> Option<Symbol> {
+        None
+    }
+
+    fn const_infer_name(&self, _: ty::ConstVid<'tcx>) -> Option<Symbol> {
+        None
+    }
+
+    fn pretty_print_dyn_existential(
+        mut self,
+        predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
+    ) -> Result<Self::DynExistential, Self::Error> {
+        // Generate the main trait ref, including associated types.
+        let mut first = true;
+
+        if let Some(principal) = predicates.principal() {
+            self = self.wrap_binder(&principal, |principal, mut cx| {
+                define_scoped_cx!(cx);
+                p!(print_def_path(principal.def_id, &[]));
+
+                let mut resugared = false;
+
+                // Special-case `Fn(...) -> ...` and re-sugar it.
+                let fn_trait_kind = cx.tcx().fn_trait_kind_from_lang_item(principal.def_id);
+                if !cx.tcx().sess.verbose() && fn_trait_kind.is_some() {
+                    if let ty::Tuple(tys) = principal.substs.type_at(0).kind() {
+                        let mut projections = predicates.projection_bounds();
+                        if let (Some(proj), None) = (projections.next(), projections.next()) {
+                            p!(pretty_fn_sig(
+                                tys,
+                                false,
+                                proj.skip_binder().term.ty().expect("Return type was a const")
+                            ));
+                            resugared = true;
+                        }
+                    }
+                }
+
+                // HACK(eddyb) this duplicates `FmtPrinter`'s `path_generic_args`,
+                // in order to place the projections inside the `<...>`.
+                if !resugared {
+                    // Use a type that can't appear in defaults of type parameters.
+                    let dummy_cx = cx.tcx().mk_ty_infer(ty::FreshTy(0));
+                    let principal = principal.with_self_ty(cx.tcx(), dummy_cx);
+
+                    let args = cx
+                        .tcx()
+                        .generics_of(principal.def_id)
+                        .own_substs_no_defaults(cx.tcx(), principal.substs);
+
+                    // Don't print `'_` if there's no unerased regions.
+                    let print_regions = args.iter().any(|arg| match arg.unpack() {
+                        GenericArgKind::Lifetime(r) => !r.is_erased(),
+                        _ => false,
+                    });
+                    let mut args = args.iter().cloned().filter(|arg| match arg.unpack() {
+                        GenericArgKind::Lifetime(_) => print_regions,
+                        _ => true,
+                    });
+                    let mut projections = predicates.projection_bounds();
+
+                    let arg0 = args.next();
+                    let projection0 = projections.next();
+                    if arg0.is_some() || projection0.is_some() {
+                        let args = arg0.into_iter().chain(args);
+                        let projections = projection0.into_iter().chain(projections);
+
+                        p!(generic_delimiters(|mut cx| {
+                            cx = cx.comma_sep(args)?;
+                            if arg0.is_some() && projection0.is_some() {
+                                write!(cx, ", ")?;
+                            }
+                            cx.comma_sep(projections)
+                        }));
+                    }
+                }
+                Ok(cx)
+            })?;
+
+            first = false;
+        }
+
+        define_scoped_cx!(self);
+
+        // Builtin bounds.
+        // FIXME(eddyb) avoid printing twice (needed to ensure
+        // that the auto traits are sorted *and* printed via cx).
+        let mut auto_traits: Vec<_> = predicates.auto_traits().collect();
+
+        // The auto traits come ordered by `DefPathHash`. While
+        // `DefPathHash` is *stable* in the sense that it depends on
+        // neither the host nor the phase of the moon, it depends
+        // "pseudorandomly" on the compiler version and the target.
+        //
+        // To avoid causing instabilities in compiletest
+        // output, sort the auto-traits alphabetically.
+        auto_traits.sort_by_cached_key(|did| self.tcx().def_path_str(*did));
+
+        for def_id in auto_traits {
+            if !first {
+                p!(" + ");
+            }
+            first = false;
+
+            p!(print_def_path(def_id, &[]));
+        }
+
+        Ok(self)
+    }
+
+    fn pretty_fn_sig(
+        mut self,
+        inputs: &[Ty<'tcx>],
+        c_variadic: bool,
+        output: Ty<'tcx>,
+    ) -> Result<Self, Self::Error> {
+        define_scoped_cx!(self);
+
+        p!("(", comma_sep(inputs.iter().copied()));
+        if c_variadic {
+            if !inputs.is_empty() {
+                p!(", ");
+            }
+            p!("...");
+        }
+        p!(")");
+        if !output.is_unit() {
+            p!(" -> ", print(output));
+        }
+
+        Ok(self)
+    }
+
+    fn pretty_print_const(
+        mut self,
+        ct: ty::Const<'tcx>,
+        print_ty: bool,
+    ) -> Result<Self::Const, Self::Error> {
+        define_scoped_cx!(self);
+
+        if self.tcx().sess.verbose() {
+            p!(write("Const({:?}: {:?})", ct.kind(), ct.ty()));
+            return Ok(self);
+        }
+
+        macro_rules! print_underscore {
+            () => {{
+                if print_ty {
+                    self = self.typed_value(
+                        |mut this| {
+                            write!(this, "_")?;
+                            Ok(this)
+                        },
+                        |this| this.print_type(ct.ty()),
+                        ": ",
+                    )?;
+                } else {
+                    write!(self, "_")?;
+                }
+            }};
+        }
+
+        match ct.kind() {
+            ty::ConstKind::Unevaluated(ty::Unevaluated {
+                def,
+                substs,
+                promoted: Some(promoted),
+            }) => {
+                p!(print_value_path(def.did, substs));
+                p!(write("::{:?}", promoted));
+            }
+            ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted: None }) => {
+                match self.tcx().def_kind(def.did) {
+                    DefKind::Static(..) | DefKind::Const | DefKind::AssocConst => {
+                        p!(print_value_path(def.did, substs))
+                    }
+                    _ => {
+                        if def.is_local() {
+                            let span = self.tcx().def_span(def.did);
+                            if let Ok(snip) = self.tcx().sess.source_map().span_to_snippet(span) {
+                                p!(write("{}", snip))
+                            } else {
+                                print_underscore!()
+                            }
+                        } else {
+                            print_underscore!()
+                        }
+                    }
+                }
+            }
+            ty::ConstKind::Infer(infer_ct) => {
+                match infer_ct {
+                    ty::InferConst::Var(ct_vid)
+                        if let Some(name) = self.const_infer_name(ct_vid) =>
+                            p!(write("{}", name)),
+                    _ => print_underscore!(),
+                }
+            }
+            ty::ConstKind::Param(ParamConst { name, .. }) => p!(write("{}", name)),
+            ty::ConstKind::Value(value) => {
+                return self.pretty_print_const_valtree(value, ct.ty(), print_ty);
+            }
+
+            ty::ConstKind::Bound(debruijn, bound_var) => {
+                self.pretty_print_bound_var(debruijn, bound_var)?
+            }
+            ty::ConstKind::Placeholder(placeholder) => p!(write("Placeholder({:?})", placeholder)),
+            ty::ConstKind::Error(_) => p!("[const error]"),
+        };
+        Ok(self)
+    }
+
+    fn pretty_print_const_scalar(
+        self,
+        scalar: Scalar,
+        ty: Ty<'tcx>,
+        print_ty: bool,
+    ) -> Result<Self::Const, Self::Error> {
+        match scalar {
+            Scalar::Ptr(ptr, _size) => self.pretty_print_const_scalar_ptr(ptr, ty, print_ty),
+            Scalar::Int(int) => self.pretty_print_const_scalar_int(int, ty, print_ty),
+        }
+    }
+
+    fn pretty_print_const_scalar_ptr(
+        mut self,
+        ptr: Pointer,
+        ty: Ty<'tcx>,
+        print_ty: bool,
+    ) -> Result<Self::Const, Self::Error> {
+        define_scoped_cx!(self);
+
+        let (alloc_id, offset) = ptr.into_parts();
+        match ty.kind() {
+            // Byte strings (&[u8; N])
+            ty::Ref(_, inner, _) => {
+                if let ty::Array(elem, len) = inner.kind() {
+                    if let ty::Uint(ty::UintTy::U8) = elem.kind() {
+                        if let ty::ConstKind::Value(ty::ValTree::Leaf(int)) = len.kind() {
+                            match self.tcx().try_get_global_alloc(alloc_id) {
+                                Some(GlobalAlloc::Memory(alloc)) => {
+                                    let len = int.assert_bits(self.tcx().data_layout.pointer_size);
+                                    let range =
+                                        AllocRange { start: offset, size: Size::from_bytes(len) };
+                                    if let Ok(byte_str) =
+                                        alloc.inner().get_bytes(&self.tcx(), range)
+                                    {
+                                        p!(pretty_print_byte_str(byte_str))
+                                    } else {
+                                        p!("<too short allocation>")
+                                    }
+                                }
+                                // FIXME: for statics, vtables, and functions, we could in principle print more detail.
+                                Some(GlobalAlloc::Static(def_id)) => {
+                                    p!(write("<static({:?})>", def_id))
+                                }
+                                Some(GlobalAlloc::Function(_)) => p!("<function>"),
+                                Some(GlobalAlloc::VTable(..)) => p!("<vtable>"),
+                                None => p!("<dangling pointer>"),
+                            }
+                            return Ok(self);
+                        }
+                    }
+                }
+            }
+            ty::FnPtr(_) => {
+                // FIXME: We should probably have a helper method to share code with the "Byte strings"
+                // printing above (which also has to handle pointers to all sorts of things).
+                if let Some(GlobalAlloc::Function(instance)) =
+                    self.tcx().try_get_global_alloc(alloc_id)
+                {
+                    self = self.typed_value(
+                        |this| this.print_value_path(instance.def_id(), instance.substs),
+                        |this| this.print_type(ty),
+                        " as ",
+                    )?;
+                    return Ok(self);
+                }
+            }
+            _ => {}
+        }
+        // Any pointer values not covered by a branch above
+        self = self.pretty_print_const_pointer(ptr, ty, print_ty)?;
+        Ok(self)
+    }
+
+    fn pretty_print_const_scalar_int(
+        mut self,
+        int: ScalarInt,
+        ty: Ty<'tcx>,
+        print_ty: bool,
+    ) -> Result<Self::Const, Self::Error> {
+        define_scoped_cx!(self);
+
+        match ty.kind() {
+            // Bool
+            ty::Bool if int == ScalarInt::FALSE => p!("false"),
+            ty::Bool if int == ScalarInt::TRUE => p!("true"),
+            // Float
+            ty::Float(ty::FloatTy::F32) => {
+                p!(write("{}f32", Single::try_from(int).unwrap()))
+            }
+            ty::Float(ty::FloatTy::F64) => {
+                p!(write("{}f64", Double::try_from(int).unwrap()))
+            }
+            // Int
+            ty::Uint(_) | ty::Int(_) => {
+                let int =
+                    ConstInt::new(int, matches!(ty.kind(), ty::Int(_)), ty.is_ptr_sized_integral());
+                if print_ty { p!(write("{:#?}", int)) } else { p!(write("{:?}", int)) }
+            }
+            // Char
+            ty::Char if char::try_from(int).is_ok() => {
+                p!(write("{:?}", char::try_from(int).unwrap()))
+            }
+            // Pointer types
+            ty::Ref(..) | ty::RawPtr(_) | ty::FnPtr(_) => {
+                let data = int.assert_bits(self.tcx().data_layout.pointer_size);
+                self = self.typed_value(
+                    |mut this| {
+                        write!(this, "0x{:x}", data)?;
+                        Ok(this)
+                    },
+                    |this| this.print_type(ty),
+                    " as ",
+                )?;
+            }
+            // Nontrivial types with scalar bit representation
+            _ => {
+                let print = |mut this: Self| {
+                    if int.size() == Size::ZERO {
+                        write!(this, "transmute(())")?;
+                    } else {
+                        write!(this, "transmute(0x{:x})", int)?;
+                    }
+                    Ok(this)
+                };
+                self = if print_ty {
+                    self.typed_value(print, |this| this.print_type(ty), ": ")?
+                } else {
+                    print(self)?
+                };
+            }
+        }
+        Ok(self)
+    }
+
+    /// This is overridden for MIR printing because we only want to hide alloc ids from users, not
+    /// from MIR where it is actually useful.
+    fn pretty_print_const_pointer<Prov: Provenance>(
+        mut self,
+        _: Pointer<Prov>,
+        ty: Ty<'tcx>,
+        print_ty: bool,
+    ) -> Result<Self::Const, Self::Error> {
+        if print_ty {
+            self.typed_value(
+                |mut this| {
+                    this.write_str("&_")?;
+                    Ok(this)
+                },
+                |this| this.print_type(ty),
+                ": ",
+            )
+        } else {
+            self.write_str("&_")?;
+            Ok(self)
+        }
+    }
+
+    fn pretty_print_byte_str(mut self, byte_str: &'tcx [u8]) -> Result<Self::Const, Self::Error> {
+        define_scoped_cx!(self);
+        p!("b\"");
+        for &c in byte_str {
+            for e in std::ascii::escape_default(c) {
+                self.write_char(e as char)?;
+            }
+        }
+        p!("\"");
+        Ok(self)
+    }
+
+    fn pretty_print_const_valtree(
+        mut self,
+        valtree: ty::ValTree<'tcx>,
+        ty: Ty<'tcx>,
+        print_ty: bool,
+    ) -> Result<Self::Const, Self::Error> {
+        define_scoped_cx!(self);
+
+        if self.tcx().sess.verbose() {
+            p!(write("ValTree({:?}: ", valtree), print(ty), ")");
+            return Ok(self);
+        }
+
+        let u8_type = self.tcx().types.u8;
+        match (valtree, ty.kind()) {
+            (ty::ValTree::Branch(_), ty::Ref(_, inner_ty, _)) => match inner_ty.kind() {
+                ty::Slice(t) if *t == u8_type => {
+                    let bytes = valtree.try_to_raw_bytes(self.tcx(), ty).unwrap_or_else(|| {
+                        bug!(
+                            "expected to convert valtree {:?} to raw bytes for type {:?}",
+                            valtree,
+                            t
+                        )
+                    });
+                    return self.pretty_print_byte_str(bytes);
+                }
+                ty::Str => {
+                    let bytes = valtree.try_to_raw_bytes(self.tcx(), ty).unwrap_or_else(|| {
+                        bug!("expected to convert valtree to raw bytes for type {:?}", ty)
+                    });
+                    p!(write("{:?}", String::from_utf8_lossy(bytes)));
+                    return Ok(self);
+                }
+                _ => {
+                    p!("&");
+                    p!(pretty_print_const_valtree(valtree, *inner_ty, print_ty));
+                    return Ok(self);
+                }
+            },
+            (ty::ValTree::Branch(_), ty::Array(t, _)) if *t == u8_type => {
+                let bytes = valtree.try_to_raw_bytes(self.tcx(), ty).unwrap_or_else(|| {
+                    bug!("expected to convert valtree to raw bytes for type {:?}", t)
+                });
+                p!("*");
+                p!(pretty_print_byte_str(bytes));
+                return Ok(self);
+            }
+            // Aggregates, printed as array/tuple/struct/variant construction syntax.
+            (ty::ValTree::Branch(_), ty::Array(..) | ty::Tuple(..) | ty::Adt(..)) => {
+                let contents =
+                    self.tcx().destructure_const(ty::Const::from_value(self.tcx(), valtree, ty));
+                let fields = contents.fields.iter().copied();
+                match *ty.kind() {
+                    ty::Array(..) => {
+                        p!("[", comma_sep(fields), "]");
+                    }
+                    ty::Tuple(..) => {
+                        p!("(", comma_sep(fields));
+                        if contents.fields.len() == 1 {
+                            p!(",");
+                        }
+                        p!(")");
+                    }
+                    ty::Adt(def, _) if def.variants().is_empty() => {
+                        self = self.typed_value(
+                            |mut this| {
+                                write!(this, "unreachable()")?;
+                                Ok(this)
+                            },
+                            |this| this.print_type(ty),
+                            ": ",
+                        )?;
+                    }
+                    ty::Adt(def, substs) => {
+                        let variant_idx =
+                            contents.variant.expect("destructed const of adt without variant idx");
+                        let variant_def = &def.variant(variant_idx);
+                        p!(print_value_path(variant_def.def_id, substs));
+                        match variant_def.ctor_kind {
+                            CtorKind::Const => {}
+                            CtorKind::Fn => {
+                                p!("(", comma_sep(fields), ")");
+                            }
+                            CtorKind::Fictive => {
+                                p!(" {{ ");
+                                let mut first = true;
+                                for (field_def, field) in iter::zip(&variant_def.fields, fields) {
+                                    if !first {
+                                        p!(", ");
+                                    }
+                                    p!(write("{}: ", field_def.name), print(field));
+                                    first = false;
+                                }
+                                p!(" }}");
+                            }
+                        }
+                    }
+                    _ => unreachable!(),
+                }
+                return Ok(self);
+            }
+            (ty::ValTree::Leaf(leaf), _) => {
+                return self.pretty_print_const_scalar_int(leaf, ty, print_ty);
+            }
+            // FIXME(oli-obk): also pretty print arrays and other aggregate constants by reading
+            // their fields instead of just dumping the memory.
+            _ => {}
+        }
+
+        // fallback
+        if valtree == ty::ValTree::zst() {
+            p!(write("<ZST>"));
+        } else {
+            p!(write("{:?}", valtree));
+        }
+        if print_ty {
+            p!(": ", print(ty));
+        }
+        Ok(self)
+    }
+}
+
+// HACK(eddyb) boxed to avoid moving around a large struct by-value.
+pub struct FmtPrinter<'a, 'tcx>(Box<FmtPrinterData<'a, 'tcx>>);
+
+pub struct FmtPrinterData<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    fmt: String,
+
+    empty_path: bool,
+    in_value: bool,
+    pub print_alloc_ids: bool,
+
+    used_region_names: FxHashSet<Symbol>,
+    region_index: usize,
+    binder_depth: usize,
+    printed_type_count: usize,
+
+    pub region_highlight_mode: RegionHighlightMode<'tcx>,
+
+    pub ty_infer_name_resolver: Option<Box<dyn Fn(ty::TyVid) -> Option<Symbol> + 'a>>,
+    pub const_infer_name_resolver: Option<Box<dyn Fn(ty::ConstVid<'tcx>) -> Option<Symbol> + 'a>>,
+}
+
+impl<'a, 'tcx> Deref for FmtPrinter<'a, 'tcx> {
+    type Target = FmtPrinterData<'a, 'tcx>;
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl DerefMut for FmtPrinter<'_, '_> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+impl<'a, 'tcx> FmtPrinter<'a, 'tcx> {
+    pub fn new(tcx: TyCtxt<'tcx>, ns: Namespace) -> Self {
+        FmtPrinter(Box::new(FmtPrinterData {
+            tcx,
+            // Estimated reasonable capacity to allocate upfront based on a few
+            // benchmarks.
+            fmt: String::with_capacity(64),
+            empty_path: false,
+            in_value: ns == Namespace::ValueNS,
+            print_alloc_ids: false,
+            used_region_names: Default::default(),
+            region_index: 0,
+            binder_depth: 0,
+            printed_type_count: 0,
+            region_highlight_mode: RegionHighlightMode::new(tcx),
+            ty_infer_name_resolver: None,
+            const_infer_name_resolver: None,
+        }))
+    }
+
+    pub fn into_buffer(self) -> String {
+        self.0.fmt
+    }
+}
+
+// HACK(eddyb) get rid of `def_path_str` and/or pass `Namespace` explicitly always
+// (but also some things just print a `DefId` generally so maybe we need this?)
+fn guess_def_namespace(tcx: TyCtxt<'_>, def_id: DefId) -> Namespace {
+    match tcx.def_key(def_id).disambiguated_data.data {
+        DefPathData::TypeNs(..) | DefPathData::CrateRoot | DefPathData::ImplTrait => {
+            Namespace::TypeNS
+        }
+
+        DefPathData::ValueNs(..)
+        | DefPathData::AnonConst
+        | DefPathData::ClosureExpr
+        | DefPathData::Ctor => Namespace::ValueNS,
+
+        DefPathData::MacroNs(..) => Namespace::MacroNS,
+
+        _ => Namespace::TypeNS,
+    }
+}
+
+impl<'t> TyCtxt<'t> {
+    /// Returns a string identifying this `DefId`. This string is
+    /// suitable for user output.
+    pub fn def_path_str(self, def_id: DefId) -> String {
+        self.def_path_str_with_substs(def_id, &[])
+    }
+
+    pub fn def_path_str_with_substs(self, def_id: DefId, substs: &'t [GenericArg<'t>]) -> String {
+        let ns = guess_def_namespace(self, def_id);
+        debug!("def_path_str: def_id={:?}, ns={:?}", def_id, ns);
+        FmtPrinter::new(self, ns).print_def_path(def_id, substs).unwrap().into_buffer()
+    }
+}
+
+impl fmt::Write for FmtPrinter<'_, '_> {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        self.fmt.push_str(s);
+        Ok(())
+    }
+}
+
+impl<'tcx> Printer<'tcx> for FmtPrinter<'_, 'tcx> {
+    type Error = fmt::Error;
+
+    type Path = Self;
+    type Region = Self;
+    type Type = Self;
+    type DynExistential = Self;
+    type Const = Self;
+
+    fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn print_def_path(
+        mut self,
+        def_id: DefId,
+        substs: &'tcx [GenericArg<'tcx>],
+    ) -> Result<Self::Path, Self::Error> {
+        define_scoped_cx!(self);
+
+        if substs.is_empty() {
+            match self.try_print_trimmed_def_path(def_id)? {
+                (cx, true) => return Ok(cx),
+                (cx, false) => self = cx,
+            }
+
+            match self.try_print_visible_def_path(def_id)? {
+                (cx, true) => return Ok(cx),
+                (cx, false) => self = cx,
+            }
+        }
+
+        let key = self.tcx.def_key(def_id);
+        if let DefPathData::Impl = key.disambiguated_data.data {
+            // Always use types for non-local impls, where types are always
+            // available, and filename/line-number is mostly uninteresting.
+            let use_types = !def_id.is_local() || {
+                // Otherwise, use filename/line-number if forced.
+                let force_no_types = FORCE_IMPL_FILENAME_LINE.with(|f| f.get());
+                !force_no_types
+            };
+
+            if !use_types {
+                // If no type info is available, fall back to
+                // pretty printing some span information. This should
+                // only occur very early in the compiler pipeline.
+                let parent_def_id = DefId { index: key.parent.unwrap(), ..def_id };
+                let span = self.tcx.def_span(def_id);
+
+                self = self.print_def_path(parent_def_id, &[])?;
+
+                // HACK(eddyb) copy of `path_append` to avoid
+                // constructing a `DisambiguatedDefPathData`.
+                if !self.empty_path {
+                    write!(self, "::")?;
+                }
+                write!(
+                    self,
+                    "<impl at {}>",
+                    // This may end up in stderr diagnostics but it may also be emitted
+                    // into MIR. Hence we use the remapped path if available
+                    self.tcx.sess.source_map().span_to_embeddable_string(span)
+                )?;
+                self.empty_path = false;
+
+                return Ok(self);
+            }
+        }
+
+        self.default_print_def_path(def_id, substs)
+    }
+
+    fn print_region(self, region: ty::Region<'tcx>) -> Result<Self::Region, Self::Error> {
+        self.pretty_print_region(region)
+    }
+
+    fn print_type(mut self, ty: Ty<'tcx>) -> Result<Self::Type, Self::Error> {
+        let type_length_limit = self.tcx.type_length_limit();
+        if type_length_limit.value_within_limit(self.printed_type_count) {
+            self.printed_type_count += 1;
+            self.pretty_print_type(ty)
+        } else {
+            write!(self, "...")?;
+            Ok(self)
+        }
+    }
+
+    fn print_dyn_existential(
+        self,
+        predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
+    ) -> Result<Self::DynExistential, Self::Error> {
+        self.pretty_print_dyn_existential(predicates)
+    }
+
+    fn print_const(self, ct: ty::Const<'tcx>) -> Result<Self::Const, Self::Error> {
+        self.pretty_print_const(ct, false)
+    }
+
+    fn path_crate(mut self, cnum: CrateNum) -> Result<Self::Path, Self::Error> {
+        self.empty_path = true;
+        if cnum == LOCAL_CRATE {
+            if self.tcx.sess.rust_2018() {
+                // We add the `crate::` keyword on Rust 2018, only when desired.
+                if SHOULD_PREFIX_WITH_CRATE.with(|flag| flag.get()) {
+                    write!(self, "{}", kw::Crate)?;
+                    self.empty_path = false;
+                }
+            }
+        } else {
+            write!(self, "{}", self.tcx.crate_name(cnum))?;
+            self.empty_path = false;
+        }
+        Ok(self)
+    }
+
+    fn path_qualified(
+        mut self,
+        self_ty: Ty<'tcx>,
+        trait_ref: Option<ty::TraitRef<'tcx>>,
+    ) -> Result<Self::Path, Self::Error> {
+        self = self.pretty_path_qualified(self_ty, trait_ref)?;
+        self.empty_path = false;
+        Ok(self)
+    }
+
+    fn path_append_impl(
+        mut self,
+        print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
+        _disambiguated_data: &DisambiguatedDefPathData,
+        self_ty: Ty<'tcx>,
+        trait_ref: Option<ty::TraitRef<'tcx>>,
+    ) -> Result<Self::Path, Self::Error> {
+        self = self.pretty_path_append_impl(
+            |mut cx| {
+                cx = print_prefix(cx)?;
+                if !cx.empty_path {
+                    write!(cx, "::")?;
+                }
+
+                Ok(cx)
+            },
+            self_ty,
+            trait_ref,
+        )?;
+        self.empty_path = false;
+        Ok(self)
+    }
+
+    fn path_append(
+        mut self,
+        print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
+        disambiguated_data: &DisambiguatedDefPathData,
+    ) -> Result<Self::Path, Self::Error> {
+        self = print_prefix(self)?;
+
+        // Skip `::{{extern}}` blocks and `::{{constructor}}` on tuple/unit structs.
+        if let DefPathData::ForeignMod | DefPathData::Ctor = disambiguated_data.data {
+            return Ok(self);
+        }
+
+        let name = disambiguated_data.data.name();
+        if !self.empty_path {
+            write!(self, "::")?;
+        }
+
+        if let DefPathDataName::Named(name) = name {
+            if Ident::with_dummy_span(name).is_raw_guess() {
+                write!(self, "r#")?;
+            }
+        }
+
+        let verbose = self.tcx.sess.verbose();
+        disambiguated_data.fmt_maybe_verbose(&mut self, verbose)?;
+
+        self.empty_path = false;
+
+        Ok(self)
+    }
+
+    fn path_generic_args(
+        mut self,
+        print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
+        args: &[GenericArg<'tcx>],
+    ) -> Result<Self::Path, Self::Error> {
+        self = print_prefix(self)?;
+
+        // Don't print `'_` if there's no unerased regions.
+        let print_regions = self.tcx.sess.verbose()
+            || args.iter().any(|arg| match arg.unpack() {
+                GenericArgKind::Lifetime(r) => !r.is_erased(),
+                _ => false,
+            });
+        let args = args.iter().cloned().filter(|arg| match arg.unpack() {
+            GenericArgKind::Lifetime(_) => print_regions,
+            _ => true,
+        });
+
+        if args.clone().next().is_some() {
+            if self.in_value {
+                write!(self, "::")?;
+            }
+            self.generic_delimiters(|cx| cx.comma_sep(args))
+        } else {
+            Ok(self)
+        }
+    }
+}
+
+impl<'tcx> PrettyPrinter<'tcx> for FmtPrinter<'_, 'tcx> {
+    fn ty_infer_name(&self, id: ty::TyVid) -> Option<Symbol> {
+        self.0.ty_infer_name_resolver.as_ref().and_then(|func| func(id))
+    }
+
+    fn const_infer_name(&self, id: ty::ConstVid<'tcx>) -> Option<Symbol> {
+        self.0.const_infer_name_resolver.as_ref().and_then(|func| func(id))
+    }
+
+    fn print_value_path(
+        mut self,
+        def_id: DefId,
+        substs: &'tcx [GenericArg<'tcx>],
+    ) -> Result<Self::Path, Self::Error> {
+        let was_in_value = std::mem::replace(&mut self.in_value, true);
+        self = self.print_def_path(def_id, substs)?;
+        self.in_value = was_in_value;
+
+        Ok(self)
+    }
+
+    fn in_binder<T>(self, value: &ty::Binder<'tcx, T>) -> Result<Self, Self::Error>
+    where
+        T: Print<'tcx, Self, Output = Self, Error = Self::Error> + TypeFoldable<'tcx>,
+    {
+        self.pretty_in_binder(value)
+    }
+
+    fn wrap_binder<T, C: FnOnce(&T, Self) -> Result<Self, Self::Error>>(
+        self,
+        value: &ty::Binder<'tcx, T>,
+        f: C,
+    ) -> Result<Self, Self::Error>
+    where
+        T: Print<'tcx, Self, Output = Self, Error = Self::Error> + TypeFoldable<'tcx>,
+    {
+        self.pretty_wrap_binder(value, f)
+    }
+
+    fn typed_value(
+        mut self,
+        f: impl FnOnce(Self) -> Result<Self, Self::Error>,
+        t: impl FnOnce(Self) -> Result<Self, Self::Error>,
+        conversion: &str,
+    ) -> Result<Self::Const, Self::Error> {
+        self.write_str("{")?;
+        self = f(self)?;
+        self.write_str(conversion)?;
+        let was_in_value = std::mem::replace(&mut self.in_value, false);
+        self = t(self)?;
+        self.in_value = was_in_value;
+        self.write_str("}")?;
+        Ok(self)
+    }
+
+    fn generic_delimiters(
+        mut self,
+        f: impl FnOnce(Self) -> Result<Self, Self::Error>,
+    ) -> Result<Self, Self::Error> {
+        write!(self, "<")?;
+
+        let was_in_value = std::mem::replace(&mut self.in_value, false);
+        let mut inner = f(self)?;
+        inner.in_value = was_in_value;
+
+        write!(inner, ">")?;
+        Ok(inner)
+    }
+
+    fn should_print_region(&self, region: ty::Region<'tcx>) -> bool {
+        let highlight = self.region_highlight_mode;
+        if highlight.region_highlighted(region).is_some() {
+            return true;
+        }
+
+        if self.tcx.sess.verbose() {
+            return true;
+        }
+
+        let identify_regions = self.tcx.sess.opts.unstable_opts.identify_regions;
+
+        match *region {
+            ty::ReEarlyBound(ref data) => {
+                data.name != kw::Empty && data.name != kw::UnderscoreLifetime
+            }
+
+            ty::ReLateBound(_, ty::BoundRegion { kind: br, .. })
+            | ty::ReFree(ty::FreeRegion { bound_region: br, .. })
+            | ty::RePlaceholder(ty::Placeholder { name: br, .. }) => {
+                if let ty::BrNamed(_, name) = br {
+                    if name != kw::Empty && name != kw::UnderscoreLifetime {
+                        return true;
+                    }
+                }
+
+                if let Some((region, _)) = highlight.highlight_bound_region {
+                    if br == region {
+                        return true;
+                    }
+                }
+
+                false
+            }
+
+            ty::ReVar(_) if identify_regions => true,
+
+            ty::ReVar(_) | ty::ReErased => false,
+
+            ty::ReStatic | ty::ReEmpty(_) => true,
+        }
+    }
+
+    fn pretty_print_const_pointer<Prov: Provenance>(
+        self,
+        p: Pointer<Prov>,
+        ty: Ty<'tcx>,
+        print_ty: bool,
+    ) -> Result<Self::Const, Self::Error> {
+        let print = |mut this: Self| {
+            define_scoped_cx!(this);
+            if this.print_alloc_ids {
+                p!(write("{:?}", p));
+            } else {
+                p!("&_");
+            }
+            Ok(this)
+        };
+        if print_ty {
+            self.typed_value(print, |this| this.print_type(ty), ": ")
+        } else {
+            print(self)
+        }
+    }
+}
+
+// HACK(eddyb) limited to `FmtPrinter` because of `region_highlight_mode`.
+impl<'tcx> FmtPrinter<'_, 'tcx> {
+    pub fn pretty_print_region(mut self, region: ty::Region<'tcx>) -> Result<Self, fmt::Error> {
+        define_scoped_cx!(self);
+
+        // Watch out for region highlights.
+        let highlight = self.region_highlight_mode;
+        if let Some(n) = highlight.region_highlighted(region) {
+            p!(write("'{}", n));
+            return Ok(self);
+        }
+
+        if self.tcx.sess.verbose() {
+            p!(write("{:?}", region));
+            return Ok(self);
+        }
+
+        let identify_regions = self.tcx.sess.opts.unstable_opts.identify_regions;
+
+        // These printouts are concise.  They do not contain all the information
+        // the user might want to diagnose an error, but there is basically no way
+        // to fit that into a short string.  Hence the recommendation to use
+        // `explain_region()` or `note_and_explain_region()`.
+        match *region {
+            ty::ReEarlyBound(ref data) => {
+                if data.name != kw::Empty {
+                    p!(write("{}", data.name));
+                    return Ok(self);
+                }
+            }
+            ty::ReLateBound(_, ty::BoundRegion { kind: br, .. })
+            | ty::ReFree(ty::FreeRegion { bound_region: br, .. })
+            | ty::RePlaceholder(ty::Placeholder { name: br, .. }) => {
+                if let ty::BrNamed(_, name) = br {
+                    if name != kw::Empty && name != kw::UnderscoreLifetime {
+                        p!(write("{}", name));
+                        return Ok(self);
+                    }
+                }
+
+                if let Some((region, counter)) = highlight.highlight_bound_region {
+                    if br == region {
+                        p!(write("'{}", counter));
+                        return Ok(self);
+                    }
+                }
+            }
+            ty::ReVar(region_vid) if identify_regions => {
+                p!(write("{:?}", region_vid));
+                return Ok(self);
+            }
+            ty::ReVar(_) => {}
+            ty::ReErased => {}
+            ty::ReStatic => {
+                p!("'static");
+                return Ok(self);
+            }
+            ty::ReEmpty(ty::UniverseIndex::ROOT) => {
+                p!("'<empty>");
+                return Ok(self);
+            }
+            ty::ReEmpty(ui) => {
+                p!(write("'<empty:{:?}>", ui));
+                return Ok(self);
+            }
+        }
+
+        p!("'_");
+
+        Ok(self)
+    }
+}
+
+/// Folds through bound vars and placeholders, naming them
+struct RegionFolder<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    current_index: ty::DebruijnIndex,
+    region_map: BTreeMap<ty::BoundRegion, ty::Region<'tcx>>,
+    name: &'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a),
+}
+
+impl<'a, 'tcx> ty::TypeFolder<'tcx> for RegionFolder<'a, 'tcx> {
+    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_binder<T: TypeFoldable<'tcx>>(
+        &mut self,
+        t: ty::Binder<'tcx, T>,
+    ) -> ty::Binder<'tcx, T> {
+        self.current_index.shift_in(1);
+        let t = t.super_fold_with(self);
+        self.current_index.shift_out(1);
+        t
+    }
+
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+        match *t.kind() {
+            _ if t.has_vars_bound_at_or_above(self.current_index) || t.has_placeholders() => {
+                return t.super_fold_with(self);
+            }
+            _ => {}
+        }
+        t
+    }
+
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        let name = &mut self.name;
+        let region = match *r {
+            ty::ReLateBound(_, br) => *self.region_map.entry(br).or_insert_with(|| name(br)),
+            ty::RePlaceholder(ty::PlaceholderRegion { name: kind, .. }) => {
+                // If this is an anonymous placeholder, don't rename. Otherwise, in some
+                // async fns, we get a `for<'r> Send` bound
+                match kind {
+                    ty::BrAnon(_) | ty::BrEnv => r,
+                    _ => {
+                        // Index doesn't matter, since this is just for naming and these never get bound
+                        let br = ty::BoundRegion { var: ty::BoundVar::from_u32(0), kind };
+                        *self.region_map.entry(br).or_insert_with(|| name(br))
+                    }
+                }
+            }
+            _ => return r,
+        };
+        if let ty::ReLateBound(debruijn1, br) = *region {
+            assert_eq!(debruijn1, ty::INNERMOST);
+            self.tcx.mk_region(ty::ReLateBound(self.current_index, br))
+        } else {
+            region
+        }
+    }
+}
+
+// HACK(eddyb) limited to `FmtPrinter` because of `binder_depth`,
+// `region_index` and `used_region_names`.
+impl<'tcx> FmtPrinter<'_, 'tcx> {
+    pub fn name_all_regions<T>(
+        mut self,
+        value: &ty::Binder<'tcx, T>,
+    ) -> Result<(Self, T, BTreeMap<ty::BoundRegion, ty::Region<'tcx>>), fmt::Error>
+    where
+        T: Print<'tcx, Self, Output = Self, Error = fmt::Error> + TypeFoldable<'tcx>,
+    {
+        fn name_by_region_index(index: usize) -> Symbol {
+            match index {
+                0 => Symbol::intern("'r"),
+                1 => Symbol::intern("'s"),
+                i => Symbol::intern(&format!("'t{}", i - 2)),
+            }
+        }
+
+        // Replace any anonymous late-bound regions with named
+        // variants, using new unique identifiers, so that we can
+        // clearly differentiate between named and unnamed regions in
+        // the output. We'll probably want to tweak this over time to
+        // decide just how much information to give.
+        if self.binder_depth == 0 {
+            self.prepare_late_bound_region_info(value);
+        }
+
+        let mut empty = true;
+        let mut start_or_continue = |cx: &mut Self, start: &str, cont: &str| {
+            let w = if empty {
+                empty = false;
+                start
+            } else {
+                cont
+            };
+            let _ = write!(cx, "{}", w);
+        };
+        let do_continue = |cx: &mut Self, cont: Symbol| {
+            let _ = write!(cx, "{}", cont);
+        };
+
+        define_scoped_cx!(self);
+
+        let mut region_index = self.region_index;
+        let mut next_name = |this: &Self| loop {
+            let name = name_by_region_index(region_index);
+            region_index += 1;
+            if !this.used_region_names.contains(&name) {
+                break name;
+            }
+        };
+
+        // If we want to print verbosely, then print *all* binders, even if they
+        // aren't named. Eventually, we might just want this as the default, but
+        // this is not *quite* right and changes the ordering of some output
+        // anyways.
+        let (new_value, map) = if self.tcx().sess.verbose() {
+            let regions: Vec<_> = value
+                .bound_vars()
+                .into_iter()
+                .map(|var| {
+                    let ty::BoundVariableKind::Region(var) = var else {
+                    // This doesn't really matter because it doesn't get used,
+                    // it's just an empty value
+                    return ty::BrAnon(0);
+                };
+                    match var {
+                        ty::BrAnon(_) | ty::BrEnv => {
+                            start_or_continue(&mut self, "for<", ", ");
+                            let name = next_name(&self);
+                            do_continue(&mut self, name);
+                            ty::BrNamed(CRATE_DEF_ID.to_def_id(), name)
+                        }
+                        ty::BrNamed(def_id, kw::UnderscoreLifetime) => {
+                            start_or_continue(&mut self, "for<", ", ");
+                            let name = next_name(&self);
+                            do_continue(&mut self, name);
+                            ty::BrNamed(def_id, name)
+                        }
+                        ty::BrNamed(def_id, name) => {
+                            start_or_continue(&mut self, "for<", ", ");
+                            do_continue(&mut self, name);
+                            ty::BrNamed(def_id, name)
+                        }
+                    }
+                })
+                .collect();
+            start_or_continue(&mut self, "", "> ");
+
+            self.tcx.replace_late_bound_regions(value.clone(), |br| {
+                let kind = regions[br.var.as_usize()];
+                self.tcx.mk_region(ty::ReLateBound(
+                    ty::INNERMOST,
+                    ty::BoundRegion { var: br.var, kind },
+                ))
+            })
+        } else {
+            let tcx = self.tcx;
+            let mut name = |br: ty::BoundRegion| {
+                start_or_continue(&mut self, "for<", ", ");
+                let kind = match br.kind {
+                    ty::BrAnon(_) | ty::BrEnv => {
+                        let name = next_name(&self);
+                        do_continue(&mut self, name);
+                        ty::BrNamed(CRATE_DEF_ID.to_def_id(), name)
+                    }
+                    ty::BrNamed(def_id, kw::UnderscoreLifetime) => {
+                        let name = next_name(&self);
+                        do_continue(&mut self, name);
+                        ty::BrNamed(def_id, name)
+                    }
+                    ty::BrNamed(_, name) => {
+                        do_continue(&mut self, name);
+                        br.kind
+                    }
+                };
+                tcx.mk_region(ty::ReLateBound(ty::INNERMOST, ty::BoundRegion { var: br.var, kind }))
+            };
+            let mut folder = RegionFolder {
+                tcx,
+                current_index: ty::INNERMOST,
+                name: &mut name,
+                region_map: BTreeMap::new(),
+            };
+            let new_value = value.clone().skip_binder().fold_with(&mut folder);
+            let region_map = folder.region_map;
+            start_or_continue(&mut self, "", "> ");
+            (new_value, region_map)
+        };
+
+        self.binder_depth += 1;
+        self.region_index = region_index;
+        Ok((self, new_value, map))
+    }
+
+    pub fn pretty_in_binder<T>(self, value: &ty::Binder<'tcx, T>) -> Result<Self, fmt::Error>
+    where
+        T: Print<'tcx, Self, Output = Self, Error = fmt::Error> + TypeFoldable<'tcx>,
+    {
+        let old_region_index = self.region_index;
+        let (new, new_value, _) = self.name_all_regions(value)?;
+        let mut inner = new_value.print(new)?;
+        inner.region_index = old_region_index;
+        inner.binder_depth -= 1;
+        Ok(inner)
+    }
+
+    pub fn pretty_wrap_binder<T, C: FnOnce(&T, Self) -> Result<Self, fmt::Error>>(
+        self,
+        value: &ty::Binder<'tcx, T>,
+        f: C,
+    ) -> Result<Self, fmt::Error>
+    where
+        T: Print<'tcx, Self, Output = Self, Error = fmt::Error> + TypeFoldable<'tcx>,
+    {
+        let old_region_index = self.region_index;
+        let (new, new_value, _) = self.name_all_regions(value)?;
+        let mut inner = f(&new_value, new)?;
+        inner.region_index = old_region_index;
+        inner.binder_depth -= 1;
+        Ok(inner)
+    }
+
+    fn prepare_late_bound_region_info<T>(&mut self, value: &ty::Binder<'tcx, T>)
+    where
+        T: TypeVisitable<'tcx>,
+    {
+        struct LateBoundRegionNameCollector<'a, 'tcx> {
+            used_region_names: &'a mut FxHashSet<Symbol>,
+            type_collector: SsoHashSet<Ty<'tcx>>,
+        }
+
+        impl<'tcx> ty::visit::TypeVisitor<'tcx> for LateBoundRegionNameCollector<'_, 'tcx> {
+            type BreakTy = ();
+
+            fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+                trace!("address: {:p}", r.0.0);
+                if let ty::ReLateBound(_, ty::BoundRegion { kind: ty::BrNamed(_, name), .. }) = *r {
+                    self.used_region_names.insert(name);
+                } else if let ty::RePlaceholder(ty::PlaceholderRegion {
+                    name: ty::BrNamed(_, name),
+                    ..
+                }) = *r
+                {
+                    self.used_region_names.insert(name);
+                }
+                r.super_visit_with(self)
+            }
+
+            // We collect types in order to prevent really large types from compiling for
+            // a really long time. See issue #83150 for why this is necessary.
+            fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+                let not_previously_inserted = self.type_collector.insert(ty);
+                if not_previously_inserted {
+                    ty.super_visit_with(self)
+                } else {
+                    ControlFlow::CONTINUE
+                }
+            }
+        }
+
+        self.used_region_names.clear();
+        let mut collector = LateBoundRegionNameCollector {
+            used_region_names: &mut self.used_region_names,
+            type_collector: SsoHashSet::new(),
+        };
+        value.visit_with(&mut collector);
+        self.region_index = 0;
+    }
+}
+
+impl<'tcx, T, P: PrettyPrinter<'tcx>> Print<'tcx, P> for ty::Binder<'tcx, T>
+where
+    T: Print<'tcx, P, Output = P, Error = P::Error> + TypeFoldable<'tcx>,
+{
+    type Output = P;
+    type Error = P::Error;
+
+    fn print(&self, cx: P) -> Result<Self::Output, Self::Error> {
+        cx.in_binder(self)
+    }
+}
+
+impl<'tcx, T, U, P: PrettyPrinter<'tcx>> Print<'tcx, P> for ty::OutlivesPredicate<T, U>
+where
+    T: Print<'tcx, P, Output = P, Error = P::Error>,
+    U: Print<'tcx, P, Output = P, Error = P::Error>,
+{
+    type Output = P;
+    type Error = P::Error;
+    fn print(&self, mut cx: P) -> Result<Self::Output, Self::Error> {
+        define_scoped_cx!(cx);
+        p!(print(self.0), ": ", print(self.1));
+        Ok(cx)
+    }
+}
+
+macro_rules! forward_display_to_print {
+    ($($ty:ty),+) => {
+        // Some of the $ty arguments may not actually use 'tcx
+        $(#[allow(unused_lifetimes)] impl<'tcx> fmt::Display for $ty {
+            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                ty::tls::with(|tcx| {
+                    let cx = tcx.lift(*self)
+                        .expect("could not lift for printing")
+                        .print(FmtPrinter::new(tcx, Namespace::TypeNS))?;
+                    f.write_str(&cx.into_buffer())?;
+                    Ok(())
+                })
+            }
+        })+
+    };
+}
+
+macro_rules! define_print_and_forward_display {
+    (($self:ident, $cx:ident): $($ty:ty $print:block)+) => {
+        $(impl<'tcx, P: PrettyPrinter<'tcx>> Print<'tcx, P> for $ty {
+            type Output = P;
+            type Error = fmt::Error;
+            fn print(&$self, $cx: P) -> Result<Self::Output, Self::Error> {
+                #[allow(unused_mut)]
+                let mut $cx = $cx;
+                define_scoped_cx!($cx);
+                let _: () = $print;
+                #[allow(unreachable_code)]
+                Ok($cx)
+            }
+        })+
+
+        forward_display_to_print!($($ty),+);
+    };
+}
+
+/// Wrapper type for `ty::TraitRef` which opts-in to pretty printing only
+/// the trait path. That is, it will print `Trait<U>` instead of
+/// `<T as Trait<U>>`.
+#[derive(Copy, Clone, TypeFoldable, TypeVisitable, Lift)]
+pub struct TraitRefPrintOnlyTraitPath<'tcx>(ty::TraitRef<'tcx>);
+
+impl<'tcx> fmt::Debug for TraitRefPrintOnlyTraitPath<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self, f)
+    }
+}
+
+/// Wrapper type for `ty::TraitRef` which opts-in to pretty printing only
+/// the trait name. That is, it will print `Trait` instead of
+/// `<T as Trait<U>>`.
+#[derive(Copy, Clone, TypeFoldable, TypeVisitable, Lift)]
+pub struct TraitRefPrintOnlyTraitName<'tcx>(ty::TraitRef<'tcx>);
+
+impl<'tcx> fmt::Debug for TraitRefPrintOnlyTraitName<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self, f)
+    }
+}
+
+impl<'tcx> ty::TraitRef<'tcx> {
+    pub fn print_only_trait_path(self) -> TraitRefPrintOnlyTraitPath<'tcx> {
+        TraitRefPrintOnlyTraitPath(self)
+    }
+
+    pub fn print_only_trait_name(self) -> TraitRefPrintOnlyTraitName<'tcx> {
+        TraitRefPrintOnlyTraitName(self)
+    }
+}
+
+impl<'tcx> ty::Binder<'tcx, ty::TraitRef<'tcx>> {
+    pub fn print_only_trait_path(self) -> ty::Binder<'tcx, TraitRefPrintOnlyTraitPath<'tcx>> {
+        self.map_bound(|tr| tr.print_only_trait_path())
+    }
+}
+
+#[derive(Copy, Clone, TypeFoldable, TypeVisitable, Lift)]
+pub struct TraitPredPrintModifiersAndPath<'tcx>(ty::TraitPredicate<'tcx>);
+
+impl<'tcx> fmt::Debug for TraitPredPrintModifiersAndPath<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self, f)
+    }
+}
+
+impl<'tcx> ty::TraitPredicate<'tcx> {
+    pub fn print_modifiers_and_trait_path(self) -> TraitPredPrintModifiersAndPath<'tcx> {
+        TraitPredPrintModifiersAndPath(self)
+    }
+}
+
+impl<'tcx> ty::PolyTraitPredicate<'tcx> {
+    pub fn print_modifiers_and_trait_path(
+        self,
+    ) -> ty::Binder<'tcx, TraitPredPrintModifiersAndPath<'tcx>> {
+        self.map_bound(TraitPredPrintModifiersAndPath)
+    }
+}
+
+forward_display_to_print! {
+    ty::Region<'tcx>,
+    Ty<'tcx>,
+    &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
+    ty::Const<'tcx>,
+
+    // HACK(eddyb) these are exhaustive instead of generic,
+    // because `for<'tcx>` isn't possible yet.
+    ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>,
+    ty::Binder<'tcx, ty::TraitRef<'tcx>>,
+    ty::Binder<'tcx, ty::ExistentialTraitRef<'tcx>>,
+    ty::Binder<'tcx, TraitRefPrintOnlyTraitPath<'tcx>>,
+    ty::Binder<'tcx, TraitRefPrintOnlyTraitName<'tcx>>,
+    ty::Binder<'tcx, ty::FnSig<'tcx>>,
+    ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
+    ty::Binder<'tcx, TraitPredPrintModifiersAndPath<'tcx>>,
+    ty::Binder<'tcx, ty::SubtypePredicate<'tcx>>,
+    ty::Binder<'tcx, ty::ProjectionPredicate<'tcx>>,
+    ty::Binder<'tcx, ty::OutlivesPredicate<Ty<'tcx>, ty::Region<'tcx>>>,
+    ty::Binder<'tcx, ty::OutlivesPredicate<ty::Region<'tcx>, ty::Region<'tcx>>>,
+
+    ty::OutlivesPredicate<Ty<'tcx>, ty::Region<'tcx>>,
+    ty::OutlivesPredicate<ty::Region<'tcx>, ty::Region<'tcx>>
+}
+
+define_print_and_forward_display! {
+    (self, cx):
+
+    &'tcx ty::List<Ty<'tcx>> {
+        p!("{{", comma_sep(self.iter()), "}}")
+    }
+
+    ty::TypeAndMut<'tcx> {
+        p!(write("{}", self.mutbl.prefix_str()), print(self.ty))
+    }
+
+    ty::ExistentialTraitRef<'tcx> {
+        // Use a type that can't appear in defaults of type parameters.
+        let dummy_self = cx.tcx().mk_ty_infer(ty::FreshTy(0));
+        let trait_ref = self.with_self_ty(cx.tcx(), dummy_self);
+        p!(print(trait_ref.print_only_trait_path()))
+    }
+
+    ty::ExistentialProjection<'tcx> {
+        let name = cx.tcx().associated_item(self.item_def_id).name;
+        p!(write("{} = ", name), print(self.term))
+    }
+
+    ty::ExistentialPredicate<'tcx> {
+        match *self {
+            ty::ExistentialPredicate::Trait(x) => p!(print(x)),
+            ty::ExistentialPredicate::Projection(x) => p!(print(x)),
+            ty::ExistentialPredicate::AutoTrait(def_id) => {
+                p!(print_def_path(def_id, &[]));
+            }
+        }
+    }
+
+    ty::FnSig<'tcx> {
+        p!(write("{}", self.unsafety.prefix_str()));
+
+        if self.abi != Abi::Rust {
+            p!(write("extern {} ", self.abi));
+        }
+
+        p!("fn", pretty_fn_sig(self.inputs(), self.c_variadic, self.output()));
+    }
+
+    ty::TraitRef<'tcx> {
+        p!(write("<{} as {}>", self.self_ty(), self.print_only_trait_path()))
+    }
+
+    TraitRefPrintOnlyTraitPath<'tcx> {
+        p!(print_def_path(self.0.def_id, self.0.substs));
+    }
+
+    TraitRefPrintOnlyTraitName<'tcx> {
+        p!(print_def_path(self.0.def_id, &[]));
+    }
+
+    TraitPredPrintModifiersAndPath<'tcx> {
+        if let ty::BoundConstness::ConstIfConst = self.0.constness {
+            p!("~const ")
+        }
+
+        if let ty::ImplPolarity::Negative = self.0.polarity {
+            p!("!")
+        }
+
+        p!(print(self.0.trait_ref.print_only_trait_path()));
+    }
+
+    ty::ParamTy {
+        p!(write("{}", self.name))
+    }
+
+    ty::ParamConst {
+        p!(write("{}", self.name))
+    }
+
+    ty::SubtypePredicate<'tcx> {
+        p!(print(self.a), " <: ", print(self.b))
+    }
+
+    ty::CoercePredicate<'tcx> {
+        p!(print(self.a), " -> ", print(self.b))
+    }
+
+    ty::TraitPredicate<'tcx> {
+        p!(print(self.trait_ref.self_ty()), ": ");
+        if let ty::BoundConstness::ConstIfConst = self.constness && cx.tcx().features().const_trait_impl {
+            p!("~const ");
+        }
+        p!(print(self.trait_ref.print_only_trait_path()))
+    }
+
+    ty::ProjectionPredicate<'tcx> {
+        p!(print(self.projection_ty), " == ", print(self.term))
+    }
+
+    ty::Term<'tcx> {
+      match self {
+        ty::Term::Ty(ty) => p!(print(ty)),
+        ty::Term::Const(c) => p!(print(c)),
+      }
+    }
+
+    ty::ProjectionTy<'tcx> {
+        p!(print_def_path(self.item_def_id, self.substs));
+    }
+
+    ty::ClosureKind {
+        match *self {
+            ty::ClosureKind::Fn => p!("Fn"),
+            ty::ClosureKind::FnMut => p!("FnMut"),
+            ty::ClosureKind::FnOnce => p!("FnOnce"),
+        }
+    }
+
+    ty::Predicate<'tcx> {
+        let binder = self.kind();
+        p!(print(binder))
+    }
+
+    ty::PredicateKind<'tcx> {
+        match *self {
+            ty::PredicateKind::Trait(ref data) => {
+                p!(print(data))
+            }
+            ty::PredicateKind::Subtype(predicate) => p!(print(predicate)),
+            ty::PredicateKind::Coerce(predicate) => p!(print(predicate)),
+            ty::PredicateKind::RegionOutlives(predicate) => p!(print(predicate)),
+            ty::PredicateKind::TypeOutlives(predicate) => p!(print(predicate)),
+            ty::PredicateKind::Projection(predicate) => p!(print(predicate)),
+            ty::PredicateKind::WellFormed(arg) => p!(print(arg), " well-formed"),
+            ty::PredicateKind::ObjectSafe(trait_def_id) => {
+                p!("the trait `", print_def_path(trait_def_id, &[]), "` is object-safe")
+            }
+            ty::PredicateKind::ClosureKind(closure_def_id, _closure_substs, kind) => {
+                p!("the closure `",
+                print_value_path(closure_def_id, &[]),
+                write("` implements the trait `{}`", kind))
+            }
+            ty::PredicateKind::ConstEvaluatable(uv) => {
+                p!("the constant `", print_value_path(uv.def.did, uv.substs), "` can be evaluated")
+            }
+            ty::PredicateKind::ConstEquate(c1, c2) => {
+                p!("the constant `", print(c1), "` equals `", print(c2), "`")
+            }
+            ty::PredicateKind::TypeWellFormedFromEnv(ty) => {
+                p!("the type `", print(ty), "` is found in the environment")
+            }
+        }
+    }
+
+    GenericArg<'tcx> {
+        match self.unpack() {
+            GenericArgKind::Lifetime(lt) => p!(print(lt)),
+            GenericArgKind::Type(ty) => p!(print(ty)),
+            GenericArgKind::Const(ct) => p!(print(ct)),
+        }
+    }
+}
+
+fn for_each_def(tcx: TyCtxt<'_>, mut collect_fn: impl for<'b> FnMut(&'b Ident, Namespace, DefId)) {
+    // Iterate all local crate items no matter where they are defined.
+    let hir = tcx.hir();
+    for id in hir.items() {
+        if matches!(tcx.def_kind(id.def_id), DefKind::Use) {
+            continue;
+        }
+
+        let item = hir.item(id);
+        if item.ident.name == kw::Empty {
+            continue;
+        }
+
+        let def_id = item.def_id.to_def_id();
+        let ns = tcx.def_kind(def_id).ns().unwrap_or(Namespace::TypeNS);
+        collect_fn(&item.ident, ns, def_id);
+    }
+
+    // Now take care of extern crate items.
+    let queue = &mut Vec::new();
+    let mut seen_defs: DefIdSet = Default::default();
+
+    for &cnum in tcx.crates(()).iter() {
+        let def_id = cnum.as_def_id();
+
+        // Ignore crates that are not direct dependencies.
+        match tcx.extern_crate(def_id) {
+            None => continue,
+            Some(extern_crate) => {
+                if !extern_crate.is_direct() {
+                    continue;
+                }
+            }
+        }
+
+        queue.push(def_id);
+    }
+
+    // Iterate external crate defs but be mindful about visibility
+    while let Some(def) = queue.pop() {
+        for child in tcx.module_children(def).iter() {
+            if !child.vis.is_public() {
+                continue;
+            }
+
+            match child.res {
+                def::Res::Def(DefKind::AssocTy, _) => {}
+                def::Res::Def(DefKind::TyAlias, _) => {}
+                def::Res::Def(defkind, def_id) => {
+                    if let Some(ns) = defkind.ns() {
+                        collect_fn(&child.ident, ns, def_id);
+                    }
+
+                    if matches!(defkind, DefKind::Mod | DefKind::Enum | DefKind::Trait)
+                        && seen_defs.insert(def_id)
+                    {
+                        queue.push(def_id);
+                    }
+                }
+                _ => {}
+            }
+        }
+    }
+}
+
+/// The purpose of this function is to collect public symbols names that are unique across all
+/// crates in the build. Later, when printing about types we can use those names instead of the
+/// full exported path to them.
+///
+/// So essentially, if a symbol name can only be imported from one place for a type, and as
+/// long as it was not glob-imported anywhere in the current crate, we can trim its printed
+/// path and print only the name.
+///
+/// This has wide implications on error messages with types, for example, shortening
+/// `std::vec::Vec` to just `Vec`, as long as there is no other `Vec` importable anywhere.
+///
+/// The implementation uses similar import discovery logic to that of 'use' suggestions.
+fn trimmed_def_paths(tcx: TyCtxt<'_>, (): ()) -> FxHashMap<DefId, Symbol> {
+    let mut map: FxHashMap<DefId, Symbol> = FxHashMap::default();
+
+    if let TrimmedDefPaths::GoodPath = tcx.sess.opts.trimmed_def_paths {
+        // For good paths causing this bug, the `rustc_middle::ty::print::with_no_trimmed_paths`
+        // wrapper can be used to suppress this query, in exchange for full paths being formatted.
+        tcx.sess.delay_good_path_bug("trimmed_def_paths constructed");
+    }
+
+    let unique_symbols_rev: &mut FxHashMap<(Namespace, Symbol), Option<DefId>> =
+        &mut FxHashMap::default();
+
+    for symbol_set in tcx.resolutions(()).glob_map.values() {
+        for symbol in symbol_set {
+            unique_symbols_rev.insert((Namespace::TypeNS, *symbol), None);
+            unique_symbols_rev.insert((Namespace::ValueNS, *symbol), None);
+            unique_symbols_rev.insert((Namespace::MacroNS, *symbol), None);
+        }
+    }
+
+    for_each_def(tcx, |ident, ns, def_id| {
+        use std::collections::hash_map::Entry::{Occupied, Vacant};
+
+        match unique_symbols_rev.entry((ns, ident.name)) {
+            Occupied(mut v) => match v.get() {
+                None => {}
+                Some(existing) => {
+                    if *existing != def_id {
+                        v.insert(None);
+                    }
+                }
+            },
+            Vacant(v) => {
+                v.insert(Some(def_id));
+            }
+        }
+    });
+
+    for ((_, symbol), opt_def_id) in unique_symbols_rev.drain() {
+        use std::collections::hash_map::Entry::{Occupied, Vacant};
+
+        if let Some(def_id) = opt_def_id {
+            match map.entry(def_id) {
+                Occupied(mut v) => {
+                    // A single DefId can be known under multiple names (e.g.,
+                    // with a `pub use ... as ...;`). We need to ensure that the
+                    // name placed in this map is chosen deterministically, so
+                    // if we find multiple names (`symbol`) resolving to the
+                    // same `def_id`, we prefer the lexicographically smallest
+                    // name.
+                    //
+                    // Any stable ordering would be fine here though.
+                    if *v.get() != symbol {
+                        if v.get().as_str() > symbol.as_str() {
+                            v.insert(symbol);
+                        }
+                    }
+                }
+                Vacant(v) => {
+                    v.insert(symbol);
+                }
+            }
+        }
+    }
+
+    map
+}
+
+pub fn provide(providers: &mut ty::query::Providers) {
+    *providers = ty::query::Providers { trimmed_def_paths, ..*providers };
+}
+
+#[derive(Default)]
+pub struct OpaqueFnEntry<'tcx> {
+    // The trait ref is already stored as a key, so just track if we have it as a real predicate
+    has_fn_once: bool,
+    fn_mut_trait_ref: Option<ty::PolyTraitRef<'tcx>>,
+    fn_trait_ref: Option<ty::PolyTraitRef<'tcx>>,
+    return_ty: Option<ty::Binder<'tcx, Term<'tcx>>>,
+}
diff --git a/compiler/rustc_middle/src/ty/query.rs b/compiler/rustc_middle/src/ty/query.rs
new file mode 100644
index 000000000..2452bcf6a
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/query.rs
@@ -0,0 +1,386 @@
+use crate::dep_graph;
+use crate::infer::canonical::{self, Canonical};
+use crate::lint::LintLevelMap;
+use crate::metadata::ModChild;
+use crate::middle::codegen_fn_attrs::CodegenFnAttrs;
+use crate::middle::exported_symbols::{ExportedSymbol, SymbolExportInfo};
+use crate::middle::lib_features::LibFeatures;
+use crate::middle::privacy::AccessLevels;
+use crate::middle::resolve_lifetime::{ObjectLifetimeDefault, Region, ResolveLifetimes};
+use crate::middle::stability::{self, DeprecationEntry};
+use crate::mir;
+use crate::mir::interpret::GlobalId;
+use crate::mir::interpret::{
+    ConstValue, EvalToAllocationRawResult, EvalToConstValueResult, EvalToValTreeResult,
+};
+use crate::mir::interpret::{LitToConstError, LitToConstInput};
+use crate::mir::mono::CodegenUnit;
+use crate::thir;
+use crate::traits::query::{
+    CanonicalPredicateGoal, CanonicalProjectionGoal, CanonicalTyGoal,
+    CanonicalTypeOpAscribeUserTypeGoal, CanonicalTypeOpEqGoal, CanonicalTypeOpNormalizeGoal,
+    CanonicalTypeOpProvePredicateGoal, CanonicalTypeOpSubtypeGoal, NoSolution,
+};
+use crate::traits::query::{
+    DropckConstraint, DropckOutlivesResult, MethodAutoderefStepsResult, NormalizationResult,
+    OutlivesBound,
+};
+use crate::traits::specialization_graph;
+use crate::traits::{self, ImplSource};
+use crate::ty::fast_reject::SimplifiedType;
+use crate::ty::layout::TyAndLayout;
+use crate::ty::subst::{GenericArg, SubstsRef};
+use crate::ty::util::AlwaysRequiresDrop;
+use crate::ty::GeneratorDiagnosticData;
+use crate::ty::{self, AdtSizedConstraint, CrateInherentImpls, ParamEnvAnd, Ty, TyCtxt};
+use rustc_ast as ast;
+use rustc_ast::expand::allocator::AllocatorKind;
+use rustc_attr as attr;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, FxIndexSet};
+use rustc_data_structures::steal::Steal;
+use rustc_data_structures::svh::Svh;
+use rustc_data_structures::sync::Lrc;
+use rustc_errors::ErrorGuaranteed;
+use rustc_hir as hir;
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, DefIdSet, LocalDefId};
+use rustc_hir::lang_items::{LangItem, LanguageItems};
+use rustc_hir::{Crate, ItemLocalId, TraitCandidate};
+use rustc_index::{bit_set::FiniteBitSet, vec::IndexVec};
+use rustc_session::config::{EntryFnType, OptLevel, OutputFilenames, SymbolManglingVersion};
+use rustc_session::cstore::{CrateDepKind, CrateSource};
+use rustc_session::cstore::{ExternCrate, ForeignModule, LinkagePreference, NativeLib};
+use rustc_session::utils::NativeLibKind;
+use rustc_session::Limits;
+use rustc_span::symbol::Symbol;
+use rustc_span::{Span, DUMMY_SP};
+use rustc_target::abi;
+use rustc_target::spec::PanicStrategy;
+use std::ops::Deref;
+use std::path::PathBuf;
+use std::sync::Arc;
+
+pub(crate) use rustc_query_system::query::QueryJobId;
+use rustc_query_system::query::*;
+
+#[derive(Copy, Clone)]
+pub struct TyCtxtAt<'tcx> {
+    pub tcx: TyCtxt<'tcx>,
+    pub span: Span,
+}
+
+impl<'tcx> Deref for TyCtxtAt<'tcx> {
+    type Target = TyCtxt<'tcx>;
+    #[inline(always)]
+    fn deref(&self) -> &Self::Target {
+        &self.tcx
+    }
+}
+
+#[derive(Copy, Clone)]
+pub struct TyCtxtEnsure<'tcx> {
+    pub tcx: TyCtxt<'tcx>,
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Returns a transparent wrapper for `TyCtxt`, which ensures queries
+    /// are executed instead of just returning their results.
+    #[inline(always)]
+    pub fn ensure(self) -> TyCtxtEnsure<'tcx> {
+        TyCtxtEnsure { tcx: self }
+    }
+
+    /// Returns a transparent wrapper for `TyCtxt` which uses
+    /// `span` as the location of queries performed through it.
+    #[inline(always)]
+    pub fn at(self, span: Span) -> TyCtxtAt<'tcx> {
+        TyCtxtAt { tcx: self, span }
+    }
+
+    pub fn try_mark_green(self, dep_node: &dep_graph::DepNode) -> bool {
+        self.queries.try_mark_green(self, dep_node)
+    }
+}
+
+/// Helper for `TyCtxtEnsure` to avoid a closure.
+#[inline(always)]
+fn noop<T>(_: &T) {}
+
+/// Helper to ensure that queries only return `Copy` types.
+#[inline(always)]
+fn copy<T: Copy>(x: &T) -> T {
+    *x
+}
+
+macro_rules! query_helper_param_ty {
+    (DefId) => { impl IntoQueryParam<DefId> };
+    ($K:ty) => { $K };
+}
+
+macro_rules! query_storage {
+    ([][$K:ty, $V:ty]) => {
+        <DefaultCacheSelector as CacheSelector<$K, $V>>::Cache
+    };
+    ([(storage $ty:ty) $($rest:tt)*][$K:ty, $V:ty]) => {
+        <$ty as CacheSelector<$K, $V>>::Cache
+    };
+    ([$other:tt $($modifiers:tt)*][$($args:tt)*]) => {
+        query_storage!([$($modifiers)*][$($args)*])
+    };
+}
+
+macro_rules! separate_provide_extern_decl {
+    ([][$name:ident]) => {
+        ()
+    };
+    ([(separate_provide_extern) $($rest:tt)*][$name:ident]) => {
+        for<'tcx> fn(
+            TyCtxt<'tcx>,
+            query_keys::$name<'tcx>,
+        ) -> query_values::$name<'tcx>
+    };
+    ([$other:tt $($modifiers:tt)*][$($args:tt)*]) => {
+        separate_provide_extern_decl!([$($modifiers)*][$($args)*])
+    };
+}
+
+macro_rules! separate_provide_extern_default {
+    ([][$name:ident]) => {
+        ()
+    };
+    ([(separate_provide_extern) $($rest:tt)*][$name:ident]) => {
+        |_, key| bug!(
+            "`tcx.{}({:?})` unsupported by its crate; \
+             perhaps the `{}` query was never assigned a provider function",
+            stringify!($name),
+            key,
+            stringify!($name),
+        )
+    };
+    ([$other:tt $($modifiers:tt)*][$($args:tt)*]) => {
+        separate_provide_extern_default!([$($modifiers)*][$($args)*])
+    };
+}
+
+macro_rules! opt_remap_env_constness {
+    ([][$name:ident]) => {};
+    ([(remap_env_constness) $($rest:tt)*][$name:ident]) => {
+        let $name = $name.without_const();
+    };
+    ([$other:tt $($modifiers:tt)*][$name:ident]) => {
+        opt_remap_env_constness!([$($modifiers)*][$name])
+    };
+}
+
+macro_rules! define_callbacks {
+    (<$tcx:tt>
+     $($(#[$attr:meta])*
+        [$($modifiers:tt)*] fn $name:ident($($K:tt)*) -> $V:ty,)*) => {
+
+        // HACK(eddyb) this is like the `impl QueryConfig for queries::$name`
+        // below, but using type aliases instead of associated types, to bypass
+        // the limitations around normalizing under HRTB - for example, this:
+        // `for<'tcx> fn(...) -> <queries::$name<'tcx> as QueryConfig<TyCtxt<'tcx>>>::Value`
+        // doesn't currently normalize to `for<'tcx> fn(...) -> query_values::$name<'tcx>`.
+        // This is primarily used by the `provide!` macro in `rustc_metadata`.
+        #[allow(nonstandard_style, unused_lifetimes)]
+        pub mod query_keys {
+            use super::*;
+
+            $(pub type $name<$tcx> = $($K)*;)*
+        }
+        #[allow(nonstandard_style, unused_lifetimes)]
+        pub mod query_values {
+            use super::*;
+
+            $(pub type $name<$tcx> = $V;)*
+        }
+        #[allow(nonstandard_style, unused_lifetimes)]
+        pub mod query_storage {
+            use super::*;
+
+            $(pub type $name<$tcx> = query_storage!([$($modifiers)*][$($K)*, $V]);)*
+        }
+        #[allow(nonstandard_style, unused_lifetimes)]
+        pub mod query_stored {
+            use super::*;
+
+            $(pub type $name<$tcx> = <query_storage::$name<$tcx> as QueryStorage>::Stored;)*
+        }
+
+        #[derive(Default)]
+        pub struct QueryCaches<$tcx> {
+            $($(#[$attr])* pub $name: query_storage::$name<$tcx>,)*
+        }
+
+        impl<$tcx> TyCtxtEnsure<$tcx> {
+            $($(#[$attr])*
+            #[inline(always)]
+            pub fn $name(self, key: query_helper_param_ty!($($K)*)) {
+                let key = key.into_query_param();
+                opt_remap_env_constness!([$($modifiers)*][key]);
+
+                let cached = try_get_cached(self.tcx, &self.tcx.query_caches.$name, &key, noop);
+
+                match cached {
+                    Ok(()) => return,
+                    Err(()) => (),
+                }
+
+                self.tcx.queries.$name(self.tcx, DUMMY_SP, key, QueryMode::Ensure);
+            })*
+        }
+
+        impl<$tcx> TyCtxt<$tcx> {
+            $($(#[$attr])*
+            #[inline(always)]
+            #[must_use]
+            pub fn $name(self, key: query_helper_param_ty!($($K)*)) -> query_stored::$name<$tcx>
+            {
+                self.at(DUMMY_SP).$name(key)
+            })*
+        }
+
+        impl<$tcx> TyCtxtAt<$tcx> {
+            $($(#[$attr])*
+            #[inline(always)]
+            pub fn $name(self, key: query_helper_param_ty!($($K)*)) -> query_stored::$name<$tcx>
+            {
+                let key = key.into_query_param();
+                opt_remap_env_constness!([$($modifiers)*][key]);
+
+                let cached = try_get_cached(self.tcx, &self.tcx.query_caches.$name, &key, copy);
+
+                match cached {
+                    Ok(value) => return value,
+                    Err(()) => (),
+                }
+
+                self.tcx.queries.$name(self.tcx, self.span, key, QueryMode::Get).unwrap()
+            })*
+        }
+
+        pub struct Providers {
+            $(pub $name: for<'tcx> fn(
+                TyCtxt<'tcx>,
+                query_keys::$name<'tcx>,
+            ) -> query_values::$name<'tcx>,)*
+        }
+
+        pub struct ExternProviders {
+            $(pub $name: separate_provide_extern_decl!([$($modifiers)*][$name]),)*
+        }
+
+        impl Default for Providers {
+            fn default() -> Self {
+                Providers {
+                    $($name: |_, key| bug!(
+                        "`tcx.{}({:?})` unsupported by its crate; \
+                         perhaps the `{}` query was never assigned a provider function",
+                        stringify!($name),
+                        key,
+                        stringify!($name),
+                    ),)*
+                }
+            }
+        }
+
+        impl Default for ExternProviders {
+            fn default() -> Self {
+                ExternProviders {
+                    $($name: separate_provide_extern_default!([$($modifiers)*][$name]),)*
+                }
+            }
+        }
+
+        impl Copy for Providers {}
+        impl Clone for Providers {
+            fn clone(&self) -> Self { *self }
+        }
+
+        impl Copy for ExternProviders {}
+        impl Clone for ExternProviders {
+            fn clone(&self) -> Self { *self }
+        }
+
+        pub trait QueryEngine<'tcx>: rustc_data_structures::sync::Sync {
+            fn as_any(&'tcx self) -> &'tcx dyn std::any::Any;
+
+            fn try_mark_green(&'tcx self, tcx: TyCtxt<'tcx>, dep_node: &dep_graph::DepNode) -> bool;
+
+            $($(#[$attr])*
+            fn $name(
+                &'tcx self,
+                tcx: TyCtxt<$tcx>,
+                span: Span,
+                key: query_keys::$name<$tcx>,
+                mode: QueryMode,
+            ) -> Option<query_stored::$name<$tcx>>;)*
+        }
+    };
+}
+
+// Each of these queries corresponds to a function pointer field in the
+// `Providers` struct for requesting a value of that type, and a method
+// on `tcx: TyCtxt` (and `tcx.at(span)`) for doing that request in a way
+// which memoizes and does dep-graph tracking, wrapping around the actual
+// `Providers` that the driver creates (using several `rustc_*` crates).
+//
+// The result type of each query must implement `Clone`, and additionally
+// `ty::query::values::Value`, which produces an appropriate placeholder
+// (error) value if the query resulted in a query cycle.
+// Queries marked with `fatal_cycle` do not need the latter implementation,
+// as they will raise an fatal error on query cycles instead.
+
+rustc_query_append! { [define_callbacks!][<'tcx>] }
+
+mod sealed {
+    use super::{DefId, LocalDefId};
+
+    /// An analogue of the `Into` trait that's intended only for query parameters.
+    ///
+    /// This exists to allow queries to accept either `DefId` or `LocalDefId` while requiring that the
+    /// user call `to_def_id` to convert between them everywhere else.
+    pub trait IntoQueryParam<P> {
+        fn into_query_param(self) -> P;
+    }
+
+    impl<P> IntoQueryParam<P> for P {
+        #[inline(always)]
+        fn into_query_param(self) -> P {
+            self
+        }
+    }
+
+    impl<'a, P: Copy> IntoQueryParam<P> for &'a P {
+        #[inline(always)]
+        fn into_query_param(self) -> P {
+            *self
+        }
+    }
+
+    impl IntoQueryParam<DefId> for LocalDefId {
+        #[inline(always)]
+        fn into_query_param(self) -> DefId {
+            self.to_def_id()
+        }
+    }
+}
+
+use sealed::IntoQueryParam;
+
+impl<'tcx> TyCtxt<'tcx> {
+    pub fn def_kind(self, def_id: impl IntoQueryParam<DefId>) -> DefKind {
+        let def_id = def_id.into_query_param();
+        self.opt_def_kind(def_id)
+            .unwrap_or_else(|| bug!("def_kind: unsupported node: {:?}", def_id))
+    }
+}
+
+impl<'tcx> TyCtxtAt<'tcx> {
+    pub fn def_kind(self, def_id: impl IntoQueryParam<DefId>) -> DefKind {
+        let def_id = def_id.into_query_param();
+        self.opt_def_kind(def_id)
+            .unwrap_or_else(|| bug!("def_kind: unsupported node: {:?}", def_id))
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/relate.rs b/compiler/rustc_middle/src/ty/relate.rs
new file mode 100644
index 000000000..818affa71
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/relate.rs
@@ -0,0 +1,841 @@
+//! Generalized type relating mechanism.
+//!
+//! A type relation `R` relates a pair of values `(A, B)`. `A and B` are usually
+//! types or regions but can be other things. Examples of type relations are
+//! subtyping, type equality, etc.
+
+use crate::ty::error::{ExpectedFound, TypeError};
+use crate::ty::subst::{GenericArg, GenericArgKind, Subst, SubstsRef};
+use crate::ty::{self, ImplSubject, Term, Ty, TyCtxt, TypeFoldable};
+use rustc_hir as ast;
+use rustc_hir::def_id::DefId;
+use rustc_span::DUMMY_SP;
+use rustc_target::spec::abi;
+use std::iter;
+
+pub type RelateResult<'tcx, T> = Result<T, TypeError<'tcx>>;
+
+#[derive(Clone, Debug)]
+pub enum Cause {
+    ExistentialRegionBound, // relating an existential region bound
+}
+
+pub trait TypeRelation<'tcx>: Sized {
+    fn tcx(&self) -> TyCtxt<'tcx>;
+
+    fn param_env(&self) -> ty::ParamEnv<'tcx>;
+
+    /// Returns a static string we can use for printouts.
+    fn tag(&self) -> &'static str;
+
+    /// Returns `true` if the value `a` is the "expected" type in the
+    /// relation. Just affects error messages.
+    fn a_is_expected(&self) -> bool;
+
+    fn with_cause<F, R>(&mut self, _cause: Cause, f: F) -> R
+    where
+        F: FnOnce(&mut Self) -> R,
+    {
+        f(self)
+    }
+
+    /// Generic relation routine suitable for most anything.
+    fn relate<T: Relate<'tcx>>(&mut self, a: T, b: T) -> RelateResult<'tcx, T> {
+        Relate::relate(self, a, b)
+    }
+
+    /// Relate the two substitutions for the given item. The default
+    /// is to look up the variance for the item and proceed
+    /// accordingly.
+    fn relate_item_substs(
+        &mut self,
+        item_def_id: DefId,
+        a_subst: SubstsRef<'tcx>,
+        b_subst: SubstsRef<'tcx>,
+    ) -> RelateResult<'tcx, SubstsRef<'tcx>> {
+        debug!(
+            "relate_item_substs(item_def_id={:?}, a_subst={:?}, b_subst={:?})",
+            item_def_id, a_subst, b_subst
+        );
+
+        let tcx = self.tcx();
+        let opt_variances = tcx.variances_of(item_def_id);
+        relate_substs_with_variances(self, item_def_id, opt_variances, a_subst, b_subst)
+    }
+
+    /// Switch variance for the purpose of relating `a` and `b`.
+    fn relate_with_variance<T: Relate<'tcx>>(
+        &mut self,
+        variance: ty::Variance,
+        info: ty::VarianceDiagInfo<'tcx>,
+        a: T,
+        b: T,
+    ) -> RelateResult<'tcx, T>;
+
+    // Overridable relations. You shouldn't typically call these
+    // directly, instead call `relate()`, which in turn calls
+    // these. This is both more uniform but also allows us to add
+    // additional hooks for other types in the future if needed
+    // without making older code, which called `relate`, obsolete.
+
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>>;
+
+    fn regions(
+        &mut self,
+        a: ty::Region<'tcx>,
+        b: ty::Region<'tcx>,
+    ) -> RelateResult<'tcx, ty::Region<'tcx>>;
+
+    fn consts(
+        &mut self,
+        a: ty::Const<'tcx>,
+        b: ty::Const<'tcx>,
+    ) -> RelateResult<'tcx, ty::Const<'tcx>>;
+
+    fn binders<T>(
+        &mut self,
+        a: ty::Binder<'tcx, T>,
+        b: ty::Binder<'tcx, T>,
+    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
+    where
+        T: Relate<'tcx>;
+}
+
+pub trait Relate<'tcx>: TypeFoldable<'tcx> + Copy {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: Self,
+        b: Self,
+    ) -> RelateResult<'tcx, Self>;
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Relate impls
+
+pub fn relate_type_and_mut<'tcx, R: TypeRelation<'tcx>>(
+    relation: &mut R,
+    a: ty::TypeAndMut<'tcx>,
+    b: ty::TypeAndMut<'tcx>,
+    base_ty: Ty<'tcx>,
+) -> RelateResult<'tcx, ty::TypeAndMut<'tcx>> {
+    debug!("{}.mts({:?}, {:?})", relation.tag(), a, b);
+    if a.mutbl != b.mutbl {
+        Err(TypeError::Mutability)
+    } else {
+        let mutbl = a.mutbl;
+        let (variance, info) = match mutbl {
+            ast::Mutability::Not => (ty::Covariant, ty::VarianceDiagInfo::None),
+            ast::Mutability::Mut => {
+                (ty::Invariant, ty::VarianceDiagInfo::Invariant { ty: base_ty, param_index: 0 })
+            }
+        };
+        let ty = relation.relate_with_variance(variance, info, a.ty, b.ty)?;
+        Ok(ty::TypeAndMut { ty, mutbl })
+    }
+}
+
+#[inline]
+pub fn relate_substs<'tcx, R: TypeRelation<'tcx>>(
+    relation: &mut R,
+    a_subst: SubstsRef<'tcx>,
+    b_subst: SubstsRef<'tcx>,
+) -> RelateResult<'tcx, SubstsRef<'tcx>> {
+    relation.tcx().mk_substs(iter::zip(a_subst, b_subst).map(|(a, b)| {
+        relation.relate_with_variance(ty::Invariant, ty::VarianceDiagInfo::default(), a, b)
+    }))
+}
+
+pub fn relate_substs_with_variances<'tcx, R: TypeRelation<'tcx>>(
+    relation: &mut R,
+    ty_def_id: DefId,
+    variances: &[ty::Variance],
+    a_subst: SubstsRef<'tcx>,
+    b_subst: SubstsRef<'tcx>,
+) -> RelateResult<'tcx, SubstsRef<'tcx>> {
+    let tcx = relation.tcx();
+
+    let mut cached_ty = None;
+    let params = iter::zip(a_subst, b_subst).enumerate().map(|(i, (a, b))| {
+        let variance = variances[i];
+        let variance_info = if variance == ty::Invariant {
+            let ty =
+                *cached_ty.get_or_insert_with(|| tcx.bound_type_of(ty_def_id).subst(tcx, a_subst));
+            ty::VarianceDiagInfo::Invariant { ty, param_index: i.try_into().unwrap() }
+        } else {
+            ty::VarianceDiagInfo::default()
+        };
+        relation.relate_with_variance(variance, variance_info, a, b)
+    });
+
+    tcx.mk_substs(params)
+}
+
+impl<'tcx> Relate<'tcx> for ty::FnSig<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::FnSig<'tcx>,
+        b: ty::FnSig<'tcx>,
+    ) -> RelateResult<'tcx, ty::FnSig<'tcx>> {
+        let tcx = relation.tcx();
+
+        if a.c_variadic != b.c_variadic {
+            return Err(TypeError::VariadicMismatch(expected_found(
+                relation,
+                a.c_variadic,
+                b.c_variadic,
+            )));
+        }
+        let unsafety = relation.relate(a.unsafety, b.unsafety)?;
+        let abi = relation.relate(a.abi, b.abi)?;
+
+        if a.inputs().len() != b.inputs().len() {
+            return Err(TypeError::ArgCount);
+        }
+
+        let inputs_and_output = iter::zip(a.inputs(), b.inputs())
+            .map(|(&a, &b)| ((a, b), false))
+            .chain(iter::once(((a.output(), b.output()), true)))
+            .map(|((a, b), is_output)| {
+                if is_output {
+                    relation.relate(a, b)
+                } else {
+                    relation.relate_with_variance(
+                        ty::Contravariant,
+                        ty::VarianceDiagInfo::default(),
+                        a,
+                        b,
+                    )
+                }
+            })
+            .enumerate()
+            .map(|(i, r)| match r {
+                Err(TypeError::Sorts(exp_found) | TypeError::ArgumentSorts(exp_found, _)) => {
+                    Err(TypeError::ArgumentSorts(exp_found, i))
+                }
+                Err(TypeError::Mutability | TypeError::ArgumentMutability(_)) => {
+                    Err(TypeError::ArgumentMutability(i))
+                }
+                r => r,
+            });
+        Ok(ty::FnSig {
+            inputs_and_output: tcx.mk_type_list(inputs_and_output)?,
+            c_variadic: a.c_variadic,
+            unsafety,
+            abi,
+        })
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::BoundConstness {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::BoundConstness,
+        b: ty::BoundConstness,
+    ) -> RelateResult<'tcx, ty::BoundConstness> {
+        if a != b {
+            Err(TypeError::ConstnessMismatch(expected_found(relation, a, b)))
+        } else {
+            Ok(a)
+        }
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ast::Unsafety {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ast::Unsafety,
+        b: ast::Unsafety,
+    ) -> RelateResult<'tcx, ast::Unsafety> {
+        if a != b {
+            Err(TypeError::UnsafetyMismatch(expected_found(relation, a, b)))
+        } else {
+            Ok(a)
+        }
+    }
+}
+
+impl<'tcx> Relate<'tcx> for abi::Abi {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: abi::Abi,
+        b: abi::Abi,
+    ) -> RelateResult<'tcx, abi::Abi> {
+        if a == b { Ok(a) } else { Err(TypeError::AbiMismatch(expected_found(relation, a, b))) }
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::ProjectionTy<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::ProjectionTy<'tcx>,
+        b: ty::ProjectionTy<'tcx>,
+    ) -> RelateResult<'tcx, ty::ProjectionTy<'tcx>> {
+        if a.item_def_id != b.item_def_id {
+            Err(TypeError::ProjectionMismatched(expected_found(
+                relation,
+                a.item_def_id,
+                b.item_def_id,
+            )))
+        } else {
+            let substs = relation.relate(a.substs, b.substs)?;
+            Ok(ty::ProjectionTy { item_def_id: a.item_def_id, substs: &substs })
+        }
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::ExistentialProjection<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::ExistentialProjection<'tcx>,
+        b: ty::ExistentialProjection<'tcx>,
+    ) -> RelateResult<'tcx, ty::ExistentialProjection<'tcx>> {
+        if a.item_def_id != b.item_def_id {
+            Err(TypeError::ProjectionMismatched(expected_found(
+                relation,
+                a.item_def_id,
+                b.item_def_id,
+            )))
+        } else {
+            let term = relation.relate_with_variance(
+                ty::Invariant,
+                ty::VarianceDiagInfo::default(),
+                a.term,
+                b.term,
+            )?;
+            let substs = relation.relate_with_variance(
+                ty::Invariant,
+                ty::VarianceDiagInfo::default(),
+                a.substs,
+                b.substs,
+            )?;
+            Ok(ty::ExistentialProjection { item_def_id: a.item_def_id, substs, term })
+        }
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::TraitRef<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::TraitRef<'tcx>,
+        b: ty::TraitRef<'tcx>,
+    ) -> RelateResult<'tcx, ty::TraitRef<'tcx>> {
+        // Different traits cannot be related.
+        if a.def_id != b.def_id {
+            Err(TypeError::Traits(expected_found(relation, a.def_id, b.def_id)))
+        } else {
+            let substs = relate_substs(relation, a.substs, b.substs)?;
+            Ok(ty::TraitRef { def_id: a.def_id, substs })
+        }
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::ExistentialTraitRef<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::ExistentialTraitRef<'tcx>,
+        b: ty::ExistentialTraitRef<'tcx>,
+    ) -> RelateResult<'tcx, ty::ExistentialTraitRef<'tcx>> {
+        // Different traits cannot be related.
+        if a.def_id != b.def_id {
+            Err(TypeError::Traits(expected_found(relation, a.def_id, b.def_id)))
+        } else {
+            let substs = relate_substs(relation, a.substs, b.substs)?;
+            Ok(ty::ExistentialTraitRef { def_id: a.def_id, substs })
+        }
+    }
+}
+
+#[derive(Copy, Debug, Clone, TypeFoldable, TypeVisitable)]
+struct GeneratorWitness<'tcx>(&'tcx ty::List<Ty<'tcx>>);
+
+impl<'tcx> Relate<'tcx> for GeneratorWitness<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: GeneratorWitness<'tcx>,
+        b: GeneratorWitness<'tcx>,
+    ) -> RelateResult<'tcx, GeneratorWitness<'tcx>> {
+        assert_eq!(a.0.len(), b.0.len());
+        let tcx = relation.tcx();
+        let types = tcx.mk_type_list(iter::zip(a.0, b.0).map(|(a, b)| relation.relate(a, b)))?;
+        Ok(GeneratorWitness(types))
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ImplSubject<'tcx> {
+    #[inline]
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ImplSubject<'tcx>,
+        b: ImplSubject<'tcx>,
+    ) -> RelateResult<'tcx, ImplSubject<'tcx>> {
+        match (a, b) {
+            (ImplSubject::Trait(trait_ref_a), ImplSubject::Trait(trait_ref_b)) => {
+                let trait_ref = ty::TraitRef::relate(relation, trait_ref_a, trait_ref_b)?;
+                Ok(ImplSubject::Trait(trait_ref))
+            }
+            (ImplSubject::Inherent(ty_a), ImplSubject::Inherent(ty_b)) => {
+                let ty = Ty::relate(relation, ty_a, ty_b)?;
+                Ok(ImplSubject::Inherent(ty))
+            }
+            (ImplSubject::Trait(_), ImplSubject::Inherent(_))
+            | (ImplSubject::Inherent(_), ImplSubject::Trait(_)) => {
+                bug!("can not relate TraitRef and Ty");
+            }
+        }
+    }
+}
+
+impl<'tcx> Relate<'tcx> for Ty<'tcx> {
+    #[inline]
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: Ty<'tcx>,
+        b: Ty<'tcx>,
+    ) -> RelateResult<'tcx, Ty<'tcx>> {
+        relation.tys(a, b)
+    }
+}
+
+/// The main "type relation" routine. Note that this does not handle
+/// inference artifacts, so you should filter those out before calling
+/// it.
+pub fn super_relate_tys<'tcx, R: TypeRelation<'tcx>>(
+    relation: &mut R,
+    a: Ty<'tcx>,
+    b: Ty<'tcx>,
+) -> RelateResult<'tcx, Ty<'tcx>> {
+    let tcx = relation.tcx();
+    debug!("super_relate_tys: a={:?} b={:?}", a, b);
+    match (a.kind(), b.kind()) {
+        (&ty::Infer(_), _) | (_, &ty::Infer(_)) => {
+            // The caller should handle these cases!
+            bug!("var types encountered in super_relate_tys")
+        }
+
+        (ty::Bound(..), _) | (_, ty::Bound(..)) => {
+            bug!("bound types encountered in super_relate_tys")
+        }
+
+        (&ty::Error(_), _) | (_, &ty::Error(_)) => Ok(tcx.ty_error()),
+
+        (&ty::Never, _)
+        | (&ty::Char, _)
+        | (&ty::Bool, _)
+        | (&ty::Int(_), _)
+        | (&ty::Uint(_), _)
+        | (&ty::Float(_), _)
+        | (&ty::Str, _)
+            if a == b =>
+        {
+            Ok(a)
+        }
+
+        (&ty::Param(ref a_p), &ty::Param(ref b_p)) if a_p.index == b_p.index => Ok(a),
+
+        (ty::Placeholder(p1), ty::Placeholder(p2)) if p1 == p2 => Ok(a),
+
+        (&ty::Adt(a_def, a_substs), &ty::Adt(b_def, b_substs)) if a_def == b_def => {
+            let substs = relation.relate_item_substs(a_def.did(), a_substs, b_substs)?;
+            Ok(tcx.mk_adt(a_def, substs))
+        }
+
+        (&ty::Foreign(a_id), &ty::Foreign(b_id)) if a_id == b_id => Ok(tcx.mk_foreign(a_id)),
+
+        (&ty::Dynamic(a_obj, a_region), &ty::Dynamic(b_obj, b_region)) => {
+            let region_bound = relation.with_cause(Cause::ExistentialRegionBound, |relation| {
+                relation.relate_with_variance(
+                    ty::Contravariant,
+                    ty::VarianceDiagInfo::default(),
+                    a_region,
+                    b_region,
+                )
+            })?;
+            Ok(tcx.mk_dynamic(relation.relate(a_obj, b_obj)?, region_bound))
+        }
+
+        (&ty::Generator(a_id, a_substs, movability), &ty::Generator(b_id, b_substs, _))
+            if a_id == b_id =>
+        {
+            // All Generator types with the same id represent
+            // the (anonymous) type of the same generator expression. So
+            // all of their regions should be equated.
+            let substs = relation.relate(a_substs, b_substs)?;
+            Ok(tcx.mk_generator(a_id, substs, movability))
+        }
+
+        (&ty::GeneratorWitness(a_types), &ty::GeneratorWitness(b_types)) => {
+            // Wrap our types with a temporary GeneratorWitness struct
+            // inside the binder so we can related them
+            let a_types = a_types.map_bound(GeneratorWitness);
+            let b_types = b_types.map_bound(GeneratorWitness);
+            // Then remove the GeneratorWitness for the result
+            let types = relation.relate(a_types, b_types)?.map_bound(|witness| witness.0);
+            Ok(tcx.mk_generator_witness(types))
+        }
+
+        (&ty::Closure(a_id, a_substs), &ty::Closure(b_id, b_substs)) if a_id == b_id => {
+            // All Closure types with the same id represent
+            // the (anonymous) type of the same closure expression. So
+            // all of their regions should be equated.
+            let substs = relation.relate(a_substs, b_substs)?;
+            Ok(tcx.mk_closure(a_id, &substs))
+        }
+
+        (&ty::RawPtr(a_mt), &ty::RawPtr(b_mt)) => {
+            let mt = relate_type_and_mut(relation, a_mt, b_mt, a)?;
+            Ok(tcx.mk_ptr(mt))
+        }
+
+        (&ty::Ref(a_r, a_ty, a_mutbl), &ty::Ref(b_r, b_ty, b_mutbl)) => {
+            let r = relation.relate_with_variance(
+                ty::Contravariant,
+                ty::VarianceDiagInfo::default(),
+                a_r,
+                b_r,
+            )?;
+            let a_mt = ty::TypeAndMut { ty: a_ty, mutbl: a_mutbl };
+            let b_mt = ty::TypeAndMut { ty: b_ty, mutbl: b_mutbl };
+            let mt = relate_type_and_mut(relation, a_mt, b_mt, a)?;
+            Ok(tcx.mk_ref(r, mt))
+        }
+
+        (&ty::Array(a_t, sz_a), &ty::Array(b_t, sz_b)) => {
+            let t = relation.relate(a_t, b_t)?;
+            match relation.relate(sz_a, sz_b) {
+                Ok(sz) => Ok(tcx.mk_ty(ty::Array(t, sz))),
+                Err(err) => {
+                    // Check whether the lengths are both concrete/known values,
+                    // but are unequal, for better diagnostics.
+                    //
+                    // It might seem dubious to eagerly evaluate these constants here,
+                    // we however cannot end up with errors in `Relate` during both
+                    // `type_of` and `predicates_of`. This means that evaluating the
+                    // constants should not cause cycle errors here.
+                    let sz_a = sz_a.try_eval_usize(tcx, relation.param_env());
+                    let sz_b = sz_b.try_eval_usize(tcx, relation.param_env());
+                    match (sz_a, sz_b) {
+                        (Some(sz_a_val), Some(sz_b_val)) if sz_a_val != sz_b_val => Err(
+                            TypeError::FixedArraySize(expected_found(relation, sz_a_val, sz_b_val)),
+                        ),
+                        _ => Err(err),
+                    }
+                }
+            }
+        }
+
+        (&ty::Slice(a_t), &ty::Slice(b_t)) => {
+            let t = relation.relate(a_t, b_t)?;
+            Ok(tcx.mk_slice(t))
+        }
+
+        (&ty::Tuple(as_), &ty::Tuple(bs)) => {
+            if as_.len() == bs.len() {
+                Ok(tcx.mk_tup(iter::zip(as_, bs).map(|(a, b)| relation.relate(a, b)))?)
+            } else if !(as_.is_empty() || bs.is_empty()) {
+                Err(TypeError::TupleSize(expected_found(relation, as_.len(), bs.len())))
+            } else {
+                Err(TypeError::Sorts(expected_found(relation, a, b)))
+            }
+        }
+
+        (&ty::FnDef(a_def_id, a_substs), &ty::FnDef(b_def_id, b_substs))
+            if a_def_id == b_def_id =>
+        {
+            let substs = relation.relate_item_substs(a_def_id, a_substs, b_substs)?;
+            Ok(tcx.mk_fn_def(a_def_id, substs))
+        }
+
+        (&ty::FnPtr(a_fty), &ty::FnPtr(b_fty)) => {
+            let fty = relation.relate(a_fty, b_fty)?;
+            Ok(tcx.mk_fn_ptr(fty))
+        }
+
+        // these two are already handled downstream in case of lazy normalization
+        (&ty::Projection(a_data), &ty::Projection(b_data)) => {
+            let projection_ty = relation.relate(a_data, b_data)?;
+            Ok(tcx.mk_projection(projection_ty.item_def_id, projection_ty.substs))
+        }
+
+        (&ty::Opaque(a_def_id, a_substs), &ty::Opaque(b_def_id, b_substs))
+            if a_def_id == b_def_id =>
+        {
+            let substs = relate_substs(relation, a_substs, b_substs)?;
+            Ok(tcx.mk_opaque(a_def_id, substs))
+        }
+
+        _ => Err(TypeError::Sorts(expected_found(relation, a, b))),
+    }
+}
+
+/// The main "const relation" routine. Note that this does not handle
+/// inference artifacts, so you should filter those out before calling
+/// it.
+pub fn super_relate_consts<'tcx, R: TypeRelation<'tcx>>(
+    relation: &mut R,
+    a: ty::Const<'tcx>,
+    b: ty::Const<'tcx>,
+) -> RelateResult<'tcx, ty::Const<'tcx>> {
+    debug!("{}.super_relate_consts(a = {:?}, b = {:?})", relation.tag(), a, b);
+    let tcx = relation.tcx();
+
+    let a_ty;
+    let b_ty;
+    if relation.tcx().features().adt_const_params {
+        a_ty = tcx.normalize_erasing_regions(relation.param_env(), a.ty());
+        b_ty = tcx.normalize_erasing_regions(relation.param_env(), b.ty());
+    } else {
+        a_ty = tcx.erase_regions(a.ty());
+        b_ty = tcx.erase_regions(b.ty());
+    }
+    if a_ty != b_ty {
+        relation.tcx().sess.delay_span_bug(
+            DUMMY_SP,
+            &format!("cannot relate constants of different types: {} != {}", a_ty, b_ty),
+        );
+    }
+
+    let eagerly_eval = |x: ty::Const<'tcx>| x.eval(tcx, relation.param_env());
+    let a = eagerly_eval(a);
+    let b = eagerly_eval(b);
+
+    // Currently, the values that can be unified are primitive types,
+    // and those that derive both `PartialEq` and `Eq`, corresponding
+    // to structural-match types.
+    let is_match = match (a.kind(), b.kind()) {
+        (ty::ConstKind::Infer(_), _) | (_, ty::ConstKind::Infer(_)) => {
+            // The caller should handle these cases!
+            bug!("var types encountered in super_relate_consts: {:?} {:?}", a, b)
+        }
+
+        (ty::ConstKind::Error(_), _) => return Ok(a),
+        (_, ty::ConstKind::Error(_)) => return Ok(b),
+
+        (ty::ConstKind::Param(a_p), ty::ConstKind::Param(b_p)) => a_p.index == b_p.index,
+        (ty::ConstKind::Placeholder(p1), ty::ConstKind::Placeholder(p2)) => p1 == p2,
+        (ty::ConstKind::Value(a_val), ty::ConstKind::Value(b_val)) => a_val == b_val,
+
+        (ty::ConstKind::Unevaluated(au), ty::ConstKind::Unevaluated(bu))
+            if tcx.features().generic_const_exprs =>
+        {
+            tcx.try_unify_abstract_consts(relation.param_env().and((au.shrink(), bu.shrink())))
+        }
+
+        // While this is slightly incorrect, it shouldn't matter for `min_const_generics`
+        // and is the better alternative to waiting until `generic_const_exprs` can
+        // be stabilized.
+        (ty::ConstKind::Unevaluated(au), ty::ConstKind::Unevaluated(bu))
+            if au.def == bu.def && au.promoted == bu.promoted =>
+        {
+            let substs = relation.relate_with_variance(
+                ty::Variance::Invariant,
+                ty::VarianceDiagInfo::default(),
+                au.substs,
+                bu.substs,
+            )?;
+            return Ok(tcx.mk_const(ty::ConstS {
+                kind: ty::ConstKind::Unevaluated(ty::Unevaluated {
+                    def: au.def,
+                    substs,
+                    promoted: au.promoted,
+                }),
+                ty: a.ty(),
+            }));
+        }
+        _ => false,
+    };
+    if is_match { Ok(a) } else { Err(TypeError::ConstMismatch(expected_found(relation, a, b))) }
+}
+
+impl<'tcx> Relate<'tcx> for &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: Self,
+        b: Self,
+    ) -> RelateResult<'tcx, Self> {
+        let tcx = relation.tcx();
+
+        // FIXME: this is wasteful, but want to do a perf run to see how slow it is.
+        // We need to perform this deduplication as we sometimes generate duplicate projections
+        // in `a`.
+        let mut a_v: Vec<_> = a.into_iter().collect();
+        let mut b_v: Vec<_> = b.into_iter().collect();
+        // `skip_binder` here is okay because `stable_cmp` doesn't look at binders
+        a_v.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
+        a_v.dedup();
+        b_v.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
+        b_v.dedup();
+        if a_v.len() != b_v.len() {
+            return Err(TypeError::ExistentialMismatch(expected_found(relation, a, b)));
+        }
+
+        let v = iter::zip(a_v, b_v).map(|(ep_a, ep_b)| {
+            use crate::ty::ExistentialPredicate::*;
+            match (ep_a.skip_binder(), ep_b.skip_binder()) {
+                (Trait(a), Trait(b)) => Ok(ep_a
+                    .rebind(Trait(relation.relate(ep_a.rebind(a), ep_b.rebind(b))?.skip_binder()))),
+                (Projection(a), Projection(b)) => Ok(ep_a.rebind(Projection(
+                    relation.relate(ep_a.rebind(a), ep_b.rebind(b))?.skip_binder(),
+                ))),
+                (AutoTrait(a), AutoTrait(b)) if a == b => Ok(ep_a.rebind(AutoTrait(a))),
+                _ => Err(TypeError::ExistentialMismatch(expected_found(relation, a, b))),
+            }
+        });
+        tcx.mk_poly_existential_predicates(v)
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::ClosureSubsts<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::ClosureSubsts<'tcx>,
+        b: ty::ClosureSubsts<'tcx>,
+    ) -> RelateResult<'tcx, ty::ClosureSubsts<'tcx>> {
+        let substs = relate_substs(relation, a.substs, b.substs)?;
+        Ok(ty::ClosureSubsts { substs })
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::GeneratorSubsts<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::GeneratorSubsts<'tcx>,
+        b: ty::GeneratorSubsts<'tcx>,
+    ) -> RelateResult<'tcx, ty::GeneratorSubsts<'tcx>> {
+        let substs = relate_substs(relation, a.substs, b.substs)?;
+        Ok(ty::GeneratorSubsts { substs })
+    }
+}
+
+impl<'tcx> Relate<'tcx> for SubstsRef<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: SubstsRef<'tcx>,
+        b: SubstsRef<'tcx>,
+    ) -> RelateResult<'tcx, SubstsRef<'tcx>> {
+        relate_substs(relation, a, b)
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::Region<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::Region<'tcx>,
+        b: ty::Region<'tcx>,
+    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+        relation.regions(a, b)
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::Const<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::Const<'tcx>,
+        b: ty::Const<'tcx>,
+    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
+        relation.consts(a, b)
+    }
+}
+
+impl<'tcx, T: Relate<'tcx>> Relate<'tcx> for ty::Binder<'tcx, T> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::Binder<'tcx, T>,
+        b: ty::Binder<'tcx, T>,
+    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>> {
+        relation.binders(a, b)
+    }
+}
+
+impl<'tcx> Relate<'tcx> for GenericArg<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: GenericArg<'tcx>,
+        b: GenericArg<'tcx>,
+    ) -> RelateResult<'tcx, GenericArg<'tcx>> {
+        match (a.unpack(), b.unpack()) {
+            (GenericArgKind::Lifetime(a_lt), GenericArgKind::Lifetime(b_lt)) => {
+                Ok(relation.relate(a_lt, b_lt)?.into())
+            }
+            (GenericArgKind::Type(a_ty), GenericArgKind::Type(b_ty)) => {
+                Ok(relation.relate(a_ty, b_ty)?.into())
+            }
+            (GenericArgKind::Const(a_ct), GenericArgKind::Const(b_ct)) => {
+                Ok(relation.relate(a_ct, b_ct)?.into())
+            }
+            (GenericArgKind::Lifetime(unpacked), x) => {
+                bug!("impossible case reached: can't relate: {:?} with {:?}", unpacked, x)
+            }
+            (GenericArgKind::Type(unpacked), x) => {
+                bug!("impossible case reached: can't relate: {:?} with {:?}", unpacked, x)
+            }
+            (GenericArgKind::Const(unpacked), x) => {
+                bug!("impossible case reached: can't relate: {:?} with {:?}", unpacked, x)
+            }
+        }
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::ImplPolarity {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::ImplPolarity,
+        b: ty::ImplPolarity,
+    ) -> RelateResult<'tcx, ty::ImplPolarity> {
+        if a != b {
+            Err(TypeError::PolarityMismatch(expected_found(relation, a, b)))
+        } else {
+            Ok(a)
+        }
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::TraitPredicate<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::TraitPredicate<'tcx>,
+        b: ty::TraitPredicate<'tcx>,
+    ) -> RelateResult<'tcx, ty::TraitPredicate<'tcx>> {
+        Ok(ty::TraitPredicate {
+            trait_ref: relation.relate(a.trait_ref, b.trait_ref)?,
+            constness: relation.relate(a.constness, b.constness)?,
+            polarity: relation.relate(a.polarity, b.polarity)?,
+        })
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::Term<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: Self,
+        b: Self,
+    ) -> RelateResult<'tcx, Self> {
+        Ok(match (a, b) {
+            (Term::Ty(a), Term::Ty(b)) => relation.relate(a, b)?.into(),
+            (Term::Const(a), Term::Const(b)) => relation.relate(a, b)?.into(),
+            _ => return Err(TypeError::Mismatch),
+        })
+    }
+}
+
+impl<'tcx> Relate<'tcx> for ty::ProjectionPredicate<'tcx> {
+    fn relate<R: TypeRelation<'tcx>>(
+        relation: &mut R,
+        a: ty::ProjectionPredicate<'tcx>,
+        b: ty::ProjectionPredicate<'tcx>,
+    ) -> RelateResult<'tcx, ty::ProjectionPredicate<'tcx>> {
+        Ok(ty::ProjectionPredicate {
+            projection_ty: relation.relate(a.projection_ty, b.projection_ty)?,
+            term: relation.relate(a.term, b.term)?,
+        })
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Error handling
+
+pub fn expected_found<'tcx, R, T>(relation: &mut R, a: T, b: T) -> ExpectedFound<T>
+where
+    R: TypeRelation<'tcx>,
+{
+    ExpectedFound::new(relation.a_is_expected(), a, b)
+}
diff --git a/compiler/rustc_middle/src/ty/rvalue_scopes.rs b/compiler/rustc_middle/src/ty/rvalue_scopes.rs
new file mode 100644
index 000000000..e86dafae3
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/rvalue_scopes.rs
@@ -0,0 +1,57 @@
+use crate::middle::region::{Scope, ScopeData, ScopeTree};
+use rustc_data_structures::fx::FxHashMap;
+use rustc_hir as hir;
+
+/// `RvalueScopes` is a mapping from sub-expressions to _extended_ lifetime as determined by
+/// rules laid out in `rustc_typeck::check::rvalue_scopes`.
+#[derive(TyEncodable, TyDecodable, Clone, Debug, Default, Eq, PartialEq, HashStable)]
+pub struct RvalueScopes {
+    map: FxHashMap<hir::ItemLocalId, Option<Scope>>,
+}
+
+impl RvalueScopes {
+    pub fn new() -> Self {
+        Self { map: <_>::default() }
+    }
+
+    /// Returns the scope when the temp created by `expr_id` will be cleaned up.
+    pub fn temporary_scope(
+        &self,
+        region_scope_tree: &ScopeTree,
+        expr_id: hir::ItemLocalId,
+    ) -> Option<Scope> {
+        // Check for a designated rvalue scope.
+        if let Some(&s) = self.map.get(&expr_id) {
+            debug!("temporary_scope({expr_id:?}) = {s:?} [custom]");
+            return s;
+        }
+
+        // Otherwise, locate the innermost terminating scope
+        // if there's one. Static items, for instance, won't
+        // have an enclosing scope, hence no scope will be
+        // returned.
+        let mut id = Scope { id: expr_id, data: ScopeData::Node };
+
+        while let Some(&(p, _)) = region_scope_tree.parent_map.get(&id) {
+            match p.data {
+                ScopeData::Destruction => {
+                    debug!("temporary_scope({expr_id:?}) = {id:?} [enclosing]");
+                    return Some(id);
+                }
+                _ => id = p,
+            }
+        }
+
+        debug!("temporary_scope({expr_id:?}) = None");
+        None
+    }
+
+    /// Make an association between a sub-expression and an extended lifetime
+    pub fn record_rvalue_scope(&mut self, var: hir::ItemLocalId, lifetime: Option<Scope>) {
+        debug!("record_rvalue_scope(var={var:?}, lifetime={lifetime:?})");
+        if let Some(lifetime) = lifetime {
+            assert!(var != lifetime.item_local_id());
+        }
+        self.map.insert(var, lifetime);
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/structural_impls.rs b/compiler/rustc_middle/src/ty/structural_impls.rs
new file mode 100644
index 000000000..7660a2f3a
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/structural_impls.rs
@@ -0,0 +1,1304 @@
+//! This module contains implements of the `Lift` and `TypeFoldable`
+//! traits for various types in the Rust compiler. Most are written by
+//! hand, though we've recently added some macros and proc-macros to help with the tedium.
+
+use crate::mir::interpret;
+use crate::mir::ProjectionKind;
+use crate::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeSuperFoldable};
+use crate::ty::print::{with_no_trimmed_paths, FmtPrinter, Printer};
+use crate::ty::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitor};
+use crate::ty::{self, InferConst, Lift, Term, Ty, TyCtxt};
+use rustc_data_structures::functor::IdFunctor;
+use rustc_hir as hir;
+use rustc_hir::def::Namespace;
+use rustc_index::vec::{Idx, IndexVec};
+
+use std::fmt;
+use std::mem::ManuallyDrop;
+use std::ops::ControlFlow;
+use std::rc::Rc;
+use std::sync::Arc;
+
+impl fmt::Debug for ty::TraitDef {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        ty::tls::with(|tcx| {
+            with_no_trimmed_paths!({
+                f.write_str(
+                    &FmtPrinter::new(tcx, Namespace::TypeNS)
+                        .print_def_path(self.def_id, &[])?
+                        .into_buffer(),
+                )
+            })
+        })
+    }
+}
+
+impl<'tcx> fmt::Debug for ty::AdtDef<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        ty::tls::with(|tcx| {
+            with_no_trimmed_paths!({
+                f.write_str(
+                    &FmtPrinter::new(tcx, Namespace::TypeNS)
+                        .print_def_path(self.did(), &[])?
+                        .into_buffer(),
+                )
+            })
+        })
+    }
+}
+
+impl fmt::Debug for ty::UpvarId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let name = ty::tls::with(|tcx| tcx.hir().name(self.var_path.hir_id));
+        write!(f, "UpvarId({:?};`{}`;{:?})", self.var_path.hir_id, name, self.closure_expr_id)
+    }
+}
+
+impl<'tcx> fmt::Debug for ty::ExistentialTraitRef<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        with_no_trimmed_paths!(fmt::Display::fmt(self, f))
+    }
+}
+
+impl<'tcx> fmt::Debug for ty::adjustment::Adjustment<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:?} -> {}", self.kind, self.target)
+    }
+}
+
+impl fmt::Debug for ty::BoundRegionKind {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            ty::BrAnon(n) => write!(f, "BrAnon({:?})", n),
+            ty::BrNamed(did, name) => {
+                if did.is_crate_root() {
+                    write!(f, "BrNamed({})", name)
+                } else {
+                    write!(f, "BrNamed({:?}, {})", did, name)
+                }
+            }
+            ty::BrEnv => write!(f, "BrEnv"),
+        }
+    }
+}
+
+impl fmt::Debug for ty::FreeRegion {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "ReFree({:?}, {:?})", self.scope, self.bound_region)
+    }
+}
+
+impl<'tcx> fmt::Debug for ty::FnSig<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "({:?}; c_variadic: {})->{:?}", self.inputs(), self.c_variadic, self.output())
+    }
+}
+
+impl<'tcx> fmt::Debug for ty::ConstVid<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "_#{}c", self.index)
+    }
+}
+
+impl fmt::Debug for ty::RegionVid {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "'_#{}r", self.index())
+    }
+}
+
+impl<'tcx> fmt::Debug for ty::TraitRef<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        with_no_trimmed_paths!(fmt::Display::fmt(self, f))
+    }
+}
+
+impl<'tcx> fmt::Debug for Ty<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        with_no_trimmed_paths!(fmt::Display::fmt(self, f))
+    }
+}
+
+impl fmt::Debug for ty::ParamTy {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}/#{}", self.name, self.index)
+    }
+}
+
+impl fmt::Debug for ty::ParamConst {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}/#{}", self.name, self.index)
+    }
+}
+
+impl<'tcx> fmt::Debug for ty::TraitPredicate<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let ty::BoundConstness::ConstIfConst = self.constness {
+            write!(f, "~const ")?;
+        }
+        write!(f, "TraitPredicate({:?}, polarity:{:?})", self.trait_ref, self.polarity)
+    }
+}
+
+impl<'tcx> fmt::Debug for ty::ProjectionPredicate<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "ProjectionPredicate({:?}, {:?})", self.projection_ty, self.term)
+    }
+}
+
+impl<'tcx> fmt::Debug for ty::Predicate<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:?}", self.kind())
+    }
+}
+
+impl<'tcx> fmt::Debug for ty::PredicateKind<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            ty::PredicateKind::Trait(ref a) => a.fmt(f),
+            ty::PredicateKind::Subtype(ref pair) => pair.fmt(f),
+            ty::PredicateKind::Coerce(ref pair) => pair.fmt(f),
+            ty::PredicateKind::RegionOutlives(ref pair) => pair.fmt(f),
+            ty::PredicateKind::TypeOutlives(ref pair) => pair.fmt(f),
+            ty::PredicateKind::Projection(ref pair) => pair.fmt(f),
+            ty::PredicateKind::WellFormed(data) => write!(f, "WellFormed({:?})", data),
+            ty::PredicateKind::ObjectSafe(trait_def_id) => {
+                write!(f, "ObjectSafe({:?})", trait_def_id)
+            }
+            ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
+                write!(f, "ClosureKind({:?}, {:?}, {:?})", closure_def_id, closure_substs, kind)
+            }
+            ty::PredicateKind::ConstEvaluatable(uv) => {
+                write!(f, "ConstEvaluatable({:?}, {:?})", uv.def, uv.substs)
+            }
+            ty::PredicateKind::ConstEquate(c1, c2) => write!(f, "ConstEquate({:?}, {:?})", c1, c2),
+            ty::PredicateKind::TypeWellFormedFromEnv(ty) => {
+                write!(f, "TypeWellFormedFromEnv({:?})", ty)
+            }
+        }
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Atomic structs
+//
+// For things that don't carry any arena-allocated data (and are
+// copy...), just add them to this list.
+
+TrivialTypeTraversalAndLiftImpls! {
+    (),
+    bool,
+    usize,
+    ::rustc_target::abi::VariantIdx,
+    u32,
+    u64,
+    String,
+    crate::middle::region::Scope,
+    crate::ty::FloatTy,
+    ::rustc_ast::InlineAsmOptions,
+    ::rustc_ast::InlineAsmTemplatePiece,
+    ::rustc_ast::NodeId,
+    ::rustc_span::symbol::Symbol,
+    ::rustc_hir::def::Res,
+    ::rustc_hir::def_id::DefId,
+    ::rustc_hir::def_id::LocalDefId,
+    ::rustc_hir::HirId,
+    ::rustc_hir::MatchSource,
+    ::rustc_hir::Mutability,
+    ::rustc_hir::Unsafety,
+    ::rustc_target::asm::InlineAsmRegOrRegClass,
+    ::rustc_target::spec::abi::Abi,
+    crate::mir::coverage::ExpressionOperandId,
+    crate::mir::coverage::CounterValueReference,
+    crate::mir::coverage::InjectedExpressionId,
+    crate::mir::coverage::InjectedExpressionIndex,
+    crate::mir::coverage::MappedExpressionIndex,
+    crate::mir::Local,
+    crate::mir::Promoted,
+    crate::traits::Reveal,
+    crate::ty::adjustment::AutoBorrowMutability,
+    crate::ty::AdtKind,
+    crate::ty::BoundConstness,
+    // Including `BoundRegionKind` is a *bit* dubious, but direct
+    // references to bound region appear in `ty::Error`, and aren't
+    // really meant to be folded. In general, we can only fold a fully
+    // general `Region`.
+    crate::ty::BoundRegionKind,
+    crate::ty::AssocItem,
+    crate::ty::AssocKind,
+    crate::ty::Placeholder<crate::ty::BoundRegionKind>,
+    crate::ty::ClosureKind,
+    crate::ty::FreeRegion,
+    crate::ty::InferTy,
+    crate::ty::IntVarValue,
+    crate::ty::ParamConst,
+    crate::ty::ParamTy,
+    crate::ty::adjustment::PointerCast,
+    crate::ty::RegionVid,
+    crate::ty::UniverseIndex,
+    crate::ty::Variance,
+    ::rustc_span::Span,
+    ::rustc_errors::ErrorGuaranteed,
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Lift implementations
+
+// FIXME(eddyb) replace all the uses of `Option::map` with `?`.
+impl<'tcx, A: Lift<'tcx>, B: Lift<'tcx>> Lift<'tcx> for (A, B) {
+    type Lifted = (A::Lifted, B::Lifted);
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        Some((tcx.lift(self.0)?, tcx.lift(self.1)?))
+    }
+}
+
+impl<'tcx, A: Lift<'tcx>, B: Lift<'tcx>, C: Lift<'tcx>> Lift<'tcx> for (A, B, C) {
+    type Lifted = (A::Lifted, B::Lifted, C::Lifted);
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        Some((tcx.lift(self.0)?, tcx.lift(self.1)?, tcx.lift(self.2)?))
+    }
+}
+
+impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for Option<T> {
+    type Lifted = Option<T::Lifted>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        match self {
+            Some(x) => tcx.lift(x).map(Some),
+            None => Some(None),
+        }
+    }
+}
+
+impl<'tcx, T: Lift<'tcx>, E: Lift<'tcx>> Lift<'tcx> for Result<T, E> {
+    type Lifted = Result<T::Lifted, E::Lifted>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        match self {
+            Ok(x) => tcx.lift(x).map(Ok),
+            Err(e) => tcx.lift(e).map(Err),
+        }
+    }
+}
+
+impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for Box<T> {
+    type Lifted = Box<T::Lifted>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift(*self).map(Box::new)
+    }
+}
+
+impl<'tcx, T: Lift<'tcx> + Clone> Lift<'tcx> for Rc<T> {
+    type Lifted = Rc<T::Lifted>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift(self.as_ref().clone()).map(Rc::new)
+    }
+}
+
+impl<'tcx, T: Lift<'tcx> + Clone> Lift<'tcx> for Arc<T> {
+    type Lifted = Arc<T::Lifted>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift(self.as_ref().clone()).map(Arc::new)
+    }
+}
+impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for Vec<T> {
+    type Lifted = Vec<T::Lifted>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        self.into_iter().map(|v| tcx.lift(v)).collect()
+    }
+}
+
+impl<'tcx, I: Idx, T: Lift<'tcx>> Lift<'tcx> for IndexVec<I, T> {
+    type Lifted = IndexVec<I, T::Lifted>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        self.into_iter().map(|e| tcx.lift(e)).collect()
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::TraitRef<'a> {
+    type Lifted = ty::TraitRef<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift(self.substs).map(|substs| ty::TraitRef { def_id: self.def_id, substs })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::ExistentialTraitRef<'a> {
+    type Lifted = ty::ExistentialTraitRef<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift(self.substs).map(|substs| ty::ExistentialTraitRef { def_id: self.def_id, substs })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::ExistentialPredicate<'a> {
+    type Lifted = ty::ExistentialPredicate<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        match self {
+            ty::ExistentialPredicate::Trait(x) => tcx.lift(x).map(ty::ExistentialPredicate::Trait),
+            ty::ExistentialPredicate::Projection(x) => {
+                tcx.lift(x).map(ty::ExistentialPredicate::Projection)
+            }
+            ty::ExistentialPredicate::AutoTrait(def_id) => {
+                Some(ty::ExistentialPredicate::AutoTrait(def_id))
+            }
+        }
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for Term<'a> {
+    type Lifted = ty::Term<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        Some(match self {
+            Term::Ty(ty) => Term::Ty(tcx.lift(ty)?),
+            Term::Const(c) => Term::Const(tcx.lift(c)?),
+        })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::TraitPredicate<'a> {
+    type Lifted = ty::TraitPredicate<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<ty::TraitPredicate<'tcx>> {
+        tcx.lift(self.trait_ref).map(|trait_ref| ty::TraitPredicate {
+            trait_ref,
+            constness: self.constness,
+            polarity: self.polarity,
+        })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::SubtypePredicate<'a> {
+    type Lifted = ty::SubtypePredicate<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<ty::SubtypePredicate<'tcx>> {
+        tcx.lift((self.a, self.b)).map(|(a, b)| ty::SubtypePredicate {
+            a_is_expected: self.a_is_expected,
+            a,
+            b,
+        })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::CoercePredicate<'a> {
+    type Lifted = ty::CoercePredicate<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<ty::CoercePredicate<'tcx>> {
+        tcx.lift((self.a, self.b)).map(|(a, b)| ty::CoercePredicate { a, b })
+    }
+}
+
+impl<'tcx, A: Copy + Lift<'tcx>, B: Copy + Lift<'tcx>> Lift<'tcx> for ty::OutlivesPredicate<A, B> {
+    type Lifted = ty::OutlivesPredicate<A::Lifted, B::Lifted>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift((self.0, self.1)).map(|(a, b)| ty::OutlivesPredicate(a, b))
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::ProjectionTy<'a> {
+    type Lifted = ty::ProjectionTy<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<ty::ProjectionTy<'tcx>> {
+        tcx.lift(self.substs)
+            .map(|substs| ty::ProjectionTy { item_def_id: self.item_def_id, substs })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::ProjectionPredicate<'a> {
+    type Lifted = ty::ProjectionPredicate<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<ty::ProjectionPredicate<'tcx>> {
+        tcx.lift((self.projection_ty, self.term))
+            .map(|(projection_ty, term)| ty::ProjectionPredicate { projection_ty, term })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::ExistentialProjection<'a> {
+    type Lifted = ty::ExistentialProjection<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift(self.substs).map(|substs| ty::ExistentialProjection {
+            substs,
+            term: tcx.lift(self.term).expect("type must lift when substs do"),
+            item_def_id: self.item_def_id,
+        })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::PredicateKind<'a> {
+    type Lifted = ty::PredicateKind<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        match self {
+            ty::PredicateKind::Trait(data) => tcx.lift(data).map(ty::PredicateKind::Trait),
+            ty::PredicateKind::Subtype(data) => tcx.lift(data).map(ty::PredicateKind::Subtype),
+            ty::PredicateKind::Coerce(data) => tcx.lift(data).map(ty::PredicateKind::Coerce),
+            ty::PredicateKind::RegionOutlives(data) => {
+                tcx.lift(data).map(ty::PredicateKind::RegionOutlives)
+            }
+            ty::PredicateKind::TypeOutlives(data) => {
+                tcx.lift(data).map(ty::PredicateKind::TypeOutlives)
+            }
+            ty::PredicateKind::Projection(data) => {
+                tcx.lift(data).map(ty::PredicateKind::Projection)
+            }
+            ty::PredicateKind::WellFormed(ty) => tcx.lift(ty).map(ty::PredicateKind::WellFormed),
+            ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
+                tcx.lift(closure_substs).map(|closure_substs| {
+                    ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind)
+                })
+            }
+            ty::PredicateKind::ObjectSafe(trait_def_id) => {
+                Some(ty::PredicateKind::ObjectSafe(trait_def_id))
+            }
+            ty::PredicateKind::ConstEvaluatable(uv) => {
+                tcx.lift(uv).map(|uv| ty::PredicateKind::ConstEvaluatable(uv))
+            }
+            ty::PredicateKind::ConstEquate(c1, c2) => {
+                tcx.lift((c1, c2)).map(|(c1, c2)| ty::PredicateKind::ConstEquate(c1, c2))
+            }
+            ty::PredicateKind::TypeWellFormedFromEnv(ty) => {
+                tcx.lift(ty).map(ty::PredicateKind::TypeWellFormedFromEnv)
+            }
+        }
+    }
+}
+
+impl<'a, 'tcx, T: Lift<'tcx>> Lift<'tcx> for ty::Binder<'a, T>
+where
+    <T as Lift<'tcx>>::Lifted: TypeVisitable<'tcx>,
+{
+    type Lifted = ty::Binder<'tcx, T::Lifted>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        let bound_vars = tcx.lift(self.bound_vars());
+        tcx.lift(self.skip_binder())
+            .zip(bound_vars)
+            .map(|(value, vars)| ty::Binder::bind_with_vars(value, vars))
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::ParamEnv<'a> {
+    type Lifted = ty::ParamEnv<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift(self.caller_bounds())
+            .map(|caller_bounds| ty::ParamEnv::new(caller_bounds, self.reveal(), self.constness()))
+    }
+}
+
+impl<'a, 'tcx, T: Lift<'tcx>> Lift<'tcx> for ty::ParamEnvAnd<'a, T> {
+    type Lifted = ty::ParamEnvAnd<'tcx, T::Lifted>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift(self.param_env).and_then(|param_env| {
+            tcx.lift(self.value).map(|value| ty::ParamEnvAnd { param_env, value })
+        })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::ClosureSubsts<'a> {
+    type Lifted = ty::ClosureSubsts<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift(self.substs).map(|substs| ty::ClosureSubsts { substs })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::GeneratorSubsts<'a> {
+    type Lifted = ty::GeneratorSubsts<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift(self.substs).map(|substs| ty::GeneratorSubsts { substs })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::adjustment::Adjustment<'a> {
+    type Lifted = ty::adjustment::Adjustment<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        let ty::adjustment::Adjustment { kind, target } = self;
+        tcx.lift(kind).and_then(|kind| {
+            tcx.lift(target).map(|target| ty::adjustment::Adjustment { kind, target })
+        })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::adjustment::Adjust<'a> {
+    type Lifted = ty::adjustment::Adjust<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        match self {
+            ty::adjustment::Adjust::NeverToAny => Some(ty::adjustment::Adjust::NeverToAny),
+            ty::adjustment::Adjust::Pointer(ptr) => Some(ty::adjustment::Adjust::Pointer(ptr)),
+            ty::adjustment::Adjust::Deref(overloaded) => {
+                tcx.lift(overloaded).map(ty::adjustment::Adjust::Deref)
+            }
+            ty::adjustment::Adjust::Borrow(autoref) => {
+                tcx.lift(autoref).map(ty::adjustment::Adjust::Borrow)
+            }
+        }
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::adjustment::OverloadedDeref<'a> {
+    type Lifted = ty::adjustment::OverloadedDeref<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift(self.region).map(|region| ty::adjustment::OverloadedDeref {
+            region,
+            mutbl: self.mutbl,
+            span: self.span,
+        })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::adjustment::AutoBorrow<'a> {
+    type Lifted = ty::adjustment::AutoBorrow<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        match self {
+            ty::adjustment::AutoBorrow::Ref(r, m) => {
+                tcx.lift(r).map(|r| ty::adjustment::AutoBorrow::Ref(r, m))
+            }
+            ty::adjustment::AutoBorrow::RawPtr(m) => Some(ty::adjustment::AutoBorrow::RawPtr(m)),
+        }
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::GenSig<'a> {
+    type Lifted = ty::GenSig<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift((self.resume_ty, self.yield_ty, self.return_ty))
+            .map(|(resume_ty, yield_ty, return_ty)| ty::GenSig { resume_ty, yield_ty, return_ty })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::FnSig<'a> {
+    type Lifted = ty::FnSig<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        tcx.lift(self.inputs_and_output).map(|x| ty::FnSig {
+            inputs_and_output: x,
+            c_variadic: self.c_variadic,
+            unsafety: self.unsafety,
+            abi: self.abi,
+        })
+    }
+}
+
+impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for ty::error::ExpectedFound<T> {
+    type Lifted = ty::error::ExpectedFound<T::Lifted>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        let ty::error::ExpectedFound { expected, found } = self;
+        tcx.lift(expected).and_then(|expected| {
+            tcx.lift(found).map(|found| ty::error::ExpectedFound { expected, found })
+        })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::error::TypeError<'a> {
+    type Lifted = ty::error::TypeError<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        use crate::ty::error::TypeError::*;
+
+        Some(match self {
+            Mismatch => Mismatch,
+            ConstnessMismatch(x) => ConstnessMismatch(x),
+            PolarityMismatch(x) => PolarityMismatch(x),
+            UnsafetyMismatch(x) => UnsafetyMismatch(x),
+            AbiMismatch(x) => AbiMismatch(x),
+            Mutability => Mutability,
+            ArgumentMutability(i) => ArgumentMutability(i),
+            TupleSize(x) => TupleSize(x),
+            FixedArraySize(x) => FixedArraySize(x),
+            ArgCount => ArgCount,
+            FieldMisMatch(x, y) => FieldMisMatch(x, y),
+            RegionsDoesNotOutlive(a, b) => {
+                return tcx.lift((a, b)).map(|(a, b)| RegionsDoesNotOutlive(a, b));
+            }
+            RegionsInsufficientlyPolymorphic(a, b) => {
+                return tcx.lift(b).map(|b| RegionsInsufficientlyPolymorphic(a, b));
+            }
+            RegionsOverlyPolymorphic(a, b) => {
+                return tcx.lift(b).map(|b| RegionsOverlyPolymorphic(a, b));
+            }
+            RegionsPlaceholderMismatch => RegionsPlaceholderMismatch,
+            IntMismatch(x) => IntMismatch(x),
+            FloatMismatch(x) => FloatMismatch(x),
+            Traits(x) => Traits(x),
+            VariadicMismatch(x) => VariadicMismatch(x),
+            CyclicTy(t) => return tcx.lift(t).map(|t| CyclicTy(t)),
+            CyclicConst(ct) => return tcx.lift(ct).map(|ct| CyclicConst(ct)),
+            ProjectionMismatched(x) => ProjectionMismatched(x),
+            ArgumentSorts(x, i) => return tcx.lift(x).map(|x| ArgumentSorts(x, i)),
+            Sorts(x) => return tcx.lift(x).map(Sorts),
+            ExistentialMismatch(x) => return tcx.lift(x).map(ExistentialMismatch),
+            ConstMismatch(x) => return tcx.lift(x).map(ConstMismatch),
+            IntrinsicCast => IntrinsicCast,
+            TargetFeatureCast(x) => TargetFeatureCast(x),
+            ObjectUnsafeCoercion(x) => return tcx.lift(x).map(ObjectUnsafeCoercion),
+        })
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for ty::InstanceDef<'a> {
+    type Lifted = ty::InstanceDef<'tcx>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        match self {
+            ty::InstanceDef::Item(def_id) => Some(ty::InstanceDef::Item(def_id)),
+            ty::InstanceDef::VTableShim(def_id) => Some(ty::InstanceDef::VTableShim(def_id)),
+            ty::InstanceDef::ReifyShim(def_id) => Some(ty::InstanceDef::ReifyShim(def_id)),
+            ty::InstanceDef::Intrinsic(def_id) => Some(ty::InstanceDef::Intrinsic(def_id)),
+            ty::InstanceDef::FnPtrShim(def_id, ty) => {
+                Some(ty::InstanceDef::FnPtrShim(def_id, tcx.lift(ty)?))
+            }
+            ty::InstanceDef::Virtual(def_id, n) => Some(ty::InstanceDef::Virtual(def_id, n)),
+            ty::InstanceDef::ClosureOnceShim { call_once, track_caller } => {
+                Some(ty::InstanceDef::ClosureOnceShim { call_once, track_caller })
+            }
+            ty::InstanceDef::DropGlue(def_id, ty) => {
+                Some(ty::InstanceDef::DropGlue(def_id, tcx.lift(ty)?))
+            }
+            ty::InstanceDef::CloneShim(def_id, ty) => {
+                Some(ty::InstanceDef::CloneShim(def_id, tcx.lift(ty)?))
+            }
+        }
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// TypeFoldable implementations.
+
+/// AdtDefs are basically the same as a DefId.
+impl<'tcx> TypeFoldable<'tcx> for ty::AdtDef<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _folder: &mut F) -> Result<Self, F::Error> {
+        Ok(self)
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for ty::AdtDef<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        ControlFlow::CONTINUE
+    }
+}
+
+impl<'tcx, T: TypeFoldable<'tcx>, U: TypeFoldable<'tcx>> TypeFoldable<'tcx> for (T, U) {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        folder: &mut F,
+    ) -> Result<(T, U), F::Error> {
+        Ok((self.0.try_fold_with(folder)?, self.1.try_fold_with(folder)?))
+    }
+}
+
+impl<'tcx, T: TypeVisitable<'tcx>, U: TypeVisitable<'tcx>> TypeVisitable<'tcx> for (T, U) {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.0.visit_with(visitor)?;
+        self.1.visit_with(visitor)
+    }
+}
+
+impl<'tcx, A: TypeFoldable<'tcx>, B: TypeFoldable<'tcx>, C: TypeFoldable<'tcx>> TypeFoldable<'tcx>
+    for (A, B, C)
+{
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        folder: &mut F,
+    ) -> Result<(A, B, C), F::Error> {
+        Ok((
+            self.0.try_fold_with(folder)?,
+            self.1.try_fold_with(folder)?,
+            self.2.try_fold_with(folder)?,
+        ))
+    }
+}
+
+impl<'tcx, A: TypeVisitable<'tcx>, B: TypeVisitable<'tcx>, C: TypeVisitable<'tcx>>
+    TypeVisitable<'tcx> for (A, B, C)
+{
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.0.visit_with(visitor)?;
+        self.1.visit_with(visitor)?;
+        self.2.visit_with(visitor)
+    }
+}
+
+EnumTypeTraversalImpl! {
+    impl<'tcx, T> TypeFoldable<'tcx> for Option<T> {
+        (Some)(a),
+        (None),
+    } where T: TypeFoldable<'tcx>
+}
+EnumTypeTraversalImpl! {
+    impl<'tcx, T> TypeVisitable<'tcx> for Option<T> {
+        (Some)(a),
+        (None),
+    } where T: TypeVisitable<'tcx>
+}
+
+EnumTypeTraversalImpl! {
+    impl<'tcx, T, E> TypeFoldable<'tcx> for Result<T, E> {
+        (Ok)(a),
+        (Err)(a),
+    } where T: TypeFoldable<'tcx>, E: TypeFoldable<'tcx>,
+}
+EnumTypeTraversalImpl! {
+    impl<'tcx, T, E> TypeVisitable<'tcx> for Result<T, E> {
+        (Ok)(a),
+        (Err)(a),
+    } where T: TypeVisitable<'tcx>, E: TypeVisitable<'tcx>,
+}
+
+impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Rc<T> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(
+        mut self,
+        folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        // We merely want to replace the contained `T`, if at all possible,
+        // so that we don't needlessly allocate a new `Rc` or indeed clone
+        // the contained type.
+        unsafe {
+            // First step is to ensure that we have a unique reference to
+            // the contained type, which `Rc::make_mut` will accomplish (by
+            // allocating a new `Rc` and cloning the `T` only if required).
+            // This is done *before* casting to `Rc<ManuallyDrop<T>>` so that
+            // panicking during `make_mut` does not leak the `T`.
+            Rc::make_mut(&mut self);
+
+            // Casting to `Rc<ManuallyDrop<T>>` is safe because `ManuallyDrop`
+            // is `repr(transparent)`.
+            let ptr = Rc::into_raw(self).cast::<ManuallyDrop<T>>();
+            let mut unique = Rc::from_raw(ptr);
+
+            // Call to `Rc::make_mut` above guarantees that `unique` is the
+            // sole reference to the contained value, so we can avoid doing
+            // a checked `get_mut` here.
+            let slot = Rc::get_mut_unchecked(&mut unique);
+
+            // Semantically move the contained type out from `unique`, fold
+            // it, then move the folded value back into `unique`.  Should
+            // folding fail, `ManuallyDrop` ensures that the "moved-out"
+            // value is not re-dropped.
+            let owned = ManuallyDrop::take(slot);
+            let folded = owned.try_fold_with(folder)?;
+            *slot = ManuallyDrop::new(folded);
+
+            // Cast back to `Rc<T>`.
+            Ok(Rc::from_raw(Rc::into_raw(unique).cast()))
+        }
+    }
+}
+
+impl<'tcx, T: TypeVisitable<'tcx>> TypeVisitable<'tcx> for Rc<T> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        (**self).visit_with(visitor)
+    }
+}
+
+impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Arc<T> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(
+        mut self,
+        folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        // We merely want to replace the contained `T`, if at all possible,
+        // so that we don't needlessly allocate a new `Arc` or indeed clone
+        // the contained type.
+        unsafe {
+            // First step is to ensure that we have a unique reference to
+            // the contained type, which `Arc::make_mut` will accomplish (by
+            // allocating a new `Arc` and cloning the `T` only if required).
+            // This is done *before* casting to `Arc<ManuallyDrop<T>>` so that
+            // panicking during `make_mut` does not leak the `T`.
+            Arc::make_mut(&mut self);
+
+            // Casting to `Arc<ManuallyDrop<T>>` is safe because `ManuallyDrop`
+            // is `repr(transparent)`.
+            let ptr = Arc::into_raw(self).cast::<ManuallyDrop<T>>();
+            let mut unique = Arc::from_raw(ptr);
+
+            // Call to `Arc::make_mut` above guarantees that `unique` is the
+            // sole reference to the contained value, so we can avoid doing
+            // a checked `get_mut` here.
+            let slot = Arc::get_mut_unchecked(&mut unique);
+
+            // Semantically move the contained type out from `unique`, fold
+            // it, then move the folded value back into `unique`.  Should
+            // folding fail, `ManuallyDrop` ensures that the "moved-out"
+            // value is not re-dropped.
+            let owned = ManuallyDrop::take(slot);
+            let folded = owned.try_fold_with(folder)?;
+            *slot = ManuallyDrop::new(folded);
+
+            // Cast back to `Arc<T>`.
+            Ok(Arc::from_raw(Arc::into_raw(unique).cast()))
+        }
+    }
+}
+
+impl<'tcx, T: TypeVisitable<'tcx>> TypeVisitable<'tcx> for Arc<T> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        (**self).visit_with(visitor)
+    }
+}
+
+impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Box<T> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        self.try_map_id(|value| value.try_fold_with(folder))
+    }
+}
+
+impl<'tcx, T: TypeVisitable<'tcx>> TypeVisitable<'tcx> for Box<T> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        (**self).visit_with(visitor)
+    }
+}
+
+impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Vec<T> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        self.try_map_id(|t| t.try_fold_with(folder))
+    }
+}
+
+impl<'tcx, T: TypeVisitable<'tcx>> TypeVisitable<'tcx> for Vec<T> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.iter().try_for_each(|t| t.visit_with(visitor))
+    }
+}
+
+impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for Box<[T]> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        self.try_map_id(|t| t.try_fold_with(folder))
+    }
+}
+
+impl<'tcx, T: TypeVisitable<'tcx>> TypeVisitable<'tcx> for Box<[T]> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.iter().try_for_each(|t| t.visit_with(visitor))
+    }
+}
+
+impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for ty::EarlyBinder<T> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        self.try_map_bound(|ty| ty.try_fold_with(folder))
+    }
+}
+
+impl<'tcx, T: TypeVisitable<'tcx>> TypeVisitable<'tcx> for ty::EarlyBinder<T> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.as_ref().0.visit_with(visitor)
+    }
+}
+
+impl<'tcx, T: TypeFoldable<'tcx>> TypeFoldable<'tcx> for ty::Binder<'tcx, T> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        folder.try_fold_binder(self)
+    }
+}
+
+impl<'tcx, T: TypeVisitable<'tcx>> TypeVisitable<'tcx> for ty::Binder<'tcx, T> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        visitor.visit_binder(self)
+    }
+}
+
+impl<'tcx, T: TypeFoldable<'tcx>> TypeSuperFoldable<'tcx> for ty::Binder<'tcx, T> {
+    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        self.try_map_bound(|ty| ty.try_fold_with(folder))
+    }
+}
+
+impl<'tcx, T: TypeVisitable<'tcx>> TypeSuperVisitable<'tcx> for ty::Binder<'tcx, T> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.as_ref().skip_binder().visit_with(visitor)
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        ty::util::fold_list(self, folder, |tcx, v| tcx.intern_poly_existential_predicates(v))
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx>
+    for &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>
+{
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.iter().try_for_each(|p| p.visit_with(visitor))
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ProjectionKind> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        ty::util::fold_list(self, folder, |tcx, v| tcx.intern_projs(v))
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for &'tcx ty::List<ProjectionKind> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.iter().try_for_each(|t| t.visit_with(visitor))
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for ty::instance::Instance<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        use crate::ty::InstanceDef::*;
+        Ok(Self {
+            substs: self.substs.try_fold_with(folder)?,
+            def: match self.def {
+                Item(def) => Item(def.try_fold_with(folder)?),
+                VTableShim(did) => VTableShim(did.try_fold_with(folder)?),
+                ReifyShim(did) => ReifyShim(did.try_fold_with(folder)?),
+                Intrinsic(did) => Intrinsic(did.try_fold_with(folder)?),
+                FnPtrShim(did, ty) => {
+                    FnPtrShim(did.try_fold_with(folder)?, ty.try_fold_with(folder)?)
+                }
+                Virtual(did, i) => Virtual(did.try_fold_with(folder)?, i),
+                ClosureOnceShim { call_once, track_caller } => {
+                    ClosureOnceShim { call_once: call_once.try_fold_with(folder)?, track_caller }
+                }
+                DropGlue(did, ty) => {
+                    DropGlue(did.try_fold_with(folder)?, ty.try_fold_with(folder)?)
+                }
+                CloneShim(did, ty) => {
+                    CloneShim(did.try_fold_with(folder)?, ty.try_fold_with(folder)?)
+                }
+            },
+        })
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for ty::instance::Instance<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        use crate::ty::InstanceDef::*;
+        self.substs.visit_with(visitor)?;
+        match self.def {
+            Item(def) => def.visit_with(visitor),
+            VTableShim(did) | ReifyShim(did) | Intrinsic(did) | Virtual(did, _) => {
+                did.visit_with(visitor)
+            }
+            FnPtrShim(did, ty) | CloneShim(did, ty) => {
+                did.visit_with(visitor)?;
+                ty.visit_with(visitor)
+            }
+            DropGlue(did, ty) => {
+                did.visit_with(visitor)?;
+                ty.visit_with(visitor)
+            }
+            ClosureOnceShim { call_once, track_caller: _ } => call_once.visit_with(visitor),
+        }
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for interpret::GlobalId<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        Ok(Self { instance: self.instance.try_fold_with(folder)?, promoted: self.promoted })
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for interpret::GlobalId<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.instance.visit_with(visitor)
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for Ty<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        folder.try_fold_ty(self)
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for Ty<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        visitor.visit_ty(*self)
+    }
+}
+
+impl<'tcx> TypeSuperFoldable<'tcx> for Ty<'tcx> {
+    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        let kind = match *self.kind() {
+            ty::RawPtr(tm) => ty::RawPtr(tm.try_fold_with(folder)?),
+            ty::Array(typ, sz) => ty::Array(typ.try_fold_with(folder)?, sz.try_fold_with(folder)?),
+            ty::Slice(typ) => ty::Slice(typ.try_fold_with(folder)?),
+            ty::Adt(tid, substs) => ty::Adt(tid, substs.try_fold_with(folder)?),
+            ty::Dynamic(trait_ty, region) => {
+                ty::Dynamic(trait_ty.try_fold_with(folder)?, region.try_fold_with(folder)?)
+            }
+            ty::Tuple(ts) => ty::Tuple(ts.try_fold_with(folder)?),
+            ty::FnDef(def_id, substs) => ty::FnDef(def_id, substs.try_fold_with(folder)?),
+            ty::FnPtr(f) => ty::FnPtr(f.try_fold_with(folder)?),
+            ty::Ref(r, ty, mutbl) => {
+                ty::Ref(r.try_fold_with(folder)?, ty.try_fold_with(folder)?, mutbl)
+            }
+            ty::Generator(did, substs, movability) => {
+                ty::Generator(did, substs.try_fold_with(folder)?, movability)
+            }
+            ty::GeneratorWitness(types) => ty::GeneratorWitness(types.try_fold_with(folder)?),
+            ty::Closure(did, substs) => ty::Closure(did, substs.try_fold_with(folder)?),
+            ty::Projection(data) => ty::Projection(data.try_fold_with(folder)?),
+            ty::Opaque(did, substs) => ty::Opaque(did, substs.try_fold_with(folder)?),
+
+            ty::Bool
+            | ty::Char
+            | ty::Str
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Error(_)
+            | ty::Infer(_)
+            | ty::Param(..)
+            | ty::Bound(..)
+            | ty::Placeholder(..)
+            | ty::Never
+            | ty::Foreign(..) => return Ok(self),
+        };
+
+        Ok(if *self.kind() == kind { self } else { folder.tcx().mk_ty(kind) })
+    }
+}
+
+impl<'tcx> TypeSuperVisitable<'tcx> for Ty<'tcx> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        match self.kind() {
+            ty::RawPtr(ref tm) => tm.visit_with(visitor),
+            ty::Array(typ, sz) => {
+                typ.visit_with(visitor)?;
+                sz.visit_with(visitor)
+            }
+            ty::Slice(typ) => typ.visit_with(visitor),
+            ty::Adt(_, substs) => substs.visit_with(visitor),
+            ty::Dynamic(ref trait_ty, ref reg) => {
+                trait_ty.visit_with(visitor)?;
+                reg.visit_with(visitor)
+            }
+            ty::Tuple(ts) => ts.visit_with(visitor),
+            ty::FnDef(_, substs) => substs.visit_with(visitor),
+            ty::FnPtr(ref f) => f.visit_with(visitor),
+            ty::Ref(r, ty, _) => {
+                r.visit_with(visitor)?;
+                ty.visit_with(visitor)
+            }
+            ty::Generator(_did, ref substs, _) => substs.visit_with(visitor),
+            ty::GeneratorWitness(ref types) => types.visit_with(visitor),
+            ty::Closure(_did, ref substs) => substs.visit_with(visitor),
+            ty::Projection(ref data) => data.visit_with(visitor),
+            ty::Opaque(_, ref substs) => substs.visit_with(visitor),
+
+            ty::Bool
+            | ty::Char
+            | ty::Str
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Error(_)
+            | ty::Infer(_)
+            | ty::Bound(..)
+            | ty::Placeholder(..)
+            | ty::Param(..)
+            | ty::Never
+            | ty::Foreign(..) => ControlFlow::CONTINUE,
+        }
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for ty::Region<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        folder.try_fold_region(self)
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for ty::Region<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        visitor.visit_region(*self)
+    }
+}
+
+impl<'tcx> TypeSuperFoldable<'tcx> for ty::Region<'tcx> {
+    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        _folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        Ok(self)
+    }
+}
+
+impl<'tcx> TypeSuperVisitable<'tcx> for ty::Region<'tcx> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        ControlFlow::CONTINUE
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for ty::Predicate<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        folder.try_fold_predicate(self)
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for ty::Predicate<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        visitor.visit_predicate(*self)
+    }
+
+    #[inline]
+    fn has_vars_bound_at_or_above(&self, binder: ty::DebruijnIndex) -> bool {
+        self.outer_exclusive_binder() > binder
+    }
+
+    #[inline]
+    fn has_type_flags(&self, flags: ty::TypeFlags) -> bool {
+        self.flags().intersects(flags)
+    }
+}
+
+impl<'tcx> TypeSuperFoldable<'tcx> for ty::Predicate<'tcx> {
+    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        let new = self.kind().try_fold_with(folder)?;
+        Ok(folder.tcx().reuse_or_mk_predicate(self, new))
+    }
+}
+
+impl<'tcx> TypeSuperVisitable<'tcx> for ty::Predicate<'tcx> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.kind().visit_with(visitor)
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ty::Predicate<'tcx>> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        ty::util::fold_list(self, folder, |tcx, v| tcx.intern_predicates(v))
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for &'tcx ty::List<ty::Predicate<'tcx>> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.iter().try_for_each(|p| p.visit_with(visitor))
+    }
+}
+
+impl<'tcx, T: TypeFoldable<'tcx>, I: Idx> TypeFoldable<'tcx> for IndexVec<I, T> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        self.try_map_id(|x| x.try_fold_with(folder))
+    }
+}
+
+impl<'tcx, T: TypeVisitable<'tcx>, I: Idx> TypeVisitable<'tcx> for IndexVec<I, T> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.iter().try_for_each(|t| t.visit_with(visitor))
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for ty::Const<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        folder.try_fold_const(self)
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for ty::Const<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        visitor.visit_const(*self)
+    }
+}
+
+impl<'tcx> TypeSuperFoldable<'tcx> for ty::Const<'tcx> {
+    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        let ty = self.ty().try_fold_with(folder)?;
+        let kind = self.kind().try_fold_with(folder)?;
+        if ty != self.ty() || kind != self.kind() {
+            Ok(folder.tcx().mk_const(ty::ConstS { ty, kind }))
+        } else {
+            Ok(self)
+        }
+    }
+}
+
+impl<'tcx> TypeSuperVisitable<'tcx> for ty::Const<'tcx> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.ty().visit_with(visitor)?;
+        self.kind().visit_with(visitor)
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for ty::ConstKind<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        Ok(match self {
+            ty::ConstKind::Infer(ic) => ty::ConstKind::Infer(ic.try_fold_with(folder)?),
+            ty::ConstKind::Param(p) => ty::ConstKind::Param(p.try_fold_with(folder)?),
+            ty::ConstKind::Unevaluated(uv) => ty::ConstKind::Unevaluated(uv.try_fold_with(folder)?),
+            ty::ConstKind::Value(_)
+            | ty::ConstKind::Bound(..)
+            | ty::ConstKind::Placeholder(..)
+            | ty::ConstKind::Error(_) => self,
+        })
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for ty::ConstKind<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        match *self {
+            ty::ConstKind::Infer(ic) => ic.visit_with(visitor),
+            ty::ConstKind::Param(p) => p.visit_with(visitor),
+            ty::ConstKind::Unevaluated(uv) => uv.visit_with(visitor),
+            ty::ConstKind::Value(_)
+            | ty::ConstKind::Bound(..)
+            | ty::ConstKind::Placeholder(_)
+            | ty::ConstKind::Error(_) => ControlFlow::CONTINUE,
+        }
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for InferConst<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _folder: &mut F) -> Result<Self, F::Error> {
+        Ok(self)
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for InferConst<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        ControlFlow::CONTINUE
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for ty::Unevaluated<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        folder.try_fold_unevaluated(self)
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for ty::Unevaluated<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        visitor.visit_unevaluated(*self)
+    }
+}
+
+impl<'tcx> TypeSuperFoldable<'tcx> for ty::Unevaluated<'tcx> {
+    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        Ok(ty::Unevaluated {
+            def: self.def,
+            substs: self.substs.try_fold_with(folder)?,
+            promoted: self.promoted,
+        })
+    }
+}
+
+impl<'tcx> TypeSuperVisitable<'tcx> for ty::Unevaluated<'tcx> {
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.substs.visit_with(visitor)
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for ty::Unevaluated<'tcx, ()> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        Ok(self.expand().try_fold_with(folder)?.shrink())
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for ty::Unevaluated<'tcx, ()> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.expand().visit_with(visitor)
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for hir::Constness {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, _: &mut F) -> Result<Self, F::Error> {
+        Ok(self)
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for hir::Constness {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> ControlFlow<V::BreakTy> {
+        ControlFlow::CONTINUE
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
new file mode 100644
index 000000000..52c3a3886
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -0,0 +1,2295 @@
+//! This module contains `TyKind` and its major components.
+
+#![allow(rustc::usage_of_ty_tykind)]
+
+use crate::infer::canonical::Canonical;
+use crate::ty::subst::{GenericArg, InternalSubsts, Subst, SubstsRef};
+use crate::ty::visit::ValidateBoundVars;
+use crate::ty::InferTy::*;
+use crate::ty::{
+    self, AdtDef, DefIdTree, Discr, Term, Ty, TyCtxt, TypeFlags, TypeSuperVisitable, TypeVisitable,
+    TypeVisitor,
+};
+use crate::ty::{List, ParamEnv};
+use polonius_engine::Atom;
+use rustc_data_structures::captures::Captures;
+use rustc_data_structures::intern::Interned;
+use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use rustc_index::vec::Idx;
+use rustc_macros::HashStable;
+use rustc_span::symbol::{kw, Symbol};
+use rustc_target::abi::VariantIdx;
+use rustc_target::spec::abi;
+use std::borrow::Cow;
+use std::cmp::Ordering;
+use std::fmt;
+use std::marker::PhantomData;
+use std::ops::{ControlFlow, Deref, Range};
+use ty::util::IntTypeExt;
+
+use rustc_type_ir::sty::TyKind::*;
+use rustc_type_ir::RegionKind as IrRegionKind;
+use rustc_type_ir::TyKind as IrTyKind;
+
+// Re-export the `TyKind` from `rustc_type_ir` here for convenience
+#[rustc_diagnostic_item = "TyKind"]
+pub type TyKind<'tcx> = IrTyKind<TyCtxt<'tcx>>;
+pub type RegionKind<'tcx> = IrRegionKind<TyCtxt<'tcx>>;
+
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub struct TypeAndMut<'tcx> {
+    pub ty: Ty<'tcx>,
+    pub mutbl: hir::Mutability,
+}
+
+#[derive(Clone, PartialEq, PartialOrd, Eq, Ord, Hash, TyEncodable, TyDecodable, Copy)]
+#[derive(HashStable)]
+/// A "free" region `fr` can be interpreted as "some region
+/// at least as big as the scope `fr.scope`".
+pub struct FreeRegion {
+    pub scope: DefId,
+    pub bound_region: BoundRegionKind,
+}
+
+#[derive(Clone, PartialEq, PartialOrd, Eq, Ord, Hash, TyEncodable, TyDecodable, Copy)]
+#[derive(HashStable)]
+pub enum BoundRegionKind {
+    /// An anonymous region parameter for a given fn (&T)
+    BrAnon(u32),
+
+    /// Named region parameters for functions (a in &'a T)
+    ///
+    /// The `DefId` is needed to distinguish free regions in
+    /// the event of shadowing.
+    BrNamed(DefId, Symbol),
+
+    /// Anonymous region for the implicit env pointer parameter
+    /// to a closure
+    BrEnv,
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash, TyEncodable, TyDecodable, Debug, PartialOrd, Ord)]
+#[derive(HashStable)]
+pub struct BoundRegion {
+    pub var: BoundVar,
+    pub kind: BoundRegionKind,
+}
+
+impl BoundRegionKind {
+    pub fn is_named(&self) -> bool {
+        match *self {
+            BoundRegionKind::BrNamed(_, name) => name != kw::UnderscoreLifetime,
+            _ => false,
+        }
+    }
+}
+
+pub trait Article {
+    fn article(&self) -> &'static str;
+}
+
+impl<'tcx> Article for TyKind<'tcx> {
+    /// Get the article ("a" or "an") to use with this type.
+    fn article(&self) -> &'static str {
+        match self {
+            Int(_) | Float(_) | Array(_, _) => "an",
+            Adt(def, _) if def.is_enum() => "an",
+            // This should never happen, but ICEing and causing the user's code
+            // to not compile felt too harsh.
+            Error(_) => "a",
+            _ => "a",
+        }
+    }
+}
+
+// `TyKind` is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+static_assert_size!(TyKind<'_>, 32);
+
+/// A closure can be modeled as a struct that looks like:
+/// ```ignore (illustrative)
+/// struct Closure<'l0...'li, T0...Tj, CK, CS, U>(...U);
+/// ```
+/// where:
+///
+/// - 'l0...'li and T0...Tj are the generic parameters
+///   in scope on the function that defined the closure,
+/// - CK represents the *closure kind* (Fn vs FnMut vs FnOnce). This
+///   is rather hackily encoded via a scalar type. See
+///   `Ty::to_opt_closure_kind` for details.
+/// - CS represents the *closure signature*, representing as a `fn()`
+///   type. For example, `fn(u32, u32) -> u32` would mean that the closure
+///   implements `CK<(u32, u32), Output = u32>`, where `CK` is the trait
+///   specified above.
+/// - U is a type parameter representing the types of its upvars, tupled up
+///   (borrowed, if appropriate; that is, if a U field represents a by-ref upvar,
+///    and the up-var has the type `Foo`, then that field of U will be `&Foo`).
+///
+/// So, for example, given this function:
+/// ```ignore (illustrative)
+/// fn foo<'a, T>(data: &'a mut T) {
+///      do(|| data.count += 1)
+/// }
+/// ```
+/// the type of the closure would be something like:
+/// ```ignore (illustrative)
+/// struct Closure<'a, T, U>(...U);
+/// ```
+/// Note that the type of the upvar is not specified in the struct.
+/// You may wonder how the impl would then be able to use the upvar,
+/// if it doesn't know it's type? The answer is that the impl is
+/// (conceptually) not fully generic over Closure but rather tied to
+/// instances with the expected upvar types:
+/// ```ignore (illustrative)
+/// impl<'b, 'a, T> FnMut() for Closure<'a, T, (&'b mut &'a mut T,)> {
+///     ...
+/// }
+/// ```
+/// You can see that the *impl* fully specified the type of the upvar
+/// and thus knows full well that `data` has type `&'b mut &'a mut T`.
+/// (Here, I am assuming that `data` is mut-borrowed.)
+///
+/// Now, the last question you may ask is: Why include the upvar types
+/// in an extra type parameter? The reason for this design is that the
+/// upvar types can reference lifetimes that are internal to the
+/// creating function. In my example above, for example, the lifetime
+/// `'b` represents the scope of the closure itself; this is some
+/// subset of `foo`, probably just the scope of the call to the to
+/// `do()`. If we just had the lifetime/type parameters from the
+/// enclosing function, we couldn't name this lifetime `'b`. Note that
+/// there can also be lifetimes in the types of the upvars themselves,
+/// if one of them happens to be a reference to something that the
+/// creating fn owns.
+///
+/// OK, you say, so why not create a more minimal set of parameters
+/// that just includes the extra lifetime parameters? The answer is
+/// primarily that it would be hard --- we don't know at the time when
+/// we create the closure type what the full types of the upvars are,
+/// nor do we know which are borrowed and which are not. In this
+/// design, we can just supply a fresh type parameter and figure that
+/// out later.
+///
+/// All right, you say, but why include the type parameters from the
+/// original function then? The answer is that codegen may need them
+/// when monomorphizing, and they may not appear in the upvars. A
+/// closure could capture no variables but still make use of some
+/// in-scope type parameter with a bound (e.g., if our example above
+/// had an extra `U: Default`, and the closure called `U::default()`).
+///
+/// There is another reason. This design (implicitly) prohibits
+/// closures from capturing themselves (except via a trait
+/// object). This simplifies closure inference considerably, since it
+/// means that when we infer the kind of a closure or its upvars, we
+/// don't have to handle cycles where the decisions we make for
+/// closure C wind up influencing the decisions we ought to make for
+/// closure C (which would then require fixed point iteration to
+/// handle). Plus it fixes an ICE. :P
+///
+/// ## Generators
+///
+/// Generators are handled similarly in `GeneratorSubsts`.  The set of
+/// type parameters is similar, but `CK` and `CS` are replaced by the
+/// following type parameters:
+///
+/// * `GS`: The generator's "resume type", which is the type of the
+///   argument passed to `resume`, and the type of `yield` expressions
+///   inside the generator.
+/// * `GY`: The "yield type", which is the type of values passed to
+///   `yield` inside the generator.
+/// * `GR`: The "return type", which is the type of value returned upon
+///   completion of the generator.
+/// * `GW`: The "generator witness".
+#[derive(Copy, Clone, Debug, TypeFoldable, TypeVisitable)]
+pub struct ClosureSubsts<'tcx> {
+    /// Lifetime and type parameters from the enclosing function,
+    /// concatenated with a tuple containing the types of the upvars.
+    ///
+    /// These are separated out because codegen wants to pass them around
+    /// when monomorphizing.
+    pub substs: SubstsRef<'tcx>,
+}
+
+/// Struct returned by `split()`.
+pub struct ClosureSubstsParts<'tcx, T> {
+    pub parent_substs: &'tcx [GenericArg<'tcx>],
+    pub closure_kind_ty: T,
+    pub closure_sig_as_fn_ptr_ty: T,
+    pub tupled_upvars_ty: T,
+}
+
+impl<'tcx> ClosureSubsts<'tcx> {
+    /// Construct `ClosureSubsts` from `ClosureSubstsParts`, containing `Substs`
+    /// for the closure parent, alongside additional closure-specific components.
+    pub fn new(
+        tcx: TyCtxt<'tcx>,
+        parts: ClosureSubstsParts<'tcx, Ty<'tcx>>,
+    ) -> ClosureSubsts<'tcx> {
+        ClosureSubsts {
+            substs: tcx.mk_substs(
+                parts.parent_substs.iter().copied().chain(
+                    [parts.closure_kind_ty, parts.closure_sig_as_fn_ptr_ty, parts.tupled_upvars_ty]
+                        .iter()
+                        .map(|&ty| ty.into()),
+                ),
+            ),
+        }
+    }
+
+    /// Divides the closure substs into their respective components.
+    /// The ordering assumed here must match that used by `ClosureSubsts::new` above.
+    fn split(self) -> ClosureSubstsParts<'tcx, GenericArg<'tcx>> {
+        match self.substs[..] {
+            [
+                ref parent_substs @ ..,
+                closure_kind_ty,
+                closure_sig_as_fn_ptr_ty,
+                tupled_upvars_ty,
+            ] => ClosureSubstsParts {
+                parent_substs,
+                closure_kind_ty,
+                closure_sig_as_fn_ptr_ty,
+                tupled_upvars_ty,
+            },
+            _ => bug!("closure substs missing synthetics"),
+        }
+    }
+
+    /// Returns `true` only if enough of the synthetic types are known to
+    /// allow using all of the methods on `ClosureSubsts` without panicking.
+    ///
+    /// Used primarily by `ty::print::pretty` to be able to handle closure
+    /// types that haven't had their synthetic types substituted in.
+    pub fn is_valid(self) -> bool {
+        self.substs.len() >= 3
+            && matches!(self.split().tupled_upvars_ty.expect_ty().kind(), Tuple(_))
+    }
+
+    /// Returns the substitutions of the closure's parent.
+    pub fn parent_substs(self) -> &'tcx [GenericArg<'tcx>] {
+        self.split().parent_substs
+    }
+
+    /// Returns an iterator over the list of types of captured paths by the closure.
+    /// In case there was a type error in figuring out the types of the captured path, an
+    /// empty iterator is returned.
+    #[inline]
+    pub fn upvar_tys(self) -> impl Iterator<Item = Ty<'tcx>> + 'tcx {
+        match self.tupled_upvars_ty().kind() {
+            TyKind::Error(_) => None,
+            TyKind::Tuple(..) => Some(self.tupled_upvars_ty().tuple_fields()),
+            TyKind::Infer(_) => bug!("upvar_tys called before capture types are inferred"),
+            ty => bug!("Unexpected representation of upvar types tuple {:?}", ty),
+        }
+        .into_iter()
+        .flatten()
+    }
+
+    /// Returns the tuple type representing the upvars for this closure.
+    #[inline]
+    pub fn tupled_upvars_ty(self) -> Ty<'tcx> {
+        self.split().tupled_upvars_ty.expect_ty()
+    }
+
+    /// Returns the closure kind for this closure; may return a type
+    /// variable during inference. To get the closure kind during
+    /// inference, use `infcx.closure_kind(substs)`.
+    pub fn kind_ty(self) -> Ty<'tcx> {
+        self.split().closure_kind_ty.expect_ty()
+    }
+
+    /// Returns the `fn` pointer type representing the closure signature for this
+    /// closure.
+    // FIXME(eddyb) this should be unnecessary, as the shallowly resolved
+    // type is known at the time of the creation of `ClosureSubsts`,
+    // see `rustc_typeck::check::closure`.
+    pub fn sig_as_fn_ptr_ty(self) -> Ty<'tcx> {
+        self.split().closure_sig_as_fn_ptr_ty.expect_ty()
+    }
+
+    /// Returns the closure kind for this closure; only usable outside
+    /// of an inference context, because in that context we know that
+    /// there are no type variables.
+    ///
+    /// If you have an inference context, use `infcx.closure_kind()`.
+    pub fn kind(self) -> ty::ClosureKind {
+        self.kind_ty().to_opt_closure_kind().unwrap()
+    }
+
+    /// Extracts the signature from the closure.
+    pub fn sig(self) -> ty::PolyFnSig<'tcx> {
+        let ty = self.sig_as_fn_ptr_ty();
+        match ty.kind() {
+            ty::FnPtr(sig) => *sig,
+            _ => bug!("closure_sig_as_fn_ptr_ty is not a fn-ptr: {:?}", ty.kind()),
+        }
+    }
+}
+
+/// Similar to `ClosureSubsts`; see the above documentation for more.
+#[derive(Copy, Clone, Debug, TypeFoldable, TypeVisitable)]
+pub struct GeneratorSubsts<'tcx> {
+    pub substs: SubstsRef<'tcx>,
+}
+
+pub struct GeneratorSubstsParts<'tcx, T> {
+    pub parent_substs: &'tcx [GenericArg<'tcx>],
+    pub resume_ty: T,
+    pub yield_ty: T,
+    pub return_ty: T,
+    pub witness: T,
+    pub tupled_upvars_ty: T,
+}
+
+impl<'tcx> GeneratorSubsts<'tcx> {
+    /// Construct `GeneratorSubsts` from `GeneratorSubstsParts`, containing `Substs`
+    /// for the generator parent, alongside additional generator-specific components.
+    pub fn new(
+        tcx: TyCtxt<'tcx>,
+        parts: GeneratorSubstsParts<'tcx, Ty<'tcx>>,
+    ) -> GeneratorSubsts<'tcx> {
+        GeneratorSubsts {
+            substs: tcx.mk_substs(
+                parts.parent_substs.iter().copied().chain(
+                    [
+                        parts.resume_ty,
+                        parts.yield_ty,
+                        parts.return_ty,
+                        parts.witness,
+                        parts.tupled_upvars_ty,
+                    ]
+                    .iter()
+                    .map(|&ty| ty.into()),
+                ),
+            ),
+        }
+    }
+
+    /// Divides the generator substs into their respective components.
+    /// The ordering assumed here must match that used by `GeneratorSubsts::new` above.
+    fn split(self) -> GeneratorSubstsParts<'tcx, GenericArg<'tcx>> {
+        match self.substs[..] {
+            [ref parent_substs @ .., resume_ty, yield_ty, return_ty, witness, tupled_upvars_ty] => {
+                GeneratorSubstsParts {
+                    parent_substs,
+                    resume_ty,
+                    yield_ty,
+                    return_ty,
+                    witness,
+                    tupled_upvars_ty,
+                }
+            }
+            _ => bug!("generator substs missing synthetics"),
+        }
+    }
+
+    /// Returns `true` only if enough of the synthetic types are known to
+    /// allow using all of the methods on `GeneratorSubsts` without panicking.
+    ///
+    /// Used primarily by `ty::print::pretty` to be able to handle generator
+    /// types that haven't had their synthetic types substituted in.
+    pub fn is_valid(self) -> bool {
+        self.substs.len() >= 5
+            && matches!(self.split().tupled_upvars_ty.expect_ty().kind(), Tuple(_))
+    }
+
+    /// Returns the substitutions of the generator's parent.
+    pub fn parent_substs(self) -> &'tcx [GenericArg<'tcx>] {
+        self.split().parent_substs
+    }
+
+    /// This describes the types that can be contained in a generator.
+    /// It will be a type variable initially and unified in the last stages of typeck of a body.
+    /// It contains a tuple of all the types that could end up on a generator frame.
+    /// The state transformation MIR pass may only produce layouts which mention types
+    /// in this tuple. Upvars are not counted here.
+    pub fn witness(self) -> Ty<'tcx> {
+        self.split().witness.expect_ty()
+    }
+
+    /// Returns an iterator over the list of types of captured paths by the generator.
+    /// In case there was a type error in figuring out the types of the captured path, an
+    /// empty iterator is returned.
+    #[inline]
+    pub fn upvar_tys(self) -> impl Iterator<Item = Ty<'tcx>> + 'tcx {
+        match self.tupled_upvars_ty().kind() {
+            TyKind::Error(_) => None,
+            TyKind::Tuple(..) => Some(self.tupled_upvars_ty().tuple_fields()),
+            TyKind::Infer(_) => bug!("upvar_tys called before capture types are inferred"),
+            ty => bug!("Unexpected representation of upvar types tuple {:?}", ty),
+        }
+        .into_iter()
+        .flatten()
+    }
+
+    /// Returns the tuple type representing the upvars for this generator.
+    #[inline]
+    pub fn tupled_upvars_ty(self) -> Ty<'tcx> {
+        self.split().tupled_upvars_ty.expect_ty()
+    }
+
+    /// Returns the type representing the resume type of the generator.
+    pub fn resume_ty(self) -> Ty<'tcx> {
+        self.split().resume_ty.expect_ty()
+    }
+
+    /// Returns the type representing the yield type of the generator.
+    pub fn yield_ty(self) -> Ty<'tcx> {
+        self.split().yield_ty.expect_ty()
+    }
+
+    /// Returns the type representing the return type of the generator.
+    pub fn return_ty(self) -> Ty<'tcx> {
+        self.split().return_ty.expect_ty()
+    }
+
+    /// Returns the "generator signature", which consists of its yield
+    /// and return types.
+    ///
+    /// N.B., some bits of the code prefers to see this wrapped in a
+    /// binder, but it never contains bound regions. Probably this
+    /// function should be removed.
+    pub fn poly_sig(self) -> PolyGenSig<'tcx> {
+        ty::Binder::dummy(self.sig())
+    }
+
+    /// Returns the "generator signature", which consists of its resume, yield
+    /// and return types.
+    pub fn sig(self) -> GenSig<'tcx> {
+        ty::GenSig {
+            resume_ty: self.resume_ty(),
+            yield_ty: self.yield_ty(),
+            return_ty: self.return_ty(),
+        }
+    }
+}
+
+impl<'tcx> GeneratorSubsts<'tcx> {
+    /// Generator has not been resumed yet.
+    pub const UNRESUMED: usize = 0;
+    /// Generator has returned or is completed.
+    pub const RETURNED: usize = 1;
+    /// Generator has been poisoned.
+    pub const POISONED: usize = 2;
+
+    const UNRESUMED_NAME: &'static str = "Unresumed";
+    const RETURNED_NAME: &'static str = "Returned";
+    const POISONED_NAME: &'static str = "Panicked";
+
+    /// The valid variant indices of this generator.
+    #[inline]
+    pub fn variant_range(&self, def_id: DefId, tcx: TyCtxt<'tcx>) -> Range<VariantIdx> {
+        // FIXME requires optimized MIR
+        let num_variants = tcx.generator_layout(def_id).unwrap().variant_fields.len();
+        VariantIdx::new(0)..VariantIdx::new(num_variants)
+    }
+
+    /// The discriminant for the given variant. Panics if the `variant_index` is
+    /// out of range.
+    #[inline]
+    pub fn discriminant_for_variant(
+        &self,
+        def_id: DefId,
+        tcx: TyCtxt<'tcx>,
+        variant_index: VariantIdx,
+    ) -> Discr<'tcx> {
+        // Generators don't support explicit discriminant values, so they are
+        // the same as the variant index.
+        assert!(self.variant_range(def_id, tcx).contains(&variant_index));
+        Discr { val: variant_index.as_usize() as u128, ty: self.discr_ty(tcx) }
+    }
+
+    /// The set of all discriminants for the generator, enumerated with their
+    /// variant indices.
+    #[inline]
+    pub fn discriminants(
+        self,
+        def_id: DefId,
+        tcx: TyCtxt<'tcx>,
+    ) -> impl Iterator<Item = (VariantIdx, Discr<'tcx>)> + Captures<'tcx> {
+        self.variant_range(def_id, tcx).map(move |index| {
+            (index, Discr { val: index.as_usize() as u128, ty: self.discr_ty(tcx) })
+        })
+    }
+
+    /// Calls `f` with a reference to the name of the enumerator for the given
+    /// variant `v`.
+    pub fn variant_name(v: VariantIdx) -> Cow<'static, str> {
+        match v.as_usize() {
+            Self::UNRESUMED => Cow::from(Self::UNRESUMED_NAME),
+            Self::RETURNED => Cow::from(Self::RETURNED_NAME),
+            Self::POISONED => Cow::from(Self::POISONED_NAME),
+            _ => Cow::from(format!("Suspend{}", v.as_usize() - 3)),
+        }
+    }
+
+    /// The type of the state discriminant used in the generator type.
+    #[inline]
+    pub fn discr_ty(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        tcx.types.u32
+    }
+
+    /// This returns the types of the MIR locals which had to be stored across suspension points.
+    /// It is calculated in rustc_mir_transform::generator::StateTransform.
+    /// All the types here must be in the tuple in GeneratorInterior.
+    ///
+    /// The locals are grouped by their variant number. Note that some locals may
+    /// be repeated in multiple variants.
+    #[inline]
+    pub fn state_tys(
+        self,
+        def_id: DefId,
+        tcx: TyCtxt<'tcx>,
+    ) -> impl Iterator<Item = impl Iterator<Item = Ty<'tcx>> + Captures<'tcx>> {
+        let layout = tcx.generator_layout(def_id).unwrap();
+        layout.variant_fields.iter().map(move |variant| {
+            variant
+                .iter()
+                .map(move |field| EarlyBinder(layout.field_tys[*field]).subst(tcx, self.substs))
+        })
+    }
+
+    /// This is the types of the fields of a generator which are not stored in a
+    /// variant.
+    #[inline]
+    pub fn prefix_tys(self) -> impl Iterator<Item = Ty<'tcx>> {
+        self.upvar_tys()
+    }
+}
+
+#[derive(Debug, Copy, Clone, HashStable)]
+pub enum UpvarSubsts<'tcx> {
+    Closure(SubstsRef<'tcx>),
+    Generator(SubstsRef<'tcx>),
+}
+
+impl<'tcx> UpvarSubsts<'tcx> {
+    /// Returns an iterator over the list of types of captured paths by the closure/generator.
+    /// In case there was a type error in figuring out the types of the captured path, an
+    /// empty iterator is returned.
+    #[inline]
+    pub fn upvar_tys(self) -> impl Iterator<Item = Ty<'tcx>> + 'tcx {
+        let tupled_tys = match self {
+            UpvarSubsts::Closure(substs) => substs.as_closure().tupled_upvars_ty(),
+            UpvarSubsts::Generator(substs) => substs.as_generator().tupled_upvars_ty(),
+        };
+
+        match tupled_tys.kind() {
+            TyKind::Error(_) => None,
+            TyKind::Tuple(..) => Some(self.tupled_upvars_ty().tuple_fields()),
+            TyKind::Infer(_) => bug!("upvar_tys called before capture types are inferred"),
+            ty => bug!("Unexpected representation of upvar types tuple {:?}", ty),
+        }
+        .into_iter()
+        .flatten()
+    }
+
+    #[inline]
+    pub fn tupled_upvars_ty(self) -> Ty<'tcx> {
+        match self {
+            UpvarSubsts::Closure(substs) => substs.as_closure().tupled_upvars_ty(),
+            UpvarSubsts::Generator(substs) => substs.as_generator().tupled_upvars_ty(),
+        }
+    }
+}
+
+/// An inline const is modeled like
+/// ```ignore (illustrative)
+/// const InlineConst<'l0...'li, T0...Tj, R>: R;
+/// ```
+/// where:
+///
+/// - 'l0...'li and T0...Tj are the generic parameters
+///   inherited from the item that defined the inline const,
+/// - R represents the type of the constant.
+///
+/// When the inline const is instantiated, `R` is substituted as the actual inferred
+/// type of the constant. The reason that `R` is represented as an extra type parameter
+/// is the same reason that [`ClosureSubsts`] have `CS` and `U` as type parameters:
+/// inline const can reference lifetimes that are internal to the creating function.
+#[derive(Copy, Clone, Debug, TypeFoldable, TypeVisitable)]
+pub struct InlineConstSubsts<'tcx> {
+    /// Generic parameters from the enclosing item,
+    /// concatenated with the inferred type of the constant.
+    pub substs: SubstsRef<'tcx>,
+}
+
+/// Struct returned by `split()`.
+pub struct InlineConstSubstsParts<'tcx, T> {
+    pub parent_substs: &'tcx [GenericArg<'tcx>],
+    pub ty: T,
+}
+
+impl<'tcx> InlineConstSubsts<'tcx> {
+    /// Construct `InlineConstSubsts` from `InlineConstSubstsParts`.
+    pub fn new(
+        tcx: TyCtxt<'tcx>,
+        parts: InlineConstSubstsParts<'tcx, Ty<'tcx>>,
+    ) -> InlineConstSubsts<'tcx> {
+        InlineConstSubsts {
+            substs: tcx.mk_substs(
+                parts.parent_substs.iter().copied().chain(std::iter::once(parts.ty.into())),
+            ),
+        }
+    }
+
+    /// Divides the inline const substs into their respective components.
+    /// The ordering assumed here must match that used by `InlineConstSubsts::new` above.
+    fn split(self) -> InlineConstSubstsParts<'tcx, GenericArg<'tcx>> {
+        match self.substs[..] {
+            [ref parent_substs @ .., ty] => InlineConstSubstsParts { parent_substs, ty },
+            _ => bug!("inline const substs missing synthetics"),
+        }
+    }
+
+    /// Returns the substitutions of the inline const's parent.
+    pub fn parent_substs(self) -> &'tcx [GenericArg<'tcx>] {
+        self.split().parent_substs
+    }
+
+    /// Returns the type of this inline const.
+    pub fn ty(self) -> Ty<'tcx> {
+        self.split().ty.expect_ty()
+    }
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, PartialOrd, Ord, Eq, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub enum ExistentialPredicate<'tcx> {
+    /// E.g., `Iterator`.
+    Trait(ExistentialTraitRef<'tcx>),
+    /// E.g., `Iterator::Item = T`.
+    Projection(ExistentialProjection<'tcx>),
+    /// E.g., `Send`.
+    AutoTrait(DefId),
+}
+
+impl<'tcx> ExistentialPredicate<'tcx> {
+    /// Compares via an ordering that will not change if modules are reordered or other changes are
+    /// made to the tree. In particular, this ordering is preserved across incremental compilations.
+    pub fn stable_cmp(&self, tcx: TyCtxt<'tcx>, other: &Self) -> Ordering {
+        use self::ExistentialPredicate::*;
+        match (*self, *other) {
+            (Trait(_), Trait(_)) => Ordering::Equal,
+            (Projection(ref a), Projection(ref b)) => {
+                tcx.def_path_hash(a.item_def_id).cmp(&tcx.def_path_hash(b.item_def_id))
+            }
+            (AutoTrait(ref a), AutoTrait(ref b)) => {
+                tcx.def_path_hash(*a).cmp(&tcx.def_path_hash(*b))
+            }
+            (Trait(_), _) => Ordering::Less,
+            (Projection(_), Trait(_)) => Ordering::Greater,
+            (Projection(_), _) => Ordering::Less,
+            (AutoTrait(_), _) => Ordering::Greater,
+        }
+    }
+}
+
+impl<'tcx> Binder<'tcx, ExistentialPredicate<'tcx>> {
+    pub fn with_self_ty(&self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> ty::Predicate<'tcx> {
+        use crate::ty::ToPredicate;
+        match self.skip_binder() {
+            ExistentialPredicate::Trait(tr) => {
+                self.rebind(tr).with_self_ty(tcx, self_ty).without_const().to_predicate(tcx)
+            }
+            ExistentialPredicate::Projection(p) => {
+                self.rebind(p.with_self_ty(tcx, self_ty)).to_predicate(tcx)
+            }
+            ExistentialPredicate::AutoTrait(did) => {
+                let trait_ref = self.rebind(ty::TraitRef {
+                    def_id: did,
+                    substs: tcx.mk_substs_trait(self_ty, &[]),
+                });
+                trait_ref.without_const().to_predicate(tcx)
+            }
+        }
+    }
+}
+
+impl<'tcx> List<ty::Binder<'tcx, ExistentialPredicate<'tcx>>> {
+    /// Returns the "principal `DefId`" of this set of existential predicates.
+    ///
+    /// A Rust trait object type consists (in addition to a lifetime bound)
+    /// of a set of trait bounds, which are separated into any number
+    /// of auto-trait bounds, and at most one non-auto-trait bound. The
+    /// non-auto-trait bound is called the "principal" of the trait
+    /// object.
+    ///
+    /// Only the principal can have methods or type parameters (because
+    /// auto traits can have neither of them). This is important, because
+    /// it means the auto traits can be treated as an unordered set (methods
+    /// would force an order for the vtable, while relating traits with
+    /// type parameters without knowing the order to relate them in is
+    /// a rather non-trivial task).
+    ///
+    /// For example, in the trait object `dyn fmt::Debug + Sync`, the
+    /// principal bound is `Some(fmt::Debug)`, while the auto-trait bounds
+    /// are the set `{Sync}`.
+    ///
+    /// It is also possible to have a "trivial" trait object that
+    /// consists only of auto traits, with no principal - for example,
+    /// `dyn Send + Sync`. In that case, the set of auto-trait bounds
+    /// is `{Send, Sync}`, while there is no principal. These trait objects
+    /// have a "trivial" vtable consisting of just the size, alignment,
+    /// and destructor.
+    pub fn principal(&self) -> Option<ty::Binder<'tcx, ExistentialTraitRef<'tcx>>> {
+        self[0]
+            .map_bound(|this| match this {
+                ExistentialPredicate::Trait(tr) => Some(tr),
+                _ => None,
+            })
+            .transpose()
+    }
+
+    pub fn principal_def_id(&self) -> Option<DefId> {
+        self.principal().map(|trait_ref| trait_ref.skip_binder().def_id)
+    }
+
+    #[inline]
+    pub fn projection_bounds<'a>(
+        &'a self,
+    ) -> impl Iterator<Item = ty::Binder<'tcx, ExistentialProjection<'tcx>>> + 'a {
+        self.iter().filter_map(|predicate| {
+            predicate
+                .map_bound(|pred| match pred {
+                    ExistentialPredicate::Projection(projection) => Some(projection),
+                    _ => None,
+                })
+                .transpose()
+        })
+    }
+
+    #[inline]
+    pub fn auto_traits<'a>(&'a self) -> impl Iterator<Item = DefId> + Captures<'tcx> + 'a {
+        self.iter().filter_map(|predicate| match predicate.skip_binder() {
+            ExistentialPredicate::AutoTrait(did) => Some(did),
+            _ => None,
+        })
+    }
+}
+
+/// A complete reference to a trait. These take numerous guises in syntax,
+/// but perhaps the most recognizable form is in a where-clause:
+/// ```ignore (illustrative)
+/// T: Foo<U>
+/// ```
+/// This would be represented by a trait-reference where the `DefId` is the
+/// `DefId` for the trait `Foo` and the substs define `T` as parameter 0,
+/// and `U` as parameter 1.
+///
+/// Trait references also appear in object types like `Foo<U>`, but in
+/// that case the `Self` parameter is absent from the substitutions.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub struct TraitRef<'tcx> {
+    pub def_id: DefId,
+    pub substs: SubstsRef<'tcx>,
+}
+
+impl<'tcx> TraitRef<'tcx> {
+    pub fn new(def_id: DefId, substs: SubstsRef<'tcx>) -> TraitRef<'tcx> {
+        TraitRef { def_id, substs }
+    }
+
+    /// Returns a `TraitRef` of the form `P0: Foo<P1..Pn>` where `Pi`
+    /// are the parameters defined on trait.
+    pub fn identity(tcx: TyCtxt<'tcx>, def_id: DefId) -> Binder<'tcx, TraitRef<'tcx>> {
+        ty::Binder::dummy(TraitRef {
+            def_id,
+            substs: InternalSubsts::identity_for_item(tcx, def_id),
+        })
+    }
+
+    #[inline]
+    pub fn self_ty(&self) -> Ty<'tcx> {
+        self.substs.type_at(0)
+    }
+
+    pub fn from_method(
+        tcx: TyCtxt<'tcx>,
+        trait_id: DefId,
+        substs: SubstsRef<'tcx>,
+    ) -> ty::TraitRef<'tcx> {
+        let defs = tcx.generics_of(trait_id);
+        ty::TraitRef { def_id: trait_id, substs: tcx.intern_substs(&substs[..defs.params.len()]) }
+    }
+}
+
+pub type PolyTraitRef<'tcx> = Binder<'tcx, TraitRef<'tcx>>;
+
+impl<'tcx> PolyTraitRef<'tcx> {
+    pub fn self_ty(&self) -> Binder<'tcx, Ty<'tcx>> {
+        self.map_bound_ref(|tr| tr.self_ty())
+    }
+
+    pub fn def_id(&self) -> DefId {
+        self.skip_binder().def_id
+    }
+
+    pub fn to_poly_trait_predicate(&self) -> ty::PolyTraitPredicate<'tcx> {
+        self.map_bound(|trait_ref| ty::TraitPredicate {
+            trait_ref,
+            constness: ty::BoundConstness::NotConst,
+            polarity: ty::ImplPolarity::Positive,
+        })
+    }
+
+    /// Same as [`PolyTraitRef::to_poly_trait_predicate`] but sets a negative polarity instead.
+    pub fn to_poly_trait_predicate_negative_polarity(&self) -> ty::PolyTraitPredicate<'tcx> {
+        self.map_bound(|trait_ref| ty::TraitPredicate {
+            trait_ref,
+            constness: ty::BoundConstness::NotConst,
+            polarity: ty::ImplPolarity::Negative,
+        })
+    }
+}
+
+/// An existential reference to a trait, where `Self` is erased.
+/// For example, the trait object `Trait<'a, 'b, X, Y>` is:
+/// ```ignore (illustrative)
+/// exists T. T: Trait<'a, 'b, X, Y>
+/// ```
+/// The substitutions don't include the erased `Self`, only trait
+/// type and lifetime parameters (`[X, Y]` and `['a, 'b]` above).
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub struct ExistentialTraitRef<'tcx> {
+    pub def_id: DefId,
+    pub substs: SubstsRef<'tcx>,
+}
+
+impl<'tcx> ExistentialTraitRef<'tcx> {
+    pub fn erase_self_ty(
+        tcx: TyCtxt<'tcx>,
+        trait_ref: ty::TraitRef<'tcx>,
+    ) -> ty::ExistentialTraitRef<'tcx> {
+        // Assert there is a Self.
+        trait_ref.substs.type_at(0);
+
+        ty::ExistentialTraitRef {
+            def_id: trait_ref.def_id,
+            substs: tcx.intern_substs(&trait_ref.substs[1..]),
+        }
+    }
+
+    /// Object types don't have a self type specified. Therefore, when
+    /// we convert the principal trait-ref into a normal trait-ref,
+    /// you must give *some* self type. A common choice is `mk_err()`
+    /// or some placeholder type.
+    pub fn with_self_ty(&self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> ty::TraitRef<'tcx> {
+        // otherwise the escaping vars would be captured by the binder
+        // debug_assert!(!self_ty.has_escaping_bound_vars());
+
+        ty::TraitRef { def_id: self.def_id, substs: tcx.mk_substs_trait(self_ty, self.substs) }
+    }
+}
+
+pub type PolyExistentialTraitRef<'tcx> = Binder<'tcx, ExistentialTraitRef<'tcx>>;
+
+impl<'tcx> PolyExistentialTraitRef<'tcx> {
+    pub fn def_id(&self) -> DefId {
+        self.skip_binder().def_id
+    }
+
+    /// Object types don't have a self type specified. Therefore, when
+    /// we convert the principal trait-ref into a normal trait-ref,
+    /// you must give *some* self type. A common choice is `mk_err()`
+    /// or some placeholder type.
+    pub fn with_self_ty(&self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> ty::PolyTraitRef<'tcx> {
+        self.map_bound(|trait_ref| trait_ref.with_self_ty(tcx, self_ty))
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
+#[derive(Encodable, Decodable, HashStable)]
+pub struct EarlyBinder<T>(pub T);
+
+impl<T> EarlyBinder<T> {
+    pub fn as_ref(&self) -> EarlyBinder<&T> {
+        EarlyBinder(&self.0)
+    }
+
+    pub fn map_bound_ref<F, U>(&self, f: F) -> EarlyBinder<U>
+    where
+        F: FnOnce(&T) -> U,
+    {
+        self.as_ref().map_bound(f)
+    }
+
+    pub fn map_bound<F, U>(self, f: F) -> EarlyBinder<U>
+    where
+        F: FnOnce(T) -> U,
+    {
+        let value = f(self.0);
+        EarlyBinder(value)
+    }
+
+    pub fn try_map_bound<F, U, E>(self, f: F) -> Result<EarlyBinder<U>, E>
+    where
+        F: FnOnce(T) -> Result<U, E>,
+    {
+        let value = f(self.0)?;
+        Ok(EarlyBinder(value))
+    }
+
+    pub fn rebind<U>(&self, value: U) -> EarlyBinder<U> {
+        EarlyBinder(value)
+    }
+}
+
+impl<T> EarlyBinder<Option<T>> {
+    pub fn transpose(self) -> Option<EarlyBinder<T>> {
+        self.0.map(|v| EarlyBinder(v))
+    }
+}
+
+impl<T, U> EarlyBinder<(T, U)> {
+    pub fn transpose_tuple2(self) -> (EarlyBinder<T>, EarlyBinder<U>) {
+        (EarlyBinder(self.0.0), EarlyBinder(self.0.1))
+    }
+}
+
+pub struct EarlyBinderIter<T> {
+    t: T,
+}
+
+impl<T: IntoIterator> EarlyBinder<T> {
+    pub fn transpose_iter(self) -> EarlyBinderIter<T::IntoIter> {
+        EarlyBinderIter { t: self.0.into_iter() }
+    }
+}
+
+impl<T: Iterator> Iterator for EarlyBinderIter<T> {
+    type Item = EarlyBinder<T::Item>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.t.next().map(|i| EarlyBinder(i))
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
+#[derive(HashStable)]
+pub enum BoundVariableKind {
+    Ty(BoundTyKind),
+    Region(BoundRegionKind),
+    Const,
+}
+
+impl BoundVariableKind {
+    pub fn expect_region(self) -> BoundRegionKind {
+        match self {
+            BoundVariableKind::Region(lt) => lt,
+            _ => bug!("expected a region, but found another kind"),
+        }
+    }
+
+    pub fn expect_ty(self) -> BoundTyKind {
+        match self {
+            BoundVariableKind::Ty(ty) => ty,
+            _ => bug!("expected a type, but found another kind"),
+        }
+    }
+
+    pub fn expect_const(self) {
+        match self {
+            BoundVariableKind::Const => (),
+            _ => bug!("expected a const, but found another kind"),
+        }
+    }
+}
+
+/// Binder is a binder for higher-ranked lifetimes or types. It is part of the
+/// compiler's representation for things like `for<'a> Fn(&'a isize)`
+/// (which would be represented by the type `PolyTraitRef ==
+/// Binder<'tcx, TraitRef>`). Note that when we instantiate,
+/// erase, or otherwise "discharge" these bound vars, we change the
+/// type from `Binder<'tcx, T>` to just `T` (see
+/// e.g., `liberate_late_bound_regions`).
+///
+/// `Decodable` and `Encodable` are implemented for `Binder<T>` using the `impl_binder_encode_decode!` macro.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
+#[derive(HashStable)]
+pub struct Binder<'tcx, T>(T, &'tcx List<BoundVariableKind>);
+
+impl<'tcx, T> Binder<'tcx, T>
+where
+    T: TypeVisitable<'tcx>,
+{
+    /// Wraps `value` in a binder, asserting that `value` does not
+    /// contain any bound vars that would be bound by the
+    /// binder. This is commonly used to 'inject' a value T into a
+    /// different binding level.
+    pub fn dummy(value: T) -> Binder<'tcx, T> {
+        assert!(!value.has_escaping_bound_vars());
+        Binder(value, ty::List::empty())
+    }
+
+    pub fn bind_with_vars(value: T, vars: &'tcx List<BoundVariableKind>) -> Binder<'tcx, T> {
+        if cfg!(debug_assertions) {
+            let mut validator = ValidateBoundVars::new(vars);
+            value.visit_with(&mut validator);
+        }
+        Binder(value, vars)
+    }
+}
+
+impl<'tcx, T> Binder<'tcx, T> {
+    /// Skips the binder and returns the "bound" value. This is a
+    /// risky thing to do because it's easy to get confused about
+    /// De Bruijn indices and the like. It is usually better to
+    /// discharge the binder using `no_bound_vars` or
+    /// `replace_late_bound_regions` or something like
+    /// that. `skip_binder` is only valid when you are either
+    /// extracting data that has nothing to do with bound vars, you
+    /// are doing some sort of test that does not involve bound
+    /// regions, or you are being very careful about your depth
+    /// accounting.
+    ///
+    /// Some examples where `skip_binder` is reasonable:
+    ///
+    /// - extracting the `DefId` from a PolyTraitRef;
+    /// - comparing the self type of a PolyTraitRef to see if it is equal to
+    ///   a type parameter `X`, since the type `X` does not reference any regions
+    pub fn skip_binder(self) -> T {
+        self.0
+    }
+
+    pub fn bound_vars(&self) -> &'tcx List<BoundVariableKind> {
+        self.1
+    }
+
+    pub fn as_ref(&self) -> Binder<'tcx, &T> {
+        Binder(&self.0, self.1)
+    }
+
+    pub fn as_deref(&self) -> Binder<'tcx, &T::Target>
+    where
+        T: Deref,
+    {
+        Binder(&self.0, self.1)
+    }
+
+    pub fn map_bound_ref_unchecked<F, U>(&self, f: F) -> Binder<'tcx, U>
+    where
+        F: FnOnce(&T) -> U,
+    {
+        let value = f(&self.0);
+        Binder(value, self.1)
+    }
+
+    pub fn map_bound_ref<F, U: TypeVisitable<'tcx>>(&self, f: F) -> Binder<'tcx, U>
+    where
+        F: FnOnce(&T) -> U,
+    {
+        self.as_ref().map_bound(f)
+    }
+
+    pub fn map_bound<F, U: TypeVisitable<'tcx>>(self, f: F) -> Binder<'tcx, U>
+    where
+        F: FnOnce(T) -> U,
+    {
+        let value = f(self.0);
+        if cfg!(debug_assertions) {
+            let mut validator = ValidateBoundVars::new(self.1);
+            value.visit_with(&mut validator);
+        }
+        Binder(value, self.1)
+    }
+
+    pub fn try_map_bound<F, U: TypeVisitable<'tcx>, E>(self, f: F) -> Result<Binder<'tcx, U>, E>
+    where
+        F: FnOnce(T) -> Result<U, E>,
+    {
+        let value = f(self.0)?;
+        if cfg!(debug_assertions) {
+            let mut validator = ValidateBoundVars::new(self.1);
+            value.visit_with(&mut validator);
+        }
+        Ok(Binder(value, self.1))
+    }
+
+    /// Wraps a `value` in a binder, using the same bound variables as the
+    /// current `Binder`. This should not be used if the new value *changes*
+    /// the bound variables. Note: the (old or new) value itself does not
+    /// necessarily need to *name* all the bound variables.
+    ///
+    /// This currently doesn't do anything different than `bind`, because we
+    /// don't actually track bound vars. However, semantically, it is different
+    /// because bound vars aren't allowed to change here, whereas they are
+    /// in `bind`. This may be (debug) asserted in the future.
+    pub fn rebind<U>(&self, value: U) -> Binder<'tcx, U>
+    where
+        U: TypeVisitable<'tcx>,
+    {
+        if cfg!(debug_assertions) {
+            let mut validator = ValidateBoundVars::new(self.bound_vars());
+            value.visit_with(&mut validator);
+        }
+        Binder(value, self.1)
+    }
+
+    /// Unwraps and returns the value within, but only if it contains
+    /// no bound vars at all. (In other words, if this binder --
+    /// and indeed any enclosing binder -- doesn't bind anything at
+    /// all.) Otherwise, returns `None`.
+    ///
+    /// (One could imagine having a method that just unwraps a single
+    /// binder, but permits late-bound vars bound by enclosing
+    /// binders, but that would require adjusting the debruijn
+    /// indices, and given the shallow binding structure we often use,
+    /// would not be that useful.)
+    pub fn no_bound_vars(self) -> Option<T>
+    where
+        T: TypeVisitable<'tcx>,
+    {
+        if self.0.has_escaping_bound_vars() { None } else { Some(self.skip_binder()) }
+    }
+
+    /// Splits the contents into two things that share the same binder
+    /// level as the original, returning two distinct binders.
+    ///
+    /// `f` should consider bound regions at depth 1 to be free, and
+    /// anything it produces with bound regions at depth 1 will be
+    /// bound in the resulting return values.
+    pub fn split<U, V, F>(self, f: F) -> (Binder<'tcx, U>, Binder<'tcx, V>)
+    where
+        F: FnOnce(T) -> (U, V),
+    {
+        let (u, v) = f(self.0);
+        (Binder(u, self.1), Binder(v, self.1))
+    }
+}
+
+impl<'tcx, T> Binder<'tcx, Option<T>> {
+    pub fn transpose(self) -> Option<Binder<'tcx, T>> {
+        let bound_vars = self.1;
+        self.0.map(|v| Binder(v, bound_vars))
+    }
+}
+
+/// Represents the projection of an associated type. In explicit UFCS
+/// form this would be written `<T as Trait<..>>::N`.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub struct ProjectionTy<'tcx> {
+    /// The parameters of the associated item.
+    pub substs: SubstsRef<'tcx>,
+
+    /// The `DefId` of the `TraitItem` for the associated type `N`.
+    ///
+    /// Note that this is not the `DefId` of the `TraitRef` containing this
+    /// associated type, which is in `tcx.associated_item(item_def_id).container`,
+    /// aka. `tcx.parent(item_def_id).unwrap()`.
+    pub item_def_id: DefId,
+}
+
+impl<'tcx> ProjectionTy<'tcx> {
+    pub fn trait_def_id(&self, tcx: TyCtxt<'tcx>) -> DefId {
+        tcx.parent(self.item_def_id)
+    }
+
+    /// Extracts the underlying trait reference and own substs from this projection.
+    /// For example, if this is a projection of `<T as StreamingIterator>::Item<'a>`,
+    /// then this function would return a `T: Iterator` trait reference and `['a]` as the own substs
+    pub fn trait_ref_and_own_substs(
+        &self,
+        tcx: TyCtxt<'tcx>,
+    ) -> (ty::TraitRef<'tcx>, &'tcx [ty::GenericArg<'tcx>]) {
+        let def_id = tcx.parent(self.item_def_id);
+        let trait_generics = tcx.generics_of(def_id);
+        (
+            ty::TraitRef { def_id, substs: self.substs.truncate_to(tcx, trait_generics) },
+            &self.substs[trait_generics.count()..],
+        )
+    }
+
+    /// Extracts the underlying trait reference from this projection.
+    /// For example, if this is a projection of `<T as Iterator>::Item`,
+    /// then this function would return a `T: Iterator` trait reference.
+    ///
+    /// WARNING: This will drop the substs for generic associated types
+    /// consider calling [Self::trait_ref_and_own_substs] to get those
+    /// as well.
+    pub fn trait_ref(&self, tcx: TyCtxt<'tcx>) -> ty::TraitRef<'tcx> {
+        let def_id = self.trait_def_id(tcx);
+        ty::TraitRef { def_id, substs: self.substs.truncate_to(tcx, tcx.generics_of(def_id)) }
+    }
+
+    pub fn self_ty(&self) -> Ty<'tcx> {
+        self.substs.type_at(0)
+    }
+}
+
+#[derive(Copy, Clone, Debug, TypeFoldable, TypeVisitable)]
+pub struct GenSig<'tcx> {
+    pub resume_ty: Ty<'tcx>,
+    pub yield_ty: Ty<'tcx>,
+    pub return_ty: Ty<'tcx>,
+}
+
+pub type PolyGenSig<'tcx> = Binder<'tcx, GenSig<'tcx>>;
+
+/// Signature of a function type, which we have arbitrarily
+/// decided to use to refer to the input/output types.
+///
+/// - `inputs`: is the list of arguments and their modes.
+/// - `output`: is the return type.
+/// - `c_variadic`: indicates whether this is a C-variadic function.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub struct FnSig<'tcx> {
+    pub inputs_and_output: &'tcx List<Ty<'tcx>>,
+    pub c_variadic: bool,
+    pub unsafety: hir::Unsafety,
+    pub abi: abi::Abi,
+}
+
+impl<'tcx> FnSig<'tcx> {
+    pub fn inputs(&self) -> &'tcx [Ty<'tcx>] {
+        &self.inputs_and_output[..self.inputs_and_output.len() - 1]
+    }
+
+    pub fn output(&self) -> Ty<'tcx> {
+        self.inputs_and_output[self.inputs_and_output.len() - 1]
+    }
+
+    // Creates a minimal `FnSig` to be used when encountering a `TyKind::Error` in a fallible
+    // method.
+    fn fake() -> FnSig<'tcx> {
+        FnSig {
+            inputs_and_output: List::empty(),
+            c_variadic: false,
+            unsafety: hir::Unsafety::Normal,
+            abi: abi::Abi::Rust,
+        }
+    }
+}
+
+pub type PolyFnSig<'tcx> = Binder<'tcx, FnSig<'tcx>>;
+
+impl<'tcx> PolyFnSig<'tcx> {
+    #[inline]
+    pub fn inputs(&self) -> Binder<'tcx, &'tcx [Ty<'tcx>]> {
+        self.map_bound_ref_unchecked(|fn_sig| fn_sig.inputs())
+    }
+    #[inline]
+    pub fn input(&self, index: usize) -> ty::Binder<'tcx, Ty<'tcx>> {
+        self.map_bound_ref(|fn_sig| fn_sig.inputs()[index])
+    }
+    pub fn inputs_and_output(&self) -> ty::Binder<'tcx, &'tcx List<Ty<'tcx>>> {
+        self.map_bound_ref(|fn_sig| fn_sig.inputs_and_output)
+    }
+    #[inline]
+    pub fn output(&self) -> ty::Binder<'tcx, Ty<'tcx>> {
+        self.map_bound_ref(|fn_sig| fn_sig.output())
+    }
+    pub fn c_variadic(&self) -> bool {
+        self.skip_binder().c_variadic
+    }
+    pub fn unsafety(&self) -> hir::Unsafety {
+        self.skip_binder().unsafety
+    }
+    pub fn abi(&self) -> abi::Abi {
+        self.skip_binder().abi
+    }
+}
+
+pub type CanonicalPolyFnSig<'tcx> = Canonical<'tcx, Binder<'tcx, FnSig<'tcx>>>;
+
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable)]
+pub struct ParamTy {
+    pub index: u32,
+    pub name: Symbol,
+}
+
+impl<'tcx> ParamTy {
+    pub fn new(index: u32, name: Symbol) -> ParamTy {
+        ParamTy { index, name }
+    }
+
+    pub fn for_def(def: &ty::GenericParamDef) -> ParamTy {
+        ParamTy::new(def.index, def.name)
+    }
+
+    #[inline]
+    pub fn to_ty(self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        tcx.mk_ty_param(self.index, self.name)
+    }
+}
+
+#[derive(Copy, Clone, Hash, TyEncodable, TyDecodable, Eq, PartialEq, Ord, PartialOrd)]
+#[derive(HashStable)]
+pub struct ParamConst {
+    pub index: u32,
+    pub name: Symbol,
+}
+
+impl ParamConst {
+    pub fn new(index: u32, name: Symbol) -> ParamConst {
+        ParamConst { index, name }
+    }
+
+    pub fn for_def(def: &ty::GenericParamDef) -> ParamConst {
+        ParamConst::new(def.index, def.name)
+    }
+}
+
+/// Use this rather than `RegionKind`, whenever possible.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, HashStable)]
+#[rustc_pass_by_value]
+pub struct Region<'tcx>(pub Interned<'tcx, RegionKind<'tcx>>);
+
+impl<'tcx> Deref for Region<'tcx> {
+    type Target = RegionKind<'tcx>;
+
+    #[inline]
+    fn deref(&self) -> &RegionKind<'tcx> {
+        &self.0.0
+    }
+}
+
+impl<'tcx> fmt::Debug for Region<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:?}", self.kind())
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash, TyEncodable, TyDecodable, PartialOrd, Ord)]
+#[derive(HashStable)]
+pub struct EarlyBoundRegion {
+    pub def_id: DefId,
+    pub index: u32,
+    pub name: Symbol,
+}
+
+impl fmt::Debug for EarlyBoundRegion {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}, {}", self.index, self.name)
+    }
+}
+
+/// A **`const`** **v**ariable **ID**.
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(HashStable, TyEncodable, TyDecodable)]
+pub struct ConstVid<'tcx> {
+    pub index: u32,
+    pub phantom: PhantomData<&'tcx ()>,
+}
+
+rustc_index::newtype_index! {
+    /// A **region** (lifetime) **v**ariable **ID**.
+    #[derive(HashStable)]
+    pub struct RegionVid {
+        DEBUG_FORMAT = custom,
+    }
+}
+
+impl Atom for RegionVid {
+    fn index(self) -> usize {
+        Idx::index(self)
+    }
+}
+
+rustc_index::newtype_index! {
+    #[derive(HashStable)]
+    pub struct BoundVar { .. }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
+#[derive(HashStable)]
+pub struct BoundTy {
+    pub var: BoundVar,
+    pub kind: BoundTyKind,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
+#[derive(HashStable)]
+pub enum BoundTyKind {
+    Anon,
+    Param(Symbol),
+}
+
+impl From<BoundVar> for BoundTy {
+    fn from(var: BoundVar) -> Self {
+        BoundTy { var, kind: BoundTyKind::Anon }
+    }
+}
+
+/// A `ProjectionPredicate` for an `ExistentialTraitRef`.
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable)]
+pub struct ExistentialProjection<'tcx> {
+    pub item_def_id: DefId,
+    pub substs: SubstsRef<'tcx>,
+    pub term: Term<'tcx>,
+}
+
+pub type PolyExistentialProjection<'tcx> = Binder<'tcx, ExistentialProjection<'tcx>>;
+
+impl<'tcx> ExistentialProjection<'tcx> {
+    /// Extracts the underlying existential trait reference from this projection.
+    /// For example, if this is a projection of `exists T. <T as Iterator>::Item == X`,
+    /// then this function would return an `exists T. T: Iterator` existential trait
+    /// reference.
+    pub fn trait_ref(&self, tcx: TyCtxt<'tcx>) -> ty::ExistentialTraitRef<'tcx> {
+        let def_id = tcx.parent(self.item_def_id);
+        let subst_count = tcx.generics_of(def_id).count() - 1;
+        let substs = tcx.intern_substs(&self.substs[..subst_count]);
+        ty::ExistentialTraitRef { def_id, substs }
+    }
+
+    pub fn with_self_ty(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        self_ty: Ty<'tcx>,
+    ) -> ty::ProjectionPredicate<'tcx> {
+        // otherwise the escaping regions would be captured by the binders
+        debug_assert!(!self_ty.has_escaping_bound_vars());
+
+        ty::ProjectionPredicate {
+            projection_ty: ty::ProjectionTy {
+                item_def_id: self.item_def_id,
+                substs: tcx.mk_substs_trait(self_ty, self.substs),
+            },
+            term: self.term,
+        }
+    }
+
+    pub fn erase_self_ty(
+        tcx: TyCtxt<'tcx>,
+        projection_predicate: ty::ProjectionPredicate<'tcx>,
+    ) -> Self {
+        // Assert there is a Self.
+        projection_predicate.projection_ty.substs.type_at(0);
+
+        Self {
+            item_def_id: projection_predicate.projection_ty.item_def_id,
+            substs: tcx.intern_substs(&projection_predicate.projection_ty.substs[1..]),
+            term: projection_predicate.term,
+        }
+    }
+}
+
+impl<'tcx> PolyExistentialProjection<'tcx> {
+    pub fn with_self_ty(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        self_ty: Ty<'tcx>,
+    ) -> ty::PolyProjectionPredicate<'tcx> {
+        self.map_bound(|p| p.with_self_ty(tcx, self_ty))
+    }
+
+    pub fn item_def_id(&self) -> DefId {
+        self.skip_binder().item_def_id
+    }
+}
+
+/// Region utilities
+impl<'tcx> Region<'tcx> {
+    pub fn kind(self) -> RegionKind<'tcx> {
+        *self.0.0
+    }
+
+    /// Is this region named by the user?
+    pub fn has_name(self) -> bool {
+        match *self {
+            ty::ReEarlyBound(ebr) => ebr.has_name(),
+            ty::ReLateBound(_, br) => br.kind.is_named(),
+            ty::ReFree(fr) => fr.bound_region.is_named(),
+            ty::ReStatic => true,
+            ty::ReVar(..) => false,
+            ty::RePlaceholder(placeholder) => placeholder.name.is_named(),
+            ty::ReEmpty(_) => false,
+            ty::ReErased => false,
+        }
+    }
+
+    #[inline]
+    pub fn is_static(self) -> bool {
+        matches!(*self, ty::ReStatic)
+    }
+
+    #[inline]
+    pub fn is_erased(self) -> bool {
+        matches!(*self, ty::ReErased)
+    }
+
+    #[inline]
+    pub fn is_late_bound(self) -> bool {
+        matches!(*self, ty::ReLateBound(..))
+    }
+
+    #[inline]
+    pub fn is_placeholder(self) -> bool {
+        matches!(*self, ty::RePlaceholder(..))
+    }
+
+    #[inline]
+    pub fn is_empty(self) -> bool {
+        matches!(*self, ty::ReEmpty(..))
+    }
+
+    #[inline]
+    pub fn bound_at_or_above_binder(self, index: ty::DebruijnIndex) -> bool {
+        match *self {
+            ty::ReLateBound(debruijn, _) => debruijn >= index,
+            _ => false,
+        }
+    }
+
+    pub fn type_flags(self) -> TypeFlags {
+        let mut flags = TypeFlags::empty();
+
+        match *self {
+            ty::ReVar(..) => {
+                flags = flags | TypeFlags::HAS_FREE_REGIONS;
+                flags = flags | TypeFlags::HAS_FREE_LOCAL_REGIONS;
+                flags = flags | TypeFlags::HAS_RE_INFER;
+            }
+            ty::RePlaceholder(..) => {
+                flags = flags | TypeFlags::HAS_FREE_REGIONS;
+                flags = flags | TypeFlags::HAS_FREE_LOCAL_REGIONS;
+                flags = flags | TypeFlags::HAS_RE_PLACEHOLDER;
+            }
+            ty::ReEarlyBound(..) => {
+                flags = flags | TypeFlags::HAS_FREE_REGIONS;
+                flags = flags | TypeFlags::HAS_FREE_LOCAL_REGIONS;
+                flags = flags | TypeFlags::HAS_RE_PARAM;
+            }
+            ty::ReFree { .. } => {
+                flags = flags | TypeFlags::HAS_FREE_REGIONS;
+                flags = flags | TypeFlags::HAS_FREE_LOCAL_REGIONS;
+            }
+            ty::ReEmpty(_) | ty::ReStatic => {
+                flags = flags | TypeFlags::HAS_FREE_REGIONS;
+            }
+            ty::ReLateBound(..) => {
+                flags = flags | TypeFlags::HAS_RE_LATE_BOUND;
+            }
+            ty::ReErased => {
+                flags = flags | TypeFlags::HAS_RE_ERASED;
+            }
+        }
+
+        debug!("type_flags({:?}) = {:?}", self, flags);
+
+        flags
+    }
+
+    /// Given an early-bound or free region, returns the `DefId` where it was bound.
+    /// For example, consider the regions in this snippet of code:
+    ///
+    /// ```ignore (illustrative)
+    /// impl<'a> Foo {
+    /// //   ^^ -- early bound, declared on an impl
+    ///
+    ///     fn bar<'b, 'c>(x: &self, y: &'b u32, z: &'c u64) where 'static: 'c
+    /// //         ^^  ^^     ^ anonymous, late-bound
+    /// //         |   early-bound, appears in where-clauses
+    /// //         late-bound, appears only in fn args
+    ///     {..}
+    /// }
+    /// ```
+    ///
+    /// Here, `free_region_binding_scope('a)` would return the `DefId`
+    /// of the impl, and for all the other highlighted regions, it
+    /// would return the `DefId` of the function. In other cases (not shown), this
+    /// function might return the `DefId` of a closure.
+    pub fn free_region_binding_scope(self, tcx: TyCtxt<'_>) -> DefId {
+        match *self {
+            ty::ReEarlyBound(br) => tcx.parent(br.def_id),
+            ty::ReFree(fr) => fr.scope,
+            _ => bug!("free_region_binding_scope invoked on inappropriate region: {:?}", self),
+        }
+    }
+
+    /// True for free regions other than `'static`.
+    pub fn is_free(self) -> bool {
+        matches!(*self, ty::ReEarlyBound(_) | ty::ReFree(_))
+    }
+
+    /// True if `self` is a free region or static.
+    pub fn is_free_or_static(self) -> bool {
+        match *self {
+            ty::ReStatic => true,
+            _ => self.is_free(),
+        }
+    }
+}
+
+/// Type utilities
+impl<'tcx> Ty<'tcx> {
+    #[inline(always)]
+    pub fn kind(self) -> &'tcx TyKind<'tcx> {
+        &self.0.0.kind
+    }
+
+    #[inline(always)]
+    pub fn flags(self) -> TypeFlags {
+        self.0.0.flags
+    }
+
+    #[inline]
+    pub fn is_unit(self) -> bool {
+        match self.kind() {
+            Tuple(ref tys) => tys.is_empty(),
+            _ => false,
+        }
+    }
+
+    #[inline]
+    pub fn is_never(self) -> bool {
+        matches!(self.kind(), Never)
+    }
+
+    #[inline]
+    pub fn is_primitive(self) -> bool {
+        self.kind().is_primitive()
+    }
+
+    #[inline]
+    pub fn is_adt(self) -> bool {
+        matches!(self.kind(), Adt(..))
+    }
+
+    #[inline]
+    pub fn is_ref(self) -> bool {
+        matches!(self.kind(), Ref(..))
+    }
+
+    #[inline]
+    pub fn is_ty_var(self) -> bool {
+        matches!(self.kind(), Infer(TyVar(_)))
+    }
+
+    #[inline]
+    pub fn ty_vid(self) -> Option<ty::TyVid> {
+        match self.kind() {
+            &Infer(TyVar(vid)) => Some(vid),
+            _ => None,
+        }
+    }
+
+    #[inline]
+    pub fn is_ty_infer(self) -> bool {
+        matches!(self.kind(), Infer(_))
+    }
+
+    #[inline]
+    pub fn is_phantom_data(self) -> bool {
+        if let Adt(def, _) = self.kind() { def.is_phantom_data() } else { false }
+    }
+
+    #[inline]
+    pub fn is_bool(self) -> bool {
+        *self.kind() == Bool
+    }
+
+    /// Returns `true` if this type is a `str`.
+    #[inline]
+    pub fn is_str(self) -> bool {
+        *self.kind() == Str
+    }
+
+    #[inline]
+    pub fn is_param(self, index: u32) -> bool {
+        match self.kind() {
+            ty::Param(ref data) => data.index == index,
+            _ => false,
+        }
+    }
+
+    #[inline]
+    pub fn is_slice(self) -> bool {
+        matches!(self.kind(), Slice(_))
+    }
+
+    #[inline]
+    pub fn is_array_slice(self) -> bool {
+        match self.kind() {
+            Slice(_) => true,
+            RawPtr(TypeAndMut { ty, .. }) | Ref(_, ty, _) => matches!(ty.kind(), Slice(_)),
+            _ => false,
+        }
+    }
+
+    #[inline]
+    pub fn is_array(self) -> bool {
+        matches!(self.kind(), Array(..))
+    }
+
+    #[inline]
+    pub fn is_simd(self) -> bool {
+        match self.kind() {
+            Adt(def, _) => def.repr().simd(),
+            _ => false,
+        }
+    }
+
+    pub fn sequence_element_type(self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        match self.kind() {
+            Array(ty, _) | Slice(ty) => *ty,
+            Str => tcx.types.u8,
+            _ => bug!("`sequence_element_type` called on non-sequence value: {}", self),
+        }
+    }
+
+    pub fn simd_size_and_type(self, tcx: TyCtxt<'tcx>) -> (u64, Ty<'tcx>) {
+        match self.kind() {
+            Adt(def, substs) => {
+                assert!(def.repr().simd(), "`simd_size_and_type` called on non-SIMD type");
+                let variant = def.non_enum_variant();
+                let f0_ty = variant.fields[0].ty(tcx, substs);
+
+                match f0_ty.kind() {
+                    // If the first field is an array, we assume it is the only field and its
+                    // elements are the SIMD components.
+                    Array(f0_elem_ty, f0_len) => {
+                        // FIXME(repr_simd): https://github.com/rust-lang/rust/pull/78863#discussion_r522784112
+                        // The way we evaluate the `N` in `[T; N]` here only works since we use
+                        // `simd_size_and_type` post-monomorphization. It will probably start to ICE
+                        // if we use it in generic code. See the `simd-array-trait` ui test.
+                        (f0_len.eval_usize(tcx, ParamEnv::empty()) as u64, *f0_elem_ty)
+                    }
+                    // Otherwise, the fields of this Adt are the SIMD components (and we assume they
+                    // all have the same type).
+                    _ => (variant.fields.len() as u64, f0_ty),
+                }
+            }
+            _ => bug!("`simd_size_and_type` called on invalid type"),
+        }
+    }
+
+    #[inline]
+    pub fn is_region_ptr(self) -> bool {
+        matches!(self.kind(), Ref(..))
+    }
+
+    #[inline]
+    pub fn is_mutable_ptr(self) -> bool {
+        matches!(
+            self.kind(),
+            RawPtr(TypeAndMut { mutbl: hir::Mutability::Mut, .. })
+                | Ref(_, _, hir::Mutability::Mut)
+        )
+    }
+
+    /// Get the mutability of the reference or `None` when not a reference
+    #[inline]
+    pub fn ref_mutability(self) -> Option<hir::Mutability> {
+        match self.kind() {
+            Ref(_, _, mutability) => Some(*mutability),
+            _ => None,
+        }
+    }
+
+    #[inline]
+    pub fn is_unsafe_ptr(self) -> bool {
+        matches!(self.kind(), RawPtr(_))
+    }
+
+    /// Tests if this is any kind of primitive pointer type (reference, raw pointer, fn pointer).
+    #[inline]
+    pub fn is_any_ptr(self) -> bool {
+        self.is_region_ptr() || self.is_unsafe_ptr() || self.is_fn_ptr()
+    }
+
+    #[inline]
+    pub fn is_box(self) -> bool {
+        match self.kind() {
+            Adt(def, _) => def.is_box(),
+            _ => false,
+        }
+    }
+
+    /// Panics if called on any type other than `Box<T>`.
+    pub fn boxed_ty(self) -> Ty<'tcx> {
+        match self.kind() {
+            Adt(def, substs) if def.is_box() => substs.type_at(0),
+            _ => bug!("`boxed_ty` is called on non-box type {:?}", self),
+        }
+    }
+
+    /// A scalar type is one that denotes an atomic datum, with no sub-components.
+    /// (A RawPtr is scalar because it represents a non-managed pointer, so its
+    /// contents are abstract to rustc.)
+    #[inline]
+    pub fn is_scalar(self) -> bool {
+        matches!(
+            self.kind(),
+            Bool | Char
+                | Int(_)
+                | Float(_)
+                | Uint(_)
+                | FnDef(..)
+                | FnPtr(_)
+                | RawPtr(_)
+                | Infer(IntVar(_) | FloatVar(_))
+        )
+    }
+
+    /// Returns `true` if this type is a floating point type.
+    #[inline]
+    pub fn is_floating_point(self) -> bool {
+        matches!(self.kind(), Float(_) | Infer(FloatVar(_)))
+    }
+
+    #[inline]
+    pub fn is_trait(self) -> bool {
+        matches!(self.kind(), Dynamic(..))
+    }
+
+    #[inline]
+    pub fn is_enum(self) -> bool {
+        matches!(self.kind(), Adt(adt_def, _) if adt_def.is_enum())
+    }
+
+    #[inline]
+    pub fn is_union(self) -> bool {
+        matches!(self.kind(), Adt(adt_def, _) if adt_def.is_union())
+    }
+
+    #[inline]
+    pub fn is_closure(self) -> bool {
+        matches!(self.kind(), Closure(..))
+    }
+
+    #[inline]
+    pub fn is_generator(self) -> bool {
+        matches!(self.kind(), Generator(..))
+    }
+
+    #[inline]
+    pub fn is_integral(self) -> bool {
+        matches!(self.kind(), Infer(IntVar(_)) | Int(_) | Uint(_))
+    }
+
+    #[inline]
+    pub fn is_fresh_ty(self) -> bool {
+        matches!(self.kind(), Infer(FreshTy(_)))
+    }
+
+    #[inline]
+    pub fn is_fresh(self) -> bool {
+        matches!(self.kind(), Infer(FreshTy(_) | FreshIntTy(_) | FreshFloatTy(_)))
+    }
+
+    #[inline]
+    pub fn is_char(self) -> bool {
+        matches!(self.kind(), Char)
+    }
+
+    #[inline]
+    pub fn is_numeric(self) -> bool {
+        self.is_integral() || self.is_floating_point()
+    }
+
+    #[inline]
+    pub fn is_signed(self) -> bool {
+        matches!(self.kind(), Int(_))
+    }
+
+    #[inline]
+    pub fn is_ptr_sized_integral(self) -> bool {
+        matches!(self.kind(), Int(ty::IntTy::Isize) | Uint(ty::UintTy::Usize))
+    }
+
+    #[inline]
+    pub fn has_concrete_skeleton(self) -> bool {
+        !matches!(self.kind(), Param(_) | Infer(_) | Error(_))
+    }
+
+    /// Checks whether a type recursively contains another type
+    ///
+    /// Example: `Option<()>` contains `()`
+    pub fn contains(self, other: Ty<'tcx>) -> bool {
+        struct ContainsTyVisitor<'tcx>(Ty<'tcx>);
+
+        impl<'tcx> TypeVisitor<'tcx> for ContainsTyVisitor<'tcx> {
+            type BreakTy = ();
+
+            fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+                if self.0 == t { ControlFlow::BREAK } else { t.super_visit_with(self) }
+            }
+        }
+
+        let cf = self.visit_with(&mut ContainsTyVisitor(other));
+        cf.is_break()
+    }
+
+    /// Returns the type and mutability of `*ty`.
+    ///
+    /// The parameter `explicit` indicates if this is an *explicit* dereference.
+    /// Some types -- notably unsafe ptrs -- can only be dereferenced explicitly.
+    pub fn builtin_deref(self, explicit: bool) -> Option<TypeAndMut<'tcx>> {
+        match self.kind() {
+            Adt(def, _) if def.is_box() => {
+                Some(TypeAndMut { ty: self.boxed_ty(), mutbl: hir::Mutability::Not })
+            }
+            Ref(_, ty, mutbl) => Some(TypeAndMut { ty: *ty, mutbl: *mutbl }),
+            RawPtr(mt) if explicit => Some(*mt),
+            _ => None,
+        }
+    }
+
+    /// Returns the type of `ty[i]`.
+    pub fn builtin_index(self) -> Option<Ty<'tcx>> {
+        match self.kind() {
+            Array(ty, _) | Slice(ty) => Some(*ty),
+            _ => None,
+        }
+    }
+
+    pub fn fn_sig(self, tcx: TyCtxt<'tcx>) -> PolyFnSig<'tcx> {
+        match self.kind() {
+            FnDef(def_id, substs) => tcx.bound_fn_sig(*def_id).subst(tcx, substs),
+            FnPtr(f) => *f,
+            Error(_) => {
+                // ignore errors (#54954)
+                ty::Binder::dummy(FnSig::fake())
+            }
+            Closure(..) => bug!(
+                "to get the signature of a closure, use `substs.as_closure().sig()` not `fn_sig()`",
+            ),
+            _ => bug!("Ty::fn_sig() called on non-fn type: {:?}", self),
+        }
+    }
+
+    #[inline]
+    pub fn is_fn(self) -> bool {
+        matches!(self.kind(), FnDef(..) | FnPtr(_))
+    }
+
+    #[inline]
+    pub fn is_fn_ptr(self) -> bool {
+        matches!(self.kind(), FnPtr(_))
+    }
+
+    #[inline]
+    pub fn is_impl_trait(self) -> bool {
+        matches!(self.kind(), Opaque(..))
+    }
+
+    #[inline]
+    pub fn ty_adt_def(self) -> Option<AdtDef<'tcx>> {
+        match self.kind() {
+            Adt(adt, _) => Some(*adt),
+            _ => None,
+        }
+    }
+
+    /// Iterates over tuple fields.
+    /// Panics when called on anything but a tuple.
+    #[inline]
+    pub fn tuple_fields(self) -> &'tcx List<Ty<'tcx>> {
+        match self.kind() {
+            Tuple(substs) => substs,
+            _ => bug!("tuple_fields called on non-tuple"),
+        }
+    }
+
+    /// If the type contains variants, returns the valid range of variant indices.
+    //
+    // FIXME: This requires the optimized MIR in the case of generators.
+    #[inline]
+    pub fn variant_range(self, tcx: TyCtxt<'tcx>) -> Option<Range<VariantIdx>> {
+        match self.kind() {
+            TyKind::Adt(adt, _) => Some(adt.variant_range()),
+            TyKind::Generator(def_id, substs, _) => {
+                Some(substs.as_generator().variant_range(*def_id, tcx))
+            }
+            _ => None,
+        }
+    }
+
+    /// If the type contains variants, returns the variant for `variant_index`.
+    /// Panics if `variant_index` is out of range.
+    //
+    // FIXME: This requires the optimized MIR in the case of generators.
+    #[inline]
+    pub fn discriminant_for_variant(
+        self,
+        tcx: TyCtxt<'tcx>,
+        variant_index: VariantIdx,
+    ) -> Option<Discr<'tcx>> {
+        match self.kind() {
+            TyKind::Adt(adt, _) if adt.variants().is_empty() => {
+                // This can actually happen during CTFE, see
+                // https://github.com/rust-lang/rust/issues/89765.
+                None
+            }
+            TyKind::Adt(adt, _) if adt.is_enum() => {
+                Some(adt.discriminant_for_variant(tcx, variant_index))
+            }
+            TyKind::Generator(def_id, substs, _) => {
+                Some(substs.as_generator().discriminant_for_variant(*def_id, tcx, variant_index))
+            }
+            _ => None,
+        }
+    }
+
+    /// Returns the type of the discriminant of this type.
+    pub fn discriminant_ty(self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        match self.kind() {
+            ty::Adt(adt, _) if adt.is_enum() => adt.repr().discr_type().to_ty(tcx),
+            ty::Generator(_, substs, _) => substs.as_generator().discr_ty(tcx),
+
+            ty::Param(_) | ty::Projection(_) | ty::Opaque(..) | ty::Infer(ty::TyVar(_)) => {
+                let assoc_items = tcx.associated_item_def_ids(
+                    tcx.require_lang_item(hir::LangItem::DiscriminantKind, None),
+                );
+                tcx.mk_projection(assoc_items[0], tcx.intern_substs(&[self.into()]))
+            }
+
+            ty::Bool
+            | ty::Char
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Adt(..)
+            | ty::Foreign(_)
+            | ty::Str
+            | ty::Array(..)
+            | ty::Slice(_)
+            | ty::RawPtr(_)
+            | ty::Ref(..)
+            | ty::FnDef(..)
+            | ty::FnPtr(..)
+            | ty::Dynamic(..)
+            | ty::Closure(..)
+            | ty::GeneratorWitness(..)
+            | ty::Never
+            | ty::Tuple(_)
+            | ty::Error(_)
+            | ty::Infer(IntVar(_) | FloatVar(_)) => tcx.types.u8,
+
+            ty::Bound(..)
+            | ty::Placeholder(_)
+            | ty::Infer(FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
+                bug!("`discriminant_ty` applied to unexpected type: {:?}", self)
+            }
+        }
+    }
+
+    /// Returns the type of metadata for (potentially fat) pointers to this type,
+    /// and a boolean signifying if this is conditional on this type being `Sized`.
+    pub fn ptr_metadata_ty(
+        self,
+        tcx: TyCtxt<'tcx>,
+        normalize: impl FnMut(Ty<'tcx>) -> Ty<'tcx>,
+    ) -> (Ty<'tcx>, bool) {
+        let tail = tcx.struct_tail_with_normalize(self, normalize, || {});
+        match tail.kind() {
+            // Sized types
+            ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
+            | ty::Uint(_)
+            | ty::Int(_)
+            | ty::Bool
+            | ty::Float(_)
+            | ty::FnDef(..)
+            | ty::FnPtr(_)
+            | ty::RawPtr(..)
+            | ty::Char
+            | ty::Ref(..)
+            | ty::Generator(..)
+            | ty::GeneratorWitness(..)
+            | ty::Array(..)
+            | ty::Closure(..)
+            | ty::Never
+            | ty::Error(_)
+            // Extern types have metadata = ().
+            | ty::Foreign(..)
+            // If returned by `struct_tail_without_normalization` this is a unit struct
+            // without any fields, or not a struct, and therefore is Sized.
+            | ty::Adt(..)
+            // If returned by `struct_tail_without_normalization` this is the empty tuple,
+            // a.k.a. unit type, which is Sized
+            | ty::Tuple(..) => (tcx.types.unit, false),
+
+            ty::Str | ty::Slice(_) => (tcx.types.usize, false),
+            ty::Dynamic(..) => {
+                let dyn_metadata = tcx.lang_items().dyn_metadata().unwrap();
+                (tcx.bound_type_of(dyn_metadata).subst(tcx, &[tail.into()]), false)
+            },
+
+            // type parameters only have unit metadata if they're sized, so return true
+            // to make sure we double check this during confirmation
+            ty::Param(_) |  ty::Projection(_) | ty::Opaque(..) => (tcx.types.unit, true),
+
+            ty::Infer(ty::TyVar(_))
+            | ty::Bound(..)
+            | ty::Placeholder(..)
+            | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
+                bug!("`ptr_metadata_ty` applied to unexpected type: {:?} (tail = {:?})", self, tail)
+            }
+        }
+    }
+
+    /// When we create a closure, we record its kind (i.e., what trait
+    /// it implements) into its `ClosureSubsts` using a type
+    /// parameter. This is kind of a phantom type, except that the
+    /// most convenient thing for us to are the integral types. This
+    /// function converts such a special type into the closure
+    /// kind. To go the other way, use
+    /// `tcx.closure_kind_ty(closure_kind)`.
+    ///
+    /// Note that during type checking, we use an inference variable
+    /// to represent the closure kind, because it has not yet been
+    /// inferred. Once upvar inference (in `rustc_typeck/src/check/upvar.rs`)
+    /// is complete, that type variable will be unified.
+    pub fn to_opt_closure_kind(self) -> Option<ty::ClosureKind> {
+        match self.kind() {
+            Int(int_ty) => match int_ty {
+                ty::IntTy::I8 => Some(ty::ClosureKind::Fn),
+                ty::IntTy::I16 => Some(ty::ClosureKind::FnMut),
+                ty::IntTy::I32 => Some(ty::ClosureKind::FnOnce),
+                _ => bug!("cannot convert type `{:?}` to a closure kind", self),
+            },
+
+            // "Bound" types appear in canonical queries when the
+            // closure type is not yet known
+            Bound(..) | Infer(_) => None,
+
+            Error(_) => Some(ty::ClosureKind::Fn),
+
+            _ => bug!("cannot convert type `{:?}` to a closure kind", self),
+        }
+    }
+
+    /// Fast path helper for testing if a type is `Sized`.
+    ///
+    /// Returning true means the type is known to be sized. Returning
+    /// `false` means nothing -- could be sized, might not be.
+    ///
+    /// Note that we could never rely on the fact that a type such as `[_]` is
+    /// trivially `!Sized` because we could be in a type environment with a
+    /// bound such as `[_]: Copy`. A function with such a bound obviously never
+    /// can be called, but that doesn't mean it shouldn't typecheck. This is why
+    /// this method doesn't return `Option<bool>`.
+    pub fn is_trivially_sized(self, tcx: TyCtxt<'tcx>) -> bool {
+        match self.kind() {
+            ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
+            | ty::Uint(_)
+            | ty::Int(_)
+            | ty::Bool
+            | ty::Float(_)
+            | ty::FnDef(..)
+            | ty::FnPtr(_)
+            | ty::RawPtr(..)
+            | ty::Char
+            | ty::Ref(..)
+            | ty::Generator(..)
+            | ty::GeneratorWitness(..)
+            | ty::Array(..)
+            | ty::Closure(..)
+            | ty::Never
+            | ty::Error(_) => true,
+
+            ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => false,
+
+            ty::Tuple(tys) => tys.iter().all(|ty| ty.is_trivially_sized(tcx)),
+
+            ty::Adt(def, _substs) => def.sized_constraint(tcx).0.is_empty(),
+
+            ty::Projection(_) | ty::Param(_) | ty::Opaque(..) => false,
+
+            ty::Infer(ty::TyVar(_)) => false,
+
+            ty::Bound(..)
+            | ty::Placeholder(..)
+            | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
+                bug!("`is_trivially_sized` applied to unexpected type: {:?}", self)
+            }
+        }
+    }
+
+    /// Fast path helper for primitives which are always `Copy` and which
+    /// have a side-effect-free `Clone` impl.
+    ///
+    /// Returning true means the type is known to be pure and `Copy+Clone`.
+    /// Returning `false` means nothing -- could be `Copy`, might not be.
+    ///
+    /// This is mostly useful for optimizations, as there are the types
+    /// on which we can replace cloning with dereferencing.
+    pub fn is_trivially_pure_clone_copy(self) -> bool {
+        match self.kind() {
+            ty::Bool | ty::Char | ty::Never => true,
+
+            // These aren't even `Clone`
+            ty::Str | ty::Slice(..) | ty::Foreign(..) | ty::Dynamic(..) => false,
+
+            ty::Int(..) | ty::Uint(..) | ty::Float(..) => true,
+
+            // The voldemort ZSTs are fine.
+            ty::FnDef(..) => true,
+
+            ty::Array(element_ty, _len) => element_ty.is_trivially_pure_clone_copy(),
+
+            // A 100-tuple isn't "trivial", so doing this only for reasonable sizes.
+            ty::Tuple(field_tys) => {
+                field_tys.len() <= 3 && field_tys.iter().all(Self::is_trivially_pure_clone_copy)
+            }
+
+            // Sometimes traits aren't implemented for every ABI or arity,
+            // because we can't be generic over everything yet.
+            ty::FnPtr(..) => false,
+
+            // Definitely absolutely not copy.
+            ty::Ref(_, _, hir::Mutability::Mut) => false,
+
+            // Thin pointers & thin shared references are pure-clone-copy, but for
+            // anything with custom metadata it might be more complicated.
+            ty::Ref(_, _, hir::Mutability::Not) | ty::RawPtr(..) => false,
+
+            ty::Generator(..) | ty::GeneratorWitness(..) => false,
+
+            // Might be, but not "trivial" so just giving the safe answer.
+            ty::Adt(..) | ty::Closure(..) | ty::Opaque(..) => false,
+
+            ty::Projection(..) | ty::Param(..) | ty::Infer(..) | ty::Error(..) => false,
+
+            ty::Bound(..) | ty::Placeholder(..) => {
+                bug!("`is_trivially_pure_clone_copy` applied to unexpected type: {:?}", self);
+            }
+        }
+    }
+}
+
+/// Extra information about why we ended up with a particular variance.
+/// This is only used to add more information to error messages, and
+/// has no effect on soundness. While choosing the 'wrong' `VarianceDiagInfo`
+/// may lead to confusing notes in error messages, it will never cause
+/// a miscompilation or unsoundness.
+///
+/// When in doubt, use `VarianceDiagInfo::default()`
+#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
+pub enum VarianceDiagInfo<'tcx> {
+    /// No additional information - this is the default.
+    /// We will not add any additional information to error messages.
+    #[default]
+    None,
+    /// We switched our variance because a generic argument occurs inside
+    /// the invariant generic argument of another type.
+    Invariant {
+        /// The generic type containing the generic parameter
+        /// that changes the variance (e.g. `*mut T`, `MyStruct<T>`)
+        ty: Ty<'tcx>,
+        /// The index of the generic parameter being used
+        /// (e.g. `0` for `*mut T`, `1` for `MyStruct<'CovariantParam, 'InvariantParam>`)
+        param_index: u32,
+    },
+}
+
+impl<'tcx> VarianceDiagInfo<'tcx> {
+    /// Mirrors `Variance::xform` - used to 'combine' the existing
+    /// and new `VarianceDiagInfo`s when our variance changes.
+    pub fn xform(self, other: VarianceDiagInfo<'tcx>) -> VarianceDiagInfo<'tcx> {
+        // For now, just use the first `VarianceDiagInfo::Invariant` that we see
+        match self {
+            VarianceDiagInfo::None => other,
+            VarianceDiagInfo::Invariant { .. } => self,
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/subst.rs b/compiler/rustc_middle/src/ty/subst.rs
new file mode 100644
index 000000000..6262aa180
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/subst.rs
@@ -0,0 +1,785 @@
+// Type substitutions.
+
+use crate::mir;
+use crate::ty::codec::{TyDecoder, TyEncoder};
+use crate::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeFolder, TypeSuperFoldable};
+use crate::ty::sty::{ClosureSubsts, GeneratorSubsts, InlineConstSubsts};
+use crate::ty::visit::{TypeVisitable, TypeVisitor};
+use crate::ty::{self, Lift, List, ParamConst, Ty, TyCtxt};
+
+use rustc_data_structures::intern::{Interned, WithStableHash};
+use rustc_hir::def_id::DefId;
+use rustc_macros::HashStable;
+use rustc_serialize::{self, Decodable, Encodable};
+use smallvec::SmallVec;
+
+use core::intrinsics;
+use std::cmp::Ordering;
+use std::fmt;
+use std::marker::PhantomData;
+use std::mem;
+use std::num::NonZeroUsize;
+use std::ops::ControlFlow;
+use std::slice;
+
+/// An entity in the Rust type system, which can be one of
+/// several kinds (types, lifetimes, and consts).
+/// To reduce memory usage, a `GenericArg` is an interned pointer,
+/// with the lowest 2 bits being reserved for a tag to
+/// indicate the type (`Ty`, `Region`, or `Const`) it points to.
+///
+/// Note: the `PartialEq`, `Eq` and `Hash` derives are only valid because `Ty`,
+/// `Region` and `Const` are all interned.
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+pub struct GenericArg<'tcx> {
+    ptr: NonZeroUsize,
+    marker: PhantomData<(Ty<'tcx>, ty::Region<'tcx>, ty::Const<'tcx>)>,
+}
+
+const TAG_MASK: usize = 0b11;
+const TYPE_TAG: usize = 0b00;
+const REGION_TAG: usize = 0b01;
+const CONST_TAG: usize = 0b10;
+
+#[derive(Debug, TyEncodable, TyDecodable, PartialEq, Eq, PartialOrd, Ord)]
+pub enum GenericArgKind<'tcx> {
+    Lifetime(ty::Region<'tcx>),
+    Type(Ty<'tcx>),
+    Const(ty::Const<'tcx>),
+}
+
+/// This function goes from `&'a [Ty<'tcx>]` to `&'a [GenericArg<'tcx>]`
+///
+/// This is sound as, for types, `GenericArg` is just
+/// `NonZeroUsize::new_unchecked(ty as *const _ as usize)` as
+/// long as we use `0` for the `TYPE_TAG`.
+pub fn ty_slice_as_generic_args<'a, 'tcx>(ts: &'a [Ty<'tcx>]) -> &'a [GenericArg<'tcx>] {
+    assert_eq!(TYPE_TAG, 0);
+    // SAFETY: the whole slice is valid and immutable.
+    // `Ty` and `GenericArg` is explained above.
+    unsafe { slice::from_raw_parts(ts.as_ptr().cast(), ts.len()) }
+}
+
+impl<'tcx> List<Ty<'tcx>> {
+    /// Allows to freely switch between `List<Ty<'tcx>>` and `List<GenericArg<'tcx>>`.
+    ///
+    /// As lists are interned, `List<Ty<'tcx>>` and `List<GenericArg<'tcx>>` have
+    /// be interned together, see `intern_type_list` for more details.
+    #[inline]
+    pub fn as_substs(&'tcx self) -> SubstsRef<'tcx> {
+        assert_eq!(TYPE_TAG, 0);
+        // SAFETY: `List<T>` is `#[repr(C)]`. `Ty` and `GenericArg` is explained above.
+        unsafe { &*(self as *const List<Ty<'tcx>> as *const List<GenericArg<'tcx>>) }
+    }
+}
+
+impl<'tcx> GenericArgKind<'tcx> {
+    #[inline]
+    fn pack(self) -> GenericArg<'tcx> {
+        let (tag, ptr) = match self {
+            GenericArgKind::Lifetime(lt) => {
+                // Ensure we can use the tag bits.
+                assert_eq!(mem::align_of_val(&*lt.0.0) & TAG_MASK, 0);
+                (REGION_TAG, lt.0.0 as *const ty::RegionKind<'tcx> as usize)
+            }
+            GenericArgKind::Type(ty) => {
+                // Ensure we can use the tag bits.
+                assert_eq!(mem::align_of_val(&*ty.0.0) & TAG_MASK, 0);
+                (TYPE_TAG, ty.0.0 as *const WithStableHash<ty::TyS<'tcx>> as usize)
+            }
+            GenericArgKind::Const(ct) => {
+                // Ensure we can use the tag bits.
+                assert_eq!(mem::align_of_val(&*ct.0.0) & TAG_MASK, 0);
+                (CONST_TAG, ct.0.0 as *const ty::ConstS<'tcx> as usize)
+            }
+        };
+
+        GenericArg { ptr: unsafe { NonZeroUsize::new_unchecked(ptr | tag) }, marker: PhantomData }
+    }
+}
+
+impl<'tcx> fmt::Debug for GenericArg<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.unpack() {
+            GenericArgKind::Lifetime(lt) => lt.fmt(f),
+            GenericArgKind::Type(ty) => ty.fmt(f),
+            GenericArgKind::Const(ct) => ct.fmt(f),
+        }
+    }
+}
+
+impl<'tcx> Ord for GenericArg<'tcx> {
+    fn cmp(&self, other: &GenericArg<'tcx>) -> Ordering {
+        self.unpack().cmp(&other.unpack())
+    }
+}
+
+impl<'tcx> PartialOrd for GenericArg<'tcx> {
+    fn partial_cmp(&self, other: &GenericArg<'tcx>) -> Option<Ordering> {
+        Some(self.cmp(&other))
+    }
+}
+
+impl<'tcx> From<ty::Region<'tcx>> for GenericArg<'tcx> {
+    #[inline]
+    fn from(r: ty::Region<'tcx>) -> GenericArg<'tcx> {
+        GenericArgKind::Lifetime(r).pack()
+    }
+}
+
+impl<'tcx> From<Ty<'tcx>> for GenericArg<'tcx> {
+    #[inline]
+    fn from(ty: Ty<'tcx>) -> GenericArg<'tcx> {
+        GenericArgKind::Type(ty).pack()
+    }
+}
+
+impl<'tcx> From<ty::Const<'tcx>> for GenericArg<'tcx> {
+    #[inline]
+    fn from(c: ty::Const<'tcx>) -> GenericArg<'tcx> {
+        GenericArgKind::Const(c).pack()
+    }
+}
+
+impl<'tcx> GenericArg<'tcx> {
+    #[inline]
+    pub fn unpack(self) -> GenericArgKind<'tcx> {
+        let ptr = self.ptr.get();
+        // SAFETY: use of `Interned::new_unchecked` here is ok because these
+        // pointers were originally created from `Interned` types in `pack()`,
+        // and this is just going in the other direction.
+        unsafe {
+            match ptr & TAG_MASK {
+                REGION_TAG => GenericArgKind::Lifetime(ty::Region(Interned::new_unchecked(
+                    &*((ptr & !TAG_MASK) as *const ty::RegionKind<'tcx>),
+                ))),
+                TYPE_TAG => GenericArgKind::Type(Ty(Interned::new_unchecked(
+                    &*((ptr & !TAG_MASK) as *const WithStableHash<ty::TyS<'tcx>>),
+                ))),
+                CONST_TAG => GenericArgKind::Const(ty::Const(Interned::new_unchecked(
+                    &*((ptr & !TAG_MASK) as *const ty::ConstS<'tcx>),
+                ))),
+                _ => intrinsics::unreachable(),
+            }
+        }
+    }
+
+    /// Unpack the `GenericArg` as a region when it is known certainly to be a region.
+    pub fn expect_region(self) -> ty::Region<'tcx> {
+        match self.unpack() {
+            GenericArgKind::Lifetime(lt) => lt,
+            _ => bug!("expected a region, but found another kind"),
+        }
+    }
+
+    /// Unpack the `GenericArg` as a type when it is known certainly to be a type.
+    /// This is true in cases where `Substs` is used in places where the kinds are known
+    /// to be limited (e.g. in tuples, where the only parameters are type parameters).
+    pub fn expect_ty(self) -> Ty<'tcx> {
+        match self.unpack() {
+            GenericArgKind::Type(ty) => ty,
+            _ => bug!("expected a type, but found another kind"),
+        }
+    }
+
+    /// Unpack the `GenericArg` as a const when it is known certainly to be a const.
+    pub fn expect_const(self) -> ty::Const<'tcx> {
+        match self.unpack() {
+            GenericArgKind::Const(c) => c,
+            _ => bug!("expected a const, but found another kind"),
+        }
+    }
+}
+
+impl<'a, 'tcx> Lift<'tcx> for GenericArg<'a> {
+    type Lifted = GenericArg<'tcx>;
+
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        match self.unpack() {
+            GenericArgKind::Lifetime(lt) => tcx.lift(lt).map(|lt| lt.into()),
+            GenericArgKind::Type(ty) => tcx.lift(ty).map(|ty| ty.into()),
+            GenericArgKind::Const(ct) => tcx.lift(ct).map(|ct| ct.into()),
+        }
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for GenericArg<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        match self.unpack() {
+            GenericArgKind::Lifetime(lt) => lt.try_fold_with(folder).map(Into::into),
+            GenericArgKind::Type(ty) => ty.try_fold_with(folder).map(Into::into),
+            GenericArgKind::Const(ct) => ct.try_fold_with(folder).map(Into::into),
+        }
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for GenericArg<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        match self.unpack() {
+            GenericArgKind::Lifetime(lt) => lt.visit_with(visitor),
+            GenericArgKind::Type(ty) => ty.visit_with(visitor),
+            GenericArgKind::Const(ct) => ct.visit_with(visitor),
+        }
+    }
+}
+
+impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> Encodable<E> for GenericArg<'tcx> {
+    fn encode(&self, e: &mut E) {
+        self.unpack().encode(e)
+    }
+}
+
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for GenericArg<'tcx> {
+    fn decode(d: &mut D) -> GenericArg<'tcx> {
+        GenericArgKind::decode(d).pack()
+    }
+}
+
+/// A substitution mapping generic parameters to new values.
+pub type InternalSubsts<'tcx> = List<GenericArg<'tcx>>;
+
+pub type SubstsRef<'tcx> = &'tcx InternalSubsts<'tcx>;
+
+impl<'tcx> InternalSubsts<'tcx> {
+    /// Checks whether all elements of this list are types, if so, transmute.
+    pub fn try_as_type_list(&'tcx self) -> Option<&'tcx List<Ty<'tcx>>> {
+        if self.iter().all(|arg| matches!(arg.unpack(), GenericArgKind::Type(_))) {
+            assert_eq!(TYPE_TAG, 0);
+            // SAFETY: All elements are types, see `List<Ty<'tcx>>::as_substs`.
+            Some(unsafe { &*(self as *const List<GenericArg<'tcx>> as *const List<Ty<'tcx>>) })
+        } else {
+            None
+        }
+    }
+
+    /// Interpret these substitutions as the substitutions of a closure type.
+    /// Closure substitutions have a particular structure controlled by the
+    /// compiler that encodes information like the signature and closure kind;
+    /// see `ty::ClosureSubsts` struct for more comments.
+    pub fn as_closure(&'tcx self) -> ClosureSubsts<'tcx> {
+        ClosureSubsts { substs: self }
+    }
+
+    /// Interpret these substitutions as the substitutions of a generator type.
+    /// Generator substitutions have a particular structure controlled by the
+    /// compiler that encodes information like the signature and generator kind;
+    /// see `ty::GeneratorSubsts` struct for more comments.
+    pub fn as_generator(&'tcx self) -> GeneratorSubsts<'tcx> {
+        GeneratorSubsts { substs: self }
+    }
+
+    /// Interpret these substitutions as the substitutions of an inline const.
+    /// Inline const substitutions have a particular structure controlled by the
+    /// compiler that encodes information like the inferred type;
+    /// see `ty::InlineConstSubsts` struct for more comments.
+    pub fn as_inline_const(&'tcx self) -> InlineConstSubsts<'tcx> {
+        InlineConstSubsts { substs: self }
+    }
+
+    /// Creates an `InternalSubsts` that maps each generic parameter to itself.
+    pub fn identity_for_item(tcx: TyCtxt<'tcx>, def_id: DefId) -> SubstsRef<'tcx> {
+        Self::for_item(tcx, def_id, |param, _| tcx.mk_param_from_def(param))
+    }
+
+    /// Creates an `InternalSubsts` for generic parameter definitions,
+    /// by calling closures to obtain each kind.
+    /// The closures get to observe the `InternalSubsts` as they're
+    /// being built, which can be used to correctly
+    /// substitute defaults of generic parameters.
+    pub fn for_item<F>(tcx: TyCtxt<'tcx>, def_id: DefId, mut mk_kind: F) -> SubstsRef<'tcx>
+    where
+        F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
+    {
+        let defs = tcx.generics_of(def_id);
+        let count = defs.count();
+        let mut substs = SmallVec::with_capacity(count);
+        Self::fill_item(&mut substs, tcx, defs, &mut mk_kind);
+        tcx.intern_substs(&substs)
+    }
+
+    pub fn extend_to<F>(&self, tcx: TyCtxt<'tcx>, def_id: DefId, mut mk_kind: F) -> SubstsRef<'tcx>
+    where
+        F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
+    {
+        Self::for_item(tcx, def_id, |param, substs| {
+            self.get(param.index as usize).cloned().unwrap_or_else(|| mk_kind(param, substs))
+        })
+    }
+
+    pub fn fill_item<F>(
+        substs: &mut SmallVec<[GenericArg<'tcx>; 8]>,
+        tcx: TyCtxt<'tcx>,
+        defs: &ty::Generics,
+        mk_kind: &mut F,
+    ) where
+        F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
+    {
+        if let Some(def_id) = defs.parent {
+            let parent_defs = tcx.generics_of(def_id);
+            Self::fill_item(substs, tcx, parent_defs, mk_kind);
+        }
+        Self::fill_single(substs, defs, mk_kind)
+    }
+
+    pub fn fill_single<F>(
+        substs: &mut SmallVec<[GenericArg<'tcx>; 8]>,
+        defs: &ty::Generics,
+        mk_kind: &mut F,
+    ) where
+        F: FnMut(&ty::GenericParamDef, &[GenericArg<'tcx>]) -> GenericArg<'tcx>,
+    {
+        substs.reserve(defs.params.len());
+        for param in &defs.params {
+            let kind = mk_kind(param, substs);
+            assert_eq!(param.index as usize, substs.len());
+            substs.push(kind);
+        }
+    }
+
+    #[inline]
+    pub fn types(&'tcx self) -> impl DoubleEndedIterator<Item = Ty<'tcx>> + 'tcx {
+        self.iter()
+            .filter_map(|k| if let GenericArgKind::Type(ty) = k.unpack() { Some(ty) } else { None })
+    }
+
+    #[inline]
+    pub fn regions(&'tcx self) -> impl DoubleEndedIterator<Item = ty::Region<'tcx>> + 'tcx {
+        self.iter().filter_map(|k| {
+            if let GenericArgKind::Lifetime(lt) = k.unpack() { Some(lt) } else { None }
+        })
+    }
+
+    #[inline]
+    pub fn consts(&'tcx self) -> impl DoubleEndedIterator<Item = ty::Const<'tcx>> + 'tcx {
+        self.iter().filter_map(|k| {
+            if let GenericArgKind::Const(ct) = k.unpack() { Some(ct) } else { None }
+        })
+    }
+
+    #[inline]
+    pub fn non_erasable_generics(
+        &'tcx self,
+    ) -> impl DoubleEndedIterator<Item = GenericArgKind<'tcx>> + 'tcx {
+        self.iter().filter_map(|k| match k.unpack() {
+            GenericArgKind::Lifetime(_) => None,
+            generic => Some(generic),
+        })
+    }
+
+    #[inline]
+    pub fn type_at(&self, i: usize) -> Ty<'tcx> {
+        if let GenericArgKind::Type(ty) = self[i].unpack() {
+            ty
+        } else {
+            bug!("expected type for param #{} in {:?}", i, self);
+        }
+    }
+
+    #[inline]
+    pub fn region_at(&self, i: usize) -> ty::Region<'tcx> {
+        if let GenericArgKind::Lifetime(lt) = self[i].unpack() {
+            lt
+        } else {
+            bug!("expected region for param #{} in {:?}", i, self);
+        }
+    }
+
+    #[inline]
+    pub fn const_at(&self, i: usize) -> ty::Const<'tcx> {
+        if let GenericArgKind::Const(ct) = self[i].unpack() {
+            ct
+        } else {
+            bug!("expected const for param #{} in {:?}", i, self);
+        }
+    }
+
+    #[inline]
+    pub fn type_for_def(&self, def: &ty::GenericParamDef) -> GenericArg<'tcx> {
+        self.type_at(def.index as usize).into()
+    }
+
+    /// Transform from substitutions for a child of `source_ancestor`
+    /// (e.g., a trait or impl) to substitutions for the same child
+    /// in a different item, with `target_substs` as the base for
+    /// the target impl/trait, with the source child-specific
+    /// parameters (e.g., method parameters) on top of that base.
+    ///
+    /// For example given:
+    ///
+    /// ```no_run
+    /// trait X<S> { fn f<T>(); }
+    /// impl<U> X<U> for U { fn f<V>() {} }
+    /// ```
+    ///
+    /// * If `self` is `[Self, S, T]`: the identity substs of `f` in the trait.
+    /// * If `source_ancestor` is the def_id of the trait.
+    /// * If `target_substs` is `[U]`, the substs for the impl.
+    /// * Then we will return `[U, T]`, the subst for `f` in the impl that
+    ///   are needed for it to match the trait.
+    pub fn rebase_onto(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        source_ancestor: DefId,
+        target_substs: SubstsRef<'tcx>,
+    ) -> SubstsRef<'tcx> {
+        let defs = tcx.generics_of(source_ancestor);
+        tcx.mk_substs(target_substs.iter().chain(self.iter().skip(defs.params.len())))
+    }
+
+    pub fn truncate_to(&self, tcx: TyCtxt<'tcx>, generics: &ty::Generics) -> SubstsRef<'tcx> {
+        tcx.mk_substs(self.iter().take(generics.count()))
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for SubstsRef<'tcx> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        // This code is hot enough that it's worth specializing for the most
+        // common length lists, to avoid the overhead of `SmallVec` creation.
+        // The match arms are in order of frequency. The 1, 2, and 0 cases are
+        // typically hit in 90--99.99% of cases. When folding doesn't change
+        // the substs, it's faster to reuse the existing substs rather than
+        // calling `intern_substs`.
+        match self.len() {
+            1 => {
+                let param0 = self[0].try_fold_with(folder)?;
+                if param0 == self[0] { Ok(self) } else { Ok(folder.tcx().intern_substs(&[param0])) }
+            }
+            2 => {
+                let param0 = self[0].try_fold_with(folder)?;
+                let param1 = self[1].try_fold_with(folder)?;
+                if param0 == self[0] && param1 == self[1] {
+                    Ok(self)
+                } else {
+                    Ok(folder.tcx().intern_substs(&[param0, param1]))
+                }
+            }
+            0 => Ok(self),
+            _ => ty::util::fold_list(self, folder, |tcx, v| tcx.intern_substs(v)),
+        }
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for SubstsRef<'tcx> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.iter().try_for_each(|t| t.visit_with(visitor))
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<Ty<'tcx>> {
+    fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
+        // This code is fairly hot, though not as hot as `SubstsRef`.
+        //
+        // When compiling stage 2, I get the following results:
+        //
+        // len |   total   |   %
+        // --- | --------- | -----
+        //  2  |  15083590 |  48.1
+        //  3  |   7540067 |  24.0
+        //  1  |   5300377 |  16.9
+        //  4  |   1351897 |   4.3
+        //  0  |   1256849 |   4.0
+        //
+        // I've tried it with some private repositories and got
+        // close to the same result, with 4 and 0 swapping places
+        // sometimes.
+        match self.len() {
+            2 => {
+                let param0 = self[0].try_fold_with(folder)?;
+                let param1 = self[1].try_fold_with(folder)?;
+                if param0 == self[0] && param1 == self[1] {
+                    Ok(self)
+                } else {
+                    Ok(folder.tcx().intern_type_list(&[param0, param1]))
+                }
+            }
+            _ => ty::util::fold_list(self, folder, |tcx, v| tcx.intern_type_list(v)),
+        }
+    }
+}
+
+impl<'tcx> TypeVisitable<'tcx> for &'tcx ty::List<Ty<'tcx>> {
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.iter().try_for_each(|t| t.visit_with(visitor))
+    }
+}
+
+// Just call `foo.subst(tcx, substs)` to perform a substitution across `foo`.
+#[rustc_on_unimplemented(message = "Calling `subst` must now be done through an `EarlyBinder`")]
+pub trait Subst<'tcx>: Sized {
+    type Inner;
+
+    fn subst(self, tcx: TyCtxt<'tcx>, substs: &[GenericArg<'tcx>]) -> Self::Inner;
+}
+
+impl<'tcx, T: TypeFoldable<'tcx>> Subst<'tcx> for ty::EarlyBinder<T> {
+    type Inner = T;
+
+    fn subst(self, tcx: TyCtxt<'tcx>, substs: &[GenericArg<'tcx>]) -> Self::Inner {
+        let mut folder = SubstFolder { tcx, substs, binders_passed: 0 };
+        self.0.fold_with(&mut folder)
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// The actual substitution engine itself is a type folder.
+
+struct SubstFolder<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    substs: &'a [GenericArg<'tcx>],
+
+    /// Number of region binders we have passed through while doing the substitution
+    binders_passed: u32,
+}
+
+impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
+    #[inline]
+    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_binder<T: TypeFoldable<'tcx>>(
+        &mut self,
+        t: ty::Binder<'tcx, T>,
+    ) -> ty::Binder<'tcx, T> {
+        self.binders_passed += 1;
+        let t = t.super_fold_with(self);
+        self.binders_passed -= 1;
+        t
+    }
+
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        #[cold]
+        #[inline(never)]
+        fn region_param_out_of_range(data: ty::EarlyBoundRegion) -> ! {
+            bug!(
+                "Region parameter out of range when substituting in region {} (index={})",
+                data.name,
+                data.index
+            )
+        }
+
+        // Note: This routine only handles regions that are bound on
+        // type declarations and other outer declarations, not those
+        // bound in *fn types*. Region substitution of the bound
+        // regions that appear in a function signature is done using
+        // the specialized routine `ty::replace_late_regions()`.
+        match *r {
+            ty::ReEarlyBound(data) => {
+                let rk = self.substs.get(data.index as usize).map(|k| k.unpack());
+                match rk {
+                    Some(GenericArgKind::Lifetime(lt)) => self.shift_region_through_binders(lt),
+                    _ => region_param_out_of_range(data),
+                }
+            }
+            _ => r,
+        }
+    }
+
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+        if !t.needs_subst() {
+            return t;
+        }
+
+        match *t.kind() {
+            ty::Param(p) => self.ty_for_param(p, t),
+            _ => t.super_fold_with(self),
+        }
+    }
+
+    fn fold_const(&mut self, c: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        if let ty::ConstKind::Param(p) = c.kind() {
+            self.const_for_param(p, c)
+        } else {
+            c.super_fold_with(self)
+        }
+    }
+
+    #[inline]
+    fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
+        c.super_fold_with(self)
+    }
+}
+
+impl<'a, 'tcx> SubstFolder<'a, 'tcx> {
+    fn ty_for_param(&self, p: ty::ParamTy, source_ty: Ty<'tcx>) -> Ty<'tcx> {
+        // Look up the type in the substitutions. It really should be in there.
+        let opt_ty = self.substs.get(p.index as usize).map(|k| k.unpack());
+        let ty = match opt_ty {
+            Some(GenericArgKind::Type(ty)) => ty,
+            Some(kind) => self.type_param_expected(p, source_ty, kind),
+            None => self.type_param_out_of_range(p, source_ty),
+        };
+
+        self.shift_vars_through_binders(ty)
+    }
+
+    #[cold]
+    #[inline(never)]
+    fn type_param_expected(&self, p: ty::ParamTy, ty: Ty<'tcx>, kind: GenericArgKind<'tcx>) -> ! {
+        bug!(
+            "expected type for `{:?}` ({:?}/{}) but found {:?} when substituting, substs={:?}",
+            p,
+            ty,
+            p.index,
+            kind,
+            self.substs,
+        )
+    }
+
+    #[cold]
+    #[inline(never)]
+    fn type_param_out_of_range(&self, p: ty::ParamTy, ty: Ty<'tcx>) -> ! {
+        bug!(
+            "type parameter `{:?}` ({:?}/{}) out of range when substituting, substs={:?}",
+            p,
+            ty,
+            p.index,
+            self.substs,
+        )
+    }
+
+    fn const_for_param(&self, p: ParamConst, source_ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        // Look up the const in the substitutions. It really should be in there.
+        let opt_ct = self.substs.get(p.index as usize).map(|k| k.unpack());
+        let ct = match opt_ct {
+            Some(GenericArgKind::Const(ct)) => ct,
+            Some(kind) => self.const_param_expected(p, source_ct, kind),
+            None => self.const_param_out_of_range(p, source_ct),
+        };
+
+        self.shift_vars_through_binders(ct)
+    }
+
+    #[cold]
+    #[inline(never)]
+    fn const_param_expected(
+        &self,
+        p: ty::ParamConst,
+        ct: ty::Const<'tcx>,
+        kind: GenericArgKind<'tcx>,
+    ) -> ! {
+        bug!(
+            "expected const for `{:?}` ({:?}/{}) but found {:?} when substituting substs={:?}",
+            p,
+            ct,
+            p.index,
+            kind,
+            self.substs,
+        )
+    }
+
+    #[cold]
+    #[inline(never)]
+    fn const_param_out_of_range(&self, p: ty::ParamConst, ct: ty::Const<'tcx>) -> ! {
+        bug!(
+            "const parameter `{:?}` ({:?}/{}) out of range when substituting substs={:?}",
+            p,
+            ct,
+            p.index,
+            self.substs,
+        )
+    }
+
+    /// It is sometimes necessary to adjust the De Bruijn indices during substitution. This occurs
+    /// when we are substituting a type with escaping bound vars into a context where we have
+    /// passed through binders. That's quite a mouthful. Let's see an example:
+    ///
+    /// ```
+    /// type Func<A> = fn(A);
+    /// type MetaFunc = for<'a> fn(Func<&'a i32>);
+    /// ```
+    ///
+    /// The type `MetaFunc`, when fully expanded, will be
+    /// ```ignore (illustrative)
+    /// for<'a> fn(fn(&'a i32))
+    /// //      ^~ ^~ ^~~
+    /// //      |  |  |
+    /// //      |  |  DebruijnIndex of 2
+    /// //      Binders
+    /// ```
+    /// Here the `'a` lifetime is bound in the outer function, but appears as an argument of the
+    /// inner one. Therefore, that appearance will have a DebruijnIndex of 2, because we must skip
+    /// over the inner binder (remember that we count De Bruijn indices from 1). However, in the
+    /// definition of `MetaFunc`, the binder is not visible, so the type `&'a i32` will have a
+    /// De Bruijn index of 1. It's only during the substitution that we can see we must increase the
+    /// depth by 1 to account for the binder that we passed through.
+    ///
+    /// As a second example, consider this twist:
+    ///
+    /// ```
+    /// type FuncTuple<A> = (A,fn(A));
+    /// type MetaFuncTuple = for<'a> fn(FuncTuple<&'a i32>);
+    /// ```
+    ///
+    /// Here the final type will be:
+    /// ```ignore (illustrative)
+    /// for<'a> fn((&'a i32, fn(&'a i32)))
+    /// //          ^~~         ^~~
+    /// //          |           |
+    /// //   DebruijnIndex of 1 |
+    /// //               DebruijnIndex of 2
+    /// ```
+    /// As indicated in the diagram, here the same type `&'a i32` is substituted once, but in the
+    /// first case we do not increase the De Bruijn index and in the second case we do. The reason
+    /// is that only in the second case have we passed through a fn binder.
+    fn shift_vars_through_binders<T: TypeFoldable<'tcx>>(&self, val: T) -> T {
+        debug!(
+            "shift_vars(val={:?}, binders_passed={:?}, has_escaping_bound_vars={:?})",
+            val,
+            self.binders_passed,
+            val.has_escaping_bound_vars()
+        );
+
+        if self.binders_passed == 0 || !val.has_escaping_bound_vars() {
+            return val;
+        }
+
+        let result = ty::fold::shift_vars(TypeFolder::tcx(self), val, self.binders_passed);
+        debug!("shift_vars: shifted result = {:?}", result);
+
+        result
+    }
+
+    fn shift_region_through_binders(&self, region: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        if self.binders_passed == 0 || !region.has_escaping_bound_vars() {
+            return region;
+        }
+        ty::fold::shift_region(self.tcx, region, self.binders_passed)
+    }
+}
+
+/// Stores the user-given substs to reach some fully qualified path
+/// (e.g., `<T>::Item` or `<T as Trait>::Item`).
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub struct UserSubsts<'tcx> {
+    /// The substitutions for the item as given by the user.
+    pub substs: SubstsRef<'tcx>,
+
+    /// The self type, in the case of a `<T>::Item` path (when applied
+    /// to an inherent impl). See `UserSelfTy` below.
+    pub user_self_ty: Option<UserSelfTy<'tcx>>,
+}
+
+/// Specifies the user-given self type. In the case of a path that
+/// refers to a member in an inherent impl, this self type is
+/// sometimes needed to constrain the type parameters on the impl. For
+/// example, in this code:
+///
+/// ```ignore (illustrative)
+/// struct Foo<T> { }
+/// impl<A> Foo<A> { fn method() { } }
+/// ```
+///
+/// when you then have a path like `<Foo<&'static u32>>::method`,
+/// this struct would carry the `DefId` of the impl along with the
+/// self type `Foo<u32>`. Then we can instantiate the parameters of
+/// the impl (with the substs from `UserSubsts`) and apply those to
+/// the self type, giving `Foo<?A>`. Finally, we unify that with
+/// the self type here, which contains `?A` to be `&'static u32`
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable, TypeFoldable, TypeVisitable, Lift)]
+pub struct UserSelfTy<'tcx> {
+    pub impl_def_id: DefId,
+    pub self_ty: Ty<'tcx>,
+}
diff --git a/compiler/rustc_middle/src/ty/trait_def.rs b/compiler/rustc_middle/src/ty/trait_def.rs
new file mode 100644
index 000000000..541dace5c
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/trait_def.rs
@@ -0,0 +1,272 @@
+use crate::traits::specialization_graph;
+use crate::ty::fast_reject::{self, SimplifiedType, TreatParams};
+use crate::ty::visit::TypeVisitable;
+use crate::ty::{Ident, Ty, TyCtxt};
+use hir::def_id::LOCAL_CRATE;
+use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use std::iter;
+
+use rustc_data_structures::fx::FxIndexMap;
+use rustc_errors::ErrorGuaranteed;
+use rustc_macros::HashStable;
+
+/// A trait's definition with type information.
+#[derive(HashStable, Encodable, Decodable)]
+pub struct TraitDef {
+    pub def_id: DefId,
+
+    pub unsafety: hir::Unsafety,
+
+    /// If `true`, then this trait had the `#[rustc_paren_sugar]`
+    /// attribute, indicating that it should be used with `Foo()`
+    /// sugar. This is a temporary thing -- eventually any trait will
+    /// be usable with the sugar (or without it).
+    pub paren_sugar: bool,
+
+    pub has_auto_impl: bool,
+
+    /// If `true`, then this trait has the `#[marker]` attribute, indicating
+    /// that all its associated items have defaults that cannot be overridden,
+    /// and thus `impl`s of it are allowed to overlap.
+    pub is_marker: bool,
+
+    /// If `true`, then this trait has the `#[rustc_skip_array_during_method_dispatch]`
+    /// attribute, indicating that editions before 2021 should not consider this trait
+    /// during method dispatch if the receiver is an array.
+    pub skip_array_during_method_dispatch: bool,
+
+    /// Used to determine whether the standard library is allowed to specialize
+    /// on this trait.
+    pub specialization_kind: TraitSpecializationKind,
+
+    /// List of functions from `#[rustc_must_implement_one_of]` attribute one of which
+    /// must be implemented.
+    pub must_implement_one_of: Option<Box<[Ident]>>,
+}
+
+/// Whether this trait is treated specially by the standard library
+/// specialization lint.
+#[derive(HashStable, PartialEq, Clone, Copy, Encodable, Decodable)]
+pub enum TraitSpecializationKind {
+    /// The default. Specializing on this trait is not allowed.
+    None,
+    /// Specializing on this trait is allowed because it doesn't have any
+    /// methods. For example `Sized` or `FusedIterator`.
+    /// Applies to traits with the `rustc_unsafe_specialization_marker`
+    /// attribute.
+    Marker,
+    /// Specializing on this trait is allowed because all of the impls of this
+    /// trait are "always applicable". Always applicable means that if
+    /// `X<'x>: T<'y>` for any lifetimes, then `for<'a, 'b> X<'a>: T<'b>`.
+    /// Applies to traits with the `rustc_specialization_trait` attribute.
+    AlwaysApplicable,
+}
+
+#[derive(Default, Debug, HashStable)]
+pub struct TraitImpls {
+    blanket_impls: Vec<DefId>,
+    /// Impls indexed by their simplified self type, for fast lookup.
+    non_blanket_impls: FxIndexMap<SimplifiedType, Vec<DefId>>,
+}
+
+impl TraitImpls {
+    pub fn blanket_impls(&self) -> &[DefId] {
+        self.blanket_impls.as_slice()
+    }
+
+    pub fn non_blanket_impls(&self) -> &FxIndexMap<SimplifiedType, Vec<DefId>> {
+        &self.non_blanket_impls
+    }
+}
+
+impl<'tcx> TraitDef {
+    pub fn new(
+        def_id: DefId,
+        unsafety: hir::Unsafety,
+        paren_sugar: bool,
+        has_auto_impl: bool,
+        is_marker: bool,
+        skip_array_during_method_dispatch: bool,
+        specialization_kind: TraitSpecializationKind,
+        must_implement_one_of: Option<Box<[Ident]>>,
+    ) -> TraitDef {
+        TraitDef {
+            def_id,
+            unsafety,
+            paren_sugar,
+            has_auto_impl,
+            is_marker,
+            skip_array_during_method_dispatch,
+            specialization_kind,
+            must_implement_one_of,
+        }
+    }
+
+    pub fn ancestors(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        of_impl: DefId,
+    ) -> Result<specialization_graph::Ancestors<'tcx>, ErrorGuaranteed> {
+        specialization_graph::ancestors(tcx, self.def_id, of_impl)
+    }
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    pub fn for_each_impl<F: FnMut(DefId)>(self, def_id: DefId, mut f: F) {
+        let impls = self.trait_impls_of(def_id);
+
+        for &impl_def_id in impls.blanket_impls.iter() {
+            f(impl_def_id);
+        }
+
+        for v in impls.non_blanket_impls.values() {
+            for &impl_def_id in v {
+                f(impl_def_id);
+            }
+        }
+    }
+
+    /// Iterate over every impl that could possibly match the
+    /// self type `self_ty`.
+    pub fn for_each_relevant_impl<F: FnMut(DefId)>(
+        self,
+        def_id: DefId,
+        self_ty: Ty<'tcx>,
+        mut f: F,
+    ) {
+        let _: Option<()> = self.find_map_relevant_impl(def_id, self_ty, |did| {
+            f(did);
+            None
+        });
+    }
+
+    pub fn non_blanket_impls_for_ty(
+        self,
+        def_id: DefId,
+        self_ty: Ty<'tcx>,
+    ) -> impl Iterator<Item = DefId> + 'tcx {
+        let impls = self.trait_impls_of(def_id);
+        if let Some(simp) = fast_reject::simplify_type(self, self_ty, TreatParams::AsInfer) {
+            if let Some(impls) = impls.non_blanket_impls.get(&simp) {
+                return impls.iter().copied();
+            }
+        }
+
+        [].iter().copied()
+    }
+
+    /// Applies function to every impl that could possibly match the self type `self_ty` and returns
+    /// the first non-none value.
+    pub fn find_map_relevant_impl<T, F: FnMut(DefId) -> Option<T>>(
+        self,
+        def_id: DefId,
+        self_ty: Ty<'tcx>,
+        mut f: F,
+    ) -> Option<T> {
+        // FIXME: This depends on the set of all impls for the trait. That is
+        // unfortunate wrt. incremental compilation.
+        //
+        // If we want to be faster, we could have separate queries for
+        // blanket and non-blanket impls, and compare them separately.
+        let impls = self.trait_impls_of(def_id);
+
+        for &impl_def_id in impls.blanket_impls.iter() {
+            if let result @ Some(_) = f(impl_def_id) {
+                return result;
+            }
+        }
+
+        // Note that we're using `TreatParams::AsPlaceholder` to query `non_blanket_impls` while using
+        // `TreatParams::AsInfer` while actually adding them.
+        //
+        // This way, when searching for some impl for `T: Trait`, we do not look at any impls
+        // whose outer level is not a parameter or projection. Especially for things like
+        // `T: Clone` this is incredibly useful as we would otherwise look at all the impls
+        // of `Clone` for `Option<T>`, `Vec<T>`, `ConcreteType` and so on.
+        if let Some(simp) = fast_reject::simplify_type(self, self_ty, TreatParams::AsPlaceholder) {
+            if let Some(impls) = impls.non_blanket_impls.get(&simp) {
+                for &impl_def_id in impls {
+                    if let result @ Some(_) = f(impl_def_id) {
+                        return result;
+                    }
+                }
+            }
+        } else {
+            for &impl_def_id in impls.non_blanket_impls.values().flatten() {
+                if let result @ Some(_) = f(impl_def_id) {
+                    return result;
+                }
+            }
+        }
+
+        None
+    }
+
+    /// Returns an iterator containing all impls
+    pub fn all_impls(self, def_id: DefId) -> impl Iterator<Item = DefId> + 'tcx {
+        let TraitImpls { blanket_impls, non_blanket_impls } = self.trait_impls_of(def_id);
+
+        blanket_impls.iter().chain(non_blanket_impls.iter().flat_map(|(_, v)| v)).cloned()
+    }
+}
+
+// Query provider for `trait_impls_of`.
+pub(super) fn trait_impls_of_provider(tcx: TyCtxt<'_>, trait_id: DefId) -> TraitImpls {
+    let mut impls = TraitImpls::default();
+
+    // Traits defined in the current crate can't have impls in upstream
+    // crates, so we don't bother querying the cstore.
+    if !trait_id.is_local() {
+        for &cnum in tcx.crates(()).iter() {
+            for &(impl_def_id, simplified_self_ty) in
+                tcx.implementations_of_trait((cnum, trait_id)).iter()
+            {
+                if let Some(simplified_self_ty) = simplified_self_ty {
+                    impls
+                        .non_blanket_impls
+                        .entry(simplified_self_ty)
+                        .or_default()
+                        .push(impl_def_id);
+                } else {
+                    impls.blanket_impls.push(impl_def_id);
+                }
+            }
+        }
+    }
+
+    for &impl_def_id in tcx.hir().trait_impls(trait_id) {
+        let impl_def_id = impl_def_id.to_def_id();
+
+        let impl_self_ty = tcx.type_of(impl_def_id);
+        if impl_self_ty.references_error() {
+            continue;
+        }
+
+        if let Some(simplified_self_ty) =
+            fast_reject::simplify_type(tcx, impl_self_ty, TreatParams::AsInfer)
+        {
+            impls.non_blanket_impls.entry(simplified_self_ty).or_default().push(impl_def_id);
+        } else {
+            impls.blanket_impls.push(impl_def_id);
+        }
+    }
+
+    impls
+}
+
+// Query provider for `incoherent_impls`.
+#[instrument(level = "debug", skip(tcx))]
+pub(super) fn incoherent_impls_provider(tcx: TyCtxt<'_>, simp: SimplifiedType) -> &[DefId] {
+    let mut impls = Vec::new();
+
+    for cnum in iter::once(LOCAL_CRATE).chain(tcx.crates(()).iter().copied()) {
+        for &impl_def_id in tcx.crate_incoherent_impls((cnum, simp)) {
+            impls.push(impl_def_id)
+        }
+    }
+
+    debug!(?impls);
+
+    tcx.arena.alloc_slice(&impls)
+}
diff --git a/compiler/rustc_middle/src/ty/util.rs b/compiler/rustc_middle/src/ty/util.rs
new file mode 100644
index 000000000..591bb7831
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/util.rs
@@ -0,0 +1,1294 @@
+//! Miscellaneous type-system utilities that are too small to deserve their own modules.
+
+use crate::middle::codegen_fn_attrs::CodegenFnAttrFlags;
+use crate::ty::layout::IntegerExt;
+use crate::ty::query::TyCtxtAt;
+use crate::ty::subst::{GenericArgKind, Subst, SubstsRef};
+use crate::ty::{
+    self, DefIdTree, FallibleTypeFolder, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable,
+    TypeVisitable,
+};
+use rustc_apfloat::Float as _;
+use rustc_ast as ast;
+use rustc_attr::{self as attr, SignedInt, UnsignedInt};
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_errors::ErrorGuaranteed;
+use rustc_hir as hir;
+use rustc_hir::def::{CtorOf, DefKind, Res};
+use rustc_hir::def_id::DefId;
+use rustc_index::bit_set::GrowableBitSet;
+use rustc_macros::HashStable;
+use rustc_span::{sym, DUMMY_SP};
+use rustc_target::abi::{Integer, Size, TargetDataLayout};
+use rustc_target::spec::abi::Abi;
+use smallvec::SmallVec;
+use std::{fmt, iter};
+
+#[derive(Copy, Clone, Debug)]
+pub struct Discr<'tcx> {
+    /// Bit representation of the discriminant (e.g., `-128i8` is `0xFF_u128`).
+    pub val: u128,
+    pub ty: Ty<'tcx>,
+}
+
+/// Used as an input to [`TyCtxt::uses_unique_generic_params`].
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum IgnoreRegions {
+    Yes,
+    No,
+}
+
+#[derive(Copy, Clone, Debug)]
+pub enum NotUniqueParam<'tcx> {
+    DuplicateParam(ty::GenericArg<'tcx>),
+    NotParam(ty::GenericArg<'tcx>),
+}
+
+impl<'tcx> fmt::Display for Discr<'tcx> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self.ty.kind() {
+            ty::Int(ity) => {
+                let size = ty::tls::with(|tcx| Integer::from_int_ty(&tcx, ity).size());
+                let x = self.val;
+                // sign extend the raw representation to be an i128
+                let x = size.sign_extend(x) as i128;
+                write!(fmt, "{}", x)
+            }
+            _ => write!(fmt, "{}", self.val),
+        }
+    }
+}
+
+fn int_size_and_signed<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> (Size, bool) {
+    let (int, signed) = match *ty.kind() {
+        ty::Int(ity) => (Integer::from_int_ty(&tcx, ity), true),
+        ty::Uint(uty) => (Integer::from_uint_ty(&tcx, uty), false),
+        _ => bug!("non integer discriminant"),
+    };
+    (int.size(), signed)
+}
+
+impl<'tcx> Discr<'tcx> {
+    /// Adds `1` to the value and wraps around if the maximum for the type is reached.
+    pub fn wrap_incr(self, tcx: TyCtxt<'tcx>) -> Self {
+        self.checked_add(tcx, 1).0
+    }
+    pub fn checked_add(self, tcx: TyCtxt<'tcx>, n: u128) -> (Self, bool) {
+        let (size, signed) = int_size_and_signed(tcx, self.ty);
+        let (val, oflo) = if signed {
+            let min = size.signed_int_min();
+            let max = size.signed_int_max();
+            let val = size.sign_extend(self.val) as i128;
+            assert!(n < (i128::MAX as u128));
+            let n = n as i128;
+            let oflo = val > max - n;
+            let val = if oflo { min + (n - (max - val) - 1) } else { val + n };
+            // zero the upper bits
+            let val = val as u128;
+            let val = size.truncate(val);
+            (val, oflo)
+        } else {
+            let max = size.unsigned_int_max();
+            let val = self.val;
+            let oflo = val > max - n;
+            let val = if oflo { n - (max - val) - 1 } else { val + n };
+            (val, oflo)
+        };
+        (Self { val, ty: self.ty }, oflo)
+    }
+}
+
+pub trait IntTypeExt {
+    fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
+    fn disr_incr<'tcx>(&self, tcx: TyCtxt<'tcx>, val: Option<Discr<'tcx>>) -> Option<Discr<'tcx>>;
+    fn initial_discriminant<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Discr<'tcx>;
+}
+
+impl IntTypeExt for attr::IntType {
+    fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        match *self {
+            SignedInt(ast::IntTy::I8) => tcx.types.i8,
+            SignedInt(ast::IntTy::I16) => tcx.types.i16,
+            SignedInt(ast::IntTy::I32) => tcx.types.i32,
+            SignedInt(ast::IntTy::I64) => tcx.types.i64,
+            SignedInt(ast::IntTy::I128) => tcx.types.i128,
+            SignedInt(ast::IntTy::Isize) => tcx.types.isize,
+            UnsignedInt(ast::UintTy::U8) => tcx.types.u8,
+            UnsignedInt(ast::UintTy::U16) => tcx.types.u16,
+            UnsignedInt(ast::UintTy::U32) => tcx.types.u32,
+            UnsignedInt(ast::UintTy::U64) => tcx.types.u64,
+            UnsignedInt(ast::UintTy::U128) => tcx.types.u128,
+            UnsignedInt(ast::UintTy::Usize) => tcx.types.usize,
+        }
+    }
+
+    fn initial_discriminant<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Discr<'tcx> {
+        Discr { val: 0, ty: self.to_ty(tcx) }
+    }
+
+    fn disr_incr<'tcx>(&self, tcx: TyCtxt<'tcx>, val: Option<Discr<'tcx>>) -> Option<Discr<'tcx>> {
+        if let Some(val) = val {
+            assert_eq!(self.to_ty(tcx), val.ty);
+            let (new, oflo) = val.checked_add(tcx, 1);
+            if oflo { None } else { Some(new) }
+        } else {
+            Some(self.initial_discriminant(tcx))
+        }
+    }
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Creates a hash of the type `Ty` which will be the same no matter what crate
+    /// context it's calculated within. This is used by the `type_id` intrinsic.
+    pub fn type_id_hash(self, ty: Ty<'tcx>) -> u64 {
+        // We want the type_id be independent of the types free regions, so we
+        // erase them. The erase_regions() call will also anonymize bound
+        // regions, which is desirable too.
+        let ty = self.erase_regions(ty);
+
+        self.with_stable_hashing_context(|mut hcx| {
+            let mut hasher = StableHasher::new();
+            hcx.while_hashing_spans(false, |hcx| ty.hash_stable(hcx, &mut hasher));
+            hasher.finish()
+        })
+    }
+
+    pub fn res_generics_def_id(self, res: Res) -> Option<DefId> {
+        match res {
+            Res::Def(DefKind::Ctor(CtorOf::Variant, _), def_id) => {
+                Some(self.parent(self.parent(def_id)))
+            }
+            Res::Def(DefKind::Variant | DefKind::Ctor(CtorOf::Struct, _), def_id) => {
+                Some(self.parent(def_id))
+            }
+            // Other `DefKind`s don't have generics and would ICE when calling
+            // `generics_of`.
+            Res::Def(
+                DefKind::Struct
+                | DefKind::Union
+                | DefKind::Enum
+                | DefKind::Trait
+                | DefKind::OpaqueTy
+                | DefKind::TyAlias
+                | DefKind::ForeignTy
+                | DefKind::TraitAlias
+                | DefKind::AssocTy
+                | DefKind::Fn
+                | DefKind::AssocFn
+                | DefKind::AssocConst
+                | DefKind::Impl,
+                def_id,
+            ) => Some(def_id),
+            Res::Err => None,
+            _ => None,
+        }
+    }
+
+    pub fn has_error_field(self, ty: Ty<'tcx>) -> bool {
+        if let ty::Adt(def, substs) = *ty.kind() {
+            for field in def.all_fields() {
+                let field_ty = field.ty(self, substs);
+                if let ty::Error(_) = field_ty.kind() {
+                    return true;
+                }
+            }
+        }
+        false
+    }
+
+    /// Attempts to returns the deeply last field of nested structures, but
+    /// does not apply any normalization in its search. Returns the same type
+    /// if input `ty` is not a structure at all.
+    pub fn struct_tail_without_normalization(self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        let tcx = self;
+        tcx.struct_tail_with_normalize(ty, |ty| ty, || {})
+    }
+
+    /// Returns the deeply last field of nested structures, or the same type if
+    /// not a structure at all. Corresponds to the only possible unsized field,
+    /// and its type can be used to determine unsizing strategy.
+    ///
+    /// Should only be called if `ty` has no inference variables and does not
+    /// need its lifetimes preserved (e.g. as part of codegen); otherwise
+    /// normalization attempt may cause compiler bugs.
+    pub fn struct_tail_erasing_lifetimes(
+        self,
+        ty: Ty<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> Ty<'tcx> {
+        let tcx = self;
+        tcx.struct_tail_with_normalize(ty, |ty| tcx.normalize_erasing_regions(param_env, ty), || {})
+    }
+
+    /// Returns the deeply last field of nested structures, or the same type if
+    /// not a structure at all. Corresponds to the only possible unsized field,
+    /// and its type can be used to determine unsizing strategy.
+    ///
+    /// This is parameterized over the normalization strategy (i.e. how to
+    /// handle `<T as Trait>::Assoc` and `impl Trait`); pass the identity
+    /// function to indicate no normalization should take place.
+    ///
+    /// See also `struct_tail_erasing_lifetimes`, which is suitable for use
+    /// during codegen.
+    pub fn struct_tail_with_normalize(
+        self,
+        mut ty: Ty<'tcx>,
+        mut normalize: impl FnMut(Ty<'tcx>) -> Ty<'tcx>,
+        // This is currently used to allow us to walk a ValTree
+        // in lockstep with the type in order to get the ValTree branch that
+        // corresponds to an unsized field.
+        mut f: impl FnMut() -> (),
+    ) -> Ty<'tcx> {
+        let recursion_limit = self.recursion_limit();
+        for iteration in 0.. {
+            if !recursion_limit.value_within_limit(iteration) {
+                return self.ty_error_with_message(
+                    DUMMY_SP,
+                    &format!("reached the recursion limit finding the struct tail for {}", ty),
+                );
+            }
+            match *ty.kind() {
+                ty::Adt(def, substs) => {
+                    if !def.is_struct() {
+                        break;
+                    }
+                    match def.non_enum_variant().fields.last() {
+                        Some(field) => {
+                            f();
+                            ty = field.ty(self, substs);
+                        }
+                        None => break,
+                    }
+                }
+
+                ty::Tuple(tys) if let Some((&last_ty, _)) = tys.split_last() => {
+                    f();
+                    ty = last_ty;
+                }
+
+                ty::Tuple(_) => break,
+
+                ty::Projection(_) | ty::Opaque(..) => {
+                    let normalized = normalize(ty);
+                    if ty == normalized {
+                        return ty;
+                    } else {
+                        ty = normalized;
+                    }
+                }
+
+                _ => {
+                    break;
+                }
+            }
+        }
+        ty
+    }
+
+    /// Same as applying `struct_tail` on `source` and `target`, but only
+    /// keeps going as long as the two types are instances of the same
+    /// structure definitions.
+    /// For `(Foo<Foo<T>>, Foo<dyn Trait>)`, the result will be `(Foo<T>, Trait)`,
+    /// whereas struct_tail produces `T`, and `Trait`, respectively.
+    ///
+    /// Should only be called if the types have no inference variables and do
+    /// not need their lifetimes preserved (e.g., as part of codegen); otherwise,
+    /// normalization attempt may cause compiler bugs.
+    pub fn struct_lockstep_tails_erasing_lifetimes(
+        self,
+        source: Ty<'tcx>,
+        target: Ty<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> (Ty<'tcx>, Ty<'tcx>) {
+        let tcx = self;
+        tcx.struct_lockstep_tails_with_normalize(source, target, |ty| {
+            tcx.normalize_erasing_regions(param_env, ty)
+        })
+    }
+
+    /// Same as applying `struct_tail` on `source` and `target`, but only
+    /// keeps going as long as the two types are instances of the same
+    /// structure definitions.
+    /// For `(Foo<Foo<T>>, Foo<dyn Trait>)`, the result will be `(Foo<T>, Trait)`,
+    /// whereas struct_tail produces `T`, and `Trait`, respectively.
+    ///
+    /// See also `struct_lockstep_tails_erasing_lifetimes`, which is suitable for use
+    /// during codegen.
+    pub fn struct_lockstep_tails_with_normalize(
+        self,
+        source: Ty<'tcx>,
+        target: Ty<'tcx>,
+        normalize: impl Fn(Ty<'tcx>) -> Ty<'tcx>,
+    ) -> (Ty<'tcx>, Ty<'tcx>) {
+        let (mut a, mut b) = (source, target);
+        loop {
+            match (&a.kind(), &b.kind()) {
+                (&ty::Adt(a_def, a_substs), &ty::Adt(b_def, b_substs))
+                    if a_def == b_def && a_def.is_struct() =>
+                {
+                    if let Some(f) = a_def.non_enum_variant().fields.last() {
+                        a = f.ty(self, a_substs);
+                        b = f.ty(self, b_substs);
+                    } else {
+                        break;
+                    }
+                }
+                (&ty::Tuple(a_tys), &ty::Tuple(b_tys)) if a_tys.len() == b_tys.len() => {
+                    if let Some(&a_last) = a_tys.last() {
+                        a = a_last;
+                        b = *b_tys.last().unwrap();
+                    } else {
+                        break;
+                    }
+                }
+                (ty::Projection(_) | ty::Opaque(..), _)
+                | (_, ty::Projection(_) | ty::Opaque(..)) => {
+                    // If either side is a projection, attempt to
+                    // progress via normalization. (Should be safe to
+                    // apply to both sides as normalization is
+                    // idempotent.)
+                    let a_norm = normalize(a);
+                    let b_norm = normalize(b);
+                    if a == a_norm && b == b_norm {
+                        break;
+                    } else {
+                        a = a_norm;
+                        b = b_norm;
+                    }
+                }
+
+                _ => break,
+            }
+        }
+        (a, b)
+    }
+
+    /// Calculate the destructor of a given type.
+    pub fn calculate_dtor(
+        self,
+        adt_did: DefId,
+        validate: impl Fn(Self, DefId) -> Result<(), ErrorGuaranteed>,
+    ) -> Option<ty::Destructor> {
+        let drop_trait = self.lang_items().drop_trait()?;
+        self.ensure().coherent_trait(drop_trait);
+
+        let ty = self.type_of(adt_did);
+        let (did, constness) = self.find_map_relevant_impl(drop_trait, ty, |impl_did| {
+            if let Some(item_id) = self.associated_item_def_ids(impl_did).first() {
+                if validate(self, impl_did).is_ok() {
+                    return Some((*item_id, self.constness(impl_did)));
+                }
+            }
+            None
+        })?;
+
+        Some(ty::Destructor { did, constness })
+    }
+
+    /// Returns the set of types that are required to be alive in
+    /// order to run the destructor of `def` (see RFCs 769 and
+    /// 1238).
+    ///
+    /// Note that this returns only the constraints for the
+    /// destructor of `def` itself. For the destructors of the
+    /// contents, you need `adt_dtorck_constraint`.
+    pub fn destructor_constraints(self, def: ty::AdtDef<'tcx>) -> Vec<ty::subst::GenericArg<'tcx>> {
+        let dtor = match def.destructor(self) {
+            None => {
+                debug!("destructor_constraints({:?}) - no dtor", def.did());
+                return vec![];
+            }
+            Some(dtor) => dtor.did,
+        };
+
+        let impl_def_id = self.parent(dtor);
+        let impl_generics = self.generics_of(impl_def_id);
+
+        // We have a destructor - all the parameters that are not
+        // pure_wrt_drop (i.e, don't have a #[may_dangle] attribute)
+        // must be live.
+
+        // We need to return the list of parameters from the ADTs
+        // generics/substs that correspond to impure parameters on the
+        // impl's generics. This is a bit ugly, but conceptually simple:
+        //
+        // Suppose our ADT looks like the following
+        //
+        //     struct S<X, Y, Z>(X, Y, Z);
+        //
+        // and the impl is
+        //
+        //     impl<#[may_dangle] P0, P1, P2> Drop for S<P1, P2, P0>
+        //
+        // We want to return the parameters (X, Y). For that, we match
+        // up the item-substs <X, Y, Z> with the substs on the impl ADT,
+        // <P1, P2, P0>, and then look up which of the impl substs refer to
+        // parameters marked as pure.
+
+        let impl_substs = match *self.type_of(impl_def_id).kind() {
+            ty::Adt(def_, substs) if def_ == def => substs,
+            _ => bug!(),
+        };
+
+        let item_substs = match *self.type_of(def.did()).kind() {
+            ty::Adt(def_, substs) if def_ == def => substs,
+            _ => bug!(),
+        };
+
+        let result = iter::zip(item_substs, impl_substs)
+            .filter(|&(_, k)| {
+                match k.unpack() {
+                    GenericArgKind::Lifetime(region) => match region.kind() {
+                        ty::ReEarlyBound(ref ebr) => {
+                            !impl_generics.region_param(ebr, self).pure_wrt_drop
+                        }
+                        // Error: not a region param
+                        _ => false,
+                    },
+                    GenericArgKind::Type(ty) => match ty.kind() {
+                        ty::Param(ref pt) => !impl_generics.type_param(pt, self).pure_wrt_drop,
+                        // Error: not a type param
+                        _ => false,
+                    },
+                    GenericArgKind::Const(ct) => match ct.kind() {
+                        ty::ConstKind::Param(ref pc) => {
+                            !impl_generics.const_param(pc, self).pure_wrt_drop
+                        }
+                        // Error: not a const param
+                        _ => false,
+                    },
+                }
+            })
+            .map(|(item_param, _)| item_param)
+            .collect();
+        debug!("destructor_constraint({:?}) = {:?}", def.did(), result);
+        result
+    }
+
+    /// Checks whether each generic argument is simply a unique generic parameter.
+    pub fn uses_unique_generic_params(
+        self,
+        substs: SubstsRef<'tcx>,
+        ignore_regions: IgnoreRegions,
+    ) -> Result<(), NotUniqueParam<'tcx>> {
+        let mut seen = GrowableBitSet::default();
+        for arg in substs {
+            match arg.unpack() {
+                GenericArgKind::Lifetime(lt) => {
+                    if ignore_regions == IgnoreRegions::No {
+                        let ty::ReEarlyBound(p) = lt.kind() else {
+                            return Err(NotUniqueParam::NotParam(lt.into()))
+                        };
+                        if !seen.insert(p.index) {
+                            return Err(NotUniqueParam::DuplicateParam(lt.into()));
+                        }
+                    }
+                }
+                GenericArgKind::Type(t) => match t.kind() {
+                    ty::Param(p) => {
+                        if !seen.insert(p.index) {
+                            return Err(NotUniqueParam::DuplicateParam(t.into()));
+                        }
+                    }
+                    _ => return Err(NotUniqueParam::NotParam(t.into())),
+                },
+                GenericArgKind::Const(c) => match c.kind() {
+                    ty::ConstKind::Param(p) => {
+                        if !seen.insert(p.index) {
+                            return Err(NotUniqueParam::DuplicateParam(c.into()));
+                        }
+                    }
+                    _ => return Err(NotUniqueParam::NotParam(c.into())),
+                },
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Returns `true` if `def_id` refers to a closure (e.g., `|x| x * 2`). Note
+    /// that closures have a `DefId`, but the closure *expression* also
+    /// has a `HirId` that is located within the context where the
+    /// closure appears (and, sadly, a corresponding `NodeId`, since
+    /// those are not yet phased out). The parent of the closure's
+    /// `DefId` will also be the context where it appears.
+    pub fn is_closure(self, def_id: DefId) -> bool {
+        matches!(self.def_kind(def_id), DefKind::Closure | DefKind::Generator)
+    }
+
+    /// Returns `true` if `def_id` refers to a definition that does not have its own
+    /// type-checking context, i.e. closure, generator or inline const.
+    pub fn is_typeck_child(self, def_id: DefId) -> bool {
+        matches!(
+            self.def_kind(def_id),
+            DefKind::Closure | DefKind::Generator | DefKind::InlineConst
+        )
+    }
+
+    /// Returns `true` if `def_id` refers to a trait (i.e., `trait Foo { ... }`).
+    pub fn is_trait(self, def_id: DefId) -> bool {
+        self.def_kind(def_id) == DefKind::Trait
+    }
+
+    /// Returns `true` if `def_id` refers to a trait alias (i.e., `trait Foo = ...;`),
+    /// and `false` otherwise.
+    pub fn is_trait_alias(self, def_id: DefId) -> bool {
+        self.def_kind(def_id) == DefKind::TraitAlias
+    }
+
+    /// Returns `true` if this `DefId` refers to the implicit constructor for
+    /// a tuple struct like `struct Foo(u32)`, and `false` otherwise.
+    pub fn is_constructor(self, def_id: DefId) -> bool {
+        matches!(self.def_kind(def_id), DefKind::Ctor(..))
+    }
+
+    /// Given the `DefId`, returns the `DefId` of the innermost item that
+    /// has its own type-checking context or "inference environment".
+    ///
+    /// For example, a closure has its own `DefId`, but it is type-checked
+    /// with the containing item. Similarly, an inline const block has its
+    /// own `DefId` but it is type-checked together with the containing item.
+    ///
+    /// Therefore, when we fetch the
+    /// `typeck` the closure, for example, we really wind up
+    /// fetching the `typeck` the enclosing fn item.
+    pub fn typeck_root_def_id(self, def_id: DefId) -> DefId {
+        let mut def_id = def_id;
+        while self.is_typeck_child(def_id) {
+            def_id = self.parent(def_id);
+        }
+        def_id
+    }
+
+    /// Given the `DefId` and substs a closure, creates the type of
+    /// `self` argument that the closure expects. For example, for a
+    /// `Fn` closure, this would return a reference type `&T` where
+    /// `T = closure_ty`.
+    ///
+    /// Returns `None` if this closure's kind has not yet been inferred.
+    /// This should only be possible during type checking.
+    ///
+    /// Note that the return value is a late-bound region and hence
+    /// wrapped in a binder.
+    pub fn closure_env_ty(
+        self,
+        closure_def_id: DefId,
+        closure_substs: SubstsRef<'tcx>,
+        env_region: ty::RegionKind<'tcx>,
+    ) -> Option<Ty<'tcx>> {
+        let closure_ty = self.mk_closure(closure_def_id, closure_substs);
+        let closure_kind_ty = closure_substs.as_closure().kind_ty();
+        let closure_kind = closure_kind_ty.to_opt_closure_kind()?;
+        let env_ty = match closure_kind {
+            ty::ClosureKind::Fn => self.mk_imm_ref(self.mk_region(env_region), closure_ty),
+            ty::ClosureKind::FnMut => self.mk_mut_ref(self.mk_region(env_region), closure_ty),
+            ty::ClosureKind::FnOnce => closure_ty,
+        };
+        Some(env_ty)
+    }
+
+    /// Returns `true` if the node pointed to by `def_id` is a `static` item.
+    #[inline]
+    pub fn is_static(self, def_id: DefId) -> bool {
+        matches!(self.def_kind(def_id), DefKind::Static(_))
+    }
+
+    #[inline]
+    pub fn static_mutability(self, def_id: DefId) -> Option<hir::Mutability> {
+        if let DefKind::Static(mt) = self.def_kind(def_id) { Some(mt) } else { None }
+    }
+
+    /// Returns `true` if this is a `static` item with the `#[thread_local]` attribute.
+    pub fn is_thread_local_static(self, def_id: DefId) -> bool {
+        self.codegen_fn_attrs(def_id).flags.contains(CodegenFnAttrFlags::THREAD_LOCAL)
+    }
+
+    /// Returns `true` if the node pointed to by `def_id` is a mutable `static` item.
+    #[inline]
+    pub fn is_mutable_static(self, def_id: DefId) -> bool {
+        self.static_mutability(def_id) == Some(hir::Mutability::Mut)
+    }
+
+    /// Get the type of the pointer to the static that we use in MIR.
+    pub fn static_ptr_ty(self, def_id: DefId) -> Ty<'tcx> {
+        // Make sure that any constants in the static's type are evaluated.
+        let static_ty = self.normalize_erasing_regions(ty::ParamEnv::empty(), self.type_of(def_id));
+
+        // Make sure that accesses to unsafe statics end up using raw pointers.
+        // For thread-locals, this needs to be kept in sync with `Rvalue::ty`.
+        if self.is_mutable_static(def_id) {
+            self.mk_mut_ptr(static_ty)
+        } else if self.is_foreign_item(def_id) {
+            self.mk_imm_ptr(static_ty)
+        } else {
+            self.mk_imm_ref(self.lifetimes.re_erased, static_ty)
+        }
+    }
+
+    /// Expands the given impl trait type, stopping if the type is recursive.
+    #[instrument(skip(self), level = "debug")]
+    pub fn try_expand_impl_trait_type(
+        self,
+        def_id: DefId,
+        substs: SubstsRef<'tcx>,
+    ) -> Result<Ty<'tcx>, Ty<'tcx>> {
+        let mut visitor = OpaqueTypeExpander {
+            seen_opaque_tys: FxHashSet::default(),
+            expanded_cache: FxHashMap::default(),
+            primary_def_id: Some(def_id),
+            found_recursion: false,
+            found_any_recursion: false,
+            check_recursion: true,
+            tcx: self,
+        };
+
+        let expanded_type = visitor.expand_opaque_ty(def_id, substs).unwrap();
+        trace!(?expanded_type);
+        if visitor.found_recursion { Err(expanded_type) } else { Ok(expanded_type) }
+    }
+
+    pub fn bound_type_of(self, def_id: DefId) -> ty::EarlyBinder<Ty<'tcx>> {
+        ty::EarlyBinder(self.type_of(def_id))
+    }
+
+    pub fn bound_fn_sig(self, def_id: DefId) -> ty::EarlyBinder<ty::PolyFnSig<'tcx>> {
+        ty::EarlyBinder(self.fn_sig(def_id))
+    }
+
+    pub fn bound_impl_trait_ref(
+        self,
+        def_id: DefId,
+    ) -> Option<ty::EarlyBinder<ty::TraitRef<'tcx>>> {
+        self.impl_trait_ref(def_id).map(|i| ty::EarlyBinder(i))
+    }
+
+    pub fn bound_explicit_item_bounds(
+        self,
+        def_id: DefId,
+    ) -> ty::EarlyBinder<&'tcx [(ty::Predicate<'tcx>, rustc_span::Span)]> {
+        ty::EarlyBinder(self.explicit_item_bounds(def_id))
+    }
+
+    pub fn bound_item_bounds(
+        self,
+        def_id: DefId,
+    ) -> ty::EarlyBinder<&'tcx ty::List<ty::Predicate<'tcx>>> {
+        ty::EarlyBinder(self.item_bounds(def_id))
+    }
+
+    pub fn bound_const_param_default(self, def_id: DefId) -> ty::EarlyBinder<ty::Const<'tcx>> {
+        ty::EarlyBinder(self.const_param_default(def_id))
+    }
+
+    pub fn bound_predicates_of(
+        self,
+        def_id: DefId,
+    ) -> ty::EarlyBinder<ty::generics::GenericPredicates<'tcx>> {
+        ty::EarlyBinder(self.predicates_of(def_id))
+    }
+
+    pub fn bound_explicit_predicates_of(
+        self,
+        def_id: DefId,
+    ) -> ty::EarlyBinder<ty::generics::GenericPredicates<'tcx>> {
+        ty::EarlyBinder(self.explicit_predicates_of(def_id))
+    }
+
+    pub fn bound_impl_subject(self, def_id: DefId) -> ty::EarlyBinder<ty::ImplSubject<'tcx>> {
+        ty::EarlyBinder(self.impl_subject(def_id))
+    }
+}
+
+struct OpaqueTypeExpander<'tcx> {
+    // Contains the DefIds of the opaque types that are currently being
+    // expanded. When we expand an opaque type we insert the DefId of
+    // that type, and when we finish expanding that type we remove the
+    // its DefId.
+    seen_opaque_tys: FxHashSet<DefId>,
+    // Cache of all expansions we've seen so far. This is a critical
+    // optimization for some large types produced by async fn trees.
+    expanded_cache: FxHashMap<(DefId, SubstsRef<'tcx>), Ty<'tcx>>,
+    primary_def_id: Option<DefId>,
+    found_recursion: bool,
+    found_any_recursion: bool,
+    /// Whether or not to check for recursive opaque types.
+    /// This is `true` when we're explicitly checking for opaque type
+    /// recursion, and 'false' otherwise to avoid unnecessary work.
+    check_recursion: bool,
+    tcx: TyCtxt<'tcx>,
+}
+
+impl<'tcx> OpaqueTypeExpander<'tcx> {
+    fn expand_opaque_ty(&mut self, def_id: DefId, substs: SubstsRef<'tcx>) -> Option<Ty<'tcx>> {
+        if self.found_any_recursion {
+            return None;
+        }
+        let substs = substs.fold_with(self);
+        if !self.check_recursion || self.seen_opaque_tys.insert(def_id) {
+            let expanded_ty = match self.expanded_cache.get(&(def_id, substs)) {
+                Some(expanded_ty) => *expanded_ty,
+                None => {
+                    let generic_ty = self.tcx.bound_type_of(def_id);
+                    let concrete_ty = generic_ty.subst(self.tcx, substs);
+                    let expanded_ty = self.fold_ty(concrete_ty);
+                    self.expanded_cache.insert((def_id, substs), expanded_ty);
+                    expanded_ty
+                }
+            };
+            if self.check_recursion {
+                self.seen_opaque_tys.remove(&def_id);
+            }
+            Some(expanded_ty)
+        } else {
+            // If another opaque type that we contain is recursive, then it
+            // will report the error, so we don't have to.
+            self.found_any_recursion = true;
+            self.found_recursion = def_id == *self.primary_def_id.as_ref().unwrap();
+            None
+        }
+    }
+}
+
+impl<'tcx> TypeFolder<'tcx> for OpaqueTypeExpander<'tcx> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+        if let ty::Opaque(def_id, substs) = *t.kind() {
+            self.expand_opaque_ty(def_id, substs).unwrap_or(t)
+        } else if t.has_opaque_types() {
+            t.super_fold_with(self)
+        } else {
+            t
+        }
+    }
+}
+
+impl<'tcx> Ty<'tcx> {
+    /// Returns the maximum value for the given numeric type (including `char`s)
+    /// or returns `None` if the type is not numeric.
+    pub fn numeric_max_val(self, tcx: TyCtxt<'tcx>) -> Option<ty::Const<'tcx>> {
+        let val = match self.kind() {
+            ty::Int(_) | ty::Uint(_) => {
+                let (size, signed) = int_size_and_signed(tcx, self);
+                let val =
+                    if signed { size.signed_int_max() as u128 } else { size.unsigned_int_max() };
+                Some(val)
+            }
+            ty::Char => Some(std::char::MAX as u128),
+            ty::Float(fty) => Some(match fty {
+                ty::FloatTy::F32 => rustc_apfloat::ieee::Single::INFINITY.to_bits(),
+                ty::FloatTy::F64 => rustc_apfloat::ieee::Double::INFINITY.to_bits(),
+            }),
+            _ => None,
+        };
+
+        val.map(|v| ty::Const::from_bits(tcx, v, ty::ParamEnv::empty().and(self)))
+    }
+
+    /// Returns the minimum value for the given numeric type (including `char`s)
+    /// or returns `None` if the type is not numeric.
+    pub fn numeric_min_val(self, tcx: TyCtxt<'tcx>) -> Option<ty::Const<'tcx>> {
+        let val = match self.kind() {
+            ty::Int(_) | ty::Uint(_) => {
+                let (size, signed) = int_size_and_signed(tcx, self);
+                let val = if signed { size.truncate(size.signed_int_min() as u128) } else { 0 };
+                Some(val)
+            }
+            ty::Char => Some(0),
+            ty::Float(fty) => Some(match fty {
+                ty::FloatTy::F32 => (-::rustc_apfloat::ieee::Single::INFINITY).to_bits(),
+                ty::FloatTy::F64 => (-::rustc_apfloat::ieee::Double::INFINITY).to_bits(),
+            }),
+            _ => None,
+        };
+
+        val.map(|v| ty::Const::from_bits(tcx, v, ty::ParamEnv::empty().and(self)))
+    }
+
+    /// Checks whether values of this type `T` are *moved* or *copied*
+    /// when referenced -- this amounts to a check for whether `T:
+    /// Copy`, but note that we **don't** consider lifetimes when
+    /// doing this check. This means that we may generate MIR which
+    /// does copies even when the type actually doesn't satisfy the
+    /// full requirements for the `Copy` trait (cc #29149) -- this
+    /// winds up being reported as an error during NLL borrow check.
+    pub fn is_copy_modulo_regions(
+        self,
+        tcx_at: TyCtxtAt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> bool {
+        self.is_trivially_pure_clone_copy() || tcx_at.is_copy_raw(param_env.and(self))
+    }
+
+    /// Checks whether values of this type `T` have a size known at
+    /// compile time (i.e., whether `T: Sized`). Lifetimes are ignored
+    /// for the purposes of this check, so it can be an
+    /// over-approximation in generic contexts, where one can have
+    /// strange rules like `<T as Foo<'static>>::Bar: Sized` that
+    /// actually carry lifetime requirements.
+    pub fn is_sized(self, tcx_at: TyCtxtAt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> bool {
+        self.is_trivially_sized(tcx_at.tcx) || tcx_at.is_sized_raw(param_env.and(self))
+    }
+
+    /// Checks whether values of this type `T` implement the `Freeze`
+    /// trait -- frozen types are those that do not contain an
+    /// `UnsafeCell` anywhere. This is a language concept used to
+    /// distinguish "true immutability", which is relevant to
+    /// optimization as well as the rules around static values. Note
+    /// that the `Freeze` trait is not exposed to end users and is
+    /// effectively an implementation detail.
+    pub fn is_freeze(self, tcx_at: TyCtxtAt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> bool {
+        self.is_trivially_freeze() || tcx_at.is_freeze_raw(param_env.and(self))
+    }
+
+    /// Fast path helper for testing if a type is `Freeze`.
+    ///
+    /// Returning true means the type is known to be `Freeze`. Returning
+    /// `false` means nothing -- could be `Freeze`, might not be.
+    fn is_trivially_freeze(self) -> bool {
+        match self.kind() {
+            ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Bool
+            | ty::Char
+            | ty::Str
+            | ty::Never
+            | ty::Ref(..)
+            | ty::RawPtr(_)
+            | ty::FnDef(..)
+            | ty::Error(_)
+            | ty::FnPtr(_) => true,
+            ty::Tuple(fields) => fields.iter().all(Self::is_trivially_freeze),
+            ty::Slice(elem_ty) | ty::Array(elem_ty, _) => elem_ty.is_trivially_freeze(),
+            ty::Adt(..)
+            | ty::Bound(..)
+            | ty::Closure(..)
+            | ty::Dynamic(..)
+            | ty::Foreign(_)
+            | ty::Generator(..)
+            | ty::GeneratorWitness(_)
+            | ty::Infer(_)
+            | ty::Opaque(..)
+            | ty::Param(_)
+            | ty::Placeholder(_)
+            | ty::Projection(_) => false,
+        }
+    }
+
+    /// Checks whether values of this type `T` implement the `Unpin` trait.
+    pub fn is_unpin(self, tcx_at: TyCtxtAt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> bool {
+        self.is_trivially_unpin() || tcx_at.is_unpin_raw(param_env.and(self))
+    }
+
+    /// Fast path helper for testing if a type is `Unpin`.
+    ///
+    /// Returning true means the type is known to be `Unpin`. Returning
+    /// `false` means nothing -- could be `Unpin`, might not be.
+    fn is_trivially_unpin(self) -> bool {
+        match self.kind() {
+            ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Bool
+            | ty::Char
+            | ty::Str
+            | ty::Never
+            | ty::Ref(..)
+            | ty::RawPtr(_)
+            | ty::FnDef(..)
+            | ty::Error(_)
+            | ty::FnPtr(_) => true,
+            ty::Tuple(fields) => fields.iter().all(Self::is_trivially_unpin),
+            ty::Slice(elem_ty) | ty::Array(elem_ty, _) => elem_ty.is_trivially_unpin(),
+            ty::Adt(..)
+            | ty::Bound(..)
+            | ty::Closure(..)
+            | ty::Dynamic(..)
+            | ty::Foreign(_)
+            | ty::Generator(..)
+            | ty::GeneratorWitness(_)
+            | ty::Infer(_)
+            | ty::Opaque(..)
+            | ty::Param(_)
+            | ty::Placeholder(_)
+            | ty::Projection(_) => false,
+        }
+    }
+
+    /// If `ty.needs_drop(...)` returns `true`, then `ty` is definitely
+    /// non-copy and *might* have a destructor attached; if it returns
+    /// `false`, then `ty` definitely has no destructor (i.e., no drop glue).
+    ///
+    /// (Note that this implies that if `ty` has a destructor attached,
+    /// then `needs_drop` will definitely return `true` for `ty`.)
+    ///
+    /// Note that this method is used to check eligible types in unions.
+    #[inline]
+    pub fn needs_drop(self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> bool {
+        // Avoid querying in simple cases.
+        match needs_drop_components(self, &tcx.data_layout) {
+            Err(AlwaysRequiresDrop) => true,
+            Ok(components) => {
+                let query_ty = match *components {
+                    [] => return false,
+                    // If we've got a single component, call the query with that
+                    // to increase the chance that we hit the query cache.
+                    [component_ty] => component_ty,
+                    _ => self,
+                };
+
+                // This doesn't depend on regions, so try to minimize distinct
+                // query keys used.
+                // If normalization fails, we just use `query_ty`.
+                let query_ty =
+                    tcx.try_normalize_erasing_regions(param_env, query_ty).unwrap_or(query_ty);
+
+                tcx.needs_drop_raw(param_env.and(query_ty))
+            }
+        }
+    }
+
+    /// Checks if `ty` has has a significant drop.
+    ///
+    /// Note that this method can return false even if `ty` has a destructor
+    /// attached; even if that is the case then the adt has been marked with
+    /// the attribute `rustc_insignificant_dtor`.
+    ///
+    /// Note that this method is used to check for change in drop order for
+    /// 2229 drop reorder migration analysis.
+    #[inline]
+    pub fn has_significant_drop(self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> bool {
+        // Avoid querying in simple cases.
+        match needs_drop_components(self, &tcx.data_layout) {
+            Err(AlwaysRequiresDrop) => true,
+            Ok(components) => {
+                let query_ty = match *components {
+                    [] => return false,
+                    // If we've got a single component, call the query with that
+                    // to increase the chance that we hit the query cache.
+                    [component_ty] => component_ty,
+                    _ => self,
+                };
+
+                // FIXME(#86868): We should be canonicalizing, or else moving this to a method of inference
+                // context, or *something* like that, but for now just avoid passing inference
+                // variables to queries that can't cope with them. Instead, conservatively
+                // return "true" (may change drop order).
+                if query_ty.needs_infer() {
+                    return true;
+                }
+
+                // This doesn't depend on regions, so try to minimize distinct
+                // query keys used.
+                let erased = tcx.normalize_erasing_regions(param_env, query_ty);
+                tcx.has_significant_drop_raw(param_env.and(erased))
+            }
+        }
+    }
+
+    /// Returns `true` if equality for this type is both reflexive and structural.
+    ///
+    /// Reflexive equality for a type is indicated by an `Eq` impl for that type.
+    ///
+    /// Primitive types (`u32`, `str`) have structural equality by definition. For composite data
+    /// types, equality for the type as a whole is structural when it is the same as equality
+    /// between all components (fields, array elements, etc.) of that type. For ADTs, structural
+    /// equality is indicated by an implementation of `PartialStructuralEq` and `StructuralEq` for
+    /// that type.
+    ///
+    /// This function is "shallow" because it may return `true` for a composite type whose fields
+    /// are not `StructuralEq`. For example, `[T; 4]` has structural equality regardless of `T`
+    /// because equality for arrays is determined by the equality of each array element. If you
+    /// want to know whether a given call to `PartialEq::eq` will proceed structurally all the way
+    /// down, you will need to use a type visitor.
+    #[inline]
+    pub fn is_structural_eq_shallow(self, tcx: TyCtxt<'tcx>) -> bool {
+        match self.kind() {
+            // Look for an impl of both `PartialStructuralEq` and `StructuralEq`.
+            ty::Adt(..) => tcx.has_structural_eq_impls(self),
+
+            // Primitive types that satisfy `Eq`.
+            ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Str | ty::Never => true,
+
+            // Composite types that satisfy `Eq` when all of their fields do.
+            //
+            // Because this function is "shallow", we return `true` for these composites regardless
+            // of the type(s) contained within.
+            ty::Ref(..) | ty::Array(..) | ty::Slice(_) | ty::Tuple(..) => true,
+
+            // Raw pointers use bitwise comparison.
+            ty::RawPtr(_) | ty::FnPtr(_) => true,
+
+            // Floating point numbers are not `Eq`.
+            ty::Float(_) => false,
+
+            // Conservatively return `false` for all others...
+
+            // Anonymous function types
+            ty::FnDef(..) | ty::Closure(..) | ty::Dynamic(..) | ty::Generator(..) => false,
+
+            // Generic or inferred types
+            //
+            // FIXME(ecstaticmorse): Maybe we should `bug` here? This should probably only be
+            // called for known, fully-monomorphized types.
+            ty::Projection(_)
+            | ty::Opaque(..)
+            | ty::Param(_)
+            | ty::Bound(..)
+            | ty::Placeholder(_)
+            | ty::Infer(_) => false,
+
+            ty::Foreign(_) | ty::GeneratorWitness(..) | ty::Error(_) => false,
+        }
+    }
+
+    /// Peel off all reference types in this type until there are none left.
+    ///
+    /// This method is idempotent, i.e. `ty.peel_refs().peel_refs() == ty.peel_refs()`.
+    ///
+    /// # Examples
+    ///
+    /// - `u8` -> `u8`
+    /// - `&'a mut u8` -> `u8`
+    /// - `&'a &'b u8` -> `u8`
+    /// - `&'a *const &'b u8 -> *const &'b u8`
+    pub fn peel_refs(self) -> Ty<'tcx> {
+        let mut ty = self;
+        while let ty::Ref(_, inner_ty, _) = ty.kind() {
+            ty = *inner_ty;
+        }
+        ty
+    }
+
+    #[inline]
+    pub fn outer_exclusive_binder(self) -> ty::DebruijnIndex {
+        self.0.outer_exclusive_binder
+    }
+}
+
+pub enum ExplicitSelf<'tcx> {
+    ByValue,
+    ByReference(ty::Region<'tcx>, hir::Mutability),
+    ByRawPointer(hir::Mutability),
+    ByBox,
+    Other,
+}
+
+impl<'tcx> ExplicitSelf<'tcx> {
+    /// Categorizes an explicit self declaration like `self: SomeType`
+    /// into either `self`, `&self`, `&mut self`, `Box<self>`, or
+    /// `Other`.
+    /// This is mainly used to require the arbitrary_self_types feature
+    /// in the case of `Other`, to improve error messages in the common cases,
+    /// and to make `Other` non-object-safe.
+    ///
+    /// Examples:
+    ///
+    /// ```ignore (illustrative)
+    /// impl<'a> Foo for &'a T {
+    ///     // Legal declarations:
+    ///     fn method1(self: &&'a T); // ExplicitSelf::ByReference
+    ///     fn method2(self: &'a T); // ExplicitSelf::ByValue
+    ///     fn method3(self: Box<&'a T>); // ExplicitSelf::ByBox
+    ///     fn method4(self: Rc<&'a T>); // ExplicitSelf::Other
+    ///
+    ///     // Invalid cases will be caught by `check_method_receiver`:
+    ///     fn method_err1(self: &'a mut T); // ExplicitSelf::Other
+    ///     fn method_err2(self: &'static T) // ExplicitSelf::ByValue
+    ///     fn method_err3(self: &&T) // ExplicitSelf::ByReference
+    /// }
+    /// ```
+    ///
+    pub fn determine<P>(self_arg_ty: Ty<'tcx>, is_self_ty: P) -> ExplicitSelf<'tcx>
+    where
+        P: Fn(Ty<'tcx>) -> bool,
+    {
+        use self::ExplicitSelf::*;
+
+        match *self_arg_ty.kind() {
+            _ if is_self_ty(self_arg_ty) => ByValue,
+            ty::Ref(region, ty, mutbl) if is_self_ty(ty) => ByReference(region, mutbl),
+            ty::RawPtr(ty::TypeAndMut { ty, mutbl }) if is_self_ty(ty) => ByRawPointer(mutbl),
+            ty::Adt(def, _) if def.is_box() && is_self_ty(self_arg_ty.boxed_ty()) => ByBox,
+            _ => Other,
+        }
+    }
+}
+
+/// Returns a list of types such that the given type needs drop if and only if
+/// *any* of the returned types need drop. Returns `Err(AlwaysRequiresDrop)` if
+/// this type always needs drop.
+pub fn needs_drop_components<'tcx>(
+    ty: Ty<'tcx>,
+    target_layout: &TargetDataLayout,
+) -> Result<SmallVec<[Ty<'tcx>; 2]>, AlwaysRequiresDrop> {
+    match ty.kind() {
+        ty::Infer(ty::FreshIntTy(_))
+        | ty::Infer(ty::FreshFloatTy(_))
+        | ty::Bool
+        | ty::Int(_)
+        | ty::Uint(_)
+        | ty::Float(_)
+        | ty::Never
+        | ty::FnDef(..)
+        | ty::FnPtr(_)
+        | ty::Char
+        | ty::GeneratorWitness(..)
+        | ty::RawPtr(_)
+        | ty::Ref(..)
+        | ty::Str => Ok(SmallVec::new()),
+
+        // Foreign types can never have destructors.
+        ty::Foreign(..) => Ok(SmallVec::new()),
+
+        ty::Dynamic(..) | ty::Error(_) => Err(AlwaysRequiresDrop),
+
+        ty::Slice(ty) => needs_drop_components(*ty, target_layout),
+        ty::Array(elem_ty, size) => {
+            match needs_drop_components(*elem_ty, target_layout) {
+                Ok(v) if v.is_empty() => Ok(v),
+                res => match size.kind().try_to_bits(target_layout.pointer_size) {
+                    // Arrays of size zero don't need drop, even if their element
+                    // type does.
+                    Some(0) => Ok(SmallVec::new()),
+                    Some(_) => res,
+                    // We don't know which of the cases above we are in, so
+                    // return the whole type and let the caller decide what to
+                    // do.
+                    None => Ok(smallvec![ty]),
+                },
+            }
+        }
+        // If any field needs drop, then the whole tuple does.
+        ty::Tuple(fields) => fields.iter().try_fold(SmallVec::new(), move |mut acc, elem| {
+            acc.extend(needs_drop_components(elem, target_layout)?);
+            Ok(acc)
+        }),
+
+        // These require checking for `Copy` bounds or `Adt` destructors.
+        ty::Adt(..)
+        | ty::Projection(..)
+        | ty::Param(_)
+        | ty::Bound(..)
+        | ty::Placeholder(..)
+        | ty::Opaque(..)
+        | ty::Infer(_)
+        | ty::Closure(..)
+        | ty::Generator(..) => Ok(smallvec![ty]),
+    }
+}
+
+pub fn is_trivially_const_drop<'tcx>(ty: Ty<'tcx>) -> bool {
+    match *ty.kind() {
+        ty::Bool
+        | ty::Char
+        | ty::Int(_)
+        | ty::Uint(_)
+        | ty::Float(_)
+        | ty::Infer(ty::IntVar(_))
+        | ty::Infer(ty::FloatVar(_))
+        | ty::Str
+        | ty::RawPtr(_)
+        | ty::Ref(..)
+        | ty::FnDef(..)
+        | ty::FnPtr(_)
+        | ty::Never
+        | ty::Foreign(_) => true,
+
+        ty::Opaque(..)
+        | ty::Dynamic(..)
+        | ty::Error(_)
+        | ty::Bound(..)
+        | ty::Param(_)
+        | ty::Placeholder(_)
+        | ty::Projection(_)
+        | ty::Infer(_) => false,
+
+        // Not trivial because they have components, and instead of looking inside,
+        // we'll just perform trait selection.
+        ty::Closure(..) | ty::Generator(..) | ty::GeneratorWitness(_) | ty::Adt(..) => false,
+
+        ty::Array(ty, _) | ty::Slice(ty) => is_trivially_const_drop(ty),
+
+        ty::Tuple(tys) => tys.iter().all(|ty| is_trivially_const_drop(ty)),
+    }
+}
+
+// Does the equivalent of
+// ```
+// let v = self.iter().map(|p| p.fold_with(folder)).collect::<SmallVec<[_; 8]>>();
+// folder.tcx().intern_*(&v)
+// ```
+pub fn fold_list<'tcx, F, T>(
+    list: &'tcx ty::List<T>,
+    folder: &mut F,
+    intern: impl FnOnce(TyCtxt<'tcx>, &[T]) -> &'tcx ty::List<T>,
+) -> Result<&'tcx ty::List<T>, F::Error>
+where
+    F: FallibleTypeFolder<'tcx>,
+    T: TypeFoldable<'tcx> + PartialEq + Copy,
+{
+    let mut iter = list.iter();
+    // Look for the first element that changed
+    match iter.by_ref().enumerate().find_map(|(i, t)| match t.try_fold_with(folder) {
+        Ok(new_t) if new_t == t => None,
+        new_t => Some((i, new_t)),
+    }) {
+        Some((i, Ok(new_t))) => {
+            // An element changed, prepare to intern the resulting list
+            let mut new_list = SmallVec::<[_; 8]>::with_capacity(list.len());
+            new_list.extend_from_slice(&list[..i]);
+            new_list.push(new_t);
+            for t in iter {
+                new_list.push(t.try_fold_with(folder)?)
+            }
+            Ok(intern(folder.tcx(), &new_list))
+        }
+        Some((_, Err(err))) => {
+            return Err(err);
+        }
+        None => Ok(list),
+    }
+}
+
+#[derive(Copy, Clone, Debug, HashStable, TyEncodable, TyDecodable)]
+pub struct AlwaysRequiresDrop;
+
+/// Normalizes all opaque types in the given value, replacing them
+/// with their underlying types.
+pub fn normalize_opaque_types<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    val: &'tcx ty::List<ty::Predicate<'tcx>>,
+) -> &'tcx ty::List<ty::Predicate<'tcx>> {
+    let mut visitor = OpaqueTypeExpander {
+        seen_opaque_tys: FxHashSet::default(),
+        expanded_cache: FxHashMap::default(),
+        primary_def_id: None,
+        found_recursion: false,
+        found_any_recursion: false,
+        check_recursion: false,
+        tcx,
+    };
+    val.fold_with(&mut visitor)
+}
+
+/// Determines whether an item is annotated with `doc(hidden)`.
+pub fn is_doc_hidden(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+    tcx.get_attrs(def_id, sym::doc)
+        .filter_map(|attr| attr.meta_item_list())
+        .any(|items| items.iter().any(|item| item.has_name(sym::hidden)))
+}
+
+/// Determines whether an item is an intrinsic by Abi.
+pub fn is_intrinsic(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+    matches!(tcx.fn_sig(def_id).abi(), Abi::RustIntrinsic | Abi::PlatformIntrinsic)
+}
+
+pub fn provide(providers: &mut ty::query::Providers) {
+    *providers =
+        ty::query::Providers { normalize_opaque_types, is_doc_hidden, is_intrinsic, ..*providers }
+}
diff --git a/compiler/rustc_middle/src/ty/visit.rs b/compiler/rustc_middle/src/ty/visit.rs
new file mode 100644
index 000000000..536506720
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/visit.rs
@@ -0,0 +1,745 @@
+//! A visiting traversal mechanism for complex data structures that contain type
+//! information.
+//!
+//! This is a read-only traversal of the data structure.
+//!
+//! This traversal has limited flexibility. Only a small number of "types of
+//! interest" within the complex data structures can receive custom
+//! visitation. These are the ones containing the most important type-related
+//! information, such as `Ty`, `Predicate`, `Region`, and `Const`.
+//!
+//! There are three groups of traits involved in each traversal.
+//! - `TypeVisitable`. This is implemented once for many types, including:
+//!   - Types of interest, for which the the methods delegate to the
+//!     visitor.
+//!   - All other types, including generic containers like `Vec` and `Option`.
+//!     It defines a "skeleton" of how they should be visited.
+//! - `TypeSuperVisitable`. This is implemented only for each type of interest,
+//!   and defines the visiting "skeleton" for these types.
+//! - `TypeVisitor`. This is implemented for each visitor. This defines how
+//!   types of interest are visited.
+//!
+//! This means each visit is a mixture of (a) generic visiting operations, and (b)
+//! custom visit operations that are specific to the visitor.
+//! - The `TypeVisitable` impls handle most of the traversal, and call into
+//!   `TypeVisitor` when they encounter a type of interest.
+//! - A `TypeVisitor` may call into another `TypeVisitable` impl, because some of
+//!   the types of interest are recursive and can contain other types of interest.
+//! - A `TypeVisitor` may also call into a `TypeSuperVisitable` impl, because each
+//!   visitor might provide custom handling only for some types of interest, or
+//!   only for some variants of each type of interest, and then use default
+//!   traversal for the remaining cases.
+//!
+//! For example, if you have `struct S(Ty, U)` where `S: TypeVisitable` and `U:
+//! TypeVisitable`, and an instance `s = S(ty, u)`, it would be visited like so:
+//! ```text
+//! s.visit_with(visitor) calls
+//! - ty.visit_with(visitor) calls
+//!   - visitor.visit_ty(ty) may call
+//!     - ty.super_visit_with(visitor)
+//! - u.visit_with(visitor)
+//! ```
+use crate::mir;
+use crate::ty::{self, flags::FlagComputation, Binder, Ty, TyCtxt, TypeFlags};
+use rustc_errors::ErrorGuaranteed;
+
+use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::sso::SsoHashSet;
+use std::fmt;
+use std::ops::ControlFlow;
+
+/// This trait is implemented for every type that can be visited,
+/// providing the skeleton of the traversal.
+///
+/// To implement this conveniently, use the derive macro located in
+/// `rustc_macros`.
+pub trait TypeVisitable<'tcx>: fmt::Debug + Clone {
+    /// The entry point for visiting. To visit a value `t` with a visitor `v`
+    /// call: `t.visit_with(v)`.
+    ///
+    /// For most types, this just traverses the value, calling `visit_with` on
+    /// each field/element.
+    ///
+    /// For types of interest (such as `Ty`), the implementation of this method
+    /// that calls a visitor method specifically for that type (such as
+    /// `V::visit_ty`). This is where control transfers from `TypeFoldable` to
+    /// `TypeVisitor`.
+    fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy>;
+
+    /// Returns `true` if `self` has any late-bound regions that are either
+    /// bound by `binder` or bound by some binder outside of `binder`.
+    /// If `binder` is `ty::INNERMOST`, this indicates whether
+    /// there are any late-bound regions that appear free.
+    fn has_vars_bound_at_or_above(&self, binder: ty::DebruijnIndex) -> bool {
+        self.visit_with(&mut HasEscapingVarsVisitor { outer_index: binder }).is_break()
+    }
+
+    /// Returns `true` if this `self` has any regions that escape `binder` (and
+    /// hence are not bound by it).
+    fn has_vars_bound_above(&self, binder: ty::DebruijnIndex) -> bool {
+        self.has_vars_bound_at_or_above(binder.shifted_in(1))
+    }
+
+    fn has_escaping_bound_vars(&self) -> bool {
+        self.has_vars_bound_at_or_above(ty::INNERMOST)
+    }
+
+    #[instrument(level = "trace")]
+    fn has_type_flags(&self, flags: TypeFlags) -> bool {
+        self.visit_with(&mut HasTypeFlagsVisitor { flags }).break_value() == Some(FoundFlags)
+    }
+    fn has_projections(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_PROJECTION)
+    }
+    fn has_opaque_types(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_TY_OPAQUE)
+    }
+    fn references_error(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_ERROR)
+    }
+    fn error_reported(&self) -> Option<ErrorGuaranteed> {
+        if self.references_error() {
+            Some(ErrorGuaranteed::unchecked_claim_error_was_emitted())
+        } else {
+            None
+        }
+    }
+    fn has_param_types_or_consts(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_TY_PARAM | TypeFlags::HAS_CT_PARAM)
+    }
+    fn has_infer_regions(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_RE_INFER)
+    }
+    fn has_infer_types(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_TY_INFER)
+    }
+    fn has_infer_types_or_consts(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_TY_INFER | TypeFlags::HAS_CT_INFER)
+    }
+    fn needs_infer(&self) -> bool {
+        self.has_type_flags(TypeFlags::NEEDS_INFER)
+    }
+    fn has_placeholders(&self) -> bool {
+        self.has_type_flags(
+            TypeFlags::HAS_RE_PLACEHOLDER
+                | TypeFlags::HAS_TY_PLACEHOLDER
+                | TypeFlags::HAS_CT_PLACEHOLDER,
+        )
+    }
+    fn needs_subst(&self) -> bool {
+        self.has_type_flags(TypeFlags::NEEDS_SUBST)
+    }
+    /// "Free" regions in this context means that it has any region
+    /// that is not (a) erased or (b) late-bound.
+    fn has_free_regions(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_FREE_REGIONS)
+    }
+
+    fn has_erased_regions(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_RE_ERASED)
+    }
+
+    /// True if there are any un-erased free regions.
+    fn has_erasable_regions(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_FREE_REGIONS)
+    }
+
+    /// Indicates whether this value references only 'global'
+    /// generic parameters that are the same regardless of what fn we are
+    /// in. This is used for caching.
+    fn is_global(&self) -> bool {
+        !self.has_type_flags(TypeFlags::HAS_FREE_LOCAL_NAMES)
+    }
+
+    /// True if there are any late-bound regions
+    fn has_late_bound_regions(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_RE_LATE_BOUND)
+    }
+
+    /// Indicates whether this value still has parameters/placeholders/inference variables
+    /// which could be replaced later, in a way that would change the results of `impl`
+    /// specialization.
+    fn still_further_specializable(&self) -> bool {
+        self.has_type_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE)
+    }
+}
+
+pub trait TypeSuperVisitable<'tcx>: TypeVisitable<'tcx> {
+    /// Provides a default visit for a type of interest. This should only be
+    /// called within `TypeVisitor` methods, when a non-custom traversal is
+    /// desired for the value of the type of interest passed to that method.
+    /// For example, in `MyVisitor::visit_ty(ty)`, it is valid to call
+    /// `ty.super_visit_with(self)`, but any other visiting should be done
+    /// with `xyz.visit_with(self)`.
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy>;
+}
+
+/// This trait is implemented for every visiting traversal. There is a visit
+/// method defined for every type of interest. Each such method has a default
+/// that recurses into the type's fields in a non-custom fashion.
+pub trait TypeVisitor<'tcx>: Sized {
+    type BreakTy = !;
+
+    fn visit_binder<T: TypeVisitable<'tcx>>(
+        &mut self,
+        t: &Binder<'tcx, T>,
+    ) -> ControlFlow<Self::BreakTy> {
+        t.super_visit_with(self)
+    }
+
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        t.super_visit_with(self)
+    }
+
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+        r.super_visit_with(self)
+    }
+
+    fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+        c.super_visit_with(self)
+    }
+
+    fn visit_unevaluated(&mut self, uv: ty::Unevaluated<'tcx>) -> ControlFlow<Self::BreakTy> {
+        uv.super_visit_with(self)
+    }
+
+    fn visit_predicate(&mut self, p: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
+        p.super_visit_with(self)
+    }
+
+    fn visit_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> ControlFlow<Self::BreakTy> {
+        c.super_visit_with(self)
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Region folder
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Invoke `callback` on every region appearing free in `value`.
+    pub fn for_each_free_region(
+        self,
+        value: &impl TypeVisitable<'tcx>,
+        mut callback: impl FnMut(ty::Region<'tcx>),
+    ) {
+        self.any_free_region_meets(value, |r| {
+            callback(r);
+            false
+        });
+    }
+
+    /// Returns `true` if `callback` returns true for every region appearing free in `value`.
+    pub fn all_free_regions_meet(
+        self,
+        value: &impl TypeVisitable<'tcx>,
+        mut callback: impl FnMut(ty::Region<'tcx>) -> bool,
+    ) -> bool {
+        !self.any_free_region_meets(value, |r| !callback(r))
+    }
+
+    /// Returns `true` if `callback` returns true for some region appearing free in `value`.
+    pub fn any_free_region_meets(
+        self,
+        value: &impl TypeVisitable<'tcx>,
+        callback: impl FnMut(ty::Region<'tcx>) -> bool,
+    ) -> bool {
+        struct RegionVisitor<F> {
+            /// The index of a binder *just outside* the things we have
+            /// traversed. If we encounter a bound region bound by this
+            /// binder or one outer to it, it appears free. Example:
+            ///
+            /// ```ignore (illustrative)
+            ///       for<'a> fn(for<'b> fn(), T)
+            /// // ^          ^          ^     ^
+            /// // |          |          |     | here, would be shifted in 1
+            /// // |          |          | here, would be shifted in 2
+            /// // |          | here, would be `INNERMOST` shifted in by 1
+            /// // | here, initially, binder would be `INNERMOST`
+            /// ```
+            ///
+            /// You see that, initially, *any* bound value is free,
+            /// because we've not traversed any binders. As we pass
+            /// through a binder, we shift the `outer_index` by 1 to
+            /// account for the new binder that encloses us.
+            outer_index: ty::DebruijnIndex,
+            callback: F,
+        }
+
+        impl<'tcx, F> TypeVisitor<'tcx> for RegionVisitor<F>
+        where
+            F: FnMut(ty::Region<'tcx>) -> bool,
+        {
+            type BreakTy = ();
+
+            fn visit_binder<T: TypeVisitable<'tcx>>(
+                &mut self,
+                t: &Binder<'tcx, T>,
+            ) -> ControlFlow<Self::BreakTy> {
+                self.outer_index.shift_in(1);
+                let result = t.super_visit_with(self);
+                self.outer_index.shift_out(1);
+                result
+            }
+
+            fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+                match *r {
+                    ty::ReLateBound(debruijn, _) if debruijn < self.outer_index => {
+                        ControlFlow::CONTINUE
+                    }
+                    _ => {
+                        if (self.callback)(r) {
+                            ControlFlow::BREAK
+                        } else {
+                            ControlFlow::CONTINUE
+                        }
+                    }
+                }
+            }
+
+            fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+                // We're only interested in types involving regions
+                if ty.flags().intersects(TypeFlags::HAS_FREE_REGIONS) {
+                    ty.super_visit_with(self)
+                } else {
+                    ControlFlow::CONTINUE
+                }
+            }
+        }
+
+        value.visit_with(&mut RegionVisitor { outer_index: ty::INNERMOST, callback }).is_break()
+    }
+
+    /// Returns a set of all late-bound regions that are constrained
+    /// by `value`, meaning that if we instantiate those LBR with
+    /// variables and equate `value` with something else, those
+    /// variables will also be equated.
+    pub fn collect_constrained_late_bound_regions<T>(
+        self,
+        value: &Binder<'tcx, T>,
+    ) -> FxHashSet<ty::BoundRegionKind>
+    where
+        T: TypeVisitable<'tcx>,
+    {
+        self.collect_late_bound_regions(value, true)
+    }
+
+    /// Returns a set of all late-bound regions that appear in `value` anywhere.
+    pub fn collect_referenced_late_bound_regions<T>(
+        self,
+        value: &Binder<'tcx, T>,
+    ) -> FxHashSet<ty::BoundRegionKind>
+    where
+        T: TypeVisitable<'tcx>,
+    {
+        self.collect_late_bound_regions(value, false)
+    }
+
+    fn collect_late_bound_regions<T>(
+        self,
+        value: &Binder<'tcx, T>,
+        just_constraint: bool,
+    ) -> FxHashSet<ty::BoundRegionKind>
+    where
+        T: TypeVisitable<'tcx>,
+    {
+        let mut collector = LateBoundRegionsCollector::new(just_constraint);
+        let result = value.as_ref().skip_binder().visit_with(&mut collector);
+        assert!(result.is_continue()); // should never have stopped early
+        collector.regions
+    }
+}
+
+pub struct ValidateBoundVars<'tcx> {
+    bound_vars: &'tcx ty::List<ty::BoundVariableKind>,
+    binder_index: ty::DebruijnIndex,
+    // We may encounter the same variable at different levels of binding, so
+    // this can't just be `Ty`
+    visited: SsoHashSet<(ty::DebruijnIndex, Ty<'tcx>)>,
+}
+
+impl<'tcx> ValidateBoundVars<'tcx> {
+    pub fn new(bound_vars: &'tcx ty::List<ty::BoundVariableKind>) -> Self {
+        ValidateBoundVars {
+            bound_vars,
+            binder_index: ty::INNERMOST,
+            visited: SsoHashSet::default(),
+        }
+    }
+}
+
+impl<'tcx> TypeVisitor<'tcx> for ValidateBoundVars<'tcx> {
+    type BreakTy = ();
+
+    fn visit_binder<T: TypeVisitable<'tcx>>(
+        &mut self,
+        t: &Binder<'tcx, T>,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.binder_index.shift_in(1);
+        let result = t.super_visit_with(self);
+        self.binder_index.shift_out(1);
+        result
+    }
+
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        if t.outer_exclusive_binder() < self.binder_index
+            || !self.visited.insert((self.binder_index, t))
+        {
+            return ControlFlow::BREAK;
+        }
+        match *t.kind() {
+            ty::Bound(debruijn, bound_ty) if debruijn == self.binder_index => {
+                if self.bound_vars.len() <= bound_ty.var.as_usize() {
+                    bug!("Not enough bound vars: {:?} not found in {:?}", t, self.bound_vars);
+                }
+                let list_var = self.bound_vars[bound_ty.var.as_usize()];
+                match list_var {
+                    ty::BoundVariableKind::Ty(kind) => {
+                        if kind != bound_ty.kind {
+                            bug!(
+                                "Mismatched type kinds: {:?} doesn't var in list {:?}",
+                                bound_ty.kind,
+                                list_var
+                            );
+                        }
+                    }
+                    _ => {
+                        bug!("Mismatched bound variable kinds! Expected type, found {:?}", list_var)
+                    }
+                }
+            }
+
+            _ => (),
+        };
+
+        t.super_visit_with(self)
+    }
+
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+        match *r {
+            ty::ReLateBound(index, br) if index == self.binder_index => {
+                if self.bound_vars.len() <= br.var.as_usize() {
+                    bug!("Not enough bound vars: {:?} not found in {:?}", br, self.bound_vars);
+                }
+                let list_var = self.bound_vars[br.var.as_usize()];
+                match list_var {
+                    ty::BoundVariableKind::Region(kind) => {
+                        if kind != br.kind {
+                            bug!(
+                                "Mismatched region kinds: {:?} doesn't match var ({:?}) in list ({:?})",
+                                br.kind,
+                                list_var,
+                                self.bound_vars
+                            );
+                        }
+                    }
+                    _ => bug!(
+                        "Mismatched bound variable kinds! Expected region, found {:?}",
+                        list_var
+                    ),
+                }
+            }
+
+            _ => (),
+        };
+
+        r.super_visit_with(self)
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+struct FoundEscapingVars;
+
+/// An "escaping var" is a bound var whose binder is not part of `t`. A bound var can be a
+/// bound region or a bound type.
+///
+/// So, for example, consider a type like the following, which has two binders:
+///
+///    for<'a> fn(x: for<'b> fn(&'a isize, &'b isize))
+///    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ outer scope
+///                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~  inner scope
+///
+/// This type has *bound regions* (`'a`, `'b`), but it does not have escaping regions, because the
+/// binders of both `'a` and `'b` are part of the type itself. However, if we consider the *inner
+/// fn type*, that type has an escaping region: `'a`.
+///
+/// Note that what I'm calling an "escaping var" is often just called a "free var". However,
+/// we already use the term "free var". It refers to the regions or types that we use to represent
+/// bound regions or type params on a fn definition while we are type checking its body.
+///
+/// To clarify, conceptually there is no particular difference between
+/// an "escaping" var and a "free" var. However, there is a big
+/// difference in practice. Basically, when "entering" a binding
+/// level, one is generally required to do some sort of processing to
+/// a bound var, such as replacing it with a fresh/placeholder
+/// var, or making an entry in the environment to represent the
+/// scope to which it is attached, etc. An escaping var represents
+/// a bound var for which this processing has not yet been done.
+struct HasEscapingVarsVisitor {
+    /// Anything bound by `outer_index` or "above" is escaping.
+    outer_index: ty::DebruijnIndex,
+}
+
+impl<'tcx> TypeVisitor<'tcx> for HasEscapingVarsVisitor {
+    type BreakTy = FoundEscapingVars;
+
+    fn visit_binder<T: TypeVisitable<'tcx>>(
+        &mut self,
+        t: &Binder<'tcx, T>,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.outer_index.shift_in(1);
+        let result = t.super_visit_with(self);
+        self.outer_index.shift_out(1);
+        result
+    }
+
+    #[inline]
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        // If the outer-exclusive-binder is *strictly greater* than
+        // `outer_index`, that means that `t` contains some content
+        // bound at `outer_index` or above (because
+        // `outer_exclusive_binder` is always 1 higher than the
+        // content in `t`). Therefore, `t` has some escaping vars.
+        if t.outer_exclusive_binder() > self.outer_index {
+            ControlFlow::Break(FoundEscapingVars)
+        } else {
+            ControlFlow::CONTINUE
+        }
+    }
+
+    #[inline]
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+        // If the region is bound by `outer_index` or anything outside
+        // of outer index, then it escapes the binders we have
+        // visited.
+        if r.bound_at_or_above_binder(self.outer_index) {
+            ControlFlow::Break(FoundEscapingVars)
+        } else {
+            ControlFlow::CONTINUE
+        }
+    }
+
+    fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+        // we don't have a `visit_infer_const` callback, so we have to
+        // hook in here to catch this case (annoying...), but
+        // otherwise we do want to remember to visit the rest of the
+        // const, as it has types/regions embedded in a lot of other
+        // places.
+        match ct.kind() {
+            ty::ConstKind::Bound(debruijn, _) if debruijn >= self.outer_index => {
+                ControlFlow::Break(FoundEscapingVars)
+            }
+            _ => ct.super_visit_with(self),
+        }
+    }
+
+    #[inline]
+    fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
+        if predicate.outer_exclusive_binder() > self.outer_index {
+            ControlFlow::Break(FoundEscapingVars)
+        } else {
+            ControlFlow::CONTINUE
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+struct FoundFlags;
+
+// FIXME: Optimize for checking for infer flags
+struct HasTypeFlagsVisitor {
+    flags: ty::TypeFlags,
+}
+
+impl std::fmt::Debug for HasTypeFlagsVisitor {
+    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.flags.fmt(fmt)
+    }
+}
+
+impl<'tcx> TypeVisitor<'tcx> for HasTypeFlagsVisitor {
+    type BreakTy = FoundFlags;
+
+    #[inline]
+    #[instrument(skip(self), level = "trace")]
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        let flags = t.flags();
+        trace!(t.flags=?t.flags());
+        if flags.intersects(self.flags) {
+            ControlFlow::Break(FoundFlags)
+        } else {
+            ControlFlow::CONTINUE
+        }
+    }
+
+    #[inline]
+    #[instrument(skip(self), level = "trace")]
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+        let flags = r.type_flags();
+        trace!(r.flags=?flags);
+        if flags.intersects(self.flags) {
+            ControlFlow::Break(FoundFlags)
+        } else {
+            ControlFlow::CONTINUE
+        }
+    }
+
+    #[inline]
+    #[instrument(level = "trace")]
+    fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+        let flags = FlagComputation::for_const(c);
+        trace!(r.flags=?flags);
+        if flags.intersects(self.flags) {
+            ControlFlow::Break(FoundFlags)
+        } else {
+            ControlFlow::CONTINUE
+        }
+    }
+
+    #[inline]
+    #[instrument(level = "trace")]
+    fn visit_unevaluated(&mut self, uv: ty::Unevaluated<'tcx>) -> ControlFlow<Self::BreakTy> {
+        let flags = FlagComputation::for_unevaluated_const(uv);
+        trace!(r.flags=?flags);
+        if flags.intersects(self.flags) {
+            ControlFlow::Break(FoundFlags)
+        } else {
+            ControlFlow::CONTINUE
+        }
+    }
+
+    #[inline]
+    #[instrument(level = "trace")]
+    fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
+        debug!(
+            "HasTypeFlagsVisitor: predicate={:?} predicate.flags={:?} self.flags={:?}",
+            predicate,
+            predicate.flags(),
+            self.flags
+        );
+        if predicate.flags().intersects(self.flags) {
+            ControlFlow::Break(FoundFlags)
+        } else {
+            ControlFlow::CONTINUE
+        }
+    }
+}
+
+/// Collects all the late-bound regions at the innermost binding level
+/// into a hash set.
+struct LateBoundRegionsCollector {
+    current_index: ty::DebruijnIndex,
+    regions: FxHashSet<ty::BoundRegionKind>,
+
+    /// `true` if we only want regions that are known to be
+    /// "constrained" when you equate this type with another type. In
+    /// particular, if you have e.g., `&'a u32` and `&'b u32`, equating
+    /// them constraints `'a == 'b`. But if you have `<&'a u32 as
+    /// Trait>::Foo` and `<&'b u32 as Trait>::Foo`, normalizing those
+    /// types may mean that `'a` and `'b` don't appear in the results,
+    /// so they are not considered *constrained*.
+    just_constrained: bool,
+}
+
+impl LateBoundRegionsCollector {
+    fn new(just_constrained: bool) -> Self {
+        LateBoundRegionsCollector {
+            current_index: ty::INNERMOST,
+            regions: Default::default(),
+            just_constrained,
+        }
+    }
+}
+
+impl<'tcx> TypeVisitor<'tcx> for LateBoundRegionsCollector {
+    fn visit_binder<T: TypeVisitable<'tcx>>(
+        &mut self,
+        t: &Binder<'tcx, T>,
+    ) -> ControlFlow<Self::BreakTy> {
+        self.current_index.shift_in(1);
+        let result = t.super_visit_with(self);
+        self.current_index.shift_out(1);
+        result
+    }
+
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        // if we are only looking for "constrained" region, we have to
+        // ignore the inputs to a projection, as they may not appear
+        // in the normalized form
+        if self.just_constrained {
+            if let ty::Projection(..) = t.kind() {
+                return ControlFlow::CONTINUE;
+            }
+        }
+
+        t.super_visit_with(self)
+    }
+
+    fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+        // if we are only looking for "constrained" region, we have to
+        // ignore the inputs of an unevaluated const, as they may not appear
+        // in the normalized form
+        if self.just_constrained {
+            if let ty::ConstKind::Unevaluated(..) = c.kind() {
+                return ControlFlow::CONTINUE;
+            }
+        }
+
+        c.super_visit_with(self)
+    }
+
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+        if let ty::ReLateBound(debruijn, br) = *r {
+            if debruijn == self.current_index {
+                self.regions.insert(br.kind);
+            }
+        }
+        ControlFlow::CONTINUE
+    }
+}
+
+/// Finds the max universe present
+pub struct MaxUniverse {
+    max_universe: ty::UniverseIndex,
+}
+
+impl MaxUniverse {
+    pub fn new() -> Self {
+        MaxUniverse { max_universe: ty::UniverseIndex::ROOT }
+    }
+
+    pub fn max_universe(self) -> ty::UniverseIndex {
+        self.max_universe
+    }
+}
+
+impl<'tcx> TypeVisitor<'tcx> for MaxUniverse {
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        if let ty::Placeholder(placeholder) = t.kind() {
+            self.max_universe = ty::UniverseIndex::from_u32(
+                self.max_universe.as_u32().max(placeholder.universe.as_u32()),
+            );
+        }
+
+        t.super_visit_with(self)
+    }
+
+    fn visit_const(&mut self, c: ty::consts::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+        if let ty::ConstKind::Placeholder(placeholder) = c.kind() {
+            self.max_universe = ty::UniverseIndex::from_u32(
+                self.max_universe.as_u32().max(placeholder.universe.as_u32()),
+            );
+        }
+
+        c.super_visit_with(self)
+    }
+
+    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+        if let ty::RePlaceholder(placeholder) = *r {
+            self.max_universe = ty::UniverseIndex::from_u32(
+                self.max_universe.as_u32().max(placeholder.universe.as_u32()),
+            );
+        }
+
+        ControlFlow::CONTINUE
+    }
+}
diff --git a/compiler/rustc_middle/src/ty/vtable.rs b/compiler/rustc_middle/src/ty/vtable.rs
new file mode 100644
index 000000000..04a9fd1f7
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/vtable.rs
@@ -0,0 +1,117 @@
+use std::convert::TryFrom;
+use std::fmt;
+
+use crate::mir::interpret::{alloc_range, AllocId, Allocation, Pointer, Scalar, ScalarMaybeUninit};
+use crate::ty::{self, Instance, PolyTraitRef, Ty, TyCtxt};
+use rustc_ast::Mutability;
+
+#[derive(Clone, Copy, PartialEq, HashStable)]
+pub enum VtblEntry<'tcx> {
+    /// destructor of this type (used in vtable header)
+    MetadataDropInPlace,
+    /// layout size of this type (used in vtable header)
+    MetadataSize,
+    /// layout align of this type (used in vtable header)
+    MetadataAlign,
+    /// non-dispatchable associated function that is excluded from trait object
+    Vacant,
+    /// dispatchable associated function
+    Method(Instance<'tcx>),
+    /// pointer to a separate supertrait vtable, can be used by trait upcasting coercion
+    TraitVPtr(PolyTraitRef<'tcx>),
+}
+
+impl<'tcx> fmt::Debug for VtblEntry<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // We want to call `Display` on `Instance` and `PolyTraitRef`,
+        // so we implement this manually.
+        match self {
+            VtblEntry::MetadataDropInPlace => write!(f, "MetadataDropInPlace"),
+            VtblEntry::MetadataSize => write!(f, "MetadataSize"),
+            VtblEntry::MetadataAlign => write!(f, "MetadataAlign"),
+            VtblEntry::Vacant => write!(f, "Vacant"),
+            VtblEntry::Method(instance) => write!(f, "Method({})", instance),
+            VtblEntry::TraitVPtr(trait_ref) => write!(f, "TraitVPtr({})", trait_ref),
+        }
+    }
+}
+
+// Needs to be associated with the `'tcx` lifetime
+impl<'tcx> TyCtxt<'tcx> {
+    pub const COMMON_VTABLE_ENTRIES: &'tcx [VtblEntry<'tcx>] =
+        &[VtblEntry::MetadataDropInPlace, VtblEntry::MetadataSize, VtblEntry::MetadataAlign];
+}
+
+pub const COMMON_VTABLE_ENTRIES_DROPINPLACE: usize = 0;
+pub const COMMON_VTABLE_ENTRIES_SIZE: usize = 1;
+pub const COMMON_VTABLE_ENTRIES_ALIGN: usize = 2;
+
+/// Retrieves an allocation that represents the contents of a vtable.
+/// Since this is a query, allocations are cached and not duplicated.
+pub(super) fn vtable_allocation_provider<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    key: (Ty<'tcx>, Option<ty::PolyExistentialTraitRef<'tcx>>),
+) -> AllocId {
+    let (ty, poly_trait_ref) = key;
+
+    let vtable_entries = if let Some(poly_trait_ref) = poly_trait_ref {
+        let trait_ref = poly_trait_ref.with_self_ty(tcx, ty);
+        let trait_ref = tcx.erase_regions(trait_ref);
+
+        tcx.vtable_entries(trait_ref)
+    } else {
+        TyCtxt::COMMON_VTABLE_ENTRIES
+    };
+
+    let layout = tcx
+        .layout_of(ty::ParamEnv::reveal_all().and(ty))
+        .expect("failed to build vtable representation");
+    assert!(!layout.is_unsized(), "can't create a vtable for an unsized type");
+    let size = layout.size.bytes();
+    let align = layout.align.abi.bytes();
+
+    let ptr_size = tcx.data_layout.pointer_size;
+    let ptr_align = tcx.data_layout.pointer_align.abi;
+
+    let vtable_size = ptr_size * u64::try_from(vtable_entries.len()).unwrap();
+    let mut vtable = Allocation::uninit(vtable_size, ptr_align, /* panic_on_fail */ true).unwrap();
+
+    // No need to do any alignment checks on the memory accesses below, because we know the
+    // allocation is correctly aligned as we created it above. Also we're only offsetting by
+    // multiples of `ptr_align`, which means that it will stay aligned to `ptr_align`.
+
+    for (idx, entry) in vtable_entries.iter().enumerate() {
+        let idx: u64 = u64::try_from(idx).unwrap();
+        let scalar = match entry {
+            VtblEntry::MetadataDropInPlace => {
+                let instance = ty::Instance::resolve_drop_in_place(tcx, ty);
+                let fn_alloc_id = tcx.create_fn_alloc(instance);
+                let fn_ptr = Pointer::from(fn_alloc_id);
+                ScalarMaybeUninit::from_pointer(fn_ptr, &tcx)
+            }
+            VtblEntry::MetadataSize => Scalar::from_uint(size, ptr_size).into(),
+            VtblEntry::MetadataAlign => Scalar::from_uint(align, ptr_size).into(),
+            VtblEntry::Vacant => continue,
+            VtblEntry::Method(instance) => {
+                // Prepare the fn ptr we write into the vtable.
+                let instance = instance.polymorphize(tcx);
+                let fn_alloc_id = tcx.create_fn_alloc(instance);
+                let fn_ptr = Pointer::from(fn_alloc_id);
+                ScalarMaybeUninit::from_pointer(fn_ptr, &tcx)
+            }
+            VtblEntry::TraitVPtr(trait_ref) => {
+                let super_trait_ref = trait_ref
+                    .map_bound(|trait_ref| ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
+                let supertrait_alloc_id = tcx.vtable_allocation((ty, Some(super_trait_ref)));
+                let vptr = Pointer::from(supertrait_alloc_id);
+                ScalarMaybeUninit::from_pointer(vptr, &tcx)
+            }
+        };
+        vtable
+            .write_scalar(&tcx, alloc_range(ptr_size * idx, ptr_size), scalar)
+            .expect("failed to build vtable representation");
+    }
+
+    vtable.mutability = Mutability::Not;
+    tcx.create_memory_alloc(tcx.intern_const_alloc(vtable))
+}
diff --git a/compiler/rustc_middle/src/ty/walk.rs b/compiler/rustc_middle/src/ty/walk.rs
new file mode 100644
index 000000000..02fe1f3a7
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/walk.rs
@@ -0,0 +1,207 @@
+//! An iterator over the type substructure.
+//! WARNING: this does not keep track of the region depth.
+
+use crate::ty::subst::{GenericArg, GenericArgKind};
+use crate::ty::{self, Ty};
+use rustc_data_structures::sso::SsoHashSet;
+use smallvec::{self, SmallVec};
+
+// The TypeWalker's stack is hot enough that it's worth going to some effort to
+// avoid heap allocations.
+type TypeWalkerStack<'tcx> = SmallVec<[GenericArg<'tcx>; 8]>;
+
+pub struct TypeWalker<'tcx> {
+    stack: TypeWalkerStack<'tcx>,
+    last_subtree: usize,
+    pub visited: SsoHashSet<GenericArg<'tcx>>,
+}
+
+/// An iterator for walking the type tree.
+///
+/// It's very easy to produce a deeply
+/// nested type tree with a lot of
+/// identical subtrees. In order to work efficiently
+/// in this situation walker only visits each type once.
+/// It maintains a set of visited types and
+/// skips any types that are already there.
+impl<'tcx> TypeWalker<'tcx> {
+    pub fn new(root: GenericArg<'tcx>) -> Self {
+        Self { stack: smallvec![root], last_subtree: 1, visited: SsoHashSet::new() }
+    }
+
+    /// Skips the subtree corresponding to the last type
+    /// returned by `next()`.
+    ///
+    /// Example: Imagine you are walking `Foo<Bar<i32>, usize>`.
+    ///
+    /// ```ignore (illustrative)
+    /// let mut iter: TypeWalker = ...;
+    /// iter.next(); // yields Foo
+    /// iter.next(); // yields Bar<i32>
+    /// iter.skip_current_subtree(); // skips i32
+    /// iter.next(); // yields usize
+    /// ```
+    pub fn skip_current_subtree(&mut self) {
+        self.stack.truncate(self.last_subtree);
+    }
+}
+
+impl<'tcx> Iterator for TypeWalker<'tcx> {
+    type Item = GenericArg<'tcx>;
+
+    fn next(&mut self) -> Option<GenericArg<'tcx>> {
+        debug!("next(): stack={:?}", self.stack);
+        loop {
+            let next = self.stack.pop()?;
+            self.last_subtree = self.stack.len();
+            if self.visited.insert(next) {
+                push_inner(&mut self.stack, next);
+                debug!("next: stack={:?}", self.stack);
+                return Some(next);
+            }
+        }
+    }
+}
+
+impl<'tcx> GenericArg<'tcx> {
+    /// Iterator that walks `self` and any types reachable from
+    /// `self`, in depth-first order. Note that just walks the types
+    /// that appear in `self`, it does not descend into the fields of
+    /// structs or variants. For example:
+    ///
+    /// ```text
+    /// isize => { isize }
+    /// Foo<Bar<isize>> => { Foo<Bar<isize>>, Bar<isize>, isize }
+    /// [isize] => { [isize], isize }
+    /// ```
+    pub fn walk(self) -> TypeWalker<'tcx> {
+        TypeWalker::new(self)
+    }
+
+    /// Iterator that walks the immediate children of `self`. Hence
+    /// `Foo<Bar<i32>, u32>` yields the sequence `[Bar<i32>, u32]`
+    /// (but not `i32`, like `walk`).
+    ///
+    /// Iterator only walks items once.
+    /// It accepts visited set, updates it with all visited types
+    /// and skips any types that are already there.
+    pub fn walk_shallow(
+        self,
+        visited: &mut SsoHashSet<GenericArg<'tcx>>,
+    ) -> impl Iterator<Item = GenericArg<'tcx>> {
+        let mut stack = SmallVec::new();
+        push_inner(&mut stack, self);
+        stack.retain(|a| visited.insert(*a));
+        stack.into_iter()
+    }
+}
+
+impl<'tcx> Ty<'tcx> {
+    /// Iterator that walks `self` and any types reachable from
+    /// `self`, in depth-first order. Note that just walks the types
+    /// that appear in `self`, it does not descend into the fields of
+    /// structs or variants. For example:
+    ///
+    /// ```text
+    /// isize => { isize }
+    /// Foo<Bar<isize>> => { Foo<Bar<isize>>, Bar<isize>, isize }
+    /// [isize] => { [isize], isize }
+    /// ```
+    pub fn walk(self) -> TypeWalker<'tcx> {
+        TypeWalker::new(self.into())
+    }
+}
+
+/// We push `GenericArg`s on the stack in reverse order so as to
+/// maintain a pre-order traversal. As of the time of this
+/// writing, the fact that the traversal is pre-order is not
+/// known to be significant to any code, but it seems like the
+/// natural order one would expect (basically, the order of the
+/// types as they are written).
+fn push_inner<'tcx>(stack: &mut TypeWalkerStack<'tcx>, parent: GenericArg<'tcx>) {
+    match parent.unpack() {
+        GenericArgKind::Type(parent_ty) => match *parent_ty.kind() {
+            ty::Bool
+            | ty::Char
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Str
+            | ty::Infer(_)
+            | ty::Param(_)
+            | ty::Never
+            | ty::Error(_)
+            | ty::Placeholder(..)
+            | ty::Bound(..)
+            | ty::Foreign(..) => {}
+
+            ty::Array(ty, len) => {
+                stack.push(len.into());
+                stack.push(ty.into());
+            }
+            ty::Slice(ty) => {
+                stack.push(ty.into());
+            }
+            ty::RawPtr(mt) => {
+                stack.push(mt.ty.into());
+            }
+            ty::Ref(lt, ty, _) => {
+                stack.push(ty.into());
+                stack.push(lt.into());
+            }
+            ty::Projection(data) => {
+                stack.extend(data.substs.iter().rev());
+            }
+            ty::Dynamic(obj, lt) => {
+                stack.push(lt.into());
+                stack.extend(obj.iter().rev().flat_map(|predicate| {
+                    let (substs, opt_ty) = match predicate.skip_binder() {
+                        ty::ExistentialPredicate::Trait(tr) => (tr.substs, None),
+                        ty::ExistentialPredicate::Projection(p) => (p.substs, Some(p.term)),
+                        ty::ExistentialPredicate::AutoTrait(_) =>
+                        // Empty iterator
+                        {
+                            (ty::InternalSubsts::empty(), None)
+                        }
+                    };
+
+                    substs.iter().rev().chain(opt_ty.map(|term| match term {
+                        ty::Term::Ty(ty) => ty.into(),
+                        ty::Term::Const(ct) => ct.into(),
+                    }))
+                }));
+            }
+            ty::Adt(_, substs)
+            | ty::Opaque(_, substs)
+            | ty::Closure(_, substs)
+            | ty::Generator(_, substs, _)
+            | ty::FnDef(_, substs) => {
+                stack.extend(substs.iter().rev());
+            }
+            ty::Tuple(ts) => stack.extend(ts.as_substs().iter().rev()),
+            ty::GeneratorWitness(ts) => {
+                stack.extend(ts.skip_binder().iter().rev().map(|ty| ty.into()));
+            }
+            ty::FnPtr(sig) => {
+                stack.push(sig.skip_binder().output().into());
+                stack.extend(sig.skip_binder().inputs().iter().copied().rev().map(|ty| ty.into()));
+            }
+        },
+        GenericArgKind::Lifetime(_) => {}
+        GenericArgKind::Const(parent_ct) => {
+            stack.push(parent_ct.ty().into());
+            match parent_ct.kind() {
+                ty::ConstKind::Infer(_)
+                | ty::ConstKind::Param(_)
+                | ty::ConstKind::Placeholder(_)
+                | ty::ConstKind::Bound(..)
+                | ty::ConstKind::Value(_)
+                | ty::ConstKind::Error(_) => {}
+
+                ty::ConstKind::Unevaluated(ct) => {
+                    stack.extend(ct.substs.iter().rev());
+                }
+            }
+        }
+    }
+}
diff --git a/compiler/rustc_middle/src/util/bug.rs b/compiler/rustc_middle/src/util/bug.rs
new file mode 100644
index 000000000..fd7045d6a
--- /dev/null
+++ b/compiler/rustc_middle/src/util/bug.rs
@@ -0,0 +1,54 @@
+// These functions are used by macro expansion for bug! and span_bug!
+
+use crate::ty::{tls, TyCtxt};
+use rustc_errors::MultiSpan;
+use rustc_span::Span;
+use std::fmt;
+use std::panic::{panic_any, Location};
+
+#[cold]
+#[inline(never)]
+#[track_caller]
+pub fn bug_fmt(args: fmt::Arguments<'_>) -> ! {
+    // this wrapper mostly exists so I don't have to write a fully
+    // qualified path of None::<Span> inside the bug!() macro definition
+    opt_span_bug_fmt(None::<Span>, args, Location::caller());
+}
+
+#[cold]
+#[inline(never)]
+#[track_caller]
+pub fn span_bug_fmt<S: Into<MultiSpan>>(span: S, args: fmt::Arguments<'_>) -> ! {
+    opt_span_bug_fmt(Some(span), args, Location::caller());
+}
+
+#[track_caller]
+fn opt_span_bug_fmt<S: Into<MultiSpan>>(
+    span: Option<S>,
+    args: fmt::Arguments<'_>,
+    location: &Location<'_>,
+) -> ! {
+    tls::with_opt(move |tcx| {
+        let msg = format!("{}: {}", location, args);
+        match (tcx, span) {
+            (Some(tcx), Some(span)) => tcx.sess.diagnostic().span_bug(span, &msg),
+            (Some(tcx), None) => tcx.sess.diagnostic().bug(&msg),
+            (None, _) => panic_any(msg),
+        }
+    });
+    unreachable!();
+}
+
+/// A query to trigger a `delay_span_bug`. Clearly, if one has a `tcx` one can already trigger a
+/// `delay_span_bug`, so what is the point of this? It exists to help us test `delay_span_bug`'s
+/// interactions with the query system and incremental.
+pub fn trigger_delay_span_bug(tcx: TyCtxt<'_>, key: rustc_hir::def_id::DefId) {
+    tcx.sess.delay_span_bug(
+        tcx.def_span(key),
+        "delayed span bug triggered by #[rustc_error(delay_span_bug_from_inside_query)]",
+    );
+}
+
+pub fn provide(providers: &mut crate::ty::query::Providers) {
+    *providers = crate::ty::query::Providers { trigger_delay_span_bug, ..*providers };
+}
diff --git a/compiler/rustc_middle/src/util/common.rs b/compiler/rustc_middle/src/util/common.rs
new file mode 100644
index 000000000..08977049d
--- /dev/null
+++ b/compiler/rustc_middle/src/util/common.rs
@@ -0,0 +1,67 @@
+use rustc_data_structures::sync::Lock;
+
+use std::fmt::Debug;
+use std::time::{Duration, Instant};
+
+#[cfg(test)]
+mod tests;
+
+pub fn to_readable_str(mut val: usize) -> String {
+    let mut groups = vec![];
+    loop {
+        let group = val % 1000;
+
+        val /= 1000;
+
+        if val == 0 {
+            groups.push(group.to_string());
+            break;
+        } else {
+            groups.push(format!("{:03}", group));
+        }
+    }
+
+    groups.reverse();
+
+    groups.join("_")
+}
+
+pub fn record_time<T, F>(accu: &Lock<Duration>, f: F) -> T
+where
+    F: FnOnce() -> T,
+{
+    let start = Instant::now();
+    let rv = f();
+    let duration = start.elapsed();
+    let mut accu = accu.lock();
+    *accu += duration;
+    rv
+}
+
+pub fn indent<R, F>(op: F) -> R
+where
+    R: Debug,
+    F: FnOnce() -> R,
+{
+    // Use in conjunction with the log post-processor like `src/etc/indenter`
+    // to make debug output more readable.
+    debug!(">>");
+    let r = op();
+    debug!("<< (Result = {:?})", r);
+    r
+}
+
+pub struct Indenter {
+    _cannot_construct_outside_of_this_module: (),
+}
+
+impl Drop for Indenter {
+    fn drop(&mut self) {
+        debug!("<<");
+    }
+}
+
+pub fn indenter() -> Indenter {
+    debug!(">>");
+    Indenter { _cannot_construct_outside_of_this_module: () }
+}
diff --git a/compiler/rustc_middle/src/util/common/tests.rs b/compiler/rustc_middle/src/util/common/tests.rs
new file mode 100644
index 000000000..9a9fb203c
--- /dev/null
+++ b/compiler/rustc_middle/src/util/common/tests.rs
@@ -0,0 +1,14 @@
+use super::*;
+
+#[test]
+fn test_to_readable_str() {
+    assert_eq!("0", to_readable_str(0));
+    assert_eq!("1", to_readable_str(1));
+    assert_eq!("99", to_readable_str(99));
+    assert_eq!("999", to_readable_str(999));
+    assert_eq!("1_000", to_readable_str(1_000));
+    assert_eq!("1_001", to_readable_str(1_001));
+    assert_eq!("999_999", to_readable_str(999_999));
+    assert_eq!("1_000_000", to_readable_str(1_000_000));
+    assert_eq!("1_234_567", to_readable_str(1_234_567));
+}