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diff --git a/compiler/rustc_middle/src/ty/adjustment.rs b/compiler/rustc_middle/src/ty/adjustment.rs
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+++ b/compiler/rustc_middle/src/ty/adjustment.rs
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+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),
+}