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

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

21 files changed, 9769 insertions, 0 deletions

diff --git a/compiler/rustc_ast/Cargo.toml b/compiler/rustc_ast/Cargo.toml
new file mode 100644
index 000000000..9822e9864
--- /dev/null
+++ b/compiler/rustc_ast/Cargo.toml
@@ -0,0 +1,18 @@
+[package]
+name = "rustc_ast"
+version = "0.0.0"
+edition = "2021"
+
+[lib]
+doctest = false
+
+[dependencies]
+rustc_serialize = { path = "../rustc_serialize" }
+tracing = "0.1"
+rustc_span = { path = "../rustc_span" }
+rustc_data_structures = { path = "../rustc_data_structures" }
+rustc_index = { path = "../rustc_index" }
+rustc_lexer = { path = "../rustc_lexer" }
+rustc_macros = { path = "../rustc_macros" }
+smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+bitflags = "1.2.1"
diff --git a/compiler/rustc_ast/README.md b/compiler/rustc_ast/README.md
new file mode 100644
index 000000000..b2b90fed0
--- /dev/null
+++ b/compiler/rustc_ast/README.md
@@ -0,0 +1,8 @@
+The `rustc_ast` crate contains those things concerned purely with syntax
+– that is, the AST ("abstract syntax tree"), along with some definitions for tokens and token streams, data structures/traits for mutating ASTs, and shared definitions for other AST-related parts of the compiler (like the lexer and macro-expansion).
+
+For more information about how these things work in rustc, see the
+rustc dev guide:
+
+- [Parsing](https://rustc-dev-guide.rust-lang.org/the-parser.html)
+- [Macro Expansion](https://rustc-dev-guide.rust-lang.org/macro-expansion.html)
diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
new file mode 100644
index 000000000..870a7c0be
--- /dev/null
+++ b/compiler/rustc_ast/src/ast.rs
@@ -0,0 +1,3051 @@
+//! The Rust abstract syntax tree module.
+//!
+//! This module contains common structures forming the language AST.
+//! Two main entities in the module are [`Item`] (which represents an AST element with
+//! additional metadata), and [`ItemKind`] (which represents a concrete type and contains
+//! information specific to the type of the item).
+//!
+//! Other module items worth mentioning:
+//! - [`Ty`] and [`TyKind`]: A parsed Rust type.
+//! - [`Expr`] and [`ExprKind`]: A parsed Rust expression.
+//! - [`Pat`] and [`PatKind`]: A parsed Rust pattern. Patterns are often dual to expressions.
+//! - [`Stmt`] and [`StmtKind`]: An executable action that does not return a value.
+//! - [`FnDecl`], [`FnHeader`] and [`Param`]: Metadata associated with a function declaration.
+//! - [`Generics`], [`GenericParam`], [`WhereClause`]: Metadata associated with generic parameters.
+//! - [`EnumDef`] and [`Variant`]: Enum declaration.
+//! - [`Lit`] and [`LitKind`]: Literal expressions.
+//! - [`MacroDef`], [`MacStmtStyle`], [`MacCall`], [`MacDelimiter`]: Macro definition and invocation.
+//! - [`Attribute`]: Metadata associated with item.
+//! - [`UnOp`], [`BinOp`], and [`BinOpKind`]: Unary and binary operators.
+
+pub use crate::util::parser::ExprPrecedence;
+pub use GenericArgs::*;
+pub use UnsafeSource::*;
+
+use crate::ptr::P;
+use crate::token::{self, CommentKind, Delimiter};
+use crate::tokenstream::{DelimSpan, LazyTokenStream, TokenStream};
+
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::stack::ensure_sufficient_stack;
+use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::thin_vec::ThinVec;
+use rustc_macros::HashStable_Generic;
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+use rustc_span::source_map::{respan, Spanned};
+use rustc_span::symbol::{kw, sym, Ident, Symbol};
+use rustc_span::{Span, DUMMY_SP};
+
+use std::cmp::Ordering;
+use std::convert::TryFrom;
+use std::fmt;
+use std::mem;
+
+/// A "Label" is an identifier of some point in sources,
+/// e.g. in the following code:
+///
+/// ```rust
+/// 'outer: loop {
+///     break 'outer;
+/// }
+/// ```
+///
+/// `'outer` is a label.
+#[derive(Clone, Encodable, Decodable, Copy, HashStable_Generic, Eq, PartialEq)]
+pub struct Label {
+    pub ident: Ident,
+}
+
+impl fmt::Debug for Label {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "label({:?})", self.ident)
+    }
+}
+
+/// A "Lifetime" is an annotation of the scope in which variable
+/// can be used, e.g. `'a` in `&'a i32`.
+#[derive(Clone, Encodable, Decodable, Copy, PartialEq, Eq)]
+pub struct Lifetime {
+    pub id: NodeId,
+    pub ident: Ident,
+}
+
+impl fmt::Debug for Lifetime {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "lifetime({}: {})", self.id, self)
+    }
+}
+
+impl fmt::Display for Lifetime {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.ident.name)
+    }
+}
+
+/// A "Path" is essentially Rust's notion of a name.
+///
+/// It's represented as a sequence of identifiers,
+/// along with a bunch of supporting information.
+///
+/// E.g., `std::cmp::PartialEq`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Path {
+    pub span: Span,
+    /// The segments in the path: the things separated by `::`.
+    /// Global paths begin with `kw::PathRoot`.
+    pub segments: Vec<PathSegment>,
+    pub tokens: Option<LazyTokenStream>,
+}
+
+impl PartialEq<Symbol> for Path {
+    #[inline]
+    fn eq(&self, symbol: &Symbol) -> bool {
+        self.segments.len() == 1 && { self.segments[0].ident.name == *symbol }
+    }
+}
+
+impl<CTX: rustc_span::HashStableContext> HashStable<CTX> for Path {
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
+        self.segments.len().hash_stable(hcx, hasher);
+        for segment in &self.segments {
+            segment.ident.hash_stable(hcx, hasher);
+        }
+    }
+}
+
+impl Path {
+    // Convert a span and an identifier to the corresponding
+    // one-segment path.
+    pub fn from_ident(ident: Ident) -> Path {
+        Path { segments: vec![PathSegment::from_ident(ident)], span: ident.span, tokens: None }
+    }
+
+    pub fn is_global(&self) -> bool {
+        !self.segments.is_empty() && self.segments[0].ident.name == kw::PathRoot
+    }
+}
+
+/// A segment of a path: an identifier, an optional lifetime, and a set of types.
+///
+/// E.g., `std`, `String` or `Box<T>`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct PathSegment {
+    /// The identifier portion of this path segment.
+    pub ident: Ident,
+
+    pub id: NodeId,
+
+    /// Type/lifetime parameters attached to this path. They come in
+    /// two flavors: `Path<A,B,C>` and `Path(A,B) -> C`.
+    /// `None` means that no parameter list is supplied (`Path`),
+    /// `Some` means that parameter list is supplied (`Path<X, Y>`)
+    /// but it can be empty (`Path<>`).
+    /// `P` is used as a size optimization for the common case with no parameters.
+    pub args: Option<P<GenericArgs>>,
+}
+
+impl PathSegment {
+    pub fn from_ident(ident: Ident) -> Self {
+        PathSegment { ident, id: DUMMY_NODE_ID, args: None }
+    }
+
+    pub fn path_root(span: Span) -> Self {
+        PathSegment::from_ident(Ident::new(kw::PathRoot, span))
+    }
+
+    pub fn span(&self) -> Span {
+        match &self.args {
+            Some(args) => self.ident.span.to(args.span()),
+            None => self.ident.span,
+        }
+    }
+}
+
+/// The arguments of a path segment.
+///
+/// E.g., `<A, B>` as in `Foo<A, B>` or `(A, B)` as in `Foo(A, B)`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum GenericArgs {
+    /// The `<'a, A, B, C>` in `foo::bar::baz::<'a, A, B, C>`.
+    AngleBracketed(AngleBracketedArgs),
+    /// The `(A, B)` and `C` in `Foo(A, B) -> C`.
+    Parenthesized(ParenthesizedArgs),
+}
+
+impl GenericArgs {
+    pub fn is_angle_bracketed(&self) -> bool {
+        matches!(self, AngleBracketed(..))
+    }
+
+    pub fn span(&self) -> Span {
+        match *self {
+            AngleBracketed(ref data) => data.span,
+            Parenthesized(ref data) => data.span,
+        }
+    }
+}
+
+/// Concrete argument in the sequence of generic args.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum GenericArg {
+    /// `'a` in `Foo<'a>`
+    Lifetime(Lifetime),
+    /// `Bar` in `Foo<Bar>`
+    Type(P<Ty>),
+    /// `1` in `Foo<1>`
+    Const(AnonConst),
+}
+
+impl GenericArg {
+    pub fn span(&self) -> Span {
+        match self {
+            GenericArg::Lifetime(lt) => lt.ident.span,
+            GenericArg::Type(ty) => ty.span,
+            GenericArg::Const(ct) => ct.value.span,
+        }
+    }
+}
+
+/// A path like `Foo<'a, T>`.
+#[derive(Clone, Encodable, Decodable, Debug, Default)]
+pub struct AngleBracketedArgs {
+    /// The overall span.
+    pub span: Span,
+    /// The comma separated parts in the `<...>`.
+    pub args: Vec<AngleBracketedArg>,
+}
+
+/// Either an argument for a parameter e.g., `'a`, `Vec<u8>`, `0`,
+/// or a constraint on an associated item, e.g., `Item = String` or `Item: Bound`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum AngleBracketedArg {
+    /// Argument for a generic parameter.
+    Arg(GenericArg),
+    /// Constraint for an associated item.
+    Constraint(AssocConstraint),
+}
+
+impl AngleBracketedArg {
+    pub fn span(&self) -> Span {
+        match self {
+            AngleBracketedArg::Arg(arg) => arg.span(),
+            AngleBracketedArg::Constraint(constraint) => constraint.span,
+        }
+    }
+}
+
+impl Into<Option<P<GenericArgs>>> for AngleBracketedArgs {
+    fn into(self) -> Option<P<GenericArgs>> {
+        Some(P(GenericArgs::AngleBracketed(self)))
+    }
+}
+
+impl Into<Option<P<GenericArgs>>> for ParenthesizedArgs {
+    fn into(self) -> Option<P<GenericArgs>> {
+        Some(P(GenericArgs::Parenthesized(self)))
+    }
+}
+
+/// A path like `Foo(A, B) -> C`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct ParenthesizedArgs {
+    /// ```text
+    /// Foo(A, B) -> C
+    /// ^^^^^^^^^^^^^^
+    /// ```
+    pub span: Span,
+
+    /// `(A, B)`
+    pub inputs: Vec<P<Ty>>,
+
+    /// ```text
+    /// Foo(A, B) -> C
+    ///    ^^^^^^
+    /// ```
+    pub inputs_span: Span,
+
+    /// `C`
+    pub output: FnRetTy,
+}
+
+impl ParenthesizedArgs {
+    pub fn as_angle_bracketed_args(&self) -> AngleBracketedArgs {
+        let args = self
+            .inputs
+            .iter()
+            .cloned()
+            .map(|input| AngleBracketedArg::Arg(GenericArg::Type(input)))
+            .collect();
+        AngleBracketedArgs { span: self.inputs_span, args }
+    }
+}
+
+pub use crate::node_id::{NodeId, CRATE_NODE_ID, DUMMY_NODE_ID};
+
+/// A modifier on a bound, e.g., `?Trait` or `~const Trait`.
+///
+/// Negative bounds should also be handled here.
+#[derive(Copy, Clone, PartialEq, Eq, Encodable, Decodable, Debug)]
+pub enum TraitBoundModifier {
+    /// No modifiers
+    None,
+
+    /// `?Trait`
+    Maybe,
+
+    /// `~const Trait`
+    MaybeConst,
+
+    /// `~const ?Trait`
+    //
+    // This parses but will be rejected during AST validation.
+    MaybeConstMaybe,
+}
+
+/// The AST represents all type param bounds as types.
+/// `typeck::collect::compute_bounds` matches these against
+/// the "special" built-in traits (see `middle::lang_items`) and
+/// detects `Copy`, `Send` and `Sync`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum GenericBound {
+    Trait(PolyTraitRef, TraitBoundModifier),
+    Outlives(Lifetime),
+}
+
+impl GenericBound {
+    pub fn span(&self) -> Span {
+        match self {
+            GenericBound::Trait(ref t, ..) => t.span,
+            GenericBound::Outlives(ref l) => l.ident.span,
+        }
+    }
+}
+
+pub type GenericBounds = Vec<GenericBound>;
+
+/// Specifies the enforced ordering for generic parameters. In the future,
+/// if we wanted to relax this order, we could override `PartialEq` and
+/// `PartialOrd`, to allow the kinds to be unordered.
+#[derive(Hash, Clone, Copy)]
+pub enum ParamKindOrd {
+    Lifetime,
+    Type,
+    Const,
+    // `Infer` is not actually constructed directly from the AST, but is implicitly constructed
+    // during HIR lowering, and `ParamKindOrd` will implicitly order inferred variables last.
+    Infer,
+}
+
+impl Ord for ParamKindOrd {
+    fn cmp(&self, other: &Self) -> Ordering {
+        use ParamKindOrd::*;
+        let to_int = |v| match v {
+            Lifetime => 0,
+            Infer | Type | Const => 1,
+        };
+
+        to_int(*self).cmp(&to_int(*other))
+    }
+}
+impl PartialOrd for ParamKindOrd {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+impl PartialEq for ParamKindOrd {
+    fn eq(&self, other: &Self) -> bool {
+        self.cmp(other) == Ordering::Equal
+    }
+}
+impl Eq for ParamKindOrd {}
+
+impl fmt::Display for ParamKindOrd {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            ParamKindOrd::Lifetime => "lifetime".fmt(f),
+            ParamKindOrd::Type => "type".fmt(f),
+            ParamKindOrd::Const { .. } => "const".fmt(f),
+            ParamKindOrd::Infer => "infer".fmt(f),
+        }
+    }
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum GenericParamKind {
+    /// A lifetime definition (e.g., `'a: 'b + 'c + 'd`).
+    Lifetime,
+    Type {
+        default: Option<P<Ty>>,
+    },
+    Const {
+        ty: P<Ty>,
+        /// Span of the `const` keyword.
+        kw_span: Span,
+        /// Optional default value for the const generic param
+        default: Option<AnonConst>,
+    },
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct GenericParam {
+    pub id: NodeId,
+    pub ident: Ident,
+    pub attrs: AttrVec,
+    pub bounds: GenericBounds,
+    pub is_placeholder: bool,
+    pub kind: GenericParamKind,
+    pub colon_span: Option<Span>,
+}
+
+impl GenericParam {
+    pub fn span(&self) -> Span {
+        match &self.kind {
+            GenericParamKind::Lifetime | GenericParamKind::Type { default: None } => {
+                self.ident.span
+            }
+            GenericParamKind::Type { default: Some(ty) } => self.ident.span.to(ty.span),
+            GenericParamKind::Const { kw_span, default: Some(default), .. } => {
+                kw_span.to(default.value.span)
+            }
+            GenericParamKind::Const { kw_span, default: None, ty } => kw_span.to(ty.span),
+        }
+    }
+}
+
+/// Represents lifetime, type and const parameters attached to a declaration of
+/// a function, enum, trait, etc.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Generics {
+    pub params: Vec<GenericParam>,
+    pub where_clause: WhereClause,
+    pub span: Span,
+}
+
+impl Default for Generics {
+    /// Creates an instance of `Generics`.
+    fn default() -> Generics {
+        Generics {
+            params: Vec::new(),
+            where_clause: WhereClause {
+                has_where_token: false,
+                predicates: Vec::new(),
+                span: DUMMY_SP,
+            },
+            span: DUMMY_SP,
+        }
+    }
+}
+
+/// A where-clause in a definition.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct WhereClause {
+    /// `true` if we ate a `where` token: this can happen
+    /// if we parsed no predicates (e.g. `struct Foo where {}`).
+    /// This allows us to pretty-print accurately.
+    pub has_where_token: bool,
+    pub predicates: Vec<WherePredicate>,
+    pub span: Span,
+}
+
+/// A single predicate in a where-clause.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum WherePredicate {
+    /// A type binding (e.g., `for<'c> Foo: Send + Clone + 'c`).
+    BoundPredicate(WhereBoundPredicate),
+    /// A lifetime predicate (e.g., `'a: 'b + 'c`).
+    RegionPredicate(WhereRegionPredicate),
+    /// An equality predicate (unsupported).
+    EqPredicate(WhereEqPredicate),
+}
+
+impl WherePredicate {
+    pub fn span(&self) -> Span {
+        match self {
+            WherePredicate::BoundPredicate(p) => p.span,
+            WherePredicate::RegionPredicate(p) => p.span,
+            WherePredicate::EqPredicate(p) => p.span,
+        }
+    }
+}
+
+/// A type bound.
+///
+/// E.g., `for<'c> Foo: Send + Clone + 'c`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct WhereBoundPredicate {
+    pub span: Span,
+    /// Any generics from a `for` binding.
+    pub bound_generic_params: Vec<GenericParam>,
+    /// The type being bounded.
+    pub bounded_ty: P<Ty>,
+    /// Trait and lifetime bounds (`Clone + Send + 'static`).
+    pub bounds: GenericBounds,
+}
+
+/// A lifetime predicate.
+///
+/// E.g., `'a: 'b + 'c`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct WhereRegionPredicate {
+    pub span: Span,
+    pub lifetime: Lifetime,
+    pub bounds: GenericBounds,
+}
+
+/// An equality predicate (unsupported).
+///
+/// E.g., `T = int`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct WhereEqPredicate {
+    pub id: NodeId,
+    pub span: Span,
+    pub lhs_ty: P<Ty>,
+    pub rhs_ty: P<Ty>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Crate {
+    pub attrs: Vec<Attribute>,
+    pub items: Vec<P<Item>>,
+    pub spans: ModSpans,
+    /// Must be equal to `CRATE_NODE_ID` after the crate root is expanded, but may hold
+    /// expansion placeholders or an unassigned value (`DUMMY_NODE_ID`) before that.
+    pub id: NodeId,
+    pub is_placeholder: bool,
+}
+
+/// Possible values inside of compile-time attribute lists.
+///
+/// E.g., the '..' in `#[name(..)]`.
+#[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum NestedMetaItem {
+    /// A full MetaItem, for recursive meta items.
+    MetaItem(MetaItem),
+    /// A literal.
+    ///
+    /// E.g., `"foo"`, `64`, `true`.
+    Literal(Lit),
+}
+
+/// A spanned compile-time attribute item.
+///
+/// E.g., `#[test]`, `#[derive(..)]`, `#[rustfmt::skip]` or `#[feature = "foo"]`.
+#[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
+pub struct MetaItem {
+    pub path: Path,
+    pub kind: MetaItemKind,
+    pub span: Span,
+}
+
+/// A compile-time attribute item.
+///
+/// E.g., `#[test]`, `#[derive(..)]` or `#[feature = "foo"]`.
+#[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum MetaItemKind {
+    /// Word meta item.
+    ///
+    /// E.g., `test` as in `#[test]`.
+    Word,
+    /// List meta item.
+    ///
+    /// E.g., `derive(..)` as in `#[derive(..)]`.
+    List(Vec<NestedMetaItem>),
+    /// Name value meta item.
+    ///
+    /// E.g., `feature = "foo"` as in `#[feature = "foo"]`.
+    NameValue(Lit),
+}
+
+/// A block (`{ .. }`).
+///
+/// E.g., `{ .. }` as in `fn foo() { .. }`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Block {
+    /// The statements in the block.
+    pub stmts: Vec<Stmt>,
+    pub id: NodeId,
+    /// Distinguishes between `unsafe { ... }` and `{ ... }`.
+    pub rules: BlockCheckMode,
+    pub span: Span,
+    pub tokens: Option<LazyTokenStream>,
+    /// The following *isn't* a parse error, but will cause multiple errors in following stages.
+    /// ```compile_fail
+    /// let x = {
+    ///     foo: var
+    /// };
+    /// ```
+    /// #34255
+    pub could_be_bare_literal: bool,
+}
+
+/// A match pattern.
+///
+/// Patterns appear in match statements and some other contexts, such as `let` and `if let`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Pat {
+    pub id: NodeId,
+    pub kind: PatKind,
+    pub span: Span,
+    pub tokens: Option<LazyTokenStream>,
+}
+
+impl Pat {
+    /// Attempt reparsing the pattern as a type.
+    /// This is intended for use by diagnostics.
+    pub fn to_ty(&self) -> Option<P<Ty>> {
+        let kind = match &self.kind {
+            // In a type expression `_` is an inference variable.
+            PatKind::Wild => TyKind::Infer,
+            // An IDENT pattern with no binding mode would be valid as path to a type. E.g. `u32`.
+            PatKind::Ident(BindingMode::ByValue(Mutability::Not), ident, None) => {
+                TyKind::Path(None, Path::from_ident(*ident))
+            }
+            PatKind::Path(qself, path) => TyKind::Path(qself.clone(), path.clone()),
+            PatKind::MacCall(mac) => TyKind::MacCall(mac.clone()),
+            // `&mut? P` can be reinterpreted as `&mut? T` where `T` is `P` reparsed as a type.
+            PatKind::Ref(pat, mutbl) => {
+                pat.to_ty().map(|ty| TyKind::Rptr(None, MutTy { ty, mutbl: *mutbl }))?
+            }
+            // A slice/array pattern `[P]` can be reparsed as `[T]`, an unsized array,
+            // when `P` can be reparsed as a type `T`.
+            PatKind::Slice(pats) if pats.len() == 1 => pats[0].to_ty().map(TyKind::Slice)?,
+            // A tuple pattern `(P0, .., Pn)` can be reparsed as `(T0, .., Tn)`
+            // assuming `T0` to `Tn` are all syntactically valid as types.
+            PatKind::Tuple(pats) => {
+                let mut tys = Vec::with_capacity(pats.len());
+                // FIXME(#48994) - could just be collected into an Option<Vec>
+                for pat in pats {
+                    tys.push(pat.to_ty()?);
+                }
+                TyKind::Tup(tys)
+            }
+            _ => return None,
+        };
+
+        Some(P(Ty { kind, id: self.id, span: self.span, tokens: None }))
+    }
+
+    /// Walk top-down and call `it` in each place where a pattern occurs
+    /// starting with the root pattern `walk` is called on. If `it` returns
+    /// false then we will descend no further but siblings will be processed.
+    pub fn walk(&self, it: &mut impl FnMut(&Pat) -> bool) {
+        if !it(self) {
+            return;
+        }
+
+        match &self.kind {
+            // Walk into the pattern associated with `Ident` (if any).
+            PatKind::Ident(_, _, Some(p)) => p.walk(it),
+
+            // Walk into each field of struct.
+            PatKind::Struct(_, _, fields, _) => fields.iter().for_each(|field| field.pat.walk(it)),
+
+            // Sequence of patterns.
+            PatKind::TupleStruct(_, _, s)
+            | PatKind::Tuple(s)
+            | PatKind::Slice(s)
+            | PatKind::Or(s) => s.iter().for_each(|p| p.walk(it)),
+
+            // Trivial wrappers over inner patterns.
+            PatKind::Box(s) | PatKind::Ref(s, _) | PatKind::Paren(s) => s.walk(it),
+
+            // These patterns do not contain subpatterns, skip.
+            PatKind::Wild
+            | PatKind::Rest
+            | PatKind::Lit(_)
+            | PatKind::Range(..)
+            | PatKind::Ident(..)
+            | PatKind::Path(..)
+            | PatKind::MacCall(_) => {}
+        }
+    }
+
+    /// Is this a `..` pattern?
+    pub fn is_rest(&self) -> bool {
+        matches!(self.kind, PatKind::Rest)
+    }
+}
+
+/// A single field in a struct pattern.
+///
+/// Patterns like the fields of `Foo { x, ref y, ref mut z }`
+/// are treated the same as `x: x, y: ref y, z: ref mut z`,
+/// except when `is_shorthand` is true.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct PatField {
+    /// The identifier for the field.
+    pub ident: Ident,
+    /// The pattern the field is destructured to.
+    pub pat: P<Pat>,
+    pub is_shorthand: bool,
+    pub attrs: AttrVec,
+    pub id: NodeId,
+    pub span: Span,
+    pub is_placeholder: bool,
+}
+
+#[derive(Clone, PartialEq, Encodable, Decodable, Debug, Copy)]
+pub enum BindingMode {
+    ByRef(Mutability),
+    ByValue(Mutability),
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum RangeEnd {
+    /// `..=` or `...`
+    Included(RangeSyntax),
+    /// `..`
+    Excluded,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum RangeSyntax {
+    /// `...`
+    DotDotDot,
+    /// `..=`
+    DotDotEq,
+}
+
+/// All the different flavors of pattern that Rust recognizes.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum PatKind {
+    /// Represents a wildcard pattern (`_`).
+    Wild,
+
+    /// A `PatKind::Ident` may either be a new bound variable (`ref mut binding @ OPT_SUBPATTERN`),
+    /// or a unit struct/variant pattern, or a const pattern (in the last two cases the third
+    /// field must be `None`). Disambiguation cannot be done with parser alone, so it happens
+    /// during name resolution.
+    Ident(BindingMode, Ident, Option<P<Pat>>),
+
+    /// A struct or struct variant pattern (e.g., `Variant {x, y, ..}`).
+    /// The `bool` is `true` in the presence of a `..`.
+    Struct(Option<QSelf>, Path, Vec<PatField>, /* recovered */ bool),
+
+    /// A tuple struct/variant pattern (`Variant(x, y, .., z)`).
+    TupleStruct(Option<QSelf>, Path, Vec<P<Pat>>),
+
+    /// An or-pattern `A | B | C`.
+    /// Invariant: `pats.len() >= 2`.
+    Or(Vec<P<Pat>>),
+
+    /// A possibly qualified path pattern.
+    /// Unqualified path patterns `A::B::C` can legally refer to variants, structs, constants
+    /// or associated constants. Qualified path patterns `<A>::B::C`/`<A as Trait>::B::C` can
+    /// only legally refer to associated constants.
+    Path(Option<QSelf>, Path),
+
+    /// A tuple pattern (`(a, b)`).
+    Tuple(Vec<P<Pat>>),
+
+    /// A `box` pattern.
+    Box(P<Pat>),
+
+    /// A reference pattern (e.g., `&mut (a, b)`).
+    Ref(P<Pat>, Mutability),
+
+    /// A literal.
+    Lit(P<Expr>),
+
+    /// A range pattern (e.g., `1...2`, `1..2`, `1..`, `..2`, `1..=2`, `..=2`).
+    Range(Option<P<Expr>>, Option<P<Expr>>, Spanned<RangeEnd>),
+
+    /// A slice pattern `[a, b, c]`.
+    Slice(Vec<P<Pat>>),
+
+    /// A rest pattern `..`.
+    ///
+    /// Syntactically it is valid anywhere.
+    ///
+    /// Semantically however, it only has meaning immediately inside:
+    /// - a slice pattern: `[a, .., b]`,
+    /// - a binding pattern immediately inside a slice pattern: `[a, r @ ..]`,
+    /// - a tuple pattern: `(a, .., b)`,
+    /// - a tuple struct/variant pattern: `$path(a, .., b)`.
+    ///
+    /// In all of these cases, an additional restriction applies,
+    /// only one rest pattern may occur in the pattern sequences.
+    Rest,
+
+    /// Parentheses in patterns used for grouping (i.e., `(PAT)`).
+    Paren(P<Pat>),
+
+    /// A macro pattern; pre-expansion.
+    MacCall(MacCall),
+}
+
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, Copy)]
+#[derive(HashStable_Generic, Encodable, Decodable)]
+pub enum Mutability {
+    Mut,
+    Not,
+}
+
+impl Mutability {
+    pub fn invert(self) -> Self {
+        match self {
+            Mutability::Mut => Mutability::Not,
+            Mutability::Not => Mutability::Mut,
+        }
+    }
+
+    pub fn prefix_str(&self) -> &'static str {
+        match self {
+            Mutability::Mut => "mut ",
+            Mutability::Not => "",
+        }
+    }
+}
+
+/// The kind of borrow in an `AddrOf` expression,
+/// e.g., `&place` or `&raw const place`.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+#[derive(Encodable, Decodable, HashStable_Generic)]
+pub enum BorrowKind {
+    /// A normal borrow, `&$expr` or `&mut $expr`.
+    /// The resulting type is either `&'a T` or `&'a mut T`
+    /// where `T = typeof($expr)` and `'a` is some lifetime.
+    Ref,
+    /// A raw borrow, `&raw const $expr` or `&raw mut $expr`.
+    /// The resulting type is either `*const T` or `*mut T`
+    /// where `T = typeof($expr)`.
+    Raw,
+}
+
+#[derive(Clone, PartialEq, Encodable, Decodable, Debug, Copy)]
+pub enum BinOpKind {
+    /// The `+` operator (addition)
+    Add,
+    /// The `-` operator (subtraction)
+    Sub,
+    /// The `*` operator (multiplication)
+    Mul,
+    /// The `/` operator (division)
+    Div,
+    /// The `%` operator (modulus)
+    Rem,
+    /// The `&&` operator (logical and)
+    And,
+    /// The `||` operator (logical or)
+    Or,
+    /// The `^` operator (bitwise xor)
+    BitXor,
+    /// The `&` operator (bitwise and)
+    BitAnd,
+    /// The `|` operator (bitwise or)
+    BitOr,
+    /// The `<<` operator (shift left)
+    Shl,
+    /// The `>>` operator (shift right)
+    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,
+}
+
+impl BinOpKind {
+    pub fn to_string(&self) -> &'static str {
+        use BinOpKind::*;
+        match *self {
+            Add => "+",
+            Sub => "-",
+            Mul => "*",
+            Div => "/",
+            Rem => "%",
+            And => "&&",
+            Or => "||",
+            BitXor => "^",
+            BitAnd => "&",
+            BitOr => "|",
+            Shl => "<<",
+            Shr => ">>",
+            Eq => "==",
+            Lt => "<",
+            Le => "<=",
+            Ne => "!=",
+            Ge => ">=",
+            Gt => ">",
+        }
+    }
+    pub fn lazy(&self) -> bool {
+        matches!(self, BinOpKind::And | BinOpKind::Or)
+    }
+
+    pub fn is_comparison(&self) -> bool {
+        use BinOpKind::*;
+        // Note for developers: please keep this as is;
+        // we want compilation to fail if another variant is added.
+        match *self {
+            Eq | Lt | Le | Ne | Gt | Ge => true,
+            And | Or | Add | Sub | Mul | Div | Rem | BitXor | BitAnd | BitOr | Shl | Shr => false,
+        }
+    }
+}
+
+pub type BinOp = Spanned<BinOpKind>;
+
+/// Unary operator.
+///
+/// Note that `&data` is not an operator, it's an `AddrOf` expression.
+#[derive(Clone, Encodable, Decodable, Debug, Copy)]
+pub enum UnOp {
+    /// The `*` operator for dereferencing
+    Deref,
+    /// The `!` operator for logical inversion
+    Not,
+    /// The `-` operator for negation
+    Neg,
+}
+
+impl UnOp {
+    pub fn to_string(op: UnOp) -> &'static str {
+        match op {
+            UnOp::Deref => "*",
+            UnOp::Not => "!",
+            UnOp::Neg => "-",
+        }
+    }
+}
+
+/// A statement
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Stmt {
+    pub id: NodeId,
+    pub kind: StmtKind,
+    pub span: Span,
+}
+
+impl Stmt {
+    pub fn has_trailing_semicolon(&self) -> bool {
+        match &self.kind {
+            StmtKind::Semi(_) => true,
+            StmtKind::MacCall(mac) => matches!(mac.style, MacStmtStyle::Semicolon),
+            _ => false,
+        }
+    }
+
+    /// Converts a parsed `Stmt` to a `Stmt` with
+    /// a trailing semicolon.
+    ///
+    /// This only modifies the parsed AST struct, not the attached
+    /// `LazyTokenStream`. The parser is responsible for calling
+    /// `CreateTokenStream::add_trailing_semi` when there is actually
+    /// a semicolon in the tokenstream.
+    pub fn add_trailing_semicolon(mut self) -> Self {
+        self.kind = match self.kind {
+            StmtKind::Expr(expr) => StmtKind::Semi(expr),
+            StmtKind::MacCall(mac) => {
+                StmtKind::MacCall(mac.map(|MacCallStmt { mac, style: _, attrs, tokens }| {
+                    MacCallStmt { mac, style: MacStmtStyle::Semicolon, attrs, tokens }
+                }))
+            }
+            kind => kind,
+        };
+
+        self
+    }
+
+    pub fn is_item(&self) -> bool {
+        matches!(self.kind, StmtKind::Item(_))
+    }
+
+    pub fn is_expr(&self) -> bool {
+        matches!(self.kind, StmtKind::Expr(_))
+    }
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum StmtKind {
+    /// A local (let) binding.
+    Local(P<Local>),
+    /// An item definition.
+    Item(P<Item>),
+    /// Expr without trailing semi-colon.
+    Expr(P<Expr>),
+    /// Expr with a trailing semi-colon.
+    Semi(P<Expr>),
+    /// Just a trailing semi-colon.
+    Empty,
+    /// Macro.
+    MacCall(P<MacCallStmt>),
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct MacCallStmt {
+    pub mac: MacCall,
+    pub style: MacStmtStyle,
+    pub attrs: AttrVec,
+    pub tokens: Option<LazyTokenStream>,
+}
+
+#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug)]
+pub enum MacStmtStyle {
+    /// The macro statement had a trailing semicolon (e.g., `foo! { ... };`
+    /// `foo!(...);`, `foo![...];`).
+    Semicolon,
+    /// The macro statement had braces (e.g., `foo! { ... }`).
+    Braces,
+    /// The macro statement had parentheses or brackets and no semicolon (e.g.,
+    /// `foo!(...)`). All of these will end up being converted into macro
+    /// expressions.
+    NoBraces,
+}
+
+/// Local represents a `let` statement, e.g., `let <pat>:<ty> = <expr>;`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Local {
+    pub id: NodeId,
+    pub pat: P<Pat>,
+    pub ty: Option<P<Ty>>,
+    pub kind: LocalKind,
+    pub span: Span,
+    pub attrs: AttrVec,
+    pub tokens: Option<LazyTokenStream>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum LocalKind {
+    /// Local declaration.
+    /// Example: `let x;`
+    Decl,
+    /// Local declaration with an initializer.
+    /// Example: `let x = y;`
+    Init(P<Expr>),
+    /// Local declaration with an initializer and an `else` clause.
+    /// Example: `let Some(x) = y else { return };`
+    InitElse(P<Expr>, P<Block>),
+}
+
+impl LocalKind {
+    pub fn init(&self) -> Option<&Expr> {
+        match self {
+            Self::Decl => None,
+            Self::Init(i) | Self::InitElse(i, _) => Some(i),
+        }
+    }
+
+    pub fn init_else_opt(&self) -> Option<(&Expr, Option<&Block>)> {
+        match self {
+            Self::Decl => None,
+            Self::Init(init) => Some((init, None)),
+            Self::InitElse(init, els) => Some((init, Some(els))),
+        }
+    }
+}
+
+/// An arm of a 'match'.
+///
+/// E.g., `0..=10 => { println!("match!") }` as in
+///
+/// ```
+/// match 123 {
+///     0..=10 => { println!("match!") },
+///     _ => { println!("no match!") },
+/// }
+/// ```
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Arm {
+    pub attrs: AttrVec,
+    /// Match arm pattern, e.g. `10` in `match foo { 10 => {}, _ => {} }`
+    pub pat: P<Pat>,
+    /// Match arm guard, e.g. `n > 10` in `match foo { n if n > 10 => {}, _ => {} }`
+    pub guard: Option<P<Expr>>,
+    /// Match arm body.
+    pub body: P<Expr>,
+    pub span: Span,
+    pub id: NodeId,
+    pub is_placeholder: bool,
+}
+
+/// A single field in a struct expression, e.g. `x: value` and `y` in `Foo { x: value, y }`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct ExprField {
+    pub attrs: AttrVec,
+    pub id: NodeId,
+    pub span: Span,
+    pub ident: Ident,
+    pub expr: P<Expr>,
+    pub is_shorthand: bool,
+    pub is_placeholder: bool,
+}
+
+#[derive(Clone, PartialEq, Encodable, Decodable, Debug, Copy)]
+pub enum BlockCheckMode {
+    Default,
+    Unsafe(UnsafeSource),
+}
+
+#[derive(Clone, PartialEq, Encodable, Decodable, Debug, Copy)]
+pub enum UnsafeSource {
+    CompilerGenerated,
+    UserProvided,
+}
+
+/// A constant (expression) that's not an item or associated item,
+/// but needs its own `DefId` for type-checking, const-eval, etc.
+/// These are usually found nested inside types (e.g., array lengths)
+/// or expressions (e.g., repeat counts), and also used to define
+/// explicit discriminant values for enum variants.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct AnonConst {
+    pub id: NodeId,
+    pub value: P<Expr>,
+}
+
+/// An expression.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Expr {
+    pub id: NodeId,
+    pub kind: ExprKind,
+    pub span: Span,
+    pub attrs: AttrVec,
+    pub tokens: Option<LazyTokenStream>,
+}
+
+impl Expr {
+    /// Returns `true` if this expression would be valid somewhere that expects a value;
+    /// for example, an `if` condition.
+    pub fn returns(&self) -> bool {
+        if let ExprKind::Block(ref block, _) = self.kind {
+            match block.stmts.last().map(|last_stmt| &last_stmt.kind) {
+                // Implicit return
+                Some(StmtKind::Expr(_)) => true,
+                // Last statement is an explicit return?
+                Some(StmtKind::Semi(expr)) => matches!(expr.kind, ExprKind::Ret(_)),
+                // This is a block that doesn't end in either an implicit or explicit return.
+                _ => false,
+            }
+        } else {
+            // This is not a block, it is a value.
+            true
+        }
+    }
+
+    /// Is this expr either `N`, or `{ N }`.
+    ///
+    /// If this is not the case, name resolution does not resolve `N` when using
+    /// `min_const_generics` as more complex expressions are not supported.
+    pub fn is_potential_trivial_const_param(&self) -> bool {
+        let this = if let ExprKind::Block(ref block, None) = self.kind {
+            if block.stmts.len() == 1 {
+                if let StmtKind::Expr(ref expr) = block.stmts[0].kind { expr } else { self }
+            } else {
+                self
+            }
+        } else {
+            self
+        };
+
+        if let ExprKind::Path(None, ref path) = this.kind {
+            if path.segments.len() == 1 && path.segments[0].args.is_none() {
+                return true;
+            }
+        }
+
+        false
+    }
+
+    pub fn to_bound(&self) -> Option<GenericBound> {
+        match &self.kind {
+            ExprKind::Path(None, path) => Some(GenericBound::Trait(
+                PolyTraitRef::new(Vec::new(), path.clone(), self.span),
+                TraitBoundModifier::None,
+            )),
+            _ => None,
+        }
+    }
+
+    pub fn peel_parens(&self) -> &Expr {
+        let mut expr = self;
+        while let ExprKind::Paren(inner) = &expr.kind {
+            expr = &inner;
+        }
+        expr
+    }
+
+    /// Attempts to reparse as `Ty` (for diagnostic purposes).
+    pub fn to_ty(&self) -> Option<P<Ty>> {
+        let kind = match &self.kind {
+            // Trivial conversions.
+            ExprKind::Path(qself, path) => TyKind::Path(qself.clone(), path.clone()),
+            ExprKind::MacCall(mac) => TyKind::MacCall(mac.clone()),
+
+            ExprKind::Paren(expr) => expr.to_ty().map(TyKind::Paren)?,
+
+            ExprKind::AddrOf(BorrowKind::Ref, mutbl, expr) => {
+                expr.to_ty().map(|ty| TyKind::Rptr(None, MutTy { ty, mutbl: *mutbl }))?
+            }
+
+            ExprKind::Repeat(expr, expr_len) => {
+                expr.to_ty().map(|ty| TyKind::Array(ty, expr_len.clone()))?
+            }
+
+            ExprKind::Array(exprs) if exprs.len() == 1 => exprs[0].to_ty().map(TyKind::Slice)?,
+
+            ExprKind::Tup(exprs) => {
+                let tys = exprs.iter().map(|expr| expr.to_ty()).collect::<Option<Vec<_>>>()?;
+                TyKind::Tup(tys)
+            }
+
+            // If binary operator is `Add` and both `lhs` and `rhs` are trait bounds,
+            // then type of result is trait object.
+            // Otherwise we don't assume the result type.
+            ExprKind::Binary(binop, lhs, rhs) if binop.node == BinOpKind::Add => {
+                if let (Some(lhs), Some(rhs)) = (lhs.to_bound(), rhs.to_bound()) {
+                    TyKind::TraitObject(vec![lhs, rhs], TraitObjectSyntax::None)
+                } else {
+                    return None;
+                }
+            }
+
+            ExprKind::Underscore => TyKind::Infer,
+
+            // This expression doesn't look like a type syntactically.
+            _ => return None,
+        };
+
+        Some(P(Ty { kind, id: self.id, span: self.span, tokens: None }))
+    }
+
+    pub fn precedence(&self) -> ExprPrecedence {
+        match self.kind {
+            ExprKind::Box(_) => ExprPrecedence::Box,
+            ExprKind::Array(_) => ExprPrecedence::Array,
+            ExprKind::ConstBlock(_) => ExprPrecedence::ConstBlock,
+            ExprKind::Call(..) => ExprPrecedence::Call,
+            ExprKind::MethodCall(..) => ExprPrecedence::MethodCall,
+            ExprKind::Tup(_) => ExprPrecedence::Tup,
+            ExprKind::Binary(op, ..) => ExprPrecedence::Binary(op.node),
+            ExprKind::Unary(..) => ExprPrecedence::Unary,
+            ExprKind::Lit(_) => ExprPrecedence::Lit,
+            ExprKind::Type(..) | ExprKind::Cast(..) => ExprPrecedence::Cast,
+            ExprKind::Let(..) => ExprPrecedence::Let,
+            ExprKind::If(..) => ExprPrecedence::If,
+            ExprKind::While(..) => ExprPrecedence::While,
+            ExprKind::ForLoop(..) => ExprPrecedence::ForLoop,
+            ExprKind::Loop(..) => ExprPrecedence::Loop,
+            ExprKind::Match(..) => ExprPrecedence::Match,
+            ExprKind::Closure(..) => ExprPrecedence::Closure,
+            ExprKind::Block(..) => ExprPrecedence::Block,
+            ExprKind::TryBlock(..) => ExprPrecedence::TryBlock,
+            ExprKind::Async(..) => ExprPrecedence::Async,
+            ExprKind::Await(..) => ExprPrecedence::Await,
+            ExprKind::Assign(..) => ExprPrecedence::Assign,
+            ExprKind::AssignOp(..) => ExprPrecedence::AssignOp,
+            ExprKind::Field(..) => ExprPrecedence::Field,
+            ExprKind::Index(..) => ExprPrecedence::Index,
+            ExprKind::Range(..) => ExprPrecedence::Range,
+            ExprKind::Underscore => ExprPrecedence::Path,
+            ExprKind::Path(..) => ExprPrecedence::Path,
+            ExprKind::AddrOf(..) => ExprPrecedence::AddrOf,
+            ExprKind::Break(..) => ExprPrecedence::Break,
+            ExprKind::Continue(..) => ExprPrecedence::Continue,
+            ExprKind::Ret(..) => ExprPrecedence::Ret,
+            ExprKind::InlineAsm(..) => ExprPrecedence::InlineAsm,
+            ExprKind::MacCall(..) => ExprPrecedence::Mac,
+            ExprKind::Struct(..) => ExprPrecedence::Struct,
+            ExprKind::Repeat(..) => ExprPrecedence::Repeat,
+            ExprKind::Paren(..) => ExprPrecedence::Paren,
+            ExprKind::Try(..) => ExprPrecedence::Try,
+            ExprKind::Yield(..) => ExprPrecedence::Yield,
+            ExprKind::Yeet(..) => ExprPrecedence::Yeet,
+            ExprKind::Err => ExprPrecedence::Err,
+        }
+    }
+
+    pub fn take(&mut self) -> Self {
+        mem::replace(
+            self,
+            Expr {
+                id: DUMMY_NODE_ID,
+                kind: ExprKind::Err,
+                span: DUMMY_SP,
+                attrs: ThinVec::new(),
+                tokens: None,
+            },
+        )
+    }
+
+    // To a first-order approximation, is this a pattern
+    pub fn is_approximately_pattern(&self) -> bool {
+        match &self.peel_parens().kind {
+            ExprKind::Box(_)
+            | ExprKind::Array(_)
+            | ExprKind::Call(_, _)
+            | ExprKind::Tup(_)
+            | ExprKind::Lit(_)
+            | ExprKind::Range(_, _, _)
+            | ExprKind::Underscore
+            | ExprKind::Path(_, _)
+            | ExprKind::Struct(_) => true,
+            _ => false,
+        }
+    }
+}
+
+/// Limit types of a range (inclusive or exclusive)
+#[derive(Copy, Clone, PartialEq, Encodable, Decodable, Debug)]
+pub enum RangeLimits {
+    /// Inclusive at the beginning, exclusive at the end
+    HalfOpen,
+    /// Inclusive at the beginning and end
+    Closed,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum StructRest {
+    /// `..x`.
+    Base(P<Expr>),
+    /// `..`.
+    Rest(Span),
+    /// No trailing `..` or expression.
+    None,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct StructExpr {
+    pub qself: Option<QSelf>,
+    pub path: Path,
+    pub fields: Vec<ExprField>,
+    pub rest: StructRest,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum ExprKind {
+    /// A `box x` expression.
+    Box(P<Expr>),
+    /// An array (`[a, b, c, d]`)
+    Array(Vec<P<Expr>>),
+    /// Allow anonymous constants from an inline `const` block
+    ConstBlock(AnonConst),
+    /// A function call
+    ///
+    /// The first field resolves to the function itself,
+    /// and the second field is the list of arguments.
+    /// This also represents calling the constructor of
+    /// tuple-like ADTs such as tuple structs and enum variants.
+    Call(P<Expr>, Vec<P<Expr>>),
+    /// A method call (`x.foo::<'static, Bar, Baz>(a, b, c, d)`)
+    ///
+    /// The `PathSegment` represents the method name and its generic arguments
+    /// (within the angle brackets).
+    /// The first element of the vector of an `Expr` is the expression that evaluates
+    /// to the object on which the method is being called on (the receiver),
+    /// and the remaining elements are the rest of the arguments.
+    /// Thus, `x.foo::<Bar, Baz>(a, b, c, d)` is represented as
+    /// `ExprKind::MethodCall(PathSegment { foo, [Bar, Baz] }, [x, a, b, c, d])`.
+    /// This `Span` is the span of the function, without the dot and receiver
+    /// (e.g. `foo(a, b)` in `x.foo(a, b)`
+    MethodCall(PathSegment, Vec<P<Expr>>, Span),
+    /// A tuple (e.g., `(a, b, c, d)`).
+    Tup(Vec<P<Expr>>),
+    /// A binary operation (e.g., `a + b`, `a * b`).
+    Binary(BinOp, P<Expr>, P<Expr>),
+    /// A unary operation (e.g., `!x`, `*x`).
+    Unary(UnOp, P<Expr>),
+    /// A literal (e.g., `1`, `"foo"`).
+    Lit(Lit),
+    /// A cast (e.g., `foo as f64`).
+    Cast(P<Expr>, P<Ty>),
+    /// A type ascription (e.g., `42: usize`).
+    Type(P<Expr>, P<Ty>),
+    /// A `let pat = expr` expression that is only semantically allowed in the condition
+    /// of `if` / `while` expressions. (e.g., `if let 0 = x { .. }`).
+    ///
+    /// `Span` represents the whole `let pat = expr` statement.
+    Let(P<Pat>, P<Expr>, Span),
+    /// An `if` block, with an optional `else` block.
+    ///
+    /// `if expr { block } else { expr }`
+    If(P<Expr>, P<Block>, Option<P<Expr>>),
+    /// A while loop, with an optional label.
+    ///
+    /// `'label: while expr { block }`
+    While(P<Expr>, P<Block>, Option<Label>),
+    /// A `for` loop, with an optional label.
+    ///
+    /// `'label: for pat in expr { block }`
+    ///
+    /// This is desugared to a combination of `loop` and `match` expressions.
+    ForLoop(P<Pat>, P<Expr>, P<Block>, Option<Label>),
+    /// Conditionless loop (can be exited with `break`, `continue`, or `return`).
+    ///
+    /// `'label: loop { block }`
+    Loop(P<Block>, Option<Label>),
+    /// A `match` block.
+    Match(P<Expr>, Vec<Arm>),
+    /// A closure (e.g., `move |a, b, c| a + b + c`).
+    ///
+    /// The final span is the span of the argument block `|...|`.
+    Closure(ClosureBinder, CaptureBy, Async, Movability, P<FnDecl>, P<Expr>, Span),
+    /// A block (`'label: { ... }`).
+    Block(P<Block>, Option<Label>),
+    /// An async block (`async move { ... }`).
+    ///
+    /// The `NodeId` is the `NodeId` for the closure that results from
+    /// desugaring an async block, just like the NodeId field in the
+    /// `Async::Yes` variant. This is necessary in order to create a def for the
+    /// closure which can be used as a parent of any child defs. Defs
+    /// created during lowering cannot be made the parent of any other
+    /// preexisting defs.
+    Async(CaptureBy, NodeId, P<Block>),
+    /// An await expression (`my_future.await`).
+    Await(P<Expr>),
+
+    /// A try block (`try { ... }`).
+    TryBlock(P<Block>),
+
+    /// An assignment (`a = foo()`).
+    /// The `Span` argument is the span of the `=` token.
+    Assign(P<Expr>, P<Expr>, Span),
+    /// An assignment with an operator.
+    ///
+    /// E.g., `a += 1`.
+    AssignOp(BinOp, P<Expr>, P<Expr>),
+    /// Access of a named (e.g., `obj.foo`) or unnamed (e.g., `obj.0`) struct field.
+    Field(P<Expr>, Ident),
+    /// An indexing operation (e.g., `foo[2]`).
+    Index(P<Expr>, P<Expr>),
+    /// A range (e.g., `1..2`, `1..`, `..2`, `1..=2`, `..=2`; and `..` in destructuring assignment).
+    Range(Option<P<Expr>>, Option<P<Expr>>, RangeLimits),
+    /// An underscore, used in destructuring assignment to ignore a value.
+    Underscore,
+
+    /// Variable reference, possibly containing `::` and/or type
+    /// parameters (e.g., `foo::bar::<baz>`).
+    ///
+    /// Optionally "qualified" (e.g., `<Vec<T> as SomeTrait>::SomeType`).
+    Path(Option<QSelf>, Path),
+
+    /// A referencing operation (`&a`, `&mut a`, `&raw const a` or `&raw mut a`).
+    AddrOf(BorrowKind, Mutability, P<Expr>),
+    /// A `break`, with an optional label to break, and an optional expression.
+    Break(Option<Label>, Option<P<Expr>>),
+    /// A `continue`, with an optional label.
+    Continue(Option<Label>),
+    /// A `return`, with an optional value to be returned.
+    Ret(Option<P<Expr>>),
+
+    /// Output of the `asm!()` macro.
+    InlineAsm(P<InlineAsm>),
+
+    /// A macro invocation; pre-expansion.
+    MacCall(MacCall),
+
+    /// A struct literal expression.
+    ///
+    /// E.g., `Foo {x: 1, y: 2}`, or `Foo {x: 1, .. rest}`.
+    Struct(P<StructExpr>),
+
+    /// An array literal constructed from one repeated element.
+    ///
+    /// E.g., `[1; 5]`. The expression is the element to be
+    /// repeated; the constant is the number of times to repeat it.
+    Repeat(P<Expr>, AnonConst),
+
+    /// No-op: used solely so we can pretty-print faithfully.
+    Paren(P<Expr>),
+
+    /// A try expression (`expr?`).
+    Try(P<Expr>),
+
+    /// A `yield`, with an optional value to be yielded.
+    Yield(Option<P<Expr>>),
+
+    /// A `do yeet` (aka `throw`/`fail`/`bail`/`raise`/whatever),
+    /// with an optional value to be returned.
+    Yeet(Option<P<Expr>>),
+
+    /// Placeholder for an expression that wasn't syntactically well formed in some way.
+    Err,
+}
+
+/// The explicit `Self` type in a "qualified path". The actual
+/// path, including the trait and the associated item, is stored
+/// separately. `position` represents the index of the associated
+/// item qualified with this `Self` type.
+///
+/// ```ignore (only-for-syntax-highlight)
+/// <Vec<T> as a::b::Trait>::AssociatedItem
+///  ^~~~~     ~~~~~~~~~~~~~~^
+///  ty        position = 3
+///
+/// <Vec<T>>::AssociatedItem
+///  ^~~~~    ^
+///  ty       position = 0
+/// ```
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct QSelf {
+    pub ty: P<Ty>,
+
+    /// The span of `a::b::Trait` in a path like `<Vec<T> as
+    /// a::b::Trait>::AssociatedItem`; in the case where `position ==
+    /// 0`, this is an empty span.
+    pub path_span: Span,
+    pub position: usize,
+}
+
+/// A capture clause used in closures and `async` blocks.
+#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum CaptureBy {
+    /// `move |x| y + x`.
+    Value,
+    /// `move` keyword was not specified.
+    Ref,
+}
+
+/// The movability of a generator / closure literal:
+/// whether a generator contains self-references, causing it to be `!Unpin`.
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Encodable, Decodable, Debug, Copy)]
+#[derive(HashStable_Generic)]
+pub enum Movability {
+    /// May contain self-references, `!Unpin`.
+    Static,
+    /// Must not contain self-references, `Unpin`.
+    Movable,
+}
+
+/// Closure lifetime binder, `for<'a, 'b>` in `for<'a, 'b> |_: &'a (), _: &'b ()|`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum ClosureBinder {
+    /// The binder is not present, all closure lifetimes are inferred.
+    NotPresent,
+    /// The binder is present.
+    For {
+        /// Span of the whole `for<>` clause
+        ///
+        /// ```text
+        /// for<'a, 'b> |_: &'a (), _: &'b ()| { ... }
+        /// ^^^^^^^^^^^ -- this
+        /// ```
+        span: Span,
+
+        /// Lifetimes in the `for<>` closure
+        ///
+        /// ```text
+        /// for<'a, 'b> |_: &'a (), _: &'b ()| { ... }
+        ///     ^^^^^^ -- this
+        /// ```
+        generic_params: P<[GenericParam]>,
+    },
+}
+
+/// Represents a macro invocation. The `path` indicates which macro
+/// is being invoked, and the `args` are arguments passed to it.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct MacCall {
+    pub path: Path,
+    pub args: P<MacArgs>,
+    pub prior_type_ascription: Option<(Span, bool)>,
+}
+
+impl MacCall {
+    pub fn span(&self) -> Span {
+        self.path.span.to(self.args.span().unwrap_or(self.path.span))
+    }
+}
+
+/// Arguments passed to an attribute or a function-like macro.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum MacArgs {
+    /// No arguments - `#[attr]`.
+    Empty,
+    /// Delimited arguments - `#[attr()/[]/{}]` or `mac!()/[]/{}`.
+    Delimited(DelimSpan, MacDelimiter, TokenStream),
+    /// Arguments of a key-value attribute - `#[attr = "value"]`.
+    Eq(
+        /// Span of the `=` token.
+        Span,
+        /// The "value".
+        MacArgsEq,
+    ),
+}
+
+// The RHS of a `MacArgs::Eq` starts out as an expression. Once macro expansion
+// is completed, all cases end up either as a literal, which is the form used
+// after lowering to HIR, or as an error.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum MacArgsEq {
+    Ast(P<Expr>),
+    Hir(Lit),
+}
+
+impl MacArgs {
+    pub fn delim(&self) -> Option<Delimiter> {
+        match self {
+            MacArgs::Delimited(_, delim, _) => Some(delim.to_token()),
+            MacArgs::Empty | MacArgs::Eq(..) => None,
+        }
+    }
+
+    pub fn span(&self) -> Option<Span> {
+        match self {
+            MacArgs::Empty => None,
+            MacArgs::Delimited(dspan, ..) => Some(dspan.entire()),
+            MacArgs::Eq(eq_span, MacArgsEq::Ast(expr)) => Some(eq_span.to(expr.span)),
+            MacArgs::Eq(_, MacArgsEq::Hir(lit)) => {
+                unreachable!("in literal form when getting span: {:?}", lit);
+            }
+        }
+    }
+
+    /// Tokens inside the delimiters or after `=`.
+    /// Proc macros see these tokens, for example.
+    pub fn inner_tokens(&self) -> TokenStream {
+        match self {
+            MacArgs::Empty => TokenStream::default(),
+            MacArgs::Delimited(.., tokens) => tokens.clone(),
+            MacArgs::Eq(_, MacArgsEq::Ast(expr)) => TokenStream::from_ast(expr),
+            MacArgs::Eq(_, MacArgsEq::Hir(lit)) => {
+                unreachable!("in literal form when getting inner tokens: {:?}", lit)
+            }
+        }
+    }
+
+    /// Whether a macro with these arguments needs a semicolon
+    /// when used as a standalone item or statement.
+    pub fn need_semicolon(&self) -> bool {
+        !matches!(self, MacArgs::Delimited(_, MacDelimiter::Brace, _))
+    }
+}
+
+impl<CTX> HashStable<CTX> for MacArgs
+where
+    CTX: crate::HashStableContext,
+{
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
+        mem::discriminant(self).hash_stable(ctx, hasher);
+        match self {
+            MacArgs::Empty => {}
+            MacArgs::Delimited(dspan, delim, tokens) => {
+                dspan.hash_stable(ctx, hasher);
+                delim.hash_stable(ctx, hasher);
+                tokens.hash_stable(ctx, hasher);
+            }
+            MacArgs::Eq(_eq_span, MacArgsEq::Ast(expr)) => {
+                unreachable!("hash_stable {:?}", expr);
+            }
+            MacArgs::Eq(eq_span, MacArgsEq::Hir(lit)) => {
+                eq_span.hash_stable(ctx, hasher);
+                lit.hash_stable(ctx, hasher);
+            }
+        }
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum MacDelimiter {
+    Parenthesis,
+    Bracket,
+    Brace,
+}
+
+impl MacDelimiter {
+    pub fn to_token(self) -> Delimiter {
+        match self {
+            MacDelimiter::Parenthesis => Delimiter::Parenthesis,
+            MacDelimiter::Bracket => Delimiter::Bracket,
+            MacDelimiter::Brace => Delimiter::Brace,
+        }
+    }
+
+    pub fn from_token(delim: Delimiter) -> Option<MacDelimiter> {
+        match delim {
+            Delimiter::Parenthesis => Some(MacDelimiter::Parenthesis),
+            Delimiter::Bracket => Some(MacDelimiter::Bracket),
+            Delimiter::Brace => Some(MacDelimiter::Brace),
+            Delimiter::Invisible => None,
+        }
+    }
+}
+
+/// Represents a macro definition.
+#[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
+pub struct MacroDef {
+    pub body: P<MacArgs>,
+    /// `true` if macro was defined with `macro_rules`.
+    pub macro_rules: bool,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug, Copy, Hash, Eq, PartialEq)]
+#[derive(HashStable_Generic)]
+pub enum StrStyle {
+    /// A regular string, like `"foo"`.
+    Cooked,
+    /// A raw string, like `r##"foo"##`.
+    ///
+    /// The value is the number of `#` symbols used.
+    Raw(u8),
+}
+
+/// An AST literal.
+#[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
+pub struct Lit {
+    /// The original literal token as written in source code.
+    pub token: token::Lit,
+    /// The "semantic" representation of the literal lowered from the original tokens.
+    /// Strings are unescaped, hexadecimal forms are eliminated, etc.
+    /// FIXME: Remove this and only create the semantic representation during lowering to HIR.
+    pub kind: LitKind,
+    pub span: Span,
+}
+
+/// Same as `Lit`, but restricted to string literals.
+#[derive(Clone, Copy, Encodable, Decodable, Debug)]
+pub struct StrLit {
+    /// The original literal token as written in source code.
+    pub style: StrStyle,
+    pub symbol: Symbol,
+    pub suffix: Option<Symbol>,
+    pub span: Span,
+    /// The unescaped "semantic" representation of the literal lowered from the original token.
+    /// FIXME: Remove this and only create the semantic representation during lowering to HIR.
+    pub symbol_unescaped: Symbol,
+}
+
+impl StrLit {
+    pub fn as_lit(&self) -> Lit {
+        let token_kind = match self.style {
+            StrStyle::Cooked => token::Str,
+            StrStyle::Raw(n) => token::StrRaw(n),
+        };
+        Lit {
+            token: token::Lit::new(token_kind, self.symbol, self.suffix),
+            span: self.span,
+            kind: LitKind::Str(self.symbol_unescaped, self.style),
+        }
+    }
+}
+
+/// Type of the integer literal based on provided suffix.
+#[derive(Clone, Copy, Encodable, Decodable, Debug, Hash, Eq, PartialEq)]
+#[derive(HashStable_Generic)]
+pub enum LitIntType {
+    /// e.g. `42_i32`.
+    Signed(IntTy),
+    /// e.g. `42_u32`.
+    Unsigned(UintTy),
+    /// e.g. `42`.
+    Unsuffixed,
+}
+
+/// Type of the float literal based on provided suffix.
+#[derive(Clone, Copy, Encodable, Decodable, Debug, Hash, Eq, PartialEq)]
+#[derive(HashStable_Generic)]
+pub enum LitFloatType {
+    /// A float literal with a suffix (`1f32` or `1E10f32`).
+    Suffixed(FloatTy),
+    /// A float literal without a suffix (`1.0 or 1.0E10`).
+    Unsuffixed,
+}
+
+/// Literal kind.
+///
+/// E.g., `"foo"`, `42`, `12.34`, or `bool`.
+#[derive(Clone, Encodable, Decodable, Debug, Hash, Eq, PartialEq, HashStable_Generic)]
+pub enum LitKind {
+    /// A string literal (`"foo"`).
+    Str(Symbol, StrStyle),
+    /// A byte string (`b"foo"`).
+    ByteStr(Lrc<[u8]>),
+    /// A byte char (`b'f'`).
+    Byte(u8),
+    /// A character literal (`'a'`).
+    Char(char),
+    /// An integer literal (`1`).
+    Int(u128, LitIntType),
+    /// A float literal (`1f64` or `1E10f64`).
+    Float(Symbol, LitFloatType),
+    /// A boolean literal.
+    Bool(bool),
+    /// Placeholder for a literal that wasn't well-formed in some way.
+    Err(Symbol),
+}
+
+impl LitKind {
+    /// Returns `true` if this literal is a string.
+    pub fn is_str(&self) -> bool {
+        matches!(self, LitKind::Str(..))
+    }
+
+    /// Returns `true` if this literal is byte literal string.
+    pub fn is_bytestr(&self) -> bool {
+        matches!(self, LitKind::ByteStr(_))
+    }
+
+    /// Returns `true` if this is a numeric literal.
+    pub fn is_numeric(&self) -> bool {
+        matches!(self, LitKind::Int(..) | LitKind::Float(..))
+    }
+
+    /// Returns `true` if this literal has no suffix.
+    /// Note: this will return true for literals with prefixes such as raw strings and byte strings.
+    pub fn is_unsuffixed(&self) -> bool {
+        !self.is_suffixed()
+    }
+
+    /// Returns `true` if this literal has a suffix.
+    pub fn is_suffixed(&self) -> bool {
+        match *self {
+            // suffixed variants
+            LitKind::Int(_, LitIntType::Signed(..) | LitIntType::Unsigned(..))
+            | LitKind::Float(_, LitFloatType::Suffixed(..)) => true,
+            // unsuffixed variants
+            LitKind::Str(..)
+            | LitKind::ByteStr(..)
+            | LitKind::Byte(..)
+            | LitKind::Char(..)
+            | LitKind::Int(_, LitIntType::Unsuffixed)
+            | LitKind::Float(_, LitFloatType::Unsuffixed)
+            | LitKind::Bool(..)
+            | LitKind::Err(..) => false,
+        }
+    }
+}
+
+// N.B., If you change this, you'll probably want to change the corresponding
+// type structure in `middle/ty.rs` as well.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct MutTy {
+    pub ty: P<Ty>,
+    pub mutbl: Mutability,
+}
+
+/// Represents a function's signature in a trait declaration,
+/// trait implementation, or free function.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct FnSig {
+    pub header: FnHeader,
+    pub decl: P<FnDecl>,
+    pub span: Span,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
+#[derive(Encodable, Decodable, HashStable_Generic)]
+pub enum FloatTy {
+    F32,
+    F64,
+}
+
+impl FloatTy {
+    pub fn name_str(self) -> &'static str {
+        match self {
+            FloatTy::F32 => "f32",
+            FloatTy::F64 => "f64",
+        }
+    }
+
+    pub fn name(self) -> Symbol {
+        match self {
+            FloatTy::F32 => sym::f32,
+            FloatTy::F64 => sym::f64,
+        }
+    }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
+#[derive(Encodable, Decodable, HashStable_Generic)]
+pub enum IntTy {
+    Isize,
+    I8,
+    I16,
+    I32,
+    I64,
+    I128,
+}
+
+impl IntTy {
+    pub fn name_str(&self) -> &'static str {
+        match *self {
+            IntTy::Isize => "isize",
+            IntTy::I8 => "i8",
+            IntTy::I16 => "i16",
+            IntTy::I32 => "i32",
+            IntTy::I64 => "i64",
+            IntTy::I128 => "i128",
+        }
+    }
+
+    pub fn name(&self) -> Symbol {
+        match *self {
+            IntTy::Isize => sym::isize,
+            IntTy::I8 => sym::i8,
+            IntTy::I16 => sym::i16,
+            IntTy::I32 => sym::i32,
+            IntTy::I64 => sym::i64,
+            IntTy::I128 => sym::i128,
+        }
+    }
+}
+
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Copy, Debug)]
+#[derive(Encodable, Decodable, HashStable_Generic)]
+pub enum UintTy {
+    Usize,
+    U8,
+    U16,
+    U32,
+    U64,
+    U128,
+}
+
+impl UintTy {
+    pub fn name_str(&self) -> &'static str {
+        match *self {
+            UintTy::Usize => "usize",
+            UintTy::U8 => "u8",
+            UintTy::U16 => "u16",
+            UintTy::U32 => "u32",
+            UintTy::U64 => "u64",
+            UintTy::U128 => "u128",
+        }
+    }
+
+    pub fn name(&self) -> Symbol {
+        match *self {
+            UintTy::Usize => sym::usize,
+            UintTy::U8 => sym::u8,
+            UintTy::U16 => sym::u16,
+            UintTy::U32 => sym::u32,
+            UintTy::U64 => sym::u64,
+            UintTy::U128 => sym::u128,
+        }
+    }
+}
+
+/// A constraint on an associated type (e.g., `A = Bar` in `Foo<A = Bar>` or
+/// `A: TraitA + TraitB` in `Foo<A: TraitA + TraitB>`).
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct AssocConstraint {
+    pub id: NodeId,
+    pub ident: Ident,
+    pub gen_args: Option<GenericArgs>,
+    pub kind: AssocConstraintKind,
+    pub span: Span,
+}
+
+/// The kinds of an `AssocConstraint`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum Term {
+    Ty(P<Ty>),
+    Const(AnonConst),
+}
+
+impl From<P<Ty>> for Term {
+    fn from(v: P<Ty>) -> Self {
+        Term::Ty(v)
+    }
+}
+
+impl From<AnonConst> for Term {
+    fn from(v: AnonConst) -> Self {
+        Term::Const(v)
+    }
+}
+
+/// The kinds of an `AssocConstraint`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum AssocConstraintKind {
+    /// E.g., `A = Bar`, `A = 3` in `Foo<A = Bar>` where A is an associated type.
+    Equality { term: Term },
+    /// E.g. `A: TraitA + TraitB` in `Foo<A: TraitA + TraitB>`.
+    Bound { bounds: GenericBounds },
+}
+
+#[derive(Encodable, Decodable, Debug)]
+pub struct Ty {
+    pub id: NodeId,
+    pub kind: TyKind,
+    pub span: Span,
+    pub tokens: Option<LazyTokenStream>,
+}
+
+impl Clone for Ty {
+    fn clone(&self) -> Self {
+        ensure_sufficient_stack(|| Self {
+            id: self.id,
+            kind: self.kind.clone(),
+            span: self.span,
+            tokens: self.tokens.clone(),
+        })
+    }
+}
+
+impl Ty {
+    pub fn peel_refs(&self) -> &Self {
+        let mut final_ty = self;
+        while let TyKind::Rptr(_, MutTy { ty, .. }) = &final_ty.kind {
+            final_ty = &ty;
+        }
+        final_ty
+    }
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct BareFnTy {
+    pub unsafety: Unsafe,
+    pub ext: Extern,
+    pub generic_params: Vec<GenericParam>,
+    pub decl: P<FnDecl>,
+    /// Span of the `fn(...) -> ...` part.
+    pub decl_span: Span,
+}
+
+/// The various kinds of type recognized by the compiler.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum TyKind {
+    /// A variable-length slice (`[T]`).
+    Slice(P<Ty>),
+    /// A fixed length array (`[T; n]`).
+    Array(P<Ty>, AnonConst),
+    /// A raw pointer (`*const T` or `*mut T`).
+    Ptr(MutTy),
+    /// A reference (`&'a T` or `&'a mut T`).
+    Rptr(Option<Lifetime>, MutTy),
+    /// A bare function (e.g., `fn(usize) -> bool`).
+    BareFn(P<BareFnTy>),
+    /// The never type (`!`).
+    Never,
+    /// A tuple (`(A, B, C, D,...)`).
+    Tup(Vec<P<Ty>>),
+    /// A path (`module::module::...::Type`), optionally
+    /// "qualified", e.g., `<Vec<T> as SomeTrait>::SomeType`.
+    ///
+    /// Type parameters are stored in the `Path` itself.
+    Path(Option<QSelf>, Path),
+    /// A trait object type `Bound1 + Bound2 + Bound3`
+    /// where `Bound` is a trait or a lifetime.
+    TraitObject(GenericBounds, TraitObjectSyntax),
+    /// An `impl Bound1 + Bound2 + Bound3` type
+    /// where `Bound` is a trait or a lifetime.
+    ///
+    /// The `NodeId` exists to prevent lowering from having to
+    /// generate `NodeId`s on the fly, which would complicate
+    /// the generation of opaque `type Foo = impl Trait` items significantly.
+    ImplTrait(NodeId, GenericBounds),
+    /// No-op; kept solely so that we can pretty-print faithfully.
+    Paren(P<Ty>),
+    /// Unused for now.
+    Typeof(AnonConst),
+    /// This means the type should be inferred instead of it having been
+    /// specified. This can appear anywhere in a type.
+    Infer,
+    /// Inferred type of a `self` or `&self` argument in a method.
+    ImplicitSelf,
+    /// A macro in the type position.
+    MacCall(MacCall),
+    /// Placeholder for a kind that has failed to be defined.
+    Err,
+    /// Placeholder for a `va_list`.
+    CVarArgs,
+}
+
+impl TyKind {
+    pub fn is_implicit_self(&self) -> bool {
+        matches!(self, TyKind::ImplicitSelf)
+    }
+
+    pub fn is_unit(&self) -> bool {
+        matches!(self, TyKind::Tup(tys) if tys.is_empty())
+    }
+
+    pub fn is_simple_path(&self) -> Option<Symbol> {
+        if let TyKind::Path(None, Path { segments, .. }) = &self && segments.len() == 1 {
+            Some(segments[0].ident.name)
+        } else {
+            None
+        }
+    }
+}
+
+/// Syntax used to declare a trait object.
+#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum TraitObjectSyntax {
+    Dyn,
+    None,
+}
+
+/// Inline assembly operand explicit register or register class.
+///
+/// E.g., `"eax"` as in `asm!("mov eax, 2", out("eax") result)`.
+#[derive(Clone, Copy, Encodable, Decodable, Debug)]
+pub enum InlineAsmRegOrRegClass {
+    Reg(Symbol),
+    RegClass(Symbol),
+}
+
+bitflags::bitflags! {
+    #[derive(Encodable, Decodable, HashStable_Generic)]
+    pub struct InlineAsmOptions: u16 {
+        const PURE = 1 << 0;
+        const NOMEM = 1 << 1;
+        const READONLY = 1 << 2;
+        const PRESERVES_FLAGS = 1 << 3;
+        const NORETURN = 1 << 4;
+        const NOSTACK = 1 << 5;
+        const ATT_SYNTAX = 1 << 6;
+        const RAW = 1 << 7;
+        const MAY_UNWIND = 1 << 8;
+    }
+}
+
+#[derive(Clone, PartialEq, Encodable, Decodable, Debug, Hash, HashStable_Generic)]
+pub enum InlineAsmTemplatePiece {
+    String(String),
+    Placeholder { operand_idx: usize, modifier: Option<char>, span: Span },
+}
+
+impl fmt::Display for InlineAsmTemplatePiece {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::String(s) => {
+                for c in s.chars() {
+                    match c {
+                        '{' => f.write_str("{{")?,
+                        '}' => f.write_str("}}")?,
+                        _ => c.fmt(f)?,
+                    }
+                }
+                Ok(())
+            }
+            Self::Placeholder { operand_idx, modifier: Some(modifier), .. } => {
+                write!(f, "{{{}:{}}}", operand_idx, modifier)
+            }
+            Self::Placeholder { operand_idx, modifier: None, .. } => {
+                write!(f, "{{{}}}", operand_idx)
+            }
+        }
+    }
+}
+
+impl InlineAsmTemplatePiece {
+    /// Rebuilds the asm template string from its pieces.
+    pub fn to_string(s: &[Self]) -> String {
+        use fmt::Write;
+        let mut out = String::new();
+        for p in s.iter() {
+            let _ = write!(out, "{}", p);
+        }
+        out
+    }
+}
+
+/// Inline assembly symbol operands get their own AST node that is somewhat
+/// similar to `AnonConst`.
+///
+/// The main difference is that we specifically don't assign it `DefId` in
+/// `DefCollector`. Instead this is deferred until AST lowering where we
+/// lower it to an `AnonConst` (for functions) or a `Path` (for statics)
+/// depending on what the path resolves to.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct InlineAsmSym {
+    pub id: NodeId,
+    pub qself: Option<QSelf>,
+    pub path: Path,
+}
+
+/// Inline assembly operand.
+///
+/// E.g., `out("eax") result` as in `asm!("mov eax, 2", out("eax") result)`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum InlineAsmOperand {
+    In {
+        reg: InlineAsmRegOrRegClass,
+        expr: P<Expr>,
+    },
+    Out {
+        reg: InlineAsmRegOrRegClass,
+        late: bool,
+        expr: Option<P<Expr>>,
+    },
+    InOut {
+        reg: InlineAsmRegOrRegClass,
+        late: bool,
+        expr: P<Expr>,
+    },
+    SplitInOut {
+        reg: InlineAsmRegOrRegClass,
+        late: bool,
+        in_expr: P<Expr>,
+        out_expr: Option<P<Expr>>,
+    },
+    Const {
+        anon_const: AnonConst,
+    },
+    Sym {
+        sym: InlineAsmSym,
+    },
+}
+
+/// Inline assembly.
+///
+/// E.g., `asm!("NOP");`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct InlineAsm {
+    pub template: Vec<InlineAsmTemplatePiece>,
+    pub template_strs: Box<[(Symbol, Option<Symbol>, Span)]>,
+    pub operands: Vec<(InlineAsmOperand, Span)>,
+    pub clobber_abis: Vec<(Symbol, Span)>,
+    pub options: InlineAsmOptions,
+    pub line_spans: Vec<Span>,
+}
+
+/// A parameter in a function header.
+///
+/// E.g., `bar: usize` as in `fn foo(bar: usize)`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Param {
+    pub attrs: AttrVec,
+    pub ty: P<Ty>,
+    pub pat: P<Pat>,
+    pub id: NodeId,
+    pub span: Span,
+    pub is_placeholder: bool,
+}
+
+/// Alternative representation for `Arg`s describing `self` parameter of methods.
+///
+/// E.g., `&mut self` as in `fn foo(&mut self)`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum SelfKind {
+    /// `self`, `mut self`
+    Value(Mutability),
+    /// `&'lt self`, `&'lt mut self`
+    Region(Option<Lifetime>, Mutability),
+    /// `self: TYPE`, `mut self: TYPE`
+    Explicit(P<Ty>, Mutability),
+}
+
+pub type ExplicitSelf = Spanned<SelfKind>;
+
+impl Param {
+    /// Attempts to cast parameter to `ExplicitSelf`.
+    pub fn to_self(&self) -> Option<ExplicitSelf> {
+        if let PatKind::Ident(BindingMode::ByValue(mutbl), ident, _) = self.pat.kind {
+            if ident.name == kw::SelfLower {
+                return match self.ty.kind {
+                    TyKind::ImplicitSelf => Some(respan(self.pat.span, SelfKind::Value(mutbl))),
+                    TyKind::Rptr(lt, MutTy { ref ty, mutbl }) if ty.kind.is_implicit_self() => {
+                        Some(respan(self.pat.span, SelfKind::Region(lt, mutbl)))
+                    }
+                    _ => Some(respan(
+                        self.pat.span.to(self.ty.span),
+                        SelfKind::Explicit(self.ty.clone(), mutbl),
+                    )),
+                };
+            }
+        }
+        None
+    }
+
+    /// Returns `true` if parameter is `self`.
+    pub fn is_self(&self) -> bool {
+        if let PatKind::Ident(_, ident, _) = self.pat.kind {
+            ident.name == kw::SelfLower
+        } else {
+            false
+        }
+    }
+
+    /// Builds a `Param` object from `ExplicitSelf`.
+    pub fn from_self(attrs: AttrVec, eself: ExplicitSelf, eself_ident: Ident) -> Param {
+        let span = eself.span.to(eself_ident.span);
+        let infer_ty = P(Ty { id: DUMMY_NODE_ID, kind: TyKind::ImplicitSelf, span, tokens: None });
+        let param = |mutbl, ty| Param {
+            attrs,
+            pat: P(Pat {
+                id: DUMMY_NODE_ID,
+                kind: PatKind::Ident(BindingMode::ByValue(mutbl), eself_ident, None),
+                span,
+                tokens: None,
+            }),
+            span,
+            ty,
+            id: DUMMY_NODE_ID,
+            is_placeholder: false,
+        };
+        match eself.node {
+            SelfKind::Explicit(ty, mutbl) => param(mutbl, ty),
+            SelfKind::Value(mutbl) => param(mutbl, infer_ty),
+            SelfKind::Region(lt, mutbl) => param(
+                Mutability::Not,
+                P(Ty {
+                    id: DUMMY_NODE_ID,
+                    kind: TyKind::Rptr(lt, MutTy { ty: infer_ty, mutbl }),
+                    span,
+                    tokens: None,
+                }),
+            ),
+        }
+    }
+}
+
+/// A signature (not the body) of a function declaration.
+///
+/// E.g., `fn foo(bar: baz)`.
+///
+/// Please note that it's different from `FnHeader` structure
+/// which contains metadata about function safety, asyncness, constness and ABI.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct FnDecl {
+    pub inputs: Vec<Param>,
+    pub output: FnRetTy,
+}
+
+impl FnDecl {
+    pub fn has_self(&self) -> bool {
+        self.inputs.get(0).map_or(false, Param::is_self)
+    }
+    pub fn c_variadic(&self) -> bool {
+        self.inputs.last().map_or(false, |arg| matches!(arg.ty.kind, TyKind::CVarArgs))
+    }
+}
+
+/// Is the trait definition an auto trait?
+#[derive(Copy, Clone, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum IsAuto {
+    Yes,
+    No,
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Encodable, Decodable, Debug)]
+#[derive(HashStable_Generic)]
+pub enum Unsafe {
+    Yes(Span),
+    No,
+}
+
+#[derive(Copy, Clone, Encodable, Decodable, Debug)]
+pub enum Async {
+    Yes { span: Span, closure_id: NodeId, return_impl_trait_id: NodeId },
+    No,
+}
+
+impl Async {
+    pub fn is_async(self) -> bool {
+        matches!(self, Async::Yes { .. })
+    }
+
+    /// In this case this is an `async` return, the `NodeId` for the generated `impl Trait` item.
+    pub fn opt_return_id(self) -> Option<NodeId> {
+        match self {
+            Async::Yes { return_impl_trait_id, .. } => Some(return_impl_trait_id),
+            Async::No => None,
+        }
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Encodable, Decodable, Debug)]
+#[derive(HashStable_Generic)]
+pub enum Const {
+    Yes(Span),
+    No,
+}
+
+/// Item defaultness.
+/// For details see the [RFC #2532](https://github.com/rust-lang/rfcs/pull/2532).
+#[derive(Copy, Clone, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum Defaultness {
+    Default(Span),
+    Final,
+}
+
+#[derive(Copy, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
+pub enum ImplPolarity {
+    /// `impl Trait for Type`
+    Positive,
+    /// `impl !Trait for Type`
+    Negative(Span),
+}
+
+impl fmt::Debug for ImplPolarity {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            ImplPolarity::Positive => "positive".fmt(f),
+            ImplPolarity::Negative(_) => "negative".fmt(f),
+        }
+    }
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum FnRetTy {
+    /// Returns type is not specified.
+    ///
+    /// Functions default to `()` and closures default to inference.
+    /// Span points to where return type would be inserted.
+    Default(Span),
+    /// Everything else.
+    Ty(P<Ty>),
+}
+
+impl FnRetTy {
+    pub fn span(&self) -> Span {
+        match *self {
+            FnRetTy::Default(span) => span,
+            FnRetTy::Ty(ref ty) => ty.span,
+        }
+    }
+}
+
+#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug)]
+pub enum Inline {
+    Yes,
+    No,
+}
+
+/// Module item kind.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum ModKind {
+    /// Module with inlined definition `mod foo { ... }`,
+    /// or with definition outlined to a separate file `mod foo;` and already loaded from it.
+    /// The inner span is from the first token past `{` to the last token until `}`,
+    /// or from the first to the last token in the loaded file.
+    Loaded(Vec<P<Item>>, Inline, ModSpans),
+    /// Module with definition outlined to a separate file `mod foo;` but not yet loaded from it.
+    Unloaded,
+}
+
+#[derive(Copy, Clone, Encodable, Decodable, Debug)]
+pub struct ModSpans {
+    /// `inner_span` covers the body of the module; for a file module, its the whole file.
+    /// For an inline module, its the span inside the `{ ... }`, not including the curly braces.
+    pub inner_span: Span,
+    pub inject_use_span: Span,
+}
+
+impl Default for ModSpans {
+    fn default() -> ModSpans {
+        ModSpans { inner_span: Default::default(), inject_use_span: Default::default() }
+    }
+}
+
+/// Foreign module declaration.
+///
+/// E.g., `extern { .. }` or `extern "C" { .. }`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct ForeignMod {
+    /// `unsafe` keyword accepted syntactically for macro DSLs, but not
+    /// semantically by Rust.
+    pub unsafety: Unsafe,
+    pub abi: Option<StrLit>,
+    pub items: Vec<P<ForeignItem>>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct EnumDef {
+    pub variants: Vec<Variant>,
+}
+/// Enum variant.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Variant {
+    /// Attributes of the variant.
+    pub attrs: AttrVec,
+    /// Id of the variant (not the constructor, see `VariantData::ctor_id()`).
+    pub id: NodeId,
+    /// Span
+    pub span: Span,
+    /// The visibility of the variant. Syntactically accepted but not semantically.
+    pub vis: Visibility,
+    /// Name of the variant.
+    pub ident: Ident,
+
+    /// Fields and constructor id of the variant.
+    pub data: VariantData,
+    /// Explicit discriminant, e.g., `Foo = 1`.
+    pub disr_expr: Option<AnonConst>,
+    /// Is a macro placeholder
+    pub is_placeholder: bool,
+}
+
+/// Part of `use` item to the right of its prefix.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum UseTreeKind {
+    /// `use prefix` or `use prefix as rename`
+    ///
+    /// The extra `NodeId`s are for HIR lowering, when additional statements are created for each
+    /// namespace.
+    Simple(Option<Ident>, NodeId, NodeId),
+    /// `use prefix::{...}`
+    Nested(Vec<(UseTree, NodeId)>),
+    /// `use prefix::*`
+    Glob,
+}
+
+/// A tree of paths sharing common prefixes.
+/// Used in `use` items both at top-level and inside of braces in import groups.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct UseTree {
+    pub prefix: Path,
+    pub kind: UseTreeKind,
+    pub span: Span,
+}
+
+impl UseTree {
+    pub fn ident(&self) -> Ident {
+        match self.kind {
+            UseTreeKind::Simple(Some(rename), ..) => rename,
+            UseTreeKind::Simple(None, ..) => {
+                self.prefix.segments.last().expect("empty prefix in a simple import").ident
+            }
+            _ => panic!("`UseTree::ident` can only be used on a simple import"),
+        }
+    }
+}
+
+/// Distinguishes between `Attribute`s that decorate items and Attributes that
+/// are contained as statements within items. These two cases need to be
+/// distinguished for pretty-printing.
+#[derive(Clone, PartialEq, Encodable, Decodable, Debug, Copy, HashStable_Generic)]
+pub enum AttrStyle {
+    Outer,
+    Inner,
+}
+
+rustc_index::newtype_index! {
+    pub struct AttrId {
+        ENCODABLE = custom
+        DEBUG_FORMAT = "AttrId({})"
+    }
+}
+
+impl<S: Encoder> Encodable<S> for AttrId {
+    fn encode(&self, _s: &mut S) {}
+}
+
+impl<D: Decoder> Decodable<D> for AttrId {
+    fn decode(_: &mut D) -> AttrId {
+        crate::attr::mk_attr_id()
+    }
+}
+
+#[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
+pub struct AttrItem {
+    pub path: Path,
+    pub args: MacArgs,
+    pub tokens: Option<LazyTokenStream>,
+}
+
+/// A list of attributes.
+pub type AttrVec = ThinVec<Attribute>;
+
+/// Metadata associated with an item.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Attribute {
+    pub kind: AttrKind,
+    pub id: AttrId,
+    /// Denotes if the attribute decorates the following construct (outer)
+    /// or the construct this attribute is contained within (inner).
+    pub style: AttrStyle,
+    pub span: Span,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum AttrKind {
+    /// A normal attribute.
+    Normal(AttrItem, Option<LazyTokenStream>),
+
+    /// A doc comment (e.g. `/// ...`, `//! ...`, `/** ... */`, `/*! ... */`).
+    /// Doc attributes (e.g. `#[doc="..."]`) are represented with the `Normal`
+    /// variant (which is much less compact and thus more expensive).
+    DocComment(CommentKind, Symbol),
+}
+
+/// `TraitRef`s appear in impls.
+///
+/// Resolution maps each `TraitRef`'s `ref_id` to its defining trait; that's all
+/// that the `ref_id` is for. The `impl_id` maps to the "self type" of this impl.
+/// If this impl is an `ItemKind::Impl`, the `impl_id` is redundant (it could be the
+/// same as the impl's `NodeId`).
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct TraitRef {
+    pub path: Path,
+    pub ref_id: NodeId,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct PolyTraitRef {
+    /// The `'a` in `for<'a> Foo<&'a T>`.
+    pub bound_generic_params: Vec<GenericParam>,
+
+    /// The `Foo<&'a T>` in `<'a> Foo<&'a T>`.
+    pub trait_ref: TraitRef,
+
+    pub span: Span,
+}
+
+impl PolyTraitRef {
+    pub fn new(generic_params: Vec<GenericParam>, path: Path, span: Span) -> Self {
+        PolyTraitRef {
+            bound_generic_params: generic_params,
+            trait_ref: TraitRef { path, ref_id: DUMMY_NODE_ID },
+            span,
+        }
+    }
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Visibility {
+    pub kind: VisibilityKind,
+    pub span: Span,
+    pub tokens: Option<LazyTokenStream>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum VisibilityKind {
+    Public,
+    Restricted { path: P<Path>, id: NodeId },
+    Inherited,
+}
+
+impl VisibilityKind {
+    pub fn is_pub(&self) -> bool {
+        matches!(self, VisibilityKind::Public)
+    }
+}
+
+/// Field definition in a struct, variant or union.
+///
+/// E.g., `bar: usize` as in `struct Foo { bar: usize }`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct FieldDef {
+    pub attrs: AttrVec,
+    pub id: NodeId,
+    pub span: Span,
+    pub vis: Visibility,
+    pub ident: Option<Ident>,
+
+    pub ty: P<Ty>,
+    pub is_placeholder: bool,
+}
+
+/// Fields and constructor ids of enum variants and structs.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum VariantData {
+    /// Struct variant.
+    ///
+    /// E.g., `Bar { .. }` as in `enum Foo { Bar { .. } }`.
+    Struct(Vec<FieldDef>, bool),
+    /// Tuple variant.
+    ///
+    /// E.g., `Bar(..)` as in `enum Foo { Bar(..) }`.
+    Tuple(Vec<FieldDef>, NodeId),
+    /// Unit variant.
+    ///
+    /// E.g., `Bar = ..` as in `enum Foo { Bar = .. }`.
+    Unit(NodeId),
+}
+
+impl VariantData {
+    /// Return the fields of this variant.
+    pub fn fields(&self) -> &[FieldDef] {
+        match *self {
+            VariantData::Struct(ref fields, ..) | VariantData::Tuple(ref fields, _) => fields,
+            _ => &[],
+        }
+    }
+
+    /// Return the `NodeId` of this variant's constructor, if it has one.
+    pub fn ctor_id(&self) -> Option<NodeId> {
+        match *self {
+            VariantData::Struct(..) => None,
+            VariantData::Tuple(_, id) | VariantData::Unit(id) => Some(id),
+        }
+    }
+}
+
+/// An item definition.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Item<K = ItemKind> {
+    pub attrs: Vec<Attribute>,
+    pub id: NodeId,
+    pub span: Span,
+    pub vis: Visibility,
+    /// The name of the item.
+    /// It might be a dummy name in case of anonymous items.
+    pub ident: Ident,
+
+    pub kind: K,
+
+    /// Original tokens this item was parsed from. This isn't necessarily
+    /// available for all items, although over time more and more items should
+    /// have this be `Some`. Right now this is primarily used for procedural
+    /// macros, notably custom attributes.
+    ///
+    /// Note that the tokens here do not include the outer attributes, but will
+    /// include inner attributes.
+    pub tokens: Option<LazyTokenStream>,
+}
+
+impl Item {
+    /// Return the span that encompasses the attributes.
+    pub fn span_with_attributes(&self) -> Span {
+        self.attrs.iter().fold(self.span, |acc, attr| acc.to(attr.span))
+    }
+}
+
+/// `extern` qualifier on a function item or function type.
+#[derive(Clone, Copy, Encodable, Decodable, Debug)]
+pub enum Extern {
+    None,
+    Implicit(Span),
+    Explicit(StrLit, Span),
+}
+
+impl Extern {
+    pub fn from_abi(abi: Option<StrLit>, span: Span) -> Extern {
+        match abi {
+            Some(name) => Extern::Explicit(name, span),
+            None => Extern::Implicit(span),
+        }
+    }
+}
+
+/// A function header.
+///
+/// All the information between the visibility and the name of the function is
+/// included in this struct (e.g., `async unsafe fn` or `const extern "C" fn`).
+#[derive(Clone, Copy, Encodable, Decodable, Debug)]
+pub struct FnHeader {
+    pub unsafety: Unsafe,
+    pub asyncness: Async,
+    pub constness: Const,
+    pub ext: Extern,
+}
+
+impl FnHeader {
+    /// Does this function header have any qualifiers or is it empty?
+    pub fn has_qualifiers(&self) -> bool {
+        let Self { unsafety, asyncness, constness, ext } = self;
+        matches!(unsafety, Unsafe::Yes(_))
+            || asyncness.is_async()
+            || matches!(constness, Const::Yes(_))
+            || !matches!(ext, Extern::None)
+    }
+}
+
+impl Default for FnHeader {
+    fn default() -> FnHeader {
+        FnHeader {
+            unsafety: Unsafe::No,
+            asyncness: Async::No,
+            constness: Const::No,
+            ext: Extern::None,
+        }
+    }
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Trait {
+    pub unsafety: Unsafe,
+    pub is_auto: IsAuto,
+    pub generics: Generics,
+    pub bounds: GenericBounds,
+    pub items: Vec<P<AssocItem>>,
+}
+
+/// The location of a where clause on a `TyAlias` (`Span`) and whether there was
+/// a `where` keyword (`bool`). This is split out from `WhereClause`, since there
+/// are two locations for where clause on type aliases, but their predicates
+/// are concatenated together.
+///
+/// Take this example:
+/// ```ignore (only-for-syntax-highlight)
+/// trait Foo {
+///   type Assoc<'a, 'b> where Self: 'a, Self: 'b;
+/// }
+/// impl Foo for () {
+///   type Assoc<'a, 'b> where Self: 'a = () where Self: 'b;
+///   //                 ^^^^^^^^^^^^^^ first where clause
+///   //                                     ^^^^^^^^^^^^^^ second where clause
+/// }
+/// ```
+///
+/// If there is no where clause, then this is `false` with `DUMMY_SP`.
+#[derive(Copy, Clone, Encodable, Decodable, Debug, Default)]
+pub struct TyAliasWhereClause(pub bool, pub Span);
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct TyAlias {
+    pub defaultness: Defaultness,
+    pub generics: Generics,
+    /// The span information for the two where clauses (before equals, after equals)
+    pub where_clauses: (TyAliasWhereClause, TyAliasWhereClause),
+    /// The index in `generics.where_clause.predicates` that would split into
+    /// predicates from the where clause before the equals and the predicates
+    /// from the where clause after the equals
+    pub where_predicates_split: usize,
+    pub bounds: GenericBounds,
+    pub ty: Option<P<Ty>>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Impl {
+    pub defaultness: Defaultness,
+    pub unsafety: Unsafe,
+    pub generics: Generics,
+    pub constness: Const,
+    pub polarity: ImplPolarity,
+    /// The trait being implemented, if any.
+    pub of_trait: Option<TraitRef>,
+    pub self_ty: P<Ty>,
+    pub items: Vec<P<AssocItem>>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct Fn {
+    pub defaultness: Defaultness,
+    pub generics: Generics,
+    pub sig: FnSig,
+    pub body: Option<P<Block>>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum ItemKind {
+    /// An `extern crate` item, with the optional *original* crate name if the crate was renamed.
+    ///
+    /// E.g., `extern crate foo` or `extern crate foo_bar as foo`.
+    ExternCrate(Option<Symbol>),
+    /// A use declaration item (`use`).
+    ///
+    /// E.g., `use foo;`, `use foo::bar;` or `use foo::bar as FooBar;`.
+    Use(UseTree),
+    /// A static item (`static`).
+    ///
+    /// E.g., `static FOO: i32 = 42;` or `static FOO: &'static str = "bar";`.
+    Static(P<Ty>, Mutability, Option<P<Expr>>),
+    /// A constant item (`const`).
+    ///
+    /// E.g., `const FOO: i32 = 42;`.
+    Const(Defaultness, P<Ty>, Option<P<Expr>>),
+    /// A function declaration (`fn`).
+    ///
+    /// E.g., `fn foo(bar: usize) -> usize { .. }`.
+    Fn(Box<Fn>),
+    /// A module declaration (`mod`).
+    ///
+    /// E.g., `mod foo;` or `mod foo { .. }`.
+    /// `unsafe` keyword on modules is accepted syntactically for macro DSLs, but not
+    /// semantically by Rust.
+    Mod(Unsafe, ModKind),
+    /// An external module (`extern`).
+    ///
+    /// E.g., `extern {}` or `extern "C" {}`.
+    ForeignMod(ForeignMod),
+    /// Module-level inline assembly (from `global_asm!()`).
+    GlobalAsm(Box<InlineAsm>),
+    /// A type alias (`type`).
+    ///
+    /// E.g., `type Foo = Bar<u8>;`.
+    TyAlias(Box<TyAlias>),
+    /// An enum definition (`enum`).
+    ///
+    /// E.g., `enum Foo<A, B> { C<A>, D<B> }`.
+    Enum(EnumDef, Generics),
+    /// A struct definition (`struct`).
+    ///
+    /// E.g., `struct Foo<A> { x: A }`.
+    Struct(VariantData, Generics),
+    /// A union definition (`union`).
+    ///
+    /// E.g., `union Foo<A, B> { x: A, y: B }`.
+    Union(VariantData, Generics),
+    /// A trait declaration (`trait`).
+    ///
+    /// E.g., `trait Foo { .. }`, `trait Foo<T> { .. }` or `auto trait Foo {}`.
+    Trait(Box<Trait>),
+    /// Trait alias
+    ///
+    /// E.g., `trait Foo = Bar + Quux;`.
+    TraitAlias(Generics, GenericBounds),
+    /// An implementation.
+    ///
+    /// E.g., `impl<A> Foo<A> { .. }` or `impl<A> Trait for Foo<A> { .. }`.
+    Impl(Box<Impl>),
+    /// A macro invocation.
+    ///
+    /// E.g., `foo!(..)`.
+    MacCall(MacCall),
+
+    /// A macro definition.
+    MacroDef(MacroDef),
+}
+
+impl ItemKind {
+    pub fn article(&self) -> &str {
+        use ItemKind::*;
+        match self {
+            Use(..) | Static(..) | Const(..) | Fn(..) | Mod(..) | GlobalAsm(..) | TyAlias(..)
+            | Struct(..) | Union(..) | Trait(..) | TraitAlias(..) | MacroDef(..) => "a",
+            ExternCrate(..) | ForeignMod(..) | MacCall(..) | Enum(..) | Impl { .. } => "an",
+        }
+    }
+
+    pub fn descr(&self) -> &str {
+        match self {
+            ItemKind::ExternCrate(..) => "extern crate",
+            ItemKind::Use(..) => "`use` import",
+            ItemKind::Static(..) => "static item",
+            ItemKind::Const(..) => "constant item",
+            ItemKind::Fn(..) => "function",
+            ItemKind::Mod(..) => "module",
+            ItemKind::ForeignMod(..) => "extern block",
+            ItemKind::GlobalAsm(..) => "global asm item",
+            ItemKind::TyAlias(..) => "type alias",
+            ItemKind::Enum(..) => "enum",
+            ItemKind::Struct(..) => "struct",
+            ItemKind::Union(..) => "union",
+            ItemKind::Trait(..) => "trait",
+            ItemKind::TraitAlias(..) => "trait alias",
+            ItemKind::MacCall(..) => "item macro invocation",
+            ItemKind::MacroDef(..) => "macro definition",
+            ItemKind::Impl { .. } => "implementation",
+        }
+    }
+
+    pub fn generics(&self) -> Option<&Generics> {
+        match self {
+            Self::Fn(box Fn { generics, .. })
+            | Self::TyAlias(box TyAlias { generics, .. })
+            | Self::Enum(_, generics)
+            | Self::Struct(_, generics)
+            | Self::Union(_, generics)
+            | Self::Trait(box Trait { generics, .. })
+            | Self::TraitAlias(generics, _)
+            | Self::Impl(box Impl { generics, .. }) => Some(generics),
+            _ => None,
+        }
+    }
+}
+
+/// Represents associated items.
+/// These include items in `impl` and `trait` definitions.
+pub type AssocItem = Item<AssocItemKind>;
+
+/// Represents associated item kinds.
+///
+/// The term "provided" in the variants below refers to the item having a default
+/// definition / body. Meanwhile, a "required" item lacks a definition / body.
+/// In an implementation, all items must be provided.
+/// The `Option`s below denote the bodies, where `Some(_)`
+/// means "provided" and conversely `None` means "required".
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum AssocItemKind {
+    /// An associated constant, `const $ident: $ty $def?;` where `def ::= "=" $expr? ;`.
+    /// If `def` is parsed, then the constant is provided, and otherwise required.
+    Const(Defaultness, P<Ty>, Option<P<Expr>>),
+    /// An associated function.
+    Fn(Box<Fn>),
+    /// An associated type.
+    TyAlias(Box<TyAlias>),
+    /// A macro expanding to associated items.
+    MacCall(MacCall),
+}
+
+impl AssocItemKind {
+    pub fn defaultness(&self) -> Defaultness {
+        match *self {
+            Self::Const(defaultness, ..)
+            | Self::Fn(box Fn { defaultness, .. })
+            | Self::TyAlias(box TyAlias { defaultness, .. }) => defaultness,
+            Self::MacCall(..) => Defaultness::Final,
+        }
+    }
+}
+
+impl From<AssocItemKind> for ItemKind {
+    fn from(assoc_item_kind: AssocItemKind) -> ItemKind {
+        match assoc_item_kind {
+            AssocItemKind::Const(a, b, c) => ItemKind::Const(a, b, c),
+            AssocItemKind::Fn(fn_kind) => ItemKind::Fn(fn_kind),
+            AssocItemKind::TyAlias(ty_alias_kind) => ItemKind::TyAlias(ty_alias_kind),
+            AssocItemKind::MacCall(a) => ItemKind::MacCall(a),
+        }
+    }
+}
+
+impl TryFrom<ItemKind> for AssocItemKind {
+    type Error = ItemKind;
+
+    fn try_from(item_kind: ItemKind) -> Result<AssocItemKind, ItemKind> {
+        Ok(match item_kind {
+            ItemKind::Const(a, b, c) => AssocItemKind::Const(a, b, c),
+            ItemKind::Fn(fn_kind) => AssocItemKind::Fn(fn_kind),
+            ItemKind::TyAlias(ty_alias_kind) => AssocItemKind::TyAlias(ty_alias_kind),
+            ItemKind::MacCall(a) => AssocItemKind::MacCall(a),
+            _ => return Err(item_kind),
+        })
+    }
+}
+
+/// An item in `extern` block.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum ForeignItemKind {
+    /// A foreign static item (`static FOO: u8`).
+    Static(P<Ty>, Mutability, Option<P<Expr>>),
+    /// An foreign function.
+    Fn(Box<Fn>),
+    /// An foreign type.
+    TyAlias(Box<TyAlias>),
+    /// A macro expanding to foreign items.
+    MacCall(MacCall),
+}
+
+impl From<ForeignItemKind> for ItemKind {
+    fn from(foreign_item_kind: ForeignItemKind) -> ItemKind {
+        match foreign_item_kind {
+            ForeignItemKind::Static(a, b, c) => ItemKind::Static(a, b, c),
+            ForeignItemKind::Fn(fn_kind) => ItemKind::Fn(fn_kind),
+            ForeignItemKind::TyAlias(ty_alias_kind) => ItemKind::TyAlias(ty_alias_kind),
+            ForeignItemKind::MacCall(a) => ItemKind::MacCall(a),
+        }
+    }
+}
+
+impl TryFrom<ItemKind> for ForeignItemKind {
+    type Error = ItemKind;
+
+    fn try_from(item_kind: ItemKind) -> Result<ForeignItemKind, ItemKind> {
+        Ok(match item_kind {
+            ItemKind::Static(a, b, c) => ForeignItemKind::Static(a, b, c),
+            ItemKind::Fn(fn_kind) => ForeignItemKind::Fn(fn_kind),
+            ItemKind::TyAlias(ty_alias_kind) => ForeignItemKind::TyAlias(ty_alias_kind),
+            ItemKind::MacCall(a) => ForeignItemKind::MacCall(a),
+            _ => return Err(item_kind),
+        })
+    }
+}
+
+pub type ForeignItem = Item<ForeignItemKind>;
+
+// 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!(AssocItemKind, 72);
+    rustc_data_structures::static_assert_size!(Attribute, 152);
+    rustc_data_structures::static_assert_size!(Block, 48);
+    rustc_data_structures::static_assert_size!(Expr, 104);
+    rustc_data_structures::static_assert_size!(Fn, 192);
+    rustc_data_structures::static_assert_size!(ForeignItemKind, 72);
+    rustc_data_structures::static_assert_size!(GenericBound, 88);
+    rustc_data_structures::static_assert_size!(Generics, 72);
+    rustc_data_structures::static_assert_size!(Impl, 200);
+    rustc_data_structures::static_assert_size!(Item, 200);
+    rustc_data_structures::static_assert_size!(ItemKind, 112);
+    rustc_data_structures::static_assert_size!(Lit, 48);
+    rustc_data_structures::static_assert_size!(Pat, 120);
+    rustc_data_structures::static_assert_size!(Path, 40);
+    rustc_data_structures::static_assert_size!(PathSegment, 24);
+    rustc_data_structures::static_assert_size!(Stmt, 32);
+    rustc_data_structures::static_assert_size!(Ty, 96);
+}
diff --git a/compiler/rustc_ast/src/ast_traits.rs b/compiler/rustc_ast/src/ast_traits.rs
new file mode 100644
index 000000000..5c30a75a1
--- /dev/null
+++ b/compiler/rustc_ast/src/ast_traits.rs
@@ -0,0 +1,442 @@
+//! A set of traits implemented for various AST nodes,
+//! typically those used in AST fragments during macro expansion.
+//! The traits are not implemented exhaustively, only when actually necessary.
+
+use crate::ptr::P;
+use crate::token::Nonterminal;
+use crate::tokenstream::LazyTokenStream;
+use crate::{Arm, Crate, ExprField, FieldDef, GenericParam, Param, PatField, Variant};
+use crate::{AssocItem, Expr, ForeignItem, Item, NodeId};
+use crate::{AttrItem, AttrKind, Block, Pat, Path, Ty, Visibility};
+use crate::{AttrVec, Attribute, Stmt, StmtKind};
+
+use rustc_span::Span;
+
+use std::fmt;
+use std::marker::PhantomData;
+
+/// A utility trait to reduce boilerplate.
+/// Standard `Deref(Mut)` cannot be reused due to coherence.
+pub trait AstDeref {
+    type Target;
+    fn ast_deref(&self) -> &Self::Target;
+    fn ast_deref_mut(&mut self) -> &mut Self::Target;
+}
+
+macro_rules! impl_not_ast_deref {
+    ($($T:ty),+ $(,)?) => {
+        $(
+            impl !AstDeref for $T {}
+        )+
+    };
+}
+
+impl_not_ast_deref!(AssocItem, Expr, ForeignItem, Item, Stmt);
+
+impl<T> AstDeref for P<T> {
+    type Target = T;
+    fn ast_deref(&self) -> &Self::Target {
+        self
+    }
+    fn ast_deref_mut(&mut self) -> &mut Self::Target {
+        self
+    }
+}
+
+/// A trait for AST nodes having an ID.
+pub trait HasNodeId {
+    fn node_id(&self) -> NodeId;
+    fn node_id_mut(&mut self) -> &mut NodeId;
+}
+
+macro_rules! impl_has_node_id {
+    ($($T:ty),+ $(,)?) => {
+        $(
+            impl HasNodeId for $T {
+                fn node_id(&self) -> NodeId {
+                    self.id
+                }
+                fn node_id_mut(&mut self) -> &mut NodeId {
+                    &mut self.id
+                }
+            }
+        )+
+    };
+}
+
+impl_has_node_id!(
+    Arm,
+    AssocItem,
+    Crate,
+    Expr,
+    ExprField,
+    FieldDef,
+    ForeignItem,
+    GenericParam,
+    Item,
+    Param,
+    Pat,
+    PatField,
+    Stmt,
+    Ty,
+    Variant,
+);
+
+impl<T: AstDeref<Target: HasNodeId>> HasNodeId for T {
+    fn node_id(&self) -> NodeId {
+        self.ast_deref().node_id()
+    }
+    fn node_id_mut(&mut self) -> &mut NodeId {
+        self.ast_deref_mut().node_id_mut()
+    }
+}
+
+/// A trait for AST nodes having a span.
+pub trait HasSpan {
+    fn span(&self) -> Span;
+}
+
+macro_rules! impl_has_span {
+    ($($T:ty),+ $(,)?) => {
+        $(
+            impl HasSpan for $T {
+                fn span(&self) -> Span {
+                    self.span
+                }
+            }
+        )+
+    };
+}
+
+impl_has_span!(AssocItem, Block, Expr, ForeignItem, Item, Pat, Path, Stmt, Ty, Visibility);
+
+impl<T: AstDeref<Target: HasSpan>> HasSpan for T {
+    fn span(&self) -> Span {
+        self.ast_deref().span()
+    }
+}
+
+impl HasSpan for AttrItem {
+    fn span(&self) -> Span {
+        self.span()
+    }
+}
+
+/// A trait for AST nodes having (or not having) collected tokens.
+pub trait HasTokens {
+    fn tokens(&self) -> Option<&LazyTokenStream>;
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>>;
+}
+
+macro_rules! impl_has_tokens {
+    ($($T:ty),+ $(,)?) => {
+        $(
+            impl HasTokens for $T {
+                fn tokens(&self) -> Option<&LazyTokenStream> {
+                    self.tokens.as_ref()
+                }
+                fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+                    Some(&mut self.tokens)
+                }
+            }
+        )+
+    };
+}
+
+macro_rules! impl_has_tokens_none {
+    ($($T:ty),+ $(,)?) => {
+        $(
+            impl HasTokens for $T {
+                fn tokens(&self) -> Option<&LazyTokenStream> {
+                    None
+                }
+                fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+                    None
+                }
+            }
+        )+
+    };
+}
+
+impl_has_tokens!(AssocItem, AttrItem, Block, Expr, ForeignItem, Item, Pat, Path, Ty, Visibility);
+impl_has_tokens_none!(Arm, ExprField, FieldDef, GenericParam, Param, PatField, Variant);
+
+impl<T: AstDeref<Target: HasTokens>> HasTokens for T {
+    fn tokens(&self) -> Option<&LazyTokenStream> {
+        self.ast_deref().tokens()
+    }
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+        self.ast_deref_mut().tokens_mut()
+    }
+}
+
+impl<T: HasTokens> HasTokens for Option<T> {
+    fn tokens(&self) -> Option<&LazyTokenStream> {
+        self.as_ref().and_then(|inner| inner.tokens())
+    }
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+        self.as_mut().and_then(|inner| inner.tokens_mut())
+    }
+}
+
+impl HasTokens for StmtKind {
+    fn tokens(&self) -> Option<&LazyTokenStream> {
+        match self {
+            StmtKind::Local(local) => local.tokens.as_ref(),
+            StmtKind::Item(item) => item.tokens(),
+            StmtKind::Expr(expr) | StmtKind::Semi(expr) => expr.tokens(),
+            StmtKind::Empty => return None,
+            StmtKind::MacCall(mac) => mac.tokens.as_ref(),
+        }
+    }
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+        match self {
+            StmtKind::Local(local) => Some(&mut local.tokens),
+            StmtKind::Item(item) => item.tokens_mut(),
+            StmtKind::Expr(expr) | StmtKind::Semi(expr) => expr.tokens_mut(),
+            StmtKind::Empty => return None,
+            StmtKind::MacCall(mac) => Some(&mut mac.tokens),
+        }
+    }
+}
+
+impl HasTokens for Stmt {
+    fn tokens(&self) -> Option<&LazyTokenStream> {
+        self.kind.tokens()
+    }
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+        self.kind.tokens_mut()
+    }
+}
+
+impl HasTokens for Attribute {
+    fn tokens(&self) -> Option<&LazyTokenStream> {
+        match &self.kind {
+            AttrKind::Normal(_, tokens) => tokens.as_ref(),
+            kind @ AttrKind::DocComment(..) => {
+                panic!("Called tokens on doc comment attr {:?}", kind)
+            }
+        }
+    }
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+        Some(match &mut self.kind {
+            AttrKind::Normal(_, tokens) => tokens,
+            kind @ AttrKind::DocComment(..) => {
+                panic!("Called tokens_mut on doc comment attr {:?}", kind)
+            }
+        })
+    }
+}
+
+impl HasTokens for Nonterminal {
+    fn tokens(&self) -> Option<&LazyTokenStream> {
+        match self {
+            Nonterminal::NtItem(item) => item.tokens(),
+            Nonterminal::NtStmt(stmt) => stmt.tokens(),
+            Nonterminal::NtExpr(expr) | Nonterminal::NtLiteral(expr) => expr.tokens(),
+            Nonterminal::NtPat(pat) => pat.tokens(),
+            Nonterminal::NtTy(ty) => ty.tokens(),
+            Nonterminal::NtMeta(attr_item) => attr_item.tokens(),
+            Nonterminal::NtPath(path) => path.tokens(),
+            Nonterminal::NtVis(vis) => vis.tokens(),
+            Nonterminal::NtBlock(block) => block.tokens(),
+            Nonterminal::NtIdent(..) | Nonterminal::NtLifetime(..) => None,
+        }
+    }
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+        match self {
+            Nonterminal::NtItem(item) => item.tokens_mut(),
+            Nonterminal::NtStmt(stmt) => stmt.tokens_mut(),
+            Nonterminal::NtExpr(expr) | Nonterminal::NtLiteral(expr) => expr.tokens_mut(),
+            Nonterminal::NtPat(pat) => pat.tokens_mut(),
+            Nonterminal::NtTy(ty) => ty.tokens_mut(),
+            Nonterminal::NtMeta(attr_item) => attr_item.tokens_mut(),
+            Nonterminal::NtPath(path) => path.tokens_mut(),
+            Nonterminal::NtVis(vis) => vis.tokens_mut(),
+            Nonterminal::NtBlock(block) => block.tokens_mut(),
+            Nonterminal::NtIdent(..) | Nonterminal::NtLifetime(..) => None,
+        }
+    }
+}
+
+/// A trait for AST nodes having (or not having) attributes.
+pub trait HasAttrs {
+    /// This is `true` if this `HasAttrs` might support 'custom' (proc-macro) inner
+    /// attributes. Attributes like `#![cfg]` and `#![cfg_attr]` are not
+    /// considered 'custom' attributes.
+    ///
+    /// If this is `false`, then this `HasAttrs` definitely does
+    /// not support 'custom' inner attributes, which enables some optimizations
+    /// during token collection.
+    const SUPPORTS_CUSTOM_INNER_ATTRS: bool;
+    fn attrs(&self) -> &[Attribute];
+    fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>));
+}
+
+macro_rules! impl_has_attrs {
+    (const SUPPORTS_CUSTOM_INNER_ATTRS: bool = $inner:literal, $($T:ty),+ $(,)?) => {
+        $(
+            impl HasAttrs for $T {
+                const SUPPORTS_CUSTOM_INNER_ATTRS: bool = $inner;
+
+                fn attrs(&self) -> &[Attribute] {
+                    &self.attrs
+                }
+
+                fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
+                    VecOrAttrVec::visit(&mut self.attrs, f)
+                }
+            }
+        )+
+    };
+}
+
+macro_rules! impl_has_attrs_none {
+    ($($T:ty),+ $(,)?) => {
+        $(
+            impl HasAttrs for $T {
+                const SUPPORTS_CUSTOM_INNER_ATTRS: bool = false;
+                fn attrs(&self) -> &[Attribute] {
+                    &[]
+                }
+                fn visit_attrs(&mut self, _f: impl FnOnce(&mut Vec<Attribute>)) {}
+            }
+        )+
+    };
+}
+
+impl_has_attrs!(
+    const SUPPORTS_CUSTOM_INNER_ATTRS: bool = true,
+    AssocItem,
+    ForeignItem,
+    Item,
+);
+impl_has_attrs!(
+    const SUPPORTS_CUSTOM_INNER_ATTRS: bool = false,
+    Arm,
+    Crate,
+    Expr,
+    ExprField,
+    FieldDef,
+    GenericParam,
+    Param,
+    PatField,
+    Variant,
+);
+impl_has_attrs_none!(Attribute, AttrItem, Block, Pat, Path, Ty, Visibility);
+
+impl<T: AstDeref<Target: HasAttrs>> HasAttrs for T {
+    const SUPPORTS_CUSTOM_INNER_ATTRS: bool = T::Target::SUPPORTS_CUSTOM_INNER_ATTRS;
+    fn attrs(&self) -> &[Attribute] {
+        self.ast_deref().attrs()
+    }
+    fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
+        self.ast_deref_mut().visit_attrs(f)
+    }
+}
+
+impl<T: HasAttrs> HasAttrs for Option<T> {
+    const SUPPORTS_CUSTOM_INNER_ATTRS: bool = T::SUPPORTS_CUSTOM_INNER_ATTRS;
+    fn attrs(&self) -> &[Attribute] {
+        self.as_ref().map(|inner| inner.attrs()).unwrap_or(&[])
+    }
+    fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
+        if let Some(inner) = self.as_mut() {
+            inner.visit_attrs(f);
+        }
+    }
+}
+
+impl HasAttrs for StmtKind {
+    // This might be a `StmtKind::Item`, which contains
+    // an item that supports inner attrs.
+    const SUPPORTS_CUSTOM_INNER_ATTRS: bool = true;
+
+    fn attrs(&self) -> &[Attribute] {
+        match self {
+            StmtKind::Local(local) => &local.attrs,
+            StmtKind::Expr(expr) | StmtKind::Semi(expr) => expr.attrs(),
+            StmtKind::Item(item) => item.attrs(),
+            StmtKind::Empty => &[],
+            StmtKind::MacCall(mac) => &mac.attrs,
+        }
+    }
+
+    fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
+        match self {
+            StmtKind::Local(local) => visit_attrvec(&mut local.attrs, f),
+            StmtKind::Expr(expr) | StmtKind::Semi(expr) => expr.visit_attrs(f),
+            StmtKind::Item(item) => item.visit_attrs(f),
+            StmtKind::Empty => {}
+            StmtKind::MacCall(mac) => visit_attrvec(&mut mac.attrs, f),
+        }
+    }
+}
+
+impl HasAttrs for Stmt {
+    const SUPPORTS_CUSTOM_INNER_ATTRS: bool = StmtKind::SUPPORTS_CUSTOM_INNER_ATTRS;
+    fn attrs(&self) -> &[Attribute] {
+        self.kind.attrs()
+    }
+    fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
+        self.kind.visit_attrs(f);
+    }
+}
+
+/// Helper trait for the impls above. Abstracts over
+/// the two types of attribute fields that AST nodes
+/// may have (`Vec<Attribute>` or `AttrVec`).
+trait VecOrAttrVec {
+    fn visit(&mut self, f: impl FnOnce(&mut Vec<Attribute>));
+}
+
+impl VecOrAttrVec for Vec<Attribute> {
+    fn visit(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
+        f(self)
+    }
+}
+
+impl VecOrAttrVec for AttrVec {
+    fn visit(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
+        visit_attrvec(self, f)
+    }
+}
+
+fn visit_attrvec(attrs: &mut AttrVec, f: impl FnOnce(&mut Vec<Attribute>)) {
+    crate::mut_visit::visit_clobber(attrs, |attrs| {
+        let mut vec = attrs.into();
+        f(&mut vec);
+        vec.into()
+    });
+}
+
+/// A newtype around an AST node that implements the traits above if the node implements them.
+pub struct AstNodeWrapper<Wrapped, Tag> {
+    pub wrapped: Wrapped,
+    pub tag: PhantomData<Tag>,
+}
+
+impl<Wrapped, Tag> AstNodeWrapper<Wrapped, Tag> {
+    pub fn new(wrapped: Wrapped, _tag: Tag) -> AstNodeWrapper<Wrapped, Tag> {
+        AstNodeWrapper { wrapped, tag: Default::default() }
+    }
+}
+
+impl<Wrapped, Tag> AstDeref for AstNodeWrapper<Wrapped, Tag> {
+    type Target = Wrapped;
+    fn ast_deref(&self) -> &Self::Target {
+        &self.wrapped
+    }
+    fn ast_deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.wrapped
+    }
+}
+
+impl<Wrapped: fmt::Debug, Tag> fmt::Debug for AstNodeWrapper<Wrapped, Tag> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("AstNodeWrapper")
+            .field("wrapped", &self.wrapped)
+            .field("tag", &self.tag)
+            .finish()
+    }
+}
diff --git a/compiler/rustc_ast/src/attr/mod.rs b/compiler/rustc_ast/src/attr/mod.rs
new file mode 100644
index 000000000..86af7769d
--- /dev/null
+++ b/compiler/rustc_ast/src/attr/mod.rs
@@ -0,0 +1,634 @@
+//! Functions dealing with attributes and meta items.
+
+use crate::ast;
+use crate::ast::{AttrId, AttrItem, AttrKind, AttrStyle, Attribute};
+use crate::ast::{Lit, LitKind};
+use crate::ast::{MacArgs, MacArgsEq, MacDelimiter, MetaItem, MetaItemKind, NestedMetaItem};
+use crate::ast::{Path, PathSegment};
+use crate::ptr::P;
+use crate::token::{self, CommentKind, Delimiter, Token};
+use crate::tokenstream::{AttrAnnotatedTokenStream, AttrAnnotatedTokenTree};
+use crate::tokenstream::{DelimSpan, Spacing, TokenTree};
+use crate::tokenstream::{LazyTokenStream, TokenStream};
+use crate::util::comments;
+
+use rustc_data_structures::thin_vec::ThinVec;
+use rustc_index::bit_set::GrowableBitSet;
+use rustc_span::source_map::BytePos;
+use rustc_span::symbol::{sym, Ident, Symbol};
+use rustc_span::Span;
+
+use std::iter;
+
+pub struct MarkedAttrs(GrowableBitSet<AttrId>);
+
+impl MarkedAttrs {
+    // We have no idea how many attributes there will be, so just
+    // initiate the vectors with 0 bits. We'll grow them as necessary.
+    pub fn new() -> Self {
+        MarkedAttrs(GrowableBitSet::new_empty())
+    }
+
+    pub fn mark(&mut self, attr: &Attribute) {
+        self.0.insert(attr.id);
+    }
+
+    pub fn is_marked(&self, attr: &Attribute) -> bool {
+        self.0.contains(attr.id)
+    }
+}
+
+impl NestedMetaItem {
+    /// Returns the `MetaItem` if `self` is a `NestedMetaItem::MetaItem`.
+    pub fn meta_item(&self) -> Option<&MetaItem> {
+        match *self {
+            NestedMetaItem::MetaItem(ref item) => Some(item),
+            _ => None,
+        }
+    }
+
+    /// Returns the `Lit` if `self` is a `NestedMetaItem::Literal`s.
+    pub fn literal(&self) -> Option<&Lit> {
+        match *self {
+            NestedMetaItem::Literal(ref lit) => Some(lit),
+            _ => None,
+        }
+    }
+
+    /// Returns `true` if this list item is a MetaItem with a name of `name`.
+    pub fn has_name(&self, name: Symbol) -> bool {
+        self.meta_item().map_or(false, |meta_item| meta_item.has_name(name))
+    }
+
+    /// For a single-segment meta item, returns its name; otherwise, returns `None`.
+    pub fn ident(&self) -> Option<Ident> {
+        self.meta_item().and_then(|meta_item| meta_item.ident())
+    }
+    pub fn name_or_empty(&self) -> Symbol {
+        self.ident().unwrap_or_else(Ident::empty).name
+    }
+
+    /// Gets the string value if `self` is a `MetaItem` and the `MetaItem` is a
+    /// `MetaItemKind::NameValue` variant containing a string, otherwise `None`.
+    pub fn value_str(&self) -> Option<Symbol> {
+        self.meta_item().and_then(|meta_item| meta_item.value_str())
+    }
+
+    /// Returns a name and single literal value tuple of the `MetaItem`.
+    pub fn name_value_literal(&self) -> Option<(Symbol, &Lit)> {
+        self.meta_item().and_then(|meta_item| {
+            meta_item.meta_item_list().and_then(|meta_item_list| {
+                if meta_item_list.len() == 1
+                    && let Some(ident) = meta_item.ident()
+                    && let Some(lit) = meta_item_list[0].literal()
+                {
+                    return Some((ident.name, lit));
+                }
+                None
+            })
+        })
+    }
+
+    /// Gets a list of inner meta items from a list `MetaItem` type.
+    pub fn meta_item_list(&self) -> Option<&[NestedMetaItem]> {
+        self.meta_item().and_then(|meta_item| meta_item.meta_item_list())
+    }
+
+    /// Returns `true` if the variant is `MetaItem`.
+    pub fn is_meta_item(&self) -> bool {
+        self.meta_item().is_some()
+    }
+
+    /// Returns `true` if `self` is a `MetaItem` and the meta item is a word.
+    pub fn is_word(&self) -> bool {
+        self.meta_item().map_or(false, |meta_item| meta_item.is_word())
+    }
+
+    /// See [`MetaItem::name_value_literal_span`].
+    pub fn name_value_literal_span(&self) -> Option<Span> {
+        self.meta_item()?.name_value_literal_span()
+    }
+}
+
+impl Attribute {
+    #[inline]
+    pub fn has_name(&self, name: Symbol) -> bool {
+        match self.kind {
+            AttrKind::Normal(ref item, _) => item.path == name,
+            AttrKind::DocComment(..) => false,
+        }
+    }
+
+    /// For a single-segment attribute, returns its name; otherwise, returns `None`.
+    pub fn ident(&self) -> Option<Ident> {
+        match self.kind {
+            AttrKind::Normal(ref item, _) => {
+                if item.path.segments.len() == 1 {
+                    Some(item.path.segments[0].ident)
+                } else {
+                    None
+                }
+            }
+            AttrKind::DocComment(..) => None,
+        }
+    }
+    pub fn name_or_empty(&self) -> Symbol {
+        self.ident().unwrap_or_else(Ident::empty).name
+    }
+
+    pub fn value_str(&self) -> Option<Symbol> {
+        match self.kind {
+            AttrKind::Normal(ref item, _) => item.meta_kind().and_then(|kind| kind.value_str()),
+            AttrKind::DocComment(..) => None,
+        }
+    }
+
+    pub fn meta_item_list(&self) -> Option<Vec<NestedMetaItem>> {
+        match self.kind {
+            AttrKind::Normal(ref item, _) => match item.meta_kind() {
+                Some(MetaItemKind::List(list)) => Some(list),
+                _ => None,
+            },
+            AttrKind::DocComment(..) => None,
+        }
+    }
+
+    pub fn is_word(&self) -> bool {
+        if let AttrKind::Normal(item, _) = &self.kind {
+            matches!(item.args, MacArgs::Empty)
+        } else {
+            false
+        }
+    }
+}
+
+impl MetaItem {
+    /// For a single-segment meta item, returns its name; otherwise, returns `None`.
+    pub fn ident(&self) -> Option<Ident> {
+        if self.path.segments.len() == 1 { Some(self.path.segments[0].ident) } else { None }
+    }
+    pub fn name_or_empty(&self) -> Symbol {
+        self.ident().unwrap_or_else(Ident::empty).name
+    }
+
+    // Example:
+    //     #[attribute(name = "value")]
+    //                 ^^^^^^^^^^^^^^
+    pub fn name_value_literal(&self) -> Option<&Lit> {
+        match &self.kind {
+            MetaItemKind::NameValue(v) => Some(v),
+            _ => None,
+        }
+    }
+
+    pub fn value_str(&self) -> Option<Symbol> {
+        match self.kind {
+            MetaItemKind::NameValue(ref v) => match v.kind {
+                LitKind::Str(ref s, _) => Some(*s),
+                _ => None,
+            },
+            _ => None,
+        }
+    }
+
+    pub fn meta_item_list(&self) -> Option<&[NestedMetaItem]> {
+        match self.kind {
+            MetaItemKind::List(ref l) => Some(&l[..]),
+            _ => None,
+        }
+    }
+
+    pub fn is_word(&self) -> bool {
+        matches!(self.kind, MetaItemKind::Word)
+    }
+
+    pub fn has_name(&self, name: Symbol) -> bool {
+        self.path == name
+    }
+
+    /// This is used in case you want the value span instead of the whole attribute. Example:
+    ///
+    /// ```text
+    /// #[doc(alias = "foo")]
+    /// ```
+    ///
+    /// In here, it'll return a span for `"foo"`.
+    pub fn name_value_literal_span(&self) -> Option<Span> {
+        Some(self.name_value_literal()?.span)
+    }
+}
+
+impl AttrItem {
+    pub fn span(&self) -> Span {
+        self.args.span().map_or(self.path.span, |args_span| self.path.span.to(args_span))
+    }
+
+    pub fn meta(&self, span: Span) -> Option<MetaItem> {
+        Some(MetaItem {
+            path: self.path.clone(),
+            kind: MetaItemKind::from_mac_args(&self.args)?,
+            span,
+        })
+    }
+
+    pub fn meta_kind(&self) -> Option<MetaItemKind> {
+        MetaItemKind::from_mac_args(&self.args)
+    }
+}
+
+impl Attribute {
+    pub fn is_doc_comment(&self) -> bool {
+        match self.kind {
+            AttrKind::Normal(..) => false,
+            AttrKind::DocComment(..) => true,
+        }
+    }
+
+    pub fn doc_str_and_comment_kind(&self) -> Option<(Symbol, CommentKind)> {
+        match self.kind {
+            AttrKind::DocComment(kind, data) => Some((data, kind)),
+            AttrKind::Normal(ref item, _) if item.path == sym::doc => item
+                .meta_kind()
+                .and_then(|kind| kind.value_str())
+                .map(|data| (data, CommentKind::Line)),
+            _ => None,
+        }
+    }
+
+    pub fn doc_str(&self) -> Option<Symbol> {
+        match self.kind {
+            AttrKind::DocComment(.., data) => Some(data),
+            AttrKind::Normal(ref item, _) if item.path == sym::doc => {
+                item.meta_kind().and_then(|kind| kind.value_str())
+            }
+            _ => None,
+        }
+    }
+
+    pub fn may_have_doc_links(&self) -> bool {
+        self.doc_str().map_or(false, |s| comments::may_have_doc_links(s.as_str()))
+    }
+
+    pub fn get_normal_item(&self) -> &AttrItem {
+        match self.kind {
+            AttrKind::Normal(ref item, _) => item,
+            AttrKind::DocComment(..) => panic!("unexpected doc comment"),
+        }
+    }
+
+    pub fn unwrap_normal_item(self) -> AttrItem {
+        match self.kind {
+            AttrKind::Normal(item, _) => item,
+            AttrKind::DocComment(..) => panic!("unexpected doc comment"),
+        }
+    }
+
+    /// Extracts the MetaItem from inside this Attribute.
+    pub fn meta(&self) -> Option<MetaItem> {
+        match self.kind {
+            AttrKind::Normal(ref item, _) => item.meta(self.span),
+            AttrKind::DocComment(..) => None,
+        }
+    }
+
+    pub fn meta_kind(&self) -> Option<MetaItemKind> {
+        match self.kind {
+            AttrKind::Normal(ref item, _) => item.meta_kind(),
+            AttrKind::DocComment(..) => None,
+        }
+    }
+
+    pub fn tokens(&self) -> AttrAnnotatedTokenStream {
+        match self.kind {
+            AttrKind::Normal(_, ref tokens) => tokens
+                .as_ref()
+                .unwrap_or_else(|| panic!("attribute is missing tokens: {:?}", self))
+                .create_token_stream(),
+            AttrKind::DocComment(comment_kind, data) => AttrAnnotatedTokenStream::from((
+                AttrAnnotatedTokenTree::Token(Token::new(
+                    token::DocComment(comment_kind, self.style, data),
+                    self.span,
+                )),
+                Spacing::Alone,
+            )),
+        }
+    }
+}
+
+/* Constructors */
+
+pub fn mk_name_value_item_str(ident: Ident, str: Symbol, str_span: Span) -> MetaItem {
+    let lit_kind = LitKind::Str(str, ast::StrStyle::Cooked);
+    mk_name_value_item(ident, lit_kind, str_span)
+}
+
+pub fn mk_name_value_item(ident: Ident, lit_kind: LitKind, lit_span: Span) -> MetaItem {
+    let lit = Lit::from_lit_kind(lit_kind, lit_span);
+    let span = ident.span.to(lit_span);
+    MetaItem { path: Path::from_ident(ident), span, kind: MetaItemKind::NameValue(lit) }
+}
+
+pub fn mk_list_item(ident: Ident, items: Vec<NestedMetaItem>) -> MetaItem {
+    MetaItem { path: Path::from_ident(ident), span: ident.span, kind: MetaItemKind::List(items) }
+}
+
+pub fn mk_word_item(ident: Ident) -> MetaItem {
+    MetaItem { path: Path::from_ident(ident), span: ident.span, kind: MetaItemKind::Word }
+}
+
+pub fn mk_nested_word_item(ident: Ident) -> NestedMetaItem {
+    NestedMetaItem::MetaItem(mk_word_item(ident))
+}
+
+pub(crate) fn mk_attr_id() -> AttrId {
+    use std::sync::atomic::AtomicU32;
+    use std::sync::atomic::Ordering;
+
+    static NEXT_ATTR_ID: AtomicU32 = AtomicU32::new(0);
+
+    let id = NEXT_ATTR_ID.fetch_add(1, Ordering::SeqCst);
+    assert!(id != u32::MAX);
+    AttrId::from_u32(id)
+}
+
+pub fn mk_attr(style: AttrStyle, path: Path, args: MacArgs, span: Span) -> Attribute {
+    mk_attr_from_item(AttrItem { path, args, tokens: None }, None, style, span)
+}
+
+pub fn mk_attr_from_item(
+    item: AttrItem,
+    tokens: Option<LazyTokenStream>,
+    style: AttrStyle,
+    span: Span,
+) -> Attribute {
+    Attribute { kind: AttrKind::Normal(item, tokens), id: mk_attr_id(), style, span }
+}
+
+/// Returns an inner attribute with the given value and span.
+pub fn mk_attr_inner(item: MetaItem) -> Attribute {
+    mk_attr(AttrStyle::Inner, item.path, item.kind.mac_args(item.span), item.span)
+}
+
+/// Returns an outer attribute with the given value and span.
+pub fn mk_attr_outer(item: MetaItem) -> Attribute {
+    mk_attr(AttrStyle::Outer, item.path, item.kind.mac_args(item.span), item.span)
+}
+
+pub fn mk_doc_comment(
+    comment_kind: CommentKind,
+    style: AttrStyle,
+    data: Symbol,
+    span: Span,
+) -> Attribute {
+    Attribute { kind: AttrKind::DocComment(comment_kind, data), id: mk_attr_id(), style, span }
+}
+
+pub fn list_contains_name(items: &[NestedMetaItem], name: Symbol) -> bool {
+    items.iter().any(|item| item.has_name(name))
+}
+
+impl MetaItem {
+    fn token_trees(&self) -> Vec<TokenTree> {
+        let mut idents = vec![];
+        let mut last_pos = BytePos(0_u32);
+        for (i, segment) in self.path.segments.iter().enumerate() {
+            let is_first = i == 0;
+            if !is_first {
+                let mod_sep_span =
+                    Span::new(last_pos, segment.ident.span.lo(), segment.ident.span.ctxt(), None);
+                idents.push(TokenTree::token_alone(token::ModSep, mod_sep_span));
+            }
+            idents.push(TokenTree::Token(Token::from_ast_ident(segment.ident), Spacing::Alone));
+            last_pos = segment.ident.span.hi();
+        }
+        idents.extend(self.kind.token_trees(self.span));
+        idents
+    }
+
+    fn from_tokens<I>(tokens: &mut iter::Peekable<I>) -> Option<MetaItem>
+    where
+        I: Iterator<Item = TokenTree>,
+    {
+        // FIXME: Share code with `parse_path`.
+        let path = match tokens.next().map(TokenTree::uninterpolate) {
+            Some(TokenTree::Token(
+                Token { kind: kind @ (token::Ident(..) | token::ModSep), span },
+                _,
+            )) => 'arm: {
+                let mut segments = if let token::Ident(name, _) = kind {
+                    if let Some(TokenTree::Token(Token { kind: token::ModSep, .. }, _)) =
+                        tokens.peek()
+                    {
+                        tokens.next();
+                        vec![PathSegment::from_ident(Ident::new(name, span))]
+                    } else {
+                        break 'arm Path::from_ident(Ident::new(name, span));
+                    }
+                } else {
+                    vec![PathSegment::path_root(span)]
+                };
+                loop {
+                    if let Some(TokenTree::Token(Token { kind: token::Ident(name, _), span }, _)) =
+                        tokens.next().map(TokenTree::uninterpolate)
+                    {
+                        segments.push(PathSegment::from_ident(Ident::new(name, span)));
+                    } else {
+                        return None;
+                    }
+                    if let Some(TokenTree::Token(Token { kind: token::ModSep, .. }, _)) =
+                        tokens.peek()
+                    {
+                        tokens.next();
+                    } else {
+                        break;
+                    }
+                }
+                let span = span.with_hi(segments.last().unwrap().ident.span.hi());
+                Path { span, segments, tokens: None }
+            }
+            Some(TokenTree::Token(Token { kind: token::Interpolated(nt), .. }, _)) => match *nt {
+                token::Nonterminal::NtMeta(ref item) => return item.meta(item.path.span),
+                token::Nonterminal::NtPath(ref path) => (**path).clone(),
+                _ => return None,
+            },
+            _ => return None,
+        };
+        let list_closing_paren_pos = tokens.peek().map(|tt| tt.span().hi());
+        let kind = MetaItemKind::from_tokens(tokens)?;
+        let hi = match kind {
+            MetaItemKind::NameValue(ref lit) => lit.span.hi(),
+            MetaItemKind::List(..) => list_closing_paren_pos.unwrap_or(path.span.hi()),
+            _ => path.span.hi(),
+        };
+        let span = path.span.with_hi(hi);
+        Some(MetaItem { path, kind, span })
+    }
+}
+
+impl MetaItemKind {
+    pub fn value_str(&self) -> Option<Symbol> {
+        match self {
+            MetaItemKind::NameValue(ref v) => match v.kind {
+                LitKind::Str(ref s, _) => Some(*s),
+                _ => None,
+            },
+            _ => None,
+        }
+    }
+
+    pub fn mac_args(&self, span: Span) -> MacArgs {
+        match self {
+            MetaItemKind::Word => MacArgs::Empty,
+            MetaItemKind::NameValue(lit) => {
+                let expr = P(ast::Expr {
+                    id: ast::DUMMY_NODE_ID,
+                    kind: ast::ExprKind::Lit(lit.clone()),
+                    span: lit.span,
+                    attrs: ThinVec::new(),
+                    tokens: None,
+                });
+                MacArgs::Eq(span, MacArgsEq::Ast(expr))
+            }
+            MetaItemKind::List(list) => {
+                let mut tts = Vec::new();
+                for (i, item) in list.iter().enumerate() {
+                    if i > 0 {
+                        tts.push(TokenTree::token_alone(token::Comma, span));
+                    }
+                    tts.extend(item.token_trees())
+                }
+                MacArgs::Delimited(
+                    DelimSpan::from_single(span),
+                    MacDelimiter::Parenthesis,
+                    TokenStream::new(tts),
+                )
+            }
+        }
+    }
+
+    fn token_trees(&self, span: Span) -> Vec<TokenTree> {
+        match *self {
+            MetaItemKind::Word => vec![],
+            MetaItemKind::NameValue(ref lit) => {
+                vec![
+                    TokenTree::token_alone(token::Eq, span),
+                    TokenTree::Token(lit.to_token(), Spacing::Alone),
+                ]
+            }
+            MetaItemKind::List(ref list) => {
+                let mut tokens = Vec::new();
+                for (i, item) in list.iter().enumerate() {
+                    if i > 0 {
+                        tokens.push(TokenTree::token_alone(token::Comma, span));
+                    }
+                    tokens.extend(item.token_trees())
+                }
+                vec![TokenTree::Delimited(
+                    DelimSpan::from_single(span),
+                    Delimiter::Parenthesis,
+                    TokenStream::new(tokens),
+                )]
+            }
+        }
+    }
+
+    fn list_from_tokens(tokens: TokenStream) -> Option<MetaItemKind> {
+        let mut tokens = tokens.into_trees().peekable();
+        let mut result = Vec::new();
+        while tokens.peek().is_some() {
+            let item = NestedMetaItem::from_tokens(&mut tokens)?;
+            result.push(item);
+            match tokens.next() {
+                None | Some(TokenTree::Token(Token { kind: token::Comma, .. }, _)) => {}
+                _ => return None,
+            }
+        }
+        Some(MetaItemKind::List(result))
+    }
+
+    fn name_value_from_tokens(
+        tokens: &mut impl Iterator<Item = TokenTree>,
+    ) -> Option<MetaItemKind> {
+        match tokens.next() {
+            Some(TokenTree::Delimited(_, Delimiter::Invisible, inner_tokens)) => {
+                MetaItemKind::name_value_from_tokens(&mut inner_tokens.into_trees())
+            }
+            Some(TokenTree::Token(token, _)) => {
+                Lit::from_token(&token).ok().map(MetaItemKind::NameValue)
+            }
+            _ => None,
+        }
+    }
+
+    fn from_mac_args(args: &MacArgs) -> Option<MetaItemKind> {
+        match args {
+            MacArgs::Empty => Some(MetaItemKind::Word),
+            MacArgs::Delimited(_, MacDelimiter::Parenthesis, tokens) => {
+                MetaItemKind::list_from_tokens(tokens.clone())
+            }
+            MacArgs::Delimited(..) => None,
+            MacArgs::Eq(_, MacArgsEq::Ast(expr)) => match &expr.kind {
+                ast::ExprKind::Lit(lit) => Some(MetaItemKind::NameValue(lit.clone())),
+                _ => None,
+            },
+            MacArgs::Eq(_, MacArgsEq::Hir(lit)) => Some(MetaItemKind::NameValue(lit.clone())),
+        }
+    }
+
+    fn from_tokens(
+        tokens: &mut iter::Peekable<impl Iterator<Item = TokenTree>>,
+    ) -> Option<MetaItemKind> {
+        match tokens.peek() {
+            Some(TokenTree::Delimited(_, Delimiter::Parenthesis, inner_tokens)) => {
+                let inner_tokens = inner_tokens.clone();
+                tokens.next();
+                MetaItemKind::list_from_tokens(inner_tokens)
+            }
+            Some(TokenTree::Delimited(..)) => None,
+            Some(TokenTree::Token(Token { kind: token::Eq, .. }, _)) => {
+                tokens.next();
+                MetaItemKind::name_value_from_tokens(tokens)
+            }
+            _ => Some(MetaItemKind::Word),
+        }
+    }
+}
+
+impl NestedMetaItem {
+    pub fn span(&self) -> Span {
+        match *self {
+            NestedMetaItem::MetaItem(ref item) => item.span,
+            NestedMetaItem::Literal(ref lit) => lit.span,
+        }
+    }
+
+    fn token_trees(&self) -> Vec<TokenTree> {
+        match *self {
+            NestedMetaItem::MetaItem(ref item) => item.token_trees(),
+            NestedMetaItem::Literal(ref lit) => {
+                vec![TokenTree::Token(lit.to_token(), Spacing::Alone)]
+            }
+        }
+    }
+
+    fn from_tokens<I>(tokens: &mut iter::Peekable<I>) -> Option<NestedMetaItem>
+    where
+        I: Iterator<Item = TokenTree>,
+    {
+        match tokens.peek() {
+            Some(TokenTree::Token(token, _))
+                if let Ok(lit) = Lit::from_token(token) =>
+            {
+                tokens.next();
+                return Some(NestedMetaItem::Literal(lit));
+            }
+            Some(TokenTree::Delimited(_, Delimiter::Invisible, inner_tokens)) => {
+                let inner_tokens = inner_tokens.clone();
+                tokens.next();
+                return NestedMetaItem::from_tokens(&mut inner_tokens.into_trees().peekable());
+            }
+            _ => {}
+        }
+        MetaItem::from_tokens(tokens).map(NestedMetaItem::MetaItem)
+    }
+}
diff --git a/compiler/rustc_ast/src/entry.rs b/compiler/rustc_ast/src/entry.rs
new file mode 100644
index 000000000..337014619
--- /dev/null
+++ b/compiler/rustc_ast/src/entry.rs
@@ -0,0 +1,8 @@
+#[derive(Debug)]
+pub enum EntryPointType {
+    None,
+    MainNamed,
+    RustcMainAttr,
+    Start,
+    OtherMain, // Not an entry point, but some other function named main
+}
diff --git a/compiler/rustc_ast/src/expand/allocator.rs b/compiler/rustc_ast/src/expand/allocator.rs
new file mode 100644
index 000000000..1976e4ad3
--- /dev/null
+++ b/compiler/rustc_ast/src/expand/allocator.rs
@@ -0,0 +1,53 @@
+use rustc_span::symbol::{sym, Symbol};
+
+#[derive(Clone, Debug, Copy, HashStable_Generic)]
+pub enum AllocatorKind {
+    Global,
+    Default,
+}
+
+impl AllocatorKind {
+    pub fn fn_name(&self, base: Symbol) -> String {
+        match *self {
+            AllocatorKind::Global => format!("__rg_{}", base),
+            AllocatorKind::Default => format!("__rdl_{}", base),
+        }
+    }
+}
+
+pub enum AllocatorTy {
+    Layout,
+    Ptr,
+    ResultPtr,
+    Unit,
+    Usize,
+}
+
+pub struct AllocatorMethod {
+    pub name: Symbol,
+    pub inputs: &'static [AllocatorTy],
+    pub output: AllocatorTy,
+}
+
+pub static ALLOCATOR_METHODS: &[AllocatorMethod] = &[
+    AllocatorMethod {
+        name: sym::alloc,
+        inputs: &[AllocatorTy::Layout],
+        output: AllocatorTy::ResultPtr,
+    },
+    AllocatorMethod {
+        name: sym::dealloc,
+        inputs: &[AllocatorTy::Ptr, AllocatorTy::Layout],
+        output: AllocatorTy::Unit,
+    },
+    AllocatorMethod {
+        name: sym::realloc,
+        inputs: &[AllocatorTy::Ptr, AllocatorTy::Layout, AllocatorTy::Usize],
+        output: AllocatorTy::ResultPtr,
+    },
+    AllocatorMethod {
+        name: sym::alloc_zeroed,
+        inputs: &[AllocatorTy::Layout],
+        output: AllocatorTy::ResultPtr,
+    },
+];
diff --git a/compiler/rustc_ast/src/expand/mod.rs b/compiler/rustc_ast/src/expand/mod.rs
new file mode 100644
index 000000000..2ee1bfe0a
--- /dev/null
+++ b/compiler/rustc_ast/src/expand/mod.rs
@@ -0,0 +1,3 @@
+//! Definitions shared by macros / syntax extensions and e.g. `rustc_middle`.
+
+pub mod allocator;
diff --git a/compiler/rustc_ast/src/lib.rs b/compiler/rustc_ast/src/lib.rs
new file mode 100644
index 000000000..4b94ec0d6
--- /dev/null
+++ b/compiler/rustc_ast/src/lib.rs
@@ -0,0 +1,63 @@
+//! The Rust Abstract Syntax Tree (AST).
+//!
+//! # Note
+//!
+//! This API is completely unstable and subject to change.
+
+#![doc(
+    html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
+    test(attr(deny(warnings)))
+)]
+#![feature(associated_type_bounds)]
+#![feature(box_patterns)]
+#![feature(const_default_impls)]
+#![feature(const_trait_impl)]
+#![feature(if_let_guard)]
+#![feature(label_break_value)]
+#![feature(let_chains)]
+#![feature(min_specialization)]
+#![feature(negative_impls)]
+#![feature(slice_internals)]
+#![feature(stmt_expr_attributes)]
+#![recursion_limit = "256"]
+
+#[macro_use]
+extern crate rustc_macros;
+
+pub mod util {
+    pub mod classify;
+    pub mod comments;
+    pub mod literal;
+    pub mod parser;
+    pub mod unicode;
+}
+
+pub mod ast;
+pub mod ast_traits;
+pub mod attr;
+pub mod entry;
+pub mod expand;
+pub mod mut_visit;
+pub mod node_id;
+pub mod ptr;
+pub mod token;
+pub mod tokenstream;
+pub mod visit;
+
+pub use self::ast::*;
+pub use self::ast_traits::{AstDeref, AstNodeWrapper, HasAttrs, HasNodeId, HasSpan, HasTokens};
+
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+
+/// Requirements for a `StableHashingContext` to be used in this crate.
+/// This is a hack to allow using the `HashStable_Generic` derive macro
+/// instead of implementing everything in `rustc_middle`.
+pub trait HashStableContext: rustc_span::HashStableContext {
+    fn hash_attr(&mut self, _: &ast::Attribute, hasher: &mut StableHasher);
+}
+
+impl<AstCtx: crate::HashStableContext> HashStable<AstCtx> for ast::Attribute {
+    fn hash_stable(&self, hcx: &mut AstCtx, hasher: &mut StableHasher) {
+        hcx.hash_attr(self, hasher)
+    }
+}
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
new file mode 100644
index 000000000..01bd498b3
--- /dev/null
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -0,0 +1,1601 @@
+//! A `MutVisitor` represents an AST modification; it accepts an AST piece and
+//! mutates it in place. So, for instance, macro expansion is a `MutVisitor`
+//! that walks over an AST and modifies it.
+//!
+//! Note: using a `MutVisitor` (other than the `MacroExpander` `MutVisitor`) on
+//! an AST before macro expansion is probably a bad idea. For instance,
+//! a `MutVisitor` renaming item names in a module will miss all of those
+//! that are created by the expansion of a macro.
+
+use crate::ast::*;
+use crate::ptr::P;
+use crate::token::{self, Token};
+use crate::tokenstream::*;
+
+use rustc_data_structures::map_in_place::MapInPlace;
+use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::thin_vec::ThinVec;
+use rustc_span::source_map::Spanned;
+use rustc_span::symbol::Ident;
+use rustc_span::Span;
+
+use smallvec::{smallvec, Array, SmallVec};
+use std::ops::DerefMut;
+use std::{panic, ptr};
+
+pub trait ExpectOne<A: Array> {
+    fn expect_one(self, err: &'static str) -> A::Item;
+}
+
+impl<A: Array> ExpectOne<A> for SmallVec<A> {
+    fn expect_one(self, err: &'static str) -> A::Item {
+        assert!(self.len() == 1, "{}", err);
+        self.into_iter().next().unwrap()
+    }
+}
+
+pub trait MutVisitor: Sized {
+    /// Mutable token visiting only exists for the `macro_rules` token marker and should not be
+    /// used otherwise. Token visitor would be entirely separate from the regular visitor if
+    /// the marker didn't have to visit AST fragments in nonterminal tokens.
+    const VISIT_TOKENS: bool = false;
+
+    // Methods in this trait have one of three forms:
+    //
+    //   fn visit_t(&mut self, t: &mut T);                      // common
+    //   fn flat_map_t(&mut self, t: T) -> SmallVec<[T; 1]>;    // rare
+    //   fn filter_map_t(&mut self, t: T) -> Option<T>;         // rarest
+    //
+    // Any additions to this trait should happen in form of a call to a public
+    // `noop_*` function that only calls out to the visitor again, not other
+    // `noop_*` functions. This is a necessary API workaround to the problem of
+    // not being able to call out to the super default method in an overridden
+    // default method.
+    //
+    // When writing these methods, it is better to use destructuring like this:
+    //
+    //   fn visit_abc(&mut self, ABC { a, b, c: _ }: &mut ABC) {
+    //       visit_a(a);
+    //       visit_b(b);
+    //   }
+    //
+    // than to use field access like this:
+    //
+    //   fn visit_abc(&mut self, abc: &mut ABC) {
+    //       visit_a(&mut abc.a);
+    //       visit_b(&mut abc.b);
+    //       // ignore abc.c
+    //   }
+    //
+    // As well as being more concise, the former is explicit about which fields
+    // are skipped. Furthermore, if a new field is added, the destructuring
+    // version will cause a compile error, which is good. In comparison, the
+    // field access version will continue working and it would be easy to
+    // forget to add handling for it.
+
+    fn visit_crate(&mut self, c: &mut Crate) {
+        noop_visit_crate(c, self)
+    }
+
+    fn visit_meta_list_item(&mut self, list_item: &mut NestedMetaItem) {
+        noop_visit_meta_list_item(list_item, self);
+    }
+
+    fn visit_meta_item(&mut self, meta_item: &mut MetaItem) {
+        noop_visit_meta_item(meta_item, self);
+    }
+
+    fn visit_use_tree(&mut self, use_tree: &mut UseTree) {
+        noop_visit_use_tree(use_tree, self);
+    }
+
+    fn flat_map_foreign_item(&mut self, ni: P<ForeignItem>) -> SmallVec<[P<ForeignItem>; 1]> {
+        noop_flat_map_foreign_item(ni, self)
+    }
+
+    fn flat_map_item(&mut self, i: P<Item>) -> SmallVec<[P<Item>; 1]> {
+        noop_flat_map_item(i, self)
+    }
+
+    fn visit_fn_header(&mut self, header: &mut FnHeader) {
+        noop_visit_fn_header(header, self);
+    }
+
+    fn flat_map_field_def(&mut self, fd: FieldDef) -> SmallVec<[FieldDef; 1]> {
+        noop_flat_map_field_def(fd, self)
+    }
+
+    fn visit_item_kind(&mut self, i: &mut ItemKind) {
+        noop_visit_item_kind(i, self);
+    }
+
+    fn flat_map_trait_item(&mut self, i: P<AssocItem>) -> SmallVec<[P<AssocItem>; 1]> {
+        noop_flat_map_assoc_item(i, self)
+    }
+
+    fn flat_map_impl_item(&mut self, i: P<AssocItem>) -> SmallVec<[P<AssocItem>; 1]> {
+        noop_flat_map_assoc_item(i, self)
+    }
+
+    fn visit_fn_decl(&mut self, d: &mut P<FnDecl>) {
+        noop_visit_fn_decl(d, self);
+    }
+
+    fn visit_asyncness(&mut self, a: &mut Async) {
+        noop_visit_asyncness(a, self);
+    }
+
+    fn visit_closure_binder(&mut self, b: &mut ClosureBinder) {
+        noop_visit_closure_binder(b, self);
+    }
+
+    fn visit_block(&mut self, b: &mut P<Block>) {
+        noop_visit_block(b, self);
+    }
+
+    fn flat_map_stmt(&mut self, s: Stmt) -> SmallVec<[Stmt; 1]> {
+        noop_flat_map_stmt(s, self)
+    }
+
+    fn flat_map_arm(&mut self, arm: Arm) -> SmallVec<[Arm; 1]> {
+        noop_flat_map_arm(arm, self)
+    }
+
+    fn visit_pat(&mut self, p: &mut P<Pat>) {
+        noop_visit_pat(p, self);
+    }
+
+    fn visit_anon_const(&mut self, c: &mut AnonConst) {
+        noop_visit_anon_const(c, self);
+    }
+
+    fn visit_expr(&mut self, e: &mut P<Expr>) {
+        noop_visit_expr(e, self);
+    }
+
+    fn filter_map_expr(&mut self, e: P<Expr>) -> Option<P<Expr>> {
+        noop_filter_map_expr(e, self)
+    }
+
+    fn visit_generic_arg(&mut self, arg: &mut GenericArg) {
+        noop_visit_generic_arg(arg, self);
+    }
+
+    fn visit_ty(&mut self, t: &mut P<Ty>) {
+        noop_visit_ty(t, self);
+    }
+
+    fn visit_lifetime(&mut self, l: &mut Lifetime) {
+        noop_visit_lifetime(l, self);
+    }
+
+    fn visit_constraint(&mut self, t: &mut AssocConstraint) {
+        noop_visit_constraint(t, self);
+    }
+
+    fn visit_foreign_mod(&mut self, nm: &mut ForeignMod) {
+        noop_visit_foreign_mod(nm, self);
+    }
+
+    fn flat_map_variant(&mut self, v: Variant) -> SmallVec<[Variant; 1]> {
+        noop_flat_map_variant(v, self)
+    }
+
+    fn visit_ident(&mut self, i: &mut Ident) {
+        noop_visit_ident(i, self);
+    }
+
+    fn visit_path(&mut self, p: &mut Path) {
+        noop_visit_path(p, self);
+    }
+
+    fn visit_qself(&mut self, qs: &mut Option<QSelf>) {
+        noop_visit_qself(qs, self);
+    }
+
+    fn visit_generic_args(&mut self, p: &mut GenericArgs) {
+        noop_visit_generic_args(p, self);
+    }
+
+    fn visit_angle_bracketed_parameter_data(&mut self, p: &mut AngleBracketedArgs) {
+        noop_visit_angle_bracketed_parameter_data(p, self);
+    }
+
+    fn visit_parenthesized_parameter_data(&mut self, p: &mut ParenthesizedArgs) {
+        noop_visit_parenthesized_parameter_data(p, self);
+    }
+
+    fn visit_local(&mut self, l: &mut P<Local>) {
+        noop_visit_local(l, self);
+    }
+
+    fn visit_mac_call(&mut self, mac: &mut MacCall) {
+        noop_visit_mac(mac, self);
+    }
+
+    fn visit_macro_def(&mut self, def: &mut MacroDef) {
+        noop_visit_macro_def(def, self);
+    }
+
+    fn visit_label(&mut self, label: &mut Label) {
+        noop_visit_label(label, self);
+    }
+
+    fn visit_attribute(&mut self, at: &mut Attribute) {
+        noop_visit_attribute(at, self);
+    }
+
+    fn flat_map_param(&mut self, param: Param) -> SmallVec<[Param; 1]> {
+        noop_flat_map_param(param, self)
+    }
+
+    fn visit_generics(&mut self, generics: &mut Generics) {
+        noop_visit_generics(generics, self);
+    }
+
+    fn visit_trait_ref(&mut self, tr: &mut TraitRef) {
+        noop_visit_trait_ref(tr, self);
+    }
+
+    fn visit_poly_trait_ref(&mut self, p: &mut PolyTraitRef) {
+        noop_visit_poly_trait_ref(p, self);
+    }
+
+    fn visit_variant_data(&mut self, vdata: &mut VariantData) {
+        noop_visit_variant_data(vdata, self);
+    }
+
+    fn flat_map_generic_param(&mut self, param: GenericParam) -> SmallVec<[GenericParam; 1]> {
+        noop_flat_map_generic_param(param, self)
+    }
+
+    fn visit_param_bound(&mut self, tpb: &mut GenericBound) {
+        noop_visit_param_bound(tpb, self);
+    }
+
+    fn visit_mt(&mut self, mt: &mut MutTy) {
+        noop_visit_mt(mt, self);
+    }
+
+    fn flat_map_expr_field(&mut self, f: ExprField) -> SmallVec<[ExprField; 1]> {
+        noop_flat_map_expr_field(f, self)
+    }
+
+    fn visit_where_clause(&mut self, where_clause: &mut WhereClause) {
+        noop_visit_where_clause(where_clause, self);
+    }
+
+    fn visit_where_predicate(&mut self, where_predicate: &mut WherePredicate) {
+        noop_visit_where_predicate(where_predicate, self);
+    }
+
+    fn visit_vis(&mut self, vis: &mut Visibility) {
+        noop_visit_vis(vis, self);
+    }
+
+    fn visit_id(&mut self, _id: &mut NodeId) {
+        // Do nothing.
+    }
+
+    fn visit_span(&mut self, _sp: &mut Span) {
+        // Do nothing.
+    }
+
+    fn flat_map_pat_field(&mut self, fp: PatField) -> SmallVec<[PatField; 1]> {
+        noop_flat_map_pat_field(fp, self)
+    }
+
+    fn visit_inline_asm(&mut self, asm: &mut InlineAsm) {
+        noop_visit_inline_asm(asm, self)
+    }
+
+    fn visit_inline_asm_sym(&mut self, sym: &mut InlineAsmSym) {
+        noop_visit_inline_asm_sym(sym, self)
+    }
+}
+
+/// Use a map-style function (`FnOnce(T) -> T`) to overwrite a `&mut T`. Useful
+/// when using a `flat_map_*` or `filter_map_*` method within a `visit_`
+/// method.
+//
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_clobber<T: DummyAstNode>(t: &mut T, f: impl FnOnce(T) -> T) {
+    unsafe {
+        // Safe because `t` is used in a read-only fashion by `read()` before
+        // being overwritten by `write()`.
+        let old_t = ptr::read(t);
+        let new_t =
+            panic::catch_unwind(panic::AssertUnwindSafe(|| f(old_t))).unwrap_or_else(|err| {
+                // Set `t` to some valid but possible meaningless value,
+                // and pass the fatal error further.
+                ptr::write(t, T::dummy());
+                panic::resume_unwind(err);
+            });
+        ptr::write(t, new_t);
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+#[inline]
+pub fn visit_vec<T, F>(elems: &mut Vec<T>, mut visit_elem: F)
+where
+    F: FnMut(&mut T),
+{
+    for elem in elems {
+        visit_elem(elem);
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+#[inline]
+pub fn visit_opt<T, F>(opt: &mut Option<T>, mut visit_elem: F)
+where
+    F: FnMut(&mut T),
+{
+    if let Some(elem) = opt {
+        visit_elem(elem);
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_attrs<T: MutVisitor>(attrs: &mut Vec<Attribute>, vis: &mut T) {
+    visit_vec(attrs, |attr| vis.visit_attribute(attr));
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_thin_attrs<T: MutVisitor>(attrs: &mut AttrVec, vis: &mut T) {
+    for attr in attrs.iter_mut() {
+        vis.visit_attribute(attr);
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_exprs<T: MutVisitor>(exprs: &mut Vec<P<Expr>>, vis: &mut T) {
+    exprs.flat_map_in_place(|expr| vis.filter_map_expr(expr))
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_bounds<T: MutVisitor>(bounds: &mut GenericBounds, vis: &mut T) {
+    visit_vec(bounds, |bound| vis.visit_param_bound(bound));
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_fn_sig<T: MutVisitor>(FnSig { header, decl, span }: &mut FnSig, vis: &mut T) {
+    vis.visit_fn_header(header);
+    vis.visit_fn_decl(decl);
+    vis.visit_span(span);
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_mac_args<T: MutVisitor>(args: &mut MacArgs, vis: &mut T) {
+    match args {
+        MacArgs::Empty => {}
+        MacArgs::Delimited(dspan, _delim, tokens) => {
+            visit_delim_span(dspan, vis);
+            visit_tts(tokens, vis);
+        }
+        MacArgs::Eq(eq_span, MacArgsEq::Ast(expr)) => {
+            vis.visit_span(eq_span);
+            vis.visit_expr(expr);
+        }
+        MacArgs::Eq(_, MacArgsEq::Hir(lit)) => {
+            unreachable!("in literal form when visiting mac args eq: {:?}", lit)
+        }
+    }
+}
+
+pub fn visit_delim_span<T: MutVisitor>(dspan: &mut DelimSpan, vis: &mut T) {
+    vis.visit_span(&mut dspan.open);
+    vis.visit_span(&mut dspan.close);
+}
+
+pub fn noop_flat_map_pat_field<T: MutVisitor>(
+    mut fp: PatField,
+    vis: &mut T,
+) -> SmallVec<[PatField; 1]> {
+    let PatField { attrs, id, ident, is_placeholder: _, is_shorthand: _, pat, span } = &mut fp;
+    vis.visit_id(id);
+    vis.visit_ident(ident);
+    vis.visit_pat(pat);
+    vis.visit_span(span);
+    visit_thin_attrs(attrs, vis);
+    smallvec![fp]
+}
+
+pub fn noop_visit_use_tree<T: MutVisitor>(use_tree: &mut UseTree, vis: &mut T) {
+    let UseTree { prefix, kind, span } = use_tree;
+    vis.visit_path(prefix);
+    match kind {
+        UseTreeKind::Simple(rename, id1, id2) => {
+            visit_opt(rename, |rename| vis.visit_ident(rename));
+            vis.visit_id(id1);
+            vis.visit_id(id2);
+        }
+        UseTreeKind::Nested(items) => {
+            for (tree, id) in items {
+                vis.visit_use_tree(tree);
+                vis.visit_id(id);
+            }
+        }
+        UseTreeKind::Glob => {}
+    }
+    vis.visit_span(span);
+}
+
+pub fn noop_flat_map_arm<T: MutVisitor>(mut arm: Arm, vis: &mut T) -> SmallVec<[Arm; 1]> {
+    let Arm { attrs, pat, guard, body, span, id, is_placeholder: _ } = &mut arm;
+    visit_thin_attrs(attrs, vis);
+    vis.visit_id(id);
+    vis.visit_pat(pat);
+    visit_opt(guard, |guard| vis.visit_expr(guard));
+    vis.visit_expr(body);
+    vis.visit_span(span);
+    smallvec![arm]
+}
+
+pub fn noop_visit_constraint<T: MutVisitor>(
+    AssocConstraint { id, ident, gen_args, kind, span }: &mut AssocConstraint,
+    vis: &mut T,
+) {
+    vis.visit_id(id);
+    vis.visit_ident(ident);
+    if let Some(ref mut gen_args) = gen_args {
+        vis.visit_generic_args(gen_args);
+    }
+    match kind {
+        AssocConstraintKind::Equality { ref mut term } => match term {
+            Term::Ty(ty) => vis.visit_ty(ty),
+            Term::Const(c) => vis.visit_anon_const(c),
+        },
+        AssocConstraintKind::Bound { ref mut bounds } => visit_bounds(bounds, vis),
+    }
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_ty<T: MutVisitor>(ty: &mut P<Ty>, vis: &mut T) {
+    let Ty { id, kind, span, tokens } = ty.deref_mut();
+    vis.visit_id(id);
+    match kind {
+        TyKind::Infer | TyKind::ImplicitSelf | TyKind::Err | TyKind::Never | TyKind::CVarArgs => {}
+        TyKind::Slice(ty) => vis.visit_ty(ty),
+        TyKind::Ptr(mt) => vis.visit_mt(mt),
+        TyKind::Rptr(lt, mt) => {
+            visit_opt(lt, |lt| noop_visit_lifetime(lt, vis));
+            vis.visit_mt(mt);
+        }
+        TyKind::BareFn(bft) => {
+            let BareFnTy { unsafety, ext: _, generic_params, decl, decl_span } = bft.deref_mut();
+            visit_unsafety(unsafety, vis);
+            generic_params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
+            vis.visit_fn_decl(decl);
+            vis.visit_span(decl_span);
+        }
+        TyKind::Tup(tys) => visit_vec(tys, |ty| vis.visit_ty(ty)),
+        TyKind::Paren(ty) => vis.visit_ty(ty),
+        TyKind::Path(qself, path) => {
+            vis.visit_qself(qself);
+            vis.visit_path(path);
+        }
+        TyKind::Array(ty, length) => {
+            vis.visit_ty(ty);
+            vis.visit_anon_const(length);
+        }
+        TyKind::Typeof(expr) => vis.visit_anon_const(expr),
+        TyKind::TraitObject(bounds, _syntax) => {
+            visit_vec(bounds, |bound| vis.visit_param_bound(bound))
+        }
+        TyKind::ImplTrait(id, bounds) => {
+            vis.visit_id(id);
+            visit_vec(bounds, |bound| vis.visit_param_bound(bound));
+        }
+        TyKind::MacCall(mac) => vis.visit_mac_call(mac),
+    }
+    vis.visit_span(span);
+    visit_lazy_tts(tokens, vis);
+}
+
+pub fn noop_visit_foreign_mod<T: MutVisitor>(foreign_mod: &mut ForeignMod, vis: &mut T) {
+    let ForeignMod { unsafety, abi: _, items } = foreign_mod;
+    visit_unsafety(unsafety, vis);
+    items.flat_map_in_place(|item| vis.flat_map_foreign_item(item));
+}
+
+pub fn noop_flat_map_variant<T: MutVisitor>(
+    mut variant: Variant,
+    visitor: &mut T,
+) -> SmallVec<[Variant; 1]> {
+    let Variant { ident, vis, attrs, id, data, disr_expr, span, is_placeholder: _ } = &mut variant;
+    visitor.visit_ident(ident);
+    visitor.visit_vis(vis);
+    visit_thin_attrs(attrs, visitor);
+    visitor.visit_id(id);
+    visitor.visit_variant_data(data);
+    visit_opt(disr_expr, |disr_expr| visitor.visit_anon_const(disr_expr));
+    visitor.visit_span(span);
+    smallvec![variant]
+}
+
+pub fn noop_visit_ident<T: MutVisitor>(Ident { name: _, span }: &mut Ident, vis: &mut T) {
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_path<T: MutVisitor>(Path { segments, span, tokens }: &mut Path, vis: &mut T) {
+    vis.visit_span(span);
+    for PathSegment { ident, id, args } in segments {
+        vis.visit_ident(ident);
+        vis.visit_id(id);
+        visit_opt(args, |args| vis.visit_generic_args(args));
+    }
+    visit_lazy_tts(tokens, vis);
+}
+
+pub fn noop_visit_qself<T: MutVisitor>(qself: &mut Option<QSelf>, vis: &mut T) {
+    visit_opt(qself, |QSelf { ty, path_span, position: _ }| {
+        vis.visit_ty(ty);
+        vis.visit_span(path_span);
+    })
+}
+
+pub fn noop_visit_generic_args<T: MutVisitor>(generic_args: &mut GenericArgs, vis: &mut T) {
+    match generic_args {
+        GenericArgs::AngleBracketed(data) => vis.visit_angle_bracketed_parameter_data(data),
+        GenericArgs::Parenthesized(data) => vis.visit_parenthesized_parameter_data(data),
+    }
+}
+
+pub fn noop_visit_generic_arg<T: MutVisitor>(arg: &mut GenericArg, vis: &mut T) {
+    match arg {
+        GenericArg::Lifetime(lt) => vis.visit_lifetime(lt),
+        GenericArg::Type(ty) => vis.visit_ty(ty),
+        GenericArg::Const(ct) => vis.visit_anon_const(ct),
+    }
+}
+
+pub fn noop_visit_angle_bracketed_parameter_data<T: MutVisitor>(
+    data: &mut AngleBracketedArgs,
+    vis: &mut T,
+) {
+    let AngleBracketedArgs { args, span } = data;
+    visit_vec(args, |arg| match arg {
+        AngleBracketedArg::Arg(arg) => vis.visit_generic_arg(arg),
+        AngleBracketedArg::Constraint(constraint) => vis.visit_constraint(constraint),
+    });
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_parenthesized_parameter_data<T: MutVisitor>(
+    args: &mut ParenthesizedArgs,
+    vis: &mut T,
+) {
+    let ParenthesizedArgs { inputs, output, span, .. } = args;
+    visit_vec(inputs, |input| vis.visit_ty(input));
+    noop_visit_fn_ret_ty(output, vis);
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_local<T: MutVisitor>(local: &mut P<Local>, vis: &mut T) {
+    let Local { id, pat, ty, kind, span, attrs, tokens } = local.deref_mut();
+    vis.visit_id(id);
+    vis.visit_pat(pat);
+    visit_opt(ty, |ty| vis.visit_ty(ty));
+    match kind {
+        LocalKind::Decl => {}
+        LocalKind::Init(init) => {
+            vis.visit_expr(init);
+        }
+        LocalKind::InitElse(init, els) => {
+            vis.visit_expr(init);
+            vis.visit_block(els);
+        }
+    }
+    vis.visit_span(span);
+    visit_thin_attrs(attrs, vis);
+    visit_lazy_tts(tokens, vis);
+}
+
+pub fn noop_visit_attribute<T: MutVisitor>(attr: &mut Attribute, vis: &mut T) {
+    let Attribute { kind, id: _, style: _, span } = attr;
+    match kind {
+        AttrKind::Normal(AttrItem { path, args, tokens }, attr_tokens) => {
+            vis.visit_path(path);
+            visit_mac_args(args, vis);
+            visit_lazy_tts(tokens, vis);
+            visit_lazy_tts(attr_tokens, vis);
+        }
+        AttrKind::DocComment(..) => {}
+    }
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_mac<T: MutVisitor>(mac: &mut MacCall, vis: &mut T) {
+    let MacCall { path, args, prior_type_ascription: _ } = mac;
+    vis.visit_path(path);
+    visit_mac_args(args, vis);
+}
+
+pub fn noop_visit_macro_def<T: MutVisitor>(macro_def: &mut MacroDef, vis: &mut T) {
+    let MacroDef { body, macro_rules: _ } = macro_def;
+    visit_mac_args(body, vis);
+}
+
+pub fn noop_visit_meta_list_item<T: MutVisitor>(li: &mut NestedMetaItem, vis: &mut T) {
+    match li {
+        NestedMetaItem::MetaItem(mi) => vis.visit_meta_item(mi),
+        NestedMetaItem::Literal(_lit) => {}
+    }
+}
+
+pub fn noop_visit_meta_item<T: MutVisitor>(mi: &mut MetaItem, vis: &mut T) {
+    let MetaItem { path: _, kind, span } = mi;
+    match kind {
+        MetaItemKind::Word => {}
+        MetaItemKind::List(mis) => visit_vec(mis, |mi| vis.visit_meta_list_item(mi)),
+        MetaItemKind::NameValue(_s) => {}
+    }
+    vis.visit_span(span);
+}
+
+pub fn noop_flat_map_param<T: MutVisitor>(mut param: Param, vis: &mut T) -> SmallVec<[Param; 1]> {
+    let Param { attrs, id, pat, span, ty, is_placeholder: _ } = &mut param;
+    vis.visit_id(id);
+    visit_thin_attrs(attrs, vis);
+    vis.visit_pat(pat);
+    vis.visit_span(span);
+    vis.visit_ty(ty);
+    smallvec![param]
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_attr_annotated_tt<T: MutVisitor>(tt: &mut AttrAnnotatedTokenTree, vis: &mut T) {
+    match tt {
+        AttrAnnotatedTokenTree::Token(token) => {
+            visit_token(token, vis);
+        }
+        AttrAnnotatedTokenTree::Delimited(DelimSpan { open, close }, _delim, tts) => {
+            vis.visit_span(open);
+            vis.visit_span(close);
+            visit_attr_annotated_tts(tts, vis);
+        }
+        AttrAnnotatedTokenTree::Attributes(data) => {
+            for attr in &mut *data.attrs {
+                match &mut attr.kind {
+                    AttrKind::Normal(_, attr_tokens) => {
+                        visit_lazy_tts(attr_tokens, vis);
+                    }
+                    AttrKind::DocComment(..) => {
+                        vis.visit_span(&mut attr.span);
+                    }
+                }
+            }
+            visit_lazy_tts_opt_mut(Some(&mut data.tokens), vis);
+        }
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_tt<T: MutVisitor>(tt: &mut TokenTree, vis: &mut T) {
+    match tt {
+        TokenTree::Token(token, _) => {
+            visit_token(token, vis);
+        }
+        TokenTree::Delimited(DelimSpan { open, close }, _delim, tts) => {
+            vis.visit_span(open);
+            vis.visit_span(close);
+            visit_tts(tts, vis);
+        }
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_tts<T: MutVisitor>(TokenStream(tts): &mut TokenStream, vis: &mut T) {
+    if T::VISIT_TOKENS && !tts.is_empty() {
+        let tts = Lrc::make_mut(tts);
+        visit_vec(tts, |tree| visit_tt(tree, vis));
+    }
+}
+
+pub fn visit_attr_annotated_tts<T: MutVisitor>(
+    AttrAnnotatedTokenStream(tts): &mut AttrAnnotatedTokenStream,
+    vis: &mut T,
+) {
+    if T::VISIT_TOKENS && !tts.is_empty() {
+        let tts = Lrc::make_mut(tts);
+        visit_vec(tts, |(tree, _is_joint)| visit_attr_annotated_tt(tree, vis));
+    }
+}
+
+pub fn visit_lazy_tts_opt_mut<T: MutVisitor>(lazy_tts: Option<&mut LazyTokenStream>, vis: &mut T) {
+    if T::VISIT_TOKENS {
+        if let Some(lazy_tts) = lazy_tts {
+            let mut tts = lazy_tts.create_token_stream();
+            visit_attr_annotated_tts(&mut tts, vis);
+            *lazy_tts = LazyTokenStream::new(tts);
+        }
+    }
+}
+
+pub fn visit_lazy_tts<T: MutVisitor>(lazy_tts: &mut Option<LazyTokenStream>, vis: &mut T) {
+    visit_lazy_tts_opt_mut(lazy_tts.as_mut(), vis);
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+// Applies ident visitor if it's an ident; applies other visits to interpolated nodes.
+// In practice the ident part is not actually used by specific visitors right now,
+// but there's a test below checking that it works.
+pub fn visit_token<T: MutVisitor>(t: &mut Token, vis: &mut T) {
+    let Token { kind, span } = t;
+    match kind {
+        token::Ident(name, _) | token::Lifetime(name) => {
+            let mut ident = Ident::new(*name, *span);
+            vis.visit_ident(&mut ident);
+            *name = ident.name;
+            *span = ident.span;
+            return; // Avoid visiting the span for the second time.
+        }
+        token::Interpolated(nt) => {
+            let mut nt = Lrc::make_mut(nt);
+            visit_nonterminal(&mut nt, vis);
+        }
+        _ => {}
+    }
+    vis.visit_span(span);
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+/// Applies the visitor to elements of interpolated nodes.
+//
+// N.B., this can occur only when applying a visitor to partially expanded
+// code, where parsed pieces have gotten implanted ito *other* macro
+// invocations. This is relevant for macro hygiene, but possibly not elsewhere.
+//
+// One problem here occurs because the types for flat_map_item, flat_map_stmt,
+// etc., allow the visitor to return *multiple* items; this is a problem for the
+// nodes here, because they insist on having exactly one piece. One solution
+// would be to mangle the MutVisitor trait to include one-to-many and
+// one-to-one versions of these entry points, but that would probably confuse a
+// lot of people and help very few. Instead, I'm just going to put in dynamic
+// checks. I think the performance impact of this will be pretty much
+// nonexistent. The danger is that someone will apply a `MutVisitor` to a
+// partially expanded node, and will be confused by the fact that their
+// `flat_map_item` or `flat_map_stmt` isn't getting called on `NtItem` or `NtStmt`
+// nodes. Hopefully they'll wind up reading this comment, and doing something
+// appropriate.
+//
+// BTW, design choice: I considered just changing the type of, e.g., `NtItem` to
+// contain multiple items, but decided against it when I looked at
+// `parse_item_or_view_item` and tried to figure out what I would do with
+// multiple items there....
+pub fn visit_nonterminal<T: MutVisitor>(nt: &mut token::Nonterminal, vis: &mut T) {
+    match nt {
+        token::NtItem(item) => visit_clobber(item, |item| {
+            // This is probably okay, because the only visitors likely to
+            // peek inside interpolated nodes will be renamings/markings,
+            // which map single items to single items.
+            vis.flat_map_item(item).expect_one("expected visitor to produce exactly one item")
+        }),
+        token::NtBlock(block) => vis.visit_block(block),
+        token::NtStmt(stmt) => visit_clobber(stmt, |stmt| {
+            // See reasoning above.
+            stmt.map(|stmt| {
+                vis.flat_map_stmt(stmt).expect_one("expected visitor to produce exactly one item")
+            })
+        }),
+        token::NtPat(pat) => vis.visit_pat(pat),
+        token::NtExpr(expr) => vis.visit_expr(expr),
+        token::NtTy(ty) => vis.visit_ty(ty),
+        token::NtIdent(ident, _is_raw) => vis.visit_ident(ident),
+        token::NtLifetime(ident) => vis.visit_ident(ident),
+        token::NtLiteral(expr) => vis.visit_expr(expr),
+        token::NtMeta(item) => {
+            let AttrItem { path, args, tokens } = item.deref_mut();
+            vis.visit_path(path);
+            visit_mac_args(args, vis);
+            visit_lazy_tts(tokens, vis);
+        }
+        token::NtPath(path) => vis.visit_path(path),
+        token::NtVis(visib) => vis.visit_vis(visib),
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_defaultness<T: MutVisitor>(defaultness: &mut Defaultness, vis: &mut T) {
+    match defaultness {
+        Defaultness::Default(span) => vis.visit_span(span),
+        Defaultness::Final => {}
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_unsafety<T: MutVisitor>(unsafety: &mut Unsafe, vis: &mut T) {
+    match unsafety {
+        Unsafe::Yes(span) => vis.visit_span(span),
+        Unsafe::No => {}
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_polarity<T: MutVisitor>(polarity: &mut ImplPolarity, vis: &mut T) {
+    match polarity {
+        ImplPolarity::Positive => {}
+        ImplPolarity::Negative(span) => vis.visit_span(span),
+    }
+}
+
+// No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_constness<T: MutVisitor>(constness: &mut Const, vis: &mut T) {
+    match constness {
+        Const::Yes(span) => vis.visit_span(span),
+        Const::No => {}
+    }
+}
+
+pub fn noop_visit_closure_binder<T: MutVisitor>(binder: &mut ClosureBinder, vis: &mut T) {
+    match binder {
+        ClosureBinder::NotPresent => {}
+        ClosureBinder::For { span: _, generic_params } => {
+            let mut vec = std::mem::take(generic_params).into_vec();
+            vec.flat_map_in_place(|param| vis.flat_map_generic_param(param));
+            *generic_params = P::from_vec(vec);
+        }
+    }
+}
+
+pub fn noop_visit_asyncness<T: MutVisitor>(asyncness: &mut Async, vis: &mut T) {
+    match asyncness {
+        Async::Yes { span: _, closure_id, return_impl_trait_id } => {
+            vis.visit_id(closure_id);
+            vis.visit_id(return_impl_trait_id);
+        }
+        Async::No => {}
+    }
+}
+
+pub fn noop_visit_fn_decl<T: MutVisitor>(decl: &mut P<FnDecl>, vis: &mut T) {
+    let FnDecl { inputs, output } = decl.deref_mut();
+    inputs.flat_map_in_place(|param| vis.flat_map_param(param));
+    noop_visit_fn_ret_ty(output, vis);
+}
+
+pub fn noop_visit_fn_ret_ty<T: MutVisitor>(fn_ret_ty: &mut FnRetTy, vis: &mut T) {
+    match fn_ret_ty {
+        FnRetTy::Default(span) => vis.visit_span(span),
+        FnRetTy::Ty(ty) => vis.visit_ty(ty),
+    }
+}
+
+pub fn noop_visit_param_bound<T: MutVisitor>(pb: &mut GenericBound, vis: &mut T) {
+    match pb {
+        GenericBound::Trait(ty, _modifier) => vis.visit_poly_trait_ref(ty),
+        GenericBound::Outlives(lifetime) => noop_visit_lifetime(lifetime, vis),
+    }
+}
+
+pub fn noop_flat_map_generic_param<T: MutVisitor>(
+    mut param: GenericParam,
+    vis: &mut T,
+) -> SmallVec<[GenericParam; 1]> {
+    let GenericParam { id, ident, attrs, bounds, kind, colon_span, is_placeholder: _ } = &mut param;
+    vis.visit_id(id);
+    vis.visit_ident(ident);
+    if let Some(ref mut colon_span) = colon_span {
+        vis.visit_span(colon_span);
+    }
+    visit_thin_attrs(attrs, vis);
+    visit_vec(bounds, |bound| noop_visit_param_bound(bound, vis));
+    match kind {
+        GenericParamKind::Lifetime => {}
+        GenericParamKind::Type { default } => {
+            visit_opt(default, |default| vis.visit_ty(default));
+        }
+        GenericParamKind::Const { ty, kw_span: _, default } => {
+            vis.visit_ty(ty);
+            visit_opt(default, |default| vis.visit_anon_const(default));
+        }
+    }
+    smallvec![param]
+}
+
+pub fn noop_visit_label<T: MutVisitor>(Label { ident }: &mut Label, vis: &mut T) {
+    vis.visit_ident(ident);
+}
+
+fn noop_visit_lifetime<T: MutVisitor>(Lifetime { id, ident }: &mut Lifetime, vis: &mut T) {
+    vis.visit_id(id);
+    vis.visit_ident(ident);
+}
+
+pub fn noop_visit_generics<T: MutVisitor>(generics: &mut Generics, vis: &mut T) {
+    let Generics { params, where_clause, span } = generics;
+    params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
+    vis.visit_where_clause(where_clause);
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_where_clause<T: MutVisitor>(wc: &mut WhereClause, vis: &mut T) {
+    let WhereClause { has_where_token: _, predicates, span } = wc;
+    visit_vec(predicates, |predicate| vis.visit_where_predicate(predicate));
+    vis.visit_span(span);
+}
+
+pub fn noop_visit_where_predicate<T: MutVisitor>(pred: &mut WherePredicate, vis: &mut T) {
+    match pred {
+        WherePredicate::BoundPredicate(bp) => {
+            let WhereBoundPredicate { span, bound_generic_params, bounded_ty, bounds } = bp;
+            vis.visit_span(span);
+            bound_generic_params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
+            vis.visit_ty(bounded_ty);
+            visit_vec(bounds, |bound| vis.visit_param_bound(bound));
+        }
+        WherePredicate::RegionPredicate(rp) => {
+            let WhereRegionPredicate { span, lifetime, bounds } = rp;
+            vis.visit_span(span);
+            noop_visit_lifetime(lifetime, vis);
+            visit_vec(bounds, |bound| noop_visit_param_bound(bound, vis));
+        }
+        WherePredicate::EqPredicate(ep) => {
+            let WhereEqPredicate { id, span, lhs_ty, rhs_ty } = ep;
+            vis.visit_id(id);
+            vis.visit_span(span);
+            vis.visit_ty(lhs_ty);
+            vis.visit_ty(rhs_ty);
+        }
+    }
+}
+
+pub fn noop_visit_variant_data<T: MutVisitor>(vdata: &mut VariantData, vis: &mut T) {
+    match vdata {
+        VariantData::Struct(fields, ..) => {
+            fields.flat_map_in_place(|field| vis.flat_map_field_def(field));
+        }
+        VariantData::Tuple(fields, id) => {
+            fields.flat_map_in_place(|field| vis.flat_map_field_def(field));
+            vis.visit_id(id);
+        }
+        VariantData::Unit(id) => vis.visit_id(id),
+    }
+}
+
+pub fn noop_visit_trait_ref<T: MutVisitor>(TraitRef { path, ref_id }: &mut TraitRef, vis: &mut T) {
+    vis.visit_path(path);
+    vis.visit_id(ref_id);
+}
+
+pub fn noop_visit_poly_trait_ref<T: MutVisitor>(p: &mut PolyTraitRef, vis: &mut T) {
+    let PolyTraitRef { bound_generic_params, trait_ref, span } = p;
+    bound_generic_params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
+    vis.visit_trait_ref(trait_ref);
+    vis.visit_span(span);
+}
+
+pub fn noop_flat_map_field_def<T: MutVisitor>(
+    mut fd: FieldDef,
+    visitor: &mut T,
+) -> SmallVec<[FieldDef; 1]> {
+    let FieldDef { span, ident, vis, id, ty, attrs, is_placeholder: _ } = &mut fd;
+    visitor.visit_span(span);
+    visit_opt(ident, |ident| visitor.visit_ident(ident));
+    visitor.visit_vis(vis);
+    visitor.visit_id(id);
+    visitor.visit_ty(ty);
+    visit_thin_attrs(attrs, visitor);
+    smallvec![fd]
+}
+
+pub fn noop_flat_map_expr_field<T: MutVisitor>(
+    mut f: ExprField,
+    vis: &mut T,
+) -> SmallVec<[ExprField; 1]> {
+    let ExprField { ident, expr, span, is_shorthand: _, attrs, id, is_placeholder: _ } = &mut f;
+    vis.visit_ident(ident);
+    vis.visit_expr(expr);
+    vis.visit_id(id);
+    vis.visit_span(span);
+    visit_thin_attrs(attrs, vis);
+    smallvec![f]
+}
+
+pub fn noop_visit_mt<T: MutVisitor>(MutTy { ty, mutbl: _ }: &mut MutTy, vis: &mut T) {
+    vis.visit_ty(ty);
+}
+
+pub fn noop_visit_block<T: MutVisitor>(block: &mut P<Block>, vis: &mut T) {
+    let Block { id, stmts, rules: _, span, tokens, could_be_bare_literal: _ } = block.deref_mut();
+    vis.visit_id(id);
+    stmts.flat_map_in_place(|stmt| vis.flat_map_stmt(stmt));
+    vis.visit_span(span);
+    visit_lazy_tts(tokens, vis);
+}
+
+pub fn noop_visit_item_kind<T: MutVisitor>(kind: &mut ItemKind, vis: &mut T) {
+    match kind {
+        ItemKind::ExternCrate(_orig_name) => {}
+        ItemKind::Use(use_tree) => vis.visit_use_tree(use_tree),
+        ItemKind::Static(ty, _, expr) => {
+            vis.visit_ty(ty);
+            visit_opt(expr, |expr| vis.visit_expr(expr));
+        }
+        ItemKind::Const(defaultness, ty, expr) => {
+            visit_defaultness(defaultness, vis);
+            vis.visit_ty(ty);
+            visit_opt(expr, |expr| vis.visit_expr(expr));
+        }
+        ItemKind::Fn(box Fn { defaultness, generics, sig, body }) => {
+            visit_defaultness(defaultness, vis);
+            visit_fn_sig(sig, vis);
+            vis.visit_generics(generics);
+            visit_opt(body, |body| vis.visit_block(body));
+        }
+        ItemKind::Mod(unsafety, mod_kind) => {
+            visit_unsafety(unsafety, vis);
+            match mod_kind {
+                ModKind::Loaded(items, _inline, ModSpans { inner_span, inject_use_span }) => {
+                    vis.visit_span(inner_span);
+                    vis.visit_span(inject_use_span);
+                    items.flat_map_in_place(|item| vis.flat_map_item(item));
+                }
+                ModKind::Unloaded => {}
+            }
+        }
+        ItemKind::ForeignMod(nm) => vis.visit_foreign_mod(nm),
+        ItemKind::GlobalAsm(asm) => vis.visit_inline_asm(asm),
+        ItemKind::TyAlias(box TyAlias {
+            defaultness, generics, where_clauses, bounds, ty, ..
+        }) => {
+            visit_defaultness(defaultness, vis);
+            vis.visit_generics(generics);
+            vis.visit_span(&mut where_clauses.0.1);
+            vis.visit_span(&mut where_clauses.1.1);
+            visit_bounds(bounds, vis);
+            visit_opt(ty, |ty| vis.visit_ty(ty));
+        }
+        ItemKind::Enum(EnumDef { variants }, generics) => {
+            variants.flat_map_in_place(|variant| vis.flat_map_variant(variant));
+            vis.visit_generics(generics);
+        }
+        ItemKind::Struct(variant_data, generics) | ItemKind::Union(variant_data, generics) => {
+            vis.visit_variant_data(variant_data);
+            vis.visit_generics(generics);
+        }
+        ItemKind::Impl(box Impl {
+            defaultness,
+            unsafety,
+            generics,
+            constness,
+            polarity,
+            of_trait,
+            self_ty,
+            items,
+        }) => {
+            visit_defaultness(defaultness, vis);
+            visit_unsafety(unsafety, vis);
+            vis.visit_generics(generics);
+            visit_constness(constness, vis);
+            visit_polarity(polarity, vis);
+            visit_opt(of_trait, |trait_ref| vis.visit_trait_ref(trait_ref));
+            vis.visit_ty(self_ty);
+            items.flat_map_in_place(|item| vis.flat_map_impl_item(item));
+        }
+        ItemKind::Trait(box Trait { unsafety, is_auto: _, generics, bounds, items }) => {
+            visit_unsafety(unsafety, vis);
+            vis.visit_generics(generics);
+            visit_bounds(bounds, vis);
+            items.flat_map_in_place(|item| vis.flat_map_trait_item(item));
+        }
+        ItemKind::TraitAlias(generics, bounds) => {
+            vis.visit_generics(generics);
+            visit_bounds(bounds, vis);
+        }
+        ItemKind::MacCall(m) => vis.visit_mac_call(m),
+        ItemKind::MacroDef(def) => vis.visit_macro_def(def),
+    }
+}
+
+pub fn noop_flat_map_assoc_item<T: MutVisitor>(
+    mut item: P<AssocItem>,
+    visitor: &mut T,
+) -> SmallVec<[P<AssocItem>; 1]> {
+    let Item { id, ident, vis, attrs, kind, span, tokens } = item.deref_mut();
+    visitor.visit_id(id);
+    visitor.visit_ident(ident);
+    visitor.visit_vis(vis);
+    visit_attrs(attrs, visitor);
+    match kind {
+        AssocItemKind::Const(defaultness, ty, expr) => {
+            visit_defaultness(defaultness, visitor);
+            visitor.visit_ty(ty);
+            visit_opt(expr, |expr| visitor.visit_expr(expr));
+        }
+        AssocItemKind::Fn(box Fn { defaultness, generics, sig, body }) => {
+            visit_defaultness(defaultness, visitor);
+            visitor.visit_generics(generics);
+            visit_fn_sig(sig, visitor);
+            visit_opt(body, |body| visitor.visit_block(body));
+        }
+        AssocItemKind::TyAlias(box TyAlias {
+            defaultness,
+            generics,
+            where_clauses,
+            bounds,
+            ty,
+            ..
+        }) => {
+            visit_defaultness(defaultness, visitor);
+            visitor.visit_generics(generics);
+            visitor.visit_span(&mut where_clauses.0.1);
+            visitor.visit_span(&mut where_clauses.1.1);
+            visit_bounds(bounds, visitor);
+            visit_opt(ty, |ty| visitor.visit_ty(ty));
+        }
+        AssocItemKind::MacCall(mac) => visitor.visit_mac_call(mac),
+    }
+    visitor.visit_span(span);
+    visit_lazy_tts(tokens, visitor);
+    smallvec![item]
+}
+
+pub fn noop_visit_fn_header<T: MutVisitor>(header: &mut FnHeader, vis: &mut T) {
+    let FnHeader { unsafety, asyncness, constness, ext: _ } = header;
+    visit_constness(constness, vis);
+    vis.visit_asyncness(asyncness);
+    visit_unsafety(unsafety, vis);
+}
+
+pub fn noop_visit_crate<T: MutVisitor>(krate: &mut Crate, vis: &mut T) {
+    let Crate { attrs, items, spans, id, is_placeholder: _ } = krate;
+    vis.visit_id(id);
+    visit_attrs(attrs, vis);
+    items.flat_map_in_place(|item| vis.flat_map_item(item));
+    let ModSpans { inner_span, inject_use_span } = spans;
+    vis.visit_span(inner_span);
+    vis.visit_span(inject_use_span);
+}
+
+// Mutates one item into possibly many items.
+pub fn noop_flat_map_item<T: MutVisitor>(
+    mut item: P<Item>,
+    visitor: &mut T,
+) -> SmallVec<[P<Item>; 1]> {
+    let Item { ident, attrs, id, kind, vis, span, tokens } = item.deref_mut();
+    visitor.visit_ident(ident);
+    visit_attrs(attrs, visitor);
+    visitor.visit_id(id);
+    visitor.visit_item_kind(kind);
+    visitor.visit_vis(vis);
+    visitor.visit_span(span);
+    visit_lazy_tts(tokens, visitor);
+
+    smallvec![item]
+}
+
+pub fn noop_flat_map_foreign_item<T: MutVisitor>(
+    mut item: P<ForeignItem>,
+    visitor: &mut T,
+) -> SmallVec<[P<ForeignItem>; 1]> {
+    let Item { ident, attrs, id, kind, vis, span, tokens } = item.deref_mut();
+    visitor.visit_id(id);
+    visitor.visit_ident(ident);
+    visitor.visit_vis(vis);
+    visit_attrs(attrs, visitor);
+    match kind {
+        ForeignItemKind::Static(ty, _, expr) => {
+            visitor.visit_ty(ty);
+            visit_opt(expr, |expr| visitor.visit_expr(expr));
+        }
+        ForeignItemKind::Fn(box Fn { defaultness, generics, sig, body }) => {
+            visit_defaultness(defaultness, visitor);
+            visitor.visit_generics(generics);
+            visit_fn_sig(sig, visitor);
+            visit_opt(body, |body| visitor.visit_block(body));
+        }
+        ForeignItemKind::TyAlias(box TyAlias {
+            defaultness,
+            generics,
+            where_clauses,
+            bounds,
+            ty,
+            ..
+        }) => {
+            visit_defaultness(defaultness, visitor);
+            visitor.visit_generics(generics);
+            visitor.visit_span(&mut where_clauses.0.1);
+            visitor.visit_span(&mut where_clauses.1.1);
+            visit_bounds(bounds, visitor);
+            visit_opt(ty, |ty| visitor.visit_ty(ty));
+        }
+        ForeignItemKind::MacCall(mac) => visitor.visit_mac_call(mac),
+    }
+    visitor.visit_span(span);
+    visit_lazy_tts(tokens, visitor);
+    smallvec![item]
+}
+
+pub fn noop_visit_pat<T: MutVisitor>(pat: &mut P<Pat>, vis: &mut T) {
+    let Pat { id, kind, span, tokens } = pat.deref_mut();
+    vis.visit_id(id);
+    match kind {
+        PatKind::Wild | PatKind::Rest => {}
+        PatKind::Ident(_binding_mode, ident, sub) => {
+            vis.visit_ident(ident);
+            visit_opt(sub, |sub| vis.visit_pat(sub));
+        }
+        PatKind::Lit(e) => vis.visit_expr(e),
+        PatKind::TupleStruct(qself, path, elems) => {
+            vis.visit_qself(qself);
+            vis.visit_path(path);
+            visit_vec(elems, |elem| vis.visit_pat(elem));
+        }
+        PatKind::Path(qself, path) => {
+            vis.visit_qself(qself);
+            vis.visit_path(path);
+        }
+        PatKind::Struct(qself, path, fields, _etc) => {
+            vis.visit_qself(qself);
+            vis.visit_path(path);
+            fields.flat_map_in_place(|field| vis.flat_map_pat_field(field));
+        }
+        PatKind::Box(inner) => vis.visit_pat(inner),
+        PatKind::Ref(inner, _mutbl) => vis.visit_pat(inner),
+        PatKind::Range(e1, e2, Spanned { span: _, node: _ }) => {
+            visit_opt(e1, |e| vis.visit_expr(e));
+            visit_opt(e2, |e| vis.visit_expr(e));
+            vis.visit_span(span);
+        }
+        PatKind::Tuple(elems) | PatKind::Slice(elems) | PatKind::Or(elems) => {
+            visit_vec(elems, |elem| vis.visit_pat(elem))
+        }
+        PatKind::Paren(inner) => vis.visit_pat(inner),
+        PatKind::MacCall(mac) => vis.visit_mac_call(mac),
+    }
+    vis.visit_span(span);
+    visit_lazy_tts(tokens, vis);
+}
+
+pub fn noop_visit_anon_const<T: MutVisitor>(AnonConst { id, value }: &mut AnonConst, vis: &mut T) {
+    vis.visit_id(id);
+    vis.visit_expr(value);
+}
+
+pub fn noop_visit_inline_asm<T: MutVisitor>(asm: &mut InlineAsm, vis: &mut T) {
+    for (op, _) in &mut asm.operands {
+        match op {
+            InlineAsmOperand::In { expr, .. }
+            | InlineAsmOperand::Out { expr: Some(expr), .. }
+            | InlineAsmOperand::InOut { expr, .. } => vis.visit_expr(expr),
+            InlineAsmOperand::Out { expr: None, .. } => {}
+            InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
+                vis.visit_expr(in_expr);
+                if let Some(out_expr) = out_expr {
+                    vis.visit_expr(out_expr);
+                }
+            }
+            InlineAsmOperand::Const { anon_const } => vis.visit_anon_const(anon_const),
+            InlineAsmOperand::Sym { sym } => vis.visit_inline_asm_sym(sym),
+        }
+    }
+}
+
+pub fn noop_visit_inline_asm_sym<T: MutVisitor>(
+    InlineAsmSym { id, qself, path }: &mut InlineAsmSym,
+    vis: &mut T,
+) {
+    vis.visit_id(id);
+    vis.visit_qself(qself);
+    vis.visit_path(path);
+}
+
+pub fn noop_visit_expr<T: MutVisitor>(
+    Expr { kind, id, span, attrs, tokens }: &mut Expr,
+    vis: &mut T,
+) {
+    match kind {
+        ExprKind::Box(expr) => vis.visit_expr(expr),
+        ExprKind::Array(exprs) => visit_exprs(exprs, vis),
+        ExprKind::ConstBlock(anon_const) => {
+            vis.visit_anon_const(anon_const);
+        }
+        ExprKind::Repeat(expr, count) => {
+            vis.visit_expr(expr);
+            vis.visit_anon_const(count);
+        }
+        ExprKind::Tup(exprs) => visit_exprs(exprs, vis),
+        ExprKind::Call(f, args) => {
+            vis.visit_expr(f);
+            visit_exprs(args, vis);
+        }
+        ExprKind::MethodCall(PathSegment { ident, id, args }, exprs, span) => {
+            vis.visit_ident(ident);
+            vis.visit_id(id);
+            visit_opt(args, |args| vis.visit_generic_args(args));
+            visit_exprs(exprs, vis);
+            vis.visit_span(span);
+        }
+        ExprKind::Binary(_binop, lhs, rhs) => {
+            vis.visit_expr(lhs);
+            vis.visit_expr(rhs);
+        }
+        ExprKind::Unary(_unop, ohs) => vis.visit_expr(ohs),
+        ExprKind::Cast(expr, ty) => {
+            vis.visit_expr(expr);
+            vis.visit_ty(ty);
+        }
+        ExprKind::Type(expr, ty) => {
+            vis.visit_expr(expr);
+            vis.visit_ty(ty);
+        }
+        ExprKind::AddrOf(_, _, ohs) => vis.visit_expr(ohs),
+        ExprKind::Let(pat, scrutinee, _) => {
+            vis.visit_pat(pat);
+            vis.visit_expr(scrutinee);
+        }
+        ExprKind::If(cond, tr, fl) => {
+            vis.visit_expr(cond);
+            vis.visit_block(tr);
+            visit_opt(fl, |fl| vis.visit_expr(fl));
+        }
+        ExprKind::While(cond, body, label) => {
+            vis.visit_expr(cond);
+            vis.visit_block(body);
+            visit_opt(label, |label| vis.visit_label(label));
+        }
+        ExprKind::ForLoop(pat, iter, body, label) => {
+            vis.visit_pat(pat);
+            vis.visit_expr(iter);
+            vis.visit_block(body);
+            visit_opt(label, |label| vis.visit_label(label));
+        }
+        ExprKind::Loop(body, label) => {
+            vis.visit_block(body);
+            visit_opt(label, |label| vis.visit_label(label));
+        }
+        ExprKind::Match(expr, arms) => {
+            vis.visit_expr(expr);
+            arms.flat_map_in_place(|arm| vis.flat_map_arm(arm));
+        }
+        ExprKind::Closure(binder, _capture_by, asyncness, _movability, decl, body, span) => {
+            vis.visit_closure_binder(binder);
+            vis.visit_asyncness(asyncness);
+            vis.visit_fn_decl(decl);
+            vis.visit_expr(body);
+            vis.visit_span(span);
+        }
+        ExprKind::Block(blk, label) => {
+            vis.visit_block(blk);
+            visit_opt(label, |label| vis.visit_label(label));
+        }
+        ExprKind::Async(_capture_by, node_id, body) => {
+            vis.visit_id(node_id);
+            vis.visit_block(body);
+        }
+        ExprKind::Await(expr) => vis.visit_expr(expr),
+        ExprKind::Assign(el, er, _) => {
+            vis.visit_expr(el);
+            vis.visit_expr(er);
+        }
+        ExprKind::AssignOp(_op, el, er) => {
+            vis.visit_expr(el);
+            vis.visit_expr(er);
+        }
+        ExprKind::Field(el, ident) => {
+            vis.visit_expr(el);
+            vis.visit_ident(ident);
+        }
+        ExprKind::Index(el, er) => {
+            vis.visit_expr(el);
+            vis.visit_expr(er);
+        }
+        ExprKind::Range(e1, e2, _lim) => {
+            visit_opt(e1, |e1| vis.visit_expr(e1));
+            visit_opt(e2, |e2| vis.visit_expr(e2));
+        }
+        ExprKind::Underscore => {}
+        ExprKind::Path(qself, path) => {
+            vis.visit_qself(qself);
+            vis.visit_path(path);
+        }
+        ExprKind::Break(label, expr) => {
+            visit_opt(label, |label| vis.visit_label(label));
+            visit_opt(expr, |expr| vis.visit_expr(expr));
+        }
+        ExprKind::Continue(label) => {
+            visit_opt(label, |label| vis.visit_label(label));
+        }
+        ExprKind::Ret(expr) => {
+            visit_opt(expr, |expr| vis.visit_expr(expr));
+        }
+        ExprKind::Yeet(expr) => {
+            visit_opt(expr, |expr| vis.visit_expr(expr));
+        }
+        ExprKind::InlineAsm(asm) => vis.visit_inline_asm(asm),
+        ExprKind::MacCall(mac) => vis.visit_mac_call(mac),
+        ExprKind::Struct(se) => {
+            let StructExpr { qself, path, fields, rest } = se.deref_mut();
+            vis.visit_qself(qself);
+            vis.visit_path(path);
+            fields.flat_map_in_place(|field| vis.flat_map_expr_field(field));
+            match rest {
+                StructRest::Base(expr) => vis.visit_expr(expr),
+                StructRest::Rest(_span) => {}
+                StructRest::None => {}
+            }
+        }
+        ExprKind::Paren(expr) => {
+            vis.visit_expr(expr);
+        }
+        ExprKind::Yield(expr) => {
+            visit_opt(expr, |expr| vis.visit_expr(expr));
+        }
+        ExprKind::Try(expr) => vis.visit_expr(expr),
+        ExprKind::TryBlock(body) => vis.visit_block(body),
+        ExprKind::Lit(_) | ExprKind::Err => {}
+    }
+    vis.visit_id(id);
+    vis.visit_span(span);
+    visit_thin_attrs(attrs, vis);
+    visit_lazy_tts(tokens, vis);
+}
+
+pub fn noop_filter_map_expr<T: MutVisitor>(mut e: P<Expr>, vis: &mut T) -> Option<P<Expr>> {
+    Some({
+        vis.visit_expr(&mut e);
+        e
+    })
+}
+
+pub fn noop_flat_map_stmt<T: MutVisitor>(
+    Stmt { kind, mut span, mut id }: Stmt,
+    vis: &mut T,
+) -> SmallVec<[Stmt; 1]> {
+    vis.visit_id(&mut id);
+    vis.visit_span(&mut span);
+    let stmts: SmallVec<_> = noop_flat_map_stmt_kind(kind, vis)
+        .into_iter()
+        .map(|kind| Stmt { id, kind, span })
+        .collect();
+    if stmts.len() > 1 {
+        panic!(
+            "cloning statement `NodeId`s is prohibited by default, \
+             the visitor should implement custom statement visiting"
+        );
+    }
+    stmts
+}
+
+pub fn noop_flat_map_stmt_kind<T: MutVisitor>(
+    kind: StmtKind,
+    vis: &mut T,
+) -> SmallVec<[StmtKind; 1]> {
+    match kind {
+        StmtKind::Local(mut local) => smallvec![StmtKind::Local({
+            vis.visit_local(&mut local);
+            local
+        })],
+        StmtKind::Item(item) => vis.flat_map_item(item).into_iter().map(StmtKind::Item).collect(),
+        StmtKind::Expr(expr) => vis.filter_map_expr(expr).into_iter().map(StmtKind::Expr).collect(),
+        StmtKind::Semi(expr) => vis.filter_map_expr(expr).into_iter().map(StmtKind::Semi).collect(),
+        StmtKind::Empty => smallvec![StmtKind::Empty],
+        StmtKind::MacCall(mut mac) => {
+            let MacCallStmt { mac: mac_, style: _, attrs, tokens } = mac.deref_mut();
+            vis.visit_mac_call(mac_);
+            visit_thin_attrs(attrs, vis);
+            visit_lazy_tts(tokens, vis);
+            smallvec![StmtKind::MacCall(mac)]
+        }
+    }
+}
+
+pub fn noop_visit_vis<T: MutVisitor>(visibility: &mut Visibility, vis: &mut T) {
+    match &mut visibility.kind {
+        VisibilityKind::Public | VisibilityKind::Inherited => {}
+        VisibilityKind::Restricted { path, id } => {
+            vis.visit_path(path);
+            vis.visit_id(id);
+        }
+    }
+    vis.visit_span(&mut visibility.span);
+}
+
+/// Some value for the AST node that is valid but possibly meaningless.
+pub trait DummyAstNode {
+    fn dummy() -> Self;
+}
+
+impl<T> DummyAstNode for Option<T> {
+    fn dummy() -> Self {
+        Default::default()
+    }
+}
+
+impl<T: DummyAstNode + 'static> DummyAstNode for P<T> {
+    fn dummy() -> Self {
+        P(DummyAstNode::dummy())
+    }
+}
+
+impl<T> DummyAstNode for ThinVec<T> {
+    fn dummy() -> Self {
+        Default::default()
+    }
+}
+
+impl DummyAstNode for Item {
+    fn dummy() -> Self {
+        Item {
+            attrs: Default::default(),
+            id: DUMMY_NODE_ID,
+            span: Default::default(),
+            vis: Visibility {
+                kind: VisibilityKind::Public,
+                span: Default::default(),
+                tokens: Default::default(),
+            },
+            ident: Ident::empty(),
+            kind: ItemKind::ExternCrate(None),
+            tokens: Default::default(),
+        }
+    }
+}
+
+impl DummyAstNode for Expr {
+    fn dummy() -> Self {
+        Expr {
+            id: DUMMY_NODE_ID,
+            kind: ExprKind::Err,
+            span: Default::default(),
+            attrs: Default::default(),
+            tokens: Default::default(),
+        }
+    }
+}
+
+impl DummyAstNode for Ty {
+    fn dummy() -> Self {
+        Ty {
+            id: DUMMY_NODE_ID,
+            kind: TyKind::Err,
+            span: Default::default(),
+            tokens: Default::default(),
+        }
+    }
+}
+
+impl DummyAstNode for Pat {
+    fn dummy() -> Self {
+        Pat {
+            id: DUMMY_NODE_ID,
+            kind: PatKind::Wild,
+            span: Default::default(),
+            tokens: Default::default(),
+        }
+    }
+}
+
+impl DummyAstNode for Stmt {
+    fn dummy() -> Self {
+        Stmt { id: DUMMY_NODE_ID, kind: StmtKind::Empty, span: Default::default() }
+    }
+}
+
+impl DummyAstNode for Block {
+    fn dummy() -> Self {
+        Block {
+            stmts: Default::default(),
+            id: DUMMY_NODE_ID,
+            rules: BlockCheckMode::Default,
+            span: Default::default(),
+            tokens: Default::default(),
+            could_be_bare_literal: Default::default(),
+        }
+    }
+}
+
+impl DummyAstNode for Crate {
+    fn dummy() -> Self {
+        Crate {
+            attrs: Default::default(),
+            items: Default::default(),
+            spans: Default::default(),
+            id: DUMMY_NODE_ID,
+            is_placeholder: Default::default(),
+        }
+    }
+}
diff --git a/compiler/rustc_ast/src/node_id.rs b/compiler/rustc_ast/src/node_id.rs
new file mode 100644
index 000000000..7f928cb57
--- /dev/null
+++ b/compiler/rustc_ast/src/node_id.rs
@@ -0,0 +1,40 @@
+use rustc_span::LocalExpnId;
+use std::fmt;
+
+rustc_index::newtype_index! {
+    /// Identifies an AST node.
+    ///
+    /// This identifies top-level definitions, expressions, and everything in between.
+    /// This is later turned into [`DefId`] and `HirId` for the HIR.
+    ///
+    /// [`DefId`]: rustc_span::def_id::DefId
+    pub struct NodeId {
+        DEBUG_FORMAT = "NodeId({})"
+    }
+}
+
+rustc_data_structures::define_id_collections!(NodeMap, NodeSet, NodeId);
+
+/// The [`NodeId`] used to represent the root of the crate.
+pub const CRATE_NODE_ID: NodeId = NodeId::from_u32(0);
+
+/// When parsing and at the beginning of doing expansions, we initially give all AST nodes
+/// this dummy AST [`NodeId`]. Then, during a later phase of expansion, we renumber them
+/// to have small, positive IDs.
+pub const DUMMY_NODE_ID: NodeId = NodeId::MAX;
+
+impl NodeId {
+    pub fn placeholder_from_expn_id(expn_id: LocalExpnId) -> Self {
+        NodeId::from_u32(expn_id.as_u32())
+    }
+
+    pub fn placeholder_to_expn_id(self) -> LocalExpnId {
+        LocalExpnId::from_u32(self.as_u32())
+    }
+}
+
+impl fmt::Display for NodeId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(&self.as_u32(), f)
+    }
+}
diff --git a/compiler/rustc_ast/src/ptr.rs b/compiler/rustc_ast/src/ptr.rs
new file mode 100644
index 000000000..30481eddf
--- /dev/null
+++ b/compiler/rustc_ast/src/ptr.rs
@@ -0,0 +1,212 @@
+//! The AST pointer.
+//!
+//! Provides `P<T>`, a frozen owned smart pointer.
+//!
+//! # Motivations and benefits
+//!
+//! * **Identity**: sharing AST nodes is problematic for the various analysis
+//!   passes (e.g., one may be able to bypass the borrow checker with a shared
+//!   `ExprKind::AddrOf` node taking a mutable borrow).
+//!
+//! * **Immutability**: `P<T>` disallows mutating its inner `T`, unlike `Box<T>`
+//!   (unless it contains an `Unsafe` interior, but that may be denied later).
+//!   This mainly prevents mistakes, but also enforces a kind of "purity".
+//!
+//! * **Efficiency**: folding can reuse allocation space for `P<T>` and `Vec<T>`,
+//!   the latter even when the input and output types differ (as it would be the
+//!   case with arenas or a GADT AST using type parameters to toggle features).
+//!
+//! * **Maintainability**: `P<T>` provides a fixed interface - `Deref`,
+//!   `and_then` and `map` - which can remain fully functional even if the
+//!   implementation changes (using a special thread-local heap, for example).
+//!   Moreover, a switch to, e.g., `P<'a, T>` would be easy and mostly automated.
+
+use std::fmt::{self, Debug, Display};
+use std::iter::FromIterator;
+use std::ops::{Deref, DerefMut};
+use std::{slice, vec};
+
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+/// An owned smart pointer.
+pub struct P<T: ?Sized> {
+    ptr: Box<T>,
+}
+
+/// Construct a `P<T>` from a `T` value.
+#[allow(non_snake_case)]
+pub fn P<T: 'static>(value: T) -> P<T> {
+    P { ptr: Box::new(value) }
+}
+
+impl<T: 'static> P<T> {
+    /// Move out of the pointer.
+    /// Intended for chaining transformations not covered by `map`.
+    pub fn and_then<U, F>(self, f: F) -> U
+    where
+        F: FnOnce(T) -> U,
+    {
+        f(*self.ptr)
+    }
+
+    /// Equivalent to `and_then(|x| x)`.
+    pub fn into_inner(self) -> T {
+        *self.ptr
+    }
+
+    /// Produce a new `P<T>` from `self` without reallocating.
+    pub fn map<F>(mut self, f: F) -> P<T>
+    where
+        F: FnOnce(T) -> T,
+    {
+        let x = f(*self.ptr);
+        *self.ptr = x;
+
+        self
+    }
+
+    /// Optionally produce a new `P<T>` from `self` without reallocating.
+    pub fn filter_map<F>(mut self, f: F) -> Option<P<T>>
+    where
+        F: FnOnce(T) -> Option<T>,
+    {
+        *self.ptr = f(*self.ptr)?;
+        Some(self)
+    }
+}
+
+impl<T: ?Sized> Deref for P<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.ptr
+    }
+}
+
+impl<T: ?Sized> DerefMut for P<T> {
+    fn deref_mut(&mut self) -> &mut T {
+        &mut self.ptr
+    }
+}
+
+impl<T: 'static + Clone> Clone for P<T> {
+    fn clone(&self) -> P<T> {
+        P((**self).clone())
+    }
+}
+
+impl<T: ?Sized + Debug> Debug for P<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        Debug::fmt(&self.ptr, f)
+    }
+}
+
+impl<T: Display> Display for P<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        Display::fmt(&**self, f)
+    }
+}
+
+impl<T> fmt::Pointer for P<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Pointer::fmt(&self.ptr, f)
+    }
+}
+
+impl<D: Decoder, T: 'static + Decodable<D>> Decodable<D> for P<T> {
+    fn decode(d: &mut D) -> P<T> {
+        P(Decodable::decode(d))
+    }
+}
+
+impl<S: Encoder, T: Encodable<S>> Encodable<S> for P<T> {
+    fn encode(&self, s: &mut S) {
+        (**self).encode(s);
+    }
+}
+
+impl<T> P<[T]> {
+    pub const fn new() -> P<[T]> {
+        P { ptr: Box::default() }
+    }
+
+    #[inline(never)]
+    pub fn from_vec(v: Vec<T>) -> P<[T]> {
+        P { ptr: v.into_boxed_slice() }
+    }
+
+    #[inline(never)]
+    pub fn into_vec(self) -> Vec<T> {
+        self.ptr.into_vec()
+    }
+}
+
+impl<T> Default for P<[T]> {
+    /// Creates an empty `P<[T]>`.
+    fn default() -> P<[T]> {
+        P::new()
+    }
+}
+
+impl<T: Clone> Clone for P<[T]> {
+    fn clone(&self) -> P<[T]> {
+        P::from_vec(self.to_vec())
+    }
+}
+
+impl<T> From<Vec<T>> for P<[T]> {
+    fn from(v: Vec<T>) -> Self {
+        P::from_vec(v)
+    }
+}
+
+impl<T> Into<Vec<T>> for P<[T]> {
+    fn into(self) -> Vec<T> {
+        self.into_vec()
+    }
+}
+
+impl<T> FromIterator<T> for P<[T]> {
+    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> P<[T]> {
+        P::from_vec(iter.into_iter().collect())
+    }
+}
+
+impl<T> IntoIterator for P<[T]> {
+    type Item = T;
+    type IntoIter = vec::IntoIter<T>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.into_vec().into_iter()
+    }
+}
+
+impl<'a, T> IntoIterator for &'a P<[T]> {
+    type Item = &'a T;
+    type IntoIter = slice::Iter<'a, T>;
+    fn into_iter(self) -> Self::IntoIter {
+        self.ptr.into_iter()
+    }
+}
+
+impl<S: Encoder, T: Encodable<S>> Encodable<S> for P<[T]> {
+    fn encode(&self, s: &mut S) {
+        Encodable::encode(&**self, s);
+    }
+}
+
+impl<D: Decoder, T: Decodable<D>> Decodable<D> for P<[T]> {
+    fn decode(d: &mut D) -> P<[T]> {
+        P::from_vec(Decodable::decode(d))
+    }
+}
+
+impl<CTX, T> HashStable<CTX> for P<T>
+where
+    T: ?Sized + HashStable<CTX>,
+{
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
+        (**self).hash_stable(hcx, hasher);
+    }
+}
diff --git a/compiler/rustc_ast/src/token.rs b/compiler/rustc_ast/src/token.rs
new file mode 100644
index 000000000..85d9687c6
--- /dev/null
+++ b/compiler/rustc_ast/src/token.rs
@@ -0,0 +1,851 @@
+pub use BinOpToken::*;
+pub use LitKind::*;
+pub use Nonterminal::*;
+pub use TokenKind::*;
+
+use crate::ast;
+use crate::ptr::P;
+
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::Lrc;
+use rustc_macros::HashStable_Generic;
+use rustc_span::symbol::{kw, sym};
+use rustc_span::symbol::{Ident, Symbol};
+use rustc_span::{self, edition::Edition, Span, DUMMY_SP};
+use std::borrow::Cow;
+use std::{fmt, mem};
+
+#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum CommentKind {
+    Line,
+    Block,
+}
+
+#[derive(Clone, PartialEq, Encodable, Decodable, Hash, Debug, Copy)]
+#[derive(HashStable_Generic)]
+pub enum BinOpToken {
+    Plus,
+    Minus,
+    Star,
+    Slash,
+    Percent,
+    Caret,
+    And,
+    Or,
+    Shl,
+    Shr,
+}
+
+/// Describes how a sequence of token trees is delimited.
+/// Cannot use `proc_macro::Delimiter` directly because this
+/// structure should implement some additional traits.
+/// The `None` variant is also renamed to `Invisible` to be
+/// less confusing and better convey the semantics.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[derive(Encodable, Decodable, Hash, HashStable_Generic)]
+pub enum Delimiter {
+    /// `( ... )`
+    Parenthesis,
+    /// `{ ... }`
+    Brace,
+    /// `[ ... ]`
+    Bracket,
+    /// `Ø ... Ø`
+    /// An invisible delimiter, that may, for example, appear around tokens coming from a
+    /// "macro variable" `$var`. It is important to preserve operator priorities in cases like
+    /// `$var * 3` where `$var` is `1 + 2`.
+    /// Invisible delimiters might not survive roundtrip of a token stream through a string.
+    Invisible,
+}
+
+#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum LitKind {
+    Bool, // AST only, must never appear in a `Token`
+    Byte,
+    Char,
+    Integer,
+    Float,
+    Str,
+    StrRaw(u8), // raw string delimited by `n` hash symbols
+    ByteStr,
+    ByteStrRaw(u8), // raw byte string delimited by `n` hash symbols
+    Err,
+}
+
+/// A literal token.
+#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub struct Lit {
+    pub kind: LitKind,
+    pub symbol: Symbol,
+    pub suffix: Option<Symbol>,
+}
+
+impl fmt::Display for Lit {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Lit { kind, symbol, suffix } = *self;
+        match kind {
+            Byte => write!(f, "b'{}'", symbol)?,
+            Char => write!(f, "'{}'", symbol)?,
+            Str => write!(f, "\"{}\"", symbol)?,
+            StrRaw(n) => write!(
+                f,
+                "r{delim}\"{string}\"{delim}",
+                delim = "#".repeat(n as usize),
+                string = symbol
+            )?,
+            ByteStr => write!(f, "b\"{}\"", symbol)?,
+            ByteStrRaw(n) => write!(
+                f,
+                "br{delim}\"{string}\"{delim}",
+                delim = "#".repeat(n as usize),
+                string = symbol
+            )?,
+            Integer | Float | Bool | Err => write!(f, "{}", symbol)?,
+        }
+
+        if let Some(suffix) = suffix {
+            write!(f, "{}", suffix)?;
+        }
+
+        Ok(())
+    }
+}
+
+impl LitKind {
+    /// An English article for the literal token kind.
+    pub fn article(self) -> &'static str {
+        match self {
+            Integer | Err => "an",
+            _ => "a",
+        }
+    }
+
+    pub fn descr(self) -> &'static str {
+        match self {
+            Bool => panic!("literal token contains `Lit::Bool`"),
+            Byte => "byte",
+            Char => "char",
+            Integer => "integer",
+            Float => "float",
+            Str | StrRaw(..) => "string",
+            ByteStr | ByteStrRaw(..) => "byte string",
+            Err => "error",
+        }
+    }
+
+    pub(crate) fn may_have_suffix(self) -> bool {
+        matches!(self, Integer | Float | Err)
+    }
+}
+
+impl Lit {
+    pub fn new(kind: LitKind, symbol: Symbol, suffix: Option<Symbol>) -> Lit {
+        Lit { kind, symbol, suffix }
+    }
+}
+
+pub fn ident_can_begin_expr(name: Symbol, span: Span, is_raw: bool) -> bool {
+    let ident_token = Token::new(Ident(name, is_raw), span);
+
+    !ident_token.is_reserved_ident()
+        || ident_token.is_path_segment_keyword()
+        || [
+            kw::Async,
+            kw::Do,
+            kw::Box,
+            kw::Break,
+            kw::Const,
+            kw::Continue,
+            kw::False,
+            kw::For,
+            kw::If,
+            kw::Let,
+            kw::Loop,
+            kw::Match,
+            kw::Move,
+            kw::Return,
+            kw::True,
+            kw::Try,
+            kw::Unsafe,
+            kw::While,
+            kw::Yield,
+            kw::Static,
+        ]
+        .contains(&name)
+}
+
+fn ident_can_begin_type(name: Symbol, span: Span, is_raw: bool) -> bool {
+    let ident_token = Token::new(Ident(name, is_raw), span);
+
+    !ident_token.is_reserved_ident()
+        || ident_token.is_path_segment_keyword()
+        || [kw::Underscore, kw::For, kw::Impl, kw::Fn, kw::Unsafe, kw::Extern, kw::Typeof, kw::Dyn]
+            .contains(&name)
+}
+
+#[derive(Clone, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub enum TokenKind {
+    /* Expression-operator symbols. */
+    Eq,
+    Lt,
+    Le,
+    EqEq,
+    Ne,
+    Ge,
+    Gt,
+    AndAnd,
+    OrOr,
+    Not,
+    Tilde,
+    BinOp(BinOpToken),
+    BinOpEq(BinOpToken),
+
+    /* Structural symbols */
+    At,
+    Dot,
+    DotDot,
+    DotDotDot,
+    DotDotEq,
+    Comma,
+    Semi,
+    Colon,
+    ModSep,
+    RArrow,
+    LArrow,
+    FatArrow,
+    Pound,
+    Dollar,
+    Question,
+    /// Used by proc macros for representing lifetimes, not generated by lexer right now.
+    SingleQuote,
+    /// An opening delimiter (e.g., `{`).
+    OpenDelim(Delimiter),
+    /// A closing delimiter (e.g., `}`).
+    CloseDelim(Delimiter),
+
+    /* Literals */
+    Literal(Lit),
+
+    /// Identifier token.
+    /// Do not forget about `NtIdent` when you want to match on identifiers.
+    /// It's recommended to use `Token::(ident,uninterpolate,uninterpolated_span)` to
+    /// treat regular and interpolated identifiers in the same way.
+    Ident(Symbol, /* is_raw */ bool),
+    /// Lifetime identifier token.
+    /// Do not forget about `NtLifetime` when you want to match on lifetime identifiers.
+    /// It's recommended to use `Token::(lifetime,uninterpolate,uninterpolated_span)` to
+    /// treat regular and interpolated lifetime identifiers in the same way.
+    Lifetime(Symbol),
+
+    /// An embedded AST node, as produced by a macro. This only exists for
+    /// historical reasons. We'd like to get rid of it, for multiple reasons.
+    /// - It's conceptually very strange. Saying a token can contain an AST
+    ///   node is like saying, in natural language, that a word can contain a
+    ///   sentence.
+    /// - It requires special handling in a bunch of places in the parser.
+    /// - It prevents `Token` from implementing `Copy`.
+    /// It adds complexity and likely slows things down. Please don't add new
+    /// occurrences of this token kind!
+    Interpolated(Lrc<Nonterminal>),
+
+    /// A doc comment token.
+    /// `Symbol` is the doc comment's data excluding its "quotes" (`///`, `/**`, etc)
+    /// similarly to symbols in string literal tokens.
+    DocComment(CommentKind, ast::AttrStyle, Symbol),
+
+    Eof,
+}
+
+// `TokenKind` is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+rustc_data_structures::static_assert_size!(TokenKind, 16);
+
+#[derive(Clone, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+pub struct Token {
+    pub kind: TokenKind,
+    pub span: Span,
+}
+
+impl TokenKind {
+    pub fn lit(kind: LitKind, symbol: Symbol, suffix: Option<Symbol>) -> TokenKind {
+        Literal(Lit::new(kind, symbol, suffix))
+    }
+
+    // An approximation to proc-macro-style single-character operators used by rustc parser.
+    // If the operator token can be broken into two tokens, the first of which is single-character,
+    // then this function performs that operation, otherwise it returns `None`.
+    pub fn break_two_token_op(&self) -> Option<(TokenKind, TokenKind)> {
+        Some(match *self {
+            Le => (Lt, Eq),
+            EqEq => (Eq, Eq),
+            Ne => (Not, Eq),
+            Ge => (Gt, Eq),
+            AndAnd => (BinOp(And), BinOp(And)),
+            OrOr => (BinOp(Or), BinOp(Or)),
+            BinOp(Shl) => (Lt, Lt),
+            BinOp(Shr) => (Gt, Gt),
+            BinOpEq(Plus) => (BinOp(Plus), Eq),
+            BinOpEq(Minus) => (BinOp(Minus), Eq),
+            BinOpEq(Star) => (BinOp(Star), Eq),
+            BinOpEq(Slash) => (BinOp(Slash), Eq),
+            BinOpEq(Percent) => (BinOp(Percent), Eq),
+            BinOpEq(Caret) => (BinOp(Caret), Eq),
+            BinOpEq(And) => (BinOp(And), Eq),
+            BinOpEq(Or) => (BinOp(Or), Eq),
+            BinOpEq(Shl) => (Lt, Le),
+            BinOpEq(Shr) => (Gt, Ge),
+            DotDot => (Dot, Dot),
+            DotDotDot => (Dot, DotDot),
+            ModSep => (Colon, Colon),
+            RArrow => (BinOp(Minus), Gt),
+            LArrow => (Lt, BinOp(Minus)),
+            FatArrow => (Eq, Gt),
+            _ => return None,
+        })
+    }
+
+    /// Returns tokens that are likely to be typed accidentally instead of the current token.
+    /// Enables better error recovery when the wrong token is found.
+    pub fn similar_tokens(&self) -> Option<Vec<TokenKind>> {
+        match *self {
+            Comma => Some(vec![Dot, Lt, Semi]),
+            Semi => Some(vec![Colon, Comma]),
+            FatArrow => Some(vec![Eq, RArrow]),
+            _ => None,
+        }
+    }
+
+    pub fn should_end_const_arg(&self) -> bool {
+        matches!(self, Gt | Ge | BinOp(Shr) | BinOpEq(Shr))
+    }
+}
+
+impl Token {
+    pub fn new(kind: TokenKind, span: Span) -> Self {
+        Token { kind, span }
+    }
+
+    /// Some token that will be thrown away later.
+    pub fn dummy() -> Self {
+        Token::new(TokenKind::Question, DUMMY_SP)
+    }
+
+    /// Recovers a `Token` from an `Ident`. This creates a raw identifier if necessary.
+    pub fn from_ast_ident(ident: Ident) -> Self {
+        Token::new(Ident(ident.name, ident.is_raw_guess()), ident.span)
+    }
+
+    /// Return this token by value and leave a dummy token in its place.
+    pub fn take(&mut self) -> Self {
+        mem::replace(self, Token::dummy())
+    }
+
+    /// For interpolated tokens, returns a span of the fragment to which the interpolated
+    /// token refers. For all other tokens this is just a regular span.
+    /// It is particularly important to use this for identifiers and lifetimes
+    /// for which spans affect name resolution and edition checks.
+    /// Note that keywords are also identifiers, so they should use this
+    /// if they keep spans or perform edition checks.
+    pub fn uninterpolated_span(&self) -> Span {
+        match &self.kind {
+            Interpolated(nt) => nt.span(),
+            _ => self.span,
+        }
+    }
+
+    pub fn is_op(&self) -> bool {
+        !matches!(
+            self.kind,
+            OpenDelim(..)
+                | CloseDelim(..)
+                | Literal(..)
+                | DocComment(..)
+                | Ident(..)
+                | Lifetime(..)
+                | Interpolated(..)
+                | Eof
+        )
+    }
+
+    pub fn is_like_plus(&self) -> bool {
+        matches!(self.kind, BinOp(Plus) | BinOpEq(Plus))
+    }
+
+    /// Returns `true` if the token can appear at the start of an expression.
+    pub fn can_begin_expr(&self) -> bool {
+        match self.uninterpolate().kind {
+            Ident(name, is_raw)              =>
+                ident_can_begin_expr(name, self.span, is_raw), // value name or keyword
+            OpenDelim(..)                     | // tuple, array or block
+            Literal(..)                       | // literal
+            Not                               | // operator not
+            BinOp(Minus)                      | // unary minus
+            BinOp(Star)                       | // dereference
+            BinOp(Or) | OrOr                  | // closure
+            BinOp(And)                        | // reference
+            AndAnd                            | // double reference
+            // DotDotDot is no longer supported, but we need some way to display the error
+            DotDot | DotDotDot | DotDotEq     | // range notation
+            Lt | BinOp(Shl)                   | // associated path
+            ModSep                            | // global path
+            Lifetime(..)                      | // labeled loop
+            Pound                             => true, // expression attributes
+            Interpolated(ref nt) => matches!(**nt, NtLiteral(..) |
+                NtExpr(..)    |
+                NtBlock(..)   |
+                NtPath(..)),
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if the token can appear at the start of a type.
+    pub fn can_begin_type(&self) -> bool {
+        match self.uninterpolate().kind {
+            Ident(name, is_raw)        =>
+                ident_can_begin_type(name, self.span, is_raw), // type name or keyword
+            OpenDelim(Delimiter::Parenthesis) | // tuple
+            OpenDelim(Delimiter::Bracket)     | // array
+            Not                         | // never
+            BinOp(Star)                 | // raw pointer
+            BinOp(And)                  | // reference
+            AndAnd                      | // double reference
+            Question                    | // maybe bound in trait object
+            Lifetime(..)                | // lifetime bound in trait object
+            Lt | BinOp(Shl)             | // associated path
+            ModSep                      => true, // global path
+            Interpolated(ref nt) => matches!(**nt, NtTy(..) | NtPath(..)),
+            _ => false,
+        }
+    }
+
+    /// Returns `true` if the token can appear at the start of a const param.
+    pub fn can_begin_const_arg(&self) -> bool {
+        match self.kind {
+            OpenDelim(Delimiter::Brace) => true,
+            Interpolated(ref nt) => matches!(**nt, NtExpr(..) | NtBlock(..) | NtLiteral(..)),
+            _ => self.can_begin_literal_maybe_minus(),
+        }
+    }
+
+    /// Returns `true` if the token can appear at the start of a generic bound.
+    pub fn can_begin_bound(&self) -> bool {
+        self.is_path_start()
+            || self.is_lifetime()
+            || self.is_keyword(kw::For)
+            || self == &Question
+            || self == &OpenDelim(Delimiter::Parenthesis)
+    }
+
+    /// Returns `true` if the token is any literal.
+    pub fn is_lit(&self) -> bool {
+        matches!(self.kind, Literal(..))
+    }
+
+    /// Returns `true` if the token is any literal, a minus (which can prefix a literal,
+    /// for example a '-42', or one of the boolean idents).
+    ///
+    /// In other words, would this token be a valid start of `parse_literal_maybe_minus`?
+    ///
+    /// Keep this in sync with and `Lit::from_token`, excluding unary negation.
+    pub fn can_begin_literal_maybe_minus(&self) -> bool {
+        match self.uninterpolate().kind {
+            Literal(..) | BinOp(Minus) => true,
+            Ident(name, false) if name.is_bool_lit() => true,
+            Interpolated(ref nt) => match &**nt {
+                NtLiteral(_) => true,
+                NtExpr(e) => match &e.kind {
+                    ast::ExprKind::Lit(_) => true,
+                    ast::ExprKind::Unary(ast::UnOp::Neg, e) => {
+                        matches!(&e.kind, ast::ExprKind::Lit(_))
+                    }
+                    _ => false,
+                },
+                _ => false,
+            },
+            _ => false,
+        }
+    }
+
+    // A convenience function for matching on identifiers during parsing.
+    // Turns interpolated identifier (`$i: ident`) or lifetime (`$l: lifetime`) token
+    // into the regular identifier or lifetime token it refers to,
+    // otherwise returns the original token.
+    pub fn uninterpolate(&self) -> Cow<'_, Token> {
+        match &self.kind {
+            Interpolated(nt) => match **nt {
+                NtIdent(ident, is_raw) => {
+                    Cow::Owned(Token::new(Ident(ident.name, is_raw), ident.span))
+                }
+                NtLifetime(ident) => Cow::Owned(Token::new(Lifetime(ident.name), ident.span)),
+                _ => Cow::Borrowed(self),
+            },
+            _ => Cow::Borrowed(self),
+        }
+    }
+
+    /// Returns an identifier if this token is an identifier.
+    #[inline]
+    pub fn ident(&self) -> Option<(Ident, /* is_raw */ bool)> {
+        // We avoid using `Token::uninterpolate` here because it's slow.
+        match &self.kind {
+            &Ident(name, is_raw) => Some((Ident::new(name, self.span), is_raw)),
+            Interpolated(nt) => match **nt {
+                NtIdent(ident, is_raw) => Some((ident, is_raw)),
+                _ => None,
+            },
+            _ => None,
+        }
+    }
+
+    /// Returns a lifetime identifier if this token is a lifetime.
+    #[inline]
+    pub fn lifetime(&self) -> Option<Ident> {
+        // We avoid using `Token::uninterpolate` here because it's slow.
+        match &self.kind {
+            &Lifetime(name) => Some(Ident::new(name, self.span)),
+            Interpolated(nt) => match **nt {
+                NtLifetime(ident) => Some(ident),
+                _ => None,
+            },
+            _ => None,
+        }
+    }
+
+    /// Returns `true` if the token is an identifier.
+    pub fn is_ident(&self) -> bool {
+        self.ident().is_some()
+    }
+
+    /// Returns `true` if the token is a lifetime.
+    pub fn is_lifetime(&self) -> bool {
+        self.lifetime().is_some()
+    }
+
+    /// Returns `true` if the token is an identifier whose name is the given
+    /// string slice.
+    pub fn is_ident_named(&self, name: Symbol) -> bool {
+        self.ident().map_or(false, |(ident, _)| ident.name == name)
+    }
+
+    /// Returns `true` if the token is an interpolated path.
+    fn is_path(&self) -> bool {
+        if let Interpolated(ref nt) = self.kind && let NtPath(..) = **nt {
+            return true;
+        }
+        false
+    }
+
+    /// Would `maybe_whole_expr` in `parser.rs` return `Ok(..)`?
+    /// That is, is this a pre-parsed expression dropped into the token stream
+    /// (which happens while parsing the result of macro expansion)?
+    pub fn is_whole_expr(&self) -> bool {
+        if let Interpolated(ref nt) = self.kind
+            && let NtExpr(_) | NtLiteral(_) | NtPath(_) | NtBlock(_) = **nt
+        {
+            return true;
+        }
+
+        false
+    }
+
+    // Is the token an interpolated block (`$b:block`)?
+    pub fn is_whole_block(&self) -> bool {
+        if let Interpolated(ref nt) = self.kind && let NtBlock(..) = **nt {
+            return true;
+        }
+        false
+    }
+
+    /// Returns `true` if the token is either the `mut` or `const` keyword.
+    pub fn is_mutability(&self) -> bool {
+        self.is_keyword(kw::Mut) || self.is_keyword(kw::Const)
+    }
+
+    pub fn is_qpath_start(&self) -> bool {
+        self == &Lt || self == &BinOp(Shl)
+    }
+
+    pub fn is_path_start(&self) -> bool {
+        self == &ModSep
+            || self.is_qpath_start()
+            || self.is_path()
+            || self.is_path_segment_keyword()
+            || self.is_ident() && !self.is_reserved_ident()
+    }
+
+    /// Returns `true` if the token is a given keyword, `kw`.
+    pub fn is_keyword(&self, kw: Symbol) -> bool {
+        self.is_non_raw_ident_where(|id| id.name == kw)
+    }
+
+    pub fn is_path_segment_keyword(&self) -> bool {
+        self.is_non_raw_ident_where(Ident::is_path_segment_keyword)
+    }
+
+    // Returns true for reserved identifiers used internally for elided lifetimes,
+    // unnamed method parameters, crate root module, error recovery etc.
+    pub fn is_special_ident(&self) -> bool {
+        self.is_non_raw_ident_where(Ident::is_special)
+    }
+
+    /// Returns `true` if the token is a keyword used in the language.
+    pub fn is_used_keyword(&self) -> bool {
+        self.is_non_raw_ident_where(Ident::is_used_keyword)
+    }
+
+    /// Returns `true` if the token is a keyword reserved for possible future use.
+    pub fn is_unused_keyword(&self) -> bool {
+        self.is_non_raw_ident_where(Ident::is_unused_keyword)
+    }
+
+    /// Returns `true` if the token is either a special identifier or a keyword.
+    pub fn is_reserved_ident(&self) -> bool {
+        self.is_non_raw_ident_where(Ident::is_reserved)
+    }
+
+    /// Returns `true` if the token is the identifier `true` or `false`.
+    pub fn is_bool_lit(&self) -> bool {
+        self.is_non_raw_ident_where(|id| id.name.is_bool_lit())
+    }
+
+    pub fn is_numeric_lit(&self) -> bool {
+        matches!(
+            self.kind,
+            Literal(Lit { kind: LitKind::Integer, .. }) | Literal(Lit { kind: LitKind::Float, .. })
+        )
+    }
+
+    /// Returns `true` if the token is a non-raw identifier for which `pred` holds.
+    pub fn is_non_raw_ident_where(&self, pred: impl FnOnce(Ident) -> bool) -> bool {
+        match self.ident() {
+            Some((id, false)) => pred(id),
+            _ => false,
+        }
+    }
+
+    pub fn glue(&self, joint: &Token) -> Option<Token> {
+        let kind = match self.kind {
+            Eq => match joint.kind {
+                Eq => EqEq,
+                Gt => FatArrow,
+                _ => return None,
+            },
+            Lt => match joint.kind {
+                Eq => Le,
+                Lt => BinOp(Shl),
+                Le => BinOpEq(Shl),
+                BinOp(Minus) => LArrow,
+                _ => return None,
+            },
+            Gt => match joint.kind {
+                Eq => Ge,
+                Gt => BinOp(Shr),
+                Ge => BinOpEq(Shr),
+                _ => return None,
+            },
+            Not => match joint.kind {
+                Eq => Ne,
+                _ => return None,
+            },
+            BinOp(op) => match joint.kind {
+                Eq => BinOpEq(op),
+                BinOp(And) if op == And => AndAnd,
+                BinOp(Or) if op == Or => OrOr,
+                Gt if op == Minus => RArrow,
+                _ => return None,
+            },
+            Dot => match joint.kind {
+                Dot => DotDot,
+                DotDot => DotDotDot,
+                _ => return None,
+            },
+            DotDot => match joint.kind {
+                Dot => DotDotDot,
+                Eq => DotDotEq,
+                _ => return None,
+            },
+            Colon => match joint.kind {
+                Colon => ModSep,
+                _ => return None,
+            },
+            SingleQuote => match joint.kind {
+                Ident(name, false) => Lifetime(Symbol::intern(&format!("'{}", name))),
+                _ => return None,
+            },
+
+            Le | EqEq | Ne | Ge | AndAnd | OrOr | Tilde | BinOpEq(..) | At | DotDotDot
+            | DotDotEq | Comma | Semi | ModSep | RArrow | LArrow | FatArrow | Pound | Dollar
+            | Question | OpenDelim(..) | CloseDelim(..) | Literal(..) | Ident(..)
+            | Lifetime(..) | Interpolated(..) | DocComment(..) | Eof => return None,
+        };
+
+        Some(Token::new(kind, self.span.to(joint.span)))
+    }
+}
+
+impl PartialEq<TokenKind> for Token {
+    fn eq(&self, rhs: &TokenKind) -> bool {
+        self.kind == *rhs
+    }
+}
+
+#[derive(Clone, Encodable, Decodable)]
+/// For interpolation during macro expansion.
+pub enum Nonterminal {
+    NtItem(P<ast::Item>),
+    NtBlock(P<ast::Block>),
+    NtStmt(P<ast::Stmt>),
+    NtPat(P<ast::Pat>),
+    NtExpr(P<ast::Expr>),
+    NtTy(P<ast::Ty>),
+    NtIdent(Ident, /* is_raw */ bool),
+    NtLifetime(Ident),
+    NtLiteral(P<ast::Expr>),
+    /// Stuff inside brackets for attributes
+    NtMeta(P<ast::AttrItem>),
+    NtPath(P<ast::Path>),
+    NtVis(P<ast::Visibility>),
+}
+
+// `Nonterminal` is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+rustc_data_structures::static_assert_size!(Nonterminal, 16);
+
+#[derive(Debug, Copy, Clone, PartialEq, Encodable, Decodable)]
+pub enum NonterminalKind {
+    Item,
+    Block,
+    Stmt,
+    PatParam {
+        /// Keep track of whether the user used `:pat_param` or `:pat` and we inferred it from the
+        /// edition of the span. This is used for diagnostics.
+        inferred: bool,
+    },
+    PatWithOr,
+    Expr,
+    Ty,
+    Ident,
+    Lifetime,
+    Literal,
+    Meta,
+    Path,
+    Vis,
+    TT,
+}
+
+impl NonterminalKind {
+    /// The `edition` closure is used to get the edition for the given symbol. Doing
+    /// `span.edition()` is expensive, so we do it lazily.
+    pub fn from_symbol(
+        symbol: Symbol,
+        edition: impl FnOnce() -> Edition,
+    ) -> Option<NonterminalKind> {
+        Some(match symbol {
+            sym::item => NonterminalKind::Item,
+            sym::block => NonterminalKind::Block,
+            sym::stmt => NonterminalKind::Stmt,
+            sym::pat => match edition() {
+                Edition::Edition2015 | Edition::Edition2018 => {
+                    NonterminalKind::PatParam { inferred: true }
+                }
+                Edition::Edition2021 | Edition::Edition2024 => NonterminalKind::PatWithOr,
+            },
+            sym::pat_param => NonterminalKind::PatParam { inferred: false },
+            sym::expr => NonterminalKind::Expr,
+            sym::ty => NonterminalKind::Ty,
+            sym::ident => NonterminalKind::Ident,
+            sym::lifetime => NonterminalKind::Lifetime,
+            sym::literal => NonterminalKind::Literal,
+            sym::meta => NonterminalKind::Meta,
+            sym::path => NonterminalKind::Path,
+            sym::vis => NonterminalKind::Vis,
+            sym::tt => NonterminalKind::TT,
+            _ => return None,
+        })
+    }
+    fn symbol(self) -> Symbol {
+        match self {
+            NonterminalKind::Item => sym::item,
+            NonterminalKind::Block => sym::block,
+            NonterminalKind::Stmt => sym::stmt,
+            NonterminalKind::PatParam { inferred: false } => sym::pat_param,
+            NonterminalKind::PatParam { inferred: true } | NonterminalKind::PatWithOr => sym::pat,
+            NonterminalKind::Expr => sym::expr,
+            NonterminalKind::Ty => sym::ty,
+            NonterminalKind::Ident => sym::ident,
+            NonterminalKind::Lifetime => sym::lifetime,
+            NonterminalKind::Literal => sym::literal,
+            NonterminalKind::Meta => sym::meta,
+            NonterminalKind::Path => sym::path,
+            NonterminalKind::Vis => sym::vis,
+            NonterminalKind::TT => sym::tt,
+        }
+    }
+}
+
+impl fmt::Display for NonterminalKind {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.symbol())
+    }
+}
+
+impl Nonterminal {
+    pub fn span(&self) -> Span {
+        match self {
+            NtItem(item) => item.span,
+            NtBlock(block) => block.span,
+            NtStmt(stmt) => stmt.span,
+            NtPat(pat) => pat.span,
+            NtExpr(expr) | NtLiteral(expr) => expr.span,
+            NtTy(ty) => ty.span,
+            NtIdent(ident, _) | NtLifetime(ident) => ident.span,
+            NtMeta(attr_item) => attr_item.span(),
+            NtPath(path) => path.span,
+            NtVis(vis) => vis.span,
+        }
+    }
+}
+
+impl PartialEq for Nonterminal {
+    fn eq(&self, rhs: &Self) -> bool {
+        match (self, rhs) {
+            (NtIdent(ident_lhs, is_raw_lhs), NtIdent(ident_rhs, is_raw_rhs)) => {
+                ident_lhs == ident_rhs && is_raw_lhs == is_raw_rhs
+            }
+            (NtLifetime(ident_lhs), NtLifetime(ident_rhs)) => ident_lhs == ident_rhs,
+            // FIXME: Assume that all "complex" nonterminal are not equal, we can't compare them
+            // correctly based on data from AST. This will prevent them from matching each other
+            // in macros. The comparison will become possible only when each nonterminal has an
+            // attached token stream from which it was parsed.
+            _ => false,
+        }
+    }
+}
+
+impl fmt::Debug for Nonterminal {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            NtItem(..) => f.pad("NtItem(..)"),
+            NtBlock(..) => f.pad("NtBlock(..)"),
+            NtStmt(..) => f.pad("NtStmt(..)"),
+            NtPat(..) => f.pad("NtPat(..)"),
+            NtExpr(..) => f.pad("NtExpr(..)"),
+            NtTy(..) => f.pad("NtTy(..)"),
+            NtIdent(..) => f.pad("NtIdent(..)"),
+            NtLiteral(..) => f.pad("NtLiteral(..)"),
+            NtMeta(..) => f.pad("NtMeta(..)"),
+            NtPath(..) => f.pad("NtPath(..)"),
+            NtVis(..) => f.pad("NtVis(..)"),
+            NtLifetime(..) => f.pad("NtLifetime(..)"),
+        }
+    }
+}
+
+impl<CTX> HashStable<CTX> for Nonterminal
+where
+    CTX: crate::HashStableContext,
+{
+    fn hash_stable(&self, _hcx: &mut CTX, _hasher: &mut StableHasher) {
+        panic!("interpolated tokens should not be present in the HIR")
+    }
+}
diff --git a/compiler/rustc_ast/src/tokenstream.rs b/compiler/rustc_ast/src/tokenstream.rs
new file mode 100644
index 000000000..9e4a22e1f
--- /dev/null
+++ b/compiler/rustc_ast/src/tokenstream.rs
@@ -0,0 +1,681 @@
+//! # Token Streams
+//!
+//! `TokenStream`s represent syntactic objects before they are converted into ASTs.
+//! A `TokenStream` is, roughly speaking, a sequence of [`TokenTree`]s,
+//! which are themselves a single [`Token`] or a `Delimited` subsequence of tokens.
+//!
+//! ## Ownership
+//!
+//! `TokenStream`s are persistent data structures constructed as ropes with reference
+//! counted-children. In general, this means that calling an operation on a `TokenStream`
+//! (such as `slice`) produces an entirely new `TokenStream` from the borrowed reference to
+//! the original. This essentially coerces `TokenStream`s into "views" of their subparts,
+//! and a borrowed `TokenStream` is sufficient to build an owned `TokenStream` without taking
+//! ownership of the original.
+
+use crate::ast::StmtKind;
+use crate::ast_traits::{HasAttrs, HasSpan, HasTokens};
+use crate::token::{self, Delimiter, Nonterminal, Token, TokenKind};
+use crate::AttrVec;
+
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::{self, Lrc};
+use rustc_macros::HashStable_Generic;
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+use rustc_span::{Span, DUMMY_SP};
+use smallvec::{smallvec, SmallVec};
+
+use std::{fmt, iter};
+
+/// When the main Rust parser encounters a syntax-extension invocation, it
+/// parses the arguments to the invocation as a token tree. This is a very
+/// loose structure, such that all sorts of different AST fragments can
+/// be passed to syntax extensions using a uniform type.
+///
+/// If the syntax extension is an MBE macro, it will attempt to match its
+/// LHS token tree against the provided token tree, and if it finds a
+/// match, will transcribe the RHS token tree, splicing in any captured
+/// `macro_parser::matched_nonterminals` into the `SubstNt`s it finds.
+///
+/// The RHS of an MBE macro is the only place `SubstNt`s are substituted.
+/// Nothing special happens to misnamed or misplaced `SubstNt`s.
+#[derive(Debug, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
+pub enum TokenTree {
+    /// A single token.
+    Token(Token, Spacing),
+    /// A delimited sequence of token trees.
+    Delimited(DelimSpan, Delimiter, TokenStream),
+}
+
+// This type is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+rustc_data_structures::static_assert_size!(TokenTree, 32);
+
+// Ensure all fields of `TokenTree` is `Send` and `Sync`.
+#[cfg(parallel_compiler)]
+fn _dummy()
+where
+    Token: Send + Sync,
+    DelimSpan: Send + Sync,
+    Delimiter: Send + Sync,
+    TokenStream: Send + Sync,
+{
+}
+
+impl TokenTree {
+    /// Checks if this `TokenTree` is equal to the other, regardless of span information.
+    pub fn eq_unspanned(&self, other: &TokenTree) -> bool {
+        match (self, other) {
+            (TokenTree::Token(token, _), TokenTree::Token(token2, _)) => token.kind == token2.kind,
+            (TokenTree::Delimited(_, delim, tts), TokenTree::Delimited(_, delim2, tts2)) => {
+                delim == delim2 && tts.eq_unspanned(&tts2)
+            }
+            _ => false,
+        }
+    }
+
+    /// Retrieves the `TokenTree`'s span.
+    pub fn span(&self) -> Span {
+        match self {
+            TokenTree::Token(token, _) => token.span,
+            TokenTree::Delimited(sp, ..) => sp.entire(),
+        }
+    }
+
+    /// Modify the `TokenTree`'s span in-place.
+    pub fn set_span(&mut self, span: Span) {
+        match self {
+            TokenTree::Token(token, _) => token.span = span,
+            TokenTree::Delimited(dspan, ..) => *dspan = DelimSpan::from_single(span),
+        }
+    }
+
+    // Create a `TokenTree::Token` with alone spacing.
+    pub fn token_alone(kind: TokenKind, span: Span) -> TokenTree {
+        TokenTree::Token(Token::new(kind, span), Spacing::Alone)
+    }
+
+    // Create a `TokenTree::Token` with joint spacing.
+    pub fn token_joint(kind: TokenKind, span: Span) -> TokenTree {
+        TokenTree::Token(Token::new(kind, span), Spacing::Joint)
+    }
+
+    pub fn uninterpolate(self) -> TokenTree {
+        match self {
+            TokenTree::Token(token, spacing) => {
+                TokenTree::Token(token.uninterpolate().into_owned(), spacing)
+            }
+            tt => tt,
+        }
+    }
+}
+
+impl<CTX> HashStable<CTX> for TokenStream
+where
+    CTX: crate::HashStableContext,
+{
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
+        for sub_tt in self.trees() {
+            sub_tt.hash_stable(hcx, hasher);
+        }
+    }
+}
+
+pub trait CreateTokenStream: sync::Send + sync::Sync {
+    fn create_token_stream(&self) -> AttrAnnotatedTokenStream;
+}
+
+impl CreateTokenStream for AttrAnnotatedTokenStream {
+    fn create_token_stream(&self) -> AttrAnnotatedTokenStream {
+        self.clone()
+    }
+}
+
+/// A lazy version of [`TokenStream`], which defers creation
+/// of an actual `TokenStream` until it is needed.
+/// `Box` is here only to reduce the structure size.
+#[derive(Clone)]
+pub struct LazyTokenStream(Lrc<Box<dyn CreateTokenStream>>);
+
+impl LazyTokenStream {
+    pub fn new(inner: impl CreateTokenStream + 'static) -> LazyTokenStream {
+        LazyTokenStream(Lrc::new(Box::new(inner)))
+    }
+
+    pub fn create_token_stream(&self) -> AttrAnnotatedTokenStream {
+        self.0.create_token_stream()
+    }
+}
+
+impl fmt::Debug for LazyTokenStream {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "LazyTokenStream({:?})", self.create_token_stream())
+    }
+}
+
+impl<S: Encoder> Encodable<S> for LazyTokenStream {
+    fn encode(&self, s: &mut S) {
+        // Used by AST json printing.
+        Encodable::encode(&self.create_token_stream(), s);
+    }
+}
+
+impl<D: Decoder> Decodable<D> for LazyTokenStream {
+    fn decode(_d: &mut D) -> Self {
+        panic!("Attempted to decode LazyTokenStream");
+    }
+}
+
+impl<CTX> HashStable<CTX> for LazyTokenStream {
+    fn hash_stable(&self, _hcx: &mut CTX, _hasher: &mut StableHasher) {
+        panic!("Attempted to compute stable hash for LazyTokenStream");
+    }
+}
+
+/// A `AttrAnnotatedTokenStream` is similar to a `TokenStream`, but with extra
+/// information about the tokens for attribute targets. This is used
+/// during expansion to perform early cfg-expansion, and to process attributes
+/// during proc-macro invocations.
+#[derive(Clone, Debug, Default, Encodable, Decodable)]
+pub struct AttrAnnotatedTokenStream(pub Lrc<Vec<(AttrAnnotatedTokenTree, Spacing)>>);
+
+/// Like `TokenTree`, but for `AttrAnnotatedTokenStream`
+#[derive(Clone, Debug, Encodable, Decodable)]
+pub enum AttrAnnotatedTokenTree {
+    Token(Token),
+    Delimited(DelimSpan, Delimiter, AttrAnnotatedTokenStream),
+    /// Stores the attributes for an attribute target,
+    /// along with the tokens for that attribute target.
+    /// See `AttributesData` for more information
+    Attributes(AttributesData),
+}
+
+impl AttrAnnotatedTokenStream {
+    pub fn new(tokens: Vec<(AttrAnnotatedTokenTree, Spacing)>) -> AttrAnnotatedTokenStream {
+        AttrAnnotatedTokenStream(Lrc::new(tokens))
+    }
+
+    /// Converts this `AttrAnnotatedTokenStream` to a plain `TokenStream
+    /// During conversion, `AttrAnnotatedTokenTree::Attributes` get 'flattened'
+    /// back to a `TokenStream` of the form `outer_attr attr_target`.
+    /// If there are inner attributes, they are inserted into the proper
+    /// place in the attribute target tokens.
+    pub fn to_tokenstream(&self) -> TokenStream {
+        let trees: Vec<_> = self
+            .0
+            .iter()
+            .flat_map(|tree| match &tree.0 {
+                AttrAnnotatedTokenTree::Token(inner) => {
+                    smallvec![TokenTree::Token(inner.clone(), tree.1)].into_iter()
+                }
+                AttrAnnotatedTokenTree::Delimited(span, delim, stream) => {
+                    smallvec![TokenTree::Delimited(*span, *delim, stream.to_tokenstream()),]
+                        .into_iter()
+                }
+                AttrAnnotatedTokenTree::Attributes(data) => {
+                    let mut outer_attrs = Vec::new();
+                    let mut inner_attrs = Vec::new();
+                    for attr in &data.attrs {
+                        match attr.style {
+                            crate::AttrStyle::Outer => {
+                                outer_attrs.push(attr);
+                            }
+                            crate::AttrStyle::Inner => {
+                                inner_attrs.push(attr);
+                            }
+                        }
+                    }
+
+                    let mut target_tokens: Vec<_> = data
+                        .tokens
+                        .create_token_stream()
+                        .to_tokenstream()
+                        .0
+                        .iter()
+                        .cloned()
+                        .collect();
+                    if !inner_attrs.is_empty() {
+                        let mut found = false;
+                        // Check the last two trees (to account for a trailing semi)
+                        for tree in target_tokens.iter_mut().rev().take(2) {
+                            if let TokenTree::Delimited(span, delim, delim_tokens) = tree {
+                                // Inner attributes are only supported on extern blocks, functions, impls,
+                                // and modules. All of these have their inner attributes placed at
+                                // the beginning of the rightmost outermost braced group:
+                                // e.g. fn foo() { #![my_attr} }
+                                //
+                                // Therefore, we can insert them back into the right location
+                                // without needing to do any extra position tracking.
+                                //
+                                // Note: Outline modules are an exception - they can
+                                // have attributes like `#![my_attr]` at the start of a file.
+                                // Support for custom attributes in this position is not
+                                // properly implemented - we always synthesize fake tokens,
+                                // so we never reach this code.
+
+                                let mut builder = TokenStreamBuilder::new();
+                                for inner_attr in inner_attrs {
+                                    builder.push(inner_attr.tokens().to_tokenstream());
+                                }
+                                builder.push(delim_tokens.clone());
+                                *tree = TokenTree::Delimited(*span, *delim, builder.build());
+                                found = true;
+                                break;
+                            }
+                        }
+
+                        assert!(
+                            found,
+                            "Failed to find trailing delimited group in: {:?}",
+                            target_tokens
+                        );
+                    }
+                    let mut flat: SmallVec<[_; 1]> = SmallVec::new();
+                    for attr in outer_attrs {
+                        // FIXME: Make this more efficient
+                        flat.extend(attr.tokens().to_tokenstream().0.clone().iter().cloned());
+                    }
+                    flat.extend(target_tokens);
+                    flat.into_iter()
+                }
+            })
+            .collect();
+        TokenStream::new(trees)
+    }
+}
+
+/// Stores the tokens for an attribute target, along
+/// with its attributes.
+///
+/// This is constructed during parsing when we need to capture
+/// tokens.
+///
+/// For example, `#[cfg(FALSE)] struct Foo {}` would
+/// have an `attrs` field containing the `#[cfg(FALSE)]` attr,
+/// and a `tokens` field storing the (unparsed) tokens `struct Foo {}`
+#[derive(Clone, Debug, Encodable, Decodable)]
+pub struct AttributesData {
+    /// Attributes, both outer and inner.
+    /// These are stored in the original order that they were parsed in.
+    pub attrs: AttrVec,
+    /// The underlying tokens for the attribute target that `attrs`
+    /// are applied to
+    pub tokens: LazyTokenStream,
+}
+
+/// A `TokenStream` is an abstract sequence of tokens, organized into [`TokenTree`]s.
+///
+/// The goal is for procedural macros to work with `TokenStream`s and `TokenTree`s
+/// instead of a representation of the abstract syntax tree.
+/// Today's `TokenTree`s can still contain AST via `token::Interpolated` for
+/// backwards compatibility.
+#[derive(Clone, Debug, Default, Encodable, Decodable)]
+pub struct TokenStream(pub(crate) Lrc<Vec<TokenTree>>);
+
+// `TokenStream` is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+rustc_data_structures::static_assert_size!(TokenStream, 8);
+
+#[derive(Clone, Copy, Debug, PartialEq, Encodable, Decodable, HashStable_Generic)]
+pub enum Spacing {
+    Alone,
+    Joint,
+}
+
+impl TokenStream {
+    /// Given a `TokenStream` with a `Stream` of only two arguments, return a new `TokenStream`
+    /// separating the two arguments with a comma for diagnostic suggestions.
+    pub fn add_comma(&self) -> Option<(TokenStream, Span)> {
+        // Used to suggest if a user writes `foo!(a b);`
+        let mut suggestion = None;
+        let mut iter = self.0.iter().enumerate().peekable();
+        while let Some((pos, ts)) = iter.next() {
+            if let Some((_, next)) = iter.peek() {
+                let sp = match (&ts, &next) {
+                    (_, TokenTree::Token(Token { kind: token::Comma, .. }, _)) => continue,
+                    (
+                        TokenTree::Token(token_left, Spacing::Alone),
+                        TokenTree::Token(token_right, _),
+                    ) if ((token_left.is_ident() && !token_left.is_reserved_ident())
+                        || token_left.is_lit())
+                        && ((token_right.is_ident() && !token_right.is_reserved_ident())
+                            || token_right.is_lit()) =>
+                    {
+                        token_left.span
+                    }
+                    (TokenTree::Delimited(sp, ..), _) => sp.entire(),
+                    _ => continue,
+                };
+                let sp = sp.shrink_to_hi();
+                let comma = TokenTree::token_alone(token::Comma, sp);
+                suggestion = Some((pos, comma, sp));
+            }
+        }
+        if let Some((pos, comma, sp)) = suggestion {
+            let mut new_stream = Vec::with_capacity(self.0.len() + 1);
+            let parts = self.0.split_at(pos + 1);
+            new_stream.extend_from_slice(parts.0);
+            new_stream.push(comma);
+            new_stream.extend_from_slice(parts.1);
+            return Some((TokenStream::new(new_stream), sp));
+        }
+        None
+    }
+}
+
+impl From<(AttrAnnotatedTokenTree, Spacing)> for AttrAnnotatedTokenStream {
+    fn from((tree, spacing): (AttrAnnotatedTokenTree, Spacing)) -> AttrAnnotatedTokenStream {
+        AttrAnnotatedTokenStream::new(vec![(tree, spacing)])
+    }
+}
+
+impl iter::FromIterator<TokenTree> for TokenStream {
+    fn from_iter<I: IntoIterator<Item = TokenTree>>(iter: I) -> Self {
+        TokenStream::new(iter.into_iter().collect::<Vec<TokenTree>>())
+    }
+}
+
+impl Eq for TokenStream {}
+
+impl PartialEq<TokenStream> for TokenStream {
+    fn eq(&self, other: &TokenStream) -> bool {
+        self.trees().eq(other.trees())
+    }
+}
+
+impl TokenStream {
+    pub fn new(streams: Vec<TokenTree>) -> TokenStream {
+        TokenStream(Lrc::new(streams))
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    pub fn trees(&self) -> CursorRef<'_> {
+        CursorRef::new(self)
+    }
+
+    pub fn into_trees(self) -> Cursor {
+        Cursor::new(self)
+    }
+
+    /// Compares two `TokenStream`s, checking equality without regarding span information.
+    pub fn eq_unspanned(&self, other: &TokenStream) -> bool {
+        let mut t1 = self.trees();
+        let mut t2 = other.trees();
+        for (t1, t2) in iter::zip(&mut t1, &mut t2) {
+            if !t1.eq_unspanned(&t2) {
+                return false;
+            }
+        }
+        t1.next().is_none() && t2.next().is_none()
+    }
+
+    pub fn map_enumerated<F: FnMut(usize, &TokenTree) -> TokenTree>(self, mut f: F) -> TokenStream {
+        TokenStream(Lrc::new(self.0.iter().enumerate().map(|(i, tree)| f(i, tree)).collect()))
+    }
+
+    fn opt_from_ast(node: &(impl HasAttrs + HasTokens)) -> Option<TokenStream> {
+        let tokens = node.tokens()?;
+        let attrs = node.attrs();
+        let attr_annotated = if attrs.is_empty() {
+            tokens.create_token_stream()
+        } else {
+            let attr_data = AttributesData { attrs: attrs.to_vec().into(), tokens: tokens.clone() };
+            AttrAnnotatedTokenStream::new(vec![(
+                AttrAnnotatedTokenTree::Attributes(attr_data),
+                Spacing::Alone,
+            )])
+        };
+        Some(attr_annotated.to_tokenstream())
+    }
+
+    // Create a token stream containing a single token with alone spacing.
+    pub fn token_alone(kind: TokenKind, span: Span) -> TokenStream {
+        TokenStream::new(vec![TokenTree::token_alone(kind, span)])
+    }
+
+    // Create a token stream containing a single token with joint spacing.
+    pub fn token_joint(kind: TokenKind, span: Span) -> TokenStream {
+        TokenStream::new(vec![TokenTree::token_joint(kind, span)])
+    }
+
+    // Create a token stream containing a single `Delimited`.
+    pub fn delimited(span: DelimSpan, delim: Delimiter, tts: TokenStream) -> TokenStream {
+        TokenStream::new(vec![TokenTree::Delimited(span, delim, tts)])
+    }
+
+    pub fn from_ast(node: &(impl HasAttrs + HasSpan + HasTokens + fmt::Debug)) -> TokenStream {
+        TokenStream::opt_from_ast(node)
+            .unwrap_or_else(|| panic!("missing tokens for node at {:?}: {:?}", node.span(), node))
+    }
+
+    pub fn from_nonterminal_ast(nt: &Nonterminal) -> TokenStream {
+        match nt {
+            Nonterminal::NtIdent(ident, is_raw) => {
+                TokenStream::token_alone(token::Ident(ident.name, *is_raw), ident.span)
+            }
+            Nonterminal::NtLifetime(ident) => {
+                TokenStream::token_alone(token::Lifetime(ident.name), ident.span)
+            }
+            Nonterminal::NtItem(item) => TokenStream::from_ast(item),
+            Nonterminal::NtBlock(block) => TokenStream::from_ast(block),
+            Nonterminal::NtStmt(stmt) if let StmtKind::Empty = stmt.kind => {
+                // FIXME: Properly collect tokens for empty statements.
+                TokenStream::token_alone(token::Semi, stmt.span)
+            }
+            Nonterminal::NtStmt(stmt) => TokenStream::from_ast(stmt),
+            Nonterminal::NtPat(pat) => TokenStream::from_ast(pat),
+            Nonterminal::NtTy(ty) => TokenStream::from_ast(ty),
+            Nonterminal::NtMeta(attr) => TokenStream::from_ast(attr),
+            Nonterminal::NtPath(path) => TokenStream::from_ast(path),
+            Nonterminal::NtVis(vis) => TokenStream::from_ast(vis),
+            Nonterminal::NtExpr(expr) | Nonterminal::NtLiteral(expr) => TokenStream::from_ast(expr),
+        }
+    }
+
+    fn flatten_token(token: &Token, spacing: Spacing) -> TokenTree {
+        match &token.kind {
+            token::Interpolated(nt) if let token::NtIdent(ident, is_raw) = **nt => {
+                TokenTree::Token(Token::new(token::Ident(ident.name, is_raw), ident.span), spacing)
+            }
+            token::Interpolated(nt) => TokenTree::Delimited(
+                DelimSpan::from_single(token.span),
+                Delimiter::Invisible,
+                TokenStream::from_nonterminal_ast(&nt).flattened(),
+            ),
+            _ => TokenTree::Token(token.clone(), spacing),
+        }
+    }
+
+    fn flatten_token_tree(tree: &TokenTree) -> TokenTree {
+        match tree {
+            TokenTree::Token(token, spacing) => TokenStream::flatten_token(token, *spacing),
+            TokenTree::Delimited(span, delim, tts) => {
+                TokenTree::Delimited(*span, *delim, tts.flattened())
+            }
+        }
+    }
+
+    #[must_use]
+    pub fn flattened(&self) -> TokenStream {
+        fn can_skip(stream: &TokenStream) -> bool {
+            stream.trees().all(|tree| match tree {
+                TokenTree::Token(token, _) => !matches!(token.kind, token::Interpolated(_)),
+                TokenTree::Delimited(_, _, inner) => can_skip(inner),
+            })
+        }
+
+        if can_skip(self) {
+            return self.clone();
+        }
+
+        self.trees().map(|tree| TokenStream::flatten_token_tree(tree)).collect()
+    }
+}
+
+// 99.5%+ of the time we have 1 or 2 elements in this vector.
+#[derive(Clone)]
+pub struct TokenStreamBuilder(SmallVec<[TokenStream; 2]>);
+
+impl TokenStreamBuilder {
+    pub fn new() -> TokenStreamBuilder {
+        TokenStreamBuilder(SmallVec::new())
+    }
+
+    pub fn push(&mut self, stream: TokenStream) {
+        self.0.push(stream);
+    }
+
+    pub fn build(self) -> TokenStream {
+        let mut streams = self.0;
+        match streams.len() {
+            0 => TokenStream::default(),
+            1 => streams.pop().unwrap(),
+            _ => {
+                // We will extend the first stream in `streams` with the
+                // elements from the subsequent streams. This requires using
+                // `make_mut()` on the first stream, and in practice this
+                // doesn't cause cloning 99.9% of the time.
+                //
+                // One very common use case is when `streams` has two elements,
+                // where the first stream has any number of elements within
+                // (often 1, but sometimes many more) and the second stream has
+                // a single element within.
+
+                // Determine how much the first stream will be extended.
+                // Needed to avoid quadratic blow up from on-the-fly
+                // reallocations (#57735).
+                let num_appends = streams.iter().skip(1).map(|ts| ts.len()).sum();
+
+                // Get the first stream, which will become the result stream.
+                // If it's `None`, create an empty stream.
+                let mut iter = streams.drain(..);
+                let mut res_stream_lrc = iter.next().unwrap().0;
+
+                // Append the subsequent elements to the result stream, after
+                // reserving space for them.
+                let res_vec_mut = Lrc::make_mut(&mut res_stream_lrc);
+                res_vec_mut.reserve(num_appends);
+                for stream in iter {
+                    let stream_iter = stream.0.iter().cloned();
+
+                    // If (a) `res_mut_vec` is not empty and the last tree
+                    // within it is a token tree marked with `Joint`, and (b)
+                    // `stream` is not empty and the first tree within it is a
+                    // token tree, and (c) the two tokens can be glued
+                    // together...
+                    if let Some(TokenTree::Token(last_tok, Spacing::Joint)) = res_vec_mut.last()
+                        && let Some(TokenTree::Token(tok, spacing)) = stream.0.first()
+                        && let Some(glued_tok) = last_tok.glue(&tok)
+                    {
+                        // ...then overwrite the last token tree in
+                        // `res_vec_mut` with the glued token, and skip the
+                        // first token tree from `stream`.
+                        *res_vec_mut.last_mut().unwrap() = TokenTree::Token(glued_tok, *spacing);
+                        res_vec_mut.extend(stream_iter.skip(1));
+                    } else {
+                        // Append all of `stream`.
+                        res_vec_mut.extend(stream_iter);
+                    }
+                }
+
+                TokenStream(res_stream_lrc)
+            }
+        }
+    }
+}
+
+/// By-reference iterator over a [`TokenStream`].
+#[derive(Clone)]
+pub struct CursorRef<'t> {
+    stream: &'t TokenStream,
+    index: usize,
+}
+
+impl<'t> CursorRef<'t> {
+    fn new(stream: &'t TokenStream) -> Self {
+        CursorRef { stream, index: 0 }
+    }
+
+    pub fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
+        self.stream.0.get(self.index + n)
+    }
+}
+
+impl<'t> Iterator for CursorRef<'t> {
+    type Item = &'t TokenTree;
+
+    fn next(&mut self) -> Option<&'t TokenTree> {
+        self.stream.0.get(self.index).map(|tree| {
+            self.index += 1;
+            tree
+        })
+    }
+}
+
+/// Owning by-value iterator over a [`TokenStream`].
+// FIXME: Many uses of this can be replaced with by-reference iterator to avoid clones.
+#[derive(Clone)]
+pub struct Cursor {
+    pub stream: TokenStream,
+    index: usize,
+}
+
+impl Iterator for Cursor {
+    type Item = TokenTree;
+
+    fn next(&mut self) -> Option<TokenTree> {
+        self.stream.0.get(self.index).map(|tree| {
+            self.index += 1;
+            tree.clone()
+        })
+    }
+}
+
+impl Cursor {
+    fn new(stream: TokenStream) -> Self {
+        Cursor { stream, index: 0 }
+    }
+
+    #[inline]
+    pub fn next_ref(&mut self) -> Option<&TokenTree> {
+        self.stream.0.get(self.index).map(|tree| {
+            self.index += 1;
+            tree
+        })
+    }
+
+    pub fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
+        self.stream.0.get(self.index + n)
+    }
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
+pub struct DelimSpan {
+    pub open: Span,
+    pub close: Span,
+}
+
+impl DelimSpan {
+    pub fn from_single(sp: Span) -> Self {
+        DelimSpan { open: sp, close: sp }
+    }
+
+    pub fn from_pair(open: Span, close: Span) -> Self {
+        DelimSpan { open, close }
+    }
+
+    pub fn dummy() -> Self {
+        Self::from_single(DUMMY_SP)
+    }
+
+    pub fn entire(self) -> Span {
+        self.open.with_hi(self.close.hi())
+    }
+}
diff --git a/compiler/rustc_ast/src/util/classify.rs b/compiler/rustc_ast/src/util/classify.rs
new file mode 100644
index 000000000..6ea3db6d3
--- /dev/null
+++ b/compiler/rustc_ast/src/util/classify.rs
@@ -0,0 +1,52 @@
+//! Routines the parser uses to classify AST nodes
+
+// Predicates on exprs and stmts that the pretty-printer and parser use
+
+use crate::ast;
+
+/// Does this expression require a semicolon to be treated
+/// as a statement? The negation of this: 'can this expression
+/// be used as a statement without a semicolon' -- is used
+/// as an early-bail-out in the parser so that, for instance,
+///     if true {...} else {...}
+///      |x| 5
+/// isn't parsed as (if true {...} else {...} | x) | 5
+pub fn expr_requires_semi_to_be_stmt(e: &ast::Expr) -> bool {
+    !matches!(
+        e.kind,
+        ast::ExprKind::If(..)
+            | ast::ExprKind::Match(..)
+            | ast::ExprKind::Block(..)
+            | ast::ExprKind::While(..)
+            | ast::ExprKind::Loop(..)
+            | ast::ExprKind::ForLoop(..)
+            | ast::ExprKind::TryBlock(..)
+    )
+}
+
+/// If an expression ends with `}`, returns the innermost expression ending in the `}`
+pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<&ast::Expr> {
+    use ast::ExprKind::*;
+
+    loop {
+        match &expr.kind {
+            AddrOf(_, _, e)
+            | Assign(_, e, _)
+            | AssignOp(_, _, e)
+            | Binary(_, _, e)
+            | Box(e)
+            | Break(_, Some(e))
+            | Closure(.., e, _)
+            | Let(_, e, _)
+            | Range(_, Some(e), _)
+            | Ret(Some(e))
+            | Unary(_, e)
+            | Yield(Some(e)) => {
+                expr = e;
+            }
+            Async(..) | Block(..) | ForLoop(..) | If(..) | Loop(..) | Match(..) | Struct(..)
+            | TryBlock(..) | While(..) => break Some(expr),
+            _ => break None,
+        }
+    }
+}
diff --git a/compiler/rustc_ast/src/util/comments.rs b/compiler/rustc_ast/src/util/comments.rs
new file mode 100644
index 000000000..c96474ccb
--- /dev/null
+++ b/compiler/rustc_ast/src/util/comments.rs
@@ -0,0 +1,255 @@
+use crate::token::CommentKind;
+use rustc_span::source_map::SourceMap;
+use rustc_span::{BytePos, CharPos, FileName, Pos, Symbol};
+
+#[cfg(test)]
+mod tests;
+
+#[derive(Clone, Copy, PartialEq, Debug)]
+pub enum CommentStyle {
+    /// No code on either side of each line of the comment
+    Isolated,
+    /// Code exists to the left of the comment
+    Trailing,
+    /// Code before /* foo */ and after the comment
+    Mixed,
+    /// Just a manual blank line "\n\n", for layout
+    BlankLine,
+}
+
+#[derive(Clone)]
+pub struct Comment {
+    pub style: CommentStyle,
+    pub lines: Vec<String>,
+    pub pos: BytePos,
+}
+
+/// A fast conservative estimate on whether the string can contain documentation links.
+/// A pair of square brackets `[]` must exist in the string, but we only search for the
+/// opening bracket because brackets always go in pairs in practice.
+#[inline]
+pub fn may_have_doc_links(s: &str) -> bool {
+    s.contains('[')
+}
+
+/// Makes a doc string more presentable to users.
+/// Used by rustdoc and perhaps other tools, but not by rustc.
+pub fn beautify_doc_string(data: Symbol, kind: CommentKind) -> Symbol {
+    fn get_vertical_trim(lines: &[&str]) -> Option<(usize, usize)> {
+        let mut i = 0;
+        let mut j = lines.len();
+        // first line of all-stars should be omitted
+        if !lines.is_empty() && lines[0].chars().all(|c| c == '*') {
+            i += 1;
+        }
+
+        // like the first, a last line of all stars should be omitted
+        if j > i && !lines[j - 1].is_empty() && lines[j - 1].chars().all(|c| c == '*') {
+            j -= 1;
+        }
+
+        if i != 0 || j != lines.len() { Some((i, j)) } else { None }
+    }
+
+    fn get_horizontal_trim<'a>(lines: &'a [&str], kind: CommentKind) -> Option<String> {
+        let mut i = usize::MAX;
+        let mut first = true;
+
+        // In case we have doc comments like `/**` or `/*!`, we want to remove stars if they are
+        // present. However, we first need to strip the empty lines so they don't get in the middle
+        // when we try to compute the "horizontal trim".
+        let lines = if kind == CommentKind::Block {
+            // Whatever happens, we skip the first line.
+            let mut i = lines
+                .get(0)
+                .map(|l| if l.trim_start().starts_with('*') { 0 } else { 1 })
+                .unwrap_or(0);
+            let mut j = lines.len();
+
+            while i < j && lines[i].trim().is_empty() {
+                i += 1;
+            }
+            while j > i && lines[j - 1].trim().is_empty() {
+                j -= 1;
+            }
+            &lines[i..j]
+        } else {
+            lines
+        };
+
+        for line in lines {
+            for (j, c) in line.chars().enumerate() {
+                if j > i || !"* \t".contains(c) {
+                    return None;
+                }
+                if c == '*' {
+                    if first {
+                        i = j;
+                        first = false;
+                    } else if i != j {
+                        return None;
+                    }
+                    break;
+                }
+            }
+            if i >= line.len() {
+                return None;
+            }
+        }
+        if lines.is_empty() { None } else { Some(lines[0][..i].into()) }
+    }
+
+    let data_s = data.as_str();
+    if data_s.contains('\n') {
+        let mut lines = data_s.lines().collect::<Vec<&str>>();
+        let mut changes = false;
+        let lines = if let Some((i, j)) = get_vertical_trim(&lines) {
+            changes = true;
+            // remove whitespace-only lines from the start/end of lines
+            &mut lines[i..j]
+        } else {
+            &mut lines
+        };
+        if let Some(horizontal) = get_horizontal_trim(&lines, kind) {
+            changes = true;
+            // remove a "[ \t]*\*" block from each line, if possible
+            for line in lines.iter_mut() {
+                if let Some(tmp) = line.strip_prefix(&horizontal) {
+                    *line = tmp;
+                    if kind == CommentKind::Block
+                        && (*line == "*" || line.starts_with("* ") || line.starts_with("**"))
+                    {
+                        *line = &line[1..];
+                    }
+                }
+            }
+        }
+        if changes {
+            return Symbol::intern(&lines.join("\n"));
+        }
+    }
+    data
+}
+
+/// Returns `None` if the first `col` chars of `s` contain a non-whitespace char.
+/// Otherwise returns `Some(k)` where `k` is first char offset after that leading
+/// whitespace. Note that `k` may be outside bounds of `s`.
+fn all_whitespace(s: &str, col: CharPos) -> Option<usize> {
+    let mut idx = 0;
+    for (i, ch) in s.char_indices().take(col.to_usize()) {
+        if !ch.is_whitespace() {
+            return None;
+        }
+        idx = i + ch.len_utf8();
+    }
+    Some(idx)
+}
+
+fn trim_whitespace_prefix(s: &str, col: CharPos) -> &str {
+    let len = s.len();
+    match all_whitespace(&s, col) {
+        Some(col) => {
+            if col < len {
+                &s[col..]
+            } else {
+                ""
+            }
+        }
+        None => s,
+    }
+}
+
+fn split_block_comment_into_lines(text: &str, col: CharPos) -> Vec<String> {
+    let mut res: Vec<String> = vec![];
+    let mut lines = text.lines();
+    // just push the first line
+    res.extend(lines.next().map(|it| it.to_string()));
+    // for other lines, strip common whitespace prefix
+    for line in lines {
+        res.push(trim_whitespace_prefix(line, col).to_string())
+    }
+    res
+}
+
+// it appears this function is called only from pprust... that's
+// probably not a good thing.
+pub fn gather_comments(sm: &SourceMap, path: FileName, src: String) -> Vec<Comment> {
+    let sm = SourceMap::new(sm.path_mapping().clone());
+    let source_file = sm.new_source_file(path, src);
+    let text = (*source_file.src.as_ref().unwrap()).clone();
+
+    let text: &str = text.as_str();
+    let start_bpos = source_file.start_pos;
+    let mut pos = 0;
+    let mut comments: Vec<Comment> = Vec::new();
+    let mut code_to_the_left = false;
+
+    if let Some(shebang_len) = rustc_lexer::strip_shebang(text) {
+        comments.push(Comment {
+            style: CommentStyle::Isolated,
+            lines: vec![text[..shebang_len].to_string()],
+            pos: start_bpos,
+        });
+        pos += shebang_len;
+    }
+
+    for token in rustc_lexer::tokenize(&text[pos..]) {
+        let token_text = &text[pos..pos + token.len as usize];
+        match token.kind {
+            rustc_lexer::TokenKind::Whitespace => {
+                if let Some(mut idx) = token_text.find('\n') {
+                    code_to_the_left = false;
+                    while let Some(next_newline) = &token_text[idx + 1..].find('\n') {
+                        idx += 1 + next_newline;
+                        comments.push(Comment {
+                            style: CommentStyle::BlankLine,
+                            lines: vec![],
+                            pos: start_bpos + BytePos((pos + idx) as u32),
+                        });
+                    }
+                }
+            }
+            rustc_lexer::TokenKind::BlockComment { doc_style, .. } => {
+                if doc_style.is_none() {
+                    let code_to_the_right = !matches!(
+                        text[pos + token.len as usize..].chars().next(),
+                        Some('\r' | '\n')
+                    );
+                    let style = match (code_to_the_left, code_to_the_right) {
+                        (_, true) => CommentStyle::Mixed,
+                        (false, false) => CommentStyle::Isolated,
+                        (true, false) => CommentStyle::Trailing,
+                    };
+
+                    // Count the number of chars since the start of the line by rescanning.
+                    let pos_in_file = start_bpos + BytePos(pos as u32);
+                    let line_begin_in_file = source_file.line_begin_pos(pos_in_file);
+                    let line_begin_pos = (line_begin_in_file - start_bpos).to_usize();
+                    let col = CharPos(text[line_begin_pos..pos].chars().count());
+
+                    let lines = split_block_comment_into_lines(token_text, col);
+                    comments.push(Comment { style, lines, pos: pos_in_file })
+                }
+            }
+            rustc_lexer::TokenKind::LineComment { doc_style } => {
+                if doc_style.is_none() {
+                    comments.push(Comment {
+                        style: if code_to_the_left {
+                            CommentStyle::Trailing
+                        } else {
+                            CommentStyle::Isolated
+                        },
+                        lines: vec![token_text.to_string()],
+                        pos: start_bpos + BytePos(pos as u32),
+                    })
+                }
+            }
+            _ => {
+                code_to_the_left = true;
+            }
+        }
+        pos += token.len as usize;
+    }
+
+    comments
+}
diff --git a/compiler/rustc_ast/src/util/comments/tests.rs b/compiler/rustc_ast/src/util/comments/tests.rs
new file mode 100644
index 000000000..11d50603a
--- /dev/null
+++ b/compiler/rustc_ast/src/util/comments/tests.rs
@@ -0,0 +1,61 @@
+use super::*;
+use rustc_span::create_default_session_globals_then;
+
+#[test]
+fn test_block_doc_comment_1() {
+    create_default_session_globals_then(|| {
+        let comment = "\n * Test \n **  Test\n *   Test\n";
+        let stripped = beautify_doc_string(Symbol::intern(comment), CommentKind::Block);
+        assert_eq!(stripped.as_str(), " Test \n*  Test\n   Test");
+    })
+}
+
+#[test]
+fn test_block_doc_comment_2() {
+    create_default_session_globals_then(|| {
+        let comment = "\n * Test\n *  Test\n";
+        let stripped = beautify_doc_string(Symbol::intern(comment), CommentKind::Block);
+        assert_eq!(stripped.as_str(), " Test\n  Test");
+    })
+}
+
+#[test]
+fn test_block_doc_comment_3() {
+    create_default_session_globals_then(|| {
+        let comment = "\n let a: *i32;\n *a = 5;\n";
+        let stripped = beautify_doc_string(Symbol::intern(comment), CommentKind::Block);
+        assert_eq!(stripped.as_str(), "let a: *i32;\n*a = 5;");
+    })
+}
+
+#[test]
+fn test_line_doc_comment() {
+    create_default_session_globals_then(|| {
+        let stripped = beautify_doc_string(Symbol::intern(" test"), CommentKind::Line);
+        assert_eq!(stripped.as_str(), " test");
+        let stripped = beautify_doc_string(Symbol::intern("! test"), CommentKind::Line);
+        assert_eq!(stripped.as_str(), "! test");
+        let stripped = beautify_doc_string(Symbol::intern("test"), CommentKind::Line);
+        assert_eq!(stripped.as_str(), "test");
+        let stripped = beautify_doc_string(Symbol::intern("!test"), CommentKind::Line);
+        assert_eq!(stripped.as_str(), "!test");
+    })
+}
+
+#[test]
+fn test_doc_blocks() {
+    create_default_session_globals_then(|| {
+        let stripped =
+            beautify_doc_string(Symbol::intern(" # Returns\n     *\n     "), CommentKind::Block);
+        assert_eq!(stripped.as_str(), " # Returns\n\n");
+
+        let stripped = beautify_doc_string(
+            Symbol::intern("\n     * # Returns\n     *\n     "),
+            CommentKind::Block,
+        );
+        assert_eq!(stripped.as_str(), " # Returns\n\n");
+
+        let stripped = beautify_doc_string(Symbol::intern("\n *     a\n "), CommentKind::Block);
+        assert_eq!(stripped.as_str(), "     a\n");
+    })
+}
diff --git a/compiler/rustc_ast/src/util/literal.rs b/compiler/rustc_ast/src/util/literal.rs
new file mode 100644
index 000000000..9c18f55c0
--- /dev/null
+++ b/compiler/rustc_ast/src/util/literal.rs
@@ -0,0 +1,336 @@
+//! Code related to parsing literals.
+
+use crate::ast::{self, Lit, LitKind};
+use crate::token::{self, Token};
+
+use rustc_lexer::unescape::{unescape_byte, unescape_char};
+use rustc_lexer::unescape::{unescape_byte_literal, unescape_literal, Mode};
+use rustc_span::symbol::{kw, sym, Symbol};
+use rustc_span::Span;
+
+use std::ascii;
+use tracing::debug;
+
+pub enum LitError {
+    NotLiteral,
+    LexerError,
+    InvalidSuffix,
+    InvalidIntSuffix,
+    InvalidFloatSuffix,
+    NonDecimalFloat(u32),
+    IntTooLarge,
+}
+
+impl LitKind {
+    /// Converts literal token into a semantic literal.
+    pub fn from_lit_token(lit: token::Lit) -> Result<LitKind, LitError> {
+        let token::Lit { kind, symbol, suffix } = lit;
+        if suffix.is_some() && !kind.may_have_suffix() {
+            return Err(LitError::InvalidSuffix);
+        }
+
+        Ok(match kind {
+            token::Bool => {
+                assert!(symbol.is_bool_lit());
+                LitKind::Bool(symbol == kw::True)
+            }
+            token::Byte => {
+                return unescape_byte(symbol.as_str())
+                    .map(LitKind::Byte)
+                    .map_err(|_| LitError::LexerError);
+            }
+            token::Char => {
+                return unescape_char(symbol.as_str())
+                    .map(LitKind::Char)
+                    .map_err(|_| LitError::LexerError);
+            }
+
+            // There are some valid suffixes for integer and float literals,
+            // so all the handling is done internally.
+            token::Integer => return integer_lit(symbol, suffix),
+            token::Float => return float_lit(symbol, suffix),
+
+            token::Str => {
+                // If there are no characters requiring special treatment we can
+                // reuse the symbol from the token. Otherwise, we must generate a
+                // new symbol because the string in the LitKind is different to the
+                // string in the token.
+                let s = symbol.as_str();
+                let symbol = if s.contains(&['\\', '\r']) {
+                    let mut buf = String::with_capacity(s.len());
+                    let mut error = Ok(());
+                    // Force-inlining here is aggressive but the closure is
+                    // called on every char in the string, so it can be
+                    // hot in programs with many long strings.
+                    unescape_literal(
+                        &s,
+                        Mode::Str,
+                        &mut #[inline(always)]
+                        |_, unescaped_char| match unescaped_char {
+                            Ok(c) => buf.push(c),
+                            Err(err) => {
+                                if err.is_fatal() {
+                                    error = Err(LitError::LexerError);
+                                }
+                            }
+                        },
+                    );
+                    error?;
+                    Symbol::intern(&buf)
+                } else {
+                    symbol
+                };
+                LitKind::Str(symbol, ast::StrStyle::Cooked)
+            }
+            token::StrRaw(n) => {
+                // Ditto.
+                let s = symbol.as_str();
+                let symbol =
+                    if s.contains('\r') {
+                        let mut buf = String::with_capacity(s.len());
+                        let mut error = Ok(());
+                        unescape_literal(&s, Mode::RawStr, &mut |_, unescaped_char| {
+                            match unescaped_char {
+                                Ok(c) => buf.push(c),
+                                Err(err) => {
+                                    if err.is_fatal() {
+                                        error = Err(LitError::LexerError);
+                                    }
+                                }
+                            }
+                        });
+                        error?;
+                        Symbol::intern(&buf)
+                    } else {
+                        symbol
+                    };
+                LitKind::Str(symbol, ast::StrStyle::Raw(n))
+            }
+            token::ByteStr => {
+                let s = symbol.as_str();
+                let mut buf = Vec::with_capacity(s.len());
+                let mut error = Ok(());
+                unescape_byte_literal(&s, Mode::ByteStr, &mut |_, unescaped_byte| {
+                    match unescaped_byte {
+                        Ok(c) => buf.push(c),
+                        Err(err) => {
+                            if err.is_fatal() {
+                                error = Err(LitError::LexerError);
+                            }
+                        }
+                    }
+                });
+                error?;
+                LitKind::ByteStr(buf.into())
+            }
+            token::ByteStrRaw(_) => {
+                let s = symbol.as_str();
+                let bytes = if s.contains('\r') {
+                    let mut buf = Vec::with_capacity(s.len());
+                    let mut error = Ok(());
+                    unescape_byte_literal(&s, Mode::RawByteStr, &mut |_, unescaped_byte| {
+                        match unescaped_byte {
+                            Ok(c) => buf.push(c),
+                            Err(err) => {
+                                if err.is_fatal() {
+                                    error = Err(LitError::LexerError);
+                                }
+                            }
+                        }
+                    });
+                    error?;
+                    buf
+                } else {
+                    symbol.to_string().into_bytes()
+                };
+
+                LitKind::ByteStr(bytes.into())
+            }
+            token::Err => LitKind::Err(symbol),
+        })
+    }
+
+    /// Attempts to recover a token from semantic literal.
+    /// This function is used when the original token doesn't exist (e.g. the literal is created
+    /// by an AST-based macro) or unavailable (e.g. from HIR pretty-printing).
+    pub fn to_lit_token(&self) -> token::Lit {
+        let (kind, symbol, suffix) = match *self {
+            LitKind::Str(symbol, ast::StrStyle::Cooked) => {
+                // Don't re-intern unless the escaped string is different.
+                let s = symbol.as_str();
+                let escaped = s.escape_default().to_string();
+                let symbol = if s == escaped { symbol } else { Symbol::intern(&escaped) };
+                (token::Str, symbol, None)
+            }
+            LitKind::Str(symbol, ast::StrStyle::Raw(n)) => (token::StrRaw(n), symbol, None),
+            LitKind::ByteStr(ref bytes) => {
+                let string = bytes
+                    .iter()
+                    .cloned()
+                    .flat_map(ascii::escape_default)
+                    .map(Into::<char>::into)
+                    .collect::<String>();
+                (token::ByteStr, Symbol::intern(&string), None)
+            }
+            LitKind::Byte(byte) => {
+                let string: String = ascii::escape_default(byte).map(Into::<char>::into).collect();
+                (token::Byte, Symbol::intern(&string), None)
+            }
+            LitKind::Char(ch) => {
+                let string: String = ch.escape_default().map(Into::<char>::into).collect();
+                (token::Char, Symbol::intern(&string), None)
+            }
+            LitKind::Int(n, ty) => {
+                let suffix = match ty {
+                    ast::LitIntType::Unsigned(ty) => Some(ty.name()),
+                    ast::LitIntType::Signed(ty) => Some(ty.name()),
+                    ast::LitIntType::Unsuffixed => None,
+                };
+                (token::Integer, sym::integer(n), suffix)
+            }
+            LitKind::Float(symbol, ty) => {
+                let suffix = match ty {
+                    ast::LitFloatType::Suffixed(ty) => Some(ty.name()),
+                    ast::LitFloatType::Unsuffixed => None,
+                };
+                (token::Float, symbol, suffix)
+            }
+            LitKind::Bool(value) => {
+                let symbol = if value { kw::True } else { kw::False };
+                (token::Bool, symbol, None)
+            }
+            LitKind::Err(symbol) => (token::Err, symbol, None),
+        };
+
+        token::Lit::new(kind, symbol, suffix)
+    }
+}
+
+impl Lit {
+    /// Converts literal token into an AST literal.
+    pub fn from_lit_token(token: token::Lit, span: Span) -> Result<Lit, LitError> {
+        Ok(Lit { token, kind: LitKind::from_lit_token(token)?, span })
+    }
+
+    /// Converts arbitrary token into an AST literal.
+    ///
+    /// Keep this in sync with `Token::can_begin_literal_or_bool` excluding unary negation.
+    pub fn from_token(token: &Token) -> Result<Lit, LitError> {
+        let lit = match token.uninterpolate().kind {
+            token::Ident(name, false) if name.is_bool_lit() => {
+                token::Lit::new(token::Bool, name, None)
+            }
+            token::Literal(lit) => lit,
+            token::Interpolated(ref nt) => {
+                if let token::NtExpr(expr) | token::NtLiteral(expr) = &**nt
+                    && let ast::ExprKind::Lit(lit) = &expr.kind
+                {
+                    return Ok(lit.clone());
+                }
+                return Err(LitError::NotLiteral);
+            }
+            _ => return Err(LitError::NotLiteral),
+        };
+
+        Lit::from_lit_token(lit, token.span)
+    }
+
+    /// Attempts to recover an AST literal from semantic literal.
+    /// This function is used when the original token doesn't exist (e.g. the literal is created
+    /// by an AST-based macro) or unavailable (e.g. from HIR pretty-printing).
+    pub fn from_lit_kind(kind: LitKind, span: Span) -> Lit {
+        Lit { token: kind.to_lit_token(), kind, span }
+    }
+
+    /// Losslessly convert an AST literal into a token.
+    pub fn to_token(&self) -> Token {
+        let kind = match self.token.kind {
+            token::Bool => token::Ident(self.token.symbol, false),
+            _ => token::Literal(self.token),
+        };
+        Token::new(kind, self.span)
+    }
+}
+
+fn strip_underscores(symbol: Symbol) -> Symbol {
+    // Do not allocate a new string unless necessary.
+    let s = symbol.as_str();
+    if s.contains('_') {
+        let mut s = s.to_string();
+        s.retain(|c| c != '_');
+        return Symbol::intern(&s);
+    }
+    symbol
+}
+
+fn filtered_float_lit(
+    symbol: Symbol,
+    suffix: Option<Symbol>,
+    base: u32,
+) -> Result<LitKind, LitError> {
+    debug!("filtered_float_lit: {:?}, {:?}, {:?}", symbol, suffix, base);
+    if base != 10 {
+        return Err(LitError::NonDecimalFloat(base));
+    }
+    Ok(match suffix {
+        Some(suf) => LitKind::Float(
+            symbol,
+            ast::LitFloatType::Suffixed(match suf {
+                sym::f32 => ast::FloatTy::F32,
+                sym::f64 => ast::FloatTy::F64,
+                _ => return Err(LitError::InvalidFloatSuffix),
+            }),
+        ),
+        None => LitKind::Float(symbol, ast::LitFloatType::Unsuffixed),
+    })
+}
+
+fn float_lit(symbol: Symbol, suffix: Option<Symbol>) -> Result<LitKind, LitError> {
+    debug!("float_lit: {:?}, {:?}", symbol, suffix);
+    filtered_float_lit(strip_underscores(symbol), suffix, 10)
+}
+
+fn integer_lit(symbol: Symbol, suffix: Option<Symbol>) -> Result<LitKind, LitError> {
+    debug!("integer_lit: {:?}, {:?}", symbol, suffix);
+    let symbol = strip_underscores(symbol);
+    let s = symbol.as_str();
+
+    let base = match s.as_bytes() {
+        [b'0', b'x', ..] => 16,
+        [b'0', b'o', ..] => 8,
+        [b'0', b'b', ..] => 2,
+        _ => 10,
+    };
+
+    let ty = match suffix {
+        Some(suf) => match suf {
+            sym::isize => ast::LitIntType::Signed(ast::IntTy::Isize),
+            sym::i8 => ast::LitIntType::Signed(ast::IntTy::I8),
+            sym::i16 => ast::LitIntType::Signed(ast::IntTy::I16),
+            sym::i32 => ast::LitIntType::Signed(ast::IntTy::I32),
+            sym::i64 => ast::LitIntType::Signed(ast::IntTy::I64),
+            sym::i128 => ast::LitIntType::Signed(ast::IntTy::I128),
+            sym::usize => ast::LitIntType::Unsigned(ast::UintTy::Usize),
+            sym::u8 => ast::LitIntType::Unsigned(ast::UintTy::U8),
+            sym::u16 => ast::LitIntType::Unsigned(ast::UintTy::U16),
+            sym::u32 => ast::LitIntType::Unsigned(ast::UintTy::U32),
+            sym::u64 => ast::LitIntType::Unsigned(ast::UintTy::U64),
+            sym::u128 => ast::LitIntType::Unsigned(ast::UintTy::U128),
+            // `1f64` and `2f32` etc. are valid float literals, and
+            // `fxxx` looks more like an invalid float literal than invalid integer literal.
+            _ if suf.as_str().starts_with('f') => return filtered_float_lit(symbol, suffix, base),
+            _ => return Err(LitError::InvalidIntSuffix),
+        },
+        _ => ast::LitIntType::Unsuffixed,
+    };
+
+    let s = &s[if base != 10 { 2 } else { 0 }..];
+    u128::from_str_radix(s, base).map(|i| LitKind::Int(i, ty)).map_err(|_| {
+        // Small bases are lexed as if they were base 10, e.g, the string
+        // might be `0b10201`. This will cause the conversion above to fail,
+        // but these kinds of errors are already reported by the lexer.
+        let from_lexer =
+            base < 10 && s.chars().any(|c| c.to_digit(10).map_or(false, |d| d >= base));
+        if from_lexer { LitError::LexerError } else { LitError::IntTooLarge }
+    })
+}
diff --git a/compiler/rustc_ast/src/util/parser.rs b/compiler/rustc_ast/src/util/parser.rs
new file mode 100644
index 000000000..74b7fe9e2
--- /dev/null
+++ b/compiler/rustc_ast/src/util/parser.rs
@@ -0,0 +1,406 @@
+use crate::ast::{self, BinOpKind};
+use crate::token::{self, BinOpToken, Token};
+use rustc_span::symbol::kw;
+
+/// Associative operator with precedence.
+///
+/// This is the enum which specifies operator precedence and fixity to the parser.
+#[derive(Copy, Clone, PartialEq, Debug)]
+pub enum AssocOp {
+    /// `+`
+    Add,
+    /// `-`
+    Subtract,
+    /// `*`
+    Multiply,
+    /// `/`
+    Divide,
+    /// `%`
+    Modulus,
+    /// `&&`
+    LAnd,
+    /// `||`
+    LOr,
+    /// `^`
+    BitXor,
+    /// `&`
+    BitAnd,
+    /// `|`
+    BitOr,
+    /// `<<`
+    ShiftLeft,
+    /// `>>`
+    ShiftRight,
+    /// `==`
+    Equal,
+    /// `<`
+    Less,
+    /// `<=`
+    LessEqual,
+    /// `!=`
+    NotEqual,
+    /// `>`
+    Greater,
+    /// `>=`
+    GreaterEqual,
+    /// `=`
+    Assign,
+    /// `?=` where ? is one of the BinOpToken
+    AssignOp(BinOpToken),
+    /// `as`
+    As,
+    /// `..` range
+    DotDot,
+    /// `..=` range
+    DotDotEq,
+    /// `:`
+    Colon,
+}
+
+#[derive(PartialEq, Debug)]
+pub enum Fixity {
+    /// The operator is left-associative
+    Left,
+    /// The operator is right-associative
+    Right,
+    /// The operator is not associative
+    None,
+}
+
+impl AssocOp {
+    /// Creates a new AssocOP from a token
+    pub fn from_token(t: &Token) -> Option<AssocOp> {
+        use AssocOp::*;
+        match t.kind {
+            token::BinOpEq(k) => Some(AssignOp(k)),
+            token::Eq => Some(Assign),
+            token::BinOp(BinOpToken::Star) => Some(Multiply),
+            token::BinOp(BinOpToken::Slash) => Some(Divide),
+            token::BinOp(BinOpToken::Percent) => Some(Modulus),
+            token::BinOp(BinOpToken::Plus) => Some(Add),
+            token::BinOp(BinOpToken::Minus) => Some(Subtract),
+            token::BinOp(BinOpToken::Shl) => Some(ShiftLeft),
+            token::BinOp(BinOpToken::Shr) => Some(ShiftRight),
+            token::BinOp(BinOpToken::And) => Some(BitAnd),
+            token::BinOp(BinOpToken::Caret) => Some(BitXor),
+            token::BinOp(BinOpToken::Or) => Some(BitOr),
+            token::Lt => Some(Less),
+            token::Le => Some(LessEqual),
+            token::Ge => Some(GreaterEqual),
+            token::Gt => Some(Greater),
+            token::EqEq => Some(Equal),
+            token::Ne => Some(NotEqual),
+            token::AndAnd => Some(LAnd),
+            token::OrOr => Some(LOr),
+            token::DotDot => Some(DotDot),
+            token::DotDotEq => Some(DotDotEq),
+            // DotDotDot is no longer supported, but we need some way to display the error
+            token::DotDotDot => Some(DotDotEq),
+            token::Colon => Some(Colon),
+            // `<-` should probably be `< -`
+            token::LArrow => Some(Less),
+            _ if t.is_keyword(kw::As) => Some(As),
+            _ => None,
+        }
+    }
+
+    /// Creates a new AssocOp from ast::BinOpKind.
+    pub fn from_ast_binop(op: BinOpKind) -> Self {
+        use AssocOp::*;
+        match op {
+            BinOpKind::Lt => Less,
+            BinOpKind::Gt => Greater,
+            BinOpKind::Le => LessEqual,
+            BinOpKind::Ge => GreaterEqual,
+            BinOpKind::Eq => Equal,
+            BinOpKind::Ne => NotEqual,
+            BinOpKind::Mul => Multiply,
+            BinOpKind::Div => Divide,
+            BinOpKind::Rem => Modulus,
+            BinOpKind::Add => Add,
+            BinOpKind::Sub => Subtract,
+            BinOpKind::Shl => ShiftLeft,
+            BinOpKind::Shr => ShiftRight,
+            BinOpKind::BitAnd => BitAnd,
+            BinOpKind::BitXor => BitXor,
+            BinOpKind::BitOr => BitOr,
+            BinOpKind::And => LAnd,
+            BinOpKind::Or => LOr,
+        }
+    }
+
+    /// Gets the precedence of this operator
+    pub fn precedence(&self) -> usize {
+        use AssocOp::*;
+        match *self {
+            As | Colon => 14,
+            Multiply | Divide | Modulus => 13,
+            Add | Subtract => 12,
+            ShiftLeft | ShiftRight => 11,
+            BitAnd => 10,
+            BitXor => 9,
+            BitOr => 8,
+            Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual => 7,
+            LAnd => 6,
+            LOr => 5,
+            DotDot | DotDotEq => 4,
+            Assign | AssignOp(_) => 2,
+        }
+    }
+
+    /// Gets the fixity of this operator
+    pub fn fixity(&self) -> Fixity {
+        use AssocOp::*;
+        // NOTE: it is a bug to have an operators that has same precedence but different fixities!
+        match *self {
+            Assign | AssignOp(_) => Fixity::Right,
+            As | Multiply | Divide | Modulus | Add | Subtract | ShiftLeft | ShiftRight | BitAnd
+            | BitXor | BitOr | Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual
+            | LAnd | LOr | Colon => Fixity::Left,
+            DotDot | DotDotEq => Fixity::None,
+        }
+    }
+
+    pub fn is_comparison(&self) -> bool {
+        use AssocOp::*;
+        match *self {
+            Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual => true,
+            Assign | AssignOp(_) | As | Multiply | Divide | Modulus | Add | Subtract
+            | ShiftLeft | ShiftRight | BitAnd | BitXor | BitOr | LAnd | LOr | DotDot | DotDotEq
+            | Colon => false,
+        }
+    }
+
+    pub fn is_assign_like(&self) -> bool {
+        use AssocOp::*;
+        match *self {
+            Assign | AssignOp(_) => true,
+            Less | Greater | LessEqual | GreaterEqual | Equal | NotEqual | As | Multiply
+            | Divide | Modulus | Add | Subtract | ShiftLeft | ShiftRight | BitAnd | BitXor
+            | BitOr | LAnd | LOr | DotDot | DotDotEq | Colon => false,
+        }
+    }
+
+    pub fn to_ast_binop(&self) -> Option<BinOpKind> {
+        use AssocOp::*;
+        match *self {
+            Less => Some(BinOpKind::Lt),
+            Greater => Some(BinOpKind::Gt),
+            LessEqual => Some(BinOpKind::Le),
+            GreaterEqual => Some(BinOpKind::Ge),
+            Equal => Some(BinOpKind::Eq),
+            NotEqual => Some(BinOpKind::Ne),
+            Multiply => Some(BinOpKind::Mul),
+            Divide => Some(BinOpKind::Div),
+            Modulus => Some(BinOpKind::Rem),
+            Add => Some(BinOpKind::Add),
+            Subtract => Some(BinOpKind::Sub),
+            ShiftLeft => Some(BinOpKind::Shl),
+            ShiftRight => Some(BinOpKind::Shr),
+            BitAnd => Some(BinOpKind::BitAnd),
+            BitXor => Some(BinOpKind::BitXor),
+            BitOr => Some(BinOpKind::BitOr),
+            LAnd => Some(BinOpKind::And),
+            LOr => Some(BinOpKind::Or),
+            Assign | AssignOp(_) | As | DotDot | DotDotEq | Colon => None,
+        }
+    }
+
+    /// This operator could be used to follow a block unambiguously.
+    ///
+    /// This is used for error recovery at the moment, providing a suggestion to wrap blocks with
+    /// parentheses while having a high degree of confidence on the correctness of the suggestion.
+    pub fn can_continue_expr_unambiguously(&self) -> bool {
+        use AssocOp::*;
+        matches!(
+            self,
+            BitXor | // `{ 42 } ^ 3`
+            Assign | // `{ 42 } = { 42 }`
+            Divide | // `{ 42 } / 42`
+            Modulus | // `{ 42 } % 2`
+            ShiftRight | // `{ 42 } >> 2`
+            LessEqual | // `{ 42 } <= 3`
+            Greater | // `{ 42 } > 3`
+            GreaterEqual | // `{ 42 } >= 3`
+            AssignOp(_) | // `{ 42 } +=`
+            As | // `{ 42 } as usize`
+            // Equal | // `{ 42 } == { 42 }`    Accepting these here would regress incorrect
+            // NotEqual | // `{ 42 } != { 42 }  struct literals parser recovery.
+            Colon, // `{ 42 }: usize`
+        )
+    }
+}
+
+pub const PREC_CLOSURE: i8 = -40;
+pub const PREC_JUMP: i8 = -30;
+pub const PREC_RANGE: i8 = -10;
+// The range 2..=14 is reserved for AssocOp binary operator precedences.
+pub const PREC_PREFIX: i8 = 50;
+pub const PREC_POSTFIX: i8 = 60;
+pub const PREC_PAREN: i8 = 99;
+pub const PREC_FORCE_PAREN: i8 = 100;
+
+#[derive(Debug, Clone, Copy)]
+pub enum ExprPrecedence {
+    Closure,
+    Break,
+    Continue,
+    Ret,
+    Yield,
+    Yeet,
+
+    Range,
+
+    Binary(BinOpKind),
+
+    Cast,
+    Type,
+
+    Assign,
+    AssignOp,
+
+    Box,
+    AddrOf,
+    Let,
+    Unary,
+
+    Call,
+    MethodCall,
+    Field,
+    Index,
+    Try,
+    InlineAsm,
+    Mac,
+
+    Array,
+    Repeat,
+    Tup,
+    Lit,
+    Path,
+    Paren,
+    If,
+    While,
+    ForLoop,
+    Loop,
+    Match,
+    ConstBlock,
+    Block,
+    TryBlock,
+    Struct,
+    Async,
+    Await,
+    Err,
+}
+
+impl ExprPrecedence {
+    pub fn order(self) -> i8 {
+        match self {
+            ExprPrecedence::Closure => PREC_CLOSURE,
+
+            ExprPrecedence::Break |
+            ExprPrecedence::Continue |
+            ExprPrecedence::Ret |
+            ExprPrecedence::Yield |
+            ExprPrecedence::Yeet => PREC_JUMP,
+
+            // `Range` claims to have higher precedence than `Assign`, but `x .. x = x` fails to
+            // parse, instead of parsing as `(x .. x) = x`.  Giving `Range` a lower precedence
+            // ensures that `pprust` will add parentheses in the right places to get the desired
+            // parse.
+            ExprPrecedence::Range => PREC_RANGE,
+
+            // Binop-like expr kinds, handled by `AssocOp`.
+            ExprPrecedence::Binary(op) => AssocOp::from_ast_binop(op).precedence() as i8,
+            ExprPrecedence::Cast => AssocOp::As.precedence() as i8,
+            ExprPrecedence::Type => AssocOp::Colon.precedence() as i8,
+
+            ExprPrecedence::Assign |
+            ExprPrecedence::AssignOp => AssocOp::Assign.precedence() as i8,
+
+            // Unary, prefix
+            ExprPrecedence::Box |
+            ExprPrecedence::AddrOf |
+            // Here `let pats = expr` has `let pats =` as a "unary" prefix of `expr`.
+            // However, this is not exactly right. When `let _ = a` is the LHS of a binop we
+            // need parens sometimes. E.g. we can print `(let _ = a) && b` as `let _ = a && b`
+            // but we need to print `(let _ = a) < b` as-is with parens.
+            ExprPrecedence::Let |
+            ExprPrecedence::Unary => PREC_PREFIX,
+
+            // Unary, postfix
+            ExprPrecedence::Await |
+            ExprPrecedence::Call |
+            ExprPrecedence::MethodCall |
+            ExprPrecedence::Field |
+            ExprPrecedence::Index |
+            ExprPrecedence::Try |
+            ExprPrecedence::InlineAsm |
+            ExprPrecedence::Mac => PREC_POSTFIX,
+
+            // Never need parens
+            ExprPrecedence::Array |
+            ExprPrecedence::Repeat |
+            ExprPrecedence::Tup |
+            ExprPrecedence::Lit |
+            ExprPrecedence::Path |
+            ExprPrecedence::Paren |
+            ExprPrecedence::If |
+            ExprPrecedence::While |
+            ExprPrecedence::ForLoop |
+            ExprPrecedence::Loop |
+            ExprPrecedence::Match |
+            ExprPrecedence::ConstBlock |
+            ExprPrecedence::Block |
+            ExprPrecedence::TryBlock |
+            ExprPrecedence::Async |
+            ExprPrecedence::Struct |
+            ExprPrecedence::Err => PREC_PAREN,
+        }
+    }
+}
+
+/// In `let p = e`, operators with precedence `<=` this one requires parentheses in `e`.
+pub fn prec_let_scrutinee_needs_par() -> usize {
+    AssocOp::LAnd.precedence()
+}
+
+/// Suppose we have `let _ = e` and the `order` of `e`.
+/// Is the `order` such that `e` in `let _ = e` needs parentheses when it is on the RHS?
+///
+/// Conversely, suppose that we have `(let _ = a) OP b` and `order` is that of `OP`.
+/// Can we print this as `let _ = a OP b`?
+pub fn needs_par_as_let_scrutinee(order: i8) -> bool {
+    order <= prec_let_scrutinee_needs_par() as i8
+}
+
+/// Expressions that syntactically contain an "exterior" struct literal i.e., not surrounded by any
+/// parens or other delimiters, e.g., `X { y: 1 }`, `X { y: 1 }.method()`, `foo == X { y: 1 }` and
+/// `X { y: 1 } == foo` all do, but `(X { y: 1 }) == foo` does not.
+pub fn contains_exterior_struct_lit(value: &ast::Expr) -> bool {
+    match value.kind {
+        ast::ExprKind::Struct(..) => true,
+
+        ast::ExprKind::Assign(ref lhs, ref rhs, _)
+        | ast::ExprKind::AssignOp(_, ref lhs, ref rhs)
+        | ast::ExprKind::Binary(_, ref lhs, ref rhs) => {
+            // X { y: 1 } + X { y: 2 }
+            contains_exterior_struct_lit(&lhs) || contains_exterior_struct_lit(&rhs)
+        }
+        ast::ExprKind::Await(ref x)
+        | ast::ExprKind::Unary(_, ref x)
+        | ast::ExprKind::Cast(ref x, _)
+        | ast::ExprKind::Type(ref x, _)
+        | ast::ExprKind::Field(ref x, _)
+        | ast::ExprKind::Index(ref x, _) => {
+            // &X { y: 1 }, X { y: 1 }.y
+            contains_exterior_struct_lit(&x)
+        }
+
+        ast::ExprKind::MethodCall(.., ref exprs, _) => {
+            // X { y: 1 }.bar(...)
+            contains_exterior_struct_lit(&exprs[0])
+        }
+
+        _ => false,
+    }
+}
diff --git a/compiler/rustc_ast/src/util/unicode.rs b/compiler/rustc_ast/src/util/unicode.rs
new file mode 100644
index 000000000..f009f7b30
--- /dev/null
+++ b/compiler/rustc_ast/src/util/unicode.rs
@@ -0,0 +1,35 @@
+pub const TEXT_FLOW_CONTROL_CHARS: &[char] = &[
+    '\u{202A}', '\u{202B}', '\u{202D}', '\u{202E}', '\u{2066}', '\u{2067}', '\u{2068}', '\u{202C}',
+    '\u{2069}',
+];
+
+#[inline]
+pub fn contains_text_flow_control_chars(s: &str) -> bool {
+    // Char   - UTF-8
+    // U+202A - E2 80 AA
+    // U+202B - E2 80 AB
+    // U+202C - E2 80 AC
+    // U+202D - E2 80 AD
+    // U+202E - E2 80 AE
+    // U+2066 - E2 81 A6
+    // U+2067 - E2 81 A7
+    // U+2068 - E2 81 A8
+    // U+2069 - E2 81 A9
+    let mut bytes = s.as_bytes();
+    loop {
+        match core::slice::memchr::memchr(0xE2, &bytes) {
+            Some(idx) => {
+                // bytes are valid UTF-8 -> E2 must be followed by two bytes
+                let ch = &bytes[idx..idx + 3];
+                match ch {
+                    [_, 0x80, 0xAA..=0xAE] | [_, 0x81, 0xA6..=0xA9] => break true,
+                    _ => {}
+                }
+                bytes = &bytes[idx + 3..];
+            }
+            None => {
+                break false;
+            }
+        }
+    }
+}
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
new file mode 100644
index 000000000..d9594b323
--- /dev/null
+++ b/compiler/rustc_ast/src/visit.rs
@@ -0,0 +1,959 @@
+//! AST walker. Each overridden visit method has full control over what
+//! happens with its node, it can do its own traversal of the node's children,
+//! call `visit::walk_*` to apply the default traversal algorithm, or prevent
+//! deeper traversal by doing nothing.
+//!
+//! Note: it is an important invariant that the default visitor walks the body
+//! of a function in "execution order" (more concretely, reverse post-order
+//! with respect to the CFG implied by the AST), meaning that if AST node A may
+//! execute before AST node B, then A is visited first. The borrow checker in
+//! particular relies on this property.
+//!
+//! Note: walking an AST before macro expansion is probably a bad idea. For
+//! instance, a walker looking for item names in a module will miss all of
+//! those that are created by the expansion of a macro.
+
+use crate::ast::*;
+
+use rustc_span::symbol::Ident;
+use rustc_span::Span;
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum AssocCtxt {
+    Trait,
+    Impl,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum FnCtxt {
+    Free,
+    Foreign,
+    Assoc(AssocCtxt),
+}
+
+#[derive(Copy, Clone, Debug)]
+pub enum BoundKind {
+    /// Trait bounds in generics bounds and type/trait alias.
+    /// E.g., `<T: Bound>`, `type A: Bound`, or `where T: Bound`.
+    Bound,
+
+    /// Trait bounds in `impl` type.
+    /// E.g., `type Foo = impl Bound1 + Bound2 + Bound3`.
+    Impl,
+
+    /// Trait bounds in trait object type.
+    /// E.g., `dyn Bound1 + Bound2 + Bound3`.
+    TraitObject,
+
+    /// Super traits of a trait.
+    /// E.g., `trait A: B`
+    SuperTraits,
+}
+
+#[derive(Copy, Clone, Debug)]
+pub enum FnKind<'a> {
+    /// E.g., `fn foo()`, `fn foo(&self)`, or `extern "Abi" fn foo()`.
+    Fn(FnCtxt, Ident, &'a FnSig, &'a Visibility, &'a Generics, Option<&'a Block>),
+
+    /// E.g., `|x, y| body`.
+    Closure(&'a ClosureBinder, &'a FnDecl, &'a Expr),
+}
+
+impl<'a> FnKind<'a> {
+    pub fn header(&self) -> Option<&'a FnHeader> {
+        match *self {
+            FnKind::Fn(_, _, sig, _, _, _) => Some(&sig.header),
+            FnKind::Closure(_, _, _) => None,
+        }
+    }
+
+    pub fn ident(&self) -> Option<&Ident> {
+        match self {
+            FnKind::Fn(_, ident, ..) => Some(ident),
+            _ => None,
+        }
+    }
+
+    pub fn decl(&self) -> &'a FnDecl {
+        match self {
+            FnKind::Fn(_, _, sig, _, _, _) => &sig.decl,
+            FnKind::Closure(_, decl, _) => decl,
+        }
+    }
+
+    pub fn ctxt(&self) -> Option<FnCtxt> {
+        match self {
+            FnKind::Fn(ctxt, ..) => Some(*ctxt),
+            FnKind::Closure(..) => None,
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+pub enum LifetimeCtxt {
+    /// Appears in a reference type.
+    Rptr,
+    /// Appears as a bound on a type or another lifetime.
+    Bound,
+    /// Appears as a generic argument.
+    GenericArg,
+}
+
+/// Each method of the `Visitor` trait is a hook to be potentially
+/// overridden. Each method's default implementation recursively visits
+/// the substructure of the input via the corresponding `walk` method;
+/// e.g., the `visit_item` method by default calls `visit::walk_item`.
+///
+/// If you want to ensure that your code handles every variant
+/// explicitly, you need to override each method. (And you also need
+/// to monitor future changes to `Visitor` in case a new method with a
+/// new default implementation gets introduced.)
+pub trait Visitor<'ast>: Sized {
+    fn visit_ident(&mut self, _ident: Ident) {}
+    fn visit_foreign_item(&mut self, i: &'ast ForeignItem) {
+        walk_foreign_item(self, i)
+    }
+    fn visit_item(&mut self, i: &'ast Item) {
+        walk_item(self, i)
+    }
+    fn visit_local(&mut self, l: &'ast Local) {
+        walk_local(self, l)
+    }
+    fn visit_block(&mut self, b: &'ast Block) {
+        walk_block(self, b)
+    }
+    fn visit_stmt(&mut self, s: &'ast Stmt) {
+        walk_stmt(self, s)
+    }
+    fn visit_param(&mut self, param: &'ast Param) {
+        walk_param(self, param)
+    }
+    fn visit_arm(&mut self, a: &'ast Arm) {
+        walk_arm(self, a)
+    }
+    fn visit_pat(&mut self, p: &'ast Pat) {
+        walk_pat(self, p)
+    }
+    fn visit_anon_const(&mut self, c: &'ast AnonConst) {
+        walk_anon_const(self, c)
+    }
+    fn visit_expr(&mut self, ex: &'ast Expr) {
+        walk_expr(self, ex)
+    }
+    fn visit_expr_post(&mut self, _ex: &'ast Expr) {}
+    fn visit_ty(&mut self, t: &'ast Ty) {
+        walk_ty(self, t)
+    }
+    fn visit_generic_param(&mut self, param: &'ast GenericParam) {
+        walk_generic_param(self, param)
+    }
+    fn visit_generics(&mut self, g: &'ast Generics) {
+        walk_generics(self, g)
+    }
+    fn visit_closure_binder(&mut self, b: &'ast ClosureBinder) {
+        walk_closure_binder(self, b)
+    }
+    fn visit_where_predicate(&mut self, p: &'ast WherePredicate) {
+        walk_where_predicate(self, p)
+    }
+    fn visit_fn(&mut self, fk: FnKind<'ast>, s: Span, _: NodeId) {
+        walk_fn(self, fk, s)
+    }
+    fn visit_assoc_item(&mut self, i: &'ast AssocItem, ctxt: AssocCtxt) {
+        walk_assoc_item(self, i, ctxt)
+    }
+    fn visit_trait_ref(&mut self, t: &'ast TraitRef) {
+        walk_trait_ref(self, t)
+    }
+    fn visit_param_bound(&mut self, bounds: &'ast GenericBound, _ctxt: BoundKind) {
+        walk_param_bound(self, bounds)
+    }
+    fn visit_poly_trait_ref(&mut self, t: &'ast PolyTraitRef, m: &'ast TraitBoundModifier) {
+        walk_poly_trait_ref(self, t, m)
+    }
+    fn visit_variant_data(&mut self, s: &'ast VariantData) {
+        walk_struct_def(self, s)
+    }
+    fn visit_field_def(&mut self, s: &'ast FieldDef) {
+        walk_field_def(self, s)
+    }
+    fn visit_enum_def(
+        &mut self,
+        enum_definition: &'ast EnumDef,
+        generics: &'ast Generics,
+        item_id: NodeId,
+        _: Span,
+    ) {
+        walk_enum_def(self, enum_definition, generics, item_id)
+    }
+    fn visit_variant(&mut self, v: &'ast Variant) {
+        walk_variant(self, v)
+    }
+    fn visit_label(&mut self, label: &'ast Label) {
+        walk_label(self, label)
+    }
+    fn visit_lifetime(&mut self, lifetime: &'ast Lifetime, _: LifetimeCtxt) {
+        walk_lifetime(self, lifetime)
+    }
+    fn visit_mac_call(&mut self, mac: &'ast MacCall) {
+        walk_mac(self, mac)
+    }
+    fn visit_mac_def(&mut self, _mac: &'ast MacroDef, _id: NodeId) {
+        // Nothing to do
+    }
+    fn visit_path(&mut self, path: &'ast Path, _id: NodeId) {
+        walk_path(self, path)
+    }
+    fn visit_use_tree(&mut self, use_tree: &'ast UseTree, id: NodeId, _nested: bool) {
+        walk_use_tree(self, use_tree, id)
+    }
+    fn visit_path_segment(&mut self, path_span: Span, path_segment: &'ast PathSegment) {
+        walk_path_segment(self, path_span, path_segment)
+    }
+    fn visit_generic_args(&mut self, path_span: Span, generic_args: &'ast GenericArgs) {
+        walk_generic_args(self, path_span, generic_args)
+    }
+    fn visit_generic_arg(&mut self, generic_arg: &'ast GenericArg) {
+        walk_generic_arg(self, generic_arg)
+    }
+    fn visit_assoc_constraint(&mut self, constraint: &'ast AssocConstraint) {
+        walk_assoc_constraint(self, constraint)
+    }
+    fn visit_attribute(&mut self, attr: &'ast Attribute) {
+        walk_attribute(self, attr)
+    }
+    fn visit_vis(&mut self, vis: &'ast Visibility) {
+        walk_vis(self, vis)
+    }
+    fn visit_fn_ret_ty(&mut self, ret_ty: &'ast FnRetTy) {
+        walk_fn_ret_ty(self, ret_ty)
+    }
+    fn visit_fn_header(&mut self, _header: &'ast FnHeader) {
+        // Nothing to do
+    }
+    fn visit_expr_field(&mut self, f: &'ast ExprField) {
+        walk_expr_field(self, f)
+    }
+    fn visit_pat_field(&mut self, fp: &'ast PatField) {
+        walk_pat_field(self, fp)
+    }
+    fn visit_crate(&mut self, krate: &'ast Crate) {
+        walk_crate(self, krate)
+    }
+    fn visit_inline_asm(&mut self, asm: &'ast InlineAsm) {
+        walk_inline_asm(self, asm)
+    }
+    fn visit_inline_asm_sym(&mut self, sym: &'ast InlineAsmSym) {
+        walk_inline_asm_sym(self, sym)
+    }
+}
+
+#[macro_export]
+macro_rules! walk_list {
+    ($visitor: expr, $method: ident, $list: expr) => {
+        for elem in $list {
+            $visitor.$method(elem)
+        }
+    };
+    ($visitor: expr, $method: ident, $list: expr, $($extra_args: expr),*) => {
+        for elem in $list {
+            $visitor.$method(elem, $($extra_args,)*)
+        }
+    }
+}
+
+pub fn walk_crate<'a, V: Visitor<'a>>(visitor: &mut V, krate: &'a Crate) {
+    walk_list!(visitor, visit_item, &krate.items);
+    walk_list!(visitor, visit_attribute, &krate.attrs);
+}
+
+pub fn walk_local<'a, V: Visitor<'a>>(visitor: &mut V, local: &'a Local) {
+    for attr in local.attrs.iter() {
+        visitor.visit_attribute(attr);
+    }
+    visitor.visit_pat(&local.pat);
+    walk_list!(visitor, visit_ty, &local.ty);
+    if let Some((init, els)) = local.kind.init_else_opt() {
+        visitor.visit_expr(init);
+        walk_list!(visitor, visit_block, els);
+    }
+}
+
+pub fn walk_label<'a, V: Visitor<'a>>(visitor: &mut V, label: &'a Label) {
+    visitor.visit_ident(label.ident);
+}
+
+pub fn walk_lifetime<'a, V: Visitor<'a>>(visitor: &mut V, lifetime: &'a Lifetime) {
+    visitor.visit_ident(lifetime.ident);
+}
+
+pub fn walk_poly_trait_ref<'a, V>(
+    visitor: &mut V,
+    trait_ref: &'a PolyTraitRef,
+    _: &TraitBoundModifier,
+) where
+    V: Visitor<'a>,
+{
+    walk_list!(visitor, visit_generic_param, &trait_ref.bound_generic_params);
+    visitor.visit_trait_ref(&trait_ref.trait_ref);
+}
+
+pub fn walk_trait_ref<'a, V: Visitor<'a>>(visitor: &mut V, trait_ref: &'a TraitRef) {
+    visitor.visit_path(&trait_ref.path, trait_ref.ref_id)
+}
+
+pub fn walk_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a Item) {
+    visitor.visit_vis(&item.vis);
+    visitor.visit_ident(item.ident);
+    match item.kind {
+        ItemKind::ExternCrate(_) => {}
+        ItemKind::Use(ref use_tree) => visitor.visit_use_tree(use_tree, item.id, false),
+        ItemKind::Static(ref typ, _, ref expr) | ItemKind::Const(_, ref typ, ref expr) => {
+            visitor.visit_ty(typ);
+            walk_list!(visitor, visit_expr, expr);
+        }
+        ItemKind::Fn(box Fn { defaultness: _, ref generics, ref sig, ref body }) => {
+            let kind =
+                FnKind::Fn(FnCtxt::Free, item.ident, sig, &item.vis, generics, body.as_deref());
+            visitor.visit_fn(kind, item.span, item.id)
+        }
+        ItemKind::Mod(_unsafety, ref mod_kind) => match mod_kind {
+            ModKind::Loaded(items, _inline, _inner_span) => {
+                walk_list!(visitor, visit_item, items)
+            }
+            ModKind::Unloaded => {}
+        },
+        ItemKind::ForeignMod(ref foreign_module) => {
+            walk_list!(visitor, visit_foreign_item, &foreign_module.items);
+        }
+        ItemKind::GlobalAsm(ref asm) => visitor.visit_inline_asm(asm),
+        ItemKind::TyAlias(box TyAlias { ref generics, ref bounds, ref ty, .. }) => {
+            visitor.visit_generics(generics);
+            walk_list!(visitor, visit_param_bound, bounds, BoundKind::Bound);
+            walk_list!(visitor, visit_ty, ty);
+        }
+        ItemKind::Enum(ref enum_definition, ref generics) => {
+            visitor.visit_generics(generics);
+            visitor.visit_enum_def(enum_definition, generics, item.id, item.span)
+        }
+        ItemKind::Impl(box Impl {
+            defaultness: _,
+            unsafety: _,
+            ref generics,
+            constness: _,
+            polarity: _,
+            ref of_trait,
+            ref self_ty,
+            ref items,
+        }) => {
+            visitor.visit_generics(generics);
+            walk_list!(visitor, visit_trait_ref, of_trait);
+            visitor.visit_ty(self_ty);
+            walk_list!(visitor, visit_assoc_item, items, AssocCtxt::Impl);
+        }
+        ItemKind::Struct(ref struct_definition, ref generics)
+        | ItemKind::Union(ref struct_definition, ref generics) => {
+            visitor.visit_generics(generics);
+            visitor.visit_variant_data(struct_definition);
+        }
+        ItemKind::Trait(box Trait {
+            unsafety: _,
+            is_auto: _,
+            ref generics,
+            ref bounds,
+            ref items,
+        }) => {
+            visitor.visit_generics(generics);
+            walk_list!(visitor, visit_param_bound, bounds, BoundKind::SuperTraits);
+            walk_list!(visitor, visit_assoc_item, items, AssocCtxt::Trait);
+        }
+        ItemKind::TraitAlias(ref generics, ref bounds) => {
+            visitor.visit_generics(generics);
+            walk_list!(visitor, visit_param_bound, bounds, BoundKind::Bound);
+        }
+        ItemKind::MacCall(ref mac) => visitor.visit_mac_call(mac),
+        ItemKind::MacroDef(ref ts) => visitor.visit_mac_def(ts, item.id),
+    }
+    walk_list!(visitor, visit_attribute, &item.attrs);
+}
+
+pub fn walk_enum_def<'a, V: Visitor<'a>>(
+    visitor: &mut V,
+    enum_definition: &'a EnumDef,
+    _: &'a Generics,
+    _: NodeId,
+) {
+    walk_list!(visitor, visit_variant, &enum_definition.variants);
+}
+
+pub fn walk_variant<'a, V: Visitor<'a>>(visitor: &mut V, variant: &'a Variant)
+where
+    V: Visitor<'a>,
+{
+    visitor.visit_ident(variant.ident);
+    visitor.visit_vis(&variant.vis);
+    visitor.visit_variant_data(&variant.data);
+    walk_list!(visitor, visit_anon_const, &variant.disr_expr);
+    walk_list!(visitor, visit_attribute, &variant.attrs);
+}
+
+pub fn walk_expr_field<'a, V: Visitor<'a>>(visitor: &mut V, f: &'a ExprField) {
+    visitor.visit_expr(&f.expr);
+    visitor.visit_ident(f.ident);
+    walk_list!(visitor, visit_attribute, f.attrs.iter());
+}
+
+pub fn walk_pat_field<'a, V: Visitor<'a>>(visitor: &mut V, fp: &'a PatField) {
+    visitor.visit_ident(fp.ident);
+    visitor.visit_pat(&fp.pat);
+    walk_list!(visitor, visit_attribute, fp.attrs.iter());
+}
+
+pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) {
+    match typ.kind {
+        TyKind::Slice(ref ty) | TyKind::Paren(ref ty) => visitor.visit_ty(ty),
+        TyKind::Ptr(ref mutable_type) => visitor.visit_ty(&mutable_type.ty),
+        TyKind::Rptr(ref opt_lifetime, ref mutable_type) => {
+            walk_list!(visitor, visit_lifetime, opt_lifetime, LifetimeCtxt::Rptr);
+            visitor.visit_ty(&mutable_type.ty)
+        }
+        TyKind::Tup(ref tuple_element_types) => {
+            walk_list!(visitor, visit_ty, tuple_element_types);
+        }
+        TyKind::BareFn(ref function_declaration) => {
+            walk_list!(visitor, visit_generic_param, &function_declaration.generic_params);
+            walk_fn_decl(visitor, &function_declaration.decl);
+        }
+        TyKind::Path(ref maybe_qself, ref path) => {
+            if let Some(ref qself) = *maybe_qself {
+                visitor.visit_ty(&qself.ty);
+            }
+            visitor.visit_path(path, typ.id);
+        }
+        TyKind::Array(ref ty, ref length) => {
+            visitor.visit_ty(ty);
+            visitor.visit_anon_const(length)
+        }
+        TyKind::TraitObject(ref bounds, ..) => {
+            walk_list!(visitor, visit_param_bound, bounds, BoundKind::TraitObject);
+        }
+        TyKind::ImplTrait(_, ref bounds) => {
+            walk_list!(visitor, visit_param_bound, bounds, BoundKind::Impl);
+        }
+        TyKind::Typeof(ref expression) => visitor.visit_anon_const(expression),
+        TyKind::Infer | TyKind::ImplicitSelf | TyKind::Err => {}
+        TyKind::MacCall(ref mac) => visitor.visit_mac_call(mac),
+        TyKind::Never | TyKind::CVarArgs => {}
+    }
+}
+
+pub fn walk_path<'a, V: Visitor<'a>>(visitor: &mut V, path: &'a Path) {
+    for segment in &path.segments {
+        visitor.visit_path_segment(path.span, segment);
+    }
+}
+
+pub fn walk_use_tree<'a, V: Visitor<'a>>(visitor: &mut V, use_tree: &'a UseTree, id: NodeId) {
+    visitor.visit_path(&use_tree.prefix, id);
+    match use_tree.kind {
+        UseTreeKind::Simple(rename, ..) => {
+            // The extra IDs are handled during HIR lowering.
+            if let Some(rename) = rename {
+                visitor.visit_ident(rename);
+            }
+        }
+        UseTreeKind::Glob => {}
+        UseTreeKind::Nested(ref use_trees) => {
+            for &(ref nested_tree, nested_id) in use_trees {
+                visitor.visit_use_tree(nested_tree, nested_id, true);
+            }
+        }
+    }
+}
+
+pub fn walk_path_segment<'a, V: Visitor<'a>>(
+    visitor: &mut V,
+    path_span: Span,
+    segment: &'a PathSegment,
+) {
+    visitor.visit_ident(segment.ident);
+    if let Some(ref args) = segment.args {
+        visitor.visit_generic_args(path_span, args);
+    }
+}
+
+pub fn walk_generic_args<'a, V>(visitor: &mut V, _path_span: Span, generic_args: &'a GenericArgs)
+where
+    V: Visitor<'a>,
+{
+    match *generic_args {
+        GenericArgs::AngleBracketed(ref data) => {
+            for arg in &data.args {
+                match arg {
+                    AngleBracketedArg::Arg(a) => visitor.visit_generic_arg(a),
+                    AngleBracketedArg::Constraint(c) => visitor.visit_assoc_constraint(c),
+                }
+            }
+        }
+        GenericArgs::Parenthesized(ref data) => {
+            walk_list!(visitor, visit_ty, &data.inputs);
+            walk_fn_ret_ty(visitor, &data.output);
+        }
+    }
+}
+
+pub fn walk_generic_arg<'a, V>(visitor: &mut V, generic_arg: &'a GenericArg)
+where
+    V: Visitor<'a>,
+{
+    match generic_arg {
+        GenericArg::Lifetime(lt) => visitor.visit_lifetime(lt, LifetimeCtxt::GenericArg),
+        GenericArg::Type(ty) => visitor.visit_ty(ty),
+        GenericArg::Const(ct) => visitor.visit_anon_const(ct),
+    }
+}
+
+pub fn walk_assoc_constraint<'a, V: Visitor<'a>>(visitor: &mut V, constraint: &'a AssocConstraint) {
+    visitor.visit_ident(constraint.ident);
+    if let Some(ref gen_args) = constraint.gen_args {
+        visitor.visit_generic_args(gen_args.span(), gen_args);
+    }
+    match constraint.kind {
+        AssocConstraintKind::Equality { ref term } => match term {
+            Term::Ty(ty) => visitor.visit_ty(ty),
+            Term::Const(c) => visitor.visit_anon_const(c),
+        },
+        AssocConstraintKind::Bound { ref bounds } => {
+            walk_list!(visitor, visit_param_bound, bounds, BoundKind::Bound);
+        }
+    }
+}
+
+pub fn walk_pat<'a, V: Visitor<'a>>(visitor: &mut V, pattern: &'a Pat) {
+    match pattern.kind {
+        PatKind::TupleStruct(ref opt_qself, ref path, ref elems) => {
+            if let Some(ref qself) = *opt_qself {
+                visitor.visit_ty(&qself.ty);
+            }
+            visitor.visit_path(path, pattern.id);
+            walk_list!(visitor, visit_pat, elems);
+        }
+        PatKind::Path(ref opt_qself, ref path) => {
+            if let Some(ref qself) = *opt_qself {
+                visitor.visit_ty(&qself.ty);
+            }
+            visitor.visit_path(path, pattern.id)
+        }
+        PatKind::Struct(ref opt_qself, ref path, ref fields, _) => {
+            if let Some(ref qself) = *opt_qself {
+                visitor.visit_ty(&qself.ty);
+            }
+            visitor.visit_path(path, pattern.id);
+            walk_list!(visitor, visit_pat_field, fields);
+        }
+        PatKind::Box(ref subpattern)
+        | PatKind::Ref(ref subpattern, _)
+        | PatKind::Paren(ref subpattern) => visitor.visit_pat(subpattern),
+        PatKind::Ident(_, ident, ref optional_subpattern) => {
+            visitor.visit_ident(ident);
+            walk_list!(visitor, visit_pat, optional_subpattern);
+        }
+        PatKind::Lit(ref expression) => visitor.visit_expr(expression),
+        PatKind::Range(ref lower_bound, ref upper_bound, _) => {
+            walk_list!(visitor, visit_expr, lower_bound);
+            walk_list!(visitor, visit_expr, upper_bound);
+        }
+        PatKind::Wild | PatKind::Rest => {}
+        PatKind::Tuple(ref elems) | PatKind::Slice(ref elems) | PatKind::Or(ref elems) => {
+            walk_list!(visitor, visit_pat, elems);
+        }
+        PatKind::MacCall(ref mac) => visitor.visit_mac_call(mac),
+    }
+}
+
+pub fn walk_foreign_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a ForeignItem) {
+    let Item { id, span, ident, ref vis, ref attrs, ref kind, tokens: _ } = *item;
+    visitor.visit_vis(vis);
+    visitor.visit_ident(ident);
+    walk_list!(visitor, visit_attribute, attrs);
+    match kind {
+        ForeignItemKind::Static(ty, _, expr) => {
+            visitor.visit_ty(ty);
+            walk_list!(visitor, visit_expr, expr);
+        }
+        ForeignItemKind::Fn(box Fn { defaultness: _, ref generics, ref sig, ref body }) => {
+            let kind = FnKind::Fn(FnCtxt::Foreign, ident, sig, vis, generics, body.as_deref());
+            visitor.visit_fn(kind, span, id);
+        }
+        ForeignItemKind::TyAlias(box TyAlias { generics, bounds, ty, .. }) => {
+            visitor.visit_generics(generics);
+            walk_list!(visitor, visit_param_bound, bounds, BoundKind::Bound);
+            walk_list!(visitor, visit_ty, ty);
+        }
+        ForeignItemKind::MacCall(mac) => {
+            visitor.visit_mac_call(mac);
+        }
+    }
+}
+
+pub fn walk_param_bound<'a, V: Visitor<'a>>(visitor: &mut V, bound: &'a GenericBound) {
+    match *bound {
+        GenericBound::Trait(ref typ, ref modifier) => visitor.visit_poly_trait_ref(typ, modifier),
+        GenericBound::Outlives(ref lifetime) => {
+            visitor.visit_lifetime(lifetime, LifetimeCtxt::Bound)
+        }
+    }
+}
+
+pub fn walk_generic_param<'a, V: Visitor<'a>>(visitor: &mut V, param: &'a GenericParam) {
+    visitor.visit_ident(param.ident);
+    walk_list!(visitor, visit_attribute, param.attrs.iter());
+    walk_list!(visitor, visit_param_bound, &param.bounds, BoundKind::Bound);
+    match param.kind {
+        GenericParamKind::Lifetime => (),
+        GenericParamKind::Type { ref default } => walk_list!(visitor, visit_ty, default),
+        GenericParamKind::Const { ref ty, ref default, .. } => {
+            visitor.visit_ty(ty);
+            if let Some(default) = default {
+                visitor.visit_anon_const(default);
+            }
+        }
+    }
+}
+
+pub fn walk_generics<'a, V: Visitor<'a>>(visitor: &mut V, generics: &'a Generics) {
+    walk_list!(visitor, visit_generic_param, &generics.params);
+    walk_list!(visitor, visit_where_predicate, &generics.where_clause.predicates);
+}
+
+pub fn walk_closure_binder<'a, V: Visitor<'a>>(visitor: &mut V, binder: &'a ClosureBinder) {
+    match binder {
+        ClosureBinder::NotPresent => {}
+        ClosureBinder::For { generic_params, span: _ } => {
+            walk_list!(visitor, visit_generic_param, generic_params)
+        }
+    }
+}
+
+pub fn walk_where_predicate<'a, V: Visitor<'a>>(visitor: &mut V, predicate: &'a WherePredicate) {
+    match *predicate {
+        WherePredicate::BoundPredicate(WhereBoundPredicate {
+            ref bounded_ty,
+            ref bounds,
+            ref bound_generic_params,
+            ..
+        }) => {
+            visitor.visit_ty(bounded_ty);
+            walk_list!(visitor, visit_param_bound, bounds, BoundKind::Bound);
+            walk_list!(visitor, visit_generic_param, bound_generic_params);
+        }
+        WherePredicate::RegionPredicate(WhereRegionPredicate {
+            ref lifetime, ref bounds, ..
+        }) => {
+            visitor.visit_lifetime(lifetime, LifetimeCtxt::Bound);
+            walk_list!(visitor, visit_param_bound, bounds, BoundKind::Bound);
+        }
+        WherePredicate::EqPredicate(WhereEqPredicate { ref lhs_ty, ref rhs_ty, .. }) => {
+            visitor.visit_ty(lhs_ty);
+            visitor.visit_ty(rhs_ty);
+        }
+    }
+}
+
+pub fn walk_fn_ret_ty<'a, V: Visitor<'a>>(visitor: &mut V, ret_ty: &'a FnRetTy) {
+    if let FnRetTy::Ty(ref output_ty) = *ret_ty {
+        visitor.visit_ty(output_ty)
+    }
+}
+
+pub fn walk_fn_decl<'a, V: Visitor<'a>>(visitor: &mut V, function_declaration: &'a FnDecl) {
+    for param in &function_declaration.inputs {
+        visitor.visit_param(param);
+    }
+    visitor.visit_fn_ret_ty(&function_declaration.output);
+}
+
+pub fn walk_fn<'a, V: Visitor<'a>>(visitor: &mut V, kind: FnKind<'a>, _span: Span) {
+    match kind {
+        FnKind::Fn(_, _, sig, _, generics, body) => {
+            visitor.visit_generics(generics);
+            visitor.visit_fn_header(&sig.header);
+            walk_fn_decl(visitor, &sig.decl);
+            walk_list!(visitor, visit_block, body);
+        }
+        FnKind::Closure(binder, decl, body) => {
+            visitor.visit_closure_binder(binder);
+            walk_fn_decl(visitor, decl);
+            visitor.visit_expr(body);
+        }
+    }
+}
+
+pub fn walk_assoc_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a AssocItem, ctxt: AssocCtxt) {
+    let Item { id, span, ident, ref vis, ref attrs, ref kind, tokens: _ } = *item;
+    visitor.visit_vis(vis);
+    visitor.visit_ident(ident);
+    walk_list!(visitor, visit_attribute, attrs);
+    match kind {
+        AssocItemKind::Const(_, ty, expr) => {
+            visitor.visit_ty(ty);
+            walk_list!(visitor, visit_expr, expr);
+        }
+        AssocItemKind::Fn(box Fn { defaultness: _, ref generics, ref sig, ref body }) => {
+            let kind = FnKind::Fn(FnCtxt::Assoc(ctxt), ident, sig, vis, generics, body.as_deref());
+            visitor.visit_fn(kind, span, id);
+        }
+        AssocItemKind::TyAlias(box TyAlias { generics, bounds, ty, .. }) => {
+            visitor.visit_generics(generics);
+            walk_list!(visitor, visit_param_bound, bounds, BoundKind::Bound);
+            walk_list!(visitor, visit_ty, ty);
+        }
+        AssocItemKind::MacCall(mac) => {
+            visitor.visit_mac_call(mac);
+        }
+    }
+}
+
+pub fn walk_struct_def<'a, V: Visitor<'a>>(visitor: &mut V, struct_definition: &'a VariantData) {
+    walk_list!(visitor, visit_field_def, struct_definition.fields());
+}
+
+pub fn walk_field_def<'a, V: Visitor<'a>>(visitor: &mut V, field: &'a FieldDef) {
+    visitor.visit_vis(&field.vis);
+    if let Some(ident) = field.ident {
+        visitor.visit_ident(ident);
+    }
+    visitor.visit_ty(&field.ty);
+    walk_list!(visitor, visit_attribute, &field.attrs);
+}
+
+pub fn walk_block<'a, V: Visitor<'a>>(visitor: &mut V, block: &'a Block) {
+    walk_list!(visitor, visit_stmt, &block.stmts);
+}
+
+pub fn walk_stmt<'a, V: Visitor<'a>>(visitor: &mut V, statement: &'a Stmt) {
+    match statement.kind {
+        StmtKind::Local(ref local) => visitor.visit_local(local),
+        StmtKind::Item(ref item) => visitor.visit_item(item),
+        StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => visitor.visit_expr(expr),
+        StmtKind::Empty => {}
+        StmtKind::MacCall(ref mac) => {
+            let MacCallStmt { ref mac, style: _, ref attrs, tokens: _ } = **mac;
+            visitor.visit_mac_call(mac);
+            for attr in attrs.iter() {
+                visitor.visit_attribute(attr);
+            }
+        }
+    }
+}
+
+pub fn walk_mac<'a, V: Visitor<'a>>(visitor: &mut V, mac: &'a MacCall) {
+    visitor.visit_path(&mac.path, DUMMY_NODE_ID);
+}
+
+pub fn walk_anon_const<'a, V: Visitor<'a>>(visitor: &mut V, constant: &'a AnonConst) {
+    visitor.visit_expr(&constant.value);
+}
+
+pub fn walk_inline_asm<'a, V: Visitor<'a>>(visitor: &mut V, asm: &'a InlineAsm) {
+    for (op, _) in &asm.operands {
+        match op {
+            InlineAsmOperand::In { expr, .. }
+            | InlineAsmOperand::Out { expr: Some(expr), .. }
+            | InlineAsmOperand::InOut { expr, .. } => visitor.visit_expr(expr),
+            InlineAsmOperand::Out { expr: None, .. } => {}
+            InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
+                visitor.visit_expr(in_expr);
+                if let Some(out_expr) = out_expr {
+                    visitor.visit_expr(out_expr);
+                }
+            }
+            InlineAsmOperand::Const { anon_const, .. } => visitor.visit_anon_const(anon_const),
+            InlineAsmOperand::Sym { sym } => visitor.visit_inline_asm_sym(sym),
+        }
+    }
+}
+
+pub fn walk_inline_asm_sym<'a, V: Visitor<'a>>(visitor: &mut V, sym: &'a InlineAsmSym) {
+    if let Some(ref qself) = sym.qself {
+        visitor.visit_ty(&qself.ty);
+    }
+    visitor.visit_path(&sym.path, sym.id);
+}
+
+pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
+    walk_list!(visitor, visit_attribute, expression.attrs.iter());
+
+    match expression.kind {
+        ExprKind::Box(ref subexpression) => visitor.visit_expr(subexpression),
+        ExprKind::Array(ref subexpressions) => {
+            walk_list!(visitor, visit_expr, subexpressions);
+        }
+        ExprKind::ConstBlock(ref anon_const) => visitor.visit_anon_const(anon_const),
+        ExprKind::Repeat(ref element, ref count) => {
+            visitor.visit_expr(element);
+            visitor.visit_anon_const(count)
+        }
+        ExprKind::Struct(ref se) => {
+            if let Some(ref qself) = se.qself {
+                visitor.visit_ty(&qself.ty);
+            }
+            visitor.visit_path(&se.path, expression.id);
+            walk_list!(visitor, visit_expr_field, &se.fields);
+            match &se.rest {
+                StructRest::Base(expr) => visitor.visit_expr(expr),
+                StructRest::Rest(_span) => {}
+                StructRest::None => {}
+            }
+        }
+        ExprKind::Tup(ref subexpressions) => {
+            walk_list!(visitor, visit_expr, subexpressions);
+        }
+        ExprKind::Call(ref callee_expression, ref arguments) => {
+            visitor.visit_expr(callee_expression);
+            walk_list!(visitor, visit_expr, arguments);
+        }
+        ExprKind::MethodCall(ref segment, ref arguments, _span) => {
+            visitor.visit_path_segment(expression.span, segment);
+            walk_list!(visitor, visit_expr, arguments);
+        }
+        ExprKind::Binary(_, ref left_expression, ref right_expression) => {
+            visitor.visit_expr(left_expression);
+            visitor.visit_expr(right_expression)
+        }
+        ExprKind::AddrOf(_, _, ref subexpression) | ExprKind::Unary(_, ref subexpression) => {
+            visitor.visit_expr(subexpression)
+        }
+        ExprKind::Cast(ref subexpression, ref typ) | ExprKind::Type(ref subexpression, ref typ) => {
+            visitor.visit_expr(subexpression);
+            visitor.visit_ty(typ)
+        }
+        ExprKind::Let(ref pat, ref expr, _) => {
+            visitor.visit_pat(pat);
+            visitor.visit_expr(expr);
+        }
+        ExprKind::If(ref head_expression, ref if_block, ref optional_else) => {
+            visitor.visit_expr(head_expression);
+            visitor.visit_block(if_block);
+            walk_list!(visitor, visit_expr, optional_else);
+        }
+        ExprKind::While(ref subexpression, ref block, ref opt_label) => {
+            walk_list!(visitor, visit_label, opt_label);
+            visitor.visit_expr(subexpression);
+            visitor.visit_block(block);
+        }
+        ExprKind::ForLoop(ref pattern, ref subexpression, ref block, ref opt_label) => {
+            walk_list!(visitor, visit_label, opt_label);
+            visitor.visit_pat(pattern);
+            visitor.visit_expr(subexpression);
+            visitor.visit_block(block);
+        }
+        ExprKind::Loop(ref block, ref opt_label) => {
+            walk_list!(visitor, visit_label, opt_label);
+            visitor.visit_block(block);
+        }
+        ExprKind::Match(ref subexpression, ref arms) => {
+            visitor.visit_expr(subexpression);
+            walk_list!(visitor, visit_arm, arms);
+        }
+        ExprKind::Closure(ref binder, _, _, _, ref decl, ref body, _decl_span) => {
+            visitor.visit_fn(FnKind::Closure(binder, decl, body), expression.span, expression.id)
+        }
+        ExprKind::Block(ref block, ref opt_label) => {
+            walk_list!(visitor, visit_label, opt_label);
+            visitor.visit_block(block);
+        }
+        ExprKind::Async(_, _, ref body) => {
+            visitor.visit_block(body);
+        }
+        ExprKind::Await(ref expr) => visitor.visit_expr(expr),
+        ExprKind::Assign(ref lhs, ref rhs, _) => {
+            visitor.visit_expr(lhs);
+            visitor.visit_expr(rhs);
+        }
+        ExprKind::AssignOp(_, ref left_expression, ref right_expression) => {
+            visitor.visit_expr(left_expression);
+            visitor.visit_expr(right_expression);
+        }
+        ExprKind::Field(ref subexpression, ident) => {
+            visitor.visit_expr(subexpression);
+            visitor.visit_ident(ident);
+        }
+        ExprKind::Index(ref main_expression, ref index_expression) => {
+            visitor.visit_expr(main_expression);
+            visitor.visit_expr(index_expression)
+        }
+        ExprKind::Range(ref start, ref end, _) => {
+            walk_list!(visitor, visit_expr, start);
+            walk_list!(visitor, visit_expr, end);
+        }
+        ExprKind::Underscore => {}
+        ExprKind::Path(ref maybe_qself, ref path) => {
+            if let Some(ref qself) = *maybe_qself {
+                visitor.visit_ty(&qself.ty);
+            }
+            visitor.visit_path(path, expression.id)
+        }
+        ExprKind::Break(ref opt_label, ref opt_expr) => {
+            walk_list!(visitor, visit_label, opt_label);
+            walk_list!(visitor, visit_expr, opt_expr);
+        }
+        ExprKind::Continue(ref opt_label) => {
+            walk_list!(visitor, visit_label, opt_label);
+        }
+        ExprKind::Ret(ref optional_expression) => {
+            walk_list!(visitor, visit_expr, optional_expression);
+        }
+        ExprKind::Yeet(ref optional_expression) => {
+            walk_list!(visitor, visit_expr, optional_expression);
+        }
+        ExprKind::MacCall(ref mac) => visitor.visit_mac_call(mac),
+        ExprKind::Paren(ref subexpression) => visitor.visit_expr(subexpression),
+        ExprKind::InlineAsm(ref asm) => visitor.visit_inline_asm(asm),
+        ExprKind::Yield(ref optional_expression) => {
+            walk_list!(visitor, visit_expr, optional_expression);
+        }
+        ExprKind::Try(ref subexpression) => visitor.visit_expr(subexpression),
+        ExprKind::TryBlock(ref body) => visitor.visit_block(body),
+        ExprKind::Lit(_) | ExprKind::Err => {}
+    }
+
+    visitor.visit_expr_post(expression)
+}
+
+pub fn walk_param<'a, V: Visitor<'a>>(visitor: &mut V, param: &'a Param) {
+    walk_list!(visitor, visit_attribute, param.attrs.iter());
+    visitor.visit_pat(&param.pat);
+    visitor.visit_ty(&param.ty);
+}
+
+pub fn walk_arm<'a, V: Visitor<'a>>(visitor: &mut V, arm: &'a Arm) {
+    visitor.visit_pat(&arm.pat);
+    walk_list!(visitor, visit_expr, &arm.guard);
+    visitor.visit_expr(&arm.body);
+    walk_list!(visitor, visit_attribute, &arm.attrs);
+}
+
+pub fn walk_vis<'a, V: Visitor<'a>>(visitor: &mut V, vis: &'a Visibility) {
+    if let VisibilityKind::Restricted { ref path, id } = vis.kind {
+        visitor.visit_path(path, id);
+    }
+}
+
+pub fn walk_attribute<'a, V: Visitor<'a>>(visitor: &mut V, attr: &'a Attribute) {
+    match attr.kind {
+        AttrKind::Normal(ref item, ref _tokens) => walk_mac_args(visitor, &item.args),
+        AttrKind::DocComment(..) => {}
+    }
+}
+
+pub fn walk_mac_args<'a, V: Visitor<'a>>(visitor: &mut V, args: &'a MacArgs) {
+    match args {
+        MacArgs::Empty => {}
+        MacArgs::Delimited(_dspan, _delim, _tokens) => {}
+        MacArgs::Eq(_eq_span, MacArgsEq::Ast(expr)) => visitor.visit_expr(expr),
+        MacArgs::Eq(_, MacArgsEq::Hir(lit)) => {
+            unreachable!("in literal form when walking mac args eq: {:?}", lit)
+        }
+    }
+}