Merging upstream version 1.69.0+dfsg1.

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

11 files changed, 759 insertions, 401 deletions

diff --git a/compiler/rustc_ast/Cargo.toml b/compiler/rustc_ast/Cargo.toml
index 9253b7e68..f0632ac92 100644
--- a/compiler/rustc_ast/Cargo.toml
+++ b/compiler/rustc_ast/Cargo.toml
@@ -7,6 +7,7 @@ edition = "2021"
 
 [dependencies]
 bitflags = "1.2.1"
+memchr = "2.5.0"
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_index = { path = "../rustc_index" }
 rustc_lexer = { path = "../rustc_lexer" }
@@ -14,5 +15,5 @@ rustc_macros = { path = "../rustc_macros" }
 rustc_serialize = { path = "../rustc_serialize" }
 rustc_span = { path = "../rustc_span" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
-thin-vec = "0.2.9"
+thin-vec = "0.2.12"
 tracing = "0.1"
diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 9317579f7..03c375c46 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -18,6 +18,7 @@
 //! - [`Attribute`]: Metadata associated with item.
 //! - [`UnOp`], [`BinOp`], and [`BinOpKind`]: Unary and binary operators.
 
+pub use crate::format::*;
 pub use crate::util::parser::ExprPrecedence;
 pub use GenericArgs::*;
 pub use UnsafeSource::*;
@@ -208,7 +209,7 @@ pub struct AngleBracketedArgs {
     /// The overall span.
     pub span: Span,
     /// The comma separated parts in the `<...>`.
-    pub args: Vec<AngleBracketedArg>,
+    pub args: ThinVec<AngleBracketedArg>,
 }
 
 /// Either an argument for a parameter e.g., `'a`, `Vec<u8>`, `0`,
@@ -252,7 +253,7 @@ pub struct ParenthesizedArgs {
     pub span: Span,
 
     /// `(A, B)`
-    pub inputs: Vec<P<Ty>>,
+    pub inputs: ThinVec<P<Ty>>,
 
     /// ```text
     /// Foo(A, B) -> C
@@ -383,7 +384,7 @@ impl GenericParam {
 /// a function, enum, trait, etc.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct Generics {
-    pub params: Vec<GenericParam>,
+    pub params: ThinVec<GenericParam>,
     pub where_clause: WhereClause,
     pub span: Span,
 }
@@ -391,7 +392,7 @@ pub struct Generics {
 impl Default for Generics {
     /// Creates an instance of `Generics`.
     fn default() -> Generics {
-        Generics { params: Vec::new(), where_clause: Default::default(), span: DUMMY_SP }
+        Generics { params: ThinVec::new(), where_clause: Default::default(), span: DUMMY_SP }
     }
 }
 
@@ -402,13 +403,13 @@ pub struct WhereClause {
     /// 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 predicates: ThinVec<WherePredicate>,
     pub span: Span,
 }
 
 impl Default for WhereClause {
     fn default() -> WhereClause {
-        WhereClause { has_where_token: false, predicates: Vec::new(), span: DUMMY_SP }
+        WhereClause { has_where_token: false, predicates: ThinVec::new(), span: DUMMY_SP }
     }
 }
 
@@ -440,7 +441,7 @@ impl WherePredicate {
 pub struct WhereBoundPredicate {
     pub span: Span,
     /// Any generics from a `for` binding.
-    pub bound_generic_params: Vec<GenericParam>,
+    pub bound_generic_params: ThinVec<GenericParam>,
     /// The type being bounded.
     pub bounded_ty: P<Ty>,
     /// Trait and lifetime bounds (`Clone + Send + 'static`).
@@ -470,7 +471,7 @@ pub struct WhereEqPredicate {
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct Crate {
     pub attrs: AttrVec,
-    pub items: Vec<P<Item>>,
+    pub items: ThinVec<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.
@@ -502,7 +503,7 @@ pub enum MetaItemKind {
     /// List meta item.
     ///
     /// E.g., `#[derive(..)]`, where the field represents the `..`.
-    List(Vec<NestedMetaItem>),
+    List(ThinVec<NestedMetaItem>),
 
     /// Name value meta item.
     ///
@@ -530,7 +531,7 @@ pub enum NestedMetaItem {
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct Block {
     /// The statements in the block.
-    pub stmts: Vec<Stmt>,
+    pub stmts: ThinVec<Stmt>,
     pub id: NodeId,
     /// Distinguishes between `unsafe { ... }` and `{ ... }`.
     pub rules: BlockCheckMode,
@@ -580,7 +581,7 @@ impl Pat {
             // 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());
+                let mut tys = ThinVec::with_capacity(pats.len());
                 // FIXME(#48994) - could just be collected into an Option<Vec>
                 for pat in pats {
                     tys.push(pat.to_ty()?);
@@ -721,14 +722,14 @@ pub enum PatKind {
 
     /// A struct or struct variant pattern (e.g., `Variant {x, y, ..}`).
     /// The `bool` is `true` in the presence of a `..`.
-    Struct(Option<P<QSelf>>, Path, Vec<PatField>, /* recovered */ bool),
+    Struct(Option<P<QSelf>>, Path, ThinVec<PatField>, /* recovered */ bool),
 
     /// A tuple struct/variant pattern (`Variant(x, y, .., z)`).
-    TupleStruct(Option<P<QSelf>>, Path, Vec<P<Pat>>),
+    TupleStruct(Option<P<QSelf>>, Path, ThinVec<P<Pat>>),
 
     /// An or-pattern `A | B | C`.
     /// Invariant: `pats.len() >= 2`.
-    Or(Vec<P<Pat>>),
+    Or(ThinVec<P<Pat>>),
 
     /// A possibly qualified path pattern.
     /// Unqualified path patterns `A::B::C` can legally refer to variants, structs, constants
@@ -737,7 +738,7 @@ pub enum PatKind {
     Path(Option<P<QSelf>>, Path),
 
     /// A tuple pattern (`(a, b)`).
-    Tuple(Vec<P<Pat>>),
+    Tuple(ThinVec<P<Pat>>),
 
     /// A `box` pattern.
     Box(P<Pat>),
@@ -752,7 +753,7 @@ pub enum PatKind {
     Range(Option<P<Expr>>, Option<P<Expr>>, Spanned<RangeEnd>),
 
     /// A slice pattern `[a, b, c]`.
-    Slice(Vec<P<Pat>>),
+    Slice(ThinVec<P<Pat>>),
 
     /// A rest pattern `..`.
     ///
@@ -1168,7 +1169,7 @@ impl Expr {
     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),
+                PolyTraitRef::new(ThinVec::new(), path.clone(), self.span),
                 TraitBoundModifier::None,
             )),
             _ => None,
@@ -1203,7 +1204,7 @@ impl Expr {
             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<_>>>()?;
+                let tys = exprs.iter().map(|expr| expr.to_ty()).collect::<Option<ThinVec<_>>>()?;
                 TyKind::Tup(tys)
             }
 
@@ -1269,6 +1270,7 @@ impl Expr {
             ExprKind::Try(..) => ExprPrecedence::Try,
             ExprKind::Yield(..) => ExprPrecedence::Yield,
             ExprKind::Yeet(..) => ExprPrecedence::Yeet,
+            ExprKind::FormatArgs(..) => ExprPrecedence::FormatArgs,
             ExprKind::Err => ExprPrecedence::Err,
         }
     }
@@ -1335,7 +1337,7 @@ pub struct MethodCall {
     /// The receiver, e.g. `x`.
     pub receiver: P<Expr>,
     /// The arguments, e.g. `a, b, c`.
-    pub args: Vec<P<Expr>>,
+    pub args: ThinVec<P<Expr>>,
     /// The span of the function, without the dot and receiver e.g. `foo::<Bar,
     /// Baz>(a, b, c)`.
     pub span: Span,
@@ -1355,7 +1357,7 @@ pub enum StructRest {
 pub struct StructExpr {
     pub qself: Option<P<QSelf>>,
     pub path: Path,
-    pub fields: Vec<ExprField>,
+    pub fields: ThinVec<ExprField>,
     pub rest: StructRest,
 }
 
@@ -1364,7 +1366,7 @@ pub enum ExprKind {
     /// A `box x` expression.
     Box(P<Expr>),
     /// An array (`[a, b, c, d]`)
-    Array(Vec<P<Expr>>),
+    Array(ThinVec<P<Expr>>),
     /// Allow anonymous constants from an inline `const` block
     ConstBlock(AnonConst),
     /// A function call
@@ -1373,11 +1375,11 @@ pub enum ExprKind {
     /// 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>>),
+    Call(P<Expr>, ThinVec<P<Expr>>),
     /// A method call (e.g. `x.foo::<Bar, Baz>(a, b, c)`).
     MethodCall(Box<MethodCall>),
     /// A tuple (e.g., `(a, b, c, d)`).
-    Tup(Vec<P<Expr>>),
+    Tup(ThinVec<P<Expr>>),
     /// A binary operation (e.g., `a + b`, `a * b`).
     Binary(BinOp, P<Expr>, P<Expr>),
     /// A unary operation (e.g., `!x`, `*x`).
@@ -1412,7 +1414,7 @@ pub enum ExprKind {
     /// `'label: loop { block }`
     Loop(P<Block>, Option<Label>, Span),
     /// A `match` block.
-    Match(P<Expr>, Vec<Arm>),
+    Match(P<Expr>, ThinVec<Arm>),
     /// A closure (e.g., `move |a, b, c| a + b + c`).
     Closure(Box<Closure>),
     /// A block (`'label: { ... }`).
@@ -1499,6 +1501,9 @@ pub enum ExprKind {
     /// with a `ByteStr` literal.
     IncludedBytes(Lrc<[u8]>),
 
+    /// A `format_args!()` expression.
+    FormatArgs(P<FormatArgs>),
+
     /// Placeholder for an expression that wasn't syntactically well formed in some way.
     Err,
 }
@@ -1569,7 +1574,7 @@ pub enum ClosureBinder {
         /// for<'a, 'b> |_: &'a (), _: &'b ()| { ... }
         ///     ^^^^^^ -- this
         /// ```
-        generic_params: P<[GenericParam]>,
+        generic_params: ThinVec<GenericParam>,
     },
 }
 
@@ -1821,6 +1826,13 @@ pub enum LitKind {
 }
 
 impl LitKind {
+    pub fn str(&self) -> Option<Symbol> {
+        match *self {
+            LitKind::Str(s, _) => Some(s),
+            _ => None,
+        }
+    }
+
     /// Returns `true` if this literal is a string.
     pub fn is_str(&self) -> bool {
         matches!(self, LitKind::Str(..))
@@ -2044,7 +2056,7 @@ impl Ty {
 pub struct BareFnTy {
     pub unsafety: Unsafe,
     pub ext: Extern,
-    pub generic_params: Vec<GenericParam>,
+    pub generic_params: ThinVec<GenericParam>,
     pub decl: P<FnDecl>,
     /// Span of the `fn(...) -> ...` part.
     pub decl_span: Span,
@@ -2066,7 +2078,7 @@ pub enum TyKind {
     /// The never type (`!`).
     Never,
     /// A tuple (`(A, B, C, D,...)`).
-    Tup(Vec<P<Ty>>),
+    Tup(ThinVec<P<Ty>>),
     /// A path (`module::module::...::Type`), optionally
     /// "qualified", e.g., `<Vec<T> as SomeTrait>::SomeType`.
     ///
@@ -2351,7 +2363,7 @@ impl Param {
 /// which contains metadata about function safety, asyncness, constness and ABI.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct FnDecl {
-    pub inputs: Vec<Param>,
+    pub inputs: ThinVec<Param>,
     pub output: FnRetTy,
 }
 
@@ -2463,7 +2475,7 @@ pub enum ModKind {
     /// 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),
+    Loaded(ThinVec<P<Item>>, Inline, ModSpans),
     /// Module with definition outlined to a separate file `mod foo;` but not yet loaded from it.
     Unloaded,
 }
@@ -2485,12 +2497,12 @@ pub struct ForeignMod {
     /// semantically by Rust.
     pub unsafety: Unsafe,
     pub abi: Option<StrLit>,
-    pub items: Vec<P<ForeignItem>>,
+    pub items: ThinVec<P<ForeignItem>>,
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct EnumDef {
-    pub variants: Vec<Variant>,
+    pub variants: ThinVec<Variant>,
 }
 /// Enum variant.
 #[derive(Clone, Encodable, Decodable, Debug)]
@@ -2520,7 +2532,7 @@ pub enum UseTreeKind {
     /// `use prefix` or `use prefix as rename`
     Simple(Option<Ident>),
     /// `use prefix::{...}`
-    Nested(Vec<(UseTree, NodeId)>),
+    Nested(ThinVec<(UseTree, NodeId)>),
     /// `use prefix::*`
     Glob,
 }
@@ -2624,7 +2636,7 @@ pub struct TraitRef {
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct PolyTraitRef {
     /// The `'a` in `for<'a> Foo<&'a T>`.
-    pub bound_generic_params: Vec<GenericParam>,
+    pub bound_generic_params: ThinVec<GenericParam>,
 
     /// The `Foo<&'a T>` in `<'a> Foo<&'a T>`.
     pub trait_ref: TraitRef,
@@ -2633,7 +2645,7 @@ pub struct PolyTraitRef {
 }
 
 impl PolyTraitRef {
-    pub fn new(generic_params: Vec<GenericParam>, path: Path, span: Span) -> Self {
+    pub fn new(generic_params: ThinVec<GenericParam>, path: Path, span: Span) -> Self {
         PolyTraitRef {
             bound_generic_params: generic_params,
             trait_ref: TraitRef { path, ref_id: DUMMY_NODE_ID },
@@ -2683,11 +2695,11 @@ pub enum VariantData {
     /// Struct variant.
     ///
     /// E.g., `Bar { .. }` as in `enum Foo { Bar { .. } }`.
-    Struct(Vec<FieldDef>, bool),
+    Struct(ThinVec<FieldDef>, bool),
     /// Tuple variant.
     ///
     /// E.g., `Bar(..)` as in `enum Foo { Bar(..) }`.
-    Tuple(Vec<FieldDef>, NodeId),
+    Tuple(ThinVec<FieldDef>, NodeId),
     /// Unit variant.
     ///
     /// E.g., `Bar = ..` as in `enum Foo { Bar = .. }`.
@@ -2814,7 +2826,7 @@ pub struct Trait {
     pub is_auto: IsAuto,
     pub generics: Generics,
     pub bounds: GenericBounds,
-    pub items: Vec<P<AssocItem>>,
+    pub items: ThinVec<P<AssocItem>>,
 }
 
 /// The location of a where clause on a `TyAlias` (`Span`) and whether there was
@@ -2862,7 +2874,7 @@ pub struct Impl {
     /// The trait being implemented, if any.
     pub of_trait: Option<TraitRef>,
     pub self_ty: P<Ty>,
-    pub items: Vec<P<AssocItem>>,
+    pub items: ThinVec<P<AssocItem>>,
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]
@@ -3100,26 +3112,26 @@ mod size_asserts {
     static_assert_size!(AssocItem, 104);
     static_assert_size!(AssocItemKind, 32);
     static_assert_size!(Attribute, 32);
-    static_assert_size!(Block, 48);
+    static_assert_size!(Block, 32);
     static_assert_size!(Expr, 72);
     static_assert_size!(ExprKind, 40);
-    static_assert_size!(Fn, 184);
+    static_assert_size!(Fn, 152);
     static_assert_size!(ForeignItem, 96);
     static_assert_size!(ForeignItemKind, 24);
     static_assert_size!(GenericArg, 24);
-    static_assert_size!(GenericBound, 72);
-    static_assert_size!(Generics, 72);
-    static_assert_size!(Impl, 184);
-    static_assert_size!(Item, 184);
-    static_assert_size!(ItemKind, 112);
+    static_assert_size!(GenericBound, 56);
+    static_assert_size!(Generics, 40);
+    static_assert_size!(Impl, 136);
+    static_assert_size!(Item, 136);
+    static_assert_size!(ItemKind, 64);
     static_assert_size!(LitKind, 24);
     static_assert_size!(Local, 72);
     static_assert_size!(MetaItemLit, 40);
     static_assert_size!(Param, 40);
-    static_assert_size!(Pat, 88);
+    static_assert_size!(Pat, 72);
     static_assert_size!(Path, 24);
     static_assert_size!(PathSegment, 24);
-    static_assert_size!(PatKind, 64);
+    static_assert_size!(PatKind, 48);
     static_assert_size!(Stmt, 32);
     static_assert_size!(StmtKind, 16);
     static_assert_size!(Ty, 64);
diff --git a/compiler/rustc_ast/src/attr/mod.rs b/compiler/rustc_ast/src/attr/mod.rs
index c6b6207b3..2e83b3e62 100644
--- a/compiler/rustc_ast/src/attr/mod.rs
+++ b/compiler/rustc_ast/src/attr/mod.rs
@@ -20,7 +20,7 @@ use std::iter;
 use std::ops::BitXor;
 #[cfg(debug_assertions)]
 use std::sync::atomic::{AtomicU32, Ordering};
-use thin_vec::thin_vec;
+use thin_vec::{thin_vec, ThinVec};
 
 pub struct MarkedAttrs(GrowableBitSet<AttrId>);
 
@@ -40,84 +40,65 @@ impl MarkedAttrs {
     }
 }
 
-impl NestedMetaItem {
-    /// Returns the `MetaItem` if `self` is a `NestedMetaItem::MetaItem`.
-    pub fn meta_item(&self) -> Option<&MetaItem> {
-        match self {
-            NestedMetaItem::MetaItem(item) => Some(item),
-            _ => None,
-        }
-    }
+pub struct AttrIdGenerator(WorkerLocal<Cell<u32>>);
 
-    /// Returns the `MetaItemLit` if `self` is a `NestedMetaItem::Literal`s.
-    pub fn lit(&self) -> Option<&MetaItemLit> {
-        match self {
-            NestedMetaItem::Lit(lit) => Some(lit),
-            _ => None,
-        }
-    }
+#[cfg(debug_assertions)]
+static MAX_ATTR_ID: AtomicU32 = AtomicU32::new(u32::MAX);
 
-    /// 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))
-    }
+impl AttrIdGenerator {
+    pub fn new() -> Self {
+        // We use `(index as u32).reverse_bits()` to initialize the
+        // starting value of AttrId in each worker thread.
+        // The `index` is the index of the worker thread.
+        // This ensures that the AttrId generated in each thread is unique.
+        AttrIdGenerator(WorkerLocal::new(|index| {
+            let index: u32 = index.try_into().unwrap();
 
-    /// 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
-    }
+            #[cfg(debug_assertions)]
+            {
+                let max_id = ((index + 1).next_power_of_two() - 1).bitxor(u32::MAX).reverse_bits();
+                MAX_ATTR_ID.fetch_min(max_id, Ordering::Release);
+            }
 
-    /// 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())
+            Cell::new(index.reverse_bits())
+        }))
     }
 
-    /// Returns a name and single literal value tuple of the `MetaItem`.
-    pub fn name_value_literal(&self) -> Option<(Symbol, &MetaItemLit)> {
-        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].lit()
-                {
-                    return Some((ident.name, lit));
-                }
-                None
-            })
-        })
-    }
+    pub fn mk_attr_id(&self) -> AttrId {
+        let id = self.0.get();
 
-    /// 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())
-    }
+        // Ensure the assigned attr_id does not overlap the bits
+        // representing the number of threads.
+        #[cfg(debug_assertions)]
+        assert!(id <= MAX_ATTR_ID.load(Ordering::Acquire));
 
-    /// Returns `true` if the variant is `MetaItem`.
-    pub fn is_meta_item(&self) -> bool {
-        self.meta_item().is_some()
+        self.0.set(id + 1);
+        AttrId::from_u32(id)
     }
+}
 
-    /// 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())
+impl Attribute {
+    pub fn get_normal_item(&self) -> &AttrItem {
+        match &self.kind {
+            AttrKind::Normal(normal) => &normal.item,
+            AttrKind::DocComment(..) => panic!("unexpected doc comment"),
+        }
     }
 
-    /// See [`MetaItem::name_value_literal_span`].
-    pub fn name_value_literal_span(&self) -> Option<Span> {
-        self.meta_item()?.name_value_literal_span()
+    pub fn unwrap_normal_item(self) -> AttrItem {
+        match self.kind {
+            AttrKind::Normal(normal) => normal.into_inner().item,
+            AttrKind::DocComment(..) => panic!("unexpected doc comment"),
+        }
     }
-}
 
-impl Attribute {
-    #[inline]
-    pub fn has_name(&self, name: Symbol) -> bool {
-        match &self.kind {
-            AttrKind::Normal(normal) => normal.item.path == name,
-            AttrKind::DocComment(..) => false,
+    /// Returns `true` if it is a sugared doc comment (`///` or `//!` for example).
+    /// So `#[doc = "doc"]` (which is a doc comment) and `#[doc(...)]` (which is not
+    /// a doc comment) will return `false`.
+    pub fn is_doc_comment(&self) -> bool {
+        match self.kind {
+            AttrKind::Normal(..) => false,
+            AttrKind::DocComment(..) => true,
         }
     }
 
@@ -138,20 +119,11 @@ impl Attribute {
         self.ident().unwrap_or_else(Ident::empty).name
     }
 
-    pub fn value_str(&self) -> Option<Symbol> {
-        match &self.kind {
-            AttrKind::Normal(normal) => normal.item.meta_kind().and_then(|kind| kind.value_str()),
-            AttrKind::DocComment(..) => None,
-        }
-    }
-
-    pub fn meta_item_list(&self) -> Option<Vec<NestedMetaItem>> {
+    #[inline]
+    pub fn has_name(&self, name: Symbol) -> bool {
         match &self.kind {
-            AttrKind::Normal(normal) => match normal.item.meta_kind() {
-                Some(MetaItemKind::List(list)) => Some(list),
-                _ => None,
-            },
-            AttrKind::DocComment(..) => None,
+            AttrKind::Normal(normal) => normal.item.path == name,
+            AttrKind::DocComment(..) => false,
         }
     }
 
@@ -162,82 +134,18 @@ impl Attribute {
             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
-    }
 
-    /// ```text
-    /// Example:
-    ///     #[attribute(name = "value")]
-    ///                 ^^^^^^^^^^^^^^
-    /// ```
-    pub fn name_value_literal(&self) -> Option<&MetaItemLit> {
+    pub fn meta_item_list(&self) -> Option<ThinVec<NestedMetaItem>> {
         match &self.kind {
-            MetaItemKind::NameValue(v) => Some(v),
-            _ => None,
+            AttrKind::Normal(normal) => normal.item.meta_item_list(),
+            AttrKind::DocComment(..) => None,
         }
     }
 
     pub fn value_str(&self) -> Option<Symbol> {
-        self.kind.value_str()
-    }
-
-    pub fn meta_item_list(&self) -> Option<&[NestedMetaItem]> {
         match &self.kind {
-            MetaItemKind::List(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: self.meta_kind()?, span })
-    }
-
-    pub fn meta_kind(&self) -> Option<MetaItemKind> {
-        MetaItemKind::from_attr_args(&self.args)
-    }
-}
-
-impl Attribute {
-    /// Returns `true` if it is a sugared doc comment (`///` or `//!` for example).
-    /// So `#[doc = "doc"]` (which is a doc comment) and `#[doc(...)]` (which is not
-    /// a doc comment) will return `false`.
-    pub fn is_doc_comment(&self) -> bool {
-        match self.kind {
-            AttrKind::Normal(..) => false,
-            AttrKind::DocComment(..) => true,
+            AttrKind::Normal(normal) => normal.item.value_str(),
+            AttrKind::DocComment(..) => None,
         }
     }
 
@@ -247,13 +155,11 @@ impl Attribute {
     /// * `#[doc = "doc"]` returns `Some(("doc", CommentKind::Line))`.
     /// * `#[doc(...)]` returns `None`.
     pub fn doc_str_and_comment_kind(&self) -> Option<(Symbol, CommentKind)> {
-        match self.kind {
-            AttrKind::DocComment(kind, data) => Some((data, kind)),
-            AttrKind::Normal(ref normal) if normal.item.path == sym::doc => normal
-                .item
-                .meta_kind()
-                .and_then(|kind| kind.value_str())
-                .map(|data| (data, CommentKind::Line)),
+        match &self.kind {
+            AttrKind::DocComment(kind, data) => Some((*data, *kind)),
+            AttrKind::Normal(normal) if normal.item.path == sym::doc => {
+                normal.item.value_str().map(|s| (s, CommentKind::Line))
+            }
             _ => None,
         }
     }
@@ -265,9 +171,7 @@ impl Attribute {
     pub fn doc_str(&self) -> Option<Symbol> {
         match &self.kind {
             AttrKind::DocComment(.., data) => Some(*data),
-            AttrKind::Normal(normal) if normal.item.path == sym::doc => {
-                normal.item.meta_kind().and_then(|kind| kind.value_str())
-            }
+            AttrKind::Normal(normal) if normal.item.path == sym::doc => normal.item.value_str(),
             _ => None,
         }
     }
@@ -276,20 +180,6 @@ impl Attribute {
         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(normal) => &normal.item,
-            AttrKind::DocComment(..) => panic!("unexpected doc comment"),
-        }
-    }
-
-    pub fn unwrap_normal_item(self) -> AttrItem {
-        match self.kind {
-            AttrKind::Normal(normal) => normal.into_inner().item,
-            AttrKind::DocComment(..) => panic!("unexpected doc comment"),
-        }
-    }
-
     /// Extracts the MetaItem from inside this Attribute.
     pub fn meta(&self) -> Option<MetaItem> {
         match &self.kind {
@@ -321,130 +211,102 @@ impl Attribute {
     }
 }
 
-pub struct AttrIdGenerator(WorkerLocal<Cell<u32>>);
-
-#[cfg(debug_assertions)]
-static MAX_ATTR_ID: AtomicU32 = AtomicU32::new(u32::MAX);
-
-impl AttrIdGenerator {
-    pub fn new() -> Self {
-        // We use `(index as u32).reverse_bits()` to initialize the
-        // starting value of AttrId in each worker thread.
-        // The `index` is the index of the worker thread.
-        // This ensures that the AttrId generated in each thread is unique.
-        AttrIdGenerator(WorkerLocal::new(|index| {
-            let index: u32 = index.try_into().unwrap();
+impl AttrItem {
+    pub fn span(&self) -> Span {
+        self.args.span().map_or(self.path.span, |args_span| self.path.span.to(args_span))
+    }
 
-            #[cfg(debug_assertions)]
-            {
-                let max_id = ((index + 1).next_power_of_two() - 1).bitxor(u32::MAX).reverse_bits();
-                MAX_ATTR_ID.fetch_min(max_id, Ordering::Release);
+    fn meta_item_list(&self) -> Option<ThinVec<NestedMetaItem>> {
+        match &self.args {
+            AttrArgs::Delimited(args) if args.delim == MacDelimiter::Parenthesis => {
+                MetaItemKind::list_from_tokens(args.tokens.clone())
             }
-
-            Cell::new(index.reverse_bits())
-        }))
+            AttrArgs::Delimited(_) | AttrArgs::Eq(..) | AttrArgs::Empty => None,
+        }
     }
 
-    pub fn mk_attr_id(&self) -> AttrId {
-        let id = self.0.get();
+    fn value_str(&self) -> Option<Symbol> {
+        match &self.args {
+            AttrArgs::Eq(_, args) => args.value_str(),
+            AttrArgs::Delimited(_) | AttrArgs::Empty => None,
+        }
+    }
 
-        // Ensure the assigned attr_id does not overlap the bits
-        // representing the number of threads.
-        #[cfg(debug_assertions)]
-        assert!(id <= MAX_ATTR_ID.load(Ordering::Acquire));
+    pub fn meta(&self, span: Span) -> Option<MetaItem> {
+        Some(MetaItem { path: self.path.clone(), kind: self.meta_kind()?, span })
+    }
 
-        self.0.set(id + 1);
-        AttrId::from_u32(id)
+    pub fn meta_kind(&self) -> Option<MetaItemKind> {
+        MetaItemKind::from_attr_args(&self.args)
     }
 }
 
-pub fn mk_attr(
-    g: &AttrIdGenerator,
-    style: AttrStyle,
-    path: Path,
-    args: AttrArgs,
-    span: Span,
-) -> Attribute {
-    mk_attr_from_item(g, AttrItem { path, args, tokens: None }, None, style, span)
+impl AttrArgsEq {
+    fn value_str(&self) -> Option<Symbol> {
+        match self {
+            AttrArgsEq::Ast(expr) => match expr.kind {
+                ExprKind::Lit(token_lit) => {
+                    LitKind::from_token_lit(token_lit).ok().and_then(|lit| lit.str())
+                }
+                _ => None,
+            },
+            AttrArgsEq::Hir(lit) => lit.kind.str(),
+        }
+    }
 }
 
-pub fn mk_attr_from_item(
-    g: &AttrIdGenerator,
-    item: AttrItem,
-    tokens: Option<LazyAttrTokenStream>,
-    style: AttrStyle,
-    span: Span,
-) -> Attribute {
-    Attribute {
-        kind: AttrKind::Normal(P(NormalAttr { item, tokens })),
-        id: g.mk_attr_id(),
-        style,
-        span,
+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 mk_attr_word(g: &AttrIdGenerator, style: AttrStyle, name: Symbol, span: Span) -> Attribute {
-    let path = Path::from_ident(Ident::new(name, span));
-    let args = AttrArgs::Empty;
-    mk_attr(g, style, path, args, span)
-}
+    pub fn name_or_empty(&self) -> Symbol {
+        self.ident().unwrap_or_else(Ident::empty).name
+    }
 
-pub fn mk_attr_name_value_str(
-    g: &AttrIdGenerator,
-    style: AttrStyle,
-    name: Symbol,
-    val: Symbol,
-    span: Span,
-) -> Attribute {
-    let lit = token::Lit::new(token::Str, escape_string_symbol(val), None);
-    let expr = P(Expr {
-        id: DUMMY_NODE_ID,
-        kind: ExprKind::Lit(lit),
-        span,
-        attrs: AttrVec::new(),
-        tokens: None,
-    });
-    let path = Path::from_ident(Ident::new(name, span));
-    let args = AttrArgs::Eq(span, AttrArgsEq::Ast(expr));
-    mk_attr(g, style, path, args, span)
-}
+    pub fn has_name(&self, name: Symbol) -> bool {
+        self.path == name
+    }
 
-pub fn mk_attr_nested_word(
-    g: &AttrIdGenerator,
-    style: AttrStyle,
-    outer: Symbol,
-    inner: Symbol,
-    span: Span,
-) -> Attribute {
-    let inner_tokens = TokenStream::new(vec![TokenTree::Token(
-        Token::from_ast_ident(Ident::new(inner, span)),
-        Spacing::Alone,
-    )]);
-    let outer_ident = Ident::new(outer, span);
-    let path = Path::from_ident(outer_ident);
-    let attr_args = AttrArgs::Delimited(DelimArgs {
-        dspan: DelimSpan::from_single(span),
-        delim: MacDelimiter::Parenthesis,
-        tokens: inner_tokens,
-    });
-    mk_attr(g, style, path, attr_args, span)
-}
+    pub fn is_word(&self) -> bool {
+        matches!(self.kind, MetaItemKind::Word)
+    }
 
-pub fn mk_doc_comment(
-    g: &AttrIdGenerator,
-    comment_kind: CommentKind,
-    style: AttrStyle,
-    data: Symbol,
-    span: Span,
-) -> Attribute {
-    Attribute { kind: AttrKind::DocComment(comment_kind, data), id: g.mk_attr_id(), style, span }
-}
+    pub fn meta_item_list(&self) -> Option<&[NestedMetaItem]> {
+        match &self.kind {
+            MetaItemKind::List(l) => Some(&**l),
+            _ => None,
+        }
+    }
 
-pub fn list_contains_name(items: &[NestedMetaItem], name: Symbol) -> bool {
-    items.iter().any(|item| item.has_name(name))
-}
+    /// ```text
+    /// Example:
+    ///     #[attribute(name = "value")]
+    ///                 ^^^^^^^^^^^^^^
+    /// ```
+    pub fn name_value_literal(&self) -> Option<&MetaItemLit> {
+        match &self.kind {
+            MetaItemKind::NameValue(v) => Some(v),
+            _ => None,
+        }
+    }
+
+    /// 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)
+    }
+
+    pub fn value_str(&self) -> Option<Symbol> {
+        self.kind.value_str()
+    }
 
-impl MetaItem {
     fn from_tokens<I>(tokens: &mut iter::Peekable<I>) -> Option<MetaItem>
     where
         I: Iterator<Item = TokenTree>,
@@ -508,17 +370,14 @@ impl MetaItem {
 impl MetaItemKind {
     pub fn value_str(&self) -> Option<Symbol> {
         match self {
-            MetaItemKind::NameValue(v) => match v.kind {
-                LitKind::Str(s, _) => Some(s),
-                _ => None,
-            },
+            MetaItemKind::NameValue(v) => v.kind.str(),
             _ => None,
         }
     }
 
-    fn list_from_tokens(tokens: TokenStream) -> Option<MetaItemKind> {
+    fn list_from_tokens(tokens: TokenStream) -> Option<ThinVec<NestedMetaItem>> {
         let mut tokens = tokens.into_trees().peekable();
-        let mut result = Vec::new();
+        let mut result = ThinVec::new();
         while tokens.peek().is_some() {
             let item = NestedMetaItem::from_tokens(&mut tokens)?;
             result.push(item);
@@ -527,7 +386,7 @@ impl MetaItemKind {
                 _ => return None,
             }
         }
-        Some(MetaItemKind::List(result))
+        Some(result)
     }
 
     fn name_value_from_tokens(
@@ -544,6 +403,24 @@ impl MetaItemKind {
         }
     }
 
+    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).map(MetaItemKind::List)
+            }
+            Some(TokenTree::Delimited(..)) => None,
+            Some(TokenTree::Token(Token { kind: token::Eq, .. }, _)) => {
+                tokens.next();
+                MetaItemKind::name_value_from_tokens(tokens)
+            }
+            _ => Some(MetaItemKind::Word),
+        }
+    }
+
     fn from_attr_args(args: &AttrArgs) -> Option<MetaItemKind> {
         match args {
             AttrArgs::Empty => Some(MetaItemKind::Word),
@@ -551,7 +428,7 @@ impl MetaItemKind {
                 dspan: _,
                 delim: MacDelimiter::Parenthesis,
                 tokens,
-            }) => MetaItemKind::list_from_tokens(tokens.clone()),
+            }) => MetaItemKind::list_from_tokens(tokens.clone()).map(MetaItemKind::List),
             AttrArgs::Delimited(..) => None,
             AttrArgs::Eq(_, AttrArgsEq::Ast(expr)) => match expr.kind {
                 ExprKind::Lit(token_lit) => {
@@ -565,24 +442,6 @@ impl MetaItemKind {
             AttrArgs::Eq(_, AttrArgsEq::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 {
@@ -593,6 +452,77 @@ impl NestedMetaItem {
         }
     }
 
+    /// 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
+    }
+
+    /// 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))
+    }
+
+    /// 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())
+    }
+
+    /// 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 a name and single literal value tuple of the `MetaItem`.
+    pub fn name_value_literal(&self) -> Option<(Symbol, &MetaItemLit)> {
+        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].lit()
+                {
+                    return Some((ident.name, lit));
+                }
+                None
+            })
+        })
+    }
+
+    /// See [`MetaItem::name_value_literal_span`].
+    pub fn name_value_literal_span(&self) -> Option<Span> {
+        self.meta_item()?.name_value_literal_span()
+    }
+
+    /// 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 the `MetaItemLit` if `self` is a `NestedMetaItem::Literal`s.
+    pub fn lit(&self) -> Option<&MetaItemLit> {
+        match self {
+            NestedMetaItem::Lit(lit) => Some(lit),
+            _ => None,
+        }
+    }
+
+    /// Returns the `MetaItem` if `self` is a `NestedMetaItem::MetaItem`.
+    pub fn meta_item(&self) -> Option<&MetaItem> {
+        match self {
+            NestedMetaItem::MetaItem(item) => Some(item),
+            _ => None,
+        }
+    }
+
+    /// Returns `true` if the variant is `MetaItem`.
+    pub fn is_meta_item(&self) -> bool {
+        self.meta_item().is_some()
+    }
+
     fn from_tokens<I>(tokens: &mut iter::Peekable<I>) -> Option<NestedMetaItem>
     where
         I: Iterator<Item = TokenTree>,
@@ -614,3 +544,89 @@ impl NestedMetaItem {
         MetaItem::from_tokens(tokens).map(NestedMetaItem::MetaItem)
     }
 }
+
+pub fn mk_doc_comment(
+    g: &AttrIdGenerator,
+    comment_kind: CommentKind,
+    style: AttrStyle,
+    data: Symbol,
+    span: Span,
+) -> Attribute {
+    Attribute { kind: AttrKind::DocComment(comment_kind, data), id: g.mk_attr_id(), style, span }
+}
+
+pub fn mk_attr(
+    g: &AttrIdGenerator,
+    style: AttrStyle,
+    path: Path,
+    args: AttrArgs,
+    span: Span,
+) -> Attribute {
+    mk_attr_from_item(g, AttrItem { path, args, tokens: None }, None, style, span)
+}
+
+pub fn mk_attr_from_item(
+    g: &AttrIdGenerator,
+    item: AttrItem,
+    tokens: Option<LazyAttrTokenStream>,
+    style: AttrStyle,
+    span: Span,
+) -> Attribute {
+    Attribute {
+        kind: AttrKind::Normal(P(NormalAttr { item, tokens })),
+        id: g.mk_attr_id(),
+        style,
+        span,
+    }
+}
+
+pub fn mk_attr_word(g: &AttrIdGenerator, style: AttrStyle, name: Symbol, span: Span) -> Attribute {
+    let path = Path::from_ident(Ident::new(name, span));
+    let args = AttrArgs::Empty;
+    mk_attr(g, style, path, args, span)
+}
+
+pub fn mk_attr_nested_word(
+    g: &AttrIdGenerator,
+    style: AttrStyle,
+    outer: Symbol,
+    inner: Symbol,
+    span: Span,
+) -> Attribute {
+    let inner_tokens = TokenStream::new(vec![TokenTree::Token(
+        Token::from_ast_ident(Ident::new(inner, span)),
+        Spacing::Alone,
+    )]);
+    let outer_ident = Ident::new(outer, span);
+    let path = Path::from_ident(outer_ident);
+    let attr_args = AttrArgs::Delimited(DelimArgs {
+        dspan: DelimSpan::from_single(span),
+        delim: MacDelimiter::Parenthesis,
+        tokens: inner_tokens,
+    });
+    mk_attr(g, style, path, attr_args, span)
+}
+
+pub fn mk_attr_name_value_str(
+    g: &AttrIdGenerator,
+    style: AttrStyle,
+    name: Symbol,
+    val: Symbol,
+    span: Span,
+) -> Attribute {
+    let lit = token::Lit::new(token::Str, escape_string_symbol(val), None);
+    let expr = P(Expr {
+        id: DUMMY_NODE_ID,
+        kind: ExprKind::Lit(lit),
+        span,
+        attrs: AttrVec::new(),
+        tokens: None,
+    });
+    let path = Path::from_ident(Ident::new(name, span));
+    let args = AttrArgs::Eq(span, AttrArgsEq::Ast(expr));
+    mk_attr(g, style, path, args, span)
+}
+
+pub fn list_contains_name(items: &[NestedMetaItem], name: Symbol) -> bool {
+    items.iter().any(|item| item.has_name(name))
+}
diff --git a/compiler/rustc_ast/src/format.rs b/compiler/rustc_ast/src/format.rs
new file mode 100644
index 000000000..d021bea5e
--- /dev/null
+++ b/compiler/rustc_ast/src/format.rs
@@ -0,0 +1,272 @@
+use crate::ptr::P;
+use crate::Expr;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_span::symbol::{Ident, Symbol};
+use rustc_span::Span;
+
+// Definitions:
+//
+// format_args!("hello {abc:.xyz$}!!", abc="world");
+// └──────────────────────────────────────────────┘
+//                     FormatArgs
+//
+// format_args!("hello {abc:.xyz$}!!", abc="world");
+//                                     └─────────┘
+//                                      argument
+//
+// format_args!("hello {abc:.xyz$}!!", abc="world");
+//              └───────────────────┘
+//                     template
+//
+// format_args!("hello {abc:.xyz$}!!", abc="world");
+//               └────┘└─────────┘└┘
+//                      pieces
+//
+// format_args!("hello {abc:.xyz$}!!", abc="world");
+//               └────┘           └┘
+//                   literal pieces
+//
+// format_args!("hello {abc:.xyz$}!!", abc="world");
+//                     └─────────┘
+//                     placeholder
+//
+// format_args!("hello {abc:.xyz$}!!", abc="world");
+//                      └─┘  └─┘
+//                      positions (could be names, numbers, empty, or `*`)
+
+/// (Parsed) format args.
+///
+/// Basically the "AST" for a complete `format_args!()`.
+///
+/// E.g., `format_args!("hello {name}");`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct FormatArgs {
+    pub span: Span,
+    pub template: Vec<FormatArgsPiece>,
+    pub arguments: FormatArguments,
+}
+
+/// A piece of a format template string.
+///
+/// E.g. "hello" or "{name}".
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum FormatArgsPiece {
+    Literal(Symbol),
+    Placeholder(FormatPlaceholder),
+}
+
+/// The arguments to format_args!().
+///
+/// E.g. `1, 2, name="ferris", n=3`,
+/// but also implicit captured arguments like `x` in `format_args!("{x}")`.
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct FormatArguments {
+    arguments: Vec<FormatArgument>,
+    num_unnamed_args: usize,
+    num_explicit_args: usize,
+    names: FxHashMap<Symbol, usize>,
+}
+
+// FIXME: Rustdoc has trouble proving Send/Sync for this. See #106930.
+#[cfg(parallel_compiler)]
+unsafe impl Sync for FormatArguments {}
+#[cfg(parallel_compiler)]
+unsafe impl Send for FormatArguments {}
+
+impl FormatArguments {
+    pub fn new() -> Self {
+        Self {
+            arguments: Vec::new(),
+            names: FxHashMap::default(),
+            num_unnamed_args: 0,
+            num_explicit_args: 0,
+        }
+    }
+
+    pub fn add(&mut self, arg: FormatArgument) -> usize {
+        let index = self.arguments.len();
+        if let Some(name) = arg.kind.ident() {
+            self.names.insert(name.name, index);
+        } else if self.names.is_empty() {
+            // Only count the unnamed args before the first named arg.
+            // (Any later ones are errors.)
+            self.num_unnamed_args += 1;
+        }
+        if !matches!(arg.kind, FormatArgumentKind::Captured(..)) {
+            // This is an explicit argument.
+            // Make sure that all arguments so far are explcit.
+            assert_eq!(
+                self.num_explicit_args,
+                self.arguments.len(),
+                "captured arguments must be added last"
+            );
+            self.num_explicit_args += 1;
+        }
+        self.arguments.push(arg);
+        index
+    }
+
+    pub fn by_name(&self, name: Symbol) -> Option<(usize, &FormatArgument)> {
+        let i = *self.names.get(&name)?;
+        Some((i, &self.arguments[i]))
+    }
+
+    pub fn by_index(&self, i: usize) -> Option<&FormatArgument> {
+        (i < self.num_explicit_args).then(|| &self.arguments[i])
+    }
+
+    pub fn unnamed_args(&self) -> &[FormatArgument] {
+        &self.arguments[..self.num_unnamed_args]
+    }
+
+    pub fn named_args(&self) -> &[FormatArgument] {
+        &self.arguments[self.num_unnamed_args..self.num_explicit_args]
+    }
+
+    pub fn explicit_args(&self) -> &[FormatArgument] {
+        &self.arguments[..self.num_explicit_args]
+    }
+
+    pub fn all_args(&self) -> &[FormatArgument] {
+        &self.arguments[..]
+    }
+
+    pub fn all_args_mut(&mut self) -> &mut [FormatArgument] {
+        &mut self.arguments[..]
+    }
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct FormatArgument {
+    pub kind: FormatArgumentKind,
+    pub expr: P<Expr>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum FormatArgumentKind {
+    /// `format_args(…, arg)`
+    Normal,
+    /// `format_args(…, arg = 1)`
+    Named(Ident),
+    /// `format_args("… {arg} …")`
+    Captured(Ident),
+}
+
+impl FormatArgumentKind {
+    pub fn ident(&self) -> Option<Ident> {
+        match self {
+            &Self::Normal => None,
+            &Self::Named(id) => Some(id),
+            &Self::Captured(id) => Some(id),
+        }
+    }
+}
+
+#[derive(Clone, Encodable, Decodable, Debug, PartialEq, Eq)]
+pub struct FormatPlaceholder {
+    /// Index into [`FormatArgs::arguments`].
+    pub argument: FormatArgPosition,
+    /// The span inside the format string for the full `{…}` placeholder.
+    pub span: Option<Span>,
+    /// `{}`, `{:?}`, or `{:x}`, etc.
+    pub format_trait: FormatTrait,
+    /// `{}` or `{:.5}` or `{:-^20}`, etc.
+    pub format_options: FormatOptions,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug, PartialEq, Eq)]
+pub struct FormatArgPosition {
+    /// Which argument this position refers to (Ok),
+    /// or would've referred to if it existed (Err).
+    pub index: Result<usize, usize>,
+    /// What kind of position this is. See [`FormatArgPositionKind`].
+    pub kind: FormatArgPositionKind,
+    /// The span of the name or number.
+    pub span: Option<Span>,
+}
+
+#[derive(Copy, Clone, Encodable, Decodable, Debug, PartialEq, Eq)]
+pub enum FormatArgPositionKind {
+    /// `{}` or `{:.*}`
+    Implicit,
+    /// `{1}` or `{:1$}` or `{:.1$}`
+    Number,
+    /// `{a}` or `{:a$}` or `{:.a$}`
+    Named,
+}
+
+#[derive(Copy, Clone, Encodable, Decodable, Debug, PartialEq, Eq, Hash)]
+pub enum FormatTrait {
+    /// `{}`
+    Display,
+    /// `{:?}`
+    Debug,
+    /// `{:e}`
+    LowerExp,
+    /// `{:E}`
+    UpperExp,
+    /// `{:o}`
+    Octal,
+    /// `{:p}`
+    Pointer,
+    /// `{:b}`
+    Binary,
+    /// `{:x}`
+    LowerHex,
+    /// `{:X}`
+    UpperHex,
+}
+
+#[derive(Clone, Encodable, Decodable, Default, Debug, PartialEq, Eq)]
+pub struct FormatOptions {
+    /// The width. E.g. `{:5}` or `{:width$}`.
+    pub width: Option<FormatCount>,
+    /// The precision. E.g. `{:.5}` or `{:.precision$}`.
+    pub precision: Option<FormatCount>,
+    /// The alignment. E.g. `{:>}` or `{:<}` or `{:^}`.
+    pub alignment: Option<FormatAlignment>,
+    /// The fill character. E.g. the `.` in `{:.>10}`.
+    pub fill: Option<char>,
+    /// The `+` or `-` flag.
+    pub sign: Option<FormatSign>,
+    /// The `#` flag.
+    pub alternate: bool,
+    /// The `0` flag. E.g. the `0` in `{:02x}`.
+    pub zero_pad: bool,
+    /// The `x` or `X` flag (for `Debug` only). E.g. the `x` in `{:x?}`.
+    pub debug_hex: Option<FormatDebugHex>,
+}
+
+#[derive(Copy, Clone, Encodable, Decodable, Debug, PartialEq, Eq)]
+pub enum FormatSign {
+    /// The `+` flag.
+    Plus,
+    /// The `-` flag.
+    Minus,
+}
+
+#[derive(Copy, Clone, Encodable, Decodable, Debug, PartialEq, Eq)]
+pub enum FormatDebugHex {
+    /// The `x` flag in `{:x?}`.
+    Lower,
+    /// The `X` flag in `{:X?}`.
+    Upper,
+}
+
+#[derive(Copy, Clone, Encodable, Decodable, Debug, PartialEq, Eq)]
+pub enum FormatAlignment {
+    /// `{:<}`
+    Left,
+    /// `{:>}`
+    Right,
+    /// `{:^}`
+    Center,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug, PartialEq, Eq)]
+pub enum FormatCount {
+    /// `{:5}` or `{:.5}`
+    Literal(usize),
+    /// `{:.*}`, `{:.5$}`, or `{:a$}`, etc.
+    Argument(FormatArgPosition),
+}
diff --git a/compiler/rustc_ast/src/lib.rs b/compiler/rustc_ast/src/lib.rs
index 9c1dfeb1a..23c32fa96 100644
--- a/compiler/rustc_ast/src/lib.rs
+++ b/compiler/rustc_ast/src/lib.rs
@@ -16,7 +16,6 @@
 #![feature(let_chains)]
 #![feature(min_specialization)]
 #![feature(negative_impls)]
-#![feature(slice_internals)]
 #![feature(stmt_expr_attributes)]
 #![recursion_limit = "256"]
 #![deny(rustc::untranslatable_diagnostic)]
@@ -42,6 +41,7 @@ pub mod ast_traits;
 pub mod attr;
 pub mod entry;
 pub mod expand;
+pub mod format;
 pub mod mut_visit;
 pub mod node_id;
 pub mod ptr;
@@ -51,6 +51,7 @@ pub mod visit;
 
 pub use self::ast::*;
 pub use self::ast_traits::{AstDeref, AstNodeWrapper, HasAttrs, HasNodeId, HasSpan, HasTokens};
+pub use self::format::*;
 
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index 77f342d1e..7dcb03b4c 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -17,10 +17,10 @@ use rustc_data_structures::sync::Lrc;
 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};
+use thin_vec::ThinVec;
 
 pub trait ExpectOne<A: Array> {
     fn expect_one(self, err: &'static str) -> A::Item;
@@ -297,6 +297,10 @@ pub trait MutVisitor: Sized {
     fn visit_inline_asm_sym(&mut self, sym: &mut InlineAsmSym) {
         noop_visit_inline_asm_sym(sym, self)
     }
+
+    fn visit_format_args(&mut self, fmt: &mut FormatArgs) {
+        noop_visit_format_args(fmt, self)
+    }
 }
 
 /// Use a map-style function (`FnOnce(T) -> T`) to overwrite a `&mut T`. Useful
@@ -333,6 +337,17 @@ where
 
 // No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
 #[inline]
+pub fn visit_thin_vec<T, F>(elems: &mut ThinVec<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),
@@ -355,6 +370,11 @@ pub fn visit_exprs<T: MutVisitor>(exprs: &mut Vec<P<Expr>>, vis: &mut T) {
 }
 
 // No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
+pub fn visit_thin_exprs<T: MutVisitor>(exprs: &mut ThinVec<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));
 }
@@ -470,7 +490,7 @@ pub fn noop_visit_ty<T: MutVisitor>(ty: &mut P<Ty>, vis: &mut T) {
             vis.visit_fn_decl(decl);
             vis.visit_span(decl_span);
         }
-        TyKind::Tup(tys) => visit_vec(tys, |ty| vis.visit_ty(ty)),
+        TyKind::Tup(tys) => visit_thin_vec(tys, |ty| vis.visit_ty(ty)),
         TyKind::Paren(ty) => vis.visit_ty(ty),
         TyKind::Path(qself, path) => {
             vis.visit_qself(qself);
@@ -557,7 +577,7 @@ pub fn noop_visit_angle_bracketed_parameter_data<T: MutVisitor>(
     vis: &mut T,
 ) {
     let AngleBracketedArgs { args, span } = data;
-    visit_vec(args, |arg| match arg {
+    visit_thin_vec(args, |arg| match arg {
         AngleBracketedArg::Arg(arg) => vis.visit_generic_arg(arg),
         AngleBracketedArg::Constraint(constraint) => vis.visit_constraint(constraint),
     });
@@ -569,7 +589,7 @@ pub fn noop_visit_parenthesized_parameter_data<T: MutVisitor>(
     vis: &mut T,
 ) {
     let ParenthesizedArgs { inputs, output, span, .. } = args;
-    visit_vec(inputs, |input| vis.visit_ty(input));
+    visit_thin_vec(inputs, |input| vis.visit_ty(input));
     noop_visit_fn_ret_ty(output, vis);
     vis.visit_span(span);
 }
@@ -632,7 +652,7 @@ 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::List(mis) => visit_thin_vec(mis, |mi| vis.visit_meta_list_item(mi)),
         MetaItemKind::NameValue(_s) => {}
     }
     vis.visit_span(span);
@@ -835,9 +855,7 @@ pub fn noop_visit_closure_binder<T: MutVisitor>(binder: &mut ClosureBinder, vis:
     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);
+            generic_params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
         }
     }
 }
@@ -915,7 +933,7 @@ pub fn noop_visit_generics<T: MutVisitor>(generics: &mut Generics, vis: &mut T)
 
 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));
+    visit_thin_vec(predicates, |predicate| vis.visit_where_predicate(predicate));
     vis.visit_span(span);
 }
 
@@ -1223,7 +1241,7 @@ pub fn noop_visit_pat<T: MutVisitor>(pat: &mut P<Pat>, vis: &mut T) {
         PatKind::TupleStruct(qself, path, elems) => {
             vis.visit_qself(qself);
             vis.visit_path(path);
-            visit_vec(elems, |elem| vis.visit_pat(elem));
+            visit_thin_vec(elems, |elem| vis.visit_pat(elem));
         }
         PatKind::Path(qself, path) => {
             vis.visit_qself(qself);
@@ -1242,7 +1260,7 @@ pub fn noop_visit_pat<T: MutVisitor>(pat: &mut P<Pat>, vis: &mut T) {
             vis.visit_span(span);
         }
         PatKind::Tuple(elems) | PatKind::Slice(elems) | PatKind::Or(elems) => {
-            visit_vec(elems, |elem| vis.visit_pat(elem))
+            visit_thin_vec(elems, |elem| vis.visit_pat(elem))
         }
         PatKind::Paren(inner) => vis.visit_pat(inner),
         PatKind::MacCall(mac) => vis.visit_mac_call(mac),
@@ -1284,13 +1302,22 @@ pub fn noop_visit_inline_asm_sym<T: MutVisitor>(
     vis.visit_path(path);
 }
 
+pub fn noop_visit_format_args<T: MutVisitor>(fmt: &mut FormatArgs, vis: &mut T) {
+    for arg in fmt.arguments.all_args_mut() {
+        if let FormatArgumentKind::Named(name) = &mut arg.kind {
+            vis.visit_ident(name);
+        }
+        vis.visit_expr(&mut arg.expr);
+    }
+}
+
 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::Array(exprs) => visit_thin_exprs(exprs, vis),
         ExprKind::ConstBlock(anon_const) => {
             vis.visit_anon_const(anon_const);
         }
@@ -1298,10 +1325,10 @@ pub fn noop_visit_expr<T: MutVisitor>(
             vis.visit_expr(expr);
             vis.visit_anon_const(count);
         }
-        ExprKind::Tup(exprs) => visit_exprs(exprs, vis),
+        ExprKind::Tup(exprs) => visit_thin_exprs(exprs, vis),
         ExprKind::Call(f, args) => {
             vis.visit_expr(f);
-            visit_exprs(args, vis);
+            visit_thin_exprs(args, vis);
         }
         ExprKind::MethodCall(box MethodCall {
             seg: PathSegment { ident, id, args: seg_args },
@@ -1313,7 +1340,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
             vis.visit_id(id);
             visit_opt(seg_args, |args| vis.visit_generic_args(args));
             vis.visit_method_receiver_expr(receiver);
-            visit_exprs(call_args, vis);
+            visit_thin_exprs(call_args, vis);
             vis.visit_span(span);
         }
         ExprKind::Binary(_binop, lhs, rhs) => {
@@ -1425,6 +1452,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
             visit_opt(expr, |expr| vis.visit_expr(expr));
         }
         ExprKind::InlineAsm(asm) => vis.visit_inline_asm(asm),
+        ExprKind::FormatArgs(fmt) => vis.visit_format_args(fmt),
         ExprKind::MacCall(mac) => vis.visit_mac_call(mac),
         ExprKind::Struct(se) => {
             let StructExpr { qself, path, fields, rest } = se.deref_mut();
diff --git a/compiler/rustc_ast/src/tokenstream.rs b/compiler/rustc_ast/src/tokenstream.rs
index fabd43a16..f0a6a5e07 100644
--- a/compiler/rustc_ast/src/tokenstream.rs
+++ b/compiler/rustc_ast/src/tokenstream.rs
@@ -41,7 +41,8 @@ use std::{fmt, iter};
 /// Nothing special happens to misnamed or misplaced `SubstNt`s.
 #[derive(Debug, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
 pub enum TokenTree {
-    /// A single token.
+    /// A single token. Should never be `OpenDelim` or `CloseDelim`, because
+    /// delimiters are implicitly represented by `Delimited`.
     Token(Token, Spacing),
     /// A delimited sequence of token trees.
     Delimited(DelimSpan, Delimiter, TokenStream),
@@ -388,12 +389,12 @@ impl TokenStream {
         self.0.len()
     }
 
-    pub fn trees(&self) -> CursorRef<'_> {
-        CursorRef::new(self)
+    pub fn trees(&self) -> RefTokenTreeCursor<'_> {
+        RefTokenTreeCursor::new(self)
     }
 
-    pub fn into_trees(self) -> Cursor {
-        Cursor::new(self)
+    pub fn into_trees(self) -> TokenTreeCursor {
+        TokenTreeCursor::new(self)
     }
 
     /// Compares two `TokenStream`s, checking equality without regarding span information.
@@ -551,16 +552,17 @@ impl TokenStream {
     }
 }
 
-/// By-reference iterator over a [`TokenStream`].
+/// By-reference iterator over a [`TokenStream`], that produces `&TokenTree`
+/// items.
 #[derive(Clone)]
-pub struct CursorRef<'t> {
+pub struct RefTokenTreeCursor<'t> {
     stream: &'t TokenStream,
     index: usize,
 }
 
-impl<'t> CursorRef<'t> {
+impl<'t> RefTokenTreeCursor<'t> {
     fn new(stream: &'t TokenStream) -> Self {
-        CursorRef { stream, index: 0 }
+        RefTokenTreeCursor { stream, index: 0 }
     }
 
     pub fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
@@ -568,7 +570,7 @@ impl<'t> CursorRef<'t> {
     }
 }
 
-impl<'t> Iterator for CursorRef<'t> {
+impl<'t> Iterator for RefTokenTreeCursor<'t> {
     type Item = &'t TokenTree;
 
     fn next(&mut self) -> Option<&'t TokenTree> {
@@ -579,15 +581,16 @@ impl<'t> Iterator for CursorRef<'t> {
     }
 }
 
-/// Owning by-value iterator over a [`TokenStream`].
+/// Owning by-value iterator over a [`TokenStream`], that produces `TokenTree`
+/// items.
 // FIXME: Many uses of this can be replaced with by-reference iterator to avoid clones.
 #[derive(Clone)]
-pub struct Cursor {
+pub struct TokenTreeCursor {
     pub stream: TokenStream,
     index: usize,
 }
 
-impl Iterator for Cursor {
+impl Iterator for TokenTreeCursor {
     type Item = TokenTree;
 
     fn next(&mut self) -> Option<TokenTree> {
@@ -598,9 +601,9 @@ impl Iterator for Cursor {
     }
 }
 
-impl Cursor {
+impl TokenTreeCursor {
     fn new(stream: TokenStream) -> Self {
-        Cursor { stream, index: 0 }
+        TokenTreeCursor { stream, index: 0 }
     }
 
     #[inline]
@@ -614,6 +617,15 @@ impl Cursor {
     pub fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
         self.stream.0.get(self.index + n)
     }
+
+    // Replace the previously obtained token tree with `tts`, and rewind to
+    // just before them.
+    pub fn replace_prev_and_rewind(&mut self, tts: Vec<TokenTree>) {
+        assert!(self.index > 0);
+        self.index -= 1;
+        let stream = Lrc::make_mut(&mut self.stream.0);
+        stream.splice(self.index..self.index + 1, tts);
+    }
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
diff --git a/compiler/rustc_ast/src/util/comments.rs b/compiler/rustc_ast/src/util/comments.rs
index 275ed02c2..eece99a3e 100644
--- a/compiler/rustc_ast/src/util/comments.rs
+++ b/compiler/rustc_ast/src/util/comments.rs
@@ -58,23 +58,24 @@ pub fn beautify_doc_string(data: Symbol, kind: CommentKind) -> Symbol {
         // 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;
+        let lines = match 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]
             }
-            &lines[i..j]
-        } else {
-            lines
+            CommentKind::Line => lines,
         };
 
         for line in lines {
diff --git a/compiler/rustc_ast/src/util/parser.rs b/compiler/rustc_ast/src/util/parser.rs
index 4f7099c7b..3a0af04f9 100644
--- a/compiler/rustc_ast/src/util/parser.rs
+++ b/compiler/rustc_ast/src/util/parser.rs
@@ -225,7 +225,7 @@ impl AssocOp {
             AssignOp(_) | // `{ 42 } +=`
             As | // `{ 42 } as usize`
             // Equal | // `{ 42 } == { 42 }`    Accepting these here would regress incorrect
-            // NotEqual | // `{ 42 } != { 42 }  struct literals parser recovery.
+            // NotEqual | // `{ 42 } != { 42 }` struct literals parser recovery.
             Colon, // `{ 42 }: usize`
         )
     }
@@ -271,6 +271,7 @@ pub enum ExprPrecedence {
     Try,
     InlineAsm,
     Mac,
+    FormatArgs,
 
     Array,
     Repeat,
@@ -335,7 +336,8 @@ impl ExprPrecedence {
             | ExprPrecedence::Index
             | ExprPrecedence::Try
             | ExprPrecedence::InlineAsm
-            | ExprPrecedence::Mac => PREC_POSTFIX,
+            | ExprPrecedence::Mac
+            | ExprPrecedence::FormatArgs => PREC_POSTFIX,
 
             // Never need parens
             ExprPrecedence::Array
diff --git a/compiler/rustc_ast/src/util/unicode.rs b/compiler/rustc_ast/src/util/unicode.rs
index 0eae791b2..6f57d66b2 100644
--- a/compiler/rustc_ast/src/util/unicode.rs
+++ b/compiler/rustc_ast/src/util/unicode.rs
@@ -17,7 +17,7 @@ pub fn contains_text_flow_control_chars(s: &str) -> bool {
     // U+2069 - E2 81 A9
     let mut bytes = s.as_bytes();
     loop {
-        match core::slice::memchr::memchr(0xE2, bytes) {
+        match memchr::memchr(0xE2, bytes) {
             Some(idx) => {
                 // bytes are valid UTF-8 -> E2 must be followed by two bytes
                 let ch = &bytes[idx..idx + 3];
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index e8823eff8..bdb1879ec 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -242,6 +242,9 @@ pub trait Visitor<'ast>: Sized {
     fn visit_inline_asm(&mut self, asm: &'ast InlineAsm) {
         walk_inline_asm(self, asm)
     }
+    fn visit_format_args(&mut self, fmt: &'ast FormatArgs) {
+        walk_format_args(self, fmt)
+    }
     fn visit_inline_asm_sym(&mut self, sym: &'ast InlineAsmSym) {
         walk_inline_asm_sym(self, sym)
     }
@@ -756,6 +759,15 @@ pub fn walk_inline_asm_sym<'a, V: Visitor<'a>>(visitor: &mut V, sym: &'a InlineA
     visitor.visit_path(&sym.path, sym.id);
 }
 
+pub fn walk_format_args<'a, V: Visitor<'a>>(visitor: &mut V, fmt: &'a FormatArgs) {
+    for arg in fmt.arguments.all_args() {
+        if let FormatArgumentKind::Named(name) = arg.kind {
+            visitor.visit_ident(name);
+        }
+        visitor.visit_expr(&arg.expr);
+    }
+}
+
 pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
     walk_list!(visitor, visit_attribute, expression.attrs.iter());
 
@@ -896,6 +908,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::MacCall(mac) => visitor.visit_mac_call(mac),
         ExprKind::Paren(subexpression) => visitor.visit_expr(subexpression),
         ExprKind::InlineAsm(asm) => visitor.visit_inline_asm(asm),
+        ExprKind::FormatArgs(f) => visitor.visit_format_args(f),
         ExprKind::Yield(optional_expression) => {
             walk_list!(visitor, visit_expr, optional_expression);
         }