Adding upstream version 1.65.0+dfsg1.upstream/1.65.0+dfsg1

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

32 files changed, 1927 insertions, 1394 deletions

diff --git a/compiler/rustc_ast/Cargo.toml b/compiler/rustc_ast/Cargo.toml
index 9822e9864..c24180bac 100644
--- a/compiler/rustc_ast/Cargo.toml
+++ b/compiler/rustc_ast/Cargo.toml
@@ -7,12 +7,13 @@ edition = "2021"
 doctest = false
 
 [dependencies]
-rustc_serialize = { path = "../rustc_serialize" }
-tracing = "0.1"
-rustc_span = { path = "../rustc_span" }
+bitflags = "1.2.1"
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_index = { path = "../rustc_index" }
 rustc_lexer = { path = "../rustc_lexer" }
 rustc_macros = { path = "../rustc_macros" }
+rustc_serialize = { path = "../rustc_serialize" }
+rustc_span = { path = "../rustc_span" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
-bitflags = "1.2.1"
+thin-vec = "0.2.8"
+tracing = "0.1"
diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 870a7c0be..d86db8f8b 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -24,22 +24,19 @@ pub use UnsafeSource::*;
 
 use crate::ptr::P;
 use crate::token::{self, CommentKind, Delimiter};
-use crate::tokenstream::{DelimSpan, LazyTokenStream, TokenStream};
-
+use crate::tokenstream::{DelimSpan, LazyAttrTokenStream, 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;
+use thin_vec::ThinVec;
 
 /// A "Label" is an identifier of some point in sources,
 /// e.g. in the following code:
@@ -94,7 +91,7 @@ pub struct Path {
     /// The segments in the path: the things separated by `::`.
     /// Global paths begin with `kw::PathRoot`.
     pub segments: Vec<PathSegment>,
-    pub tokens: Option<LazyTokenStream>,
+    pub tokens: Option<LazyAttrTokenStream>,
 }
 
 impl PartialEq<Symbol> for Path {
@@ -326,46 +323,17 @@ 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)]
+#[derive(Hash, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 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))
-    }
+    TypeOrConst,
 }
-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),
+            ParamKindOrd::TypeOrConst => "type and const".fmt(f),
         }
     }
 }
@@ -497,7 +465,6 @@ pub struct WhereRegionPredicate {
 /// 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>,
@@ -505,7 +472,7 @@ pub struct WhereEqPredicate {
 
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct Crate {
-    pub attrs: Vec<Attribute>,
+    pub attrs: AttrVec,
     pub items: Vec<P<Item>>,
     pub spans: ModSpans,
     /// Must be equal to `CRATE_NODE_ID` after the crate root is expanded, but may hold
@@ -567,7 +534,7 @@ pub struct Block {
     /// Distinguishes between `unsafe { ... }` and `{ ... }`.
     pub rules: BlockCheckMode,
     pub span: Span,
-    pub tokens: Option<LazyTokenStream>,
+    pub tokens: Option<LazyAttrTokenStream>,
     /// The following *isn't* a parse error, but will cause multiple errors in following stages.
     /// ```compile_fail
     /// let x = {
@@ -586,7 +553,7 @@ pub struct Pat {
     pub id: NodeId,
     pub kind: PatKind,
     pub span: Span,
-    pub tokens: Option<LazyTokenStream>,
+    pub tokens: Option<LazyAttrTokenStream>,
 }
 
 impl Pat {
@@ -597,7 +564,7 @@ impl Pat {
             // 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) => {
+            PatKind::Ident(BindingAnnotation::NONE, ident, None) => {
                 TyKind::Path(None, Path::from_ident(*ident))
             }
             PatKind::Path(qself, path) => TyKind::Path(qself.clone(), path.clone()),
@@ -684,10 +651,43 @@ pub struct PatField {
     pub is_placeholder: bool,
 }
 
-#[derive(Clone, PartialEq, Encodable, Decodable, Debug, Copy)]
-pub enum BindingMode {
-    ByRef(Mutability),
-    ByValue(Mutability),
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+#[derive(Encodable, Decodable, HashStable_Generic)]
+pub enum ByRef {
+    Yes,
+    No,
+}
+
+impl From<bool> for ByRef {
+    fn from(b: bool) -> ByRef {
+        match b {
+            false => ByRef::No,
+            true => ByRef::Yes,
+        }
+    }
+}
+
+/// Explicit binding annotations given in the HIR for a binding. Note
+/// that this is not the final binding *mode* that we infer after type
+/// inference.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+#[derive(Encodable, Decodable, HashStable_Generic)]
+pub struct BindingAnnotation(pub ByRef, pub Mutability);
+
+impl BindingAnnotation {
+    pub const NONE: Self = Self(ByRef::No, Mutability::Not);
+    pub const REF: Self = Self(ByRef::Yes, Mutability::Not);
+    pub const MUT: Self = Self(ByRef::No, Mutability::Mut);
+    pub const REF_MUT: Self = Self(ByRef::Yes, Mutability::Mut);
+
+    pub fn prefix_str(self) -> &'static str {
+        match self {
+            Self::NONE => "",
+            Self::REF => "ref ",
+            Self::MUT => "mut ",
+            Self::REF_MUT => "ref mut ",
+        }
+    }
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]
@@ -716,7 +716,7 @@ pub enum PatKind {
     /// 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>>),
+    Ident(BindingAnnotation, Ident, Option<P<Pat>>),
 
     /// A struct or struct variant pattern (e.g., `Variant {x, y, ..}`).
     /// The `bool` is `true` in the presence of a `..`.
@@ -771,7 +771,7 @@ pub enum PatKind {
     Paren(P<Pat>),
 
     /// A macro pattern; pre-expansion.
-    MacCall(MacCall),
+    MacCall(P<MacCall>),
 }
 
 #[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, Copy)]
@@ -937,8 +937,8 @@ impl Stmt {
     /// 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
+    /// `LazyAttrTokenStream`. The parser is responsible for calling
+    /// `ToAttrTokenStream::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 {
@@ -981,10 +981,10 @@ pub enum StmtKind {
 
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct MacCallStmt {
-    pub mac: MacCall,
+    pub mac: P<MacCall>,
     pub style: MacStmtStyle,
     pub attrs: AttrVec,
-    pub tokens: Option<LazyTokenStream>,
+    pub tokens: Option<LazyAttrTokenStream>,
 }
 
 #[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug)]
@@ -1009,7 +1009,7 @@ pub struct Local {
     pub kind: LocalKind,
     pub span: Span,
     pub attrs: AttrVec,
-    pub tokens: Option<LazyTokenStream>,
+    pub tokens: Option<LazyAttrTokenStream>,
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]
@@ -1108,7 +1108,7 @@ pub struct Expr {
     pub kind: ExprKind,
     pub span: Span,
     pub attrs: AttrVec,
-    pub tokens: Option<LazyTokenStream>,
+    pub tokens: Option<LazyAttrTokenStream>,
 }
 
 impl Expr {
@@ -1269,7 +1269,7 @@ impl Expr {
                 id: DUMMY_NODE_ID,
                 kind: ExprKind::Err,
                 span: DUMMY_SP,
-                attrs: ThinVec::new(),
+                attrs: AttrVec::new(),
                 tokens: None,
             },
         )
@@ -1439,7 +1439,7 @@ pub enum ExprKind {
     InlineAsm(P<InlineAsm>),
 
     /// A macro invocation; pre-expansion.
-    MacCall(MacCall),
+    MacCall(P<MacCall>),
 
     /// A struct literal expression.
     ///
@@ -1691,7 +1691,7 @@ pub enum StrStyle {
 #[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
 pub struct Lit {
     /// The original literal token as written in source code.
-    pub token: token::Lit,
+    pub token_lit: 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.
@@ -1719,7 +1719,7 @@ impl StrLit {
             StrStyle::Raw(n) => token::StrRaw(n),
         };
         Lit {
-            token: token::Lit::new(token_kind, self.symbol, self.suffix),
+            token_lit: token::Lit::new(token_kind, self.symbol, self.suffix),
             span: self.span,
             kind: LitKind::Str(self.symbol_unescaped, self.style),
         }
@@ -1753,7 +1753,8 @@ pub enum LitFloatType {
 /// 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"`).
+    /// A string literal (`"foo"`). The symbol is unescaped, and so may differ
+    /// from the original token's symbol.
     Str(Symbol, StrStyle),
     /// A byte string (`b"foo"`).
     ByteStr(Lrc<[u8]>),
@@ -1763,12 +1764,13 @@ pub enum LitKind {
     Char(char),
     /// An integer literal (`1`).
     Int(u128, LitIntType),
-    /// A float literal (`1f64` or `1E10f64`).
+    /// A float literal (`1f64` or `1E10f64`). Stored as a symbol rather than
+    /// `f64` so that `LitKind` can impl `Eq` and `Hash`.
     Float(Symbol, LitFloatType),
     /// A boolean literal.
     Bool(bool),
     /// Placeholder for a literal that wasn't well-formed in some way.
-    Err(Symbol),
+    Err,
 }
 
 impl LitKind {
@@ -1807,7 +1809,7 @@ impl LitKind {
             | LitKind::Int(_, LitIntType::Unsuffixed)
             | LitKind::Float(_, LitFloatType::Unsuffixed)
             | LitKind::Bool(..)
-            | LitKind::Err(..) => false,
+            | LitKind::Err => false,
         }
     }
 }
@@ -1966,7 +1968,7 @@ pub struct Ty {
     pub id: NodeId,
     pub kind: TyKind,
     pub span: Span,
-    pub tokens: Option<LazyTokenStream>,
+    pub tokens: Option<LazyAttrTokenStream>,
 }
 
 impl Clone for Ty {
@@ -2042,7 +2044,7 @@ pub enum TyKind {
     /// Inferred type of a `self` or `&self` argument in a method.
     ImplicitSelf,
     /// A macro in the type position.
-    MacCall(MacCall),
+    MacCall(P<MacCall>),
     /// Placeholder for a kind that has failed to be defined.
     Err,
     /// Placeholder for a `va_list`.
@@ -2059,8 +2061,11 @@ impl TyKind {
     }
 
     pub fn is_simple_path(&self) -> Option<Symbol> {
-        if let TyKind::Path(None, Path { segments, .. }) = &self && segments.len() == 1 {
-            Some(segments[0].ident.name)
+        if let TyKind::Path(None, Path { segments, .. }) = &self
+            && let [segment] = &segments[..]
+            && segment.args.is_none()
+        {
+            Some(segment.ident.name)
         } else {
             None
         }
@@ -2071,6 +2076,7 @@ impl TyKind {
 #[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
 pub enum TraitObjectSyntax {
     Dyn,
+    DynStar,
     None,
 }
 
@@ -2086,15 +2092,15 @@ pub enum InlineAsmRegOrRegClass {
 bitflags::bitflags! {
     #[derive(Encodable, Decodable, HashStable_Generic)]
     pub struct InlineAsmOptions: u16 {
-        const PURE = 1 << 0;
-        const NOMEM = 1 << 1;
-        const READONLY = 1 << 2;
+        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;
+        const NORETURN        = 1 << 4;
+        const NOSTACK         = 1 << 5;
+        const ATT_SYNTAX      = 1 << 6;
+        const RAW             = 1 << 7;
+        const MAY_UNWIND      = 1 << 8;
     }
 }
 
@@ -2230,7 +2236,7 @@ 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 let PatKind::Ident(BindingAnnotation(ByRef::No, 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))),
@@ -2260,11 +2266,24 @@ impl Param {
     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 {
+        let (mutbl, ty) = match eself.node {
+            SelfKind::Explicit(ty, mutbl) => (mutbl, ty),
+            SelfKind::Value(mutbl) => (mutbl, infer_ty),
+            SelfKind::Region(lt, mutbl) => (
+                Mutability::Not,
+                P(Ty {
+                    id: DUMMY_NODE_ID,
+                    kind: TyKind::Rptr(lt, MutTy { ty: infer_ty, mutbl }),
+                    span,
+                    tokens: None,
+                }),
+            ),
+        };
+        Param {
             attrs,
             pat: P(Pat {
                 id: DUMMY_NODE_ID,
-                kind: PatKind::Ident(BindingMode::ByValue(mutbl), eself_ident, None),
+                kind: PatKind::Ident(BindingAnnotation(ByRef::No, mutbl), eself_ident, None),
                 span,
                 tokens: None,
             }),
@@ -2272,19 +2291,6 @@ impl Param {
             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,
-                }),
-            ),
         }
     }
 }
@@ -2336,9 +2342,9 @@ impl Async {
     }
 
     /// In this case this is an `async` return, the `NodeId` for the generated `impl Trait` item.
-    pub fn opt_return_id(self) -> Option<NodeId> {
+    pub fn opt_return_id(self) -> Option<(NodeId, Span)> {
         match self {
-            Async::Yes { return_impl_trait_id, .. } => Some(return_impl_trait_id),
+            Async::Yes { return_impl_trait_id, span, .. } => Some((return_impl_trait_id, span)),
             Async::No => None,
         }
     }
@@ -2522,8 +2528,8 @@ impl<S: Encoder> Encodable<S> for AttrId {
 }
 
 impl<D: Decoder> Decodable<D> for AttrId {
-    fn decode(_: &mut D) -> AttrId {
-        crate::attr::mk_attr_id()
+    default fn decode(_: &mut D) -> AttrId {
+        panic!("cannot decode `AttrId` with `{}`", std::any::type_name::<D>());
     }
 }
 
@@ -2531,7 +2537,7 @@ impl<D: Decoder> Decodable<D> for AttrId {
 pub struct AttrItem {
     pub path: Path,
     pub args: MacArgs,
-    pub tokens: Option<LazyTokenStream>,
+    pub tokens: Option<LazyAttrTokenStream>,
 }
 
 /// A list of attributes.
@@ -2549,9 +2555,15 @@ pub struct Attribute {
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]
+pub struct NormalAttr {
+    pub item: AttrItem,
+    pub tokens: Option<LazyAttrTokenStream>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug)]
 pub enum AttrKind {
     /// A normal attribute.
-    Normal(AttrItem, Option<LazyTokenStream>),
+    Normal(P<NormalAttr>),
 
     /// A doc comment (e.g. `/// ...`, `//! ...`, `/** ... */`, `/*! ... */`).
     /// Doc attributes (e.g. `#[doc="..."]`) are represented with the `Normal`
@@ -2596,13 +2608,13 @@ impl PolyTraitRef {
 pub struct Visibility {
     pub kind: VisibilityKind,
     pub span: Span,
-    pub tokens: Option<LazyTokenStream>,
+    pub tokens: Option<LazyAttrTokenStream>,
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum VisibilityKind {
     Public,
-    Restricted { path: P<Path>, id: NodeId },
+    Restricted { path: P<Path>, id: NodeId, shorthand: bool },
     Inherited,
 }
 
@@ -2665,7 +2677,7 @@ impl VariantData {
 /// An item definition.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct Item<K = ItemKind> {
-    pub attrs: Vec<Attribute>,
+    pub attrs: AttrVec,
     pub id: NodeId,
     pub span: Span,
     pub vis: Visibility,
@@ -2682,7 +2694,7 @@ pub struct Item<K = ItemKind> {
     ///
     /// Note that the tokens here do not include the outer attributes, but will
     /// include inner attributes.
-    pub tokens: Option<LazyTokenStream>,
+    pub tokens: Option<LazyAttrTokenStream>,
 }
 
 impl Item {
@@ -2873,7 +2885,7 @@ pub enum ItemKind {
     /// A macro invocation.
     ///
     /// E.g., `foo!(..)`.
-    MacCall(MacCall),
+    MacCall(P<MacCall>),
 
     /// A macro definition.
     MacroDef(MacroDef),
@@ -2947,7 +2959,7 @@ pub enum AssocItemKind {
     /// An associated type.
     TyAlias(Box<TyAlias>),
     /// A macro expanding to associated items.
-    MacCall(MacCall),
+    MacCall(P<MacCall>),
 }
 
 impl AssocItemKind {
@@ -2996,7 +3008,7 @@ pub enum ForeignItemKind {
     /// An foreign type.
     TyAlias(Box<TyAlias>),
     /// A macro expanding to foreign items.
-    MacCall(MacCall),
+    MacCall(P<MacCall>),
 }
 
 impl From<ForeignItemKind> for ItemKind {
@@ -3030,22 +3042,34 @@ pub type ForeignItem = Item<ForeignItemKind>;
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 mod size_asserts {
     use super::*;
+    use rustc_data_structures::static_assert_size;
     // 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);
+    static_assert_size!(AssocItem, 104);
+    static_assert_size!(AssocItemKind, 32);
+    static_assert_size!(Attribute, 32);
+    static_assert_size!(Block, 48);
+    static_assert_size!(Expr, 104);
+    static_assert_size!(ExprKind, 72);
+    #[cfg(not(bootstrap))]
+    static_assert_size!(Fn, 184);
+    static_assert_size!(ForeignItem, 96);
+    static_assert_size!(ForeignItemKind, 24);
+    static_assert_size!(GenericArg, 24);
+    static_assert_size!(GenericBound, 88);
+    static_assert_size!(Generics, 72);
+    static_assert_size!(Impl, 200);
+    static_assert_size!(Item, 184);
+    static_assert_size!(ItemKind, 112);
+    static_assert_size!(Lit, 48);
+    static_assert_size!(LitKind, 24);
+    static_assert_size!(Local, 72);
+    static_assert_size!(Param, 40);
+    static_assert_size!(Pat, 120);
+    static_assert_size!(PatKind, 96);
+    static_assert_size!(Path, 40);
+    static_assert_size!(PathSegment, 24);
+    static_assert_size!(Stmt, 32);
+    static_assert_size!(StmtKind, 16);
+    static_assert_size!(Ty, 96);
+    static_assert_size!(TyKind, 72);
 }
diff --git a/compiler/rustc_ast/src/ast_traits.rs b/compiler/rustc_ast/src/ast_traits.rs
index 5c30a75a1..1b31be07f 100644
--- a/compiler/rustc_ast/src/ast_traits.rs
+++ b/compiler/rustc_ast/src/ast_traits.rs
@@ -4,7 +4,7 @@
 
 use crate::ptr::P;
 use crate::token::Nonterminal;
-use crate::tokenstream::LazyTokenStream;
+use crate::tokenstream::LazyAttrTokenStream;
 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};
@@ -124,18 +124,18 @@ impl HasSpan for AttrItem {
 
 /// 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>>;
+    fn tokens(&self) -> Option<&LazyAttrTokenStream>;
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyAttrTokenStream>>;
 }
 
 macro_rules! impl_has_tokens {
     ($($T:ty),+ $(,)?) => {
         $(
             impl HasTokens for $T {
-                fn tokens(&self) -> Option<&LazyTokenStream> {
+                fn tokens(&self) -> Option<&LazyAttrTokenStream> {
                     self.tokens.as_ref()
                 }
-                fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+                fn tokens_mut(&mut self) -> Option<&mut Option<LazyAttrTokenStream>> {
                     Some(&mut self.tokens)
                 }
             }
@@ -147,10 +147,10 @@ macro_rules! impl_has_tokens_none {
     ($($T:ty),+ $(,)?) => {
         $(
             impl HasTokens for $T {
-                fn tokens(&self) -> Option<&LazyTokenStream> {
+                fn tokens(&self) -> Option<&LazyAttrTokenStream> {
                     None
                 }
-                fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+                fn tokens_mut(&mut self) -> Option<&mut Option<LazyAttrTokenStream>> {
                     None
                 }
             }
@@ -162,25 +162,25 @@ impl_has_tokens!(AssocItem, AttrItem, Block, Expr, ForeignItem, Item, Pat, Path,
 impl_has_tokens_none!(Arm, ExprField, FieldDef, GenericParam, Param, PatField, Variant);
 
 impl<T: AstDeref<Target: HasTokens>> HasTokens for T {
-    fn tokens(&self) -> Option<&LazyTokenStream> {
+    fn tokens(&self) -> Option<&LazyAttrTokenStream> {
         self.ast_deref().tokens()
     }
-    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyAttrTokenStream>> {
         self.ast_deref_mut().tokens_mut()
     }
 }
 
 impl<T: HasTokens> HasTokens for Option<T> {
-    fn tokens(&self) -> Option<&LazyTokenStream> {
+    fn tokens(&self) -> Option<&LazyAttrTokenStream> {
         self.as_ref().and_then(|inner| inner.tokens())
     }
-    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyAttrTokenStream>> {
         self.as_mut().and_then(|inner| inner.tokens_mut())
     }
 }
 
 impl HasTokens for StmtKind {
-    fn tokens(&self) -> Option<&LazyTokenStream> {
+    fn tokens(&self) -> Option<&LazyAttrTokenStream> {
         match self {
             StmtKind::Local(local) => local.tokens.as_ref(),
             StmtKind::Item(item) => item.tokens(),
@@ -189,7 +189,7 @@ impl HasTokens for StmtKind {
             StmtKind::MacCall(mac) => mac.tokens.as_ref(),
         }
     }
-    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyAttrTokenStream>> {
         match self {
             StmtKind::Local(local) => Some(&mut local.tokens),
             StmtKind::Item(item) => item.tokens_mut(),
@@ -201,26 +201,26 @@ impl HasTokens for StmtKind {
 }
 
 impl HasTokens for Stmt {
-    fn tokens(&self) -> Option<&LazyTokenStream> {
+    fn tokens(&self) -> Option<&LazyAttrTokenStream> {
         self.kind.tokens()
     }
-    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyAttrTokenStream>> {
         self.kind.tokens_mut()
     }
 }
 
 impl HasTokens for Attribute {
-    fn tokens(&self) -> Option<&LazyTokenStream> {
+    fn tokens(&self) -> Option<&LazyAttrTokenStream> {
         match &self.kind {
-            AttrKind::Normal(_, tokens) => tokens.as_ref(),
+            AttrKind::Normal(normal) => normal.tokens.as_ref(),
             kind @ AttrKind::DocComment(..) => {
                 panic!("Called tokens on doc comment attr {:?}", kind)
             }
         }
     }
-    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyAttrTokenStream>> {
         Some(match &mut self.kind {
-            AttrKind::Normal(_, tokens) => tokens,
+            AttrKind::Normal(normal) => &mut normal.tokens,
             kind @ AttrKind::DocComment(..) => {
                 panic!("Called tokens_mut on doc comment attr {:?}", kind)
             }
@@ -229,7 +229,7 @@ impl HasTokens for Attribute {
 }
 
 impl HasTokens for Nonterminal {
-    fn tokens(&self) -> Option<&LazyTokenStream> {
+    fn tokens(&self) -> Option<&LazyAttrTokenStream> {
         match self {
             Nonterminal::NtItem(item) => item.tokens(),
             Nonterminal::NtStmt(stmt) => stmt.tokens(),
@@ -243,7 +243,7 @@ impl HasTokens for Nonterminal {
             Nonterminal::NtIdent(..) | Nonterminal::NtLifetime(..) => None,
         }
     }
-    fn tokens_mut(&mut self) -> Option<&mut Option<LazyTokenStream>> {
+    fn tokens_mut(&mut self) -> Option<&mut Option<LazyAttrTokenStream>> {
         match self {
             Nonterminal::NtItem(item) => item.tokens_mut(),
             Nonterminal::NtStmt(stmt) => stmt.tokens_mut(),
@@ -270,7 +270,7 @@ pub trait HasAttrs {
     /// during token collection.
     const SUPPORTS_CUSTOM_INNER_ATTRS: bool;
     fn attrs(&self) -> &[Attribute];
-    fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>));
+    fn visit_attrs(&mut self, f: impl FnOnce(&mut AttrVec));
 }
 
 macro_rules! impl_has_attrs {
@@ -279,12 +279,13 @@ macro_rules! impl_has_attrs {
             impl HasAttrs for $T {
                 const SUPPORTS_CUSTOM_INNER_ATTRS: bool = $inner;
 
+                #[inline]
                 fn attrs(&self) -> &[Attribute] {
                     &self.attrs
                 }
 
-                fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
-                    VecOrAttrVec::visit(&mut self.attrs, f)
+                fn visit_attrs(&mut self, f: impl FnOnce(&mut AttrVec)) {
+                    f(&mut self.attrs)
                 }
             }
         )+
@@ -299,7 +300,7 @@ macro_rules! impl_has_attrs_none {
                 fn attrs(&self) -> &[Attribute] {
                     &[]
                 }
-                fn visit_attrs(&mut self, _f: impl FnOnce(&mut Vec<Attribute>)) {}
+                fn visit_attrs(&mut self, _f: impl FnOnce(&mut AttrVec)) {}
             }
         )+
     };
@@ -330,7 +331,7 @@ impl<T: AstDeref<Target: HasAttrs>> HasAttrs for T {
     fn attrs(&self) -> &[Attribute] {
         self.ast_deref().attrs()
     }
-    fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
+    fn visit_attrs(&mut self, f: impl FnOnce(&mut AttrVec)) {
         self.ast_deref_mut().visit_attrs(f)
     }
 }
@@ -340,7 +341,7 @@ impl<T: HasAttrs> HasAttrs for Option<T> {
     fn attrs(&self) -> &[Attribute] {
         self.as_ref().map(|inner| inner.attrs()).unwrap_or(&[])
     }
-    fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
+    fn visit_attrs(&mut self, f: impl FnOnce(&mut AttrVec)) {
         if let Some(inner) = self.as_mut() {
             inner.visit_attrs(f);
         }
@@ -362,13 +363,13 @@ impl HasAttrs for StmtKind {
         }
     }
 
-    fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
+    fn visit_attrs(&mut self, f: impl FnOnce(&mut AttrVec)) {
         match self {
-            StmtKind::Local(local) => visit_attrvec(&mut local.attrs, f),
+            StmtKind::Local(local) => f(&mut local.attrs),
             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),
+            StmtKind::MacCall(mac) => f(&mut mac.attrs),
         }
     }
 }
@@ -378,38 +379,11 @@ impl HasAttrs for Stmt {
     fn attrs(&self) -> &[Attribute] {
         self.kind.attrs()
     }
-    fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
+    fn visit_attrs(&mut self, f: impl FnOnce(&mut AttrVec)) {
         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,
diff --git a/compiler/rustc_ast/src/attr/mod.rs b/compiler/rustc_ast/src/attr/mod.rs
index 86af7769d..990f4f8f1 100644
--- a/compiler/rustc_ast/src/attr/mod.rs
+++ b/compiler/rustc_ast/src/attr/mod.rs
@@ -7,18 +7,22 @@ use crate::ast::{MacArgs, MacArgsEq, MacDelimiter, MetaItem, MetaItemKind, Neste
 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::tokenstream::{LazyAttrTokenStream, TokenStream};
 use crate::util::comments;
 
-use rustc_data_structures::thin_vec::ThinVec;
+use rustc_data_structures::sync::WorkerLocal;
 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::cell::Cell;
 use std::iter;
+#[cfg(debug_assertions)]
+use std::ops::BitXor;
+#[cfg(debug_assertions)]
+use std::sync::atomic::{AtomicU32, Ordering};
 
 pub struct MarkedAttrs(GrowableBitSet<AttrId>);
 
@@ -114,7 +118,7 @@ impl Attribute {
     #[inline]
     pub fn has_name(&self, name: Symbol) -> bool {
         match self.kind {
-            AttrKind::Normal(ref item, _) => item.path == name,
+            AttrKind::Normal(ref normal) => normal.item.path == name,
             AttrKind::DocComment(..) => false,
         }
     }
@@ -122,9 +126,9 @@ impl Attribute {
     /// 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)
+            AttrKind::Normal(ref normal) => {
+                if normal.item.path.segments.len() == 1 {
+                    Some(normal.item.path.segments[0].ident)
                 } else {
                     None
                 }
@@ -138,14 +142,16 @@ impl Attribute {
 
     pub fn value_str(&self) -> Option<Symbol> {
         match self.kind {
-            AttrKind::Normal(ref item, _) => item.meta_kind().and_then(|kind| kind.value_str()),
+            AttrKind::Normal(ref normal) => {
+                normal.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() {
+            AttrKind::Normal(ref normal) => match normal.item.meta_kind() {
                 Some(MetaItemKind::List(list)) => Some(list),
                 _ => None,
             },
@@ -154,8 +160,8 @@ impl Attribute {
     }
 
     pub fn is_word(&self) -> bool {
-        if let AttrKind::Normal(item, _) = &self.kind {
-            matches!(item.args, MacArgs::Empty)
+        if let AttrKind::Normal(normal) = &self.kind {
+            matches!(normal.item.args, MacArgs::Empty)
         } else {
             false
         }
@@ -182,13 +188,7 @@ impl MetaItem {
     }
 
     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,
-        }
+        self.kind.value_str()
     }
 
     pub fn meta_item_list(&self) -> Option<&[NestedMetaItem]> {
@@ -237,6 +237,9 @@ impl AttrItem {
 }
 
 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,
@@ -244,10 +247,16 @@ impl Attribute {
         }
     }
 
+    /// Returns the documentation and its kind if this is a doc comment or a sugared doc comment.
+    /// * `///doc` returns `Some(("doc", CommentKind::Line))`.
+    /// * `/** doc */` returns `Some(("doc", CommentKind::Block))`.
+    /// * `#[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 item, _) if item.path == sym::doc => item
+            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)),
@@ -255,11 +264,15 @@ impl Attribute {
         }
     }
 
+    /// Returns the documentation if this is a doc comment or a sugared doc comment.
+    /// * `///doc` returns `Some("doc")`.
+    /// * `#[doc = "doc"]` returns `Some("doc")`.
+    /// * `#[doc(...)]` returns `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())
+            AttrKind::Normal(ref normal) if normal.item.path == sym::doc => {
+                normal.item.meta_kind().and_then(|kind| kind.value_str())
             }
             _ => None,
         }
@@ -271,14 +284,14 @@ impl Attribute {
 
     pub fn get_normal_item(&self) -> &AttrItem {
         match self.kind {
-            AttrKind::Normal(ref item, _) => item,
+            AttrKind::Normal(ref normal) => &normal.item,
             AttrKind::DocComment(..) => panic!("unexpected doc comment"),
         }
     }
 
     pub fn unwrap_normal_item(self) -> AttrItem {
         match self.kind {
-            AttrKind::Normal(item, _) => item,
+            AttrKind::Normal(normal) => normal.into_inner().item,
             AttrKind::DocComment(..) => panic!("unexpected doc comment"),
         }
     }
@@ -286,31 +299,30 @@ impl Attribute {
     /// 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::Normal(ref normal) => normal.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::Normal(ref normal) => normal.item.meta_kind(),
             AttrKind::DocComment(..) => None,
         }
     }
 
-    pub fn tokens(&self) -> AttrAnnotatedTokenStream {
+    pub fn tokens(&self) -> TokenStream {
         match self.kind {
-            AttrKind::Normal(_, ref tokens) => tokens
+            AttrKind::Normal(ref normal) => normal
+                .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,
-                )),
+                .to_attr_token_stream()
+                .to_tokenstream(),
+            AttrKind::DocComment(comment_kind, data) => TokenStream::new(vec![TokenTree::Token(
+                Token::new(token::DocComment(comment_kind, self.style, data), self.span),
                 Spacing::Alone,
-            )),
+            )]),
         }
     }
 }
@@ -340,47 +352,86 @@ 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;
+pub struct AttrIdGenerator(WorkerLocal<Cell<u32>>);
 
-    static NEXT_ATTR_ID: AtomicU32 = AtomicU32::new(0);
+#[cfg(debug_assertions)]
+static MAX_ATTR_ID: AtomicU32 = AtomicU32::new(u32::MAX);
 
-    let id = NEXT_ATTR_ID.fetch_add(1, Ordering::SeqCst);
-    assert!(id != u32::MAX);
-    AttrId::from_u32(id)
+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();
+
+            #[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);
+            }
+
+            Cell::new(index.reverse_bits())
+        }))
+    }
+
+    pub fn mk_attr_id(&self) -> AttrId {
+        let id = self.0.get();
+
+        // 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));
+
+        self.0.set(id + 1);
+        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(
+    g: &AttrIdGenerator,
+    style: AttrStyle,
+    path: Path,
+    args: MacArgs,
+    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<LazyTokenStream>,
+    tokens: Option<LazyAttrTokenStream>,
     style: AttrStyle,
     span: Span,
 ) -> Attribute {
-    Attribute { kind: AttrKind::Normal(item, tokens), id: mk_attr_id(), style, span }
+    Attribute {
+        kind: AttrKind::Normal(P(ast::NormalAttr { item, tokens })),
+        id: g.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)
+pub fn mk_attr_inner(g: &AttrIdGenerator, item: MetaItem) -> Attribute {
+    mk_attr(g, 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_attr_outer(g: &AttrIdGenerator, item: MetaItem) -> Attribute {
+    mk_attr(g, AttrStyle::Outer, item.path, item.kind.mac_args(item.span), item.span)
 }
 
 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: mk_attr_id(), style, span }
+    Attribute { kind: AttrKind::DocComment(comment_kind, data), id: g.mk_attr_id(), style, span }
 }
 
 pub fn list_contains_name(items: &[NestedMetaItem], name: Symbol) -> bool {
@@ -484,7 +535,7 @@ impl MetaItemKind {
                     id: ast::DUMMY_NODE_ID,
                     kind: ast::ExprKind::Lit(lit.clone()),
                     span: lit.span,
-                    attrs: ThinVec::new(),
+                    attrs: ast::AttrVec::new(),
                     tokens: None,
                 });
                 MacArgs::Eq(span, MacArgsEq::Ast(expr))
diff --git a/compiler/rustc_ast/src/lib.rs b/compiler/rustc_ast/src/lib.rs
index 4b94ec0d6..bd7a85b07 100644
--- a/compiler/rustc_ast/src/lib.rs
+++ b/compiler/rustc_ast/src/lib.rs
@@ -13,17 +13,23 @@
 #![feature(const_default_impls)]
 #![feature(const_trait_impl)]
 #![feature(if_let_guard)]
-#![feature(label_break_value)]
+#![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(let_chains)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(slice_internals)]
 #![feature(stmt_expr_attributes)]
 #![recursion_limit = "256"]
+#![deny(rustc::untranslatable_diagnostic)]
+#![deny(rustc::diagnostic_outside_of_impl)]
 
 #[macro_use]
 extern crate rustc_macros;
 
+#[macro_use]
+extern crate tracing;
+
 pub mod util {
     pub mod classify;
     pub mod comments;
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index 01bd498b3..9fd0b63c4 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -14,7 +14,6 @@ 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;
@@ -338,12 +337,7 @@ where
 }
 
 // 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) {
+pub fn visit_attrs<T: MutVisitor>(attrs: &mut AttrVec, vis: &mut T) {
     for attr in attrs.iter_mut() {
         vis.visit_attribute(attr);
     }
@@ -398,7 +392,7 @@ pub fn noop_flat_map_pat_field<T: MutVisitor>(
     vis.visit_ident(ident);
     vis.visit_pat(pat);
     vis.visit_span(span);
-    visit_thin_attrs(attrs, vis);
+    visit_attrs(attrs, vis);
     smallvec![fp]
 }
 
@@ -424,7 +418,7 @@ pub fn noop_visit_use_tree<T: MutVisitor>(use_tree: &mut UseTree, vis: &mut T) {
 
 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);
+    visit_attrs(attrs, vis);
     vis.visit_id(id);
     vis.visit_pat(pat);
     visit_opt(guard, |guard| vis.visit_expr(guard));
@@ -507,7 +501,7 @@ pub fn noop_flat_map_variant<T: MutVisitor>(
     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);
+    visit_attrs(attrs, visitor);
     visitor.visit_id(id);
     visitor.visit_variant_data(data);
     visit_opt(disr_expr, |disr_expr| visitor.visit_anon_const(disr_expr));
@@ -589,14 +583,16 @@ pub fn noop_visit_local<T: MutVisitor>(local: &mut P<Local>, vis: &mut T) {
         }
     }
     vis.visit_span(span);
-    visit_thin_attrs(attrs, vis);
+    visit_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) => {
+        AttrKind::Normal(normal) => {
+            let NormalAttr { item: AttrItem { path, args, tokens }, tokens: attr_tokens } =
+                &mut **normal;
             vis.visit_path(path);
             visit_mac_args(args, vis);
             visit_lazy_tts(tokens, vis);
@@ -638,7 +634,7 @@ pub fn noop_visit_meta_item<T: MutVisitor>(mi: &mut MetaItem, vis: &mut T) {
 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);
+    visit_attrs(attrs, vis);
     vis.visit_pat(pat);
     vis.visit_span(span);
     vis.visit_ty(ty);
@@ -646,21 +642,21 @@ pub fn noop_flat_map_param<T: MutVisitor>(mut param: Param, vis: &mut T) -> Smal
 }
 
 // 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) {
+pub fn visit_attr_tt<T: MutVisitor>(tt: &mut AttrTokenTree, vis: &mut T) {
     match tt {
-        AttrAnnotatedTokenTree::Token(token) => {
+        AttrTokenTree::Token(token, _) => {
             visit_token(token, vis);
         }
-        AttrAnnotatedTokenTree::Delimited(DelimSpan { open, close }, _delim, tts) => {
+        AttrTokenTree::Delimited(DelimSpan { open, close }, _delim, tts) => {
             vis.visit_span(open);
             vis.visit_span(close);
-            visit_attr_annotated_tts(tts, vis);
+            visit_attr_tts(tts, vis);
         }
-        AttrAnnotatedTokenTree::Attributes(data) => {
+        AttrTokenTree::Attributes(data) => {
             for attr in &mut *data.attrs {
                 match &mut attr.kind {
-                    AttrKind::Normal(_, attr_tokens) => {
-                        visit_lazy_tts(attr_tokens, vis);
+                    AttrKind::Normal(normal) => {
+                        visit_lazy_tts(&mut normal.tokens, vis);
                     }
                     AttrKind::DocComment(..) => {
                         vis.visit_span(&mut attr.span);
@@ -694,27 +690,27 @@ pub fn visit_tts<T: MutVisitor>(TokenStream(tts): &mut TokenStream, vis: &mut T)
     }
 }
 
-pub fn visit_attr_annotated_tts<T: MutVisitor>(
-    AttrAnnotatedTokenStream(tts): &mut AttrAnnotatedTokenStream,
-    vis: &mut T,
-) {
+pub fn visit_attr_tts<T: MutVisitor>(AttrTokenStream(tts): &mut AttrTokenStream, 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));
+        visit_vec(tts, |tree| visit_attr_tt(tree, vis));
     }
 }
 
-pub fn visit_lazy_tts_opt_mut<T: MutVisitor>(lazy_tts: Option<&mut LazyTokenStream>, vis: &mut T) {
+pub fn visit_lazy_tts_opt_mut<T: MutVisitor>(
+    lazy_tts: Option<&mut LazyAttrTokenStream>,
+    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);
+            let mut tts = lazy_tts.to_attr_token_stream();
+            visit_attr_tts(&mut tts, vis);
+            *lazy_tts = LazyAttrTokenStream::new(tts);
         }
     }
 }
 
-pub fn visit_lazy_tts<T: MutVisitor>(lazy_tts: &mut Option<LazyTokenStream>, vis: &mut T) {
+pub fn visit_lazy_tts<T: MutVisitor>(lazy_tts: &mut Option<LazyAttrTokenStream>, vis: &mut T) {
     visit_lazy_tts_opt_mut(lazy_tts.as_mut(), vis);
 }
 
@@ -880,7 +876,7 @@ pub fn noop_flat_map_generic_param<T: MutVisitor>(
     if let Some(ref mut colon_span) = colon_span {
         vis.visit_span(colon_span);
     }
-    visit_thin_attrs(attrs, vis);
+    visit_attrs(attrs, vis);
     visit_vec(bounds, |bound| noop_visit_param_bound(bound, vis));
     match kind {
         GenericParamKind::Lifetime => {}
@@ -933,8 +929,7 @@ pub fn noop_visit_where_predicate<T: MutVisitor>(pred: &mut WherePredicate, 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);
+            let WhereEqPredicate { span, lhs_ty, rhs_ty } = ep;
             vis.visit_span(span);
             vis.visit_ty(lhs_ty);
             vis.visit_ty(rhs_ty);
@@ -977,7 +972,7 @@ pub fn noop_flat_map_field_def<T: MutVisitor>(
     visitor.visit_vis(vis);
     visitor.visit_id(id);
     visitor.visit_ty(ty);
-    visit_thin_attrs(attrs, visitor);
+    visit_attrs(attrs, visitor);
     smallvec![fd]
 }
 
@@ -990,7 +985,7 @@ pub fn noop_flat_map_expr_field<T: MutVisitor>(
     vis.visit_expr(expr);
     vis.visit_id(id);
     vis.visit_span(span);
-    visit_thin_attrs(attrs, vis);
+    visit_attrs(attrs, vis);
     smallvec![f]
 }
 
@@ -1430,7 +1425,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
     }
     vis.visit_id(id);
     vis.visit_span(span);
-    visit_thin_attrs(attrs, vis);
+    visit_attrs(attrs, vis);
     visit_lazy_tts(tokens, vis);
 }
 
@@ -1476,7 +1471,7 @@ pub fn noop_flat_map_stmt_kind<T: MutVisitor>(
         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_attrs(attrs, vis);
             visit_lazy_tts(tokens, vis);
             smallvec![StmtKind::MacCall(mac)]
         }
@@ -1486,7 +1481,7 @@ pub fn noop_flat_map_stmt_kind<T: MutVisitor>(
 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 } => {
+        VisibilityKind::Restricted { path, id, shorthand: _ } => {
             vis.visit_path(path);
             vis.visit_id(id);
         }
@@ -1511,12 +1506,6 @@ impl<T: DummyAstNode + 'static> DummyAstNode for P<T> {
     }
 }
 
-impl<T> DummyAstNode for ThinVec<T> {
-    fn dummy() -> Self {
-        Default::default()
-    }
-}
-
 impl DummyAstNode for Item {
     fn dummy() -> Self {
         Item {
diff --git a/compiler/rustc_ast/src/node_id.rs b/compiler/rustc_ast/src/node_id.rs
index 7f928cb57..7b5acc3f4 100644
--- a/compiler/rustc_ast/src/node_id.rs
+++ b/compiler/rustc_ast/src/node_id.rs
@@ -13,7 +13,7 @@ rustc_index::newtype_index! {
     }
 }
 
-rustc_data_structures::define_id_collections!(NodeMap, NodeSet, NodeId);
+rustc_data_structures::define_id_collections!(NodeMap, NodeSet, NodeMapEntry, NodeId);
 
 /// The [`NodeId`] used to represent the root of the crate.
 pub const CRATE_NODE_ID: NodeId = NodeId::from_u32(0);
diff --git a/compiler/rustc_ast/src/token.rs b/compiler/rustc_ast/src/token.rs
index 85d9687c6..97dfb7837 100644
--- a/compiler/rustc_ast/src/token.rs
+++ b/compiler/rustc_ast/src/token.rs
@@ -398,6 +398,30 @@ impl Token {
         }
     }
 
+    /// Returns `true` if the token can appear at the start of an pattern.
+    ///
+    /// Shamelessly borrowed from `can_begin_expr`, only used for diagnostics right now.
+    pub fn can_begin_pattern(&self) -> bool {
+        match self.uninterpolate().kind {
+            Ident(name, is_raw)              =>
+                ident_can_begin_expr(name, self.span, is_raw), // value name or keyword
+            | OpenDelim(Delimiter::Bracket | Delimiter::Parenthesis)  // tuple or array
+            | Literal(..)                        // literal
+            | BinOp(Minus)                       // unary minus
+            | BinOp(And)                         // reference
+            | AndAnd                             // double reference
+            // DotDotDot is no longer supported
+            | DotDot | DotDotDot | DotDotEq      // ranges
+            | Lt | BinOp(Shl)                    // associated path
+            | ModSep                    => true, // global path
+            Interpolated(ref nt) => matches!(**nt, NtLiteral(..) |
+                NtPat(..)     |
+                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 {
@@ -436,6 +460,31 @@ impl Token {
             || self == &OpenDelim(Delimiter::Parenthesis)
     }
 
+    /// Returns `true` if the token can appear at the start of an item.
+    pub fn can_begin_item(&self) -> bool {
+        match self.kind {
+            Ident(name, _) => [
+                kw::Fn,
+                kw::Use,
+                kw::Struct,
+                kw::Enum,
+                kw::Pub,
+                kw::Trait,
+                kw::Extern,
+                kw::Impl,
+                kw::Unsafe,
+                kw::Const,
+                kw::Static,
+                kw::Union,
+                kw::Macro,
+                kw::Mod,
+                kw::Type,
+            ]
+            .contains(&name),
+            _ => false,
+        }
+    }
+
     /// Returns `true` if the token is any literal.
     pub fn is_lit(&self) -> bool {
         matches!(self.kind, Literal(..))
diff --git a/compiler/rustc_ast/src/tokenstream.rs b/compiler/rustc_ast/src/tokenstream.rs
index 9e4a22e1f..875cd620d 100644
--- a/compiler/rustc_ast/src/tokenstream.rs
+++ b/compiler/rustc_ast/src/tokenstream.rs
@@ -121,12 +121,12 @@ where
     }
 }
 
-pub trait CreateTokenStream: sync::Send + sync::Sync {
-    fn create_token_stream(&self) -> AttrAnnotatedTokenStream;
+pub trait ToAttrTokenStream: sync::Send + sync::Sync {
+    fn to_attr_token_stream(&self) -> AttrTokenStream;
 }
 
-impl CreateTokenStream for AttrAnnotatedTokenStream {
-    fn create_token_stream(&self) -> AttrAnnotatedTokenStream {
+impl ToAttrTokenStream for AttrTokenStream {
+    fn to_attr_token_stream(&self) -> AttrTokenStream {
         self.clone()
     }
 }
@@ -135,68 +135,68 @@ impl CreateTokenStream for AttrAnnotatedTokenStream {
 /// 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>>);
+pub struct LazyAttrTokenStream(Lrc<Box<dyn ToAttrTokenStream>>);
 
-impl LazyTokenStream {
-    pub fn new(inner: impl CreateTokenStream + 'static) -> LazyTokenStream {
-        LazyTokenStream(Lrc::new(Box::new(inner)))
+impl LazyAttrTokenStream {
+    pub fn new(inner: impl ToAttrTokenStream + 'static) -> LazyAttrTokenStream {
+        LazyAttrTokenStream(Lrc::new(Box::new(inner)))
     }
 
-    pub fn create_token_stream(&self) -> AttrAnnotatedTokenStream {
-        self.0.create_token_stream()
+    pub fn to_attr_token_stream(&self) -> AttrTokenStream {
+        self.0.to_attr_token_stream()
     }
 }
 
-impl fmt::Debug for LazyTokenStream {
+impl fmt::Debug for LazyAttrTokenStream {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "LazyTokenStream({:?})", self.create_token_stream())
+        write!(f, "LazyAttrTokenStream({:?})", self.to_attr_token_stream())
     }
 }
 
-impl<S: Encoder> Encodable<S> for LazyTokenStream {
+impl<S: Encoder> Encodable<S> for LazyAttrTokenStream {
     fn encode(&self, s: &mut S) {
         // Used by AST json printing.
-        Encodable::encode(&self.create_token_stream(), s);
+        Encodable::encode(&self.to_attr_token_stream(), s);
     }
 }
 
-impl<D: Decoder> Decodable<D> for LazyTokenStream {
+impl<D: Decoder> Decodable<D> for LazyAttrTokenStream {
     fn decode(_d: &mut D) -> Self {
-        panic!("Attempted to decode LazyTokenStream");
+        panic!("Attempted to decode LazyAttrTokenStream");
     }
 }
 
-impl<CTX> HashStable<CTX> for LazyTokenStream {
+impl<CTX> HashStable<CTX> for LazyAttrTokenStream {
     fn hash_stable(&self, _hcx: &mut CTX, _hasher: &mut StableHasher) {
-        panic!("Attempted to compute stable hash for LazyTokenStream");
+        panic!("Attempted to compute stable hash for LazyAttrTokenStream");
     }
 }
 
-/// A `AttrAnnotatedTokenStream` is similar to a `TokenStream`, but with extra
+/// An `AttrTokenStream` 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)>>);
+pub struct AttrTokenStream(pub Lrc<Vec<AttrTokenTree>>);
 
-/// Like `TokenTree`, but for `AttrAnnotatedTokenStream`
+/// Like `TokenTree`, but for `AttrTokenStream`.
 #[derive(Clone, Debug, Encodable, Decodable)]
-pub enum AttrAnnotatedTokenTree {
-    Token(Token),
-    Delimited(DelimSpan, Delimiter, AttrAnnotatedTokenStream),
+pub enum AttrTokenTree {
+    Token(Token, Spacing),
+    Delimited(DelimSpan, Delimiter, AttrTokenStream),
     /// 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))
+impl AttrTokenStream {
+    pub fn new(tokens: Vec<AttrTokenTree>) -> AttrTokenStream {
+        AttrTokenStream(Lrc::new(tokens))
     }
 
-    /// Converts this `AttrAnnotatedTokenStream` to a plain `TokenStream
-    /// During conversion, `AttrAnnotatedTokenTree::Attributes` get 'flattened'
+    /// Converts this `AttrTokenStream` to a plain `TokenStream`.
+    /// During conversion, `AttrTokenTree::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.
@@ -204,31 +204,27 @@ impl AttrAnnotatedTokenStream {
         let trees: Vec<_> = self
             .0
             .iter()
-            .flat_map(|tree| match &tree.0 {
-                AttrAnnotatedTokenTree::Token(inner) => {
-                    smallvec![TokenTree::Token(inner.clone(), tree.1)].into_iter()
+            .flat_map(|tree| match &tree {
+                AttrTokenTree::Token(inner, spacing) => {
+                    smallvec![TokenTree::Token(inner.clone(), *spacing)].into_iter()
                 }
-                AttrAnnotatedTokenTree::Delimited(span, delim, stream) => {
+                AttrTokenTree::Delimited(span, delim, stream) => {
                     smallvec![TokenTree::Delimited(*span, *delim, stream.to_tokenstream()),]
                         .into_iter()
                 }
-                AttrAnnotatedTokenTree::Attributes(data) => {
+                AttrTokenTree::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);
-                            }
+                            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_attr_token_stream()
                         .to_tokenstream()
                         .0
                         .iter()
@@ -239,9 +235,9 @@ impl AttrAnnotatedTokenStream {
                         // 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:
+                                // 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
@@ -255,7 +251,7 @@ impl AttrAnnotatedTokenStream {
 
                                 let mut builder = TokenStreamBuilder::new();
                                 for inner_attr in inner_attrs {
-                                    builder.push(inner_attr.tokens().to_tokenstream());
+                                    builder.push(inner_attr.tokens());
                                 }
                                 builder.push(delim_tokens.clone());
                                 *tree = TokenTree::Delimited(*span, *delim, builder.build());
@@ -273,7 +269,7 @@ impl AttrAnnotatedTokenStream {
                     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(attr.tokens().0.clone().iter().cloned());
                     }
                     flat.extend(target_tokens);
                     flat.into_iter()
@@ -300,7 +296,7 @@ pub struct AttributesData {
     pub attrs: AttrVec,
     /// The underlying tokens for the attribute target that `attrs`
     /// are applied to
-    pub tokens: LazyTokenStream,
+    pub tokens: LazyAttrTokenStream,
 }
 
 /// A `TokenStream` is an abstract sequence of tokens, organized into [`TokenTree`]s.
@@ -363,12 +359,6 @@ impl TokenStream {
     }
 }
 
-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>>())
@@ -420,21 +410,6 @@ impl 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)])
@@ -451,8 +426,18 @@ impl TokenStream {
     }
 
     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))
+        let Some(tokens) = node.tokens() else {
+            panic!("missing tokens for node at {:?}: {:?}", node.span(), node);
+        };
+        let attrs = node.attrs();
+        let attr_stream = if attrs.is_empty() {
+            tokens.to_attr_token_stream()
+        } else {
+            let attr_data =
+                AttributesData { attrs: attrs.iter().cloned().collect(), tokens: tokens.clone() };
+            AttrTokenStream::new(vec![AttrTokenTree::Attributes(attr_data)])
+        };
+        attr_stream.to_tokenstream()
     }
 
     pub fn from_nonterminal_ast(nt: &Nonterminal) -> TokenStream {
@@ -555,7 +540,7 @@ impl TokenStreamBuilder {
 
                 // 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 iter = streams.into_iter();
                 let mut res_stream_lrc = iter.next().unwrap().0;
 
                 // Append the subsequent elements to the result stream, after
diff --git a/compiler/rustc_ast/src/util/literal.rs b/compiler/rustc_ast/src/util/literal.rs
index 9c18f55c0..536b38560 100644
--- a/compiler/rustc_ast/src/util/literal.rs
+++ b/compiler/rustc_ast/src/util/literal.rs
@@ -9,7 +9,6 @@ use rustc_span::symbol::{kw, sym, Symbol};
 use rustc_span::Span;
 
 use std::ascii;
-use tracing::debug;
 
 pub enum LitError {
     NotLiteral,
@@ -23,7 +22,7 @@ pub enum LitError {
 
 impl LitKind {
     /// Converts literal token into a semantic literal.
-    pub fn from_lit_token(lit: token::Lit) -> Result<LitKind, LitError> {
+    pub fn from_token_lit(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);
@@ -146,14 +145,14 @@ impl LitKind {
 
                 LitKind::ByteStr(bytes.into())
             }
-            token::Err => LitKind::Err(symbol),
+            token::Err => LitKind::Err,
         })
     }
 
     /// 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 {
+    pub fn to_token_lit(&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.
@@ -164,12 +163,7 @@ impl LitKind {
             }
             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>();
+                let string = bytes.escape_ascii().to_string();
                 (token::ByteStr, Symbol::intern(&string), None)
             }
             LitKind::Byte(byte) => {
@@ -199,7 +193,9 @@ impl LitKind {
                 let symbol = if value { kw::True } else { kw::False };
                 (token::Bool, symbol, None)
             }
-            LitKind::Err(symbol) => (token::Err, symbol, None),
+            // This only shows up in places like `-Zunpretty=hir` output, so we
+            // don't bother to produce something useful.
+            LitKind::Err => (token::Err, Symbol::intern("<bad-literal>"), None),
         };
 
         token::Lit::new(kind, symbol, suffix)
@@ -208,8 +204,8 @@ impl LitKind {
 
 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 })
+    pub fn from_token_lit(token_lit: token::Lit, span: Span) -> Result<Lit, LitError> {
+        Ok(Lit { token_lit, kind: LitKind::from_token_lit(token_lit)?, span })
     }
 
     /// Converts arbitrary token into an AST literal.
@@ -232,21 +228,21 @@ impl Lit {
             _ => return Err(LitError::NotLiteral),
         };
 
-        Lit::from_lit_token(lit, token.span)
+        Lit::from_token_lit(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 }
+        Lit { token_lit: kind.to_token_lit(), 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),
+        let kind = match self.token_lit.kind {
+            token::Bool => token::Ident(self.token_lit.symbol, false),
+            _ => token::Literal(self.token_lit),
         };
         Token::new(kind, self.span)
     }
diff --git a/compiler/rustc_ast/src/util/parser.rs b/compiler/rustc_ast/src/util/parser.rs
index 74b7fe9e2..6c5c7f66f 100644
--- a/compiler/rustc_ast/src/util/parser.rs
+++ b/compiler/rustc_ast/src/util/parser.rs
@@ -297,11 +297,11 @@ impl ExprPrecedence {
         match self {
             ExprPrecedence::Closure => PREC_CLOSURE,
 
-            ExprPrecedence::Break |
-            ExprPrecedence::Continue |
-            ExprPrecedence::Ret |
-            ExprPrecedence::Yield |
-            ExprPrecedence::Yeet => PREC_JUMP,
+            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
@@ -318,43 +318,43 @@ impl ExprPrecedence {
             ExprPrecedence::AssignOp => AssocOp::Assign.precedence() as i8,
 
             // Unary, prefix
-            ExprPrecedence::Box |
-            ExprPrecedence::AddrOf |
+            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,
+            | 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,
+            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,
+            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,
         }
     }
 }
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index d9594b323..1d0de5a4b 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -156,8 +156,8 @@ pub trait Visitor<'ast>: Sized {
     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_fn(&mut self, fk: FnKind<'ast>, _: Span, _: NodeId) {
+        walk_fn(self, fk)
     }
     fn visit_assoc_item(&mut self, i: &'ast AssocItem, ctxt: AssocCtxt) {
         walk_assoc_item(self, i, ctxt)
@@ -168,8 +168,8 @@ pub trait Visitor<'ast>: Sized {
     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_poly_trait_ref(&mut self, t: &'ast PolyTraitRef) {
+        walk_poly_trait_ref(self, t)
     }
     fn visit_variant_data(&mut self, s: &'ast VariantData) {
         walk_struct_def(self, s)
@@ -177,14 +177,8 @@ pub trait Visitor<'ast>: Sized {
     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_enum_def(&mut self, enum_definition: &'ast EnumDef) {
+        walk_enum_def(self, enum_definition)
     }
     fn visit_variant(&mut self, v: &'ast Variant) {
         walk_variant(self, v)
@@ -207,11 +201,11 @@ pub trait Visitor<'ast>: Sized {
     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_path_segment(&mut self, path_segment: &'ast PathSegment) {
+        walk_path_segment(self, 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_args(&mut self, generic_args: &'ast GenericArgs) {
+        walk_generic_args(self, generic_args)
     }
     fn visit_generic_arg(&mut self, generic_arg: &'ast GenericArg) {
         walk_generic_arg(self, generic_arg)
@@ -287,11 +281,8 @@ 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
+pub fn walk_poly_trait_ref<'a, V>(visitor: &mut V, trait_ref: &'a PolyTraitRef)
+where
     V: Visitor<'a>,
 {
     walk_list!(visitor, visit_generic_param, &trait_ref.bound_generic_params);
@@ -334,7 +325,7 @@ pub fn walk_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a Item) {
         }
         ItemKind::Enum(ref enum_definition, ref generics) => {
             visitor.visit_generics(generics);
-            visitor.visit_enum_def(enum_definition, generics, item.id, item.span)
+            visitor.visit_enum_def(enum_definition)
         }
         ItemKind::Impl(box Impl {
             defaultness: _,
@@ -377,12 +368,7 @@ pub fn walk_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a Item) {
     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,
-) {
+pub fn walk_enum_def<'a, V: Visitor<'a>>(visitor: &mut V, enum_definition: &'a EnumDef) {
     walk_list!(visitor, visit_variant, &enum_definition.variants);
 }
 
@@ -449,7 +435,7 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) {
 
 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);
+        visitor.visit_path_segment(segment);
     }
 }
 
@@ -471,18 +457,14 @@ pub fn walk_use_tree<'a, V: Visitor<'a>>(visitor: &mut V, use_tree: &'a UseTree,
     }
 }
 
-pub fn walk_path_segment<'a, V: Visitor<'a>>(
-    visitor: &mut V,
-    path_span: Span,
-    segment: &'a PathSegment,
-) {
+pub fn walk_path_segment<'a, V: Visitor<'a>>(visitor: &mut V, segment: &'a PathSegment) {
     visitor.visit_ident(segment.ident);
     if let Some(ref args) = segment.args {
-        visitor.visit_generic_args(path_span, args);
+        visitor.visit_generic_args(args);
     }
 }
 
-pub fn walk_generic_args<'a, V>(visitor: &mut V, _path_span: Span, generic_args: &'a GenericArgs)
+pub fn walk_generic_args<'a, V>(visitor: &mut V, generic_args: &'a GenericArgs)
 where
     V: Visitor<'a>,
 {
@@ -516,7 +498,7 @@ where
 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);
+        visitor.visit_generic_args(gen_args);
     }
     match constraint.kind {
         AssocConstraintKind::Equality { ref term } => match term {
@@ -598,7 +580,7 @@ pub fn walk_foreign_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a ForeignI
 
 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::Trait(ref typ, ref _modifier) => visitor.visit_poly_trait_ref(typ),
         GenericBound::Outlives(ref lifetime) => {
             visitor.visit_lifetime(lifetime, LifetimeCtxt::Bound)
         }
@@ -673,7 +655,7 @@ pub fn walk_fn_decl<'a, V: Visitor<'a>>(visitor: &mut V, function_declaration: &
     visitor.visit_fn_ret_ty(&function_declaration.output);
 }
 
-pub fn walk_fn<'a, V: Visitor<'a>>(visitor: &mut V, kind: FnKind<'a>, _span: Span) {
+pub fn walk_fn<'a, V: Visitor<'a>>(visitor: &mut V, kind: FnKind<'a>) {
     match kind {
         FnKind::Fn(_, _, sig, _, generics, body) => {
             visitor.visit_generics(generics);
@@ -814,7 +796,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
             walk_list!(visitor, visit_expr, arguments);
         }
         ExprKind::MethodCall(ref segment, ref arguments, _span) => {
-            visitor.visit_path_segment(expression.span, segment);
+            visitor.visit_path_segment(segment);
             walk_list!(visitor, visit_expr, arguments);
         }
         ExprKind::Binary(_, ref left_expression, ref right_expression) => {
@@ -935,14 +917,14 @@ pub fn walk_arm<'a, V: Visitor<'a>>(visitor: &mut V, arm: &'a Arm) {
 }
 
 pub fn walk_vis<'a, V: Visitor<'a>>(visitor: &mut V, vis: &'a Visibility) {
-    if let VisibilityKind::Restricted { ref path, id } = vis.kind {
+    if let VisibilityKind::Restricted { ref path, id, shorthand: _ } = 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::Normal(ref normal) => walk_mac_args(visitor, &normal.item.args),
         AttrKind::DocComment(..) => {}
     }
 }
diff --git a/compiler/rustc_ast_lowering/Cargo.toml b/compiler/rustc_ast_lowering/Cargo.toml
index 39ba62ef2..ce1c8d499 100644
--- a/compiler/rustc_ast_lowering/Cargo.toml
+++ b/compiler/rustc_ast_lowering/Cargo.toml
@@ -8,16 +8,18 @@ doctest = false
 
 [dependencies]
 rustc_arena = { path = "../rustc_arena" }
-tracing = "0.1"
+rustc_ast = { path = "../rustc_ast" }
 rustc_ast_pretty = { path = "../rustc_ast_pretty" }
-rustc_hir = { path = "../rustc_hir" }
-rustc_target = { path = "../rustc_target" }
 rustc_data_structures = { path = "../rustc_data_structures" }
+rustc_errors = { path = "../rustc_errors" }
+rustc_hir = { path = "../rustc_hir" }
 rustc_index = { path = "../rustc_index" }
 rustc_middle = { path = "../rustc_middle" }
+rustc_macros = { path = "../rustc_macros" }
 rustc_query_system = { path = "../rustc_query_system" }
-rustc_span = { path = "../rustc_span" }
-rustc_errors = { path = "../rustc_errors" }
 rustc_session = { path = "../rustc_session" }
-rustc_ast = { path = "../rustc_ast" }
+rustc_span = { path = "../rustc_span" }
+rustc_target = { path = "../rustc_target" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+thin-vec = "0.2.8"
+tracing = "0.1"
diff --git a/compiler/rustc_ast_lowering/src/asm.rs b/compiler/rustc_ast_lowering/src/asm.rs
index 4166b4fc2..24672efc6 100644
--- a/compiler/rustc_ast_lowering/src/asm.rs
+++ b/compiler/rustc_ast_lowering/src/asm.rs
@@ -1,11 +1,17 @@
 use crate::{ImplTraitContext, ImplTraitPosition, ParamMode, ResolverAstLoweringExt};
 
+use super::errors::{
+    AbiSpecifiedMultipleTimes, AttSyntaxOnlyX86, ClobberAbiNotSupported,
+    InlineAsmUnsupportedTarget, InvalidAbiClobberAbi, InvalidAsmTemplateModifierConst,
+    InvalidAsmTemplateModifierRegClass, InvalidAsmTemplateModifierRegClassSub,
+    InvalidAsmTemplateModifierSym, InvalidRegister, InvalidRegisterClass, RegisterClassOnlyClobber,
+    RegisterConflict,
+};
 use super::LoweringContext;
 
 use rustc_ast::ptr::P;
 use rustc_ast::*;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
-use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::definitions::DefPathData;
@@ -26,13 +32,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let asm_arch =
             if self.tcx.sess.opts.actually_rustdoc { None } else { self.tcx.sess.asm_arch };
         if asm_arch.is_none() && !self.tcx.sess.opts.actually_rustdoc {
-            struct_span_err!(
-                self.tcx.sess,
-                sp,
-                E0472,
-                "inline assembly is unsupported on this target"
-            )
-            .emit();
+            self.tcx.sess.emit_err(InlineAsmUnsupportedTarget { span: sp });
         }
         if let Some(asm_arch) = asm_arch {
             // Inline assembly is currently only stable for these architectures.
@@ -59,10 +59,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             && !matches!(asm_arch, Some(asm::InlineAsmArch::X86 | asm::InlineAsmArch::X86_64))
             && !self.tcx.sess.opts.actually_rustdoc
         {
-            self.tcx
-                .sess
-                .struct_span_err(sp, "the `att_syntax` option is only supported on x86")
-                .emit();
+            self.tcx.sess.emit_err(AttSyntaxOnlyX86 { span: sp });
         }
         if asm.options.contains(InlineAsmOptions::MAY_UNWIND) && !self.tcx.features().asm_unwind {
             feature_err(
@@ -82,51 +79,37 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         // If the abi was already in the list, emit an error
                         match clobber_abis.get(&abi) {
                             Some((prev_name, prev_sp)) => {
-                                let mut err = self.tcx.sess.struct_span_err(
-                                    *abi_span,
-                                    &format!("`{}` ABI specified multiple times", prev_name),
-                                );
-                                err.span_label(*prev_sp, "previously specified here");
-
                                 // Multiple different abi names may actually be the same ABI
                                 // If the specified ABIs are not the same name, alert the user that they resolve to the same ABI
                                 let source_map = self.tcx.sess.source_map();
-                                if source_map.span_to_snippet(*prev_sp)
-                                    != source_map.span_to_snippet(*abi_span)
-                                {
-                                    err.note("these ABIs are equivalent on the current target");
-                                }
+                                let equivalent = (source_map.span_to_snippet(*prev_sp)
+                                    != source_map.span_to_snippet(*abi_span))
+                                .then_some(());
 
-                                err.emit();
+                                self.tcx.sess.emit_err(AbiSpecifiedMultipleTimes {
+                                    abi_span: *abi_span,
+                                    prev_name: *prev_name,
+                                    prev_span: *prev_sp,
+                                    equivalent,
+                                });
                             }
                             None => {
-                                clobber_abis.insert(abi, (abi_name, *abi_span));
+                                clobber_abis.insert(abi, (*abi_name, *abi_span));
                             }
                         }
                     }
                     Err(&[]) => {
-                        self.tcx
-                            .sess
-                            .struct_span_err(
-                                *abi_span,
-                                "`clobber_abi` is not supported on this target",
-                            )
-                            .emit();
+                        self.tcx.sess.emit_err(ClobberAbiNotSupported { abi_span: *abi_span });
                     }
                     Err(supported_abis) => {
-                        let mut err = self
-                            .tcx
-                            .sess
-                            .struct_span_err(*abi_span, "invalid ABI for `clobber_abi`");
                         let mut abis = format!("`{}`", supported_abis[0]);
                         for m in &supported_abis[1..] {
                             let _ = write!(abis, ", `{}`", m);
                         }
-                        err.note(&format!(
-                            "the following ABIs are supported on this target: {}",
-                            abis
-                        ));
-                        err.emit();
+                        self.tcx.sess.emit_err(InvalidAbiClobberAbi {
+                            abi_span: *abi_span,
+                            supported_abis: abis,
+                        });
                     }
                 }
             }
@@ -141,24 +124,28 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             .iter()
             .map(|(op, op_sp)| {
                 let lower_reg = |reg| match reg {
-                    InlineAsmRegOrRegClass::Reg(s) => {
+                    InlineAsmRegOrRegClass::Reg(reg) => {
                         asm::InlineAsmRegOrRegClass::Reg(if let Some(asm_arch) = asm_arch {
-                            asm::InlineAsmReg::parse(asm_arch, s).unwrap_or_else(|e| {
-                                let msg = format!("invalid register `{}`: {}", s, e);
-                                sess.struct_span_err(*op_sp, &msg).emit();
+                            asm::InlineAsmReg::parse(asm_arch, reg).unwrap_or_else(|error| {
+                                sess.emit_err(InvalidRegister { op_span: *op_sp, reg, error });
                                 asm::InlineAsmReg::Err
                             })
                         } else {
                             asm::InlineAsmReg::Err
                         })
                     }
-                    InlineAsmRegOrRegClass::RegClass(s) => {
+                    InlineAsmRegOrRegClass::RegClass(reg_class) => {
                         asm::InlineAsmRegOrRegClass::RegClass(if let Some(asm_arch) = asm_arch {
-                            asm::InlineAsmRegClass::parse(asm_arch, s).unwrap_or_else(|e| {
-                                let msg = format!("invalid register class `{}`: {}", s, e);
-                                sess.struct_span_err(*op_sp, &msg).emit();
-                                asm::InlineAsmRegClass::Err
-                            })
+                            asm::InlineAsmRegClass::parse(asm_arch, reg_class).unwrap_or_else(
+                                |error| {
+                                    sess.emit_err(InvalidRegisterClass {
+                                        op_span: *op_sp,
+                                        reg_class,
+                                        error,
+                                    });
+                                    asm::InlineAsmRegClass::Err
+                                },
+                            )
                         } else {
                             asm::InlineAsmRegClass::Err
                         })
@@ -168,26 +155,26 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 let op = match *op {
                     InlineAsmOperand::In { reg, ref expr } => hir::InlineAsmOperand::In {
                         reg: lower_reg(reg),
-                        expr: self.lower_expr_mut(expr),
+                        expr: self.lower_expr(expr),
                     },
                     InlineAsmOperand::Out { reg, late, ref expr } => hir::InlineAsmOperand::Out {
                         reg: lower_reg(reg),
                         late,
-                        expr: expr.as_ref().map(|expr| self.lower_expr_mut(expr)),
+                        expr: expr.as_ref().map(|expr| self.lower_expr(expr)),
                     },
                     InlineAsmOperand::InOut { reg, late, ref expr } => {
                         hir::InlineAsmOperand::InOut {
                             reg: lower_reg(reg),
                             late,
-                            expr: self.lower_expr_mut(expr),
+                            expr: self.lower_expr(expr),
                         }
                     }
                     InlineAsmOperand::SplitInOut { reg, late, ref in_expr, ref out_expr } => {
                         hir::InlineAsmOperand::SplitInOut {
                             reg: lower_reg(reg),
                             late,
-                            in_expr: self.lower_expr_mut(in_expr),
-                            out_expr: out_expr.as_ref().map(|expr| self.lower_expr_mut(expr)),
+                            in_expr: self.lower_expr(in_expr),
+                            out_expr: out_expr.as_ref().map(|expr| self.lower_expr(expr)),
                         }
                     }
                     InlineAsmOperand::Const { ref anon_const } => {
@@ -233,7 +220,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                 &sym.qself,
                                 &sym.path,
                                 ParamMode::Optional,
-                                ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                                &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                             );
                             hir::InlineAsmOperand::SymStatic { path, def_id }
                         } else {
@@ -282,50 +269,39 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         }
                         let valid_modifiers = class.valid_modifiers(asm_arch.unwrap());
                         if !valid_modifiers.contains(&modifier) {
-                            let mut err = sess.struct_span_err(
-                                placeholder_span,
-                                "invalid asm template modifier for this register class",
-                            );
-                            err.span_label(placeholder_span, "template modifier");
-                            err.span_label(op_sp, "argument");
-                            if !valid_modifiers.is_empty() {
+                            let sub = if !valid_modifiers.is_empty() {
                                 let mut mods = format!("`{}`", valid_modifiers[0]);
                                 for m in &valid_modifiers[1..] {
                                     let _ = write!(mods, ", `{}`", m);
                                 }
-                                err.note(&format!(
-                                    "the `{}` register class supports \
-                                     the following template modifiers: {}",
-                                    class.name(),
-                                    mods
-                                ));
+                                InvalidAsmTemplateModifierRegClassSub::SupportModifier {
+                                    class_name: class.name(),
+                                    modifiers: mods,
+                                }
                             } else {
-                                err.note(&format!(
-                                    "the `{}` register class does not support template modifiers",
-                                    class.name()
-                                ));
-                            }
-                            err.emit();
+                                InvalidAsmTemplateModifierRegClassSub::DoesNotSupportModifier {
+                                    class_name: class.name(),
+                                }
+                            };
+                            sess.emit_err(InvalidAsmTemplateModifierRegClass {
+                                placeholder_span,
+                                op_span: op_sp,
+                                sub,
+                            });
                         }
                     }
                     hir::InlineAsmOperand::Const { .. } => {
-                        let mut err = sess.struct_span_err(
+                        sess.emit_err(InvalidAsmTemplateModifierConst {
                             placeholder_span,
-                            "asm template modifiers are not allowed for `const` arguments",
-                        );
-                        err.span_label(placeholder_span, "template modifier");
-                        err.span_label(op_sp, "argument");
-                        err.emit();
+                            op_span: op_sp,
+                        });
                     }
                     hir::InlineAsmOperand::SymFn { .. }
                     | hir::InlineAsmOperand::SymStatic { .. } => {
-                        let mut err = sess.struct_span_err(
+                        sess.emit_err(InvalidAsmTemplateModifierSym {
                             placeholder_span,
-                            "asm template modifiers are not allowed for `sym` arguments",
-                        );
-                        err.span_label(placeholder_span, "template modifier");
-                        err.span_label(op_sp, "argument");
-                        err.emit();
+                            op_span: op_sp,
+                        });
                     }
                 }
             }
@@ -346,12 +322,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 // require that the operand name an explicit register, not a
                 // register class.
                 if reg_class.is_clobber_only(asm_arch.unwrap()) && !op.is_clobber() {
-                    let msg = format!(
-                        "register class `{}` can only be used as a clobber, \
-                             not as an input or output",
-                        reg_class.name()
-                    );
-                    sess.struct_span_err(op_sp, &msg).emit();
+                    sess.emit_err(RegisterClassOnlyClobber {
+                        op_span: op_sp,
+                        reg_class_name: reg_class.name(),
+                    });
                     continue;
                 }
 
@@ -391,16 +365,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                         unreachable!();
                                     };
 
-                                    let msg = format!(
-                                        "register `{}` conflicts with register `{}`",
-                                        reg.name(),
-                                        reg2.name()
-                                    );
-                                    let mut err = sess.struct_span_err(op_sp, &msg);
-                                    err.span_label(op_sp, &format!("register `{}`", reg.name()));
-                                    err.span_label(op_sp2, &format!("register `{}`", reg2.name()));
-
-                                    match (op, op2) {
+                                    let in_out = match (op, op2) {
                                         (
                                             hir::InlineAsmOperand::In { .. },
                                             hir::InlineAsmOperand::Out { late, .. },
@@ -411,14 +376,18 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                         ) => {
                                             assert!(!*late);
                                             let out_op_sp = if input { op_sp2 } else { op_sp };
-                                            let msg = "use `lateout` instead of \
-                                                       `out` to avoid conflict";
-                                            err.span_help(out_op_sp, msg);
-                                        }
-                                        _ => {}
-                                    }
+                                            Some(out_op_sp)
+                                        },
+                                        _ => None,
+                                    };
 
-                                    err.emit();
+                                    sess.emit_err(RegisterConflict {
+                                        op_span1: op_sp,
+                                        op_span2: op_sp2,
+                                        reg1_name: reg.name(),
+                                        reg2_name: reg2.name(),
+                                        in_out
+                                    });
                                 }
                                 Entry::Vacant(v) => {
                                     if r == reg {
diff --git a/compiler/rustc_ast_lowering/src/block.rs b/compiler/rustc_ast_lowering/src/block.rs
index 7cbfe143b..12a0cc0d2 100644
--- a/compiler/rustc_ast_lowering/src/block.rs
+++ b/compiler/rustc_ast_lowering/src/block.rs
@@ -1,8 +1,6 @@
 use crate::{ImplTraitContext, ImplTraitPosition, LoweringContext};
 use rustc_ast::{Block, BlockCheckMode, Local, LocalKind, Stmt, StmtKind};
 use rustc_hir as hir;
-use rustc_session::parse::feature_err;
-use rustc_span::sym;
 
 use smallvec::SmallVec;
 
@@ -87,20 +85,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let ty = l
             .ty
             .as_ref()
-            .map(|t| self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Variable)));
+            .map(|t| self.lower_ty(t, &ImplTraitContext::Disallowed(ImplTraitPosition::Variable)));
         let init = l.kind.init().map(|init| self.lower_expr(init));
         let hir_id = self.lower_node_id(l.id);
         let pat = self.lower_pat(&l.pat);
         let els = if let LocalKind::InitElse(_, els) = &l.kind {
-            if !self.tcx.features().let_else {
-                feature_err(
-                    &self.tcx.sess.parse_sess,
-                    sym::let_else,
-                    l.span,
-                    "`let...else` statements are unstable",
-                )
-                .emit();
-            }
             Some(self.lower_block(els, false))
         } else {
             None
diff --git a/compiler/rustc_ast_lowering/src/errors.rs b/compiler/rustc_ast_lowering/src/errors.rs
new file mode 100644
index 000000000..c87d0ca96
--- /dev/null
+++ b/compiler/rustc_ast_lowering/src/errors.rs
@@ -0,0 +1,347 @@
+use rustc_errors::{fluent, AddSubdiagnostic, Applicability, Diagnostic, DiagnosticArgFromDisplay};
+use rustc_macros::{SessionDiagnostic, SessionSubdiagnostic};
+use rustc_span::{symbol::Ident, Span, Symbol};
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::generic_type_with_parentheses, code = "E0214")]
+pub struct GenericTypeWithParentheses {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[subdiagnostic]
+    pub sub: Option<UseAngleBrackets>,
+}
+
+#[derive(Clone, Copy)]
+pub struct UseAngleBrackets {
+    pub open_param: Span,
+    pub close_param: Span,
+}
+
+impl AddSubdiagnostic for UseAngleBrackets {
+    fn add_to_diagnostic(self, diag: &mut Diagnostic) {
+        diag.multipart_suggestion(
+            fluent::ast_lowering::use_angle_brackets,
+            vec![(self.open_param, String::from("<")), (self.close_param, String::from(">"))],
+            Applicability::MaybeIncorrect,
+        );
+    }
+}
+
+#[derive(SessionDiagnostic)]
+#[help]
+#[diag(ast_lowering::invalid_abi, code = "E0703")]
+pub struct InvalidAbi {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub abi: Symbol,
+    pub valid_abis: String,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::assoc_ty_parentheses)]
+pub struct AssocTyParentheses {
+    #[primary_span]
+    pub span: Span,
+    #[subdiagnostic]
+    pub sub: AssocTyParenthesesSub,
+}
+
+#[derive(Clone, Copy)]
+pub enum AssocTyParenthesesSub {
+    Empty { parentheses_span: Span },
+    NotEmpty { open_param: Span, close_param: Span },
+}
+
+impl AddSubdiagnostic for AssocTyParenthesesSub {
+    fn add_to_diagnostic(self, diag: &mut Diagnostic) {
+        match self {
+            Self::Empty { parentheses_span } => diag.multipart_suggestion(
+                fluent::ast_lowering::remove_parentheses,
+                vec![(parentheses_span, String::new())],
+                Applicability::MaybeIncorrect,
+            ),
+            Self::NotEmpty { open_param, close_param } => diag.multipart_suggestion(
+                fluent::ast_lowering::use_angle_brackets,
+                vec![(open_param, String::from("<")), (close_param, String::from(">"))],
+                Applicability::MaybeIncorrect,
+            ),
+        };
+    }
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_lowering::misplaced_impl_trait, code = "E0562")]
+pub struct MisplacedImplTrait<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub position: DiagnosticArgFromDisplay<'a>,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::rustc_box_attribute_error)]
+pub struct RustcBoxAttributeError {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::underscore_expr_lhs_assign)]
+pub struct UnderscoreExprLhsAssign {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::base_expression_double_dot)]
+pub struct BaseExpressionDoubleDot {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::await_only_in_async_fn_and_blocks, code = "E0728")]
+pub struct AwaitOnlyInAsyncFnAndBlocks {
+    #[primary_span]
+    #[label]
+    pub dot_await_span: Span,
+    #[label(ast_lowering::this_not_async)]
+    pub item_span: Option<Span>,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::generator_too_many_parameters, code = "E0628")]
+pub struct GeneratorTooManyParameters {
+    #[primary_span]
+    pub fn_decl_span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::closure_cannot_be_static, code = "E0697")]
+pub struct ClosureCannotBeStatic {
+    #[primary_span]
+    pub fn_decl_span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[help]
+#[diag(ast_lowering::async_non_move_closure_not_supported, code = "E0708")]
+pub struct AsyncNonMoveClosureNotSupported {
+    #[primary_span]
+    pub fn_decl_span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::functional_record_update_destructuring_assignment)]
+pub struct FunctionalRecordUpdateDestructuringAssignemnt {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "machine-applicable")]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::async_generators_not_supported, code = "E0727")]
+pub struct AsyncGeneratorsNotSupported {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::inline_asm_unsupported_target, code = "E0472")]
+pub struct InlineAsmUnsupportedTarget {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::att_syntax_only_x86)]
+pub struct AttSyntaxOnlyX86 {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::abi_specified_multiple_times)]
+pub struct AbiSpecifiedMultipleTimes {
+    #[primary_span]
+    pub abi_span: Span,
+    pub prev_name: Symbol,
+    #[label]
+    pub prev_span: Span,
+    #[note]
+    pub equivalent: Option<()>,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::clobber_abi_not_supported)]
+pub struct ClobberAbiNotSupported {
+    #[primary_span]
+    pub abi_span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[note]
+#[diag(ast_lowering::invalid_abi_clobber_abi)]
+pub struct InvalidAbiClobberAbi {
+    #[primary_span]
+    pub abi_span: Span,
+    pub supported_abis: String,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::invalid_register)]
+pub struct InvalidRegister<'a> {
+    #[primary_span]
+    pub op_span: Span,
+    pub reg: Symbol,
+    pub error: &'a str,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::invalid_register_class)]
+pub struct InvalidRegisterClass<'a> {
+    #[primary_span]
+    pub op_span: Span,
+    pub reg_class: Symbol,
+    pub error: &'a str,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_lowering::invalid_asm_template_modifier_reg_class)]
+pub struct InvalidAsmTemplateModifierRegClass {
+    #[primary_span]
+    #[label(ast_lowering::template_modifier)]
+    pub placeholder_span: Span,
+    #[label(ast_lowering::argument)]
+    pub op_span: Span,
+    #[subdiagnostic]
+    pub sub: InvalidAsmTemplateModifierRegClassSub,
+}
+
+#[derive(SessionSubdiagnostic)]
+pub enum InvalidAsmTemplateModifierRegClassSub {
+    #[note(ast_lowering::support_modifiers)]
+    SupportModifier { class_name: Symbol, modifiers: String },
+    #[note(ast_lowering::does_not_support_modifiers)]
+    DoesNotSupportModifier { class_name: Symbol },
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::invalid_asm_template_modifier_const)]
+pub struct InvalidAsmTemplateModifierConst {
+    #[primary_span]
+    #[label(ast_lowering::template_modifier)]
+    pub placeholder_span: Span,
+    #[label(ast_lowering::argument)]
+    pub op_span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::invalid_asm_template_modifier_sym)]
+pub struct InvalidAsmTemplateModifierSym {
+    #[primary_span]
+    #[label(ast_lowering::template_modifier)]
+    pub placeholder_span: Span,
+    #[label(ast_lowering::argument)]
+    pub op_span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::register_class_only_clobber)]
+pub struct RegisterClassOnlyClobber {
+    #[primary_span]
+    pub op_span: Span,
+    pub reg_class_name: Symbol,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::register_conflict)]
+pub struct RegisterConflict<'a> {
+    #[primary_span]
+    #[label(ast_lowering::register1)]
+    pub op_span1: Span,
+    #[label(ast_lowering::register2)]
+    pub op_span2: Span,
+    pub reg1_name: &'a str,
+    pub reg2_name: &'a str,
+    #[help]
+    pub in_out: Option<Span>,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[help]
+#[diag(ast_lowering::sub_tuple_binding)]
+pub struct SubTupleBinding<'a> {
+    #[primary_span]
+    #[label]
+    #[suggestion_verbose(
+        ast_lowering::sub_tuple_binding_suggestion,
+        code = "..",
+        applicability = "maybe-incorrect"
+    )]
+    pub span: Span,
+    pub ident: Ident,
+    pub ident_name: Symbol,
+    pub ctx: &'a str,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::extra_double_dot)]
+pub struct ExtraDoubleDot<'a> {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[label(ast_lowering::previously_used_here)]
+    pub prev_span: Span,
+    pub ctx: &'a str,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[note]
+#[diag(ast_lowering::misplaced_double_dot)]
+pub struct MisplacedDoubleDot {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::misplaced_relax_trait_bound)]
+pub struct MisplacedRelaxTraitBound {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::not_supported_for_lifetime_binder_async_closure)]
+pub struct NotSupportedForLifetimeBinderAsyncClosure {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::arbitrary_expression_in_pattern)]
+pub struct ArbitraryExpressionInPattern {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::inclusive_range_with_no_end)]
+pub struct InclusiveRangeWithNoEnd {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::trait_fn_async, code = "E0706")]
+#[note]
+#[note(ast_lowering::note2)]
+pub struct TraitFnAsync {
+    #[primary_span]
+    pub fn_span: Span,
+    #[label]
+    pub span: Span,
+}
diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index fb6715ff1..7b8070d3c 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -1,19 +1,23 @@
+use super::errors::{
+    AsyncGeneratorsNotSupported, AsyncNonMoveClosureNotSupported, AwaitOnlyInAsyncFnAndBlocks,
+    BaseExpressionDoubleDot, ClosureCannotBeStatic, FunctionalRecordUpdateDestructuringAssignemnt,
+    GeneratorTooManyParameters, InclusiveRangeWithNoEnd, NotSupportedForLifetimeBinderAsyncClosure,
+    RustcBoxAttributeError, UnderscoreExprLhsAssign,
+};
 use super::ResolverAstLoweringExt;
 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
 use crate::{FnDeclKind, ImplTraitPosition};
-
 use rustc_ast::attr;
 use rustc_ast::ptr::P as AstP;
 use rustc_ast::*;
 use rustc_data_structures::stack::ensure_sufficient_stack;
-use rustc_data_structures::thin_vec::ThinVec;
-use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::def::Res;
 use rustc_hir::definitions::DefPathData;
 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::DUMMY_SP;
+use thin_vec::thin_vec;
 
 impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
@@ -46,13 +50,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             let hir_id = self.lower_node_id(e.id);
                             return hir::Expr { hir_id, kind, span: self.lower_span(e.span) };
                         } else {
-                            self.tcx.sess
-                                .struct_span_err(
-                                    e.span,
-                                    "#[rustc_box] requires precisely one argument \
-                                    and no other attributes are allowed",
-                                )
-                                .emit();
+                            self.tcx.sess.emit_err(RustcBoxAttributeError { span: e.span });
                             hir::ExprKind::Err
                         }
                     } else if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
@@ -68,10 +66,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         seg,
                         ParamMode::Optional,
                         ParenthesizedGenericArgs::Err,
-                        ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     ));
-                    let args = self.lower_exprs(args);
-                    hir::ExprKind::MethodCall(hir_seg, args, self.lower_span(span))
+                    let receiver = self.lower_expr(&args[0]);
+                    let args =
+                        self.arena.alloc_from_iter(args[1..].iter().map(|x| self.lower_expr_mut(x)));
+                    hir::ExprKind::MethodCall(hir_seg, receiver, args, self.lower_span(span))
                 }
                 ExprKind::Binary(binop, ref lhs, ref rhs) => {
                     let binop = self.lower_binop(binop);
@@ -90,13 +90,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ExprKind::Cast(ref expr, ref ty) => {
                     let expr = self.lower_expr(expr);
                     let ty =
-                        self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                        self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                     hir::ExprKind::Cast(expr, ty)
                 }
                 ExprKind::Type(ref expr, ref ty) => {
                     let expr = self.lower_expr(expr);
                     let ty =
-                        self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                        self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                     hir::ExprKind::Type(expr, ty)
                 }
                 ExprKind::AddrOf(k, m, ref ohs) => {
@@ -146,13 +146,19 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
                     ),
                 ExprKind::Await(ref expr) => {
-                    let span = if expr.span.hi() < e.span.hi() {
-                        expr.span.shrink_to_hi().with_hi(e.span.hi())
+                    let dot_await_span = if expr.span.hi() < e.span.hi() {
+                        let span_with_whitespace = self
+                            .tcx
+                            .sess
+                            .source_map()
+                            .span_extend_while(expr.span, char::is_whitespace)
+                            .unwrap_or(expr.span);
+                        span_with_whitespace.shrink_to_hi().with_hi(e.span.hi())
                     } else {
                         // this is a recovered `await expr`
                         e.span
                     };
-                    self.lower_expr_await(span, expr)
+                    self.lower_expr_await(dot_await_span, expr)
                 }
                 ExprKind::Closure(
                     ref binder,
@@ -210,13 +216,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
                 }
                 ExprKind::Underscore => {
-                    self.tcx
-                        .sess.struct_span_err(
-                            e.span,
-                            "in expressions, `_` can only be used on the left-hand side of an assignment",
-                        )
-                        .span_label(e.span, "`_` not allowed here")
-                        .emit();
+                    self.tcx.sess.emit_err(UnderscoreExprLhsAssign { span: e.span });
                     hir::ExprKind::Err
                 }
                 ExprKind::Path(ref qself, ref path) => {
@@ -225,7 +225,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         qself,
                         path,
                         ParamMode::Optional,
-                        ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     );
                     hir::ExprKind::Path(qpath)
                 }
@@ -248,11 +248,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     let rest = match &se.rest {
                         StructRest::Base(e) => Some(self.lower_expr(e)),
                         StructRest::Rest(sp) => {
-                            self.tcx
-                                .sess
-                                .struct_span_err(*sp, "base expression required after `..`")
-                                .span_label(*sp, "add a base expression here")
-                                .emit();
+                            self.tcx.sess.emit_err(BaseExpressionDoubleDot { span: *sp });
                             Some(&*self.arena.alloc(self.expr_err(*sp)))
                         }
                         StructRest::None => None,
@@ -263,7 +259,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             &se.qself,
                             &se.path,
                             ParamMode::Optional,
-                            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         )),
                         self.arena
                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
@@ -446,12 +442,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
         let new_cond = self.manage_let_cond(lowered_cond);
         let then = self.lower_block_expr(body);
-        let expr_break = self.expr_break(span, ThinVec::new());
+        let expr_break = self.expr_break(span, AttrVec::new());
         let stmt_break = self.stmt_expr(span, expr_break);
         let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
-        let else_expr = self.arena.alloc(self.expr_block(else_blk, ThinVec::new()));
+        let else_expr = self.arena.alloc(self.expr_block(else_blk, AttrVec::new()));
         let if_kind = hir::ExprKind::If(new_cond, self.arena.alloc(then), Some(else_expr));
-        let if_expr = self.expr(span, if_kind, ThinVec::new());
+        let if_expr = self.expr(span, if_kind, AttrVec::new());
         let block = self.block_expr(self.arena.alloc(if_expr));
         let span = self.lower_span(span.with_hi(cond.span.hi()));
         let opt_label = self.lower_label(opt_label);
@@ -510,7 +506,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let constructor = self.arena.alloc(self.expr_lang_item_path(
             method_span,
             lang_item,
-            ThinVec::new(),
+            AttrVec::new(),
             None,
         ));
         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
@@ -562,7 +558,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     ) -> hir::ExprKind<'hir> {
         let output = match ret_ty {
             Some(ty) => hir::FnRetTy::Return(
-                self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock)),
+                self.lower_ty(&ty, &ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock)),
             ),
             None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
         };
@@ -587,7 +583,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let (pat, task_context_hid) = self.pat_ident_binding_mode(
             span,
             Ident::with_dummy_span(sym::_task_context),
-            hir::BindingAnnotation::Mutable,
+            hir::BindingAnnotation::MUT,
         );
         let param = hir::Param {
             hir_id: self.next_id(),
@@ -633,7 +629,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let gen_future = self.expr_lang_item_path(
             unstable_span,
             hir::LangItem::FromGenerator,
-            ThinVec::new(),
+            AttrVec::new(),
             None,
         );
 
@@ -661,17 +657,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
         match self.generator_kind {
             Some(hir::GeneratorKind::Async(_)) => {}
             Some(hir::GeneratorKind::Gen) | None => {
-                let mut err = struct_span_err!(
-                    self.tcx.sess,
+                self.tcx.sess.emit_err(AwaitOnlyInAsyncFnAndBlocks {
                     dot_await_span,
-                    E0728,
-                    "`await` is only allowed inside `async` functions and blocks"
-                );
-                err.span_label(dot_await_span, "only allowed inside `async` functions and blocks");
-                if let Some(item_sp) = self.current_item {
-                    err.span_label(item_sp, "this is not `async`");
-                }
-                err.emit();
+                    item_span: self.current_item,
+                });
             }
         }
         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
@@ -688,7 +677,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // this name to identify what is being awaited by a suspended async functions.
         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
         let (awaitee_pat, awaitee_pat_hid) =
-            self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::Mutable);
+            self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::MUT);
 
         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 
@@ -745,7 +734,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             let break_x = self.with_loop_scope(loop_node_id, move |this| {
                 let expr_break =
                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
-                this.arena.alloc(this.expr(gen_future_span, expr_break, ThinVec::new()))
+                this.arena.alloc(this.expr(gen_future_span, expr_break, AttrVec::new()))
             });
             self.arm(ready_pat, break_x)
         };
@@ -778,7 +767,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             let yield_expr = self.expr(
                 span,
                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
-                ThinVec::new(),
+                AttrVec::new(),
             );
             let yield_expr = self.arena.alloc(yield_expr);
 
@@ -866,7 +855,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
         // Lower outside new scope to preserve `is_in_loop_condition`.
-        let fn_decl = self.lower_fn_decl(decl, None, FnDeclKind::Closure, None);
+        let fn_decl = self.lower_fn_decl(decl, None, fn_decl_span, FnDeclKind::Closure, None);
 
         let c = self.arena.alloc(hir::Closure {
             binder: binder_clause,
@@ -891,13 +880,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         match generator_kind {
             Some(hir::GeneratorKind::Gen) => {
                 if decl.inputs.len() > 1 {
-                    struct_span_err!(
-                        self.tcx.sess,
-                        fn_decl_span,
-                        E0628,
-                        "too many parameters for a generator (expected 0 or 1 parameters)"
-                    )
-                    .emit();
+                    self.tcx.sess.emit_err(GeneratorTooManyParameters { fn_decl_span });
                 }
                 Some(movability)
             }
@@ -906,13 +889,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }
             None => {
                 if movability == Movability::Static {
-                    struct_span_err!(
-                        self.tcx.sess,
-                        fn_decl_span,
-                        E0697,
-                        "closures cannot be static"
-                    )
-                    .emit();
+                    self.tcx.sess.emit_err(ClosureCannotBeStatic { fn_decl_span });
                 }
                 None
             }
@@ -945,10 +922,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         fn_decl_span: Span,
     ) -> hir::ExprKind<'hir> {
         if let &ClosureBinder::For { span, .. } = binder {
-            self.tcx.sess.span_err(
-                span,
-                "`for<...>` binders on `async` closures are not currently supported",
-            );
+            self.tcx.sess.emit_err(NotSupportedForLifetimeBinderAsyncClosure { span });
         }
 
         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
@@ -959,17 +933,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let body = self.with_new_scopes(|this| {
             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
-                struct_span_err!(
-                    this.tcx.sess,
-                    fn_decl_span,
-                    E0708,
-                    "`async` non-`move` closures with parameters are not currently supported",
-                )
-                .help(
-                    "consider using `let` statements to manually capture \
-                    variables by reference before entering an `async move` closure",
-                )
-                .emit();
+                this.tcx.sess.emit_err(AsyncNonMoveClosureNotSupported { fn_decl_span });
             }
 
             // Transform `async |x: u8| -> X { ... }` into
@@ -985,17 +949,17 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     hir::AsyncGeneratorKind::Closure,
                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
                 );
-                this.expr(fn_decl_span, async_body, ThinVec::new())
+                this.expr(fn_decl_span, async_body, AttrVec::new())
             });
             body_id
         });
 
         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
-
         // We need to lower the declaration outside the new scope, because we
         // have to conserve the state of being inside a loop condition for the
         // closure argument types.
-        let fn_decl = self.lower_fn_decl(&outer_decl, None, FnDeclKind::Closure, None);
+        let fn_decl =
+            self.lower_fn_decl(&outer_decl, None, fn_decl_span, FnDeclKind::Closure, None);
 
         let c = self.arena.alloc(hir::Closure {
             binder: binder_clause,
@@ -1165,11 +1129,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         qself,
                         path,
                         ParamMode::Optional,
-                        ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     );
                     // Destructure like a tuple struct.
-                    let tuple_struct_pat =
-                        hir::PatKind::TupleStruct(qpath, pats, rest.map(|r| r.0));
+                    let tuple_struct_pat = hir::PatKind::TupleStruct(
+                        qpath,
+                        pats,
+                        hir::DotDotPos::new(rest.map(|r| r.0)),
+                    );
                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
                 }
             }
@@ -1181,7 +1148,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         qself,
                         path,
                         ParamMode::Optional,
-                        ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     );
                     // Destructure like a unit struct.
                     let unit_struct_pat = hir::PatKind::Path(qpath);
@@ -1205,24 +1172,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     &se.qself,
                     &se.path,
                     ParamMode::Optional,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                 );
                 let fields_omitted = match &se.rest {
                     StructRest::Base(e) => {
-                        self.tcx
-                            .sess
-                            .struct_span_err(
-                                e.span,
-                                "functional record updates are not allowed in destructuring \
-                                    assignments",
-                            )
-                            .span_suggestion(
-                                e.span,
-                                "consider removing the trailing pattern",
-                                "",
-                                rustc_errors::Applicability::MachineApplicable,
-                            )
-                            .emit();
+                        self.tcx.sess.emit_err(FunctionalRecordUpdateDestructuringAssignemnt {
+                            span: e.span,
+                        });
                         true
                     }
                     StructRest::Rest(_) => true,
@@ -1235,13 +1191,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
             ExprKind::Tup(elements) => {
                 let (pats, rest) =
                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
-                let tuple_pat = hir::PatKind::Tuple(pats, rest.map(|r| r.0));
+                let tuple_pat = hir::PatKind::Tuple(pats, hir::DotDotPos::new(rest.map(|r| r.0)));
                 return self.pat_without_dbm(lhs.span, tuple_pat);
             }
             ExprKind::Paren(e) => {
                 // We special-case `(..)` for consistency with patterns.
                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
-                    let tuple_pat = hir::PatKind::Tuple(&[], Some(0));
+                    let tuple_pat = hir::PatKind::Tuple(&[], hir::DotDotPos::new(Some(0)));
                     return self.pat_without_dbm(lhs.span, tuple_pat);
                 } else {
                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
@@ -1255,7 +1211,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let ident = self.expr_ident(lhs.span, ident, binding);
         let assign =
             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
-        let expr = self.expr(lhs.span, assign, ThinVec::new());
+        let expr = self.expr(lhs.span, assign, AttrVec::new());
         assignments.push(self.stmt_expr(lhs.span, expr));
         pat
     }
@@ -1297,7 +1253,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let fn_path =
             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
         let fn_expr =
-            self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
+            self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), AttrVec::new()));
         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
     }
 
@@ -1317,7 +1273,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
             (Some(..), Some(..), Closed) => unreachable!(),
-            (_, None, Closed) => self.diagnostic().span_fatal(span, "inclusive range with no end"),
+            (start, None, Closed) => {
+                self.tcx.sess.emit_err(InclusiveRangeWithNoEnd { span });
+                match start {
+                    Some(..) => hir::LangItem::RangeFrom,
+                    None => hir::LangItem::RangeFull,
+                }
+            }
         };
 
         let fields = self.arena.alloc_from_iter(
@@ -1404,8 +1366,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
+        let hir_id = self.lower_node_id(f.id);
+        self.lower_attrs(hir_id, &f.attrs);
         hir::ExprField {
-            hir_id: self.next_id(),
+            hir_id,
             ident: self.lower_ident(f.ident),
             expr: self.lower_expr(&f.expr),
             span: self.lower_span(f.span),
@@ -1417,13 +1381,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         match self.generator_kind {
             Some(hir::GeneratorKind::Gen) => {}
             Some(hir::GeneratorKind::Async(_)) => {
-                struct_span_err!(
-                    self.tcx.sess,
-                    span,
-                    E0727,
-                    "`async` generators are not yet supported"
-                )
-                .emit();
+                self.tcx.sess.emit_err(AsyncGeneratorsNotSupported { span });
             }
             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
         }
@@ -1468,7 +1426,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // `None => break`
         let none_arm = {
             let break_expr =
-                self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, ThinVec::new()));
+                self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, AttrVec::new()));
             let pat = self.pat_none(for_span);
             self.arm(pat, break_expr)
         };
@@ -1477,14 +1435,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let some_arm = {
             let some_pat = self.pat_some(pat_span, pat);
             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
-            let body_expr = self.arena.alloc(self.expr_block(body_block, ThinVec::new()));
+            let body_expr = self.arena.alloc(self.expr_block(body_block, AttrVec::new()));
             self.arm(some_pat, body_expr)
         };
 
         // `mut iter`
         let iter = Ident::with_dummy_span(sym::iter);
         let (iter_pat, iter_pat_nid) =
-            self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::Mutable);
+            self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::MUT);
 
         // `match Iterator::next(&mut iter) { ... }`
         let match_expr = {
@@ -1534,15 +1492,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
             hir::MatchSource::ForLoopDesugar,
         ));
 
-        let attrs: Vec<_> = e.attrs.iter().map(|a| self.lower_attr(a)).collect();
-
         // This is effectively `{ let _result = ...; _result }`.
         // The construct was introduced in #21984 and is necessary to make sure that
         // temporaries in the `head` expression are dropped and do not leak to the
         // surrounding scope of the `match` since the `match` is not a terminating scope.
         //
         // Also, add the attributes to the outer returned expr node.
-        self.expr_drop_temps_mut(for_span, match_expr, attrs.into())
+        self.expr_drop_temps_mut(for_span, match_expr, e.attrs.clone())
     }
 
     /// Desugar `ExprKind::Try` from: `<expr>?` into:
@@ -1592,9 +1548,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let uc_nested = attr::mk_nested_word_item(uc_ident);
                 attr::mk_list_item(allow_ident, vec![uc_nested])
             };
-            attr::mk_attr_outer(allow)
+            attr::mk_attr_outer(&self.tcx.sess.parse_sess.attr_id_generator, allow)
         };
-        let attrs = vec![attr];
+        let attrs: AttrVec = thin_vec![attr];
 
         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
         let continue_arm = {
@@ -1604,7 +1560,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 span,
                 val_ident,
                 val_pat_nid,
-                ThinVec::from(attrs.clone()),
+                attrs.clone(),
             ));
             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
             self.arm(continue_pat, val_expr)
@@ -1623,7 +1579,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 self.arena.alloc(residual_expr),
                 unstable_span,
             );
-            let thin_attrs = ThinVec::from(attrs);
             let ret_expr = if let Some(catch_node) = self.catch_scope {
                 let target_id = Ok(self.lower_node_id(catch_node));
                 self.arena.alloc(self.expr(
@@ -1632,13 +1587,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         hir::Destination { label: None, target_id },
                         Some(from_residual_expr),
                     ),
-                    thin_attrs,
+                    attrs,
                 ))
             } else {
                 self.arena.alloc(self.expr(
                     try_span,
                     hir::ExprKind::Ret(Some(from_residual_expr)),
-                    thin_attrs,
+                    attrs,
                 ))
             };
 
@@ -1726,7 +1681,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         arms: &'hir [hir::Arm<'hir>],
         source: hir::MatchSource,
     ) -> hir::Expr<'hir> {
-        self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
+        self.expr(span, hir::ExprKind::Match(arg, arms, source), AttrVec::new())
     }
 
     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
@@ -1743,12 +1698,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.expr(
             span,
             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
-            ThinVec::new(),
+            AttrVec::new(),
         )
     }
 
     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
-        self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
+        self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), AttrVec::new()))
     }
 
     fn expr_call_mut(
@@ -1757,7 +1712,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         e: &'hir hir::Expr<'hir>,
         args: &'hir [hir::Expr<'hir>],
     ) -> hir::Expr<'hir> {
-        self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new())
+        self.expr(span, hir::ExprKind::Call(e, args), AttrVec::new())
     }
 
     fn expr_call(
@@ -1777,7 +1732,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         hir_id: Option<hir::HirId>,
     ) -> hir::Expr<'hir> {
         let path =
-            self.arena.alloc(self.expr_lang_item_path(span, lang_item, ThinVec::new(), hir_id));
+            self.arena.alloc(self.expr_lang_item_path(span, lang_item, AttrVec::new(), hir_id));
         self.expr_call_mut(span, path, args)
     }
 
@@ -1820,7 +1775,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         ident: Ident,
         binding: hir::HirId,
     ) -> hir::Expr<'hir> {
-        self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
+        self.expr_ident_with_attrs(sp, ident, binding, AttrVec::new())
     }
 
     fn expr_ident_with_attrs(
@@ -1830,12 +1785,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
         binding: hir::HirId,
         attrs: AttrVec,
     ) -> hir::Expr<'hir> {
+        let hir_id = self.next_id();
+        let res = Res::Local(binding);
         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
             None,
             self.arena.alloc(hir::Path {
                 span: self.lower_span(span),
-                res: Res::Local(binding),
-                segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
+                res,
+                segments: arena_vec![self; hir::PathSegment::new(ident, hir_id, res)],
             }),
         ));
 
@@ -1858,13 +1815,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }),
                 None,
             ),
-            ThinVec::new(),
+            AttrVec::new(),
         )
     }
 
     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
         let blk = self.block_all(span, &[], None);
-        let expr = self.expr_block(blk, ThinVec::new());
+        let expr = self.expr_block(blk, AttrVec::new());
         self.arena.alloc(expr)
     }
 
diff --git a/compiler/rustc_ast_lowering/src/index.rs b/compiler/rustc_ast_lowering/src/index.rs
index d5af74d47..85846b567 100644
--- a/compiler/rustc_ast_lowering/src/index.rs
+++ b/compiler/rustc_ast_lowering/src/index.rs
@@ -11,8 +11,6 @@ use rustc_session::Session;
 use rustc_span::source_map::SourceMap;
 use rustc_span::{Span, DUMMY_SP};
 
-use tracing::debug;
-
 /// A visitor that walks over the HIR and collects `Node`s into a HIR map.
 pub(super) struct NodeCollector<'a, 'hir> {
     /// Source map
@@ -31,7 +29,7 @@ pub(super) struct NodeCollector<'a, 'hir> {
     definitions: &'a definitions::Definitions,
 }
 
-#[tracing::instrument(level = "debug", skip(sess, definitions, bodies))]
+#[instrument(level = "debug", skip(sess, definitions, bodies))]
 pub(super) fn index_hir<'hir>(
     sess: &Session,
     definitions: &definitions::Definitions,
@@ -67,10 +65,11 @@ pub(super) fn index_hir<'hir>(
 }
 
 impl<'a, 'hir> NodeCollector<'a, 'hir> {
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn insert(&mut self, span: Span, hir_id: HirId, node: Node<'hir>) {
         debug_assert_eq!(self.owner, hir_id.owner);
         debug_assert_ne!(hir_id.local_id.as_u32(), 0);
+        debug_assert_ne!(hir_id.local_id, self.parent_node);
 
         // Make sure that the DepNode of some node coincides with the HirId
         // owner of that node.
@@ -142,7 +141,7 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         });
     }
 
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn visit_item(&mut self, i: &'hir Item<'hir>) {
         debug_assert_eq!(i.def_id, self.owner);
         self.with_parent(i.hir_id(), |this| {
@@ -156,7 +155,7 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         });
     }
 
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn visit_foreign_item(&mut self, fi: &'hir ForeignItem<'hir>) {
         debug_assert_eq!(fi.def_id, self.owner);
         self.with_parent(fi.hir_id(), |this| {
@@ -175,7 +174,7 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         })
     }
 
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn visit_trait_item(&mut self, ti: &'hir TraitItem<'hir>) {
         debug_assert_eq!(ti.def_id, self.owner);
         self.with_parent(ti.hir_id(), |this| {
@@ -183,7 +182,7 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         });
     }
 
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn visit_impl_item(&mut self, ii: &'hir ImplItem<'hir>) {
         debug_assert_eq!(ii.def_id, self.owner);
         self.with_parent(ii.hir_id(), |this| {
@@ -199,6 +198,13 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         });
     }
 
+    fn visit_pat_field(&mut self, field: &'hir PatField<'hir>) {
+        self.insert(field.span, field.hir_id, Node::PatField(field));
+        self.with_parent(field.hir_id, |this| {
+            intravisit::walk_pat_field(this, field);
+        });
+    }
+
     fn visit_arm(&mut self, arm: &'hir Arm<'hir>) {
         let node = Node::Arm(arm);
 
@@ -225,6 +231,13 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         });
     }
 
+    fn visit_expr_field(&mut self, field: &'hir ExprField<'hir>) {
+        self.insert(field.span, field.hir_id, Node::ExprField(field));
+        self.with_parent(field.hir_id, |this| {
+            intravisit::walk_expr_field(this, field);
+        });
+    }
+
     fn visit_stmt(&mut self, stmt: &'hir Stmt<'hir>) {
         self.insert(stmt.span, stmt.hir_id, Node::Stmt(stmt));
 
@@ -233,11 +246,9 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         });
     }
 
-    fn visit_path_segment(&mut self, path_span: Span, path_segment: &'hir PathSegment<'hir>) {
-        if let Some(hir_id) = path_segment.hir_id {
-            self.insert(path_span, hir_id, Node::PathSegment(path_segment));
-        }
-        intravisit::walk_path_segment(self, path_span, path_segment);
+    fn visit_path_segment(&mut self, path_segment: &'hir PathSegment<'hir>) {
+        self.insert(path_segment.ident.span, path_segment.hir_id, Node::PathSegment(path_segment));
+        intravisit::walk_path_segment(self, path_segment);
     }
 
     fn visit_ty(&mut self, ty: &'hir Ty<'hir>) {
@@ -269,12 +280,12 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         fk: intravisit::FnKind<'hir>,
         fd: &'hir FnDecl<'hir>,
         b: BodyId,
-        s: Span,
+        _: Span,
         id: HirId,
     ) {
         assert_eq!(self.owner, id.owner);
         assert_eq!(self.parent_node, id.local_id);
-        intravisit::walk_fn(self, fk, fd, b, s, id);
+        intravisit::walk_fn(self, fk, fd, b, id);
     }
 
     fn visit_block(&mut self, block: &'hir Block<'hir>) {
@@ -295,14 +306,14 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         self.insert(lifetime.span, lifetime.hir_id, Node::Lifetime(lifetime));
     }
 
-    fn visit_variant(&mut self, v: &'hir Variant<'hir>, g: &'hir Generics<'hir>, item_id: HirId) {
+    fn visit_variant(&mut self, v: &'hir Variant<'hir>) {
         self.insert(v.span, v.id, Node::Variant(v));
         self.with_parent(v.id, |this| {
             // Register the constructor of this variant.
             if let Some(ctor_hir_id) = v.data.ctor_hir_id() {
                 this.insert(v.span, ctor_hir_id, Node::Ctor(&v.data));
             }
-            intravisit::walk_variant(this, v, g, item_id);
+            intravisit::walk_variant(this, v);
         });
     }
 
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index ee4c0036f..550833275 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -1,5 +1,6 @@
+use super::errors::{InvalidAbi, MisplacedRelaxTraitBound};
 use super::ResolverAstLoweringExt;
-use super::{AstOwner, ImplTraitContext, ImplTraitPosition};
+use super::{Arena, AstOwner, ImplTraitContext, ImplTraitPosition};
 use super::{FnDeclKind, LoweringContext, ParamMode};
 
 use rustc_ast::ptr::P;
@@ -7,7 +8,6 @@ use rustc_ast::visit::AssocCtxt;
 use rustc_ast::*;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sorted_map::SortedMap;
-use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
@@ -25,6 +25,7 @@ use std::iter;
 pub(super) struct ItemLowerer<'a, 'hir> {
     pub(super) tcx: TyCtxt<'hir>,
     pub(super) resolver: &'a mut ResolverAstLowering,
+    pub(super) ast_arena: &'a Arena<'static>,
     pub(super) ast_index: &'a IndexVec<LocalDefId, AstOwner<'a>>,
     pub(super) owners: &'a mut IndexVec<LocalDefId, hir::MaybeOwner<&'hir hir::OwnerInfo<'hir>>>,
 }
@@ -60,6 +61,7 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
             tcx: self.tcx,
             resolver: self.resolver,
             arena: self.tcx.hir_arena,
+            ast_arena: self.ast_arena,
 
             // HirId handling.
             bodies: Vec::new(),
@@ -85,6 +87,7 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
             allow_try_trait: Some([sym::try_trait_v2, sym::yeet_desugar_details][..].into()),
             allow_gen_future: Some([sym::gen_future][..].into()),
             allow_into_future: Some([sym::into_future][..].into()),
+            generics_def_id_map: Default::default(),
         };
         lctx.with_hir_id_owner(owner, |lctx| f(lctx));
 
@@ -120,7 +123,7 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
         self.with_lctx(CRATE_NODE_ID, |lctx| {
             let module = lctx.lower_mod(&c.items, &c.spans);
             lctx.lower_attrs(hir::CRATE_HIR_ID, &c.attrs);
-            hir::OwnerNode::Crate(lctx.arena.alloc(module))
+            hir::OwnerNode::Crate(module)
         })
     }
 
@@ -158,14 +161,18 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
 }
 
 impl<'hir> LoweringContext<'_, 'hir> {
-    pub(super) fn lower_mod(&mut self, items: &[P<Item>], spans: &ModSpans) -> hir::Mod<'hir> {
-        hir::Mod {
+    pub(super) fn lower_mod(
+        &mut self,
+        items: &[P<Item>],
+        spans: &ModSpans,
+    ) -> &'hir hir::Mod<'hir> {
+        self.arena.alloc(hir::Mod {
             spans: hir::ModSpans {
                 inner_span: self.lower_span(spans.inner_span),
                 inject_use_span: self.lower_span(spans.inject_use_span),
             },
             item_ids: self.arena.alloc_from_iter(items.iter().flat_map(|x| self.lower_item_ref(x))),
-        }
+        })
     }
 
     pub(super) fn lower_item_ref(&mut self, i: &Item) -> SmallVec<[hir::ItemId; 1]> {
@@ -259,10 +266,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     let body_id =
                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 
-                    let itctx = ImplTraitContext::Universal;
-                    let (generics, decl) = this.lower_generics(generics, id, itctx, |this| {
+                    let mut itctx = ImplTraitContext::Universal;
+                    let (generics, decl) = this.lower_generics(generics, id, &mut itctx, |this| {
                         let ret_id = asyncness.opt_return_id();
-                        this.lower_fn_decl(&decl, Some(id), FnDeclKind::Fn, ret_id)
+                        this.lower_fn_decl(&decl, Some(id), fn_sig_span, FnDeclKind::Fn, ret_id)
                     });
                     let sig = hir::FnSig {
                         decl,
@@ -306,8 +313,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, ty) = self.lower_generics(
                     &generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
-                    |this| this.lower_ty(ty, ImplTraitContext::TypeAliasesOpaqueTy),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    |this| this.lower_ty(ty, &ImplTraitContext::TypeAliasesOpaqueTy),
                 );
                 hir::ItemKind::TyAlias(ty, generics)
             }
@@ -319,7 +326,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, ty) = self.lower_generics(
                     &generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.arena.alloc(this.ty(span, hir::TyKind::Err)),
                 );
                 hir::ItemKind::TyAlias(ty, generics)
@@ -328,7 +335,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, variants) = self.lower_generics(
                     generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         this.arena.alloc_from_iter(
                             enum_definition.variants.iter().map(|x| this.lower_variant(x)),
@@ -341,7 +348,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, struct_def) = self.lower_generics(
                     generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.lower_variant_data(hir_id, struct_def),
                 );
                 hir::ItemKind::Struct(struct_def, generics)
@@ -350,7 +357,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, vdata) = self.lower_generics(
                     generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.lower_variant_data(hir_id, vdata),
                 );
                 hir::ItemKind::Union(vdata, generics)
@@ -378,18 +385,18 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 // method, it will not be considered an in-band
                 // lifetime to be added, but rather a reference to a
                 // parent lifetime.
-                let itctx = ImplTraitContext::Universal;
+                let mut itctx = ImplTraitContext::Universal;
                 let (generics, (trait_ref, lowered_ty)) =
-                    self.lower_generics(ast_generics, id, itctx, |this| {
+                    self.lower_generics(ast_generics, id, &mut itctx, |this| {
                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
                             this.lower_trait_ref(
                                 trait_ref,
-                                ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
+                                &ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
                             )
                         });
 
                         let lowered_ty = this
-                            .lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                            .lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 
                         (trait_ref, lowered_ty)
                     });
@@ -428,11 +435,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, (unsafety, items, bounds)) = self.lower_generics(
                     generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         let bounds = this.lower_param_bounds(
                             bounds,
-                            ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
+                            &ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                         );
                         let items = this.arena.alloc_from_iter(
                             items.iter().map(|item| this.lower_trait_item_ref(item)),
@@ -447,11 +454,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, bounds) = self.lower_generics(
                     generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         this.lower_param_bounds(
                             bounds,
-                            ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
+                            &ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                         )
                     },
                 );
@@ -474,7 +481,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         span: Span,
         body: Option<&Expr>,
     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
-        let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+        let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
         (ty, self.lower_const_body(span, body))
     }
 
@@ -647,12 +654,18 @@ impl<'hir> LoweringContext<'_, 'hir> {
             kind: match i.kind {
                 ForeignItemKind::Fn(box Fn { ref sig, ref generics, .. }) => {
                     let fdec = &sig.decl;
-                    let itctx = ImplTraitContext::Universal;
+                    let mut itctx = ImplTraitContext::Universal;
                     let (generics, (fn_dec, fn_args)) =
-                        self.lower_generics(generics, i.id, itctx, |this| {
+                        self.lower_generics(generics, i.id, &mut itctx, |this| {
                             (
                                 // Disallow `impl Trait` in foreign items.
-                                this.lower_fn_decl(fdec, None, FnDeclKind::ExternFn, None),
+                                this.lower_fn_decl(
+                                    fdec,
+                                    None,
+                                    sig.span,
+                                    FnDeclKind::ExternFn,
+                                    None,
+                                ),
                                 this.lower_fn_params_to_names(fdec),
                             )
                         });
@@ -661,7 +674,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }
                 ForeignItemKind::Static(ref t, m, _) => {
                     let ty =
-                        self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                        self.lower_ty(t, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                     hir::ForeignItemKind::Static(ty, m)
                 }
                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
@@ -729,11 +742,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 qself,
                 path,
                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
-                ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
             );
             self.arena.alloc(t)
         } else {
-            self.lower_ty(&f.ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type))
+            self.lower_ty(&f.ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type))
         };
         let hir_id = self.lower_node_id(f.id);
         self.lower_attrs(hir_id, &f.attrs);
@@ -756,14 +769,20 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let (generics, kind, has_default) = match i.kind {
             AssocItemKind::Const(_, ref ty, ref default) => {
-                let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body), body.is_some())
             }
             AssocItemKind::Fn(box Fn { ref sig, ref generics, body: None, .. }) => {
+                let asyncness = sig.header.asyncness;
                 let names = self.lower_fn_params_to_names(&sig.decl);
-                let (generics, sig) =
-                    self.lower_method_sig(generics, sig, i.id, FnDeclKind::Trait, None);
+                let (generics, sig) = self.lower_method_sig(
+                    generics,
+                    sig,
+                    i.id,
+                    FnDeclKind::Trait,
+                    asyncness.opt_return_id(),
+                );
                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)), false)
             }
             AssocItemKind::Fn(box Fn { ref sig, ref generics, body: Some(ref body), .. }) => {
@@ -791,15 +810,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, kind) = self.lower_generics(
                     &generics,
                     i.id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         let ty = ty.as_ref().map(|x| {
-                            this.lower_ty(x, ImplTraitContext::Disallowed(ImplTraitPosition::Type))
+                            this.lower_ty(x, &ImplTraitContext::Disallowed(ImplTraitPosition::Type))
                         });
                         hir::TraitItemKind::Type(
                             this.lower_param_bounds(
                                 bounds,
-                                ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                                &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                             ),
                             ty,
                         )
@@ -852,7 +871,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let (generics, kind) = match &i.kind {
             AssocItemKind::Const(_, ty, expr) => {
-                let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                 (
                     hir::Generics::empty(),
                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
@@ -879,14 +898,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 self.lower_generics(
                     &generics,
                     i.id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| match ty {
                         None => {
                             let ty = this.arena.alloc(this.ty(i.span, hir::TyKind::Err));
                             hir::ImplItemKind::TyAlias(ty)
                         }
                         Some(ty) => {
-                            let ty = this.lower_ty(ty, ImplTraitContext::TypeAliasesOpaqueTy);
+                            let ty = this.lower_ty(ty, &ImplTraitContext::TypeAliasesOpaqueTy);
                             hir::ImplItemKind::TyAlias(ty)
                         }
                     },
@@ -947,7 +966,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
         params: &'hir [hir::Param<'hir>],
         value: hir::Expr<'hir>,
     ) -> hir::BodyId {
-        let body = hir::Body { generator_kind: self.generator_kind, params, value };
+        let body = hir::Body {
+            generator_kind: self.generator_kind,
+            params,
+            value: self.arena.alloc(value),
+        };
         let id = body.id();
         debug_assert_eq!(id.hir_id.owner, self.current_hir_id_owner);
         self.bodies.push((id.hir_id.local_id, self.arena.alloc(body)));
@@ -1074,12 +1097,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
                 let (ident, is_simple_parameter) = match parameter.pat.kind {
-                    hir::PatKind::Binding(
-                        hir::BindingAnnotation::Unannotated | hir::BindingAnnotation::Mutable,
-                        _,
-                        ident,
-                        _,
-                    ) => (ident, true),
+                    hir::PatKind::Binding(hir::BindingAnnotation(ByRef::No, _), _, ident, _) => {
+                        (ident, true)
+                    }
                     // For `ref mut` or wildcard arguments, we can't reuse the binding, but
                     // we can keep the same name for the parameter.
                     // This lets rustdoc render it correctly in documentation.
@@ -1144,7 +1164,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     let (move_pat, move_id) = this.pat_ident_binding_mode(
                         desugared_span,
                         ident,
-                        hir::BindingAnnotation::Mutable,
+                        hir::BindingAnnotation::MUT,
                     );
                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
                     let move_stmt = this.stmt_let_pat(
@@ -1225,12 +1245,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
         sig: &FnSig,
         id: NodeId,
         kind: FnDeclKind,
-        is_async: Option<NodeId>,
+        is_async: Option<(NodeId, Span)>,
     ) -> (&'hir hir::Generics<'hir>, hir::FnSig<'hir>) {
         let header = self.lower_fn_header(sig.header);
-        let itctx = ImplTraitContext::Universal;
-        let (generics, decl) = self.lower_generics(generics, id, itctx, |this| {
-            this.lower_fn_decl(&sig.decl, Some(id), kind, is_async)
+        let mut itctx = ImplTraitContext::Universal;
+        let (generics, decl) = self.lower_generics(generics, id, &mut itctx, |this| {
+            this.lower_fn_decl(&sig.decl, Some(id), sig.span, kind, is_async)
         });
         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
     }
@@ -1260,10 +1280,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     fn error_on_invalid_abi(&self, abi: StrLit) {
-        struct_span_err!(self.tcx.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
-            .span_label(abi.span, "invalid ABI")
-            .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
-            .emit();
+        self.tcx.sess.emit_err(InvalidAbi {
+            span: abi.span,
+            abi: abi.symbol,
+            valid_abis: abi::all_names().join(", "),
+        });
     }
 
     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
@@ -1294,7 +1315,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         &mut self,
         generics: &Generics,
         parent_node_id: NodeId,
-        itctx: ImplTraitContext,
+        itctx: &ImplTraitContext,
         f: impl FnOnce(&mut Self) -> T,
     ) -> (&'hir hir::Generics<'hir>, T) {
         debug_assert!(self.impl_trait_defs.is_empty());
@@ -1338,11 +1359,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }
                 let is_param = *is_param.get_or_insert_with(compute_is_param);
                 if !is_param {
-                    self.diagnostic().span_err(
-                        bound.span(),
-                        "`?Trait` bounds are only permitted at the \
-                        point where a type parameter is declared",
-                    );
+                    self.tcx.sess.emit_err(MisplacedRelaxTraitBound { span: bound.span() });
                 }
             }
         }
@@ -1403,7 +1420,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         id: NodeId,
         kind: &GenericParamKind,
         bounds: &[GenericBound],
-        itctx: ImplTraitContext,
+        itctx: &ImplTraitContext,
         origin: PredicateOrigin,
     ) -> Option<hir::WherePredicate<'hir>> {
         // Do not create a clause if we do not have anything inside it.
@@ -1434,10 +1451,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
             GenericParamKind::Const { .. } => None,
             GenericParamKind::Type { .. } => {
                 let def_id = self.local_def_id(id).to_def_id();
+                let hir_id = self.next_id();
+                let res = Res::Def(DefKind::TyParam, def_id);
                 let ty_path = self.arena.alloc(hir::Path {
                     span: param_span,
-                    res: Res::Def(DefKind::TyParam, def_id),
-                    segments: self.arena.alloc_from_iter([hir::PathSegment::from_ident(ident)]),
+                    res,
+                    segments: self
+                        .arena
+                        .alloc_from_iter([hir::PathSegment::new(ident, hir_id, res)]),
                 });
                 let ty_id = self.next_id();
                 let bounded_ty =
@@ -1475,11 +1496,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }) => hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
                 bound_generic_params: self.lower_generic_params(bound_generic_params),
                 bounded_ty: self
-                    .lower_ty(bounded_ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
+                    .lower_ty(bounded_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
                 bounds: self.arena.alloc_from_iter(bounds.iter().map(|bound| {
                     self.lower_param_bound(
                         bound,
-                        ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
+                        &ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                     )
                 })),
                 span: self.lower_span(span),
@@ -1494,17 +1515,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 lifetime: self.lower_lifetime(lifetime),
                 bounds: self.lower_param_bounds(
                     bounds,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                 ),
                 in_where_clause: true,
             }),
-            WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
+            WherePredicate::EqPredicate(WhereEqPredicate { ref lhs_ty, ref rhs_ty, span }) => {
                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
-                    hir_id: self.lower_node_id(id),
                     lhs_ty: self
-                        .lower_ty(lhs_ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
+                        .lower_ty(lhs_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
                     rhs_ty: self
-                        .lower_ty(rhs_ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
+                        .lower_ty(rhs_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
                     span: self.lower_span(span),
                 })
             }
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 224dc3c23..9012aa704 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -32,14 +32,20 @@
 
 #![feature(box_patterns)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(never_type)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]
+#![deny(rustc::untranslatable_diagnostic)]
+#![deny(rustc::diagnostic_outside_of_impl)]
 
 #[macro_use]
 extern crate tracing;
 
+use crate::errors::{AssocTyParentheses, AssocTyParenthesesSub, MisplacedImplTrait, TraitFnAsync};
+
+use rustc_arena::declare_arena;
+use rustc_ast::ptr::P;
 use rustc_ast::visit;
 use rustc_ast::{self as ast, *};
 use rustc_ast_pretty::pprust;
@@ -49,7 +55,7 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{struct_span_err, Applicability, Handler};
+use rustc_errors::{DiagnosticArgFromDisplay, Handler, StashKey};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, LifetimeRes, Namespace, PartialRes, PerNS, Res};
 use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
@@ -75,6 +81,7 @@ macro_rules! arena_vec {
 
 mod asm;
 mod block;
+mod errors;
 mod expr;
 mod index;
 mod item;
@@ -89,6 +96,13 @@ struct LoweringContext<'a, 'hir> {
     /// Used to allocate HIR nodes.
     arena: &'hir hir::Arena<'hir>,
 
+    /// Used to allocate temporary AST nodes for use during lowering.
+    /// This allows us to create "fake" AST -- these nodes can sometimes
+    /// be allocated on the stack, but other times we need them to live longer
+    /// than the current stack frame, so they can be collected into vectors
+    /// and things like that.
+    ast_arena: &'a Arena<'static>,
+
     /// Bodies inside the owner being lowered.
     bodies: Vec<(hir::ItemLocalId, &'hir hir::Body<'hir>)>,
     /// Attributes inside the owner being lowered.
@@ -126,8 +140,23 @@ struct LoweringContext<'a, 'hir> {
     allow_try_trait: Option<Lrc<[Symbol]>>,
     allow_gen_future: Option<Lrc<[Symbol]>>,
     allow_into_future: Option<Lrc<[Symbol]>>,
+
+    /// Mapping from generics `def_id`s to TAIT generics `def_id`s.
+    /// For each captured lifetime (e.g., 'a), we create a new lifetime parameter that is a generic
+    /// defined on the TAIT, so we have type Foo<'a1> = ... and we establish a mapping in this
+    /// field from the original parameter 'a to the new parameter 'a1.
+    generics_def_id_map: Vec<FxHashMap<LocalDefId, LocalDefId>>,
 }
 
+declare_arena!([
+    [] tys: rustc_ast::Ty,
+    [] aba: rustc_ast::AngleBracketedArgs,
+    [] ptr: rustc_ast::PolyTraitRef,
+    // This _marker field is needed because `declare_arena` creates `Arena<'tcx>` and we need to
+    // use `'tcx`. If we don't have this we get a compile error.
+    [] _marker: std::marker::PhantomData<&'tcx ()>,
+]);
+
 trait ResolverAstLoweringExt {
     fn legacy_const_generic_args(&self, expr: &Expr) -> Option<Vec<usize>>;
     fn get_partial_res(&self, id: NodeId) -> Option<PartialRes>;
@@ -136,12 +165,6 @@ trait ResolverAstLoweringExt {
     fn get_lifetime_res(&self, id: NodeId) -> Option<LifetimeRes>;
     fn take_extra_lifetime_params(&mut self, id: NodeId) -> Vec<(Ident, NodeId, LifetimeRes)>;
     fn decl_macro_kind(&self, def_id: LocalDefId) -> MacroKind;
-    /// Record the map from `from` local def id to `to` local def id, on `generics_def_id_map`
-    /// field.
-    fn record_def_id_remap(&mut self, from: LocalDefId, to: LocalDefId);
-    /// Get the previously recorded `to` local def id given the `from` local def id, obtained using
-    /// `generics_def_id_map` field.
-    fn get_remapped_def_id(&self, local_def_id: LocalDefId) -> LocalDefId;
 }
 
 impl ResolverAstLoweringExt for ResolverAstLowering {
@@ -209,41 +232,6 @@ impl ResolverAstLoweringExt for ResolverAstLowering {
     fn decl_macro_kind(&self, def_id: LocalDefId) -> MacroKind {
         self.builtin_macro_kinds.get(&def_id).copied().unwrap_or(MacroKind::Bang)
     }
-
-    /// Push a remapping into the top-most map.
-    /// Panics if no map has been pushed.
-    /// Remapping is used when creating lowering `-> impl Trait` return
-    /// types to create the resulting opaque type.
-    #[tracing::instrument(level = "debug", skip(self))]
-    fn record_def_id_remap(&mut self, from: LocalDefId, to: LocalDefId) {
-        self.generics_def_id_map.last_mut().expect("no map pushed").insert(from, to);
-    }
-
-    fn get_remapped_def_id(&self, mut local_def_id: LocalDefId) -> LocalDefId {
-        // `generics_def_id_map` is a stack of mappings. As we go deeper in impl traits nesting we
-        // push new mappings so we need to try first the latest mappings, hence `iter().rev()`.
-        //
-        // Consider:
-        //
-        // `fn test<'a, 'b>() -> impl Trait<&'a u8, Ty = impl Sized + 'b> {}`
-        //
-        // We would end with a generics_def_id_map like:
-        //
-        // `[[fn#'b -> impl_trait#'b], [fn#'b -> impl_sized#'b]]`
-        //
-        // for the opaque type generated on `impl Sized + 'b`, We want the result to be:
-        // impl_sized#'b, so iterating forward is the wrong thing to do.
-        for map in self.generics_def_id_map.iter().rev() {
-            if let Some(r) = map.get(&local_def_id) {
-                debug!("def_id_remapper: remapping from `{local_def_id:?}` to `{r:?}`");
-                local_def_id = *r;
-            } else {
-                debug!("def_id_remapper: no remapping for `{local_def_id:?}` found in map");
-            }
-        }
-
-        local_def_id
-    }
 }
 
 /// Context of `impl Trait` in code, which determines whether it is allowed in an HIR subtree,
@@ -264,6 +252,7 @@ enum ImplTraitContext {
     ReturnPositionOpaqueTy {
         /// Origin: Either OpaqueTyOrigin::FnReturn or OpaqueTyOrigin::AsyncFn,
         origin: hir::OpaqueTyOrigin,
+        in_trait: bool,
     },
     /// Impl trait in type aliases.
     TypeAliasesOpaqueTy,
@@ -323,7 +312,7 @@ impl std::fmt::Display for ImplTraitPosition {
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, PartialEq, Eq)]
 enum FnDeclKind {
     Fn,
     Inherent,
@@ -335,9 +324,17 @@ enum FnDeclKind {
 }
 
 impl FnDeclKind {
-    fn impl_trait_return_allowed(&self) -> bool {
+    fn impl_trait_return_allowed(&self, tcx: TyCtxt<'_>) -> bool {
         match self {
             FnDeclKind::Fn | FnDeclKind::Inherent => true,
+            FnDeclKind::Impl if tcx.features().return_position_impl_trait_in_trait => true,
+            _ => false,
+        }
+    }
+
+    fn impl_trait_in_trait_allowed(&self, tcx: TyCtxt<'_>) -> bool {
+        match self {
+            FnDeclKind::Trait if tcx.features().return_position_impl_trait_in_trait => true,
             _ => false,
         }
     }
@@ -430,10 +427,13 @@ pub fn lower_to_hir<'hir>(tcx: TyCtxt<'hir>, (): ()) -> hir::Crate<'hir> {
         tcx.definitions_untracked().def_index_count(),
     );
 
+    let ast_arena = Arena::default();
+
     for def_id in ast_index.indices() {
         item::ItemLowerer {
             tcx,
             resolver: &mut resolver,
+            ast_arena: &ast_arena,
             ast_index: &ast_index,
             owners: &mut owners,
         }
@@ -500,6 +500,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     /// Given the id of some node in the AST, finds the `LocalDefId` associated with it by the name
+    /// resolver (if any).
+    fn orig_opt_local_def_id(&self, node: NodeId) -> Option<LocalDefId> {
+        self.resolver.node_id_to_def_id.get(&node).map(|local_def_id| *local_def_id)
+    }
+
+    fn orig_local_def_id(&self, node: NodeId) -> LocalDefId {
+        self.orig_opt_local_def_id(node)
+            .unwrap_or_else(|| panic!("no entry for node id: `{:?}`", node))
+    }
+
+    /// Given the id of some node in the AST, finds the `LocalDefId` associated with it by the name
     /// resolver (if any), after applying any remapping from `get_remapped_def_id`.
     ///
     /// For example, in a function like `fn foo<'a>(x: &'a u32)`,
@@ -513,16 +524,36 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     /// we would create an opaque type `type FooReturn<'a1> = impl Debug + 'a1`.
     /// When lowering the `Debug + 'a` bounds, we add a remapping to map `'a` to `'a1`.
     fn opt_local_def_id(&self, node: NodeId) -> Option<LocalDefId> {
-        self.resolver
-            .node_id_to_def_id
-            .get(&node)
-            .map(|local_def_id| self.resolver.get_remapped_def_id(*local_def_id))
+        self.orig_opt_local_def_id(node).map(|local_def_id| self.get_remapped_def_id(local_def_id))
     }
 
     fn local_def_id(&self, node: NodeId) -> LocalDefId {
         self.opt_local_def_id(node).unwrap_or_else(|| panic!("no entry for node id: `{:?}`", node))
     }
 
+    /// Get the previously recorded `to` local def id given the `from` local def id, obtained using
+    /// `generics_def_id_map` field.
+    fn get_remapped_def_id(&self, local_def_id: LocalDefId) -> LocalDefId {
+        // `generics_def_id_map` is a stack of mappings. As we go deeper in impl traits nesting we
+        // push new mappings, so we first need to get the latest (innermost) mappings, hence `iter().rev()`.
+        //
+        // Consider:
+        //
+        // `fn test<'a, 'b>() -> impl Trait<&'a u8, Ty = impl Sized + 'b> {}`
+        //
+        // We would end with a generics_def_id_map like:
+        //
+        // `[[fn#'b -> impl_trait#'b], [fn#'b -> impl_sized#'b]]`
+        //
+        // for the opaque type generated on `impl Sized + 'b`, we want the result to be: impl_sized#'b.
+        // So, if we were trying to find first from the start (outermost) would give the wrong result, impl_trait#'b.
+        self.generics_def_id_map
+            .iter()
+            .rev()
+            .find_map(|map| map.get(&local_def_id).map(|local_def_id| *local_def_id))
+            .unwrap_or(local_def_id)
+    }
+
     /// Freshen the `LoweringContext` and ready it to lower a nested item.
     /// The lowered item is registered into `self.children`.
     ///
@@ -591,9 +622,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         remap: FxHashMap<LocalDefId, LocalDefId>,
         f: impl FnOnce(&mut Self) -> R,
     ) -> R {
-        self.resolver.generics_def_id_map.push(remap);
+        self.generics_def_id_map.push(remap);
         let res = f(self);
-        self.resolver.generics_def_id_map.pop();
+        self.generics_def_id_map.pop();
         res
     }
 
@@ -644,14 +675,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     ) -> (Fingerprint, Fingerprint) {
         self.tcx.with_stable_hashing_context(|mut hcx| {
             let mut stable_hasher = StableHasher::new();
-            hcx.with_hir_bodies(true, node.def_id(), bodies, |hcx| {
+            hcx.with_hir_bodies(node.def_id(), bodies, |hcx| {
                 node.hash_stable(hcx, &mut stable_hasher)
             });
             let hash_including_bodies = stable_hasher.finish();
             let mut stable_hasher = StableHasher::new();
-            hcx.with_hir_bodies(false, node.def_id(), bodies, |hcx| {
-                node.hash_stable(hcx, &mut stable_hasher)
-            });
+            hcx.without_hir_bodies(|hcx| node.hash_stable(hcx, &mut stable_hasher));
             let hash_without_bodies = stable_hasher.finish();
             (hash_including_bodies, hash_without_bodies)
         })
@@ -663,6 +692,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     /// actually used in the HIR, as that would trigger an assertion in the
     /// `HirIdValidator` later on, which makes sure that all `NodeId`s got mapped
     /// properly. Calling the method twice with the same `NodeId` is fine though.
+    #[instrument(level = "debug", skip(self), ret)]
     fn lower_node_id(&mut self, ast_node_id: NodeId) -> hir::HirId {
         assert_ne!(ast_node_id, DUMMY_NODE_ID);
 
@@ -696,6 +726,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     /// Generate a new `HirId` without a backing `NodeId`.
+    #[instrument(level = "debug", skip(self), ret)]
     fn next_id(&mut self) -> hir::HirId {
         let owner = self.current_hir_id_owner;
         let local_id = self.item_local_id_counter;
@@ -767,7 +798,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     /// Converts a lifetime into a new generic parameter.
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn lifetime_res_to_generic_param(
         &mut self,
         ident: Ident,
@@ -811,7 +842,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     /// name resolver owing to lifetime elision; this also populates the resolver's node-id->def-id
     /// map, so that later calls to `opt_node_id_to_def_id` that refer to these extra lifetime
     /// parameters will be successful.
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     #[inline]
     fn lower_lifetime_binder(
         &mut self,
@@ -874,14 +905,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         // the `HirId`s. We don't actually need HIR version of attributes anyway.
         // Tokens are also not needed after macro expansion and parsing.
         let kind = match attr.kind {
-            AttrKind::Normal(ref item, _) => AttrKind::Normal(
-                AttrItem {
-                    path: item.path.clone(),
-                    args: self.lower_mac_args(&item.args),
+            AttrKind::Normal(ref normal) => AttrKind::Normal(P(NormalAttr {
+                item: AttrItem {
+                    path: normal.item.path.clone(),
+                    args: self.lower_mac_args(&normal.item.args),
                     tokens: None,
                 },
-                None,
-            ),
+                tokens: None,
+            })),
             AttrKind::DocComment(comment_kind, data) => AttrKind::DocComment(comment_kind, data),
         };
 
@@ -929,8 +960,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     lit.clone()
                 } else {
                     Lit {
-                        token: token::Lit::new(token::LitKind::Err, kw::Empty, None),
-                        kind: LitKind::Err(kw::Empty),
+                        token_lit: token::Lit::new(token::LitKind::Err, kw::Empty, None),
+                        kind: LitKind::Err,
                         span: DUMMY_SP,
                     }
                 };
@@ -956,7 +987,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_assoc_ty_constraint(
         &mut self,
         constraint: &AssocConstraint,
-        itctx: ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::TypeBinding<'hir> {
         debug!("lower_assoc_ty_constraint(constraint={:?}, itctx={:?})", constraint, itctx);
         // lower generic arguments of identifier in constraint
@@ -967,18 +998,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 }
                 GenericArgs::Parenthesized(ref data) => {
                     self.emit_bad_parenthesized_trait_in_assoc_ty(data);
-                    self.lower_angle_bracketed_parameter_data(
-                        &data.as_angle_bracketed_args(),
-                        ParamMode::Explicit,
-                        itctx,
-                    )
-                    .0
+                    let aba = self.ast_arena.aba.alloc(data.as_angle_bracketed_args());
+                    self.lower_angle_bracketed_parameter_data(aba, ParamMode::Explicit, itctx).0
                 }
             };
             gen_args_ctor.into_generic_args(self)
         } else {
             self.arena.alloc(hir::GenericArgs::none())
         };
+        let itctx_tait = &ImplTraitContext::TypeAliasesOpaqueTy;
 
         let kind = match constraint.kind {
             AssocConstraintKind::Equality { ref term } => {
@@ -1016,9 +1044,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     // then to an opaque type).
                     //
                     // FIXME: this is only needed until `impl Trait` is allowed in type aliases.
-                    ImplTraitContext::Disallowed(_) if self.is_in_dyn_type => {
-                        (true, ImplTraitContext::TypeAliasesOpaqueTy)
-                    }
+                    ImplTraitContext::Disallowed(_) if self.is_in_dyn_type => (true, itctx_tait),
 
                     // We are in the parameter position, but not within a dyn type:
                     //
@@ -1040,15 +1066,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
                     self.with_dyn_type_scope(false, |this| {
                         let node_id = this.next_node_id();
-                        let ty = this.lower_ty(
-                            &Ty {
-                                id: node_id,
-                                kind: TyKind::ImplTrait(impl_trait_node_id, bounds.clone()),
-                                span: this.lower_span(constraint.span),
-                                tokens: None,
-                            },
-                            itctx,
-                        );
+                        let ty = this.ast_arena.tys.alloc(Ty {
+                            id: node_id,
+                            kind: TyKind::ImplTrait(impl_trait_node_id, bounds.clone()),
+                            span: this.lower_span(constraint.span),
+                            tokens: None,
+                        });
+                        let ty = this.lower_ty(ty, itctx);
 
                         hir::TypeBindingKind::Equality { term: ty.into() }
                     })
@@ -1072,19 +1096,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     fn emit_bad_parenthesized_trait_in_assoc_ty(&self, data: &ParenthesizedArgs) {
-        let mut err = self.tcx.sess.struct_span_err(
-            data.span,
-            "parenthesized generic arguments cannot be used in associated type constraints",
-        );
         // Suggest removing empty parentheses: "Trait()" -> "Trait"
-        if data.inputs.is_empty() {
+        let sub = if data.inputs.is_empty() {
             let parentheses_span =
                 data.inputs_span.shrink_to_lo().to(data.inputs_span.shrink_to_hi());
-            err.multipart_suggestion(
-                "remove these parentheses",
-                vec![(parentheses_span, String::new())],
-                Applicability::MaybeIncorrect,
-            );
+            AssocTyParenthesesSub::Empty { parentheses_span }
         }
         // Suggest replacing parentheses with angle brackets `Trait(params...)` to `Trait<params...>`
         else {
@@ -1098,20 +1114,16 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             // End of last argument to end of parameters
             let close_param =
                 data.inputs.last().unwrap().span.shrink_to_hi().to(data.inputs_span.shrink_to_hi());
-            err.multipart_suggestion(
-                &format!("use angle brackets instead",),
-                vec![(open_param, String::from("<")), (close_param, String::from(">"))],
-                Applicability::MaybeIncorrect,
-            );
-        }
-        err.emit();
+            AssocTyParenthesesSub::NotEmpty { open_param, close_param }
+        };
+        self.tcx.sess.emit_err(AssocTyParentheses { span: data.span, sub });
     }
 
     #[instrument(level = "debug", skip(self))]
     fn lower_generic_arg(
         &mut self,
         arg: &ast::GenericArg,
-        itctx: ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::GenericArg<'hir> {
         match arg {
             ast::GenericArg::Lifetime(lt) => GenericArg::Lifetime(self.lower_lifetime(&lt)),
@@ -1163,7 +1175,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     }
                     _ => {}
                 }
-                GenericArg::Type(self.lower_ty_direct(&ty, itctx))
+                GenericArg::Type(self.lower_ty(&ty, itctx))
             }
             ast::GenericArg::Const(ct) => GenericArg::Const(ConstArg {
                 value: self.lower_anon_const(&ct),
@@ -1173,7 +1185,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     #[instrument(level = "debug", skip(self))]
-    fn lower_ty(&mut self, t: &Ty, itctx: ImplTraitContext) -> &'hir hir::Ty<'hir> {
+    fn lower_ty(&mut self, t: &Ty, itctx: &ImplTraitContext) -> &'hir hir::Ty<'hir> {
         self.arena.alloc(self.lower_ty_direct(t, itctx))
     }
 
@@ -1183,11 +1195,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         qself: &Option<QSelf>,
         path: &Path,
         param_mode: ParamMode,
-        itctx: ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::Ty<'hir> {
         // Check whether we should interpret this as a bare trait object.
         // This check mirrors the one in late resolution.  We only introduce this special case in
-        // the rare occurence we need to lower `Fresh` anonymous lifetimes.
+        // the rare occurrence we need to lower `Fresh` anonymous lifetimes.
         // The other cases when a qpath should be opportunistically made a trait object are handled
         // by `ty_path`.
         if qself.is_none()
@@ -1196,19 +1208,20 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             && let Res::Def(DefKind::Trait | DefKind::TraitAlias, _) = partial_res.base_res()
         {
             let (bounds, lifetime_bound) = self.with_dyn_type_scope(true, |this| {
+                let poly_trait_ref = this.ast_arena.ptr.alloc(PolyTraitRef {
+                    bound_generic_params: vec![],
+                    trait_ref: TraitRef { path: path.clone(), ref_id: t.id },
+                    span: t.span
+                });
                 let bound = this.lower_poly_trait_ref(
-                    &PolyTraitRef {
-                        bound_generic_params: vec![],
-                        trait_ref: TraitRef { path: path.clone(), ref_id: t.id },
-                        span: t.span
-                    },
+                    poly_trait_ref,
                     itctx,
                 );
                 let bounds = this.arena.alloc_from_iter([bound]);
                 let lifetime_bound = this.elided_dyn_bound(t.span);
                 (bounds, lifetime_bound)
             });
-            let kind = hir::TyKind::TraitObject(bounds, lifetime_bound, TraitObjectSyntax::None);
+            let kind = hir::TyKind::TraitObject(bounds, &lifetime_bound, TraitObjectSyntax::None);
             return hir::Ty { kind, span: self.lower_span(t.span), hir_id: self.next_id() };
         }
 
@@ -1225,7 +1238,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.ty(span, hir::TyKind::Tup(tys))
     }
 
-    fn lower_ty_direct(&mut self, t: &Ty, itctx: ImplTraitContext) -> hir::Ty<'hir> {
+    fn lower_ty_direct(&mut self, t: &Ty, itctx: &ImplTraitContext) -> hir::Ty<'hir> {
         let kind = match t.kind {
             TyKind::Infer => hir::TyKind::Infer,
             TyKind::Err => hir::TyKind::Err,
@@ -1241,7 +1254,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     } else {
                         self.next_node_id()
                     };
-                    let span = self.tcx.sess.source_map().next_point(t.span.shrink_to_lo());
+                    let span = self.tcx.sess.source_map().start_point(t.span);
                     Lifetime { ident: Ident::new(kw::UnderscoreLifetime, span), id }
                 });
                 let lifetime = self.lower_lifetime(&region);
@@ -1253,7 +1266,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     generic_params,
                     unsafety: self.lower_unsafety(f.unsafety),
                     abi: self.lower_extern(f.ext),
-                    decl: self.lower_fn_decl(&f.decl, None, FnDeclKind::Pointer, None),
+                    decl: self.lower_fn_decl(&f.decl, None, t.span, FnDeclKind::Pointer, None),
                     param_names: self.lower_fn_params_to_names(&f.decl),
                 }))
             }
@@ -1268,14 +1281,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 return self.lower_path_ty(t, qself, path, ParamMode::Explicit, itctx);
             }
             TyKind::ImplicitSelf => {
+                let hir_id = self.next_id();
                 let res = self.expect_full_res(t.id);
                 let res = self.lower_res(res);
                 hir::TyKind::Path(hir::QPath::Resolved(
                     None,
                     self.arena.alloc(hir::Path {
                         res,
-                        segments: arena_vec![self; hir::PathSegment::from_ident(
-                            Ident::with_dummy_span(kw::SelfUpper)
+                        segments: arena_vec![self; hir::PathSegment::new(
+                            Ident::with_dummy_span(kw::SelfUpper),
+                            hir_id,
+                            res
                         )],
                         span: self.lower_span(t.span),
                     }),
@@ -1318,19 +1334,23 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             TyKind::ImplTrait(def_node_id, ref bounds) => {
                 let span = t.span;
                 match itctx {
-                    ImplTraitContext::ReturnPositionOpaqueTy { origin } => {
-                        self.lower_opaque_impl_trait(span, origin, def_node_id, bounds, itctx)
-                    }
-                    ImplTraitContext::TypeAliasesOpaqueTy => {
-                        let nested_itctx = ImplTraitContext::TypeAliasesOpaqueTy;
-                        self.lower_opaque_impl_trait(
+                    ImplTraitContext::ReturnPositionOpaqueTy { origin, in_trait } => self
+                        .lower_opaque_impl_trait(
                             span,
-                            hir::OpaqueTyOrigin::TyAlias,
+                            *origin,
                             def_node_id,
                             bounds,
-                            nested_itctx,
-                        )
-                    }
+                            *in_trait,
+                            itctx,
+                        ),
+                    ImplTraitContext::TypeAliasesOpaqueTy => self.lower_opaque_impl_trait(
+                        span,
+                        hir::OpaqueTyOrigin::TyAlias,
+                        def_node_id,
+                        bounds,
+                        false,
+                        &ImplTraitContext::TypeAliasesOpaqueTy,
+                    ),
                     ImplTraitContext::Universal => {
                         let span = t.span;
                         let ident = Ident::from_str_and_span(&pprust::ty_to_string(t), span);
@@ -1342,15 +1362,26 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         }
                         path
                     }
+                    ImplTraitContext::Disallowed(
+                        position @ (ImplTraitPosition::TraitReturn | ImplTraitPosition::ImplReturn),
+                    ) => {
+                        self.tcx
+                            .sess
+                            .create_feature_err(
+                                MisplacedImplTrait {
+                                    span: t.span,
+                                    position: DiagnosticArgFromDisplay(&position),
+                                },
+                                sym::return_position_impl_trait_in_trait,
+                            )
+                            .emit();
+                        hir::TyKind::Err
+                    }
                     ImplTraitContext::Disallowed(position) => {
-                        let mut err = struct_span_err!(
-                            self.tcx.sess,
-                            t.span,
-                            E0562,
-                            "`impl Trait` only allowed in function and inherent method return types, not in {}",
-                            position
-                        );
-                        err.emit();
+                        self.tcx.sess.emit_err(MisplacedImplTrait {
+                            span: t.span,
+                            position: DiagnosticArgFromDisplay(&position),
+                        });
                         hir::TyKind::Err
                     }
                 }
@@ -1397,14 +1428,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     /// added explicitly in the HIR). But this includes all the lifetimes, and we only want to
     /// capture the lifetimes that are referenced in the bounds. Therefore, we add *extra* lifetime parameters
     /// for the lifetimes that get captured (`'x`, in our example above) and reference those.
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self), ret)]
     fn lower_opaque_impl_trait(
         &mut self,
         span: Span,
         origin: hir::OpaqueTyOrigin,
         opaque_ty_node_id: NodeId,
         bounds: &GenericBounds,
-        itctx: ImplTraitContext,
+        in_trait: bool,
+        itctx: &ImplTraitContext,
     ) -> hir::TyKind<'hir> {
         // Make sure we know that some funky desugaring has been going on here.
         // This is a first: there is code in other places like for loop
@@ -1492,6 +1524,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     }),
                     bounds: hir_bounds,
                     origin,
+                    in_trait,
                 };
                 debug!(?opaque_ty_item);
 
@@ -1518,7 +1551,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         debug!(?lifetimes);
 
         // `impl Trait` now just becomes `Foo<'a, 'b, ..>`.
-        hir::TyKind::OpaqueDef(hir::ItemId { def_id: opaque_ty_def_id }, lifetimes)
+        hir::TyKind::OpaqueDef(hir::ItemId { def_id: opaque_ty_def_id }, lifetimes, in_trait)
     }
 
     /// Registers a new opaque type with the proper `NodeId`s and
@@ -1577,7 +1610,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
                 LifetimeRes::Fresh { param, binder: _ } => {
                     debug_assert_eq!(lifetime.ident.name, kw::UnderscoreLifetime);
-                    if let Some(old_def_id) = self.opt_local_def_id(param) && remapping.get(&old_def_id).is_none() {
+                    if let Some(old_def_id) = self.orig_opt_local_def_id(param) && remapping.get(&old_def_id).is_none() {
                         let node_id = self.next_node_id();
 
                         let new_def_id = self.create_def(
@@ -1626,19 +1659,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     // `fn_def_id`: if `Some`, impl Trait arguments are lowered into generic parameters on the
     //      given DefId, otherwise impl Trait is disallowed. Must be `Some` if
     //      `make_ret_async` is also `Some`.
-    // `impl_trait_return_allow`: determines whether `impl Trait` can be used in return position.
-    //      This guards against trait declarations and implementations where `impl Trait` is
-    //      disallowed.
     // `make_ret_async`: if `Some`, converts `-> T` into `-> impl Future<Output = T>` in the
     //      return type. This is used for `async fn` declarations. The `NodeId` is the ID of the
-    //      return type `impl Trait` item.
-    #[tracing::instrument(level = "debug", skip(self))]
+    //      return type `impl Trait` item, and the `Span` points to the `async` keyword.
+    #[instrument(level = "debug", skip(self))]
     fn lower_fn_decl(
         &mut self,
         decl: &FnDecl,
         fn_node_id: Option<NodeId>,
+        fn_span: Span,
         kind: FnDeclKind,
-        make_ret_async: Option<NodeId>,
+        make_ret_async: Option<(NodeId, Span)>,
     ) -> &'hir hir::FnDecl<'hir> {
         let c_variadic = decl.c_variadic();
 
@@ -1651,11 +1682,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
         let inputs = self.arena.alloc_from_iter(inputs.iter().map(|param| {
             if fn_node_id.is_some() {
-                self.lower_ty_direct(&param.ty, ImplTraitContext::Universal)
+                self.lower_ty_direct(&param.ty, &ImplTraitContext::Universal)
             } else {
                 self.lower_ty_direct(
                     &param.ty,
-                    ImplTraitContext::Disallowed(match kind {
+                    &ImplTraitContext::Disallowed(match kind {
                         FnDeclKind::Fn | FnDeclKind::Inherent => {
                             unreachable!("fn should allow in-band lifetimes")
                         }
@@ -1669,20 +1700,63 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             }
         }));
 
-        let output = if let Some(ret_id) = make_ret_async {
-            self.lower_async_fn_ret_ty(
-                &decl.output,
-                fn_node_id.expect("`make_ret_async` but no `fn_def_id`"),
-                ret_id,
-            )
+        let output = if let Some((ret_id, span)) = make_ret_async {
+            match kind {
+                FnDeclKind::Trait => {
+                    if !kind.impl_trait_in_trait_allowed(self.tcx) {
+                        self.tcx
+                            .sess
+                            .create_feature_err(
+                                TraitFnAsync { fn_span, span },
+                                sym::return_position_impl_trait_in_trait,
+                            )
+                            .emit();
+                    }
+                    self.lower_async_fn_ret_ty(
+                        &decl.output,
+                        fn_node_id.expect("`make_ret_async` but no `fn_def_id`"),
+                        ret_id,
+                        true,
+                    )
+                }
+                _ => {
+                    if !kind.impl_trait_return_allowed(self.tcx) {
+                        if kind == FnDeclKind::Impl {
+                            self.tcx
+                                .sess
+                                .create_feature_err(
+                                    TraitFnAsync { fn_span, span },
+                                    sym::return_position_impl_trait_in_trait,
+                                )
+                                .emit();
+                        } else {
+                            self.tcx.sess.emit_err(TraitFnAsync { fn_span, span });
+                        }
+                    }
+                    self.lower_async_fn_ret_ty(
+                        &decl.output,
+                        fn_node_id.expect("`make_ret_async` but no `fn_def_id`"),
+                        ret_id,
+                        false,
+                    )
+                }
+            }
         } else {
             match decl.output {
                 FnRetTy::Ty(ref ty) => {
-                    let context = match fn_node_id {
-                        Some(fn_node_id) if kind.impl_trait_return_allowed() => {
+                    let mut context = match fn_node_id {
+                        Some(fn_node_id) if kind.impl_trait_return_allowed(self.tcx) => {
+                            let fn_def_id = self.local_def_id(fn_node_id);
+                            ImplTraitContext::ReturnPositionOpaqueTy {
+                                origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
+                                in_trait: false,
+                            }
+                        }
+                        Some(fn_node_id) if kind.impl_trait_in_trait_allowed(self.tcx) => {
                             let fn_def_id = self.local_def_id(fn_node_id);
                             ImplTraitContext::ReturnPositionOpaqueTy {
                                 origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
+                                in_trait: true,
                             }
                         }
                         _ => ImplTraitContext::Disallowed(match kind {
@@ -1696,7 +1770,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             FnDeclKind::Impl => ImplTraitPosition::ImplReturn,
                         }),
                     };
-                    hir::FnRetTy::Return(self.lower_ty(ty, context))
+                    hir::FnRetTy::Return(self.lower_ty(ty, &mut context))
                 }
                 FnRetTy::Default(span) => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
             }
@@ -1707,10 +1781,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             output,
             c_variadic,
             implicit_self: decl.inputs.get(0).map_or(hir::ImplicitSelfKind::None, |arg| {
-                use BindingMode::{ByRef, ByValue};
                 let is_mutable_pat = matches!(
                     arg.pat.kind,
-                    PatKind::Ident(ByValue(Mutability::Mut) | ByRef(Mutability::Mut), ..)
+                    PatKind::Ident(hir::BindingAnnotation(_, Mutability::Mut), ..)
                 );
 
                 match arg.ty.kind {
@@ -1741,12 +1814,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     // `output`: unlowered output type (`T` in `-> T`)
     // `fn_def_id`: `DefId` of the parent function (used to create child impl trait definition)
     // `opaque_ty_node_id`: `NodeId` of the opaque `impl Trait` type that should be created
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn lower_async_fn_ret_ty(
         &mut self,
         output: &FnRetTy,
         fn_node_id: NodeId,
         opaque_ty_node_id: NodeId,
+        in_trait: bool,
     ) -> hir::FnRetTy<'hir> {
         let span = output.span();
 
@@ -1805,7 +1879,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let extra_lifetime_params = self.resolver.take_extra_lifetime_params(opaque_ty_node_id);
         debug!(?extra_lifetime_params);
         for (ident, outer_node_id, outer_res) in extra_lifetime_params {
-            let outer_def_id = self.local_def_id(outer_node_id);
+            let outer_def_id = self.orig_local_def_id(outer_node_id);
             let inner_node_id = self.next_node_id();
 
             // Add a definition for the in scope lifetime def.
@@ -1873,8 +1947,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 //
                 // Then, we will create `fn foo(..) -> Foo<'_, '_>`, and
                 // hence the elision takes place at the fn site.
-                let future_bound =
-                    this.lower_async_fn_output_type_to_future_bound(output, fn_def_id, span);
+                let future_bound = this.lower_async_fn_output_type_to_future_bound(
+                    output,
+                    span,
+                    ImplTraitContext::ReturnPositionOpaqueTy {
+                        origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
+                        in_trait,
+                    },
+                );
 
                 let generic_params = this.arena.alloc_from_iter(collected_lifetimes.iter().map(
                     |&(new_node_id, lifetime, _)| {
@@ -1912,6 +1992,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     }),
                     bounds: arena_vec![this; future_bound],
                     origin: hir::OpaqueTyOrigin::AsyncFn(fn_def_id),
+                    in_trait,
                 };
 
                 trace!("exist ty from async fn def id: {:#?}", opaque_ty_def_id);
@@ -1948,8 +2029,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 let res = res.unwrap_or(
                     self.resolver.get_lifetime_res(lifetime.id).unwrap_or(LifetimeRes::Error),
                 );
-                let l = self.new_named_lifetime_with_res(id, span, ident, res);
-                hir::GenericArg::Lifetime(l)
+                hir::GenericArg::Lifetime(self.new_named_lifetime_with_res(id, span, ident, res))
             },
         ));
 
@@ -1957,8 +2037,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         // Foo = impl Trait` is, internally, created as a child of the
         // async fn, so the *type parameters* are inherited.  It's
         // only the lifetime parameters that we must supply.
-        let opaque_ty_ref =
-            hir::TyKind::OpaqueDef(hir::ItemId { def_id: opaque_ty_def_id }, generic_args);
+        let opaque_ty_ref = hir::TyKind::OpaqueDef(
+            hir::ItemId { def_id: opaque_ty_def_id },
+            generic_args,
+            in_trait,
+        );
         let opaque_ty = self.ty(opaque_ty_span, opaque_ty_ref);
         hir::FnRetTy::Return(self.arena.alloc(opaque_ty))
     }
@@ -1967,8 +2050,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_async_fn_output_type_to_future_bound(
         &mut self,
         output: &FnRetTy,
-        fn_def_id: LocalDefId,
         span: Span,
+        mut nested_impl_trait_context: ImplTraitContext,
     ) -> hir::GenericBound<'hir> {
         // Compute the `T` in `Future<Output = T>` from the return type.
         let output_ty = match output {
@@ -1976,10 +2059,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 // Not `OpaqueTyOrigin::AsyncFn`: that's only used for the
                 // `impl Future` opaque type that `async fn` implicitly
                 // generates.
-                let context = ImplTraitContext::ReturnPositionOpaqueTy {
-                    origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
-                };
-                self.lower_ty(ty, context)
+                self.lower_ty(ty, &mut nested_impl_trait_context)
             }
             FnRetTy::Default(ret_ty_span) => self.arena.alloc(self.ty_tup(*ret_ty_span, &[])),
         };
@@ -2005,7 +2085,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_param_bound(
         &mut self,
         tpb: &GenericBound,
-        itctx: ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::GenericBound<'hir> {
         match tpb {
             GenericBound::Trait(p, modifier) => hir::GenericBound::Trait(
@@ -2018,24 +2098,24 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
-    fn lower_lifetime(&mut self, l: &Lifetime) -> hir::Lifetime {
+    fn lower_lifetime(&mut self, l: &Lifetime) -> &'hir hir::Lifetime {
         let span = self.lower_span(l.ident.span);
         let ident = self.lower_ident(l.ident);
         self.new_named_lifetime(l.id, l.id, span, ident)
     }
 
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn new_named_lifetime_with_res(
         &mut self,
         id: NodeId,
         span: Span,
         ident: Ident,
         res: LifetimeRes,
-    ) -> hir::Lifetime {
+    ) -> &'hir hir::Lifetime {
         let name = match res {
             LifetimeRes::Param { param, .. } => {
                 let p_name = ParamName::Plain(ident);
-                let param = self.resolver.get_remapped_def_id(param);
+                let param = self.get_remapped_def_id(param);
 
                 hir::LifetimeName::Param(param, p_name)
             }
@@ -2052,17 +2132,21 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         };
 
         debug!(?name);
-        hir::Lifetime { hir_id: self.lower_node_id(id), span: self.lower_span(span), name }
+        self.arena.alloc(hir::Lifetime {
+            hir_id: self.lower_node_id(id),
+            span: self.lower_span(span),
+            name,
+        })
     }
 
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn new_named_lifetime(
         &mut self,
         id: NodeId,
         new_id: NodeId,
         span: Span,
         ident: Ident,
-    ) -> hir::Lifetime {
+    ) -> &'hir hir::Lifetime {
         let res = self.resolver.get_lifetime_res(id).unwrap_or(LifetimeRes::Error);
         self.new_named_lifetime_with_res(new_id, span, ident, res)
     }
@@ -2117,7 +2201,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             GenericParamKind::Type { ref default, .. } => {
                 let kind = hir::GenericParamKind::Type {
                     default: default.as_ref().map(|x| {
-                        self.lower_ty(x, ImplTraitContext::Disallowed(ImplTraitPosition::Type))
+                        self.lower_ty(x, &ImplTraitContext::Disallowed(ImplTraitPosition::Type))
                     }),
                     synthetic: false,
                 };
@@ -2125,7 +2209,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 (hir::ParamName::Plain(self.lower_ident(param.ident)), kind)
             }
             GenericParamKind::Const { ref ty, kw_span: _, ref default } => {
-                let ty = self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                let ty = self.lower_ty(&ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                 let default = default.as_ref().map(|def| self.lower_anon_const(def));
                 (
                     hir::ParamName::Plain(self.lower_ident(param.ident)),
@@ -2135,7 +2219,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
-    fn lower_trait_ref(&mut self, p: &TraitRef, itctx: ImplTraitContext) -> hir::TraitRef<'hir> {
+    fn lower_trait_ref(&mut self, p: &TraitRef, itctx: &ImplTraitContext) -> hir::TraitRef<'hir> {
         let path = match self.lower_qpath(p.ref_id, &None, &p.path, ParamMode::Explicit, itctx) {
             hir::QPath::Resolved(None, path) => path,
             qpath => panic!("lower_trait_ref: unexpected QPath `{:?}`", qpath),
@@ -2143,11 +2227,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         hir::TraitRef { path, hir_ref_id: self.lower_node_id(p.ref_id) }
     }
 
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self))]
     fn lower_poly_trait_ref(
         &mut self,
         p: &PolyTraitRef,
-        itctx: ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::PolyTraitRef<'hir> {
         let bound_generic_params =
             self.lower_lifetime_binder(p.trait_ref.ref_id, &p.bound_generic_params);
@@ -2155,14 +2239,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         hir::PolyTraitRef { bound_generic_params, trait_ref, span: self.lower_span(p.span) }
     }
 
-    fn lower_mt(&mut self, mt: &MutTy, itctx: ImplTraitContext) -> hir::MutTy<'hir> {
+    fn lower_mt(&mut self, mt: &MutTy, itctx: &ImplTraitContext) -> hir::MutTy<'hir> {
         hir::MutTy { ty: self.lower_ty(&mt.ty, itctx), mutbl: mt.mutbl }
     }
 
+    #[instrument(level = "debug", skip(self), ret)]
     fn lower_param_bounds(
         &mut self,
         bounds: &[GenericBound],
-        itctx: ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::GenericBounds<'hir> {
         self.arena.alloc_from_iter(self.lower_param_bounds_mut(bounds, itctx))
     }
@@ -2170,11 +2255,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_param_bounds_mut<'s>(
         &'s mut self,
         bounds: &'s [GenericBound],
-        itctx: ImplTraitContext,
+        itctx: &'s ImplTraitContext,
     ) -> impl Iterator<Item = hir::GenericBound<'hir>> + Captures<'s> + Captures<'a> {
         bounds.iter().map(move |bound| self.lower_param_bound(bound, itctx))
     }
 
+    #[instrument(level = "debug", skip(self), ret)]
     fn lower_generic_and_bounds(
         &mut self,
         node_id: NodeId,
@@ -2200,16 +2286,19 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             node_id,
             &GenericParamKind::Type { default: None },
             bounds,
-            ImplTraitContext::Universal,
+            &ImplTraitContext::Universal,
             hir::PredicateOrigin::ImplTrait,
         );
 
+        let hir_id = self.next_id();
+        let res = Res::Def(DefKind::TyParam, def_id.to_def_id());
         let ty = hir::TyKind::Path(hir::QPath::Resolved(
             None,
             self.arena.alloc(hir::Path {
                 span: self.lower_span(span),
-                res: Res::Def(DefKind::TyParam, def_id.to_def_id()),
-                segments: arena_vec![self; hir::PathSegment::from_ident(self.lower_ident(ident))],
+                res,
+                segments:
+                    arena_vec![self; hir::PathSegment::new(self.lower_ident(ident), hir_id, res)],
             }),
         ));
 
@@ -2235,7 +2324,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         c.value.span,
                         "using `_` for array lengths is unstable",
                     )
-                    .emit();
+                    .stash(c.value.span, StashKey::UnderscoreForArrayLengths);
                     hir::ArrayLen::Body(self.lower_anon_const(c))
                 }
             }
@@ -2372,11 +2461,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     fn pat_ident(&mut self, span: Span, ident: Ident) -> (&'hir hir::Pat<'hir>, hir::HirId) {
-        self.pat_ident_binding_mode(span, ident, hir::BindingAnnotation::Unannotated)
+        self.pat_ident_binding_mode(span, ident, hir::BindingAnnotation::NONE)
     }
 
     fn pat_ident_mut(&mut self, span: Span, ident: Ident) -> (hir::Pat<'hir>, hir::HirId) {
-        self.pat_ident_binding_mode_mut(span, ident, hir::BindingAnnotation::Unannotated)
+        self.pat_ident_binding_mode_mut(span, ident, hir::BindingAnnotation::NONE)
     }
 
     fn pat_ident_binding_mode(
@@ -2465,14 +2554,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     /// bound, like the bound in `Box<dyn Debug>`. This method is not invoked
     /// when the bound is written, even if it is written with `'_` like in
     /// `Box<dyn Debug + '_>`. In those cases, `lower_lifetime` is invoked.
-    fn elided_dyn_bound(&mut self, span: Span) -> hir::Lifetime {
+    fn elided_dyn_bound(&mut self, span: Span) -> &'hir hir::Lifetime {
         let r = hir::Lifetime {
             hir_id: self.next_id(),
             span: self.lower_span(span),
             name: hir::LifetimeName::ImplicitObjectLifetimeDefault,
         };
         debug!("elided_dyn_bound: r={:?}", r);
-        r
+        self.arena.alloc(r)
     }
 }
 
diff --git a/compiler/rustc_ast_lowering/src/lifetime_collector.rs b/compiler/rustc_ast_lowering/src/lifetime_collector.rs
index 81006e00f..914fc5f58 100644
--- a/compiler/rustc_ast_lowering/src/lifetime_collector.rs
+++ b/compiler/rustc_ast_lowering/src/lifetime_collector.rs
@@ -1,9 +1,6 @@
 use super::ResolverAstLoweringExt;
 use rustc_ast::visit::{self, BoundKind, LifetimeCtxt, Visitor};
-use rustc_ast::{
-    FnRetTy, GenericBounds, Lifetime, NodeId, PathSegment, PolyTraitRef, TraitBoundModifier, Ty,
-    TyKind,
-};
+use rustc_ast::{FnRetTy, GenericBounds, Lifetime, NodeId, PathSegment, PolyTraitRef, Ty, TyKind};
 use rustc_hir::def::LifetimeRes;
 use rustc_middle::span_bug;
 use rustc_middle::ty::ResolverAstLowering;
@@ -66,15 +63,15 @@ impl<'ast> Visitor<'ast> for LifetimeCollectVisitor<'ast> {
         self.record_lifetime_use(*lifetime);
     }
 
-    fn visit_path_segment(&mut self, path_span: Span, path_segment: &'ast PathSegment) {
-        self.record_elided_anchor(path_segment.id, path_span);
-        visit::walk_path_segment(self, path_span, path_segment);
+    fn visit_path_segment(&mut self, path_segment: &'ast PathSegment) {
+        self.record_elided_anchor(path_segment.id, path_segment.ident.span);
+        visit::walk_path_segment(self, path_segment);
     }
 
-    fn visit_poly_trait_ref(&mut self, t: &'ast PolyTraitRef, m: &'ast TraitBoundModifier) {
+    fn visit_poly_trait_ref(&mut self, t: &'ast PolyTraitRef) {
         self.current_binders.push(t.trait_ref.ref_id);
 
-        visit::walk_poly_trait_ref(self, t, m);
+        visit::walk_poly_trait_ref(self, t);
 
         self.current_binders.pop();
     }
diff --git a/compiler/rustc_ast_lowering/src/pat.rs b/compiler/rustc_ast_lowering/src/pat.rs
index bd2e76e55..1ea76fdbf 100644
--- a/compiler/rustc_ast_lowering/src/pat.rs
+++ b/compiler/rustc_ast_lowering/src/pat.rs
@@ -1,3 +1,6 @@
+use super::errors::{
+    ArbitraryExpressionInPattern, ExtraDoubleDot, MisplacedDoubleDot, SubTupleBinding,
+};
 use super::ResolverAstLoweringExt;
 use super::{ImplTraitContext, LoweringContext, ParamMode};
 use crate::ImplTraitPosition;
@@ -5,7 +8,6 @@ use crate::ImplTraitPosition;
 use rustc_ast::ptr::P;
 use rustc_ast::*;
 use rustc_data_structures::stack::ensure_sufficient_stack;
-use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_hir::def::Res;
 use rustc_span::symbol::Ident;
@@ -22,7 +24,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             let node = loop {
                 match pattern.kind {
                     PatKind::Wild => break hir::PatKind::Wild,
-                    PatKind::Ident(ref binding_mode, ident, ref sub) => {
+                    PatKind::Ident(binding_mode, ident, ref sub) => {
                         let lower_sub = |this: &mut Self| sub.as_ref().map(|s| this.lower_pat(&*s));
                         break self.lower_pat_ident(pattern, binding_mode, ident, lower_sub);
                     }
@@ -35,7 +37,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             qself,
                             path,
                             ParamMode::Optional,
-                            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         );
                         let (pats, ddpos) = self.lower_pat_tuple(pats, "tuple struct");
                         break hir::PatKind::TupleStruct(qpath, pats, ddpos);
@@ -51,7 +53,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             qself,
                             path,
                             ParamMode::Optional,
-                            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         );
                         break hir::PatKind::Path(qpath);
                     }
@@ -61,15 +63,20 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             qself,
                             path,
                             ParamMode::Optional,
-                            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         );
 
-                        let fs = self.arena.alloc_from_iter(fields.iter().map(|f| hir::PatField {
-                            hir_id: self.next_id(),
-                            ident: self.lower_ident(f.ident),
-                            pat: self.lower_pat(&f.pat),
-                            is_shorthand: f.is_shorthand,
-                            span: self.lower_span(f.span),
+                        let fs = self.arena.alloc_from_iter(fields.iter().map(|f| {
+                            let hir_id = self.lower_node_id(f.id);
+                            self.lower_attrs(hir_id, &f.attrs);
+
+                            hir::PatField {
+                                hir_id,
+                                ident: self.lower_ident(f.ident),
+                                pat: self.lower_pat(&f.pat),
+                                is_shorthand: f.is_shorthand,
+                                span: self.lower_span(f.span),
+                            }
                         }));
                         break hir::PatKind::Struct(qpath, fs, etc);
                     }
@@ -109,7 +116,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         &mut self,
         pats: &[P<Pat>],
         ctx: &str,
-    ) -> (&'hir [hir::Pat<'hir>], Option<usize>) {
+    ) -> (&'hir [hir::Pat<'hir>], hir::DotDotPos) {
         let mut elems = Vec::with_capacity(pats.len());
         let mut rest = None;
 
@@ -129,20 +136,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 // This is not allowed as a sub-tuple pattern
                 PatKind::Ident(ref _bm, ident, Some(ref sub)) if sub.is_rest() => {
                     let sp = pat.span;
-                    self.diagnostic()
-                        .struct_span_err(
-                            sp,
-                            &format!("`{} @` is not allowed in a {}", ident.name, ctx),
-                        )
-                        .span_label(sp, "this is only allowed in slice patterns")
-                        .help("remove this and bind each tuple field independently")
-                        .span_suggestion_verbose(
-                            sp,
-                            &format!("if you don't need to use the contents of {}, discard the tuple's remaining fields", ident),
-                            "..",
-                            Applicability::MaybeIncorrect,
-                        )
-                        .emit();
+                    self.tcx.sess.emit_err(SubTupleBinding {
+                        span: sp,
+                        ident_name: ident.name,
+                        ident,
+                        ctx,
+                    });
                 }
                 _ => {}
             }
@@ -161,7 +160,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             }
         }
 
-        (self.arena.alloc_from_iter(elems), rest.map(|(ddpos, _)| ddpos))
+        (self.arena.alloc_from_iter(elems), hir::DotDotPos::new(rest.map(|(ddpos, _)| ddpos)))
     }
 
     /// Lower a slice pattern of form `[pat_0, ..., pat_n]` into
@@ -177,9 +176,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let mut prev_rest_span = None;
 
         // Lowers `$bm $ident @ ..` to `$bm $ident @ _`.
-        let lower_rest_sub = |this: &mut Self, pat, bm, ident, sub| {
+        let lower_rest_sub = |this: &mut Self, pat, ann, ident, sub| {
             let lower_sub = |this: &mut Self| Some(this.pat_wild_with_node_id_of(sub));
-            let node = this.lower_pat_ident(pat, bm, ident, lower_sub);
+            let node = this.lower_pat_ident(pat, ann, ident, lower_sub);
             this.pat_with_node_id_of(pat, node)
         };
 
@@ -195,9 +194,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 }
                 // Found a sub-slice pattern `$binding_mode $ident @ ..`.
                 // Record, lower it to `$binding_mode $ident @ _`, and stop here.
-                PatKind::Ident(ref bm, ident, Some(ref sub)) if sub.is_rest() => {
+                PatKind::Ident(ann, ident, Some(ref sub)) if sub.is_rest() => {
                     prev_rest_span = Some(sub.span);
-                    slice = Some(self.arena.alloc(lower_rest_sub(self, pat, bm, ident, sub)));
+                    slice = Some(self.arena.alloc(lower_rest_sub(self, pat, ann, ident, sub)));
                     break;
                 }
                 // It was not a subslice pattern so lower it normally.
@@ -210,9 +209,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             // There was a previous subslice pattern; make sure we don't allow more.
             let rest_span = match pat.kind {
                 PatKind::Rest => Some(pat.span),
-                PatKind::Ident(ref bm, ident, Some(ref sub)) if sub.is_rest() => {
+                PatKind::Ident(ann, ident, Some(ref sub)) if sub.is_rest() => {
                     // #69103: Lower into `binding @ _` as above to avoid ICEs.
-                    after.push(lower_rest_sub(self, pat, bm, ident, sub));
+                    after.push(lower_rest_sub(self, pat, ann, ident, sub));
                     Some(sub.span)
                 }
                 _ => None,
@@ -236,7 +235,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_pat_ident(
         &mut self,
         p: &Pat,
-        binding_mode: &BindingMode,
+        annotation: BindingAnnotation,
         ident: Ident,
         lower_sub: impl FnOnce(&mut Self) -> Option<&'hir hir::Pat<'hir>>,
     ) -> hir::PatKind<'hir> {
@@ -249,29 +248,24 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 };
 
                 hir::PatKind::Binding(
-                    self.lower_binding_mode(binding_mode),
+                    annotation,
                     self.lower_node_id(canonical_id),
                     self.lower_ident(ident),
                     lower_sub(self),
                 )
             }
-            Some(res) => hir::PatKind::Path(hir::QPath::Resolved(
-                None,
-                self.arena.alloc(hir::Path {
-                    span: self.lower_span(ident.span),
-                    res: self.lower_res(res),
-                    segments: arena_vec![self; hir::PathSegment::from_ident(self.lower_ident(ident))],
-                }),
-            )),
-        }
-    }
-
-    fn lower_binding_mode(&mut self, b: &BindingMode) -> hir::BindingAnnotation {
-        match *b {
-            BindingMode::ByValue(Mutability::Not) => hir::BindingAnnotation::Unannotated,
-            BindingMode::ByRef(Mutability::Not) => hir::BindingAnnotation::Ref,
-            BindingMode::ByValue(Mutability::Mut) => hir::BindingAnnotation::Mutable,
-            BindingMode::ByRef(Mutability::Mut) => hir::BindingAnnotation::RefMut,
+            Some(res) => {
+                let hir_id = self.next_id();
+                let res = self.lower_res(res);
+                hir::PatKind::Path(hir::QPath::Resolved(
+                    None,
+                    self.arena.alloc(hir::Path {
+                        span: self.lower_span(ident.span),
+                        res,
+                        segments: arena_vec![self; hir::PathSegment::new(self.lower_ident(ident), hir_id, res)],
+                    }),
+            ))
+            }
         }
     }
 
@@ -291,19 +285,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
     /// Emit a friendly error for extra `..` patterns in a tuple/tuple struct/slice pattern.
     pub(crate) fn ban_extra_rest_pat(&self, sp: Span, prev_sp: Span, ctx: &str) {
-        self.diagnostic()
-            .struct_span_err(sp, &format!("`..` can only be used once per {} pattern", ctx))
-            .span_label(sp, &format!("can only be used once per {} pattern", ctx))
-            .span_label(prev_sp, "previously used here")
-            .emit();
+        self.tcx.sess.emit_err(ExtraDoubleDot { span: sp, prev_span: prev_sp, ctx });
     }
 
     /// Used to ban the `..` pattern in places it shouldn't be semantically.
     fn ban_illegal_rest_pat(&self, sp: Span) -> hir::PatKind<'hir> {
-        self.diagnostic()
-            .struct_span_err(sp, "`..` patterns are not allowed here")
-            .note("only allowed in tuple, tuple struct, and slice patterns")
-            .emit();
+        self.tcx.sess.emit_err(MisplacedDoubleDot { span: sp });
 
         // We're not in a list context so `..` can be reasonably treated
         // as `_` because it should always be valid and roughly matches the
@@ -340,8 +327,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             ExprKind::Path(..) if allow_paths => {}
             ExprKind::Unary(UnOp::Neg, ref inner) if matches!(inner.kind, ExprKind::Lit(_)) => {}
             _ => {
-                self.diagnostic()
-                    .span_err(expr.span, "arbitrary expressions aren't allowed in patterns");
+                self.tcx.sess.emit_err(ArbitraryExpressionInPattern { span: expr.span });
                 return self.arena.alloc(self.expr_err(expr.span));
             }
         }
diff --git a/compiler/rustc_ast_lowering/src/path.rs b/compiler/rustc_ast_lowering/src/path.rs
index 393be3b45..6bb1bb9ea 100644
--- a/compiler/rustc_ast_lowering/src/path.rs
+++ b/compiler/rustc_ast_lowering/src/path.rs
@@ -1,11 +1,11 @@
 use crate::ImplTraitPosition;
 
+use super::errors::{GenericTypeWithParentheses, UseAngleBrackets};
 use super::ResolverAstLoweringExt;
 use super::{GenericArgsCtor, LifetimeRes, ParenthesizedGenericArgs};
 use super::{ImplTraitContext, LoweringContext, ParamMode};
 
 use rustc_ast::{self as ast, *};
-use rustc_errors::{struct_span_err, Applicability};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, PartialRes, Res};
 use rustc_hir::GenericArg;
@@ -13,7 +13,6 @@ use rustc_span::symbol::{kw, Ident};
 use rustc_span::{BytePos, Span, DUMMY_SP};
 
 use smallvec::smallvec;
-use tracing::debug;
 
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
     #[instrument(level = "trace", skip(self))]
@@ -23,7 +22,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         qself: &Option<QSelf>,
         p: &Path,
         param_mode: ParamMode,
-        itctx: ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::QPath<'hir> {
         let qself_position = qself.as_ref().map(|q| q.position);
         let qself = qself.as_ref().map(|q| self.lower_ty(&q.ty, itctx));
@@ -157,7 +156,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     segment,
                     param_mode,
                     ParenthesizedGenericArgs::Err,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                 )
             })),
             span: self.lower_span(p.span),
@@ -181,11 +180,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         segment: &PathSegment,
         param_mode: ParamMode,
         parenthesized_generic_args: ParenthesizedGenericArgs,
-        itctx: ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::PathSegment<'hir> {
         debug!("path_span: {:?}, lower_path_segment(segment: {:?})", path_span, segment,);
         let (mut generic_args, infer_args) = if let Some(ref generic_args) = segment.args {
-            let msg = "parenthesized type parameters may only be used with a `Fn` trait";
             match **generic_args {
                 GenericArgs::AngleBracketed(ref data) => {
                     self.lower_angle_bracketed_parameter_data(data, param_mode, itctx)
@@ -193,10 +191,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 GenericArgs::Parenthesized(ref data) => match parenthesized_generic_args {
                     ParenthesizedGenericArgs::Ok => self.lower_parenthesized_parameter_data(data),
                     ParenthesizedGenericArgs::Err => {
-                        let mut err = struct_span_err!(self.tcx.sess, data.span, E0214, "{}", msg);
-                        err.span_label(data.span, "only `Fn` traits may use parentheses");
                         // Suggest replacing parentheses with angle brackets `Trait(params...)` to `Trait<params...>`
-                        if !data.inputs.is_empty() {
+                        let sub = if !data.inputs.is_empty() {
                             // Start of the span to the 1st character of 1st argument
                             let open_param = data.inputs_span.shrink_to_lo().to(data
                                 .inputs
@@ -212,16 +208,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                 .span
                                 .shrink_to_hi()
                                 .to(data.inputs_span.shrink_to_hi());
-                            err.multipart_suggestion(
-                                &format!("use angle brackets instead",),
-                                vec![
-                                    (open_param, String::from("<")),
-                                    (close_param, String::from(">")),
-                                ],
-                                Applicability::MaybeIncorrect,
-                            );
-                        }
-                        err.emit();
+
+                            Some(UseAngleBrackets { open_param, close_param })
+                        } else {
+                            None
+                        };
+                        self.tcx.sess.emit_err(GenericTypeWithParentheses { span: data.span, sub });
                         (
                             self.lower_angle_bracketed_parameter_data(
                                 &data.as_angle_bracketed_args(),
@@ -258,16 +250,16 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
 
         let res = self.expect_full_res(segment.id);
-        let id = self.lower_node_id(segment.id);
+        let hir_id = self.lower_node_id(segment.id);
         debug!(
             "lower_path_segment: ident={:?} original-id={:?} new-id={:?}",
-            segment.ident, segment.id, id,
+            segment.ident, segment.id, hir_id,
         );
 
         hir::PathSegment {
             ident: self.lower_ident(segment.ident),
-            hir_id: Some(id),
-            res: Some(self.lower_res(res)),
+            hir_id,
+            res: self.lower_res(res),
             infer_args,
             args: if generic_args.is_empty() && generic_args.span.is_empty() {
                 None
@@ -324,7 +316,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         &mut self,
         data: &AngleBracketedArgs,
         param_mode: ParamMode,
-        itctx: ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> (GenericArgsCtor<'hir>, bool) {
         let has_non_lt_args = data.args.iter().any(|arg| match arg {
             AngleBracketedArg::Arg(ast::GenericArg::Lifetime(_))
@@ -358,15 +350,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         // we generally don't permit such things (see #51008).
         let ParenthesizedArgs { span, inputs, inputs_span, output } = data;
         let inputs = self.arena.alloc_from_iter(inputs.iter().map(|ty| {
-            self.lower_ty_direct(ty, ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitParam))
+            self.lower_ty_direct(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitParam))
         }));
         let output_ty = match output {
             FnRetTy::Ty(ty) => {
-                self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitReturn))
+                self.lower_ty(&ty, &ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitReturn))
             }
             FnRetTy::Default(_) => self.arena.alloc(self.ty_tup(*span, &[])),
         };
-        let args = smallvec![GenericArg::Type(self.ty_tup(*inputs_span, inputs))];
+        let args = smallvec![GenericArg::Type(self.arena.alloc(self.ty_tup(*inputs_span, inputs)))];
         let binding = self.output_ty_binding(output_ty.span, output_ty);
         (
             GenericArgsCtor {
diff --git a/compiler/rustc_ast_passes/Cargo.toml b/compiler/rustc_ast_passes/Cargo.toml
index 22742b2ad..37eff9207 100644
--- a/compiler/rustc_ast_passes/Cargo.toml
+++ b/compiler/rustc_ast_passes/Cargo.toml
@@ -11,6 +11,7 @@ rustc_attr = { path = "../rustc_attr" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
 rustc_feature = { path = "../rustc_feature" }
+rustc_macros = { path = "../rustc_macros" }
 rustc_parse = { path = "../rustc_parse" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index 2d9d0073f..b1d10e07a 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -13,9 +13,7 @@ use rustc_ast::walk_list;
 use rustc_ast::*;
 use rustc_ast_pretty::pprust::{self, State};
 use rustc_data_structures::fx::FxHashMap;
-use rustc_errors::{
-    error_code, pluralize, struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed,
-};
+use rustc_errors::{error_code, fluent, pluralize, struct_span_err, Applicability};
 use rustc_parse::validate_attr;
 use rustc_session::lint::builtin::{
     DEPRECATED_WHERE_CLAUSE_LOCATION, MISSING_ABI, PATTERNS_IN_FNS_WITHOUT_BODY,
@@ -29,6 +27,8 @@ use rustc_target::spec::abi;
 use std::mem;
 use std::ops::{Deref, DerefMut};
 
+use crate::errors::*;
+
 const MORE_EXTERN: &str =
     "for more information, visit https://doc.rust-lang.org/std/keyword.extern.html";
 
@@ -121,30 +121,9 @@ impl<'a> AstValidator<'a> {
     fn ban_let_expr(&self, expr: &'a Expr, forbidden_let_reason: ForbiddenLetReason) {
         let sess = &self.session;
         if sess.opts.unstable_features.is_nightly_build() {
-            let err = "`let` expressions are not supported here";
-            let mut diag = sess.struct_span_err(expr.span, err);
-            diag.note("only supported directly in conditions of `if` and `while` expressions");
-            match forbidden_let_reason {
-                ForbiddenLetReason::GenericForbidden => {}
-                ForbiddenLetReason::NotSupportedOr(span) => {
-                    diag.span_note(
-                        span,
-                        "`||` operators are not supported in let chain expressions",
-                    );
-                }
-                ForbiddenLetReason::NotSupportedParentheses(span) => {
-                    diag.span_note(
-                        span,
-                        "`let`s wrapped in parentheses are not supported in a context with let \
-                        chains",
-                    );
-                }
-            }
-            diag.emit();
+            sess.emit_err(ForbiddenLet { span: expr.span, reason: forbidden_let_reason });
         } else {
-            sess.struct_span_err(expr.span, "expected expression, found statement (`let`)")
-                .note("variable declaration using `let` is a statement")
-                .emit();
+            sess.emit_err(ForbiddenLetStable { span: expr.span });
         }
     }
 
@@ -175,7 +154,7 @@ impl<'a> AstValidator<'a> {
                 DEPRECATED_WHERE_CLAUSE_LOCATION,
                 id,
                 where_clauses.0.1,
-                "where clause not allowed here",
+                fluent::ast_passes::deprecated_where_clause_location,
                 BuiltinLintDiagnostics::DeprecatedWhereclauseLocation(
                     where_clauses.1.1.shrink_to_hi(),
                     suggestion,
@@ -205,10 +184,7 @@ impl<'a> AstValidator<'a> {
             AssocConstraintKind::Equality { .. } => {}
             AssocConstraintKind::Bound { .. } => {
                 if self.is_assoc_ty_bound_banned {
-                    self.err_handler().span_err(
-                        constraint.span,
-                        "associated type bounds are not allowed within structs, enums, or unions",
-                    );
+                    self.session.emit_err(ForbiddenAssocConstraint { span: constraint.span });
                 }
             }
         }
@@ -247,11 +223,9 @@ impl<'a> AstValidator<'a> {
                 for (i, segment) in path.segments.iter().enumerate() {
                     // Allow `impl Trait` iff we're on the final path segment
                     if i == path.segments.len() - 1 {
-                        self.visit_path_segment(path.span, segment);
+                        self.visit_path_segment(segment);
                     } else {
-                        self.with_banned_impl_trait(|this| {
-                            this.visit_path_segment(path.span, segment)
-                        });
+                        self.with_banned_impl_trait(|this| this.visit_path_segment(segment));
                     }
                 }
             }
@@ -280,38 +254,33 @@ impl<'a> AstValidator<'a> {
     fn check_lifetime(&self, ident: Ident) {
         let valid_names = [kw::UnderscoreLifetime, kw::StaticLifetime, kw::Empty];
         if !valid_names.contains(&ident.name) && ident.without_first_quote().is_reserved() {
-            self.err_handler().span_err(ident.span, "lifetimes cannot use keyword names");
+            self.session.emit_err(KeywordLifetime { span: ident.span });
         }
     }
 
     fn check_label(&self, ident: Ident) {
         if ident.without_first_quote().is_reserved() {
-            self.err_handler()
-                .span_err(ident.span, &format!("invalid label name `{}`", ident.name));
+            self.session.emit_err(InvalidLabel { span: ident.span, name: ident.name });
         }
     }
 
-    fn invalid_visibility(&self, vis: &Visibility, note: Option<&str>) {
+    fn invalid_visibility(&self, vis: &Visibility, note: Option<InvalidVisibilityNote>) {
         if let VisibilityKind::Inherited = vis.kind {
             return;
         }
 
-        let mut err =
-            struct_span_err!(self.session, vis.span, E0449, "unnecessary visibility qualifier");
-        if vis.kind.is_pub() {
-            err.span_label(vis.span, "`pub` not permitted here because it's implied");
-        }
-        if let Some(note) = note {
-            err.note(note);
-        }
-        err.emit();
+        self.session.emit_err(InvalidVisibility {
+            span: vis.span,
+            implied: if vis.kind.is_pub() { Some(vis.span) } else { None },
+            note,
+        });
     }
 
     fn check_decl_no_pat(decl: &FnDecl, mut report_err: impl FnMut(Span, Option<Ident>, bool)) {
         for Param { pat, .. } in &decl.inputs {
             match pat.kind {
-                PatKind::Ident(BindingMode::ByValue(Mutability::Not), _, None) | PatKind::Wild => {}
-                PatKind::Ident(BindingMode::ByValue(Mutability::Mut), ident, None) => {
+                PatKind::Ident(BindingAnnotation::NONE, _, None) | PatKind::Wild => {}
+                PatKind::Ident(BindingAnnotation::MUT, ident, None) => {
                     report_err(pat.span, Some(ident), true)
                 }
                 _ => report_err(pat.span, None, false),
@@ -319,31 +288,9 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn check_trait_fn_not_async(&self, fn_span: Span, asyncness: Async) {
-        if let Async::Yes { span, .. } = asyncness {
-            struct_span_err!(
-                self.session,
-                fn_span,
-                E0706,
-                "functions in traits cannot be declared `async`"
-            )
-            .span_label(span, "`async` because of this")
-            .note("`async` trait functions are not currently supported")
-            .note("consider using the `async-trait` crate: https://crates.io/crates/async-trait")
-            .emit();
-        }
-    }
-
     fn check_trait_fn_not_const(&self, constness: Const) {
         if let Const::Yes(span) = constness {
-            struct_span_err!(
-                self.session,
-                span,
-                E0379,
-                "functions in traits cannot be declared const"
-            )
-            .span_label(span, "functions in traits cannot be const")
-            .emit();
+            self.session.emit_err(TraitFnConst { span });
         }
     }
 
@@ -356,8 +303,7 @@ impl<'a> AstValidator<'a> {
                 GenericParamKind::Lifetime { .. } => {
                     if !param.bounds.is_empty() {
                         let spans: Vec<_> = param.bounds.iter().map(|b| b.span()).collect();
-                        self.err_handler()
-                            .span_err(spans, "lifetime bounds cannot be used in this context");
+                        self.session.emit_err(ForbiddenLifetimeBound { spans });
                     }
                     None
                 }
@@ -365,10 +311,7 @@ impl<'a> AstValidator<'a> {
             })
             .collect();
         if !non_lt_param_spans.is_empty() {
-            self.err_handler().span_err(
-                non_lt_param_spans,
-                "only lifetime parameters can be used in this context",
-            );
+            self.session.emit_err(ForbiddenNonLifetimeParam { spans: non_lt_param_spans });
         }
     }
 
@@ -385,10 +328,7 @@ impl<'a> AstValidator<'a> {
         let max_num_args: usize = u16::MAX.into();
         if fn_decl.inputs.len() > max_num_args {
             let Param { span, .. } = fn_decl.inputs[0];
-            self.err_handler().span_fatal(
-                span,
-                &format!("function can not have more than {} arguments", max_num_args),
-            );
+            self.session.emit_fatal(FnParamTooMany { span, max_num_args });
         }
     }
 
@@ -396,19 +336,13 @@ impl<'a> AstValidator<'a> {
         match &*fn_decl.inputs {
             [Param { ty, span, .. }] => {
                 if let TyKind::CVarArgs = ty.kind {
-                    self.err_handler().span_err(
-                        *span,
-                        "C-variadic function must be declared with at least one named argument",
-                    );
+                    self.session.emit_err(FnParamCVarArgsOnly { span: *span });
                 }
             }
             [ps @ .., _] => {
                 for Param { ty, span, .. } in ps {
                     if let TyKind::CVarArgs = ty.kind {
-                        self.err_handler().span_err(
-                            *span,
-                            "`...` must be the last argument of a C-variadic function",
-                        );
+                        self.session.emit_err(FnParamCVarArgsNotLast { span: *span });
                     }
                 }
             }
@@ -435,19 +369,9 @@ impl<'a> AstValidator<'a> {
             })
             .for_each(|attr| {
                 if attr.is_doc_comment() {
-                    self.err_handler()
-                        .struct_span_err(
-                            attr.span,
-                            "documentation comments cannot be applied to function parameters",
-                        )
-                        .span_label(attr.span, "doc comments are not allowed here")
-                        .emit();
+                    self.session.emit_err(FnParamDocComment { span: attr.span });
                 } else {
-                    self.err_handler().span_err(
-                        attr.span,
-                        "allow, cfg, cfg_attr, deny, expect, \
-                forbid, and warn are the only allowed built-in attributes in function parameters",
-                    );
+                    self.session.emit_err(FnParamForbiddenAttr { span: attr.span });
                 }
             });
     }
@@ -455,14 +379,7 @@ impl<'a> AstValidator<'a> {
     fn check_decl_self_param(&self, fn_decl: &FnDecl, self_semantic: SelfSemantic) {
         if let (SelfSemantic::No, [param, ..]) = (self_semantic, &*fn_decl.inputs) {
             if param.is_self() {
-                self.err_handler()
-                    .struct_span_err(
-                        param.span,
-                        "`self` parameter is only allowed in associated functions",
-                    )
-                    .span_label(param.span, "not semantically valid as function parameter")
-                    .note("associated functions are those in `impl` or `trait` definitions")
-                    .emit();
+                self.session.emit_err(FnParamForbiddenSelf { span: param.span });
             }
         }
     }
@@ -470,47 +387,20 @@ impl<'a> AstValidator<'a> {
     fn check_defaultness(&self, span: Span, defaultness: Defaultness) {
         if let Defaultness::Default(def_span) = defaultness {
             let span = self.session.source_map().guess_head_span(span);
-            self.err_handler()
-                .struct_span_err(span, "`default` is only allowed on items in trait impls")
-                .span_label(def_span, "`default` because of this")
-                .emit();
+            self.session.emit_err(ForbiddenDefault { span, def_span });
         }
     }
 
-    fn error_item_without_body(&self, sp: Span, ctx: &str, msg: &str, sugg: &str) {
-        self.error_item_without_body_with_help(sp, ctx, msg, sugg, |_| ());
-    }
-
-    fn error_item_without_body_with_help(
-        &self,
-        sp: Span,
-        ctx: &str,
-        msg: &str,
-        sugg: &str,
-        help: impl FnOnce(&mut DiagnosticBuilder<'_, ErrorGuaranteed>),
-    ) {
+    /// If `sp` ends with a semicolon, returns it as a `Span`
+    /// Otherwise, returns `sp.shrink_to_hi()`
+    fn ending_semi_or_hi(&self, sp: Span) -> Span {
         let source_map = self.session.source_map();
         let end = source_map.end_point(sp);
-        let replace_span = if source_map.span_to_snippet(end).map(|s| s == ";").unwrap_or(false) {
+
+        if source_map.span_to_snippet(end).map(|s| s == ";").unwrap_or(false) {
             end
         } else {
             sp.shrink_to_hi()
-        };
-        let mut err = self.err_handler().struct_span_err(sp, msg);
-        err.span_suggestion(
-            replace_span,
-            &format!("provide a definition for the {}", ctx),
-            sugg,
-            Applicability::HasPlaceholders,
-        );
-        help(&mut err);
-        err.emit();
-    }
-
-    fn check_impl_item_provided<T>(&self, sp: Span, body: &Option<T>, ctx: &str, sugg: &str) {
-        if body.is_none() {
-            let msg = format!("associated {} in `impl` without body", ctx);
-            self.error_item_without_body(sp, ctx, &msg, sugg);
         }
     }
 
@@ -947,10 +837,10 @@ fn validate_generic_param_order(
         let (kind, bounds, span) = (&param.kind, &param.bounds, ident.span);
         let (ord_kind, ident) = match &param.kind {
             GenericParamKind::Lifetime => (ParamKindOrd::Lifetime, ident.to_string()),
-            GenericParamKind::Type { default: _ } => (ParamKindOrd::Type, ident.to_string()),
+            GenericParamKind::Type { default: _ } => (ParamKindOrd::TypeOrConst, ident.to_string()),
             GenericParamKind::Const { ref ty, kw_span: _, default: _ } => {
                 let ty = pprust::ty_to_string(ty);
-                (ParamKindOrd::Const, format!("const {}: {}", ident, ty))
+                (ParamKindOrd::TypeOrConst, format!("const {}: {}", ident, ty))
             }
         };
         param_idents.push((kind, ord_kind, bounds, idx, ident));
@@ -1180,7 +1070,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 
                 self.invalid_visibility(
                     &item.vis,
-                    Some("place qualifiers on individual impl items instead"),
+                    Some(InvalidVisibilityNote::IndividualImplItems),
                 );
                 if let Unsafe::Yes(span) = unsafety {
                     error(span, "unsafe").code(error_code!(E0197)).emit();
@@ -1203,37 +1093,23 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 self.check_defaultness(item.span, defaultness);
 
                 if body.is_none() {
-                    let msg = "free function without a body";
-                    let ext = sig.header.ext;
-
-                    let f = |e: &mut DiagnosticBuilder<'_, _>| {
-                        if let Extern::Implicit(start_span) | Extern::Explicit(_, start_span) = &ext
-                        {
-                            let start_suggestion = if let Extern::Explicit(abi, _) = ext {
-                                format!("extern \"{}\" {{", abi.symbol_unescaped)
-                            } else {
-                                "extern {".to_owned()
-                            };
-
-                            let end_suggestion = " }".to_owned();
-                            let end_span = item.span.shrink_to_hi();
-
-                            e
-                            .multipart_suggestion(
-                                "if you meant to declare an externally defined function, use an `extern` block",
-                                vec![(*start_span, start_suggestion), (end_span, end_suggestion)],
-                                Applicability::MaybeIncorrect,
-                             );
-                        }
-                    };
-
-                    self.error_item_without_body_with_help(
-                        item.span,
-                        "function",
-                        msg,
-                        " { <body> }",
-                        f,
-                    );
+                    self.session.emit_err(FnWithoutBody {
+                        span: item.span,
+                        replace_span: self.ending_semi_or_hi(item.span),
+                        extern_block_suggestion: match sig.header.ext {
+                            Extern::None => None,
+                            Extern::Implicit(start_span) => Some(ExternBlockSuggestion {
+                                start_span,
+                                end_span: item.span.shrink_to_hi(),
+                                abi: None,
+                            }),
+                            Extern::Explicit(abi, start_span) => Some(ExternBlockSuggestion {
+                                start_span,
+                                end_span: item.span.shrink_to_hi(),
+                                abi: Some(abi.symbol_unescaped),
+                            }),
+                        },
+                    });
                 }
 
                 self.visit_vis(&item.vis);
@@ -1248,7 +1124,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 let old_item = mem::replace(&mut self.extern_mod, Some(item));
                 self.invalid_visibility(
                     &item.vis,
-                    Some("place qualifiers on individual foreign items instead"),
+                    Some(InvalidVisibilityNote::IndividualForeignItems),
                 );
                 if let Unsafe::Yes(span) = unsafety {
                     self.err_handler().span_err(span, "extern block cannot be declared unsafe");
@@ -1339,12 +1215,16 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             }
             ItemKind::Const(def, .., None) => {
                 self.check_defaultness(item.span, def);
-                let msg = "free constant item without body";
-                self.error_item_without_body(item.span, "constant", msg, " = <expr>;");
+                self.session.emit_err(ConstWithoutBody {
+                    span: item.span,
+                    replace_span: self.ending_semi_or_hi(item.span),
+                });
             }
             ItemKind::Static(.., None) => {
-                let msg = "free static item without body";
-                self.error_item_without_body(item.span, "static", msg, " = <expr>;");
+                self.session.emit_err(StaticWithoutBody {
+                    span: item.span,
+                    replace_span: self.ending_semi_or_hi(item.span),
+                });
             }
             ItemKind::TyAlias(box TyAlias {
                 defaultness,
@@ -1355,8 +1235,10 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             }) => {
                 self.check_defaultness(item.span, defaultness);
                 if ty.is_none() {
-                    let msg = "free type alias without body";
-                    self.error_item_without_body(item.span, "type", msg, " = <type>;");
+                    self.session.emit_err(TyAliasWithoutBody {
+                        span: item.span,
+                        replace_span: self.ending_semi_or_hi(item.span),
+                    });
                 }
                 self.check_type_no_bounds(bounds, "this context");
                 if where_clauses.1.0 {
@@ -1409,7 +1291,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
     }
 
     // Mirrors `visit::walk_generic_args`, but tracks relevant state.
-    fn visit_generic_args(&mut self, _: Span, generic_args: &'a GenericArgs) {
+    fn visit_generic_args(&mut self, generic_args: &'a GenericArgs) {
         match *generic_args {
             GenericArgs::AngleBracketed(ref data) => {
                 self.check_generic_args_before_constraints(data);
@@ -1548,25 +1430,17 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
         visit::walk_param_bound(self, bound)
     }
 
-    fn visit_poly_trait_ref(&mut self, t: &'a PolyTraitRef, m: &'a TraitBoundModifier) {
+    fn visit_poly_trait_ref(&mut self, t: &'a PolyTraitRef) {
         self.check_late_bound_lifetime_defs(&t.bound_generic_params);
-        visit::walk_poly_trait_ref(self, t, m);
+        visit::walk_poly_trait_ref(self, t);
     }
 
     fn visit_variant_data(&mut self, s: &'a VariantData) {
         self.with_banned_assoc_ty_bound(|this| visit::walk_struct_def(this, s))
     }
 
-    fn visit_enum_def(
-        &mut self,
-        enum_definition: &'a EnumDef,
-        generics: &'a Generics,
-        item_id: NodeId,
-        _: Span,
-    ) {
-        self.with_banned_assoc_ty_bound(|this| {
-            visit::walk_enum_def(this, enum_definition, generics, item_id)
-        })
+    fn visit_enum_def(&mut self, enum_definition: &'a EnumDef) {
+        self.with_banned_assoc_ty_bound(|this| visit::walk_enum_def(this, enum_definition))
     }
 
     fn visit_fn(&mut self, fk: FnKind<'a>, span: Span, id: NodeId) {
@@ -1653,7 +1527,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             matches!(fk.header(), Some(FnHeader { constness: Const::Yes(_), .. }))
                 || matches!(fk.ctxt(), Some(FnCtxt::Assoc(_)));
 
-        self.with_tilde_const(tilde_const_allowed, |this| visit::walk_fn(this, fk, span));
+        self.with_tilde_const(tilde_const_allowed, |this| visit::walk_fn(this, fk));
     }
 
     fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
@@ -1668,10 +1542,20 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
         if ctxt == AssocCtxt::Impl {
             match &item.kind {
                 AssocItemKind::Const(_, _, body) => {
-                    self.check_impl_item_provided(item.span, body, "constant", " = <expr>;");
+                    if body.is_none() {
+                        self.session.emit_err(AssocConstWithoutBody {
+                            span: item.span,
+                            replace_span: self.ending_semi_or_hi(item.span),
+                        });
+                    }
                 }
                 AssocItemKind::Fn(box Fn { body, .. }) => {
-                    self.check_impl_item_provided(item.span, body, "function", " { <body> }");
+                    if body.is_none() {
+                        self.session.emit_err(AssocFnWithoutBody {
+                            span: item.span,
+                            replace_span: self.ending_semi_or_hi(item.span),
+                        });
+                    }
                 }
                 AssocItemKind::TyAlias(box TyAlias {
                     generics,
@@ -1681,7 +1565,12 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     ty,
                     ..
                 }) => {
-                    self.check_impl_item_provided(item.span, ty, "type", " = <type>;");
+                    if ty.is_none() {
+                        self.session.emit_err(AssocTypeWithoutBody {
+                            span: item.span,
+                            replace_span: self.ending_semi_or_hi(item.span),
+                        });
+                    }
                     self.check_type_no_bounds(bounds, "`impl`s");
                     if ty.is_some() {
                         self.check_gat_where(
@@ -1699,7 +1588,6 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             self.invalid_visibility(&item.vis, None);
             if let AssocItemKind::Fn(box Fn { sig, .. }) = &item.kind {
                 self.check_trait_fn_not_const(sig.header.constness);
-                self.check_trait_fn_not_async(item.span, sig.header.asyncness);
             }
         }
 
@@ -1896,14 +1784,14 @@ pub fn check_crate(session: &Session, krate: &Crate, lints: &mut LintBuffer) ->
 
 /// Used to forbid `let` expressions in certain syntactic locations.
 #[derive(Clone, Copy)]
-enum ForbiddenLetReason {
+pub(crate) enum ForbiddenLetReason {
     /// `let` is not valid and the source environment is not important
     GenericForbidden,
     /// A let chain with the `||` operator
     NotSupportedOr(Span),
     /// A let chain with invalid parentheses
     ///
-    /// For exemple, `let 1 = 1 && (expr && expr)` is allowed
+    /// For example, `let 1 = 1 && (expr && expr)` is allowed
     /// but `(let 1 = 1 && (let 1 = 1 && (let 1 = 1))) && let a = 1` is not
     NotSupportedParentheses(Span),
 }
diff --git a/compiler/rustc_ast_passes/src/errors.rs b/compiler/rustc_ast_passes/src/errors.rs
new file mode 100644
index 000000000..4f3b09c58
--- /dev/null
+++ b/compiler/rustc_ast_passes/src/errors.rs
@@ -0,0 +1,245 @@
+//! Errors emitted by ast_passes.
+
+use rustc_errors::{fluent, AddSubdiagnostic, Applicability, Diagnostic};
+use rustc_macros::{SessionDiagnostic, SessionSubdiagnostic};
+use rustc_span::{Span, Symbol};
+
+use crate::ast_validation::ForbiddenLetReason;
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::forbidden_let)]
+#[note]
+pub struct ForbiddenLet {
+    #[primary_span]
+    pub span: Span,
+    #[subdiagnostic]
+    pub(crate) reason: ForbiddenLetReason,
+}
+
+impl AddSubdiagnostic for ForbiddenLetReason {
+    fn add_to_diagnostic(self, diag: &mut Diagnostic) {
+        match self {
+            Self::GenericForbidden => {}
+            Self::NotSupportedOr(span) => {
+                diag.span_note(span, fluent::ast_passes::not_supported_or);
+            }
+            Self::NotSupportedParentheses(span) => {
+                diag.span_note(span, fluent::ast_passes::not_supported_parentheses);
+            }
+        }
+    }
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::forbidden_let_stable)]
+#[note]
+pub struct ForbiddenLetStable {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::forbidden_assoc_constraint)]
+pub struct ForbiddenAssocConstraint {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::keyword_lifetime)]
+pub struct KeywordLifetime {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::invalid_label)]
+pub struct InvalidLabel {
+    #[primary_span]
+    pub span: Span,
+    pub name: Symbol,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::invalid_visibility, code = "E0449")]
+pub struct InvalidVisibility {
+    #[primary_span]
+    pub span: Span,
+    #[label(ast_passes::implied)]
+    pub implied: Option<Span>,
+    #[subdiagnostic]
+    pub note: Option<InvalidVisibilityNote>,
+}
+
+#[derive(SessionSubdiagnostic)]
+pub enum InvalidVisibilityNote {
+    #[note(ast_passes::individual_impl_items)]
+    IndividualImplItems,
+    #[note(ast_passes::individual_foreign_items)]
+    IndividualForeignItems,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::trait_fn_const, code = "E0379")]
+pub struct TraitFnConst {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::forbidden_lifetime_bound)]
+pub struct ForbiddenLifetimeBound {
+    #[primary_span]
+    pub spans: Vec<Span>,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::forbidden_non_lifetime_param)]
+pub struct ForbiddenNonLifetimeParam {
+    #[primary_span]
+    pub spans: Vec<Span>,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::fn_param_too_many)]
+pub struct FnParamTooMany {
+    #[primary_span]
+    pub span: Span,
+    pub max_num_args: usize,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::fn_param_c_var_args_only)]
+pub struct FnParamCVarArgsOnly {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::fn_param_c_var_args_not_last)]
+pub struct FnParamCVarArgsNotLast {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::fn_param_doc_comment)]
+pub struct FnParamDocComment {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::fn_param_forbidden_attr)]
+pub struct FnParamForbiddenAttr {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::fn_param_forbidden_self)]
+#[note]
+pub struct FnParamForbiddenSelf {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::forbidden_default)]
+pub struct ForbiddenDefault {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub def_span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::assoc_const_without_body)]
+pub struct AssocConstWithoutBody {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = " = <expr>;", applicability = "has-placeholders")]
+    pub replace_span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::assoc_fn_without_body)]
+pub struct AssocFnWithoutBody {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = " {{ <body> }}", applicability = "has-placeholders")]
+    pub replace_span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::assoc_type_without_body)]
+pub struct AssocTypeWithoutBody {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = " = <type>;", applicability = "has-placeholders")]
+    pub replace_span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::const_without_body)]
+pub struct ConstWithoutBody {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = " = <expr>;", applicability = "has-placeholders")]
+    pub replace_span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::static_without_body)]
+pub struct StaticWithoutBody {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = " = <expr>;", applicability = "has-placeholders")]
+    pub replace_span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::ty_alias_without_body)]
+pub struct TyAliasWithoutBody {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = " = <type>;", applicability = "has-placeholders")]
+    pub replace_span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::fn_without_body)]
+pub struct FnWithoutBody {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = " {{ <body> }}", applicability = "has-placeholders")]
+    pub replace_span: Span,
+    #[subdiagnostic]
+    pub extern_block_suggestion: Option<ExternBlockSuggestion>,
+}
+
+pub struct ExternBlockSuggestion {
+    pub start_span: Span,
+    pub end_span: Span,
+    pub abi: Option<Symbol>,
+}
+
+impl AddSubdiagnostic for ExternBlockSuggestion {
+    fn add_to_diagnostic(self, diag: &mut Diagnostic) {
+        let start_suggestion = if let Some(abi) = self.abi {
+            format!("extern \"{}\" {{", abi)
+        } else {
+            "extern {".to_owned()
+        };
+        let end_suggestion = " }".to_owned();
+
+        diag.multipart_suggestion(
+            fluent::ast_passes::extern_block_suggestion,
+            vec![(self.start_span, start_suggestion), (self.end_span, end_suggestion)],
+            Applicability::MaybeIncorrect,
+        );
+    }
+}
diff --git a/compiler/rustc_ast_passes/src/feature_gate.rs b/compiler/rustc_ast_passes/src/feature_gate.rs
index 789eca1f0..aeff73c5b 100644
--- a/compiler/rustc_ast_passes/src/feature_gate.rs
+++ b/compiler/rustc_ast_passes/src/feature_gate.rs
@@ -2,17 +2,15 @@ use rustc_ast as ast;
 use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
 use rustc_ast::{AssocConstraint, AssocConstraintKind, NodeId};
 use rustc_ast::{PatKind, RangeEnd, VariantData};
-use rustc_errors::{struct_span_err, Applicability};
+use rustc_errors::{struct_span_err, Applicability, StashKey};
+use rustc_feature::Features;
 use rustc_feature::{AttributeGate, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP};
-use rustc_feature::{Features, GateIssue};
-use rustc_session::parse::{feature_err, feature_err_issue};
+use rustc_session::parse::{feature_err, feature_warn};
 use rustc_session::Session;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::sym;
 use rustc_span::Span;
 
-use tracing::debug;
-
 macro_rules! gate_feature_fn {
     ($visitor: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr, $help: expr) => {{
         let (visitor, has_feature, span, name, explain, help) =
@@ -20,9 +18,7 @@ macro_rules! gate_feature_fn {
         let has_feature: bool = has_feature(visitor.features);
         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
         if !has_feature && !span.allows_unstable($name) {
-            feature_err_issue(&visitor.sess.parse_sess, name, span, GateIssue::Language, explain)
-                .help(help)
-                .emit();
+            feature_err(&visitor.sess.parse_sess, name, span, explain).help(help).emit();
         }
     }};
     ($visitor: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr) => {{
@@ -31,8 +27,19 @@ macro_rules! gate_feature_fn {
         let has_feature: bool = has_feature(visitor.features);
         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
         if !has_feature && !span.allows_unstable($name) {
-            feature_err_issue(&visitor.sess.parse_sess, name, span, GateIssue::Language, explain)
-                .emit();
+            feature_err(&visitor.sess.parse_sess, name, span, explain).emit();
+        }
+    }};
+    (future_incompatible; $visitor: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr) => {{
+        let (visitor, has_feature, span, name, explain) =
+            (&*$visitor, $has_feature, $span, $name, $explain);
+        let has_feature: bool = has_feature(visitor.features);
+        debug!(
+            "gate_feature(feature = {:?}, span = {:?}); has? {} (future_incompatible)",
+            name, span, has_feature
+        );
+        if !has_feature && !span.allows_unstable($name) {
+            feature_warn(&visitor.sess.parse_sess, name, span, explain);
         }
     }};
 }
@@ -44,6 +51,9 @@ macro_rules! gate_feature_post {
     ($visitor: expr, $feature: ident, $span: expr, $explain: expr) => {
         gate_feature_fn!($visitor, |x: &Features| x.$feature, $span, sym::$feature, $explain)
     };
+    (future_incompatible; $visitor: expr, $feature: ident, $span: expr, $explain: expr) => {
+        gate_feature_fn!(future_incompatible; $visitor, |x: &Features| x.$feature, $span, sym::$feature, $explain)
+    };
 }
 
 pub fn check_attribute(attr: &ast::Attribute, sess: &Session, features: &Features) {
@@ -330,25 +340,6 @@ impl<'a> PostExpansionVisitor<'a> {
         }
     }
 
-    fn check_gat(&self, generics: &ast::Generics, span: Span) {
-        if !generics.params.is_empty() {
-            gate_feature_post!(
-                &self,
-                generic_associated_types,
-                span,
-                "generic associated types are unstable"
-            );
-        }
-        if !generics.where_clause.predicates.is_empty() {
-            gate_feature_post!(
-                &self,
-                generic_associated_types,
-                span,
-                "where clauses on associated types are unstable"
-            );
-        }
-    }
-
     /// Feature gate `impl Trait` inside `type Alias = $type_expr;`.
     fn check_impl_trait(&self, ty: &ast::Ty) {
         struct ImplTraitVisitor<'a> {
@@ -417,6 +408,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 || attr.has_name(sym::stable)
                 || attr.has_name(sym::rustc_const_unstable)
                 || attr.has_name(sym::rustc_const_stable)
+                || attr.has_name(sym::rustc_default_body_unstable)
             {
                 struct_span_err!(
                     self.sess,
@@ -562,6 +554,9 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             ast::TyKind::Never => {
                 gate_feature_post!(&self, never_type, ty.span, "the `!` type is experimental");
             }
+            ast::TyKind::TraitObject(_, ast::TraitObjectSyntax::DynStar, ..) => {
+                gate_feature_post!(&self, dyn_star, ty.span, "dyn* trait objects are unstable");
+            }
             _ => {}
         }
         visit::walk_ty(self, ty)
@@ -587,11 +582,10 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         {
             // When we encounter a statement of the form `foo: Ty = val;`, this will emit a type
             // ascription error, but the likely intention was to write a `let` statement. (#78907).
-            feature_err_issue(
+            feature_err(
                 &self.sess.parse_sess,
                 sym::type_ascription,
                 lhs.span,
-                GateIssue::Language,
                 "type ascription is experimental",
             ).span_suggestion_verbose(
                 lhs.span.shrink_to_lo(),
@@ -614,28 +608,27 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 );
             }
             ast::ExprKind::Type(..) => {
-                // To avoid noise about type ascription in common syntax errors, only emit if it
-                // is the *only* error.
                 if self.sess.parse_sess.span_diagnostic.err_count() == 0 {
+                    // To avoid noise about type ascription in common syntax errors,
+                    // only emit if it is the *only* error.
                     gate_feature_post!(
                         &self,
                         type_ascription,
                         e.span,
                         "type ascription is experimental"
                     );
+                } else {
+                    // And if it isn't, cancel the early-pass warning.
+                    self.sess
+                        .parse_sess
+                        .span_diagnostic
+                        .steal_diagnostic(e.span, StashKey::EarlySyntaxWarning)
+                        .map(|err| err.cancel());
                 }
             }
             ast::ExprKind::TryBlock(_) => {
                 gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
             }
-            ast::ExprKind::Block(_, Some(label)) => {
-                gate_feature_post!(
-                    &self,
-                    label_break_value,
-                    label.ident.span,
-                    "labels on blocks are unstable"
-                );
-            }
             _ => {}
         }
         visit::walk_expr(self, e)
@@ -690,7 +683,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             gate_feature_post!(&self, c_variadic, span, "C-variadic functions are unstable");
         }
 
-        visit::walk_fn(self, fn_kind, span)
+        visit::walk_fn(self, fn_kind)
     }
 
     fn visit_assoc_constraint(&mut self, constraint: &'a AssocConstraint) {
@@ -708,7 +701,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
     fn visit_assoc_item(&mut self, i: &'a ast::AssocItem, ctxt: AssocCtxt) {
         let is_fn = match i.kind {
             ast::AssocItemKind::Fn(_) => true,
-            ast::AssocItemKind::TyAlias(box ast::TyAlias { ref generics, ref ty, .. }) => {
+            ast::AssocItemKind::TyAlias(box ast::TyAlias { ref ty, .. }) => {
                 if let (Some(_), AssocCtxt::Trait) = (ty, ctxt) {
                     gate_feature_post!(
                         &self,
@@ -720,7 +713,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 if let Some(ty) = ty {
                     self.check_impl_trait(ty);
                 }
-                self.check_gat(generics, i.span);
                 false
             }
             _ => false,
@@ -789,14 +781,12 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).
+    // We emit an early future-incompatible warning for these.
+    // New syntax gates should go above here to get a hard error gate.
     macro_rules! gate_all {
         ($gate:ident, $msg:literal) => {
-            // FIXME(eddyb) do something more useful than always
-            // disabling these uses of early feature-gatings.
-            if false {
-                for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
-                    gate_feature_post!(&visitor, $gate, *span, $msg);
-                }
+            for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
+                gate_feature_post!(future_incompatible; &visitor, $gate, *span, $msg);
             }
         };
     }
@@ -807,13 +797,8 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(box_patterns, "box pattern syntax is experimental");
     gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");
     gate_all!(try_blocks, "`try` blocks are unstable");
-    gate_all!(label_break_value, "labels on blocks are unstable");
     gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
-    // To avoid noise about type ascription in common syntax errors,
-    // only emit if it is the *only* error. (Also check it last.)
-    if sess.parse_sess.span_diagnostic.err_count() == 0 {
-        gate_all!(type_ascription, "type ascription is experimental");
-    }
+    gate_all!(type_ascription, "type ascription is experimental");
 
     visit::walk_crate(&mut visitor, krate);
 }
diff --git a/compiler/rustc_ast_passes/src/lib.rs b/compiler/rustc_ast_passes/src/lib.rs
index 9d52c3288..8aa9d57f0 100644
--- a/compiler/rustc_ast_passes/src/lib.rs
+++ b/compiler/rustc_ast_passes/src/lib.rs
@@ -9,10 +9,14 @@
 #![feature(if_let_guard)]
 #![feature(iter_is_partitioned)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![recursion_limit = "256"]
 
+#[macro_use]
+extern crate tracing;
+
 pub mod ast_validation;
+mod errors;
 pub mod feature_gate;
 pub mod node_count;
 pub mod show_span;
diff --git a/compiler/rustc_ast_passes/src/node_count.rs b/compiler/rustc_ast_passes/src/node_count.rs
index 9c7369c83..fa42f8778 100644
--- a/compiler/rustc_ast_passes/src/node_count.rs
+++ b/compiler/rustc_ast_passes/src/node_count.rs
@@ -63,9 +63,9 @@ impl<'ast> Visitor<'ast> for NodeCounter {
         self.count += 1;
         walk_generics(self, g)
     }
-    fn visit_fn(&mut self, fk: visit::FnKind<'_>, s: Span, _: NodeId) {
+    fn visit_fn(&mut self, fk: visit::FnKind<'_>, _: Span, _: NodeId) {
         self.count += 1;
-        walk_fn(self, fk, s)
+        walk_fn(self, fk)
     }
     fn visit_assoc_item(&mut self, ti: &AssocItem, ctxt: AssocCtxt) {
         self.count += 1;
@@ -79,9 +79,9 @@ impl<'ast> Visitor<'ast> for NodeCounter {
         self.count += 1;
         walk_param_bound(self, bounds)
     }
-    fn visit_poly_trait_ref(&mut self, t: &PolyTraitRef, m: &TraitBoundModifier) {
+    fn visit_poly_trait_ref(&mut self, t: &PolyTraitRef) {
         self.count += 1;
-        walk_poly_trait_ref(self, t, m)
+        walk_poly_trait_ref(self, t)
     }
     fn visit_variant_data(&mut self, s: &VariantData) {
         self.count += 1;
@@ -91,15 +91,9 @@ impl<'ast> Visitor<'ast> for NodeCounter {
         self.count += 1;
         walk_field_def(self, s)
     }
-    fn visit_enum_def(
-        &mut self,
-        enum_definition: &EnumDef,
-        generics: &Generics,
-        item_id: NodeId,
-        _: Span,
-    ) {
+    fn visit_enum_def(&mut self, enum_definition: &EnumDef) {
         self.count += 1;
-        walk_enum_def(self, enum_definition, generics, item_id)
+        walk_enum_def(self, enum_definition)
     }
     fn visit_variant(&mut self, v: &Variant) {
         self.count += 1;
@@ -121,9 +115,9 @@ impl<'ast> Visitor<'ast> for NodeCounter {
         self.count += 1;
         walk_use_tree(self, use_tree, id)
     }
-    fn visit_generic_args(&mut self, path_span: Span, generic_args: &GenericArgs) {
+    fn visit_generic_args(&mut self, generic_args: &GenericArgs) {
         self.count += 1;
-        walk_generic_args(self, path_span, generic_args)
+        walk_generic_args(self, generic_args)
     }
     fn visit_assoc_constraint(&mut self, constraint: &AssocConstraint) {
         self.count += 1;
diff --git a/compiler/rustc_ast_pretty/src/lib.rs b/compiler/rustc_ast_pretty/src/lib.rs
index 79178830b..bf094af5f 100644
--- a/compiler/rustc_ast_pretty/src/lib.rs
+++ b/compiler/rustc_ast_pretty/src/lib.rs
@@ -1,3 +1,5 @@
+#![deny(rustc::untranslatable_diagnostic)]
+#![deny(rustc::diagnostic_outside_of_impl)]
 #![feature(associated_type_bounds)]
 #![feature(box_patterns)]
 #![feature(with_negative_coherence)]
diff --git a/compiler/rustc_ast_pretty/src/pprust/state.rs b/compiler/rustc_ast_pretty/src/pprust/state.rs
index 5eb7bf634..b87c6f78d 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/state.rs
@@ -11,8 +11,8 @@ use rustc_ast::tokenstream::{TokenStream, TokenTree};
 use rustc_ast::util::classify;
 use rustc_ast::util::comments::{gather_comments, Comment, CommentStyle};
 use rustc_ast::util::parser;
-use rustc_ast::{self as ast, BlockCheckMode, PatKind, RangeEnd, RangeSyntax};
-use rustc_ast::{attr, Term};
+use rustc_ast::{self as ast, BlockCheckMode, Mutability, PatKind, RangeEnd, RangeSyntax};
+use rustc_ast::{attr, BindingAnnotation, ByRef, Term};
 use rustc_ast::{GenericArg, MacArgs, MacArgsEq};
 use rustc_ast::{GenericBound, SelfKind, TraitBoundModifier};
 use rustc_ast::{InlineAsmOperand, InlineAsmRegOrRegClass};
@@ -22,6 +22,7 @@ use rustc_span::source_map::{SourceMap, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, IdentPrinter, Symbol};
 use rustc_span::{BytePos, FileName, Span};
 
+use rustc_ast::attr::AttrIdGenerator;
 use std::borrow::Cow;
 
 pub use self::delimited::IterDelimited;
@@ -107,6 +108,7 @@ pub fn print_crate<'a>(
     ann: &'a dyn PpAnn,
     is_expanded: bool,
     edition: Edition,
+    g: &AttrIdGenerator,
 ) -> String {
     let mut s =
         State { s: pp::Printer::new(), comments: Some(Comments::new(sm, filename, input)), ann };
@@ -120,7 +122,7 @@ pub fn print_crate<'a>(
         // `#![feature(prelude_import)]`
         let pi_nested = attr::mk_nested_word_item(Ident::with_dummy_span(sym::prelude_import));
         let list = attr::mk_list_item(Ident::with_dummy_span(sym::feature), vec![pi_nested]);
-        let fake_attr = attr::mk_attr_inner(list);
+        let fake_attr = attr::mk_attr_inner(g, list);
         s.print_attribute(&fake_attr);
 
         // Currently, in Rust 2018 we don't have `extern crate std;` at the crate
@@ -128,7 +130,7 @@ pub fn print_crate<'a>(
         if edition == Edition::Edition2015 {
             // `#![no_std]`
             let no_std_meta = attr::mk_word_item(Ident::with_dummy_span(sym::no_std));
-            let fake_attr = attr::mk_attr_inner(no_std_meta);
+            let fake_attr = attr::mk_attr_inner(g, no_std_meta);
             s.print_attribute(&fake_attr);
         }
     }
@@ -372,7 +374,7 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
 
     fn print_literal(&mut self, lit: &ast::Lit) {
         self.maybe_print_comment(lit.span.lo());
-        self.word(lit.token.to_string())
+        self.word(lit.token_lit.to_string())
     }
 
     fn print_string(&mut self, st: &str, style: ast::StrStyle) {
@@ -442,12 +444,12 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
         }
         self.maybe_print_comment(attr.span.lo());
         match attr.kind {
-            ast::AttrKind::Normal(ref item, _) => {
+            ast::AttrKind::Normal(ref normal) => {
                 match attr.style {
                     ast::AttrStyle::Inner => self.word("#!["),
                     ast::AttrStyle::Outer => self.word("#["),
                 }
-                self.print_attr_item(&item, attr.span);
+                self.print_attr_item(&normal.item, attr.span);
                 self.word("]");
             }
             ast::AttrKind::DocComment(comment_kind, data) => {
@@ -1399,16 +1401,12 @@ impl<'a> State<'a> {
         is that it doesn't matter */
         match pat.kind {
             PatKind::Wild => self.word("_"),
-            PatKind::Ident(binding_mode, ident, ref sub) => {
-                match binding_mode {
-                    ast::BindingMode::ByRef(mutbl) => {
-                        self.word_nbsp("ref");
-                        self.print_mutability(mutbl, false);
-                    }
-                    ast::BindingMode::ByValue(ast::Mutability::Not) => {}
-                    ast::BindingMode::ByValue(ast::Mutability::Mut) => {
-                        self.word_nbsp("mut");
-                    }
+            PatKind::Ident(BindingAnnotation(by_ref, mutbl), ident, ref sub) => {
+                if by_ref == ByRef::Yes {
+                    self.word_nbsp("ref");
+                }
+                if mutbl == Mutability::Mut {
+                    self.word_nbsp("mut");
                 }
                 self.print_ident(ident);
                 if let Some(ref p) = *sub {
@@ -1487,12 +1485,10 @@ impl<'a> State<'a> {
             }
             PatKind::Ref(ref inner, mutbl) => {
                 self.word("&");
-                if mutbl == ast::Mutability::Mut {
+                if mutbl == Mutability::Mut {
                     self.word("mut ");
                 }
-                if let PatKind::Ident(ast::BindingMode::ByValue(ast::Mutability::Mut), ..) =
-                    inner.kind
-                {
+                if let PatKind::Ident(ast::BindingAnnotation::MUT, ..) = inner.kind {
                     self.popen();
                     self.print_pat(inner);
                     self.pclose();
diff --git a/compiler/rustc_ast_pretty/src/pprust/state/item.rs b/compiler/rustc_ast_pretty/src/pprust/state/item.rs
index f1caf22f3..54bac29a6 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state/item.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/state/item.rs
@@ -218,6 +218,8 @@ impl<'a> State<'a> {
             ast::ItemKind::GlobalAsm(ref asm) => {
                 self.head(visibility_qualified(&item.vis, "global_asm!"));
                 self.print_inline_asm(asm);
+                self.word(";");
+                self.end();
                 self.end();
             }
             ast::ItemKind::TyAlias(box ast::TyAlias {
@@ -412,9 +414,9 @@ impl<'a> State<'a> {
     pub(crate) fn print_visibility(&mut self, vis: &ast::Visibility) {
         match vis.kind {
             ast::VisibilityKind::Public => self.word_nbsp("pub"),
-            ast::VisibilityKind::Restricted { ref path, .. } => {
+            ast::VisibilityKind::Restricted { ref path, id: _, shorthand } => {
                 let path = Self::to_string(|s| s.print_path(path, false, 0));
-                if path == "crate" || path == "self" || path == "super" {
+                if shorthand && (path == "crate" || path == "self" || path == "super") {
                     self.word_nbsp(format!("pub({})", path))
                 } else {
                     self.word_nbsp(format!("pub(in {})", path))