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

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

20 files changed, 11703 insertions, 0 deletions

diff --git a/compiler/rustc_expand/src/base.rs b/compiler/rustc_expand/src/base.rs
new file mode 100644
index 000000000..6e093811f
--- /dev/null
+++ b/compiler/rustc_expand/src/base.rs
@@ -0,0 +1,1444 @@
+use crate::expand::{self, AstFragment, Invocation};
+use crate::module::DirOwnership;
+
+use rustc_ast::attr::MarkedAttrs;
+use rustc_ast::ptr::P;
+use rustc_ast::token::{self, Nonterminal};
+use rustc_ast::tokenstream::TokenStream;
+use rustc_ast::visit::{AssocCtxt, Visitor};
+use rustc_ast::{self as ast, Attribute, HasAttrs, Item, NodeId, PatKind};
+use rustc_attr::{self as attr, Deprecation, Stability};
+use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
+use rustc_data_structures::sync::{self, Lrc};
+use rustc_errors::{Applicability, DiagnosticBuilder, ErrorGuaranteed, MultiSpan, PResult};
+use rustc_lint_defs::builtin::PROC_MACRO_BACK_COMPAT;
+use rustc_lint_defs::{BufferedEarlyLint, BuiltinLintDiagnostics};
+use rustc_parse::{self, parser, MACRO_ARGUMENTS};
+use rustc_session::{parse::ParseSess, Limit, Session, SessionDiagnostic};
+use rustc_span::def_id::{CrateNum, DefId, LocalDefId};
+use rustc_span::edition::Edition;
+use rustc_span::hygiene::{AstPass, ExpnData, ExpnKind, LocalExpnId};
+use rustc_span::source_map::SourceMap;
+use rustc_span::symbol::{kw, sym, Ident, Symbol};
+use rustc_span::{FileName, Span, DUMMY_SP};
+use smallvec::{smallvec, SmallVec};
+
+use std::default::Default;
+use std::iter;
+use std::path::PathBuf;
+use std::rc::Rc;
+
+pub(crate) use rustc_span::hygiene::MacroKind;
+
+// When adding new variants, make sure to
+// adjust the `visit_*` / `flat_map_*` calls in `InvocationCollector`
+// to use `assign_id!`
+#[derive(Debug, Clone)]
+pub enum Annotatable {
+    Item(P<ast::Item>),
+    TraitItem(P<ast::AssocItem>),
+    ImplItem(P<ast::AssocItem>),
+    ForeignItem(P<ast::ForeignItem>),
+    Stmt(P<ast::Stmt>),
+    Expr(P<ast::Expr>),
+    Arm(ast::Arm),
+    ExprField(ast::ExprField),
+    PatField(ast::PatField),
+    GenericParam(ast::GenericParam),
+    Param(ast::Param),
+    FieldDef(ast::FieldDef),
+    Variant(ast::Variant),
+    Crate(ast::Crate),
+}
+
+impl Annotatable {
+    pub fn span(&self) -> Span {
+        match *self {
+            Annotatable::Item(ref item) => item.span,
+            Annotatable::TraitItem(ref trait_item) => trait_item.span,
+            Annotatable::ImplItem(ref impl_item) => impl_item.span,
+            Annotatable::ForeignItem(ref foreign_item) => foreign_item.span,
+            Annotatable::Stmt(ref stmt) => stmt.span,
+            Annotatable::Expr(ref expr) => expr.span,
+            Annotatable::Arm(ref arm) => arm.span,
+            Annotatable::ExprField(ref field) => field.span,
+            Annotatable::PatField(ref fp) => fp.pat.span,
+            Annotatable::GenericParam(ref gp) => gp.ident.span,
+            Annotatable::Param(ref p) => p.span,
+            Annotatable::FieldDef(ref sf) => sf.span,
+            Annotatable::Variant(ref v) => v.span,
+            Annotatable::Crate(ref c) => c.spans.inner_span,
+        }
+    }
+
+    pub fn visit_attrs(&mut self, f: impl FnOnce(&mut Vec<Attribute>)) {
+        match self {
+            Annotatable::Item(item) => item.visit_attrs(f),
+            Annotatable::TraitItem(trait_item) => trait_item.visit_attrs(f),
+            Annotatable::ImplItem(impl_item) => impl_item.visit_attrs(f),
+            Annotatable::ForeignItem(foreign_item) => foreign_item.visit_attrs(f),
+            Annotatable::Stmt(stmt) => stmt.visit_attrs(f),
+            Annotatable::Expr(expr) => expr.visit_attrs(f),
+            Annotatable::Arm(arm) => arm.visit_attrs(f),
+            Annotatable::ExprField(field) => field.visit_attrs(f),
+            Annotatable::PatField(fp) => fp.visit_attrs(f),
+            Annotatable::GenericParam(gp) => gp.visit_attrs(f),
+            Annotatable::Param(p) => p.visit_attrs(f),
+            Annotatable::FieldDef(sf) => sf.visit_attrs(f),
+            Annotatable::Variant(v) => v.visit_attrs(f),
+            Annotatable::Crate(c) => c.visit_attrs(f),
+        }
+    }
+
+    pub fn visit_with<'a, V: Visitor<'a>>(&'a self, visitor: &mut V) {
+        match self {
+            Annotatable::Item(item) => visitor.visit_item(item),
+            Annotatable::TraitItem(item) => visitor.visit_assoc_item(item, AssocCtxt::Trait),
+            Annotatable::ImplItem(item) => visitor.visit_assoc_item(item, AssocCtxt::Impl),
+            Annotatable::ForeignItem(foreign_item) => visitor.visit_foreign_item(foreign_item),
+            Annotatable::Stmt(stmt) => visitor.visit_stmt(stmt),
+            Annotatable::Expr(expr) => visitor.visit_expr(expr),
+            Annotatable::Arm(arm) => visitor.visit_arm(arm),
+            Annotatable::ExprField(field) => visitor.visit_expr_field(field),
+            Annotatable::PatField(fp) => visitor.visit_pat_field(fp),
+            Annotatable::GenericParam(gp) => visitor.visit_generic_param(gp),
+            Annotatable::Param(p) => visitor.visit_param(p),
+            Annotatable::FieldDef(sf) => visitor.visit_field_def(sf),
+            Annotatable::Variant(v) => visitor.visit_variant(v),
+            Annotatable::Crate(c) => visitor.visit_crate(c),
+        }
+    }
+
+    pub fn to_tokens(&self) -> TokenStream {
+        match self {
+            Annotatable::Item(node) => TokenStream::from_ast(node),
+            Annotatable::TraitItem(node) | Annotatable::ImplItem(node) => {
+                TokenStream::from_ast(node)
+            }
+            Annotatable::ForeignItem(node) => TokenStream::from_ast(node),
+            Annotatable::Stmt(node) => {
+                assert!(!matches!(node.kind, ast::StmtKind::Empty));
+                TokenStream::from_ast(node)
+            }
+            Annotatable::Expr(node) => TokenStream::from_ast(node),
+            Annotatable::Arm(..)
+            | Annotatable::ExprField(..)
+            | Annotatable::PatField(..)
+            | Annotatable::GenericParam(..)
+            | Annotatable::Param(..)
+            | Annotatable::FieldDef(..)
+            | Annotatable::Variant(..)
+            | Annotatable::Crate(..) => panic!("unexpected annotatable"),
+        }
+    }
+
+    pub fn expect_item(self) -> P<ast::Item> {
+        match self {
+            Annotatable::Item(i) => i,
+            _ => panic!("expected Item"),
+        }
+    }
+
+    pub fn expect_trait_item(self) -> P<ast::AssocItem> {
+        match self {
+            Annotatable::TraitItem(i) => i,
+            _ => panic!("expected Item"),
+        }
+    }
+
+    pub fn expect_impl_item(self) -> P<ast::AssocItem> {
+        match self {
+            Annotatable::ImplItem(i) => i,
+            _ => panic!("expected Item"),
+        }
+    }
+
+    pub fn expect_foreign_item(self) -> P<ast::ForeignItem> {
+        match self {
+            Annotatable::ForeignItem(i) => i,
+            _ => panic!("expected foreign item"),
+        }
+    }
+
+    pub fn expect_stmt(self) -> ast::Stmt {
+        match self {
+            Annotatable::Stmt(stmt) => stmt.into_inner(),
+            _ => panic!("expected statement"),
+        }
+    }
+
+    pub fn expect_expr(self) -> P<ast::Expr> {
+        match self {
+            Annotatable::Expr(expr) => expr,
+            _ => panic!("expected expression"),
+        }
+    }
+
+    pub fn expect_arm(self) -> ast::Arm {
+        match self {
+            Annotatable::Arm(arm) => arm,
+            _ => panic!("expected match arm"),
+        }
+    }
+
+    pub fn expect_expr_field(self) -> ast::ExprField {
+        match self {
+            Annotatable::ExprField(field) => field,
+            _ => panic!("expected field"),
+        }
+    }
+
+    pub fn expect_pat_field(self) -> ast::PatField {
+        match self {
+            Annotatable::PatField(fp) => fp,
+            _ => panic!("expected field pattern"),
+        }
+    }
+
+    pub fn expect_generic_param(self) -> ast::GenericParam {
+        match self {
+            Annotatable::GenericParam(gp) => gp,
+            _ => panic!("expected generic parameter"),
+        }
+    }
+
+    pub fn expect_param(self) -> ast::Param {
+        match self {
+            Annotatable::Param(param) => param,
+            _ => panic!("expected parameter"),
+        }
+    }
+
+    pub fn expect_field_def(self) -> ast::FieldDef {
+        match self {
+            Annotatable::FieldDef(sf) => sf,
+            _ => panic!("expected struct field"),
+        }
+    }
+
+    pub fn expect_variant(self) -> ast::Variant {
+        match self {
+            Annotatable::Variant(v) => v,
+            _ => panic!("expected variant"),
+        }
+    }
+
+    pub fn expect_crate(self) -> ast::Crate {
+        match self {
+            Annotatable::Crate(krate) => krate,
+            _ => panic!("expected krate"),
+        }
+    }
+}
+
+/// Result of an expansion that may need to be retried.
+/// Consider using this for non-`MultiItemModifier` expanders as well.
+pub enum ExpandResult<T, U> {
+    /// Expansion produced a result (possibly dummy).
+    Ready(T),
+    /// Expansion could not produce a result and needs to be retried.
+    Retry(U),
+}
+
+// `meta_item` is the attribute, and `item` is the item being modified.
+pub trait MultiItemModifier {
+    fn expand(
+        &self,
+        ecx: &mut ExtCtxt<'_>,
+        span: Span,
+        meta_item: &ast::MetaItem,
+        item: Annotatable,
+    ) -> ExpandResult<Vec<Annotatable>, Annotatable>;
+}
+
+impl<F> MultiItemModifier for F
+where
+    F: Fn(&mut ExtCtxt<'_>, Span, &ast::MetaItem, Annotatable) -> Vec<Annotatable>,
+{
+    fn expand(
+        &self,
+        ecx: &mut ExtCtxt<'_>,
+        span: Span,
+        meta_item: &ast::MetaItem,
+        item: Annotatable,
+    ) -> ExpandResult<Vec<Annotatable>, Annotatable> {
+        ExpandResult::Ready(self(ecx, span, meta_item, item))
+    }
+}
+
+pub trait BangProcMacro {
+    fn expand<'cx>(
+        &self,
+        ecx: &'cx mut ExtCtxt<'_>,
+        span: Span,
+        ts: TokenStream,
+    ) -> Result<TokenStream, ErrorGuaranteed>;
+}
+
+impl<F> BangProcMacro for F
+where
+    F: Fn(TokenStream) -> TokenStream,
+{
+    fn expand<'cx>(
+        &self,
+        _ecx: &'cx mut ExtCtxt<'_>,
+        _span: Span,
+        ts: TokenStream,
+    ) -> Result<TokenStream, ErrorGuaranteed> {
+        // FIXME setup implicit context in TLS before calling self.
+        Ok(self(ts))
+    }
+}
+
+pub trait AttrProcMacro {
+    fn expand<'cx>(
+        &self,
+        ecx: &'cx mut ExtCtxt<'_>,
+        span: Span,
+        annotation: TokenStream,
+        annotated: TokenStream,
+    ) -> Result<TokenStream, ErrorGuaranteed>;
+}
+
+impl<F> AttrProcMacro for F
+where
+    F: Fn(TokenStream, TokenStream) -> TokenStream,
+{
+    fn expand<'cx>(
+        &self,
+        _ecx: &'cx mut ExtCtxt<'_>,
+        _span: Span,
+        annotation: TokenStream,
+        annotated: TokenStream,
+    ) -> Result<TokenStream, ErrorGuaranteed> {
+        // FIXME setup implicit context in TLS before calling self.
+        Ok(self(annotation, annotated))
+    }
+}
+
+/// Represents a thing that maps token trees to Macro Results
+pub trait TTMacroExpander {
+    fn expand<'cx>(
+        &self,
+        ecx: &'cx mut ExtCtxt<'_>,
+        span: Span,
+        input: TokenStream,
+    ) -> Box<dyn MacResult + 'cx>;
+}
+
+pub type MacroExpanderFn =
+    for<'cx> fn(&'cx mut ExtCtxt<'_>, Span, TokenStream) -> Box<dyn MacResult + 'cx>;
+
+impl<F> TTMacroExpander for F
+where
+    F: for<'cx> Fn(&'cx mut ExtCtxt<'_>, Span, TokenStream) -> Box<dyn MacResult + 'cx>,
+{
+    fn expand<'cx>(
+        &self,
+        ecx: &'cx mut ExtCtxt<'_>,
+        span: Span,
+        input: TokenStream,
+    ) -> Box<dyn MacResult + 'cx> {
+        self(ecx, span, input)
+    }
+}
+
+// Use a macro because forwarding to a simple function has type system issues
+macro_rules! make_stmts_default {
+    ($me:expr) => {
+        $me.make_expr().map(|e| {
+            smallvec![ast::Stmt {
+                id: ast::DUMMY_NODE_ID,
+                span: e.span,
+                kind: ast::StmtKind::Expr(e),
+            }]
+        })
+    };
+}
+
+/// The result of a macro expansion. The return values of the various
+/// methods are spliced into the AST at the callsite of the macro.
+pub trait MacResult {
+    /// Creates an expression.
+    fn make_expr(self: Box<Self>) -> Option<P<ast::Expr>> {
+        None
+    }
+
+    /// Creates zero or more items.
+    fn make_items(self: Box<Self>) -> Option<SmallVec<[P<ast::Item>; 1]>> {
+        None
+    }
+
+    /// Creates zero or more impl items.
+    fn make_impl_items(self: Box<Self>) -> Option<SmallVec<[P<ast::AssocItem>; 1]>> {
+        None
+    }
+
+    /// Creates zero or more trait items.
+    fn make_trait_items(self: Box<Self>) -> Option<SmallVec<[P<ast::AssocItem>; 1]>> {
+        None
+    }
+
+    /// Creates zero or more items in an `extern {}` block
+    fn make_foreign_items(self: Box<Self>) -> Option<SmallVec<[P<ast::ForeignItem>; 1]>> {
+        None
+    }
+
+    /// Creates a pattern.
+    fn make_pat(self: Box<Self>) -> Option<P<ast::Pat>> {
+        None
+    }
+
+    /// Creates zero or more statements.
+    ///
+    /// By default this attempts to create an expression statement,
+    /// returning None if that fails.
+    fn make_stmts(self: Box<Self>) -> Option<SmallVec<[ast::Stmt; 1]>> {
+        make_stmts_default!(self)
+    }
+
+    fn make_ty(self: Box<Self>) -> Option<P<ast::Ty>> {
+        None
+    }
+
+    fn make_arms(self: Box<Self>) -> Option<SmallVec<[ast::Arm; 1]>> {
+        None
+    }
+
+    fn make_expr_fields(self: Box<Self>) -> Option<SmallVec<[ast::ExprField; 1]>> {
+        None
+    }
+
+    fn make_pat_fields(self: Box<Self>) -> Option<SmallVec<[ast::PatField; 1]>> {
+        None
+    }
+
+    fn make_generic_params(self: Box<Self>) -> Option<SmallVec<[ast::GenericParam; 1]>> {
+        None
+    }
+
+    fn make_params(self: Box<Self>) -> Option<SmallVec<[ast::Param; 1]>> {
+        None
+    }
+
+    fn make_field_defs(self: Box<Self>) -> Option<SmallVec<[ast::FieldDef; 1]>> {
+        None
+    }
+
+    fn make_variants(self: Box<Self>) -> Option<SmallVec<[ast::Variant; 1]>> {
+        None
+    }
+
+    fn make_crate(self: Box<Self>) -> Option<ast::Crate> {
+        // Fn-like macros cannot produce a crate.
+        unreachable!()
+    }
+}
+
+macro_rules! make_MacEager {
+    ( $( $fld:ident: $t:ty, )* ) => {
+        /// `MacResult` implementation for the common case where you've already
+        /// built each form of AST that you might return.
+        #[derive(Default)]
+        pub struct MacEager {
+            $(
+                pub $fld: Option<$t>,
+            )*
+        }
+
+        impl MacEager {
+            $(
+                pub fn $fld(v: $t) -> Box<dyn MacResult> {
+                    Box::new(MacEager {
+                        $fld: Some(v),
+                        ..Default::default()
+                    })
+                }
+            )*
+        }
+    }
+}
+
+make_MacEager! {
+    expr: P<ast::Expr>,
+    pat: P<ast::Pat>,
+    items: SmallVec<[P<ast::Item>; 1]>,
+    impl_items: SmallVec<[P<ast::AssocItem>; 1]>,
+    trait_items: SmallVec<[P<ast::AssocItem>; 1]>,
+    foreign_items: SmallVec<[P<ast::ForeignItem>; 1]>,
+    stmts: SmallVec<[ast::Stmt; 1]>,
+    ty: P<ast::Ty>,
+}
+
+impl MacResult for MacEager {
+    fn make_expr(self: Box<Self>) -> Option<P<ast::Expr>> {
+        self.expr
+    }
+
+    fn make_items(self: Box<Self>) -> Option<SmallVec<[P<ast::Item>; 1]>> {
+        self.items
+    }
+
+    fn make_impl_items(self: Box<Self>) -> Option<SmallVec<[P<ast::AssocItem>; 1]>> {
+        self.impl_items
+    }
+
+    fn make_trait_items(self: Box<Self>) -> Option<SmallVec<[P<ast::AssocItem>; 1]>> {
+        self.trait_items
+    }
+
+    fn make_foreign_items(self: Box<Self>) -> Option<SmallVec<[P<ast::ForeignItem>; 1]>> {
+        self.foreign_items
+    }
+
+    fn make_stmts(self: Box<Self>) -> Option<SmallVec<[ast::Stmt; 1]>> {
+        match self.stmts.as_ref().map_or(0, |s| s.len()) {
+            0 => make_stmts_default!(self),
+            _ => self.stmts,
+        }
+    }
+
+    fn make_pat(self: Box<Self>) -> Option<P<ast::Pat>> {
+        if let Some(p) = self.pat {
+            return Some(p);
+        }
+        if let Some(e) = self.expr {
+            if let ast::ExprKind::Lit(_) = e.kind {
+                return Some(P(ast::Pat {
+                    id: ast::DUMMY_NODE_ID,
+                    span: e.span,
+                    kind: PatKind::Lit(e),
+                    tokens: None,
+                }));
+            }
+        }
+        None
+    }
+
+    fn make_ty(self: Box<Self>) -> Option<P<ast::Ty>> {
+        self.ty
+    }
+}
+
+/// Fill-in macro expansion result, to allow compilation to continue
+/// after hitting errors.
+#[derive(Copy, Clone)]
+pub struct DummyResult {
+    is_error: bool,
+    span: Span,
+}
+
+impl DummyResult {
+    /// Creates a default MacResult that can be anything.
+    ///
+    /// Use this as a return value after hitting any errors and
+    /// calling `span_err`.
+    pub fn any(span: Span) -> Box<dyn MacResult + 'static> {
+        Box::new(DummyResult { is_error: true, span })
+    }
+
+    /// Same as `any`, but must be a valid fragment, not error.
+    pub fn any_valid(span: Span) -> Box<dyn MacResult + 'static> {
+        Box::new(DummyResult { is_error: false, span })
+    }
+
+    /// A plain dummy expression.
+    pub fn raw_expr(sp: Span, is_error: bool) -> P<ast::Expr> {
+        P(ast::Expr {
+            id: ast::DUMMY_NODE_ID,
+            kind: if is_error { ast::ExprKind::Err } else { ast::ExprKind::Tup(Vec::new()) },
+            span: sp,
+            attrs: ast::AttrVec::new(),
+            tokens: None,
+        })
+    }
+
+    /// A plain dummy pattern.
+    pub fn raw_pat(sp: Span) -> ast::Pat {
+        ast::Pat { id: ast::DUMMY_NODE_ID, kind: PatKind::Wild, span: sp, tokens: None }
+    }
+
+    /// A plain dummy type.
+    pub fn raw_ty(sp: Span, is_error: bool) -> P<ast::Ty> {
+        P(ast::Ty {
+            id: ast::DUMMY_NODE_ID,
+            kind: if is_error { ast::TyKind::Err } else { ast::TyKind::Tup(Vec::new()) },
+            span: sp,
+            tokens: None,
+        })
+    }
+}
+
+impl MacResult for DummyResult {
+    fn make_expr(self: Box<DummyResult>) -> Option<P<ast::Expr>> {
+        Some(DummyResult::raw_expr(self.span, self.is_error))
+    }
+
+    fn make_pat(self: Box<DummyResult>) -> Option<P<ast::Pat>> {
+        Some(P(DummyResult::raw_pat(self.span)))
+    }
+
+    fn make_items(self: Box<DummyResult>) -> Option<SmallVec<[P<ast::Item>; 1]>> {
+        Some(SmallVec::new())
+    }
+
+    fn make_impl_items(self: Box<DummyResult>) -> Option<SmallVec<[P<ast::AssocItem>; 1]>> {
+        Some(SmallVec::new())
+    }
+
+    fn make_trait_items(self: Box<DummyResult>) -> Option<SmallVec<[P<ast::AssocItem>; 1]>> {
+        Some(SmallVec::new())
+    }
+
+    fn make_foreign_items(self: Box<Self>) -> Option<SmallVec<[P<ast::ForeignItem>; 1]>> {
+        Some(SmallVec::new())
+    }
+
+    fn make_stmts(self: Box<DummyResult>) -> Option<SmallVec<[ast::Stmt; 1]>> {
+        Some(smallvec![ast::Stmt {
+            id: ast::DUMMY_NODE_ID,
+            kind: ast::StmtKind::Expr(DummyResult::raw_expr(self.span, self.is_error)),
+            span: self.span,
+        }])
+    }
+
+    fn make_ty(self: Box<DummyResult>) -> Option<P<ast::Ty>> {
+        Some(DummyResult::raw_ty(self.span, self.is_error))
+    }
+
+    fn make_arms(self: Box<DummyResult>) -> Option<SmallVec<[ast::Arm; 1]>> {
+        Some(SmallVec::new())
+    }
+
+    fn make_expr_fields(self: Box<DummyResult>) -> Option<SmallVec<[ast::ExprField; 1]>> {
+        Some(SmallVec::new())
+    }
+
+    fn make_pat_fields(self: Box<DummyResult>) -> Option<SmallVec<[ast::PatField; 1]>> {
+        Some(SmallVec::new())
+    }
+
+    fn make_generic_params(self: Box<DummyResult>) -> Option<SmallVec<[ast::GenericParam; 1]>> {
+        Some(SmallVec::new())
+    }
+
+    fn make_params(self: Box<DummyResult>) -> Option<SmallVec<[ast::Param; 1]>> {
+        Some(SmallVec::new())
+    }
+
+    fn make_field_defs(self: Box<DummyResult>) -> Option<SmallVec<[ast::FieldDef; 1]>> {
+        Some(SmallVec::new())
+    }
+
+    fn make_variants(self: Box<DummyResult>) -> Option<SmallVec<[ast::Variant; 1]>> {
+        Some(SmallVec::new())
+    }
+}
+
+/// A syntax extension kind.
+pub enum SyntaxExtensionKind {
+    /// A token-based function-like macro.
+    Bang(
+        /// An expander with signature TokenStream -> TokenStream.
+        Box<dyn BangProcMacro + sync::Sync + sync::Send>,
+    ),
+
+    /// An AST-based function-like macro.
+    LegacyBang(
+        /// An expander with signature TokenStream -> AST.
+        Box<dyn TTMacroExpander + sync::Sync + sync::Send>,
+    ),
+
+    /// A token-based attribute macro.
+    Attr(
+        /// An expander with signature (TokenStream, TokenStream) -> TokenStream.
+        /// The first TokenSteam is the attribute itself, the second is the annotated item.
+        /// The produced TokenSteam replaces the input TokenSteam.
+        Box<dyn AttrProcMacro + sync::Sync + sync::Send>,
+    ),
+
+    /// An AST-based attribute macro.
+    LegacyAttr(
+        /// An expander with signature (AST, AST) -> AST.
+        /// The first AST fragment is the attribute itself, the second is the annotated item.
+        /// The produced AST fragment replaces the input AST fragment.
+        Box<dyn MultiItemModifier + sync::Sync + sync::Send>,
+    ),
+
+    /// A trivial attribute "macro" that does nothing,
+    /// only keeps the attribute and marks it as inert,
+    /// thus making it ineligible for further expansion.
+    NonMacroAttr,
+
+    /// A token-based derive macro.
+    Derive(
+        /// An expander with signature TokenStream -> TokenStream (not yet).
+        /// The produced TokenSteam is appended to the input TokenSteam.
+        Box<dyn MultiItemModifier + sync::Sync + sync::Send>,
+    ),
+
+    /// An AST-based derive macro.
+    LegacyDerive(
+        /// An expander with signature AST -> AST.
+        /// The produced AST fragment is appended to the input AST fragment.
+        Box<dyn MultiItemModifier + sync::Sync + sync::Send>,
+    ),
+}
+
+/// A struct representing a macro definition in "lowered" form ready for expansion.
+pub struct SyntaxExtension {
+    /// A syntax extension kind.
+    pub kind: SyntaxExtensionKind,
+    /// Span of the macro definition.
+    pub span: Span,
+    /// List of unstable features that are treated as stable inside this macro.
+    pub allow_internal_unstable: Option<Lrc<[Symbol]>>,
+    /// Suppresses the `unsafe_code` lint for code produced by this macro.
+    pub allow_internal_unsafe: bool,
+    /// Enables the macro helper hack (`ident!(...)` -> `$crate::ident!(...)`) for this macro.
+    pub local_inner_macros: bool,
+    /// The macro's stability info.
+    pub stability: Option<Stability>,
+    /// The macro's deprecation info.
+    pub deprecation: Option<Deprecation>,
+    /// Names of helper attributes registered by this macro.
+    pub helper_attrs: Vec<Symbol>,
+    /// Edition of the crate in which this macro is defined.
+    pub edition: Edition,
+    /// Built-in macros have a couple of special properties like availability
+    /// in `#[no_implicit_prelude]` modules, so we have to keep this flag.
+    pub builtin_name: Option<Symbol>,
+}
+
+impl SyntaxExtension {
+    /// Returns which kind of macro calls this syntax extension.
+    pub fn macro_kind(&self) -> MacroKind {
+        match self.kind {
+            SyntaxExtensionKind::Bang(..) | SyntaxExtensionKind::LegacyBang(..) => MacroKind::Bang,
+            SyntaxExtensionKind::Attr(..)
+            | SyntaxExtensionKind::LegacyAttr(..)
+            | SyntaxExtensionKind::NonMacroAttr => MacroKind::Attr,
+            SyntaxExtensionKind::Derive(..) | SyntaxExtensionKind::LegacyDerive(..) => {
+                MacroKind::Derive
+            }
+        }
+    }
+
+    /// Constructs a syntax extension with default properties.
+    pub fn default(kind: SyntaxExtensionKind, edition: Edition) -> SyntaxExtension {
+        SyntaxExtension {
+            span: DUMMY_SP,
+            allow_internal_unstable: None,
+            allow_internal_unsafe: false,
+            local_inner_macros: false,
+            stability: None,
+            deprecation: None,
+            helper_attrs: Vec::new(),
+            edition,
+            builtin_name: None,
+            kind,
+        }
+    }
+
+    /// Constructs a syntax extension with the given properties
+    /// and other properties converted from attributes.
+    pub fn new(
+        sess: &Session,
+        kind: SyntaxExtensionKind,
+        span: Span,
+        helper_attrs: Vec<Symbol>,
+        edition: Edition,
+        name: Symbol,
+        attrs: &[ast::Attribute],
+    ) -> SyntaxExtension {
+        let allow_internal_unstable =
+            attr::allow_internal_unstable(sess, &attrs).collect::<Vec<Symbol>>();
+
+        let mut local_inner_macros = false;
+        if let Some(macro_export) = sess.find_by_name(attrs, sym::macro_export) {
+            if let Some(l) = macro_export.meta_item_list() {
+                local_inner_macros = attr::list_contains_name(&l, sym::local_inner_macros);
+            }
+        }
+
+        let (builtin_name, helper_attrs) = sess
+            .find_by_name(attrs, sym::rustc_builtin_macro)
+            .map(|attr| {
+                // Override `helper_attrs` passed above if it's a built-in macro,
+                // marking `proc_macro_derive` macros as built-in is not a realistic use case.
+                parse_macro_name_and_helper_attrs(sess.diagnostic(), attr, "built-in").map_or_else(
+                    || (Some(name), Vec::new()),
+                    |(name, helper_attrs)| (Some(name), helper_attrs),
+                )
+            })
+            .unwrap_or_else(|| (None, helper_attrs));
+        let (stability, const_stability) = attr::find_stability(&sess, attrs, span);
+        if let Some((_, sp)) = const_stability {
+            sess.parse_sess
+                .span_diagnostic
+                .struct_span_err(sp, "macros cannot have const stability attributes")
+                .span_label(sp, "invalid const stability attribute")
+                .span_label(
+                    sess.source_map().guess_head_span(span),
+                    "const stability attribute affects this macro",
+                )
+                .emit();
+        }
+
+        SyntaxExtension {
+            kind,
+            span,
+            allow_internal_unstable: (!allow_internal_unstable.is_empty())
+                .then(|| allow_internal_unstable.into()),
+            allow_internal_unsafe: sess.contains_name(attrs, sym::allow_internal_unsafe),
+            local_inner_macros,
+            stability: stability.map(|(s, _)| s),
+            deprecation: attr::find_deprecation(&sess, attrs).map(|(d, _)| d),
+            helper_attrs,
+            edition,
+            builtin_name,
+        }
+    }
+
+    pub fn dummy_bang(edition: Edition) -> SyntaxExtension {
+        fn expander<'cx>(
+            _: &'cx mut ExtCtxt<'_>,
+            span: Span,
+            _: TokenStream,
+        ) -> Box<dyn MacResult + 'cx> {
+            DummyResult::any(span)
+        }
+        SyntaxExtension::default(SyntaxExtensionKind::LegacyBang(Box::new(expander)), edition)
+    }
+
+    pub fn dummy_derive(edition: Edition) -> SyntaxExtension {
+        fn expander(
+            _: &mut ExtCtxt<'_>,
+            _: Span,
+            _: &ast::MetaItem,
+            _: Annotatable,
+        ) -> Vec<Annotatable> {
+            Vec::new()
+        }
+        SyntaxExtension::default(SyntaxExtensionKind::Derive(Box::new(expander)), edition)
+    }
+
+    pub fn non_macro_attr(edition: Edition) -> SyntaxExtension {
+        SyntaxExtension::default(SyntaxExtensionKind::NonMacroAttr, edition)
+    }
+
+    pub fn expn_data(
+        &self,
+        parent: LocalExpnId,
+        call_site: Span,
+        descr: Symbol,
+        macro_def_id: Option<DefId>,
+        parent_module: Option<DefId>,
+    ) -> ExpnData {
+        ExpnData::new(
+            ExpnKind::Macro(self.macro_kind(), descr),
+            parent.to_expn_id(),
+            call_site,
+            self.span,
+            self.allow_internal_unstable.clone(),
+            self.allow_internal_unsafe,
+            self.local_inner_macros,
+            self.edition,
+            macro_def_id,
+            parent_module,
+        )
+    }
+}
+
+/// Error type that denotes indeterminacy.
+pub struct Indeterminate;
+
+pub type DeriveResolutions = Vec<(ast::Path, Annotatable, Option<Lrc<SyntaxExtension>>)>;
+
+pub trait ResolverExpand {
+    fn next_node_id(&mut self) -> NodeId;
+    fn invocation_parent(&self, id: LocalExpnId) -> LocalDefId;
+
+    fn resolve_dollar_crates(&mut self);
+    fn visit_ast_fragment_with_placeholders(
+        &mut self,
+        expn_id: LocalExpnId,
+        fragment: &AstFragment,
+    );
+    fn register_builtin_macro(&mut self, name: Symbol, ext: SyntaxExtensionKind);
+
+    fn expansion_for_ast_pass(
+        &mut self,
+        call_site: Span,
+        pass: AstPass,
+        features: &[Symbol],
+        parent_module_id: Option<NodeId>,
+    ) -> LocalExpnId;
+
+    fn resolve_imports(&mut self);
+
+    fn resolve_macro_invocation(
+        &mut self,
+        invoc: &Invocation,
+        eager_expansion_root: LocalExpnId,
+        force: bool,
+    ) -> Result<Lrc<SyntaxExtension>, Indeterminate>;
+
+    fn record_macro_rule_usage(&mut self, mac_id: NodeId, rule_index: usize);
+
+    fn check_unused_macros(&mut self);
+
+    // Resolver interfaces for specific built-in macros.
+    /// Does `#[derive(...)]` attribute with the given `ExpnId` have built-in `Copy` inside it?
+    fn has_derive_copy(&self, expn_id: LocalExpnId) -> bool;
+    /// Resolve paths inside the `#[derive(...)]` attribute with the given `ExpnId`.
+    fn resolve_derives(
+        &mut self,
+        expn_id: LocalExpnId,
+        force: bool,
+        derive_paths: &dyn Fn() -> DeriveResolutions,
+    ) -> Result<(), Indeterminate>;
+    /// Take resolutions for paths inside the `#[derive(...)]` attribute with the given `ExpnId`
+    /// back from resolver.
+    fn take_derive_resolutions(&mut self, expn_id: LocalExpnId) -> Option<DeriveResolutions>;
+    /// Path resolution logic for `#[cfg_accessible(path)]`.
+    fn cfg_accessible(
+        &mut self,
+        expn_id: LocalExpnId,
+        path: &ast::Path,
+    ) -> Result<bool, Indeterminate>;
+
+    /// Decodes the proc-macro quoted span in the specified crate, with the specified id.
+    /// No caching is performed.
+    fn get_proc_macro_quoted_span(&self, krate: CrateNum, id: usize) -> Span;
+
+    /// The order of items in the HIR is unrelated to the order of
+    /// items in the AST. However, we generate proc macro harnesses
+    /// based on the AST order, and later refer to these harnesses
+    /// from the HIR. This field keeps track of the order in which
+    /// we generated proc macros harnesses, so that we can map
+    /// HIR proc macros items back to their harness items.
+    fn declare_proc_macro(&mut self, id: NodeId);
+
+    /// Tools registered with `#![register_tool]` and used by tool attributes and lints.
+    fn registered_tools(&self) -> &FxHashSet<Ident>;
+}
+
+pub trait LintStoreExpand {
+    fn pre_expansion_lint(
+        &self,
+        sess: &Session,
+        registered_tools: &FxHashSet<Ident>,
+        node_id: NodeId,
+        attrs: &[Attribute],
+        items: &[P<Item>],
+        name: &str,
+    );
+}
+
+type LintStoreExpandDyn<'a> = Option<&'a (dyn LintStoreExpand + 'a)>;
+
+#[derive(Clone, Default)]
+pub struct ModuleData {
+    /// Path to the module starting from the crate name, like `my_crate::foo::bar`.
+    pub mod_path: Vec<Ident>,
+    /// Stack of paths to files loaded by out-of-line module items,
+    /// used to detect and report recursive module inclusions.
+    pub file_path_stack: Vec<PathBuf>,
+    /// Directory to search child module files in,
+    /// often (but not necessarily) the parent of the top file path on the `file_path_stack`.
+    pub dir_path: PathBuf,
+}
+
+impl ModuleData {
+    pub fn with_dir_path(&self, dir_path: PathBuf) -> ModuleData {
+        ModuleData {
+            mod_path: self.mod_path.clone(),
+            file_path_stack: self.file_path_stack.clone(),
+            dir_path,
+        }
+    }
+}
+
+#[derive(Clone)]
+pub struct ExpansionData {
+    pub id: LocalExpnId,
+    pub depth: usize,
+    pub module: Rc<ModuleData>,
+    pub dir_ownership: DirOwnership,
+    pub prior_type_ascription: Option<(Span, bool)>,
+    /// Some parent node that is close to this macro call
+    pub lint_node_id: NodeId,
+    pub is_trailing_mac: bool,
+}
+
+/// One of these is made during expansion and incrementally updated as we go;
+/// when a macro expansion occurs, the resulting nodes have the `backtrace()
+/// -> expn_data` of their expansion context stored into their span.
+pub struct ExtCtxt<'a> {
+    pub sess: &'a Session,
+    pub ecfg: expand::ExpansionConfig<'a>,
+    pub reduced_recursion_limit: Option<Limit>,
+    pub root_path: PathBuf,
+    pub resolver: &'a mut dyn ResolverExpand,
+    pub current_expansion: ExpansionData,
+    /// Error recovery mode entered when expansion is stuck
+    /// (or during eager expansion, but that's a hack).
+    pub force_mode: bool,
+    pub expansions: FxIndexMap<Span, Vec<String>>,
+    /// Used for running pre-expansion lints on freshly loaded modules.
+    pub(super) lint_store: LintStoreExpandDyn<'a>,
+    /// Used for storing lints generated during expansion, like `NAMED_ARGUMENTS_USED_POSITIONALLY`
+    pub buffered_early_lint: Vec<BufferedEarlyLint>,
+    /// When we 'expand' an inert attribute, we leave it
+    /// in the AST, but insert it here so that we know
+    /// not to expand it again.
+    pub(super) expanded_inert_attrs: MarkedAttrs,
+}
+
+impl<'a> ExtCtxt<'a> {
+    pub fn new(
+        sess: &'a Session,
+        ecfg: expand::ExpansionConfig<'a>,
+        resolver: &'a mut dyn ResolverExpand,
+        lint_store: LintStoreExpandDyn<'a>,
+    ) -> ExtCtxt<'a> {
+        ExtCtxt {
+            sess,
+            ecfg,
+            reduced_recursion_limit: None,
+            resolver,
+            lint_store,
+            root_path: PathBuf::new(),
+            current_expansion: ExpansionData {
+                id: LocalExpnId::ROOT,
+                depth: 0,
+                module: Default::default(),
+                dir_ownership: DirOwnership::Owned { relative: None },
+                prior_type_ascription: None,
+                lint_node_id: ast::CRATE_NODE_ID,
+                is_trailing_mac: false,
+            },
+            force_mode: false,
+            expansions: FxIndexMap::default(),
+            expanded_inert_attrs: MarkedAttrs::new(),
+            buffered_early_lint: vec![],
+        }
+    }
+
+    /// Returns a `Folder` for deeply expanding all macros in an AST node.
+    pub fn expander<'b>(&'b mut self) -> expand::MacroExpander<'b, 'a> {
+        expand::MacroExpander::new(self, false)
+    }
+
+    /// Returns a `Folder` that deeply expands all macros and assigns all `NodeId`s in an AST node.
+    /// Once `NodeId`s are assigned, the node may not be expanded, removed, or otherwise modified.
+    pub fn monotonic_expander<'b>(&'b mut self) -> expand::MacroExpander<'b, 'a> {
+        expand::MacroExpander::new(self, true)
+    }
+    pub fn new_parser_from_tts(&self, stream: TokenStream) -> parser::Parser<'a> {
+        rustc_parse::stream_to_parser(&self.sess.parse_sess, stream, MACRO_ARGUMENTS)
+    }
+    pub fn source_map(&self) -> &'a SourceMap {
+        self.sess.parse_sess.source_map()
+    }
+    pub fn parse_sess(&self) -> &'a ParseSess {
+        &self.sess.parse_sess
+    }
+    pub fn call_site(&self) -> Span {
+        self.current_expansion.id.expn_data().call_site
+    }
+
+    /// Returns the current expansion kind's description.
+    pub(crate) fn expansion_descr(&self) -> String {
+        let expn_data = self.current_expansion.id.expn_data();
+        expn_data.kind.descr()
+    }
+
+    /// Equivalent of `Span::def_site` from the proc macro API,
+    /// except that the location is taken from the span passed as an argument.
+    pub fn with_def_site_ctxt(&self, span: Span) -> Span {
+        span.with_def_site_ctxt(self.current_expansion.id.to_expn_id())
+    }
+
+    /// Equivalent of `Span::call_site` from the proc macro API,
+    /// except that the location is taken from the span passed as an argument.
+    pub fn with_call_site_ctxt(&self, span: Span) -> Span {
+        span.with_call_site_ctxt(self.current_expansion.id.to_expn_id())
+    }
+
+    /// Equivalent of `Span::mixed_site` from the proc macro API,
+    /// except that the location is taken from the span passed as an argument.
+    pub fn with_mixed_site_ctxt(&self, span: Span) -> Span {
+        span.with_mixed_site_ctxt(self.current_expansion.id.to_expn_id())
+    }
+
+    /// Returns span for the macro which originally caused the current expansion to happen.
+    ///
+    /// Stops backtracing at include! boundary.
+    pub fn expansion_cause(&self) -> Option<Span> {
+        self.current_expansion.id.expansion_cause()
+    }
+
+    #[rustc_lint_diagnostics]
+    pub fn struct_span_err<S: Into<MultiSpan>>(
+        &self,
+        sp: S,
+        msg: &str,
+    ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
+        self.sess.parse_sess.span_diagnostic.struct_span_err(sp, msg)
+    }
+
+    pub fn create_err(
+        &self,
+        err: impl SessionDiagnostic<'a>,
+    ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
+        self.sess.create_err(err)
+    }
+
+    pub fn emit_err(&self, err: impl SessionDiagnostic<'a>) -> ErrorGuaranteed {
+        self.sess.emit_err(err)
+    }
+
+    /// Emit `msg` attached to `sp`, without immediately stopping
+    /// compilation.
+    ///
+    /// Compilation will be stopped in the near future (at the end of
+    /// the macro expansion phase).
+    #[rustc_lint_diagnostics]
+    pub fn span_err<S: Into<MultiSpan>>(&self, sp: S, msg: &str) {
+        self.sess.parse_sess.span_diagnostic.span_err(sp, msg);
+    }
+    #[rustc_lint_diagnostics]
+    pub fn span_warn<S: Into<MultiSpan>>(&self, sp: S, msg: &str) {
+        self.sess.parse_sess.span_diagnostic.span_warn(sp, msg);
+    }
+    pub fn span_bug<S: Into<MultiSpan>>(&self, sp: S, msg: &str) -> ! {
+        self.sess.parse_sess.span_diagnostic.span_bug(sp, msg);
+    }
+    pub fn trace_macros_diag(&mut self) {
+        for (sp, notes) in self.expansions.iter() {
+            let mut db = self.sess.parse_sess.span_diagnostic.span_note_diag(*sp, "trace_macro");
+            for note in notes {
+                db.note(note);
+            }
+            db.emit();
+        }
+        // Fixme: does this result in errors?
+        self.expansions.clear();
+    }
+    pub fn bug(&self, msg: &str) -> ! {
+        self.sess.parse_sess.span_diagnostic.bug(msg);
+    }
+    pub fn trace_macros(&self) -> bool {
+        self.ecfg.trace_mac
+    }
+    pub fn set_trace_macros(&mut self, x: bool) {
+        self.ecfg.trace_mac = x
+    }
+    pub fn std_path(&self, components: &[Symbol]) -> Vec<Ident> {
+        let def_site = self.with_def_site_ctxt(DUMMY_SP);
+        iter::once(Ident::new(kw::DollarCrate, def_site))
+            .chain(components.iter().map(|&s| Ident::with_dummy_span(s)))
+            .collect()
+    }
+    pub fn def_site_path(&self, components: &[Symbol]) -> Vec<Ident> {
+        let def_site = self.with_def_site_ctxt(DUMMY_SP);
+        components.iter().map(|&s| Ident::new(s, def_site)).collect()
+    }
+
+    pub fn check_unused_macros(&mut self) {
+        self.resolver.check_unused_macros();
+    }
+}
+
+/// Resolves a `path` mentioned inside Rust code, returning an absolute path.
+///
+/// This unifies the logic used for resolving `include_X!`.
+pub fn resolve_path(
+    parse_sess: &ParseSess,
+    path: impl Into<PathBuf>,
+    span: Span,
+) -> PResult<'_, PathBuf> {
+    let path = path.into();
+
+    // Relative paths are resolved relative to the file in which they are found
+    // after macro expansion (that is, they are unhygienic).
+    if !path.is_absolute() {
+        let callsite = span.source_callsite();
+        let mut result = match parse_sess.source_map().span_to_filename(callsite) {
+            FileName::Real(name) => name
+                .into_local_path()
+                .expect("attempting to resolve a file path in an external file"),
+            FileName::DocTest(path, _) => path,
+            other => {
+                return Err(parse_sess.span_diagnostic.struct_span_err(
+                    span,
+                    &format!(
+                        "cannot resolve relative path in non-file source `{}`",
+                        parse_sess.source_map().filename_for_diagnostics(&other)
+                    ),
+                ));
+            }
+        };
+        result.pop();
+        result.push(path);
+        Ok(result)
+    } else {
+        Ok(path)
+    }
+}
+
+/// Extracts a string literal from the macro expanded version of `expr`,
+/// returning a diagnostic error of `err_msg` if `expr` is not a string literal.
+/// The returned bool indicates whether an applicable suggestion has already been
+/// added to the diagnostic to avoid emitting multiple suggestions. `Err(None)`
+/// indicates that an ast error was encountered.
+pub fn expr_to_spanned_string<'a>(
+    cx: &'a mut ExtCtxt<'_>,
+    expr: P<ast::Expr>,
+    err_msg: &str,
+) -> Result<(Symbol, ast::StrStyle, Span), Option<(DiagnosticBuilder<'a, ErrorGuaranteed>, bool)>> {
+    // Perform eager expansion on the expression.
+    // We want to be able to handle e.g., `concat!("foo", "bar")`.
+    let expr = cx.expander().fully_expand_fragment(AstFragment::Expr(expr)).make_expr();
+
+    Err(match expr.kind {
+        ast::ExprKind::Lit(ref l) => match l.kind {
+            ast::LitKind::Str(s, style) => return Ok((s, style, expr.span)),
+            ast::LitKind::ByteStr(_) => {
+                let mut err = cx.struct_span_err(l.span, err_msg);
+                err.span_suggestion(
+                    expr.span.shrink_to_lo(),
+                    "consider removing the leading `b`",
+                    "",
+                    Applicability::MaybeIncorrect,
+                );
+                Some((err, true))
+            }
+            ast::LitKind::Err(_) => None,
+            _ => Some((cx.struct_span_err(l.span, err_msg), false)),
+        },
+        ast::ExprKind::Err => None,
+        _ => Some((cx.struct_span_err(expr.span, err_msg), false)),
+    })
+}
+
+/// Extracts a string literal from the macro expanded version of `expr`,
+/// emitting `err_msg` if `expr` is not a string literal. This does not stop
+/// compilation on error, merely emits a non-fatal error and returns `None`.
+pub fn expr_to_string(
+    cx: &mut ExtCtxt<'_>,
+    expr: P<ast::Expr>,
+    err_msg: &str,
+) -> Option<(Symbol, ast::StrStyle)> {
+    expr_to_spanned_string(cx, expr, err_msg)
+        .map_err(|err| {
+            err.map(|(mut err, _)| {
+                err.emit();
+            })
+        })
+        .ok()
+        .map(|(symbol, style, _)| (symbol, style))
+}
+
+/// Non-fatally assert that `tts` is empty. Note that this function
+/// returns even when `tts` is non-empty, macros that *need* to stop
+/// compilation should call
+/// `cx.parse_sess.span_diagnostic.abort_if_errors()` (this should be
+/// done as rarely as possible).
+pub fn check_zero_tts(cx: &ExtCtxt<'_>, sp: Span, tts: TokenStream, name: &str) {
+    if !tts.is_empty() {
+        cx.span_err(sp, &format!("{} takes no arguments", name));
+    }
+}
+
+/// Parse an expression. On error, emit it, advancing to `Eof`, and return `None`.
+pub fn parse_expr(p: &mut parser::Parser<'_>) -> Option<P<ast::Expr>> {
+    match p.parse_expr() {
+        Ok(e) => return Some(e),
+        Err(mut err) => {
+            err.emit();
+        }
+    }
+    while p.token != token::Eof {
+        p.bump();
+    }
+    None
+}
+
+/// Interpreting `tts` as a comma-separated sequence of expressions,
+/// expect exactly one string literal, or emit an error and return `None`.
+pub fn get_single_str_from_tts(
+    cx: &mut ExtCtxt<'_>,
+    sp: Span,
+    tts: TokenStream,
+    name: &str,
+) -> Option<Symbol> {
+    let mut p = cx.new_parser_from_tts(tts);
+    if p.token == token::Eof {
+        cx.span_err(sp, &format!("{} takes 1 argument", name));
+        return None;
+    }
+    let ret = parse_expr(&mut p)?;
+    let _ = p.eat(&token::Comma);
+
+    if p.token != token::Eof {
+        cx.span_err(sp, &format!("{} takes 1 argument", name));
+    }
+    expr_to_string(cx, ret, "argument must be a string literal").map(|(s, _)| s)
+}
+
+/// Extracts comma-separated expressions from `tts`.
+/// On error, emit it, and return `None`.
+pub fn get_exprs_from_tts(
+    cx: &mut ExtCtxt<'_>,
+    sp: Span,
+    tts: TokenStream,
+) -> Option<Vec<P<ast::Expr>>> {
+    let mut p = cx.new_parser_from_tts(tts);
+    let mut es = Vec::new();
+    while p.token != token::Eof {
+        let expr = parse_expr(&mut p)?;
+
+        // Perform eager expansion on the expression.
+        // We want to be able to handle e.g., `concat!("foo", "bar")`.
+        let expr = cx.expander().fully_expand_fragment(AstFragment::Expr(expr)).make_expr();
+
+        es.push(expr);
+        if p.eat(&token::Comma) {
+            continue;
+        }
+        if p.token != token::Eof {
+            cx.span_err(sp, "expected token: `,`");
+            return None;
+        }
+    }
+    Some(es)
+}
+
+pub fn parse_macro_name_and_helper_attrs(
+    diag: &rustc_errors::Handler,
+    attr: &Attribute,
+    descr: &str,
+) -> Option<(Symbol, Vec<Symbol>)> {
+    // Once we've located the `#[proc_macro_derive]` attribute, verify
+    // that it's of the form `#[proc_macro_derive(Foo)]` or
+    // `#[proc_macro_derive(Foo, attributes(A, ..))]`
+    let list = attr.meta_item_list()?;
+    if list.len() != 1 && list.len() != 2 {
+        diag.span_err(attr.span, "attribute must have either one or two arguments");
+        return None;
+    }
+    let Some(trait_attr) = list[0].meta_item() else {
+        diag.span_err(list[0].span(), "not a meta item");
+        return None;
+    };
+    let trait_ident = match trait_attr.ident() {
+        Some(trait_ident) if trait_attr.is_word() => trait_ident,
+        _ => {
+            diag.span_err(trait_attr.span, "must only be one word");
+            return None;
+        }
+    };
+
+    if !trait_ident.name.can_be_raw() {
+        diag.span_err(
+            trait_attr.span,
+            &format!("`{}` cannot be a name of {} macro", trait_ident, descr),
+        );
+    }
+
+    let attributes_attr = list.get(1);
+    let proc_attrs: Vec<_> = if let Some(attr) = attributes_attr {
+        if !attr.has_name(sym::attributes) {
+            diag.span_err(attr.span(), "second argument must be `attributes`");
+        }
+        attr.meta_item_list()
+            .unwrap_or_else(|| {
+                diag.span_err(attr.span(), "attribute must be of form: `attributes(foo, bar)`");
+                &[]
+            })
+            .iter()
+            .filter_map(|attr| {
+                let Some(attr) = attr.meta_item() else {
+                    diag.span_err(attr.span(), "not a meta item");
+                    return None;
+                };
+
+                let ident = match attr.ident() {
+                    Some(ident) if attr.is_word() => ident,
+                    _ => {
+                        diag.span_err(attr.span, "must only be one word");
+                        return None;
+                    }
+                };
+                if !ident.name.can_be_raw() {
+                    diag.span_err(
+                        attr.span,
+                        &format!("`{}` cannot be a name of derive helper attribute", ident),
+                    );
+                }
+
+                Some(ident.name)
+            })
+            .collect()
+    } else {
+        Vec::new()
+    };
+
+    Some((trait_ident.name, proc_attrs))
+}
+
+/// This nonterminal looks like some specific enums from
+/// `proc-macro-hack` and `procedural-masquerade` crates.
+/// We need to maintain some special pretty-printing behavior for them due to incorrect
+/// asserts in old versions of those crates and their wide use in the ecosystem.
+/// See issue #73345 for more details.
+/// FIXME(#73933): Remove this eventually.
+fn pretty_printing_compatibility_hack(item: &Item, sess: &ParseSess) -> bool {
+    let name = item.ident.name;
+    if name == sym::ProceduralMasqueradeDummyType {
+        if let ast::ItemKind::Enum(enum_def, _) = &item.kind {
+            if let [variant] = &*enum_def.variants {
+                if variant.ident.name == sym::Input {
+                    sess.buffer_lint_with_diagnostic(
+                        &PROC_MACRO_BACK_COMPAT,
+                        item.ident.span,
+                        ast::CRATE_NODE_ID,
+                        "using `procedural-masquerade` crate",
+                        BuiltinLintDiagnostics::ProcMacroBackCompat(
+                        "The `procedural-masquerade` crate has been unnecessary since Rust 1.30.0. \
+                        Versions of this crate below 0.1.7 will eventually stop compiling.".to_string())
+                    );
+                    return true;
+                }
+            }
+        }
+    }
+    false
+}
+
+pub(crate) fn ann_pretty_printing_compatibility_hack(ann: &Annotatable, sess: &ParseSess) -> bool {
+    let item = match ann {
+        Annotatable::Item(item) => item,
+        Annotatable::Stmt(stmt) => match &stmt.kind {
+            ast::StmtKind::Item(item) => item,
+            _ => return false,
+        },
+        _ => return false,
+    };
+    pretty_printing_compatibility_hack(item, sess)
+}
+
+pub(crate) fn nt_pretty_printing_compatibility_hack(nt: &Nonterminal, sess: &ParseSess) -> bool {
+    let item = match nt {
+        Nonterminal::NtItem(item) => item,
+        Nonterminal::NtStmt(stmt) => match &stmt.kind {
+            ast::StmtKind::Item(item) => item,
+            _ => return false,
+        },
+        _ => return false,
+    };
+    pretty_printing_compatibility_hack(item, sess)
+}
diff --git a/compiler/rustc_expand/src/build.rs b/compiler/rustc_expand/src/build.rs
new file mode 100644
index 000000000..fa3e2a4a5
--- /dev/null
+++ b/compiler/rustc_expand/src/build.rs
@@ -0,0 +1,616 @@
+use crate::base::ExtCtxt;
+
+use rustc_ast::attr;
+use rustc_ast::ptr::P;
+use rustc_ast::{self as ast, AttrVec, BlockCheckMode, Expr, LocalKind, PatKind, UnOp};
+use rustc_span::source_map::Spanned;
+use rustc_span::symbol::{kw, sym, Ident, Symbol};
+
+use rustc_span::Span;
+
+impl<'a> ExtCtxt<'a> {
+    pub fn path(&self, span: Span, strs: Vec<Ident>) -> ast::Path {
+        self.path_all(span, false, strs, vec![])
+    }
+    pub fn path_ident(&self, span: Span, id: Ident) -> ast::Path {
+        self.path(span, vec![id])
+    }
+    pub fn path_global(&self, span: Span, strs: Vec<Ident>) -> ast::Path {
+        self.path_all(span, true, strs, vec![])
+    }
+    pub fn path_all(
+        &self,
+        span: Span,
+        global: bool,
+        mut idents: Vec<Ident>,
+        args: Vec<ast::GenericArg>,
+    ) -> ast::Path {
+        assert!(!idents.is_empty());
+        let add_root = global && !idents[0].is_path_segment_keyword();
+        let mut segments = Vec::with_capacity(idents.len() + add_root as usize);
+        if add_root {
+            segments.push(ast::PathSegment::path_root(span));
+        }
+        let last_ident = idents.pop().unwrap();
+        segments.extend(
+            idents.into_iter().map(|ident| ast::PathSegment::from_ident(ident.with_span_pos(span))),
+        );
+        let args = if !args.is_empty() {
+            let args = args.into_iter().map(ast::AngleBracketedArg::Arg).collect();
+            ast::AngleBracketedArgs { args, span }.into()
+        } else {
+            None
+        };
+        segments.push(ast::PathSegment {
+            ident: last_ident.with_span_pos(span),
+            id: ast::DUMMY_NODE_ID,
+            args,
+        });
+        ast::Path { span, segments, tokens: None }
+    }
+
+    pub fn ty_mt(&self, ty: P<ast::Ty>, mutbl: ast::Mutability) -> ast::MutTy {
+        ast::MutTy { ty, mutbl }
+    }
+
+    pub fn ty(&self, span: Span, kind: ast::TyKind) -> P<ast::Ty> {
+        P(ast::Ty { id: ast::DUMMY_NODE_ID, span, kind, tokens: None })
+    }
+
+    pub fn ty_infer(&self, span: Span) -> P<ast::Ty> {
+        self.ty(span, ast::TyKind::Infer)
+    }
+
+    pub fn ty_path(&self, path: ast::Path) -> P<ast::Ty> {
+        self.ty(path.span, ast::TyKind::Path(None, path))
+    }
+
+    // Might need to take bounds as an argument in the future, if you ever want
+    // to generate a bounded existential trait type.
+    pub fn ty_ident(&self, span: Span, ident: Ident) -> P<ast::Ty> {
+        self.ty_path(self.path_ident(span, ident))
+    }
+
+    pub fn anon_const(&self, span: Span, kind: ast::ExprKind) -> ast::AnonConst {
+        ast::AnonConst {
+            id: ast::DUMMY_NODE_ID,
+            value: P(ast::Expr {
+                id: ast::DUMMY_NODE_ID,
+                kind,
+                span,
+                attrs: AttrVec::new(),
+                tokens: None,
+            }),
+        }
+    }
+
+    pub fn const_ident(&self, span: Span, ident: Ident) -> ast::AnonConst {
+        self.anon_const(span, ast::ExprKind::Path(None, self.path_ident(span, ident)))
+    }
+
+    pub fn ty_rptr(
+        &self,
+        span: Span,
+        ty: P<ast::Ty>,
+        lifetime: Option<ast::Lifetime>,
+        mutbl: ast::Mutability,
+    ) -> P<ast::Ty> {
+        self.ty(span, ast::TyKind::Rptr(lifetime, self.ty_mt(ty, mutbl)))
+    }
+
+    pub fn ty_ptr(&self, span: Span, ty: P<ast::Ty>, mutbl: ast::Mutability) -> P<ast::Ty> {
+        self.ty(span, ast::TyKind::Ptr(self.ty_mt(ty, mutbl)))
+    }
+
+    pub fn typaram(
+        &self,
+        span: Span,
+        ident: Ident,
+        attrs: Vec<ast::Attribute>,
+        bounds: ast::GenericBounds,
+        default: Option<P<ast::Ty>>,
+    ) -> ast::GenericParam {
+        ast::GenericParam {
+            ident: ident.with_span_pos(span),
+            id: ast::DUMMY_NODE_ID,
+            attrs: attrs.into(),
+            bounds,
+            kind: ast::GenericParamKind::Type { default },
+            is_placeholder: false,
+            colon_span: None,
+        }
+    }
+
+    pub fn trait_ref(&self, path: ast::Path) -> ast::TraitRef {
+        ast::TraitRef { path, ref_id: ast::DUMMY_NODE_ID }
+    }
+
+    pub fn poly_trait_ref(&self, span: Span, path: ast::Path) -> ast::PolyTraitRef {
+        ast::PolyTraitRef {
+            bound_generic_params: Vec::new(),
+            trait_ref: self.trait_ref(path),
+            span,
+        }
+    }
+
+    pub fn trait_bound(&self, path: ast::Path) -> ast::GenericBound {
+        ast::GenericBound::Trait(
+            self.poly_trait_ref(path.span, path),
+            ast::TraitBoundModifier::None,
+        )
+    }
+
+    pub fn lifetime(&self, span: Span, ident: Ident) -> ast::Lifetime {
+        ast::Lifetime { id: ast::DUMMY_NODE_ID, ident: ident.with_span_pos(span) }
+    }
+
+    pub fn lifetime_static(&self, span: Span) -> ast::Lifetime {
+        self.lifetime(span, Ident::new(kw::StaticLifetime, span))
+    }
+
+    pub fn stmt_expr(&self, expr: P<ast::Expr>) -> ast::Stmt {
+        ast::Stmt { id: ast::DUMMY_NODE_ID, span: expr.span, kind: ast::StmtKind::Expr(expr) }
+    }
+
+    pub fn stmt_let_pat(&self, sp: Span, pat: P<ast::Pat>, ex: P<ast::Expr>) -> ast::Stmt {
+        let local = P(ast::Local {
+            pat,
+            ty: None,
+            id: ast::DUMMY_NODE_ID,
+            kind: LocalKind::Init(ex),
+            span: sp,
+            attrs: AttrVec::new(),
+            tokens: None,
+        });
+        self.stmt_local(local, sp)
+    }
+
+    pub fn stmt_let(&self, sp: Span, mutbl: bool, ident: Ident, ex: P<ast::Expr>) -> ast::Stmt {
+        self.stmt_let_ty(sp, mutbl, ident, None, ex)
+    }
+
+    pub fn stmt_let_ty(
+        &self,
+        sp: Span,
+        mutbl: bool,
+        ident: Ident,
+        ty: Option<P<ast::Ty>>,
+        ex: P<ast::Expr>,
+    ) -> ast::Stmt {
+        let pat = if mutbl {
+            let binding_mode = ast::BindingMode::ByValue(ast::Mutability::Mut);
+            self.pat_ident_binding_mode(sp, ident, binding_mode)
+        } else {
+            self.pat_ident(sp, ident)
+        };
+        let local = P(ast::Local {
+            pat,
+            ty,
+            id: ast::DUMMY_NODE_ID,
+            kind: LocalKind::Init(ex),
+            span: sp,
+            attrs: AttrVec::new(),
+            tokens: None,
+        });
+        self.stmt_local(local, sp)
+    }
+
+    // Generates `let _: Type;`, which is usually used for type assertions.
+    pub fn stmt_let_type_only(&self, span: Span, ty: P<ast::Ty>) -> ast::Stmt {
+        let local = P(ast::Local {
+            pat: self.pat_wild(span),
+            ty: Some(ty),
+            id: ast::DUMMY_NODE_ID,
+            kind: LocalKind::Decl,
+            span,
+            attrs: AttrVec::new(),
+            tokens: None,
+        });
+        self.stmt_local(local, span)
+    }
+
+    pub fn stmt_local(&self, local: P<ast::Local>, span: Span) -> ast::Stmt {
+        ast::Stmt { id: ast::DUMMY_NODE_ID, kind: ast::StmtKind::Local(local), span }
+    }
+
+    pub fn stmt_item(&self, sp: Span, item: P<ast::Item>) -> ast::Stmt {
+        ast::Stmt { id: ast::DUMMY_NODE_ID, kind: ast::StmtKind::Item(item), span: sp }
+    }
+
+    pub fn block_expr(&self, expr: P<ast::Expr>) -> P<ast::Block> {
+        self.block(
+            expr.span,
+            vec![ast::Stmt {
+                id: ast::DUMMY_NODE_ID,
+                span: expr.span,
+                kind: ast::StmtKind::Expr(expr),
+            }],
+        )
+    }
+    pub fn block(&self, span: Span, stmts: Vec<ast::Stmt>) -> P<ast::Block> {
+        P(ast::Block {
+            stmts,
+            id: ast::DUMMY_NODE_ID,
+            rules: BlockCheckMode::Default,
+            span,
+            tokens: None,
+            could_be_bare_literal: false,
+        })
+    }
+
+    pub fn expr(&self, span: Span, kind: ast::ExprKind) -> P<ast::Expr> {
+        P(ast::Expr { id: ast::DUMMY_NODE_ID, kind, span, attrs: AttrVec::new(), tokens: None })
+    }
+
+    pub fn expr_path(&self, path: ast::Path) -> P<ast::Expr> {
+        self.expr(path.span, ast::ExprKind::Path(None, path))
+    }
+
+    pub fn expr_ident(&self, span: Span, id: Ident) -> P<ast::Expr> {
+        self.expr_path(self.path_ident(span, id))
+    }
+    pub fn expr_self(&self, span: Span) -> P<ast::Expr> {
+        self.expr_ident(span, Ident::with_dummy_span(kw::SelfLower))
+    }
+
+    pub fn expr_binary(
+        &self,
+        sp: Span,
+        op: ast::BinOpKind,
+        lhs: P<ast::Expr>,
+        rhs: P<ast::Expr>,
+    ) -> P<ast::Expr> {
+        self.expr(sp, ast::ExprKind::Binary(Spanned { node: op, span: sp }, lhs, rhs))
+    }
+
+    pub fn expr_deref(&self, sp: Span, e: P<ast::Expr>) -> P<ast::Expr> {
+        self.expr(sp, ast::ExprKind::Unary(UnOp::Deref, e))
+    }
+
+    pub fn expr_addr_of(&self, sp: Span, e: P<ast::Expr>) -> P<ast::Expr> {
+        self.expr(sp, ast::ExprKind::AddrOf(ast::BorrowKind::Ref, ast::Mutability::Not, e))
+    }
+
+    pub fn expr_call(
+        &self,
+        span: Span,
+        expr: P<ast::Expr>,
+        args: Vec<P<ast::Expr>>,
+    ) -> P<ast::Expr> {
+        self.expr(span, ast::ExprKind::Call(expr, args))
+    }
+    pub fn expr_call_ident(&self, span: Span, id: Ident, args: Vec<P<ast::Expr>>) -> P<ast::Expr> {
+        self.expr(span, ast::ExprKind::Call(self.expr_ident(span, id), args))
+    }
+    pub fn expr_call_global(
+        &self,
+        sp: Span,
+        fn_path: Vec<Ident>,
+        args: Vec<P<ast::Expr>>,
+    ) -> P<ast::Expr> {
+        let pathexpr = self.expr_path(self.path_global(sp, fn_path));
+        self.expr_call(sp, pathexpr, args)
+    }
+    pub fn expr_block(&self, b: P<ast::Block>) -> P<ast::Expr> {
+        self.expr(b.span, ast::ExprKind::Block(b, None))
+    }
+    pub fn field_imm(&self, span: Span, ident: Ident, e: P<ast::Expr>) -> ast::ExprField {
+        ast::ExprField {
+            ident: ident.with_span_pos(span),
+            expr: e,
+            span,
+            is_shorthand: false,
+            attrs: AttrVec::new(),
+            id: ast::DUMMY_NODE_ID,
+            is_placeholder: false,
+        }
+    }
+    pub fn expr_struct(
+        &self,
+        span: Span,
+        path: ast::Path,
+        fields: Vec<ast::ExprField>,
+    ) -> P<ast::Expr> {
+        self.expr(
+            span,
+            ast::ExprKind::Struct(P(ast::StructExpr {
+                qself: None,
+                path,
+                fields,
+                rest: ast::StructRest::None,
+            })),
+        )
+    }
+    pub fn expr_struct_ident(
+        &self,
+        span: Span,
+        id: Ident,
+        fields: Vec<ast::ExprField>,
+    ) -> P<ast::Expr> {
+        self.expr_struct(span, self.path_ident(span, id), fields)
+    }
+
+    pub fn expr_lit(&self, span: Span, lit_kind: ast::LitKind) -> P<ast::Expr> {
+        let lit = ast::Lit::from_lit_kind(lit_kind, span);
+        self.expr(span, ast::ExprKind::Lit(lit))
+    }
+    pub fn expr_usize(&self, span: Span, i: usize) -> P<ast::Expr> {
+        self.expr_lit(
+            span,
+            ast::LitKind::Int(i as u128, ast::LitIntType::Unsigned(ast::UintTy::Usize)),
+        )
+    }
+    pub fn expr_u32(&self, sp: Span, u: u32) -> P<ast::Expr> {
+        self.expr_lit(sp, ast::LitKind::Int(u as u128, ast::LitIntType::Unsigned(ast::UintTy::U32)))
+    }
+    pub fn expr_bool(&self, sp: Span, value: bool) -> P<ast::Expr> {
+        self.expr_lit(sp, ast::LitKind::Bool(value))
+    }
+
+    /// `[expr1, expr2, ...]`
+    pub fn expr_array(&self, sp: Span, exprs: Vec<P<ast::Expr>>) -> P<ast::Expr> {
+        self.expr(sp, ast::ExprKind::Array(exprs))
+    }
+
+    /// `&[expr1, expr2, ...]`
+    pub fn expr_array_ref(&self, sp: Span, exprs: Vec<P<ast::Expr>>) -> P<ast::Expr> {
+        self.expr_addr_of(sp, self.expr_array(sp, exprs))
+    }
+
+    pub fn expr_str(&self, sp: Span, s: Symbol) -> P<ast::Expr> {
+        self.expr_lit(sp, ast::LitKind::Str(s, ast::StrStyle::Cooked))
+    }
+
+    pub fn expr_cast(&self, sp: Span, expr: P<ast::Expr>, ty: P<ast::Ty>) -> P<ast::Expr> {
+        self.expr(sp, ast::ExprKind::Cast(expr, ty))
+    }
+
+    pub fn expr_some(&self, sp: Span, expr: P<ast::Expr>) -> P<ast::Expr> {
+        let some = self.std_path(&[sym::option, sym::Option, sym::Some]);
+        self.expr_call_global(sp, some, vec![expr])
+    }
+
+    pub fn expr_none(&self, sp: Span) -> P<ast::Expr> {
+        let none = self.std_path(&[sym::option, sym::Option, sym::None]);
+        self.expr_path(self.path_global(sp, none))
+    }
+    pub fn expr_tuple(&self, sp: Span, exprs: Vec<P<ast::Expr>>) -> P<ast::Expr> {
+        self.expr(sp, ast::ExprKind::Tup(exprs))
+    }
+
+    pub fn expr_fail(&self, span: Span, msg: Symbol) -> P<ast::Expr> {
+        self.expr_call_global(
+            span,
+            [sym::std, sym::rt, sym::begin_panic].iter().map(|s| Ident::new(*s, span)).collect(),
+            vec![self.expr_str(span, msg)],
+        )
+    }
+
+    pub fn expr_unreachable(&self, span: Span) -> P<ast::Expr> {
+        self.expr_fail(span, Symbol::intern("internal error: entered unreachable code"))
+    }
+
+    pub fn expr_ok(&self, sp: Span, expr: P<ast::Expr>) -> P<ast::Expr> {
+        let ok = self.std_path(&[sym::result, sym::Result, sym::Ok]);
+        self.expr_call_global(sp, ok, vec![expr])
+    }
+
+    pub fn expr_try(&self, sp: Span, head: P<ast::Expr>) -> P<ast::Expr> {
+        let ok = self.std_path(&[sym::result, sym::Result, sym::Ok]);
+        let ok_path = self.path_global(sp, ok);
+        let err = self.std_path(&[sym::result, sym::Result, sym::Err]);
+        let err_path = self.path_global(sp, err);
+
+        let binding_variable = Ident::new(sym::__try_var, sp);
+        let binding_pat = self.pat_ident(sp, binding_variable);
+        let binding_expr = self.expr_ident(sp, binding_variable);
+
+        // `Ok(__try_var)` pattern
+        let ok_pat = self.pat_tuple_struct(sp, ok_path, vec![binding_pat.clone()]);
+
+        // `Err(__try_var)` (pattern and expression respectively)
+        let err_pat = self.pat_tuple_struct(sp, err_path.clone(), vec![binding_pat]);
+        let err_inner_expr =
+            self.expr_call(sp, self.expr_path(err_path), vec![binding_expr.clone()]);
+        // `return Err(__try_var)`
+        let err_expr = self.expr(sp, ast::ExprKind::Ret(Some(err_inner_expr)));
+
+        // `Ok(__try_var) => __try_var`
+        let ok_arm = self.arm(sp, ok_pat, binding_expr);
+        // `Err(__try_var) => return Err(__try_var)`
+        let err_arm = self.arm(sp, err_pat, err_expr);
+
+        // `match head { Ok() => ..., Err() => ... }`
+        self.expr_match(sp, head, vec![ok_arm, err_arm])
+    }
+
+    pub fn pat(&self, span: Span, kind: PatKind) -> P<ast::Pat> {
+        P(ast::Pat { id: ast::DUMMY_NODE_ID, kind, span, tokens: None })
+    }
+    pub fn pat_wild(&self, span: Span) -> P<ast::Pat> {
+        self.pat(span, PatKind::Wild)
+    }
+    pub fn pat_lit(&self, span: Span, expr: P<ast::Expr>) -> P<ast::Pat> {
+        self.pat(span, PatKind::Lit(expr))
+    }
+    pub fn pat_ident(&self, span: Span, ident: Ident) -> P<ast::Pat> {
+        let binding_mode = ast::BindingMode::ByValue(ast::Mutability::Not);
+        self.pat_ident_binding_mode(span, ident, binding_mode)
+    }
+
+    pub fn pat_ident_binding_mode(
+        &self,
+        span: Span,
+        ident: Ident,
+        bm: ast::BindingMode,
+    ) -> P<ast::Pat> {
+        let pat = PatKind::Ident(bm, ident.with_span_pos(span), None);
+        self.pat(span, pat)
+    }
+    pub fn pat_path(&self, span: Span, path: ast::Path) -> P<ast::Pat> {
+        self.pat(span, PatKind::Path(None, path))
+    }
+    pub fn pat_tuple_struct(
+        &self,
+        span: Span,
+        path: ast::Path,
+        subpats: Vec<P<ast::Pat>>,
+    ) -> P<ast::Pat> {
+        self.pat(span, PatKind::TupleStruct(None, path, subpats))
+    }
+    pub fn pat_struct(
+        &self,
+        span: Span,
+        path: ast::Path,
+        field_pats: Vec<ast::PatField>,
+    ) -> P<ast::Pat> {
+        self.pat(span, PatKind::Struct(None, path, field_pats, false))
+    }
+    pub fn pat_tuple(&self, span: Span, pats: Vec<P<ast::Pat>>) -> P<ast::Pat> {
+        self.pat(span, PatKind::Tuple(pats))
+    }
+
+    pub fn pat_some(&self, span: Span, pat: P<ast::Pat>) -> P<ast::Pat> {
+        let some = self.std_path(&[sym::option, sym::Option, sym::Some]);
+        let path = self.path_global(span, some);
+        self.pat_tuple_struct(span, path, vec![pat])
+    }
+
+    pub fn arm(&self, span: Span, pat: P<ast::Pat>, expr: P<ast::Expr>) -> ast::Arm {
+        ast::Arm {
+            attrs: AttrVec::new(),
+            pat,
+            guard: None,
+            body: expr,
+            span,
+            id: ast::DUMMY_NODE_ID,
+            is_placeholder: false,
+        }
+    }
+
+    pub fn arm_unreachable(&self, span: Span) -> ast::Arm {
+        self.arm(span, self.pat_wild(span), self.expr_unreachable(span))
+    }
+
+    pub fn expr_match(&self, span: Span, arg: P<ast::Expr>, arms: Vec<ast::Arm>) -> P<Expr> {
+        self.expr(span, ast::ExprKind::Match(arg, arms))
+    }
+
+    pub fn expr_if(
+        &self,
+        span: Span,
+        cond: P<ast::Expr>,
+        then: P<ast::Expr>,
+        els: Option<P<ast::Expr>>,
+    ) -> P<ast::Expr> {
+        let els = els.map(|x| self.expr_block(self.block_expr(x)));
+        self.expr(span, ast::ExprKind::If(cond, self.block_expr(then), els))
+    }
+
+    pub fn lambda(&self, span: Span, ids: Vec<Ident>, body: P<ast::Expr>) -> P<ast::Expr> {
+        let fn_decl = self.fn_decl(
+            ids.iter().map(|id| self.param(span, *id, self.ty(span, ast::TyKind::Infer))).collect(),
+            ast::FnRetTy::Default(span),
+        );
+
+        // FIXME -- We are using `span` as the span of the `|...|`
+        // part of the lambda, but it probably (maybe?) corresponds to
+        // the entire lambda body. Probably we should extend the API
+        // here, but that's not entirely clear.
+        self.expr(
+            span,
+            ast::ExprKind::Closure(
+                ast::ClosureBinder::NotPresent,
+                ast::CaptureBy::Ref,
+                ast::Async::No,
+                ast::Movability::Movable,
+                fn_decl,
+                body,
+                span,
+            ),
+        )
+    }
+
+    pub fn lambda0(&self, span: Span, body: P<ast::Expr>) -> P<ast::Expr> {
+        self.lambda(span, Vec::new(), body)
+    }
+
+    pub fn lambda1(&self, span: Span, body: P<ast::Expr>, ident: Ident) -> P<ast::Expr> {
+        self.lambda(span, vec![ident], body)
+    }
+
+    pub fn lambda_stmts_1(&self, span: Span, stmts: Vec<ast::Stmt>, ident: Ident) -> P<ast::Expr> {
+        self.lambda1(span, self.expr_block(self.block(span, stmts)), ident)
+    }
+
+    pub fn param(&self, span: Span, ident: Ident, ty: P<ast::Ty>) -> ast::Param {
+        let arg_pat = self.pat_ident(span, ident);
+        ast::Param {
+            attrs: AttrVec::default(),
+            id: ast::DUMMY_NODE_ID,
+            pat: arg_pat,
+            span,
+            ty,
+            is_placeholder: false,
+        }
+    }
+
+    // `self` is unused but keep it as method for the convenience use.
+    pub fn fn_decl(&self, inputs: Vec<ast::Param>, output: ast::FnRetTy) -> P<ast::FnDecl> {
+        P(ast::FnDecl { inputs, output })
+    }
+
+    pub fn item(
+        &self,
+        span: Span,
+        name: Ident,
+        attrs: Vec<ast::Attribute>,
+        kind: ast::ItemKind,
+    ) -> P<ast::Item> {
+        // FIXME: Would be nice if our generated code didn't violate
+        // Rust coding conventions
+        P(ast::Item {
+            ident: name,
+            attrs,
+            id: ast::DUMMY_NODE_ID,
+            kind,
+            vis: ast::Visibility {
+                span: span.shrink_to_lo(),
+                kind: ast::VisibilityKind::Inherited,
+                tokens: None,
+            },
+            span,
+            tokens: None,
+        })
+    }
+
+    pub fn item_static(
+        &self,
+        span: Span,
+        name: Ident,
+        ty: P<ast::Ty>,
+        mutbl: ast::Mutability,
+        expr: P<ast::Expr>,
+    ) -> P<ast::Item> {
+        self.item(span, name, Vec::new(), ast::ItemKind::Static(ty, mutbl, Some(expr)))
+    }
+
+    pub fn item_const(
+        &self,
+        span: Span,
+        name: Ident,
+        ty: P<ast::Ty>,
+        expr: P<ast::Expr>,
+    ) -> P<ast::Item> {
+        let def = ast::Defaultness::Final;
+        self.item(span, name, Vec::new(), ast::ItemKind::Const(def, ty, Some(expr)))
+    }
+
+    pub fn attribute(&self, mi: ast::MetaItem) -> ast::Attribute {
+        attr::mk_attr_outer(mi)
+    }
+
+    pub fn meta_word(&self, sp: Span, w: Symbol) -> ast::MetaItem {
+        attr::mk_word_item(Ident::new(w, sp))
+    }
+}
diff --git a/compiler/rustc_expand/src/config.rs b/compiler/rustc_expand/src/config.rs
new file mode 100644
index 000000000..3e1acf438
--- /dev/null
+++ b/compiler/rustc_expand/src/config.rs
@@ -0,0 +1,535 @@
+//! Conditional compilation stripping.
+
+use rustc_ast::ptr::P;
+use rustc_ast::token::{Delimiter, Token, TokenKind};
+use rustc_ast::tokenstream::{AttrAnnotatedTokenStream, AttrAnnotatedTokenTree};
+use rustc_ast::tokenstream::{DelimSpan, Spacing};
+use rustc_ast::tokenstream::{LazyTokenStream, TokenTree};
+use rustc_ast::NodeId;
+use rustc_ast::{self as ast, AttrStyle, Attribute, HasAttrs, HasTokens, MetaItem};
+use rustc_attr as attr;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::map_in_place::MapInPlace;
+use rustc_errors::{error_code, struct_span_err, Applicability, Handler};
+use rustc_feature::{Feature, Features, State as FeatureState};
+use rustc_feature::{
+    ACCEPTED_FEATURES, ACTIVE_FEATURES, REMOVED_FEATURES, STABLE_REMOVED_FEATURES,
+};
+use rustc_parse::validate_attr;
+use rustc_session::parse::feature_err;
+use rustc_session::Session;
+use rustc_span::edition::{Edition, ALL_EDITIONS};
+use rustc_span::symbol::{sym, Symbol};
+use rustc_span::{Span, DUMMY_SP};
+
+/// A folder that strips out items that do not belong in the current configuration.
+pub struct StripUnconfigured<'a> {
+    pub sess: &'a Session,
+    pub features: Option<&'a Features>,
+    /// If `true`, perform cfg-stripping on attached tokens.
+    /// This is only used for the input to derive macros,
+    /// which needs eager expansion of `cfg` and `cfg_attr`
+    pub config_tokens: bool,
+    pub lint_node_id: NodeId,
+}
+
+fn get_features(
+    sess: &Session,
+    span_handler: &Handler,
+    krate_attrs: &[ast::Attribute],
+) -> Features {
+    fn feature_removed(span_handler: &Handler, span: Span, reason: Option<&str>) {
+        let mut err = struct_span_err!(span_handler, span, E0557, "feature has been removed");
+        err.span_label(span, "feature has been removed");
+        if let Some(reason) = reason {
+            err.note(reason);
+        }
+        err.emit();
+    }
+
+    fn active_features_up_to(edition: Edition) -> impl Iterator<Item = &'static Feature> {
+        ACTIVE_FEATURES.iter().filter(move |feature| {
+            if let Some(feature_edition) = feature.edition {
+                feature_edition <= edition
+            } else {
+                false
+            }
+        })
+    }
+
+    let mut features = Features::default();
+    let mut edition_enabled_features = FxHashMap::default();
+    let crate_edition = sess.edition();
+
+    for &edition in ALL_EDITIONS {
+        if edition <= crate_edition {
+            // The `crate_edition` implies its respective umbrella feature-gate
+            // (i.e., `#![feature(rust_20XX_preview)]` isn't needed on edition 20XX).
+            edition_enabled_features.insert(edition.feature_name(), edition);
+        }
+    }
+
+    for feature in active_features_up_to(crate_edition) {
+        feature.set(&mut features, DUMMY_SP);
+        edition_enabled_features.insert(feature.name, crate_edition);
+    }
+
+    // Process the edition umbrella feature-gates first, to ensure
+    // `edition_enabled_features` is completed before it's queried.
+    for attr in krate_attrs {
+        if !attr.has_name(sym::feature) {
+            continue;
+        }
+
+        let Some(list) = attr.meta_item_list() else {
+            continue;
+        };
+
+        for mi in list {
+            if !mi.is_word() {
+                continue;
+            }
+
+            let name = mi.name_or_empty();
+
+            let edition = ALL_EDITIONS.iter().find(|e| name == e.feature_name()).copied();
+            if let Some(edition) = edition {
+                if edition <= crate_edition {
+                    continue;
+                }
+
+                for feature in active_features_up_to(edition) {
+                    // FIXME(Manishearth) there is currently no way to set
+                    // lib features by edition
+                    feature.set(&mut features, DUMMY_SP);
+                    edition_enabled_features.insert(feature.name, edition);
+                }
+            }
+        }
+    }
+
+    for attr in krate_attrs {
+        if !attr.has_name(sym::feature) {
+            continue;
+        }
+
+        let Some(list) = attr.meta_item_list() else {
+            continue;
+        };
+
+        let bad_input = |span| {
+            struct_span_err!(span_handler, span, E0556, "malformed `feature` attribute input")
+        };
+
+        for mi in list {
+            let name = match mi.ident() {
+                Some(ident) if mi.is_word() => ident.name,
+                Some(ident) => {
+                    bad_input(mi.span())
+                        .span_suggestion(
+                            mi.span(),
+                            "expected just one word",
+                            ident.name,
+                            Applicability::MaybeIncorrect,
+                        )
+                        .emit();
+                    continue;
+                }
+                None => {
+                    bad_input(mi.span()).span_label(mi.span(), "expected just one word").emit();
+                    continue;
+                }
+            };
+
+            if let Some(edition) = edition_enabled_features.get(&name) {
+                let msg =
+                    &format!("the feature `{}` is included in the Rust {} edition", name, edition);
+                span_handler.struct_span_warn_with_code(mi.span(), msg, error_code!(E0705)).emit();
+                continue;
+            }
+
+            if ALL_EDITIONS.iter().any(|e| name == e.feature_name()) {
+                // Handled in the separate loop above.
+                continue;
+            }
+
+            let removed = REMOVED_FEATURES.iter().find(|f| name == f.name);
+            let stable_removed = STABLE_REMOVED_FEATURES.iter().find(|f| name == f.name);
+            if let Some(Feature { state, .. }) = removed.or(stable_removed) {
+                if let FeatureState::Removed { reason } | FeatureState::Stabilized { reason } =
+                    state
+                {
+                    feature_removed(span_handler, mi.span(), *reason);
+                    continue;
+                }
+            }
+
+            if let Some(Feature { since, .. }) = ACCEPTED_FEATURES.iter().find(|f| name == f.name) {
+                let since = Some(Symbol::intern(since));
+                features.declared_lang_features.push((name, mi.span(), since));
+                features.active_features.insert(name);
+                continue;
+            }
+
+            if let Some(allowed) = sess.opts.unstable_opts.allow_features.as_ref() {
+                if allowed.iter().all(|f| name.as_str() != f) {
+                    struct_span_err!(
+                        span_handler,
+                        mi.span(),
+                        E0725,
+                        "the feature `{}` is not in the list of allowed features",
+                        name
+                    )
+                    .emit();
+                    continue;
+                }
+            }
+
+            if let Some(f) = ACTIVE_FEATURES.iter().find(|f| name == f.name) {
+                f.set(&mut features, mi.span());
+                features.declared_lang_features.push((name, mi.span(), None));
+                features.active_features.insert(name);
+                continue;
+            }
+
+            features.declared_lib_features.push((name, mi.span()));
+            features.active_features.insert(name);
+        }
+    }
+
+    features
+}
+
+// `cfg_attr`-process the crate's attributes and compute the crate's features.
+pub fn features(
+    sess: &Session,
+    mut krate: ast::Crate,
+    lint_node_id: NodeId,
+) -> (ast::Crate, Features) {
+    let mut strip_unconfigured =
+        StripUnconfigured { sess, features: None, config_tokens: false, lint_node_id };
+
+    let unconfigured_attrs = krate.attrs.clone();
+    let diag = &sess.parse_sess.span_diagnostic;
+    let err_count = diag.err_count();
+    let features = match strip_unconfigured.configure_krate_attrs(krate.attrs) {
+        None => {
+            // The entire crate is unconfigured.
+            krate.attrs = Vec::new();
+            krate.items = Vec::new();
+            Features::default()
+        }
+        Some(attrs) => {
+            krate.attrs = attrs;
+            let features = get_features(sess, diag, &krate.attrs);
+            if err_count == diag.err_count() {
+                // Avoid reconfiguring malformed `cfg_attr`s.
+                strip_unconfigured.features = Some(&features);
+                // Run configuration again, this time with features available
+                // so that we can perform feature-gating.
+                strip_unconfigured.configure_krate_attrs(unconfigured_attrs);
+            }
+            features
+        }
+    };
+    (krate, features)
+}
+
+#[macro_export]
+macro_rules! configure {
+    ($this:ident, $node:ident) => {
+        match $this.configure($node) {
+            Some(node) => node,
+            None => return Default::default(),
+        }
+    };
+}
+
+impl<'a> StripUnconfigured<'a> {
+    pub fn configure<T: HasAttrs + HasTokens>(&self, mut node: T) -> Option<T> {
+        self.process_cfg_attrs(&mut node);
+        if self.in_cfg(node.attrs()) {
+            self.try_configure_tokens(&mut node);
+            Some(node)
+        } else {
+            None
+        }
+    }
+
+    fn try_configure_tokens<T: HasTokens>(&self, node: &mut T) {
+        if self.config_tokens {
+            if let Some(Some(tokens)) = node.tokens_mut() {
+                let attr_annotated_tokens = tokens.create_token_stream();
+                *tokens = LazyTokenStream::new(self.configure_tokens(&attr_annotated_tokens));
+            }
+        }
+    }
+
+    fn configure_krate_attrs(&self, mut attrs: Vec<ast::Attribute>) -> Option<Vec<ast::Attribute>> {
+        attrs.flat_map_in_place(|attr| self.process_cfg_attr(attr));
+        if self.in_cfg(&attrs) { Some(attrs) } else { None }
+    }
+
+    /// Performs cfg-expansion on `stream`, producing a new `AttrAnnotatedTokenStream`.
+    /// This is only used during the invocation of `derive` proc-macros,
+    /// which require that we cfg-expand their entire input.
+    /// Normal cfg-expansion operates on parsed AST nodes via the `configure` method
+    fn configure_tokens(&self, stream: &AttrAnnotatedTokenStream) -> AttrAnnotatedTokenStream {
+        fn can_skip(stream: &AttrAnnotatedTokenStream) -> bool {
+            stream.0.iter().all(|(tree, _spacing)| match tree {
+                AttrAnnotatedTokenTree::Attributes(_) => false,
+                AttrAnnotatedTokenTree::Token(_) => true,
+                AttrAnnotatedTokenTree::Delimited(_, _, inner) => can_skip(inner),
+            })
+        }
+
+        if can_skip(stream) {
+            return stream.clone();
+        }
+
+        let trees: Vec<_> = stream
+            .0
+            .iter()
+            .flat_map(|(tree, spacing)| match tree.clone() {
+                AttrAnnotatedTokenTree::Attributes(mut data) => {
+                    let mut attrs: Vec<_> = std::mem::take(&mut data.attrs).into();
+                    attrs.flat_map_in_place(|attr| self.process_cfg_attr(attr));
+                    data.attrs = attrs.into();
+
+                    if self.in_cfg(&data.attrs) {
+                        data.tokens = LazyTokenStream::new(
+                            self.configure_tokens(&data.tokens.create_token_stream()),
+                        );
+                        Some((AttrAnnotatedTokenTree::Attributes(data), *spacing)).into_iter()
+                    } else {
+                        None.into_iter()
+                    }
+                }
+                AttrAnnotatedTokenTree::Delimited(sp, delim, mut inner) => {
+                    inner = self.configure_tokens(&inner);
+                    Some((AttrAnnotatedTokenTree::Delimited(sp, delim, inner), *spacing))
+                        .into_iter()
+                }
+                AttrAnnotatedTokenTree::Token(ref token) if let TokenKind::Interpolated(ref nt) = token.kind => {
+                    panic!(
+                        "Nonterminal should have been flattened at {:?}: {:?}",
+                        token.span, nt
+                    );
+                }
+                AttrAnnotatedTokenTree::Token(token) => {
+                    Some((AttrAnnotatedTokenTree::Token(token), *spacing)).into_iter()
+                }
+            })
+            .collect();
+        AttrAnnotatedTokenStream::new(trees)
+    }
+
+    /// Parse and expand all `cfg_attr` attributes into a list of attributes
+    /// that are within each `cfg_attr` that has a true configuration predicate.
+    ///
+    /// Gives compiler warnings if any `cfg_attr` does not contain any
+    /// attributes and is in the original source code. Gives compiler errors if
+    /// the syntax of any `cfg_attr` is incorrect.
+    fn process_cfg_attrs<T: HasAttrs>(&self, node: &mut T) {
+        node.visit_attrs(|attrs| {
+            attrs.flat_map_in_place(|attr| self.process_cfg_attr(attr));
+        });
+    }
+
+    fn process_cfg_attr(&self, attr: Attribute) -> Vec<Attribute> {
+        if attr.has_name(sym::cfg_attr) { self.expand_cfg_attr(attr, true) } else { vec![attr] }
+    }
+
+    /// Parse and expand a single `cfg_attr` attribute into a list of attributes
+    /// when the configuration predicate is true, or otherwise expand into an
+    /// empty list of attributes.
+    ///
+    /// Gives a compiler warning when the `cfg_attr` contains no attributes and
+    /// is in the original source file. Gives a compiler error if the syntax of
+    /// the attribute is incorrect.
+    pub(crate) fn expand_cfg_attr(&self, attr: Attribute, recursive: bool) -> Vec<Attribute> {
+        let Some((cfg_predicate, expanded_attrs)) =
+            rustc_parse::parse_cfg_attr(&attr, &self.sess.parse_sess) else {
+                return vec![];
+            };
+
+        // Lint on zero attributes in source.
+        if expanded_attrs.is_empty() {
+            self.sess.parse_sess.buffer_lint(
+                rustc_lint_defs::builtin::UNUSED_ATTRIBUTES,
+                attr.span,
+                ast::CRATE_NODE_ID,
+                "`#[cfg_attr]` does not expand to any attributes",
+            );
+        }
+
+        if !attr::cfg_matches(
+            &cfg_predicate,
+            &self.sess.parse_sess,
+            self.lint_node_id,
+            self.features,
+        ) {
+            return vec![];
+        }
+
+        if recursive {
+            // We call `process_cfg_attr` recursively in case there's a
+            // `cfg_attr` inside of another `cfg_attr`. E.g.
+            //  `#[cfg_attr(false, cfg_attr(true, some_attr))]`.
+            expanded_attrs
+                .into_iter()
+                .flat_map(|item| self.process_cfg_attr(self.expand_cfg_attr_item(&attr, item)))
+                .collect()
+        } else {
+            expanded_attrs.into_iter().map(|item| self.expand_cfg_attr_item(&attr, item)).collect()
+        }
+    }
+
+    fn expand_cfg_attr_item(
+        &self,
+        attr: &Attribute,
+        (item, item_span): (ast::AttrItem, Span),
+    ) -> Attribute {
+        let orig_tokens = attr.tokens().to_tokenstream();
+
+        // We are taking an attribute of the form `#[cfg_attr(pred, attr)]`
+        // and producing an attribute of the form `#[attr]`. We
+        // have captured tokens for `attr` itself, but we need to
+        // synthesize tokens for the wrapper `#` and `[]`, which
+        // we do below.
+
+        // Use the `#` in `#[cfg_attr(pred, attr)]` as the `#` token
+        // for `attr` when we expand it to `#[attr]`
+        let mut orig_trees = orig_tokens.into_trees();
+        let TokenTree::Token(pound_token @ Token { kind: TokenKind::Pound, .. }, _) = orig_trees.next().unwrap() else {
+            panic!("Bad tokens for attribute {:?}", attr);
+        };
+        let pound_span = pound_token.span;
+
+        let mut trees = vec![(AttrAnnotatedTokenTree::Token(pound_token), Spacing::Alone)];
+        if attr.style == AttrStyle::Inner {
+            // For inner attributes, we do the same thing for the `!` in `#![some_attr]`
+            let TokenTree::Token(bang_token @ Token { kind: TokenKind::Not, .. }, _) = orig_trees.next().unwrap() else {
+                panic!("Bad tokens for attribute {:?}", attr);
+            };
+            trees.push((AttrAnnotatedTokenTree::Token(bang_token), Spacing::Alone));
+        }
+        // We don't really have a good span to use for the synthesized `[]`
+        // in `#[attr]`, so just use the span of the `#` token.
+        let bracket_group = AttrAnnotatedTokenTree::Delimited(
+            DelimSpan::from_single(pound_span),
+            Delimiter::Bracket,
+            item.tokens
+                .as_ref()
+                .unwrap_or_else(|| panic!("Missing tokens for {:?}", item))
+                .create_token_stream(),
+        );
+        trees.push((bracket_group, Spacing::Alone));
+        let tokens = Some(LazyTokenStream::new(AttrAnnotatedTokenStream::new(trees)));
+        let attr = attr::mk_attr_from_item(item, tokens, attr.style, item_span);
+        if attr.has_name(sym::crate_type) {
+            self.sess.parse_sess.buffer_lint(
+                rustc_lint_defs::builtin::DEPRECATED_CFG_ATTR_CRATE_TYPE_NAME,
+                attr.span,
+                ast::CRATE_NODE_ID,
+                "`crate_type` within an `#![cfg_attr] attribute is deprecated`",
+            );
+        }
+        if attr.has_name(sym::crate_name) {
+            self.sess.parse_sess.buffer_lint(
+                rustc_lint_defs::builtin::DEPRECATED_CFG_ATTR_CRATE_TYPE_NAME,
+                attr.span,
+                ast::CRATE_NODE_ID,
+                "`crate_name` within an `#![cfg_attr] attribute is deprecated`",
+            );
+        }
+        attr
+    }
+
+    /// Determines if a node with the given attributes should be included in this configuration.
+    fn in_cfg(&self, attrs: &[Attribute]) -> bool {
+        attrs.iter().all(|attr| !is_cfg(attr) || self.cfg_true(attr))
+    }
+
+    pub(crate) fn cfg_true(&self, attr: &Attribute) -> bool {
+        let meta_item = match validate_attr::parse_meta(&self.sess.parse_sess, attr) {
+            Ok(meta_item) => meta_item,
+            Err(mut err) => {
+                err.emit();
+                return true;
+            }
+        };
+        parse_cfg(&meta_item, &self.sess).map_or(true, |meta_item| {
+            attr::cfg_matches(&meta_item, &self.sess.parse_sess, self.lint_node_id, self.features)
+        })
+    }
+
+    /// If attributes are not allowed on expressions, emit an error for `attr`
+    pub(crate) fn maybe_emit_expr_attr_err(&self, attr: &Attribute) {
+        if !self.features.map_or(true, |features| features.stmt_expr_attributes) {
+            let mut err = feature_err(
+                &self.sess.parse_sess,
+                sym::stmt_expr_attributes,
+                attr.span,
+                "attributes on expressions are experimental",
+            );
+
+            if attr.is_doc_comment() {
+                err.help("`///` is for documentation comments. For a plain comment, use `//`.");
+            }
+
+            err.emit();
+        }
+    }
+
+    pub fn configure_expr(&self, expr: &mut P<ast::Expr>) {
+        for attr in expr.attrs.iter() {
+            self.maybe_emit_expr_attr_err(attr);
+        }
+
+        // If an expr is valid to cfg away it will have been removed by the
+        // outer stmt or expression folder before descending in here.
+        // Anything else is always required, and thus has to error out
+        // in case of a cfg attr.
+        //
+        // N.B., this is intentionally not part of the visit_expr() function
+        //     in order for filter_map_expr() to be able to avoid this check
+        if let Some(attr) = expr.attrs().iter().find(|a| is_cfg(*a)) {
+            let msg = "removing an expression is not supported in this position";
+            self.sess.parse_sess.span_diagnostic.span_err(attr.span, msg);
+        }
+
+        self.process_cfg_attrs(expr);
+        self.try_configure_tokens(&mut *expr);
+    }
+}
+
+pub fn parse_cfg<'a>(meta_item: &'a MetaItem, sess: &Session) -> Option<&'a MetaItem> {
+    let error = |span, msg, suggestion: &str| {
+        let mut err = sess.parse_sess.span_diagnostic.struct_span_err(span, msg);
+        if !suggestion.is_empty() {
+            err.span_suggestion(
+                span,
+                "expected syntax is",
+                suggestion,
+                Applicability::HasPlaceholders,
+            );
+        }
+        err.emit();
+        None
+    };
+    let span = meta_item.span;
+    match meta_item.meta_item_list() {
+        None => error(span, "`cfg` is not followed by parentheses", "cfg(/* predicate */)"),
+        Some([]) => error(span, "`cfg` predicate is not specified", ""),
+        Some([_, .., l]) => error(l.span(), "multiple `cfg` predicates are specified", ""),
+        Some([single]) => match single.meta_item() {
+            Some(meta_item) => Some(meta_item),
+            None => error(single.span(), "`cfg` predicate key cannot be a literal", ""),
+        },
+    }
+}
+
+fn is_cfg(attr: &Attribute) -> bool {
+    attr.has_name(sym::cfg)
+}
diff --git a/compiler/rustc_expand/src/expand.rs b/compiler/rustc_expand/src/expand.rs
new file mode 100644
index 000000000..93eeca5b2
--- /dev/null
+++ b/compiler/rustc_expand/src/expand.rs
@@ -0,0 +1,1888 @@
+use crate::base::*;
+use crate::config::StripUnconfigured;
+use crate::hygiene::SyntaxContext;
+use crate::mbe::macro_rules::annotate_err_with_kind;
+use crate::module::{mod_dir_path, parse_external_mod, DirOwnership, ParsedExternalMod};
+use crate::placeholders::{placeholder, PlaceholderExpander};
+
+use rustc_ast as ast;
+use rustc_ast::mut_visit::*;
+use rustc_ast::ptr::P;
+use rustc_ast::token::{self, Delimiter};
+use rustc_ast::tokenstream::TokenStream;
+use rustc_ast::visit::{self, AssocCtxt, Visitor};
+use rustc_ast::{AssocItemKind, AstNodeWrapper, AttrStyle, ExprKind, ForeignItemKind};
+use rustc_ast::{HasAttrs, HasNodeId};
+use rustc_ast::{Inline, ItemKind, MacArgs, MacStmtStyle, MetaItemKind, ModKind};
+use rustc_ast::{NestedMetaItem, NodeId, PatKind, StmtKind, TyKind};
+use rustc_ast_pretty::pprust;
+use rustc_data_structures::map_in_place::MapInPlace;
+use rustc_data_structures::sync::Lrc;
+use rustc_errors::{Applicability, PResult};
+use rustc_feature::Features;
+use rustc_parse::parser::{
+    AttemptLocalParseRecovery, CommaRecoveryMode, ForceCollect, Parser, RecoverColon, RecoverComma,
+};
+use rustc_parse::validate_attr;
+use rustc_session::lint::builtin::{UNUSED_ATTRIBUTES, UNUSED_DOC_COMMENTS};
+use rustc_session::lint::BuiltinLintDiagnostics;
+use rustc_session::parse::{feature_err, ParseSess};
+use rustc_session::Limit;
+use rustc_span::symbol::{sym, Ident};
+use rustc_span::{FileName, LocalExpnId, Span};
+
+use smallvec::SmallVec;
+use std::ops::Deref;
+use std::path::PathBuf;
+use std::rc::Rc;
+use std::{iter, mem};
+
+macro_rules! ast_fragments {
+    (
+        $($Kind:ident($AstTy:ty) {
+            $kind_name:expr;
+            $(one fn $mut_visit_ast:ident; fn $visit_ast:ident;)?
+            $(many fn $flat_map_ast_elt:ident; fn $visit_ast_elt:ident($($args:tt)*);)?
+            fn $make_ast:ident;
+        })*
+    ) => {
+        /// A fragment of AST that can be produced by a single macro expansion.
+        /// Can also serve as an input and intermediate result for macro expansion operations.
+        pub enum AstFragment {
+            OptExpr(Option<P<ast::Expr>>),
+            $($Kind($AstTy),)*
+        }
+
+        /// "Discriminant" of an AST fragment.
+        #[derive(Copy, Clone, PartialEq, Eq)]
+        pub enum AstFragmentKind {
+            OptExpr,
+            $($Kind,)*
+        }
+
+        impl AstFragmentKind {
+            pub fn name(self) -> &'static str {
+                match self {
+                    AstFragmentKind::OptExpr => "expression",
+                    $(AstFragmentKind::$Kind => $kind_name,)*
+                }
+            }
+
+            fn make_from<'a>(self, result: Box<dyn MacResult + 'a>) -> Option<AstFragment> {
+                match self {
+                    AstFragmentKind::OptExpr =>
+                        result.make_expr().map(Some).map(AstFragment::OptExpr),
+                    $(AstFragmentKind::$Kind => result.$make_ast().map(AstFragment::$Kind),)*
+                }
+            }
+        }
+
+        impl AstFragment {
+            pub fn add_placeholders(&mut self, placeholders: &[NodeId]) {
+                if placeholders.is_empty() {
+                    return;
+                }
+                match self {
+                    $($(AstFragment::$Kind(ast) => ast.extend(placeholders.iter().flat_map(|id| {
+                        ${ignore(flat_map_ast_elt)}
+                        placeholder(AstFragmentKind::$Kind, *id, None).$make_ast()
+                    })),)?)*
+                    _ => panic!("unexpected AST fragment kind")
+                }
+            }
+
+            pub fn make_opt_expr(self) -> Option<P<ast::Expr>> {
+                match self {
+                    AstFragment::OptExpr(expr) => expr,
+                    _ => panic!("AstFragment::make_* called on the wrong kind of fragment"),
+                }
+            }
+
+            $(pub fn $make_ast(self) -> $AstTy {
+                match self {
+                    AstFragment::$Kind(ast) => ast,
+                    _ => panic!("AstFragment::make_* called on the wrong kind of fragment"),
+                }
+            })*
+
+            fn make_ast<T: InvocationCollectorNode>(self) -> T::OutputTy {
+                T::fragment_to_output(self)
+            }
+
+            pub fn mut_visit_with<F: MutVisitor>(&mut self, vis: &mut F) {
+                match self {
+                    AstFragment::OptExpr(opt_expr) => {
+                        visit_clobber(opt_expr, |opt_expr| {
+                            if let Some(expr) = opt_expr {
+                                vis.filter_map_expr(expr)
+                            } else {
+                                None
+                            }
+                        });
+                    }
+                    $($(AstFragment::$Kind(ast) => vis.$mut_visit_ast(ast),)?)*
+                    $($(AstFragment::$Kind(ast) =>
+                        ast.flat_map_in_place(|ast| vis.$flat_map_ast_elt(ast)),)?)*
+                }
+            }
+
+            pub fn visit_with<'a, V: Visitor<'a>>(&'a self, visitor: &mut V) {
+                match *self {
+                    AstFragment::OptExpr(Some(ref expr)) => visitor.visit_expr(expr),
+                    AstFragment::OptExpr(None) => {}
+                    $($(AstFragment::$Kind(ref ast) => visitor.$visit_ast(ast),)?)*
+                    $($(AstFragment::$Kind(ref ast) => for ast_elt in &ast[..] {
+                        visitor.$visit_ast_elt(ast_elt, $($args)*);
+                    })?)*
+                }
+            }
+        }
+
+        impl<'a> MacResult for crate::mbe::macro_rules::ParserAnyMacro<'a> {
+            $(fn $make_ast(self: Box<crate::mbe::macro_rules::ParserAnyMacro<'a>>)
+                           -> Option<$AstTy> {
+                Some(self.make(AstFragmentKind::$Kind).$make_ast())
+            })*
+        }
+    }
+}
+
+ast_fragments! {
+    Expr(P<ast::Expr>) { "expression"; one fn visit_expr; fn visit_expr; fn make_expr; }
+    Pat(P<ast::Pat>) { "pattern"; one fn visit_pat; fn visit_pat; fn make_pat; }
+    Ty(P<ast::Ty>) { "type"; one fn visit_ty; fn visit_ty; fn make_ty; }
+    Stmts(SmallVec<[ast::Stmt; 1]>) {
+        "statement"; many fn flat_map_stmt; fn visit_stmt(); fn make_stmts;
+    }
+    Items(SmallVec<[P<ast::Item>; 1]>) {
+        "item"; many fn flat_map_item; fn visit_item(); fn make_items;
+    }
+    TraitItems(SmallVec<[P<ast::AssocItem>; 1]>) {
+        "trait item";
+        many fn flat_map_trait_item;
+        fn visit_assoc_item(AssocCtxt::Trait);
+        fn make_trait_items;
+    }
+    ImplItems(SmallVec<[P<ast::AssocItem>; 1]>) {
+        "impl item";
+        many fn flat_map_impl_item;
+        fn visit_assoc_item(AssocCtxt::Impl);
+        fn make_impl_items;
+    }
+    ForeignItems(SmallVec<[P<ast::ForeignItem>; 1]>) {
+        "foreign item";
+        many fn flat_map_foreign_item;
+        fn visit_foreign_item();
+        fn make_foreign_items;
+    }
+    Arms(SmallVec<[ast::Arm; 1]>) {
+        "match arm"; many fn flat_map_arm; fn visit_arm(); fn make_arms;
+    }
+    ExprFields(SmallVec<[ast::ExprField; 1]>) {
+        "field expression"; many fn flat_map_expr_field; fn visit_expr_field(); fn make_expr_fields;
+    }
+    PatFields(SmallVec<[ast::PatField; 1]>) {
+        "field pattern";
+        many fn flat_map_pat_field;
+        fn visit_pat_field();
+        fn make_pat_fields;
+    }
+    GenericParams(SmallVec<[ast::GenericParam; 1]>) {
+        "generic parameter";
+        many fn flat_map_generic_param;
+        fn visit_generic_param();
+        fn make_generic_params;
+    }
+    Params(SmallVec<[ast::Param; 1]>) {
+        "function parameter"; many fn flat_map_param; fn visit_param(); fn make_params;
+    }
+    FieldDefs(SmallVec<[ast::FieldDef; 1]>) {
+        "field";
+        many fn flat_map_field_def;
+        fn visit_field_def();
+        fn make_field_defs;
+    }
+    Variants(SmallVec<[ast::Variant; 1]>) {
+        "variant"; many fn flat_map_variant; fn visit_variant(); fn make_variants;
+    }
+    Crate(ast::Crate) { "crate"; one fn visit_crate; fn visit_crate; fn make_crate; }
+}
+
+pub enum SupportsMacroExpansion {
+    No,
+    Yes { supports_inner_attrs: bool },
+}
+
+impl AstFragmentKind {
+    pub(crate) fn dummy(self, span: Span) -> AstFragment {
+        self.make_from(DummyResult::any(span)).expect("couldn't create a dummy AST fragment")
+    }
+
+    pub fn supports_macro_expansion(self) -> SupportsMacroExpansion {
+        match self {
+            AstFragmentKind::OptExpr
+            | AstFragmentKind::Expr
+            | AstFragmentKind::Stmts
+            | AstFragmentKind::Ty
+            | AstFragmentKind::Pat => SupportsMacroExpansion::Yes { supports_inner_attrs: false },
+            AstFragmentKind::Items
+            | AstFragmentKind::TraitItems
+            | AstFragmentKind::ImplItems
+            | AstFragmentKind::ForeignItems
+            | AstFragmentKind::Crate => SupportsMacroExpansion::Yes { supports_inner_attrs: true },
+            AstFragmentKind::Arms
+            | AstFragmentKind::ExprFields
+            | AstFragmentKind::PatFields
+            | AstFragmentKind::GenericParams
+            | AstFragmentKind::Params
+            | AstFragmentKind::FieldDefs
+            | AstFragmentKind::Variants => SupportsMacroExpansion::No,
+        }
+    }
+
+    fn expect_from_annotatables<I: IntoIterator<Item = Annotatable>>(
+        self,
+        items: I,
+    ) -> AstFragment {
+        let mut items = items.into_iter();
+        match self {
+            AstFragmentKind::Arms => {
+                AstFragment::Arms(items.map(Annotatable::expect_arm).collect())
+            }
+            AstFragmentKind::ExprFields => {
+                AstFragment::ExprFields(items.map(Annotatable::expect_expr_field).collect())
+            }
+            AstFragmentKind::PatFields => {
+                AstFragment::PatFields(items.map(Annotatable::expect_pat_field).collect())
+            }
+            AstFragmentKind::GenericParams => {
+                AstFragment::GenericParams(items.map(Annotatable::expect_generic_param).collect())
+            }
+            AstFragmentKind::Params => {
+                AstFragment::Params(items.map(Annotatable::expect_param).collect())
+            }
+            AstFragmentKind::FieldDefs => {
+                AstFragment::FieldDefs(items.map(Annotatable::expect_field_def).collect())
+            }
+            AstFragmentKind::Variants => {
+                AstFragment::Variants(items.map(Annotatable::expect_variant).collect())
+            }
+            AstFragmentKind::Items => {
+                AstFragment::Items(items.map(Annotatable::expect_item).collect())
+            }
+            AstFragmentKind::ImplItems => {
+                AstFragment::ImplItems(items.map(Annotatable::expect_impl_item).collect())
+            }
+            AstFragmentKind::TraitItems => {
+                AstFragment::TraitItems(items.map(Annotatable::expect_trait_item).collect())
+            }
+            AstFragmentKind::ForeignItems => {
+                AstFragment::ForeignItems(items.map(Annotatable::expect_foreign_item).collect())
+            }
+            AstFragmentKind::Stmts => {
+                AstFragment::Stmts(items.map(Annotatable::expect_stmt).collect())
+            }
+            AstFragmentKind::Expr => AstFragment::Expr(
+                items.next().expect("expected exactly one expression").expect_expr(),
+            ),
+            AstFragmentKind::OptExpr => {
+                AstFragment::OptExpr(items.next().map(Annotatable::expect_expr))
+            }
+            AstFragmentKind::Crate => {
+                AstFragment::Crate(items.next().expect("expected exactly one crate").expect_crate())
+            }
+            AstFragmentKind::Pat | AstFragmentKind::Ty => {
+                panic!("patterns and types aren't annotatable")
+            }
+        }
+    }
+}
+
+pub struct Invocation {
+    pub kind: InvocationKind,
+    pub fragment_kind: AstFragmentKind,
+    pub expansion_data: ExpansionData,
+}
+
+pub enum InvocationKind {
+    Bang {
+        mac: ast::MacCall,
+        span: Span,
+    },
+    Attr {
+        attr: ast::Attribute,
+        // Re-insertion position for inert attributes.
+        pos: usize,
+        item: Annotatable,
+        // Required for resolving derive helper attributes.
+        derives: Vec<ast::Path>,
+    },
+    Derive {
+        path: ast::Path,
+        item: Annotatable,
+    },
+}
+
+impl InvocationKind {
+    fn placeholder_visibility(&self) -> Option<ast::Visibility> {
+        // HACK: For unnamed fields placeholders should have the same visibility as the actual
+        // fields because for tuple structs/variants resolve determines visibilities of their
+        // constructor using these field visibilities before attributes on them are are expanded.
+        // The assumption is that the attribute expansion cannot change field visibilities,
+        // and it holds because only inert attributes are supported in this position.
+        match self {
+            InvocationKind::Attr { item: Annotatable::FieldDef(field), .. }
+            | InvocationKind::Derive { item: Annotatable::FieldDef(field), .. }
+                if field.ident.is_none() =>
+            {
+                Some(field.vis.clone())
+            }
+            _ => None,
+        }
+    }
+}
+
+impl Invocation {
+    pub fn span(&self) -> Span {
+        match &self.kind {
+            InvocationKind::Bang { span, .. } => *span,
+            InvocationKind::Attr { attr, .. } => attr.span,
+            InvocationKind::Derive { path, .. } => path.span,
+        }
+    }
+}
+
+pub struct MacroExpander<'a, 'b> {
+    pub cx: &'a mut ExtCtxt<'b>,
+    monotonic: bool, // cf. `cx.monotonic_expander()`
+}
+
+impl<'a, 'b> MacroExpander<'a, 'b> {
+    pub fn new(cx: &'a mut ExtCtxt<'b>, monotonic: bool) -> Self {
+        MacroExpander { cx, monotonic }
+    }
+
+    pub fn expand_crate(&mut self, krate: ast::Crate) -> ast::Crate {
+        let file_path = match self.cx.source_map().span_to_filename(krate.spans.inner_span) {
+            FileName::Real(name) => name
+                .into_local_path()
+                .expect("attempting to resolve a file path in an external file"),
+            other => PathBuf::from(other.prefer_local().to_string()),
+        };
+        let dir_path = file_path.parent().unwrap_or(&file_path).to_owned();
+        self.cx.root_path = dir_path.clone();
+        self.cx.current_expansion.module = Rc::new(ModuleData {
+            mod_path: vec![Ident::from_str(&self.cx.ecfg.crate_name)],
+            file_path_stack: vec![file_path],
+            dir_path,
+        });
+        let krate = self.fully_expand_fragment(AstFragment::Crate(krate)).make_crate();
+        assert_eq!(krate.id, ast::CRATE_NODE_ID);
+        self.cx.trace_macros_diag();
+        krate
+    }
+
+    // Recursively expand all macro invocations in this AST fragment.
+    pub fn fully_expand_fragment(&mut self, input_fragment: AstFragment) -> AstFragment {
+        let orig_expansion_data = self.cx.current_expansion.clone();
+        let orig_force_mode = self.cx.force_mode;
+
+        // Collect all macro invocations and replace them with placeholders.
+        let (mut fragment_with_placeholders, mut invocations) =
+            self.collect_invocations(input_fragment, &[]);
+
+        // Optimization: if we resolve all imports now,
+        // we'll be able to immediately resolve most of imported macros.
+        self.resolve_imports();
+
+        // Resolve paths in all invocations and produce output expanded fragments for them, but
+        // do not insert them into our input AST fragment yet, only store in `expanded_fragments`.
+        // The output fragments also go through expansion recursively until no invocations are left.
+        // Unresolved macros produce dummy outputs as a recovery measure.
+        invocations.reverse();
+        let mut expanded_fragments = Vec::new();
+        let mut undetermined_invocations = Vec::new();
+        let (mut progress, mut force) = (false, !self.monotonic);
+        loop {
+            let Some((invoc, ext)) = invocations.pop() else {
+                self.resolve_imports();
+                if undetermined_invocations.is_empty() {
+                    break;
+                }
+                invocations = mem::take(&mut undetermined_invocations);
+                force = !mem::replace(&mut progress, false);
+                if force && self.monotonic {
+                    self.cx.sess.delay_span_bug(
+                        invocations.last().unwrap().0.span(),
+                        "expansion entered force mode without producing any errors",
+                    );
+                }
+                continue;
+            };
+
+            let ext = match ext {
+                Some(ext) => ext,
+                None => {
+                    let eager_expansion_root = if self.monotonic {
+                        invoc.expansion_data.id
+                    } else {
+                        orig_expansion_data.id
+                    };
+                    match self.cx.resolver.resolve_macro_invocation(
+                        &invoc,
+                        eager_expansion_root,
+                        force,
+                    ) {
+                        Ok(ext) => ext,
+                        Err(Indeterminate) => {
+                            // Cannot resolve, will retry this invocation later.
+                            undetermined_invocations.push((invoc, None));
+                            continue;
+                        }
+                    }
+                }
+            };
+
+            let ExpansionData { depth, id: expn_id, .. } = invoc.expansion_data;
+            let depth = depth - orig_expansion_data.depth;
+            self.cx.current_expansion = invoc.expansion_data.clone();
+            self.cx.force_mode = force;
+
+            let fragment_kind = invoc.fragment_kind;
+            let (expanded_fragment, new_invocations) = match self.expand_invoc(invoc, &ext.kind) {
+                ExpandResult::Ready(fragment) => {
+                    let mut derive_invocations = Vec::new();
+                    let derive_placeholders = self
+                        .cx
+                        .resolver
+                        .take_derive_resolutions(expn_id)
+                        .map(|derives| {
+                            derive_invocations.reserve(derives.len());
+                            derives
+                                .into_iter()
+                                .map(|(path, item, _exts)| {
+                                    // FIXME: Consider using the derive resolutions (`_exts`)
+                                    // instead of enqueuing the derives to be resolved again later.
+                                    let expn_id = LocalExpnId::fresh_empty();
+                                    derive_invocations.push((
+                                        Invocation {
+                                            kind: InvocationKind::Derive { path, item },
+                                            fragment_kind,
+                                            expansion_data: ExpansionData {
+                                                id: expn_id,
+                                                ..self.cx.current_expansion.clone()
+                                            },
+                                        },
+                                        None,
+                                    ));
+                                    NodeId::placeholder_from_expn_id(expn_id)
+                                })
+                                .collect::<Vec<_>>()
+                        })
+                        .unwrap_or_default();
+
+                    let (fragment, collected_invocations) =
+                        self.collect_invocations(fragment, &derive_placeholders);
+                    // We choose to expand any derive invocations associated with this macro invocation
+                    // *before* any macro invocations collected from the output fragment
+                    derive_invocations.extend(collected_invocations);
+                    (fragment, derive_invocations)
+                }
+                ExpandResult::Retry(invoc) => {
+                    if force {
+                        self.cx.span_bug(
+                            invoc.span(),
+                            "expansion entered force mode but is still stuck",
+                        );
+                    } else {
+                        // Cannot expand, will retry this invocation later.
+                        undetermined_invocations.push((invoc, Some(ext)));
+                        continue;
+                    }
+                }
+            };
+
+            progress = true;
+            if expanded_fragments.len() < depth {
+                expanded_fragments.push(Vec::new());
+            }
+            expanded_fragments[depth - 1].push((expn_id, expanded_fragment));
+            invocations.extend(new_invocations.into_iter().rev());
+        }
+
+        self.cx.current_expansion = orig_expansion_data;
+        self.cx.force_mode = orig_force_mode;
+
+        // Finally incorporate all the expanded macros into the input AST fragment.
+        let mut placeholder_expander = PlaceholderExpander::default();
+        while let Some(expanded_fragments) = expanded_fragments.pop() {
+            for (expn_id, expanded_fragment) in expanded_fragments.into_iter().rev() {
+                placeholder_expander
+                    .add(NodeId::placeholder_from_expn_id(expn_id), expanded_fragment);
+            }
+        }
+        fragment_with_placeholders.mut_visit_with(&mut placeholder_expander);
+        fragment_with_placeholders
+    }
+
+    fn resolve_imports(&mut self) {
+        if self.monotonic {
+            self.cx.resolver.resolve_imports();
+        }
+    }
+
+    /// Collects all macro invocations reachable at this time in this AST fragment, and replace
+    /// them with "placeholders" - dummy macro invocations with specially crafted `NodeId`s.
+    /// Then call into resolver that builds a skeleton ("reduced graph") of the fragment and
+    /// prepares data for resolving paths of macro invocations.
+    fn collect_invocations(
+        &mut self,
+        mut fragment: AstFragment,
+        extra_placeholders: &[NodeId],
+    ) -> (AstFragment, Vec<(Invocation, Option<Lrc<SyntaxExtension>>)>) {
+        // Resolve `$crate`s in the fragment for pretty-printing.
+        self.cx.resolver.resolve_dollar_crates();
+
+        let mut invocations = {
+            let mut collector = InvocationCollector {
+                // Non-derive macro invocations cannot see the results of cfg expansion - they
+                // will either be removed along with the item, or invoked before the cfg/cfg_attr
+                // attribute is expanded. Therefore, we don't need to configure the tokens
+                // Derive macros *can* see the results of cfg-expansion - they are handled
+                // specially in `fully_expand_fragment`
+                cx: self.cx,
+                invocations: Vec::new(),
+                monotonic: self.monotonic,
+            };
+            fragment.mut_visit_with(&mut collector);
+            fragment.add_placeholders(extra_placeholders);
+            collector.invocations
+        };
+
+        if self.monotonic {
+            self.cx
+                .resolver
+                .visit_ast_fragment_with_placeholders(self.cx.current_expansion.id, &fragment);
+
+            if self.cx.sess.opts.unstable_opts.incremental_relative_spans {
+                for (invoc, _) in invocations.iter_mut() {
+                    let expn_id = invoc.expansion_data.id;
+                    let parent_def = self.cx.resolver.invocation_parent(expn_id);
+                    let span = match &mut invoc.kind {
+                        InvocationKind::Bang { ref mut span, .. } => span,
+                        InvocationKind::Attr { attr, .. } => &mut attr.span,
+                        InvocationKind::Derive { path, .. } => &mut path.span,
+                    };
+                    *span = span.with_parent(Some(parent_def));
+                }
+            }
+        }
+
+        (fragment, invocations)
+    }
+
+    fn error_recursion_limit_reached(&mut self) {
+        let expn_data = self.cx.current_expansion.id.expn_data();
+        let suggested_limit = match self.cx.ecfg.recursion_limit {
+            Limit(0) => Limit(2),
+            limit => limit * 2,
+        };
+        self.cx
+            .struct_span_err(
+                expn_data.call_site,
+                &format!("recursion limit reached while expanding `{}`", expn_data.kind.descr()),
+            )
+            .help(&format!(
+                "consider increasing the recursion limit by adding a \
+                 `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
+                suggested_limit, self.cx.ecfg.crate_name,
+            ))
+            .emit();
+        self.cx.trace_macros_diag();
+    }
+
+    /// A macro's expansion does not fit in this fragment kind.
+    /// For example, a non-type macro in a type position.
+    fn error_wrong_fragment_kind(&mut self, kind: AstFragmentKind, mac: &ast::MacCall, span: Span) {
+        let msg = format!(
+            "non-{kind} macro in {kind} position: {path}",
+            kind = kind.name(),
+            path = pprust::path_to_string(&mac.path),
+        );
+        self.cx.span_err(span, &msg);
+        self.cx.trace_macros_diag();
+    }
+
+    fn expand_invoc(
+        &mut self,
+        invoc: Invocation,
+        ext: &SyntaxExtensionKind,
+    ) -> ExpandResult<AstFragment, Invocation> {
+        let recursion_limit =
+            self.cx.reduced_recursion_limit.unwrap_or(self.cx.ecfg.recursion_limit);
+        if !recursion_limit.value_within_limit(self.cx.current_expansion.depth) {
+            if self.cx.reduced_recursion_limit.is_none() {
+                self.error_recursion_limit_reached();
+            }
+
+            // Reduce the recursion limit by half each time it triggers.
+            self.cx.reduced_recursion_limit = Some(recursion_limit / 2);
+
+            return ExpandResult::Ready(invoc.fragment_kind.dummy(invoc.span()));
+        }
+
+        let (fragment_kind, span) = (invoc.fragment_kind, invoc.span());
+        ExpandResult::Ready(match invoc.kind {
+            InvocationKind::Bang { mac, .. } => match ext {
+                SyntaxExtensionKind::Bang(expander) => {
+                    let Ok(tok_result) = expander.expand(self.cx, span, mac.args.inner_tokens()) else {
+                        return ExpandResult::Ready(fragment_kind.dummy(span));
+                    };
+                    self.parse_ast_fragment(tok_result, fragment_kind, &mac.path, span)
+                }
+                SyntaxExtensionKind::LegacyBang(expander) => {
+                    let prev = self.cx.current_expansion.prior_type_ascription;
+                    self.cx.current_expansion.prior_type_ascription = mac.prior_type_ascription;
+                    let tok_result = expander.expand(self.cx, span, mac.args.inner_tokens());
+                    let result = if let Some(result) = fragment_kind.make_from(tok_result) {
+                        result
+                    } else {
+                        self.error_wrong_fragment_kind(fragment_kind, &mac, span);
+                        fragment_kind.dummy(span)
+                    };
+                    self.cx.current_expansion.prior_type_ascription = prev;
+                    result
+                }
+                _ => unreachable!(),
+            },
+            InvocationKind::Attr { attr, pos, mut item, derives } => match ext {
+                SyntaxExtensionKind::Attr(expander) => {
+                    self.gate_proc_macro_input(&item);
+                    self.gate_proc_macro_attr_item(span, &item);
+                    let tokens = match &item {
+                        // FIXME: Collect tokens and use them instead of generating
+                        // fake ones. These are unstable, so it needs to be
+                        // fixed prior to stabilization
+                        // Fake tokens when we are invoking an inner attribute, and
+                        // we are invoking it on an out-of-line module or crate.
+                        Annotatable::Crate(krate) => rustc_parse::fake_token_stream_for_crate(
+                            &self.cx.sess.parse_sess,
+                            krate,
+                        ),
+                        Annotatable::Item(item_inner)
+                            if matches!(attr.style, AttrStyle::Inner)
+                                && matches!(
+                                    item_inner.kind,
+                                    ItemKind::Mod(
+                                        _,
+                                        ModKind::Unloaded | ModKind::Loaded(_, Inline::No, _),
+                                    )
+                                ) =>
+                        {
+                            rustc_parse::fake_token_stream_for_item(
+                                &self.cx.sess.parse_sess,
+                                item_inner,
+                            )
+                        }
+                        _ => item.to_tokens(),
+                    };
+                    let attr_item = attr.unwrap_normal_item();
+                    if let MacArgs::Eq(..) = attr_item.args {
+                        self.cx.span_err(span, "key-value macro attributes are not supported");
+                    }
+                    let inner_tokens = attr_item.args.inner_tokens();
+                    let Ok(tok_result) = expander.expand(self.cx, span, inner_tokens, tokens) else {
+                        return ExpandResult::Ready(fragment_kind.dummy(span));
+                    };
+                    self.parse_ast_fragment(tok_result, fragment_kind, &attr_item.path, span)
+                }
+                SyntaxExtensionKind::LegacyAttr(expander) => {
+                    match validate_attr::parse_meta(&self.cx.sess.parse_sess, &attr) {
+                        Ok(meta) => {
+                            let items = match expander.expand(self.cx, span, &meta, item) {
+                                ExpandResult::Ready(items) => items,
+                                ExpandResult::Retry(item) => {
+                                    // Reassemble the original invocation for retrying.
+                                    return ExpandResult::Retry(Invocation {
+                                        kind: InvocationKind::Attr { attr, pos, item, derives },
+                                        ..invoc
+                                    });
+                                }
+                            };
+                            if fragment_kind == AstFragmentKind::Expr && items.is_empty() {
+                                let msg =
+                                    "removing an expression is not supported in this position";
+                                self.cx.span_err(span, msg);
+                                fragment_kind.dummy(span)
+                            } else {
+                                fragment_kind.expect_from_annotatables(items)
+                            }
+                        }
+                        Err(mut err) => {
+                            err.emit();
+                            fragment_kind.dummy(span)
+                        }
+                    }
+                }
+                SyntaxExtensionKind::NonMacroAttr => {
+                    self.cx.expanded_inert_attrs.mark(&attr);
+                    item.visit_attrs(|attrs| attrs.insert(pos, attr));
+                    fragment_kind.expect_from_annotatables(iter::once(item))
+                }
+                _ => unreachable!(),
+            },
+            InvocationKind::Derive { path, item } => match ext {
+                SyntaxExtensionKind::Derive(expander)
+                | SyntaxExtensionKind::LegacyDerive(expander) => {
+                    if let SyntaxExtensionKind::Derive(..) = ext {
+                        self.gate_proc_macro_input(&item);
+                    }
+                    let meta = ast::MetaItem { kind: MetaItemKind::Word, span, path };
+                    let items = match expander.expand(self.cx, span, &meta, item) {
+                        ExpandResult::Ready(items) => items,
+                        ExpandResult::Retry(item) => {
+                            // Reassemble the original invocation for retrying.
+                            return ExpandResult::Retry(Invocation {
+                                kind: InvocationKind::Derive { path: meta.path, item },
+                                ..invoc
+                            });
+                        }
+                    };
+                    fragment_kind.expect_from_annotatables(items)
+                }
+                _ => unreachable!(),
+            },
+        })
+    }
+
+    fn gate_proc_macro_attr_item(&self, span: Span, item: &Annotatable) {
+        let kind = match item {
+            Annotatable::Item(_)
+            | Annotatable::TraitItem(_)
+            | Annotatable::ImplItem(_)
+            | Annotatable::ForeignItem(_)
+            | Annotatable::Crate(..) => return,
+            Annotatable::Stmt(stmt) => {
+                // Attributes are stable on item statements,
+                // but unstable on all other kinds of statements
+                if stmt.is_item() {
+                    return;
+                }
+                "statements"
+            }
+            Annotatable::Expr(_) => "expressions",
+            Annotatable::Arm(..)
+            | Annotatable::ExprField(..)
+            | Annotatable::PatField(..)
+            | Annotatable::GenericParam(..)
+            | Annotatable::Param(..)
+            | Annotatable::FieldDef(..)
+            | Annotatable::Variant(..) => panic!("unexpected annotatable"),
+        };
+        if self.cx.ecfg.proc_macro_hygiene() {
+            return;
+        }
+        feature_err(
+            &self.cx.sess.parse_sess,
+            sym::proc_macro_hygiene,
+            span,
+            &format!("custom attributes cannot be applied to {}", kind),
+        )
+        .emit();
+    }
+
+    fn gate_proc_macro_input(&self, annotatable: &Annotatable) {
+        struct GateProcMacroInput<'a> {
+            parse_sess: &'a ParseSess,
+        }
+
+        impl<'ast, 'a> Visitor<'ast> for GateProcMacroInput<'a> {
+            fn visit_item(&mut self, item: &'ast ast::Item) {
+                match &item.kind {
+                    ItemKind::Mod(_, mod_kind)
+                        if !matches!(mod_kind, ModKind::Loaded(_, Inline::Yes, _)) =>
+                    {
+                        feature_err(
+                            self.parse_sess,
+                            sym::proc_macro_hygiene,
+                            item.span,
+                            "non-inline modules in proc macro input are unstable",
+                        )
+                        .emit();
+                    }
+                    _ => {}
+                }
+
+                visit::walk_item(self, item);
+            }
+        }
+
+        if !self.cx.ecfg.proc_macro_hygiene() {
+            annotatable
+                .visit_with(&mut GateProcMacroInput { parse_sess: &self.cx.sess.parse_sess });
+        }
+    }
+
+    fn parse_ast_fragment(
+        &mut self,
+        toks: TokenStream,
+        kind: AstFragmentKind,
+        path: &ast::Path,
+        span: Span,
+    ) -> AstFragment {
+        let mut parser = self.cx.new_parser_from_tts(toks);
+        match parse_ast_fragment(&mut parser, kind) {
+            Ok(fragment) => {
+                ensure_complete_parse(&mut parser, path, kind.name(), span);
+                fragment
+            }
+            Err(mut err) => {
+                if err.span.is_dummy() {
+                    err.set_span(span);
+                }
+                annotate_err_with_kind(&mut err, kind, span);
+                err.emit();
+                self.cx.trace_macros_diag();
+                kind.dummy(span)
+            }
+        }
+    }
+}
+
+pub fn parse_ast_fragment<'a>(
+    this: &mut Parser<'a>,
+    kind: AstFragmentKind,
+) -> PResult<'a, AstFragment> {
+    Ok(match kind {
+        AstFragmentKind::Items => {
+            let mut items = SmallVec::new();
+            while let Some(item) = this.parse_item(ForceCollect::No)? {
+                items.push(item);
+            }
+            AstFragment::Items(items)
+        }
+        AstFragmentKind::TraitItems => {
+            let mut items = SmallVec::new();
+            while let Some(item) = this.parse_trait_item(ForceCollect::No)? {
+                items.extend(item);
+            }
+            AstFragment::TraitItems(items)
+        }
+        AstFragmentKind::ImplItems => {
+            let mut items = SmallVec::new();
+            while let Some(item) = this.parse_impl_item(ForceCollect::No)? {
+                items.extend(item);
+            }
+            AstFragment::ImplItems(items)
+        }
+        AstFragmentKind::ForeignItems => {
+            let mut items = SmallVec::new();
+            while let Some(item) = this.parse_foreign_item(ForceCollect::No)? {
+                items.extend(item);
+            }
+            AstFragment::ForeignItems(items)
+        }
+        AstFragmentKind::Stmts => {
+            let mut stmts = SmallVec::new();
+            // Won't make progress on a `}`.
+            while this.token != token::Eof && this.token != token::CloseDelim(Delimiter::Brace) {
+                if let Some(stmt) = this.parse_full_stmt(AttemptLocalParseRecovery::Yes)? {
+                    stmts.push(stmt);
+                }
+            }
+            AstFragment::Stmts(stmts)
+        }
+        AstFragmentKind::Expr => AstFragment::Expr(this.parse_expr()?),
+        AstFragmentKind::OptExpr => {
+            if this.token != token::Eof {
+                AstFragment::OptExpr(Some(this.parse_expr()?))
+            } else {
+                AstFragment::OptExpr(None)
+            }
+        }
+        AstFragmentKind::Ty => AstFragment::Ty(this.parse_ty()?),
+        AstFragmentKind::Pat => AstFragment::Pat(this.parse_pat_allow_top_alt(
+            None,
+            RecoverComma::No,
+            RecoverColon::Yes,
+            CommaRecoveryMode::LikelyTuple,
+        )?),
+        AstFragmentKind::Crate => AstFragment::Crate(this.parse_crate_mod()?),
+        AstFragmentKind::Arms
+        | AstFragmentKind::ExprFields
+        | AstFragmentKind::PatFields
+        | AstFragmentKind::GenericParams
+        | AstFragmentKind::Params
+        | AstFragmentKind::FieldDefs
+        | AstFragmentKind::Variants => panic!("unexpected AST fragment kind"),
+    })
+}
+
+pub fn ensure_complete_parse<'a>(
+    this: &mut Parser<'a>,
+    macro_path: &ast::Path,
+    kind_name: &str,
+    span: Span,
+) {
+    if this.token != token::Eof {
+        let token = pprust::token_to_string(&this.token);
+        let msg = format!("macro expansion ignores token `{}` and any following", token);
+        // Avoid emitting backtrace info twice.
+        let def_site_span = this.token.span.with_ctxt(SyntaxContext::root());
+        let mut err = this.struct_span_err(def_site_span, &msg);
+        err.span_label(span, "caused by the macro expansion here");
+        let msg = format!(
+            "the usage of `{}!` is likely invalid in {} context",
+            pprust::path_to_string(macro_path),
+            kind_name,
+        );
+        err.note(&msg);
+        let semi_span = this.sess.source_map().next_point(span);
+
+        let semi_full_span = semi_span.to(this.sess.source_map().next_point(semi_span));
+        match this.sess.source_map().span_to_snippet(semi_full_span) {
+            Ok(ref snippet) if &snippet[..] != ";" && kind_name == "expression" => {
+                err.span_suggestion(
+                    semi_span,
+                    "you might be missing a semicolon here",
+                    ";",
+                    Applicability::MaybeIncorrect,
+                );
+            }
+            _ => {}
+        }
+        err.emit();
+    }
+}
+
+/// Wraps a call to `noop_visit_*` / `noop_flat_map_*`
+/// for an AST node that supports attributes
+/// (see the `Annotatable` enum)
+/// This method assigns a `NodeId`, and sets that `NodeId`
+/// as our current 'lint node id'. If a macro call is found
+/// inside this AST node, we will use this AST node's `NodeId`
+/// to emit lints associated with that macro (allowing
+/// `#[allow]` / `#[deny]` to be applied close to
+/// the macro invocation).
+///
+/// Do *not* call this for a macro AST node
+/// (e.g. `ExprKind::MacCall`) - we cannot emit lints
+/// at these AST nodes, since they are removed and
+/// replaced with the result of macro expansion.
+///
+/// All other `NodeId`s are assigned by `visit_id`.
+/// * `self` is the 'self' parameter for the current method,
+/// * `id` is a mutable reference to the `NodeId` field
+///    of the current AST node.
+/// * `closure` is a closure that executes the
+///   `noop_visit_*` / `noop_flat_map_*` method
+///   for the current AST node.
+macro_rules! assign_id {
+    ($self:ident, $id:expr, $closure:expr) => {{
+        let old_id = $self.cx.current_expansion.lint_node_id;
+        if $self.monotonic {
+            debug_assert_eq!(*$id, ast::DUMMY_NODE_ID);
+            let new_id = $self.cx.resolver.next_node_id();
+            *$id = new_id;
+            $self.cx.current_expansion.lint_node_id = new_id;
+        }
+        let ret = ($closure)();
+        $self.cx.current_expansion.lint_node_id = old_id;
+        ret
+    }};
+}
+
+enum AddSemicolon {
+    Yes,
+    No,
+}
+
+/// A trait implemented for all `AstFragment` nodes and providing all pieces
+/// of functionality used by `InvocationCollector`.
+trait InvocationCollectorNode: HasAttrs + HasNodeId + Sized {
+    type OutputTy = SmallVec<[Self; 1]>;
+    type AttrsTy: Deref<Target = [ast::Attribute]> = Vec<ast::Attribute>;
+    const KIND: AstFragmentKind;
+    fn to_annotatable(self) -> Annotatable;
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy;
+    fn descr() -> &'static str {
+        unreachable!()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, _visitor: &mut V) -> Self::OutputTy {
+        unreachable!()
+    }
+    fn noop_visit<V: MutVisitor>(&mut self, _visitor: &mut V) {
+        unreachable!()
+    }
+    fn is_mac_call(&self) -> bool {
+        false
+    }
+    fn take_mac_call(self) -> (ast::MacCall, Self::AttrsTy, AddSemicolon) {
+        unreachable!()
+    }
+    fn pre_flat_map_node_collect_attr(_cfg: &StripUnconfigured<'_>, _attr: &ast::Attribute) {}
+    fn post_flat_map_node_collect_bang(_output: &mut Self::OutputTy, _add_semicolon: AddSemicolon) {
+    }
+    fn wrap_flat_map_node_noop_flat_map(
+        node: Self,
+        collector: &mut InvocationCollector<'_, '_>,
+        noop_flat_map: impl FnOnce(Self, &mut InvocationCollector<'_, '_>) -> Self::OutputTy,
+    ) -> Result<Self::OutputTy, Self> {
+        Ok(noop_flat_map(node, collector))
+    }
+}
+
+impl InvocationCollectorNode for P<ast::Item> {
+    const KIND: AstFragmentKind = AstFragmentKind::Items;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::Item(self)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_items()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        noop_flat_map_item(self, visitor)
+    }
+    fn is_mac_call(&self) -> bool {
+        matches!(self.kind, ItemKind::MacCall(..))
+    }
+    fn take_mac_call(self) -> (ast::MacCall, Self::AttrsTy, AddSemicolon) {
+        let node = self.into_inner();
+        match node.kind {
+            ItemKind::MacCall(mac) => (mac, node.attrs, AddSemicolon::No),
+            _ => unreachable!(),
+        }
+    }
+    fn wrap_flat_map_node_noop_flat_map(
+        mut node: Self,
+        collector: &mut InvocationCollector<'_, '_>,
+        noop_flat_map: impl FnOnce(Self, &mut InvocationCollector<'_, '_>) -> Self::OutputTy,
+    ) -> Result<Self::OutputTy, Self> {
+        if !matches!(node.kind, ItemKind::Mod(..)) {
+            return Ok(noop_flat_map(node, collector));
+        }
+
+        // Work around borrow checker not seeing through `P`'s deref.
+        let (ident, span, mut attrs) = (node.ident, node.span, mem::take(&mut node.attrs));
+        let ItemKind::Mod(_, mod_kind) = &mut node.kind else {
+            unreachable!()
+        };
+
+        let ecx = &mut collector.cx;
+        let (file_path, dir_path, dir_ownership) = match mod_kind {
+            ModKind::Loaded(_, inline, _) => {
+                // Inline `mod foo { ... }`, but we still need to push directories.
+                let (dir_path, dir_ownership) = mod_dir_path(
+                    &ecx.sess,
+                    ident,
+                    &attrs,
+                    &ecx.current_expansion.module,
+                    ecx.current_expansion.dir_ownership,
+                    *inline,
+                );
+                node.attrs = attrs;
+                (None, dir_path, dir_ownership)
+            }
+            ModKind::Unloaded => {
+                // We have an outline `mod foo;` so we need to parse the file.
+                let old_attrs_len = attrs.len();
+                let ParsedExternalMod { items, spans, file_path, dir_path, dir_ownership } =
+                    parse_external_mod(
+                        &ecx.sess,
+                        ident,
+                        span,
+                        &ecx.current_expansion.module,
+                        ecx.current_expansion.dir_ownership,
+                        &mut attrs,
+                    );
+
+                if let Some(lint_store) = ecx.lint_store {
+                    lint_store.pre_expansion_lint(
+                        ecx.sess,
+                        ecx.resolver.registered_tools(),
+                        ecx.current_expansion.lint_node_id,
+                        &attrs,
+                        &items,
+                        ident.name.as_str(),
+                    );
+                }
+
+                *mod_kind = ModKind::Loaded(items, Inline::No, spans);
+                node.attrs = attrs;
+                if node.attrs.len() > old_attrs_len {
+                    // If we loaded an out-of-line module and added some inner attributes,
+                    // then we need to re-configure it and re-collect attributes for
+                    // resolution and expansion.
+                    return Err(node);
+                }
+                (Some(file_path), dir_path, dir_ownership)
+            }
+        };
+
+        // Set the module info before we flat map.
+        let mut module = ecx.current_expansion.module.with_dir_path(dir_path);
+        module.mod_path.push(ident);
+        if let Some(file_path) = file_path {
+            module.file_path_stack.push(file_path);
+        }
+
+        let orig_module = mem::replace(&mut ecx.current_expansion.module, Rc::new(module));
+        let orig_dir_ownership =
+            mem::replace(&mut ecx.current_expansion.dir_ownership, dir_ownership);
+
+        let res = Ok(noop_flat_map(node, collector));
+
+        collector.cx.current_expansion.dir_ownership = orig_dir_ownership;
+        collector.cx.current_expansion.module = orig_module;
+        res
+    }
+}
+
+struct TraitItemTag;
+impl InvocationCollectorNode for AstNodeWrapper<P<ast::AssocItem>, TraitItemTag> {
+    type OutputTy = SmallVec<[P<ast::AssocItem>; 1]>;
+    const KIND: AstFragmentKind = AstFragmentKind::TraitItems;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::TraitItem(self.wrapped)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_trait_items()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        noop_flat_map_assoc_item(self.wrapped, visitor)
+    }
+    fn is_mac_call(&self) -> bool {
+        matches!(self.wrapped.kind, AssocItemKind::MacCall(..))
+    }
+    fn take_mac_call(self) -> (ast::MacCall, Self::AttrsTy, AddSemicolon) {
+        let item = self.wrapped.into_inner();
+        match item.kind {
+            AssocItemKind::MacCall(mac) => (mac, item.attrs, AddSemicolon::No),
+            _ => unreachable!(),
+        }
+    }
+}
+
+struct ImplItemTag;
+impl InvocationCollectorNode for AstNodeWrapper<P<ast::AssocItem>, ImplItemTag> {
+    type OutputTy = SmallVec<[P<ast::AssocItem>; 1]>;
+    const KIND: AstFragmentKind = AstFragmentKind::ImplItems;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::ImplItem(self.wrapped)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_impl_items()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        noop_flat_map_assoc_item(self.wrapped, visitor)
+    }
+    fn is_mac_call(&self) -> bool {
+        matches!(self.wrapped.kind, AssocItemKind::MacCall(..))
+    }
+    fn take_mac_call(self) -> (ast::MacCall, Self::AttrsTy, AddSemicolon) {
+        let item = self.wrapped.into_inner();
+        match item.kind {
+            AssocItemKind::MacCall(mac) => (mac, item.attrs, AddSemicolon::No),
+            _ => unreachable!(),
+        }
+    }
+}
+
+impl InvocationCollectorNode for P<ast::ForeignItem> {
+    const KIND: AstFragmentKind = AstFragmentKind::ForeignItems;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::ForeignItem(self)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_foreign_items()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        noop_flat_map_foreign_item(self, visitor)
+    }
+    fn is_mac_call(&self) -> bool {
+        matches!(self.kind, ForeignItemKind::MacCall(..))
+    }
+    fn take_mac_call(self) -> (ast::MacCall, Self::AttrsTy, AddSemicolon) {
+        let node = self.into_inner();
+        match node.kind {
+            ForeignItemKind::MacCall(mac) => (mac, node.attrs, AddSemicolon::No),
+            _ => unreachable!(),
+        }
+    }
+}
+
+impl InvocationCollectorNode for ast::Variant {
+    const KIND: AstFragmentKind = AstFragmentKind::Variants;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::Variant(self)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_variants()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        noop_flat_map_variant(self, visitor)
+    }
+}
+
+impl InvocationCollectorNode for ast::FieldDef {
+    const KIND: AstFragmentKind = AstFragmentKind::FieldDefs;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::FieldDef(self)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_field_defs()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        noop_flat_map_field_def(self, visitor)
+    }
+}
+
+impl InvocationCollectorNode for ast::PatField {
+    const KIND: AstFragmentKind = AstFragmentKind::PatFields;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::PatField(self)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_pat_fields()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        noop_flat_map_pat_field(self, visitor)
+    }
+}
+
+impl InvocationCollectorNode for ast::ExprField {
+    const KIND: AstFragmentKind = AstFragmentKind::ExprFields;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::ExprField(self)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_expr_fields()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        noop_flat_map_expr_field(self, visitor)
+    }
+}
+
+impl InvocationCollectorNode for ast::Param {
+    const KIND: AstFragmentKind = AstFragmentKind::Params;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::Param(self)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_params()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        noop_flat_map_param(self, visitor)
+    }
+}
+
+impl InvocationCollectorNode for ast::GenericParam {
+    const KIND: AstFragmentKind = AstFragmentKind::GenericParams;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::GenericParam(self)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_generic_params()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        noop_flat_map_generic_param(self, visitor)
+    }
+}
+
+impl InvocationCollectorNode for ast::Arm {
+    const KIND: AstFragmentKind = AstFragmentKind::Arms;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::Arm(self)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_arms()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        noop_flat_map_arm(self, visitor)
+    }
+}
+
+impl InvocationCollectorNode for ast::Stmt {
+    type AttrsTy = ast::AttrVec;
+    const KIND: AstFragmentKind = AstFragmentKind::Stmts;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::Stmt(P(self))
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_stmts()
+    }
+    fn noop_flat_map<V: MutVisitor>(self, visitor: &mut V) -> Self::OutputTy {
+        noop_flat_map_stmt(self, visitor)
+    }
+    fn is_mac_call(&self) -> bool {
+        match &self.kind {
+            StmtKind::MacCall(..) => true,
+            StmtKind::Item(item) => matches!(item.kind, ItemKind::MacCall(..)),
+            StmtKind::Semi(expr) => matches!(expr.kind, ExprKind::MacCall(..)),
+            StmtKind::Expr(..) => unreachable!(),
+            StmtKind::Local(..) | StmtKind::Empty => false,
+        }
+    }
+    fn take_mac_call(self) -> (ast::MacCall, Self::AttrsTy, AddSemicolon) {
+        // We pull macro invocations (both attributes and fn-like macro calls) out of their
+        // `StmtKind`s and treat them as statement macro invocations, not as items or expressions.
+        let (add_semicolon, mac, attrs) = match self.kind {
+            StmtKind::MacCall(mac) => {
+                let ast::MacCallStmt { mac, style, attrs, .. } = mac.into_inner();
+                (style == MacStmtStyle::Semicolon, mac, attrs)
+            }
+            StmtKind::Item(item) => match item.into_inner() {
+                ast::Item { kind: ItemKind::MacCall(mac), attrs, .. } => {
+                    (mac.args.need_semicolon(), mac, attrs.into())
+                }
+                _ => unreachable!(),
+            },
+            StmtKind::Semi(expr) => match expr.into_inner() {
+                ast::Expr { kind: ExprKind::MacCall(mac), attrs, .. } => {
+                    (mac.args.need_semicolon(), mac, attrs)
+                }
+                _ => unreachable!(),
+            },
+            _ => unreachable!(),
+        };
+        (mac, attrs, if add_semicolon { AddSemicolon::Yes } else { AddSemicolon::No })
+    }
+    fn post_flat_map_node_collect_bang(stmts: &mut Self::OutputTy, add_semicolon: AddSemicolon) {
+        // If this is a macro invocation with a semicolon, then apply that
+        // semicolon to the final statement produced by expansion.
+        if matches!(add_semicolon, AddSemicolon::Yes) {
+            if let Some(stmt) = stmts.pop() {
+                stmts.push(stmt.add_trailing_semicolon());
+            }
+        }
+    }
+}
+
+impl InvocationCollectorNode for ast::Crate {
+    type OutputTy = ast::Crate;
+    const KIND: AstFragmentKind = AstFragmentKind::Crate;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::Crate(self)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_crate()
+    }
+    fn noop_visit<V: MutVisitor>(&mut self, visitor: &mut V) {
+        noop_visit_crate(self, visitor)
+    }
+}
+
+impl InvocationCollectorNode for P<ast::Ty> {
+    type OutputTy = P<ast::Ty>;
+    const KIND: AstFragmentKind = AstFragmentKind::Ty;
+    fn to_annotatable(self) -> Annotatable {
+        unreachable!()
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_ty()
+    }
+    fn noop_visit<V: MutVisitor>(&mut self, visitor: &mut V) {
+        noop_visit_ty(self, visitor)
+    }
+    fn is_mac_call(&self) -> bool {
+        matches!(self.kind, ast::TyKind::MacCall(..))
+    }
+    fn take_mac_call(self) -> (ast::MacCall, Self::AttrsTy, AddSemicolon) {
+        let node = self.into_inner();
+        match node.kind {
+            TyKind::MacCall(mac) => (mac, Vec::new(), AddSemicolon::No),
+            _ => unreachable!(),
+        }
+    }
+}
+
+impl InvocationCollectorNode for P<ast::Pat> {
+    type OutputTy = P<ast::Pat>;
+    const KIND: AstFragmentKind = AstFragmentKind::Pat;
+    fn to_annotatable(self) -> Annotatable {
+        unreachable!()
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_pat()
+    }
+    fn noop_visit<V: MutVisitor>(&mut self, visitor: &mut V) {
+        noop_visit_pat(self, visitor)
+    }
+    fn is_mac_call(&self) -> bool {
+        matches!(self.kind, PatKind::MacCall(..))
+    }
+    fn take_mac_call(self) -> (ast::MacCall, Self::AttrsTy, AddSemicolon) {
+        let node = self.into_inner();
+        match node.kind {
+            PatKind::MacCall(mac) => (mac, Vec::new(), AddSemicolon::No),
+            _ => unreachable!(),
+        }
+    }
+}
+
+impl InvocationCollectorNode for P<ast::Expr> {
+    type OutputTy = P<ast::Expr>;
+    type AttrsTy = ast::AttrVec;
+    const KIND: AstFragmentKind = AstFragmentKind::Expr;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::Expr(self)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_expr()
+    }
+    fn descr() -> &'static str {
+        "an expression"
+    }
+    fn noop_visit<V: MutVisitor>(&mut self, visitor: &mut V) {
+        noop_visit_expr(self, visitor)
+    }
+    fn is_mac_call(&self) -> bool {
+        matches!(self.kind, ExprKind::MacCall(..))
+    }
+    fn take_mac_call(self) -> (ast::MacCall, Self::AttrsTy, AddSemicolon) {
+        let node = self.into_inner();
+        match node.kind {
+            ExprKind::MacCall(mac) => (mac, node.attrs, AddSemicolon::No),
+            _ => unreachable!(),
+        }
+    }
+}
+
+struct OptExprTag;
+impl InvocationCollectorNode for AstNodeWrapper<P<ast::Expr>, OptExprTag> {
+    type OutputTy = Option<P<ast::Expr>>;
+    type AttrsTy = ast::AttrVec;
+    const KIND: AstFragmentKind = AstFragmentKind::OptExpr;
+    fn to_annotatable(self) -> Annotatable {
+        Annotatable::Expr(self.wrapped)
+    }
+    fn fragment_to_output(fragment: AstFragment) -> Self::OutputTy {
+        fragment.make_opt_expr()
+    }
+    fn noop_flat_map<V: MutVisitor>(mut self, visitor: &mut V) -> Self::OutputTy {
+        noop_visit_expr(&mut self.wrapped, visitor);
+        Some(self.wrapped)
+    }
+    fn is_mac_call(&self) -> bool {
+        matches!(self.wrapped.kind, ast::ExprKind::MacCall(..))
+    }
+    fn take_mac_call(self) -> (ast::MacCall, Self::AttrsTy, AddSemicolon) {
+        let node = self.wrapped.into_inner();
+        match node.kind {
+            ExprKind::MacCall(mac) => (mac, node.attrs, AddSemicolon::No),
+            _ => unreachable!(),
+        }
+    }
+    fn pre_flat_map_node_collect_attr(cfg: &StripUnconfigured<'_>, attr: &ast::Attribute) {
+        cfg.maybe_emit_expr_attr_err(&attr);
+    }
+}
+
+struct InvocationCollector<'a, 'b> {
+    cx: &'a mut ExtCtxt<'b>,
+    invocations: Vec<(Invocation, Option<Lrc<SyntaxExtension>>)>,
+    monotonic: bool,
+}
+
+impl<'a, 'b> InvocationCollector<'a, 'b> {
+    fn cfg(&self) -> StripUnconfigured<'_> {
+        StripUnconfigured {
+            sess: &self.cx.sess,
+            features: self.cx.ecfg.features,
+            config_tokens: false,
+            lint_node_id: self.cx.current_expansion.lint_node_id,
+        }
+    }
+
+    fn collect(&mut self, fragment_kind: AstFragmentKind, kind: InvocationKind) -> AstFragment {
+        let expn_id = LocalExpnId::fresh_empty();
+        let vis = kind.placeholder_visibility();
+        self.invocations.push((
+            Invocation {
+                kind,
+                fragment_kind,
+                expansion_data: ExpansionData {
+                    id: expn_id,
+                    depth: self.cx.current_expansion.depth + 1,
+                    ..self.cx.current_expansion.clone()
+                },
+            },
+            None,
+        ));
+        placeholder(fragment_kind, NodeId::placeholder_from_expn_id(expn_id), vis)
+    }
+
+    fn collect_bang(&mut self, mac: ast::MacCall, kind: AstFragmentKind) -> AstFragment {
+        // cache the macro call span so that it can be
+        // easily adjusted for incremental compilation
+        let span = mac.span();
+        self.collect(kind, InvocationKind::Bang { mac, span })
+    }
+
+    fn collect_attr(
+        &mut self,
+        (attr, pos, derives): (ast::Attribute, usize, Vec<ast::Path>),
+        item: Annotatable,
+        kind: AstFragmentKind,
+    ) -> AstFragment {
+        self.collect(kind, InvocationKind::Attr { attr, pos, item, derives })
+    }
+
+    /// If `item` is an attribute invocation, remove the attribute and return it together with
+    /// its position and derives following it. We have to collect the derives in order to resolve
+    /// legacy derive helpers (helpers written before derives that introduce them).
+    fn take_first_attr(
+        &self,
+        item: &mut impl HasAttrs,
+    ) -> Option<(ast::Attribute, usize, Vec<ast::Path>)> {
+        let mut attr = None;
+
+        let mut cfg_pos = None;
+        let mut attr_pos = None;
+        for (pos, attr) in item.attrs().iter().enumerate() {
+            if !attr.is_doc_comment() && !self.cx.expanded_inert_attrs.is_marked(attr) {
+                let name = attr.ident().map(|ident| ident.name);
+                if name == Some(sym::cfg) || name == Some(sym::cfg_attr) {
+                    cfg_pos = Some(pos); // a cfg attr found, no need to search anymore
+                    break;
+                } else if attr_pos.is_none()
+                    && !name.map_or(false, rustc_feature::is_builtin_attr_name)
+                {
+                    attr_pos = Some(pos); // a non-cfg attr found, still may find a cfg attr
+                }
+            }
+        }
+
+        item.visit_attrs(|attrs| {
+            attr = Some(match (cfg_pos, attr_pos) {
+                (Some(pos), _) => (attrs.remove(pos), pos, Vec::new()),
+                (_, Some(pos)) => {
+                    let attr = attrs.remove(pos);
+                    let following_derives = attrs[pos..]
+                        .iter()
+                        .filter(|a| a.has_name(sym::derive))
+                        .flat_map(|a| a.meta_item_list().unwrap_or_default())
+                        .filter_map(|nested_meta| match nested_meta {
+                            NestedMetaItem::MetaItem(ast::MetaItem {
+                                kind: MetaItemKind::Word,
+                                path,
+                                ..
+                            }) => Some(path),
+                            _ => None,
+                        })
+                        .collect();
+
+                    (attr, pos, following_derives)
+                }
+                _ => return,
+            });
+        });
+
+        attr
+    }
+
+    // Detect use of feature-gated or invalid attributes on macro invocations
+    // since they will not be detected after macro expansion.
+    fn check_attributes(&self, attrs: &[ast::Attribute], call: &ast::MacCall) {
+        let features = self.cx.ecfg.features.unwrap();
+        let mut attrs = attrs.iter().peekable();
+        let mut span: Option<Span> = None;
+        while let Some(attr) = attrs.next() {
+            rustc_ast_passes::feature_gate::check_attribute(attr, self.cx.sess, features);
+            validate_attr::check_meta(&self.cx.sess.parse_sess, attr);
+
+            let current_span = if let Some(sp) = span { sp.to(attr.span) } else { attr.span };
+            span = Some(current_span);
+
+            if attrs.peek().map_or(false, |next_attr| next_attr.doc_str().is_some()) {
+                continue;
+            }
+
+            if attr.is_doc_comment() {
+                self.cx.sess.parse_sess.buffer_lint_with_diagnostic(
+                    &UNUSED_DOC_COMMENTS,
+                    current_span,
+                    self.cx.current_expansion.lint_node_id,
+                    "unused doc comment",
+                    BuiltinLintDiagnostics::UnusedDocComment(attr.span),
+                );
+            } else if rustc_attr::is_builtin_attr(attr) {
+                let attr_name = attr.ident().unwrap().name;
+                // `#[cfg]` and `#[cfg_attr]` are special - they are
+                // eagerly evaluated.
+                if attr_name != sym::cfg && attr_name != sym::cfg_attr {
+                    self.cx.sess.parse_sess.buffer_lint_with_diagnostic(
+                        &UNUSED_ATTRIBUTES,
+                        attr.span,
+                        self.cx.current_expansion.lint_node_id,
+                        &format!("unused attribute `{}`", attr_name),
+                        BuiltinLintDiagnostics::UnusedBuiltinAttribute {
+                            attr_name,
+                            macro_name: pprust::path_to_string(&call.path),
+                            invoc_span: call.path.span,
+                        },
+                    );
+                }
+            }
+        }
+    }
+
+    fn expand_cfg_true(
+        &mut self,
+        node: &mut impl HasAttrs,
+        attr: ast::Attribute,
+        pos: usize,
+    ) -> bool {
+        let res = self.cfg().cfg_true(&attr);
+        if res {
+            // FIXME: `cfg(TRUE)` attributes do not currently remove themselves during expansion,
+            // and some tools like rustdoc and clippy rely on that. Find a way to remove them
+            // while keeping the tools working.
+            self.cx.expanded_inert_attrs.mark(&attr);
+            node.visit_attrs(|attrs| attrs.insert(pos, attr));
+        }
+        res
+    }
+
+    fn expand_cfg_attr(&self, node: &mut impl HasAttrs, attr: ast::Attribute, pos: usize) {
+        node.visit_attrs(|attrs| {
+            attrs.splice(pos..pos, self.cfg().expand_cfg_attr(attr, false));
+        });
+    }
+
+    fn flat_map_node<Node: InvocationCollectorNode<OutputTy: Default>>(
+        &mut self,
+        mut node: Node,
+    ) -> Node::OutputTy {
+        loop {
+            return match self.take_first_attr(&mut node) {
+                Some((attr, pos, derives)) => match attr.name_or_empty() {
+                    sym::cfg => {
+                        if self.expand_cfg_true(&mut node, attr, pos) {
+                            continue;
+                        }
+                        Default::default()
+                    }
+                    sym::cfg_attr => {
+                        self.expand_cfg_attr(&mut node, attr, pos);
+                        continue;
+                    }
+                    _ => {
+                        Node::pre_flat_map_node_collect_attr(&self.cfg(), &attr);
+                        self.collect_attr((attr, pos, derives), node.to_annotatable(), Node::KIND)
+                            .make_ast::<Node>()
+                    }
+                },
+                None if node.is_mac_call() => {
+                    let (mac, attrs, add_semicolon) = node.take_mac_call();
+                    self.check_attributes(&attrs, &mac);
+                    let mut res = self.collect_bang(mac, Node::KIND).make_ast::<Node>();
+                    Node::post_flat_map_node_collect_bang(&mut res, add_semicolon);
+                    res
+                }
+                None => {
+                    match Node::wrap_flat_map_node_noop_flat_map(node, self, |mut node, this| {
+                        assign_id!(this, node.node_id_mut(), || node.noop_flat_map(this))
+                    }) {
+                        Ok(output) => output,
+                        Err(returned_node) => {
+                            node = returned_node;
+                            continue;
+                        }
+                    }
+                }
+            };
+        }
+    }
+
+    fn visit_node<Node: InvocationCollectorNode<OutputTy = Node> + DummyAstNode>(
+        &mut self,
+        node: &mut Node,
+    ) {
+        loop {
+            return match self.take_first_attr(node) {
+                Some((attr, pos, derives)) => match attr.name_or_empty() {
+                    sym::cfg => {
+                        let span = attr.span;
+                        if self.expand_cfg_true(node, attr, pos) {
+                            continue;
+                        }
+                        let msg =
+                            format!("removing {} is not supported in this position", Node::descr());
+                        self.cx.span_err(span, &msg);
+                        continue;
+                    }
+                    sym::cfg_attr => {
+                        self.expand_cfg_attr(node, attr, pos);
+                        continue;
+                    }
+                    _ => visit_clobber(node, |node| {
+                        self.collect_attr((attr, pos, derives), node.to_annotatable(), Node::KIND)
+                            .make_ast::<Node>()
+                    }),
+                },
+                None if node.is_mac_call() => {
+                    visit_clobber(node, |node| {
+                        // Do not clobber unless it's actually a macro (uncommon case).
+                        let (mac, attrs, _) = node.take_mac_call();
+                        self.check_attributes(&attrs, &mac);
+                        self.collect_bang(mac, Node::KIND).make_ast::<Node>()
+                    })
+                }
+                None => {
+                    assign_id!(self, node.node_id_mut(), || node.noop_visit(self))
+                }
+            };
+        }
+    }
+}
+
+impl<'a, 'b> MutVisitor for InvocationCollector<'a, 'b> {
+    fn flat_map_item(&mut self, node: P<ast::Item>) -> SmallVec<[P<ast::Item>; 1]> {
+        self.flat_map_node(node)
+    }
+
+    fn flat_map_trait_item(&mut self, node: P<ast::AssocItem>) -> SmallVec<[P<ast::AssocItem>; 1]> {
+        self.flat_map_node(AstNodeWrapper::new(node, TraitItemTag))
+    }
+
+    fn flat_map_impl_item(&mut self, node: P<ast::AssocItem>) -> SmallVec<[P<ast::AssocItem>; 1]> {
+        self.flat_map_node(AstNodeWrapper::new(node, ImplItemTag))
+    }
+
+    fn flat_map_foreign_item(
+        &mut self,
+        node: P<ast::ForeignItem>,
+    ) -> SmallVec<[P<ast::ForeignItem>; 1]> {
+        self.flat_map_node(node)
+    }
+
+    fn flat_map_variant(&mut self, node: ast::Variant) -> SmallVec<[ast::Variant; 1]> {
+        self.flat_map_node(node)
+    }
+
+    fn flat_map_field_def(&mut self, node: ast::FieldDef) -> SmallVec<[ast::FieldDef; 1]> {
+        self.flat_map_node(node)
+    }
+
+    fn flat_map_pat_field(&mut self, node: ast::PatField) -> SmallVec<[ast::PatField; 1]> {
+        self.flat_map_node(node)
+    }
+
+    fn flat_map_expr_field(&mut self, node: ast::ExprField) -> SmallVec<[ast::ExprField; 1]> {
+        self.flat_map_node(node)
+    }
+
+    fn flat_map_param(&mut self, node: ast::Param) -> SmallVec<[ast::Param; 1]> {
+        self.flat_map_node(node)
+    }
+
+    fn flat_map_generic_param(
+        &mut self,
+        node: ast::GenericParam,
+    ) -> SmallVec<[ast::GenericParam; 1]> {
+        self.flat_map_node(node)
+    }
+
+    fn flat_map_arm(&mut self, node: ast::Arm) -> SmallVec<[ast::Arm; 1]> {
+        self.flat_map_node(node)
+    }
+
+    fn flat_map_stmt(&mut self, node: ast::Stmt) -> SmallVec<[ast::Stmt; 1]> {
+        // FIXME: invocations in semicolon-less expressions positions are expanded as expressions,
+        // changing that requires some compatibility measures.
+        if node.is_expr() {
+            // The only way that we can end up with a `MacCall` expression statement,
+            // (as opposed to a `StmtKind::MacCall`) is if we have a macro as the
+            // trailing expression in a block (e.g. `fn foo() { my_macro!() }`).
+            // Record this information, so that we can report a more specific
+            // `SEMICOLON_IN_EXPRESSIONS_FROM_MACROS` lint if needed.
+            // See #78991 for an investigation of treating macros in this position
+            // as statements, rather than expressions, during parsing.
+            return match &node.kind {
+                StmtKind::Expr(expr)
+                    if matches!(**expr, ast::Expr { kind: ExprKind::MacCall(..), .. }) =>
+                {
+                    self.cx.current_expansion.is_trailing_mac = true;
+                    // Don't use `assign_id` for this statement - it may get removed
+                    // entirely due to a `#[cfg]` on the contained expression
+                    let res = noop_flat_map_stmt(node, self);
+                    self.cx.current_expansion.is_trailing_mac = false;
+                    res
+                }
+                _ => noop_flat_map_stmt(node, self),
+            };
+        }
+
+        self.flat_map_node(node)
+    }
+
+    fn visit_crate(&mut self, node: &mut ast::Crate) {
+        self.visit_node(node)
+    }
+
+    fn visit_ty(&mut self, node: &mut P<ast::Ty>) {
+        self.visit_node(node)
+    }
+
+    fn visit_pat(&mut self, node: &mut P<ast::Pat>) {
+        self.visit_node(node)
+    }
+
+    fn visit_expr(&mut self, node: &mut P<ast::Expr>) {
+        // FIXME: Feature gating is performed inconsistently between `Expr` and `OptExpr`.
+        if let Some(attr) = node.attrs.first() {
+            self.cfg().maybe_emit_expr_attr_err(attr);
+        }
+        self.visit_node(node)
+    }
+
+    fn filter_map_expr(&mut self, node: P<ast::Expr>) -> Option<P<ast::Expr>> {
+        self.flat_map_node(AstNodeWrapper::new(node, OptExprTag))
+    }
+
+    fn visit_block(&mut self, node: &mut P<ast::Block>) {
+        let orig_dir_ownership = mem::replace(
+            &mut self.cx.current_expansion.dir_ownership,
+            DirOwnership::UnownedViaBlock,
+        );
+        noop_visit_block(node, self);
+        self.cx.current_expansion.dir_ownership = orig_dir_ownership;
+    }
+
+    fn visit_id(&mut self, id: &mut NodeId) {
+        // We may have already assigned a `NodeId`
+        // by calling `assign_id`
+        if self.monotonic && *id == ast::DUMMY_NODE_ID {
+            *id = self.cx.resolver.next_node_id();
+        }
+    }
+}
+
+pub struct ExpansionConfig<'feat> {
+    pub crate_name: String,
+    pub features: Option<&'feat Features>,
+    pub recursion_limit: Limit,
+    pub trace_mac: bool,
+    pub should_test: bool,          // If false, strip `#[test]` nodes
+    pub span_debug: bool,           // If true, use verbose debugging for `proc_macro::Span`
+    pub proc_macro_backtrace: bool, // If true, show backtraces for proc-macro panics
+}
+
+impl<'feat> ExpansionConfig<'feat> {
+    pub fn default(crate_name: String) -> ExpansionConfig<'static> {
+        ExpansionConfig {
+            crate_name,
+            features: None,
+            recursion_limit: Limit::new(1024),
+            trace_mac: false,
+            should_test: false,
+            span_debug: false,
+            proc_macro_backtrace: false,
+        }
+    }
+
+    fn proc_macro_hygiene(&self) -> bool {
+        self.features.map_or(false, |features| features.proc_macro_hygiene)
+    }
+}
diff --git a/compiler/rustc_expand/src/lib.rs b/compiler/rustc_expand/src/lib.rs
new file mode 100644
index 000000000..9d0232822
--- /dev/null
+++ b/compiler/rustc_expand/src/lib.rs
@@ -0,0 +1,53 @@
+#![feature(array_windows)]
+#![feature(associated_type_bounds)]
+#![feature(associated_type_defaults)]
+#![feature(if_let_guard)]
+#![feature(let_chains)]
+#![feature(let_else)]
+#![feature(macro_metavar_expr)]
+#![feature(proc_macro_diagnostic)]
+#![feature(proc_macro_internals)]
+#![feature(proc_macro_span)]
+#![feature(rustc_attrs)]
+#![feature(try_blocks)]
+#![recursion_limit = "256"]
+
+#[macro_use]
+extern crate rustc_macros;
+
+extern crate proc_macro as pm;
+
+mod placeholders;
+mod proc_macro_server;
+
+pub use mbe::macro_rules::compile_declarative_macro;
+pub(crate) use rustc_span::hygiene;
+pub mod base;
+pub mod build;
+#[macro_use]
+pub mod config;
+pub mod expand;
+pub mod module;
+pub mod proc_macro;
+
+pub(crate) mod mbe;
+
+// HACK(Centril, #64197): These shouldn't really be here.
+// Rather, they should be with their respective modules which are defined in other crates.
+// However, since for now constructing a `ParseSess` sorta requires `config` from this crate,
+// these tests will need to live here in the interim.
+
+#[cfg(test)]
+mod tests;
+#[cfg(test)]
+mod parse {
+    mod tests;
+}
+#[cfg(test)]
+mod tokenstream {
+    mod tests;
+}
+#[cfg(test)]
+mod mut_visit {
+    mod tests;
+}
diff --git a/compiler/rustc_expand/src/mbe.rs b/compiler/rustc_expand/src/mbe.rs
new file mode 100644
index 000000000..f42576b16
--- /dev/null
+++ b/compiler/rustc_expand/src/mbe.rs
@@ -0,0 +1,110 @@
+//! This module implements declarative macros: old `macro_rules` and the newer
+//! `macro`. Declarative macros are also known as "macro by example", and that's
+//! why we call this module `mbe`. For external documentation, prefer the
+//! official terminology: "declarative macros".
+
+pub(crate) mod macro_check;
+pub(crate) mod macro_parser;
+pub(crate) mod macro_rules;
+pub(crate) mod metavar_expr;
+pub(crate) mod quoted;
+pub(crate) mod transcribe;
+
+use metavar_expr::MetaVarExpr;
+use rustc_ast::token::{Delimiter, NonterminalKind, Token, TokenKind};
+use rustc_ast::tokenstream::DelimSpan;
+use rustc_span::symbol::Ident;
+use rustc_span::Span;
+
+/// Contains the sub-token-trees of a "delimited" token tree such as `(a b c)`.
+/// The delimiters are not represented explicitly in the `tts` vector.
+#[derive(PartialEq, Encodable, Decodable, Debug)]
+struct Delimited {
+    delim: Delimiter,
+    /// FIXME: #67062 has details about why this is sub-optimal.
+    tts: Vec<TokenTree>,
+}
+
+#[derive(PartialEq, Encodable, Decodable, Debug)]
+struct SequenceRepetition {
+    /// The sequence of token trees
+    tts: Vec<TokenTree>,
+    /// The optional separator
+    separator: Option<Token>,
+    /// Whether the sequence can be repeated zero (*), or one or more times (+)
+    kleene: KleeneToken,
+    /// The number of `Match`s that appear in the sequence (and subsequences)
+    num_captures: usize,
+}
+
+#[derive(Clone, PartialEq, Encodable, Decodable, Debug, Copy)]
+struct KleeneToken {
+    span: Span,
+    op: KleeneOp,
+}
+
+impl KleeneToken {
+    fn new(op: KleeneOp, span: Span) -> KleeneToken {
+        KleeneToken { span, op }
+    }
+}
+
+/// A Kleene-style [repetition operator](https://en.wikipedia.org/wiki/Kleene_star)
+/// for token sequences.
+#[derive(Clone, PartialEq, Encodable, Decodable, Debug, Copy)]
+enum KleeneOp {
+    /// Kleene star (`*`) for zero or more repetitions
+    ZeroOrMore,
+    /// Kleene plus (`+`) for one or more repetitions
+    OneOrMore,
+    /// Kleene optional (`?`) for zero or one repetitions
+    ZeroOrOne,
+}
+
+/// Similar to `tokenstream::TokenTree`, except that `Sequence`, `MetaVar`, `MetaVarDecl`, and
+/// `MetaVarExpr` are "first-class" token trees. Useful for parsing macros.
+#[derive(Debug, PartialEq, Encodable, Decodable)]
+enum TokenTree {
+    Token(Token),
+    /// A delimited sequence, e.g. `($e:expr)` (RHS) or `{ $e }` (LHS).
+    Delimited(DelimSpan, Delimited),
+    /// A kleene-style repetition sequence, e.g. `$($e:expr)*` (RHS) or `$($e),*` (LHS).
+    Sequence(DelimSpan, SequenceRepetition),
+    /// e.g., `$var`.
+    MetaVar(Span, Ident),
+    /// e.g., `$var:expr`. Only appears on the LHS.
+    MetaVarDecl(Span, Ident /* name to bind */, Option<NonterminalKind>),
+    /// A meta-variable expression inside `${...}`.
+    MetaVarExpr(DelimSpan, MetaVarExpr),
+}
+
+impl TokenTree {
+    /// Returns `true` if the given token tree is delimited.
+    fn is_delimited(&self) -> bool {
+        matches!(*self, TokenTree::Delimited(..))
+    }
+
+    /// Returns `true` if the given token tree is a token of the given kind.
+    fn is_token(&self, expected_kind: &TokenKind) -> bool {
+        match self {
+            TokenTree::Token(Token { kind: actual_kind, .. }) => actual_kind == expected_kind,
+            _ => false,
+        }
+    }
+
+    /// Retrieves the `TokenTree`'s span.
+    fn span(&self) -> Span {
+        match *self {
+            TokenTree::Token(Token { span, .. })
+            | TokenTree::MetaVar(span, _)
+            | TokenTree::MetaVarDecl(span, _, _) => span,
+            TokenTree::Delimited(span, _)
+            | TokenTree::MetaVarExpr(span, _)
+            | TokenTree::Sequence(span, _) => span.entire(),
+        }
+    }
+
+    fn token(kind: TokenKind, span: Span) -> TokenTree {
+        TokenTree::Token(Token::new(kind, span))
+    }
+}
diff --git a/compiler/rustc_expand/src/mbe/macro_check.rs b/compiler/rustc_expand/src/mbe/macro_check.rs
new file mode 100644
index 000000000..8994a2f78
--- /dev/null
+++ b/compiler/rustc_expand/src/mbe/macro_check.rs
@@ -0,0 +1,652 @@
+//! Checks that meta-variables in macro definition are correctly declared and used.
+//!
+//! # What is checked
+//!
+//! ## Meta-variables must not be bound twice
+//!
+//! ```compile_fail
+//! macro_rules! foo { ($x:tt $x:tt) => { $x }; }
+//! ```
+//!
+//! This check is sound (no false-negative) and complete (no false-positive).
+//!
+//! ## Meta-variables must not be free
+//!
+//! ```
+//! macro_rules! foo { () => { $x }; }
+//! ```
+//!
+//! This check is also done at macro instantiation but only if the branch is taken.
+//!
+//! ## Meta-variables must repeat at least as many times as their binder
+//!
+//! ```
+//! macro_rules! foo { ($($x:tt)*) => { $x }; }
+//! ```
+//!
+//! This check is also done at macro instantiation but only if the branch is taken.
+//!
+//! ## Meta-variables must repeat with the same Kleene operators as their binder
+//!
+//! ```
+//! macro_rules! foo { ($($x:tt)+) => { $($x)* }; }
+//! ```
+//!
+//! This check is not done at macro instantiation.
+//!
+//! # Disclaimer
+//!
+//! In the presence of nested macros (a macro defined in a macro), those checks may have false
+//! positives and false negatives. We try to detect those cases by recognizing potential macro
+//! definitions in RHSes, but nested macros may be hidden through the use of particular values of
+//! meta-variables.
+//!
+//! ## Examples of false positive
+//!
+//! False positives can come from cases where we don't recognize a nested macro, because it depends
+//! on particular values of meta-variables. In the following example, we think both instances of
+//! `$x` are free, which is a correct statement if `$name` is anything but `macro_rules`. But when
+//! `$name` is `macro_rules`, like in the instantiation below, then `$x:tt` is actually a binder of
+//! the nested macro and `$x` is bound to it.
+//!
+//! ```
+//! macro_rules! foo { ($name:ident) => { $name! bar { ($x:tt) => { $x }; } }; }
+//! foo!(macro_rules);
+//! ```
+//!
+//! False positives can also come from cases where we think there is a nested macro while there
+//! isn't. In the following example, we think `$x` is free, which is incorrect because `bar` is not
+//! a nested macro since it is not evaluated as code by `stringify!`.
+//!
+//! ```
+//! macro_rules! foo { () => { stringify!(macro_rules! bar { () => { $x }; }) }; }
+//! ```
+//!
+//! ## Examples of false negative
+//!
+//! False negatives can come from cases where we don't recognize a meta-variable, because it depends
+//! on particular values of meta-variables. In the following examples, we don't see that if `$d` is
+//! instantiated with `$` then `$d z` becomes `$z` in the nested macro definition and is thus a free
+//! meta-variable. Note however, that if `foo` is instantiated, then we would check the definition
+//! of `bar` and would see the issue.
+//!
+//! ```
+//! macro_rules! foo { ($d:tt) => { macro_rules! bar { ($y:tt) => { $d z }; } }; }
+//! ```
+//!
+//! # How it is checked
+//!
+//! There are 3 main functions: `check_binders`, `check_occurrences`, and `check_nested_macro`. They
+//! all need some kind of environment.
+//!
+//! ## Environments
+//!
+//! Environments are used to pass information.
+//!
+//! ### From LHS to RHS
+//!
+//! When checking a LHS with `check_binders`, we produce (and use) an environment for binders,
+//! namely `Binders`. This is a mapping from binder name to information about that binder: the span
+//! of the binder for error messages and the stack of Kleene operators under which it was bound in
+//! the LHS.
+//!
+//! This environment is used by both the LHS and RHS. The LHS uses it to detect duplicate binders.
+//! The RHS uses it to detect the other errors.
+//!
+//! ### From outer macro to inner macro
+//!
+//! When checking the RHS of an outer macro and we detect a nested macro definition, we push the
+//! current state, namely `MacroState`, to an environment of nested macro definitions. Each state
+//! stores the LHS binders when entering the macro definition as well as the stack of Kleene
+//! operators under which the inner macro is defined in the RHS.
+//!
+//! This environment is a stack representing the nesting of macro definitions. As such, the stack of
+//! Kleene operators under which a meta-variable is repeating is the concatenation of the stacks
+//! stored when entering a macro definition starting from the state in which the meta-variable is
+//! bound.
+use crate::mbe::{KleeneToken, TokenTree};
+
+use rustc_ast::token::{Delimiter, Token, TokenKind};
+use rustc_ast::{NodeId, DUMMY_NODE_ID};
+use rustc_data_structures::fx::FxHashMap;
+use rustc_errors::MultiSpan;
+use rustc_session::lint::builtin::{META_VARIABLE_MISUSE, MISSING_FRAGMENT_SPECIFIER};
+use rustc_session::parse::ParseSess;
+use rustc_span::symbol::kw;
+use rustc_span::{symbol::MacroRulesNormalizedIdent, Span};
+
+use smallvec::SmallVec;
+
+use std::iter;
+
+/// Stack represented as linked list.
+///
+/// Those are used for environments because they grow incrementally and are not mutable.
+enum Stack<'a, T> {
+    /// Empty stack.
+    Empty,
+    /// A non-empty stack.
+    Push {
+        /// The top element.
+        top: T,
+        /// The previous elements.
+        prev: &'a Stack<'a, T>,
+    },
+}
+
+impl<'a, T> Stack<'a, T> {
+    /// Returns whether a stack is empty.
+    fn is_empty(&self) -> bool {
+        matches!(*self, Stack::Empty)
+    }
+
+    /// Returns a new stack with an element of top.
+    fn push(&'a self, top: T) -> Stack<'a, T> {
+        Stack::Push { top, prev: self }
+    }
+}
+
+impl<'a, T> Iterator for &'a Stack<'a, T> {
+    type Item = &'a T;
+
+    // Iterates from top to bottom of the stack.
+    fn next(&mut self) -> Option<&'a T> {
+        match *self {
+            Stack::Empty => None,
+            Stack::Push { ref top, ref prev } => {
+                *self = prev;
+                Some(top)
+            }
+        }
+    }
+}
+
+impl From<&Stack<'_, KleeneToken>> for SmallVec<[KleeneToken; 1]> {
+    fn from(ops: &Stack<'_, KleeneToken>) -> SmallVec<[KleeneToken; 1]> {
+        let mut ops: SmallVec<[KleeneToken; 1]> = ops.cloned().collect();
+        // The stack is innermost on top. We want outermost first.
+        ops.reverse();
+        ops
+    }
+}
+
+/// Information attached to a meta-variable binder in LHS.
+struct BinderInfo {
+    /// The span of the meta-variable in LHS.
+    span: Span,
+    /// The stack of Kleene operators (outermost first).
+    ops: SmallVec<[KleeneToken; 1]>,
+}
+
+/// An environment of meta-variables to their binder information.
+type Binders = FxHashMap<MacroRulesNormalizedIdent, BinderInfo>;
+
+/// The state at which we entered a macro definition in the RHS of another macro definition.
+struct MacroState<'a> {
+    /// The binders of the branch where we entered the macro definition.
+    binders: &'a Binders,
+    /// The stack of Kleene operators (outermost first) where we entered the macro definition.
+    ops: SmallVec<[KleeneToken; 1]>,
+}
+
+/// Checks that meta-variables are used correctly in a macro definition.
+///
+/// Arguments:
+/// - `sess` is used to emit diagnostics and lints
+/// - `node_id` is used to emit lints
+/// - `span` is used when no spans are available
+/// - `lhses` and `rhses` should have the same length and represent the macro definition
+pub(super) fn check_meta_variables(
+    sess: &ParseSess,
+    node_id: NodeId,
+    span: Span,
+    lhses: &[TokenTree],
+    rhses: &[TokenTree],
+) -> bool {
+    if lhses.len() != rhses.len() {
+        sess.span_diagnostic.span_bug(span, "length mismatch between LHSes and RHSes")
+    }
+    let mut valid = true;
+    for (lhs, rhs) in iter::zip(lhses, rhses) {
+        let mut binders = Binders::default();
+        check_binders(sess, node_id, lhs, &Stack::Empty, &mut binders, &Stack::Empty, &mut valid);
+        check_occurrences(sess, node_id, rhs, &Stack::Empty, &binders, &Stack::Empty, &mut valid);
+    }
+    valid
+}
+
+/// Checks `lhs` as part of the LHS of a macro definition, extends `binders` with new binders, and
+/// sets `valid` to false in case of errors.
+///
+/// Arguments:
+/// - `sess` is used to emit diagnostics and lints
+/// - `node_id` is used to emit lints
+/// - `lhs` is checked as part of a LHS
+/// - `macros` is the stack of possible outer macros
+/// - `binders` contains the binders of the LHS
+/// - `ops` is the stack of Kleene operators from the LHS
+/// - `valid` is set in case of errors
+fn check_binders(
+    sess: &ParseSess,
+    node_id: NodeId,
+    lhs: &TokenTree,
+    macros: &Stack<'_, MacroState<'_>>,
+    binders: &mut Binders,
+    ops: &Stack<'_, KleeneToken>,
+    valid: &mut bool,
+) {
+    match *lhs {
+        TokenTree::Token(..) => {}
+        // This can only happen when checking a nested macro because this LHS is then in the RHS of
+        // the outer macro. See ui/macros/macro-of-higher-order.rs where $y:$fragment in the
+        // LHS of the nested macro (and RHS of the outer macro) is parsed as MetaVar(y) Colon
+        // MetaVar(fragment) and not as MetaVarDecl(y, fragment).
+        TokenTree::MetaVar(span, name) => {
+            if macros.is_empty() {
+                sess.span_diagnostic.span_bug(span, "unexpected MetaVar in lhs");
+            }
+            let name = MacroRulesNormalizedIdent::new(name);
+            // There are 3 possibilities:
+            if let Some(prev_info) = binders.get(&name) {
+                // 1. The meta-variable is already bound in the current LHS: This is an error.
+                let mut span = MultiSpan::from_span(span);
+                span.push_span_label(prev_info.span, "previous declaration");
+                buffer_lint(sess, span, node_id, "duplicate matcher binding");
+            } else if get_binder_info(macros, binders, name).is_none() {
+                // 2. The meta-variable is free: This is a binder.
+                binders.insert(name, BinderInfo { span, ops: ops.into() });
+            } else {
+                // 3. The meta-variable is bound: This is an occurrence.
+                check_occurrences(sess, node_id, lhs, macros, binders, ops, valid);
+            }
+        }
+        // Similarly, this can only happen when checking a toplevel macro.
+        TokenTree::MetaVarDecl(span, name, kind) => {
+            if kind.is_none() && node_id != DUMMY_NODE_ID {
+                // FIXME: Report this as a hard error eventually and remove equivalent errors from
+                // `parse_tt_inner` and `nameize`. Until then the error may be reported twice, once
+                // as a hard error and then once as a buffered lint.
+                sess.buffer_lint(
+                    MISSING_FRAGMENT_SPECIFIER,
+                    span,
+                    node_id,
+                    "missing fragment specifier",
+                );
+            }
+            if !macros.is_empty() {
+                sess.span_diagnostic.span_bug(span, "unexpected MetaVarDecl in nested lhs");
+            }
+            let name = MacroRulesNormalizedIdent::new(name);
+            if let Some(prev_info) = get_binder_info(macros, binders, name) {
+                // Duplicate binders at the top-level macro definition are errors. The lint is only
+                // for nested macro definitions.
+                sess.span_diagnostic
+                    .struct_span_err(span, "duplicate matcher binding")
+                    .span_label(span, "duplicate binding")
+                    .span_label(prev_info.span, "previous binding")
+                    .emit();
+                *valid = false;
+            } else {
+                binders.insert(name, BinderInfo { span, ops: ops.into() });
+            }
+        }
+        // `MetaVarExpr` can not appear in the LHS of a macro arm
+        TokenTree::MetaVarExpr(..) => {}
+        TokenTree::Delimited(_, ref del) => {
+            for tt in &del.tts {
+                check_binders(sess, node_id, tt, macros, binders, ops, valid);
+            }
+        }
+        TokenTree::Sequence(_, ref seq) => {
+            let ops = ops.push(seq.kleene);
+            for tt in &seq.tts {
+                check_binders(sess, node_id, tt, macros, binders, &ops, valid);
+            }
+        }
+    }
+}
+
+/// Returns the binder information of a meta-variable.
+///
+/// Arguments:
+/// - `macros` is the stack of possible outer macros
+/// - `binders` contains the current binders
+/// - `name` is the name of the meta-variable we are looking for
+fn get_binder_info<'a>(
+    mut macros: &'a Stack<'a, MacroState<'a>>,
+    binders: &'a Binders,
+    name: MacroRulesNormalizedIdent,
+) -> Option<&'a BinderInfo> {
+    binders.get(&name).or_else(|| macros.find_map(|state| state.binders.get(&name)))
+}
+
+/// Checks `rhs` as part of the RHS of a macro definition and sets `valid` to false in case of
+/// errors.
+///
+/// Arguments:
+/// - `sess` is used to emit diagnostics and lints
+/// - `node_id` is used to emit lints
+/// - `rhs` is checked as part of a RHS
+/// - `macros` is the stack of possible outer macros
+/// - `binders` contains the binders of the associated LHS
+/// - `ops` is the stack of Kleene operators from the RHS
+/// - `valid` is set in case of errors
+fn check_occurrences(
+    sess: &ParseSess,
+    node_id: NodeId,
+    rhs: &TokenTree,
+    macros: &Stack<'_, MacroState<'_>>,
+    binders: &Binders,
+    ops: &Stack<'_, KleeneToken>,
+    valid: &mut bool,
+) {
+    match *rhs {
+        TokenTree::Token(..) => {}
+        TokenTree::MetaVarDecl(span, _name, _kind) => {
+            sess.span_diagnostic.span_bug(span, "unexpected MetaVarDecl in rhs")
+        }
+        TokenTree::MetaVar(span, name) => {
+            let name = MacroRulesNormalizedIdent::new(name);
+            check_ops_is_prefix(sess, node_id, macros, binders, ops, span, name);
+        }
+        TokenTree::MetaVarExpr(dl, ref mve) => {
+            let Some(name) = mve.ident().map(MacroRulesNormalizedIdent::new) else {
+                return;
+            };
+            check_ops_is_prefix(sess, node_id, macros, binders, ops, dl.entire(), name);
+        }
+        TokenTree::Delimited(_, ref del) => {
+            check_nested_occurrences(sess, node_id, &del.tts, macros, binders, ops, valid);
+        }
+        TokenTree::Sequence(_, ref seq) => {
+            let ops = ops.push(seq.kleene);
+            check_nested_occurrences(sess, node_id, &seq.tts, macros, binders, &ops, valid);
+        }
+    }
+}
+
+/// Represents the processed prefix of a nested macro.
+#[derive(Clone, Copy, PartialEq, Eq)]
+enum NestedMacroState {
+    /// Nothing that matches a nested macro definition was processed yet.
+    Empty,
+    /// The token `macro_rules` was processed.
+    MacroRules,
+    /// The tokens `macro_rules!` were processed.
+    MacroRulesNot,
+    /// The tokens `macro_rules!` followed by a name were processed. The name may be either directly
+    /// an identifier or a meta-variable (that hopefully would be instantiated by an identifier).
+    MacroRulesNotName,
+    /// The keyword `macro` was processed.
+    Macro,
+    /// The keyword `macro` followed by a name was processed.
+    MacroName,
+    /// The keyword `macro` followed by a name and a token delimited by parentheses was processed.
+    MacroNameParen,
+}
+
+/// Checks `tts` as part of the RHS of a macro definition, tries to recognize nested macro
+/// definitions, and sets `valid` to false in case of errors.
+///
+/// Arguments:
+/// - `sess` is used to emit diagnostics and lints
+/// - `node_id` is used to emit lints
+/// - `tts` is checked as part of a RHS and may contain macro definitions
+/// - `macros` is the stack of possible outer macros
+/// - `binders` contains the binders of the associated LHS
+/// - `ops` is the stack of Kleene operators from the RHS
+/// - `valid` is set in case of errors
+fn check_nested_occurrences(
+    sess: &ParseSess,
+    node_id: NodeId,
+    tts: &[TokenTree],
+    macros: &Stack<'_, MacroState<'_>>,
+    binders: &Binders,
+    ops: &Stack<'_, KleeneToken>,
+    valid: &mut bool,
+) {
+    let mut state = NestedMacroState::Empty;
+    let nested_macros = macros.push(MacroState { binders, ops: ops.into() });
+    let mut nested_binders = Binders::default();
+    for tt in tts {
+        match (state, tt) {
+            (
+                NestedMacroState::Empty,
+                &TokenTree::Token(Token { kind: TokenKind::Ident(name, false), .. }),
+            ) => {
+                if name == kw::MacroRules {
+                    state = NestedMacroState::MacroRules;
+                } else if name == kw::Macro {
+                    state = NestedMacroState::Macro;
+                }
+            }
+            (
+                NestedMacroState::MacroRules,
+                &TokenTree::Token(Token { kind: TokenKind::Not, .. }),
+            ) => {
+                state = NestedMacroState::MacroRulesNot;
+            }
+            (
+                NestedMacroState::MacroRulesNot,
+                &TokenTree::Token(Token { kind: TokenKind::Ident(..), .. }),
+            ) => {
+                state = NestedMacroState::MacroRulesNotName;
+            }
+            (NestedMacroState::MacroRulesNot, &TokenTree::MetaVar(..)) => {
+                state = NestedMacroState::MacroRulesNotName;
+                // We check that the meta-variable is correctly used.
+                check_occurrences(sess, node_id, tt, macros, binders, ops, valid);
+            }
+            (NestedMacroState::MacroRulesNotName, &TokenTree::Delimited(_, ref del))
+            | (NestedMacroState::MacroName, &TokenTree::Delimited(_, ref del))
+                if del.delim == Delimiter::Brace =>
+            {
+                let macro_rules = state == NestedMacroState::MacroRulesNotName;
+                state = NestedMacroState::Empty;
+                let rest =
+                    check_nested_macro(sess, node_id, macro_rules, &del.tts, &nested_macros, valid);
+                // If we did not check the whole macro definition, then check the rest as if outside
+                // the macro definition.
+                check_nested_occurrences(
+                    sess,
+                    node_id,
+                    &del.tts[rest..],
+                    macros,
+                    binders,
+                    ops,
+                    valid,
+                );
+            }
+            (
+                NestedMacroState::Macro,
+                &TokenTree::Token(Token { kind: TokenKind::Ident(..), .. }),
+            ) => {
+                state = NestedMacroState::MacroName;
+            }
+            (NestedMacroState::Macro, &TokenTree::MetaVar(..)) => {
+                state = NestedMacroState::MacroName;
+                // We check that the meta-variable is correctly used.
+                check_occurrences(sess, node_id, tt, macros, binders, ops, valid);
+            }
+            (NestedMacroState::MacroName, &TokenTree::Delimited(_, ref del))
+                if del.delim == Delimiter::Parenthesis =>
+            {
+                state = NestedMacroState::MacroNameParen;
+                nested_binders = Binders::default();
+                check_binders(
+                    sess,
+                    node_id,
+                    tt,
+                    &nested_macros,
+                    &mut nested_binders,
+                    &Stack::Empty,
+                    valid,
+                );
+            }
+            (NestedMacroState::MacroNameParen, &TokenTree::Delimited(_, ref del))
+                if del.delim == Delimiter::Brace =>
+            {
+                state = NestedMacroState::Empty;
+                check_occurrences(
+                    sess,
+                    node_id,
+                    tt,
+                    &nested_macros,
+                    &nested_binders,
+                    &Stack::Empty,
+                    valid,
+                );
+            }
+            (_, ref tt) => {
+                state = NestedMacroState::Empty;
+                check_occurrences(sess, node_id, tt, macros, binders, ops, valid);
+            }
+        }
+    }
+}
+
+/// Checks the body of nested macro, returns where the check stopped, and sets `valid` to false in
+/// case of errors.
+///
+/// The token trees are checked as long as they look like a list of (LHS) => {RHS} token trees. This
+/// check is a best-effort to detect a macro definition. It returns the position in `tts` where we
+/// stopped checking because we detected we were not in a macro definition anymore.
+///
+/// Arguments:
+/// - `sess` is used to emit diagnostics and lints
+/// - `node_id` is used to emit lints
+/// - `macro_rules` specifies whether the macro is `macro_rules`
+/// - `tts` is checked as a list of (LHS) => {RHS}
+/// - `macros` is the stack of outer macros
+/// - `valid` is set in case of errors
+fn check_nested_macro(
+    sess: &ParseSess,
+    node_id: NodeId,
+    macro_rules: bool,
+    tts: &[TokenTree],
+    macros: &Stack<'_, MacroState<'_>>,
+    valid: &mut bool,
+) -> usize {
+    let n = tts.len();
+    let mut i = 0;
+    let separator = if macro_rules { TokenKind::Semi } else { TokenKind::Comma };
+    loop {
+        // We expect 3 token trees: `(LHS) => {RHS}`. The separator is checked after.
+        if i + 2 >= n
+            || !tts[i].is_delimited()
+            || !tts[i + 1].is_token(&TokenKind::FatArrow)
+            || !tts[i + 2].is_delimited()
+        {
+            break;
+        }
+        let lhs = &tts[i];
+        let rhs = &tts[i + 2];
+        let mut binders = Binders::default();
+        check_binders(sess, node_id, lhs, macros, &mut binders, &Stack::Empty, valid);
+        check_occurrences(sess, node_id, rhs, macros, &binders, &Stack::Empty, valid);
+        // Since the last semicolon is optional for `macro_rules` macros and decl_macro are not terminated,
+        // we increment our checked position by how many token trees we already checked (the 3
+        // above) before checking for the separator.
+        i += 3;
+        if i == n || !tts[i].is_token(&separator) {
+            break;
+        }
+        // We increment our checked position for the semicolon.
+        i += 1;
+    }
+    i
+}
+
+/// Checks that a meta-variable occurrence is valid.
+///
+/// Arguments:
+/// - `sess` is used to emit diagnostics and lints
+/// - `node_id` is used to emit lints
+/// - `macros` is the stack of possible outer macros
+/// - `binders` contains the binders of the associated LHS
+/// - `ops` is the stack of Kleene operators from the RHS
+/// - `span` is the span of the meta-variable to check
+/// - `name` is the name of the meta-variable to check
+fn check_ops_is_prefix(
+    sess: &ParseSess,
+    node_id: NodeId,
+    macros: &Stack<'_, MacroState<'_>>,
+    binders: &Binders,
+    ops: &Stack<'_, KleeneToken>,
+    span: Span,
+    name: MacroRulesNormalizedIdent,
+) {
+    let macros = macros.push(MacroState { binders, ops: ops.into() });
+    // Accumulates the stacks the operators of each state until (and including when) the
+    // meta-variable is found. The innermost stack is first.
+    let mut acc: SmallVec<[&SmallVec<[KleeneToken; 1]>; 1]> = SmallVec::new();
+    for state in &macros {
+        acc.push(&state.ops);
+        if let Some(binder) = state.binders.get(&name) {
+            // This variable concatenates the stack of operators from the RHS of the LHS where the
+            // meta-variable was defined to where it is used (in possibly nested macros). The
+            // outermost operator is first.
+            let mut occurrence_ops: SmallVec<[KleeneToken; 2]> = SmallVec::new();
+            // We need to iterate from the end to start with outermost stack.
+            for ops in acc.iter().rev() {
+                occurrence_ops.extend_from_slice(ops);
+            }
+            ops_is_prefix(sess, node_id, span, name, &binder.ops, &occurrence_ops);
+            return;
+        }
+    }
+    buffer_lint(sess, span.into(), node_id, &format!("unknown macro variable `{}`", name));
+}
+
+/// Returns whether `binder_ops` is a prefix of `occurrence_ops`.
+///
+/// The stack of Kleene operators of a meta-variable occurrence just needs to have the stack of
+/// Kleene operators of its binder as a prefix.
+///
+/// Consider $i in the following example:
+/// ```ignore (illustrative)
+/// ( $( $i:ident = $($j:ident),+ );* ) => { $($( $i += $j; )+)* }
+/// ```
+/// It occurs under the Kleene stack ["*", "+"] and is bound under ["*"] only.
+///
+/// Arguments:
+/// - `sess` is used to emit diagnostics and lints
+/// - `node_id` is used to emit lints
+/// - `span` is the span of the meta-variable being check
+/// - `name` is the name of the meta-variable being check
+/// - `binder_ops` is the stack of Kleene operators for the binder
+/// - `occurrence_ops` is the stack of Kleene operators for the occurrence
+fn ops_is_prefix(
+    sess: &ParseSess,
+    node_id: NodeId,
+    span: Span,
+    name: MacroRulesNormalizedIdent,
+    binder_ops: &[KleeneToken],
+    occurrence_ops: &[KleeneToken],
+) {
+    for (i, binder) in binder_ops.iter().enumerate() {
+        if i >= occurrence_ops.len() {
+            let mut span = MultiSpan::from_span(span);
+            span.push_span_label(binder.span, "expected repetition");
+            let message = &format!("variable '{}' is still repeating at this depth", name);
+            buffer_lint(sess, span, node_id, message);
+            return;
+        }
+        let occurrence = &occurrence_ops[i];
+        if occurrence.op != binder.op {
+            let mut span = MultiSpan::from_span(span);
+            span.push_span_label(binder.span, "expected repetition");
+            span.push_span_label(occurrence.span, "conflicting repetition");
+            let message = "meta-variable repeats with different Kleene operator";
+            buffer_lint(sess, span, node_id, message);
+            return;
+        }
+    }
+}
+
+fn buffer_lint(sess: &ParseSess, span: MultiSpan, node_id: NodeId, message: &str) {
+    // Macros loaded from other crates have dummy node ids.
+    if node_id != DUMMY_NODE_ID {
+        sess.buffer_lint(&META_VARIABLE_MISUSE, span, node_id, message);
+    }
+}
diff --git a/compiler/rustc_expand/src/mbe/macro_parser.rs b/compiler/rustc_expand/src/mbe/macro_parser.rs
new file mode 100644
index 000000000..4fa91dfea
--- /dev/null
+++ b/compiler/rustc_expand/src/mbe/macro_parser.rs
@@ -0,0 +1,704 @@
+//! This is an NFA-based parser, which calls out to the main Rust parser for named non-terminals
+//! (which it commits to fully when it hits one in a grammar). There's a set of current NFA threads
+//! and a set of next ones. Instead of NTs, we have a special case for Kleene star. The big-O, in
+//! pathological cases, is worse than traditional use of NFA or Earley parsing, but it's an easier
+//! fit for Macro-by-Example-style rules.
+//!
+//! (In order to prevent the pathological case, we'd need to lazily construct the resulting
+//! `NamedMatch`es at the very end. It'd be a pain, and require more memory to keep around old
+//! matcher positions, but it would also save overhead)
+//!
+//! We don't say this parser uses the Earley algorithm, because it's unnecessarily inaccurate.
+//! The macro parser restricts itself to the features of finite state automata. Earley parsers
+//! can be described as an extension of NFAs with completion rules, prediction rules, and recursion.
+//!
+//! Quick intro to how the parser works:
+//!
+//! A "matcher position" (a.k.a. "position" or "mp") is a dot in the middle of a matcher, usually
+//! written as a `·`. For example `· a $( a )* a b` is one, as is `a $( · a )* a b`.
+//!
+//! The parser walks through the input a token at a time, maintaining a list
+//! of threads consistent with the current position in the input string: `cur_mps`.
+//!
+//! As it processes them, it fills up `eof_mps` with threads that would be valid if
+//! the macro invocation is now over, `bb_mps` with threads that are waiting on
+//! a Rust non-terminal like `$e:expr`, and `next_mps` with threads that are waiting
+//! on a particular token. Most of the logic concerns moving the · through the
+//! repetitions indicated by Kleene stars. The rules for moving the · without
+//! consuming any input are called epsilon transitions. It only advances or calls
+//! out to the real Rust parser when no `cur_mps` threads remain.
+//!
+//! Example:
+//!
+//! ```text, ignore
+//! Start parsing a a a a b against [· a $( a )* a b].
+//!
+//! Remaining input: a a a a b
+//! next: [· a $( a )* a b]
+//!
+//! - - - Advance over an a. - - -
+//!
+//! Remaining input: a a a b
+//! cur: [a · $( a )* a b]
+//! Descend/Skip (first position).
+//! next: [a $( · a )* a b]  [a $( a )* · a b].
+//!
+//! - - - Advance over an a. - - -
+//!
+//! Remaining input: a a b
+//! cur: [a $( a · )* a b]  [a $( a )* a · b]
+//! Follow epsilon transition: Finish/Repeat (first position)
+//! next: [a $( a )* · a b]  [a $( · a )* a b]  [a $( a )* a · b]
+//!
+//! - - - Advance over an a. - - - (this looks exactly like the last step)
+//!
+//! Remaining input: a b
+//! cur: [a $( a · )* a b]  [a $( a )* a · b]
+//! Follow epsilon transition: Finish/Repeat (first position)
+//! next: [a $( a )* · a b]  [a $( · a )* a b]  [a $( a )* a · b]
+//!
+//! - - - Advance over an a. - - - (this looks exactly like the last step)
+//!
+//! Remaining input: b
+//! cur: [a $( a · )* a b]  [a $( a )* a · b]
+//! Follow epsilon transition: Finish/Repeat (first position)
+//! next: [a $( a )* · a b]  [a $( · a )* a b]  [a $( a )* a · b]
+//!
+//! - - - Advance over a b. - - -
+//!
+//! Remaining input: ''
+//! eof: [a $( a )* a b ·]
+//! ```
+
+pub(crate) use NamedMatch::*;
+pub(crate) use ParseResult::*;
+
+use crate::mbe::{KleeneOp, TokenTree};
+
+use rustc_ast::token::{self, DocComment, Nonterminal, NonterminalKind, Token};
+use rustc_lint_defs::pluralize;
+use rustc_parse::parser::{NtOrTt, Parser};
+use rustc_span::symbol::MacroRulesNormalizedIdent;
+use rustc_span::Span;
+
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::sync::Lrc;
+use rustc_span::symbol::Ident;
+use std::borrow::Cow;
+use std::collections::hash_map::Entry::{Occupied, Vacant};
+
+/// A unit within a matcher that a `MatcherPos` can refer to. Similar to (and derived from)
+/// `mbe::TokenTree`, but designed specifically for fast and easy traversal during matching.
+/// Notable differences to `mbe::TokenTree`:
+/// - It is non-recursive, i.e. there is no nesting.
+/// - The end pieces of each sequence (the separator, if present, and the Kleene op) are
+///   represented explicitly, as is the very end of the matcher.
+///
+/// This means a matcher can be represented by `&[MatcherLoc]`, and traversal mostly involves
+/// simply incrementing the current matcher position index by one.
+pub(super) enum MatcherLoc {
+    Token {
+        token: Token,
+    },
+    Delimited,
+    Sequence {
+        op: KleeneOp,
+        num_metavar_decls: usize,
+        idx_first_after: usize,
+        next_metavar: usize,
+        seq_depth: usize,
+    },
+    SequenceKleeneOpNoSep {
+        op: KleeneOp,
+        idx_first: usize,
+    },
+    SequenceSep {
+        separator: Token,
+    },
+    SequenceKleeneOpAfterSep {
+        idx_first: usize,
+    },
+    MetaVarDecl {
+        span: Span,
+        bind: Ident,
+        kind: Option<NonterminalKind>,
+        next_metavar: usize,
+        seq_depth: usize,
+    },
+    Eof,
+}
+
+pub(super) fn compute_locs(matcher: &[TokenTree]) -> Vec<MatcherLoc> {
+    fn inner(
+        tts: &[TokenTree],
+        locs: &mut Vec<MatcherLoc>,
+        next_metavar: &mut usize,
+        seq_depth: usize,
+    ) {
+        for tt in tts {
+            match tt {
+                TokenTree::Token(token) => {
+                    locs.push(MatcherLoc::Token { token: token.clone() });
+                }
+                TokenTree::Delimited(span, delimited) => {
+                    let open_token = Token::new(token::OpenDelim(delimited.delim), span.open);
+                    let close_token = Token::new(token::CloseDelim(delimited.delim), span.close);
+
+                    locs.push(MatcherLoc::Delimited);
+                    locs.push(MatcherLoc::Token { token: open_token });
+                    inner(&delimited.tts, locs, next_metavar, seq_depth);
+                    locs.push(MatcherLoc::Token { token: close_token });
+                }
+                TokenTree::Sequence(_, seq) => {
+                    // We can't determine `idx_first_after` and construct the final
+                    // `MatcherLoc::Sequence` until after `inner()` is called and the sequence end
+                    // pieces are processed. So we push a dummy value (`Eof` is cheapest to
+                    // construct) now, and overwrite it with the proper value below.
+                    let dummy = MatcherLoc::Eof;
+                    locs.push(dummy);
+
+                    let next_metavar_orig = *next_metavar;
+                    let op = seq.kleene.op;
+                    let idx_first = locs.len();
+                    let idx_seq = idx_first - 1;
+                    inner(&seq.tts, locs, next_metavar, seq_depth + 1);
+
+                    if let Some(separator) = &seq.separator {
+                        locs.push(MatcherLoc::SequenceSep { separator: separator.clone() });
+                        locs.push(MatcherLoc::SequenceKleeneOpAfterSep { idx_first });
+                    } else {
+                        locs.push(MatcherLoc::SequenceKleeneOpNoSep { op, idx_first });
+                    }
+
+                    // Overwrite the dummy value pushed above with the proper value.
+                    locs[idx_seq] = MatcherLoc::Sequence {
+                        op,
+                        num_metavar_decls: seq.num_captures,
+                        idx_first_after: locs.len(),
+                        next_metavar: next_metavar_orig,
+                        seq_depth,
+                    };
+                }
+                &TokenTree::MetaVarDecl(span, bind, kind) => {
+                    locs.push(MatcherLoc::MetaVarDecl {
+                        span,
+                        bind,
+                        kind,
+                        next_metavar: *next_metavar,
+                        seq_depth,
+                    });
+                    *next_metavar += 1;
+                }
+                TokenTree::MetaVar(..) | TokenTree::MetaVarExpr(..) => unreachable!(),
+            }
+        }
+    }
+
+    let mut locs = vec![];
+    let mut next_metavar = 0;
+    inner(matcher, &mut locs, &mut next_metavar, /* seq_depth */ 0);
+
+    // A final entry is needed for eof.
+    locs.push(MatcherLoc::Eof);
+
+    locs
+}
+
+/// A single matcher position, representing the state of matching.
+struct MatcherPos {
+    /// The index into `TtParser::locs`, which represents the "dot".
+    idx: usize,
+
+    /// The matches made against metavar decls so far. On a successful match, this vector ends up
+    /// with one element per metavar decl in the matcher. Each element records token trees matched
+    /// against the relevant metavar by the black box parser. An element will be a `MatchedSeq` if
+    /// the corresponding metavar decl is within a sequence.
+    ///
+    /// It is critical to performance that this is an `Lrc`, because it gets cloned frequently when
+    /// processing sequences. Mostly for sequence-ending possibilities that must be tried but end
+    /// up failing.
+    matches: Lrc<Vec<NamedMatch>>,
+}
+
+// This type is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+rustc_data_structures::static_assert_size!(MatcherPos, 16);
+
+impl MatcherPos {
+    /// Adds `m` as a named match for the `metavar_idx`-th metavar. There are only two call sites,
+    /// and both are hot enough to be always worth inlining.
+    #[inline(always)]
+    fn push_match(&mut self, metavar_idx: usize, seq_depth: usize, m: NamedMatch) {
+        let matches = Lrc::make_mut(&mut self.matches);
+        match seq_depth {
+            0 => {
+                // We are not within a sequence. Just append `m`.
+                assert_eq!(metavar_idx, matches.len());
+                matches.push(m);
+            }
+            _ => {
+                // We are within a sequence. Find the final `MatchedSeq` at the appropriate depth
+                // and append `m` to its vector.
+                let mut curr = &mut matches[metavar_idx];
+                for _ in 0..seq_depth - 1 {
+                    match curr {
+                        MatchedSeq(seq) => curr = seq.last_mut().unwrap(),
+                        _ => unreachable!(),
+                    }
+                }
+                match curr {
+                    MatchedSeq(seq) => seq.push(m),
+                    _ => unreachable!(),
+                }
+            }
+        }
+    }
+}
+
+enum EofMatcherPositions {
+    None,
+    One(MatcherPos),
+    Multiple,
+}
+
+/// Represents the possible results of an attempted parse.
+pub(crate) enum ParseResult<T> {
+    /// Parsed successfully.
+    Success(T),
+    /// Arm failed to match. If the second parameter is `token::Eof`, it indicates an unexpected
+    /// end of macro invocation. Otherwise, it indicates that no rules expected the given token.
+    Failure(Token, &'static str),
+    /// Fatal error (malformed macro?). Abort compilation.
+    Error(rustc_span::Span, String),
+    ErrorReported,
+}
+
+/// A `ParseResult` where the `Success` variant contains a mapping of
+/// `MacroRulesNormalizedIdent`s to `NamedMatch`es. This represents the mapping
+/// of metavars to the token trees they bind to.
+pub(crate) type NamedParseResult = ParseResult<FxHashMap<MacroRulesNormalizedIdent, NamedMatch>>;
+
+/// Count how many metavars declarations are in `matcher`.
+pub(super) fn count_metavar_decls(matcher: &[TokenTree]) -> usize {
+    matcher
+        .iter()
+        .map(|tt| match tt {
+            TokenTree::MetaVarDecl(..) => 1,
+            TokenTree::Sequence(_, seq) => seq.num_captures,
+            TokenTree::Delimited(_, delim) => count_metavar_decls(&delim.tts),
+            TokenTree::Token(..) => 0,
+            TokenTree::MetaVar(..) | TokenTree::MetaVarExpr(..) => unreachable!(),
+        })
+        .sum()
+}
+
+/// `NamedMatch` is a pattern-match result for a single metavar. All
+/// `MatchedNonterminal`s in the `NamedMatch` have the same non-terminal type
+/// (expr, item, etc).
+///
+/// The in-memory structure of a particular `NamedMatch` represents the match
+/// that occurred when a particular subset of a matcher was applied to a
+/// particular token tree.
+///
+/// The width of each `MatchedSeq` in the `NamedMatch`, and the identity of
+/// the `MatchedNtNonTts`s, will depend on the token tree it was applied
+/// to: each `MatchedSeq` corresponds to a single repetition in the originating
+/// token tree. The depth of the `NamedMatch` structure will therefore depend
+/// only on the nesting depth of repetitions in the originating token tree it
+/// was derived from.
+///
+/// In layperson's terms: `NamedMatch` will form a tree representing nested matches of a particular
+/// meta variable. For example, if we are matching the following macro against the following
+/// invocation...
+///
+/// ```rust
+/// macro_rules! foo {
+///   ($($($x:ident),+);+) => {}
+/// }
+///
+/// foo!(a, b, c, d; a, b, c, d, e);
+/// ```
+///
+/// Then, the tree will have the following shape:
+///
+/// ```ignore (private-internal)
+/// # use NamedMatch::*;
+/// MatchedSeq([
+///   MatchedSeq([
+///     MatchedNonterminal(a),
+///     MatchedNonterminal(b),
+///     MatchedNonterminal(c),
+///     MatchedNonterminal(d),
+///   ]),
+///   MatchedSeq([
+///     MatchedNonterminal(a),
+///     MatchedNonterminal(b),
+///     MatchedNonterminal(c),
+///     MatchedNonterminal(d),
+///     MatchedNonterminal(e),
+///   ])
+/// ])
+/// ```
+#[derive(Debug, Clone)]
+pub(crate) enum NamedMatch {
+    MatchedSeq(Vec<NamedMatch>),
+
+    // A metavar match of type `tt`.
+    MatchedTokenTree(rustc_ast::tokenstream::TokenTree),
+
+    // A metavar match of any type other than `tt`.
+    MatchedNonterminal(Lrc<Nonterminal>),
+}
+
+/// Performs a token equality check, ignoring syntax context (that is, an unhygienic comparison)
+fn token_name_eq(t1: &Token, t2: &Token) -> bool {
+    if let (Some((ident1, is_raw1)), Some((ident2, is_raw2))) = (t1.ident(), t2.ident()) {
+        ident1.name == ident2.name && is_raw1 == is_raw2
+    } else if let (Some(ident1), Some(ident2)) = (t1.lifetime(), t2.lifetime()) {
+        ident1.name == ident2.name
+    } else {
+        t1.kind == t2.kind
+    }
+}
+
+// Note: the vectors could be created and dropped within `parse_tt`, but to avoid excess
+// allocations we have a single vector for each kind that is cleared and reused repeatedly.
+pub struct TtParser {
+    macro_name: Ident,
+
+    /// The set of current mps to be processed. This should be empty by the end of a successful
+    /// execution of `parse_tt_inner`.
+    cur_mps: Vec<MatcherPos>,
+
+    /// The set of newly generated mps. These are used to replenish `cur_mps` in the function
+    /// `parse_tt`.
+    next_mps: Vec<MatcherPos>,
+
+    /// The set of mps that are waiting for the black-box parser.
+    bb_mps: Vec<MatcherPos>,
+
+    /// Pre-allocate an empty match array, so it can be cloned cheaply for macros with many rules
+    /// that have no metavars.
+    empty_matches: Lrc<Vec<NamedMatch>>,
+}
+
+impl TtParser {
+    pub(super) fn new(macro_name: Ident) -> TtParser {
+        TtParser {
+            macro_name,
+            cur_mps: vec![],
+            next_mps: vec![],
+            bb_mps: vec![],
+            empty_matches: Lrc::new(vec![]),
+        }
+    }
+
+    /// Process the matcher positions of `cur_mps` until it is empty. In the process, this will
+    /// produce more mps in `next_mps` and `bb_mps`.
+    ///
+    /// # Returns
+    ///
+    /// `Some(result)` if everything is finished, `None` otherwise. Note that matches are kept
+    /// track of through the mps generated.
+    fn parse_tt_inner(
+        &mut self,
+        matcher: &[MatcherLoc],
+        token: &Token,
+    ) -> Option<NamedParseResult> {
+        // Matcher positions that would be valid if the macro invocation was over now. Only
+        // modified if `token == Eof`.
+        let mut eof_mps = EofMatcherPositions::None;
+
+        while let Some(mut mp) = self.cur_mps.pop() {
+            match &matcher[mp.idx] {
+                MatcherLoc::Token { token: t } => {
+                    // If it's a doc comment, we just ignore it and move on to the next tt in the
+                    // matcher. This is a bug, but #95267 showed that existing programs rely on
+                    // this behaviour, and changing it would require some care and a transition
+                    // period.
+                    //
+                    // If the token matches, we can just advance the parser.
+                    //
+                    // Otherwise, this match has failed, there is nothing to do, and hopefully
+                    // another mp in `cur_mps` will match.
+                    if matches!(t, Token { kind: DocComment(..), .. }) {
+                        mp.idx += 1;
+                        self.cur_mps.push(mp);
+                    } else if token_name_eq(&t, token) {
+                        mp.idx += 1;
+                        self.next_mps.push(mp);
+                    }
+                }
+                MatcherLoc::Delimited => {
+                    // Entering the delimeter is trivial.
+                    mp.idx += 1;
+                    self.cur_mps.push(mp);
+                }
+                &MatcherLoc::Sequence {
+                    op,
+                    num_metavar_decls,
+                    idx_first_after,
+                    next_metavar,
+                    seq_depth,
+                } => {
+                    // Install an empty vec for each metavar within the sequence.
+                    for metavar_idx in next_metavar..next_metavar + num_metavar_decls {
+                        mp.push_match(metavar_idx, seq_depth, MatchedSeq(vec![]));
+                    }
+
+                    if op == KleeneOp::ZeroOrMore || op == KleeneOp::ZeroOrOne {
+                        // Try zero matches of this sequence, by skipping over it.
+                        self.cur_mps.push(MatcherPos {
+                            idx: idx_first_after,
+                            matches: mp.matches.clone(), // a cheap clone
+                        });
+                    }
+
+                    // Try one or more matches of this sequence, by entering it.
+                    mp.idx += 1;
+                    self.cur_mps.push(mp);
+                }
+                &MatcherLoc::SequenceKleeneOpNoSep { op, idx_first } => {
+                    // We are past the end of a sequence with no separator. Try ending the
+                    // sequence. If that's not possible, `ending_mp` will fail quietly when it is
+                    // processed next time around the loop.
+                    let ending_mp = MatcherPos {
+                        idx: mp.idx + 1,             // +1 skips the Kleene op
+                        matches: mp.matches.clone(), // a cheap clone
+                    };
+                    self.cur_mps.push(ending_mp);
+
+                    if op != KleeneOp::ZeroOrOne {
+                        // Try another repetition.
+                        mp.idx = idx_first;
+                        self.cur_mps.push(mp);
+                    }
+                }
+                MatcherLoc::SequenceSep { separator } => {
+                    // We are past the end of a sequence with a separator but we haven't seen the
+                    // separator yet. Try ending the sequence. If that's not possible, `ending_mp`
+                    // will fail quietly when it is processed next time around the loop.
+                    let ending_mp = MatcherPos {
+                        idx: mp.idx + 2,             // +2 skips the separator and the Kleene op
+                        matches: mp.matches.clone(), // a cheap clone
+                    };
+                    self.cur_mps.push(ending_mp);
+
+                    if token_name_eq(token, separator) {
+                        // The separator matches the current token. Advance past it.
+                        mp.idx += 1;
+                        self.next_mps.push(mp);
+                    }
+                }
+                &MatcherLoc::SequenceKleeneOpAfterSep { idx_first } => {
+                    // We are past the sequence separator. This can't be a `?` Kleene op, because
+                    // they don't permit separators. Try another repetition.
+                    mp.idx = idx_first;
+                    self.cur_mps.push(mp);
+                }
+                &MatcherLoc::MetaVarDecl { span, kind, .. } => {
+                    // Built-in nonterminals never start with these tokens, so we can eliminate
+                    // them from consideration. We use the span of the metavariable declaration
+                    // to determine any edition-specific matching behavior for non-terminals.
+                    if let Some(kind) = kind {
+                        if Parser::nonterminal_may_begin_with(kind, token) {
+                            self.bb_mps.push(mp);
+                        }
+                    } else {
+                        // E.g. `$e` instead of `$e:expr`, reported as a hard error if actually used.
+                        // Both this check and the one in `nameize` are necessary, surprisingly.
+                        return Some(Error(span, "missing fragment specifier".to_string()));
+                    }
+                }
+                MatcherLoc::Eof => {
+                    // We are past the matcher's end, and not in a sequence. Try to end things.
+                    debug_assert_eq!(mp.idx, matcher.len() - 1);
+                    if *token == token::Eof {
+                        eof_mps = match eof_mps {
+                            EofMatcherPositions::None => EofMatcherPositions::One(mp),
+                            EofMatcherPositions::One(_) | EofMatcherPositions::Multiple => {
+                                EofMatcherPositions::Multiple
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        // If we reached the end of input, check that there is EXACTLY ONE possible matcher.
+        // Otherwise, either the parse is ambiguous (which is an error) or there is a syntax error.
+        if *token == token::Eof {
+            Some(match eof_mps {
+                EofMatcherPositions::One(mut eof_mp) => {
+                    // Need to take ownership of the matches from within the `Lrc`.
+                    Lrc::make_mut(&mut eof_mp.matches);
+                    let matches = Lrc::try_unwrap(eof_mp.matches).unwrap().into_iter();
+                    self.nameize(matcher, matches)
+                }
+                EofMatcherPositions::Multiple => {
+                    Error(token.span, "ambiguity: multiple successful parses".to_string())
+                }
+                EofMatcherPositions::None => Failure(
+                    Token::new(
+                        token::Eof,
+                        if token.span.is_dummy() { token.span } else { token.span.shrink_to_hi() },
+                    ),
+                    "missing tokens in macro arguments",
+                ),
+            })
+        } else {
+            None
+        }
+    }
+
+    /// Match the token stream from `parser` against `matcher`.
+    pub(super) fn parse_tt(
+        &mut self,
+        parser: &mut Cow<'_, Parser<'_>>,
+        matcher: &[MatcherLoc],
+    ) -> NamedParseResult {
+        // A queue of possible matcher positions. We initialize it with the matcher position in
+        // which the "dot" is before the first token of the first token tree in `matcher`.
+        // `parse_tt_inner` then processes all of these possible matcher positions and produces
+        // possible next positions into `next_mps`. After some post-processing, the contents of
+        // `next_mps` replenish `cur_mps` and we start over again.
+        self.cur_mps.clear();
+        self.cur_mps.push(MatcherPos { idx: 0, matches: self.empty_matches.clone() });
+
+        loop {
+            self.next_mps.clear();
+            self.bb_mps.clear();
+
+            // Process `cur_mps` until either we have finished the input or we need to get some
+            // parsing from the black-box parser done.
+            if let Some(res) = self.parse_tt_inner(matcher, &parser.token) {
+                return res;
+            }
+
+            // `parse_tt_inner` handled all of `cur_mps`, so it's empty.
+            assert!(self.cur_mps.is_empty());
+
+            // Error messages here could be improved with links to original rules.
+            match (self.next_mps.len(), self.bb_mps.len()) {
+                (0, 0) => {
+                    // There are no possible next positions AND we aren't waiting for the black-box
+                    // parser: syntax error.
+                    return Failure(
+                        parser.token.clone(),
+                        "no rules expected this token in macro call",
+                    );
+                }
+
+                (_, 0) => {
+                    // Dump all possible `next_mps` into `cur_mps` for the next iteration. Then
+                    // process the next token.
+                    self.cur_mps.append(&mut self.next_mps);
+                    parser.to_mut().bump();
+                }
+
+                (0, 1) => {
+                    // We need to call the black-box parser to get some nonterminal.
+                    let mut mp = self.bb_mps.pop().unwrap();
+                    let loc = &matcher[mp.idx];
+                    if let &MatcherLoc::MetaVarDecl {
+                        span,
+                        kind: Some(kind),
+                        next_metavar,
+                        seq_depth,
+                        ..
+                    } = loc
+                    {
+                        // We use the span of the metavariable declaration to determine any
+                        // edition-specific matching behavior for non-terminals.
+                        let nt = match parser.to_mut().parse_nonterminal(kind) {
+                            Err(mut err) => {
+                                err.span_label(
+                                    span,
+                                    format!(
+                                        "while parsing argument for this `{kind}` macro fragment"
+                                    ),
+                                )
+                                .emit();
+                                return ErrorReported;
+                            }
+                            Ok(nt) => nt,
+                        };
+                        let m = match nt {
+                            NtOrTt::Nt(nt) => MatchedNonterminal(Lrc::new(nt)),
+                            NtOrTt::Tt(tt) => MatchedTokenTree(tt),
+                        };
+                        mp.push_match(next_metavar, seq_depth, m);
+                        mp.idx += 1;
+                    } else {
+                        unreachable!()
+                    }
+                    self.cur_mps.push(mp);
+                }
+
+                (_, _) => {
+                    // Too many possibilities!
+                    return self.ambiguity_error(matcher, parser.token.span);
+                }
+            }
+
+            assert!(!self.cur_mps.is_empty());
+        }
+    }
+
+    fn ambiguity_error(
+        &self,
+        matcher: &[MatcherLoc],
+        token_span: rustc_span::Span,
+    ) -> NamedParseResult {
+        let nts = self
+            .bb_mps
+            .iter()
+            .map(|mp| match &matcher[mp.idx] {
+                MatcherLoc::MetaVarDecl { bind, kind: Some(kind), .. } => {
+                    format!("{} ('{}')", kind, bind)
+                }
+                _ => unreachable!(),
+            })
+            .collect::<Vec<String>>()
+            .join(" or ");
+
+        Error(
+            token_span,
+            format!(
+                "local ambiguity when calling macro `{}`: multiple parsing options: {}",
+                self.macro_name,
+                match self.next_mps.len() {
+                    0 => format!("built-in NTs {}.", nts),
+                    n => format!("built-in NTs {} or {n} other option{s}.", nts, s = pluralize!(n)),
+                }
+            ),
+        )
+    }
+
+    fn nameize<I: Iterator<Item = NamedMatch>>(
+        &self,
+        matcher: &[MatcherLoc],
+        mut res: I,
+    ) -> NamedParseResult {
+        // Make that each metavar has _exactly one_ binding. If so, insert the binding into the
+        // `NamedParseResult`. Otherwise, it's an error.
+        let mut ret_val = FxHashMap::default();
+        for loc in matcher {
+            if let &MatcherLoc::MetaVarDecl { span, bind, kind, .. } = loc {
+                if kind.is_some() {
+                    match ret_val.entry(MacroRulesNormalizedIdent::new(bind)) {
+                        Vacant(spot) => spot.insert(res.next().unwrap()),
+                        Occupied(..) => {
+                            return Error(span, format!("duplicated bind name: {}", bind));
+                        }
+                    };
+                } else {
+                    // E.g. `$e` instead of `$e:expr`, reported as a hard error if actually used.
+                    // Both this check and the one in `parse_tt_inner` are necessary, surprisingly.
+                    return Error(span, "missing fragment specifier".to_string());
+                }
+            }
+        }
+        Success(ret_val)
+    }
+}
diff --git a/compiler/rustc_expand/src/mbe/macro_rules.rs b/compiler/rustc_expand/src/mbe/macro_rules.rs
new file mode 100644
index 000000000..f7e1575af
--- /dev/null
+++ b/compiler/rustc_expand/src/mbe/macro_rules.rs
@@ -0,0 +1,1420 @@
+use crate::base::{DummyResult, ExtCtxt, MacResult, TTMacroExpander};
+use crate::base::{SyntaxExtension, SyntaxExtensionKind};
+use crate::expand::{ensure_complete_parse, parse_ast_fragment, AstFragment, AstFragmentKind};
+use crate::mbe;
+use crate::mbe::macro_check;
+use crate::mbe::macro_parser::{Error, ErrorReported, Failure, Success, TtParser};
+use crate::mbe::macro_parser::{MatchedSeq, MatchedTokenTree, MatcherLoc};
+use crate::mbe::transcribe::transcribe;
+
+use rustc_ast as ast;
+use rustc_ast::token::{self, Delimiter, NonterminalKind, Token, TokenKind, TokenKind::*};
+use rustc_ast::tokenstream::{DelimSpan, TokenStream};
+use rustc_ast::{NodeId, DUMMY_NODE_ID};
+use rustc_ast_pretty::pprust;
+use rustc_attr::{self as attr, TransparencyError};
+use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
+use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_feature::Features;
+use rustc_lint_defs::builtin::{
+    RUST_2021_INCOMPATIBLE_OR_PATTERNS, SEMICOLON_IN_EXPRESSIONS_FROM_MACROS,
+};
+use rustc_lint_defs::BuiltinLintDiagnostics;
+use rustc_parse::parser::Parser;
+use rustc_session::parse::ParseSess;
+use rustc_session::Session;
+use rustc_span::edition::Edition;
+use rustc_span::hygiene::Transparency;
+use rustc_span::source_map::SourceMap;
+use rustc_span::symbol::{kw, sym, Ident, MacroRulesNormalizedIdent};
+use rustc_span::Span;
+
+use std::borrow::Cow;
+use std::collections::hash_map::Entry;
+use std::{mem, slice};
+use tracing::debug;
+
+pub(crate) struct ParserAnyMacro<'a> {
+    parser: Parser<'a>,
+
+    /// Span of the expansion site of the macro this parser is for
+    site_span: Span,
+    /// The ident of the macro we're parsing
+    macro_ident: Ident,
+    lint_node_id: NodeId,
+    is_trailing_mac: bool,
+    arm_span: Span,
+    /// Whether or not this macro is defined in the current crate
+    is_local: bool,
+}
+
+pub(crate) fn annotate_err_with_kind(err: &mut Diagnostic, kind: AstFragmentKind, span: Span) {
+    match kind {
+        AstFragmentKind::Ty => {
+            err.span_label(span, "this macro call doesn't expand to a type");
+        }
+        AstFragmentKind::Pat => {
+            err.span_label(span, "this macro call doesn't expand to a pattern");
+        }
+        _ => {}
+    };
+}
+
+fn emit_frag_parse_err(
+    mut e: DiagnosticBuilder<'_, rustc_errors::ErrorGuaranteed>,
+    parser: &Parser<'_>,
+    orig_parser: &mut Parser<'_>,
+    site_span: Span,
+    arm_span: Span,
+    kind: AstFragmentKind,
+) {
+    // FIXME(davidtwco): avoid depending on the error message text
+    if parser.token == token::Eof && e.message[0].0.expect_str().ends_with(", found `<eof>`") {
+        if !e.span.is_dummy() {
+            // early end of macro arm (#52866)
+            e.replace_span_with(parser.sess.source_map().next_point(parser.token.span));
+        }
+        let msg = &e.message[0];
+        e.message[0] = (
+            rustc_errors::DiagnosticMessage::Str(format!(
+                "macro expansion ends with an incomplete expression: {}",
+                msg.0.expect_str().replace(", found `<eof>`", ""),
+            )),
+            msg.1,
+        );
+    }
+    if e.span.is_dummy() {
+        // Get around lack of span in error (#30128)
+        e.replace_span_with(site_span);
+        if !parser.sess.source_map().is_imported(arm_span) {
+            e.span_label(arm_span, "in this macro arm");
+        }
+    } else if parser.sess.source_map().is_imported(parser.token.span) {
+        e.span_label(site_span, "in this macro invocation");
+    }
+    match kind {
+        // Try a statement if an expression is wanted but failed and suggest adding `;` to call.
+        AstFragmentKind::Expr => match parse_ast_fragment(orig_parser, AstFragmentKind::Stmts) {
+            Err(err) => err.cancel(),
+            Ok(_) => {
+                e.note(
+                    "the macro call doesn't expand to an expression, but it can expand to a statement",
+                );
+                e.span_suggestion_verbose(
+                    site_span.shrink_to_hi(),
+                    "add `;` to interpret the expansion as a statement",
+                    ";",
+                    Applicability::MaybeIncorrect,
+                );
+            }
+        },
+        _ => annotate_err_with_kind(&mut e, kind, site_span),
+    };
+    e.emit();
+}
+
+impl<'a> ParserAnyMacro<'a> {
+    pub(crate) fn make(mut self: Box<ParserAnyMacro<'a>>, kind: AstFragmentKind) -> AstFragment {
+        let ParserAnyMacro {
+            site_span,
+            macro_ident,
+            ref mut parser,
+            lint_node_id,
+            arm_span,
+            is_trailing_mac,
+            is_local,
+        } = *self;
+        let snapshot = &mut parser.create_snapshot_for_diagnostic();
+        let fragment = match parse_ast_fragment(parser, kind) {
+            Ok(f) => f,
+            Err(err) => {
+                emit_frag_parse_err(err, parser, snapshot, site_span, arm_span, kind);
+                return kind.dummy(site_span);
+            }
+        };
+
+        // We allow semicolons at the end of expressions -- e.g., the semicolon in
+        // `macro_rules! m { () => { panic!(); } }` isn't parsed by `.parse_expr()`,
+        // but `m!()` is allowed in expression positions (cf. issue #34706).
+        if kind == AstFragmentKind::Expr && parser.token == token::Semi {
+            if is_local {
+                parser.sess.buffer_lint_with_diagnostic(
+                    SEMICOLON_IN_EXPRESSIONS_FROM_MACROS,
+                    parser.token.span,
+                    lint_node_id,
+                    "trailing semicolon in macro used in expression position",
+                    BuiltinLintDiagnostics::TrailingMacro(is_trailing_mac, macro_ident),
+                );
+            }
+            parser.bump();
+        }
+
+        // Make sure we don't have any tokens left to parse so we don't silently drop anything.
+        let path = ast::Path::from_ident(macro_ident.with_span_pos(site_span));
+        ensure_complete_parse(parser, &path, kind.name(), site_span);
+        fragment
+    }
+}
+
+struct MacroRulesMacroExpander {
+    node_id: NodeId,
+    name: Ident,
+    span: Span,
+    transparency: Transparency,
+    lhses: Vec<Vec<MatcherLoc>>,
+    rhses: Vec<mbe::TokenTree>,
+    valid: bool,
+}
+
+impl TTMacroExpander for MacroRulesMacroExpander {
+    fn expand<'cx>(
+        &self,
+        cx: &'cx mut ExtCtxt<'_>,
+        sp: Span,
+        input: TokenStream,
+    ) -> Box<dyn MacResult + 'cx> {
+        if !self.valid {
+            return DummyResult::any(sp);
+        }
+        expand_macro(
+            cx,
+            sp,
+            self.span,
+            self.node_id,
+            self.name,
+            self.transparency,
+            input,
+            &self.lhses,
+            &self.rhses,
+        )
+    }
+}
+
+fn macro_rules_dummy_expander<'cx>(
+    _: &'cx mut ExtCtxt<'_>,
+    span: Span,
+    _: TokenStream,
+) -> Box<dyn MacResult + 'cx> {
+    DummyResult::any(span)
+}
+
+fn trace_macros_note(cx_expansions: &mut FxIndexMap<Span, Vec<String>>, sp: Span, message: String) {
+    let sp = sp.macro_backtrace().last().map_or(sp, |trace| trace.call_site);
+    cx_expansions.entry(sp).or_default().push(message);
+}
+
+/// Expands the rules based macro defined by `lhses` and `rhses` for a given
+/// input `arg`.
+fn expand_macro<'cx>(
+    cx: &'cx mut ExtCtxt<'_>,
+    sp: Span,
+    def_span: Span,
+    node_id: NodeId,
+    name: Ident,
+    transparency: Transparency,
+    arg: TokenStream,
+    lhses: &[Vec<MatcherLoc>],
+    rhses: &[mbe::TokenTree],
+) -> Box<dyn MacResult + 'cx> {
+    let sess = &cx.sess.parse_sess;
+    // Macros defined in the current crate have a real node id,
+    // whereas macros from an external crate have a dummy id.
+    let is_local = node_id != DUMMY_NODE_ID;
+
+    if cx.trace_macros() {
+        let msg = format!("expanding `{}! {{ {} }}`", name, pprust::tts_to_string(&arg));
+        trace_macros_note(&mut cx.expansions, sp, msg);
+    }
+
+    // Which arm's failure should we report? (the one furthest along)
+    let mut best_failure: Option<(Token, &str)> = None;
+
+    // We create a base parser that can be used for the "black box" parts.
+    // Every iteration needs a fresh copy of that parser. However, the parser
+    // is not mutated on many of the iterations, particularly when dealing with
+    // macros like this:
+    //
+    // macro_rules! foo {
+    //     ("a") => (A);
+    //     ("b") => (B);
+    //     ("c") => (C);
+    //     // ... etc. (maybe hundreds more)
+    // }
+    //
+    // as seen in the `html5ever` benchmark. We use a `Cow` so that the base
+    // parser is only cloned when necessary (upon mutation). Furthermore, we
+    // reinitialize the `Cow` with the base parser at the start of every
+    // iteration, so that any mutated parsers are not reused. This is all quite
+    // hacky, but speeds up the `html5ever` benchmark significantly. (Issue
+    // 68836 suggests a more comprehensive but more complex change to deal with
+    // this situation.)
+    let parser = parser_from_cx(sess, arg.clone());
+
+    // Try each arm's matchers.
+    let mut tt_parser = TtParser::new(name);
+    for (i, lhs) in lhses.iter().enumerate() {
+        // Take a snapshot of the state of pre-expansion gating at this point.
+        // This is used so that if a matcher is not `Success(..)`ful,
+        // then the spans which became gated when parsing the unsuccessful matcher
+        // are not recorded. On the first `Success(..)`ful matcher, the spans are merged.
+        let mut gated_spans_snapshot = mem::take(&mut *sess.gated_spans.spans.borrow_mut());
+
+        match tt_parser.parse_tt(&mut Cow::Borrowed(&parser), lhs) {
+            Success(named_matches) => {
+                // The matcher was `Success(..)`ful.
+                // Merge the gated spans from parsing the matcher with the pre-existing ones.
+                sess.gated_spans.merge(gated_spans_snapshot);
+
+                let (rhs, rhs_span): (&mbe::Delimited, DelimSpan) = match &rhses[i] {
+                    mbe::TokenTree::Delimited(span, delimited) => (&delimited, *span),
+                    _ => cx.span_bug(sp, "malformed macro rhs"),
+                };
+                let arm_span = rhses[i].span();
+
+                let rhs_spans = rhs.tts.iter().map(|t| t.span()).collect::<Vec<_>>();
+                // rhs has holes ( `$id` and `$(...)` that need filled)
+                let mut tts = match transcribe(cx, &named_matches, &rhs, rhs_span, transparency) {
+                    Ok(tts) => tts,
+                    Err(mut err) => {
+                        err.emit();
+                        return DummyResult::any(arm_span);
+                    }
+                };
+
+                // Replace all the tokens for the corresponding positions in the macro, to maintain
+                // proper positions in error reporting, while maintaining the macro_backtrace.
+                if rhs_spans.len() == tts.len() {
+                    tts = tts.map_enumerated(|i, tt| {
+                        let mut tt = tt.clone();
+                        let mut sp = rhs_spans[i];
+                        sp = sp.with_ctxt(tt.span().ctxt());
+                        tt.set_span(sp);
+                        tt
+                    });
+                }
+
+                if cx.trace_macros() {
+                    let msg = format!("to `{}`", pprust::tts_to_string(&tts));
+                    trace_macros_note(&mut cx.expansions, sp, msg);
+                }
+
+                let mut p = Parser::new(sess, tts, false, None);
+                p.last_type_ascription = cx.current_expansion.prior_type_ascription;
+
+                if is_local {
+                    cx.resolver.record_macro_rule_usage(node_id, i);
+                }
+
+                // Let the context choose how to interpret the result.
+                // Weird, but useful for X-macros.
+                return Box::new(ParserAnyMacro {
+                    parser: p,
+
+                    // Pass along the original expansion site and the name of the macro
+                    // so we can print a useful error message if the parse of the expanded
+                    // macro leaves unparsed tokens.
+                    site_span: sp,
+                    macro_ident: name,
+                    lint_node_id: cx.current_expansion.lint_node_id,
+                    is_trailing_mac: cx.current_expansion.is_trailing_mac,
+                    arm_span,
+                    is_local,
+                });
+            }
+            Failure(token, msg) => match best_failure {
+                Some((ref best_token, _)) if best_token.span.lo() >= token.span.lo() => {}
+                _ => best_failure = Some((token, msg)),
+            },
+            Error(err_sp, ref msg) => {
+                let span = err_sp.substitute_dummy(sp);
+                cx.struct_span_err(span, &msg).emit();
+                return DummyResult::any(span);
+            }
+            ErrorReported => return DummyResult::any(sp),
+        }
+
+        // The matcher was not `Success(..)`ful.
+        // Restore to the state before snapshotting and maybe try again.
+        mem::swap(&mut gated_spans_snapshot, &mut sess.gated_spans.spans.borrow_mut());
+    }
+    drop(parser);
+
+    let (token, label) = best_failure.expect("ran no matchers");
+    let span = token.span.substitute_dummy(sp);
+    let mut err = cx.struct_span_err(span, &parse_failure_msg(&token));
+    err.span_label(span, label);
+    if !def_span.is_dummy() && !cx.source_map().is_imported(def_span) {
+        err.span_label(cx.source_map().guess_head_span(def_span), "when calling this macro");
+    }
+    annotate_doc_comment(&mut err, sess.source_map(), span);
+    // Check whether there's a missing comma in this macro call, like `println!("{}" a);`
+    if let Some((arg, comma_span)) = arg.add_comma() {
+        for lhs in lhses {
+            let parser = parser_from_cx(sess, arg.clone());
+            if let Success(_) = tt_parser.parse_tt(&mut Cow::Borrowed(&parser), lhs) {
+                if comma_span.is_dummy() {
+                    err.note("you might be missing a comma");
+                } else {
+                    err.span_suggestion_short(
+                        comma_span,
+                        "missing comma here",
+                        ", ",
+                        Applicability::MachineApplicable,
+                    );
+                }
+            }
+        }
+    }
+    err.emit();
+    cx.trace_macros_diag();
+    DummyResult::any(sp)
+}
+
+// Note that macro-by-example's input is also matched against a token tree:
+//                   $( $lhs:tt => $rhs:tt );+
+//
+// Holy self-referential!
+
+/// Converts a macro item into a syntax extension.
+pub fn compile_declarative_macro(
+    sess: &Session,
+    features: &Features,
+    def: &ast::Item,
+    edition: Edition,
+) -> (SyntaxExtension, Vec<(usize, Span)>) {
+    debug!("compile_declarative_macro: {:?}", def);
+    let mk_syn_ext = |expander| {
+        SyntaxExtension::new(
+            sess,
+            SyntaxExtensionKind::LegacyBang(expander),
+            def.span,
+            Vec::new(),
+            edition,
+            def.ident.name,
+            &def.attrs,
+        )
+    };
+    let dummy_syn_ext = || (mk_syn_ext(Box::new(macro_rules_dummy_expander)), Vec::new());
+
+    let diag = &sess.parse_sess.span_diagnostic;
+    let lhs_nm = Ident::new(sym::lhs, def.span);
+    let rhs_nm = Ident::new(sym::rhs, def.span);
+    let tt_spec = Some(NonterminalKind::TT);
+
+    // Parse the macro_rules! invocation
+    let (macro_rules, body) = match &def.kind {
+        ast::ItemKind::MacroDef(def) => (def.macro_rules, def.body.inner_tokens()),
+        _ => unreachable!(),
+    };
+
+    // The pattern that macro_rules matches.
+    // The grammar for macro_rules! is:
+    // $( $lhs:tt => $rhs:tt );+
+    // ...quasiquoting this would be nice.
+    // These spans won't matter, anyways
+    let argument_gram = vec![
+        mbe::TokenTree::Sequence(
+            DelimSpan::dummy(),
+            mbe::SequenceRepetition {
+                tts: vec![
+                    mbe::TokenTree::MetaVarDecl(def.span, lhs_nm, tt_spec),
+                    mbe::TokenTree::token(token::FatArrow, def.span),
+                    mbe::TokenTree::MetaVarDecl(def.span, rhs_nm, tt_spec),
+                ],
+                separator: Some(Token::new(
+                    if macro_rules { token::Semi } else { token::Comma },
+                    def.span,
+                )),
+                kleene: mbe::KleeneToken::new(mbe::KleeneOp::OneOrMore, def.span),
+                num_captures: 2,
+            },
+        ),
+        // to phase into semicolon-termination instead of semicolon-separation
+        mbe::TokenTree::Sequence(
+            DelimSpan::dummy(),
+            mbe::SequenceRepetition {
+                tts: vec![mbe::TokenTree::token(
+                    if macro_rules { token::Semi } else { token::Comma },
+                    def.span,
+                )],
+                separator: None,
+                kleene: mbe::KleeneToken::new(mbe::KleeneOp::ZeroOrMore, def.span),
+                num_captures: 0,
+            },
+        ),
+    ];
+    // Convert it into `MatcherLoc` form.
+    let argument_gram = mbe::macro_parser::compute_locs(&argument_gram);
+
+    let parser = Parser::new(&sess.parse_sess, body, true, rustc_parse::MACRO_ARGUMENTS);
+    let mut tt_parser =
+        TtParser::new(Ident::with_dummy_span(if macro_rules { kw::MacroRules } else { kw::Macro }));
+    let argument_map = match tt_parser.parse_tt(&mut Cow::Borrowed(&parser), &argument_gram) {
+        Success(m) => m,
+        Failure(token, msg) => {
+            let s = parse_failure_msg(&token);
+            let sp = token.span.substitute_dummy(def.span);
+            let mut err = sess.parse_sess.span_diagnostic.struct_span_err(sp, &s);
+            err.span_label(sp, msg);
+            annotate_doc_comment(&mut err, sess.source_map(), sp);
+            err.emit();
+            return dummy_syn_ext();
+        }
+        Error(sp, msg) => {
+            sess.parse_sess
+                .span_diagnostic
+                .struct_span_err(sp.substitute_dummy(def.span), &msg)
+                .emit();
+            return dummy_syn_ext();
+        }
+        ErrorReported => {
+            return dummy_syn_ext();
+        }
+    };
+
+    let mut valid = true;
+
+    // Extract the arguments:
+    let lhses = match argument_map[&MacroRulesNormalizedIdent::new(lhs_nm)] {
+        MatchedSeq(ref s) => s
+            .iter()
+            .map(|m| {
+                if let MatchedTokenTree(ref tt) = *m {
+                    let tt = mbe::quoted::parse(
+                        TokenStream::new(vec![tt.clone()]),
+                        true,
+                        &sess.parse_sess,
+                        def.id,
+                        features,
+                        edition,
+                    )
+                    .pop()
+                    .unwrap();
+                    valid &= check_lhs_nt_follows(&sess.parse_sess, &def, &tt);
+                    return tt;
+                }
+                sess.parse_sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs")
+            })
+            .collect::<Vec<mbe::TokenTree>>(),
+        _ => sess.parse_sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs"),
+    };
+
+    let rhses = match argument_map[&MacroRulesNormalizedIdent::new(rhs_nm)] {
+        MatchedSeq(ref s) => s
+            .iter()
+            .map(|m| {
+                if let MatchedTokenTree(ref tt) = *m {
+                    return mbe::quoted::parse(
+                        TokenStream::new(vec![tt.clone()]),
+                        false,
+                        &sess.parse_sess,
+                        def.id,
+                        features,
+                        edition,
+                    )
+                    .pop()
+                    .unwrap();
+                }
+                sess.parse_sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs")
+            })
+            .collect::<Vec<mbe::TokenTree>>(),
+        _ => sess.parse_sess.span_diagnostic.span_bug(def.span, "wrong-structured rhs"),
+    };
+
+    for rhs in &rhses {
+        valid &= check_rhs(&sess.parse_sess, rhs);
+    }
+
+    // don't abort iteration early, so that errors for multiple lhses can be reported
+    for lhs in &lhses {
+        valid &= check_lhs_no_empty_seq(&sess.parse_sess, slice::from_ref(lhs));
+    }
+
+    valid &= macro_check::check_meta_variables(&sess.parse_sess, def.id, def.span, &lhses, &rhses);
+
+    let (transparency, transparency_error) = attr::find_transparency(&def.attrs, macro_rules);
+    match transparency_error {
+        Some(TransparencyError::UnknownTransparency(value, span)) => {
+            diag.span_err(span, &format!("unknown macro transparency: `{}`", value));
+        }
+        Some(TransparencyError::MultipleTransparencyAttrs(old_span, new_span)) => {
+            diag.span_err(vec![old_span, new_span], "multiple macro transparency attributes");
+        }
+        None => {}
+    }
+
+    // Compute the spans of the macro rules for unused rule linting.
+    // To avoid warning noise, only consider the rules of this
+    // macro for the lint, if all rules are valid.
+    // Also, we are only interested in non-foreign macros.
+    let rule_spans = if valid && def.id != DUMMY_NODE_ID {
+        lhses
+            .iter()
+            .zip(rhses.iter())
+            .enumerate()
+            // If the rhs contains an invocation like compile_error!,
+            // don't consider the rule for the unused rule lint.
+            .filter(|(_idx, (_lhs, rhs))| !has_compile_error_macro(rhs))
+            // We only take the span of the lhs here,
+            // so that the spans of created warnings are smaller.
+            .map(|(idx, (lhs, _rhs))| (idx, lhs.span()))
+            .collect::<Vec<_>>()
+    } else {
+        Vec::new()
+    };
+
+    // Convert the lhses into `MatcherLoc` form, which is better for doing the
+    // actual matching. Unless the matcher is invalid.
+    let lhses = if valid {
+        lhses
+            .iter()
+            .map(|lhs| {
+                // Ignore the delimiters around the matcher.
+                match lhs {
+                    mbe::TokenTree::Delimited(_, delimited) => {
+                        mbe::macro_parser::compute_locs(&delimited.tts)
+                    }
+                    _ => sess.parse_sess.span_diagnostic.span_bug(def.span, "malformed macro lhs"),
+                }
+            })
+            .collect()
+    } else {
+        vec![]
+    };
+
+    let expander = Box::new(MacroRulesMacroExpander {
+        name: def.ident,
+        span: def.span,
+        node_id: def.id,
+        transparency,
+        lhses,
+        rhses,
+        valid,
+    });
+    (mk_syn_ext(expander), rule_spans)
+}
+
+#[derive(SessionSubdiagnostic)]
+enum ExplainDocComment {
+    #[label(expand::explain_doc_comment_inner)]
+    Inner {
+        #[primary_span]
+        span: Span,
+    },
+    #[label(expand::explain_doc_comment_outer)]
+    Outer {
+        #[primary_span]
+        span: Span,
+    },
+}
+
+fn annotate_doc_comment(
+    err: &mut DiagnosticBuilder<'_, ErrorGuaranteed>,
+    sm: &SourceMap,
+    span: Span,
+) {
+    if let Ok(src) = sm.span_to_snippet(span) {
+        if src.starts_with("///") || src.starts_with("/**") {
+            err.subdiagnostic(ExplainDocComment::Outer { span });
+        } else if src.starts_with("//!") || src.starts_with("/*!") {
+            err.subdiagnostic(ExplainDocComment::Inner { span });
+        }
+    }
+}
+
+fn check_lhs_nt_follows(sess: &ParseSess, def: &ast::Item, lhs: &mbe::TokenTree) -> bool {
+    // lhs is going to be like TokenTree::Delimited(...), where the
+    // entire lhs is those tts. Or, it can be a "bare sequence", not wrapped in parens.
+    if let mbe::TokenTree::Delimited(_, delimited) = lhs {
+        check_matcher(sess, def, &delimited.tts)
+    } else {
+        let msg = "invalid macro matcher; matchers must be contained in balanced delimiters";
+        sess.span_diagnostic.span_err(lhs.span(), msg);
+        false
+    }
+    // we don't abort on errors on rejection, the driver will do that for us
+    // after parsing/expansion. we can report every error in every macro this way.
+}
+
+/// Checks that the lhs contains no repetition which could match an empty token
+/// tree, because then the matcher would hang indefinitely.
+fn check_lhs_no_empty_seq(sess: &ParseSess, tts: &[mbe::TokenTree]) -> bool {
+    use mbe::TokenTree;
+    for tt in tts {
+        match *tt {
+            TokenTree::Token(..)
+            | TokenTree::MetaVar(..)
+            | TokenTree::MetaVarDecl(..)
+            | TokenTree::MetaVarExpr(..) => (),
+            TokenTree::Delimited(_, ref del) => {
+                if !check_lhs_no_empty_seq(sess, &del.tts) {
+                    return false;
+                }
+            }
+            TokenTree::Sequence(span, ref seq) => {
+                if seq.separator.is_none()
+                    && seq.tts.iter().all(|seq_tt| match *seq_tt {
+                        TokenTree::MetaVarDecl(_, _, Some(NonterminalKind::Vis)) => true,
+                        TokenTree::Sequence(_, ref sub_seq) => {
+                            sub_seq.kleene.op == mbe::KleeneOp::ZeroOrMore
+                                || sub_seq.kleene.op == mbe::KleeneOp::ZeroOrOne
+                        }
+                        _ => false,
+                    })
+                {
+                    let sp = span.entire();
+                    sess.span_diagnostic.span_err(sp, "repetition matches empty token tree");
+                    return false;
+                }
+                if !check_lhs_no_empty_seq(sess, &seq.tts) {
+                    return false;
+                }
+            }
+        }
+    }
+
+    true
+}
+
+fn check_rhs(sess: &ParseSess, rhs: &mbe::TokenTree) -> bool {
+    match *rhs {
+        mbe::TokenTree::Delimited(..) => return true,
+        _ => {
+            sess.span_diagnostic.span_err(rhs.span(), "macro rhs must be delimited");
+        }
+    }
+    false
+}
+
+fn check_matcher(sess: &ParseSess, def: &ast::Item, matcher: &[mbe::TokenTree]) -> bool {
+    let first_sets = FirstSets::new(matcher);
+    let empty_suffix = TokenSet::empty();
+    let err = sess.span_diagnostic.err_count();
+    check_matcher_core(sess, def, &first_sets, matcher, &empty_suffix);
+    err == sess.span_diagnostic.err_count()
+}
+
+fn has_compile_error_macro(rhs: &mbe::TokenTree) -> bool {
+    match rhs {
+        mbe::TokenTree::Delimited(_sp, d) => {
+            let has_compile_error = d.tts.array_windows::<3>().any(|[ident, bang, args]| {
+                if let mbe::TokenTree::Token(ident) = ident &&
+                        let TokenKind::Ident(ident, _) = ident.kind &&
+                        ident == sym::compile_error &&
+                        let mbe::TokenTree::Token(bang) = bang &&
+                        let TokenKind::Not = bang.kind &&
+                        let mbe::TokenTree::Delimited(_, del) = args &&
+                        del.delim != Delimiter::Invisible
+                    {
+                        true
+                    } else {
+                        false
+                    }
+            });
+            if has_compile_error { true } else { d.tts.iter().any(has_compile_error_macro) }
+        }
+        _ => false,
+    }
+}
+
+// `The FirstSets` for a matcher is a mapping from subsequences in the
+// matcher to the FIRST set for that subsequence.
+//
+// This mapping is partially precomputed via a backwards scan over the
+// token trees of the matcher, which provides a mapping from each
+// repetition sequence to its *first* set.
+//
+// (Hypothetically, sequences should be uniquely identifiable via their
+// spans, though perhaps that is false, e.g., for macro-generated macros
+// that do not try to inject artificial span information. My plan is
+// to try to catch such cases ahead of time and not include them in
+// the precomputed mapping.)
+struct FirstSets<'tt> {
+    // this maps each TokenTree::Sequence `$(tt ...) SEP OP` that is uniquely identified by its
+    // span in the original matcher to the First set for the inner sequence `tt ...`.
+    //
+    // If two sequences have the same span in a matcher, then map that
+    // span to None (invalidating the mapping here and forcing the code to
+    // use a slow path).
+    first: FxHashMap<Span, Option<TokenSet<'tt>>>,
+}
+
+impl<'tt> FirstSets<'tt> {
+    fn new(tts: &'tt [mbe::TokenTree]) -> FirstSets<'tt> {
+        use mbe::TokenTree;
+
+        let mut sets = FirstSets { first: FxHashMap::default() };
+        build_recur(&mut sets, tts);
+        return sets;
+
+        // walks backward over `tts`, returning the FIRST for `tts`
+        // and updating `sets` at the same time for all sequence
+        // substructure we find within `tts`.
+        fn build_recur<'tt>(sets: &mut FirstSets<'tt>, tts: &'tt [TokenTree]) -> TokenSet<'tt> {
+            let mut first = TokenSet::empty();
+            for tt in tts.iter().rev() {
+                match *tt {
+                    TokenTree::Token(..)
+                    | TokenTree::MetaVar(..)
+                    | TokenTree::MetaVarDecl(..)
+                    | TokenTree::MetaVarExpr(..) => {
+                        first.replace_with(TtHandle::TtRef(tt));
+                    }
+                    TokenTree::Delimited(span, ref delimited) => {
+                        build_recur(sets, &delimited.tts);
+                        first.replace_with(TtHandle::from_token_kind(
+                            token::OpenDelim(delimited.delim),
+                            span.open,
+                        ));
+                    }
+                    TokenTree::Sequence(sp, ref seq_rep) => {
+                        let subfirst = build_recur(sets, &seq_rep.tts);
+
+                        match sets.first.entry(sp.entire()) {
+                            Entry::Vacant(vac) => {
+                                vac.insert(Some(subfirst.clone()));
+                            }
+                            Entry::Occupied(mut occ) => {
+                                // if there is already an entry, then a span must have collided.
+                                // This should not happen with typical macro_rules macros,
+                                // but syntax extensions need not maintain distinct spans,
+                                // so distinct syntax trees can be assigned the same span.
+                                // In such a case, the map cannot be trusted; so mark this
+                                // entry as unusable.
+                                occ.insert(None);
+                            }
+                        }
+
+                        // If the sequence contents can be empty, then the first
+                        // token could be the separator token itself.
+
+                        if let (Some(sep), true) = (&seq_rep.separator, subfirst.maybe_empty) {
+                            first.add_one_maybe(TtHandle::from_token(sep.clone()));
+                        }
+
+                        // Reverse scan: Sequence comes before `first`.
+                        if subfirst.maybe_empty
+                            || seq_rep.kleene.op == mbe::KleeneOp::ZeroOrMore
+                            || seq_rep.kleene.op == mbe::KleeneOp::ZeroOrOne
+                        {
+                            // If sequence is potentially empty, then
+                            // union them (preserving first emptiness).
+                            first.add_all(&TokenSet { maybe_empty: true, ..subfirst });
+                        } else {
+                            // Otherwise, sequence guaranteed
+                            // non-empty; replace first.
+                            first = subfirst;
+                        }
+                    }
+                }
+            }
+
+            first
+        }
+    }
+
+    // walks forward over `tts` until all potential FIRST tokens are
+    // identified.
+    fn first(&self, tts: &'tt [mbe::TokenTree]) -> TokenSet<'tt> {
+        use mbe::TokenTree;
+
+        let mut first = TokenSet::empty();
+        for tt in tts.iter() {
+            assert!(first.maybe_empty);
+            match *tt {
+                TokenTree::Token(..)
+                | TokenTree::MetaVar(..)
+                | TokenTree::MetaVarDecl(..)
+                | TokenTree::MetaVarExpr(..) => {
+                    first.add_one(TtHandle::TtRef(tt));
+                    return first;
+                }
+                TokenTree::Delimited(span, ref delimited) => {
+                    first.add_one(TtHandle::from_token_kind(
+                        token::OpenDelim(delimited.delim),
+                        span.open,
+                    ));
+                    return first;
+                }
+                TokenTree::Sequence(sp, ref seq_rep) => {
+                    let subfirst_owned;
+                    let subfirst = match self.first.get(&sp.entire()) {
+                        Some(&Some(ref subfirst)) => subfirst,
+                        Some(&None) => {
+                            subfirst_owned = self.first(&seq_rep.tts);
+                            &subfirst_owned
+                        }
+                        None => {
+                            panic!("We missed a sequence during FirstSets construction");
+                        }
+                    };
+
+                    // If the sequence contents can be empty, then the first
+                    // token could be the separator token itself.
+                    if let (Some(sep), true) = (&seq_rep.separator, subfirst.maybe_empty) {
+                        first.add_one_maybe(TtHandle::from_token(sep.clone()));
+                    }
+
+                    assert!(first.maybe_empty);
+                    first.add_all(subfirst);
+                    if subfirst.maybe_empty
+                        || seq_rep.kleene.op == mbe::KleeneOp::ZeroOrMore
+                        || seq_rep.kleene.op == mbe::KleeneOp::ZeroOrOne
+                    {
+                        // Continue scanning for more first
+                        // tokens, but also make sure we
+                        // restore empty-tracking state.
+                        first.maybe_empty = true;
+                        continue;
+                    } else {
+                        return first;
+                    }
+                }
+            }
+        }
+
+        // we only exit the loop if `tts` was empty or if every
+        // element of `tts` matches the empty sequence.
+        assert!(first.maybe_empty);
+        first
+    }
+}
+
+// Most `mbe::TokenTree`s are pre-existing in the matcher, but some are defined
+// implicitly, such as opening/closing delimiters and sequence repetition ops.
+// This type encapsulates both kinds. It implements `Clone` while avoiding the
+// need for `mbe::TokenTree` to implement `Clone`.
+#[derive(Debug)]
+enum TtHandle<'tt> {
+    /// This is used in most cases.
+    TtRef(&'tt mbe::TokenTree),
+
+    /// This is only used for implicit token trees. The `mbe::TokenTree` *must*
+    /// be `mbe::TokenTree::Token`. No other variants are allowed. We store an
+    /// `mbe::TokenTree` rather than a `Token` so that `get()` can return a
+    /// `&mbe::TokenTree`.
+    Token(mbe::TokenTree),
+}
+
+impl<'tt> TtHandle<'tt> {
+    fn from_token(tok: Token) -> Self {
+        TtHandle::Token(mbe::TokenTree::Token(tok))
+    }
+
+    fn from_token_kind(kind: TokenKind, span: Span) -> Self {
+        TtHandle::from_token(Token::new(kind, span))
+    }
+
+    // Get a reference to a token tree.
+    fn get(&'tt self) -> &'tt mbe::TokenTree {
+        match self {
+            TtHandle::TtRef(tt) => tt,
+            TtHandle::Token(token_tt) => &token_tt,
+        }
+    }
+}
+
+impl<'tt> PartialEq for TtHandle<'tt> {
+    fn eq(&self, other: &TtHandle<'tt>) -> bool {
+        self.get() == other.get()
+    }
+}
+
+impl<'tt> Clone for TtHandle<'tt> {
+    fn clone(&self) -> Self {
+        match self {
+            TtHandle::TtRef(tt) => TtHandle::TtRef(tt),
+
+            // This variant *must* contain a `mbe::TokenTree::Token`, and not
+            // any other variant of `mbe::TokenTree`.
+            TtHandle::Token(mbe::TokenTree::Token(tok)) => {
+                TtHandle::Token(mbe::TokenTree::Token(tok.clone()))
+            }
+
+            _ => unreachable!(),
+        }
+    }
+}
+
+// A set of `mbe::TokenTree`s, which may include `TokenTree::Match`s
+// (for macro-by-example syntactic variables). It also carries the
+// `maybe_empty` flag; that is true if and only if the matcher can
+// match an empty token sequence.
+//
+// The First set is computed on submatchers like `$($a:expr b),* $(c)* d`,
+// which has corresponding FIRST = {$a:expr, c, d}.
+// Likewise, `$($a:expr b),* $(c)+ d` has FIRST = {$a:expr, c}.
+//
+// (Notably, we must allow for *-op to occur zero times.)
+#[derive(Clone, Debug)]
+struct TokenSet<'tt> {
+    tokens: Vec<TtHandle<'tt>>,
+    maybe_empty: bool,
+}
+
+impl<'tt> TokenSet<'tt> {
+    // Returns a set for the empty sequence.
+    fn empty() -> Self {
+        TokenSet { tokens: Vec::new(), maybe_empty: true }
+    }
+
+    // Returns the set `{ tok }` for the single-token (and thus
+    // non-empty) sequence [tok].
+    fn singleton(tt: TtHandle<'tt>) -> Self {
+        TokenSet { tokens: vec![tt], maybe_empty: false }
+    }
+
+    // Changes self to be the set `{ tok }`.
+    // Since `tok` is always present, marks self as non-empty.
+    fn replace_with(&mut self, tt: TtHandle<'tt>) {
+        self.tokens.clear();
+        self.tokens.push(tt);
+        self.maybe_empty = false;
+    }
+
+    // Changes self to be the empty set `{}`; meant for use when
+    // the particular token does not matter, but we want to
+    // record that it occurs.
+    fn replace_with_irrelevant(&mut self) {
+        self.tokens.clear();
+        self.maybe_empty = false;
+    }
+
+    // Adds `tok` to the set for `self`, marking sequence as non-empy.
+    fn add_one(&mut self, tt: TtHandle<'tt>) {
+        if !self.tokens.contains(&tt) {
+            self.tokens.push(tt);
+        }
+        self.maybe_empty = false;
+    }
+
+    // Adds `tok` to the set for `self`. (Leaves `maybe_empty` flag alone.)
+    fn add_one_maybe(&mut self, tt: TtHandle<'tt>) {
+        if !self.tokens.contains(&tt) {
+            self.tokens.push(tt);
+        }
+    }
+
+    // Adds all elements of `other` to this.
+    //
+    // (Since this is a set, we filter out duplicates.)
+    //
+    // If `other` is potentially empty, then preserves the previous
+    // setting of the empty flag of `self`. If `other` is guaranteed
+    // non-empty, then `self` is marked non-empty.
+    fn add_all(&mut self, other: &Self) {
+        for tt in &other.tokens {
+            if !self.tokens.contains(tt) {
+                self.tokens.push(tt.clone());
+            }
+        }
+        if !other.maybe_empty {
+            self.maybe_empty = false;
+        }
+    }
+}
+
+// Checks that `matcher` is internally consistent and that it
+// can legally be followed by a token `N`, for all `N` in `follow`.
+// (If `follow` is empty, then it imposes no constraint on
+// the `matcher`.)
+//
+// Returns the set of NT tokens that could possibly come last in
+// `matcher`. (If `matcher` matches the empty sequence, then
+// `maybe_empty` will be set to true.)
+//
+// Requires that `first_sets` is pre-computed for `matcher`;
+// see `FirstSets::new`.
+fn check_matcher_core<'tt>(
+    sess: &ParseSess,
+    def: &ast::Item,
+    first_sets: &FirstSets<'tt>,
+    matcher: &'tt [mbe::TokenTree],
+    follow: &TokenSet<'tt>,
+) -> TokenSet<'tt> {
+    use mbe::TokenTree;
+
+    let mut last = TokenSet::empty();
+
+    // 2. For each token and suffix  [T, SUFFIX] in M:
+    // ensure that T can be followed by SUFFIX, and if SUFFIX may be empty,
+    // then ensure T can also be followed by any element of FOLLOW.
+    'each_token: for i in 0..matcher.len() {
+        let token = &matcher[i];
+        let suffix = &matcher[i + 1..];
+
+        let build_suffix_first = || {
+            let mut s = first_sets.first(suffix);
+            if s.maybe_empty {
+                s.add_all(follow);
+            }
+            s
+        };
+
+        // (we build `suffix_first` on demand below; you can tell
+        // which cases are supposed to fall through by looking for the
+        // initialization of this variable.)
+        let suffix_first;
+
+        // First, update `last` so that it corresponds to the set
+        // of NT tokens that might end the sequence `... token`.
+        match *token {
+            TokenTree::Token(..)
+            | TokenTree::MetaVar(..)
+            | TokenTree::MetaVarDecl(..)
+            | TokenTree::MetaVarExpr(..) => {
+                if token_can_be_followed_by_any(token) {
+                    // don't need to track tokens that work with any,
+                    last.replace_with_irrelevant();
+                    // ... and don't need to check tokens that can be
+                    // followed by anything against SUFFIX.
+                    continue 'each_token;
+                } else {
+                    last.replace_with(TtHandle::TtRef(token));
+                    suffix_first = build_suffix_first();
+                }
+            }
+            TokenTree::Delimited(span, ref d) => {
+                let my_suffix = TokenSet::singleton(TtHandle::from_token_kind(
+                    token::CloseDelim(d.delim),
+                    span.close,
+                ));
+                check_matcher_core(sess, def, first_sets, &d.tts, &my_suffix);
+                // don't track non NT tokens
+                last.replace_with_irrelevant();
+
+                // also, we don't need to check delimited sequences
+                // against SUFFIX
+                continue 'each_token;
+            }
+            TokenTree::Sequence(_, ref seq_rep) => {
+                suffix_first = build_suffix_first();
+                // The trick here: when we check the interior, we want
+                // to include the separator (if any) as a potential
+                // (but not guaranteed) element of FOLLOW. So in that
+                // case, we make a temp copy of suffix and stuff
+                // delimiter in there.
+                //
+                // FIXME: Should I first scan suffix_first to see if
+                // delimiter is already in it before I go through the
+                // work of cloning it? But then again, this way I may
+                // get a "tighter" span?
+                let mut new;
+                let my_suffix = if let Some(sep) = &seq_rep.separator {
+                    new = suffix_first.clone();
+                    new.add_one_maybe(TtHandle::from_token(sep.clone()));
+                    &new
+                } else {
+                    &suffix_first
+                };
+
+                // At this point, `suffix_first` is built, and
+                // `my_suffix` is some TokenSet that we can use
+                // for checking the interior of `seq_rep`.
+                let next = check_matcher_core(sess, def, first_sets, &seq_rep.tts, my_suffix);
+                if next.maybe_empty {
+                    last.add_all(&next);
+                } else {
+                    last = next;
+                }
+
+                // the recursive call to check_matcher_core already ran the 'each_last
+                // check below, so we can just keep going forward here.
+                continue 'each_token;
+            }
+        }
+
+        // (`suffix_first` guaranteed initialized once reaching here.)
+
+        // Now `last` holds the complete set of NT tokens that could
+        // end the sequence before SUFFIX. Check that every one works with `suffix`.
+        for tt in &last.tokens {
+            if let &TokenTree::MetaVarDecl(span, name, Some(kind)) = tt.get() {
+                for next_token in &suffix_first.tokens {
+                    let next_token = next_token.get();
+
+                    // Check if the old pat is used and the next token is `|`
+                    // to warn about incompatibility with Rust 2021.
+                    // We only emit this lint if we're parsing the original
+                    // definition of this macro_rules, not while (re)parsing
+                    // the macro when compiling another crate that is using the
+                    // macro. (See #86567.)
+                    // Macros defined in the current crate have a real node id,
+                    // whereas macros from an external crate have a dummy id.
+                    if def.id != DUMMY_NODE_ID
+                        && matches!(kind, NonterminalKind::PatParam { inferred: true })
+                        && matches!(next_token, TokenTree::Token(token) if token.kind == BinOp(token::BinOpToken::Or))
+                    {
+                        // It is suggestion to use pat_param, for example: $x:pat -> $x:pat_param.
+                        let suggestion = quoted_tt_to_string(&TokenTree::MetaVarDecl(
+                            span,
+                            name,
+                            Some(NonterminalKind::PatParam { inferred: false }),
+                        ));
+                        sess.buffer_lint_with_diagnostic(
+                            &RUST_2021_INCOMPATIBLE_OR_PATTERNS,
+                            span,
+                            ast::CRATE_NODE_ID,
+                            "the meaning of the `pat` fragment specifier is changing in Rust 2021, which may affect this macro",
+                            BuiltinLintDiagnostics::OrPatternsBackCompat(span, suggestion),
+                        );
+                    }
+                    match is_in_follow(next_token, kind) {
+                        IsInFollow::Yes => {}
+                        IsInFollow::No(possible) => {
+                            let may_be = if last.tokens.len() == 1 && suffix_first.tokens.len() == 1
+                            {
+                                "is"
+                            } else {
+                                "may be"
+                            };
+
+                            let sp = next_token.span();
+                            let mut err = sess.span_diagnostic.struct_span_err(
+                                sp,
+                                &format!(
+                                    "`${name}:{frag}` {may_be} followed by `{next}`, which \
+                                     is not allowed for `{frag}` fragments",
+                                    name = name,
+                                    frag = kind,
+                                    next = quoted_tt_to_string(next_token),
+                                    may_be = may_be
+                                ),
+                            );
+                            err.span_label(sp, format!("not allowed after `{}` fragments", kind));
+
+                            if kind == NonterminalKind::PatWithOr
+                                && sess.edition.rust_2021()
+                                && next_token.is_token(&BinOp(token::BinOpToken::Or))
+                            {
+                                let suggestion = quoted_tt_to_string(&TokenTree::MetaVarDecl(
+                                    span,
+                                    name,
+                                    Some(NonterminalKind::PatParam { inferred: false }),
+                                ));
+                                err.span_suggestion(
+                                    span,
+                                    "try a `pat_param` fragment specifier instead",
+                                    suggestion,
+                                    Applicability::MaybeIncorrect,
+                                );
+                            }
+
+                            let msg = "allowed there are: ";
+                            match possible {
+                                &[] => {}
+                                &[t] => {
+                                    err.note(&format!(
+                                        "only {} is allowed after `{}` fragments",
+                                        t, kind,
+                                    ));
+                                }
+                                ts => {
+                                    err.note(&format!(
+                                        "{}{} or {}",
+                                        msg,
+                                        ts[..ts.len() - 1]
+                                            .iter()
+                                            .copied()
+                                            .collect::<Vec<_>>()
+                                            .join(", "),
+                                        ts[ts.len() - 1],
+                                    ));
+                                }
+                            }
+                            err.emit();
+                        }
+                    }
+                }
+            }
+        }
+    }
+    last
+}
+
+fn token_can_be_followed_by_any(tok: &mbe::TokenTree) -> bool {
+    if let mbe::TokenTree::MetaVarDecl(_, _, Some(kind)) = *tok {
+        frag_can_be_followed_by_any(kind)
+    } else {
+        // (Non NT's can always be followed by anything in matchers.)
+        true
+    }
+}
+
+/// Returns `true` if a fragment of type `frag` can be followed by any sort of
+/// token. We use this (among other things) as a useful approximation
+/// for when `frag` can be followed by a repetition like `$(...)*` or
+/// `$(...)+`. In general, these can be a bit tricky to reason about,
+/// so we adopt a conservative position that says that any fragment
+/// specifier which consumes at most one token tree can be followed by
+/// a fragment specifier (indeed, these fragments can be followed by
+/// ANYTHING without fear of future compatibility hazards).
+fn frag_can_be_followed_by_any(kind: NonterminalKind) -> bool {
+    matches!(
+        kind,
+        NonterminalKind::Item           // always terminated by `}` or `;`
+        | NonterminalKind::Block        // exactly one token tree
+        | NonterminalKind::Ident        // exactly one token tree
+        | NonterminalKind::Literal      // exactly one token tree
+        | NonterminalKind::Meta         // exactly one token tree
+        | NonterminalKind::Lifetime     // exactly one token tree
+        | NonterminalKind::TT // exactly one token tree
+    )
+}
+
+enum IsInFollow {
+    Yes,
+    No(&'static [&'static str]),
+}
+
+/// Returns `true` if `frag` can legally be followed by the token `tok`. For
+/// fragments that can consume an unbounded number of tokens, `tok`
+/// must be within a well-defined follow set. This is intended to
+/// guarantee future compatibility: for example, without this rule, if
+/// we expanded `expr` to include a new binary operator, we might
+/// break macros that were relying on that binary operator as a
+/// separator.
+// when changing this do not forget to update doc/book/macros.md!
+fn is_in_follow(tok: &mbe::TokenTree, kind: NonterminalKind) -> IsInFollow {
+    use mbe::TokenTree;
+
+    if let TokenTree::Token(Token { kind: token::CloseDelim(_), .. }) = *tok {
+        // closing a token tree can never be matched by any fragment;
+        // iow, we always require that `(` and `)` match, etc.
+        IsInFollow::Yes
+    } else {
+        match kind {
+            NonterminalKind::Item => {
+                // since items *must* be followed by either a `;` or a `}`, we can
+                // accept anything after them
+                IsInFollow::Yes
+            }
+            NonterminalKind::Block => {
+                // anything can follow block, the braces provide an easy boundary to
+                // maintain
+                IsInFollow::Yes
+            }
+            NonterminalKind::Stmt | NonterminalKind::Expr => {
+                const TOKENS: &[&str] = &["`=>`", "`,`", "`;`"];
+                match tok {
+                    TokenTree::Token(token) => match token.kind {
+                        FatArrow | Comma | Semi => IsInFollow::Yes,
+                        _ => IsInFollow::No(TOKENS),
+                    },
+                    _ => IsInFollow::No(TOKENS),
+                }
+            }
+            NonterminalKind::PatParam { .. } => {
+                const TOKENS: &[&str] = &["`=>`", "`,`", "`=`", "`|`", "`if`", "`in`"];
+                match tok {
+                    TokenTree::Token(token) => match token.kind {
+                        FatArrow | Comma | Eq | BinOp(token::Or) => IsInFollow::Yes,
+                        Ident(name, false) if name == kw::If || name == kw::In => IsInFollow::Yes,
+                        _ => IsInFollow::No(TOKENS),
+                    },
+                    _ => IsInFollow::No(TOKENS),
+                }
+            }
+            NonterminalKind::PatWithOr { .. } => {
+                const TOKENS: &[&str] = &["`=>`", "`,`", "`=`", "`if`", "`in`"];
+                match tok {
+                    TokenTree::Token(token) => match token.kind {
+                        FatArrow | Comma | Eq => IsInFollow::Yes,
+                        Ident(name, false) if name == kw::If || name == kw::In => IsInFollow::Yes,
+                        _ => IsInFollow::No(TOKENS),
+                    },
+                    _ => IsInFollow::No(TOKENS),
+                }
+            }
+            NonterminalKind::Path | NonterminalKind::Ty => {
+                const TOKENS: &[&str] = &[
+                    "`{`", "`[`", "`=>`", "`,`", "`>`", "`=`", "`:`", "`;`", "`|`", "`as`",
+                    "`where`",
+                ];
+                match tok {
+                    TokenTree::Token(token) => match token.kind {
+                        OpenDelim(Delimiter::Brace)
+                        | OpenDelim(Delimiter::Bracket)
+                        | Comma
+                        | FatArrow
+                        | Colon
+                        | Eq
+                        | Gt
+                        | BinOp(token::Shr)
+                        | Semi
+                        | BinOp(token::Or) => IsInFollow::Yes,
+                        Ident(name, false) if name == kw::As || name == kw::Where => {
+                            IsInFollow::Yes
+                        }
+                        _ => IsInFollow::No(TOKENS),
+                    },
+                    TokenTree::MetaVarDecl(_, _, Some(NonterminalKind::Block)) => IsInFollow::Yes,
+                    _ => IsInFollow::No(TOKENS),
+                }
+            }
+            NonterminalKind::Ident | NonterminalKind::Lifetime => {
+                // being a single token, idents and lifetimes are harmless
+                IsInFollow::Yes
+            }
+            NonterminalKind::Literal => {
+                // literals may be of a single token, or two tokens (negative numbers)
+                IsInFollow::Yes
+            }
+            NonterminalKind::Meta | NonterminalKind::TT => {
+                // being either a single token or a delimited sequence, tt is
+                // harmless
+                IsInFollow::Yes
+            }
+            NonterminalKind::Vis => {
+                // Explicitly disallow `priv`, on the off chance it comes back.
+                const TOKENS: &[&str] = &["`,`", "an ident", "a type"];
+                match tok {
+                    TokenTree::Token(token) => match token.kind {
+                        Comma => IsInFollow::Yes,
+                        Ident(name, is_raw) if is_raw || name != kw::Priv => IsInFollow::Yes,
+                        _ => {
+                            if token.can_begin_type() {
+                                IsInFollow::Yes
+                            } else {
+                                IsInFollow::No(TOKENS)
+                            }
+                        }
+                    },
+                    TokenTree::MetaVarDecl(
+                        _,
+                        _,
+                        Some(NonterminalKind::Ident | NonterminalKind::Ty | NonterminalKind::Path),
+                    ) => IsInFollow::Yes,
+                    _ => IsInFollow::No(TOKENS),
+                }
+            }
+        }
+    }
+}
+
+fn quoted_tt_to_string(tt: &mbe::TokenTree) -> String {
+    match *tt {
+        mbe::TokenTree::Token(ref token) => pprust::token_to_string(&token).into(),
+        mbe::TokenTree::MetaVar(_, name) => format!("${}", name),
+        mbe::TokenTree::MetaVarDecl(_, name, Some(kind)) => format!("${}:{}", name, kind),
+        mbe::TokenTree::MetaVarDecl(_, name, None) => format!("${}:", name),
+        _ => panic!(
+            "{}",
+            "unexpected mbe::TokenTree::{Sequence or Delimited} \
+             in follow set checker"
+        ),
+    }
+}
+
+fn parser_from_cx(sess: &ParseSess, tts: TokenStream) -> Parser<'_> {
+    Parser::new(sess, tts, true, rustc_parse::MACRO_ARGUMENTS)
+}
+
+/// Generates an appropriate parsing failure message. For EOF, this is "unexpected end...". For
+/// other tokens, this is "unexpected token...".
+fn parse_failure_msg(tok: &Token) -> String {
+    match tok.kind {
+        token::Eof => "unexpected end of macro invocation".to_string(),
+        _ => format!("no rules expected the token `{}`", pprust::token_to_string(tok),),
+    }
+}
diff --git a/compiler/rustc_expand/src/mbe/metavar_expr.rs b/compiler/rustc_expand/src/mbe/metavar_expr.rs
new file mode 100644
index 000000000..fc808401a
--- /dev/null
+++ b/compiler/rustc_expand/src/mbe/metavar_expr.rs
@@ -0,0 +1,161 @@
+use rustc_ast::token::{self, Delimiter};
+use rustc_ast::tokenstream::{CursorRef, TokenStream, TokenTree};
+use rustc_ast::{LitIntType, LitKind};
+use rustc_ast_pretty::pprust;
+use rustc_errors::{Applicability, PResult};
+use rustc_session::parse::ParseSess;
+use rustc_span::symbol::Ident;
+use rustc_span::Span;
+
+/// A meta-variable expression, for expansions based on properties of meta-variables.
+#[derive(Debug, Clone, PartialEq, Encodable, Decodable)]
+pub(crate) enum MetaVarExpr {
+    /// The number of repetitions of an identifier, optionally limited to a number
+    /// of outer-most repetition depths. If the depth limit is `None` then the depth is unlimited.
+    Count(Ident, Option<usize>),
+
+    /// Ignore a meta-variable for repetition without expansion.
+    Ignore(Ident),
+
+    /// The index of the repetition at a particular depth, where 0 is the inner-most
+    /// repetition. The `usize` is the depth.
+    Index(usize),
+
+    /// The length of the repetition at a particular depth, where 0 is the inner-most
+    /// repetition. The `usize` is the depth.
+    Length(usize),
+}
+
+impl MetaVarExpr {
+    /// Attempt to parse a meta-variable expression from a token stream.
+    pub(crate) fn parse<'sess>(
+        input: &TokenStream,
+        outer_span: Span,
+        sess: &'sess ParseSess,
+    ) -> PResult<'sess, MetaVarExpr> {
+        let mut tts = input.trees();
+        let ident = parse_ident(&mut tts, sess, outer_span)?;
+        let Some(TokenTree::Delimited(_, Delimiter::Parenthesis, args)) = tts.next() else {
+            let msg = "meta-variable expression parameter must be wrapped in parentheses";
+            return Err(sess.span_diagnostic.struct_span_err(ident.span, msg));
+        };
+        check_trailing_token(&mut tts, sess)?;
+        let mut iter = args.trees();
+        let rslt = match &*ident.as_str() {
+            "count" => parse_count(&mut iter, sess, ident.span)?,
+            "ignore" => MetaVarExpr::Ignore(parse_ident(&mut iter, sess, ident.span)?),
+            "index" => MetaVarExpr::Index(parse_depth(&mut iter, sess, ident.span)?),
+            "length" => MetaVarExpr::Length(parse_depth(&mut iter, sess, ident.span)?),
+            _ => {
+                let err_msg = "unrecognized meta-variable expression";
+                let mut err = sess.span_diagnostic.struct_span_err(ident.span, err_msg);
+                err.span_suggestion(
+                    ident.span,
+                    "supported expressions are count, ignore, index and length",
+                    "",
+                    Applicability::MachineApplicable,
+                );
+                return Err(err);
+            }
+        };
+        check_trailing_token(&mut iter, sess)?;
+        Ok(rslt)
+    }
+
+    pub(crate) fn ident(&self) -> Option<Ident> {
+        match *self {
+            MetaVarExpr::Count(ident, _) | MetaVarExpr::Ignore(ident) => Some(ident),
+            MetaVarExpr::Index(..) | MetaVarExpr::Length(..) => None,
+        }
+    }
+}
+
+// Checks if there are any remaining tokens. For example, `${ignore(ident ... a b c ...)}`
+fn check_trailing_token<'sess>(
+    iter: &mut CursorRef<'_>,
+    sess: &'sess ParseSess,
+) -> PResult<'sess, ()> {
+    if let Some(tt) = iter.next() {
+        let mut diag = sess
+            .span_diagnostic
+            .struct_span_err(tt.span(), &format!("unexpected token: {}", pprust::tt_to_string(tt)));
+        diag.span_note(tt.span(), "meta-variable expression must not have trailing tokens");
+        Err(diag)
+    } else {
+        Ok(())
+    }
+}
+
+/// Parse a meta-variable `count` expression: `count(ident[, depth])`
+fn parse_count<'sess>(
+    iter: &mut CursorRef<'_>,
+    sess: &'sess ParseSess,
+    span: Span,
+) -> PResult<'sess, MetaVarExpr> {
+    let ident = parse_ident(iter, sess, span)?;
+    let depth = if try_eat_comma(iter) { Some(parse_depth(iter, sess, span)?) } else { None };
+    Ok(MetaVarExpr::Count(ident, depth))
+}
+
+/// Parses the depth used by index(depth) and length(depth).
+fn parse_depth<'sess>(
+    iter: &mut CursorRef<'_>,
+    sess: &'sess ParseSess,
+    span: Span,
+) -> PResult<'sess, usize> {
+    let Some(tt) = iter.next() else { return Ok(0) };
+    let TokenTree::Token(token::Token {
+        kind: token::TokenKind::Literal(lit), ..
+    }, _) = tt else {
+        return Err(sess.span_diagnostic.struct_span_err(
+            span,
+            "meta-variable expression depth must be a literal"
+        ));
+    };
+    if let Ok(lit_kind) = LitKind::from_lit_token(*lit)
+        && let LitKind::Int(n_u128, LitIntType::Unsuffixed) = lit_kind
+        && let Ok(n_usize) = usize::try_from(n_u128)
+    {
+        Ok(n_usize)
+    }
+    else {
+        let msg = "only unsuffixes integer literals are supported in meta-variable expressions";
+        Err(sess.span_diagnostic.struct_span_err(span, msg))
+    }
+}
+
+/// Parses an generic ident
+fn parse_ident<'sess>(
+    iter: &mut CursorRef<'_>,
+    sess: &'sess ParseSess,
+    span: Span,
+) -> PResult<'sess, Ident> {
+    if let Some(tt) = iter.next() && let TokenTree::Token(token, _) = tt {
+        if let Some((elem, false)) = token.ident() {
+            return Ok(elem);
+        }
+        let token_str = pprust::token_to_string(token);
+        let mut err = sess.span_diagnostic.struct_span_err(
+            span,
+            &format!("expected identifier, found `{}`", &token_str)
+        );
+        err.span_suggestion(
+            token.span,
+            &format!("try removing `{}`", &token_str),
+            "",
+            Applicability::MaybeIncorrect,
+        );
+        return Err(err);
+    }
+    Err(sess.span_diagnostic.struct_span_err(span, "expected identifier"))
+}
+
+/// Tries to move the iterator forward returning `true` if there is a comma. If not, then the
+/// iterator is not modified and the result is `false`.
+fn try_eat_comma(iter: &mut CursorRef<'_>) -> bool {
+    if let Some(TokenTree::Token(token::Token { kind: token::Comma, .. }, _)) = iter.look_ahead(0) {
+        let _ = iter.next();
+        return true;
+    }
+    false
+}
diff --git a/compiler/rustc_expand/src/mbe/quoted.rs b/compiler/rustc_expand/src/mbe/quoted.rs
new file mode 100644
index 000000000..ee17d54f6
--- /dev/null
+++ b/compiler/rustc_expand/src/mbe/quoted.rs
@@ -0,0 +1,366 @@
+use crate::mbe::macro_parser::count_metavar_decls;
+use crate::mbe::{Delimited, KleeneOp, KleeneToken, MetaVarExpr, SequenceRepetition, TokenTree};
+
+use rustc_ast::token::{self, Delimiter, Token};
+use rustc_ast::{tokenstream, NodeId};
+use rustc_ast_pretty::pprust;
+use rustc_feature::Features;
+use rustc_session::parse::{feature_err, ParseSess};
+use rustc_span::symbol::{kw, sym, Ident};
+
+use rustc_span::edition::Edition;
+use rustc_span::{Span, SyntaxContext};
+
+const VALID_FRAGMENT_NAMES_MSG: &str = "valid fragment specifiers are \
+                                        `ident`, `block`, `stmt`, `expr`, `pat`, `ty`, `lifetime`, \
+                                        `literal`, `path`, `meta`, `tt`, `item` and `vis`";
+
+/// Takes a `tokenstream::TokenStream` and returns a `Vec<self::TokenTree>`. Specifically, this
+/// takes a generic `TokenStream`, such as is used in the rest of the compiler, and returns a
+/// collection of `TokenTree` for use in parsing a macro.
+///
+/// # Parameters
+///
+/// - `input`: a token stream to read from, the contents of which we are parsing.
+/// - `parsing_patterns`: `parse` can be used to parse either the "patterns" or the "body" of a
+///   macro. Both take roughly the same form _except_ that:
+///   - In a pattern, metavars are declared with their "matcher" type. For example `$var:expr` or
+///     `$id:ident`. In this example, `expr` and `ident` are "matchers". They are not present in the
+///     body of a macro rule -- just in the pattern.
+///   - Metavariable expressions are only valid in the "body", not the "pattern".
+/// - `sess`: the parsing session. Any errors will be emitted to this session.
+/// - `node_id`: the NodeId of the macro we are parsing.
+/// - `features`: language features so we can do feature gating.
+///
+/// # Returns
+///
+/// A collection of `self::TokenTree`. There may also be some errors emitted to `sess`.
+pub(super) fn parse(
+    input: tokenstream::TokenStream,
+    parsing_patterns: bool,
+    sess: &ParseSess,
+    node_id: NodeId,
+    features: &Features,
+    edition: Edition,
+) -> Vec<TokenTree> {
+    // Will contain the final collection of `self::TokenTree`
+    let mut result = Vec::new();
+
+    // For each token tree in `input`, parse the token into a `self::TokenTree`, consuming
+    // additional trees if need be.
+    let mut trees = input.into_trees();
+    while let Some(tree) = trees.next() {
+        // Given the parsed tree, if there is a metavar and we are expecting matchers, actually
+        // parse out the matcher (i.e., in `$id:ident` this would parse the `:` and `ident`).
+        let tree = parse_tree(tree, &mut trees, parsing_patterns, sess, node_id, features, edition);
+        match tree {
+            TokenTree::MetaVar(start_sp, ident) if parsing_patterns => {
+                let span = match trees.next() {
+                    Some(tokenstream::TokenTree::Token(Token { kind: token::Colon, span }, _)) => {
+                        match trees.next() {
+                            Some(tokenstream::TokenTree::Token(token, _)) => match token.ident() {
+                                Some((frag, _)) => {
+                                    let span = token.span.with_lo(start_sp.lo());
+
+                                    let kind =
+                                        token::NonterminalKind::from_symbol(frag.name, || {
+                                            // FIXME(#85708) - once we properly decode a foreign
+                                            // crate's `SyntaxContext::root`, then we can replace
+                                            // this with just `span.edition()`. A
+                                            // `SyntaxContext::root()` from the current crate will
+                                            // have the edition of the current crate, and a
+                                            // `SyntaxContext::root()` from a foreign crate will
+                                            // have the edition of that crate (which we manually
+                                            // retrieve via the `edition` parameter).
+                                            if span.ctxt() == SyntaxContext::root() {
+                                                edition
+                                            } else {
+                                                span.edition()
+                                            }
+                                        })
+                                        .unwrap_or_else(
+                                            || {
+                                                let msg = format!(
+                                                    "invalid fragment specifier `{}`",
+                                                    frag.name
+                                                );
+                                                sess.span_diagnostic
+                                                    .struct_span_err(span, &msg)
+                                                    .help(VALID_FRAGMENT_NAMES_MSG)
+                                                    .emit();
+                                                token::NonterminalKind::Ident
+                                            },
+                                        );
+                                    result.push(TokenTree::MetaVarDecl(span, ident, Some(kind)));
+                                    continue;
+                                }
+                                _ => token.span,
+                            },
+                            tree => tree.as_ref().map_or(span, tokenstream::TokenTree::span),
+                        }
+                    }
+                    tree => tree.as_ref().map_or(start_sp, tokenstream::TokenTree::span),
+                };
+
+                result.push(TokenTree::MetaVarDecl(span, ident, None));
+            }
+
+            // Not a metavar or no matchers allowed, so just return the tree
+            _ => result.push(tree),
+        }
+    }
+    result
+}
+
+/// Asks for the `macro_metavar_expr` feature if it is not already declared
+fn maybe_emit_macro_metavar_expr_feature(features: &Features, sess: &ParseSess, span: Span) {
+    if !features.macro_metavar_expr {
+        let msg = "meta-variable expressions are unstable";
+        feature_err(&sess, sym::macro_metavar_expr, span, msg).emit();
+    }
+}
+
+/// Takes a `tokenstream::TokenTree` and returns a `self::TokenTree`. Specifically, this takes a
+/// generic `TokenTree`, such as is used in the rest of the compiler, and returns a `TokenTree`
+/// for use in parsing a macro.
+///
+/// Converting the given tree may involve reading more tokens.
+///
+/// # Parameters
+///
+/// - `tree`: the tree we wish to convert.
+/// - `outer_trees`: an iterator over trees. We may need to read more tokens from it in order to finish
+///   converting `tree`
+/// - `parsing_patterns`: same as [parse].
+/// - `sess`: the parsing session. Any errors will be emitted to this session.
+/// - `features`: language features so we can do feature gating.
+fn parse_tree(
+    tree: tokenstream::TokenTree,
+    outer_trees: &mut impl Iterator<Item = tokenstream::TokenTree>,
+    parsing_patterns: bool,
+    sess: &ParseSess,
+    node_id: NodeId,
+    features: &Features,
+    edition: Edition,
+) -> TokenTree {
+    // Depending on what `tree` is, we could be parsing different parts of a macro
+    match tree {
+        // `tree` is a `$` token. Look at the next token in `trees`
+        tokenstream::TokenTree::Token(Token { kind: token::Dollar, span }, _) => {
+            // FIXME: Handle `Invisible`-delimited groups in a more systematic way
+            // during parsing.
+            let mut next = outer_trees.next();
+            let mut trees: Box<dyn Iterator<Item = tokenstream::TokenTree>>;
+            if let Some(tokenstream::TokenTree::Delimited(_, Delimiter::Invisible, tts)) = next {
+                trees = Box::new(tts.into_trees());
+                next = trees.next();
+            } else {
+                trees = Box::new(outer_trees);
+            }
+
+            match next {
+                // `tree` is followed by a delimited set of token trees.
+                Some(tokenstream::TokenTree::Delimited(delim_span, delim, tts)) => {
+                    if parsing_patterns {
+                        if delim != Delimiter::Parenthesis {
+                            span_dollar_dollar_or_metavar_in_the_lhs_err(
+                                sess,
+                                &Token { kind: token::OpenDelim(delim), span: delim_span.entire() },
+                            );
+                        }
+                    } else {
+                        match delim {
+                            Delimiter::Brace => {
+                                // The delimiter is `{`.  This indicates the beginning
+                                // of a meta-variable expression (e.g. `${count(ident)}`).
+                                // Try to parse the meta-variable expression.
+                                match MetaVarExpr::parse(&tts, delim_span.entire(), sess) {
+                                    Err(mut err) => {
+                                        err.emit();
+                                        // Returns early the same read `$` to avoid spanning
+                                        // unrelated diagnostics that could be performed afterwards
+                                        return TokenTree::token(token::Dollar, span);
+                                    }
+                                    Ok(elem) => {
+                                        maybe_emit_macro_metavar_expr_feature(
+                                            features,
+                                            sess,
+                                            delim_span.entire(),
+                                        );
+                                        return TokenTree::MetaVarExpr(delim_span, elem);
+                                    }
+                                }
+                            }
+                            Delimiter::Parenthesis => {}
+                            _ => {
+                                let tok = pprust::token_kind_to_string(&token::OpenDelim(delim));
+                                let msg = format!("expected `(` or `{{`, found `{}`", tok);
+                                sess.span_diagnostic.span_err(delim_span.entire(), &msg);
+                            }
+                        }
+                    }
+                    // If we didn't find a metavar expression above, then we must have a
+                    // repetition sequence in the macro (e.g. `$(pat)*`).  Parse the
+                    // contents of the sequence itself
+                    let sequence = parse(tts, parsing_patterns, sess, node_id, features, edition);
+                    // Get the Kleene operator and optional separator
+                    let (separator, kleene) =
+                        parse_sep_and_kleene_op(&mut trees, delim_span.entire(), sess);
+                    // Count the number of captured "names" (i.e., named metavars)
+                    let num_captures =
+                        if parsing_patterns { count_metavar_decls(&sequence) } else { 0 };
+                    TokenTree::Sequence(
+                        delim_span,
+                        SequenceRepetition { tts: sequence, separator, kleene, num_captures },
+                    )
+                }
+
+                // `tree` is followed by an `ident`. This could be `$meta_var` or the `$crate`
+                // special metavariable that names the crate of the invocation.
+                Some(tokenstream::TokenTree::Token(token, _)) if token.is_ident() => {
+                    let (ident, is_raw) = token.ident().unwrap();
+                    let span = ident.span.with_lo(span.lo());
+                    if ident.name == kw::Crate && !is_raw {
+                        TokenTree::token(token::Ident(kw::DollarCrate, is_raw), span)
+                    } else {
+                        TokenTree::MetaVar(span, ident)
+                    }
+                }
+
+                // `tree` is followed by another `$`. This is an escaped `$`.
+                Some(tokenstream::TokenTree::Token(Token { kind: token::Dollar, span }, _)) => {
+                    if parsing_patterns {
+                        span_dollar_dollar_or_metavar_in_the_lhs_err(
+                            sess,
+                            &Token { kind: token::Dollar, span },
+                        );
+                    } else {
+                        maybe_emit_macro_metavar_expr_feature(features, sess, span);
+                    }
+                    TokenTree::token(token::Dollar, span)
+                }
+
+                // `tree` is followed by some other token. This is an error.
+                Some(tokenstream::TokenTree::Token(token, _)) => {
+                    let msg = format!(
+                        "expected identifier, found `{}`",
+                        pprust::token_to_string(&token),
+                    );
+                    sess.span_diagnostic.span_err(token.span, &msg);
+                    TokenTree::MetaVar(token.span, Ident::empty())
+                }
+
+                // There are no more tokens. Just return the `$` we already have.
+                None => TokenTree::token(token::Dollar, span),
+            }
+        }
+
+        // `tree` is an arbitrary token. Keep it.
+        tokenstream::TokenTree::Token(token, _) => TokenTree::Token(token),
+
+        // `tree` is the beginning of a delimited set of tokens (e.g., `(` or `{`). We need to
+        // descend into the delimited set and further parse it.
+        tokenstream::TokenTree::Delimited(span, delim, tts) => TokenTree::Delimited(
+            span,
+            Delimited {
+                delim,
+                tts: parse(tts, parsing_patterns, sess, node_id, features, edition),
+            },
+        ),
+    }
+}
+
+/// Takes a token and returns `Some(KleeneOp)` if the token is `+` `*` or `?`. Otherwise, return
+/// `None`.
+fn kleene_op(token: &Token) -> Option<KleeneOp> {
+    match token.kind {
+        token::BinOp(token::Star) => Some(KleeneOp::ZeroOrMore),
+        token::BinOp(token::Plus) => Some(KleeneOp::OneOrMore),
+        token::Question => Some(KleeneOp::ZeroOrOne),
+        _ => None,
+    }
+}
+
+/// Parse the next token tree of the input looking for a KleeneOp. Returns
+///
+/// - Ok(Ok((op, span))) if the next token tree is a KleeneOp
+/// - Ok(Err(tok, span)) if the next token tree is a token but not a KleeneOp
+/// - Err(span) if the next token tree is not a token
+fn parse_kleene_op(
+    input: &mut impl Iterator<Item = tokenstream::TokenTree>,
+    span: Span,
+) -> Result<Result<(KleeneOp, Span), Token>, Span> {
+    match input.next() {
+        Some(tokenstream::TokenTree::Token(token, _)) => match kleene_op(&token) {
+            Some(op) => Ok(Ok((op, token.span))),
+            None => Ok(Err(token)),
+        },
+        tree => Err(tree.as_ref().map_or(span, tokenstream::TokenTree::span)),
+    }
+}
+
+/// Attempt to parse a single Kleene star, possibly with a separator.
+///
+/// For example, in a pattern such as `$(a),*`, `a` is the pattern to be repeated, `,` is the
+/// separator, and `*` is the Kleene operator. This function is specifically concerned with parsing
+/// the last two tokens of such a pattern: namely, the optional separator and the Kleene operator
+/// itself. Note that here we are parsing the _macro_ itself, rather than trying to match some
+/// stream of tokens in an invocation of a macro.
+///
+/// This function will take some input iterator `input` corresponding to `span` and a parsing
+/// session `sess`. If the next one (or possibly two) tokens in `input` correspond to a Kleene
+/// operator and separator, then a tuple with `(separator, KleeneOp)` is returned. Otherwise, an
+/// error with the appropriate span is emitted to `sess` and a dummy value is returned.
+fn parse_sep_and_kleene_op(
+    input: &mut impl Iterator<Item = tokenstream::TokenTree>,
+    span: Span,
+    sess: &ParseSess,
+) -> (Option<Token>, KleeneToken) {
+    // We basically look at two token trees here, denoted as #1 and #2 below
+    let span = match parse_kleene_op(input, span) {
+        // #1 is a `?`, `+`, or `*` KleeneOp
+        Ok(Ok((op, span))) => return (None, KleeneToken::new(op, span)),
+
+        // #1 is a separator followed by #2, a KleeneOp
+        Ok(Err(token)) => match parse_kleene_op(input, token.span) {
+            // #2 is the `?` Kleene op, which does not take a separator (error)
+            Ok(Ok((KleeneOp::ZeroOrOne, span))) => {
+                // Error!
+                sess.span_diagnostic.span_err(
+                    token.span,
+                    "the `?` macro repetition operator does not take a separator",
+                );
+
+                // Return a dummy
+                return (None, KleeneToken::new(KleeneOp::ZeroOrMore, span));
+            }
+
+            // #2 is a KleeneOp :D
+            Ok(Ok((op, span))) => return (Some(token), KleeneToken::new(op, span)),
+
+            // #2 is a random token or not a token at all :(
+            Ok(Err(Token { span, .. })) | Err(span) => span,
+        },
+
+        // #1 is not a token
+        Err(span) => span,
+    };
+
+    // If we ever get to this point, we have experienced an "unexpected token" error
+    sess.span_diagnostic.span_err(span, "expected one of: `*`, `+`, or `?`");
+
+    // Return a dummy
+    (None, KleeneToken::new(KleeneOp::ZeroOrMore, span))
+}
+
+// `$$` or a meta-variable is the lhs of a macro but shouldn't.
+//
+// For example, `macro_rules! foo { ( ${length()} ) => {} }`
+fn span_dollar_dollar_or_metavar_in_the_lhs_err<'sess>(sess: &'sess ParseSess, token: &Token) {
+    sess.span_diagnostic
+        .span_err(token.span, &format!("unexpected token: {}", pprust::token_to_string(token)));
+    sess.span_diagnostic.span_note_without_error(
+        token.span,
+        "`$$` and meta-variable expressions are not allowed inside macro parameter definitions",
+    );
+}
diff --git a/compiler/rustc_expand/src/mbe/transcribe.rs b/compiler/rustc_expand/src/mbe/transcribe.rs
new file mode 100644
index 000000000..e47ea83ac
--- /dev/null
+++ b/compiler/rustc_expand/src/mbe/transcribe.rs
@@ -0,0 +1,580 @@
+use crate::base::ExtCtxt;
+use crate::mbe::macro_parser::{MatchedNonterminal, MatchedSeq, MatchedTokenTree, NamedMatch};
+use crate::mbe::{self, MetaVarExpr};
+use rustc_ast::mut_visit::{self, MutVisitor};
+use rustc_ast::token::{self, Delimiter, Token, TokenKind};
+use rustc_ast::tokenstream::{DelimSpan, Spacing, TokenStream, TokenTree};
+use rustc_data_structures::fx::FxHashMap;
+use rustc_errors::{pluralize, PResult};
+use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed};
+use rustc_span::hygiene::{LocalExpnId, Transparency};
+use rustc_span::symbol::{sym, Ident, MacroRulesNormalizedIdent};
+use rustc_span::Span;
+
+use smallvec::{smallvec, SmallVec};
+use std::mem;
+
+// A Marker adds the given mark to the syntax context.
+struct Marker(LocalExpnId, Transparency);
+
+impl MutVisitor for Marker {
+    const VISIT_TOKENS: bool = true;
+
+    fn visit_span(&mut self, span: &mut Span) {
+        *span = span.apply_mark(self.0.to_expn_id(), self.1)
+    }
+}
+
+/// An iterator over the token trees in a delimited token tree (`{ ... }`) or a sequence (`$(...)`).
+enum Frame<'a> {
+    Delimited { tts: &'a [mbe::TokenTree], idx: usize, delim: Delimiter, span: DelimSpan },
+    Sequence { tts: &'a [mbe::TokenTree], idx: usize, sep: Option<Token> },
+}
+
+impl<'a> Frame<'a> {
+    /// Construct a new frame around the delimited set of tokens.
+    fn new(src: &'a mbe::Delimited, span: DelimSpan) -> Frame<'a> {
+        Frame::Delimited { tts: &src.tts, idx: 0, delim: src.delim, span }
+    }
+}
+
+impl<'a> Iterator for Frame<'a> {
+    type Item = &'a mbe::TokenTree;
+
+    fn next(&mut self) -> Option<&'a mbe::TokenTree> {
+        match self {
+            Frame::Delimited { tts, ref mut idx, .. }
+            | Frame::Sequence { tts, ref mut idx, .. } => {
+                let res = tts.get(*idx);
+                *idx += 1;
+                res
+            }
+        }
+    }
+}
+
+/// This can do Macro-By-Example transcription.
+/// - `interp` is a map of meta-variables to the tokens (non-terminals) they matched in the
+///   invocation. We are assuming we already know there is a match.
+/// - `src` is the RHS of the MBE, that is, the "example" we are filling in.
+///
+/// For example,
+///
+/// ```rust
+/// macro_rules! foo {
+///     ($id:ident) => { println!("{}", stringify!($id)); }
+/// }
+///
+/// foo!(bar);
+/// ```
+///
+/// `interp` would contain `$id => bar` and `src` would contain `println!("{}", stringify!($id));`.
+///
+/// `transcribe` would return a `TokenStream` containing `println!("{}", stringify!(bar));`.
+///
+/// Along the way, we do some additional error checking.
+pub(super) fn transcribe<'a>(
+    cx: &ExtCtxt<'a>,
+    interp: &FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
+    src: &mbe::Delimited,
+    src_span: DelimSpan,
+    transparency: Transparency,
+) -> PResult<'a, TokenStream> {
+    // Nothing for us to transcribe...
+    if src.tts.is_empty() {
+        return Ok(TokenStream::default());
+    }
+
+    // We descend into the RHS (`src`), expanding things as we go. This stack contains the things
+    // we have yet to expand/are still expanding. We start the stack off with the whole RHS.
+    let mut stack: SmallVec<[Frame<'_>; 1]> = smallvec![Frame::new(&src, src_span)];
+
+    // As we descend in the RHS, we will need to be able to match nested sequences of matchers.
+    // `repeats` keeps track of where we are in matching at each level, with the last element being
+    // the most deeply nested sequence. This is used as a stack.
+    let mut repeats = Vec::new();
+
+    // `result` contains resulting token stream from the TokenTree we just finished processing. At
+    // the end, this will contain the full result of transcription, but at arbitrary points during
+    // `transcribe`, `result` will contain subsets of the final result.
+    //
+    // Specifically, as we descend into each TokenTree, we will push the existing results onto the
+    // `result_stack` and clear `results`. We will then produce the results of transcribing the
+    // TokenTree into `results`. Then, as we unwind back out of the `TokenTree`, we will pop the
+    // `result_stack` and append `results` too it to produce the new `results` up to that point.
+    //
+    // Thus, if we try to pop the `result_stack` and it is empty, we have reached the top-level
+    // again, and we are done transcribing.
+    let mut result: Vec<TokenTree> = Vec::new();
+    let mut result_stack = Vec::new();
+    let mut marker = Marker(cx.current_expansion.id, transparency);
+
+    loop {
+        // Look at the last frame on the stack.
+        // If it still has a TokenTree we have not looked at yet, use that tree.
+        let Some(tree) = stack.last_mut().unwrap().next() else {
+            // This else-case never produces a value for `tree` (it `continue`s or `return`s).
+
+            // Otherwise, if we have just reached the end of a sequence and we can keep repeating,
+            // go back to the beginning of the sequence.
+            if let Frame::Sequence { idx, sep, .. } = stack.last_mut().unwrap() {
+                let (repeat_idx, repeat_len) = repeats.last_mut().unwrap();
+                *repeat_idx += 1;
+                if repeat_idx < repeat_len {
+                    *idx = 0;
+                    if let Some(sep) = sep {
+                        result.push(TokenTree::Token(sep.clone(), Spacing::Alone));
+                    }
+                    continue;
+                }
+            }
+
+            // We are done with the top of the stack. Pop it. Depending on what it was, we do
+            // different things. Note that the outermost item must be the delimited, wrapped RHS
+            // that was passed in originally to `transcribe`.
+            match stack.pop().unwrap() {
+                // Done with a sequence. Pop from repeats.
+                Frame::Sequence { .. } => {
+                    repeats.pop();
+                }
+
+                // We are done processing a Delimited. If this is the top-level delimited, we are
+                // done. Otherwise, we unwind the result_stack to append what we have produced to
+                // any previous results.
+                Frame::Delimited { delim, span, .. } => {
+                    if result_stack.is_empty() {
+                        // No results left to compute! We are back at the top-level.
+                        return Ok(TokenStream::new(result));
+                    }
+
+                    // Step back into the parent Delimited.
+                    let tree = TokenTree::Delimited(span, delim, TokenStream::new(result));
+                    result = result_stack.pop().unwrap();
+                    result.push(tree);
+                }
+            }
+            continue;
+        };
+
+        // At this point, we know we are in the middle of a TokenTree (the last one on `stack`).
+        // `tree` contains the next `TokenTree` to be processed.
+        match tree {
+            // We are descending into a sequence. We first make sure that the matchers in the RHS
+            // and the matches in `interp` have the same shape. Otherwise, either the caller or the
+            // macro writer has made a mistake.
+            seq @ mbe::TokenTree::Sequence(_, delimited) => {
+                match lockstep_iter_size(&seq, interp, &repeats) {
+                    LockstepIterSize::Unconstrained => {
+                        return Err(cx.struct_span_err(
+                            seq.span(), /* blame macro writer */
+                            "attempted to repeat an expression containing no syntax variables \
+                             matched as repeating at this depth",
+                        ));
+                    }
+
+                    LockstepIterSize::Contradiction(msg) => {
+                        // FIXME: this really ought to be caught at macro definition time... It
+                        // happens when two meta-variables are used in the same repetition in a
+                        // sequence, but they come from different sequence matchers and repeat
+                        // different amounts.
+                        return Err(cx.struct_span_err(seq.span(), &msg));
+                    }
+
+                    LockstepIterSize::Constraint(len, _) => {
+                        // We do this to avoid an extra clone above. We know that this is a
+                        // sequence already.
+                        let mbe::TokenTree::Sequence(sp, seq) = seq else {
+                            unreachable!()
+                        };
+
+                        // Is the repetition empty?
+                        if len == 0 {
+                            if seq.kleene.op == mbe::KleeneOp::OneOrMore {
+                                // FIXME: this really ought to be caught at macro definition
+                                // time... It happens when the Kleene operator in the matcher and
+                                // the body for the same meta-variable do not match.
+                                return Err(cx.struct_span_err(
+                                    sp.entire(),
+                                    "this must repeat at least once",
+                                ));
+                            }
+                        } else {
+                            // 0 is the initial counter (we have done 0 repetitions so far). `len`
+                            // is the total number of repetitions we should generate.
+                            repeats.push((0, len));
+
+                            // The first time we encounter the sequence we push it to the stack. It
+                            // then gets reused (see the beginning of the loop) until we are done
+                            // repeating.
+                            stack.push(Frame::Sequence {
+                                idx: 0,
+                                sep: seq.separator.clone(),
+                                tts: &delimited.tts,
+                            });
+                        }
+                    }
+                }
+            }
+
+            // Replace the meta-var with the matched token tree from the invocation.
+            mbe::TokenTree::MetaVar(mut sp, mut original_ident) => {
+                // Find the matched nonterminal from the macro invocation, and use it to replace
+                // the meta-var.
+                let ident = MacroRulesNormalizedIdent::new(original_ident);
+                if let Some(cur_matched) = lookup_cur_matched(ident, interp, &repeats) {
+                    match cur_matched {
+                        MatchedTokenTree(ref tt) => {
+                            // `tt`s are emitted into the output stream directly as "raw tokens",
+                            // without wrapping them into groups.
+                            let token = tt.clone();
+                            result.push(token);
+                        }
+                        MatchedNonterminal(ref nt) => {
+                            // Other variables are emitted into the output stream as groups with
+                            // `Delimiter::Invisible` to maintain parsing priorities.
+                            // `Interpolated` is currently used for such groups in rustc parser.
+                            marker.visit_span(&mut sp);
+                            let token = TokenTree::token_alone(token::Interpolated(nt.clone()), sp);
+                            result.push(token);
+                        }
+                        MatchedSeq(..) => {
+                            // We were unable to descend far enough. This is an error.
+                            return Err(cx.struct_span_err(
+                                sp, /* blame the macro writer */
+                                &format!("variable '{}' is still repeating at this depth", ident),
+                            ));
+                        }
+                    }
+                } else {
+                    // If we aren't able to match the meta-var, we push it back into the result but
+                    // with modified syntax context. (I believe this supports nested macros).
+                    marker.visit_span(&mut sp);
+                    marker.visit_ident(&mut original_ident);
+                    result.push(TokenTree::token_alone(token::Dollar, sp));
+                    result.push(TokenTree::Token(
+                        Token::from_ast_ident(original_ident),
+                        Spacing::Alone,
+                    ));
+                }
+            }
+
+            // Replace meta-variable expressions with the result of their expansion.
+            mbe::TokenTree::MetaVarExpr(sp, expr) => {
+                transcribe_metavar_expr(cx, expr, interp, &mut marker, &repeats, &mut result, &sp)?;
+            }
+
+            // If we are entering a new delimiter, we push its contents to the `stack` to be
+            // processed, and we push all of the currently produced results to the `result_stack`.
+            // We will produce all of the results of the inside of the `Delimited` and then we will
+            // jump back out of the Delimited, pop the result_stack and add the new results back to
+            // the previous results (from outside the Delimited).
+            mbe::TokenTree::Delimited(mut span, delimited) => {
+                mut_visit::visit_delim_span(&mut span, &mut marker);
+                stack.push(Frame::Delimited {
+                    tts: &delimited.tts,
+                    delim: delimited.delim,
+                    idx: 0,
+                    span,
+                });
+                result_stack.push(mem::take(&mut result));
+            }
+
+            // Nothing much to do here. Just push the token to the result, being careful to
+            // preserve syntax context.
+            mbe::TokenTree::Token(token) => {
+                let mut token = token.clone();
+                mut_visit::visit_token(&mut token, &mut marker);
+                let tt = TokenTree::Token(token, Spacing::Alone);
+                result.push(tt);
+            }
+
+            // There should be no meta-var declarations in the invocation of a macro.
+            mbe::TokenTree::MetaVarDecl(..) => panic!("unexpected `TokenTree::MetaVarDecl"),
+        }
+    }
+}
+
+/// Lookup the meta-var named `ident` and return the matched token tree from the invocation using
+/// the set of matches `interpolations`.
+///
+/// See the definition of `repeats` in the `transcribe` function. `repeats` is used to descend
+/// into the right place in nested matchers. If we attempt to descend too far, the macro writer has
+/// made a mistake, and we return `None`.
+fn lookup_cur_matched<'a>(
+    ident: MacroRulesNormalizedIdent,
+    interpolations: &'a FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
+    repeats: &[(usize, usize)],
+) -> Option<&'a NamedMatch> {
+    interpolations.get(&ident).map(|matched| {
+        let mut matched = matched;
+        for &(idx, _) in repeats {
+            match matched {
+                MatchedTokenTree(_) | MatchedNonterminal(_) => break,
+                MatchedSeq(ref ads) => matched = ads.get(idx).unwrap(),
+            }
+        }
+
+        matched
+    })
+}
+
+/// An accumulator over a TokenTree to be used with `fold`. During transcription, we need to make
+/// sure that the size of each sequence and all of its nested sequences are the same as the sizes
+/// of all the matched (nested) sequences in the macro invocation. If they don't match, somebody
+/// has made a mistake (either the macro writer or caller).
+#[derive(Clone)]
+enum LockstepIterSize {
+    /// No constraints on length of matcher. This is true for any TokenTree variants except a
+    /// `MetaVar` with an actual `MatchedSeq` (as opposed to a `MatchedNonterminal`).
+    Unconstrained,
+
+    /// A `MetaVar` with an actual `MatchedSeq`. The length of the match and the name of the
+    /// meta-var are returned.
+    Constraint(usize, MacroRulesNormalizedIdent),
+
+    /// Two `Constraint`s on the same sequence had different lengths. This is an error.
+    Contradiction(String),
+}
+
+impl LockstepIterSize {
+    /// Find incompatibilities in matcher/invocation sizes.
+    /// - `Unconstrained` is compatible with everything.
+    /// - `Contradiction` is incompatible with everything.
+    /// - `Constraint(len)` is only compatible with other constraints of the same length.
+    fn with(self, other: LockstepIterSize) -> LockstepIterSize {
+        match self {
+            LockstepIterSize::Unconstrained => other,
+            LockstepIterSize::Contradiction(_) => self,
+            LockstepIterSize::Constraint(l_len, ref l_id) => match other {
+                LockstepIterSize::Unconstrained => self,
+                LockstepIterSize::Contradiction(_) => other,
+                LockstepIterSize::Constraint(r_len, _) if l_len == r_len => self,
+                LockstepIterSize::Constraint(r_len, r_id) => {
+                    let msg = format!(
+                        "meta-variable `{}` repeats {} time{}, but `{}` repeats {} time{}",
+                        l_id,
+                        l_len,
+                        pluralize!(l_len),
+                        r_id,
+                        r_len,
+                        pluralize!(r_len),
+                    );
+                    LockstepIterSize::Contradiction(msg)
+                }
+            },
+        }
+    }
+}
+
+/// Given a `tree`, make sure that all sequences have the same length as the matches for the
+/// appropriate meta-vars in `interpolations`.
+///
+/// Note that if `repeats` does not match the exact correct depth of a meta-var,
+/// `lookup_cur_matched` will return `None`, which is why this still works even in the presence of
+/// multiple nested matcher sequences.
+///
+/// Example: `$($($x $y)+*);+` -- we need to make sure that `x` and `y` repeat the same amount as
+/// each other at the given depth when the macro was invoked. If they don't it might mean they were
+/// declared at unequal depths or there was a compile bug. For example, if we have 3 repetitions of
+/// the outer sequence and 4 repetitions of the inner sequence for `x`, we should have the same for
+/// `y`; otherwise, we can't transcribe them both at the given depth.
+fn lockstep_iter_size(
+    tree: &mbe::TokenTree,
+    interpolations: &FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
+    repeats: &[(usize, usize)],
+) -> LockstepIterSize {
+    use mbe::TokenTree;
+    match *tree {
+        TokenTree::Delimited(_, ref delimited) => {
+            delimited.tts.iter().fold(LockstepIterSize::Unconstrained, |size, tt| {
+                size.with(lockstep_iter_size(tt, interpolations, repeats))
+            })
+        }
+        TokenTree::Sequence(_, ref seq) => {
+            seq.tts.iter().fold(LockstepIterSize::Unconstrained, |size, tt| {
+                size.with(lockstep_iter_size(tt, interpolations, repeats))
+            })
+        }
+        TokenTree::MetaVar(_, name) | TokenTree::MetaVarDecl(_, name, _) => {
+            let name = MacroRulesNormalizedIdent::new(name);
+            match lookup_cur_matched(name, interpolations, repeats) {
+                Some(matched) => match matched {
+                    MatchedTokenTree(_) | MatchedNonterminal(_) => LockstepIterSize::Unconstrained,
+                    MatchedSeq(ref ads) => LockstepIterSize::Constraint(ads.len(), name),
+                },
+                _ => LockstepIterSize::Unconstrained,
+            }
+        }
+        TokenTree::MetaVarExpr(_, ref expr) => {
+            let default_rslt = LockstepIterSize::Unconstrained;
+            let Some(ident) = expr.ident() else { return default_rslt; };
+            let name = MacroRulesNormalizedIdent::new(ident);
+            match lookup_cur_matched(name, interpolations, repeats) {
+                Some(MatchedSeq(ref ads)) => {
+                    default_rslt.with(LockstepIterSize::Constraint(ads.len(), name))
+                }
+                _ => default_rslt,
+            }
+        }
+        TokenTree::Token(..) => LockstepIterSize::Unconstrained,
+    }
+}
+
+/// Used solely by the `count` meta-variable expression, counts the outer-most repetitions at a
+/// given optional nested depth.
+///
+/// For example, a macro parameter of `$( { $( $foo:ident ),* } )*` called with `{ a, b } { c }`:
+///
+/// * `[ $( ${count(foo)} ),* ]` will return [2, 1] with a, b = 2 and c = 1
+/// * `[ $( ${count(foo, 0)} ),* ]` will be the same as `[ $( ${count(foo)} ),* ]`
+/// * `[ $( ${count(foo, 1)} ),* ]` will return an error because `${count(foo, 1)}` is
+///   declared inside a single repetition and the index `1` implies two nested repetitions.
+fn count_repetitions<'a>(
+    cx: &ExtCtxt<'a>,
+    depth_opt: Option<usize>,
+    mut matched: &NamedMatch,
+    repeats: &[(usize, usize)],
+    sp: &DelimSpan,
+) -> PResult<'a, usize> {
+    // Recursively count the number of matches in `matched` at given depth
+    // (or at the top-level of `matched` if no depth is given).
+    fn count<'a>(
+        cx: &ExtCtxt<'a>,
+        declared_lhs_depth: usize,
+        depth_opt: Option<usize>,
+        matched: &NamedMatch,
+        sp: &DelimSpan,
+    ) -> PResult<'a, usize> {
+        match matched {
+            MatchedTokenTree(_) | MatchedNonterminal(_) => {
+                if declared_lhs_depth == 0 {
+                    return Err(cx.struct_span_err(
+                        sp.entire(),
+                        "`count` can not be placed inside the inner-most repetition",
+                    ));
+                }
+                match depth_opt {
+                    None => Ok(1),
+                    Some(_) => Err(out_of_bounds_err(cx, declared_lhs_depth, sp.entire(), "count")),
+                }
+            }
+            MatchedSeq(ref named_matches) => {
+                let new_declared_lhs_depth = declared_lhs_depth + 1;
+                match depth_opt {
+                    None => named_matches
+                        .iter()
+                        .map(|elem| count(cx, new_declared_lhs_depth, None, elem, sp))
+                        .sum(),
+                    Some(0) => Ok(named_matches.len()),
+                    Some(depth) => named_matches
+                        .iter()
+                        .map(|elem| count(cx, new_declared_lhs_depth, Some(depth - 1), elem, sp))
+                        .sum(),
+                }
+            }
+        }
+    }
+    // `repeats` records all of the nested levels at which we are currently
+    // matching meta-variables. The meta-var-expr `count($x)` only counts
+    // matches that occur in this "subtree" of the `NamedMatch` where we
+    // are currently transcribing, so we need to descend to that subtree
+    // before we start counting. `matched` contains the various levels of the
+    // tree as we descend, and its final value is the subtree we are currently at.
+    for &(idx, _) in repeats {
+        if let MatchedSeq(ref ads) = matched {
+            matched = &ads[idx];
+        }
+    }
+    count(cx, 0, depth_opt, matched, sp)
+}
+
+/// Returns a `NamedMatch` item declared on the LHS given an arbitrary [Ident]
+fn matched_from_ident<'ctx, 'interp, 'rslt>(
+    cx: &ExtCtxt<'ctx>,
+    ident: Ident,
+    interp: &'interp FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
+) -> PResult<'ctx, &'rslt NamedMatch>
+where
+    'interp: 'rslt,
+{
+    let span = ident.span;
+    let key = MacroRulesNormalizedIdent::new(ident);
+    interp.get(&key).ok_or_else(|| {
+        cx.struct_span_err(
+            span,
+            &format!("variable `{}` is not recognized in meta-variable expression", key),
+        )
+    })
+}
+
+/// Used by meta-variable expressions when an user input is out of the actual declared bounds. For
+/// example, index(999999) in an repetition of only three elements.
+fn out_of_bounds_err<'a>(
+    cx: &ExtCtxt<'a>,
+    max: usize,
+    span: Span,
+    ty: &str,
+) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
+    let msg = if max == 0 {
+        format!(
+            "meta-variable expression `{ty}` with depth parameter \
+             must be called inside of a macro repetition"
+        )
+    } else {
+        format!(
+            "depth parameter on meta-variable expression `{ty}` \
+             must be less than {max}"
+        )
+    };
+    cx.struct_span_err(span, &msg)
+}
+
+fn transcribe_metavar_expr<'a>(
+    cx: &ExtCtxt<'a>,
+    expr: &MetaVarExpr,
+    interp: &FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
+    marker: &mut Marker,
+    repeats: &[(usize, usize)],
+    result: &mut Vec<TokenTree>,
+    sp: &DelimSpan,
+) -> PResult<'a, ()> {
+    let mut visited_span = || {
+        let mut span = sp.entire();
+        marker.visit_span(&mut span);
+        span
+    };
+    match *expr {
+        MetaVarExpr::Count(original_ident, depth_opt) => {
+            let matched = matched_from_ident(cx, original_ident, interp)?;
+            let count = count_repetitions(cx, depth_opt, matched, &repeats, sp)?;
+            let tt = TokenTree::token_alone(
+                TokenKind::lit(token::Integer, sym::integer(count), None),
+                visited_span(),
+            );
+            result.push(tt);
+        }
+        MetaVarExpr::Ignore(original_ident) => {
+            // Used to ensure that `original_ident` is present in the LHS
+            let _ = matched_from_ident(cx, original_ident, interp)?;
+        }
+        MetaVarExpr::Index(depth) => match repeats.iter().nth_back(depth) {
+            Some((index, _)) => {
+                result.push(TokenTree::token_alone(
+                    TokenKind::lit(token::Integer, sym::integer(*index), None),
+                    visited_span(),
+                ));
+            }
+            None => return Err(out_of_bounds_err(cx, repeats.len(), sp.entire(), "index")),
+        },
+        MetaVarExpr::Length(depth) => match repeats.iter().nth_back(depth) {
+            Some((_, length)) => {
+                result.push(TokenTree::token_alone(
+                    TokenKind::lit(token::Integer, sym::integer(*length), None),
+                    visited_span(),
+                ));
+            }
+            None => return Err(out_of_bounds_err(cx, repeats.len(), sp.entire(), "length")),
+        },
+    }
+    Ok(())
+}
diff --git a/compiler/rustc_expand/src/module.rs b/compiler/rustc_expand/src/module.rs
new file mode 100644
index 000000000..0315d1163
--- /dev/null
+++ b/compiler/rustc_expand/src/module.rs
@@ -0,0 +1,298 @@
+use crate::base::ModuleData;
+use rustc_ast::ptr::P;
+use rustc_ast::{token, Attribute, Inline, Item, ModSpans};
+use rustc_errors::{struct_span_err, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_parse::new_parser_from_file;
+use rustc_parse::validate_attr;
+use rustc_session::parse::ParseSess;
+use rustc_session::Session;
+use rustc_span::symbol::{sym, Ident};
+use rustc_span::Span;
+
+use std::path::{self, Path, PathBuf};
+
+#[derive(Copy, Clone)]
+pub enum DirOwnership {
+    Owned {
+        // None if `mod.rs`, `Some("foo")` if we're in `foo.rs`.
+        relative: Option<Ident>,
+    },
+    UnownedViaBlock,
+}
+
+// Public for rustfmt usage.
+pub struct ModulePathSuccess {
+    pub file_path: PathBuf,
+    pub dir_ownership: DirOwnership,
+}
+
+pub(crate) struct ParsedExternalMod {
+    pub items: Vec<P<Item>>,
+    pub spans: ModSpans,
+    pub file_path: PathBuf,
+    pub dir_path: PathBuf,
+    pub dir_ownership: DirOwnership,
+}
+
+pub enum ModError<'a> {
+    CircularInclusion(Vec<PathBuf>),
+    ModInBlock(Option<Ident>),
+    FileNotFound(Ident, PathBuf, PathBuf),
+    MultipleCandidates(Ident, PathBuf, PathBuf),
+    ParserError(DiagnosticBuilder<'a, ErrorGuaranteed>),
+}
+
+pub(crate) fn parse_external_mod(
+    sess: &Session,
+    ident: Ident,
+    span: Span, // The span to blame on errors.
+    module: &ModuleData,
+    mut dir_ownership: DirOwnership,
+    attrs: &mut Vec<Attribute>,
+) -> ParsedExternalMod {
+    // We bail on the first error, but that error does not cause a fatal error... (1)
+    let result: Result<_, ModError<'_>> = try {
+        // Extract the file path and the new ownership.
+        let mp = mod_file_path(sess, ident, &attrs, &module.dir_path, dir_ownership)?;
+        dir_ownership = mp.dir_ownership;
+
+        // Ensure file paths are acyclic.
+        if let Some(pos) = module.file_path_stack.iter().position(|p| p == &mp.file_path) {
+            Err(ModError::CircularInclusion(module.file_path_stack[pos..].to_vec()))?;
+        }
+
+        // Actually parse the external file as a module.
+        let mut parser = new_parser_from_file(&sess.parse_sess, &mp.file_path, Some(span));
+        let (mut inner_attrs, items, inner_span) =
+            parser.parse_mod(&token::Eof).map_err(|err| ModError::ParserError(err))?;
+        attrs.append(&mut inner_attrs);
+        (items, inner_span, mp.file_path)
+    };
+    // (1) ...instead, we return a dummy module.
+    let (items, spans, file_path) =
+        result.map_err(|err| err.report(sess, span)).unwrap_or_default();
+
+    // Extract the directory path for submodules of the module.
+    let dir_path = file_path.parent().unwrap_or(&file_path).to_owned();
+
+    ParsedExternalMod { items, spans, file_path, dir_path, dir_ownership }
+}
+
+pub(crate) fn mod_dir_path(
+    sess: &Session,
+    ident: Ident,
+    attrs: &[Attribute],
+    module: &ModuleData,
+    mut dir_ownership: DirOwnership,
+    inline: Inline,
+) -> (PathBuf, DirOwnership) {
+    match inline {
+        Inline::Yes if let Some(file_path) = mod_file_path_from_attr(sess, attrs, &module.dir_path) => {
+            // For inline modules file path from `#[path]` is actually the directory path
+            // for historical reasons, so we don't pop the last segment here.
+            (file_path, DirOwnership::Owned { relative: None })
+        }
+        Inline::Yes => {
+            // We have to push on the current module name in the case of relative
+            // paths in order to ensure that any additional module paths from inline
+            // `mod x { ... }` come after the relative extension.
+            //
+            // For example, a `mod z { ... }` inside `x/y.rs` should set the current
+            // directory path to `/x/y/z`, not `/x/z` with a relative offset of `y`.
+            let mut dir_path = module.dir_path.clone();
+            if let DirOwnership::Owned { relative } = &mut dir_ownership {
+                if let Some(ident) = relative.take() {
+                    // Remove the relative offset.
+                    dir_path.push(ident.as_str());
+                }
+            }
+            dir_path.push(ident.as_str());
+
+            (dir_path, dir_ownership)
+        }
+        Inline::No => {
+            // FIXME: This is a subset of `parse_external_mod` without actual parsing,
+            // check whether the logic for unloaded, loaded and inline modules can be unified.
+            let file_path = mod_file_path(sess, ident, &attrs, &module.dir_path, dir_ownership)
+                .map(|mp| {
+                    dir_ownership = mp.dir_ownership;
+                    mp.file_path
+                })
+                .unwrap_or_default();
+
+            // Extract the directory path for submodules of the module.
+            let dir_path = file_path.parent().unwrap_or(&file_path).to_owned();
+
+            (dir_path, dir_ownership)
+        }
+    }
+}
+
+fn mod_file_path<'a>(
+    sess: &'a Session,
+    ident: Ident,
+    attrs: &[Attribute],
+    dir_path: &Path,
+    dir_ownership: DirOwnership,
+) -> Result<ModulePathSuccess, ModError<'a>> {
+    if let Some(file_path) = mod_file_path_from_attr(sess, attrs, dir_path) {
+        // All `#[path]` files are treated as though they are a `mod.rs` file.
+        // This means that `mod foo;` declarations inside `#[path]`-included
+        // files are siblings,
+        //
+        // Note that this will produce weirdness when a file named `foo.rs` is
+        // `#[path]` included and contains a `mod foo;` declaration.
+        // If you encounter this, it's your own darn fault :P
+        let dir_ownership = DirOwnership::Owned { relative: None };
+        return Ok(ModulePathSuccess { file_path, dir_ownership });
+    }
+
+    let relative = match dir_ownership {
+        DirOwnership::Owned { relative } => relative,
+        DirOwnership::UnownedViaBlock => None,
+    };
+    let result = default_submod_path(&sess.parse_sess, ident, relative, dir_path);
+    match dir_ownership {
+        DirOwnership::Owned { .. } => result,
+        DirOwnership::UnownedViaBlock => Err(ModError::ModInBlock(match result {
+            Ok(_) | Err(ModError::MultipleCandidates(..)) => Some(ident),
+            _ => None,
+        })),
+    }
+}
+
+/// Derive a submodule path from the first found `#[path = "path_string"]`.
+/// The provided `dir_path` is joined with the `path_string`.
+fn mod_file_path_from_attr(
+    sess: &Session,
+    attrs: &[Attribute],
+    dir_path: &Path,
+) -> Option<PathBuf> {
+    // Extract path string from first `#[path = "path_string"]` attribute.
+    let first_path = attrs.iter().find(|at| at.has_name(sym::path))?;
+    let Some(path_sym) = first_path.value_str() else {
+        // This check is here mainly to catch attempting to use a macro,
+        // such as #[path = concat!(...)]. This isn't currently supported
+        // because otherwise the InvocationCollector would need to defer
+        // loading a module until the #[path] attribute was expanded, and
+        // it doesn't support that (and would likely add a bit of
+        // complexity). Usually bad forms are checked in AstValidator (via
+        // `check_builtin_attribute`), but by the time that runs the macro
+        // is expanded, and it doesn't give an error.
+        validate_attr::emit_fatal_malformed_builtin_attribute(
+            &sess.parse_sess,
+            first_path,
+            sym::path,
+        );
+    };
+
+    let path_str = path_sym.as_str();
+
+    // On windows, the base path might have the form
+    // `\\?\foo\bar` in which case it does not tolerate
+    // mixed `/` and `\` separators, so canonicalize
+    // `/` to `\`.
+    #[cfg(windows)]
+    let path_str = path_str.replace("/", "\\");
+
+    Some(dir_path.join(path_str))
+}
+
+/// Returns a path to a module.
+// Public for rustfmt usage.
+pub fn default_submod_path<'a>(
+    sess: &'a ParseSess,
+    ident: Ident,
+    relative: Option<Ident>,
+    dir_path: &Path,
+) -> Result<ModulePathSuccess, ModError<'a>> {
+    // If we're in a foo.rs file instead of a mod.rs file,
+    // we need to look for submodules in
+    // `./foo/<ident>.rs` and `./foo/<ident>/mod.rs` rather than
+    // `./<ident>.rs` and `./<ident>/mod.rs`.
+    let relative_prefix_string;
+    let relative_prefix = if let Some(ident) = relative {
+        relative_prefix_string = format!("{}{}", ident.name, path::MAIN_SEPARATOR);
+        &relative_prefix_string
+    } else {
+        ""
+    };
+
+    let default_path_str = format!("{}{}.rs", relative_prefix, ident.name);
+    let secondary_path_str =
+        format!("{}{}{}mod.rs", relative_prefix, ident.name, path::MAIN_SEPARATOR);
+    let default_path = dir_path.join(&default_path_str);
+    let secondary_path = dir_path.join(&secondary_path_str);
+    let default_exists = sess.source_map().file_exists(&default_path);
+    let secondary_exists = sess.source_map().file_exists(&secondary_path);
+
+    match (default_exists, secondary_exists) {
+        (true, false) => Ok(ModulePathSuccess {
+            file_path: default_path,
+            dir_ownership: DirOwnership::Owned { relative: Some(ident) },
+        }),
+        (false, true) => Ok(ModulePathSuccess {
+            file_path: secondary_path,
+            dir_ownership: DirOwnership::Owned { relative: None },
+        }),
+        (false, false) => Err(ModError::FileNotFound(ident, default_path, secondary_path)),
+        (true, true) => Err(ModError::MultipleCandidates(ident, default_path, secondary_path)),
+    }
+}
+
+impl ModError<'_> {
+    fn report(self, sess: &Session, span: Span) -> ErrorGuaranteed {
+        let diag = &sess.parse_sess.span_diagnostic;
+        match self {
+            ModError::CircularInclusion(file_paths) => {
+                let mut msg = String::from("circular modules: ");
+                for file_path in &file_paths {
+                    msg.push_str(&file_path.display().to_string());
+                    msg.push_str(" -> ");
+                }
+                msg.push_str(&file_paths[0].display().to_string());
+                diag.struct_span_err(span, &msg)
+            }
+            ModError::ModInBlock(ident) => {
+                let msg = "cannot declare a non-inline module inside a block unless it has a path attribute";
+                let mut err = diag.struct_span_err(span, msg);
+                if let Some(ident) = ident {
+                    let note =
+                        format!("maybe `use` the module `{}` instead of redeclaring it", ident);
+                    err.span_note(span, &note);
+                }
+                err
+            }
+            ModError::FileNotFound(ident, default_path, secondary_path) => {
+                let mut err = struct_span_err!(
+                    diag,
+                    span,
+                    E0583,
+                    "file not found for module `{}`",
+                    ident,
+                );
+                err.help(&format!(
+                    "to create the module `{}`, create file \"{}\" or \"{}\"",
+                    ident,
+                    default_path.display(),
+                    secondary_path.display(),
+                ));
+                err
+            }
+            ModError::MultipleCandidates(ident, default_path, secondary_path) => {
+                let mut err = struct_span_err!(
+                    diag,
+                    span,
+                    E0761,
+                    "file for module `{}` found at both \"{}\" and \"{}\"",
+                    ident,
+                    default_path.display(),
+                    secondary_path.display(),
+                );
+                err.help("delete or rename one of them to remove the ambiguity");
+                err
+            }
+            ModError::ParserError(err) => err,
+        }.emit()
+    }
+}
diff --git a/compiler/rustc_expand/src/mut_visit/tests.rs b/compiler/rustc_expand/src/mut_visit/tests.rs
new file mode 100644
index 000000000..8974d45b4
--- /dev/null
+++ b/compiler/rustc_expand/src/mut_visit/tests.rs
@@ -0,0 +1,72 @@
+use crate::tests::{matches_codepattern, string_to_crate};
+
+use rustc_ast as ast;
+use rustc_ast::mut_visit::MutVisitor;
+use rustc_ast_pretty::pprust;
+use rustc_span::create_default_session_globals_then;
+use rustc_span::symbol::Ident;
+
+// This version doesn't care about getting comments or doc-strings in.
+fn print_crate_items(krate: &ast::Crate) -> String {
+    krate.items.iter().map(|i| pprust::item_to_string(i)).collect::<Vec<_>>().join(" ")
+}
+
+// Change every identifier to "zz".
+struct ToZzIdentMutVisitor;
+
+impl MutVisitor for ToZzIdentMutVisitor {
+    const VISIT_TOKENS: bool = true;
+
+    fn visit_ident(&mut self, ident: &mut Ident) {
+        *ident = Ident::from_str("zz");
+    }
+}
+
+// Maybe add to `expand.rs`.
+macro_rules! assert_pred {
+    ($pred:expr, $predname:expr, $a:expr , $b:expr) => {{
+        let pred_val = $pred;
+        let a_val = $a;
+        let b_val = $b;
+        if !(pred_val(&a_val, &b_val)) {
+            panic!("expected args satisfying {}, got {} and {}", $predname, a_val, b_val);
+        }
+    }};
+}
+
+// Make sure idents get transformed everywhere.
+#[test]
+fn ident_transformation() {
+    create_default_session_globals_then(|| {
+        let mut zz_visitor = ToZzIdentMutVisitor;
+        let mut krate =
+            string_to_crate("#[a] mod b {fn c (d : e, f : g) {h!(i,j,k);l;m}}".to_string());
+        zz_visitor.visit_crate(&mut krate);
+        assert_pred!(
+            matches_codepattern,
+            "matches_codepattern",
+            print_crate_items(&krate),
+            "#[zz]mod zz{fn zz(zz:zz,zz:zz){zz!(zz,zz,zz);zz;zz}}".to_string()
+        );
+    })
+}
+
+// Make sure idents get transformed even inside macro defs.
+#[test]
+fn ident_transformation_in_defs() {
+    create_default_session_globals_then(|| {
+        let mut zz_visitor = ToZzIdentMutVisitor;
+        let mut krate = string_to_crate(
+            "macro_rules! a {(b $c:expr $(d $e:token)f+ => \
+            (g $(d $d $e)+))} "
+                .to_string(),
+        );
+        zz_visitor.visit_crate(&mut krate);
+        assert_pred!(
+            matches_codepattern,
+            "matches_codepattern",
+            print_crate_items(&krate),
+            "macro_rules! zz{(zz$zz:zz$(zz $zz:zz)zz+=>(zz$(zz$zz$zz)+))}".to_string()
+        );
+    })
+}
diff --git a/compiler/rustc_expand/src/parse/tests.rs b/compiler/rustc_expand/src/parse/tests.rs
new file mode 100644
index 000000000..a3c631d33
--- /dev/null
+++ b/compiler/rustc_expand/src/parse/tests.rs
@@ -0,0 +1,358 @@
+use crate::tests::{matches_codepattern, string_to_stream, with_error_checking_parse};
+
+use rustc_ast::ptr::P;
+use rustc_ast::token::{self, Delimiter, Token};
+use rustc_ast::tokenstream::{DelimSpan, TokenStream, TokenTree};
+use rustc_ast::visit;
+use rustc_ast::{self as ast, PatKind};
+use rustc_ast_pretty::pprust::item_to_string;
+use rustc_errors::PResult;
+use rustc_parse::new_parser_from_source_str;
+use rustc_parse::parser::ForceCollect;
+use rustc_session::parse::ParseSess;
+use rustc_span::create_default_session_globals_then;
+use rustc_span::source_map::FilePathMapping;
+use rustc_span::symbol::{kw, sym, Symbol};
+use rustc_span::{BytePos, FileName, Pos, Span};
+
+use std::path::PathBuf;
+
+fn sess() -> ParseSess {
+    ParseSess::new(FilePathMapping::empty())
+}
+
+/// Parses an item.
+///
+/// Returns `Ok(Some(item))` when successful, `Ok(None)` when no item was found, and `Err`
+/// when a syntax error occurred.
+fn parse_item_from_source_str(
+    name: FileName,
+    source: String,
+    sess: &ParseSess,
+) -> PResult<'_, Option<P<ast::Item>>> {
+    new_parser_from_source_str(sess, name, source).parse_item(ForceCollect::No)
+}
+
+// Produces a `rustc_span::span`.
+fn sp(a: u32, b: u32) -> Span {
+    Span::with_root_ctxt(BytePos(a), BytePos(b))
+}
+
+/// Parses a string, return an expression.
+fn string_to_expr(source_str: String) -> P<ast::Expr> {
+    with_error_checking_parse(source_str, &sess(), |p| p.parse_expr())
+}
+
+/// Parses a string, returns an item.
+fn string_to_item(source_str: String) -> Option<P<ast::Item>> {
+    with_error_checking_parse(source_str, &sess(), |p| p.parse_item(ForceCollect::No))
+}
+
+#[should_panic]
+#[test]
+fn bad_path_expr_1() {
+    create_default_session_globals_then(|| {
+        string_to_expr("::abc::def::return".to_string());
+    })
+}
+
+// Checks the token-tree-ization of macros.
+#[test]
+fn string_to_tts_macro() {
+    create_default_session_globals_then(|| {
+        let tts: Vec<_> =
+            string_to_stream("macro_rules! zip (($a)=>($a))".to_string()).into_trees().collect();
+        let tts: &[TokenTree] = &tts[..];
+
+        match tts {
+            [
+                TokenTree::Token(Token { kind: token::Ident(name_macro_rules, false), .. }, _),
+                TokenTree::Token(Token { kind: token::Not, .. }, _),
+                TokenTree::Token(Token { kind: token::Ident(name_zip, false), .. }, _),
+                TokenTree::Delimited(_, macro_delim, macro_tts),
+            ] if name_macro_rules == &kw::MacroRules && name_zip.as_str() == "zip" => {
+                let tts = &macro_tts.trees().collect::<Vec<_>>();
+                match &tts[..] {
+                    [
+                        TokenTree::Delimited(_, first_delim, first_tts),
+                        TokenTree::Token(Token { kind: token::FatArrow, .. }, _),
+                        TokenTree::Delimited(_, second_delim, second_tts),
+                    ] if macro_delim == &Delimiter::Parenthesis => {
+                        let tts = &first_tts.trees().collect::<Vec<_>>();
+                        match &tts[..] {
+                            [
+                                TokenTree::Token(Token { kind: token::Dollar, .. }, _),
+                                TokenTree::Token(Token { kind: token::Ident(name, false), .. }, _),
+                            ] if first_delim == &Delimiter::Parenthesis && name.as_str() == "a" => {
+                            }
+                            _ => panic!("value 3: {:?} {:?}", first_delim, first_tts),
+                        }
+                        let tts = &second_tts.trees().collect::<Vec<_>>();
+                        match &tts[..] {
+                            [
+                                TokenTree::Token(Token { kind: token::Dollar, .. }, _),
+                                TokenTree::Token(Token { kind: token::Ident(name, false), .. }, _),
+                            ] if second_delim == &Delimiter::Parenthesis
+                                && name.as_str() == "a" => {}
+                            _ => panic!("value 4: {:?} {:?}", second_delim, second_tts),
+                        }
+                    }
+                    _ => panic!("value 2: {:?} {:?}", macro_delim, macro_tts),
+                }
+            }
+            _ => panic!("value: {:?}", tts),
+        }
+    })
+}
+
+#[test]
+fn string_to_tts_1() {
+    create_default_session_globals_then(|| {
+        let tts = string_to_stream("fn a (b : i32) { b; }".to_string());
+
+        let expected = TokenStream::new(vec![
+            TokenTree::token_alone(token::Ident(kw::Fn, false), sp(0, 2)),
+            TokenTree::token_alone(token::Ident(Symbol::intern("a"), false), sp(3, 4)),
+            TokenTree::Delimited(
+                DelimSpan::from_pair(sp(5, 6), sp(13, 14)),
+                Delimiter::Parenthesis,
+                TokenStream::new(vec![
+                    TokenTree::token_alone(token::Ident(Symbol::intern("b"), false), sp(6, 7)),
+                    TokenTree::token_alone(token::Colon, sp(8, 9)),
+                    TokenTree::token_alone(token::Ident(sym::i32, false), sp(10, 13)),
+                ])
+                .into(),
+            ),
+            TokenTree::Delimited(
+                DelimSpan::from_pair(sp(15, 16), sp(20, 21)),
+                Delimiter::Brace,
+                TokenStream::new(vec![
+                    TokenTree::token_joint(token::Ident(Symbol::intern("b"), false), sp(17, 18)),
+                    TokenTree::token_alone(token::Semi, sp(18, 19)),
+                ])
+                .into(),
+            ),
+        ]);
+
+        assert_eq!(tts, expected);
+    })
+}
+
+#[test]
+fn parse_use() {
+    create_default_session_globals_then(|| {
+        let use_s = "use foo::bar::baz;";
+        let vitem = string_to_item(use_s.to_string()).unwrap();
+        let vitem_s = item_to_string(&vitem);
+        assert_eq!(&vitem_s[..], use_s);
+
+        let use_s = "use foo::bar as baz;";
+        let vitem = string_to_item(use_s.to_string()).unwrap();
+        let vitem_s = item_to_string(&vitem);
+        assert_eq!(&vitem_s[..], use_s);
+    })
+}
+
+#[test]
+fn parse_extern_crate() {
+    create_default_session_globals_then(|| {
+        let ex_s = "extern crate foo;";
+        let vitem = string_to_item(ex_s.to_string()).unwrap();
+        let vitem_s = item_to_string(&vitem);
+        assert_eq!(&vitem_s[..], ex_s);
+
+        let ex_s = "extern crate foo as bar;";
+        let vitem = string_to_item(ex_s.to_string()).unwrap();
+        let vitem_s = item_to_string(&vitem);
+        assert_eq!(&vitem_s[..], ex_s);
+    })
+}
+
+fn get_spans_of_pat_idents(src: &str) -> Vec<Span> {
+    let item = string_to_item(src.to_string()).unwrap();
+
+    struct PatIdentVisitor {
+        spans: Vec<Span>,
+    }
+    impl<'a> visit::Visitor<'a> for PatIdentVisitor {
+        fn visit_pat(&mut self, p: &'a ast::Pat) {
+            match p.kind {
+                PatKind::Ident(_, ref ident, _) => {
+                    self.spans.push(ident.span.clone());
+                }
+                _ => {
+                    visit::walk_pat(self, p);
+                }
+            }
+        }
+    }
+    let mut v = PatIdentVisitor { spans: Vec::new() };
+    visit::walk_item(&mut v, &item);
+    return v.spans;
+}
+
+#[test]
+fn span_of_self_arg_pat_idents_are_correct() {
+    create_default_session_globals_then(|| {
+        let srcs = [
+            "impl z { fn a (&self, &myarg: i32) {} }",
+            "impl z { fn a (&mut self, &myarg: i32) {} }",
+            "impl z { fn a (&'a self, &myarg: i32) {} }",
+            "impl z { fn a (self, &myarg: i32) {} }",
+            "impl z { fn a (self: Foo, &myarg: i32) {} }",
+        ];
+
+        for src in srcs {
+            let spans = get_spans_of_pat_idents(src);
+            let (lo, hi) = (spans[0].lo(), spans[0].hi());
+            assert!(
+                "self" == &src[lo.to_usize()..hi.to_usize()],
+                "\"{}\" != \"self\". src=\"{}\"",
+                &src[lo.to_usize()..hi.to_usize()],
+                src
+            )
+        }
+    })
+}
+
+#[test]
+fn parse_exprs() {
+    create_default_session_globals_then(|| {
+        // just make sure that they parse....
+        string_to_expr("3 + 4".to_string());
+        string_to_expr("a::z.froob(b,&(987+3))".to_string());
+    })
+}
+
+#[test]
+fn attrs_fix_bug() {
+    create_default_session_globals_then(|| {
+        string_to_item(
+            "pub fn mk_file_writer(path: &Path, flags: &[FileFlag])
+                -> Result<Box<Writer>, String> {
+#[cfg(windows)]
+fn wb() -> c_int {
+    (O_WRONLY | libc::consts::os::extra::O_BINARY) as c_int
+}
+
+#[cfg(unix)]
+fn wb() -> c_int { O_WRONLY as c_int }
+
+let mut fflags: c_int = wb();
+}"
+            .to_string(),
+        );
+    })
+}
+
+#[test]
+fn crlf_doc_comments() {
+    create_default_session_globals_then(|| {
+        let sess = sess();
+
+        let name_1 = FileName::Custom("crlf_source_1".to_string());
+        let source = "/// doc comment\r\nfn foo() {}".to_string();
+        let item = parse_item_from_source_str(name_1, source, &sess).unwrap().unwrap();
+        let doc = item.attrs.iter().filter_map(|at| at.doc_str()).next().unwrap();
+        assert_eq!(doc.as_str(), " doc comment");
+
+        let name_2 = FileName::Custom("crlf_source_2".to_string());
+        let source = "/// doc comment\r\n/// line 2\r\nfn foo() {}".to_string();
+        let item = parse_item_from_source_str(name_2, source, &sess).unwrap().unwrap();
+        let docs = item.attrs.iter().filter_map(|at| at.doc_str()).collect::<Vec<_>>();
+        let b: &[_] = &[Symbol::intern(" doc comment"), Symbol::intern(" line 2")];
+        assert_eq!(&docs[..], b);
+
+        let name_3 = FileName::Custom("clrf_source_3".to_string());
+        let source = "/** doc comment\r\n *  with CRLF */\r\nfn foo() {}".to_string();
+        let item = parse_item_from_source_str(name_3, source, &sess).unwrap().unwrap();
+        let doc = item.attrs.iter().filter_map(|at| at.doc_str()).next().unwrap();
+        assert_eq!(doc.as_str(), " doc comment\n *  with CRLF ");
+    });
+}
+
+#[test]
+fn ttdelim_span() {
+    fn parse_expr_from_source_str(
+        name: FileName,
+        source: String,
+        sess: &ParseSess,
+    ) -> PResult<'_, P<ast::Expr>> {
+        new_parser_from_source_str(sess, name, source).parse_expr()
+    }
+
+    create_default_session_globals_then(|| {
+        let sess = sess();
+        let expr = parse_expr_from_source_str(
+            PathBuf::from("foo").into(),
+            "foo!( fn main() { body } )".to_string(),
+            &sess,
+        )
+        .unwrap();
+
+        let tts: Vec<_> = match expr.kind {
+            ast::ExprKind::MacCall(ref mac) => mac.args.inner_tokens().into_trees().collect(),
+            _ => panic!("not a macro"),
+        };
+
+        let span = tts.iter().rev().next().unwrap().span();
+
+        match sess.source_map().span_to_snippet(span) {
+            Ok(s) => assert_eq!(&s[..], "{ body }"),
+            Err(_) => panic!("could not get snippet"),
+        }
+    });
+}
+
+// This tests that when parsing a string (rather than a file) we don't try
+// and read in a file for a module declaration and just parse a stub.
+// See `recurse_into_file_modules` in the parser.
+#[test]
+fn out_of_line_mod() {
+    create_default_session_globals_then(|| {
+        let item = parse_item_from_source_str(
+            PathBuf::from("foo").into(),
+            "mod foo { struct S; mod this_does_not_exist; }".to_owned(),
+            &sess(),
+        )
+        .unwrap()
+        .unwrap();
+
+        if let ast::ItemKind::Mod(_, ref mod_kind) = item.kind {
+            assert!(matches!(mod_kind, ast::ModKind::Loaded(items, ..) if items.len() == 2));
+        } else {
+            panic!();
+        }
+    });
+}
+
+#[test]
+fn eqmodws() {
+    assert_eq!(matches_codepattern("", ""), true);
+    assert_eq!(matches_codepattern("", "a"), false);
+    assert_eq!(matches_codepattern("a", ""), false);
+    assert_eq!(matches_codepattern("a", "a"), true);
+    assert_eq!(matches_codepattern("a b", "a   \n\t\r  b"), true);
+    assert_eq!(matches_codepattern("a b ", "a   \n\t\r  b"), true);
+    assert_eq!(matches_codepattern("a b", "a   \n\t\r  b "), false);
+    assert_eq!(matches_codepattern("a   b", "a b"), true);
+    assert_eq!(matches_codepattern("ab", "a b"), false);
+    assert_eq!(matches_codepattern("a   b", "ab"), true);
+    assert_eq!(matches_codepattern(" a   b", "ab"), true);
+}
+
+#[test]
+fn pattern_whitespace() {
+    assert_eq!(matches_codepattern("", "\x0C"), false);
+    assert_eq!(matches_codepattern("a b ", "a   \u{0085}\n\t\r  b"), true);
+    assert_eq!(matches_codepattern("a b", "a   \u{0085}\n\t\r  b "), false);
+}
+
+#[test]
+fn non_pattern_whitespace() {
+    // These have the property 'White_Space' but not 'Pattern_White_Space'
+    assert_eq!(matches_codepattern("a b", "a\u{2002}b"), false);
+    assert_eq!(matches_codepattern("a   b", "a\u{2002}b"), false);
+    assert_eq!(matches_codepattern("\u{205F}a   b", "ab"), false);
+    assert_eq!(matches_codepattern("a  \u{3000}b", "ab"), false);
+}
diff --git a/compiler/rustc_expand/src/placeholders.rs b/compiler/rustc_expand/src/placeholders.rs
new file mode 100644
index 000000000..0d5d6ee07
--- /dev/null
+++ b/compiler/rustc_expand/src/placeholders.rs
@@ -0,0 +1,373 @@
+use crate::expand::{AstFragment, AstFragmentKind};
+
+use rustc_ast as ast;
+use rustc_ast::mut_visit::*;
+use rustc_ast::ptr::P;
+use rustc_span::source_map::DUMMY_SP;
+use rustc_span::symbol::Ident;
+
+use smallvec::{smallvec, SmallVec};
+
+use rustc_data_structures::fx::FxHashMap;
+
+pub fn placeholder(
+    kind: AstFragmentKind,
+    id: ast::NodeId,
+    vis: Option<ast::Visibility>,
+) -> AstFragment {
+    fn mac_placeholder() -> ast::MacCall {
+        ast::MacCall {
+            path: ast::Path { span: DUMMY_SP, segments: Vec::new(), tokens: None },
+            args: P(ast::MacArgs::Empty),
+            prior_type_ascription: None,
+        }
+    }
+
+    let ident = Ident::empty();
+    let attrs = Vec::new();
+    let vis = vis.unwrap_or(ast::Visibility {
+        span: DUMMY_SP,
+        kind: ast::VisibilityKind::Inherited,
+        tokens: None,
+    });
+    let span = DUMMY_SP;
+    let expr_placeholder = || {
+        P(ast::Expr {
+            id,
+            span,
+            attrs: ast::AttrVec::new(),
+            kind: ast::ExprKind::MacCall(mac_placeholder()),
+            tokens: None,
+        })
+    };
+    let ty =
+        || P(ast::Ty { id, kind: ast::TyKind::MacCall(mac_placeholder()), span, tokens: None });
+    let pat =
+        || P(ast::Pat { id, kind: ast::PatKind::MacCall(mac_placeholder()), span, tokens: None });
+
+    match kind {
+        AstFragmentKind::Crate => AstFragment::Crate(ast::Crate {
+            attrs: Default::default(),
+            items: Default::default(),
+            spans: ast::ModSpans { inner_span: span, ..Default::default() },
+            id,
+            is_placeholder: true,
+        }),
+        AstFragmentKind::Expr => AstFragment::Expr(expr_placeholder()),
+        AstFragmentKind::OptExpr => AstFragment::OptExpr(Some(expr_placeholder())),
+        AstFragmentKind::Items => AstFragment::Items(smallvec![P(ast::Item {
+            id,
+            span,
+            ident,
+            vis,
+            attrs,
+            kind: ast::ItemKind::MacCall(mac_placeholder()),
+            tokens: None,
+        })]),
+        AstFragmentKind::TraitItems => AstFragment::TraitItems(smallvec![P(ast::AssocItem {
+            id,
+            span,
+            ident,
+            vis,
+            attrs,
+            kind: ast::AssocItemKind::MacCall(mac_placeholder()),
+            tokens: None,
+        })]),
+        AstFragmentKind::ImplItems => AstFragment::ImplItems(smallvec![P(ast::AssocItem {
+            id,
+            span,
+            ident,
+            vis,
+            attrs,
+            kind: ast::AssocItemKind::MacCall(mac_placeholder()),
+            tokens: None,
+        })]),
+        AstFragmentKind::ForeignItems => {
+            AstFragment::ForeignItems(smallvec![P(ast::ForeignItem {
+                id,
+                span,
+                ident,
+                vis,
+                attrs,
+                kind: ast::ForeignItemKind::MacCall(mac_placeholder()),
+                tokens: None,
+            })])
+        }
+        AstFragmentKind::Pat => AstFragment::Pat(P(ast::Pat {
+            id,
+            span,
+            kind: ast::PatKind::MacCall(mac_placeholder()),
+            tokens: None,
+        })),
+        AstFragmentKind::Ty => AstFragment::Ty(P(ast::Ty {
+            id,
+            span,
+            kind: ast::TyKind::MacCall(mac_placeholder()),
+            tokens: None,
+        })),
+        AstFragmentKind::Stmts => AstFragment::Stmts(smallvec![{
+            let mac = P(ast::MacCallStmt {
+                mac: mac_placeholder(),
+                style: ast::MacStmtStyle::Braces,
+                attrs: ast::AttrVec::new(),
+                tokens: None,
+            });
+            ast::Stmt { id, span, kind: ast::StmtKind::MacCall(mac) }
+        }]),
+        AstFragmentKind::Arms => AstFragment::Arms(smallvec![ast::Arm {
+            attrs: Default::default(),
+            body: expr_placeholder(),
+            guard: None,
+            id,
+            pat: pat(),
+            span,
+            is_placeholder: true,
+        }]),
+        AstFragmentKind::ExprFields => AstFragment::ExprFields(smallvec![ast::ExprField {
+            attrs: Default::default(),
+            expr: expr_placeholder(),
+            id,
+            ident,
+            is_shorthand: false,
+            span,
+            is_placeholder: true,
+        }]),
+        AstFragmentKind::PatFields => AstFragment::PatFields(smallvec![ast::PatField {
+            attrs: Default::default(),
+            id,
+            ident,
+            is_shorthand: false,
+            pat: pat(),
+            span,
+            is_placeholder: true,
+        }]),
+        AstFragmentKind::GenericParams => AstFragment::GenericParams(smallvec![{
+            ast::GenericParam {
+                attrs: Default::default(),
+                bounds: Default::default(),
+                id,
+                ident,
+                is_placeholder: true,
+                kind: ast::GenericParamKind::Lifetime,
+                colon_span: None,
+            }
+        }]),
+        AstFragmentKind::Params => AstFragment::Params(smallvec![ast::Param {
+            attrs: Default::default(),
+            id,
+            pat: pat(),
+            span,
+            ty: ty(),
+            is_placeholder: true,
+        }]),
+        AstFragmentKind::FieldDefs => AstFragment::FieldDefs(smallvec![ast::FieldDef {
+            attrs: Default::default(),
+            id,
+            ident: None,
+            span,
+            ty: ty(),
+            vis,
+            is_placeholder: true,
+        }]),
+        AstFragmentKind::Variants => AstFragment::Variants(smallvec![ast::Variant {
+            attrs: Default::default(),
+            data: ast::VariantData::Struct(Default::default(), false),
+            disr_expr: None,
+            id,
+            ident,
+            span,
+            vis,
+            is_placeholder: true,
+        }]),
+    }
+}
+
+#[derive(Default)]
+pub struct PlaceholderExpander {
+    expanded_fragments: FxHashMap<ast::NodeId, AstFragment>,
+}
+
+impl PlaceholderExpander {
+    pub fn add(&mut self, id: ast::NodeId, mut fragment: AstFragment) {
+        fragment.mut_visit_with(self);
+        self.expanded_fragments.insert(id, fragment);
+    }
+
+    fn remove(&mut self, id: ast::NodeId) -> AstFragment {
+        self.expanded_fragments.remove(&id).unwrap()
+    }
+}
+
+impl MutVisitor for PlaceholderExpander {
+    fn flat_map_arm(&mut self, arm: ast::Arm) -> SmallVec<[ast::Arm; 1]> {
+        if arm.is_placeholder {
+            self.remove(arm.id).make_arms()
+        } else {
+            noop_flat_map_arm(arm, self)
+        }
+    }
+
+    fn flat_map_expr_field(&mut self, field: ast::ExprField) -> SmallVec<[ast::ExprField; 1]> {
+        if field.is_placeholder {
+            self.remove(field.id).make_expr_fields()
+        } else {
+            noop_flat_map_expr_field(field, self)
+        }
+    }
+
+    fn flat_map_pat_field(&mut self, fp: ast::PatField) -> SmallVec<[ast::PatField; 1]> {
+        if fp.is_placeholder {
+            self.remove(fp.id).make_pat_fields()
+        } else {
+            noop_flat_map_pat_field(fp, self)
+        }
+    }
+
+    fn flat_map_generic_param(
+        &mut self,
+        param: ast::GenericParam,
+    ) -> SmallVec<[ast::GenericParam; 1]> {
+        if param.is_placeholder {
+            self.remove(param.id).make_generic_params()
+        } else {
+            noop_flat_map_generic_param(param, self)
+        }
+    }
+
+    fn flat_map_param(&mut self, p: ast::Param) -> SmallVec<[ast::Param; 1]> {
+        if p.is_placeholder {
+            self.remove(p.id).make_params()
+        } else {
+            noop_flat_map_param(p, self)
+        }
+    }
+
+    fn flat_map_field_def(&mut self, sf: ast::FieldDef) -> SmallVec<[ast::FieldDef; 1]> {
+        if sf.is_placeholder {
+            self.remove(sf.id).make_field_defs()
+        } else {
+            noop_flat_map_field_def(sf, self)
+        }
+    }
+
+    fn flat_map_variant(&mut self, variant: ast::Variant) -> SmallVec<[ast::Variant; 1]> {
+        if variant.is_placeholder {
+            self.remove(variant.id).make_variants()
+        } else {
+            noop_flat_map_variant(variant, self)
+        }
+    }
+
+    fn flat_map_item(&mut self, item: P<ast::Item>) -> SmallVec<[P<ast::Item>; 1]> {
+        match item.kind {
+            ast::ItemKind::MacCall(_) => self.remove(item.id).make_items(),
+            _ => noop_flat_map_item(item, self),
+        }
+    }
+
+    fn flat_map_trait_item(&mut self, item: P<ast::AssocItem>) -> SmallVec<[P<ast::AssocItem>; 1]> {
+        match item.kind {
+            ast::AssocItemKind::MacCall(_) => self.remove(item.id).make_trait_items(),
+            _ => noop_flat_map_assoc_item(item, self),
+        }
+    }
+
+    fn flat_map_impl_item(&mut self, item: P<ast::AssocItem>) -> SmallVec<[P<ast::AssocItem>; 1]> {
+        match item.kind {
+            ast::AssocItemKind::MacCall(_) => self.remove(item.id).make_impl_items(),
+            _ => noop_flat_map_assoc_item(item, self),
+        }
+    }
+
+    fn flat_map_foreign_item(
+        &mut self,
+        item: P<ast::ForeignItem>,
+    ) -> SmallVec<[P<ast::ForeignItem>; 1]> {
+        match item.kind {
+            ast::ForeignItemKind::MacCall(_) => self.remove(item.id).make_foreign_items(),
+            _ => noop_flat_map_foreign_item(item, self),
+        }
+    }
+
+    fn visit_expr(&mut self, expr: &mut P<ast::Expr>) {
+        match expr.kind {
+            ast::ExprKind::MacCall(_) => *expr = self.remove(expr.id).make_expr(),
+            _ => noop_visit_expr(expr, self),
+        }
+    }
+
+    fn filter_map_expr(&mut self, expr: P<ast::Expr>) -> Option<P<ast::Expr>> {
+        match expr.kind {
+            ast::ExprKind::MacCall(_) => self.remove(expr.id).make_opt_expr(),
+            _ => noop_filter_map_expr(expr, self),
+        }
+    }
+
+    fn flat_map_stmt(&mut self, stmt: ast::Stmt) -> SmallVec<[ast::Stmt; 1]> {
+        let (style, mut stmts) = match stmt.kind {
+            ast::StmtKind::MacCall(mac) => (mac.style, self.remove(stmt.id).make_stmts()),
+            _ => return noop_flat_map_stmt(stmt, self),
+        };
+
+        if style == ast::MacStmtStyle::Semicolon {
+            // Implement the proposal described in
+            // https://github.com/rust-lang/rust/issues/61733#issuecomment-509626449
+            //
+            // The macro invocation expands to the list of statements. If the
+            // list of statements is empty, then 'parse' the trailing semicolon
+            // on the original invocation as an empty statement. That is:
+            //
+            // `empty();` is parsed as a single `StmtKind::Empty`
+            //
+            // If the list of statements is non-empty, see if the final
+            // statement already has a trailing semicolon.
+            //
+            // If it doesn't have a semicolon, then 'parse' the trailing
+            // semicolon from the invocation as part of the final statement,
+            // using `stmt.add_trailing_semicolon()`
+            //
+            // If it does have a semicolon, then 'parse' the trailing semicolon
+            // from the invocation as a new StmtKind::Empty
+
+            // FIXME: We will need to preserve the original semicolon token and
+            // span as part of #15701
+            let empty_stmt =
+                ast::Stmt { id: ast::DUMMY_NODE_ID, kind: ast::StmtKind::Empty, span: DUMMY_SP };
+
+            if let Some(stmt) = stmts.pop() {
+                if stmt.has_trailing_semicolon() {
+                    stmts.push(stmt);
+                    stmts.push(empty_stmt);
+                } else {
+                    stmts.push(stmt.add_trailing_semicolon());
+                }
+            } else {
+                stmts.push(empty_stmt);
+            }
+        }
+
+        stmts
+    }
+
+    fn visit_pat(&mut self, pat: &mut P<ast::Pat>) {
+        match pat.kind {
+            ast::PatKind::MacCall(_) => *pat = self.remove(pat.id).make_pat(),
+            _ => noop_visit_pat(pat, self),
+        }
+    }
+
+    fn visit_ty(&mut self, ty: &mut P<ast::Ty>) {
+        match ty.kind {
+            ast::TyKind::MacCall(_) => *ty = self.remove(ty.id).make_ty(),
+            _ => noop_visit_ty(ty, self),
+        }
+    }
+
+    fn visit_crate(&mut self, krate: &mut ast::Crate) {
+        if krate.is_placeholder {
+            *krate = self.remove(krate.id).make_crate();
+        } else {
+            noop_visit_crate(krate, self)
+        }
+    }
+}
diff --git a/compiler/rustc_expand/src/proc_macro.rs b/compiler/rustc_expand/src/proc_macro.rs
new file mode 100644
index 000000000..1a2ab9d19
--- /dev/null
+++ b/compiler/rustc_expand/src/proc_macro.rs
@@ -0,0 +1,181 @@
+use crate::base::{self, *};
+use crate::proc_macro_server;
+
+use rustc_ast as ast;
+use rustc_ast::ptr::P;
+use rustc_ast::token;
+use rustc_ast::tokenstream::TokenStream;
+use rustc_data_structures::sync::Lrc;
+use rustc_errors::ErrorGuaranteed;
+use rustc_parse::parser::ForceCollect;
+use rustc_session::config::ProcMacroExecutionStrategy;
+use rustc_span::profiling::SpannedEventArgRecorder;
+use rustc_span::{Span, DUMMY_SP};
+
+struct CrossbeamMessagePipe<T> {
+    tx: crossbeam_channel::Sender<T>,
+    rx: crossbeam_channel::Receiver<T>,
+}
+
+impl<T> pm::bridge::server::MessagePipe<T> for CrossbeamMessagePipe<T> {
+    fn new() -> (Self, Self) {
+        let (tx1, rx1) = crossbeam_channel::bounded(1);
+        let (tx2, rx2) = crossbeam_channel::bounded(1);
+        (CrossbeamMessagePipe { tx: tx1, rx: rx2 }, CrossbeamMessagePipe { tx: tx2, rx: rx1 })
+    }
+
+    fn send(&mut self, value: T) {
+        self.tx.send(value).unwrap();
+    }
+
+    fn recv(&mut self) -> Option<T> {
+        self.rx.recv().ok()
+    }
+}
+
+fn exec_strategy(ecx: &ExtCtxt<'_>) -> impl pm::bridge::server::ExecutionStrategy {
+    pm::bridge::server::MaybeCrossThread::<CrossbeamMessagePipe<_>>::new(
+        ecx.sess.opts.unstable_opts.proc_macro_execution_strategy
+            == ProcMacroExecutionStrategy::CrossThread,
+    )
+}
+
+pub struct BangProcMacro {
+    pub client: pm::bridge::client::Client<pm::TokenStream, pm::TokenStream>,
+}
+
+impl base::BangProcMacro for BangProcMacro {
+    fn expand<'cx>(
+        &self,
+        ecx: &'cx mut ExtCtxt<'_>,
+        span: Span,
+        input: TokenStream,
+    ) -> Result<TokenStream, ErrorGuaranteed> {
+        let _timer =
+            ecx.sess.prof.generic_activity_with_arg_recorder("expand_proc_macro", |recorder| {
+                recorder.record_arg_with_span(ecx.expansion_descr(), span);
+            });
+
+        let proc_macro_backtrace = ecx.ecfg.proc_macro_backtrace;
+        let strategy = exec_strategy(ecx);
+        let server = proc_macro_server::Rustc::new(ecx);
+        self.client.run(&strategy, server, input, proc_macro_backtrace).map_err(|e| {
+            let mut err = ecx.struct_span_err(span, "proc macro panicked");
+            if let Some(s) = e.as_str() {
+                err.help(&format!("message: {}", s));
+            }
+            err.emit()
+        })
+    }
+}
+
+pub struct AttrProcMacro {
+    pub client: pm::bridge::client::Client<(pm::TokenStream, pm::TokenStream), pm::TokenStream>,
+}
+
+impl base::AttrProcMacro for AttrProcMacro {
+    fn expand<'cx>(
+        &self,
+        ecx: &'cx mut ExtCtxt<'_>,
+        span: Span,
+        annotation: TokenStream,
+        annotated: TokenStream,
+    ) -> Result<TokenStream, ErrorGuaranteed> {
+        let _timer =
+            ecx.sess.prof.generic_activity_with_arg_recorder("expand_proc_macro", |recorder| {
+                recorder.record_arg_with_span(ecx.expansion_descr(), span);
+            });
+
+        let proc_macro_backtrace = ecx.ecfg.proc_macro_backtrace;
+        let strategy = exec_strategy(ecx);
+        let server = proc_macro_server::Rustc::new(ecx);
+        self.client.run(&strategy, server, annotation, annotated, proc_macro_backtrace).map_err(
+            |e| {
+                let mut err = ecx.struct_span_err(span, "custom attribute panicked");
+                if let Some(s) = e.as_str() {
+                    err.help(&format!("message: {}", s));
+                }
+                err.emit()
+            },
+        )
+    }
+}
+
+pub struct DeriveProcMacro {
+    pub client: pm::bridge::client::Client<pm::TokenStream, pm::TokenStream>,
+}
+
+impl MultiItemModifier for DeriveProcMacro {
+    fn expand(
+        &self,
+        ecx: &mut ExtCtxt<'_>,
+        span: Span,
+        _meta_item: &ast::MetaItem,
+        item: Annotatable,
+    ) -> ExpandResult<Vec<Annotatable>, Annotatable> {
+        // We need special handling for statement items
+        // (e.g. `fn foo() { #[derive(Debug)] struct Bar; }`)
+        let is_stmt = matches!(item, Annotatable::Stmt(..));
+        let hack = crate::base::ann_pretty_printing_compatibility_hack(&item, &ecx.sess.parse_sess);
+        let input = if hack {
+            let nt = match item {
+                Annotatable::Item(item) => token::NtItem(item),
+                Annotatable::Stmt(stmt) => token::NtStmt(stmt),
+                _ => unreachable!(),
+            };
+            TokenStream::token_alone(token::Interpolated(Lrc::new(nt)), DUMMY_SP)
+        } else {
+            item.to_tokens()
+        };
+
+        let stream = {
+            let _timer =
+                ecx.sess.prof.generic_activity_with_arg_recorder("expand_proc_macro", |recorder| {
+                    recorder.record_arg_with_span(ecx.expansion_descr(), span);
+                });
+            let proc_macro_backtrace = ecx.ecfg.proc_macro_backtrace;
+            let strategy = exec_strategy(ecx);
+            let server = proc_macro_server::Rustc::new(ecx);
+            match self.client.run(&strategy, server, input, proc_macro_backtrace) {
+                Ok(stream) => stream,
+                Err(e) => {
+                    let mut err = ecx.struct_span_err(span, "proc-macro derive panicked");
+                    if let Some(s) = e.as_str() {
+                        err.help(&format!("message: {}", s));
+                    }
+                    err.emit();
+                    return ExpandResult::Ready(vec![]);
+                }
+            }
+        };
+
+        let error_count_before = ecx.sess.parse_sess.span_diagnostic.err_count();
+        let mut parser =
+            rustc_parse::stream_to_parser(&ecx.sess.parse_sess, stream, Some("proc-macro derive"));
+        let mut items = vec![];
+
+        loop {
+            match parser.parse_item(ForceCollect::No) {
+                Ok(None) => break,
+                Ok(Some(item)) => {
+                    if is_stmt {
+                        items.push(Annotatable::Stmt(P(ecx.stmt_item(span, item))));
+                    } else {
+                        items.push(Annotatable::Item(item));
+                    }
+                }
+                Err(mut err) => {
+                    err.emit();
+                    break;
+                }
+            }
+        }
+
+        // fail if there have been errors emitted
+        if ecx.sess.parse_sess.span_diagnostic.err_count() > error_count_before {
+            ecx.struct_span_err(span, "proc-macro derive produced unparseable tokens").emit();
+        }
+
+        ExpandResult::Ready(items)
+    }
+}
diff --git a/compiler/rustc_expand/src/proc_macro_server.rs b/compiler/rustc_expand/src/proc_macro_server.rs
new file mode 100644
index 000000000..7d9a4aed0
--- /dev/null
+++ b/compiler/rustc_expand/src/proc_macro_server.rs
@@ -0,0 +1,766 @@
+use crate::base::ExtCtxt;
+
+use rustc_ast as ast;
+use rustc_ast::token;
+use rustc_ast::tokenstream::{self, Spacing::*, TokenStream};
+use rustc_ast_pretty::pprust;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::sync::Lrc;
+use rustc_errors::{Diagnostic, MultiSpan, PResult};
+use rustc_parse::lexer::nfc_normalize;
+use rustc_parse::parse_stream_from_source_str;
+use rustc_session::parse::ParseSess;
+use rustc_span::def_id::CrateNum;
+use rustc_span::symbol::{self, sym, Symbol};
+use rustc_span::{BytePos, FileName, Pos, SourceFile, Span};
+
+use pm::bridge::{
+    server, DelimSpan, ExpnGlobals, Group, Ident, LitKind, Literal, Punct, TokenTree,
+};
+use pm::{Delimiter, Level, LineColumn};
+use std::ops::Bound;
+
+trait FromInternal<T> {
+    fn from_internal(x: T) -> Self;
+}
+
+trait ToInternal<T> {
+    fn to_internal(self) -> T;
+}
+
+impl FromInternal<token::Delimiter> for Delimiter {
+    fn from_internal(delim: token::Delimiter) -> Delimiter {
+        match delim {
+            token::Delimiter::Parenthesis => Delimiter::Parenthesis,
+            token::Delimiter::Brace => Delimiter::Brace,
+            token::Delimiter::Bracket => Delimiter::Bracket,
+            token::Delimiter::Invisible => Delimiter::None,
+        }
+    }
+}
+
+impl ToInternal<token::Delimiter> for Delimiter {
+    fn to_internal(self) -> token::Delimiter {
+        match self {
+            Delimiter::Parenthesis => token::Delimiter::Parenthesis,
+            Delimiter::Brace => token::Delimiter::Brace,
+            Delimiter::Bracket => token::Delimiter::Bracket,
+            Delimiter::None => token::Delimiter::Invisible,
+        }
+    }
+}
+
+impl FromInternal<token::LitKind> for LitKind {
+    fn from_internal(kind: token::LitKind) -> Self {
+        match kind {
+            token::Byte => LitKind::Byte,
+            token::Char => LitKind::Char,
+            token::Integer => LitKind::Integer,
+            token::Float => LitKind::Float,
+            token::Str => LitKind::Str,
+            token::StrRaw(n) => LitKind::StrRaw(n),
+            token::ByteStr => LitKind::ByteStr,
+            token::ByteStrRaw(n) => LitKind::ByteStrRaw(n),
+            token::Err => LitKind::Err,
+            token::Bool => unreachable!(),
+        }
+    }
+}
+
+impl ToInternal<token::LitKind> for LitKind {
+    fn to_internal(self) -> token::LitKind {
+        match self {
+            LitKind::Byte => token::Byte,
+            LitKind::Char => token::Char,
+            LitKind::Integer => token::Integer,
+            LitKind::Float => token::Float,
+            LitKind::Str => token::Str,
+            LitKind::StrRaw(n) => token::StrRaw(n),
+            LitKind::ByteStr => token::ByteStr,
+            LitKind::ByteStrRaw(n) => token::ByteStrRaw(n),
+            LitKind::Err => token::Err,
+        }
+    }
+}
+
+impl FromInternal<(TokenStream, &mut Rustc<'_, '_>)> for Vec<TokenTree<TokenStream, Span, Symbol>> {
+    fn from_internal((stream, rustc): (TokenStream, &mut Rustc<'_, '_>)) -> Self {
+        use rustc_ast::token::*;
+
+        // Estimate the capacity as `stream.len()` rounded up to the next power
+        // of two to limit the number of required reallocations.
+        let mut trees = Vec::with_capacity(stream.len().next_power_of_two());
+        let mut cursor = stream.into_trees();
+
+        while let Some(tree) = cursor.next() {
+            let (Token { kind, span }, joint) = match tree {
+                tokenstream::TokenTree::Delimited(span, delim, tts) => {
+                    let delimiter = pm::Delimiter::from_internal(delim);
+                    trees.push(TokenTree::Group(Group {
+                        delimiter,
+                        stream: Some(tts),
+                        span: DelimSpan {
+                            open: span.open,
+                            close: span.close,
+                            entire: span.entire(),
+                        },
+                    }));
+                    continue;
+                }
+                tokenstream::TokenTree::Token(token, spacing) => (token, spacing == Joint),
+            };
+
+            let mut op = |s: &str| {
+                assert!(s.is_ascii());
+                trees.extend(s.as_bytes().iter().enumerate().map(|(idx, &ch)| {
+                    TokenTree::Punct(Punct { ch, joint: joint || idx != s.len() - 1, span })
+                }));
+            };
+
+            match kind {
+                Eq => op("="),
+                Lt => op("<"),
+                Le => op("<="),
+                EqEq => op("=="),
+                Ne => op("!="),
+                Ge => op(">="),
+                Gt => op(">"),
+                AndAnd => op("&&"),
+                OrOr => op("||"),
+                Not => op("!"),
+                Tilde => op("~"),
+                BinOp(Plus) => op("+"),
+                BinOp(Minus) => op("-"),
+                BinOp(Star) => op("*"),
+                BinOp(Slash) => op("/"),
+                BinOp(Percent) => op("%"),
+                BinOp(Caret) => op("^"),
+                BinOp(And) => op("&"),
+                BinOp(Or) => op("|"),
+                BinOp(Shl) => op("<<"),
+                BinOp(Shr) => op(">>"),
+                BinOpEq(Plus) => op("+="),
+                BinOpEq(Minus) => op("-="),
+                BinOpEq(Star) => op("*="),
+                BinOpEq(Slash) => op("/="),
+                BinOpEq(Percent) => op("%="),
+                BinOpEq(Caret) => op("^="),
+                BinOpEq(And) => op("&="),
+                BinOpEq(Or) => op("|="),
+                BinOpEq(Shl) => op("<<="),
+                BinOpEq(Shr) => op(">>="),
+                At => op("@"),
+                Dot => op("."),
+                DotDot => op(".."),
+                DotDotDot => op("..."),
+                DotDotEq => op("..="),
+                Comma => op(","),
+                Semi => op(";"),
+                Colon => op(":"),
+                ModSep => op("::"),
+                RArrow => op("->"),
+                LArrow => op("<-"),
+                FatArrow => op("=>"),
+                Pound => op("#"),
+                Dollar => op("$"),
+                Question => op("?"),
+                SingleQuote => op("'"),
+
+                Ident(sym, is_raw) => trees.push(TokenTree::Ident(Ident { sym, is_raw, span })),
+                Lifetime(name) => {
+                    let ident = symbol::Ident::new(name, span).without_first_quote();
+                    trees.extend([
+                        TokenTree::Punct(Punct { ch: b'\'', joint: true, span }),
+                        TokenTree::Ident(Ident { sym: ident.name, is_raw: false, span }),
+                    ]);
+                }
+                Literal(token::Lit { kind, symbol, suffix }) => {
+                    trees.push(TokenTree::Literal(self::Literal {
+                        kind: FromInternal::from_internal(kind),
+                        symbol,
+                        suffix,
+                        span,
+                    }));
+                }
+                DocComment(_, attr_style, data) => {
+                    let mut escaped = String::new();
+                    for ch in data.as_str().chars() {
+                        escaped.extend(ch.escape_debug());
+                    }
+                    let stream = [
+                        Ident(sym::doc, false),
+                        Eq,
+                        TokenKind::lit(token::Str, Symbol::intern(&escaped), None),
+                    ]
+                    .into_iter()
+                    .map(|kind| tokenstream::TokenTree::token_alone(kind, span))
+                    .collect();
+                    trees.push(TokenTree::Punct(Punct { ch: b'#', joint: false, span }));
+                    if attr_style == ast::AttrStyle::Inner {
+                        trees.push(TokenTree::Punct(Punct { ch: b'!', joint: false, span }));
+                    }
+                    trees.push(TokenTree::Group(Group {
+                        delimiter: pm::Delimiter::Bracket,
+                        stream: Some(stream),
+                        span: DelimSpan::from_single(span),
+                    }));
+                }
+
+                Interpolated(nt) if let NtIdent(ident, is_raw) = *nt => {
+                    trees.push(TokenTree::Ident(Ident { sym: ident.name, is_raw, span: ident.span }))
+                }
+
+                Interpolated(nt) => {
+                    let stream = TokenStream::from_nonterminal_ast(&nt);
+                    // A hack used to pass AST fragments to attribute and derive
+                    // macros as a single nonterminal token instead of a token
+                    // stream.  Such token needs to be "unwrapped" and not
+                    // represented as a delimited group.
+                    // FIXME: It needs to be removed, but there are some
+                    // compatibility issues (see #73345).
+                    if crate::base::nt_pretty_printing_compatibility_hack(&nt, rustc.sess()) {
+                        trees.extend(Self::from_internal((stream, rustc)));
+                    } else {
+                        trees.push(TokenTree::Group(Group {
+                            delimiter: pm::Delimiter::None,
+                            stream: Some(stream),
+                            span: DelimSpan::from_single(span),
+                        }))
+                    }
+                }
+
+                OpenDelim(..) | CloseDelim(..) => unreachable!(),
+                Eof => unreachable!(),
+            }
+        }
+        trees
+    }
+}
+
+impl ToInternal<TokenStream> for (TokenTree<TokenStream, Span, Symbol>, &mut Rustc<'_, '_>) {
+    fn to_internal(self) -> TokenStream {
+        use rustc_ast::token::*;
+
+        let (tree, rustc) = self;
+        let (ch, joint, span) = match tree {
+            TokenTree::Punct(Punct { ch, joint, span }) => (ch, joint, span),
+            TokenTree::Group(Group { delimiter, stream, span: DelimSpan { open, close, .. } }) => {
+                return tokenstream::TokenStream::delimited(
+                    tokenstream::DelimSpan { open, close },
+                    delimiter.to_internal(),
+                    stream.unwrap_or_default(),
+                );
+            }
+            TokenTree::Ident(self::Ident { sym, is_raw, span }) => {
+                rustc.sess().symbol_gallery.insert(sym, span);
+                return tokenstream::TokenStream::token_alone(Ident(sym, is_raw), span);
+            }
+            TokenTree::Literal(self::Literal {
+                kind: self::LitKind::Integer,
+                symbol,
+                suffix,
+                span,
+            }) if symbol.as_str().starts_with('-') => {
+                let minus = BinOp(BinOpToken::Minus);
+                let symbol = Symbol::intern(&symbol.as_str()[1..]);
+                let integer = TokenKind::lit(token::Integer, symbol, suffix);
+                let a = tokenstream::TokenTree::token_alone(minus, span);
+                let b = tokenstream::TokenTree::token_alone(integer, span);
+                return [a, b].into_iter().collect();
+            }
+            TokenTree::Literal(self::Literal {
+                kind: self::LitKind::Float,
+                symbol,
+                suffix,
+                span,
+            }) if symbol.as_str().starts_with('-') => {
+                let minus = BinOp(BinOpToken::Minus);
+                let symbol = Symbol::intern(&symbol.as_str()[1..]);
+                let float = TokenKind::lit(token::Float, symbol, suffix);
+                let a = tokenstream::TokenTree::token_alone(minus, span);
+                let b = tokenstream::TokenTree::token_alone(float, span);
+                return [a, b].into_iter().collect();
+            }
+            TokenTree::Literal(self::Literal { kind, symbol, suffix, span }) => {
+                return tokenstream::TokenStream::token_alone(
+                    TokenKind::lit(kind.to_internal(), symbol, suffix),
+                    span,
+                );
+            }
+        };
+
+        let kind = match ch {
+            b'=' => Eq,
+            b'<' => Lt,
+            b'>' => Gt,
+            b'!' => Not,
+            b'~' => Tilde,
+            b'+' => BinOp(Plus),
+            b'-' => BinOp(Minus),
+            b'*' => BinOp(Star),
+            b'/' => BinOp(Slash),
+            b'%' => BinOp(Percent),
+            b'^' => BinOp(Caret),
+            b'&' => BinOp(And),
+            b'|' => BinOp(Or),
+            b'@' => At,
+            b'.' => Dot,
+            b',' => Comma,
+            b';' => Semi,
+            b':' => Colon,
+            b'#' => Pound,
+            b'$' => Dollar,
+            b'?' => Question,
+            b'\'' => SingleQuote,
+            _ => unreachable!(),
+        };
+
+        if joint {
+            tokenstream::TokenStream::token_joint(kind, span)
+        } else {
+            tokenstream::TokenStream::token_alone(kind, span)
+        }
+    }
+}
+
+impl ToInternal<rustc_errors::Level> for Level {
+    fn to_internal(self) -> rustc_errors::Level {
+        match self {
+            Level::Error => rustc_errors::Level::Error { lint: false },
+            Level::Warning => rustc_errors::Level::Warning(None),
+            Level::Note => rustc_errors::Level::Note,
+            Level::Help => rustc_errors::Level::Help,
+            _ => unreachable!("unknown proc_macro::Level variant: {:?}", self),
+        }
+    }
+}
+
+pub struct FreeFunctions;
+
+pub(crate) struct Rustc<'a, 'b> {
+    ecx: &'a mut ExtCtxt<'b>,
+    def_site: Span,
+    call_site: Span,
+    mixed_site: Span,
+    krate: CrateNum,
+    rebased_spans: FxHashMap<usize, Span>,
+}
+
+impl<'a, 'b> Rustc<'a, 'b> {
+    pub fn new(ecx: &'a mut ExtCtxt<'b>) -> Self {
+        let expn_data = ecx.current_expansion.id.expn_data();
+        Rustc {
+            def_site: ecx.with_def_site_ctxt(expn_data.def_site),
+            call_site: ecx.with_call_site_ctxt(expn_data.call_site),
+            mixed_site: ecx.with_mixed_site_ctxt(expn_data.call_site),
+            krate: expn_data.macro_def_id.unwrap().krate,
+            rebased_spans: FxHashMap::default(),
+            ecx,
+        }
+    }
+
+    fn sess(&self) -> &ParseSess {
+        self.ecx.parse_sess()
+    }
+}
+
+impl server::Types for Rustc<'_, '_> {
+    type FreeFunctions = FreeFunctions;
+    type TokenStream = TokenStream;
+    type SourceFile = Lrc<SourceFile>;
+    type MultiSpan = Vec<Span>;
+    type Diagnostic = Diagnostic;
+    type Span = Span;
+    type Symbol = Symbol;
+}
+
+impl server::FreeFunctions for Rustc<'_, '_> {
+    fn track_env_var(&mut self, var: &str, value: Option<&str>) {
+        self.sess()
+            .env_depinfo
+            .borrow_mut()
+            .insert((Symbol::intern(var), value.map(Symbol::intern)));
+    }
+
+    fn track_path(&mut self, path: &str) {
+        self.sess().file_depinfo.borrow_mut().insert(Symbol::intern(path));
+    }
+
+    fn literal_from_str(&mut self, s: &str) -> Result<Literal<Self::Span, Self::Symbol>, ()> {
+        let name = FileName::proc_macro_source_code(s);
+        let mut parser = rustc_parse::new_parser_from_source_str(self.sess(), name, s.to_owned());
+
+        let first_span = parser.token.span.data();
+        let minus_present = parser.eat(&token::BinOp(token::Minus));
+
+        let lit_span = parser.token.span.data();
+        let token::Literal(mut lit) = parser.token.kind else {
+            return Err(());
+        };
+
+        // Check no comment or whitespace surrounding the (possibly negative)
+        // literal, or more tokens after it.
+        if (lit_span.hi.0 - first_span.lo.0) as usize != s.len() {
+            return Err(());
+        }
+
+        if minus_present {
+            // If minus is present, check no comment or whitespace in between it
+            // and the literal token.
+            if first_span.hi.0 != lit_span.lo.0 {
+                return Err(());
+            }
+
+            // Check literal is a kind we allow to be negated in a proc macro token.
+            match lit.kind {
+                token::LitKind::Bool
+                | token::LitKind::Byte
+                | token::LitKind::Char
+                | token::LitKind::Str
+                | token::LitKind::StrRaw(_)
+                | token::LitKind::ByteStr
+                | token::LitKind::ByteStrRaw(_)
+                | token::LitKind::Err => return Err(()),
+                token::LitKind::Integer | token::LitKind::Float => {}
+            }
+
+            // Synthesize a new symbol that includes the minus sign.
+            let symbol = Symbol::intern(&s[..1 + lit.symbol.as_str().len()]);
+            lit = token::Lit::new(lit.kind, symbol, lit.suffix);
+        }
+        let token::Lit { kind, symbol, suffix } = lit;
+        Ok(Literal {
+            kind: FromInternal::from_internal(kind),
+            symbol,
+            suffix,
+            span: self.call_site,
+        })
+    }
+}
+
+impl server::TokenStream for Rustc<'_, '_> {
+    fn is_empty(&mut self, stream: &Self::TokenStream) -> bool {
+        stream.is_empty()
+    }
+
+    fn from_str(&mut self, src: &str) -> Self::TokenStream {
+        parse_stream_from_source_str(
+            FileName::proc_macro_source_code(src),
+            src.to_string(),
+            self.sess(),
+            Some(self.call_site),
+        )
+    }
+
+    fn to_string(&mut self, stream: &Self::TokenStream) -> String {
+        pprust::tts_to_string(stream)
+    }
+
+    fn expand_expr(&mut self, stream: &Self::TokenStream) -> Result<Self::TokenStream, ()> {
+        // Parse the expression from our tokenstream.
+        let expr: PResult<'_, _> = try {
+            let mut p = rustc_parse::stream_to_parser(
+                self.sess(),
+                stream.clone(),
+                Some("proc_macro expand expr"),
+            );
+            let expr = p.parse_expr()?;
+            if p.token != token::Eof {
+                p.unexpected()?;
+            }
+            expr
+        };
+        let expr = expr.map_err(|mut err| {
+            err.emit();
+        })?;
+
+        // Perform eager expansion on the expression.
+        let expr = self
+            .ecx
+            .expander()
+            .fully_expand_fragment(crate::expand::AstFragment::Expr(expr))
+            .make_expr();
+
+        // NOTE: For now, limit `expand_expr` to exclusively expand to literals.
+        // This may be relaxed in the future.
+        // We don't use `TokenStream::from_ast` as the tokenstream currently cannot
+        // be recovered in the general case.
+        match &expr.kind {
+            ast::ExprKind::Lit(l) if l.token.kind == token::Bool => Ok(
+                tokenstream::TokenStream::token_alone(token::Ident(l.token.symbol, false), l.span),
+            ),
+            ast::ExprKind::Lit(l) => {
+                Ok(tokenstream::TokenStream::token_alone(token::Literal(l.token), l.span))
+            }
+            ast::ExprKind::Unary(ast::UnOp::Neg, e) => match &e.kind {
+                ast::ExprKind::Lit(l) => match l.token {
+                    token::Lit { kind: token::Integer | token::Float, .. } => {
+                        Ok(Self::TokenStream::from_iter([
+                            // FIXME: The span of the `-` token is lost when
+                            // parsing, so we cannot faithfully recover it here.
+                            tokenstream::TokenTree::token_alone(token::BinOp(token::Minus), e.span),
+                            tokenstream::TokenTree::token_alone(token::Literal(l.token), l.span),
+                        ]))
+                    }
+                    _ => Err(()),
+                },
+                _ => Err(()),
+            },
+            _ => Err(()),
+        }
+    }
+
+    fn from_token_tree(
+        &mut self,
+        tree: TokenTree<Self::TokenStream, Self::Span, Self::Symbol>,
+    ) -> Self::TokenStream {
+        (tree, &mut *self).to_internal()
+    }
+
+    fn concat_trees(
+        &mut self,
+        base: Option<Self::TokenStream>,
+        trees: Vec<TokenTree<Self::TokenStream, Self::Span, Self::Symbol>>,
+    ) -> Self::TokenStream {
+        let mut builder = tokenstream::TokenStreamBuilder::new();
+        if let Some(base) = base {
+            builder.push(base);
+        }
+        for tree in trees {
+            builder.push((tree, &mut *self).to_internal());
+        }
+        builder.build()
+    }
+
+    fn concat_streams(
+        &mut self,
+        base: Option<Self::TokenStream>,
+        streams: Vec<Self::TokenStream>,
+    ) -> Self::TokenStream {
+        let mut builder = tokenstream::TokenStreamBuilder::new();
+        if let Some(base) = base {
+            builder.push(base);
+        }
+        for stream in streams {
+            builder.push(stream);
+        }
+        builder.build()
+    }
+
+    fn into_trees(
+        &mut self,
+        stream: Self::TokenStream,
+    ) -> Vec<TokenTree<Self::TokenStream, Self::Span, Self::Symbol>> {
+        FromInternal::from_internal((stream, self))
+    }
+}
+
+impl server::SourceFile for Rustc<'_, '_> {
+    fn eq(&mut self, file1: &Self::SourceFile, file2: &Self::SourceFile) -> bool {
+        Lrc::ptr_eq(file1, file2)
+    }
+
+    fn path(&mut self, file: &Self::SourceFile) -> String {
+        match file.name {
+            FileName::Real(ref name) => name
+                .local_path()
+                .expect("attempting to get a file path in an imported file in `proc_macro::SourceFile::path`")
+                .to_str()
+                .expect("non-UTF8 file path in `proc_macro::SourceFile::path`")
+                .to_string(),
+            _ => file.name.prefer_local().to_string(),
+        }
+    }
+
+    fn is_real(&mut self, file: &Self::SourceFile) -> bool {
+        file.is_real_file()
+    }
+}
+
+impl server::MultiSpan for Rustc<'_, '_> {
+    fn new(&mut self) -> Self::MultiSpan {
+        vec![]
+    }
+
+    fn push(&mut self, spans: &mut Self::MultiSpan, span: Self::Span) {
+        spans.push(span)
+    }
+}
+
+impl server::Diagnostic for Rustc<'_, '_> {
+    fn new(&mut self, level: Level, msg: &str, spans: Self::MultiSpan) -> Self::Diagnostic {
+        let mut diag = Diagnostic::new(level.to_internal(), msg);
+        diag.set_span(MultiSpan::from_spans(spans));
+        diag
+    }
+
+    fn sub(
+        &mut self,
+        diag: &mut Self::Diagnostic,
+        level: Level,
+        msg: &str,
+        spans: Self::MultiSpan,
+    ) {
+        diag.sub(level.to_internal(), msg, MultiSpan::from_spans(spans), None);
+    }
+
+    fn emit(&mut self, mut diag: Self::Diagnostic) {
+        self.sess().span_diagnostic.emit_diagnostic(&mut diag);
+    }
+}
+
+impl server::Span for Rustc<'_, '_> {
+    fn debug(&mut self, span: Self::Span) -> String {
+        if self.ecx.ecfg.span_debug {
+            format!("{:?}", span)
+        } else {
+            format!("{:?} bytes({}..{})", span.ctxt(), span.lo().0, span.hi().0)
+        }
+    }
+
+    fn source_file(&mut self, span: Self::Span) -> Self::SourceFile {
+        self.sess().source_map().lookup_char_pos(span.lo()).file
+    }
+
+    fn parent(&mut self, span: Self::Span) -> Option<Self::Span> {
+        span.parent_callsite()
+    }
+
+    fn source(&mut self, span: Self::Span) -> Self::Span {
+        span.source_callsite()
+    }
+
+    fn start(&mut self, span: Self::Span) -> LineColumn {
+        let loc = self.sess().source_map().lookup_char_pos(span.lo());
+        LineColumn { line: loc.line, column: loc.col.to_usize() }
+    }
+
+    fn end(&mut self, span: Self::Span) -> LineColumn {
+        let loc = self.sess().source_map().lookup_char_pos(span.hi());
+        LineColumn { line: loc.line, column: loc.col.to_usize() }
+    }
+
+    fn before(&mut self, span: Self::Span) -> Self::Span {
+        span.shrink_to_lo()
+    }
+
+    fn after(&mut self, span: Self::Span) -> Self::Span {
+        span.shrink_to_hi()
+    }
+
+    fn join(&mut self, first: Self::Span, second: Self::Span) -> Option<Self::Span> {
+        let self_loc = self.sess().source_map().lookup_char_pos(first.lo());
+        let other_loc = self.sess().source_map().lookup_char_pos(second.lo());
+
+        if self_loc.file.name != other_loc.file.name {
+            return None;
+        }
+
+        Some(first.to(second))
+    }
+
+    fn subspan(
+        &mut self,
+        span: Self::Span,
+        start: Bound<usize>,
+        end: Bound<usize>,
+    ) -> Option<Self::Span> {
+        let length = span.hi().to_usize() - span.lo().to_usize();
+
+        let start = match start {
+            Bound::Included(lo) => lo,
+            Bound::Excluded(lo) => lo.checked_add(1)?,
+            Bound::Unbounded => 0,
+        };
+
+        let end = match end {
+            Bound::Included(hi) => hi.checked_add(1)?,
+            Bound::Excluded(hi) => hi,
+            Bound::Unbounded => length,
+        };
+
+        // Bounds check the values, preventing addition overflow and OOB spans.
+        if start > u32::MAX as usize
+            || end > u32::MAX as usize
+            || (u32::MAX - start as u32) < span.lo().to_u32()
+            || (u32::MAX - end as u32) < span.lo().to_u32()
+            || start >= end
+            || end > length
+        {
+            return None;
+        }
+
+        let new_lo = span.lo() + BytePos::from_usize(start);
+        let new_hi = span.lo() + BytePos::from_usize(end);
+        Some(span.with_lo(new_lo).with_hi(new_hi))
+    }
+
+    fn resolved_at(&mut self, span: Self::Span, at: Self::Span) -> Self::Span {
+        span.with_ctxt(at.ctxt())
+    }
+
+    fn source_text(&mut self, span: Self::Span) -> Option<String> {
+        self.sess().source_map().span_to_snippet(span).ok()
+    }
+    /// Saves the provided span into the metadata of
+    /// *the crate we are currently compiling*, which must
+    /// be a proc-macro crate. This id can be passed to
+    /// `recover_proc_macro_span` when our current crate
+    /// is *run* as a proc-macro.
+    ///
+    /// Let's suppose that we have two crates - `my_client`
+    /// and `my_proc_macro`. The `my_proc_macro` crate
+    /// contains a procedural macro `my_macro`, which
+    /// is implemented as: `quote! { "hello" }`
+    ///
+    /// When we *compile* `my_proc_macro`, we will execute
+    /// the `quote` proc-macro. This will save the span of
+    /// "hello" into the metadata of `my_proc_macro`. As a result,
+    /// the body of `my_proc_macro` (after expansion) will end
+    /// up containing a call that looks like this:
+    /// `proc_macro::Ident::new("hello", proc_macro::Span::recover_proc_macro_span(0))`
+    ///
+    /// where `0` is the id returned by this function.
+    /// When `my_proc_macro` *executes* (during the compilation of `my_client`),
+    /// the call to `recover_proc_macro_span` will load the corresponding
+    /// span from the metadata of `my_proc_macro` (which we have access to,
+    /// since we've loaded `my_proc_macro` from disk in order to execute it).
+    /// In this way, we have obtained a span pointing into `my_proc_macro`
+    fn save_span(&mut self, span: Self::Span) -> usize {
+        self.sess().save_proc_macro_span(span)
+    }
+
+    fn recover_proc_macro_span(&mut self, id: usize) -> Self::Span {
+        let (resolver, krate, def_site) = (&*self.ecx.resolver, self.krate, self.def_site);
+        *self.rebased_spans.entry(id).or_insert_with(|| {
+            // FIXME: `SyntaxContext` for spans from proc macro crates is lost during encoding,
+            // replace it with a def-site context until we are encoding it properly.
+            resolver.get_proc_macro_quoted_span(krate, id).with_ctxt(def_site.ctxt())
+        })
+    }
+}
+
+impl server::Symbol for Rustc<'_, '_> {
+    fn normalize_and_validate_ident(&mut self, string: &str) -> Result<Self::Symbol, ()> {
+        let sym = nfc_normalize(string);
+        if rustc_lexer::is_ident(sym.as_str()) { Ok(sym) } else { Err(()) }
+    }
+}
+
+impl server::Server for Rustc<'_, '_> {
+    fn globals(&mut self) -> ExpnGlobals<Self::Span> {
+        ExpnGlobals {
+            def_site: self.def_site,
+            call_site: self.call_site,
+            mixed_site: self.mixed_site,
+        }
+    }
+
+    fn intern_symbol(string: &str) -> Self::Symbol {
+        Symbol::intern(string)
+    }
+
+    fn with_symbol_string(symbol: &Self::Symbol, f: impl FnOnce(&str)) {
+        f(&symbol.as_str())
+    }
+}
diff --git a/compiler/rustc_expand/src/tests.rs b/compiler/rustc_expand/src/tests.rs
new file mode 100644
index 000000000..e44f06081
--- /dev/null
+++ b/compiler/rustc_expand/src/tests.rs
@@ -0,0 +1,1016 @@
+use rustc_ast as ast;
+use rustc_ast::tokenstream::TokenStream;
+use rustc_parse::{new_parser_from_source_str, parser::Parser, source_file_to_stream};
+use rustc_session::parse::ParseSess;
+use rustc_span::create_default_session_if_not_set_then;
+use rustc_span::source_map::{FilePathMapping, SourceMap};
+use rustc_span::{BytePos, Span};
+
+use rustc_data_structures::sync::Lrc;
+use rustc_errors::emitter::EmitterWriter;
+use rustc_errors::{Handler, MultiSpan, PResult};
+
+use std::io;
+use std::io::prelude::*;
+use std::iter::Peekable;
+use std::path::{Path, PathBuf};
+use std::str;
+use std::sync::{Arc, Mutex};
+
+/// Map string to parser (via tts).
+fn string_to_parser(ps: &ParseSess, source_str: String) -> Parser<'_> {
+    new_parser_from_source_str(ps, PathBuf::from("bogofile").into(), source_str)
+}
+
+pub(crate) fn with_error_checking_parse<'a, T, F>(s: String, ps: &'a ParseSess, f: F) -> T
+where
+    F: FnOnce(&mut Parser<'a>) -> PResult<'a, T>,
+{
+    let mut p = string_to_parser(&ps, s);
+    let x = f(&mut p).unwrap();
+    p.sess.span_diagnostic.abort_if_errors();
+    x
+}
+
+/// Maps a string to tts, using a made-up filename.
+pub(crate) fn string_to_stream(source_str: String) -> TokenStream {
+    let ps = ParseSess::new(FilePathMapping::empty());
+    source_file_to_stream(
+        &ps,
+        ps.source_map().new_source_file(PathBuf::from("bogofile").into(), source_str),
+        None,
+    )
+    .0
+}
+
+/// Parses a string, returns a crate.
+pub(crate) fn string_to_crate(source_str: String) -> ast::Crate {
+    let ps = ParseSess::new(FilePathMapping::empty());
+    with_error_checking_parse(source_str, &ps, |p| p.parse_crate_mod())
+}
+
+/// Does the given string match the pattern? whitespace in the first string
+/// may be deleted or replaced with other whitespace to match the pattern.
+/// This function is relatively Unicode-ignorant; fortunately, the careful design
+/// of UTF-8 mitigates this ignorance. It doesn't do NKF-normalization(?).
+pub(crate) fn matches_codepattern(a: &str, b: &str) -> bool {
+    let mut a_iter = a.chars().peekable();
+    let mut b_iter = b.chars().peekable();
+
+    loop {
+        let (a, b) = match (a_iter.peek(), b_iter.peek()) {
+            (None, None) => return true,
+            (None, _) => return false,
+            (Some(&a), None) => {
+                if rustc_lexer::is_whitespace(a) {
+                    break; // Trailing whitespace check is out of loop for borrowck.
+                } else {
+                    return false;
+                }
+            }
+            (Some(&a), Some(&b)) => (a, b),
+        };
+
+        if rustc_lexer::is_whitespace(a) && rustc_lexer::is_whitespace(b) {
+            // Skip whitespace for `a` and `b`.
+            scan_for_non_ws_or_end(&mut a_iter);
+            scan_for_non_ws_or_end(&mut b_iter);
+        } else if rustc_lexer::is_whitespace(a) {
+            // Skip whitespace for `a`.
+            scan_for_non_ws_or_end(&mut a_iter);
+        } else if a == b {
+            a_iter.next();
+            b_iter.next();
+        } else {
+            return false;
+        }
+    }
+
+    // Check if a has *only* trailing whitespace.
+    a_iter.all(rustc_lexer::is_whitespace)
+}
+
+/// Advances the given peekable `Iterator` until it reaches a non-whitespace character.
+fn scan_for_non_ws_or_end<I: Iterator<Item = char>>(iter: &mut Peekable<I>) {
+    while iter.peek().copied().map(rustc_lexer::is_whitespace) == Some(true) {
+        iter.next();
+    }
+}
+
+/// Identifies a position in the text by the n'th occurrence of a string.
+struct Position {
+    string: &'static str,
+    count: usize,
+}
+
+struct SpanLabel {
+    start: Position,
+    end: Position,
+    label: &'static str,
+}
+
+pub(crate) struct Shared<T: Write> {
+    pub data: Arc<Mutex<T>>,
+}
+
+impl<T: Write> Write for Shared<T> {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.data.lock().unwrap().write(buf)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.data.lock().unwrap().flush()
+    }
+}
+
+fn test_harness(file_text: &str, span_labels: Vec<SpanLabel>, expected_output: &str) {
+    create_default_session_if_not_set_then(|_| {
+        let output = Arc::new(Mutex::new(Vec::new()));
+
+        let fallback_bundle =
+            rustc_errors::fallback_fluent_bundle(rustc_errors::DEFAULT_LOCALE_RESOURCES, false);
+        let source_map = Lrc::new(SourceMap::new(FilePathMapping::empty()));
+        source_map.new_source_file(Path::new("test.rs").to_owned().into(), file_text.to_owned());
+
+        let primary_span = make_span(&file_text, &span_labels[0].start, &span_labels[0].end);
+        let mut msp = MultiSpan::from_span(primary_span);
+        for span_label in span_labels {
+            let span = make_span(&file_text, &span_label.start, &span_label.end);
+            msp.push_span_label(span, span_label.label);
+            println!("span: {:?} label: {:?}", span, span_label.label);
+            println!("text: {:?}", source_map.span_to_snippet(span));
+        }
+
+        let emitter = EmitterWriter::new(
+            Box::new(Shared { data: output.clone() }),
+            Some(source_map.clone()),
+            None,
+            fallback_bundle,
+            false,
+            false,
+            false,
+            None,
+            false,
+        );
+        let handler = Handler::with_emitter(true, None, Box::new(emitter));
+        handler.span_err(msp, "foo");
+
+        assert!(
+            expected_output.chars().next() == Some('\n'),
+            "expected output should begin with newline"
+        );
+        let expected_output = &expected_output[1..];
+
+        let bytes = output.lock().unwrap();
+        let actual_output = str::from_utf8(&bytes).unwrap();
+        println!("expected output:\n------\n{}------", expected_output);
+        println!("actual output:\n------\n{}------", actual_output);
+
+        assert!(expected_output == actual_output)
+    })
+}
+
+fn make_span(file_text: &str, start: &Position, end: &Position) -> Span {
+    let start = make_pos(file_text, start);
+    let end = make_pos(file_text, end) + end.string.len(); // just after matching thing ends
+    assert!(start <= end);
+    Span::with_root_ctxt(BytePos(start as u32), BytePos(end as u32))
+}
+
+fn make_pos(file_text: &str, pos: &Position) -> usize {
+    let mut remainder = file_text;
+    let mut offset = 0;
+    for _ in 0..pos.count {
+        if let Some(n) = remainder.find(&pos.string) {
+            offset += n;
+            remainder = &remainder[n + 1..];
+        } else {
+            panic!("failed to find {} instances of {:?} in {:?}", pos.count, pos.string, file_text);
+        }
+    }
+    offset
+}
+
+#[test]
+fn ends_on_col0() {
+    test_harness(
+        r#"
+fn foo() {
+}
+"#,
+        vec![SpanLabel {
+            start: Position { string: "{", count: 1 },
+            end: Position { string: "}", count: 1 },
+            label: "test",
+        }],
+        r#"
+error: foo
+ --> test.rs:2:10
+  |
+2 |   fn foo() {
+  |  __________^
+3 | | }
+  | |_^ test
+
+"#,
+    );
+}
+
+#[test]
+fn ends_on_col2() {
+    test_harness(
+        r#"
+fn foo() {
+
+
+  }
+"#,
+        vec![SpanLabel {
+            start: Position { string: "{", count: 1 },
+            end: Position { string: "}", count: 1 },
+            label: "test",
+        }],
+        r#"
+error: foo
+ --> test.rs:2:10
+  |
+2 |   fn foo() {
+  |  __________^
+3 | |
+4 | |
+5 | |   }
+  | |___^ test
+
+"#,
+    );
+}
+#[test]
+fn non_nested() {
+    test_harness(
+        r#"
+fn foo() {
+  X0 Y0
+  X1 Y1
+  X2 Y2
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "X0", count: 1 },
+                end: Position { string: "X2", count: 1 },
+                label: "`X` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "Y0", count: 1 },
+                end: Position { string: "Y2", count: 1 },
+                label: "`Y` is a good letter too",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |      X0 Y0
+  |  ____^__-
+  | | ___|
+  | ||
+4 | ||   X1 Y1
+5 | ||   X2 Y2
+  | ||____^__- `Y` is a good letter too
+  |  |____|
+  |       `X` is a good letter
+
+"#,
+    );
+}
+
+#[test]
+fn nested() {
+    test_harness(
+        r#"
+fn foo() {
+  X0 Y0
+  Y1 X1
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "X0", count: 1 },
+                end: Position { string: "X1", count: 1 },
+                label: "`X` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "Y0", count: 1 },
+                end: Position { string: "Y1", count: 1 },
+                label: "`Y` is a good letter too",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |      X0 Y0
+  |  ____^__-
+  | | ___|
+  | ||
+4 | ||   Y1 X1
+  | ||____-__^ `X` is a good letter
+  | |_____|
+  |       `Y` is a good letter too
+
+"#,
+    );
+}
+
+#[test]
+fn different_overlap() {
+    test_harness(
+        r#"
+fn foo() {
+  X0 Y0 Z0
+  X1 Y1 Z1
+  X2 Y2 Z2
+  X3 Y3 Z3
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "Y0", count: 1 },
+                end: Position { string: "X2", count: 1 },
+                label: "`X` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "Z1", count: 1 },
+                end: Position { string: "X3", count: 1 },
+                label: "`Y` is a good letter too",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:6
+  |
+3 |      X0 Y0 Z0
+  |   ______^
+4 |  |   X1 Y1 Z1
+  |  |_________-
+5 | ||   X2 Y2 Z2
+  | ||____^ `X` is a good letter
+6 | |    X3 Y3 Z3
+  | |_____- `Y` is a good letter too
+
+"#,
+    );
+}
+
+#[test]
+fn triple_overlap() {
+    test_harness(
+        r#"
+fn foo() {
+  X0 Y0 Z0
+  X1 Y1 Z1
+  X2 Y2 Z2
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "X0", count: 1 },
+                end: Position { string: "X2", count: 1 },
+                label: "`X` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "Y0", count: 1 },
+                end: Position { string: "Y2", count: 1 },
+                label: "`Y` is a good letter too",
+            },
+            SpanLabel {
+                start: Position { string: "Z0", count: 1 },
+                end: Position { string: "Z2", count: 1 },
+                label: "`Z` label",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |       X0 Y0 Z0
+  |  _____^__-__-
+  | | ____|__|
+  | || ___|
+  | |||
+4 | |||   X1 Y1 Z1
+5 | |||   X2 Y2 Z2
+  | |||____^__-__- `Z` label
+  |  ||____|__|
+  |   |____|  `Y` is a good letter too
+  |        `X` is a good letter
+
+"#,
+    );
+}
+
+#[test]
+fn triple_exact_overlap() {
+    test_harness(
+        r#"
+fn foo() {
+  X0 Y0 Z0
+  X1 Y1 Z1
+  X2 Y2 Z2
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "X0", count: 1 },
+                end: Position { string: "X2", count: 1 },
+                label: "`X` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "X0", count: 1 },
+                end: Position { string: "X2", count: 1 },
+                label: "`Y` is a good letter too",
+            },
+            SpanLabel {
+                start: Position { string: "X0", count: 1 },
+                end: Position { string: "X2", count: 1 },
+                label: "`Z` label",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 | /   X0 Y0 Z0
+4 | |   X1 Y1 Z1
+5 | |   X2 Y2 Z2
+  | |    ^
+  | |    |
+  | |    `X` is a good letter
+  | |____`Y` is a good letter too
+  |      `Z` label
+
+"#,
+    );
+}
+
+#[test]
+fn minimum_depth() {
+    test_harness(
+        r#"
+fn foo() {
+  X0 Y0 Z0
+  X1 Y1 Z1
+  X2 Y2 Z2
+  X3 Y3 Z3
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "Y0", count: 1 },
+                end: Position { string: "X1", count: 1 },
+                label: "`X` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "Y1", count: 1 },
+                end: Position { string: "Z2", count: 1 },
+                label: "`Y` is a good letter too",
+            },
+            SpanLabel {
+                start: Position { string: "X2", count: 1 },
+                end: Position { string: "Y3", count: 1 },
+                label: "`Z`",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:6
+  |
+3 |      X0 Y0 Z0
+  |   ______^
+4 |  |   X1 Y1 Z1
+  |  |____^_-
+  | ||____|
+  | |     `X` is a good letter
+5 | |    X2 Y2 Z2
+  | |____-______- `Y` is a good letter too
+  |  ____|
+  | |
+6 | |    X3 Y3 Z3
+  | |________- `Z`
+
+"#,
+    );
+}
+
+#[test]
+fn non_overlaping() {
+    test_harness(
+        r#"
+fn foo() {
+  X0 Y0 Z0
+  X1 Y1 Z1
+  X2 Y2 Z2
+  X3 Y3 Z3
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "X0", count: 1 },
+                end: Position { string: "X1", count: 1 },
+                label: "`X` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "Y2", count: 1 },
+                end: Position { string: "Z3", count: 1 },
+                label: "`Y` is a good letter too",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 | /   X0 Y0 Z0
+4 | |   X1 Y1 Z1
+  | |____^ `X` is a good letter
+5 |     X2 Y2 Z2
+  |  ______-
+6 | |   X3 Y3 Z3
+  | |__________- `Y` is a good letter too
+
+"#,
+    );
+}
+
+#[test]
+fn overlaping_start_and_end() {
+    test_harness(
+        r#"
+fn foo() {
+  X0 Y0 Z0
+  X1 Y1 Z1
+  X2 Y2 Z2
+  X3 Y3 Z3
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "Y0", count: 1 },
+                end: Position { string: "X1", count: 1 },
+                label: "`X` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "Z1", count: 1 },
+                end: Position { string: "Z3", count: 1 },
+                label: "`Y` is a good letter too",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:6
+  |
+3 |      X0 Y0 Z0
+  |   ______^
+4 |  |   X1 Y1 Z1
+  |  |____^____-
+  | ||____|
+  | |     `X` is a good letter
+5 | |    X2 Y2 Z2
+6 | |    X3 Y3 Z3
+  | |___________- `Y` is a good letter too
+
+"#,
+    );
+}
+
+#[test]
+fn multiple_labels_primary_without_message() {
+    test_harness(
+        r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "b", count: 1 },
+                end: Position { string: "}", count: 1 },
+                label: "",
+            },
+            SpanLabel {
+                start: Position { string: "a", count: 1 },
+                end: Position { string: "d", count: 1 },
+                label: "`a` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "c", count: 1 },
+                end: Position { string: "c", count: 1 },
+                label: "",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:7
+  |
+3 |   a { b { c } d }
+  |   ----^^^^-^^-- `a` is a good letter
+
+"#,
+    );
+}
+
+#[test]
+fn multiple_labels_secondary_without_message() {
+    test_harness(
+        r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "a", count: 1 },
+                end: Position { string: "d", count: 1 },
+                label: "`a` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "b", count: 1 },
+                end: Position { string: "}", count: 1 },
+                label: "",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a { b { c } d }
+  |   ^^^^-------^^ `a` is a good letter
+
+"#,
+    );
+}
+
+#[test]
+fn multiple_labels_primary_without_message_2() {
+    test_harness(
+        r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "b", count: 1 },
+                end: Position { string: "}", count: 1 },
+                label: "`b` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "a", count: 1 },
+                end: Position { string: "d", count: 1 },
+                label: "",
+            },
+            SpanLabel {
+                start: Position { string: "c", count: 1 },
+                end: Position { string: "c", count: 1 },
+                label: "",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:7
+  |
+3 |   a { b { c } d }
+  |   ----^^^^-^^--
+  |       |
+  |       `b` is a good letter
+
+"#,
+    );
+}
+
+#[test]
+fn multiple_labels_secondary_without_message_2() {
+    test_harness(
+        r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "a", count: 1 },
+                end: Position { string: "d", count: 1 },
+                label: "",
+            },
+            SpanLabel {
+                start: Position { string: "b", count: 1 },
+                end: Position { string: "}", count: 1 },
+                label: "`b` is a good letter",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a { b { c } d }
+  |   ^^^^-------^^
+  |       |
+  |       `b` is a good letter
+
+"#,
+    );
+}
+
+#[test]
+fn multiple_labels_secondary_without_message_3() {
+    test_harness(
+        r#"
+fn foo() {
+  a  bc  d
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "a", count: 1 },
+                end: Position { string: "b", count: 1 },
+                label: "`a` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "c", count: 1 },
+                end: Position { string: "d", count: 1 },
+                label: "",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a  bc  d
+  |   ^^^^----
+  |   |
+  |   `a` is a good letter
+
+"#,
+    );
+}
+
+#[test]
+fn multiple_labels_without_message() {
+    test_harness(
+        r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "a", count: 1 },
+                end: Position { string: "d", count: 1 },
+                label: "",
+            },
+            SpanLabel {
+                start: Position { string: "b", count: 1 },
+                end: Position { string: "}", count: 1 },
+                label: "",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a { b { c } d }
+  |   ^^^^-------^^
+
+"#,
+    );
+}
+
+#[test]
+fn multiple_labels_without_message_2() {
+    test_harness(
+        r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "b", count: 1 },
+                end: Position { string: "}", count: 1 },
+                label: "",
+            },
+            SpanLabel {
+                start: Position { string: "a", count: 1 },
+                end: Position { string: "d", count: 1 },
+                label: "",
+            },
+            SpanLabel {
+                start: Position { string: "c", count: 1 },
+                end: Position { string: "c", count: 1 },
+                label: "",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:7
+  |
+3 |   a { b { c } d }
+  |   ----^^^^-^^--
+
+"#,
+    );
+}
+
+#[test]
+fn multiple_labels_with_message() {
+    test_harness(
+        r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "a", count: 1 },
+                end: Position { string: "d", count: 1 },
+                label: "`a` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "b", count: 1 },
+                end: Position { string: "}", count: 1 },
+                label: "`b` is a good letter",
+            },
+        ],
+        r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a { b { c } d }
+  |   ^^^^-------^^
+  |   |   |
+  |   |   `b` is a good letter
+  |   `a` is a good letter
+
+"#,
+    );
+}
+
+#[test]
+fn single_label_with_message() {
+    test_harness(
+        r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+        vec![SpanLabel {
+            start: Position { string: "a", count: 1 },
+            end: Position { string: "d", count: 1 },
+            label: "`a` is a good letter",
+        }],
+        r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a { b { c } d }
+  |   ^^^^^^^^^^^^^ `a` is a good letter
+
+"#,
+    );
+}
+
+#[test]
+fn single_label_without_message() {
+    test_harness(
+        r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+        vec![SpanLabel {
+            start: Position { string: "a", count: 1 },
+            end: Position { string: "d", count: 1 },
+            label: "",
+        }],
+        r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a { b { c } d }
+  |   ^^^^^^^^^^^^^
+
+"#,
+    );
+}
+
+#[test]
+fn long_snippet() {
+    test_harness(
+        r#"
+fn foo() {
+  X0 Y0 Z0
+  X1 Y1 Z1
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+  X2 Y2 Z2
+  X3 Y3 Z3
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "Y0", count: 1 },
+                end: Position { string: "X1", count: 1 },
+                label: "`X` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "Z1", count: 1 },
+                end: Position { string: "Z3", count: 1 },
+                label: "`Y` is a good letter too",
+            },
+        ],
+        r#"
+error: foo
+  --> test.rs:3:6
+   |
+3  |      X0 Y0 Z0
+   |   ______^
+4  |  |   X1 Y1 Z1
+   |  |____^____-
+   | ||____|
+   | |     `X` is a good letter
+5  | |  1
+6  | |  2
+7  | |  3
+...  |
+15 | |    X2 Y2 Z2
+16 | |    X3 Y3 Z3
+   | |___________- `Y` is a good letter too
+
+"#,
+    );
+}
+
+#[test]
+fn long_snippet_multiple_spans() {
+    test_harness(
+        r#"
+fn foo() {
+  X0 Y0 Z0
+1
+2
+3
+  X1 Y1 Z1
+4
+5
+6
+  X2 Y2 Z2
+7
+8
+9
+10
+  X3 Y3 Z3
+}
+"#,
+        vec![
+            SpanLabel {
+                start: Position { string: "Y0", count: 1 },
+                end: Position { string: "Y3", count: 1 },
+                label: "`Y` is a good letter",
+            },
+            SpanLabel {
+                start: Position { string: "Z1", count: 1 },
+                end: Position { string: "Z2", count: 1 },
+                label: "`Z` is a good letter too",
+            },
+        ],
+        r#"
+error: foo
+  --> test.rs:3:6
+   |
+3  |      X0 Y0 Z0
+   |   ______^
+4  |  | 1
+5  |  | 2
+6  |  | 3
+7  |  |   X1 Y1 Z1
+   |  |_________-
+8  | || 4
+9  | || 5
+10 | || 6
+11 | ||   X2 Y2 Z2
+   | ||__________- `Z` is a good letter too
+...   |
+15 |  | 10
+16 |  |   X3 Y3 Z3
+   |  |_______^ `Y` is a good letter
+
+"#,
+    );
+}
diff --git a/compiler/rustc_expand/src/tokenstream/tests.rs b/compiler/rustc_expand/src/tokenstream/tests.rs
new file mode 100644
index 000000000..eed696810
--- /dev/null
+++ b/compiler/rustc_expand/src/tokenstream/tests.rs
@@ -0,0 +1,110 @@
+use crate::tests::string_to_stream;
+
+use rustc_ast::token;
+use rustc_ast::tokenstream::{TokenStream, TokenStreamBuilder};
+use rustc_span::create_default_session_globals_then;
+use rustc_span::{BytePos, Span, Symbol};
+
+fn string_to_ts(string: &str) -> TokenStream {
+    string_to_stream(string.to_owned())
+}
+
+fn sp(a: u32, b: u32) -> Span {
+    Span::with_root_ctxt(BytePos(a), BytePos(b))
+}
+
+#[test]
+fn test_concat() {
+    create_default_session_globals_then(|| {
+        let test_res = string_to_ts("foo::bar::baz");
+        let test_fst = string_to_ts("foo::bar");
+        let test_snd = string_to_ts("::baz");
+        let mut builder = TokenStreamBuilder::new();
+        builder.push(test_fst);
+        builder.push(test_snd);
+        let eq_res = builder.build();
+        assert_eq!(test_res.trees().count(), 5);
+        assert_eq!(eq_res.trees().count(), 5);
+        assert_eq!(test_res.eq_unspanned(&eq_res), true);
+    })
+}
+
+#[test]
+fn test_to_from_bijection() {
+    create_default_session_globals_then(|| {
+        let test_start = string_to_ts("foo::bar(baz)");
+        let test_end = test_start.trees().cloned().collect();
+        assert_eq!(test_start, test_end)
+    })
+}
+
+#[test]
+fn test_eq_0() {
+    create_default_session_globals_then(|| {
+        let test_res = string_to_ts("foo");
+        let test_eqs = string_to_ts("foo");
+        assert_eq!(test_res, test_eqs)
+    })
+}
+
+#[test]
+fn test_eq_1() {
+    create_default_session_globals_then(|| {
+        let test_res = string_to_ts("::bar::baz");
+        let test_eqs = string_to_ts("::bar::baz");
+        assert_eq!(test_res, test_eqs)
+    })
+}
+
+#[test]
+fn test_eq_3() {
+    create_default_session_globals_then(|| {
+        let test_res = string_to_ts("");
+        let test_eqs = string_to_ts("");
+        assert_eq!(test_res, test_eqs)
+    })
+}
+
+#[test]
+fn test_diseq_0() {
+    create_default_session_globals_then(|| {
+        let test_res = string_to_ts("::bar::baz");
+        let test_eqs = string_to_ts("bar::baz");
+        assert_eq!(test_res == test_eqs, false)
+    })
+}
+
+#[test]
+fn test_diseq_1() {
+    create_default_session_globals_then(|| {
+        let test_res = string_to_ts("(bar,baz)");
+        let test_eqs = string_to_ts("bar,baz");
+        assert_eq!(test_res == test_eqs, false)
+    })
+}
+
+#[test]
+fn test_is_empty() {
+    create_default_session_globals_then(|| {
+        let test0 = TokenStream::default();
+        let test1 = TokenStream::token_alone(token::Ident(Symbol::intern("a"), false), sp(0, 1));
+        let test2 = string_to_ts("foo(bar::baz)");
+
+        assert_eq!(test0.is_empty(), true);
+        assert_eq!(test1.is_empty(), false);
+        assert_eq!(test2.is_empty(), false);
+    })
+}
+
+#[test]
+fn test_dotdotdot() {
+    create_default_session_globals_then(|| {
+        let mut builder = TokenStreamBuilder::new();
+        builder.push(TokenStream::token_joint(token::Dot, sp(0, 1)));
+        builder.push(TokenStream::token_joint(token::Dot, sp(1, 2)));
+        builder.push(TokenStream::token_alone(token::Dot, sp(2, 3)));
+        let stream = builder.build();
+        assert!(stream.eq_unspanned(&string_to_ts("...")));
+        assert_eq!(stream.trees().count(), 1);
+    })
+}