Auto merge of #127516 - nnethercote:simplify-LazyAttrTokenStream, r=petrochenkov

Simplify `LazyAttrTokenStream`

`LazyAttrTokenStream` is an unpleasant type: `Lrc<Box<dyn ToAttrTokenStream>>`. Why does it look like that?
- There are two `ToAttrTokenStream` impls, one for the lazy case, and one for the case where we already have an `AttrTokenStream`.
- The lazy case (`LazyAttrTokenStreamImpl`) is implemented in `rustc_parse`, but `LazyAttrTokenStream` is defined in `rustc_ast`, which does not depend on `rustc_parse`. The use of the trait lets `rustc_ast` implicitly depend on `rustc_parse`. This explains the `dyn`.
- `LazyAttrTokenStream` must have a `size_of` as small as possible, because it's used in many AST nodes. This explains the `Lrc<Box<_>>`, which keeps it to one word. (It's required `Lrc<dyn _>` would be a fat pointer.)

This PR moves `LazyAttrTokenStreamImpl` (and a few other token stream things) from `rustc_parse` to `rustc_ast`. This lets us replace the `ToAttrTokenStream` trait with a two-variant enum and also remove the `Box`, changing `LazyAttrTokenStream` to `Lrc<LazyAttrTokenStreamInner>`. Plus it does a few cleanups.

r? `@petrochenkov`

14 files changed, 399 insertions, 378 deletions

diff --git a/compiler/rustc_ast/src/lib.rs b/compiler/rustc_ast/src/lib.rs
index 1471262d2d6..e572ec99dab 100644
--- a/compiler/rustc_ast/src/lib.rs
+++ b/compiler/rustc_ast/src/lib.rs
@@ -12,6 +12,7 @@
     test(attr(deny(warnings)))
 )]
 #![doc(rust_logo)]
+#![feature(array_windows)]
 #![feature(associated_type_defaults)]
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
@@ -19,6 +20,7 @@
 #![feature(never_type)]
 #![feature(rustdoc_internals)]
 #![feature(stmt_expr_attributes)]
+#![recursion_limit = "256"]
 // tidy-alphabetical-end
 
 pub mod util {
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index cd2293423db..bef04dc4048 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -836,7 +836,7 @@ fn visit_lazy_tts_opt_mut<T: MutVisitor>(vis: &mut T, lazy_tts: Option<&mut Lazy
         if let Some(lazy_tts) = lazy_tts {
             let mut tts = lazy_tts.to_attr_token_stream();
             visit_attr_tts(vis, &mut tts);
-            *lazy_tts = LazyAttrTokenStream::new(tts);
+            *lazy_tts = LazyAttrTokenStream::new_direct(tts);
         }
     }
 }
diff --git a/compiler/rustc_ast/src/tokenstream.rs b/compiler/rustc_ast/src/tokenstream.rs
index 43d25d18075..636c26bcde0 100644
--- a/compiler/rustc_ast/src/tokenstream.rs
+++ b/compiler/rustc_ast/src/tokenstream.rs
@@ -14,14 +14,16 @@
 //! ownership of the original.
 
 use std::borrow::Cow;
+use std::ops::Range;
 use std::sync::Arc;
-use std::{cmp, fmt, iter};
+use std::{cmp, fmt, iter, mem};
 
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync;
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 use rustc_serialize::{Decodable, Encodable};
 use rustc_span::{DUMMY_SP, Span, SpanDecoder, SpanEncoder, Symbol, sym};
+use thin_vec::ThinVec;
 
 use crate::ast::AttrStyle;
 use crate::ast_traits::{HasAttrs, HasTokens};
@@ -106,25 +108,30 @@ where
     }
 }
 
-pub trait ToAttrTokenStream: sync::DynSend + sync::DynSync {
-    fn to_attr_token_stream(&self) -> AttrTokenStream;
-}
-
-impl ToAttrTokenStream for AttrTokenStream {
-    fn to_attr_token_stream(&self) -> AttrTokenStream {
-        self.clone()
-    }
-}
-
-/// A lazy version of [`TokenStream`], which defers creation
-/// of an actual `TokenStream` until it is needed.
-/// `Box` is here only to reduce the structure size.
+/// A lazy version of [`AttrTokenStream`], which defers creation of an actual
+/// `AttrTokenStream` until it is needed.
 #[derive(Clone)]
-pub struct LazyAttrTokenStream(Arc<Box<dyn ToAttrTokenStream>>);
+pub struct LazyAttrTokenStream(Arc<LazyAttrTokenStreamInner>);
 
 impl LazyAttrTokenStream {
-    pub fn new(inner: impl ToAttrTokenStream + 'static) -> LazyAttrTokenStream {
-        LazyAttrTokenStream(Arc::new(Box::new(inner)))
+    pub fn new_direct(stream: AttrTokenStream) -> LazyAttrTokenStream {
+        LazyAttrTokenStream(Arc::new(LazyAttrTokenStreamInner::Direct(stream)))
+    }
+
+    pub fn new_pending(
+        start_token: (Token, Spacing),
+        cursor_snapshot: TokenCursor,
+        num_calls: u32,
+        break_last_token: u32,
+        node_replacements: ThinVec<NodeReplacement>,
+    ) -> LazyAttrTokenStream {
+        LazyAttrTokenStream(Arc::new(LazyAttrTokenStreamInner::Pending {
+            start_token,
+            cursor_snapshot,
+            num_calls,
+            break_last_token,
+            node_replacements,
+        }))
     }
 
     pub fn to_attr_token_stream(&self) -> AttrTokenStream {
@@ -156,6 +163,184 @@ impl<CTX> HashStable<CTX> for LazyAttrTokenStream {
     }
 }
 
+/// A token range within a `Parser`'s full token stream.
+#[derive(Clone, Debug)]
+pub struct ParserRange(pub Range<u32>);
+
+/// A token range within an individual AST node's (lazy) token stream, i.e.
+/// relative to that node's first token. Distinct from `ParserRange` so the two
+/// kinds of range can't be mixed up.
+#[derive(Clone, Debug)]
+pub struct NodeRange(pub Range<u32>);
+
+/// Indicates a range of tokens that should be replaced by an `AttrsTarget`
+/// (replacement) or be replaced by nothing (deletion). This is used in two
+/// places during token collection.
+///
+/// 1. Replacement. During the parsing of an AST node that may have a
+///    `#[derive]` attribute, when we parse a nested AST node that has `#[cfg]`
+///    or `#[cfg_attr]`, we replace the entire inner AST node with
+///    `FlatToken::AttrsTarget`. This lets us perform eager cfg-expansion on an
+///    `AttrTokenStream`.
+///
+/// 2. Deletion. We delete inner attributes from all collected token streams,
+///    and instead track them through the `attrs` field on the AST node. This
+///    lets us manipulate them similarly to outer attributes. When we create a
+///    `TokenStream`, the inner attributes are inserted into the proper place
+///    in the token stream.
+///
+/// Each replacement starts off in `ParserReplacement` form but is converted to
+/// `NodeReplacement` form when it is attached to a single AST node, via
+/// `LazyAttrTokenStreamImpl`.
+pub type ParserReplacement = (ParserRange, Option<AttrsTarget>);
+
+/// See the comment on `ParserReplacement`.
+pub type NodeReplacement = (NodeRange, Option<AttrsTarget>);
+
+impl NodeRange {
+    // Converts a range within a parser's tokens to a range within a
+    // node's tokens beginning at `start_pos`.
+    //
+    // For example, imagine a parser with 50 tokens in its token stream, a
+    // function that spans `ParserRange(20..40)` and an inner attribute within
+    // that function that spans `ParserRange(30..35)`. We would find the inner
+    // attribute's range within the function's tokens by subtracting 20, which
+    // is the position of the function's start token. This gives
+    // `NodeRange(10..15)`.
+    pub fn new(ParserRange(parser_range): ParserRange, start_pos: u32) -> NodeRange {
+        assert!(!parser_range.is_empty());
+        assert!(parser_range.start >= start_pos);
+        NodeRange((parser_range.start - start_pos)..(parser_range.end - start_pos))
+    }
+}
+
+enum LazyAttrTokenStreamInner {
+    // The token stream has already been produced.
+    Direct(AttrTokenStream),
+
+    // From a value of this type we can reconstruct the `TokenStream` seen by
+    // the `f` callback passed to a call to `Parser::collect_tokens`, by
+    // replaying the getting of the tokens. This saves us producing a
+    // `TokenStream` if it is never needed, e.g. a captured `macro_rules!`
+    // argument that is never passed to a proc macro. In practice, token stream
+    // creation happens rarely compared to calls to `collect_tokens` (see some
+    // statistics in #78736) so we are doing as little up-front work as
+    // possible.
+    //
+    // This also makes `Parser` very cheap to clone, since there is no
+    // intermediate collection buffer to clone.
+    Pending {
+        start_token: (Token, Spacing),
+        cursor_snapshot: TokenCursor,
+        num_calls: u32,
+        break_last_token: u32,
+        node_replacements: ThinVec<NodeReplacement>,
+    },
+}
+
+impl LazyAttrTokenStreamInner {
+    fn to_attr_token_stream(&self) -> AttrTokenStream {
+        match self {
+            LazyAttrTokenStreamInner::Direct(stream) => stream.clone(),
+            LazyAttrTokenStreamInner::Pending {
+                start_token,
+                cursor_snapshot,
+                num_calls,
+                break_last_token,
+                node_replacements,
+            } => {
+                // The token produced by the final call to `{,inlined_}next` was not
+                // actually consumed by the callback. The combination of chaining the
+                // initial token and using `take` produces the desired result - we
+                // produce an empty `TokenStream` if no calls were made, and omit the
+                // final token otherwise.
+                let mut cursor_snapshot = cursor_snapshot.clone();
+                let tokens = iter::once(FlatToken::Token(*start_token))
+                    .chain(iter::repeat_with(|| FlatToken::Token(cursor_snapshot.next())))
+                    .take(*num_calls as usize);
+
+                if node_replacements.is_empty() {
+                    make_attr_token_stream(tokens, *break_last_token)
+                } else {
+                    let mut tokens: Vec<_> = tokens.collect();
+                    let mut node_replacements = node_replacements.to_vec();
+                    node_replacements.sort_by_key(|(range, _)| range.0.start);
+
+                    #[cfg(debug_assertions)]
+                    for [(node_range, tokens), (next_node_range, next_tokens)] in
+                        node_replacements.array_windows()
+                    {
+                        assert!(
+                            node_range.0.end <= next_node_range.0.start
+                                || node_range.0.end >= next_node_range.0.end,
+                            "Node ranges should be disjoint or nested: ({:?}, {:?}) ({:?}, {:?})",
+                            node_range,
+                            tokens,
+                            next_node_range,
+                            next_tokens,
+                        );
+                    }
+
+                    // Process the replace ranges, starting from the highest start
+                    // position and working our way back. If have tokens like:
+                    //
+                    // `#[cfg(FALSE)] struct Foo { #[cfg(FALSE)] field: bool }`
+                    //
+                    // Then we will generate replace ranges for both
+                    // the `#[cfg(FALSE)] field: bool` and the entire
+                    // `#[cfg(FALSE)] struct Foo { #[cfg(FALSE)] field: bool }`
+                    //
+                    // By starting processing from the replace range with the greatest
+                    // start position, we ensure that any (outer) replace range which
+                    // encloses another (inner) replace range will fully overwrite the
+                    // inner range's replacement.
+                    for (node_range, target) in node_replacements.into_iter().rev() {
+                        assert!(
+                            !node_range.0.is_empty(),
+                            "Cannot replace an empty node range: {:?}",
+                            node_range.0
+                        );
+
+                        // Replace the tokens in range with zero or one `FlatToken::AttrsTarget`s,
+                        // plus enough `FlatToken::Empty`s to fill up the rest of the range. This
+                        // keeps the total length of `tokens` constant throughout the replacement
+                        // process, allowing us to do all replacements without adjusting indices.
+                        let target_len = target.is_some() as usize;
+                        tokens.splice(
+                            (node_range.0.start as usize)..(node_range.0.end as usize),
+                            target.into_iter().map(|target| FlatToken::AttrsTarget(target)).chain(
+                                iter::repeat(FlatToken::Empty)
+                                    .take(node_range.0.len() - target_len),
+                            ),
+                        );
+                    }
+                    make_attr_token_stream(tokens.into_iter(), *break_last_token)
+                }
+            }
+        }
+    }
+}
+
+/// A helper struct used when building an `AttrTokenStream` from
+/// a `LazyAttrTokenStream`. Both delimiter and non-delimited tokens
+/// are stored as `FlatToken::Token`. A vector of `FlatToken`s
+/// is then 'parsed' to build up an `AttrTokenStream` with nested
+/// `AttrTokenTree::Delimited` tokens.
+#[derive(Debug, Clone)]
+enum FlatToken {
+    /// A token - this holds both delimiter (e.g. '{' and '}')
+    /// and non-delimiter tokens
+    Token((Token, Spacing)),
+    /// Holds the `AttrsTarget` for an AST node. The `AttrsTarget` is inserted
+    /// directly into the constructed `AttrTokenStream` as an
+    /// `AttrTokenTree::AttrsTarget`.
+    AttrsTarget(AttrsTarget),
+    /// A special 'empty' token that is ignored during the conversion
+    /// to an `AttrTokenStream`. This is used to simplify the
+    /// handling of replace ranges.
+    Empty,
+}
+
 /// An `AttrTokenStream` is similar to a `TokenStream`, but with extra
 /// information about the tokens for attribute targets. This is used
 /// during expansion to perform early cfg-expansion, and to process attributes
@@ -163,6 +348,71 @@ impl<CTX> HashStable<CTX> for LazyAttrTokenStream {
 #[derive(Clone, Debug, Default, Encodable, Decodable)]
 pub struct AttrTokenStream(pub Arc<Vec<AttrTokenTree>>);
 
+/// Converts a flattened iterator of tokens (including open and close delimiter tokens) into an
+/// `AttrTokenStream`, creating an `AttrTokenTree::Delimited` for each matching pair of open and
+/// close delims.
+fn make_attr_token_stream(
+    iter: impl Iterator<Item = FlatToken>,
+    break_last_token: u32,
+) -> AttrTokenStream {
+    #[derive(Debug)]
+    struct FrameData {
+        // This is `None` for the first frame, `Some` for all others.
+        open_delim_sp: Option<(Delimiter, Span, Spacing)>,
+        inner: Vec<AttrTokenTree>,
+    }
+    // The stack always has at least one element. Storing it separately makes for shorter code.
+    let mut stack_top = FrameData { open_delim_sp: None, inner: vec![] };
+    let mut stack_rest = vec![];
+    for flat_token in iter {
+        match flat_token {
+            FlatToken::Token((token @ Token { kind, span }, spacing)) => {
+                if let Some(delim) = kind.open_delim() {
+                    stack_rest.push(mem::replace(
+                        &mut stack_top,
+                        FrameData { open_delim_sp: Some((delim, span, spacing)), inner: vec![] },
+                    ));
+                } else if let Some(delim) = kind.close_delim() {
+                    let frame_data = mem::replace(&mut stack_top, stack_rest.pop().unwrap());
+                    let (open_delim, open_sp, open_spacing) = frame_data.open_delim_sp.unwrap();
+                    assert!(
+                        open_delim.eq_ignoring_invisible_origin(&delim),
+                        "Mismatched open/close delims: open={open_delim:?} close={span:?}"
+                    );
+                    let dspan = DelimSpan::from_pair(open_sp, span);
+                    let dspacing = DelimSpacing::new(open_spacing, spacing);
+                    let stream = AttrTokenStream::new(frame_data.inner);
+                    let delimited = AttrTokenTree::Delimited(dspan, dspacing, delim, stream);
+                    stack_top.inner.push(delimited);
+                } else {
+                    stack_top.inner.push(AttrTokenTree::Token(token, spacing))
+                }
+            }
+            FlatToken::AttrsTarget(target) => {
+                stack_top.inner.push(AttrTokenTree::AttrsTarget(target))
+            }
+            FlatToken::Empty => {}
+        }
+    }
+
+    if break_last_token > 0 {
+        let last_token = stack_top.inner.pop().unwrap();
+        if let AttrTokenTree::Token(last_token, spacing) = last_token {
+            let (unglued, _) = last_token.kind.break_two_token_op(break_last_token).unwrap();
+
+            // Tokens are always ASCII chars, so we can use byte arithmetic here.
+            let mut first_span = last_token.span.shrink_to_lo();
+            first_span =
+                first_span.with_hi(first_span.lo() + rustc_span::BytePos(break_last_token));
+
+            stack_top.inner.push(AttrTokenTree::Token(Token::new(unglued, first_span), spacing));
+        } else {
+            panic!("Unexpected last token {last_token:?}")
+        }
+    }
+    AttrTokenStream::new(stack_top.inner)
+}
+
 /// Like `TokenTree`, but for `AttrTokenStream`.
 #[derive(Clone, Debug, Encodable, Decodable)]
 pub enum AttrTokenTree {
@@ -641,6 +891,104 @@ impl<'t> Iterator for TokenStreamIter<'t> {
     }
 }
 
+#[derive(Clone, Debug)]
+pub struct TokenTreeCursor {
+    stream: TokenStream,
+    /// Points to the current token tree in the stream. In `TokenCursor::curr`,
+    /// this can be any token tree. In `TokenCursor::stack`, this is always a
+    /// `TokenTree::Delimited`.
+    index: usize,
+}
+
+impl TokenTreeCursor {
+    #[inline]
+    pub fn new(stream: TokenStream) -> Self {
+        TokenTreeCursor { stream, index: 0 }
+    }
+
+    #[inline]
+    pub fn curr(&self) -> Option<&TokenTree> {
+        self.stream.get(self.index)
+    }
+
+    pub fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
+        self.stream.get(self.index + n)
+    }
+
+    #[inline]
+    pub fn bump(&mut self) {
+        self.index += 1;
+    }
+}
+
+/// A `TokenStream` cursor that produces `Token`s. It's a bit odd that
+/// we (a) lex tokens into a nice tree structure (`TokenStream`), and then (b)
+/// use this type to emit them as a linear sequence. But a linear sequence is
+/// what the parser expects, for the most part.
+#[derive(Clone, Debug)]
+pub struct TokenCursor {
+    // Cursor for the current (innermost) token stream. The index within the
+    // cursor can point to any token tree in the stream (or one past the end).
+    // The delimiters for this token stream are found in `self.stack.last()`;
+    // if that is `None` we are in the outermost token stream which never has
+    // delimiters.
+    pub curr: TokenTreeCursor,
+
+    // Token streams surrounding the current one. The index within each cursor
+    // always points to a `TokenTree::Delimited`.
+    pub stack: Vec<TokenTreeCursor>,
+}
+
+impl TokenCursor {
+    pub fn next(&mut self) -> (Token, Spacing) {
+        self.inlined_next()
+    }
+
+    /// This always-inlined version should only be used on hot code paths.
+    #[inline(always)]
+    pub fn inlined_next(&mut self) -> (Token, Spacing) {
+        loop {
+            // FIXME: we currently don't return `Delimiter::Invisible` open/close delims. To fix
+            // #67062 we will need to, whereupon the `delim != Delimiter::Invisible` conditions
+            // below can be removed.
+            if let Some(tree) = self.curr.curr() {
+                match tree {
+                    &TokenTree::Token(token, spacing) => {
+                        debug_assert!(!token.kind.is_delim());
+                        let res = (token, spacing);
+                        self.curr.bump();
+                        return res;
+                    }
+                    &TokenTree::Delimited(sp, spacing, delim, ref tts) => {
+                        let trees = TokenTreeCursor::new(tts.clone());
+                        self.stack.push(mem::replace(&mut self.curr, trees));
+                        if !delim.skip() {
+                            return (Token::new(delim.as_open_token_kind(), sp.open), spacing.open);
+                        }
+                        // No open delimiter to return; continue on to the next iteration.
+                    }
+                };
+            } else if let Some(parent) = self.stack.pop() {
+                // We have exhausted this token stream. Move back to its parent token stream.
+                let Some(&TokenTree::Delimited(span, spacing, delim, _)) = parent.curr() else {
+                    panic!("parent should be Delimited")
+                };
+                self.curr = parent;
+                self.curr.bump(); // move past the `Delimited`
+                if !delim.skip() {
+                    return (Token::new(delim.as_close_token_kind(), span.close), spacing.close);
+                }
+                // No close delimiter to return; continue on to the next iteration.
+            } else {
+                // We have exhausted the outermost token stream. The use of
+                // `Spacing::Alone` is arbitrary and immaterial, because the
+                // `Eof` token's spacing is never used.
+                return (Token::new(token::Eof, DUMMY_SP), Spacing::Alone);
+            }
+        }
+    }
+}
+
 #[derive(Debug, Copy, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
 pub struct DelimSpan {
     pub open: Span,
@@ -687,6 +1035,7 @@ mod size_asserts {
     static_assert_size!(AttrTokenStream, 8);
     static_assert_size!(AttrTokenTree, 32);
     static_assert_size!(LazyAttrTokenStream, 8);
+    static_assert_size!(LazyAttrTokenStreamInner, 88);
     static_assert_size!(Option<LazyAttrTokenStream>, 8); // must be small, used in many AST nodes
     static_assert_size!(TokenStream, 8);
     static_assert_size!(TokenTree, 32);
diff --git a/compiler/rustc_attr_parsing/src/lib.rs b/compiler/rustc_attr_parsing/src/lib.rs
index b9692c01e2c..874fccf7ff6 100644
--- a/compiler/rustc_attr_parsing/src/lib.rs
+++ b/compiler/rustc_attr_parsing/src/lib.rs
@@ -80,6 +80,7 @@
 #![cfg_attr(bootstrap, feature(let_chains))]
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]
+#![recursion_limit = "256"]
 // tidy-alphabetical-end
 
 #[macro_use]
diff --git a/compiler/rustc_builtin_macros/src/lib.rs b/compiler/rustc_builtin_macros/src/lib.rs
index 70e817db2a6..c2f5bf0f457 100644
--- a/compiler/rustc_builtin_macros/src/lib.rs
+++ b/compiler/rustc_builtin_macros/src/lib.rs
@@ -18,6 +18,7 @@
 #![feature(rustdoc_internals)]
 #![feature(string_from_utf8_lossy_owned)]
 #![feature(try_blocks)]
+#![recursion_limit = "256"]
 // tidy-alphabetical-end
 
 extern crate proc_macro;
diff --git a/compiler/rustc_codegen_ssa/src/lib.rs b/compiler/rustc_codegen_ssa/src/lib.rs
index b67c871cac9..bfec208c4ae 100644
--- a/compiler/rustc_codegen_ssa/src/lib.rs
+++ b/compiler/rustc_codegen_ssa/src/lib.rs
@@ -14,6 +14,7 @@
 #![feature(string_from_utf8_lossy_owned)]
 #![feature(trait_alias)]
 #![feature(try_blocks)]
+#![recursion_limit = "256"]
 // tidy-alphabetical-end
 
 //! This crate contains codegen code that is used by all codegen backends (LLVM and others).
diff --git a/compiler/rustc_expand/src/config.rs b/compiler/rustc_expand/src/config.rs
index d2e45d717d9..2df3281568b 100644
--- a/compiler/rustc_expand/src/config.rs
+++ b/compiler/rustc_expand/src/config.rs
@@ -162,7 +162,7 @@ pub(crate) fn attr_into_trace(mut attr: Attribute, trace_name: Symbol) -> Attrib
             let NormalAttr { item, tokens } = &mut **normal;
             item.path.segments[0].ident.name = trace_name;
             // This makes the trace attributes unobservable to token-based proc macros.
-            *tokens = Some(LazyAttrTokenStream::new(AttrTokenStream::default()));
+            *tokens = Some(LazyAttrTokenStream::new_direct(AttrTokenStream::default()));
         }
         AttrKind::DocComment(..) => unreachable!(),
     }
@@ -192,7 +192,7 @@ impl<'a> StripUnconfigured<'a> {
         if self.config_tokens {
             if let Some(Some(tokens)) = node.tokens_mut() {
                 let attr_stream = tokens.to_attr_token_stream();
-                *tokens = LazyAttrTokenStream::new(self.configure_tokens(&attr_stream));
+                *tokens = LazyAttrTokenStream::new_direct(self.configure_tokens(&attr_stream));
             }
         }
     }
@@ -223,7 +223,7 @@ impl<'a> StripUnconfigured<'a> {
                     target.attrs.flat_map_in_place(|attr| self.process_cfg_attr(&attr));
 
                     if self.in_cfg(&target.attrs) {
-                        target.tokens = LazyAttrTokenStream::new(
+                        target.tokens = LazyAttrTokenStream::new_direct(
                             self.configure_tokens(&target.tokens.to_attr_token_stream()),
                         );
                         Some(AttrTokenTree::AttrsTarget(target))
@@ -361,7 +361,7 @@ impl<'a> StripUnconfigured<'a> {
                 .to_attr_token_stream(),
         ));
 
-        let tokens = Some(LazyAttrTokenStream::new(AttrTokenStream::new(trees)));
+        let tokens = Some(LazyAttrTokenStream::new_direct(AttrTokenStream::new(trees)));
         let attr = ast::attr::mk_attr_from_item(
             &self.sess.psess.attr_id_generator,
             item,
diff --git a/compiler/rustc_hir/src/lib.rs b/compiler/rustc_hir/src/lib.rs
index 32064f96dd6..5533920aee4 100644
--- a/compiler/rustc_hir/src/lib.rs
+++ b/compiler/rustc_hir/src/lib.rs
@@ -14,6 +14,7 @@
 #![feature(never_type)]
 #![feature(rustc_attrs)]
 #![feature(variant_count)]
+#![recursion_limit = "256"]
 // tidy-alphabetical-end
 
 extern crate self as rustc_hir;
diff --git a/compiler/rustc_middle/src/lib.rs b/compiler/rustc_middle/src/lib.rs
index df025aeebf0..711036865bb 100644
--- a/compiler/rustc_middle/src/lib.rs
+++ b/compiler/rustc_middle/src/lib.rs
@@ -61,6 +61,7 @@
 #![feature(try_trait_v2_yeet)]
 #![feature(type_alias_impl_trait)]
 #![feature(yeet_expr)]
+#![recursion_limit = "256"]
 // tidy-alphabetical-end
 
 #[cfg(test)]
diff --git a/compiler/rustc_parse/src/lib.rs b/compiler/rustc_parse/src/lib.rs
index e73d68e2037..d06922f1e04 100644
--- a/compiler/rustc_parse/src/lib.rs
+++ b/compiler/rustc_parse/src/lib.rs
@@ -5,13 +5,13 @@
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
 #![cfg_attr(bootstrap, feature(let_chains))]
-#![feature(array_windows)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
 #![feature(debug_closure_helpers)]
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
 #![feature(string_from_utf8_lossy_owned)]
+#![recursion_limit = "256"]
 // tidy-alphabetical-end
 
 use std::path::{Path, PathBuf};
diff --git a/compiler/rustc_parse/src/parser/attr.rs b/compiler/rustc_parse/src/parser/attr.rs
index 53614049f08..41d3889c448 100644
--- a/compiler/rustc_parse/src/parser/attr.rs
+++ b/compiler/rustc_parse/src/parser/attr.rs
@@ -1,5 +1,6 @@
 use rustc_ast as ast;
 use rustc_ast::token::{self, MetaVarKind};
+use rustc_ast::tokenstream::ParserRange;
 use rustc_ast::{Attribute, attr};
 use rustc_errors::codes::*;
 use rustc_errors::{Diag, PResult};
@@ -8,8 +9,7 @@ use thin_vec::ThinVec;
 use tracing::debug;
 
 use super::{
-    AttrWrapper, Capturing, FnParseMode, ForceCollect, Parser, ParserRange, PathStyle, Trailing,
-    UsePreAttrPos,
+    AttrWrapper, Capturing, FnParseMode, ForceCollect, Parser, PathStyle, Trailing, UsePreAttrPos,
 };
 use crate::{errors, exp, fluent_generated as fluent};
 
diff --git a/compiler/rustc_parse/src/parser/attr_wrapper.rs b/compiler/rustc_parse/src/parser/attr_wrapper.rs
index 6061c9cb485..44fdf146f9c 100644
--- a/compiler/rustc_parse/src/parser/attr_wrapper.rs
+++ b/compiler/rustc_parse/src/parser/attr_wrapper.rs
@@ -1,21 +1,18 @@
 use std::borrow::Cow;
-use std::{iter, mem};
+use std::mem;
 
-use rustc_ast::token::{Delimiter, Token};
+use rustc_ast::token::Token;
 use rustc_ast::tokenstream::{
-    AttrTokenStream, AttrTokenTree, AttrsTarget, DelimSpacing, DelimSpan, LazyAttrTokenStream,
-    Spacing, ToAttrTokenStream,
+    AttrsTarget, LazyAttrTokenStream, NodeRange, ParserRange, Spacing, TokenCursor,
 };
 use rustc_ast::{self as ast, AttrVec, Attribute, HasAttrs, HasTokens};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::PResult;
 use rustc_session::parse::ParseSess;
-use rustc_span::{DUMMY_SP, Span, sym};
+use rustc_span::{DUMMY_SP, sym};
+use thin_vec::ThinVec;
 
-use super::{
-    Capturing, FlatToken, ForceCollect, NodeRange, NodeReplacement, Parser, ParserRange,
-    TokenCursor, Trailing,
-};
+use super::{Capturing, ForceCollect, Parser, Trailing};
 
 // When collecting tokens, this fully captures the start point. Usually its
 // just after outer attributes, but occasionally it's before.
@@ -94,95 +91,6 @@ fn has_cfg_or_cfg_attr(attrs: &[Attribute]) -> bool {
     })
 }
 
-// From a value of this type we can reconstruct the `TokenStream` seen by the
-// `f` callback passed to a call to `Parser::collect_tokens`, by
-// replaying the getting of the tokens. This saves us producing a `TokenStream`
-// if it is never needed, e.g. a captured `macro_rules!` argument that is never
-// passed to a proc macro. In practice, token stream creation happens rarely
-// compared to calls to `collect_tokens` (see some statistics in #78736) so we
-// are doing as little up-front work as possible.
-//
-// This also makes `Parser` very cheap to clone, since
-// there is no intermediate collection buffer to clone.
-struct LazyAttrTokenStreamImpl {
-    start_token: (Token, Spacing),
-    cursor_snapshot: TokenCursor,
-    num_calls: u32,
-    break_last_token: u32,
-    node_replacements: Box<[NodeReplacement]>,
-}
-
-impl ToAttrTokenStream for LazyAttrTokenStreamImpl {
-    fn to_attr_token_stream(&self) -> AttrTokenStream {
-        // The token produced by the final call to `{,inlined_}next` was not
-        // actually consumed by the callback. The combination of chaining the
-        // initial token and using `take` produces the desired result - we
-        // produce an empty `TokenStream` if no calls were made, and omit the
-        // final token otherwise.
-        let mut cursor_snapshot = self.cursor_snapshot.clone();
-        let tokens = iter::once(FlatToken::Token(self.start_token))
-            .chain(iter::repeat_with(|| FlatToken::Token(cursor_snapshot.next())))
-            .take(self.num_calls as usize);
-
-        if self.node_replacements.is_empty() {
-            make_attr_token_stream(tokens, self.break_last_token)
-        } else {
-            let mut tokens: Vec<_> = tokens.collect();
-            let mut node_replacements = self.node_replacements.to_vec();
-            node_replacements.sort_by_key(|(range, _)| range.0.start);
-
-            #[cfg(debug_assertions)]
-            for [(node_range, tokens), (next_node_range, next_tokens)] in
-                node_replacements.array_windows()
-            {
-                assert!(
-                    node_range.0.end <= next_node_range.0.start
-                        || node_range.0.end >= next_node_range.0.end,
-                    "Node ranges should be disjoint or nested: ({:?}, {:?}) ({:?}, {:?})",
-                    node_range,
-                    tokens,
-                    next_node_range,
-                    next_tokens,
-                );
-            }
-
-            // Process the replace ranges, starting from the highest start
-            // position and working our way back. If have tokens like:
-            //
-            // `#[cfg(FALSE)] struct Foo { #[cfg(FALSE)] field: bool }`
-            //
-            // Then we will generate replace ranges for both
-            // the `#[cfg(FALSE)] field: bool` and the entire
-            // `#[cfg(FALSE)] struct Foo { #[cfg(FALSE)] field: bool }`
-            //
-            // By starting processing from the replace range with the greatest
-            // start position, we ensure that any (outer) replace range which
-            // encloses another (inner) replace range will fully overwrite the
-            // inner range's replacement.
-            for (node_range, target) in node_replacements.into_iter().rev() {
-                assert!(
-                    !node_range.0.is_empty(),
-                    "Cannot replace an empty node range: {:?}",
-                    node_range.0
-                );
-
-                // Replace the tokens in range with zero or one `FlatToken::AttrsTarget`s, plus
-                // enough `FlatToken::Empty`s to fill up the rest of the range. This keeps the
-                // total length of `tokens` constant throughout the replacement process, allowing
-                // us to do all replacements without adjusting indices.
-                let target_len = target.is_some() as usize;
-                tokens.splice(
-                    (node_range.0.start as usize)..(node_range.0.end as usize),
-                    target.into_iter().map(|target| FlatToken::AttrsTarget(target)).chain(
-                        iter::repeat(FlatToken::Empty).take(node_range.0.len() - target_len),
-                    ),
-                );
-            }
-            make_attr_token_stream(tokens.into_iter(), self.break_last_token)
-        }
-    }
-}
-
 impl<'a> Parser<'a> {
     pub(super) fn collect_pos(&self) -> CollectPos {
         CollectPos {
@@ -387,10 +295,10 @@ impl<'a> Parser<'a> {
 
         // This is hot enough for `deep-vector` that checking the conditions for an empty iterator
         // is measurably faster than actually executing the iterator.
-        let node_replacements: Box<[_]> = if parser_replacements_start == parser_replacements_end
+        let node_replacements = if parser_replacements_start == parser_replacements_end
             && inner_attr_parser_replacements.is_empty()
         {
-            Box::new([])
+            ThinVec::new()
         } else {
             // Grab any replace ranges that occur *inside* the current AST node. Convert them
             // from `ParserRange` form to `NodeRange` form. We will perform the actual
@@ -429,13 +337,13 @@ impl<'a> Parser<'a> {
         //     - `attrs`: includes the outer and the inner attr.
         //     - `tokens`: lazy tokens for `g` (with its inner attr deleted).
 
-        let tokens = LazyAttrTokenStream::new(LazyAttrTokenStreamImpl {
-            start_token: collect_pos.start_token,
-            cursor_snapshot: collect_pos.cursor_snapshot,
+        let tokens = LazyAttrTokenStream::new_pending(
+            collect_pos.start_token,
+            collect_pos.cursor_snapshot,
             num_calls,
-            break_last_token: self.break_last_token,
+            self.break_last_token,
             node_replacements,
-        });
+        );
         let mut tokens_used = false;
 
         // If in "definite capture mode" we need to register a replace range
@@ -483,71 +391,6 @@ impl<'a> Parser<'a> {
     }
 }
 
-/// Converts a flattened iterator of tokens (including open and close delimiter tokens) into an
-/// `AttrTokenStream`, creating an `AttrTokenTree::Delimited` for each matching pair of open and
-/// close delims.
-fn make_attr_token_stream(
-    iter: impl Iterator<Item = FlatToken>,
-    break_last_token: u32,
-) -> AttrTokenStream {
-    #[derive(Debug)]
-    struct FrameData {
-        // This is `None` for the first frame, `Some` for all others.
-        open_delim_sp: Option<(Delimiter, Span, Spacing)>,
-        inner: Vec<AttrTokenTree>,
-    }
-    // The stack always has at least one element. Storing it separately makes for shorter code.
-    let mut stack_top = FrameData { open_delim_sp: None, inner: vec![] };
-    let mut stack_rest = vec![];
-    for flat_token in iter {
-        match flat_token {
-            FlatToken::Token((token @ Token { kind, span }, spacing)) => {
-                if let Some(delim) = kind.open_delim() {
-                    stack_rest.push(mem::replace(
-                        &mut stack_top,
-                        FrameData { open_delim_sp: Some((delim, span, spacing)), inner: vec![] },
-                    ));
-                } else if let Some(delim) = kind.close_delim() {
-                    let frame_data = mem::replace(&mut stack_top, stack_rest.pop().unwrap());
-                    let (open_delim, open_sp, open_spacing) = frame_data.open_delim_sp.unwrap();
-                    assert!(
-                        open_delim.eq_ignoring_invisible_origin(&delim),
-                        "Mismatched open/close delims: open={open_delim:?} close={span:?}"
-                    );
-                    let dspan = DelimSpan::from_pair(open_sp, span);
-                    let dspacing = DelimSpacing::new(open_spacing, spacing);
-                    let stream = AttrTokenStream::new(frame_data.inner);
-                    let delimited = AttrTokenTree::Delimited(dspan, dspacing, delim, stream);
-                    stack_top.inner.push(delimited);
-                } else {
-                    stack_top.inner.push(AttrTokenTree::Token(token, spacing))
-                }
-            }
-            FlatToken::AttrsTarget(target) => {
-                stack_top.inner.push(AttrTokenTree::AttrsTarget(target))
-            }
-            FlatToken::Empty => {}
-        }
-    }
-
-    if break_last_token > 0 {
-        let last_token = stack_top.inner.pop().unwrap();
-        if let AttrTokenTree::Token(last_token, spacing) = last_token {
-            let (unglued, _) = last_token.kind.break_two_token_op(break_last_token).unwrap();
-
-            // Tokens are always ASCII chars, so we can use byte arithmetic here.
-            let mut first_span = last_token.span.shrink_to_lo();
-            first_span =
-                first_span.with_hi(first_span.lo() + rustc_span::BytePos(break_last_token));
-
-            stack_top.inner.push(AttrTokenTree::Token(Token::new(unglued, first_span), spacing));
-        } else {
-            panic!("Unexpected last token {last_token:?}")
-        }
-    }
-    AttrTokenStream::new(stack_top.inner)
-}
-
 /// Tokens are needed if:
 /// - any non-single-segment attributes (other than doc comments) are present,
 ///   e.g. `rustfmt::skip`; or
@@ -562,14 +405,3 @@ fn needs_tokens(attrs: &[ast::Attribute]) -> bool {
         }
     })
 }
-
-// Some types are used a lot. Make sure they don't unintentionally get bigger.
-#[cfg(target_pointer_width = "64")]
-mod size_asserts {
-    use rustc_data_structures::static_assert_size;
-
-    use super::*;
-    // tidy-alphabetical-start
-    static_assert_size!(LazyAttrTokenStreamImpl, 96);
-    // tidy-alphabetical-end
-}
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index 48df8b59d55..5500fba58a5 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -12,7 +12,6 @@ pub mod token_type;
 mod ty;
 
 use std::assert_matches::debug_assert_matches;
-use std::ops::Range;
 use std::{fmt, mem, slice};
 
 use attr_wrapper::{AttrWrapper, UsePreAttrPos};
@@ -25,7 +24,9 @@ use rustc_ast::ptr::P;
 use rustc_ast::token::{
     self, IdentIsRaw, InvisibleOrigin, MetaVarKind, NtExprKind, NtPatKind, Token, TokenKind,
 };
-use rustc_ast::tokenstream::{AttrsTarget, Spacing, TokenStream, TokenTree};
+use rustc_ast::tokenstream::{
+    ParserRange, ParserReplacement, Spacing, TokenCursor, TokenStream, TokenTree, TokenTreeCursor,
+};
 use rustc_ast::util::case::Case;
 use rustc_ast::{
     self as ast, AnonConst, AttrArgs, AttrId, ByRef, Const, CoroutineKind, DUMMY_NODE_ID,
@@ -37,7 +38,7 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::{Applicability, Diag, FatalError, MultiSpan, PResult};
 use rustc_index::interval::IntervalSet;
 use rustc_session::parse::ParseSess;
-use rustc_span::{DUMMY_SP, Ident, Span, Symbol, kw, sym};
+use rustc_span::{Ident, Span, Symbol, kw, sym};
 use thin_vec::ThinVec;
 use token_type::TokenTypeSet;
 pub use token_type::{ExpKeywordPair, ExpTokenPair, TokenType};
@@ -187,57 +188,6 @@ struct ClosureSpans {
     body: Span,
 }
 
-/// A token range within a `Parser`'s full token stream.
-#[derive(Clone, Debug)]
-struct ParserRange(Range<u32>);
-
-/// A token range within an individual AST node's (lazy) token stream, i.e.
-/// relative to that node's first token. Distinct from `ParserRange` so the two
-/// kinds of range can't be mixed up.
-#[derive(Clone, Debug)]
-struct NodeRange(Range<u32>);
-
-/// Indicates a range of tokens that should be replaced by an `AttrsTarget`
-/// (replacement) or be replaced by nothing (deletion). This is used in two
-/// places during token collection.
-///
-/// 1. Replacement. During the parsing of an AST node that may have a
-///    `#[derive]` attribute, when we parse a nested AST node that has `#[cfg]`
-///    or `#[cfg_attr]`, we replace the entire inner AST node with
-///    `FlatToken::AttrsTarget`. This lets us perform eager cfg-expansion on an
-///    `AttrTokenStream`.
-///
-/// 2. Deletion. We delete inner attributes from all collected token streams,
-///    and instead track them through the `attrs` field on the AST node. This
-///    lets us manipulate them similarly to outer attributes. When we create a
-///    `TokenStream`, the inner attributes are inserted into the proper place
-///    in the token stream.
-///
-/// Each replacement starts off in `ParserReplacement` form but is converted to
-/// `NodeReplacement` form when it is attached to a single AST node, via
-/// `LazyAttrTokenStreamImpl`.
-type ParserReplacement = (ParserRange, Option<AttrsTarget>);
-
-/// See the comment on `ParserReplacement`.
-type NodeReplacement = (NodeRange, Option<AttrsTarget>);
-
-impl NodeRange {
-    // Converts a range within a parser's tokens to a range within a
-    // node's tokens beginning at `start_pos`.
-    //
-    // For example, imagine a parser with 50 tokens in its token stream, a
-    // function that spans `ParserRange(20..40)` and an inner attribute within
-    // that function that spans `ParserRange(30..35)`. We would find the inner
-    // attribute's range within the function's tokens by subtracting 20, which
-    // is the position of the function's start token. This gives
-    // `NodeRange(10..15)`.
-    fn new(ParserRange(parser_range): ParserRange, start_pos: u32) -> NodeRange {
-        assert!(!parser_range.is_empty());
-        assert!(parser_range.start >= start_pos);
-        NodeRange((parser_range.start - start_pos)..(parser_range.end - start_pos))
-    }
-}
-
 /// Controls how we capture tokens. Capturing can be expensive,
 /// so we try to avoid performing capturing in cases where
 /// we will never need an `AttrTokenStream`.
@@ -260,104 +210,6 @@ struct CaptureState {
     seen_attrs: IntervalSet<AttrId>,
 }
 
-#[derive(Clone, Debug)]
-struct TokenTreeCursor {
-    stream: TokenStream,
-    /// Points to the current token tree in the stream. In `TokenCursor::curr`,
-    /// this can be any token tree. In `TokenCursor::stack`, this is always a
-    /// `TokenTree::Delimited`.
-    index: usize,
-}
-
-impl TokenTreeCursor {
-    #[inline]
-    fn new(stream: TokenStream) -> Self {
-        TokenTreeCursor { stream, index: 0 }
-    }
-
-    #[inline]
-    fn curr(&self) -> Option<&TokenTree> {
-        self.stream.get(self.index)
-    }
-
-    fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
-        self.stream.get(self.index + n)
-    }
-
-    #[inline]
-    fn bump(&mut self) {
-        self.index += 1;
-    }
-}
-
-/// A `TokenStream` cursor that produces `Token`s. It's a bit odd that
-/// we (a) lex tokens into a nice tree structure (`TokenStream`), and then (b)
-/// use this type to emit them as a linear sequence. But a linear sequence is
-/// what the parser expects, for the most part.
-#[derive(Clone, Debug)]
-struct TokenCursor {
-    // Cursor for the current (innermost) token stream. The index within the
-    // cursor can point to any token tree in the stream (or one past the end).
-    // The delimiters for this token stream are found in `self.stack.last()`;
-    // if that is `None` we are in the outermost token stream which never has
-    // delimiters.
-    curr: TokenTreeCursor,
-
-    // Token streams surrounding the current one. The index within each cursor
-    // always points to a `TokenTree::Delimited`.
-    stack: Vec<TokenTreeCursor>,
-}
-
-impl TokenCursor {
-    fn next(&mut self) -> (Token, Spacing) {
-        self.inlined_next()
-    }
-
-    /// This always-inlined version should only be used on hot code paths.
-    #[inline(always)]
-    fn inlined_next(&mut self) -> (Token, Spacing) {
-        loop {
-            // FIXME: we currently don't return `Delimiter::Invisible` open/close delims. To fix
-            // #67062 we will need to, whereupon the `delim != Delimiter::Invisible` conditions
-            // below can be removed.
-            if let Some(tree) = self.curr.curr() {
-                match tree {
-                    &TokenTree::Token(token, spacing) => {
-                        debug_assert!(!token.kind.is_delim());
-                        let res = (token, spacing);
-                        self.curr.bump();
-                        return res;
-                    }
-                    &TokenTree::Delimited(sp, spacing, delim, ref tts) => {
-                        let trees = TokenTreeCursor::new(tts.clone());
-                        self.stack.push(mem::replace(&mut self.curr, trees));
-                        if !delim.skip() {
-                            return (Token::new(delim.as_open_token_kind(), sp.open), spacing.open);
-                        }
-                        // No open delimiter to return; continue on to the next iteration.
-                    }
-                };
-            } else if let Some(parent) = self.stack.pop() {
-                // We have exhausted this token stream. Move back to its parent token stream.
-                let Some(&TokenTree::Delimited(span, spacing, delim, _)) = parent.curr() else {
-                    panic!("parent should be Delimited")
-                };
-                self.curr = parent;
-                self.curr.bump(); // move past the `Delimited`
-                if !delim.skip() {
-                    return (Token::new(delim.as_close_token_kind(), span.close), spacing.close);
-                }
-                // No close delimiter to return; continue on to the next iteration.
-            } else {
-                // We have exhausted the outermost token stream. The use of
-                // `Spacing::Alone` is arbitrary and immaterial, because the
-                // `Eof` token's spacing is never used.
-                return (Token::new(token::Eof, DUMMY_SP), Spacing::Alone);
-            }
-        }
-    }
-}
-
 /// A sequence separator.
 #[derive(Debug)]
 struct SeqSep<'a> {
@@ -1742,26 +1594,6 @@ impl<'a> Parser<'a> {
     }
 }
 
-/// A helper struct used when building an `AttrTokenStream` from
-/// a `LazyAttrTokenStream`. Both delimiter and non-delimited tokens
-/// are stored as `FlatToken::Token`. A vector of `FlatToken`s
-/// is then 'parsed' to build up an `AttrTokenStream` with nested
-/// `AttrTokenTree::Delimited` tokens.
-#[derive(Debug, Clone)]
-enum FlatToken {
-    /// A token - this holds both delimiter (e.g. '{' and '}')
-    /// and non-delimiter tokens
-    Token((Token, Spacing)),
-    /// Holds the `AttrsTarget` for an AST node. The `AttrsTarget` is inserted
-    /// directly into the constructed `AttrTokenStream` as an
-    /// `AttrTokenTree::AttrsTarget`.
-    AttrsTarget(AttrsTarget),
-    /// A special 'empty' token that is ignored during the conversion
-    /// to an `AttrTokenStream`. This is used to simplify the
-    /// handling of replace ranges.
-    Empty,
-}
-
 // Metavar captures of various kinds.
 #[derive(Clone, Debug)]
 pub enum ParseNtResult {
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 4a252a7b528..d2da3ac7d86 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -19,6 +19,7 @@
 #![feature(iter_intersperse)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
+#![recursion_limit = "256"]
 // tidy-alphabetical-end
 
 use std::cell::{Cell, RefCell};