Auto merge of #114115 - nnethercote:less-token-tree-cloning, r=petrochenkov

Less `TokenTree` cloning

`TokenTreeCursor` has this comment on it:
```
// FIXME: Many uses of this can be replaced with by-reference iterator to avoid clones.
```
This PR completes that FIXME. It doesn't have much perf effect, but at least we now know that.

r? `@petrochenkov`

9 files changed, 83 insertions, 90 deletions

diff --git a/compiler/rustc_ast/src/attr/mod.rs b/compiler/rustc_ast/src/attr/mod.rs
index 15fe29580c2..8b9bb1df5dc 100644
--- a/compiler/rustc_ast/src/attr/mod.rs
+++ b/compiler/rustc_ast/src/attr/mod.rs
@@ -285,17 +285,17 @@ impl MetaItem {
         self.kind.value_str()
     }
 
-    fn from_tokens<I>(tokens: &mut iter::Peekable<I>) -> Option<MetaItem>
+    fn from_tokens<'a, I>(tokens: &mut iter::Peekable<I>) -> Option<MetaItem>
     where
-        I: Iterator<Item = TokenTree>,
+        I: Iterator<Item = &'a TokenTree>,
     {
         // FIXME: Share code with `parse_path`.
-        let path = match tokens.next().map(TokenTree::uninterpolate) {
-            Some(TokenTree::Token(
-                Token { kind: kind @ (token::Ident(..) | token::ModSep), span },
+        let path = match tokens.next().map(|tt| TokenTree::uninterpolate(tt)).as_deref() {
+            Some(&TokenTree::Token(
+                Token { kind: ref kind @ (token::Ident(..) | token::ModSep), span },
                 _,
             )) => 'arm: {
-                let mut segments = if let token::Ident(name, _) = kind {
+                let mut segments = if let &token::Ident(name, _) = kind {
                     if let Some(TokenTree::Token(Token { kind: token::ModSep, .. }, _)) =
                         tokens.peek()
                     {
@@ -308,8 +308,8 @@ impl MetaItem {
                     thin_vec![PathSegment::path_root(span)]
                 };
                 loop {
-                    if let Some(TokenTree::Token(Token { kind: token::Ident(name, _), span }, _)) =
-                        tokens.next().map(TokenTree::uninterpolate)
+                    if let Some(&TokenTree::Token(Token { kind: token::Ident(name, _), span }, _)) =
+                        tokens.next().map(|tt| TokenTree::uninterpolate(tt)).as_deref()
                     {
                         segments.push(PathSegment::from_ident(Ident::new(name, span)));
                     } else {
@@ -326,7 +326,7 @@ impl MetaItem {
                 let span = span.with_hi(segments.last().unwrap().ident.span.hi());
                 Path { span, segments, tokens: None }
             }
-            Some(TokenTree::Token(Token { kind: token::Interpolated(nt), .. }, _)) => match &*nt {
+            Some(TokenTree::Token(Token { kind: token::Interpolated(nt), .. }, _)) => match &**nt {
                 token::Nonterminal::NtMeta(item) => return item.meta(item.path.span),
                 token::Nonterminal::NtPath(path) => (**path).clone(),
                 _ => return None,
@@ -354,7 +354,7 @@ impl MetaItemKind {
     }
 
     fn list_from_tokens(tokens: TokenStream) -> Option<ThinVec<NestedMetaItem>> {
-        let mut tokens = tokens.into_trees().peekable();
+        let mut tokens = tokens.trees().peekable();
         let mut result = ThinVec::new();
         while tokens.peek().is_some() {
             let item = NestedMetaItem::from_tokens(&mut tokens)?;
@@ -367,12 +367,12 @@ impl MetaItemKind {
         Some(result)
     }
 
-    fn name_value_from_tokens(
-        tokens: &mut impl Iterator<Item = TokenTree>,
+    fn name_value_from_tokens<'a>(
+        tokens: &mut impl Iterator<Item = &'a TokenTree>,
     ) -> Option<MetaItemKind> {
         match tokens.next() {
             Some(TokenTree::Delimited(_, Delimiter::Invisible, inner_tokens)) => {
-                MetaItemKind::name_value_from_tokens(&mut inner_tokens.into_trees())
+                MetaItemKind::name_value_from_tokens(&mut inner_tokens.trees())
             }
             Some(TokenTree::Token(token, _)) => {
                 MetaItemLit::from_token(&token).map(MetaItemKind::NameValue)
@@ -381,8 +381,8 @@ impl MetaItemKind {
         }
     }
 
-    fn from_tokens(
-        tokens: &mut iter::Peekable<impl Iterator<Item = TokenTree>>,
+    fn from_tokens<'a>(
+        tokens: &mut iter::Peekable<impl Iterator<Item = &'a TokenTree>>,
     ) -> Option<MetaItemKind> {
         match tokens.peek() {
             Some(TokenTree::Delimited(_, Delimiter::Parenthesis, inner_tokens)) => {
@@ -501,9 +501,9 @@ impl NestedMetaItem {
         self.meta_item().is_some()
     }
 
-    fn from_tokens<I>(tokens: &mut iter::Peekable<I>) -> Option<NestedMetaItem>
+    fn from_tokens<'a, I>(tokens: &mut iter::Peekable<I>) -> Option<NestedMetaItem>
     where
-        I: Iterator<Item = TokenTree>,
+        I: Iterator<Item = &'a TokenTree>,
     {
         match tokens.peek() {
             Some(TokenTree::Token(token, _))
@@ -513,9 +513,8 @@ impl NestedMetaItem {
                 return Some(NestedMetaItem::Lit(lit));
             }
             Some(TokenTree::Delimited(_, Delimiter::Invisible, inner_tokens)) => {
-                let inner_tokens = inner_tokens.clone();
                 tokens.next();
-                return NestedMetaItem::from_tokens(&mut inner_tokens.into_trees().peekable());
+                return NestedMetaItem::from_tokens(&mut inner_tokens.trees().peekable());
             }
             _ => {}
         }
diff --git a/compiler/rustc_ast/src/tokenstream.rs b/compiler/rustc_ast/src/tokenstream.rs
index ca4a739abd7..348c37c480f 100644
--- a/compiler/rustc_ast/src/tokenstream.rs
+++ b/compiler/rustc_ast/src/tokenstream.rs
@@ -25,6 +25,7 @@ use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use rustc_span::{Span, DUMMY_SP};
 use smallvec::{smallvec, SmallVec};
 
+use std::borrow::Cow;
 use std::{fmt, iter, mem};
 
 /// When the main Rust parser encounters a syntax-extension invocation, it
@@ -98,12 +99,13 @@ impl TokenTree {
         TokenTree::Token(Token::new(kind, span), Spacing::Joint)
     }
 
-    pub fn uninterpolate(self) -> TokenTree {
+    pub fn uninterpolate(&self) -> Cow<'_, TokenTree> {
         match self {
-            TokenTree::Token(token, spacing) => {
-                TokenTree::Token(token.uninterpolate().into_owned(), spacing)
-            }
-            tt => tt,
+            TokenTree::Token(token, spacing) => match token.uninterpolate() {
+                Cow::Owned(token) => Cow::Owned(TokenTree::Token(token, *spacing)),
+                Cow::Borrowed(_) => Cow::Borrowed(self),
+            },
+            _ => Cow::Borrowed(self),
         }
     }
 }
@@ -595,26 +597,21 @@ impl<'t> Iterator for RefTokenTreeCursor<'t> {
     }
 }
 
-/// Owning by-value iterator over a [`TokenStream`], that produces `TokenTree`
+/// Owning by-value iterator over a [`TokenStream`], that produces `&TokenTree`
 /// items.
-// FIXME: Many uses of this can be replaced with by-reference iterator to avoid clones.
+///
+/// Doesn't impl `Iterator` because Rust doesn't permit an owning iterator to
+/// return `&T` from `next`; the need for an explicit lifetime in the `Item`
+/// associated type gets in the way. Instead, use `next_ref` (which doesn't
+/// involve associated types) for getting individual elements, or
+/// `RefTokenTreeCursor` if you really want an `Iterator`, e.g. in a `for`
+/// loop.
 #[derive(Clone)]
 pub struct TokenTreeCursor {
     pub stream: TokenStream,
     index: usize,
 }
 
-impl Iterator for TokenTreeCursor {
-    type Item = TokenTree;
-
-    fn next(&mut self) -> Option<TokenTree> {
-        self.stream.0.get(self.index).map(|tree| {
-            self.index += 1;
-            tree.clone()
-        })
-    }
-}
-
 impl TokenTreeCursor {
     fn new(stream: TokenStream) -> Self {
         TokenTreeCursor { stream, index: 0 }
diff --git a/compiler/rustc_expand/src/config.rs b/compiler/rustc_expand/src/config.rs
index 4ec5ac22e90..aeb4f6e861b 100644
--- a/compiler/rustc_expand/src/config.rs
+++ b/compiler/rustc_expand/src/config.rs
@@ -365,9 +365,9 @@ impl<'a> StripUnconfigured<'a> {
 
         // 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 mut orig_trees = orig_tokens.trees();
         let TokenTree::Token(pound_token @ Token { kind: TokenKind::Pound, .. }, _) =
-            orig_trees.next().unwrap()
+            orig_trees.next().unwrap().clone()
         else {
             panic!("Bad tokens for attribute {:?}", attr);
         };
@@ -377,7 +377,7 @@ impl<'a> StripUnconfigured<'a> {
         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()
+                orig_trees.next().unwrap().clone()
             else {
                 panic!("Bad tokens for attribute {:?}", attr);
             };
diff --git a/compiler/rustc_expand/src/mbe/macro_rules.rs b/compiler/rustc_expand/src/mbe/macro_rules.rs
index 102bae2a744..7398a124fdb 100644
--- a/compiler/rustc_expand/src/mbe/macro_rules.rs
+++ b/compiler/rustc_expand/src/mbe/macro_rules.rs
@@ -500,7 +500,7 @@ pub fn compile_declarative_macro(
             .map(|m| {
                 if let MatchedTokenTree(tt) = m {
                     let tt = mbe::quoted::parse(
-                        TokenStream::new(vec![tt.clone()]),
+                        &TokenStream::new(vec![tt.clone()]),
                         true,
                         &sess.parse_sess,
                         def.id,
@@ -524,7 +524,7 @@ pub fn compile_declarative_macro(
             .map(|m| {
                 if let MatchedTokenTree(tt) = m {
                     return mbe::quoted::parse(
-                        TokenStream::new(vec![tt.clone()]),
+                        &TokenStream::new(vec![tt.clone()]),
                         false,
                         &sess.parse_sess,
                         def.id,
diff --git a/compiler/rustc_expand/src/mbe/quoted.rs b/compiler/rustc_expand/src/mbe/quoted.rs
index 40bfa3715be..ac862ae8c4f 100644
--- a/compiler/rustc_expand/src/mbe/quoted.rs
+++ b/compiler/rustc_expand/src/mbe/quoted.rs
@@ -36,7 +36,7 @@ const VALID_FRAGMENT_NAMES_MSG: &str = "valid fragment specifiers are \
 ///
 /// A collection of `self::TokenTree`. There may also be some errors emitted to `sess`.
 pub(super) fn parse(
-    input: tokenstream::TokenStream,
+    input: &tokenstream::TokenStream,
     parsing_patterns: bool,
     sess: &ParseSess,
     node_id: NodeId,
@@ -48,7 +48,7 @@ pub(super) fn parse(
 
     // 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();
+    let mut trees = input.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`).
@@ -56,7 +56,7 @@ pub(super) fn parse(
         match tree {
             TokenTree::MetaVar(start_sp, ident) if parsing_patterns => {
                 let span = match trees.next() {
-                    Some(tokenstream::TokenTree::Token(Token { kind: token::Colon, span }, _)) => {
+                    Some(&tokenstream::TokenTree::Token(Token { kind: token::Colon, span }, _)) => {
                         match trees.next() {
                             Some(tokenstream::TokenTree::Token(token, _)) => match token.ident() {
                                 Some((frag, _)) => {
@@ -96,10 +96,10 @@ pub(super) fn parse(
                                 }
                                 _ => token.span,
                             },
-                            tree => tree.as_ref().map_or(span, tokenstream::TokenTree::span),
+                            tree => tree.map_or(span, tokenstream::TokenTree::span),
                         }
                     }
-                    tree => tree.as_ref().map_or(start_sp, tokenstream::TokenTree::span),
+                    tree => tree.map_or(start_sp, tokenstream::TokenTree::span),
                 };
 
                 result.push(TokenTree::MetaVarDecl(span, ident, None));
@@ -134,9 +134,9 @@ fn maybe_emit_macro_metavar_expr_feature(features: &Features, sess: &ParseSess,
 /// - `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>,
+fn parse_tree<'a>(
+    tree: &'a tokenstream::TokenTree,
+    outer_trees: &mut impl Iterator<Item = &'a tokenstream::TokenTree>,
     parsing_patterns: bool,
     sess: &ParseSess,
     node_id: NodeId,
@@ -146,13 +146,13 @@ fn parse_tree(
     // 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 }, _) => {
+        &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>>;
+            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());
+                trees = Box::new(tts.trees());
                 next = trees.next();
             } else {
                 trees = Box::new(outer_trees);
@@ -160,7 +160,7 @@ fn parse_tree(
 
             match next {
                 // `tree` is followed by a delimited set of token trees.
-                Some(tokenstream::TokenTree::Delimited(delim_span, delim, tts)) => {
+                Some(&tokenstream::TokenTree::Delimited(delim_span, delim, ref tts)) => {
                     if parsing_patterns {
                         if delim != Delimiter::Parenthesis {
                             span_dollar_dollar_or_metavar_in_the_lhs_err(
@@ -228,7 +228,7 @@ fn parse_tree(
                 }
 
                 // `tree` is followed by another `$`. This is an escaped `$`.
-                Some(tokenstream::TokenTree::Token(Token { kind: token::Dollar, span }, _)) => {
+                Some(&tokenstream::TokenTree::Token(Token { kind: token::Dollar, span }, _)) => {
                     if parsing_patterns {
                         span_dollar_dollar_or_metavar_in_the_lhs_err(
                             sess,
@@ -256,11 +256,11 @@ fn parse_tree(
         }
 
         // `tree` is an arbitrary token. Keep it.
-        tokenstream::TokenTree::Token(token, _) => TokenTree::Token(token),
+        tokenstream::TokenTree::Token(token, _) => TokenTree::Token(token.clone()),
 
         // `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(
+        &tokenstream::TokenTree::Delimited(span, delim, ref tts) => TokenTree::Delimited(
             span,
             Delimited {
                 delim,
@@ -286,16 +286,16 @@ fn kleene_op(token: &Token) -> Option<KleeneOp> {
 /// - 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>,
+fn parse_kleene_op<'a>(
+    input: &mut impl Iterator<Item = &'a 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)),
+            None => Ok(Err(token.clone())),
         },
-        tree => Err(tree.as_ref().map_or(span, tokenstream::TokenTree::span)),
+        tree => Err(tree.map_or(span, tokenstream::TokenTree::span)),
     }
 }
 
@@ -311,8 +311,8 @@ fn parse_kleene_op(
 /// 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>,
+fn parse_sep_and_kleene_op<'a>(
+    input: &mut impl Iterator<Item = &'a tokenstream::TokenTree>,
     span: Span,
     sess: &ParseSess,
 ) -> (Option<Token>, KleeneToken) {
diff --git a/compiler/rustc_expand/src/parse/tests.rs b/compiler/rustc_expand/src/parse/tests.rs
index e133501c5d0..bdc20882a9d 100644
--- a/compiler/rustc_expand/src/parse/tests.rs
+++ b/compiler/rustc_expand/src/parse/tests.rs
@@ -69,9 +69,8 @@ fn bad_path_expr_1() {
 #[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[..];
+        let stream = string_to_stream("macro_rules! zip (($a)=>($a))".to_string());
+        let tts = &stream.trees().collect::<Vec<_>>()[..];
 
         match tts {
             [
@@ -300,9 +299,7 @@ fn ttdelim_span() {
         .unwrap();
 
         let ast::ExprKind::MacCall(mac) = &expr.kind else { panic!("not a macro") };
-        let tts: Vec<_> = mac.args.tokens.clone().into_trees().collect();
-
-        let span = tts.iter().rev().next().unwrap().span();
+        let span = mac.args.tokens.trees().last().unwrap().span();
 
         match sess.source_map().span_to_snippet(span) {
             Ok(s) => assert_eq!(&s[..], "{ body }"),
diff --git a/compiler/rustc_expand/src/proc_macro_server.rs b/compiler/rustc_expand/src/proc_macro_server.rs
index ecd2315112a..ac73b5d72b7 100644
--- a/compiler/rustc_expand/src/proc_macro_server.rs
+++ b/compiler/rustc_expand/src/proc_macro_server.rs
@@ -94,10 +94,10 @@ impl FromInternal<(TokenStream, &mut Rustc<'_, '_>)> for Vec<TokenTree<TokenStre
         // 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();
+        let mut cursor = stream.trees();
 
         while let Some(tree) = cursor.next() {
-            let (Token { kind, span }, joint) = match tree {
+            let (Token { kind, span }, joint) = match tree.clone() {
                 tokenstream::TokenTree::Delimited(span, delim, tts) => {
                     let delimiter = pm::Delimiter::from_internal(delim);
                     trees.push(TokenTree::Group(Group {
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index e022cec5b51..aeae1dd0570 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -2489,19 +2489,19 @@ fn get_all_import_attributes<'hir>(
 }
 
 fn filter_tokens_from_list(
-    args_tokens: TokenStream,
+    args_tokens: &TokenStream,
     should_retain: impl Fn(&TokenTree) -> bool,
 ) -> Vec<TokenTree> {
     let mut tokens = Vec::with_capacity(args_tokens.len());
     let mut skip_next_comma = false;
-    for token in args_tokens.into_trees() {
+    for token in args_tokens.trees() {
         match token {
             TokenTree::Token(Token { kind: TokenKind::Comma, .. }, _) if skip_next_comma => {
                 skip_next_comma = false;
             }
-            token if should_retain(&token) => {
+            token if should_retain(token) => {
                 skip_next_comma = false;
-                tokens.push(token);
+                tokens.push(token.clone());
             }
             _ => {
                 skip_next_comma = true;
@@ -2559,7 +2559,7 @@ fn add_without_unwanted_attributes<'hir>(
                     match normal.item.args {
                         ast::AttrArgs::Delimited(ref mut args) => {
                             let tokens =
-                                filter_tokens_from_list(args.tokens.clone(), |token| {
+                                filter_tokens_from_list(&args.tokens, |token| {
                                     !matches!(
                                         token,
                                         TokenTree::Token(
diff --git a/src/tools/rustfmt/src/macros.rs b/src/tools/rustfmt/src/macros.rs
index e9a298a2769..4f45d0c7402 100644
--- a/src/tools/rustfmt/src/macros.rs
+++ b/src/tools/rustfmt/src/macros.rs
@@ -13,7 +13,7 @@ use std::collections::HashMap;
 use std::panic::{catch_unwind, AssertUnwindSafe};
 
 use rustc_ast::token::{BinOpToken, Delimiter, Token, TokenKind};
-use rustc_ast::tokenstream::{TokenStream, TokenTree, TokenTreeCursor};
+use rustc_ast::tokenstream::{RefTokenTreeCursor, TokenStream, TokenTree};
 use rustc_ast::{ast, ptr};
 use rustc_ast_pretty::pprust;
 use rustc_span::{
@@ -394,7 +394,7 @@ pub(crate) fn rewrite_macro_def(
     }
 
     let ts = def.body.tokens.clone();
-    let mut parser = MacroParser::new(ts.into_trees());
+    let mut parser = MacroParser::new(ts.trees());
     let parsed_def = match parser.parse() {
         Some(def) => def,
         None => return snippet,
@@ -736,9 +736,9 @@ impl MacroArgParser {
         self.buf.clear();
     }
 
-    fn add_meta_variable(&mut self, iter: &mut TokenTreeCursor) -> Option<()> {
+    fn add_meta_variable(&mut self, iter: &mut RefTokenTreeCursor<'_>) -> Option<()> {
         match iter.next() {
-            Some(TokenTree::Token(
+            Some(&TokenTree::Token(
                 Token {
                     kind: TokenKind::Ident(name, _),
                     ..
@@ -768,7 +768,7 @@ impl MacroArgParser {
         &mut self,
         inner: Vec<ParsedMacroArg>,
         delim: Delimiter,
-        iter: &mut TokenTreeCursor,
+        iter: &mut RefTokenTreeCursor<'_>,
     ) -> Option<()> {
         let mut buffer = String::new();
         let mut first = true;
@@ -868,11 +868,11 @@ impl MacroArgParser {
 
     /// Returns a collection of parsed macro def's arguments.
     fn parse(mut self, tokens: TokenStream) -> Option<Vec<ParsedMacroArg>> {
-        let mut iter = tokens.into_trees();
+        let mut iter = tokens.trees();
 
         while let Some(tok) = iter.next() {
             match tok {
-                TokenTree::Token(
+                &TokenTree::Token(
                     Token {
                         kind: TokenKind::Dollar,
                         span,
@@ -901,7 +901,7 @@ impl MacroArgParser {
                     self.add_meta_variable(&mut iter)?;
                 }
                 TokenTree::Token(ref t, _) => self.update_buffer(t),
-                TokenTree::Delimited(_delimited_span, delimited, ref tts) => {
+                &TokenTree::Delimited(_delimited_span, delimited, ref tts) => {
                     if !self.buf.is_empty() {
                         if next_space(&self.last_tok.kind) == SpaceState::Always {
                             self.add_separator();
@@ -1119,12 +1119,12 @@ pub(crate) fn macro_style(mac: &ast::MacCall, context: &RewriteContext<'_>) -> D
 
 // A very simple parser that just parses a macros 2.0 definition into its branches.
 // Currently we do not attempt to parse any further than that.
-struct MacroParser {
-    toks: TokenTreeCursor,
+struct MacroParser<'a> {
+    toks: RefTokenTreeCursor<'a>,
 }
 
-impl MacroParser {
-    const fn new(toks: TokenTreeCursor) -> Self {
+impl<'a> MacroParser<'a> {
+    const fn new(toks: RefTokenTreeCursor<'a>) -> Self {
         Self { toks }
     }
 
@@ -1143,9 +1143,9 @@ impl MacroParser {
         let tok = self.toks.next()?;
         let (lo, args_paren_kind) = match tok {
             TokenTree::Token(..) => return None,
-            TokenTree::Delimited(delimited_span, d, _) => (delimited_span.open.lo(), d),
+            &TokenTree::Delimited(delimited_span, d, _) => (delimited_span.open.lo(), d),
         };
-        let args = TokenStream::new(vec![tok]);
+        let args = TokenStream::new(vec![tok.clone()]);
         match self.toks.next()? {
             TokenTree::Token(
                 Token {