Auto merge of #34652 - jseyfried:fix_expansion_perf, r=nrc

Fix expansion performance regression

**syntax-[breaking-change] cc #31645**

This fixes #34630 by reverting commit 5bf7970 of PR #33943, which landed in #34424.

By removing the `Rc<_>` wrapping around `Delimited` and `SequenceRepetition` in `TokenTree`, 5bf7970 made cloning `TokenTree`s more expensive. While this had no measurable performance impact on the compiler's crates, it caused an order of magnitude performance regression on some macro-heavy code in the wild. I believe this is due to clones of `TokenTree`s in `macro_parser.rs` and/or `macro_rules.rs`.

r? @nrc

1 files changed, 6 insertions, 4 deletions

diff --git a/src/libsyntax/tokenstream.rs b/src/libsyntax/tokenstream.rs
index 35377d14bab..f0f0a7bc580 100644
--- a/src/libsyntax/tokenstream.rs
+++ b/src/libsyntax/tokenstream.rs
@@ -21,6 +21,8 @@ use parse::lexer::comments::{doc_comment_style, strip_doc_comment_decoration};
 use parse::lexer;
 use parse::token;
 
+use std::rc::Rc;
+
 /// A delimited sequence of token trees
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub struct Delimited {
@@ -94,13 +96,13 @@ pub enum TokenTree {
     /// A single token
     Token(Span, token::Token),
     /// A delimited sequence of token trees
-    Delimited(Span, Delimited),
+    Delimited(Span, Rc<Delimited>),
 
     // This only makes sense in MBE macros.
 
     /// A kleene-style repetition sequence with a span
     // FIXME(eddyb) #12938 Use DST.
-    Sequence(Span, SequenceRepetition),
+    Sequence(Span, Rc<SequenceRepetition>),
 }
 
 impl TokenTree {
@@ -149,7 +151,7 @@ impl TokenTree {
                     Some(*cnt)
                 }).max().unwrap_or(0);
 
-                TokenTree::Delimited(sp, Delimited {
+                TokenTree::Delimited(sp, Rc::new(Delimited {
                     delim: token::Bracket,
                     open_span: sp,
                     tts: vec![TokenTree::Token(sp, token::Ident(token::str_to_ident("doc"))),
@@ -157,7 +159,7 @@ impl TokenTree {
                               TokenTree::Token(sp, token::Literal(
                                   token::StrRaw(token::intern(&stripped), num_of_hashes), None))],
                     close_span: sp,
-                })
+                }))
             }
             (&TokenTree::Delimited(_, ref delimed), _) => {
                 if index == 0 {