From f6aec82d4d7c9a75698808c28454d601aceac06f Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Mon, 14 Sep 2020 01:45:10 -0400
Subject: [PATCH] Avoid cloning the contents of a `TokenStream` in a few places

---
 compiler/rustc_ast/src/tokenstream.rs        | 10 +++++-----
 compiler/rustc_expand/src/mbe/macro_rules.rs |  3 ++-
 compiler/rustc_parse/src/parser/mod.rs       | 10 +++++-----
 3 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/compiler/rustc_ast/src/tokenstream.rs b/compiler/rustc_ast/src/tokenstream.rs
index 8acb6b2f37589..3363c7223cc48 100644
--- a/compiler/rustc_ast/src/tokenstream.rs
+++ b/compiler/rustc_ast/src/tokenstream.rs
@@ -125,7 +125,7 @@ where
 /// instead of a representation of the abstract syntax tree.
 /// Today's `TokenTree`s can still contain AST via `token::Interpolated` for back-compat.
 #[derive(Clone, Debug, Default, Encodable, Decodable)]
-pub struct TokenStream(pub Lrc<Vec<TreeAndSpacing>>);
+pub struct TokenStream(pub(crate) Lrc<Vec<TreeAndSpacing>>);
 
 pub type TreeAndSpacing = (TokenTree, Spacing);
 
@@ -286,12 +286,12 @@ impl TokenStream {
         t1.next().is_none() && t2.next().is_none()
     }
 
-    pub fn map_enumerated<F: FnMut(usize, TokenTree) -> TokenTree>(self, mut f: F) -> TokenStream {
+    pub fn map_enumerated<F: FnMut(usize, &TokenTree) -> TokenTree>(self, mut f: F) -> TokenStream {
         TokenStream(Lrc::new(
             self.0
                 .iter()
                 .enumerate()
-                .map(|(i, (tree, is_joint))| (f(i, tree.clone()), *is_joint))
+                .map(|(i, (tree, is_joint))| (f(i, tree), *is_joint))
                 .collect(),
         ))
     }
@@ -394,8 +394,8 @@ impl Cursor {
         self.index = index;
     }
 
-    pub fn look_ahead(&self, n: usize) -> Option<TokenTree> {
-        self.stream.0[self.index..].get(n).map(|(tree, _)| tree.clone())
+    pub fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
+        self.stream.0[self.index..].get(n).map(|(tree, _)| tree)
     }
 }
 
diff --git a/compiler/rustc_expand/src/mbe/macro_rules.rs b/compiler/rustc_expand/src/mbe/macro_rules.rs
index f0e6fe39a3c7f..791d2686cb5eb 100644
--- a/compiler/rustc_expand/src/mbe/macro_rules.rs
+++ b/compiler/rustc_expand/src/mbe/macro_rules.rs
@@ -288,7 +288,8 @@ fn generic_extension<'cx>(
                 // 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, mut tt| {
+                    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);
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index 7970ad36456d1..1860f1238c4b5 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -833,15 +833,15 @@ impl<'a> Parser<'a> {
         }
 
         let frame = &self.token_cursor.frame;
-        looker(&match frame.tree_cursor.look_ahead(dist - 1) {
+        match frame.tree_cursor.look_ahead(dist - 1) {
             Some(tree) => match tree {
-                TokenTree::Token(token) => token,
+                TokenTree::Token(token) => looker(token),
                 TokenTree::Delimited(dspan, delim, _) => {
-                    Token::new(token::OpenDelim(delim), dspan.open)
+                    looker(&Token::new(token::OpenDelim(*delim), dspan.open))
                 }
             },
-            None => Token::new(token::CloseDelim(frame.delim), frame.span.close),
-        })
+            None => looker(&Token::new(token::CloseDelim(frame.delim), frame.span.close)),
+        }
     }
 
     /// Returns whether any of the given keywords are `dist` tokens ahead of the current one.