perform TokenStream replacement in-place when possible in expand_macro

compiler/rustc_ast/src/tokenstream.rs

@@ -25,7 +25,7 @@ use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use rustc_span::{Span, DUMMY_SP};
 use smallvec::{smallvec, SmallVec};
 
-use std::{fmt, iter};
+use std::{fmt, iter, mem};
 
 /// When the main Rust parser encounters a syntax-extension invocation, it
 /// parses the arguments to the invocation as a token tree. This is a very
@@ -410,8 +410,17 @@ impl TokenStream {
         t1.next().is_none() && t2.next().is_none()
     }
 
-    pub fn map_enumerated<F: FnMut(usize, &TokenTree) -> TokenTree>(self, mut f: F) -> TokenStream {
-        TokenStream(Lrc::new(self.0.iter().enumerate().map(|(i, tree)| f(i, tree)).collect()))
+    /// Applies the supplied function to each `TokenTree` and its index in `self`, returning a new `TokenStream`
+    ///
+    /// It is equivalent to `TokenStream::new(self.trees().cloned().enumerate().map(|(i, tt)| f(i, tt)).collect())`.
+    pub fn map_enumerated_owned(
+        mut self,
+        mut f: impl FnMut(usize, TokenTree) -> TokenTree,
+    ) -> TokenStream {
+        let owned = Lrc::make_mut(&mut self.0); // clone if necessary
+        // rely on vec's in-place optimizations to avoid another allocation
+        *owned = mem::take(owned).into_iter().enumerate().map(|(i, tree)| f(i, tree)).collect();
+        self
     }
 
     /// Create a token stream containing a single token with alone spacing.

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -223,8 +223,7 @@ fn expand_macro<'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 tts.len() == rhs.tts.len() {
-                tts = tts.map_enumerated(|i, tt| {
-                    let mut tt = tt.clone();
+                tts = tts.map_enumerated_owned(|i, mut tt| {
                     let rhs_tt = &rhs.tts[i];
                     let ctxt = tt.span().ctxt();
                     match (&mut tt, rhs_tt) {