Use `SmallVec` in `TokenStreamBuilder`.

This reduces by 12% the number of allocations done for a "clean
incremental" of `webrender_api`, which reduces the instruction count by
about 0.5%.

It also reduces instruction counts by up to 1.4% across a range of
rustc-perf benchmark runs.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/libsyntax/parse/attr.rs b/src/libsyntax/parse/attr.rs
index 4211268f33e..e99a86e807f 100644
--- a/src/libsyntax/parse/attr.rs
+++ b/src/libsyntax/parse/attr.rs
@@ -6,6 +6,7 @@ use crate::parse::parser::{Parser, TokenType, PathStyle};
 use crate::tokenstream::{TokenStream, TokenTree};
 
 use log::debug;
+use smallvec::smallvec;
 
 #[derive(Debug)]
 enum InnerAttributeParsePolicy<'a> {
@@ -171,7 +172,7 @@ impl<'a> Parser<'a> {
                 } else {
                     self.parse_unsuffixed_lit()?.tokens()
                 };
-                TokenStream::from_streams(vec![eq.into(), tokens])
+                TokenStream::from_streams(smallvec![eq.into(), tokens])
             } else {
                 TokenStream::empty()
             };