Avoid allocations in `Decoder::read_str`.

`opaque::Decoder::read_str` is very hot within `rustc` due to its use in
the reading of crate metadata, and it currently returns a `String`. This
commit changes it to instead return a `Cow<str>`, which avoids a heap
allocation.

This change reduces the number of calls to `malloc` by almost 10% in
some benchmarks.

This is a [breaking-change] to libserialize.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index 09bc5607946..73d9695a990 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -566,7 +566,7 @@ impl PartialEq<InternedString> for str {
 
 impl Decodable for InternedString {
     fn decode<D: Decoder>(d: &mut D) -> Result<InternedString, D::Error> {
-        Ok(intern(d.read_str()?.as_ref()).as_str())
+        Ok(intern(&d.read_str()?).as_str())
     }
 }