Don't copy ByRef passed types to local stack slot when not necessary

Eg when the local is immutable **and** the type is freeze.

This makes the simple raytracer runtime benchmark 1% faster than cg_llvm
without optimizations. Before it was 2% slower.

cc #691
cc #684

1 files changed, 4 insertions, 0 deletions

diff --git a/example/example.rs b/example/example.rs
index e8ddc4aaea1..5878e8548d9 100644
--- a/example/example.rs
+++ b/example/example.rs
@@ -202,3 +202,7 @@ fn get_sized_field_ref_from_unsized_type(u: &Unsized) -> &u8 {
 fn get_unsized_field_ref_from_unsized_type(u: &Unsized) -> &str {
     &u.1
 }
+
+pub fn reuse_byref_argument_storage(a: (u8, u16, u32)) -> u8 {
+    a.0
+}