auto merge of #6260 : alexcrichton/rust/issue-3466-no-swap, r=pcwalton

There may be a more efficient implementation of `core::util::swap_ptr`. The issue mentioned using `move_val_init`, but I couldn't figure out what that did, so I just used `copy_memory` a few times instead.

I'm not exactly the best at reading LLVM generated by rust, but this does appear to be optimized away just as expected (when possible).

1 files changed, 3 insertions, 4 deletions

diff --git a/src/libstd/future.rs b/src/libstd/future.rs
index b1b2fa2cd28..ac23ea1a6e2 100644
--- a/src/libstd/future.rs
+++ b/src/libstd/future.rs
@@ -26,6 +26,7 @@ use core::cell::Cell;
 use core::comm::{PortOne, oneshot, send_one};
 use core::pipes::recv;
 use core::task;
+use core::util::replace;
 
 #[doc = "The future type"]
 #[cfg(stage0)]
@@ -77,8 +78,7 @@ pub impl<A> Future<A> {
                 }
             }
             {
-                let mut state = Evaluating;
-                self.state <-> state;
+                let state = replace(&mut self.state, Evaluating);
                 match state {
                     Forced(_) | Evaluating => fail!(~"Logic error."),
                     Pending(f) => {
@@ -108,8 +108,7 @@ pub impl<A> Future<A> {
                 }
             }
             {
-                let mut state = Evaluating;
-                self.state <-> state;
+                let state = replace(&mut self.state, Evaluating);
                 match state {
                     Forced(_) | Evaluating => fail!(~"Logic error."),
                     Pending(f) => {