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, 6 deletions

diff --git a/src/libstd/workcache.rs b/src/libstd/workcache.rs
index 9b0a6cb6226..a9e4ec50c7c 100644
--- a/src/libstd/workcache.rs
+++ b/src/libstd/workcache.rs
@@ -22,6 +22,7 @@ use core::io;
 use core::pipes::recv;
 use core::run;
 use core::to_bytes;
+use core::util::replace;
 
 /**
 *
@@ -352,9 +353,7 @@ impl TPrep for Prep {
 
             _ => {
                 let (port, chan) = oneshot();
-                let mut blk = None;
-                blk <-> bo;
-                let blk = blk.unwrap();
+                let blk = replace(&mut bo, None).unwrap();
                 let chan = Cell(chan);
 
                 do task::spawn {
@@ -386,9 +385,7 @@ fn unwrap<T:Owned +
             Decodable<json::Decoder>>( // FIXME(#5121)
         w: Work<T>) -> T {
     let mut ww = w;
-    let mut s = None;
-
-    ww.res <-> s;
+    let s = replace(&mut ww.res, None);
 
     match s {
         None => fail!(),