Fix codegen breaking aliasing rules for functions with sret results

This reverts commit a0ec902e23 "Avoid
unnecessary temporary on assignments".

Leaving out the temporary for the functions return value can lead to a
situation that conflicts with rust's aliasing rules.

Given this:

````rust
fn func(f: &mut Foo) -> Foo { /* ... */ }

fn bar() {
    let mut foo = Foo { /* ... */ };

    foo = func(&mut foo);
}
````

We effectively get two mutable references to the same variable `foo` at
the same time. One for the parameter `f`, and one for the hidden
out-pointer. So we can't just `trans_into` the destination directly, but
must use `trans` to get a new temporary slot from which the result can
be copied.

src/librustc/middle/trans/expr.rs

@@ -940,6 +940,7 @@ fn trans_rvalue_stmt_unadjusted<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
             controlflow::trans_loop(bcx, expr.id, &**body)
         }
         ast::ExprAssign(ref dst, ref src) => {
+            let src_datum = unpack_datum!(bcx, trans(bcx, &**src));
             let dst_datum = unpack_datum!(bcx, trans_to_lvalue(bcx, &**dst, "assign"));
 
             if ty::type_needs_drop(bcx.tcx(), dst_datum.ty) {
@@ -960,7 +961,6 @@ fn trans_rvalue_stmt_unadjusted<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
                 // We could avoid this intermediary with some analysis
                 // to determine whether `dst` may possibly own `src`.
                 debuginfo::set_source_location(bcx.fcx, expr.id, expr.span);
-                let src_datum = unpack_datum!(bcx, trans(bcx, &**src));
                 let src_datum = unpack_datum!(
                     bcx, src_datum.to_rvalue_datum(bcx, "ExprAssign"));
                 bcx = glue::drop_ty(bcx,
@@ -969,7 +969,7 @@ fn trans_rvalue_stmt_unadjusted<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
                                     Some(NodeInfo { id: expr.id, span: expr.span }));
                 src_datum.store_to(bcx, dst_datum.val)
             } else {
-                trans_into(bcx, &**src, SaveIn(dst_datum.to_llref()))
+                src_datum.store_to(bcx, dst_datum.val)
             }
         }
         ast::ExprAssignOp(op, ref dst, ref src) => {

src/test/run-pass/out-pointer-aliasing.rs

@@ -0,0 +1,30 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+pub struct Foo {
+    f1: int,
+    _f2: int,
+}
+
+#[inline(never)]
+pub fn foo(f: &mut Foo) -> Foo {
+    let ret = *f;
+    f.f1 = 0;
+    ret
+}
+
+pub fn main() {
+    let mut f = Foo {
+        f1: 8,
+        _f2: 9,
+    };
+    f = foo(&mut f);
+    assert_eq!(f.f1, 8);
+}