Fix codegen breaking aliasing rules for functions with sret results

This reverts commit a0ec902e239b2219edf1a18b036dd32c18d3be42 "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.

2 files changed, 32 insertions, 2 deletions

diff --git a/src/librustc/middle/trans/expr.rs b/src/librustc/middle/trans/expr.rs
index d638286a9c5..a7b9c137331 100644
--- a/src/librustc/middle/trans/expr.rs
+++ b/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) => {
diff --git a/src/test/run-pass/out-pointer-aliasing.rs b/src/test/run-pass/out-pointer-aliasing.rs
new file mode 100644
index 00000000000..2a44df7a1b5
--- /dev/null
+++ b/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);
+}