librustc: Schedule cleanups properly when coercing to a &Trait.

src/librustc/middle/trans/expr.rs

@@ -277,7 +277,7 @@ fn auto_slice_and_ref<'a>(
         auto_ref(bcx, datum, expr)
     }
 
-    fn auto_borrow_obj<'a>(bcx: &'a Block<'a>,
+    fn auto_borrow_obj<'a>(mut bcx: &'a Block<'a>,
                            expr: &ast::Expr,
                            source_datum: Datum<Expr>)
                            -> DatumBlock<'a, Expr> {
@@ -285,7 +285,11 @@ fn auto_borrow_obj<'a>(bcx: &'a Block<'a>,
         let target_obj_ty = expr_ty_adjusted(bcx, expr);
         debug!("auto_borrow_obj(target={})", target_obj_ty.repr(tcx));
 
-        let mut datum = source_datum.to_expr_datum();
+        // Arrange cleanup, if not already done. This is needed in
+        // case we are auto-borrowing a Box<Trait> to &Trait
+        let datum = unpack_datum!(
+            bcx, source_datum.to_lvalue_datum(bcx, "autoborrowobj", expr.id));
+        let mut datum = datum.to_expr_datum();
         datum.ty = target_obj_ty;
         DatumBlock::new(bcx, datum)
     }

src/test/run-pass/cleanup-auto-borrow-obj.rs

+// 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.
+
+
+// This would previously leak the Box<Trait> because we wouldn't
+// schedule cleanups when auto borrowing trait objects.
+// This program should be valgrind clean.
+
+
+static mut DROP_RAN: bool = false;
+
+struct Foo;
+impl Drop for Foo {
+    fn drop(&mut self) {
+        unsafe { DROP_RAN = true; }
+    }
+}
+
+
+trait Trait {}
+impl Trait for Foo {}
+
+pub fn main() {
+    {
+        let _x: &Trait = box Foo as Box<Trait>;
+    }
+    unsafe {
+        assert!(DROP_RAN);
+    }
+}
+