Fix soundness hole when unsizing boxes.

src/test/compile-fail/borrowck-consume-unsize-vec.rs

@@ -0,0 +1,22 @@
+// Copyright 2015 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.
+
+// Check that we report an error if an upcast box is moved twice.
+
+fn consume(_: Box<[i32]>) {
+}
+
+fn foo(b: Box<[i32;5]>) {
+    consume(b);
+    consume(b); //~ ERROR use of moved value
+}
+
+fn main() {
+}

src/test/compile-fail/borrowck-consume-upcast-box.rs

@@ -0,0 +1,24 @@
+// Copyright 2015 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.
+
+// Check that we report an error if an upcast box is moved twice.
+
+trait Foo { fn dummy(&self); }
+
+fn consume(_: Box<Foo>) {
+}
+
+fn foo(b: Box<Foo+Send>) {
+    consume(b);
+    consume(b); //~ ERROR use of moved value
+}
+
+fn main() {
+}