Auto merge of #23423 - nikomatsakis:issue-18737-trait-subtyping, r=nrc

This upcast coercion currently never requires vtable changes. It should be generalized. 

This is a [breaking-change] -- if you have an impl on an object type like `impl SomeTrait`, then this will no longer be applicable to object types like `SomeTrait+Send`. In the standard library, this primarily affected `Any`, and this PR adds impls for `Any+Send` as to keep the API the same in practice. An alternate workaround is to use UFCS form or standalone fns. For more details, see <https://github.com/rust-lang/rust/issues/18737#issuecomment-78450798>.

r? @nrc

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() {
+}

src/test/compile-fail/retslot-cast.rs

@@ -12,9 +12,15 @@
 #![allow(warnings)]
 
 pub fn fail(x: Option<& (Iterator+Send)>) -> Option<&Iterator> {
-    // This call used to trigger an LLVM assertion because the return slot had type
-    // "Option<&Iterator>"* instead of "Option<&(Iterator+Send)>"*
-    inner(x)
+    // This call used to trigger an LLVM assertion because the return
+    // slot had type "Option<&Iterator>"* instead of
+    // "Option<&(Iterator+Send)>"* -- but this now yields a
+    // compilation error and I'm not sure how to create a comparable
+    // test. To ensure that this PARTICULAR failure doesn't occur
+    // again, though, I've left this test here, so if this ever starts
+    // to compile again, we can adjust the test appropriately (clearly
+    // it should never ICE...). -nmatsakis
+    inner(x) //~ ERROR mismatched types
 }
 
 pub fn inner(x: Option<& (Iterator+Send)>) -> Option<&(Iterator+Send)> {
@@ -22,4 +28,4 @@ pub fn inner(x: Option<& (Iterator+Send)>) -> Option<&(Iterator+Send)> {
 }
 
 #[rustc_error]
-fn main() {} //~ ERROR compilation successful
+fn main() {}