Add test from #49736

Fixes #49736

Author: sapphire-arches

src/test/ui/borrowck/two-phase-multi-mut.rs

@@ -0,0 +1,26 @@
+// Copyright 2018 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.
+
+#![feature(nll)]
+
+struct Foo {
+}
+
+impl Foo {
+    fn method(&mut self, foo: &mut Foo) {
+    }
+}
+
+fn main() {
+    let mut foo = Foo { };
+    foo.method(&mut foo);
+    //~^     cannot borrow `foo` as mutable more than once at a time
+    //~^^    cannot borrow `foo` as mutable more than once at a time
+}

src/test/ui/borrowck/two-phase-multi-mut.stderr

@@ -0,0 +1,23 @@
+error[E0499]: cannot borrow `foo` as mutable more than once at a time
+  --> $DIR/two-phase-multi-mut.rs:23:16
+   |
+LL |     foo.method(&mut foo);
+   |     -----------^^^^^^^^-
+   |     |          |
+   |     |          second mutable borrow occurs here
+   |     first mutable borrow occurs here
+   |     borrow later used here
+
+error[E0499]: cannot borrow `foo` as mutable more than once at a time
+  --> $DIR/two-phase-multi-mut.rs:23:5
+   |
+LL |     foo.method(&mut foo);
+   |     ^^^^^^^^^^^--------^
+   |     |          |
+   |     |          first mutable borrow occurs here
+   |     second mutable borrow occurs here
+   |     borrow later used here
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0499`.