Clarify some borrowck errors

Closes #17263.

src/test/compile-fail/issue-17263.rs

@@ -0,0 +1,23 @@
+// 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.
+
+struct Foo { a: int, b: int }
+
+fn main() {
+    let mut x = box Foo { a: 1, b: 2 };
+    let (a, b) = (&mut x.a, &mut x.b);
+    //~^ ERROR cannot borrow `x` (here through borrowing `x.b`) as mutable more than once at a time
+    //~^^ NOTE previous borrow of `x` occurs here (through borrowing `x.a`)
+
+    let mut foo = box Foo { a: 1, b: 2 };
+    let (c, d) = (&mut foo.a, &foo.b);
+    //~^ ERROR cannot borrow `foo` (here through borrowing `foo.b`) as immutable
+    //~^^ NOTE previous borrow of `foo` occurs here (through borrowing `foo.a`)
+}