auto merge of #8797 : nikomatsakis/rust/issue-8625-assign-to-andmut-in-borrowed-loc-2, r=pcwalton

Fixes for #8625 to prevent assigning to `&mut` in borrowed or aliasable locations. The old code was insufficient in that it failed to catch bizarre cases like `& &mut &mut`. 

r? @pnkfelix

src/test/compile-fail/borrowck-assign-to-andmut-in-aliasable-loc.rs

@@ -0,0 +1,30 @@
+// Copyright 2012 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.
+
+// Test that assignments to an `&mut` pointer which is found in a
+// borrowed (but otherwise non-aliasable) location is illegal.
+
+struct S<'self> {
+    pointer: &'self mut int
+}
+
+fn a(s: &S) {
+    *s.pointer += 1; //~ ERROR cannot assign
+}
+
+fn b(s: &mut S) {
+    *s.pointer += 1;
+}
+
+fn c(s: & &mut S) {
+    *s.pointer += 1; //~ ERROR cannot assign
+}
+
+fn main() {}

src/test/compile-fail/borrowck-assign-to-andmut-in-borrowed-loc.rs

@@ -0,0 +1,31 @@
+// Copyright 2012 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.
+
+// Test that assignments to an `&mut` pointer which is found in a
+// borrowed (but otherwise non-aliasable) location is illegal.
+
+struct S<'self> {
+    pointer: &'self mut int
+}
+
+fn copy_borrowed_ptr<'a>(p: &'a mut S<'a>) -> S<'a> {
+    S { pointer: &mut *p.pointer }
+}
+
+fn main() {
+    let mut x = 1;
+
+    {
+        let mut y = S { pointer: &mut x };
+        let z = copy_borrowed_ptr(&mut y);
+        *y.pointer += 1; //~ ERROR cannot assign
+        *z.pointer += 1;
+    }
+}