Forbid coercing &T to &mut T

src/librustc/middle/infer/coercion.rs

@@ -213,7 +213,12 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
 
         let inner_ty = match a.sty {
             ty::ty_uniq(_) => return Err(ty::terr_mismatch),
-            ty::ty_rptr(_, mt_a) => mt_a.ty,
+            ty::ty_rptr(_, mt_a) => {
+                if !can_coerce_mutbls(mt_a.mutbl, mutbl_b) {
+                    return Err(ty::terr_mutability);
+                }
+                mt_a.ty
+            }
             _ => {
                 return self.subtype(a, b);
             }

src/test/compile-fail/coerce-mut.rs

@@ -0,0 +1,20 @@
+// 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.
+
+fn f(x: &mut i32) {}
+
+fn main() {
+    let x = 0;
+    f(&x);
+    //~^ ERROR mismatched types
+    //~| expected `&mut i32`
+    //~| found `&_`
+    //~| values differ in mutability
+}

src/test/compile-fail/issue-12028.rs

@@ -43,7 +43,7 @@ impl<H: StreamHasher> Hash<H> for u8 {
 
 impl<H: StreamHasher> StreamHash<H> for u8 {
     fn input_stream(&self, stream: &mut H::S) {
-        Stream::input(&*stream, &[*self]);
+        Stream::input(stream, &[*self]);
     }
 }
 

src/test/compile-fail/method-self-arg-2.rs

@@ -22,6 +22,7 @@ fn main() {
     let y = &mut x;
     Foo::bar(&x); //~ERROR cannot borrow `x`
 
-    let x = Foo;
-    Foo::baz(&x); //~ERROR cannot borrow immutable borrowed content as mutable
+    let mut x = Foo;
+    let y = &mut x;
+    Foo::baz(&mut x); //~ERROR cannot borrow `x`
 }

src/test/compile-fail/slice-mut.rs

@@ -13,5 +13,9 @@
 fn main() {
     let x: &[isize] = &[1, 2, 3, 4, 5];
     // Immutable slices are not mutable.
-    let y: &mut[_] = &x[2..4]; //~ ERROR cannot borrow immutable borrowed content as mutable
+    let y: &mut[_] = &x[2..4];
+    //~^ ERROR mismatched types
+    //~| expected `&mut [_]`
+    //~| found `&_`
+    //~| values differ in mutability
 }