Suggest not mutably borrowing a mutable reference

This commit is concerned with the case where the user tries to mutably
borrow a mutable reference, thereby triggering an error. Instead of the
existing suggestion to make the binding mutable, the compiler will now
suggest to avoid borrowing altogether.

src/test/ui/borrowck/mut-borrow-of-mut-ref.nll.stderr

@@ -0,0 +1,9 @@
+error[E0596]: cannot borrow immutable item `b` as mutable
+  --> $DIR/mut-borrow-of-mut-ref.rs:18:7
+   |
+LL |     g(&mut b) //~ ERROR cannot borrow
+   |       ^^^^^^ cannot borrow as mutable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.

src/test/ui/borrowck/mut-borrow-of-mut-ref.rs

@@ -0,0 +1,21 @@
+// Copyright 2017 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.
+
+// Suggest not mutably borrowing a mutable reference
+
+fn main() {
+    f(&mut 0)
+}
+
+fn f(b: &mut i32) {
+    g(&mut b) //~ ERROR cannot borrow
+}
+
+fn g(_: &mut i32) {}

src/test/ui/borrowck/mut-borrow-of-mut-ref.stderr

@@ -0,0 +1,13 @@
+error[E0596]: cannot borrow immutable argument `b` as mutable
+  --> $DIR/mut-borrow-of-mut-ref.rs:18:12
+   |
+LL |     g(&mut b) //~ ERROR cannot borrow
+   |            ^ cannot borrow mutably
+help: consider removing the `&mut`, as it is an immutable binding to a mutable reference
+   |
+LL |     g(b) //~ ERROR cannot borrow
+   |       ^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.

src/test/ui/did_you_mean/issue-31424.stderr

@@ -10,10 +10,12 @@ LL |         (&mut self).bar(); //~ ERROR cannot borrow
 error[E0596]: cannot borrow immutable argument `self` as mutable
   --> $DIR/issue-31424.rs:23:15
    |
-LL |     fn bar(self: &mut Self) {
-   |            --------------- consider changing this to `mut self: &mut Self`
 LL |         (&mut self).bar(); //~ ERROR cannot borrow
    |               ^^^^ cannot borrow mutably
+help: consider removing the `&mut`, as it is an immutable binding to a mutable reference
+   |
+LL |         self.bar(); //~ ERROR cannot borrow
+   |         ^^^^
 
 error: aborting due to 2 previous errors