Add `ref mut` suggestion test

src/test/ui/suggestions/suggest-ref-mut.rs

@@ -0,0 +1,31 @@
+// 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)]
+
+fn main() {
+    let ref foo = 16;
+    //~^ HELP
+    //~| SUGGESTION ref mut foo
+    *foo = 32;
+    //~^ ERROR
+    if let Some(ref bar) = Some(16) {
+        //~^ HELP
+        //~| SUGGESTION ref mut bar
+        *bar = 32;
+        //~^ ERROR
+    }
+    match 16 {
+        ref quo => { *quo = 32; },
+        //~^ ERROR
+        //~| HELP
+        //~| SUGGESTION ref mut quo
+    }
+}

src/test/ui/suggestions/suggest-ref-mut.stderr

@@ -0,0 +1,21 @@
+error[E0594]: cannot assign to `*foo` which is behind a `&` reference
+  --> $DIR/suggest-ref-mut.rs:17:5
+   |
+LL |     *foo = 32;
+   |     ^^^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `*bar` which is behind a `&` reference
+  --> $DIR/suggest-ref-mut.rs:22:9
+   |
+LL |         *bar = 32;
+   |         ^^^^^^^^^ cannot assign
+
+error[E0594]: cannot assign to `*quo` which is behind a `&` reference
+  --> $DIR/suggest-ref-mut.rs:26:22
+   |
+LL |         ref quo => { *quo = 32; },
+   |                      ^^^^^^^^^ cannot assign
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0594`.