Do not suggest to make mut binding external to Fn closure

src/test/ui/suggestions/fn-closure-mutable-capture.rs

@@ -0,0 +1,20 @@
+// 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.
+
+pub fn bar<F: Fn()>(_f: F) {}
+
+pub fn foo() {
+    let mut x = 0;
+    bar(move || x = 1);
+    //~^ ERROR cannot assign to captured outer variable in an `Fn` closure
+    //~| NOTE `Fn` closures cannot capture their enclosing environment for modifications
+}
+
+fn main() {}

src/test/ui/suggestions/fn-closure-mutable-capture.stderr

@@ -0,0 +1,15 @@
+error[E0594]: cannot assign to captured outer variable in an `Fn` closure
+  --> $DIR/fn-closure-mutable-capture.rs:15:17
+   |
+15 |     bar(move || x = 1);
+   |                 ^^^^^
+   |
+   = note: `Fn` closures cannot capture their enclosing environment for modifications
+help: consider changing this closure to take self by mutable reference
+  --> $DIR/fn-closure-mutable-capture.rs:15:9
+   |
+15 |     bar(move || x = 1);
+   |         ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+