Add additional ui tests for issue 26046

This tests the FnMut case.

src/test/ui/issue-26046-fn-mut.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.
+
+fn foo() -> Box<Fn()> {
+    let num = 5;
+
+    let closure = || {
+        num += 1;
+    };
+
+    Box::new(closure)
+}
+
+fn main() {}

src/test/ui/issue-26046-fn-mut.stderr

@@ -0,0 +1,17 @@
+error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnMut`
+  --> $DIR/issue-26046-fn-mut.rs:14:19
+   |
+14 |       let closure = || {
+   |  ___________________^
+15 | |         num += 1;
+16 | |     };
+   | |_____^
+   |
+note: closure is `FnMut` because it mutates the variable `num` here
+  --> $DIR/issue-26046-fn-mut.rs:15:9
+   |
+15 |         num += 1;
+   |         ^^^
+
+error: aborting due to previous error(s)
+