Add test for rust-lang#326 (rust-lang#950)

Author: zhassan-aws

tests/kani/Closure/fn_mut_closure.rs

@@ -0,0 +1,23 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+// Check that we can pass a FnMut closure to a stand alone
+// function definition.
+
+// kani-flags: --unwind 6
+
+fn each<T, F>(x: &[T], mut f: F)
+where
+    F: FnMut(&T),
+{
+    for val in x {
+        f(val)
+    }
+}
+
+fn main() {
+    let mut sum = 0_usize;
+    let elems = [1_usize, 2, 3, 4, 5];
+    each(&elems, |val| sum += *val);
+    assert_eq!(sum, 15);
+}