Add run-pass test for assignment to static mut

src/test/run-pass/borrowck/borrowck-assignment-to-static-mut.rs

@@ -0,0 +1,23 @@
+// Copyright 2016 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.
+
+// Test taken from #45641 (https://github.com/rust-lang/rust/issues/45641)
+
+// ignore-tidy-linelength
+// revisions: ast mir
+//[mir]compile-flags: -Z emit-end-regions -Z borrowck-mir
+
+static mut Y: u32 = 0;
+
+unsafe fn should_ok() {
+    Y = 1;
+}
+
+fn main() {}