Allow assignment to static muts

Author: KiChjang

src/librustc_mir/borrow_check.rs

@@ -640,10 +640,12 @@ impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx>
                         Mutability::Mut => return,
                     }
                 }
-                Lvalue::Static(_) => {
+                Lvalue::Static(ref static_) => {
                     // mutation of non-mut static is always illegal,
                     // independent of dataflow.
-                    self.report_assignment_to_static(context, (lvalue, span));
+                    if !self.tcx.is_static_mut(static_.def_id) {
+                        self.report_assignment_to_static(context, (lvalue, span));
+                    }
                     return;
                 }
             }

src/test/run-pass/borrowck/borrowck-unsafe-static-mutable-borrows.rs

@@ -0,0 +1,31 @@
+// 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.
+
+// revisions: ast mir
+//[mir]compile-flags: -Z emit-end-regions -Z borrowck-mir
+
+// Test file taken from issue 45129 (https://github.com/rust-lang/rust/issues/45129)
+
+struct Foo { x: [usize; 2] }
+
+static mut SFOO: Foo = Foo { x: [23, 32] };
+
+impl Foo {
+    fn x(&mut self) -> &mut usize { &mut self.x[0] }
+}
+
+fn main() {
+    unsafe {
+        let sfoo: *mut Foo = &mut SFOO;
+        let x = (*sfoo).x();
+        (*sfoo).x[1] += 1;
+        *x += 1;
+    }
+}