Fix reborrows of &mut pointers

Fixes #28839

Author: dotdash

src/librustc_typeck/check/coercion.rs

@@ -110,10 +110,6 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                a,
                b);
 
-        if a == b {
-            return Ok(None);
-        }
-
         // Consider coercing the subtype to a DST
         let unsize = self.unpack_actual_value(a, |a| {
             self.coerce_unsized(a, b)

src/test/compile-fail/borrowck-loan-of-static-data-issue-27616.rs

@@ -23,7 +23,7 @@ fn evil(mut s: &'static mut String)
     let alias: &'static mut String = s;
     let inner: &str = &alias;
     // free value
-    *s = String::new(); //~ ERROR use of moved value
+    *s = String::new(); //~ ERROR cannot assign
     let _spray = "0wned".to_owned();
     // ... and then use it
     println!("{}", inner);

src/test/run-pass/issue-28839.rs

@@ -0,0 +1,24 @@
+// Copyright 2015 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.
+
+// ignore-pretty : (#23623) problems with newlines before // comments
+
+pub struct Foo;
+
+pub fn get_foo2<'a>(foo: &'a mut Option<&'a mut Foo>) -> &'a mut Foo {
+    match foo {
+        // Ensure that this is not considered a move, but rather a reborrow.
+        &mut Some(ref mut x) => *x,
+        &mut None => panic!(),
+    }
+}
+
+fn main() {
+}