rollup merge of #23119: nikomatsakis/issue-23116-ref-mut

Don't allow upcasting to a supertype in the type of the match discriminant. Fixes #23116.

This is a [breaking-change] in that it closes a type hole that previously existed.

r? @pnkfelix

src/test/compile-fail/match-ref-mut-invariance.rs

@@ -0,0 +1,24 @@
+// Copyright 2014 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.
+
+// Check that when making a ref mut binding with type `&mut T`, the
+// type `T` must match precisely the type `U` of the value being
+// matched, and in particular cannot be some supertype of `U`. Issue
+// #23116. This test focuses on a `match`.
+
+#![allow(dead_code)]
+struct S<'b>(&'b i32);
+impl<'b> S<'b> {
+    fn bar<'a>(&'a mut self) -> &'a mut &'a i32 {
+        match self.0 { ref mut x => x } //~ ERROR mismatched types
+    }
+}
+
+fn main() {}

src/test/compile-fail/match-ref-mut-let-invariance.rs

@@ -0,0 +1,25 @@
+// Copyright 2014 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.
+
+// Check that when making a ref mut binding with type `&mut T`, the
+// type `T` must match precisely the type `U` of the value being
+// matched, and in particular cannot be some supertype of `U`. Issue
+// #23116. This test focuses on a `let`.
+
+#![allow(dead_code)]
+struct S<'b>(&'b i32);
+impl<'b> S<'b> {
+    fn bar<'a>(&'a mut self) -> &'a mut &'a i32 {
+        let ref mut x = self.0;
+        x //~ ERROR mismatched types
+    }
+}
+
+fn main() {}