Adds a test for rust-lang#48070

Resolves rust-lang#48070.
The bug itself was fixed by rust-lang#48770, but that PR didn't add a test for it.

src/test/run-pass/issue-48070.rs

@@ -0,0 +1,34 @@
+// Copyright 2018 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: lxl nll
+
+#![cfg_attr(nll, feature(nll))]
+
+struct Foo {
+    x: u32
+}
+
+impl Foo {
+    fn twiddle(&mut self) -> &mut Self { self }
+    fn twaddle(&mut self) -> &mut Self { self }
+    fn emit(&mut self) {
+        self.x += 1;
+    }
+}
+
+fn main() {
+    let mut foo = Foo { x: 0 };
+    match 22 {
+        22 => &mut foo,
+        44 => foo.twiddle(),
+        _ => foo.twaddle(),
+    }.emit();
+}