add test case showing how previous attempt was unsound

src/test/ui/borrowck/issue-52713-bug.rs

@@ -0,0 +1,29 @@
+// 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.
+
+// Regression test for a bug in #52713: this was an optimization for
+// computing liveness that wound up accidentally causing the program
+// below to be accepted.
+
+#![feature(nll)]
+
+fn foo<'a>(x: &'a mut u32) -> u32 {
+    let mut x = 22;
+    let y = &x;
+    if false {
+        return x;
+    }
+
+    x += 1; //~ ERROR
+    println!("{}", y);
+    return 0;
+}
+
+fn main() { }

src/test/ui/borrowck/issue-52713-bug.stderr

@@ -0,0 +1,14 @@
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/issue-52713-bug.rs:24:5
+   |
+LL |     let y = &x;
+   |             -- borrow of `x` occurs here
+...
+LL |     x += 1; //~ ERROR
+   |     ^^^^^^ assignment to borrowed `x` occurs here
+LL |     println!("{}", y);
+   |                    - borrow later used here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0506`.