Rollup merge of rust-lang#47689 - davidtwco:issue-45157, r=nikomatsakis

Fix borrow checker unsoundness with unions

Fixes rust-lang#45157. After discussion with @nikomatsakis on Gitter, this PR only adds a test since the original issue was resolved elsewhere.

r? @nikomatsakis

src/test/ui/issue-45157.rs

@@ -0,0 +1,43 @@
+// Copyright 2017 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.
+
+#![allow(unused)]
+#![feature(nll)]
+
+#[derive(Clone, Copy, Default)]
+struct S {
+    a: u8,
+    b: u8,
+}
+#[derive(Clone, Copy, Default)]
+struct Z {
+    c: u8,
+    d: u8,
+}
+
+union U {
+    s: S,
+    z: Z,
+}
+
+fn main() {
+    unsafe {
+        let mut u = U { s: Default::default() };
+
+        let mref = &mut u.s.a;
+        *mref = 22;
+
+        let nref = &u.z.c;
+        //~^ ERROR cannot borrow `u.z.c` as immutable because it is also borrowed as mutable [E0502]
+        println!("{} {}", mref, nref)
+        //~^ ERROR cannot borrow `u.s.a` as mutable because it is also borrowed as immutable [E0502]
+    }
+}
+

src/test/ui/issue-45157.stderr

@@ -0,0 +1,20 @@
+error[E0502]: cannot borrow `u.z.c` as immutable because it is also borrowed as mutable
+  --> $DIR/issue-45157.rs:37:20
+   |
+34 |         let mref = &mut u.s.a;
+   |                    ---------- mutable borrow occurs here
+...
+37 |         let nref = &u.z.c;
+   |                    ^^^^^^ immutable borrow occurs here
+
+error[E0502]: cannot borrow `u.s.a` as mutable because it is also borrowed as immutable
+  --> $DIR/issue-45157.rs:39:27
+   |
+37 |         let nref = &u.z.c;
+   |                    ------ immutable borrow occurs here
+38 |         //~^ ERROR cannot borrow `u.z.c` as immutable because it is also borrowed as mutable [E0502]
+39 |         println!("{} {}", mref, nref)
+   |                           ^^^^ mutable borrow occurs here
+
+error: aborting due to 2 previous errors
+