Add a test for transmuting via unions in constants

src/test/ui/const-eval/const_transmute.rs

@@ -0,0 +1,62 @@
+// 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.
+
+// must-compile-successfully
+// run-pass
+
+union Transmute<T: Copy, U: Copy> {
+    t: T,
+    u: U,
+}
+
+trait Bar {
+    fn bar(&self) -> u32;
+}
+
+struct Foo {
+    foo: u32,
+    bar: bool,
+}
+
+impl Bar for Foo {
+    fn bar(&self) -> u32 {
+        self.foo
+    }
+}
+
+impl Drop for Foo {
+    fn drop(&mut self) {
+        assert!(!self.bar);
+        self.bar = true;
+        println!("dropping Foo");
+    }
+}
+
+#[derive(Copy, Clone)]
+struct Fat<'a>(&'a Foo, &'static VTable);
+
+struct VTable {
+    drop: Option<for<'a> fn(&'a mut Foo)>,
+    size: usize,
+    align: usize,
+    bar: for<'a> fn(&'a Foo) -> u32,
+}
+
+const FOO: &Bar = &Foo { foo: 128, bar: false };
+const G: Fat = unsafe { Transmute { t: FOO }.u };
+const F: Option<for<'a> fn(&'a mut Foo)> = G.1.drop;
+const H: for<'a> fn(&'a Foo) -> u32 = G.1.bar;
+
+fn main() {
+    let mut foo = Foo { foo: 99, bar: false };
+    (F.unwrap())(&mut foo);
+    std::mem::forget(foo); // already ran the drop impl
+    assert_eq!(H(&Foo { foo: 42, bar: false }), 42);
+}