Auto merge of #50803 - varkor:never-transmute-never, r=eddyb

Fix an ICE when attempting to transmute an uninhabited type

Fixes https://github.com/rust-lang/rust/issues/50570.

src/test/compile-fail/never_transmute_never.rs

@@ -0,0 +1,33 @@
+// 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.
+
+// compile-pass
+
+#![crate_type="lib"]
+
+#![feature(never_type)]
+#![allow(dead_code)]
+#![allow(unreachable_code)]
+#![allow(unused_variables)]
+
+struct Foo;
+
+pub fn f(x: !) -> ! {
+    x
+}
+
+pub fn ub() {
+    // This is completely undefined behaviour,
+    // but we still want to make sure it compiles.
+    let x: ! = unsafe {
+        std::mem::transmute::<Foo, !>(Foo)
+    };
+    f(x)
+}