Fix an ICE when attempting to transmute an uninhabited type

src/librustc_codegen_llvm/mir/block.rs

@@ -428,9 +428,19 @@ fn codegen_terminator(&mut self,
                 let intrinsic = intrinsic.as_ref().map(|s| &s[..]);
 
                 if intrinsic == Some("transmute") {
-                    let &(ref dest, target) = destination.as_ref().unwrap();
-                    self.codegen_transmute(&bx, &args[0], dest);
-                    funclet_br(self, bx, target);
+                    if let Some(destination_ref) = destination.as_ref() {
+                        let &(ref dest, target) = destination_ref;
+                        self.trans_transmute(&bx, &args[0], dest);
+                        funclet_br(self, bx, target);
+                    } else {
+                        // If we are trying to transmute to an uninhabited type,
+                        // it is likely there is no allotted destination. In fact,
+                        // transmuting to an uninhabited type is UB, which means
+                        // we can do what we like. Here, we declare that transmuting
+                        // into an uninhabited type is impossible, so anything following
+                        // it must be unreachable.
+                        bx.unreachable();
+                    }
                     return;
                 }
 

src/test/compile-fail/never_transmute_never.rs

+// 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)
+}