Fix thinning pointers to extern types in miri

src/librustc_mir/interpret/eval_context.rs

@@ -654,21 +654,24 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M
                         if self.type_is_fat_ptr(src.ty) {
                             match (src.value, self.type_is_fat_ptr(dest_ty)) {
                                 (Value::ByRef { .. }, _) |
+                                // pointers to extern types
+                                (Value::ByVal(_),_) |
+                                // slices and trait objects to other slices/trait objects
                                 (Value::ByValPair(..), true) => {
                                     let valty = ValTy {
                                         value: src.value,
                                         ty: dest_ty,
                                     };
                                     self.write_value(valty, dest)?;
                                 }
+                                // slices and trait objects to thin pointers (dropping the metadata)
                                 (Value::ByValPair(data, _), false) => {
                                     let valty = ValTy {
                                         value: Value::ByVal(data),
                                         ty: dest_ty,
                                     };
                                     self.write_value(valty, dest)?;
                                 }
-                                (Value::ByVal(_), _) => bug!("expected fat ptr"),
                             }
                         } else {
                             let src_layout = self.layout_of(src.ty)?;

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

@@ -0,0 +1,23 @@
+// 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
+
+#![feature(extern_types)]
+
+extern {
+    type Opaque;
+}
+
+const FOO: *const u8 = &42 as *const _ as *const Opaque as *const u8;
+
+fn main() {
+    let _foo = FOO;
+}