Fix ICE when accessing mutably an immutable enum

Author: estebank

src/librustc/middle/mem_categorization.rs

@@ -202,7 +202,9 @@ impl<'tcx> cmt_<'tcx> {
             Categorization::Downcast(ref cmt, _) => {
                 if let Categorization::Local(_) = cmt.cat {
                     if let ty::TyAdt(def, _) = self.ty.sty {
-                        return def.struct_variant().find_field_named(name).map(|x| x.did);
+                        if def.is_struct() {
+                            return def.struct_variant().find_field_named(name).map(|x| x.did);
+                        }
                     }
                     None
                 } else {

src/test/ui/did_you_mean/issue-39544.rs

@@ -0,0 +1,22 @@
+// 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.
+
+enum X {
+    Y
+}
+
+struct Z {
+    x: X
+}
+
+fn main() {
+    let z = Z { x: X::Y };
+    let _ = &mut z.x;
+}

src/test/ui/did_you_mean/issue-39544.stderr

@@ -0,0 +1,8 @@
+error: cannot borrow immutable field `z.x` as mutable
+  --> $DIR/issue-39544.rs:21:18
+   |
+21 |     let _ = &mut z.x;
+   |                  ^^^
+
+error: aborting due to previous error
+