Fix ICE: don't use `struct_variant` on enums

Fix #40221 and add unittest.

src/librustc_const_eval/pattern.rs

@@ -152,7 +152,11 @@ impl<'tcx> fmt::Display for Pattern<'tcx> {
                         Some(&adt_def.variants[variant_index])
                     }
                     _ => if let ty::TyAdt(adt, _) = self.ty.sty {
-                        Some(adt.struct_variant())
+                        if adt.is_univariant() {
+                            Some(&adt.variants[0])
+                        } else {
+                            None
+                        }
                     } else {
                         None
                     }

src/test/ui/missing-items/issue-40221.rs

@@ -0,0 +1,26 @@
+// 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 P {
+    C(PC),
+}
+
+enum PC {
+    Q,
+    QA,
+}
+
+fn test(proto: P) {
+    match proto {
+        P::C(PC::Q) => (),
+    }
+}
+
+fn main() {}

src/test/ui/missing-items/issue-40221.stderr

@@ -0,0 +1,8 @@
+error[E0004]: non-exhaustive patterns: `C(QA)` not covered
+  --> $DIR/issue-40221.rs:21:11
+   |
+21 |     match proto {
+   |           ^^^^^ pattern `C(QA)` not covered
+
+error: aborting due to previous error
+