Rollup merge of #55567 - durka:derive-debug-uninhabited, r=nikomatsakis

add test for deriving Debug on uninhabited enum

Adds a test to close #38885.

Author: GuillaumeGomez

src/test/ui/derive-uninhabited-enum-38885.rs

@@ -0,0 +1,27 @@
+// 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
+// compile-flags: -Wunused
+
+// ensure there are no special warnings about uninhabited types
+// when deriving Debug on an empty enum
+
+#[derive(Debug)]
+enum Void {} //~ WARN never used
+
+#[derive(Debug)]
+enum Foo { //~ WARN never used
+    Bar(u8),
+    Void(Void),
+}
+
+fn main() {}
+

src/test/ui/derive-uninhabited-enum-38885.stderr

@@ -0,0 +1,14 @@
+warning: enum is never used: `Void`
+  --> $DIR/derive-uninhabited-enum-38885.rs:18:1
+   |
+LL | enum Void {} //~ WARN never used
+   | ^^^^^^^^^
+   |
+   = note: `-W dead-code` implied by `-W unused`
+
+warning: enum is never used: `Foo`
+  --> $DIR/derive-uninhabited-enum-38885.rs:21:1
+   |
+LL | enum Foo { //~ WARN never used
+   | ^^^^^^^^
+