Rollup merge of #21468 - sanxiyn:dead-variant, r=

This implements a wish suggested in #17410, detecting enum variants that are never constructed, even in the presence of `#[derive(Clone)]`. The implementation is general and not specific to `#[derive(Clone)]`.

r? @jakub-

src/test/compile-fail/lint-dead-code-variant.rs

@@ -0,0 +1,42 @@
+// Copyright 2015 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.
+
+#![deny(dead_code)]
+
+#[derive(Copy)]
+enum Enum {
+    Variant1, //~ ERROR: variant is never used
+    Variant2,
+    Variant3,
+}
+
+fn copy(e: Enum) -> Enum {
+    use Enum::*;
+    match e {
+        Variant1 => Variant1,
+        Variant2 => Variant2,
+        Variant3 => Variant3,
+    }
+}
+
+fn max(e: Enum) -> Enum {
+    use Enum::*;
+    match e {
+        Variant1 => Variant3,
+        Variant2 => Variant3,
+        Variant3 => Variant3,
+    }
+}
+
+fn main() {
+    let e = Enum::Variant2;
+    copy(e);
+    max(e);
+}