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/libtest/lib.rs

@@ -121,6 +121,7 @@ impl fmt::Display for TestName {
 #[derive(Clone, Copy)]
 enum NamePadding {
     PadNone,
+    #[allow(dead_code)]
     PadOnLeft,
     PadOnRight,
 }