Rollup merge of #39009 - canndrew:default-unit-warnings, r=nikomatsakis

Add warning for () to ! switch

With feature(never_type) enabled diverging type variables will default to `!` instead of `()`. This can cause breakages where a trait is resolved on such a type.

This PR emits a future-compatibility warning when it sees this happen.

src/librustc_lint/lib.rs

@@ -219,6 +219,10 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             id: LintId::of(LIFETIME_UNDERSCORE),
             reference: "issue #36892 <https://github.com/rust-lang/rust/issues/36892>",
         },
+        FutureIncompatibleInfo {
+            id: LintId::of(RESOLVE_TRAIT_ON_DEFAULTED_UNIT),
+            reference: "issue #39216 <https://github.com/rust-lang/rust/issues/39216>",
+        },
         FutureIncompatibleInfo {
             id: LintId::of(SAFE_EXTERN_STATICS),
             reference: "issue #36247 <https://github.com/rust-lang/rust/issues/35112>",