Rollup merge of rust-lang#151783 - mu001999-contrib:impl/final-method, r=fee1-dead

Implement RFC 3678: Final trait methods

Tracking: rust-lang#131179

This PR is based on rust-lang#130802, with some minor changes and conflict resolution.

Futhermore, this PR excludes final methods from the vtable of a dyn Trait.

And some excerpt from the original PR description:
> Implements the surface part of rust-lang/rfcs#3678.
>
> I'm using the word "method" in the title, but in the diagnostics and the feature gate I used "associated function", since that's more accurate.

cc @joshtriplett

Author: Zalathar

clippy_utils/src/ast_utils/mod.rs

@@ -819,7 +819,9 @@ pub fn eq_use_tree_kind(l: &UseTreeKind, r: &UseTreeKind) -> bool {
 pub fn eq_defaultness(l: Defaultness, r: Defaultness) -> bool {
     matches!(
         (l, r),
-        (Defaultness::Final, Defaultness::Final) | (Defaultness::Default(_), Defaultness::Default(_))
+        (Defaultness::Implicit, Defaultness::Implicit)
+        | (Defaultness::Default(_), Defaultness::Default(_))
+        | (Defaultness::Final(_), Defaultness::Final(_))
     )
 }