Implement RFC 3678: Final trait methods Tracking: https://github.com/rust-lang/rust/issues/131179 This PR is based on rust-lang/rust#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 https://github.com/rust-lang/rfcs/pull/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