`final fn` and vtables

It seems like there's a couple different visions for final fn related to dyn Trait, and I think we need to resolve them.

Is it:

• Always statically dispatched, like it was on an extension trait?
• Always in the vtable, like it was a normal method?
• Something in between depending whether the method is dyn-safe?

Notably, just because it can't be overridden doesn't mean that there's no reason to put it in the vtable, since the entry in the vtable can be substantially more optimized.

For example, https://doc.rust-lang.org/nightly/std/slice/fn.range.html is currently a function in the module because people depend on it for unsoundness. One reason to have final fn is that it'd let us move it into RangeBounds since people wouldn't be able to override the provided implementation and thus wouldn't be able to cause unsoundness.

But consider what happens if the dyn RangeBounds<usize> in question is actually RangeFull:

• With slice_range not in the vtable, then contains needs to make two vtable calls then match on the Bound<&usize> that comes back in order to create the Range<usize>.
• With slice_range in the vtable, then contains makes only one vtable call, which trivially returns the correct Range<usize> without needing any matching at all because the slice_range in the vtable is optimized knowing that it's a RangeFull and thus it just returns 0..bounds.end without doing any matching or comparing.

So it would be a shame, IMHO, if we made final fn have to be the things that performs worse. After all, there are other ways to add helpers, but not other ways to have things in the vtable.