Implement RFC 3678: Final trait methods

[mu001999]

Tracking: #131179

This PR is based on #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

[rustbot]

Some changes occurred in compiler/rustc_builtin_macros/src/autodiff.rs

cc @ZuseZ4

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[rustbot]

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[fmease]

Could you mark errs as a coauthor in all 6 commits, I think that's only fair. Thanks! :)

Namely: Co-authored-by: Michael Goulet <[email protected]>

[mu001999]

@fmease Done. Thanks for the reminder! :)

[joshtriplett]

What it doesn't implement are any optimizations that can be done on final trait methods, such as excluding them from the vtable of a dyn Trait.

Does this implement the requirement that final methods don't prevent a trait from being dyn-compatible?

[mu001999]

Does this implement the requirement that final methods don't prevent a trait from being dyn-compatible?

Implemented now :). Also, updated the PR description.

[mu001999]

Spurious failure

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.