Strengthen the follow-set rule for macros

Over in:

• https://github.com/rust-lang/rust/pull/130635

@compiler-errors describes this general problem:

The breakage specifically represents an inherent limitation to the "macro follow-set" formulation which is supposed to make us more resilient against breakages due to extensions to the grammar like this.

Given two macro matcher arms:

• ($ty:ty) => ...
• (($tt:tt)*) => ...

And given tokens like:

• & pin mut [...more tokens may follow...]

On nightly today, &pin gets parsed as a type. However, we run out of matchers but still have tokens left (the mut token is next), so we fall through to the next arm. Since it's written like ($tt:tt)*, everything is allowed, and we match the second arm successfully...

I think that's weird, because if this second arm were written like $ty:ty mut, that would be illegal, since mut is not in the follow-set of the :ty matcher. Thus, we can use :tt matchers to observe whether the compiler actually parses things not in our grammar that should otherwise be protected against, which seems pretty gross.

And @Noratrieb proposes a general solution:

I believe a solution to this would be the following new logic:

• after the end of a macro matcher arm has been reached
• and there are still input tokens remaining
• and if the last part of the matcher is a metavar
• ensure that the first remaining token is in the follow set of this metavar
• if it is, move on to the next arm
• if it is not, emit an error

What this semantically does is strengthen the "commit to fully matching metavars or error" behavior such that it extends past the end. I don't know how many macros rely on this, but it seems like emitting an FCW (instead of error) on such macro invocations would find all these cases and ensure that the follow-set logic is actually robust past the end. But imo this shouldn't block this PR (which should probably just ship as-is) and can be done separately.

This issue is to track the proposal for this FCW.

cc @Noratrieb @compiler-errors @eholk @rust-lang/lang

Start by reading the linked pull request 130635 and the quoted macro follow-set discussion. Trace the compiler entry point that handles a completed macro matcher arm with remaining input, then identify where follow-set validation and FCW diagnostics are tested. Done means trailing tokens outside a final metavar's follow set are diagnosed consistently rather than allowing a later matcher arm to accept them.