Need test coverage for attr edge-cases (ambiguity, ignored positions, etc)

[workingjubilee]

We have a variety of edge-cases involving attributes that are either simply untested or under-tested, and certainly not tested for every attribute where it is relevant. As most attributes are a series of special-cases (though less so now) they each need testing.

A key issue is that they have interactions with proc-macro attributes and such.

We are currently allowing this to happen and not blocking PRs on fixes in this area because we already have legacy problems in this lack of coverage and we are in the middle of a migration.

This came up in

• warn on align on fields to avoid breaking changes #143868
• Regression: align is ambiguous #143834

[workingjubilee]

cc @JonathanBrouwer @jdonszelmann

( I assigned Jana because I can't assign Jonathan directly, Jonathan you can rustbot-claim this if you want. )

[JonathanBrouwer]

@rustbot claim