Initial impl of repr_packed_without_abi

[lukaslueg]

Fixes #13375

I've added the lint next to the other attribute-related ones. Not sure if this is the correct place, since while we are looking after the packed-attribute (there is nothing we can do about types defined elsewhere), we are more concerned about the type's representation set by the attribute (instead of "duplicate attributes" and such).

The lint simply looks at the attributes themselves without concern for the item-kind, since items where repr is not allowed end up in a compile-error anyway.

I'm somewhat concerned about the level of noise this lint would cause if/when it goes into stable, although it does not come up in lintcheck.

changelog: [`repr_packed_without_abi`]: Initial implementation

[rustbot]

r? @Alexendoo

rustbot has assigned @Alexendoo.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[bors]

☔ The latest upstream changes (presumably #13286) made this pull request unmergeable. Please resolve the merge conflicts.

[Alexendoo]

Sorry for the wait!

Before 1.74 you couldn't write repr(Rust), adding an MSRV check is an option but I think leaving it out is reasonable too since you could just disable the lint if you want repr(Rust, packed) and are targeting older versions

I'll leave that choice up to you, the rest looks great

[lukaslueg]

Force-pushed a rebase. I've added a msrv filter, although I'm not sure given the docs if msrv_aliases! is supposed to only name actual feature-names as used in rustc; as far as I can see, #[repr(Rust)] didn't get a name, so I made something up. Bumped to 1.84. Had to rebase beyond 63d0ba9, and I'm unsure if PostExpansionEarlyAttributes is where this lint should live according to 63d0ba9's intent.

[Alexendoo]

It should be fine to go in Attributes, the late pass

MSRV name is 👍, just has to be understandable

[lukaslueg]

Moved it over

[Alexendoo]

Thanks!