Implement initial version of cfg(accessible(..))

[crlf0710]

Implements an initial version of cfg(accessible(..)). The code includes a few FIXMEs, and i think i need some help with understanding how things suppose to work before i can solve them. Also, maybe i need some advice on organizing these code.

I'll assign to Vadim but feel free to reassign.

cc #64797

r? @petrochenkov

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

[Urgau]

This doesn't seems to share any code with the already implemented cfg_accessible macro.

What's the plan with that earlier attempt? Replace it with this one?

[crlf0710]

What's the plan with that earlier attempt?

I guess we can decide that after the we work out the "advice on organizing these code" part :)

[tgross35]

There seem to be existing tests at tests/ui/conditional-compilation/cfg_accessible-*, maybe these should stay in the same place? Or just make a new cfg-accessible directory and move everything there.

[crlf0710]

Ack, will move those files in the next push. Any advice on the implementation?

[tgross35]

I don't know too much about this area of the compiler unfortunately, petrochenkov is probably the expert here. Just happened to notice the existing tests while doing a driveby of this feature's history.

[bors]

☔ The latest upstream changes (presumably #135726) made this pull request unmergeable. Please resolve the merge conflicts.