Report usage of lib features in ast validation

[compiler-errors]

No idea why it was split between ast validation for lang features and a later pass for lang features.

r? @Nilstrieb

[compiler-errors]

Oh, it was added in #89012. It didn't even add a test!

I kinda don't care about that, lmao. If we want to fix this the right way we can do that somehow else. I'll reopen #88802 and leave a comment that moving the validation just for library features is likely not the right solution.

[compiler-errors]

Actually, we don't halt compilation if we hit a feature-on-stable error? (At least afaict??)

So the warning that we currently issue for a library feature that is already stable is still there. I don't actually think there's any regression here at all.

[Noratrieb]

ok, this was quite confusing, but I got it eventually.
originally, all the check for whether we're using feature on stable was in ast_passes.
then, it got split, where it's checked in ast_passes when there are lang features, and not in ast_passes when there are any no lang features.
when there are no lang features, it still did a check for libs features specifically in the later pass.
this improved a diagnostic for libs features, but made the code quite confusing (as evidenced by my confusion).
so now you basically just revert that and make it nice again, where everything is checked in ast_passes.
good!

[Noratrieb]

@bors r+

[bors]

📌 Commit 153a381 has been approved by Nilstrieb

It is now in the queue for this repository.

[Noratrieb]

and don't forget to reopen the issue!

[compiler-errors]

and don't forget to reopen the issue!

Wait, did you see my last comment? I don't think this regresses the issue. I will just add a test.

@bors r-

[Noratrieb]

uh, right. yeah.

[compiler-errors]

It did in fact not regress:

error[E0554]: `#![feature]` may not be used on the dev release channel
 --> /home/michael/test.rs:1:1
  |
1 | #![feature(discriminant_value)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: the feature `discriminant_value` has been stable since 1.21.0 and no longer requires an attribute to enable
 --> /home/michael/test.rs:1:12
  |
1 | #![feature(discriminant_value)]
  |            ^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(stable_features)]` on by default

But also I can't add a test for it because we don't error for feature() on nightly, lol. Too lazy to add a cfg or something to test this. This is a custom build that disables feature in the compiler.

@bors r=Nilstrieb

[bors]

📌 Commit 153a381 has been approved by Nilstrieb

It is now in the queue for this repository.