Suggest cfg(false) instead of cfg(FALSE)

[clubby789]

Split from #149791

cc @Urgau

[rustbot]

Some changes occurred in check-cfg diagnostics

cc @Urgau

[rustbot]

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[joshtriplett]

LGTM. r=me once CI passes.

@bors delegate+

[bors]

✌️ @clubby789, you can now approve this pull request!

If @joshtriplett told you to "r=me" after making some further change, please make that change, then do @bors r=@joshtriplett

[Kivooeo]

This will not work on cases like "False" right?

[clubby789]

That's correct - this is mostly based off of FALSE being a somewhat common idiom, while False is not

[jdonszelmann]

hm, have you seen @scrabsha's work on uppercase/lowercase suggestions in the parser? I think it'd be cool if we could suggest against all mis-capitalizations here and maybe do that in a general enough way that all attribute parsers can benefit from it instead of a cfg-specific lint. At least, could you take a quick look if we can re-use the parsing infra for it? I like it when the error messages are consistent :)

[jdonszelmann]

if that doesn't help let me know and I'll approve this version. The change is a good one.

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[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[jdonszelmann]

@bors delegate-

[clubby789]

I've spent a while looking into this and I'm not sure it makes sense to factor out the exact machinery from parsing, especially as the attr parsing machinery is some distance from the check-cfg lint.
This check-cfg special case is to address the specific pattern of people using an obviously non-existent FALSE cfg, and to inform them that the new boolean literal syntax exists. Similarly, fake false and true expected cfg names to use the existing 'similar name' machinery in check-cfg would give us a more consistent but less specific error:

warning: unexpected `cfg` condition name: `FALSE`
  --> $DIR/false.rs:8:7
   |
LL | #[cfg(FALSE)]
   |       ^^^^^ help: there is a config with a similar name: `false`
   |
   = help: to expect this configuration use `--check-cfg=cfg(FALSE)`
   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
   = note: `#[warn(unexpected_cfgs)]` on by default

warning: 1 warning emitted