feat: support completions inside cfg!()

[A4-Tacks]

Example

fn main() {
    if cfg!($0)
}

Before this PR

Complete list is empty

After this PR

ba all
ba any
ba not
ba true
...

Known Problems

• Strangly expansion result, like Inline macro Expand macro recursive
true -> builtin #cfg(test), use feat: support hover on cfg predicate #23390 instead
• This implementation cannot be used for cfg_select! {}
• New false negative: let cfg!(true) = true;, maybe similar False negative: missing match arm when constant in trait #23383

[ChayimFriedman2]

The new implementation of the macro feels very wrong and I object to it. What I support instead is expanding to a builtin # cfg expression that also takes CfgPredicate (this cannot support cfg_select! either).

[A4-Tacks]

What I support instead is expanding to a builtin # cfg

There is a feature: Expand cfg!() to evaluate the results

If expanded as built-in expr, how to output the result and predicate, like builtin # cfg(true, test)?

[ChayimFriedman2]

We will not. This is not better than your approach, and I can tolerate that.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[lnicola]

[A4-Tacks]

[image]

Looks like the behavior is in line with expectations

[lnicola]

Yeah, don't worry, it's image storage for the changelog.