Attributes are parsed twice

[JonathanBrouwer]

All attributes, both using the old and new parsing infrastructure are currently parsed (at least) twice

1. To do a few checks on the attribute, here: https://github.com/rust-lang/rust/blob/cccf075eba88363269e8589ebb8d40874cc542d8/compiler/rustc_parse/src/validate_attr.rs#L34C19-L34C29
2. To lower the attribute, here:

   rust/compiler/rustc_attr_parsing/src/context.rs

   Line 616 in cccf075

   pub fn parse_attribute_list(

#[cfg] was one of the only attributes that was only parsed once. When we ported #[cfg] the the new parsing infrastructure (#143460), we sadly had to also parse it twice, losing around ~0.2% performance on some benchmarks.

This suggests that if we can merge these checks into the new attribute parser we can gain quite a bit of performance. We should do this.

cc @jdonszelmann

[JonathanBrouwer]

@rustbot claim
Will experiment a bit and see what I can come up with