Parse ~const modifier

[dtolnay]

• Introduce ~const rust-lang/rust#88328

[yaahc]

Hey, I'm interested in fixing this issue so that cargo unstable-api can start working again. I looked into it a bit and it seems like the right place to make the change would be to update TraitBoundModifier to add a MaybeConst variant or something, but this enum is pub and exhaustive, so I'm assuming this change can't be made under syn's stability guarantees.

Do you have an idea for how you are planning to add support for the const modifier? I'd be happy to implement it but I'm unsure of how to proceed so I figured I'd ask first.

[dtolnay]

You can stick it anywhere in the existing syntax tree. The syntax tree itself doesn't change until after the syntax gets stabilized.

For example any of the following would work:

• Parse the whole surrounding impl-block as Item::Verbatim instead of Item::Impl
• Convert the surrounding PredicateType's bounded_ty to a Type::Verbatim and prefix the ~const tokens onto it
• Insert ~const as a segment in the TraitBound's path