Tracking Issue for cfg_attribute_in_where

[pnkfelix]

This is a tracking issue for the RFC "Allow cfg-attributes in where clauses" (rust-lang/rfcs#3399).
The feature gate for the issue is #![feature(cfg_attribute_in_where)].

The intent, as described in the Guide-level explanation, is to enable cfg-attributes on where-clauses, like so:

impl<T> SomeTrait<T> for Thing
where
    #[cfg(something_a)] T: SomeRequirementA,
    #[cfg(something_b)] T: SomeRequirementB,
{}

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the RFC (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
• Adjust documentation (see instructions on rustc-dev-guide)
• Formatting for new syntax has been added to the Style Guide (nightly-style-procedure)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• should cfg-attributes be allowed to decorate individual bounds on the right hand side of the colon?
• rustfmt is supposed to be able to format the where clause somehow, do we expect it to (try to) put the attribute on the same line, or would it always prefer the attribute on separate lines?

Implementation history

[mj10021]

Is anyone working on this or is any help needed?

[recatek]

Help would certainly be welcome!

[mj10021]

Oh, great, I can start working on this very soon (sorry for the delayed reply btw). I'm assuming I should just ask on Zulip if I get stuck on anything?

[recatek]

Wanted to link these here for ongoing work on adding opportunities to use cfg attributes in more places.

rust-lang/rfcs#3532 -- RFC: Allow cfg-attributes on elements of tuple type #declarations
https://hackmd.io/2k0ZRofFRoqSxdJ5zwmFqg?view -- Comma-Terminated Fragments and #[cfg]