Remove PredicateKind in favor of only Binder<PredicateAtom>

[jackh726]

Proposal

In #285 it was proposed, and in implemented in rust-lang/rust#73503, to change Predicate to move the Binder(s) out of the variants and into an outer PredicateKind::ForAll, with an additional no-binding PredicateKind::Implies. The overall motivation here was based around 1) matching Chalk more closely and 2) enabling GATs. In Chalk, we've actually since removed the Implies variant (rust-lang/chalk#514). Additionally, in rust-lang/rust#76814, I've found that the ForAll/Implies split actually does make some things more complicated w.r.t. supertrait enumeration and such.

This proposal essentially aims to effectively remove the distinction between ForAll/Implies and make Predicate just container a Binder<PredicateAtom> always. This again more closely matches current Chalk.

This is already implemented in rust-lang/rust#80679 (modulo a bug).

Mentors or Reviewers

@lcnr and @nikomatsakis to review

Process

The main points of the Major Change Process is as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[nikomatsakis]

@rustbot second

[jackh726]

The PR for this has been merged and this issue can be closed.