Introduce ty::WhereClause to align chalk and rustc dyn representation

[csmoe]

Proposal

As @nikomatsakis wrote in rust-lang/types-team#30 , chalk's binder + predicate representation seems better than rustc's ExistentialPredicate.
rust-lang/rust#85466 renamed ExistentailPredicate to WhereClause, and created a binder alias:

pub enum WhereClause {
    ...
}
pub type ExistentialPredicate<'tcx> = Binder<'tcx, WhereClause<'tcx>>;

Mentors or Reviewers

@nikomatsakis

Process

The main points of the Major Change Process are 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.

You can read more about Major Change Proposals on forge.

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

[jackh726]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted