merge rigid types in `TyKind` into a single variant.

[lcnr]

Proposal

Incrementally work towards merging all variants which are rigid type constructors with the goal of having the following final enum:

enum RigidTy<'tcx> {
    Bool,
    Char,
    // ...
}

enum TyKind<'tcx> {
    Rigid(RigidTy<'tcx>, SubstsRef<'tcx>),
    /// We can keep `Projection` and `Opaque` separate for now.
    Alias(AliasTy<'tcx>, SubstsRef<'tcx>),
    Infer(...),
    Placeholder(...),
    Bound(...),
    Error(...),
}

This greatly simplifies type relations and makes it easier to use a TypeFolder for generalization, as getting variance right will be a lot easier by adding a method fn RigidTy::variances(self) -> &'tcx [ty::Variance] which we then use to relate the SubstsRef.

It should have a minimal (potentially even positive) impact on performance and a noticeable impact on readability.

Mentors or Reviewers

I am up to mentor and review this work.

Process

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

• [x] File an issue describing the proposal.
• [ ] A types 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.
  • Types 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/types

[lcnr]

closing this due to concerns over perf https://rust-lang.zulipchat.com/#narrow/stream/326866-t-types.2Fnominated/topic/merge.20rigid.20types.20in.20.60TyKind.60.20into.20a.20single.20v.E2.80.A6.20types-team.2364