Type system refactorings for further `rustc_type_ir`-ification

[compiler-errors]

Proposal

I propose the following changes that will have a large footprint, but will ultimately simplify rustc_type_ir migration:

1. Get rid of TypeAndMut and just have the tykind variant be RawPtr(Ty, Mutability), like how refs are just Ref(Region, Ty, Mutability).

2. Remove the type system's dependency on hir::Mutability and hir::Movability and copy both of these enums into rustc_type_ir (will require converting between hir::Mutability and ty::Mutability during astconv).

Mentors or Reviewers

I'll make the changes.

Process

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

• 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

[jackh726]

@rustbot second

[jackh726]

Closing as accepted.