Add higher-kinded inference variables for opaque types handling

[lcnr]

Proposal

Add higher-kinded inference variables to Rust for the sake of opaque type inference. Longterm this should also be really impactful when dealing with aliases in binders and closure signature inference.

What

A higher-kinded inference variable is a type-level function taking generic arguments as input and returning a generic arg. I am using the notation ?infer[T, U] to talk about the type-level function fn(T: Type, U: Type) -> Type. A type-level function in Rust just a higher-ranked definition: for<T, U> whatever or at an even more syntax level type Foo<T, U> = whatever.

I intend to use them to infer opaque types in their defining scope as the current implementation in the new solver is somewhat brittle.

The idea is that in the defining scope we put for<generic_args> opaque<generic_args> = ?infer[generic_args] in the environment.

Normalizing an opaque now checks whether the given opaque is in the current environment, and if so, returns the instantiated RHS. See https://rust-lang.zulipchat.com/#narrow/channel/144729-t-types/topic/higher-kinded.20inference.20and.20opaques/with/578882618 for how this should work in more detail.

References

• opaque types: consider yeeting structural lookup trait-system-refactor-initiative#271
• closure signature inference can leak placeholders and fail to infer stuff to be higher ranked lcnr/random-rust-snippets#25

Mentors or Reviewers

I intended to mentor and review this work.

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]

Important

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.

Concerns or objections can formally be registered here by adding a comment.

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cc @rust-lang/types

[BoxyUwU]

@rustbot second

[lcnr]

accepted