Tracking Issue for extended generic associated types

[jackh726]

This is a tracking issue for extensions to the minimal generic_associated_types feature, intended to contain features not intended to be stabilized during initial stabilization. This is primarily for experimentation and likely will remain "incomplete" and almost certainly unsound, at least in the short-term. Longer-term, we like split these features out into separate feature gates, as their implementation becomes more complete.

The RFC for GATs is rust-lang/rfcs#1598, but this feature gate likely will contain features not included in the original RFC.
The feature gate for the issue is #![feature(generic_associated_types_extended)].

Repository

• Detailed tracking information is available at the GAT initiative repository

Features included under this feature gate

"Lazy" associated type obligations during projection

Summary

Briefly, this ignores obligations generated during projection when they contain placeholder regions generated from higher-ranked trait bounds. Eventually, the idea is to either check these more logically or to confirm that they are checked with concrete regions.

Implementation history

• Add the generic_associated_types_extended feature #94869: First implemented
• Remove generic_associated_types_extended feature gate #133768: Removal

Unresolved questions

• Logically and eagerly prove obligations; or, lazily prove obligations?

Object safe GATs

Summary

This feature allows traits with GATs to be object safe (other object safety requirements must hold).

Implementation history

• Make GATs object safe under generic_associated_types_extended feature #94911: First implementation

Unresolved questions

• There are a number of well-formedness checks that are unimplemented. It's not quite sure how these should or need to be implemented. These are briefly mentioned here: https://github.com/rust-lang/chalk/blob/1f43e8265eb0472700286bf5c0afa30fcf4cec5a/chalk-solve/src/clauses.rs#L442

[fmease]

Since the feature has been removed, I'm going to close this tracking issue as not planned.
However, we might want to open a T-types Relevant to the types team, which will review and decide on the PR/issue. C-feature-request Category: A feature request, i.e: not implemented / a PR. A-GATs Area: Generic associated types (GATs) A-trait-objects Area: trait objects, vtable layout r-l/r¹ issue to continue tracking "GATs in dyn-Trait" in some way.

Edit: Ah: #133768 (comment)

I don't think a tracking issue for object-safe GATs is super necessary, though I guess if you disagree feel free to open one.

Footnotes

1. Or in https://github.com/rust-lang/generic-associated-types-initiative? ↩