Const Generics

[nikomatsakis]

Metadata
Point of contact	@BoxyUwU
Team(s)	lang
Goal document	2025h2/const-generics

Summary

Work towards stabilizing the remaining const generics functionality that was left out of the original min_const_generics feature.

Tasks and status

• Design meeting (lang )
• adt_const_params RFC draft (@BoxyUwU)
• Finish mgca prototype (@BoxyUwU, @camelid)

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Note: we have updated the body to match the 2025h2 goal. Your original text is preserved below.

Details

Metadata
Point of contact	@BoxyUwU
Team(s)	lang, types
Goal document	2025h1/min_generic_const_arguments

Summary

Experiment with a new min_generic_const_args implementation to address challenges found with the existing approach to supporting generic parameters in const generic arguments.

Tasks and status

• Discussion and moral support (lang, types )
• Implementation and mentoring (@BoxyUwU)
• Implementation (@camelid, @compiler-errors)
• Reviewer (@compiler-errors)

[nikomatsakis]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[BoxyUwU]

Key Developments: #125915 landed which is necessary to start implementation work on min_generic_const_args, a draft outlining the ideas behind the proposed feature has also been written
Blockers: no
Help Wanted: not currently in need of anything

[BoxyUwU]

The past ~month has been spent figuring out some implementation constraints arising from DefId creation of anon consts. My time has also been somewhat diverted to other things though that should have cleared up now :-)

[BoxyUwU]

camlid has started working on using the new lowering schema for more than just const parameters which once done will allow us to introduce a min_generic_const_args feature gate. compiler-errors has been working on removing the eval_x methods on Const that do not perform proper normalization and are incompatible with this feature

[BoxyUwU]

min_generic_const_args now exists as a feature gate (though without any functionality only some gated refactorings), shouldn't be long before it has actual functionality behind it 🤞 The refactoring to remove all the eval_x methods on ty::Const has been completed so it will now be possible to correctly implement normalization for constants.

[BoxyUwU]

(nothing this month)

[nikomatsakis]

This is a continuing project goal, and the updates below this comment will be for the new period 2025h1