"Stabilizable" prototype for expanded const generics

Summary

Experiment with a new min_generic_const_args implementation to address challenges found with the existing approach

Motivation

min_const_generics was stabilized with the restriction that const-generic arguments may not use generic parameters other than a bare const parameter, e.g. Foo<N> is legal but not Foo<{ T::ASSOC }>. This restriction is lifted under feature(generic_const_exprs) however its design is fundamentally flawed and introduces significant complexity to the compiler. A ground up rewrite of the feature with a significantly limited scope (e.g. min_generic_const_args) would give a viable path to stabilization and result in large cleanups to the compiler.

The status quo

A large amount of rust users run into the min_const_generics limitation that it is not legal to use generic parameters with const generics. It is generally a bad user experience to hit a wall where a feature is unfinished, and this limitation also prevents patterns that are highly desirable. We have always intended to lift this restriction since stabilizing min_const_generics but we did not know how.

It is possible to use generic parameters with const generics by using feature(generic_const_exprs). Unfortunately this feature has a number of fundamental issues that are hard to solve and as a result is very broken. It being so broken results in two main issues:

• When users hit a wall with min_const_generics they cannot reach for the generic_const_exprs feature because it is either broken or has no path to stabilization.
• In the compiler, to work around the fundamental issues with generic_const_exprs, we have a number of hacks which negatively affect the quality of the codebase and the general experience of contributing to the type system.

The next six months

We have a design for min_generic_const_args approach in mind but we want to validate it through implementation as const generics has a history of unforeseen issues showing up during implementation. Therefore we will pursue a prototype implementation in 2024.

As a stretch goal, we will attempt to review the design with the lang team in the form of a design meeting or RFC. Doing so will likely also involve authoring a design retrospective for generic_const_exprs in order to communicate why that design did not work out and why the constraints imposed by min_generic_const_args makes sense.

The "shiny future" we are working towards

The larger goal here is to lift most of the restrictions that const generics currently have:

• Arbitrary types can be used in const generics instead of just: integers, floats, bool and char.
  • implemented under feature(adt_const_params) and is relatively close to stabilization
• Generic parameters are allowed to be used in const generic arguments (e.g. Foo<{ <T as Trait>::ASSOC_CONST }>).
• Users can specify _ as the argument to a const generic, allowing inferring the value just like with types.
  • implemented under feature(generic_arg_infer) and is relatively close to stabilization
• Associated const items can introduce generic parameters to bring feature parity with type aliases
  • implemented under feature(generic_const_items), needs a bit of work to finish it. Becomes significantly more important after implementing min_generic_const_args
• Introduce associated const equality bounds, e.g. T: Trait<ASSOC = N> to bring feature parity with associated types
  • implemented under feature(associated_const_equality), blocked on allowing generic parameters in const generic arguments

Allowing generic parameters to be used in const generic arguments is the only part of const generics that requires significant amounts of work while also having significant benefit. Everything else is already relatively close to the point of stabilization. I chose to specify this goal to be for implementing min_generic_const_args over "stabilize the easy stuff" as I would like to know whether the implementation of min_generic_const_args will surface constraints on the other features that may not be possible to easily fix in a backwards compatible manner. Regardless I expect these features will still progress while min_generic_const_args is being implemented.

Design axioms

• Do not block future extensions to const generics
• It should not feel worse to write type system logic with const generics compared to type generics
• Avoid post-monomorphization errors
• The "minimal" subset should not feel arbitrary

Ownership and team asks

Owner: Boxy, project-const-generics lead, T-types member

This section defines the specific work items that are planned and who is expected to do them. It should also include what will be needed from Rust teams.

• Subgoal:
  • Describe the work to be done and use ↳ to mark "subitems".
• Owner(s) or team(s):
  • List the owner for this item (who will do the work) or if an owner is needed.
  • If the item is a "team ask" (i.e., approve an RFC), put and the team name(s).
• Status:
  • List if there is an owner but they need support, for example funding.
  • Other needs (e.g., complete, in FCP, etc) are also fine.

Subgoal	Owner(s) or team(s)	Notes
Discussion and moral support	lang types
Implementation and mentoring	Boxy
Implementation	Noah Lev
Reviewer	Michael Goulet

Outputs and milestones

Outputs

• A sound, fully implemented feature(min_generic_const_args) available on nightly
• All issues with generic_const_exprs's design have been comprehensively documented (stretch goal)
• RFC for min_generic_const_args's design (stretch goal)

Milestones

• Prerequisite refactorings for min_generic_const_args have taken place
• Initial implementation of min_generic_const_args lands and is useable on nightly
• All known issues are resolved with min_generic_const_args
• Document detailing generic_const_exprs issues
• RFC is written and filed for min_generic_const_args

Frequently asked questions

Do you expect min_generic_const_args to be stabilized by the end?

No. The feature should be fully implemented such that it does not need any more work to make it ready for stabilization, however I do not intend to actually set the goal of stabilizing it as it may wind up blocked on the new trait solver being stable first.