const-generics

Summary

Implement and design the const_generics feature with the initial goal of stabilizing the min_const_generics subset.

Info

• Lead: @lcnr
• Lang-team liaison: @nikomatsakis
• Charter: projects/const-generics.md
• Repository: none yet
• Zulip stream: none yet

What is this issue?

This issue represents an active project group. It is meant to be used for the group to post updates to the lang team (and others) in a lightweight fashion. Please do not use the comments here for discussion, that should be kept in the Zulip stream (discussion comments here will be marked as off-topic).

[lcnr]

2021-04-07:

news since the last update:

• feature(min_const_generics) has been stabilized
• we now have regular weekly meetings every tuesday
• feature(const_generics_defaults) is now on nightly, allowing for

#![feature(const_generics_defaults)]
struct WithDefault<const N: usize = 3> {
    inner: [u8; N],
}

current topics:

• complex generic const arguments, i.e. fn split_first<T, const N: usize>(arr: [T; N]) -> (T, [T; N - 1])
  • available on nightly using feature(const_evaluatable_checked) which is still quite broken
  • there are still some big design and implementation challenges
• working towards allowing more const param types, such as fn foo<const CONFIG: Config>
• working towards stabilization of some smaller improvements, see our meeting notes for more details

[varkor]

In the past, there have been various “procedural RFCs” that specify functionality that has already been deemed desirable, and therefore not required the full comment period (e.g. https://github.com/rust-lang/rfcs/pull/2300, https://github.com/rust-lang/rfcs/pull/2591). Perhaps this is another such situation.