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Stabilizing a subset of const generics #37

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withoutboats opened this issue Jul 16, 2020 · 4 comments
Closed

Stabilizing a subset of const generics #37

withoutboats opened this issue Jul 16, 2020 · 4 comments
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meeting-proposal Proposal for a lang team design meeting meeting-scheduled Lang team design meeting that has a scheduled date T-lang

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@withoutboats
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Summary

I want to propose that we get a subset const generics on track to stabilization in the near future. We should have a meeting to discuss the specifics and attempt to achieve consensus on this plan.

Background reading

About this issue

This issue corresponds to a lang-team design meeting proposal. It corresponds
to a possible topic of discussion that may be scheduled for deeper discussion
during one of our design meetings.
@withoutboats withoutboats added meeting-proposal Proposal for a lang team design meeting T-lang labels Jul 16, 2020
@lcnr lcnr mentioned this issue Jul 18, 2020
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@nikomatsakis
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As discussed on Zulip, we are thinking of doing this meeting this week, July 22 -- @withoutboats can you confirm availability for that slot?

@withoutboats
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Yes I plan to attend.

@nikomatsakis
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Scheduled for July 22 design meeting.

@nikomatsakis nikomatsakis added the meeting-scheduled Lang team design meeting that has a scheduled date label Jul 20, 2020
@nikomatsakis
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Minutes and recording are available: https://github.com/rust-lang/lang-team/blob/master/design-meeting-minutes/2020-07-22-Const-generics-MVP.md

This was referenced Jul 28, 2020
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bors added a commit to rust-lang-ci/rust that referenced this issue Aug 8, 2020
@bors
Auto merge of rust-lang#74877 - lcnr:min_const_generics, r=oli-obk
f9c2177 
Implement the `min_const_generics` feature gate

Implements both rust-lang/lang-team#37 and rust-lang/compiler-team#332.

Adds the new feature gate `#![feature(min_const_generics)]`.
This feature gate adds the following limitations to using const generics:
- generic parameters must only be used in types if they are trivial. (either `N` or `{ N }`)
- generic parameters must be either integers, `bool` or `char`.

We do allow arbitrary expressions in associated consts though, meaning that the following is allowed,
even if `<[u8; 0] as Foo>::ASSOC` is not const evaluatable.
```rust
trait Foo {
    const ASSOC: usize;
}

impl<const N: usize> Foo for [u8; N] {
    const ASSOC: usize = 64 / N;
}
```

r? @varkor cc @eddyb @withoutboats
@nickray nickray mentioned this issue Oct 28, 2020
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meeting-proposal Proposal for a lang team design meeting meeting-scheduled Lang team design meeting that has a scheduled date T-lang
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@nikomatsakis @withoutboats