Implement the `min_const_generics` feature gate #74877

lcnr · 2020-07-28T14:28:47Z

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.

trait Foo {
    const ASSOC: usize;
}

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

r? @varkor cc @eddyb @withoutboats

mark-i-m · 2020-07-28T18:16:38Z

@rustbot modify labels: +A-const-generics

oli-obk · 2020-08-05T13:07:16Z

r? @oli-obk

oli-obk

you got some submodule changes entangled in your PR

I think we should port all const_generics tests that can be ported to min_const_generics in order to have a good coverage of the behaviour that we're stabilizing. This may require splitting some tests into a min_const_generic test and a const_generic test.

src/librustc_ast_passes/feature_gate.rs

src/librustc_typeck/collect/type_of.rs

nikomatsakis · 2020-08-05T13:32:56Z

I don't think it should block this PR, but I would like to see us doing a better write-up of what is permitted and not permitted in the "min const generics" variant, and why, and where the tests are. I imagine this is the work we'll be doing in the project group though.

UPDATE: I realize there are a lot of write-ups already and I think other folks who are closer to what's going on have a clear picture. It's just that I feel like I don't have a 'canonical reference' to go and re-read each time I realize I'm a bit confused.

lcnr · 2020-08-05T16:42:35Z

submodules should be fixed, I ported all type-dependent tests for now without any issues,

imo the remaining tests can be done in a separate PR.

src/test/ui/const-generics/min_const_generics/type-dependent/const-arg-in-const-arg.rs

src/test/ui/const-generics/min_const_generics/type-dependent/issue-67144-1.rs

oli-obk · 2020-08-06T09:17:16Z

These tests are copied from const_generics tests, right? Can we remove the original tests now? They don't really give us anything new.

lcnr · 2020-08-06T09:34:35Z

These tests are copied from const_generics tests, right? Can we remove the original tests now? They don't really give us anything new.

They do, min const generics eagerly evaluates constants without supplying the parent generics, IMO this is different enough for us to consider running all tests both with min_const_generics and const_generics.

bors · 2020-08-07T17:20:04Z

⌛ Testing commit 644c894 with merge e4c1ad1de3ced21a64716504511e76edd7622162...

bors · 2020-08-07T17:50:54Z

💔 Test failed - checks-actions

lcnr · 2020-08-07T18:04:31Z

network failure

@bors retry

bors · 2020-08-08T01:48:40Z

⌛ Testing commit 644c894 with merge f9c2177...

bors · 2020-08-08T03:54:19Z

☀️ Test successful - checks-actions, checks-azure
Approved by: oli-obk
Pushing f9c2177 to master...

tesuji · 2020-08-08T10:37:20Z

src/librustc_feature/active.rs

@@ -579,6 +579,9 @@ declare_features! (
    /// Alloc calling `transmute` in const fn
    (active, const_fn_transmute, "1.46.0", Some(53605), None),

+    /// The smallest useful subset of `const_generics`.
+    (active, min_const_generics, "1.46.0", Some(74878), None),


This should be 1.47.0

thx, opened #75290

…n, r=jonas-schievink fix `min_const_generics` version rust-lang#74877 (comment) > This should be 1.47.0

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lcnr force-pushed the min_const_generics branch from ef8554c to 0060802 Compare July 28, 2020 14:31

lcnr mentioned this pull request Jul 28, 2020

Tracking Issue for min_const_generics #74878

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lcnr force-pushed the min_const_generics branch from a8a15da to 415548e Compare July 28, 2020 15:05

lcnr added the F-min_const_generics label Jul 28, 2020

petrochenkov self-assigned this Jul 28, 2020

rustbot added the A-const-generics Area: const generics (parameters and arguments) label Jul 28, 2020

petrochenkov removed their assignment Aug 2, 2020

rust-highfive assigned oli-obk and unassigned varkor Aug 5, 2020

oli-obk requested changes Aug 5, 2020

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lcnr force-pushed the min_const_generics branch from cdb0baa to 791752c Compare August 5, 2020 16:28

lcnr added 5 commits August 5, 2020 18:30

add min_const_generics feature gate

375bccb

forbid generic params in complex consts

289e5fc

forbid complex types for generic parameters

188bbf8

add tracking issue

a5a5ca0

impl review

0d54f57

lcnr force-pushed the min_const_generics branch from 791752c to 98e08c0 Compare August 5, 2020 16:40

lcnr force-pushed the min_const_generics branch from 42469de to b462e7d Compare August 5, 2020 17:21

oli-obk reviewed Aug 6, 2020

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 7, 2020

bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Aug 7, 2020

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 7, 2020

bors added the merged-by-bors This PR was explicitly merged by bors. label Aug 8, 2020

bors merged commit f9c2177 into rust-lang:master Aug 8, 2020

lcnr deleted the min_const_generics branch August 8, 2020 04:44

lcnr mentioned this pull request Aug 8, 2020

Use revisions to test min_const_generics #75279

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lcnr mentioned this pull request Aug 8, 2020

fix min_const_generics version #75290

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JohnTitor added a commit to JohnTitor/rust that referenced this pull request Aug 8, 2020

Rollup merge of rust-lang#75290 - rust-lang:min_const_generics-versio…

55f2490

…n, r=jonas-schievink fix `min_const_generics` version rust-lang#74877 (comment) > This should be 1.47.0

hudson-ayers mentioned this pull request Aug 11, 2020

pmp: add PMP struct tock/tock#2064

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2 tasks

jyn514 mentioned this pull request Sep 9, 2020

#![feature(const_fn)] causes rustdoc to drop the const keyword when documenting instances of const fn #76501

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lcnr mentioned this pull request Nov 17, 2020

Tracking issue for const generics (RFC 2000) #44580

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cuviper added this to the 1.47.0 milestone May 2, 2024

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Implement the `min_const_generics` feature gate #74877

Implement the `min_const_generics` feature gate #74877

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Implement the min_const_generics feature gate #74877

Implement the min_const_generics feature gate #74877

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Implement the `min_const_generics` feature gate #74877

Implement the `min_const_generics` feature gate #74877

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