Add #[rustc_legacy_const_generics] by Amanieu · Pull Request #82447 · rust-lang/rust

Amanieu · 2021-02-23T17:17:21Z

This is the first step towards removing #[rustc_args_required_const]: a new attribute is added which rewrites function calls of the form func(a, b, c) to func::<{b}>(a, c). This allows previously stabilized functions in stdarch which use rustc_args_required_const to use const generics instead.

This new attribute is not intended to ever be stabilized, it is only intended for use in stdarch as a replacement for #[rustc_args_required_const].

#[rustc_legacy_const_generics(1)]
pub fn foo<const Y: usize>(x: usize, z: usize) -> [usize; 3] {
    [x, Y, z]
}

fn main() {
    assert_eq!(foo(0 + 0, 1 + 1, 2 + 2), [0, 2, 4]);
    assert_eq!(foo::<{1 + 1}>(0 + 0, 2 + 2), [0, 2, 4]);
}

r? @oli-obk

compiler/rustc_ast_lowering/src/expr.rs

Amanieu · 2021-02-24T06:55:14Z

The current implementation is not very well optimized, let's see how bad it is...

@bors try @rust-timer queue

rust-timer · 2021-02-24T06:55:15Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

bors · 2021-02-24T06:55:24Z

⌛ Trying commit 00eca69 with merge 6d7225329e02f2f6fc73dfb977e802102a412bff...

bors · 2021-02-24T07:42:52Z

☀️ Try build successful - checks-actions
Build commit: 6d7225329e02f2f6fc73dfb977e802102a412bff (6d7225329e02f2f6fc73dfb977e802102a412bff)

rust-timer · 2021-02-24T07:42:54Z

Queued 6d7225329e02f2f6fc73dfb977e802102a412bff with parent fe1bf8e, future comparison URL.

oli-obk

I think we should ignore the same-crate situation and just not do anything there. If you can write functions with const generics you can change your call sites, too.

compiler/rustc_ast_lowering/src/expr.rs

compiler/rustc_ast_lowering/src/lib.rs

compiler/rustc_resolve/src/late.rs

src/test/ui/auxiliary/legacy-const-generics.rs

rust-timer · 2021-02-24T10:32:14Z

Finished benchmarking try commit (6d7225329e02f2f6fc73dfb977e802102a412bff): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf

oli-obk · 2021-02-24T11:43:58Z

Ok, it does have a measurable impact. So... one way to optimize this would be to write the list of all rustc_legacy_const_generics of a crate into metadata. Then, instead of looking up the attribute, we can look into that list (should probably become a hash set upon reading from metadata). Since most crates' list will be entirely empty, this should be really fast to check.

Though... I'm not sure if this is actually a bottleneck at all since there seem to be no additional calls/decodings of item_attrs, though that may just be because these aren't registered yet at these early stages where we don't have tcx yet.

Amanieu · 2021-02-24T12:09:31Z

I think part of the overhead comes from returning a Vec which requires an allocation on every call to item_attrs. I'll try changing that function to take a FnMut(&[Attributes]) closure argument instead, which should avoid the allocation.

compiler/rustc_ast_lowering/src/expr.rs

Amanieu · 2021-02-25T00:39:55Z

I've addressed the review comments and have attempted to address the performance issues using my favored strategy of throwing a HashMap at the problem.

@bors try @rust-timer queue

rust-timer · 2021-02-25T00:39:56Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

rust-timer · 2021-02-25T05:14:25Z

Finished benchmarking try commit (96bc946ebd85c25f0f05e1b979e8f996be1a1daf): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf

jyn514 · 2021-02-25T05:43:17Z

Perf looks much better now :)

Amanieu · 2021-02-25T09:05:19Z

@oli-obk Ready for review.

oli-obk · 2021-02-25T11:18:08Z

@bors r+

bors · 2021-02-25T11:18:09Z

📌 Commit 00afbe7 has been approved by oli-obk

bors · 2021-02-25T11:41:54Z

⌛ Testing commit 00afbe7 with merge 80c5a3d003b0f4610bddd0dd4237980f6f21d9a5...

bors · 2021-02-25T12:33:17Z

💔 Test failed - checks-actions

rust-log-analyzer · 2021-02-25T12:33:18Z

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

oli-obk · 2021-02-25T13:23:26Z

A lot of builders passed, let's try again

@bors retry

bors · 2021-02-25T18:14:56Z

⌛ Testing commit 00afbe7 with merge 98f8cce...

bors · 2021-02-25T21:00:48Z

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 98f8cce to master...

rust-highfive assigned oli-obk Feb 23, 2021

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Feb 23, 2021

Add #[rustc_legacy_const_generics]

d87eec1

Amanieu force-pushed the legacy_const_generics branch from f2e91d5 to d87eec1 Compare February 23, 2021 17:27

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Move tests to subdirectory

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Aaron1011 reviewed Feb 23, 2021

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jyn514 added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Feb 23, 2021

Amanieu added 2 commits February 24, 2021 02:36

TODO -> FIXME

69cde44

Properly reject non-const arguments

00eca69

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Feb 24, 2021

oli-obk requested changes Feb 24, 2021

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Feb 24, 2021

jyn514 reviewed Feb 24, 2021

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Address review comments

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Amanieu added 2 commits February 25, 2021 00:37

Fix tests

cccd779

Add a cache for rustc_legacy_const_generics

22184a0

rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Feb 25, 2021

Improve checking for attribute

00afbe7

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 Feb 25, 2021

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 Feb 25, 2021

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 Feb 25, 2021

bors added the merged-by-bors This PR was explicitly merged by bors. label Feb 25, 2021

bors merged commit 98f8cce into rust-lang:master Feb 25, 2021

rustbot added this to the 1.52.0 milestone Feb 25, 2021

This was referenced Feb 26, 2021

LLVM ERROR when test shift and insert methods in aarch64 rust-lang/stdarch#1017

Closed

Initial conversion to const generics rust-lang/stdarch#1018

Merged

lcnr added the A-const-generics Area: const generics (parameters and arguments) label Feb 27, 2021

tmiasko mentioned this pull request Feb 27, 2021

Convert some SSE2 intrinsics to const generics rust-lang/stdarch#1021

Merged

Amanieu mentioned this pull request Mar 15, 2021

Use const generics for stdarch intrinsics #83167

Closed

lcnr mentioned this pull request Dec 11, 2021

Tracking issue for const generics (RFC 2000) #44580

Closed

16 tasks

RalfJung mentioned this pull request Sep 15, 2025

Add #[rustc_legacy_const_generics] to all intrinsics with const-generic parameters rust-lang/stdarch#1916

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