Add #[rustc_legacy_const_generics]

[Amanieu]

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

[Amanieu]

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

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[bors]

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

[bors]

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

[rust-timer]

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.

[rust-timer]

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]

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]

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.

[Amanieu]

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]

Awaiting bors try build completion.

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

[rust-timer]

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]

Perf looks much better now :)

[Amanieu]

@oli-obk Ready for review.

[oli-obk]

@bors r+

[bors]

📌 Commit 00afbe7 has been approved by oli-obk

[bors]

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

[bors]

💔 Test failed - checks-actions

[rust-log-analyzer]

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]

A lot of builders passed, let's try again

@bors retry

[bors]

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

[bors]

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