Stabilise inline_const #104087

nbdd0121 · 2022-11-07T05:23:02Z

Stabilisation Report

Summary

This PR will stabilise inline_const feature in expression position. inline_const_pat is still unstable and will not be stabilised.

The feature will allow code like this:

foo(const { 1   1 })

which is roughly desugared into

struct Foo;
impl Foo {
    const FOO: i32 = 1   1;
}
foo(Foo::FOO)

This feature is from rust-lang/rfcs#2920 and is tracked in #76001 (the tracking issue should not be closed as it needs to track inline const in pattern position). The initial implementation is done in #77124.

Difference from RFC

There are two major differences (enhancements) as implemented from the RFC. First thing is that the RFC says that the type of an inline const block inferred from the content within it, but we currently can infer the type using the information from outside the const block as well. This is a frequently requested feature to the initial implementation (e.g. #89964). The inference is implemented in #89561 and is done by treating inline const similar to a closure and therefore share inference context with its parent body.

This allows code like:

let v: Vec<i32> = const { Vec::new() };

Another enhancement that differs from the RFC is that we currently allow inline consts to reference generic parameters. This is implemented in #96557.

This allows code like:

fn create_none_array<T, const N: usize>() -> [Option<T>; N] {
    [const { None::<T> }; N]
}

This enhancement also makes inline const usable as static asserts:

fn require_zst<T>() {
    const { assert!(std::mem::size_of::<T>() == 0) }
}

Documentation

Reference: rust-lang/reference#1295

Unresolved issues

We still have a few issues that are not resolved, but I don't think it necessarily has to block stabilisation:

expr fragment specifier issue: Non-terminals, e.g. expr, have diverged between parser and macro matcher #86730
const {} behaves similar to async {} but not to {} and unsafe {} (they are treated as ExpressionWithoutBlock rather than ExpressionWithBlock): https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/const.20blocks.20differ.20from.20normal.20and.20from.20unsafe.20blocks/near/290229453

Tests

There are a few tests in https://github.com/rust-lang/rust/tree/master/src/test/ui/inline-const

rustbot · 2022-11-07T05:23:08Z

r? @JohnTitor

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rustbot · 2022-11-07T05:23:11Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

Stabilizing library features
Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
Changing public documentation in ways that create new stability guarantees
Changing observable runtime behavior of library APIs

JohnTitor · 2022-11-07T21:54:09Z

I think this needs FCP, cc @rust-lang/lang

nbdd0121 · 2022-11-15T21:10:51Z

@rustbot label: -T-libs T-lang I-lang-nominated

scottmcm · 2022-11-15T22:24:50Z

Thanks for the report! It looks like the current state is enough to allow this to work:

pub fn create_none_array<T, const N: usize>() -> [Option<T>; N] {
    [const { None }; N]
}

which is the use that keeps coming up on URLO. And this also keeps coming up (1 2) on IRLO as needed (or at least wanted) for various things.

So let's do it!

@rfcbot fcp merge

cc Tracking Issue #76001, which also covers patterns so isn't closed by this PR.

scottmcm · 2022-11-15T22:26:15Z

(Sorry, compiler folks. One day I'll remember to check the labels before kicking off an FCP.)

rust-log-analyzer · 2024-04-24T17:18:38Z

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

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bors · 2024-04-24T17:23:07Z

⌛ Testing commit 8169c4c with merge 7bb4f08...

rust-log-analyzer · 2024-04-24T17:25:50Z

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

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bors · 2024-04-24T19:25:43Z

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing 7bb4f08 to master...

bors · 2024-04-24T19:25:43Z

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing 7bb4f08 to master...

rust-timer · 2024-04-24T21:32:08Z

Finished benchmarking commit (7bb4f08): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.0%	[-2.0%, -2.0%]	1
Improvements ✅ (secondary)	-2.9%	[-3.3%, -2.5%]	3
All ❌✅ (primary)	-2.0%	[-2.0%, -2.0%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 674.062s -> 672.117s (-0.29%)
Artifact size: 316.12 MiB -> 316.14 MiB (0.01%)

nbdd0121 · 2024-04-29T11:00:34Z

@rustbot label relnotes

…trieb Use inline const blocks to create arrays of `MaybeUninit`. This PR contains 2 changes enabled by the fact that [`inline_const` is now stable](rust-lang#104087), and was split out of rust-lang#125082. 1. Use inline const instead of `unsafe` to construct arrays in `MaybeUninit` examples. Rationale: Demonstrate good practice of avoiding `unsafe` code where it is not strictly necessary. 4. Use inline const instead of `unsafe` to implement `MaybeUninit::uninit_array()`. This is arguably giving the compiler more work to do, in exchange for eliminating just one single internal unsafe block, so it's less certain that this is good on net. r? `@Nilstrieb`

Rollup merge of rust-lang#125995 - kpreid:const-uninit-stable, r=Nilstrieb Use inline const blocks to create arrays of `MaybeUninit`. This PR contains 2 changes enabled by the fact that [`inline_const` is now stable](rust-lang#104087), and was split out of rust-lang#125082. 1. Use inline const instead of `unsafe` to construct arrays in `MaybeUninit` examples. Rationale: Demonstrate good practice of avoiding `unsafe` code where it is not strictly necessary. 4. Use inline const instead of `unsafe` to implement `MaybeUninit::uninit_array()`. This is arguably giving the compiler more work to do, in exchange for eliminating just one single internal unsafe block, so it's less certain that this is good on net. r? `@Nilstrieb`

Use inline const blocks to create arrays of `MaybeUninit`. This PR contains 2 changes enabled by the fact that [`inline_const` is now stable](rust-lang/rust#104087), and was split out of #125082. 1. Use inline const instead of `unsafe` to construct arrays in `MaybeUninit` examples. Rationale: Demonstrate good practice of avoiding `unsafe` code where it is not strictly necessary. 4. Use inline const instead of `unsafe` to implement `MaybeUninit::uninit_array()`. This is arguably giving the compiler more work to do, in exchange for eliminating just one single internal unsafe block, so it's less certain that this is good on net. r? `@Nilstrieb`

rustbot assigned JohnTitor Nov 7, 2022

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Nov 7, 2022

nbdd0121 force-pushed the const branch from 4b0868c to 39c3558 Compare November 7, 2022 06:03

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Document inline const/const block expression rust-lang/reference#1295

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JohnTitor added the needs-fcp This change is insta-stable, so needs a completed FCP to proceed. label Nov 7, 2022

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rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Nov 15, 2022

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rfcbot removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Nov 15, 2022

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scottmcm mentioned this pull request Nov 15, 2022

Tracking Issue for inline const patterns (RFC 2920) #76001

Open

11 tasks

bors added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Apr 24, 2024

bors added merged-by-bors This PR was explicitly merged by bors. labels Apr 24, 2024

bors merged commit 7bb4f08 into rust-lang:master Apr 24, 2024
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rustbot added this to the 1.79.0 milestone Apr 24, 2024

This was referenced Apr 24, 2024

Stabilize count, ignore, index, and length (macro_metavar_expr) #122808

Open

Stabilize hint::assert_unchecked #123588

Merged

tgross35 mentioned this pull request Apr 24, 2024

Tracking Issue for asm_const #93332

Open

joshlf mentioned this pull request Apr 24, 2024

Someday Maybe google/zerocopy#1120

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nbdd0121 deleted the const branch April 24, 2024 23:02

rustbot added the relnotes Marks issues that should be documented in the release notes of the next release. label Apr 29, 2024

apiraino removed the to-announce Announce this issue on triage meeting label Apr 29, 2024

jswrenn mentioned this pull request Apr 29, 2024

Consider compiler-erroring instead of panicking in DST APIs google/zerocopy#1149

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programmerjake mentioned this pull request Apr 30, 2024

Update rust version rust-lang/portable-simd#418

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jhpratt mentioned this pull request Apr 30, 2024

Tracking Issue for slice::array_chunks #74985

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dtolnay mentioned this pull request May 11, 2024

Fix, document, and test parser and pretty-printer edge cases related to braced macro calls #119427

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kpreid mentioned this pull request May 13, 2024

Remove MaybeUninit::uninit_array() and replace it with inline const blocks. #125082

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kpreid mentioned this pull request Jun 4, 2024

Use inline const blocks to create arrays of MaybeUninit. #125995

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Tracking Issue for MaybeUninit methods for arrays #96097

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Stabilise inline_const #104087

Stabilise inline_const #104087

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nbdd0121 commented Apr 29, 2024

Stabilise inline_const #104087

Stabilise inline_const #104087

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nbdd0121 commented Nov 7, 2022 • edited Loading

Stabilisation Report

Summary

Difference from RFC

Documentation

Unresolved issues

Tests

rustbot commented Nov 7, 2022

rustbot commented Nov 7, 2022

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rust-log-analyzer commented Apr 24, 2024

bors commented Apr 24, 2024

rust-log-analyzer commented Apr 24, 2024

bors commented Apr 24, 2024

bors commented Apr 24, 2024

rust-timer commented Apr 24, 2024

Overall result: no relevant changes - no action needed

Instruction count

Max RSS (memory usage)

Cycles

Binary size

nbdd0121 commented Apr 29, 2024

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