stabilize const_extern_fn

[folkertdev]

closes #64926

tracking issue: #64926
reference PR: rust-lang/reference#1596

Stabilizaton Report

Summary

Using const extern "Rust" and const extern "C" was already stabilized (since version 1.62.0, see #95346). This PR stabilizes the other calling conventions: it is now possible to write const unsafe extern "calling-convention" fn and const extern "calling-convention" fn for any supported calling convention:

const extern "C-unwind" fn foo1(val: u8) -> u8 { val + 1}
const extern "stdcall" fn foo2(val: u8) -> u8 { val + 1}
const unsafe extern "C-unwind" fn bar1(val: bool) -> bool { !val }
const unsafe extern "stdcall" fn bar2(val: bool) -> bool { !val }

This can be used to const-ify an extern fn, or conversely, to make a const fn callable from external code.

r? T-lang

cc @RalfJung

[rustbot]

Failed to set assignee to t-lang: invalid assignee

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

[RalfJung]

The unstable book is auto-generated, AFAIK.

Cc @rust-lang/wg-const-eval
As already mentioned in #64926, I see no reason why other ABIs would somehow behave differently here.

[traviscross]

@folkertdev: Perhaps you're working on it in parallel, but you'll want to add more in the way of a stabilization report to the PR description here (see e.g. the recent stabilizations from RalfJ for some examples).

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[folkertdev]

if anyone from clippy is following along, is there some equivalent of CURRENT_RUSTC_VERSION here? In any case given that we're close to a rust release this may or may not be accurate when merged

[folkertdev]

@traviscross allright I copied and slightly updated some text from the tracking issue. Let me know if there's something else that should be included.

[traviscross]

Why hadn't we stabilized this any sooner? I.e., are there any tricky questions here, open questions that have recently been answered, or recent implementation work done to make this ready?

(Or has this been in good shape for a long time and was just waiting on a stabilization PR?)

[RalfJung]

I have no idea why #95346 only stabilized "Rust" and "C". Maybe @Aaron1011 remembers?

[traviscross]

@rfcbot fcp merge

We talked about this in triage, and this sounded good to us all.

[rfcbot]

Team member @traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[nikomatsakis]

@rfcbot reviewed

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[bors]

☔ The latest upstream changes (presumably #130312) made this pull request unmergeable. Please resolve the merge conflicts.

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[folkertdev]

cool, just fixed a merge conflict and squashed all commits into one

[RalfJung]

r=me for the rustc side. The clippy side seems reasonably simple but I think the test can be merged with another test file now.

[RalfJung]

@bors r+

[bors]

📌 Commit a528f4e has been approved by RalfJung

It is now in the queue for this repository.

[bors]

⌛ Testing commit a528f4e with merge 4f1be92...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing 4f1be92 to master...

[rust-timer]

Finished benchmarking commit (4f1be92): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.4%)

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)	2.4%	[1.8%, 3.0%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[1.8%, 3.0%]	2

Cycles

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

Binary size

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

Bootstrap: 758.697s -> 760.09s (0.18%)
Artifact size: 340.54 MiB -> 340.51 MiB (-0.01%)