Tracking issue for `const extern fn` and `const unsafe extern fn` #64926

Aaron1011 · 2019-09-30T18:48:53Z

This is a tracking issue for implementing and stabilizing defining a const extern fn and const unsafe extern fn

Add a test according to Add support for const unsafe? extern fn #64906 (comment) (added in Ensure that we error when calling "const extern fn" with wrong convention #68370)

Stabilizaton Report

Summary

This allows writing const unsafe extern "calling-convention" fn and const extern "calling-convention" fn:

Today Rust and C calling conventions are stable (as of 1.62, see #95346). All other calling conventions are still unstable.

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

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

Test cases

Test for defining and calling in Rust code: https://github.com/rust-lang/rust/blob/master/src/test/ui/consts/const-extern-fn/const-extern-fn.rs
Test that calling with the wrong calling convention (from a const context) leads to an error: https://github.com/rust-lang/rust/blob/master/src/test/ui/consts/miri_unleashed/abi-mismatch.rs

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RalfJung · 2019-10-01T03:08:05Z

Could you explain a bit more what this means and what the point is? What does the bare extern keyword even do? extern "Rust" fn and fn are the same thing, after all. (extern fn is poorly named; AFAIK it really is about specifying the ABI and has little to do with extern visibility.)

Aaron1011 · 2019-10-01T03:10:24Z

@RalfJung: The point is to be able to define an const extern fn (i.e. const extern "C" fn), which has a C-compatible ABI and can be invoked from const contexts.

…r=Centril Add support for `const unsafe? extern fn` This works just as you might expect - an `const extern fn` is a `const fn` that is callable from foreign code. Currently, panicking is not allowed in `const`s. When rust-lang/rfcs#2345 (rust-lang#51999) is stabilized, then panicking in an `const extern fn` will produce a compile-time error when invoked at compile time, and an abort when invoked at runtime. Since this is extending the language (we're allowing the `const` keyword in a new context), I believe that this will need an FCP. However, it's a very minor change, so I didn't think that filing an RFC was necessary. This will allow libc (and other FFI crates) to make many functions `const`, without having to give up on making them `extern` as well. Tracking issue: rust-lang#64926.

Add support for `const unsafe? extern fn` This works just as you might expect - an `const extern fn` is a `const fn` that is callable from foreign code. Currently, panicking is not allowed in `const`s. When rust-lang/rfcs#2345 (#51999) is stabilized, then panicking in an `const extern fn` will produce a compile-time error when invoked at compile time, and an abort when invoked at runtime. Since this is extending the language (we're allowing the `const` keyword in a new context), I believe that this will need an FCP. However, it's a very minor change, so I didn't think that filing an RFC was necessary. This will allow libc (and other FFI crates) to make many functions `const`, without having to give up on making them `extern` as well. Tracking issue: #64926.

Centril · 2019-10-07T03:50:05Z

Implemented in #64906 (2019-10-07).

Aaron1011 · 2020-01-19T02:19:15Z

From #64906 (comment):

@Aaron1011 Could we also get a test making sure that we properly report an error when calling a const fn using the wrong ABI, such as calling an extern "C" const fn with the call site assuming it is an extern "Rust" const fn? The checks for that should already exist in the engine (for standalone Miri).

@RalfJung: Are you talking about a miri-unleashed test involved function pointers? As far I as know, that's the only way for such an ABI mismatch to occur.

RalfJung · 2020-01-19T16:21:12Z

Yes, I meant a miri-unleashed test.

…tion See rust-lang#64926

…Jung Ensure that we error when calling "const extern fn" with wrong convention See rust-lang#64926

Aaron1011 · 2020-03-16T20:43:29Z

@Centril: In #64906 (comment), you said:

However, I think we may eventually want a stabilization RFC that takes into account all of the discussion thus far and elaborates on semantics, use-cases, etc.

Since this PR was merged, rust-lang/libc#1536 was merged, which takes advantage of this feature. Do you still want an RFC prior to stabilization? If so, I'd be happy to start writing one up.

Centril · 2020-03-26T01:58:41Z

Some points:

The pre-expansion gating of the syntax no longer happens. Post expansion gating is used now instead (so you might want to use that to your advantage).
An RFC is perhaps unnecessary, but a detailed stabilization report could serve the same purpose, and we'd want that for any stabilization.
To stabilize this now would be to extend the surface arena under which the soundness hole in Default behavior of unwinding in FFI functions #58794 (comment) applies, so I would like to see that fixed before stabilizing.

RalfJung · 2020-03-26T07:30:46Z

To stabilize this now would be to extend the surface arena under which the soundness hole in #58794 (comment) applies, so I would like to see that fixed before stabilizing.

Also see #63943 where the soundness hole is tracked.

eira-fransham · 2021-07-12T13:25:18Z

All the issues that were blocking this feature from being stablised appear to have been addressed, could this be stablised now? I think any code that would want to use it could just use a non-const extern function which wraps a const fn Rust-ABI function but if there's no reason for it not to be stablised it then it might as well be.

rfcbot · 2022-03-18T18:08:31Z

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.

All other ABIs are left unstable for now. cc rust-lang#64926

… r=pnkfelix Stablize `const_extern_fn` for "Rust" and "C" All other ABIs are left unstable for now. cc rust-lang#64926

joshtriplett · 2022-06-22T17:49:36Z

It looks like "C" and "Rust" calling conventions were stabilized for the upcoming 1.62 release.

Other calling conventions could be stabilized as well; is there a blocker for doing so?

RalfJung · 2022-06-22T22:13:43Z

I am not aware of any calling convention being special here. (Other than variadics, but those are generally unstable, not just for const fn.)

stabilize `const_extern_fn` closes rust-lang/rust#64926 tracking issue: rust-lang/rust#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 rust-lang/rust#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: ```rust 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`

Aaron1011 mentioned this issue Sep 30, 2019

Meta tracking issue for const fn #57563

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Centril added the F-const_extern_fn `#![feature(const_extern_fn)]` label Sep 30, 2019

Aaron1011 mentioned this issue Sep 30, 2019

Add support for const unsafe? extern fn #64906

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Aaron1011 mentioned this issue Jan 19, 2020

Ensure that we error when calling "const extern fn" with wrong convention #68370

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Ensure that we error when calling "const extern fn" with wrong conven…

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…tion See rust-lang#64926

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Rollup merge of rust-lang#68370 - Aaron1011:const-extern-test, r=Ralf…

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…Jung Ensure that we error when calling "const extern fn" with wrong convention See rust-lang#64926

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unix: Extend UnixStream and UnixDatagram to send and receive file descriptors #69864

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Checking that function is const if marked with rustc_const_unstable #86838

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Improve the efficiency of cmsg_space! nix-rust/nix#1475

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ancillary data size and doc improvement #88425

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Aaron1011 added the S-waiting-on-team Status: Awaiting decision from the relevant subteam (see the T-<team> label). label Feb 25, 2022

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Aaron1011 mentioned this issue Mar 26, 2022

Stablize const_extern_fn for "Rust" and "C" #95346

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Aaron1011 added a commit to Aaron1011/rust that referenced this issue Mar 26, 2022

Stablize const_extern_fn for "Rust" and "C"

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All other ABIs are left unstable for now. cc rust-lang#64926

fee1-dead added a commit to fee1-dead-contrib/rust that referenced this issue Apr 15, 2022

Rollup merge of rust-lang#95346 - Aaron1011:stablize-const-extern-fn,…

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… r=pnkfelix Stablize `const_extern_fn` for "Rust" and "C" All other ABIs are left unstable for now. cc rust-lang#64926

Dylan-DPC added a commit to Dylan-DPC/rust that referenced this issue Apr 15, 2022

Rollup merge of rust-lang#95346 - Aaron1011:stablize-const-extern-fn,…

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… r=pnkfelix Stablize `const_extern_fn` for "Rust" and "C" All other ABIs are left unstable for now. cc rust-lang#64926

RalfJung added a commit to RalfJung/rust that referenced this issue Apr 15, 2022

Rollup merge of rust-lang#95346 - Aaron1011:stablize-const-extern-fn,…

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… r=pnkfelix Stablize `const_extern_fn` for "Rust" and "C" All other ABIs are left unstable for now. cc rust-lang#64926

RalfJung added a commit to RalfJung/rust that referenced this issue Apr 15, 2022

Rollup merge of rust-lang#95346 - Aaron1011:stablize-const-extern-fn,…

94351ec

… r=pnkfelix Stablize `const_extern_fn` for "Rust" and "C" All other ABIs are left unstable for now. cc rust-lang#64926

RalfJung added a commit to RalfJung/rust that referenced this issue Apr 15, 2022

Rollup merge of rust-lang#95346 - Aaron1011:stablize-const-extern-fn,…

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… r=pnkfelix Stablize `const_extern_fn` for "Rust" and "C" All other ABIs are left unstable for now. cc rust-lang#64926

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Rollup merge of rust-lang#95346 - Aaron1011:stablize-const-extern-fn,…

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… r=pnkfelix Stablize `const_extern_fn` for "Rust" and "C" All other ABIs are left unstable for now. cc rust-lang#64926

ehuss mentioned this issue Apr 21, 2022

Update for const_extern_fn rust-lang/reference#1197

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mattheww mentioned this issue May 7, 2022

Stop saying that const functions cannot use 'extern' rust-lang/reference#1207

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xiaoyuxlu mentioned this issue Aug 9, 2022

Build error const extern fn definitions are unstable. jyao1/rust-spdm#260

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This was referenced Aug 29, 2024

Use the "C-unwind" abi for functions we cannot guarantee won't panic trifectatechfoundation/zlib-rs#171

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stabilize const_extern_fn #129753

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stabilize const_extern_fn rust-lang/reference#1596

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bors closed this as completed in 4f1be92 Sep 15, 2024

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Tracking issue for experimental features in 1.49 implemented in gccrs Rust-GCC/gccrs#3210

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Tracking issue for `const extern fn` and `const unsafe extern fn` #64926

Tracking issue for `const extern fn` and `const unsafe extern fn` #64926

Aaron1011 commented Sep 30, 2019 •

edited by Mark-Simulacrum

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RalfJung commented Oct 1, 2019

Aaron1011 commented Oct 1, 2019 •

edited

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Centril commented Oct 7, 2019 •

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Aaron1011 commented Jan 19, 2020

RalfJung commented Jan 19, 2020

Aaron1011 commented Mar 16, 2020

Centril commented Mar 26, 2020

RalfJung commented Mar 26, 2020

eira-fransham commented Jul 12, 2021

rfcbot commented Mar 18, 2022

joshtriplett commented Jun 22, 2022

RalfJung commented Jun 22, 2022

Tracking issue for const extern fn and const unsafe extern fn #64926

Tracking issue for const extern fn and const unsafe extern fn #64926

Comments

Aaron1011 commented Sep 30, 2019 • edited by Mark-Simulacrum Loading

Stabilizaton Report

Summary

Test cases

RalfJung commented Oct 1, 2019

Aaron1011 commented Oct 1, 2019 • edited Loading

Centril commented Oct 7, 2019 • edited Loading

Aaron1011 commented Jan 19, 2020

RalfJung commented Jan 19, 2020

Aaron1011 commented Mar 16, 2020

Centril commented Mar 26, 2020

RalfJung commented Mar 26, 2020

eira-fransham commented Jul 12, 2021

rfcbot commented Mar 18, 2022

joshtriplett commented Jun 22, 2022

RalfJung commented Jun 22, 2022

Tracking issue for `const extern fn` and `const unsafe extern fn` #64926

Tracking issue for `const extern fn` and `const unsafe extern fn` #64926

Aaron1011 commented Sep 30, 2019 •

edited by Mark-Simulacrum

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Aaron1011 commented Oct 1, 2019 •

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Centril commented Oct 7, 2019 •

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