add `extern "custom"` functions #140770

folkertdev · 2025-05-07T20:26:31Z

tracking issue: #140829
previous discussion: #140566

In short, an extern "custom" function is a function with a custom ABI, that rust does not know about. Therefore, such functions can only be defined with #[unsafe(naked)] and naked_asm!, or via an extern "C" { /* ... */ } block. These functions cannot be called using normal rust syntax: calling them can only be done from inline assembly.

The motivation is low-level scenarios where a custom calling convention is used. Currently, we often pick extern "C", but that is a lie because the function does not actually respect the C calling convention.

At the moment "custom" seems to be the name with the most support. That name is not final, but we need to pick something to actually implement this.

r? @traviscross
cc @tgross35

try-job: x86_64-apple-2

rustbot · 2025-05-07T20:26:39Z

These commits modify compiler targets.
(See the Target Tier Policy.)

rustbot · 2025-05-07T20:53:08Z

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

compiler/rustc_smir/src/stable_mir/abi.rs

compiler/rustc_passes/src/custom_abi.rs

compiler/rustc_passes/messages.ftl

compiler/rustc_smir/src/stable_mir/abi.rs

tests/ui/abi/custom.rs

tests/ui/abi/bad-custom.rs

bjorn3 · 2025-05-08T18:39:10Z

Emit some extern "custom" errors in ast_validation:wq

Did you forget to press escape before quitting vim?

folkertdev · 2025-05-08T18:45:19Z

ah, yeah. Well it'll probably get squashed before this is ready

folkertdev · 2025-05-08T20:59:28Z

I've made a proper tracking issue at #140829

bors · 2025-05-17T07:02:22Z

☔ The latest upstream changes (presumably #141002) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2025-06-04T03:24:22Z

☔ The latest upstream changes (presumably #141984) made this pull request unmergeable. Please resolve the merge conflicts.

bjorn3 · 2025-06-12T18:10:42Z

Mach-O prefixes symbol names defined in C (and by extension Rust) with an _. You could try using a sym operand referring to the unsafe extern "custom" { fn increment(); } declaration to ensure both sides agree on the symbol name.

folkertdev · 2025-06-12T18:33:35Z

Oh, that's clever!

@bors try

bors · 2025-06-12T18:34:46Z

⌛ Trying commit 5f73ce2 with merge 33aabc7...

add `extern "custom"` functions tracking issue: #140829 previous discussion: #140566 In short, an `extern "custom"` function is a function with a custom ABI, that rust does not know about. Therefore, such functions can only be defined with `#[unsafe(naked)]` and `naked_asm!`, or via an `extern "C" { /* ... */ }` block. These functions cannot be called using normal rust syntax: calling them can only be done from inline assembly. The motivation is low-level scenarios where a custom calling convention is used. Currently, we often pick `extern "C"`, but that is a lie because the function does not actually respect the C calling convention. At the moment `"custom"` seems to be the name with the most support. That name is not final, but we need to pick something to actually implement this. r? `@traviscross` cc `@tgross35` try-job: x86_64-apple-2

rust-log-analyzer · 2025-06-12T19:25:38Z

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

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

[TIMING] core::build_steps::test::Crate { compiler: Compiler { stage: 1, host: x86_64-unknown-linux-gnu, forced_compiler: true }, target: x86_64-unknown-linux-gnu, mode: Std, crates: ["std"] } -- 69.262
Build completed successfully in 0:01:12
+ head -n 1 /tmp/browser-ui-test.version
+ npm install [email protected] --unsafe-perm=true
npm ERR! code E503
npm ERR! 503 Service Unavailable - GET https://registry.npmjs.org/@puppeteer/browsers/-/browsers-2.3.0.tgz - Service Unavailable

npm ERR! A complete log of this run can be found in: /root/.npm/_logs/2025-06-12T19_23_02_212Z-debug-0.log
  local time: Thu Jun 12 19:25:26 UTC 2025
  network time: Thu, 12 Jun 2025 19:25:26 GMT
##[error]Process completed with exit code 1.
Post job cleanup.

tgross35 · 2025-06-12T19:47:58Z

^ pretty widespread internet outages going on right now, including npm

bors · 2025-06-12T20:09:24Z

☀️ Try build successful - checks-actions
Build commit: 33aabc7 (33aabc703ff3ae8fd55fe25c96b5461fb8601fca)

folkertdev · 2025-06-12T22:52:08Z

Looks like the job that failed with the npm stuff got re-run? In any case, it's green now, the try build worked, so

@bors r=@tgross35

bors · 2025-06-12T22:52:11Z

📌 Commit 5f73ce2 has been approved by tgross35

It is now in the queue for this repository.

Rollup of 9 pull requests Successful merges: - #128425 (Make `missing_fragment_specifier` an unconditional error) - #135927 (retpoline and retpoline-external-thunk flags (target modifiers) to enable retpoline-related target features) - #140770 (add `extern "custom"` functions) - #142176 (tests: Split dont-shuffle-bswaps along opt-levels and arches) - #142248 (Add supported asm types for LoongArch32) - #142267 (assert more in release in `rustc_ast_lowering`) - #142274 (Update the stdarch submodule) - #142276 (Update dependencies in `library/Cargo.lock`) - #142308 (Upgrade `object`, `addr2line`, and `unwinding` in the standard library) Failed merges: - #140920 (Extract some shared code from codegen backend target feature handling) r? `@ghost` `@rustbot` modify labels: rollup try-job: aarch64-apple try-job: x86_64-msvc-1 try-job: x86_64-gnu try-job: dist-i586-gnu-i586-i686-musl try-job: test-various

Rollup merge of #140770 - folkertdev:custom-abi, r=tgross35 add `extern "custom"` functions tracking issue: #140829 previous discussion: #140566 In short, an `extern "custom"` function is a function with a custom ABI, that rust does not know about. Therefore, such functions can only be defined with `#[unsafe(naked)]` and `naked_asm!`, or via an `extern "C" { /* ... */ }` block. These functions cannot be called using normal rust syntax: calling them can only be done from inline assembly. The motivation is low-level scenarios where a custom calling convention is used. Currently, we often pick `extern "C"`, but that is a lie because the function does not actually respect the C calling convention. At the moment `"custom"` seems to be the name with the most support. That name is not final, but we need to pick something to actually implement this. r? `@traviscross` cc `@tgross35` try-job: x86_64-apple-2

Rollup of 9 pull requests Successful merges: - rust-lang/rust#128425 (Make `missing_fragment_specifier` an unconditional error) - rust-lang/rust#135927 (retpoline and retpoline-external-thunk flags (target modifiers) to enable retpoline-related target features) - rust-lang/rust#140770 (add `extern "custom"` functions) - rust-lang/rust#142176 (tests: Split dont-shuffle-bswaps along opt-levels and arches) - rust-lang/rust#142248 (Add supported asm types for LoongArch32) - rust-lang/rust#142267 (assert more in release in `rustc_ast_lowering`) - rust-lang/rust#142274 (Update the stdarch submodule) - rust-lang/rust#142276 (Update dependencies in `library/Cargo.lock`) - rust-lang/rust#142308 (Upgrade `object`, `addr2line`, and `unwinding` in the standard library) Failed merges: - rust-lang/rust#140920 (Extract some shared code from codegen backend target feature handling) r? `@ghost` `@rustbot` modify labels: rollup try-job: aarch64-apple try-job: x86_64-msvc-1 try-job: x86_64-gnu try-job: dist-i586-gnu-i586-i686-musl try-job: test-various

Kobzol · 2025-06-17T06:28:12Z

@rust-timer build 2bf564a

(Testing #142443)

rust-timer · 2025-06-17T09:19:05Z

Finished benchmarking commit (2bf564a): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

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

Instruction count

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

Max RSS (memory usage)

Results (primary 2.0%, secondary -1.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.0%	[2.0%, 2.0%]	1
Regressions ❌ (secondary)	3.0%	[3.0%, 3.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.7%	[-6.7%, -6.7%]	1
All ❌✅ (primary)	2.0%	[2.0%, 2.0%]	1

Cycles

Results (secondary 2.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	5.3%	[4.5%, 6.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.8%	[-1.8%, -1.8%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 755.964s -> 690.275s (-8.69%)
Artifact size: 372.26 MiB -> 372.29 MiB (0.01%)

rustbot assigned traviscross May 7, 2025

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. labels May 7, 2025

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folkertdev force-pushed the custom-abi branch from f61cada to 847b357 Compare May 7, 2025 20:53

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folkertdev force-pushed the custom-abi branch from 847b357 to 93971b9 Compare May 7, 2025 22:17

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folkertdev force-pushed the custom-abi branch from 93971b9 to a72606d Compare May 8, 2025 15:44

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folkertdev force-pushed the custom-abi branch from a72606d to 64868d3 Compare May 8, 2025 17:02

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folkertdev force-pushed the custom-abi branch from 64868d3 to 7807720 Compare May 8, 2025 17:59

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folkertdev force-pushed the custom-abi branch from 7807720 to 9d529f8 Compare May 8, 2025 18:37

folkertdev mentioned this pull request May 8, 2025

Tracking Issue for abi_custom #140829

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folkertdev force-pushed the custom-abi branch from 9d529f8 to 6ebe16c Compare May 8, 2025 20:58

folkertdev force-pushed the custom-abi branch from 6ebe16c to f9e1963 Compare May 17, 2025 19:22

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folkertdev force-pushed the custom-abi branch from 6351952 to 288e4bc Compare June 12, 2025 18:20

add extern "custom" functions

5f73ce2

folkertdev force-pushed the custom-abi branch from 288e4bc to 5f73ce2 Compare June 12, 2025 18:27

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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 12, 2025

matthiaskrgr mentioned this pull request Jun 13, 2025

Rollup of 9 pull requests #142443

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bors merged commit 7c3b2e5 into rust-lang:master Jun 13, 2025
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rustbot added this to the 1.89.0 milestone Jun 13, 2025

dtolnay mentioned this pull request Jun 13, 2025

Reduce precedence of expressions that have an outer attr #134661

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add extern "custom" functions #140770

add extern "custom" functions #140770

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rust-timer commented Jun 17, 2025

Overall result: no relevant changes - no action needed

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add `extern "custom"` functions #140770

add `extern "custom"` functions #140770

folkertdev commented May 7, 2025 •

edited by rustbot

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