add FCW to warn about wasm ABI transition #138601

RalfJung · 2025-03-17T11:18:27Z

See #122532 for context: the "C" ABI on wasm32-unk-unk will change. The goal of this lint is to warn about any function definition and calls whose behavior will be affected by the change. My understanding is the following:

scalar arguments are fine
- including 128 bit types, they get passed as two i64 arguments in both ABIs
repr(C) structs (recursively) wrapping a single scalar argument are fine (unless they have extra padding due to over-alignment attributes)
all return values are fine

@bjorn3 @alexcrichton @Manishearth is that correct?

I am making this a "show up in future compat reports" lint to maximize the chances people become aware of this. OTOH this likely means warnings for most users of Diplomat so maybe we shouldn't do this?

IIUC, wasm-bindgen should be unaffected by this lint as they only pass scalar types as arguments.

Tracking issue: #138762
Transition plan blog post: rust-lang/blog.rust-lang.org#1531

try-job: dist-various-2

rustbot · 2025-03-17T11:18:32Z

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RalfJung · 2025-03-17T11:22:19Z

tests/ui/lint/wasm_c_abi_transition.stderr

+LL | pub extern "C" fn my_fun(_x: MyType) {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!


It's not technically correct that this will become a hard error (we'll just do the ABI switch), but I don't know how to make an FCW that does not have this message...

bjorn3 · 2025-03-17T11:29:20Z

I am a bit concerned about 128bit scalar types as wasm doesn't have those. Do we have to warn about those as well?

That doesn't seem to be necessary. Both the old abi and new abi pass it as two i64 arguments.

compiler/rustc_monomorphize/src/mono_checks/abi_check.rs

hanna-kruppe · 2025-03-17T12:10:12Z

Re: i128, we just had an ABI break (independent of -Zwasm-c-abi) in 1.85 but it only changed alignment not the by-value ABI I believe #134165

alexcrichton · 2025-03-17T15:48:08Z

... @alexcrichton ... is that correct?

Honestly I'm not actually sure. The ABI bits have always been a black box to me and I don't feel confident that I would be able to enumerate the list of differences between -Zwasm-c-abi={spec,legacy}. I think the problem is that the before state is "whatever rustc/llvm together do, pending private details".

That being said what you listed matches my rough understanding. I mostly want to clarify in that I do not have certainty on the exhaustiveness of my understanding.

Would it be possible to "run both ABIs and look for a difference"? That'd help make me more confident in the exhaustiveness of the check, but I'm not sure if that's possible given the structure of the code.

What I can perhaps clarify is that the "official definition" is this for params/results. I'm not knowledgable enough of rustc ABI internals to diff that with what the previous behavior was myself though.

alexcrichton · 2025-03-17T15:51:16Z

compiler/rustc_lint_defs/src/builtin.rs

+    "detects code relying on rustc's non-spec-compliant wasm C ABI",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
+        reference: "issue #122532 <https://github.com/rust-lang/rust/issues/122532>",


Before landing this I think it might be best to open a fresh issue here. This one's got ~100 comments and is quite old. I'm happy to open a fresh issue to fill in here once we're more settled on a final transition plan.

I don't think it's 100 comments, and it is from May last year. But sure happy to update this to another issue number if you tell me the number :)

Yeah I think we should have a separate issue that helps talk about mitigation strategies.

RalfJung · 2025-03-17T16:10:38Z

Would it be possible to "run both ABIs and look for a difference"? That'd help make me more confident in the exhaustiveness of the check, but I'm not sure if that's possible given the structure of the code.

No, it's not possible unfortunately. The entire issue is that the legacy ABI relies on LLVM details rustc has no knowledge about, and on rustc_codegen_llvm details the rest of rustc has no knowledge about.

alexcrichton · 2025-03-17T16:13:11Z

Ok thanks for clarifying, in that case I think we can mitigate that by (a) adding appropriate wording to the blog post about this change and perhaps (b) calling out in the release notes for the next release of Rust "hey things are changing in wasm go read this blog post for more info". Basically just doing our best to raise awareness and get folks testing out -Zwasm-c-abi=spec. If things come up we can continue to refine the lint.

compiler/rustc_monomorphize/src/mono_checks/abi_check.rs

rustbot · 2025-03-17T22:00:12Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

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

alexcrichton · 2025-03-21T00:27:23Z

I've filed #138762 as a landing page for this lint

RalfJung · 2025-03-21T07:08:31Z

Thanks, I have updated the PR to link to that issue. The PR should be good to go then?

Nowhere in the blog post or the tracking issue do we mention that -Zwasm-c-abi=legacy suppresses the lint. That seemed to be a key part of Diplomat's transition plan, or is that not the case any more? This PR becomes simpler if we don't have that flag suppress the lint.

RalfJung · 2025-03-25T10:04:43Z

Yeah same error. No idea what is happening.

library/proc_macro/src/bridge/mod.rs

RalfJung · 2025-03-25T10:29:48Z

@bors try

add FCW to warn about wasm ABI transition See rust-lang#122532 for context: the "C" ABI on wasm32-unk-unk will change. The goal of this lint is to warn about any function definition and calls whose behavior will be affected by the change. My understanding is the following: - scalar arguments are fine - including 128 bit types, they get passed as two `i64` arguments in both ABIs - `repr(C)` structs (recursively) wrapping a single scalar argument are fine (unless they have extra padding due to over-alignment attributes) - all return values are fine `@bjorn3` `@alexcrichton` `@Manishearth` is that correct? I am making this a "show up in future compat reports" lint to maximize the chances people become aware of this. OTOH this likely means warnings for most users of Diplomat so maybe we shouldn't do this? IIUC, wasm-bindgen should be unaffected by this lint as they only pass scalar types as arguments. Tracking issue: rust-lang#138762 Transition plan blog post: rust-lang/blog.rust-lang.org#1531 try-job: dist-various-2

bors · 2025-03-25T10:31:04Z

⌛ Trying commit 782a797 with merge fde8c77...

RalfJung · 2025-03-25T10:32:10Z

@bors try

add FCW to warn about wasm ABI transition See rust-lang#122532 for context: the "C" ABI on wasm32-unk-unk will change. The goal of this lint is to warn about any function definition and calls whose behavior will be affected by the change. My understanding is the following: - scalar arguments are fine - including 128 bit types, they get passed as two `i64` arguments in both ABIs - `repr(C)` structs (recursively) wrapping a single scalar argument are fine (unless they have extra padding due to over-alignment attributes) - all return values are fine `@bjorn3` `@alexcrichton` `@Manishearth` is that correct? I am making this a "show up in future compat reports" lint to maximize the chances people become aware of this. OTOH this likely means warnings for most users of Diplomat so maybe we shouldn't do this? IIUC, wasm-bindgen should be unaffected by this lint as they only pass scalar types as arguments. Tracking issue: rust-lang#138762 Transition plan blog post: rust-lang/blog.rust-lang.org#1531 try-job: dist-various-2

bors · 2025-03-25T10:33:23Z

⌛ Trying commit e88c49c with merge c19c3fb...

RalfJung · 2025-03-25T11:18:25Z

r? @alexcrichton
Hopefully CI will pass this time... but there's a new commit someone should look at :)

bors · 2025-03-25T11:43:56Z

☀️ Try build successful - checks-actions
Build commit: c19c3fb (c19c3fbac0c3230987fe8ccfcb40573cd8ef9b14)

alexcrichton · 2025-03-25T14:38:47Z

@bors: r+

bors · 2025-03-25T14:38:51Z

📌 Commit e88c49c has been approved by alexcrichton

It is now in the queue for this repository.

bors · 2025-03-26T00:06:49Z

⌛ Testing commit e88c49c with merge 068609c...

bors · 2025-03-26T03:20:18Z

☀️ Test successful - checks-actions
Approved by: alexcrichton
Pushing 068609c to master...

github-actions · 2025-03-26T03:22:22Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 43f0014 (parent) -> 068609c (this PR)

Test differences

Show 288 test diffs

errors::verify_monomorphize_wasm_c_abi_transition_11 (stage 1): [missing] -> pass (J0)
[ui] tests/ui/lint/wasm_c_abi_transition.rs (stage 2): [missing] -> pass (J1)
[ui] tests/ui/lint/wasm_c_abi_transition.rs (stage 1): [missing] -> pass (J2)

Additionally, 285 doctest diffs were found. These are ignored, as they are noisy.

Job group index

J0: aarch64-apple, aarch64-gnu, i686-gnu-2, i686-gnu-nopt-2, i686-mingw-2, i686-mingw-3, i686-msvc-2, x86_64-apple-1, x86_64-gnu, x86_64-gnu-llvm-18-3, x86_64-gnu-llvm-19-3, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-2, x86_64-msvc-2
J1: aarch64-apple, aarch64-gnu, arm-android, armhf-gnu, dist-i586-gnu-i586-i686-musl, i686-gnu-1, i686-gnu-nopt-1, i686-msvc-1, test-various, x86_64-apple-2, x86_64-gnu, x86_64-gnu-llvm-18-1, x86_64-gnu-llvm-18-2, x86_64-gnu-llvm-19-1, x86_64-gnu-llvm-19-2, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-1, x86_64-msvc-1
J2: x86_64-gnu-llvm-18-3, x86_64-gnu-llvm-19-3

rust-timer · 2025-03-26T04:38:12Z

Finished benchmarking commit (068609c): 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)

Results (primary -1.1%, secondary -0.6%)

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)	3.4%	[3.4%, 3.4%]	1
Improvements ✅ (primary)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-2.6%	[-2.7%, -2.5%]	2
All ❌✅ (primary)	-1.1%	[-1.1%, -1.1%]	1

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Bootstrap: 777.618s -> 777.496s (-0.02%)
Artifact size: 365.81 MiB -> 365.89 MiB (0.02%)

add FCW to warn about wasm ABI transition See rust-lang#122532 for context: the "C" ABI on wasm32-unk-unk will change. The goal of this lint is to warn about any function definition and calls whose behavior will be affected by the change. My understanding is the following: - scalar arguments are fine - including 128 bit types, they get passed as two `i64` arguments in both ABIs - `repr(C)` structs (recursively) wrapping a single scalar argument are fine (unless they have extra padding due to over-alignment attributes) - all return values are fine `@bjorn3` `@alexcrichton` `@Manishearth` is that correct? I am making this a "show up in future compat reports" lint to maximize the chances people become aware of this. OTOH this likely means warnings for most users of Diplomat so maybe we shouldn't do this? IIUC, wasm-bindgen should be unaffected by this lint as they only pass scalar types as arguments. Tracking issue: rust-lang#138762 Transition plan blog post: rust-lang/blog.rust-lang.org#1531 try-job: dist-various-2

This adds a post for changes discussed in rust-lang/rust#122532 and rust-lang/rust#138601, notably as a result of this decision: rust-lang/rust#122532 (comment)

rustbot assigned wesleywiser Mar 17, 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 Mar 17, 2025

RalfJung mentioned this pull request Mar 17, 2025

Tracking Issue for perma-unstable wasm-c-abi flag #122532

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This was referenced Mar 21, 2025

Tracking issue for wasm32-unknown-unknown's future-incompat warning for C ABI changes #138762

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C ABI Changes for wasm32-unknown-unknown rust-lang/blog.rust-lang.org#1531

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acquire more accurate HirId for ABI check lints

e88c49c

RalfJung force-pushed the wasm-abi-fcw branch from 782a797 to e88c49c Compare March 25, 2025 10:31

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Remove wasm legacy abi #133952

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Add release notes for 1.86.0 #139002

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add FCW to warn about wasm ABI transition #138601

add FCW to warn about wasm ABI transition #138601

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add FCW to warn about wasm ABI transition #138601

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