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Ignore zero-sized types in wasm future-compat warning #139498

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Merged
merged 1 commit into from
Apr 18, 2025
Merged

Ignore zero-sized types in wasm future-compat warning #139498

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5 changes: 5 additions & 0 deletions compiler/rustc_monomorphize/src/mono_checks/abi_check.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -111,6 +111,11 @@ fn wasm_abi_safe<'tcx>(tcx: TyCtxt<'tcx>, arg: &ArgAbi<'tcx, Ty<'tcx>>) -> bool
}
}

// Zero-sized types are dropped in both ABIs, so they're safe
if arg.layout.is_zst() {
return true;
}
Comment on lines +114 to +117
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That's not actually a stable guarantee, nobody should rely on this...

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I think that this at least respects the status quo where the purpose of the wasm_c_abi warning is "this will change" and for ZSTs behavior isn't changing.

Otherwise though for relying on ZSTs, I know wasm-bindgen definitely does rely on ZSTs getting dropped at the ABI level, and AFAIK it's quite difficult to remove the reliance. I can't speak for other projects though.

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for ZSTs behavior isn't changing.

Yeah, that's a fair point, we don't have to warn about them.

But it seems quite concerning that wasm-bindgen would rely on unstable undocumented ABI guarantees without (apparently) any effort to make those things into stable guarantees. We don't guarantee that ZST are skipped in any form for any ABI, and I'm not even aware of an issue someone opened asking for this as a guarantee.


false
}

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6 changes: 6 additions & 0 deletions tests/ui/lint/wasm_c_abi_transition.rs
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Expand Up @@ -39,3 +39,9 @@ pub fn call_other_fun(x: MyType) {
unsafe { other_fun(x) } //~ERROR: wasm ABI transition
//~^WARN: previously accepted
}

// Zero-sized types are safe in both ABIs
#[repr(C)]
pub struct MyZstType;
#[allow(improper_ctypes_definitions)]
pub extern "C" fn zst_safe(_x: (), _y: MyZstType) {}
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