Distinguish repr(C) ZSTs from others in ABI compatibility rules#157973
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Jules-Bertholet wants to merge 1 commit into
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Distinguish repr(C) ZSTs from others in ABI compatibility rules#157973Jules-Bertholet wants to merge 1 commit into
repr(C) ZSTs from others in ABI compatibility rules#157973Jules-Bertholet wants to merge 1 commit into
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Jun 16, 2026
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| /// - Any two types fulfilling all the following conditions are ABI-compatible; | ||
| /// such types are said to have "trivial ABI": | ||
| /// - Size 0 | ||
| /// - Alignment 1 | ||
| /// - Not `repr(C)` | ||
| /// - Not a `repr(transparent)` wrapper around a type that fails to satisfy these conditions |
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One question is whether we also wish to exclude arrays of size 0. E.g., should [SomeReprCStruct; 0] be guaranteed to have trivial ABI? (C currently can't pass arrays by value, but maybe some day they will fix that)
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(Split out from compiler implementation in #156112)
Some C ABIs pass and return ZSTs by pointer. But
()should never be returned by pointer, as it must matchvoid. To account for this, we have to weaken the present guarantee of "any two types with size 0 and alignment 1 are ABI-compatible" to excluderepr(C).Fixes rust-lang/unsafe-code-guidelines#552; see also #78586, #155299.
@rustbot label T-lang A-ABI needs-fcp