other-reprs: do not make it sound like we are making ABI promises for repr(int) enums #461
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RalfJung
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CAD97
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Sep 11, 2024
| knowledge of "invalid" values at this type to optimize enum layout, such as when | ||
| this enum is wrapped in `Option`). Note that the function call ABI for these | ||
| types is still in general unspecified, except that across `extern "C"` calls they | ||
| are ABI-compatible with C enums of the same sign and size. |
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Just to point this out here as well, C has non-transitive rules that allow enum and the underlying integer type to be punned by ABI.
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Yeah I saw that comment in the issue. I don't think that's something to discuss here though -- they key point is that the Rust thing matches the C thing.
Non-transitive compatibility doesn't really make sense IMO but that's a discussion for another day.
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I've run into problems with cfi tags on methods involving enums in the signature. You probably want to be careful about what you promise here. Unfortunately I don't know what the precise rules are CFI wise.
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Until something like rust-lang/rust#128600 lands, let's be careful what we say about this.
Also mention that
repr(C)generally makes this behave the same as C ABI-wise. That's anyway what people already assume this means and are relying on in practice, so there's no point in not saying it IMO?Cc @rust-lang/opsem @chorman0773