fix field-less repr(C) enum docs #2018
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| > [!NOTE] | ||
| > The enum representation in C is implementation defined, so this is really a "best guess". In particular, this may be incorrect when the C code of interest is compiled with certain flags. | ||
| > For maximum portability, it is always preferred to set the size and alignment explicitly using a [primitive representation](#r-layout.repr.primitive.enum). |
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This is the Rust reference but here we are explicitly steering people on how to do an FFI portability thing primarily designed to be compatible with C code. I think we might as well go ahead and note how this can be done in the C source code. Please feel free to disagree, I don't want to scope creep anything unduly.
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| > For maximum portability, it is always preferred to set the size and alignment explicitly using a [primitive representation](#r-layout.repr.primitive.enum). | |
| > For maximum portability, it is always preferred to set the size and alignment explicitly using a [primitive representation](#r-layout.repr.primitive.enum). | |
| > This can be mimicked in C23 or C++17 using the syntax for enums with fixed underlying types. |
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This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed. Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers. |
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@traviscross is there anything this needs to proceed? :) |
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The docs for field-less repr(C) enums are wrong in the sense that they say "the
Crepresentation has the size and alignment of the defaultenumsize and alignment for the target platform's C ABI" which implies that there's a single size and alignment (determined by the target) that all repr(C) enums share -- which isn't true. The size of the enum depends on the discriminant values and is intended to mimic what the default C compiler for the target would do with an enum that has the same discriminant values.Also, it seems worth mentioning the type that the discriminant expressions of an enum are type-checked at: that's
isizefor all enums expect those with primitive representation.This PR presupposes that we are going ahead with rust-lang/rust#147017 and documents things as-if the FCW added there was already a hard error. This is mostly because otherwise it's unclear what we should document as the logic before that bug doesn't always match the target's C compiler (see rust-lang/rust#146504).