Guarantee repr(C) enum discriminants are stable within a compilation #1454
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…within a compilation
joshlf
changed the title
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Friendly ping 🙂 |
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Friendly ping 🙂 |
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Is there some context for this change? Has it been discussed by the lang team? |
| > discriminants will have the same size and alignment *within a given | ||
| > compilation of the program.* This is true for both field-less enums and | ||
| > for enums with fields. |
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"within a given compilation of the program" doesn't seem clear to me what that means. Certainly all types have the same size and alignment during compilation. I'm not sure I can guess what this is trying to say, can you clarify?
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Maybe I should phrase this differently. The idea is that, whatever choice rustc makes about the size and alignment of the discriminant of a repr(C) enum, it makes the same choice for all repr(C) enums within a program.
The reason this is important is that the size and alignment aren't guaranteed in any way other than to say "rustc will choose some size and alignment." This puts unsafe code in a bind: how do you know what the size and alignment of the discriminant is if you're trying to work with the layout of an enum type? With this added text, you can construct a "dummy" field-less repr(C) enum, query for its size and alignment using size_of and align_of, and take these to be equal to the size and alignment of the discriminant of the enum you're working with.
The specific context is that zerocopy's TryFromBytes trait can currently be derived on field-less enums, and we want to add support for data-ful enums, but we can't without this guarantee (or some other way of querying for the size and alignment of repr(C) enum discriminants).
No, but I figured it'd be pretty uncontroversial. Happy to CC them here if you'd like. |
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