Add a note about uninhabited-struct layout optimization #346
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RalfJung
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Aug 15, 2025
src/frequently-requested-changes.md
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| that never reaching the `assume_init` part, it's still unlikely that this occurs frequently. | ||
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| There *is* still interest in doing optimizations like this on *sum* types, however. There's more | ||
| to potentially be gained there since one variant of a `union` or `enum` being uninhabited doesn't |
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I am very surprised to see union mentioned here. As far as I am concerned those are syntactic sugar for transmutes, and I would be very surprised if we could now start doing such optimizations without breaking tons of code. (I think even the train for "fields not at offset 0" may have sailed, many people forget to set repr(C).)
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Yeah, I wrote that without thinking about it that hard, and looking again you're right, I don't see this happening on unions.
Co-authored-by: Jacob Lifshay <[email protected]>
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I think this is pretty settled but don't know that we had a good link about it, so figured I'd write this up.