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Non-exhaustive structs may be empty This is a follow-up to a discrepancy noticed in https://github.com/rust-lang/rust/pull/122792: today, the following struct is considered inhabited (non-empty) outside its defining crate: ```rust #[non_exhaustive] pub struct UninhabitedStruct { pub never: !, // other fields } ``` `#[non_exhaustive]` on a struct should mean that adding fields to it isn't a breaking change. There is no way that adding fields to this struct could make it non-empty since the `never` field must stay and is inconstructible. I suspect this was implemented this way due to confusion with `#[non_exhaustive]` enums, which indeed should be considered non-empty outside their defining crate. I propose that we consider such a struct uninhabited (empty), just like it would be without the `#[non_exhaustive]` annotation. Code that doesn't pass today and will pass after this: ```rust // In a different crate fn empty_match_on_empty_struct<T>(x: UninhabitedStruct) -> T { match x {} } ``` This is not a breaking change. r? ``@compiler-errors`` |
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