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Rollup merge of #128934 - Nadrieril:fix-empty-non-exhaustive, r=compiler-errors

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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``
This commit is contained in:
Matthias Krüger 2024-09-03 19:13:24 +02:00 • committed by GitHub
commit e7504ac704
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16 changed files with 136 additions and 208 deletions
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8
compiler/rustc_pattern_analysis/src/rustc.rs
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@ -229,17 +229,11 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
} else {
let variant =
&adt.variant(RustcPatCtxt::variant_index_for_adt(&ctor, *adt));
// In the cases of either a `#[non_exhaustive]` field list or a non-public
// field, we skip uninhabited fields in order not to reveal the
// uninhabitedness of the whole variant.
let is_non_exhaustive =
variant.is_field_list_non_exhaustive() && !adt.did().is_local();
let tys = cx.variant_sub_tys(ty, variant).map(|(field, ty)| {
let is_visible =
adt.is_enum() || field.vis.is_accessible_from(cx.module, cx.tcx);
let is_uninhabited = cx.is_uninhabited(*ty);
let skip = is_uninhabited && (!is_visible || is_non_exhaustive);
let skip = is_uninhabited && !is_visible;
(ty, PrivateUninhabitedField(skip))
});
cx.dropless_arena.alloc_from_iter(tys)