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

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``

1 files changed, 1 insertions, 7 deletions

diff --git a/compiler/rustc_pattern_analysis/src/rustc.rs b/compiler/rustc_pattern_analysis/src/rustc.rs
index d7885e05a2f..6c09f97bfe7 100644
--- a/compiler/rustc_pattern_analysis/src/rustc.rs
+++ b/compiler/rustc_pattern_analysis/src/rustc.rs
@@ -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)