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Disallow single-variant enums

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Couldn't find documentation supporting that single-variant
`#[repr(Rust)]` enums with RHS assigned work as expected with this
change.

```rust
enum Variants {
  A = 17,
} // Would this be zero sized optimized guaranteed?
```
This commit is contained in:
Arvind Mukund 2024-03-09 15:13:29 -08:00
parent 223d5eb64f
commit 014ddac9c9
3 changed files with 39 additions and 17 deletions
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10
compiler/rustc_lint/src/types.rs
View file

@ -1102,22 +1102,24 @@ fn get_nullable_type<'tcx>(
/// A type is niche_optimization_candiate iff:
/// - Is a zero-sized type with alignment 1 (a “1-ZST”).
/// - Has no fields.
/// - Does not have the #[non_exhaustive] attribute.
/// - Does not have the `#[non_exhaustive]` attribute.
fn is_niche_optimization_candidate<'tcx>(
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
ty: Ty<'tcx>,
) -> bool {
if !tcx.layout_of(param_env.and(ty)).is_ok_and(|layout| layout.is_1zst()) {
if tcx.layout_of(param_env.and(ty)).is_ok_and(|layout| !layout.is_1zst()) {
return false;
}
match ty.kind() {
ty::Adt(ty_def, _) => {
let non_exhaustive = ty_def.is_variant_list_non_exhaustive();
let contains_no_fields = ty_def.all_fields().next().is_none();
// Should single-variant enums be allowed?
let empty = (ty_def.is_struct() && ty_def.all_fields().next().is_none())
|| (ty_def.is_enum() && ty_def.variants().is_empty());
!non_exhaustive && contains_no_fields
!non_exhaustive && empty
}
ty::Tuple(tys) => tys.is_empty(),
_ => false,