clarify 'no fields'

Author: RalfJung

compiler/rustc_lint/src/types.rs

@@ -814,7 +814,7 @@ fn get_nullable_type<'tcx>(
 
 /// A type is niche-optimization candidate iff:
 /// - Is a zero-sized type with alignment 1 (a “1-ZST”).
-/// - Has no fields.
+/// - Is either a struct/tuple with no fields, or an enum with no variants.
 /// - Does not have the `#[non_exhaustive]` attribute.
 fn is_niche_optimization_candidate<'tcx>(
     tcx: TyCtxt<'tcx>,
@@ -828,7 +828,7 @@ fn is_niche_optimization_candidate<'tcx>(
     match ty.kind() {
         ty::Adt(ty_def, _) => {
             let non_exhaustive = ty_def.is_variant_list_non_exhaustive();
-            let empty = (ty_def.is_struct() && ty_def.all_fields().next().is_none())
+            let empty = (ty_def.is_struct() && ty_def.non_enum_variant().fields.is_empty())
                 || (ty_def.is_enum() && ty_def.variants().is_empty());
 
             !non_exhaustive && empty

library/core/src/result.rs

@@ -235,7 +235,7 @@
 //! [representation guarantees][opt-rep] (let's call that type `I`), and the *other* type must meet
 //! all of the following conditions:
 //! * Is a zero-sized type with alignment 1 (a "1-ZST").
-//! * Has no fields.
+//! * Is either a struct/tuple with no fields, or an enum with no variants.
 //! * Does not have the `#[non_exhaustive]` attribute.
 //!
 //! If that is the case, then `Result<T, E>` has the same size, alignment, and [function call ABI]