Structs with no fields or all-ZST fields are ZSTs

src/glossary.md

@@ -220,8 +220,10 @@ A type is zero sized (a ZST) if its size is 0. Such types have at most one possi
 - [Function items] (see [type.fn-item.intro]).
 - The constructors of [tuple-like structs] (see [type.fn-item.intro]).
 - The constructors of [tuple-like enum variants] (see [type.fn-item.intro]).
+- `repr(Rust)` [structs] with no fields or where all fields are zero-sized (see [layout.repr.rust.struct-zst]).
 - `repr(C)` [structs] with no fields or where all fields are zero-sized (see [layout.repr.c.struct.size-field-offset]).
 - `repr(transparent)` [structs] with no fields or where all fields are zero-sized (see [layout.repr.transparent.layout-abi]).
+- `repr(Rust)` [enums] (without a primitive representation specified) with a single struct-like variant with no fields or where all fields are zero-sized (see [layout.repr.rust.enum-struct-like-zst])
 - [Arrays] of zero-sized types (see [layout.array]).
 - [Arrays] of length zero (see [layout.array]).
 - [Unions] of zero-sized types (see [items.union.common-storage]).
@@ -231,6 +233,13 @@ A type is zero sized (a ZST) if its size is 0. Such types have at most one possi
 fn f() {}
 struct S(u8);
 enum E { V(u8) }
+struct UnitLike;
+struct NoFields {}
+struct OnlyZST {
+    f1: (),
+    f2: [(); 10],
+    f3: [u8; 0],
+}
 #[repr(C)]
 struct C1 {}
 #[repr(C)]
@@ -253,17 +262,24 @@ union U {
     f2: [(); 10],
     f3: [u8; 0],
 }
+enum E2 {
+    V1 { f1: (), f2: [(); 10 ] },
+}
 assert_eq!(0, size_of::<()>());
 assert_eq!(0, size_of_val(&f));
 assert_eq!(0, size_of_val(&S));
 assert_eq!(0, size_of_val(&E::V));
+assert_eq!(0, size_of::<UnitLike>());
+assert_eq!(0, size_of::<NoFields>());
+assert_eq!(0, size_of::<OnlyZST>());
 assert_eq!(0, size_of::<C1>());
 assert_eq!(0, size_of::<C2>());
 assert_eq!(0, size_of::<T1>());
 assert_eq!(0, size_of::<T2>());
 assert_eq!(0, size_of::<[(); 10]>());
 assert_eq!(0, size_of::<[u8; 0]>());
 assert_eq!(0, size_of::<U>());
+assert_eq!(0, size_of::<E2>());
 ```
 
 [`extern` blocks]: items.extern

src/type-layout.md

@@ -180,6 +180,12 @@ For [structs], it is further guaranteed that the fields do not overlap. That is,
 
 Be aware that this guarantee does not imply that the fields have distinct addresses: [zero-sized types] may have the same address as other fields in the same struct.
 
+r[layout.repr.rust.struct-zst]
+For [structs] with no fields, or where all fields are [zero-sized], it is further guaranteed that the structs are themselves [zero-sized].
+
+r[layout.repr.rust.enum-struct-like-zst]
+For [enums] (without a primitive representation specified) with a single struct-like variant with no fields or where all fields are [zero-sized], the enum itself is [zero-sized].
+
 r[layout.repr.rust.unspecified]
 There are no other guarantees of data layout made by this representation.
 
@@ -556,6 +562,7 @@ Because this representation delegates type layout to another type, it cannot be
 [`Sized`]: std::marker::Sized
 [`Copy`]: std::marker::Copy
 [dynamically sized types]: dynamically-sized-types.md
+[enums]: items/enumerations.md
 [field-less enums]: items/enumerations.md#field-less-enum
 [fn-abi-compatibility]: ../core/primitive.fn.md#abi-compatibility
 [enumerations]: items/enumerations.md