New rule layout.repr.rust.enum-struct-like-zst

Author: DanielEScherzer

src/glossary.md

@@ -223,6 +223,7 @@ A type is zero sized (a ZST) if its size is 0. Such types have at most one possi
 - `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] 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]).
@@ -261,6 +262,9 @@ 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));
@@ -275,6 +279,7 @@ 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

@@ -183,6 +183,9 @@ Be aware that this guarantee does not imply that the fields have distinct addres
 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] 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.
 
@@ -559,6 +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