New rule items.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]).
+- [Enums] with a single struct-like variant with no fields or where all fields are zero-sized (see [items.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/items/enumerations.md

@@ -77,6 +77,9 @@ enum Enum {
 }
 ```
 
+r[items.enum.struct-like-zst]
+If an enum only has a single struct-like variant with no fields or where all fields are [zero-sized], the enum itself is [zero-sized].
+
 r[items.enum.constructor-names]
 Variant constructors are similar to [struct] definitions, and can be referenced by a path from the enumeration name, including in [use declarations].
 
@@ -364,3 +367,4 @@ enum E {
 [unit-only]: #unit-only-enum
 [use declarations]: use-declarations.md
 [value namespace]: ../names/namespaces.md
+[zero-sized]: glossary.zst