New rule: items.struct.zst

Guarantee that structs with no fields, or where all fields are ZSTs, are
themselves ZSTs.

Author: DanielEScherzer

src/glossary.md

@@ -220,8 +220,9 @@ 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(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]).
+- [structs] with no fields or where all fields are zero-sized (see [items.struct.zst])
+  - Including those marked with `repr(C)` (see [layout.repr.c.struct.size-field-offset]).
+  - or marked with `repr(transparent)` (see [layout.repr.transparent.layout-abi]).
 - [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 +232,13 @@ 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)]
@@ -257,6 +265,9 @@ 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>());

src/items/structs.md

@@ -64,9 +64,13 @@ let c = [Cookie, Cookie {}, Cookie, Cookie {}];
 r[items.struct.layout]
 The precise memory layout of a struct is not specified. One can specify a particular layout using the [`repr` attribute].
 
+r[items.struct.zst]
+Structs with no fields, or where all fields are [zero-sized], are themselves guaranteed to be [zero-sized].
+
 [`repr` attribute]: ../type-layout.md#representations
 [constant]: constant-items.md
 [struct type]: ../types/struct.md
 [tuple type]: ../types/tuple.md
 [type namespace]: ../names/namespaces.md
 [value namespace]: ../names/namespaces.md
+[zero-sized]: glossary.zst