Merge pull request #2375 from DanielEScherzer/phantomdata-1-zst

PhantomData: clearly document alignment of 1, add to glossary examples

Author: traviscross

src/glossary.md

@@ -122,8 +122,10 @@ Examples of 1-ZSTs include:
 - `repr(C)` [structs] with no fields or where all fields are 1-ZSTs (see [layout.repr.c.struct.align] and [layout.repr.c.struct.size-field-offset]).
 - `repr(transparent)` [structs] with no fields or where all fields are 1-ZSTs (see [layout.repr.transparent.layout-abi]).
 - [Arrays] of 1-ZSTs (see [layout.array]).
+- [`PhantomData<T>`] for any type `T` (see [lang-types.phantom-data]).
 
 ```rust
+# use core::marker::PhantomData;
 # use core::mem::{align_of, size_of};
 #[repr(C)]
 struct C1 {}
@@ -151,6 +153,8 @@ assert!(is_1_zst::<C2>());
 assert!(is_1_zst::<[(); 10]>());
 assert!(is_1_zst::<T1>());
 assert!(is_1_zst::<T2>());
+assert!(is_1_zst::<PhantomData<()>>());
+assert!(is_1_zst::<PhantomData<u16>>());
 ```
 
 ### Name
@@ -273,8 +277,10 @@ A type is zero sized (a ZST) if its size is 0. Such types have at most one possi
 - [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]).
+- [`PhantomData<T>`] for any type `T` (see [lang-types.phantom-data]).
 
 ```rust
+# use core::marker::PhantomData;
 # use core::mem::{size_of, size_of_val};
 fn f() {}
 struct S(u8);
@@ -353,10 +359,16 @@ assert_eq!(0, size_of::<E4>());
 assert_eq!(0, size_of::<E5>());
 assert_eq!(0, size_of::<E6>());
 assert_eq!(0, size_of::<E7>());
+# // `PhantomData` for a ZST.
+assert_eq!(0, size_of::<PhantomData<()>>());
+# // `PhantomData` for a type that is not a ZST.
+assert_eq!(0, size_of::<PhantomData<u16>>());
+
 ```
 
 [`extern` blocks]: items.extern
 [`extern fn`]: items.fn.extern
+[`PhantomData<T>`]: lang-types.phantom-data
 [alignment]: type-layout.md#size-and-alignment
 [arrays]: type.array
 [associated item]: #associated-item

src/special-types-and-traits.md

@@ -51,7 +51,7 @@ It also ensures that [`static` items] which have a type with interior mutability
 r[lang-types.phantom-data]
 ## `PhantomData<T>`
 
-[`std::marker::PhantomData<T>`] is a [zero-sized], minimum alignment, type that is considered to own a `T` for the purposes of [variance], [drop check], and [auto traits](#auto-traits).
+[`std::marker::PhantomData<T>`] is a [zero-sized], [1-aligned] type that is considered to own a `T` for the purposes of [variance], [drop check], and [auto traits](#auto-traits).
 
 r[lang-types.va-list]
 ## `VaList<'_>`
@@ -195,6 +195,7 @@ These implicit `Sized` bounds may be relaxed by using the special `?Sized` bound
 [`UnwindSafe`]: std::panic::UnwindSafe
 [`Unpin`]: std::marker::Unpin
 
+[1-aligned]: layout.properties.align
 [Arrays]: types/array.md
 [associated types]: items/associated-items.md#associated-types
 [call expressions]: expressions/call-expr.md