Add documentation for MaybeUninit<T>

src/behavior-considered-undefined.md

@@ -144,6 +144,9 @@ r[undefined.validity.struct]
 r[undefined.validity.union]
 * For a `union`, the exact validity requirements are not decided yet. Obviously, all values that can be created entirely in safe code are valid. If the union has a [zero-sized] field, then every possible value is valid. Further details are [still being debated](https://github.com/rust-lang/unsafe-code-guidelines/issues/438).
 
+r[undefined.validity.maybe-uninit]
+* A [`MaybeUninit<T>`] is always valid (see [lang-types.maybe-uninit.validity]).
+
 r[undefined.validity.reference-box]
 * A reference or [`Box<T>`] must be aligned and non-null, it cannot be [dangling], and it must point to a valid value (in case of dynamically sized types, using the actual dynamic type of the pointee as determined by the [metadata]). Note that the last point (about pointing to a valid value) remains a subject of some debate.
 
@@ -203,6 +206,7 @@ r[undefined.validity.undef]
 [`target_feature`]: attributes/codegen.md#the-target_feature-attribute
 [`UnsafeCell<U>`]: std::cell::UnsafeCell
 [Rustonomicon]: ../nomicon/index.html
+[`MaybeUninit<T>`]: core::mem::MaybeUninit
 [metadata]: dynamic-sized.pointer-types
 [`NonNull<T>`]: core::ptr::NonNull
 [`NonZero<T>`]: core::num::NonZero

src/glossary.md

@@ -124,10 +124,11 @@ Examples of 1-ZSTs include:
 - `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]).
+- [`MaybeUninit<T>`] for a type `T` that is itself a 1-ZST (see [lang-types.maybe-uninit.layout]).
 
 ```rust
 # use core::marker::PhantomData;
-# use core::mem::{align_of, size_of};
+# use core::mem::{MaybeUninit, align_of, size_of};
 #[repr(C)]
 struct C1 {}
 #[repr(C)]
@@ -157,6 +158,7 @@ assert!(is_1_zst::<T1>());
 assert!(is_1_zst::<T2>());
 assert!(is_1_zst::<PhantomData<()>>());
 assert!(is_1_zst::<PhantomData<u16>>());
+assert!(is_1_zst::<MaybeUninit<[(); 2]>>());
 ```
 
 ### Name
@@ -281,10 +283,11 @@ A type is zero sized (a ZST) if its size is 0. Such types have at most one possi
 - [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]).
+- [`MaybeUninit<T>`] for a type `T` that is itself a zero-sized type (see [lang-types.maybe-uninit.layout]).
 
 ```rust
 # use core::marker::PhantomData;
-# use core::mem::{size_of, size_of_val};
+# use core::mem::{MaybeUninit, size_of, size_of_val};
 fn f() {}
 struct S(u8);
 enum E { V(u8) }
@@ -367,10 +370,12 @@ assert_eq!(0, size_of::<E7>());
 assert_eq!(0, size_of::<PhantomData<()>>());
 # // `PhantomData` for a type that is not a ZST.
 assert_eq!(0, size_of::<PhantomData<u16>>());
+assert_eq!(0, size_of::<MaybeUninit<[(); 2]>>());
 ```
 
 [`extern` blocks]: items.extern
 [`extern fn`]: items.fn.extern
+[`MaybeUninit<T>`]: lang-types.maybe-uninit
 [`PhantomData<T>`]: lang-types.phantom-data
 [alignment]: type-layout.md#size-and-alignment
 [arrays]: type.array

src/special-types-and-traits.md

@@ -48,6 +48,18 @@ r[lang-types.unsafe-cell.interior-mut]
 r[lang-types.unsafe-cell.read-only-alloc]
 It also ensures that [`static` items] which have a type with interior mutability aren't placed in memory marked as read only.
 
+r[lang-types.maybe-uninit]
+## `MaybeUninit<T>`
+
+r[lang-types.maybe-uninit.intro]
+[`MaybeUninit<T>`] is a type for values that are laid out like a `T` but need not be a valid `T`. This is useful when allocating and writing to memory intended for `T` values before that memory holds a valid `T`, as when initializing such values in place.
+
+r[lang-types.maybe-uninit.validity]
+A value of type [`MaybeUninit<T>`] is always [valid], regardless of any validity requirements for a value of type `T`.
+
+r[lang-types.maybe-uninit.layout]
+[`MaybeUninit<T>`] is guaranteed to have the same [size], [alignment], and [ABI] as `T`.
+
 r[lang-types.phantom-data]
 ## `PhantomData<T>`
 
@@ -188,6 +200,7 @@ These implicit `Sized` bounds may be relaxed by using the special `?Sized` bound
 [`Arc<Self>`]: std::sync::Arc
 [`Deref`]: std::ops::Deref
 [`DerefMut`]: std::ops::DerefMut
+[`MaybeUninit<T>`]: core::mem::MaybeUninit
 [`Pin<P>`]: std::pin::Pin
 [`Rc<Self>`]: std::rc::Rc
 [`RefUnwindSafe`]: std::panic::RefUnwindSafe
@@ -196,6 +209,8 @@ These implicit `Sized` bounds may be relaxed by using the special `?Sized` bound
 [`Unpin`]: std::marker::Unpin
 
 [1-aligned]: layout.properties.align
+[ABI]: glossary.abi
+[alignment]: layout.properties.align
 [Arrays]: types/array.md
 [associated types]: items/associated-items.md#associated-types
 [call expressions]: expressions/call-expr.md
@@ -216,12 +231,14 @@ These implicit `Sized` bounds may be relaxed by using the special `?Sized` bound
 [moved from]: expr.move.movable-place
 [operators]: expressions/operator-expr.md
 [orphan rules]: items/implementations.md#trait-implementation-coherence
+[size]: layout.properties.size
 [`static` items]: items/static-items.md
 [test functions]: attributes/testing.md#the-test-attribute
 [the standard library]: std
 [trait object]: types/trait-object.md
 [Tuples]: types/tuple.md
 [Type parameters]: types/parameters.md
+[valid]: undefined.validity
 [`VaList`]: core::ffi::VaList
 [variance]: subtyping.md#variance
 [zero-sized]: glossary.zst