From c351107cdc378dec4f855c29a3566fbd1d2ac358 Mon Sep 17 00:00:00 2001 From: rodrimati1992 Date: Sun, 31 Jan 2021 01:21:06 -0300 Subject: [PATCH] Document how `MaybeUninit` can be initialized. --- library/core/src/mem/maybe_uninit.rs | 39 +++++++++++++++++++++++++--- 1 file changed, 35 insertions(+), 4 deletions(-) diff --git a/library/core/src/mem/maybe_uninit.rs b/library/core/src/mem/maybe_uninit.rs index 05bcd90d3ca..c85b5655bdb 100644 --- a/library/core/src/mem/maybe_uninit.rs +++ b/library/core/src/mem/maybe_uninit.rs @@ -172,10 +172,41 @@ /// /// ## Initializing a struct field-by-field /// -/// There is currently no supported way to create a raw pointer or reference -/// to a field of a struct inside `MaybeUninit`. That means it is not possible -/// to create a struct by calling `MaybeUninit::uninit::()` and then writing -/// to its fields. +/// You can use `MaybeUninit`, and the [`std::ptr::addr_of_mut`] macro, to initialize structs field by field: +/// +/// ```rust +/// use std::mem::MaybeUninit; +/// use std::ptr::addr_of_mut; +/// +/// #[derive(Debug, PartialEq)] +/// pub struct Foo { +/// name: String, +/// list: Vec, +/// } +/// +/// let foo = { +/// let mut uninit: MaybeUninit = MaybeUninit::uninit(); +/// let ptr = uninit.as_mut_ptr(); +/// +/// // Initializing the `name` field +/// unsafe { addr_of_mut!((*ptr).name).write("Bob".to_string()); } +/// +/// // Initializing the `list` field +/// // If there was a panic here, then the `String` in the `name` field would be leaked. +/// unsafe { addr_of_mut!((*ptr).list).write(vec![0, 1, 2]); } +/// +/// // All the fields are initialized, so we call `assume_init` to get an initialized Foo. +/// unsafe { uninit.assume_init() } +/// }; +/// +/// assert_eq!( +/// foo, +/// Foo { +/// name: "Bob".to_string(), +/// list: vec![0, 1, 2] +/// } +/// ); +/// ``` /// /// [ub]: ../../reference/behavior-considered-undefined.html /// -- 2.44.0