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Document how MaybeUninit<Struct> can be initialized. #81580

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Feb 6, 2021
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Document how MaybeUninit<Struct> can be initialized. #81580

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c351107
Document how `MaybeUninit<Struct>` can be initialized.
rodrimati1992 Jan 31, 2021
0974026
Removed trailing whitespace
rodrimati1992 Jan 31, 2021
aa83e2a
Update maybe_uninit.rs
rodrimati1992 Jan 31, 2021
21c2343
Update comment about leaking
rodrimati1992 Jan 31, 2021
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39 changes: 35 additions & 4 deletions library/core/src/mem/maybe_uninit.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -172,10 +172,41 @@ use crate::ptr;
///
/// ## 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<Struct>`. That means it is not possible
/// to create a struct by calling `MaybeUninit::uninit::<Struct>()` and then writing
/// to its fields.
/// You can use `MaybeUninit<T>`, 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<u8>,
/// }
///
/// let foo = {
/// let mut uninit: MaybeUninit<Foo> = 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.
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Language nitpick: would be leaked --> leaks

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Maybe the leak can be avoided by reordering the code a bit?

let name = ...
let list = ...

unsafe {
   addr_of_mut!((*ptr).name).write(name);
   addr_of_mut!((*ptr).list).write(list);
}

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@the8472 In this case yes, but I wanted this to translate to how one would initialize a large struct field by field.
If the struct had been a heap allocated struct with a large array field instead of a Vec, it wouldn't be possible to construct the array on the stack before moving it to the field in debug builds, since it would overflow the stack.

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@the8472 the8472 Jan 31, 2021 •
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Well, afaik currently there is no guarantee that passing a large array directly to function arguments gets optimized any better than first assigning it to a named variable. You're at the mercy of the optimizer in either case since there's no placement API. rust-lang/rfcs#2884 is still under discussion.

I think to initialize a large array without blowing the stack in debug mode you would have to loop over each item.

/// 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
///
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