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Clarify MaybeUninit docs #138005

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126 changes: 63 additions & 63 deletions library/core/src/mem/maybe_uninit.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -5,11 +5,15 @@ use crate::{fmt, intrinsics, ptr, slice};

/// A wrapper type to construct uninitialized instances of `T`.
///
/// A `MaybeUninit<T>` is like a `T`, but without the requirement that it is properly initialized as a `T`.
/// Dropping a `MaybeUninit<T>` does nothing, even if properly initialized as a `T`, because
/// the compiler relies on the type system to decide how to drop variables. Thus, if a `MaybeUninit<T>`
/// should be dropped like a `T`, it should be converted to a `T` with `assume_init` or similar.
///
/// # Initialization invariant
///
/// The compiler, in general, assumes that a variable is properly initialized
/// according to the requirements of the variable's type. For example, a variable of
/// reference type must be aligned and non-null. This is an invariant that must
/// Every variable must be properly initialized according to the requirements of its type.
/// For example, a variable of reference type must be aligned and non-null. This is an invariant that must
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@RalfJung RalfJung Mar 5, 2025 •
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You're still dong changes here without explaining the rhyme or reason for them. What is wrong with the sentence that you changed here, the first sentence of this section?

Sorry, I'm already swamped with reviews, I really don't have the time to tease out the meaningful changes from the gratuitous rewordings in a PR like this.

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@hkBst hkBst Mar 5, 2025

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The idea here is to remove the distraction of mentioning the compiler. (Since it is really the language definition that matters, not what the compiler assumes...)

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@RalfJung RalfJung Mar 5, 2025

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That is the rigorous way of talking about it, but not the most accessible way. We have to be careful not to turn the docs into unreadable (to most of the audience) legalese.

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@hkBst hkBst Oct 24, 2025

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In what way did my change make the text less accessible here? My change simplifies the text, while also making it more correct.

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@RalfJung RalfJung Nov 7, 2025 •
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I do not remember any more what exactly I was referring to. It wasn't just this change but the entire PR.

I will note though that even your new text above says "the compiler relies", so you are not being consistent about avoiding mentioning the compiler. We also do mention the compiler quite a bit elsewhere in the docs -- it's a framing programmers understand well. I don't think it is productive to bikeshed such stylistic choices, and it is not a good use of reviewer time to arbitrarily change a small subset of the docs from one style to the other.

I can help ensure the accuracy of new factual content being added, and I can help with changes that solve a clearly stated, concrete, local problem with the existing docs. I do not have the capacity to review purely editorial changes based on largely subjective preferences. Therefore, I will see myself out of this PR.

/// *always* be upheld, even in unsafe code. As a consequence, zero-initializing a
/// variable of reference type causes instantaneous [undefined behavior][ub],
/// no matter whether that reference ever gets used to access memory:
Expand Down Expand Up @@ -392,7 +396,7 @@ impl<T> MaybeUninit<T> {
MaybeUninit { value: ManuallyDrop::new(val) }
}

/// Creates a new `MaybeUninit<T>` in an uninitialized state.
/// Creates a new uninitialized `MaybeUninit<T>`.
///
/// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
/// It is your responsibility to make sure `T` gets dropped if it got initialized.
Expand All @@ -415,8 +419,7 @@ impl<T> MaybeUninit<T> {
MaybeUninit { uninit: () }
}

/// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being
/// filled with `0` bytes. It depends on `T` whether that already makes for
/// Creates a new zero-filled `MaybeUninit<T>`. It depends on `T` whether that already makes for
/// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,
/// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not
/// be null.
Expand Down Expand Up @@ -475,14 +478,11 @@ impl<T> MaybeUninit<T> {
/// For your convenience, this also returns a mutable reference to the
/// (now safely initialized) contents of `self`.
///
/// As the content is stored inside a `ManuallyDrop`, the destructor is not
/// run for the inner data if the MaybeUninit leaves scope without a call to
/// [`assume_init`], [`assume_init_drop`], or similar. Code that receives
/// the mutable reference returned by this function needs to keep this in
/// mind. The safety model of Rust regards leaks as safe, but they are
/// usually still undesirable. This being said, the mutable reference
/// behaves like any other mutable reference would, so assigning a new value
/// to it will drop the old content.
/// Keep in mind, that the value, as it is wrapped in a `MaybeUninit`,
/// will not be dropped when its wrapper is. You can make sure the value is dropped by unwrapping
/// it with a call to [`assume_init`], or by dropping it directly with [`assume_init_drop`].
/// While the value is also dropped when the returned mutable reference is assigned a new value,
/// the new value is then subject to the same rules, as now the new value is wrapped in a `MaybeUninit`.
///
/// [`assume_init`]: Self::assume_init
/// [`assume_init_drop`]: Self::assume_init_drop
Expand Down Expand Up @@ -518,7 +518,7 @@ impl<T> MaybeUninit<T> {
/// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
/// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
/// # unsafe { MaybeUninit::assume_init_drop(&mut x); }
/// // This leaks the contained string:
/// // This leaks the initialized string:
/// x.write("hello".to_string());
/// // x is initialized now:
/// let s = unsafe { x.assume_init() };
Expand Down Expand Up @@ -593,7 +593,7 @@ impl<T> MaybeUninit<T> {
/// ```
///
/// (Notice that the rules around references to uninitialized data are not finalized yet, but
/// until they are, it is advisable to avoid them.)
/// until they are, it is advisable to avoid references to uninitialized data.)
#[stable(feature = "maybe_uninit", since = "1.36.0")]
#[rustc_const_stable(feature = "const_maybe_uninit_as_ptr", since = "1.59.0")]
#[rustc_as_ptr]
Expand Down Expand Up @@ -645,16 +645,17 @@ impl<T> MaybeUninit<T> {
self as *mut _ as *mut T
}

/// Extracts the value from the `MaybeUninit<T>` container. This is a great way
/// Converts an initialized `MaybeUninit<T>` into a `T`. This is a great way
/// to ensure that the data will get dropped, because the resulting `T` is
/// subject to the usual drop handling.
///
/// # Safety
///
/// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
/// state. Calling this when the content is not yet fully initialized causes immediate undefined
/// behavior. The [type-level documentation][inv] contains more information about
/// this initialization invariant.
/// It is up to the caller to ensure that the `MaybeUninit<T>` has been fully initialized,
/// before converting it into a `T`. Calling this when the `T` value of the `MaybeUninit<T>`
/// is not yet fully initialized causes immediate undefined behavior.
///
/// The [type-level documentation][inv] contains more information about this initialization invariant.
///
/// [inv]: #initialization-invariant
///
Expand Down Expand Up @@ -706,17 +707,19 @@ impl<T> MaybeUninit<T> {
}
}

/// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject
/// to the usual drop handling.
/// Reads the `T` value of the `MaybeUninit<T>`. The result is an ordinary `T` which,
/// just like all `T` values, is subject to the usual drop handling.
///
/// Whenever possible, it is preferable to use [`assume_init`] instead, which
/// prevents duplicating the content of the `MaybeUninit<T>`.
///
/// # Safety
///
/// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
/// state. Calling this when the content is not yet fully initialized causes undefined
/// behavior. The [type-level documentation][inv] contains more information about
/// It is up to the caller to ensure that the `MaybeUninit<T>` has been fully initialized,
/// before reading the `T` value of the `MaybeUninit<T>`. Calling this when the `T` value
/// of the `MaybeUninit<T>` is not yet fully initialized causes immediate undefined behavior.
///
/// The [type-level documentation][inv] contains more information about
/// this initialization invariant.
///
/// Moreover, similar to the [`ptr::read`] function, this function creates a
Expand Down Expand Up @@ -776,16 +779,16 @@ impl<T> MaybeUninit<T> {
}
}

/// Drops the contained value in place.
/// Drops the `T` value of the `MaybeUninit<T>` in place, like [`ptr::drop_in_place`].
///
/// If you have ownership of the `MaybeUninit`, you can also use
/// [`assume_init`] as an alternative.
///
/// # Safety
///
/// It is up to the caller to guarantee that the `MaybeUninit<T>` really is
/// in an initialized state. Calling this when the content is not yet fully
/// initialized causes undefined behavior.
/// It is up to the caller to ensure that the `MaybeUninit<T>` has been fully initialized,
/// before dropping the `T` value of the `MaybeUninit<T>`. Calling this when the `T` value
/// of the `MaybeUninit<T>` is not yet fully initialized causes immediate undefined behavior.
///
/// On top of that, all additional invariants of the type `T` must be
/// satisfied, as the `Drop` implementation of `T` (or its members) may
Expand Down Expand Up @@ -817,9 +820,9 @@ impl<T> MaybeUninit<T> {
///
/// # Safety
///
/// Calling this when the content is not yet fully initialized causes undefined
/// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
/// is in an initialized state.
/// It is up to the caller to ensure that the `MaybeUninit<T>` has been fully initialized,
/// before getting a reference to the `T` value of the `MaybeUninit<T>`. Calling this when the `T` value
/// of the `MaybeUninit<T>` is not yet fully initialized causes immediate undefined behavior.
///
/// # Examples
///
Expand Down Expand Up @@ -884,10 +887,9 @@ impl<T> MaybeUninit<T> {
///
/// # Safety
///
/// Calling this when the content is not yet fully initialized causes undefined
/// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
/// is in an initialized state. For instance, `.assume_init_mut()` cannot be used to
/// initialize a `MaybeUninit`.
/// It is up to the caller to ensure that the `MaybeUninit<T>` has been fully initialized,
/// before getting a mutable reference to the `T` value of the `MaybeUninit<T>`. Calling this when the `T` value
/// of the `MaybeUninit<T>` is not yet fully initialized causes immediate undefined behavior.
///
/// # Examples
///
Expand Down Expand Up @@ -998,7 +1000,7 @@ impl<T> MaybeUninit<T> {
/// # Safety
///
/// It is up to the caller to guarantee that all elements of the array are
/// in an initialized state.
/// properly initialized.
///
/// # Examples
///
Expand Down Expand Up @@ -1035,8 +1037,7 @@ impl<T> MaybeUninit<T> {

/// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
///
/// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
/// contain padding bytes which are left uninitialized.
/// Note that a value may still contain uninitialized padding bytes even if it has been fully initialized.
///
/// # Examples
///
Expand All @@ -1061,8 +1062,7 @@ impl<T> MaybeUninit<T> {
/// Returns the contents of this `MaybeUninit` as a mutable slice of potentially uninitialized
/// bytes.
///
/// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
/// contain padding bytes which are left uninitialized.
/// Note that a value may still contain uninitialized padding bytes even if it has been fully initialized.
///
/// # Examples
///
Expand Down Expand Up @@ -1108,8 +1108,8 @@ impl<T> MaybeUninit<T> {
}

impl<T> [MaybeUninit<T>] {
/// Copies the elements from `src` to `self`,
/// returning a mutable reference to the now initialized contents of `self`.
/// Copies all elements from a `&[T]` to this `[MaybeUninit<T>]`.
/// Returns a `&mut [T]` to the so initialized array.
///
/// If `T` does not implement `Copy`, use [`write_clone_of_slice`] instead.
///
Expand Down Expand Up @@ -1165,13 +1165,14 @@ impl<T> [MaybeUninit<T>] {
unsafe { self.assume_init_mut() }
}

/// Clones the elements from `src` to `self`,
/// returning a mutable reference to the now initialized contents of `self`.
/// Clones all elements from a `&[T]` to this `[MaybeUninit<T>]`.
/// Returns a `&mut [T]` to the so initialized array.
/// Any already initialized elements will not be dropped.
///
/// If `T` implements `Copy`, use [`write_copy_of_slice`] instead.
///
/// This is similar to [`slice::clone_from_slice`] but does not drop existing elements.
/// This is similar to [`slice::clone_from_slice`] but cannot drop existing `MaybeUninit<T>`,
/// as it cannot know if any of them was initialized.
///
/// # Panics
///
Expand Down Expand Up @@ -1244,8 +1245,9 @@ impl<T> [MaybeUninit<T>] {
unsafe { self.assume_init_mut() }
}

/// Fills a slice with elements by cloning `value`, returning a mutable reference to the now
/// initialized contents of the slice.
/// Fills a maybe-uninitialized slice with clones of the given value.
/// Returns a `&mut [T]` to the so initialized slice.
///
/// Any previously initialized elements will not be dropped.
///
/// This is similar to [`slice::fill`].
Expand Down Expand Up @@ -1278,7 +1280,7 @@ impl<T> [MaybeUninit<T>] {
unsafe { self.assume_init_mut() }
}

/// Fills a slice with elements returned by calling a closure for each index.
/// Fills a maybe-uninitialized slice with elements returned by calling a closure for each index.
///
/// This method uses a closure to create new values. If you'd rather `Clone` a given value, use
/// [slice::write_filled]. If you want to use the `Default` trait to generate values, you can
Expand Down Expand Up @@ -1319,7 +1321,7 @@ impl<T> [MaybeUninit<T>] {
unsafe { self.assume_init_mut() }
}

/// Fills a slice with elements yielded by an iterator until either all elements have been
/// Fills a maybe-uninitialized slice with elements yielded by an iterator until either all elements have been
/// initialized or the iterator is empty.
///
/// Returns two slices. The first slice contains the initialized portion of the original slice.
Expand Down Expand Up @@ -1403,8 +1405,7 @@ impl<T> [MaybeUninit<T>] {

/// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
///
/// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
/// contain padding bytes which are left uninitialized.
/// Note that a value may still contain uninitialized padding bytes even if it has been fully initialized.
///
/// # Examples
///
Expand All @@ -1430,8 +1431,7 @@ impl<T> [MaybeUninit<T>] {
/// Returns the contents of this `MaybeUninit` slice as a mutable slice of potentially
/// uninitialized bytes.
///
/// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
/// contain padding bytes which are left uninitialized.
/// Note that a value may still contain uninitialized padding bytes even if it has been fully initialized.
///
/// # Examples
///
Expand Down Expand Up @@ -1460,13 +1460,13 @@ impl<T> [MaybeUninit<T>] {
}
}

/// Drops the contained values in place.
/// Assumes all elements have been fully initialized and drops them in place, like [`ptr::drop_in_place`].
///
/// # Safety
///
/// It is up to the caller to guarantee that every `MaybeUninit<T>` in the slice
/// really is in an initialized state. Calling this when the content is not yet
/// fully initialized causes undefined behavior.
/// It is up to the caller to ensure that the `MaybeUninit<T>` has been fully initialized,
/// before calling. Calling this when any of the `T` elements of the `[MaybeUninit<T>]`
/// has not yet been fully initialized causes immediate undefined behavior.
///
/// On top of that, all additional invariants of the type `T` must be
/// satisfied, as the `Drop` implementation of `T` (or its members) may
Expand Down Expand Up @@ -1495,9 +1495,9 @@ impl<T> [MaybeUninit<T>] {
///
/// # Safety
///
/// Calling this when the content is not yet fully initialized causes undefined
/// Calling this when the elements have not been fully initialized causes undefined
/// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in
/// the slice really is in an initialized state.
/// the slice really is properly initialized.
#[unstable(feature = "maybe_uninit_slice", issue = "63569")]
#[inline(always)]
pub const unsafe fn assume_init_ref(&self) -> &[T] {
Expand All @@ -1512,9 +1512,9 @@ impl<T> [MaybeUninit<T>] {
///
/// # Safety
///
/// Calling this when the content is not yet fully initialized causes undefined
/// Calling this when the elements have not been fully initialized causes undefined
/// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in the
/// slice really is in an initialized state. For instance, `.assume_init_mut()` cannot
/// slice really is properly initialized. For instance, `.assume_init_mut()` cannot
/// be used to initialize a `MaybeUninit` slice.
#[unstable(feature = "maybe_uninit_slice", issue = "63569")]
#[inline(always)]
Expand Down
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