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ManuallyDrop != MaybeUninit #58780

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Mar 3, 2019
Merged

ManuallyDrop != MaybeUninit #58780

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4e7d4c7
ManuallyDrop != MaybeUninit
RalfJung Feb 27, 2019
f92c204
improve readability
RalfJung Feb 27, 2019
0c1a38c
Update src/libcore/mem.rs
Centril Feb 28, 2019
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8 changes: 8 additions & 0 deletions src/libcore/mem.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -900,10 +900,16 @@ pub fn discriminant<T>(v: &T) -> Discriminant<T> {
}
}

// FIXME: Reference `MaybeUninit` from these docs, once that is stable.
/// A wrapper to inhibit compiler from automatically calling `T`’s destructor.
///
/// This wrapper is 0-cost.
///
/// `ManuallyDrop<T>` is subject to the same layout optimizations as `T`.
/// As a consequence, it has *no effect* on the assumptions that the compiler makes
/// about all values being initialized at their type. In particular, initializing
/// a `ManuallyDrop<&mut T>` with [`mem::zeroed`] is undefined behavior.
///
/// # Examples
///
/// This wrapper helps with explicitly documenting the drop order dependencies between fields of
Expand Down Expand Up @@ -935,6 +941,8 @@ pub fn discriminant<T>(v: &T) -> Discriminant<T> {
/// }
/// }
/// ```
///
/// [`mem::zeroed`]: fn.zeroed.html
#[stable(feature = "manually_drop", since = "1.20.0")]
#[lang = "manually_drop"]
#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
Expand Down