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be explicit that mem::uninitialized is the same as MaybeUninit::uninit().assume_init() #67274

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Dec 13, 2019
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be explicit that mem::uninitialized is the same as MaybeUninit::uninit().assume_init() #67274

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6 changes: 5 additions & 1 deletion src/libcore/mem/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -510,7 +510,9 @@ pub unsafe fn zeroed<T>() -> T {
/// **This function is deprecated.** Use [`MaybeUninit<T>`] instead.
///
/// The reason for deprecation is that the function basically cannot be used
/// correctly: [the Rust compiler assumes][inv] that values are properly initialized.
/// correctly: it has the same effect as [`MaybeUninit::uninit().assume_init()`][uninit].
/// As the [`assume_init` documentation][assume_init] explains,
/// [the Rust compiler assumes][inv] that values are properly initialized.
/// As a consequence, calling e.g. `mem::uninitialized::<bool>()` causes immediate
/// undefined behavior for returning a `bool` that is not definitely either `true`
/// or `false`. Worse, truly uninitialized memory like what gets returned here
Expand All @@ -521,6 +523,8 @@ pub unsafe fn zeroed<T>() -> T {
/// until they are, it is advisable to avoid them.)
///
/// [`MaybeUninit<T>`]: union.MaybeUninit.html
/// [uninit]: union.MaybeUninit.html#method.uninit
/// [assume_init]: union.MaybeUninit.html#method.assume_init
/// [inv]: union.MaybeUninit.html#initialization-invariant
#[inline]
#[rustc_deprecated(since = "1.39.0", reason = "use `mem::MaybeUninit` instead")]
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