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docs: transmute<&mut T, &mut MaybeUninit<T>> is unsound when exposed to safe code #134583

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Dec 23, 2024
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docs: transmute<&mut T, &mut MaybeUninit<T>> is unsound when exposed to safe code #134583

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20 changes: 20 additions & 0 deletions library/core/src/mem/maybe_uninit.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -232,6 +232,26 @@ use crate::{fmt, intrinsics, ptr, slice};
/// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has
/// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that
/// guarantee may evolve.
///
/// Note that even though `T` and `MaybeUninit<T>` are ABI compatible it is still unsound to
/// transmute `&mut T` to `&mut MaybeUninit<T>` and expose that to safe code because it would allow
/// safe code to access uninitialized memory:
///
/// ```rust,no_run
/// use core::mem::MaybeUninit;
///
/// fn unsound_transmute<T>(val: &mut T) -> &mut MaybeUninit<T> {
/// unsafe { core::mem::transmute(val) }
/// }
///
/// fn main() {
/// let mut code = 0;
/// let code = &mut code;
/// let code2 = unsound_transmute(code);
/// *code2 = MaybeUninit::uninit();
/// std::process::exit(*code); // UB! Accessing uninitialized memory.
/// }
/// ```
#[stable(feature = "maybe_uninit", since = "1.36.0")]
// Lang item so we can wrap other types in it. This is useful for coroutines.
#[lang = "maybe_uninit"]
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