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Add Vec::get_uninit_unchecked #70967

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Add Vec::get_uninit_unchecked #70967

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836e264
Add Vec::get_uninit_unchecked
moshg Apr 10, 2020
45077aa
Reflect suggested changes* Add unsafe* Use debug_assert instead of cf…
moshg Apr 10, 2020
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38 changes: 37 additions & 1 deletion src/liballoc/vec.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -66,7 +66,7 @@ use core::hash::{self, Hash};
use core::intrinsics::{arith_offset, assume};
use core::iter::{FromIterator, FusedIterator, TrustedLen};
use core::marker::PhantomData;
use core::mem::{self, ManuallyDrop};
use core::mem::{self, ManuallyDrop, MaybeUninit};
use core::ops::Bound::{Excluded, Included, Unbounded};
use core::ops::{self, Index, IndexMut, RangeBounds};
use core::ptr::{self, NonNull};
Expand Down Expand Up @@ -854,6 +854,42 @@ impl<T> Vec<T> {
ptr
}

/// Returns a mutable reference to an element, without doing bounds
/// checking.
///
/// This is generally not recommended, use with caution!
/// Calling this method with an out-of-allocation index is *[undefined behavior]*
/// even if the resulting reference is not used.
///
/// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
///
/// # Examples
///
/// ```
/// #![feature(vec_get_uninit_unchecked, maybe_uninit_extra)]
///
/// // Allocate vector big enough for 4 elements.
/// let size = 4;
/// let mut x = Vec::with_capacity(4);
///
/// // Initialize elements via raw pointer writes, then set length.
/// unsafe {
/// for i in 0..size {
/// x.get_uninit_unchecked(i).write(i as i32);
/// }
/// x.set_len(size);
/// }
/// assert_eq!(&*x, &[0, 1, 2, 3]);
/// ```
#[unstable(feature = "vec_get_uninit_unchecked", issue = "none")]
pub fn get_uninit_unchecked(&mut self, index: usize) -> &mut MaybeUninit<T> {
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Suggested change
pub fn get_uninit_unchecked(&mut self, index: usize) -> &mut MaybeUninit<T> {
pub unsafe fn get_uninit_unchecked(&mut self, index: usize) -> &mut MaybeUninit<T> {

There are two ways in which you can violate soundness with this method:

  1. You provide index where index >= self.capacity().
  2. You do let elem = vec.get_uninit_unchecked(index); *elem = MaybeUninit::uninit(); drop(vec[index]);.

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Also, you can have a memory leak if you forget to use vec.set_len(...) afterwards in some circumstances; also worth noting in docs.

if cfg!(debug_assertions) && index >= self.capacity() {
panic!("Out of allocation access")
} else {
unsafe { &mut *(self.as_mut_ptr().add(index) as *mut MaybeUninit<T>) }
}
}

/// Forces the length of the vector to `new_len`.
///
/// This is a low-level operation that maintains none of the normal
Expand Down