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Add get_pin_ref and get_pin_mut methods to slice #78370

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Add get_pin_ref and get_pin_mut methods to slice #78370

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60 changes: 60 additions & 0 deletions library/core/src/slice/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,7 @@ use crate::num::NonZeroUsize;
use crate::ops::{FnMut, Range, RangeBounds};
use crate::option::Option;
use crate::option::Option::{None, Some};
use crate::pin::Pin;
use crate::ptr;
use crate::result::Result;
use crate::result::Result::{Err, Ok};
Expand Down Expand Up @@ -307,6 +308,65 @@ impl<T> [T] {
index.get_mut(self)
}

/// Returns a pinned reference to an element or subslice depending on the
/// type of index (see [`get`]) or `None` if the index is out of bounds.
///
/// [`get`]: #method.get
///
/// # Examples
///
/// ```
/// #![feature(slice_get_pin)]
/// use std::pin::Pin;
///
/// let v = vec![0, 1, 2].into_boxed_slice();
/// let pinned = Pin::from(v);
///
/// let x: Option<Pin<&i32>> = pinned.as_ref().get_pin_ref(1);
/// assert_eq!(&1, &*x.unwrap());
/// ```
#[unstable(feature = "slice_get_pin", issue = "none")]
#[inline]
pub fn get_pin_ref<I>(self: Pin<&Self>, index: I) -> Option<Pin<&I::Output>>
where
I: SliceIndex<Self>,
{
// SAFETY: `x` is guaranteed to be pinned because it comes from `self`
// which is pinned.
unsafe { self.get_ref().get(index).map(|x| Pin::new_unchecked(x)) }
}

/// Returns a pinned mutable reference to an element or subslice depending on the
/// type of index (see [`get`]) or `None` if the index is out of bounds.
///
/// [`get`]: #method.get
///
/// # Examples
///
/// ```
/// #![feature(slice_get_pin)]
/// use std::pin::Pin;
///
/// let v = vec![0, 1, 2].into_boxed_slice();
/// let mut pinned = Pin::from(v);
///
/// if let Some(mut elem) = pinned.as_mut().get_pin_mut(1) {
/// elem.set(10);
/// }
/// assert_eq!(&*pinned, &[0, 10, 2]);
/// ```
#[unstable(feature = "slice_get_pin", issue = "none")]
#[inline]
pub fn get_pin_mut<I>(self: Pin<&mut Self>, index: I) -> Option<Pin<&mut I::Output>>
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The existence of this method means that unsafe code needs to be careful when projecting from a Pin<&mut [T]> to a &mut T (a mutable reference to one of the slice elements). It can still be done soundly, but the &mut T cannot be moved out of if the Pin<&mut [T]> is ever exposed to safe code (which could call get_pin_mut).

I think it would be good to mention this in the docs.

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That is correct. Option::as_pin_mut also has the same issue. We need to add docs to both.

where
I: SliceIndex<Self>,
{
// SAFETY: `get_unchecked_mut` is never used to move the slice inside `self` (`SliceIndex`
// is sealed and all `SliceIndex::get_mut` implementations never move elements).
// `x` is guaranteed to be pinned because it comes from `self` which is pinned.
unsafe { self.get_unchecked_mut().get_mut(index).map(|x| Pin::new_unchecked(x)) }
}

/// Returns a reference to an element or subslice, without doing bounds
/// checking.
///
Expand Down