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Add slice take methods #88502

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Dec 1, 2021
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Add slice take methods #88502

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3 changes: 3 additions & 0 deletions library/core/src/ops/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -181,6 +181,9 @@ pub use self::range::{Range, RangeFrom, RangeFull, RangeTo};
#[stable(feature = "inclusive_range", since = "1.26.0")]
pub use self::range::{Bound, RangeBounds, RangeInclusive, RangeToInclusive};

#[unstable(feature = "one_sided_range", issue = "69780")]
pub use self::range::OneSidedRange;

#[unstable(feature = "try_trait_v2", issue = "84277")]
pub use self::try_trait::{FromResidual, Try};

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18 changes: 18 additions & 0 deletions library/core/src/ops/range.rs
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Expand Up @@ -971,3 +971,21 @@ impl<T> RangeBounds<T> for RangeToInclusive<&T> {
Included(self.end)
}
}

/// `OneSidedRange` is implemented for built-in range types that are unbounded
/// on one side. For example, `a..`, `..b` and `..=c` implement `OneSidedRange`,
/// but `..`, `d..e`, and `f..=g` do not.
///
/// Types that implement `OneSidedRange<T>` must return `Bound::Unbounded`
/// from one of `RangeBounds::start_bound` or `RangeBounds::end_bound`.
#[unstable(feature = "one_sided_range", issue = "69780")]
pub trait OneSidedRange<T: ?Sized>: RangeBounds<T> {}

#[unstable(feature = "one_sided_range", issue = "69780")]
impl<T> OneSidedRange<T> for RangeTo<T> where Self: RangeBounds<T> {}

#[unstable(feature = "one_sided_range", issue = "69780")]
impl<T> OneSidedRange<T> for RangeFrom<T> where Self: RangeBounds<T> {}

#[unstable(feature = "one_sided_range", issue = "69780")]
impl<T> OneSidedRange<T> for RangeToInclusive<T> where Self: RangeBounds<T> {}
264 changes: 263 additions & 1 deletion library/core/src/slice/mod.rs
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Expand Up @@ -10,7 +10,7 @@ use crate::cmp::Ordering::{self, Greater, Less};
use crate::marker::Copy;
use crate::mem;
use crate::num::NonZeroUsize;
use crate::ops::{FnMut, Range, RangeBounds};
use crate::ops::{Bound, FnMut, OneSidedRange, Range, RangeBounds};
use crate::option::Option;
use crate::option::Option::{None, Some};
use crate::ptr;
Expand Down Expand Up @@ -82,6 +82,29 @@ pub use index::range;
#[unstable(feature = "inherent_ascii_escape", issue = "77174")]
pub use ascii::EscapeAscii;

/// Calculates the direction and split point of a one-sided range.
///
/// This is a helper function for `take` and `take_mut` that returns
/// the direction of the split (front or back) as well as the index at
/// which to split. Returns `None` if the split index would overflow.
#[inline]
fn split_point_of(range: impl OneSidedRange<usize>) -> Option<(Direction, usize)> {
use Bound::*;

Some(match (range.start_bound(), range.end_bound()) {
(Unbounded, Excluded(i)) => (Direction::Front, *i),
(Unbounded, Included(i)) => (Direction::Front, i.checked_add(1)?),
(Excluded(i), Unbounded) => (Direction::Back, i.checked_add(1)?),
(Included(i), Unbounded) => (Direction::Back, *i),
_ => unreachable!(),
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I don't really like the presence of panic call here which optimiser will have to remove. Would it make more sense to make this a method of OneSidedRange in order to statically eliminate impossible cases?

})
}

enum Direction {
Front,
Back,
}

#[lang = "slice"]
#[cfg(not(test))]
impl<T> [T] {
Expand Down Expand Up @@ -3576,6 +3599,245 @@ impl<T> [T] {
{
self.binary_search_by(|x| if pred(x) { Less } else { Greater }).unwrap_or_else(|i| i)
}

/// Removes the subslice corresponding to the given range
/// and returns a reference to it.
///
/// Returns `None` and does not modify the slice if the given
/// range is out of bounds.
///
/// Note that this method only accepts one-sided ranges such as
/// `2..` or `..6`, but not `2..6`.
///
/// # Examples
///
/// Taking the first three elements of a slice:
///
/// ```
/// #![feature(slice_take)]
///
/// let mut slice: &[_] = &['a', 'b', 'c', 'd'];
/// let mut first_three = slice.take(..3).unwrap();
///
/// assert_eq!(slice, &['d']);
/// assert_eq!(first_three, &['a', 'b', 'c']);
/// ```
///
/// Taking the last two elements of a slice:
///
/// ```
/// #![feature(slice_take)]
///
/// let mut slice: &[_] = &['a', 'b', 'c', 'd'];
/// let mut tail = slice.take(2..).unwrap();
///
/// assert_eq!(slice, &['a', 'b']);
/// assert_eq!(tail, &['c', 'd']);
/// ```
///
/// Getting `None` when `range` is out of bounds:
///
/// ```
/// #![feature(slice_take)]
///
/// let mut slice: &[_] = &['a', 'b', 'c', 'd'];
///
/// assert_eq!(None, slice.take(5..));
/// assert_eq!(None, slice.take(..5));
/// assert_eq!(None, slice.take(..=4));
/// let expected: &[char] = &['a', 'b', 'c', 'd'];
/// assert_eq!(Some(expected), slice.take(..4));
/// ```
#[inline]
#[must_use = "method does not modify the slice if the range is out of bounds"]
#[unstable(feature = "slice_take", issue = "62280")]
pub fn take<'a, R: OneSidedRange<usize>>(self: &mut &'a Self, range: R) -> Option<&'a Self> {
let (direction, split_index) = split_point_of(range)?;
if split_index > self.len() {
return None;
}
let (front, back) = self.split_at(split_index);
match direction {
Direction::Front => {
*self = back;
Some(front)
}
Direction::Back => {
*self = front;
Some(back)
}
}
}

/// Removes the subslice corresponding to the given range
/// and returns a mutable reference to it.
///
/// Returns `None` and does not modify the slice if the given
/// range is out of bounds.
///
/// Note that this method only accepts one-sided ranges such as
/// `2..` or `..6`, but not `2..6`.
///
/// # Examples
///
/// Taking the first three elements of a slice:
///
/// ```
/// #![feature(slice_take)]
///
/// let mut slice: &mut [_] = &mut ['a', 'b', 'c', 'd'];
/// let mut first_three = slice.take_mut(..3).unwrap();
///
/// assert_eq!(slice, &mut ['d']);
/// assert_eq!(first_three, &mut ['a', 'b', 'c']);
/// ```
///
/// Taking the last two elements of a slice:
///
/// ```
/// #![feature(slice_take)]
///
/// let mut slice: &mut [_] = &mut ['a', 'b', 'c', 'd'];
/// let mut tail = slice.take_mut(2..).unwrap();
///
/// assert_eq!(slice, &mut ['a', 'b']);
/// assert_eq!(tail, &mut ['c', 'd']);
/// ```
///
/// Getting `None` when `range` is out of bounds:
///
/// ```
/// #![feature(slice_take)]
///
/// let mut slice: &mut [_] = &mut ['a', 'b', 'c', 'd'];
///
/// assert_eq!(None, slice.take_mut(5..));
/// assert_eq!(None, slice.take_mut(..5));
/// assert_eq!(None, slice.take_mut(..=4));
/// let expected: &mut [_] = &mut ['a', 'b', 'c', 'd'];
/// assert_eq!(Some(expected), slice.take_mut(..4));
/// ```
#[inline]
#[must_use = "method does not modify the slice if the range is out of bounds"]
#[unstable(feature = "slice_take", issue = "62280")]
pub fn take_mut<'a, R: OneSidedRange<usize>>(
self: &mut &'a mut Self,
range: R,
) -> Option<&'a mut Self> {
let (direction, split_index) = split_point_of(range)?;
if split_index > self.len() {
return None;
}
let (front, back) = mem::take(self).split_at_mut(split_index);
match direction {
Direction::Front => {
*self = back;
Some(front)
}
Direction::Back => {
*self = front;
Some(back)
}
}
}

/// Removes the first element of the slice and returns a reference
/// to it.
///
/// Returns `None` if the slice is empty.
///
/// # Examples
///
/// ```
/// #![feature(slice_take)]
///
/// let mut slice: &[_] = &['a', 'b', 'c'];
/// let first = slice.take_first().unwrap();
///
/// assert_eq!(slice, &['b', 'c']);
/// assert_eq!(first, &'a');
/// ```
#[inline]
#[unstable(feature = "slice_take", issue = "62280")]
pub fn take_first<'a>(self: &mut &'a Self) -> Option<&'a T> {
let (first, rem) = self.split_first()?;
*self = rem;
Some(first)
}

/// Removes the first element of the slice and returns a mutable
/// reference to it.
///
/// Returns `None` if the slice is empty.
///
/// # Examples
///
/// ```
/// #![feature(slice_take)]
///
/// let mut slice: &mut [_] = &mut ['a', 'b', 'c'];
/// let first = slice.take_first_mut().unwrap();
/// *first = 'd';
///
/// assert_eq!(slice, &['b', 'c']);
/// assert_eq!(first, &'d');
/// ```
#[inline]
#[unstable(feature = "slice_take", issue = "62280")]
pub fn take_first_mut<'a>(self: &mut &'a mut Self) -> Option<&'a mut T> {
let (first, rem) = mem::take(self).split_first_mut()?;
*self = rem;
Some(first)
}

/// Removes the last element of the slice and returns a reference
/// to it.
///
/// Returns `None` if the slice is empty.
///
/// # Examples
///
/// ```
/// #![feature(slice_take)]
///
/// let mut slice: &[_] = &['a', 'b', 'c'];
/// let last = slice.take_last().unwrap();
///
/// assert_eq!(slice, &['a', 'b']);
/// assert_eq!(last, &'c');
/// ```
#[inline]
#[unstable(feature = "slice_take", issue = "62280")]
pub fn take_last<'a>(self: &mut &'a Self) -> Option<&'a T> {
let (last, rem) = self.split_last()?;
*self = rem;
Some(last)
}

/// Removes the last element of the slice and returns a mutable
/// reference to it.
///
/// Returns `None` if the slice is empty.
///
/// # Examples
///
/// ```
/// #![feature(slice_take)]
///
/// let mut slice: &mut [_] = &mut ['a', 'b', 'c'];
/// let last = slice.take_last_mut().unwrap();
/// *last = 'd';
///
/// assert_eq!(slice, &['a', 'b']);
/// assert_eq!(last, &'d');
/// ```
#[inline]
#[unstable(feature = "slice_take", issue = "62280")]
pub fn take_last_mut<'a>(self: &mut &'a mut Self) -> Option<&'a mut T> {
let (last, rem) = mem::take(self).split_last_mut()?;
*self = rem;
Some(last)
}
}

trait CloneFromSpec<T> {
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1 change: 1 addition & 0 deletions library/core/tests/lib.rs
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Expand Up @@ -34,6 +34,7 @@
#![feature(pattern)]
#![feature(sort_internals)]
#![feature(slice_partition_at_index)]
#![feature(slice_take)]
#![feature(maybe_uninit_uninit_array)]
#![feature(maybe_uninit_array_assume_init)]
#![feature(maybe_uninit_extra)]
Expand Down
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