Take 2: Add take_... functions to slices

library/core/src/ops/mod.rs

@@ -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", issue = "42327")]
 pub use self::r#try::Try;
 

library/core/src/ops/range.rs

@@ -1004,3 +1004,21 @@ impl<T> RangeBounds<T> for RangeToInclusive<&T> {
         Included(self.end)
     }
 }
+
+/// `OneSidedRange` is implemented by Rust's built-in range types which
+/// are unbounded on one side. For example, `a..`, `..b` and `..=c` implement
+/// `OneSidedRange`, but `..`, `d..e`, and `f..=g` do not.
+///
+/// Types which implement `OneSidedRange<T>` must return `Bound::Unbounded`
+/// from exactly one of `RangeBounds::start_bound` and `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> {}

library/core/src/slice/mod.rs

@@ -12,7 +12,7 @@ use crate::cmp::Ordering::{self, Equal, 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;
@@ -73,6 +73,24 @@ pub use sort::heapsort;
 #[stable(feature = "slice_get_slice", since = "1.28.0")]
 pub use index::SliceIndex;
 
+/// Calculates the direction and split point of a one-sided range.
+///
+/// Helper for `take` and `take_mut` which returns a boolean
+/// indicating whether the front of the split is being taken
+/// (as opposed to the back), as well as a number indicating the
+/// index at which to split. Returns `None` if the split index would
+/// overflow `usize`.
+#[inline]
+fn take_split_point(range: impl OneSidedRange<usize>) -> Option<(bool, usize)> {
+    Some(match (range.start_bound(), range.end_bound()) {
+        (Bound::Unbounded, Bound::Excluded(i)) => (true, *i),
+        (Bound::Unbounded, Bound::Included(i)) => (true, i.checked_add(1)?),
+        (Bound::Excluded(i), Bound::Unbounded) => (false, i.checked_add(1)?),
+        (Bound::Included(i), Bound::Unbounded) => (false, *i),
+        _ => unreachable!(),
+    })
+}
+
 #[lang = "slice"]
 #[cfg(not(test))]
 impl<T> [T] {
@@ -3169,6 +3187,232 @@ impl<T> [T] {
 
         left
     }
+
+    /// Returns the subslice corresponding to the given range,
+    /// and modifies the slice to no longer include this subslice.
+    ///
+    /// Returns `None` and does not modify the slice if the given
+    /// range is out of bounds.
+    ///
+    /// # Examples
+    ///
+    /// Taking the first three items from a slice (via `..3`):
+    ///
+    /// ```
+    /// #![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 tail of a slice starting at index two (via `2..`):
+    ///
+    /// ```
+    /// #![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` starts or ends outside of the slice:
+    ///
+    /// ```
+    /// #![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]
+    #[unstable(feature = "slice_take", issue = "62280")]
+    pub fn take<'a, R: OneSidedRange<usize>>(self: &mut &'a Self, range: R) -> Option<&'a Self> {
+        let (taking_front, split_index) = take_split_point(range)?;
+        if split_index > self.len() {
+            return None;
+        }
+        let (front, back) = self.split_at(split_index);
+        if taking_front {
+            *self = back;
+            Some(front)
+        } else {
+            *self = front;
+            Some(back)
+        }
+    }
+
+    /// Returns the mutable subslice corresponding to the given range,
+    /// and modifies the slice to no longer include this subslice.
+    ///
+    /// Returns `None` and does not modify the slice if the given
+    /// range is out of bounds.
+    ///
+    /// # Examples
+    ///
+    /// Taking the first three items from a slice (via `..3`):
+    ///
+    /// ```
+    /// #![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 tail of a slice starting at index two (via `2..`):
+    ///
+    /// ```
+    /// #![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` starts or ends outside of the slice:
+    ///
+    /// ```
+    /// #![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]
+    #[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 (taking_front, split_index) = take_split_point(range)?;
+        if split_index > self.len() {
+            return None;
+        }
+        let original = crate::mem::take(self);
+        let (front, back) = original.split_at_mut(split_index);
+        if taking_front {
+            *self = back;
+            Some(front)
+        } else {
+            *self = front;
+            Some(back)
+        }
+    }
+
+    /// Returns a reference to the first element of the slice,
+    /// and modifies the slice to no longer include this element.
+    ///
+    /// 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)
+    }
+
+    /// Returns a mutable reference to the first element of the slice,
+    /// and modifies the slice to no longer include this element.
+    ///
+    /// 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)
+    }
+
+    /// Returns a reference to the last element of the slice,
+    /// and modifies the slice to no longer include this element.
+    ///
+    /// 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)
+    }
+
+    /// Returns a mutable reference to the last element of the slice,
+    /// and modifies the slice to no longer include this element.
+    ///
+    /// 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)
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

library/core/tests/lib.rs

@@ -39,6 +39,7 @@
 #![feature(try_trait)]
 #![feature(slice_internals)]
 #![feature(slice_partition_dedup)]
+#![feature(slice_take)]
 #![feature(int_error_matching)]
 #![feature(array_value_iter)]
 #![feature(iter_advance_by)]

library/core/tests/slice.rs

@@ -2038,3 +2038,83 @@ fn test_slice_run_destructors() {
 
     assert_eq!(x.get(), 1);
 }
+
+macro_rules! take_tests {
+    (slice: &$slice:expr, $($tts:tt)*) => {
+        take_tests!(ty: &[_], slice: &$slice, $($tts)*);
+    };
+    (slice: &mut $slice:expr, $($tts:tt)*) => {
+        take_tests!(ty: &mut [_], slice: &mut $slice, $($tts)*);
+    };
+    (ty: $ty:ty, slice: $slice:expr, method: $method:ident, $(($test_name:ident, ($($args:expr),*), $output:expr, $remaining:expr),)*) => {
+        $(
+            #[test]
+            fn $test_name() {
+                let mut slice: $ty = $slice;
+                assert_eq!($output, slice.$method($($args)*));
+                let remaining: $ty = $remaining;
+                assert_eq!(remaining, slice);
+            }
+        )*
+    };
+}
+
+take_tests! {
+    slice: &[0, 1, 2, 3], method: take,
+    (take_in_bounds_range_to, (..1), Some(&[0] as _), &[1, 2, 3]),
+    (take_in_bounds_range_to_inclusive, (..=0), Some(&[0] as _), &[1, 2, 3]),
+    (take_in_bounds_range_from, (2..), Some(&[2, 3] as _), &[0, 1]),
+    (take_oob_range_to, (..5), None, &[0, 1, 2, 3]),
+    (take_oob_range_to_inclusive, (..=4), None, &[0, 1, 2, 3]),
+    (take_oob_range_from, (5..), None, &[0, 1, 2, 3]),
+}
+
+take_tests! {
+    slice: &mut [0, 1, 2, 3], method: take_mut,
+    (take_mut_in_bounds_range_to, (..1), Some(&mut [0] as _), &mut [1, 2, 3]),
+    (take_mut_in_bounds_range_to_inclusive, (..=0), Some(&mut [0] as _), &mut [1, 2, 3]),
+    (take_mut_in_bounds_range_from, (2..), Some(&mut [2, 3] as _), &mut [0, 1]),
+    (take_mut_oob_range_to, (..5), None, &mut [0, 1, 2, 3]),
+    (take_mut_oob_range_to_inclusive, (..=4), None, &mut [0, 1, 2, 3]),
+    (take_mut_oob_range_from, (5..), None, &mut [0, 1, 2, 3]),
+}
+
+take_tests! {
+    ty: &[_], slice: &[1, 2], method: take_first,
+    (take_first_nonempty, (), Some(&1), &[2]),
+}
+
+take_tests! {
+    ty: &mut [_], slice: &mut [1, 2], method: take_first_mut,
+    (take_first_mut_nonempty, (), Some(&mut 1), &mut [2]),
+}
+
+take_tests! {
+    ty: &[_], slice: &[1, 2], method: take_last,
+    (take_last_nonempty, (), Some(&2), &[1]),
+}
+
+take_tests! {
+    ty: &mut [_], slice: &mut [1, 2], method: take_last_mut,
+    (take_last_mut_nonempty, (), Some(&mut 2), &mut [1]),
+}
+
+take_tests! {
+    ty: &[()], slice: &[], method: take_first,
+    (take_first_empty, (), None, &[]),
+}
+
+take_tests! {
+    ty: &mut [()], slice: &mut [], method: take_first_mut,
+    (take_first_mut_empty, (), None, &mut []),
+}
+
+take_tests! {
+    ty: &[()], slice: &[], method: take_last,
+    (take_last_empty, (), None, &[]),
+}
+
+take_tests! {
+    ty: &mut [()], slice: &mut [], method: take_last_mut,
+    (take_last_mut_empty, (), None, &mut []),
+}