Rollup merge of rust-lang#136555 - cramertj:split_off, r=dtolnay

Rename `slice::take...` methods to `split_off...`

This rename was discussed and recommended in a recent t-libs meeting.

cc rust-lang#62280

There's an additional commit here which modifies internals of unstable `OneSidedRange` APIs in order to implement `split_off` methods in a panic-free way (remove `unreachable!()`) as recommended in https://github.com/rust-lang/rust/pull/88502/files#r760177240. I can split this out into a separate PR if needed.

library/core/src/ops/mod.rs

@@ -182,10 +182,10 @@ pub use self::function::{Fn, FnMut, FnOnce};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::index::{Index, IndexMut};
 pub(crate) use self::index_range::IndexRange;
-#[unstable(feature = "one_sided_range", issue = "69780")]
-pub use self::range::OneSidedRange;
 #[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, OneSidedRangeBound};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::range::{Range, RangeFrom, RangeFull, RangeTo};
 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]

library/core/src/ops/range.rs

@@ -979,20 +979,58 @@ impl<T> RangeBounds<T> for RangeToInclusive<&T> {
     }
 }
 
+/// An internal helper for `split_off` functions indicating
+/// which end a `OneSidedRange` is bounded on.
+#[unstable(feature = "one_sided_range", issue = "69780")]
+#[allow(missing_debug_implementations)]
+pub enum OneSidedRangeBound {
+    /// The range is bounded inclusively from below and is unbounded above.
+    StartInclusive,
+    /// The range is bounded exclusively from above and is unbounded below.
+    End,
+    /// The range is bounded inclusively from above and is unbounded below.
+    EndInclusive,
+}
+
 /// `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> {}
+pub trait OneSidedRange<T: ?Sized>: RangeBounds<T> {
+    /// An internal-only helper function for `split_off` and
+    /// `split_off_mut` that returns the bound of the one-sided range.
+    fn bound(self) -> (OneSidedRangeBound, T);
+}
 
 #[unstable(feature = "one_sided_range", issue = "69780")]
-impl<T> OneSidedRange<T> for RangeTo<T> where Self: RangeBounds<T> {}
+impl<T> OneSidedRange<T> for RangeTo<T>
+where
+    Self: RangeBounds<T>,
+{
+    fn bound(self) -> (OneSidedRangeBound, T) {
+        (OneSidedRangeBound::End, self.end)
+    }
+}
 
 #[unstable(feature = "one_sided_range", issue = "69780")]
-impl<T> OneSidedRange<T> for RangeFrom<T> where Self: RangeBounds<T> {}
+impl<T> OneSidedRange<T> for RangeFrom<T>
+where
+    Self: RangeBounds<T>,
+{
+    fn bound(self) -> (OneSidedRangeBound, T) {
+        (OneSidedRangeBound::StartInclusive, self.start)
+    }
+}
 
 #[unstable(feature = "one_sided_range", issue = "69780")]
-impl<T> OneSidedRange<T> for RangeToInclusive<T> where Self: RangeBounds<T> {}
+impl<T> OneSidedRange<T> for RangeToInclusive<T>
+where
+    Self: RangeBounds<T>,
+{
+    fn bound(self) -> (OneSidedRangeBound, T) {
+        (OneSidedRangeBound::EndInclusive, self.end)
+    }
+}

library/core/src/slice/mod.rs

@@ -10,7 +10,7 @@ use crate::cmp::Ordering::{self, Equal, Greater, Less};
 use crate::intrinsics::{exact_div, unchecked_sub};
 use crate::mem::{self, SizedTypeProperties};
 use crate::num::NonZero;
-use crate::ops::{Bound, OneSidedRange, Range, RangeBounds, RangeInclusive};
+use crate::ops::{OneSidedRange, OneSidedRangeBound, Range, RangeBounds, RangeInclusive};
 use crate::panic::const_panic;
 use crate::simd::{self, Simd};
 use crate::ub_checks::assert_unsafe_precondition;
@@ -83,14 +83,12 @@ pub use raw::{from_raw_parts, from_raw_parts_mut};
 /// 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!(),
+    use OneSidedRangeBound::{End, EndInclusive, StartInclusive};
+
+    Some(match range.bound() {
+        (StartInclusive, i) => (Direction::Back, i),
+        (End, i) => (Direction::Front, i),
+        (EndInclusive, i) => (Direction::Front, i.checked_add(1)?),
     })
 }
 
@@ -4294,25 +4292,25 @@ impl<T> [T] {
     ///
     /// # Examples
     ///
-    /// Taking the first three elements of a slice:
+    /// Splitting off 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();
+    /// let mut first_three = slice.split_off(..3).unwrap();
     ///
     /// assert_eq!(slice, &['d']);
     /// assert_eq!(first_three, &['a', 'b', 'c']);
     /// ```
     ///
-    /// Taking the last two elements of a slice:
+    /// Splitting off the last two elements of a slice:
     ///
     /// ```
     /// #![feature(slice_take)]
     ///
     /// let mut slice: &[_] = &['a', 'b', 'c', 'd'];
-    /// let mut tail = slice.take(2..).unwrap();
+    /// let mut tail = slice.split_off(2..).unwrap();
     ///
     /// assert_eq!(slice, &['a', 'b']);
     /// assert_eq!(tail, &['c', 'd']);
@@ -4325,16 +4323,19 @@ impl<T> [T] {
     ///
     /// 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));
+    /// assert_eq!(None, slice.split_off(5..));
+    /// assert_eq!(None, slice.split_off(..5));
+    /// assert_eq!(None, slice.split_off(..=4));
     /// let expected: &[char] = &['a', 'b', 'c', 'd'];
-    /// assert_eq!(Some(expected), slice.take(..4));
+    /// assert_eq!(Some(expected), slice.split_off(..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> {
+    pub fn split_off<'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;
@@ -4363,13 +4364,13 @@ impl<T> [T] {
     ///
     /// # Examples
     ///
-    /// Taking the first three elements of a slice:
+    /// Splitting off 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();
+    /// let mut first_three = slice.split_off_mut(..3).unwrap();
     ///
     /// assert_eq!(slice, &mut ['d']);
     /// assert_eq!(first_three, &mut ['a', 'b', 'c']);
@@ -4381,7 +4382,7 @@ impl<T> [T] {
     /// #![feature(slice_take)]
     ///
     /// let mut slice: &mut [_] = &mut ['a', 'b', 'c', 'd'];
-    /// let mut tail = slice.take_mut(2..).unwrap();
+    /// let mut tail = slice.split_off_mut(2..).unwrap();
     ///
     /// assert_eq!(slice, &mut ['a', 'b']);
     /// assert_eq!(tail, &mut ['c', 'd']);
@@ -4394,16 +4395,16 @@ impl<T> [T] {
     ///
     /// 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));
+    /// assert_eq!(None, slice.split_off_mut(5..));
+    /// assert_eq!(None, slice.split_off_mut(..5));
+    /// assert_eq!(None, slice.split_off_mut(..=4));
     /// let expected: &mut [_] = &mut ['a', 'b', 'c', 'd'];
-    /// assert_eq!(Some(expected), slice.take_mut(..4));
+    /// assert_eq!(Some(expected), slice.split_off_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>>(
+    pub fn split_off_mut<'a, R: OneSidedRange<usize>>(
         self: &mut &'a mut Self,
         range: R,
     ) -> Option<&'a mut Self> {
@@ -4435,14 +4436,14 @@ impl<T> [T] {
     /// #![feature(slice_take)]
     ///
     /// let mut slice: &[_] = &['a', 'b', 'c'];
-    /// let first = slice.take_first().unwrap();
+    /// let first = slice.split_off_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> {
+    pub fn split_off_first<'a>(self: &mut &'a Self) -> Option<&'a T> {
         let (first, rem) = self.split_first()?;
         *self = rem;
         Some(first)
@@ -4459,15 +4460,15 @@ impl<T> [T] {
     /// #![feature(slice_take)]
     ///
     /// let mut slice: &mut [_] = &mut ['a', 'b', 'c'];
-    /// let first = slice.take_first_mut().unwrap();
+    /// let first = slice.split_off_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> {
+    pub fn split_off_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)
@@ -4484,14 +4485,14 @@ impl<T> [T] {
     /// #![feature(slice_take)]
     ///
     /// let mut slice: &[_] = &['a', 'b', 'c'];
-    /// let last = slice.take_last().unwrap();
+    /// let last = slice.split_off_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> {
+    pub fn split_off_last<'a>(self: &mut &'a Self) -> Option<&'a T> {
         let (last, rem) = self.split_last()?;
         *self = rem;
         Some(last)
@@ -4508,15 +4509,15 @@ impl<T> [T] {
     /// #![feature(slice_take)]
     ///
     /// let mut slice: &mut [_] = &mut ['a', 'b', 'c'];
-    /// let last = slice.take_last_mut().unwrap();
+    /// let last = slice.split_off_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> {
+    pub fn split_off_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)