Auto merge of #77853 - ijackson:slice-strip-stab, r=Amanieu

Stabilize slice::strip_prefix and slice::strip_suffix

These two methods are useful.  The corresponding methods on `str` are already stable.

I believe that stablising these now would not get in the way of, in the future, extending these to take a richer pattern API a la `str`'s patterns.

Tracking PR: #73413.  I also have an outstanding PR to improve the docs for these two functions and the corresponding ones on `str`: #75078

I have tried to follow the [instructions in the dev guide](https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#stabilization-pr).  The part to do with `compiler/rustc_feature` did not seem applicable.  I assume that's because these are just library features, so there is no corresponding machinery in rustc.

library/core/src/slice/mod.rs

@@ -1872,19 +1872,24 @@ impl<T> [T] {
     /// # Examples
     ///
     /// ```
-    /// #![feature(slice_strip)]
     /// let v = &[10, 40, 30];
     /// assert_eq!(v.strip_prefix(&[10]), Some(&[40, 30][..]));
     /// assert_eq!(v.strip_prefix(&[10, 40]), Some(&[30][..]));
     /// assert_eq!(v.strip_prefix(&[50]), None);
     /// assert_eq!(v.strip_prefix(&[10, 50]), None);
+    ///
+    /// let prefix : &str = "he";
+    /// assert_eq!(b"hello".strip_prefix(prefix.as_bytes()),
+    ///            Some(b"llo".as_ref()));
     /// ```
     #[must_use = "returns the subslice without modifying the original"]
-    #[unstable(feature = "slice_strip", issue = "73413")]
-    pub fn strip_prefix(&self, prefix: &[T]) -> Option<&[T]>
+    #[stable(feature = "slice_strip", since = "1.50.0")]
+    pub fn strip_prefix<P: SlicePattern<Item = T> + ?Sized>(&self, prefix: &P) -> Option<&[T]>
     where
         T: PartialEq,
     {
+        // This function will need rewriting if and when SlicePattern becomes more sophisticated.
+        let prefix = prefix.as_slice();
         let n = prefix.len();
         if n <= self.len() {
             let (head, tail) = self.split_at(n);
@@ -1905,7 +1910,6 @@ impl<T> [T] {
     /// # Examples
     ///
     /// ```
-    /// #![feature(slice_strip)]
     /// let v = &[10, 40, 30];
     /// assert_eq!(v.strip_suffix(&[30]), Some(&[10, 40][..]));
     /// assert_eq!(v.strip_suffix(&[40, 30]), Some(&[10][..]));
@@ -1913,11 +1917,13 @@ impl<T> [T] {
     /// assert_eq!(v.strip_suffix(&[50, 30]), None);
     /// ```
     #[must_use = "returns the subslice without modifying the original"]
-    #[unstable(feature = "slice_strip", issue = "73413")]
-    pub fn strip_suffix(&self, suffix: &[T]) -> Option<&[T]>
+    #[stable(feature = "slice_strip", since = "1.50.0")]
+    pub fn strip_suffix<P: SlicePattern<Item = T> + ?Sized>(&self, suffix: &P) -> Option<&[T]>
     where
         T: PartialEq,
     {
+        // This function will need rewriting if and when SlicePattern becomes more sophisticated.
+        let suffix = suffix.as_slice();
         let (len, n) = (self.len(), suffix.len());
         if n <= len {
             let (head, tail) = self.split_at(len - n);
@@ -3310,3 +3316,35 @@ impl<T> Default for &mut [T] {
         &mut []
     }
 }
+
+#[unstable(feature = "slice_pattern", reason = "stopgap trait for slice patterns", issue = "56345")]
+/// Patterns in slices - currently, only used by `strip_prefix` and `strip_suffix`.  At a future
+/// point, we hope to generalise `core::str::Pattern` (which at the time of writing is limited to
+/// `str`) to slices, and then this trait will be replaced or abolished.
+pub trait SlicePattern {
+    /// The element type of the slice being matched on.
+    type Item;
+
+    /// Currently, the consumers of `SlicePattern` need a slice.
+    fn as_slice(&self) -> &[Self::Item];
+}
+
+#[stable(feature = "slice_strip", since = "1.50.0")]
+impl<T> SlicePattern for [T] {
+    type Item = T;
+
+    #[inline]
+    fn as_slice(&self) -> &[Self::Item] {
+        self
+    }
+}
+
+#[stable(feature = "slice_strip", since = "1.50.0")]
+impl<T, const N: usize> SlicePattern for [T; N] {
+    type Item = T;
+
+    #[inline]
+    fn as_slice(&self) -> &[Self::Item] {
+        self
+    }
+}

library/std/src/lib.rs

@@ -307,7 +307,6 @@
 #![feature(slice_internals)]
 #![feature(slice_ptr_get)]
 #![feature(slice_ptr_len)]
-#![feature(slice_strip)]
 #![feature(staged_api)]
 #![feature(std_internals)]
 #![feature(stdsimd)]