Rollup merge of #83571 - a1phyr:feature_const_slice_first_last, r=dtolnay

Constantify some slice methods

Tracking issue: #83570

This PR constantifies the following functions under feature `const_slice_first_last`:
- `slice::first`
- `slice::split_first`
- `slice::last`
- `slice::split_last`

Blocking on `#![feature(const_mut_refs)]`:
- `slice::first_mut`
- `slice::split_first_mut`
- `slice::last_mut`
- `slice::split_last_mut`

library/core/src/slice/mod.rs

@@ -148,8 +148,9 @@ impl<T> [T] {
     /// assert_eq!(None, w.first());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
     #[inline]
-    pub fn first(&self) -> Option<&T> {
+    pub const fn first(&self) -> Option<&T> {
         if let [first, ..] = self { Some(first) } else { None }
     }
 
@@ -166,8 +167,9 @@ impl<T> [T] {
     /// assert_eq!(x, &[5, 1, 2]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
     #[inline]
-    pub fn first_mut(&mut self) -> Option<&mut T> {
+    pub const fn first_mut(&mut self) -> Option<&mut T> {
         if let [first, ..] = self { Some(first) } else { None }
     }
 
@@ -184,8 +186,9 @@ impl<T> [T] {
     /// }
     /// ```
     #[stable(feature = "slice_splits", since = "1.5.0")]
+    #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
     #[inline]
-    pub fn split_first(&self) -> Option<(&T, &[T])> {
+    pub const fn split_first(&self) -> Option<(&T, &[T])> {
         if let [first, tail @ ..] = self { Some((first, tail)) } else { None }
     }
 
@@ -204,8 +207,9 @@ impl<T> [T] {
     /// assert_eq!(x, &[3, 4, 5]);
     /// ```
     #[stable(feature = "slice_splits", since = "1.5.0")]
+    #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
     #[inline]
-    pub fn split_first_mut(&mut self) -> Option<(&mut T, &mut [T])> {
+    pub const fn split_first_mut(&mut self) -> Option<(&mut T, &mut [T])> {
         if let [first, tail @ ..] = self { Some((first, tail)) } else { None }
     }
 
@@ -222,8 +226,9 @@ impl<T> [T] {
     /// }
     /// ```
     #[stable(feature = "slice_splits", since = "1.5.0")]
+    #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
     #[inline]
-    pub fn split_last(&self) -> Option<(&T, &[T])> {
+    pub const fn split_last(&self) -> Option<(&T, &[T])> {
         if let [init @ .., last] = self { Some((last, init)) } else { None }
     }
 
@@ -242,8 +247,9 @@ impl<T> [T] {
     /// assert_eq!(x, &[4, 5, 3]);
     /// ```
     #[stable(feature = "slice_splits", since = "1.5.0")]
+    #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
     #[inline]
-    pub fn split_last_mut(&mut self) -> Option<(&mut T, &mut [T])> {
+    pub const fn split_last_mut(&mut self) -> Option<(&mut T, &mut [T])> {
         if let [init @ .., last] = self { Some((last, init)) } else { None }
     }
 
@@ -259,8 +265,9 @@ impl<T> [T] {
     /// assert_eq!(None, w.last());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
     #[inline]
-    pub fn last(&self) -> Option<&T> {
+    pub const fn last(&self) -> Option<&T> {
         if let [.., last] = self { Some(last) } else { None }
     }
 
@@ -277,8 +284,9 @@ impl<T> [T] {
     /// assert_eq!(x, &[0, 1, 10]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
     #[inline]
-    pub fn last_mut(&mut self) -> Option<&mut T> {
+    pub const fn last_mut(&mut self) -> Option<&mut T> {
         if let [.., last] = self { Some(last) } else { None }
     }