Make split_inclusive() on an empty slice yield an empty output

`[].split_inclusive()` currently yields a single, empty slice. That's
different from `"".split_inslusive()`, which yields no output at
all. I think that makes the slice version harder to use.

The case where I ran into this bug was when writing code for
generating a diff between two slices of bytes. I wanted to prefix
removed lines with "-" and a added lines with "+". Due to
`split_inclusive()`'s current behavior, that means that my code prints
just a "-" or "+" for empty files. I suspect most existing callers
have similar "bugs" (which would be fixed by this patch).

Closes #89716.

library/alloc/tests/slice.rs

@@ -863,7 +863,7 @@ fn test_splitator_inclusive() {
     assert_eq!(xs.split_inclusive(|_| true).collect::<Vec<&[i32]>>(), splits);
 
     let xs: &[i32] = &[];
-    let splits: &[&[i32]] = &[&[]];
+    let splits: &[&[i32]] = &[];
     assert_eq!(xs.split_inclusive(|x| *x == 5).collect::<Vec<&[i32]>>(), splits);
 }
 
@@ -883,7 +883,7 @@ fn test_splitator_inclusive_reverse() {
     assert_eq!(xs.split_inclusive(|_| true).rev().collect::<Vec<_>>(), splits);
 
     let xs: &[i32] = &[];
-    let splits: &[&[i32]] = &[&[]];
+    let splits: &[&[i32]] = &[];
     assert_eq!(xs.split_inclusive(|x| *x == 5).rev().collect::<Vec<_>>(), splits);
 }
 
@@ -903,7 +903,7 @@ fn test_splitator_mut_inclusive() {
     assert_eq!(xs.split_inclusive_mut(|_| true).collect::<Vec<_>>(), splits);
 
     let xs: &mut [i32] = &mut [];
-    let splits: &[&[i32]] = &[&[]];
+    let splits: &[&[i32]] = &[];
     assert_eq!(xs.split_inclusive_mut(|x| *x == 5).collect::<Vec<_>>(), splits);
 }
 
@@ -923,7 +923,7 @@ fn test_splitator_mut_inclusive_reverse() {
     assert_eq!(xs.split_inclusive_mut(|_| true).rev().collect::<Vec<_>>(), splits);
 
     let xs: &mut [i32] = &mut [];
-    let splits: &[&[i32]] = &[&[]];
+    let splits: &[&[i32]] = &[];
     assert_eq!(xs.split_inclusive_mut(|x| *x == 5).rev().collect::<Vec<_>>(), splits);
 }
 

library/core/src/slice/iter.rs

@@ -479,7 +479,8 @@ where
 impl<'a, T: 'a, P: FnMut(&T) -> bool> SplitInclusive<'a, T, P> {
     #[inline]
     pub(super) fn new(slice: &'a [T], pred: P) -> Self {
-        Self { v: slice, pred, finished: false }
+        let finished = slice.is_empty();
+        Self { v: slice, pred, finished }
     }
 }
 
@@ -727,7 +728,8 @@ where
 impl<'a, T: 'a, P: FnMut(&T) -> bool> SplitInclusiveMut<'a, T, P> {
     #[inline]
     pub(super) fn new(slice: &'a mut [T], pred: P) -> Self {
-        Self { v: slice, pred, finished: false }
+        let finished = slice.is_empty();
+        Self { v: slice, pred, finished }
     }
 }