Rollup merge of #124699 - scottmcm:split_at_unchecked_should_use_unchecked, r=Nilstrieb

Use `unchecked_sub` in `split_at`

LLVM currently isn't figuring it out on its own, even in the checked version where it hypothetically could.

Before: <https://rust.godbolt.org/z/PEY38YrKs>
```llvm
bb1:                                              ; preds = %start
  %4 = getelementptr inbounds float, ptr %x.0, i64 %n
  %5 = sub i64 %x.1, %n
```

After:
```llvm
bb1:                                              ; preds = %start
  %4 = getelementptr inbounds float, ptr %x.0, i64 %n
  %5 = sub nuw i64 %x.1, %n
```

This is not using the wrapper because there's already a ubcheck covering it, so I don't want this to get a second one once #121571 lands.

---

This is basically the same as #108763, since `split_at` is essentially doing two `get_unchecked`s.

Author: matthiaskrgr

library/core/src/slice/mod.rs

@@ -9,7 +9,7 @@
 use crate::cmp::Ordering::{self, Equal, Greater, Less};
 use crate::fmt;
 use crate::hint;
-use crate::intrinsics::exact_div;
+use crate::intrinsics::{exact_div, unchecked_sub};
 use crate::mem::{self, SizedTypeProperties};
 use crate::num::NonZero;
 use crate::ops::{Bound, OneSidedRange, Range, RangeBounds};
@@ -1983,7 +1983,7 @@ impl<T> [T] {
         );
 
         // SAFETY: Caller has to check that `0 <= mid <= self.len()`
-        unsafe { (from_raw_parts(ptr, mid), from_raw_parts(ptr.add(mid), len - mid)) }
+        unsafe { (from_raw_parts(ptr, mid), from_raw_parts(ptr.add(mid), unchecked_sub(len, mid))) }
     }
 
     /// Divides one mutable slice into two at an index, without doing bounds checking.
@@ -2035,7 +2035,12 @@ impl<T> [T] {
         //
         // `[ptr; mid]` and `[mid; len]` are not overlapping, so returning a mutable reference
         // is fine.
-        unsafe { (from_raw_parts_mut(ptr, mid), from_raw_parts_mut(ptr.add(mid), len - mid)) }
+        unsafe {
+            (
+                from_raw_parts_mut(ptr, mid),
+                from_raw_parts_mut(ptr.add(mid), unchecked_sub(len, mid)),
+            )
+        }
     }
 
     /// Divides one slice into two at an index, returning `None` if the slice is