Implement split_at_spare_mut without Deref to a slice so that the spare slice is valid

library/alloc/src/vec/mod.rs

@@ -2141,12 +2141,17 @@ impl<T, A: Allocator> Vec<T, A> {
     unsafe fn split_at_spare_mut_with_len(
         &mut self,
     ) -> (&mut [T], &mut [MaybeUninit<T>], &mut usize) {
-        let Range { start: ptr, end: spare_ptr } = self.as_mut_ptr_range();
+        let ptr = self.as_mut_ptr();
+        // SAFETY:
+        // - `ptr` is guaranteed to be valid for `self.len` elements
+        // - but the allocation extends out to `self.buf.capacity()` elements, possibly
+        // uninitialized
+        let spare_ptr = unsafe { ptr.add(self.len) };
         let spare_ptr = spare_ptr.cast::<MaybeUninit<T>>();
         let spare_len = self.buf.capacity() - self.len;
 
         // SAFETY:
-        // - `ptr` is guaranteed to be valid for `len` elements
+        // - `ptr` is guaranteed to be valid for `self.len` elements
         // - `spare_ptr` is pointing one element past the buffer, so it doesn't overlap with `initialized`
         unsafe {
             let initialized = slice::from_raw_parts_mut(ptr, self.len);