Rollup merge of rust-lang#85610 - SkiFire13:fix-copy-within-provenanc…

…e, r=oli-obk

Fix pointer provenance in <[T]>::copy_within

Previously the `self.as_mut_ptr()` invalidated the pointer created by the first `self.as_ptr()`. This also triggered miri when run with `-Zmiri-track-raw-pointers`

library/core/src/slice/mod.rs

@@ -3096,7 +3096,11 @@ impl<T> [T] {
  // SAFETY: the conditions for `ptr::copy` have all been checked above,
  // as have those for `ptr::add`.
  unsafe {
- ptr::copy(self.as_ptr().add(src_start), self.as_mut_ptr().add(dest), count);
+ // Derive both `src_ptr` and `dest_ptr` from the same loan
+ let ptr = self.as_mut_ptr();
+ let src_ptr = ptr.add(src_start);
+ let dest_ptr = ptr.add(dest);
+ ptr::copy(src_ptr, dest_ptr, count);
  }
  }