Don't do pointer arithmetic on pointers to deallocated memory

vec::Splice can invalidate the slice::Iter inside vec::Drain.
So we replace them with dangling pointers which, unlike ones to
deallocated memory, are allowed.

library/alloc/src/vec/drain.rs

@@ -223,9 +223,9 @@ impl<T, A: Allocator> Drop for Drain<'_, T, A> {
         }
 
         // as_slice() must only be called when iter.len() is > 0 because
-        // vec::Splice modifies vec::Drain fields and may grow the vec which would invalidate
-        // the iterator's internal pointers. Creating a reference to deallocated memory
-        // is invalid even when it is zero-length
+        // it also gets touched by vec::Splice which may turn it into a dangling pointer
+        // which would make it and the vec pointer point to different allocations which would
+        // lead to invalid pointer arithmetic below.
         let drop_ptr = iter.as_slice().as_ptr();
 
         unsafe {

library/alloc/src/vec/splice.rs

@@ -54,6 +54,12 @@ impl<I: Iterator, A: Allocator> ExactSizeIterator for Splice<'_, I, A> {}
 impl<I: Iterator, A: Allocator> Drop for Splice<'_, I, A> {
     fn drop(&mut self) {
         self.drain.by_ref().for_each(drop);
+        // At this point draining is done and the only remaining tasks are splicing
+        // and moving things into the final place.
+        // Which means we can replace the slice::Iter with pointers that won't point to deallocated
+        // memory, so that Drain::drop is still allowed to call iter.len(), otherwise it would break
+        // the ptr.sub_ptr contract.
+        self.drain.iter = (&[]).iter();
 
         unsafe {
             if self.drain.tail_len == 0 {