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Auto merge of #60072 - RalfJung:linked-list, r=shepmaster
fix LinkedList invalidating mutable references The test `test_insert_prev` failed in Miri due to what I consider a bug in `LinkedList`: in various places, `NonNull::as_mut` got called to modify the `prev`/`next` pointers of existing nodes. In particular, the unstable `insert_next` has to modify the `next` pointer of the node that was last handed out by the iterator; to this end it creates a mutable reference to the *entire node* that overlaps with the mutable reference to the node's content that was handed out by the iterator! Thus, the next use if said mutable reference is UB. In code: ```rust loop { match it.next() { // mutable reference handed to us None => break, Some(elt) => { it.insert_next(*elt + 1); // this invalidates `elt` because it creates an overlapping mutable reference match it.peek_next() { Some(x) => assert_eq!(*x, *elt + 2), // this use of `elt` now is a use of an invalid pointer None => assert_eq!(8, *elt), } } } } ``` This PR fixes that by using `as_ptr` instead of `as_mut`. This avoids invalidating the mutable reference that was handed to the user. I did this in all methods called by iterators, just to be sure. Cc @gankro
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