Add comment on why `RefCell::unwrap` is safe
Implement BorrowFrom<&'a T> for T (with mutable variants)
auto merge of #19137 : tbu-/rust/pr_refcell_unsafety, r=huonw
auto merge of #19146 : gereeter/rust/reference-borrow, r=aturon

This should be a more general version of #19131.
Implement into_iter() for BinaryHeap.