The following code causes UB (found by a patched miri enforcing validity invariants):

    let mut b = std::collections::BTreeSet::new();
    b.insert(42);

What happens is that BTreeSet::new sets the root node to a pointer to a static EMPTY_ROOT_NODE shared for this purpose by all B-trees that has type LeafNode<(), ()>. That step is okay.

However, in the next step, insert calls ensure_root_is_owned which indirectly turns this ptr into &LeafNode<i32, ()>. The trouble is that LeafNode<i32, ()> is bigger than LeafNode<(), ()>, so this reference is partially dangling: Its beginning points to allocated memory (the static EMPTY_ROOT_NODE), but that allocated memory ends "too early". The invariant that &LeafNode<i32, ()> be dereferencable for mem::size_of::<LeafNode<i32, ()>>() bytes is violated. Since we are passing dereferencable(n) to LLVM with n set to the full size of the struct, this means we have UB not just according to our own validity rules, but according to LLVM's rules.