Right, a Rust lambda is not a COM object on its own, but by the same token you can't just stash one in a COM object's property either- COM object properties don

Right, a Rust lambda is not a COM object on its own, but by the same token you can't just stash one in a COM object's property either- COM object properties don't have a type for "Rust lambda," or "C++ lambda" for that matter. Instead you need to convert the lambda (from whatever language) into the type of that property- in this case some kind of Delegate, at which point it is ref-counted like any other COM object. So in this case, installing the handler increments the refcount, reading the handler back out to invoke it increments it again, the handler itself decrements it, and finally whoever invoked it drops their reference and the Delegate gets freed.