This looks like it's calling .type_id() on a Box<dyn RandomSource> (not on the underlying trait object), and as a result always returns the fixed TypeId of the box. Therefore this function can never return Err(false). The documentation for it however says:

If the source has already been specified Err is returned with a bool indicating if the set failed because method was previously invoked (true) or if the default source is already being used (false).

I'm currently working on improving clippy's type_id_on_box lint and it found this instance