Function downcast_value 

fn downcast_value<I: Any, O: Sized + 'static>(input: I) -> Result<O, I>

Try to cast I (which is presumably a TAIT) to O (presumably a concrete type)

We use runtime casting, but typically the answer is known at compile time.

Astonishingly, this isn’t in any of the following:

• std
• match-downcast
• better_any (downcast:move comes close but doesn’t give you your self back)
• castaway
• mopa
• as_any