Infallible is a clunky name for the Never type in Rust, i.e. a type that has no values and cannot be instantiated. Thus, Option<Infallible> only has a single v

Infallible is a clunky name for the Never type in Rust, i.e. a type that has no values and cannot be instantiated. Thus, Option<Infallible> only has a single value, viz. None. Then Vec<Option<Infallible>> is isomorphic to Vec<()> which reduces to a length field - there's no other data associated with it.