Function expect 

pub fn expect<T: Unwrap>(msg: &str) -> impl Fn(T) -> T::Out + '_

Unwrap an Option or Result with a custom message.

This might cause a panic, use carefully.

#[derive(Valust)]
struct MustBeSome(
    #[trans(func(Option<i32> => expect("must be some")))]
    i32,
);