Macro enum_derive_2018::EnumFromInner

macro_rules! EnumFromInner {
    (() $vis:vis enum $name:ident { $($body:tt)* }) => { ... };
    (
        @expand $name:ident ($($var_names:ident($var_tys:ty),)*)
    ) => { ... };
}

Derives From<T> for each variant's payload, assuming all variants are unary.