Enum to and from concrete value

main.rs

macro_rules! convertable_enum {
    ($enum_name: ident, $enum_type: ty, {$($name: ident = $value: expr,)+}) => {
        enum $enum_name {
            $($name = $value,)+
        }
        impl std::convert::TryFrom<$enum_type> for $enum_name {
            type Error = String;
            fn try_from(value: $enum_type) -> Result<Self, Self::Error> {
                match value {
                    $($value => Ok($enum_name::$name), )+
                    _ => Err(format!("Unrecognized value {}", value)),
                }
            }
        }
    }
}

fn main() {
    use std::convert::TryFrom;
    convertable_enum!(Nums, i32, {One = 1, Two = 2,});
    assert_eq!(Nums::One as i32, 1);
    assert!(match Nums::try_from(2) {
        Ok(Nums::Two) => true,
        _ => false,
    })
}