Macro enum_with_unknown

macro_rules! enum_with_unknown {
    (
        $( #[$enum_attr:meta] )*
        pub enum $name:ident($ty:ty) {
            $(
              $( #[$variant_attr:meta] )*
              $variant:ident = $value:expr
            ),+ $(,)?
        }
    ) => { ... };
}

Derive To/From with an added “Unknown” variant catch-all for converting from numerical values that do not match a valid variant in order to avoid either panicking or cumbersome error handling.

Yoinked shamelessly (with some modification) from smoltcp.