Macro enum_from_primitive

macro_rules! enum_from_primitive {
    (
        $( #[$enum_attr:meta] )*
        enum $name:ident {
            $( $( #[$variant_attr:meta] )* $variant:ident ),+ $( = $discriminator:expr, $( $( #[$variant_two_attr:meta] )* $variant_two:ident ),+ )*
        }
    ) => { ... };
    (
        $( #[$enum_attr:meta] )*
        enum $name:ident {
            $( $( $( #[$variant_attr:meta] )* $variant:ident ),+ = $discriminator:expr ),*
        }
    ) => { ... };
    (
        $( #[$enum_attr:meta] )*
        enum $name:ident {
            $( $( #[$variant_attr:meta] )* $variant:ident ),+ $( = $discriminator:expr, $( $( #[$variant_two_attr:meta] )* $variant_two:ident ),+ )*,
        }
    ) => { ... };
    (
        $( #[$enum_attr:meta] )*
        enum $name:ident {
            $( $( $( #[$variant_attr:meta] )* $variant:ident ),+ = $discriminator:expr ),+,
        }
    ) => { ... };
    (
        $( #[$enum_attr:meta] )*
        pub enum $name:ident {
            $( $( #[$variant_attr:meta] )* $variant:ident ),+ $( = $discriminator:expr, $( $( #[$variant_two_attr:meta] )* $variant_two:ident ),+ )*
        }
    ) => { ... };
    (
        $( #[$enum_attr:meta] )*
        pub enum $name:ident {
            $( $( $( #[$variant_attr:meta] )* $variant:ident ),+ = $discriminator:expr ),*
        }
    ) => { ... };
    (
        $( #[$enum_attr:meta] )*
        pub enum $name:ident {
            $( $( #[$variant_attr:meta] )* $variant:ident ),+ $( = $discriminator:expr, $( $( #[$variant_two_attr:meta] )* $variant_two:ident ),+ )*,
        }
    ) => { ... };
    (
        $( #[$enum_attr:meta] )*
        pub enum $name:ident {
            $( $( $( #[$variant_attr:meta] )* $variant:ident ),+ = $discriminator:expr ),+,
        }
    ) => { ... };
}

Wrap this macro around an enum declaration to get an automatically generated implementation of num::FromPrimitive.