Macro custom_error

macro_rules! custom_error {
    (
        $( #[$meta_attribute:meta] )* // Attributes, like #[derive(SomeTrait)]
        $visibility:vis // `pub` marker
        $errtype:ident // Name of the error type to generate
        $( < $(
            $type_param:tt // Optional type parameters for generic error types
            ),*
        > )?
        $(
            $( #[$field_meta:meta] )* // Meta-attributes for the variant, such as a doc comment
            $field:ident // Name of an error variant
            $( { $(
                $( #[$attr_meta:meta] )* // Meta-attributes for the attribute of the error variant
                $attr_name:ident // Name of an attribute of the error variant
                :
                $attr_type:ty // type of the attribute
            ),* } )?
            =
            $( @{ $($msg_fun:tt)* } )?
            $($msg:expr)? // The human-readable error message
         ),*
         $(,)* // Trailing comma
    ) => { ... };
    (
        $( #[$meta_attribute:meta] )* // Attributes, like #[derive(SomeTrait)]
        $visibility:vis // `pub` marker
        $errtype:ident // Name of the error type to generate
        $( < $(
            $type_param:tt // Optional type parameters for generic error types
            ),*
        > )?
        { $(
            $( #[$field_meta:meta] )* // Field meta attributes, such as doc comments
            $field_name:ident // Name of an attribute of the error variant
            :
            $field_type:ty // type of the attribute
        ),* }
        =
        $( @{ $($msg_fun:tt)* } )?
        $($msg:expr)? // The human-readable error message
        $(,)* // Trailing comma
    ) => { ... };
}