Macro define_spec_handler 

macro_rules! define_spec_handler {
    (
        impl $handler:ident($this:ident, $inner:ident) {
            $(
                $(#[$custom_meta:meta])*
                fn $custom_name:ident $custom_args:tt $(-> $custom_return:ty)? $custom_body:block
            )*
        }
    ) => { ... };
    (
        impl $handler:ident($this:ident, $inner:ident) {
            $(
                $(#[$custom_meta:meta])*
                fn $custom_name:ident $custom_args:tt $(-> $custom_return:ty)? $custom_body:block
            )*
        }
        delegate {
            $(
                fn $delegated_name:ident $delegated_args:tt $(-> $delegated_return:ty)?;
            )*
        }
    ) => { ... };
}