Function rustc_expand::mbe::macro_rules::generic_extension

fn generic_extension<'cx>(
    cx: &'cx mut ExtCtxt<'_>, 
    sp: Span, 
    def_span: Span, 
    name: Ident, 
    transparency: Transparency, 
    arg: TokenStream, 
    lhses: &[TokenTree], 
    rhses: &[TokenTree], 
    is_local: bool
) -> Box<dyn MacResult + 'cx>

Given lhses and rhses, this is the new macro we create