Function expand_macro 

fn expand_macro<'cx, 'a: 'cx>(
    cx: &'cx mut ExtCtxt<'_>,
    sp: Span,
    def_span: Span,
    node_id: NodeId,
    name: Ident,
    transparency: Transparency,
    arg: TokenStream,
    rules: &'a [MacroRule],
) -> Box<dyn MacResult + 'cx>

Expands the rules based macro defined by rules for a given input arg.