Macro try_invoke 

try_invoke!() { /* proc-macro */ }

struct MaybeMacro;
try_invoke!(
    self::MaybeMacro!(my arguments);
    // Anything following is expanded if maybe_macro is not a macro
    impl ToString for MaybeMacro {
        fn to_string(&self) -> String {
            "Not a macro".to_string()
        }
    }
);
fn main() {
    println!("{}", MaybeMacro.to_string());
}