Macro __mx_condition_pair 

#[doc(hidden)]
macro_rules! __mx_condition_pair {
    (($name:literal, $value:expr $(,)?)) => { ... };
    (($name:expr, $value:expr $(,)?)) => { ... };
}

Internal: one (name, value) pair of a bracketed data = [...] list; the name goes through the reserved-slot check (compile time for literals).