Macro mutex 

macro_rules! mutex {
    ($val:expr) => { ... };
}

Create a crate::Mutex<T> around $val that works in both heap and zero-heap modes.

Use () as the value for a “no-data” lock (i.e. Mutex<()> used purely to synchronise access to other state) — matches std::sync::Mutex<()> / parking_lot::Mutex<()> convention.