Macro bench 

macro_rules! bench {
    ($name:ident, $fn:expr) => { ... };
}

Declare a benchmark function.

If cfg(benchmark) is enabled, a function named name is emitted and it receives RealBencher.

If cfg(test) is enabled, a module named name with a single test called test_bench is emitted and it receives TestBencher.

This setup allows for all benchmarking functions to run in unit test mode, allowing problems to be noticed sooner.

Note that $fn doesn’t have to be a named function - it can also be an anonymous closure.