pub fn pool_size(&mut self, size: usize) -> &mut Self {
        assert!(size > 0);
        self.pool_size = size;
        self
    }
Set size of a future ThreadPool

The size of a thread pool is the number of worker threads spawned. By
default, this is equal to the number of CPU cores.

# Panics

Panics if `pool_size == 0`.
futures-executor/src/thread_pool.rs