$ cargo check
    Checking hello v0.1.0 (file:///projects/hello)
error[E0433]: cannot find type `ThreadPool` in this scope
  --> src/main.rs:11:16
   |
11 |     let pool = ThreadPool::new(4);
   |                ^^^^^^^^^^ use of undeclared type `ThreadPool`

For more information about this error, try `rustc --explain E0433`.
error: could not compile `hello` (bin "hello") due to 1 previous error
