$ cargo check
    Checking hello v0.1.0 (file:///projects/hello)
error[E0433]: failed to resolve: use of undeclared type or module `ThreadPool`
  --> src/main.rs:10:16
   |
10 |     let pool = ThreadPool::new(4);
   |                ^^^^^^^^^^ use of undeclared type or module `ThreadPool`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0433`.
error: could not compile `hello`.

To learn more, run the command again with --verbose.
