cargo build Compiling restaurant v0.1.0 (file:///projects/restaurant) error[E0433]: failed to resolve: use of undeclared crate or module `hosting` --> src/lib.rs:11:9 | 11 | hosting::add_to_waitlist(); 动开发。下面是我们得到的第一个错误： $ cargo check Checking hello v0.1.0 (file:///projects/hello) error[E0433]: failed to resolve: use of undeclared type `ThreadPool` --> src/main.rs:11:16 | 11 | let pool = ThreadPool::new(4);