Rust treats testing as a first-class construct; all tools in the ecosystem supports testing. The compiler provides a built-in configuration attribute that designates a module for testing. There is also a test attribute that designates functions as tests. When Cargo generates a project from scratch, it sets up this boilerplate. Let's look at an example project; we will call it factorial. It will export a macro that computes the factorial given an integer. Since we have conveniently written such a macro before, we will just re-use that code here. Note that since this crate will be used as a library, this does not have a main function:
# cargo new factorial --lib
# cargo test
Compiling factorial v0.1.0 (file:///Users/Abhishek/Desktop/rust-book/src/ch2/factorial)
Finished dev [unoptimized + debuginfo] target(s) in 1.6 secs
Running target/debug/deps/factorial-364286f171614349...