What is the difference between cargo new --lib and cargo new --bin?

Hello, I am quite new to Rust, who is reading the Rust Programming Book!

I got curious what is the difference between cargo new --lib and cargo new --bin.

More specifically, if once I create a new package using cargo new --bin and later decided to add src/lib.rs, I was not able to import contents in lib.rs, using extern crate <package name>.

However, the opposite way works very well.

What is the key difference that those packages created from --lib flag and --bin flag behave differently?



There isn’t any. The only differences are in the generated files (Cargo.toml, .gitignore, src/*.rs).

The problem is that you can’t link to a binary crate because it’s a binary.

Thank you for your answer!

Then I have a different question, where are information that indicates a crate is a binary crate, is stored?

I have no idea how Rust distinguish between binary crates and library crates, if both has main.rs and lib.rs in the src directory?

I looked at Cargo.toml, but I couldn;t see any differences.

It’s implied by the source files. If there’s a src/main.rs, that is compiled as a binary. If there’s a src/lib.rs, that is compiled as a library. Each src/bin/*.rs is compiled as a binary.

You can be explicit about this, which is explained in the Cargo documentation.


So there are rules that Rust determines the crate type from the project layout.

I will take a look at the document.

Thanks a lot!