Chapter 8. Crates and Modules

This is one note in a Rust theme: systems programmers can have nice things.

Robert O’Callahan, “Random Thoughts on Rust: and IDEs”

Suppose you’re writing a program that simulates the growth of ferns, from the level of individual cells on up. Your program, like a fern, will start out very simple, with all the code, perhaps, in a single file—just the spore of an idea. As it grows, it will start to have internal structure. Different pieces will have different purposes. It will branch out into multiple files. It may cover a whole directory tree. In time it may become a significant part of a whole software ecosystem. For any program that grows beyond a few data structures or a few hundred lines, some organization is necessary.

This chapter covers the features of Rust that help keep your program organized: crates and modules. We’ll also cover other topics related to the structure and distribution of a Rust crate, including how to document and test Rust code, how to silence unwanted compiler warnings, how to use Cargo to manage project dependencies and versioning, how to publish open source libraries on Rust’s public crate repository,, how Rust evolves through language editions, and more, using the fern simulator as our running example.


Rust programs are made of crates. Each crate is a complete, cohesive unit: all the source code for a single library or executable, plus any associated tests, examples, tools, configuration, ...

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