Start with the idea
mod declares a module and use brings a path into scope, while pub decides what actually escapes. Cargo.toml describes the package around them — the same file the package manager reads to resolve your dependencies.
A module gives names and boundaries to related code. In the example, mod greeter; tells Rust to compile the module from src/greeter.rs; greeter::message() follows a path to an item inside it. The use keyword can shorten a path in the current scope, but it does not change whether an item is visible. Visibility starts private. pub opens an item to callers permitted by the module hierarchy, and a public function exposed from a library crate becomes part of the interface other crates may rely on.
Start a tiny project with src/main.rs and src/greeter.rs.
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