Filesystem Hierarchy

Omitting the module content will tell Rust to look for it in another file:

# // Copyright 2022 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
mod garden;

This tells Rust that the garden module content is found at src/garden.rs. Similarly, a garden::vegetables module can be found at src/garden/vegetables.rs.

The crate root is in:

• src/lib.rs (for a library crate)
• src/main.rs (for a binary crate)

Modules defined in files can be documented, too, using "inner doc comments". These document the item that contains them -- in this case, a module.

# // Copyright 2022 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
//! This module implements the garden, including a highly performant germination
//! implementation.

// Re-export types from this module.
pub use garden::Garden;
pub use seeds::SeedPacket;

/// Sow the given seed packets.
pub fn sow(seeds: Vec<SeedPacket>) {
    todo!()
}

/// Harvest the produce in the garden that is ready.
pub fn harvest(garden: &mut Garden) {
    todo!()
}

• Before Rust 2018, modules needed to be located at module/mod.rs instead of module.rs, and this is still a working alternative for editions after 2018.

• The main reason to introduce filename.rs as alternative to filename/mod.rs was because many files named mod.rs can be hard to distinguish in IDEs.

• Deeper nesting can use folders, even if the main module is a file:

  ignoreCopy

  src/
  ├── main.rs
  ├── top_module.rs
  └── top_module/
      └── sub_module.rs
  

• The place Rust will look for modules can be changed with a compiler directive:

  rust,ignoreCopy

  # // Copyright 2022 Google LLC
  # // SPDX-License-Identifier: Apache-2.0
  #
  #[path = "some/path.rs"]
  mod some_module;
  

  This is useful, for example, if you would like to place tests for a module in a file named some_module_test.rs, similar to the convention in Go.