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 in a garden.rs file in the same directory as the file that declared the module. If the garden module similarly declares a vegetables submodule, then the content of garden::vegetables would be found at garden/vegetables.rs.

The crate root is in:

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

This is the starting point from which the compiler identifies additional modules.

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.

// Declare submodules.
mod plant;
mod seeds;

// Re-export types from this module.
pub use plant::Plant;
pub use seeds::SeedPacket;

pub struct Garden {
    plants: Vec<Plant>,
}

/// 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!()
}

This slide should take about 5 minutes.

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:

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:
```
// 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.

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Filesystem Hierarchy