How to access nested modules/files from nested directories

Question

Let's say that I have this file structure in my project:

.
├── Cargo.lock
├── Cargo.toml
├── src
│   ├── hotel
│   │   ├── guest.rs
│   │   ├── hotel_manager.rs
│   │   └── hotel.rs
│   ├── lib.rs
│   └── main.rs

In hotel_menager I declared only that I will use hotel and guest module:

pub mod hotel;
pub mod guest;

Then inside lib.rs I declared that I will use hotel_manager module:

pub mod hotel_manager;

and in the end I wanted to use hotel and guest in my main.rs binary but I am getting this error :

file not found for module hotel_manager

Then I thought that maybe I should use use but it also didn't help at all:

use crate::hotel::hotel_manager;

unresolved import crate::hotel.

My Cargo.toml:

[package]
name = "learn_file_spliting"
version = "0.1.0"
authors = ["kaczor6418 <[email protected]>"]
edition = "2018"

[lib]
name = "lib"
path = "src/lib.rs"

[[bin]]
name = "main"
path = "src/main.rs"

How can I use this nested module in lib.rs and then use re-exported modules inside main.rs ?

I know that if I create hotel_manager in /src directory then everything will work, but I want to have this hotel_manager module inside nested directory and learn how to use nested modules.

mod hotel_manager; will look for ./hotel_manager.rs or ./hotel_manager/mod.rs, but not for ./hotel/hotel_manager.rs. — mcarton
– mcarton, Commented Sep 6, 2020 at 16:51

Lymia Kanokawa · Accepted Answer · 2020-09-06 16:50:45Z

In Rust, you can't create a module by simply having it exist in the directory tree. You have to explicitly declare it with mod module_name; statement. This doesn't only declare that a module named module_name is used, it declares that such a module exists at all.

pub mod hotel_manager; doesn't work in src/lib.rs because this tries to creates a submodule named hotel_manager in the crate's root module, which would have to exist in either src/hotel_manager.rs or src/hotel_manager/mod.rs -- and neither exist.

Importing crate::hotel::hotel_manager in src/lib.rs doesn't work because you have never declared a pub mod hotel; in src/lib.rs, and therefore, no module named crate::hotel exists at this point.

Basically, for the hotel_manager module to exist at src/hotel/hotel_manager.rs, it would have to be declared as a submodule of the a crate::hotel module, which itself would need to be declared in the root module.

The way to do what you want is to write the following in src/lib.rs:

pub mod hotel;

This submodule named hotel would have to exist at either src/hotel.rs or src/hotel/mod.rs. Since we want it to have further submodules, by convention, the latter is used. So create a file named src/hotel/mod.rs, with the following contents:

pub mod guest;
pub mod hotel_manager;
pub mod hotel;

This creates a proper module structure we want with hotel as a submodule of the root module, which contains further submodules named guest, hotel_manager and hotel.

Then, in the crate::hotel::hotel_manager module, instead of using mod statements, you should use the following statements, which import the modules that have been declared elsewhere into the current scope:

use crate::hotel::guest;
use crate::hotel::hotel;

Note that in Rust, you can import items smaller than just the whole module. For example, if you have a struct named Hotel in the crate::hotel::hotel module, you can import only that with a statement like use crate::hotel::hotel::Hotel;

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