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In Chapter 7 of rustbook, Section 7.2 -> Modules Cheat Sheet -> Declaring modules:, the preference of the compiler for module declarations is clearly defined. However, when I tried experimenting with all the three ways, namely: 1 of 3) Inline, within curly brackets that replace the semicolon following mod garden 2 of 3) In the file src/garden.rs 3 of 3) In the file src/garden/mod.rs I observed inconsistency in the compiler's behaviour when using multiple ways.

To check how these three ways might conflict with each other, I first tried 1 of 3) and 2 of 3) above. The behaviour of the compiler was to silently (as in, no error or warning) prefer 1 of 3) over 2 of 3), effectively ignoring 2 of 3). So, the compilation was successful.

However, when I tried 2 of 3) and 3 of 3), the compiler threw error[E0761], complaining about the module being found in multiple places.

Code (1 of 3) and 2 of 3))

restaurant$ tree
.
├── Cargo.lock
├── Cargo.toml
└── src
    ├── front_of_house
    │   └── hosting.rs
    ├── front_of_house.rs
    ├── lib.rs
    └── main.rs

2 directories, 6 files

restaurant$ cat Cargo.toml 
[package]
name = "restaurant"
version = "0.1.0"
edition = "2021"

[dependencies]

restaurant$ cat src/main.rs 
use restaurant::eat_at_restaurant;

fn main() {
    println!("{}:{}: Hello, world!", file!(), line!());
    eat_at_restaurant();
}

restaurant$ cat src/lib.rs 
mod front_of_house;

pub use crate::front_of_house::hosting;

pub fn eat_at_restaurant() {
    hosting::add_to_waitlist();

restaurant$ cat src/front_of_house.rs 
pub mod hosting {
    pub fn add_to_waitlist() {
        println!("{}:{}: I was called!", file!(), line!());
    }
}

restaurant$ cat src/front_of_house/hosting.rs 
pub fn add_to_waitlist() {
    println!("{}:{}: I was called!", file!(), line!());
}

restaurant$ cargo run
   Compiling restaurant v0.1.0 (/media/nibu/ext4_data/ext4_progprac/Basics/restaurant)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.16s
     Running `target/debug/restaurant`
src/main.rs:4: Hello, world!
src/front_of_house.rs:3: I was called!

Code (2 of 3) and 3 of 3))

restaurant$ tree
.
├── Cargo.lock
├── Cargo.toml
└── src
    ├── front_of_house
    │   ├── hosting
    │   │   └── mod.rs
    │   └── hosting.rs
    ├── front_of_house.rs
    ├── lib.rs
    └── main.rs

3 directories, 7 files

restaurant$ cat Cargo.toml 
[package]
name = "restaurant"
version = "0.1.0"
edition = "2021"

[dependencies]

restaurant$ cat src/main.rs 
use restaurant::eat_at_restaurant;

fn main() {
    println!("{}:{}: Hello, world!", file!(), line!());
    eat_at_restaurant();
}

restaurant$ cat src/lib.rs 
mod front_of_house;

pub use crate::front_of_house::hosting;

pub fn eat_at_restaurant() {
    hosting::add_to_waitlist();

restaurant$ cat src/front_of_house.rs 
pub mod hosting;

restaurant$ cat src/front_of_house/hosting.rs 
pub fn add_to_waitlist() {
    println!("{}:{}: I was called!", file!(), line!());
}

restaurant$ cat src/front_of_house/hosting/mod.rs 
pub fn add_to_waitlist() {
    println!("{}:{}: I was called!", file!(), line!());
}

restaurant$ cargo run
   Compiling restaurant v0.1.0 (/home/user/restaurant)
error[E0761]: file for module `hosting` found at both "src/front_of_house/hosting.rs" and "src/front_of_house/hosting/mod.rs"
 --> src/front_of_house.rs:1:1
  |
1 | pub mod hosting;
  | ^^^^^^^^^^^^^^^^
  |
  = help: delete or rename one of them to remove the ambiguity

error[E0425]: cannot find function `add_to_waitlist` in module `hosting`
 --> src/lib.rs:6:14
  |
6 |     hosting::add_to_waitlist();
  |              ^^^^^^^^^^^^^^^ not found in `hosting`

Some errors have detailed explanations: E0425, E0761.
For more information about an error, try `rustc --explain E0425`.
error: could not compile `restaurant` (lib) due to 2 previous errors

rustc --version --verbose:

rustc 1.84.0-nightly (32b17d56e 2024-10-28)
binary: rustc
commit-hash: 32b17d56eb02495f9865028e1f7271a3a48c0b9b
commit-date: 2024-10-28
host: x86_64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.1

cargo --version --verbose:

cargo 1.84.0-nightly (e75214ea4 2024-10-25)
release: 1.84.0-nightly
commit-hash: e75214ea4936d2f2c909a71a1237042cc0e14b07
commit-date: 2024-10-25
host: x86_64-unknown-linux-gnu
libgit2: 1.8.1 (sys:0.19.0 vendored)
libcurl: 8.9.0-DEV (sys:0.4.74+curl-8.9.0 vendored ssl:OpenSSL/3.0.2)
os: Ubuntu 22.4.0 (jammy) [64-bit]

I had originally raised this issue as an internal rust compiler issue due to the biased/inconsistent behaviour of the compiler. However, I was advised to open this issue here instead: https://github.com/rust-lang/rust/issues/139685

Thanks!