Allowing binary in the same crate to use library's modules but disallowing external crates to use the same modules

Question

I have three files under the src directory:

src
|____ keys.rs // a 'sub-module' under `lib.rs`
|____ lib.rs
|____ main.rs // binary file

In my cargo.toml, I specify specifically the paths to the lib and bin, i.e.:

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

[[bin]]
name = "urs" // name of my binary
path = "src/main.rs"

In keys.rs, I defined several structs and functions.

In lib.rs, I declare mod keys;, i.e.:

pub(crate) mod keys;

use [...]

In main.rs, I want to use the module keys from lib, so I do:

use lib::keys;

use [...]

I want to expose the module keys to be used anywhere within the crate but disallow its use outside of it, hence, I use the visibility modifier pub(crate) when declaring pub(crate) mod keys; in lib.rs.

However, cargo is telling me this:

error[E0603]: module `keys` is private
 --> src/main.rs:5:10
  |
5 | use lib::keys;
  |          ^^^^ private module
  |
note: the module `keys` is defined here
 --> F:\rust_projects\sui-utils-rust\src/lib.rs:1:1
  |
1 | pub(crate) mod keys;
  | ^^^^^^^^^^^^^^^^^^^

My questions:

Is it possible to achieve what I intend to achieve? I.e. by declaring pub mod keys; I'll be allowing external crates to use stuff within the keys module, how can I prevent that while retaining crate-exclusive access to the keys module?

Answer 1

You can do this simply by changing your project structure like this:

1. Make keys.rs a shared module: Instead of making keys.rs a submodule of lib.rs, you can make it a shared module that both lib.rs and main.rs can access.
2. Modifying lib.rs: Remove the pub(crate) mod keys; declaration from lib.rs.
3. Modifying main.rs: Instead of using use lib::keys;, directly reference keys.rs as a module.

Here's how you can modify your files:

src/keys.rs

// Your structs and functions

src/lib.rs

mod keys;
// Rest of your library code

src/main.rs

mod keys;
// Rest of your binary code

fn main() {
    // You can use items from keys here
}

This makes the keys module a shared module between your binary and library within the same crate. This way, it's not exposed to external crates, but both your binary and library can use it.