The Rust Programming Language

We resolved the error messages shown in Listing 7-4 by moving the network and network::server code into the src/network/mod.rs and src/network/server.rs files, respectively. At that point, cargo build was able to build our project, but we still get warning messages about the client::connect, network::connect, and network::server::connect functions not being used:

warning: function is never used: `connect`, #[warn(dead_code)] on by default
src/client.rs:1:1
  |
1 | fn connect() {
  | ^

warning: function is never used: `connect`, #[warn(dead_code)] on by default
 --> src/network/mod.rs:1:1
  |
1 | fn connect() {
  | ^

warning: function is never used: `connect`, #[warn(dead_code)] on by default
 --> src/network/server.rs:1:1
  |
1 | fn connect() {
  | ^

So why are we receiving these warnings? After all, we’re building a library with functions that are intended to be used by our users, not necessarily by us within our own project, so it shouldn’t matter that these connect functions go unused. The point of creating them is that they will be used by another project, not our own.

To understand why this program invokes these warnings, let’s try using the connect library from another project, calling it externally. To do that, we’ll create a binary crate in the same directory as our library crate by making a src/main.rs file containing this code:

Filename: src/main.rs

extern crate communicator;

fn main() {
    communicator::client::connect();
}

We use the extern crate command to bring the communicator library crate into scope. Our package now contains two crates. Cargo treats src/main.rs as the root file of a binary crate, which is separate from the existing library crate whose root file is src/lib.rs. This pattern is quite common for executable projects: most functionality is in a library crate, and the binary crate uses that library crate. As a result, other programs can also use the library crate, and it’s a nice separation of concerns.

From the point of view of a crate outside the communicator library looking in, all the modules we’ve been creating are within a module that has the same name as the crate, communicator. We call the top-level module of a crate the root module.

Also note that even if we’re using an external crate within a submodule of our project, the extern crate should go in our root module (so in src/main.rs or src/lib.rs). Then, in our submodules, we can refer to items from external crates as if the items are top-level modules.

Right now, our binary crate just calls our library’s connect function from the client module. However, invoking cargo build will now give us an error after the warnings:

error: module `client` is private
 --> src/main.rs:4:5
  |
4 |     communicator::client::connect();
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Ah ha! This error tells us that the client module is private, which is the crux of the warnings. It’s also the first time we’ve run into the concepts of public and private in the context of Rust. The default state of all code in Rust is private: no one else is allowed to use the code. If you don’t use a private function within your program, because your program is the only code allowed to use that function, Rust will warn you that the function has gone unused.

After we specify that a function like client::connect is public, not only will our call to that function from our binary crate be allowed, but the warning that the function is unused will go away. Marking a function as public lets Rust know that the function will be used by code outside of our program. Rust considers the theoretical external usage that’s now possible as the function “being used.” Thus, when something is marked public, Rust will not require that it be used in our program and will stop warning that the item is unused.

To tell Rust to make something public, we add the pub keyword to the start of the declaration of the item we want to make public. We’ll focus on fixing the warning that indicates client::connect has gone unused for now, as well as the module `client` is private error from our binary crate. Modify src/lib.rs to make the client module public, like so:

Filename: src/lib.rs

pub mod client;

mod network;

The pub keyword is placed right before mod. Let’s try building again:

error: function `connect` is private
 --> src/main.rs:4:5
  |
4 |     communicator::client::connect();
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Hooray! We have a different error! Yes, different error messages are a cause for celebration. The new error shows function `connect` is private, so let’s edit src/client.rs to make client::connect public too:

Filename: src/client.rs

# #![allow(unused_variables)]
#fn main() {
pub fn connect() {
}
#}

Now run cargo build again:

warning: function is never used: `connect`, #[warn(dead_code)] on by default
 --> src/network/mod.rs:1:1
  |
1 | fn connect() {
  | ^

warning: function is never used: `connect`, #[warn(dead_code)] on by default
 --> src/network/server.rs:1:1
  |
1 | fn connect() {
  | ^

The code compiled, and the warning about client::connect not being used is gone!

Unused code warnings don’t always indicate that an item in your code needs to be made public: if you didn’t want these functions to be part of your public API, unused code warnings could be alerting you to code you no longer need that you can safely delete. They could also be alerting you to a bug if you had just accidentally removed all places within your library where this function is called.

But in this case, we do want the other two functions to be part of our crate’s public API, so let’s mark them as pub as well to get rid of the remaining warnings. Modify src/network/mod.rs to look like the following:

Filename: src/network/mod.rs

pub fn connect() {
}

mod server;

Then compile the code:

warning: function is never used: `connect`, #[warn(dead_code)] on by default
 --> src/network/mod.rs:1:1
  |
1 | pub fn connect() {
  | ^

warning: function is never used: `connect`, #[warn(dead_code)] on by default
 --> src/network/server.rs:1:1
  |
1 | fn connect() {
  | ^

Hmmm, we’re still getting an unused function warning, even though network::connect is set to pub. The reason is that the function is public within the module, but the network module that the function resides in is not public. We’re working from the interior of the library out this time, whereas with client::connect we worked from the outside in. We need to change src/lib.rs to make network public too, like so:

Filename: src/lib.rs

pub mod client;

pub mod network;

Now when we compile, that warning is gone:

warning: function is never used: `connect`, #[warn(dead_code)] on by default
 --> src/network/server.rs:1:1
  |
1 | fn connect() {
  | ^

Only one warning is left! Try to fix this one on your own!

Overall, these are the rules for item visibility:

1. If an item is public, it can be accessed through any of its parent modules.
2. If an item is private, it can be accessed only by its immediate parent module and any of the parent’s child modules.

Let’s look at a few more privacy examples to get some practice. Create a new library project and enter the code in Listing 7-5 into your new project’s src/lib.rs:

Filename: src/lib.rs

mod outermost {
    pub fn middle_function() {}

    fn middle_secret_function() {}

    mod inside {
        pub fn inner_function() {}

        fn secret_function() {}
    }
}

fn try_me() {
    outermost::middle_function();
    outermost::middle_secret_function();
    outermost::inside::inner_function();
    outermost::inside::secret_function();
}

Listing 7-5: Examples of private and public functions, some of which are incorrect

Before you try to compile this code, make a guess about which lines in the try_me function will have errors. Then, try compiling the code to see whether you were right, and read on for the discussion of the errors!

The try_me function is in the root module of our project. The module named outermost is private, but the second privacy rule states that the try_me function is allowed to access the outermost module because outermost is in the current (root) module, as is try_me.

The call to outermost::middle_function will work because middle_function is public, and try_me is accessing middle_function through its parent module outermost. We determined in the previous paragraph that this module is accessible.

The call to outermost::middle_secret_function will cause a compilation error. middle_secret_function is private, so the second rule applies. The root module is neither the current module of middle_secret_function (outermost is), nor is it a child module of the current module of middle_secret_function.

The module named inside is private and has no child modules, so it can only be accessed by its current module outermost. That means the try_me function is not allowed to call outermost::inside::inner_function or outermost::inside::secret_function.

Here are some suggestions for changing the code in an attempt to fix the errors. Before you try each one, make a guess as to whether it will fix the errors, and then compile the code to see whether or not you’re right, using the privacy rules to understand why.

• What if the inside module was public?
• What if outermost was public and inside was private?
• What if, in the body of inner_function, you called ::outermost::middle_secret_function()? (The two colons at the beginning mean that we want to refer to the modules starting from the root module.)

Feel free to design more experiments and try them out!

Next, let’s talk about bringing items into scope with the use keyword.