The Rust Programming Language

In Chapter 12, we built a package that included a binary crate and a library crate. As your project develops, you might find that the library crate continues to get bigger and you want to split up your package further into multiple library crates. In this situation, Cargo offers a feature called workspaces that can help manage multiple related packages that are developed in tandem.

A workspace is a set of packages that share the same Cargo.lock and output directory. Let’s make a project using a workspace—we’ll use trivial code so we can concentrate on the structure of the workspace. There are multiple ways to structure a workspace; we’re going to show one common way. We’ll have a workspace containing a binary and two libraries. The binary, which will provide the main functionality, will depend on the two libraries. One library will provide an add_one function, and a second library an add_two function. These three crates will be part of the same workspace. We’ll start by creating a new directory for the workspace:

$ mkdir add
$ cd add

Next, in the add directory, we create the Cargo.toml file that will configure the entire workspace. This file won’t have a [package] section or the metadata we’ve seen in other Cargo.toml files. Instead, it will start with a [workspace] section that will allow us to add members to the workspace by specifying the path to our binary crate; in this case, that path is adder:

Filename: Cargo.toml

[workspace]

members = [
    "adder",
]

Next, we’ll create the adder binary crate by running cargo new within the add directory:

$ cargo new adder
     Created binary (application) `adder` project

At this point, we can build the workspace by running cargo build. The files in your add directory should look like this:

├── Cargo.lock
├── Cargo.toml
├── adder
│   ├── Cargo.toml
│   └── src
│       └── main.rs
└── target

The workspace has one target directory at the top level for the compiled artifacts to be placed into; the adder crate doesn’t have its own target directory. Even if we were to run cargo build from inside the adder directory, the compiled artifacts would still end up in add/target rather than add/adder/target. Cargo structures the target directory in a workspace like this because the crates in a workspace are meant to depend on each other. If each crate had its own target directory, each crate would have to recompile each of the other crates in the workspace to have the artifacts in its own target directory. By sharing one target directory, the crates can avoid unnecessary rebuilding.

Next, let’s create another member crate in the workspace and call it add-one. Change the top-level Cargo.toml to specify the add-one path in the members list:

Filename: Cargo.toml

[workspace]

members = [
    "adder",
    "add-one",
]

Then generate a new library crate named add-one:

$ cargo new add-one --lib
     Created library `add-one` project

Your add directory should now have these directories and files:

├── Cargo.lock
├── Cargo.toml
├── add-one
│   ├── Cargo.toml
│   └── src
│       └── lib.rs
├── adder
│   ├── Cargo.toml
│   └── src
│       └── main.rs
└── target

In the add-one/src/lib.rs file, let’s add an add_one function:

Filename: add-one/src/lib.rs

# #![allow(unused_variables)]
#fn main() {
pub fn add_one(x: i32) -> i32 {
    x + 1
}
#}

Now that we have a library crate in the workspace, we can have the binary crate adder depend on the library crate add-one. First, we’ll need to add a path dependency on add-one to adder/Cargo.toml.

Filename: adder/Cargo.toml

[dependencies]

add-one = { path = "../add-one" }

Cargo doesn’t assume that crates in a workspace will depend on each other, so we need to be explicit about the dependency relationships between the crates.

Next, let’s use the add_one function from the add-one crate in the adder crate. Open the adder/src/main.rs file and add a use line at the top to bring the new add-one library crate into scope. Then change the main function to call the add_one function, as in Listing 14-7.

Filename: adder/src/main.rs

use add_one;

fn main() {
    let num = 10;
    println!("Hello, world! {} plus one is {}!", num, add_one::add_one(num));
}

Listing 14-7: Using the add-one library crate from the adder crate

Let’s build the workspace by running cargo build in the top-level add directory!

$ cargo build
   Compiling add-one v0.1.0 (file:///projects/add/add-one)
   Compiling adder v0.1.0 (file:///projects/add/adder)
    Finished dev [unoptimized + debuginfo] target(s) in 0.68 secs

To run the binary crate from the add directory, we need to specify which package in the workspace we want to use by using the -p argument and the package name with cargo run:

$ cargo run -p adder
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/adder`
Hello, world! 10 plus one is 11!

This runs the code in adder/src/main.rs, which depends on the add-one crate.

Notice that the workspace has only one Cargo.lock file at the top level of the workspace rather than having a Cargo.lock in each crate’s directory. This ensures that all crates are using the same version of all dependencies. If we add the rand crate to the adder/Cargo.toml and add-one/Cargo.toml files, Cargo will resolve both of those to one version of rand and record that in the one Cargo.lock. Making all crates in the workspace use the same dependencies means the crates in the workspace will always be compatible with each other. Let’s add the rand crate to the [dependencies] section in the add-one/Cargo.toml file to be able to use the rand crate in the add-one crate:

Filename: add-one/Cargo.toml

[dependencies]

rand = "0.3.14"

We can now add use rand; to the add-one/src/lib.rs file, and building the whole workspace by running cargo build in the add directory will bring in and compile the rand crate:

$ cargo build
    Updating registry `https://github.com/rust-lang/crates.io-index`
 Downloading rand v0.3.14
   --snip--
   Compiling rand v0.3.14
   Compiling add-one v0.1.0 (file:///projects/add/add-one)
   Compiling adder v0.1.0 (file:///projects/add/adder)
    Finished dev [unoptimized + debuginfo] target(s) in 10.18 secs

The top-level Cargo.lock now contains information about the dependency of add-one on rand. However, even though rand is used somewhere in the workspace, we can’t use it in other crates in the workspace unless we add rand to their Cargo.toml files as well. For example, if we add use rand; to the adder/src/main.rs file for the adder crate, we’ll get an error:

$ cargo build
   Compiling adder v0.1.0 (file:///projects/add/adder)
error: use of unstable library feature 'rand': use `rand` from crates.io (see
issue #27703)
 --> adder/src/main.rs:1:1
  |
1 | use rand;

To fix this, edit the Cargo.toml file for the adder crate and indicate that rand is a dependency for that crate as well. Building the adder crate will add rand to the list of dependencies for adder in Cargo.lock, but no additional copies of rand will be downloaded. Cargo has ensured that every crate in the workspace using the rand crate will be using the same version. Using the same version of rand across the workspace saves space because we won’t have multiple copies and ensures that the crates in the workspace will be compatible with each other.

For another enhancement, let’s add a test of the add_one::add_one function within the add_one crate:

Filename: add-one/src/lib.rs

# #![allow(unused_variables)]
#fn main() {
pub fn add_one(x: i32) -> i32 {
    x + 1
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn it_works() {
        assert_eq!(3, add_one(2));
    }
}
#}

Now run cargo test in the top-level add directory:

$ cargo test
   Compiling add-one v0.1.0 (file:///projects/add/add-one)
   Compiling adder v0.1.0 (file:///projects/add/adder)
    Finished dev [unoptimized + debuginfo] target(s) in 0.27 secs
     Running target/debug/deps/add_one-f0253159197f7841

running 1 test
test tests::it_works ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

     Running target/debug/deps/adder-f88af9d2cc175a5e

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

   Doc-tests add-one

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

The first section of the output shows that the it_works test in the add-one crate passed. The next section shows that zero tests were found in the adder crate, and then the last section shows zero documentation tests were found in the add-one crate. Running cargo test in a workspace structured like this one will run the tests for all the crates in the workspace.

We can also run tests for one particular crate in a workspace from the top-level directory by using the -p flag and specifying the name of the crate we want to test:

$ cargo test -p add-one
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running target/debug/deps/add_one-b3235fea9a156f74

running 1 test
test tests::it_works ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

   Doc-tests add-one

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

This output shows cargo test only ran the tests for the add-one crate and didn’t run the adder crate tests.

If you publish the crates in the workspace to https://crates.io/, each crate in the workspace will need to be published separately. The cargo publish command does not have an --all flag or a -p flag, so you must change to each crate’s directory and run cargo publish on each crate in the workspace to publish the crates.

For additional practice, add an add-two crate to this workspace in a similar way as the add-one crate!

As your project grows, consider using a workspace: it’s easier to understand smaller, individual components than one big blob of code. Furthermore, keeping the crates in a workspace can make coordination between them easier if they are often changed at the same time.