Cargo Workspaces - The Rust Programming Language

## Cargo Workspaces

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.

### Creating a Workspace

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.

### Creating the Second Crate in the Workspace

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 an `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.

#### Depending on an External Crate in a Workspace

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 `extern crate 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.

#### Adding a Test to a Workspace

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.