The Rust Programming Language

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

A workspace is a set of packages that will all share the same Cargo.lock and output directory. Let’s make a project using a workspace, using trivial code so we can concentrate on the structure of a workspace. We’ll have a binary that uses two libraries: one library that will provide an add_one function and a second library that will provide an add_two function. These three crates will all be part of the same workspace. We’ll start by creating a new crate for the binary:

$ cargo new --bin adder
     Created binary (application) `adder` project
$ cd adder

We need to modify the binary package’s Cargo.toml and add a [workspace] section to tell Cargo the adder package is a workspace. Add this at the bottom of the file:

[workspace]

Like many Cargo features, workspaces support convention over configuration: we don’t need to add anything more than this to Cargo.toml to define our workspace as long as we follow the convention.

The workspace convention says any crates in any subdirectories that the top-level crate depends on are part of the workspace. Any crate, whether in a workspace or not, can specify that it has a dependency on a crate in a local directory by using the path attribute on the dependency specification in Cargo.toml. If a crate has the [workspace] key and we specify path dependencies where the paths are subdirectories of the crate’s directory, those dependent crates will be considered part of the workspace. Let’s specify in the Cargo.toml for the top-level adder crate that it will have a dependency on an add-one crate that will be in the add-one subdirectory, by changing Cargo.toml to look like this:

[dependencies]
add-one = { path = "add-one" }

If we add dependencies to Cargo.toml that don’t have a path specified, those dependencies will be normal dependencies that aren’t in this workspace and are assumed to come from Crates.io.

Next, while in the adder directory, generate an add-one crate:

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

Your adder directory should now have these directories and files:

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

In add-one/src/lib.rs, 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
}
#}

Open up src/main.rs for adder and add an extern crate line at the top of the file to bring the new add-one library crate into scope. Then change the main function to call the add_one function, as in Listing 14-12:

extern crate add_one;

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

Listing 14-12: Using the add-one library crate from the adder crate

Let’s build the adder crate by running cargo build in the adder directory!

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

Note that this builds both the adder crate and the add-one crate in adder/add-one. Now your adder directory should have these files:

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

The workspace has one target directory at the top level; add-one doesn’t have its own target directory. Even if we go into the add-one directory and run cargo build, the compiled artifacts end up in adder/target rather than adder/add-one/target. The crates in a workspace depend on each other. If each crate had its own target directory, each crate in the workspace would have to recompile each other crate in the workspace in order to have the artifacts in its own target directory. By sharing one target directory, the crates in the workspace can avoid rebuilding the other crates in the workspace more than necessary.

Also notice the workspace only has one Cargo.lock, rather than having a top-level Cargo.lock and add-one/Cargo.lock. This ensures that all crates are using the same version of all dependencies. If we add the rand crate to both Cargo.toml and add-one/Cargo.toml, 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 try this out now.

Let’s add the rand crate to the [dependencies] section in add-one/Cargo.toml in order 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 extern crate rand; to add-one/src/lib.rs, and building the whole workspace by running cargo build in the adder 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/adder/add-one)
   Compiling adder v0.1.0 (file:///projects/adder)
    Finished dev [unoptimized + debuginfo] target(s) in 10.18 secs

The top level Cargo.lock now contains information about add-one’s dependency 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 as well. If we add extern crate rand; to src/main.rs for the top level adder crate, for example, we’ll get an error:

$ cargo build
   Compiling adder v0.1.0 (file:///projects/adder)
error[E0463]: can't find crate for `rand`
 --> src/main.rs:1:1
  |
1 | extern crate rand;
  | ^^^^^^^^^^^^^^^^^^^ can't find crate

To fix this, edit Cargo.toml for the top level 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 for us that any 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 since 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 adder directory:

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

running 0 tests

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

Wait a second, zero tests? We just added one! If we look at the output, we can see that cargo test in a workspace only runs tests for the top level crate. To run tests for all of the crates in the workspace, we need to pass the --all flag:

$ cargo test --all
    Finished dev [unoptimized + debuginfo] target(s) in 0.37 secs
     Running target/debug/deps/add_one-abcabcabc

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-abcabcabc

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

When passing --all, cargo test will run the tests for all of the crates in the workspace. We can also choose to 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 choose to publish the crates in the workspace to crates.io, each crate in the workspace will get published separately. The cargo publish command does not have an --all flag or a -p flag, so it is necessary to change to each crate’s directory and run cargo publish on each crate in the workspace in order to publish them.

Now try adding an add-two crate to this workspace in a similar way as the add-one crate for some more practice!

As your project grows, consider using a workspace: smaller components are easier to understand individually than one big blob of code. Keeping the crates in a workspace can make coordination among them easier if they work together and are often changed at the same time.