[TOC] # More about Cargo and Crates.io We've used some features of Cargo in this book so far, but only the most basic ones. We've used Cargo to build, run, and test our code, but it can do a lot more. Let's go over some of its other features now. Cargo can do even more than what we will cover in this chapter; for a full explanation, see its documentation. We're going to cover: * Customizing your build through release profiles * Publishing libraries on crates.io * Organizing larger projects with workspaces * Installing binaries from crates.io * Extending Cargo with your own custom commands ## Release profiles Cargo supports a notion of *release profiles*. These profiles control various options for compiling your code and let you configure each profile independently of the others. You've seen a hint of this feature in the output of your builds: ```text $ cargo build Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs $ cargo build --release Finished release [optimized] target(s) in 0.0 secs ``` The "debug" and "release" notifications here indicate that the compiler is using different profiles. Cargo supports four profiles: * `dev`: used for `cargo build` * `release` used for `cargo build --release` * `test` used for `cargo test` * `doc` used for `cargo doc` We can customize our `Cargo.toml` file with `[profile.*]` sections to tweak various compiler options for these profiles. For example, here's one of the default options for the `dev` and `release` profiles: ```toml [profile.dev] opt-level = 0 [profile.release] opt-level = 3 ``` The `opt-level` setting controls how many optimizations Rust will apply to your code. The setting goes from zero to three. Applying more optimizations takes more time. When you're compiling very often in development, you'd usually want compiling to be fast at the expense of the resulting code running slower. When you're ready to release, it's better to spend more time compiling the one time that you build your code to trade off for code that will run faster every time you use that compiled code. We could override these defaults by changing them in `Cargo.toml`. For example, if we wanted to use optimization level 1 in development: ```toml [profile.dev] opt-level = 1 ``` This overrides the default setting of `0`, and now our development builds will use more optimizations. Not as much as a release build, but a little bit more. For the full list of settings and the defaults for each profile, see Cargo's documentation. at *http://doc.crates.io/* ## Publishing a Crate to Crates.io We've added crates from crates.io as dependencies of our project. We can choose to share our code for other people to use as well. Crates.io distributes the source code of your packages, so it is primarily used to distribute code that's open source. Rust and Cargo have some features that can make your published package easier for people to find and use. We'll talk about some of those features, then cover how to publish a package. ### Documentation Comments In Chapter 3, we saw comments in Rust that start with `//`. Rust also has a second kind of comment: the *documentation comment*. While comments can be useful if someone is reading your code, you can generate HTML documentation that displays the contents of documentation comments for public API items meant for someone who's interested in knowing how to *use* your crate, as opposed to how your crate is *implemented*. Note that documentation is only generated for library crates, since binary crates don't have a public API that people need to know how to use. Documentation comments use `///` instead of `//` and support Markdown notation inside. They go just before the item they are documenting. Here's documentation comments for an `add_one` function:
Filename: src/lib.rs ````rust /// Adds one to the number given. /// /// # Examples /// /// ``` /// let five = 5; /// /// assert_eq!(6, add_one(5)); /// # fn add_one(x: i32) -> i32 { /// # x + 1 /// # } /// ``` pub fn add_one(x: i32) -> i32 { x + 1 } ````
Listing 14-1: A documentation comment for a function
`cargo doc` runs a tool distributed with Rust, `rustdoc`, to generate HTML documentation from these comments. To try this out locally, you can run `cargo doc --open`, which will build the documentation for your current crate (as well as all of your crate's dependencies) and open it in a web browser. Navigate to the `add_one` function and you'll see how the text in the documentation comments gets rendered. Adding examples in code blocks in your documentation comments is a way to clearly demonstrate how to use your library. There's an additional bonus reason to do this: `cargo test` will run the code examples in your documentation as tests! Nothing is better than documentation with examples. Nothing is worse than examples that don't actually work because the code has changed since the documentation has been written. Try running `cargo test` with the documentation for the `add_one` function in Listing 14-1; you'll see a section in the test results like this: ```test Doc-tests add-one running 1 test test add_one_0 ... ok test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured ``` Try changing the function or the example to see that `cargo test` will catch that the example no longer works! There's another style of doc comment, `//!`, to comment containing items (e.g. crates, modules or functions), instead of the items following it. These are typically used inside the crate root (lib.rs) or a module's root (mod.rs) to document the crate or the module as a whole, respectively. Here's the documentation within the `libstd` module that contains the entire standard library: ``` //! # The Rust Standard Library //! //! The Rust Standard Library provides the essential runtime //! functionality for building portable Rust software. ``` ### Exporting a Convenient Public API with `pub use` In Chapter 7, we covered how to organize our code into modules with the `mod` keyword, how to make items public with the `pub` keyword, and how to bring items into a scope with the `use` keyword. When publishing a crate for people unfamiliar with the implementation to use, it's worth taking time to consider if the structure of your crate that's useful for you as you're developing is what would be useful for people depending on your crate. If the structure isn't convenient to use from another library, you don't have to rearrange your internal organization: you can choose to re-export items to make a different public structure with `pub use`. For example, say that we made a library named `art` consisting of a `kinds` module containing an enum named `Color` and a `utils` module containing a function named `mix` as shown in Listing 14-2:
Filename: src/lib.rs ```rust //! # Art //! //! A library for modeling artistic concepts. pub mod kinds { /// The primary colors according to the RYB color model. pub enum PrimaryColor { Red, Yellow, Blue, } /// The secondary colors according to the RYB color model. pub enum SecondaryColor { Orange, Green, Purple, } } pub mod utils { use kinds::*; /// Combines two primary colors in equal amounts to create /// a secondary color. pub fn mix(c1: PrimaryColor, c2: PrimaryColor) -> SecondaryColor { // ...snip... } } ```
Listing 14-2: An `art` library with items organized into `kinds` and `utils` modules
In order to use this library, another crate would have `use` statements as in Listing 14-3:
Filename: src/main.rs ```rust,ignore extern crate art; use art::kinds::PrimaryColor; use art::utils::mix; fn main() { let red = PrimaryColor::Red; let yellow = PrimaryColor::Yellow; mix(red, yellow); } ```
Listing 14-3: A program using the `art` crate's items with its internal structure exported
Users of this crate shouldn't need to know that `PrimaryColor` and `SecondaryColor` are in the `kinds` module, and `mix` is in the `utils` module; that structure might be useful for internal organization but doesn't have much meaning from the outside looking in. To change this, we can add the following `pub use` statements to the code from Listing 14-2 to re-export the types at the top level, as shown in Listing 14-4:
Filename: src/lib.rs ```rust //! # Art //! //! A library for modeling artistic concepts. pub use kinds::PrimaryColor; pub use kinds::SecondaryColor; pub use utils::mix; pub mod kinds { // ...snip... ```
Listing 14-4: Adding `pub use` statements to re-export items
Re-exports are listed and linked on the front page of the crate's API documentation. Users of the `art` crate can still see and choose to use the internal structure as in Listing 14-3, or they can use the more convenient structure from Listing 14-4, as shown in Listing 14-5:
Filename: src/main.rs ```rust,ignore extern crate art; use art::PrimaryColor; use art::mix; fn main() { // ...snip... } ```
Listing 14-5: Using the re-exported items from the `art` crate
Creating a useful public API structure is more of an art than a science. Choosing `pub use` gives you flexibility in how you expose your crate's internal structure to users. Take a look at some of the code of crates you've installed to see if their internal structure differs from their public API. ### Before Your First Publish Before being able to publish any crates, you'll need to create an account on crates.io at *https://crates.io* and get an API token. To do so, visit the home page at *https://crates.io* and log in via a GitHub account. A GitHub account is a requirement for now, but the site might support other ways of creating an account in the future. Once you're logged in, visit your Account Settings at *https://crates.io/me* page and run the `cargo login` command with the API key as the page specifies, which will look something like this: ```text $ cargo login abcdefghijklmnopqrstuvwxyz012345 ``` This command will inform Cargo of your API token and store it locally in *~/.cargo/config*. Note that this token is a **secret** and should not be shared with anyone else. If it gets shared with anyone for any reason, you should regenerate it immediately. ### Before Publishing a New Crate First, your crate will need a unique name. While you're working on a crate locally, you may name a crate whatever you'd like, but crate names on crates.io at *https://crates.io* are allocated on a first-come-first- serve basis. Once a crate name is taken, it cannot be used for another crate, so check on the site that the name you'd like is available. If you try to publish a crate as generated by `cargo new`, you'll get a warning and then an error: ```text $ cargo publish Updating registry `https://github.com/rust-lang/crates.io-index` warning: manifest has no description, license, license-file, documentation, homepage or repository. ...snip... error: api errors: missing or empty metadata fields: description, license. Please see http://doc.crates.io/manifest.html#package-metadata for how to upload metadata ``` We can include more information about our package in *Cargo.toml*. Some of these fields are optional, but a description and a license are required in order to publish so that people will know what your crate does and under what terms they may use it. The description appears with your crate in search results and on your crate's page. Descriptions are usually a sentence or two. The `license` field takes a license identifier value, and the possible values have been specified by the Linux Foundation's Software Package Data Exchange (SPDX) at *http://spdx.org/licenses/*. If you would like to use a license that doesn't appear there, instead of the `license` key, you can use `license-file` to specify the name of a file in your project that contains the text of the license you want to use. Guidance on which license is right for your project is out of scope for this book. Many people in the Rust community choose to license their projects in the same way as Rust itself, with a dual license of `MIT/Apache-2.0`, which demonstrates that you can specify multiple license identifiers separated by a slash. So the *Cargo.toml* for a project that is ready to publish might look like this: ```toml [package] name = "guessing_game" version = "0.1.0" authors = ["Your Name "] description = "A fun game where you guess what number the computer has chosen." license = "MIT/Apache-2.0" [dependencies] ``` Be sure to check out the documentation on crates.io at *http://doc.crates.io/manifest.html#package-metadata* that describes other metadata you can specify to ensure your crate can be discovered and used more easily! ### Publishing to Crates.io Now that we've created an account, saved our API token, chosen a name for our crate, and specified the required metadata, we're ready to publish! Publishing a crate is when a specific version is uploaded to be hosted on crates.io. Take care when publishing a crate, because a publish is **permanent**. The version can never be overwritten, and the code cannot be deleted. However, there is no limit to the number of versions which can be published. Let's run the `cargo publish` command, which should succeed this time since we've now specified the required metadata: ```text $ cargo publish Updating registry `https://github.com/rust-lang/crates.io-index` Packaging guessing_game v0.1.0 (file:///projects/guessing_game) Verifying guessing_game v0.1.0 (file:///projects/guessing_game) Compiling guessing_game v0.1.0 (file:///projects/guessing_game/target/package/guessing_game-0.1.0) Finished debug [unoptimized + debuginfo] target(s) in 0.19 secs Uploading guessing_game v0.1.0 (file:///projects/guessing_game) ``` Congratulations! You've now shared your code with the Rust community, and anyone can easily add your crate as a dependency to their project. ### Publishing a New Version of an Existing Crate When you've made changes to your crate and are ready to release a new version, change the `version` value specified in your *Cargo.toml*. Use the Semantic Versioning rules at *http://semver.org/* to decide what an appropriate next version number is based on the kinds of changes you've made. Then run `cargo publish` to upload the new version. ### Removing Versions from Crates.io with `cargo yank` Occasions may arise where you publish a version of a crate that actually ends up being broken for one reason or another, such as a syntax error or forgetting to include a file. For situations such as this, Cargo supports *yanking* a version of a crate. Marking a version of a crate as yanked means that no projects will be able to start depending on that version, but all existing projects that depend on that version will continue to be allowed to download and depend on that version. One of the major goals of crates.io is to act as a permanent archive of code so that builds of all projects will continue to work, and allowing deletion of a version would go against this goal. Essentially, a yank means that all projects with a *Cargo.lock* will not break, while any future *Cargo.lock* files generated will not use the yanked version. A yank **does not** delete any code. The yank feature is not intended for deleting accidentally uploaded secrets, for example. If that happens, you must reset those secrets immediately. To yank a version of a crate, run `cargo yank` and specify which version you want to yank: ```text $ cargo yank --vers 1.0.1 ``` You can also undo a yank, and allow projects to start depending on a version again, by adding `--undo` to the command: ```text $ cargo yank --vers 1.0.1 --undo ``` ## Cargo Workspaces In Chapter 12, we built a package that included both a binary crate and a library crate. But what if the library crate continues to get bigger and we want to split our package up further into multiple library crates? As packages grow, separating out major components can be quite useful. In this situation, Cargo has a feature called *workspaces* that can help us 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 where the code will be trivial so that we can concentrate on the structure of a workspace. We'll have a binary that uses two libraries: one that will provide an `add_one` method and a second that will provide an `add_two` method. Let's start by creating a new crate for the binary: ```text $ cargo new --bin adder Created binary (application) `adder` project $ cd adder ``` We need to modify the binary package's *Cargo.toml* to tell Cargo the `adder` package is a workspace. Add this at the bottom of the file: ```toml [workspace] ``` Like many Cargo features, workspaces support convention over configuration: we don't need to say anything more than this as long as we follow the convention. The convention is that any crates that we depend on as sub-directories will be part of the workspace. Let's add a path dependency to the `adder` crate by changing the `[dependencies]` section of *Cargo.toml* to look like this: ```toml [dependencies] add-one = { path = "add-one" } ``` If we add dependencies that don't have a `path` specified, those will be normal dependencies that aren't in this workspace. Next, generate the `add-one` crate within the `adder` directory: ```text $ cargo new add-one Created library `add-one` project ``` Your `adder` directory should now have these directories and files: ```text ├── Cargo.toml ├── add-one │   ├── Cargo.toml │   └── src │   └── lib.rs └── src └── main.rs ``` In *add-one/src/lib.rs*, let's add an implementation of an `add_one` function: Filename: add-one/src/lib.rs ```rust pub fn add_one(x: i32) -> i32 { x + 1 } ``` Open up *src/main.rs* for `adder` and add an `extern crate` line to bring the new `add-one` library crate into scope, and change the `main` function to use the `add_one` function: ```rust,ignore extern crate add_one; fn main() { let num = 10; println!("Hello, world! {} plus one is {}!", num, add_one::add_one(num)); } ``` Let's build it! ```text $ cargo build Compiling add-one v0.1.0 (file:///projects/adder/add-one) Compiling adder v0.1.0 (file:///projects/adder) Finished debug [unoptimized + debuginfo] target(s) in 0.68 secs ``` Note that running `cargo build` in the *adder* directory built both that crate and the `add-one` crate in *adder/add-one*, but created only one *Cargo.lock* and one *target* directory, both in the *adder* directory. See if you can add an `add-two` crate in the same way. Let's now say that we'd like to use the `rand` crate in our `add-one` crate. As usual, we'll add it to the `[dependencies]` section in the `Cargo.toml` for that crate: Filename: add-one/Cargo.toml ```toml [dependencies] rand = "0.3.14" ``` And if we add `extern crate rand;` to *add-one/src/lib.rs* then run `cargo build`, it will succeed: ```text $ 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 debug [unoptimized + debuginfo] target(s) in 10.18 secs ``` The top level *Cargo.lock* now contains information about the dependency `add-one` has 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: ```text $ 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 and indicate that `rand` is a dependency for the `adder` crate. For another enhancement, let's add a test of the `add_one::add_one` function within that crate: Filename: add-one/src/lib.rs ```rust 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: ```text $ cargo test Compiling adder v0.1.0 (file:///projects/adder) Finished debug [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 the tests for the top level crate. To run tests for the other crates, we need to use the `-p` argument to indicate we want to run tests for a particular package: ```text $ cargo test -p add-one Finished debug [unoptimized + debuginfo] target(s) in 0.0 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 Doc-tests add-one running 0 tests test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured ``` Similarly, if you choose to publish the workspace to crates.io, each crate in the workspace will get published separately. As your project grows, consider 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. ## Installing Binaries from Crates.io with `cargo install` The `cargo install` command allows you to install and use binary crates locally. This isn't intended to replace system packages; it's meant to be a convenient way for Rust developers to install tools that others have shared on crates.io. Only packages which have binary targets can be installed, and all binaries are installed into the installation root's *bin* folder. If you installed Rust using *rustup.rs* and don't have any custom configurations, this will be `$HOME/.cargo/bin`. Add that directory to your `$PATH` to be able to run programs you've gotten through `cargo install`. For example, we mentioned in Chapter 12 that there's a Rust implementation of the `grep` tool for searching files called `ripgrep`. If we want to install `ripgrep`, we can run: ```text $ cargo install ripgrep Updating registry `https://github.com/rust-lang/crates.io-index` Downloading ripgrep v0.3.2 ...snip... Compiling ripgrep v0.3.2 Finished release [optimized + debuginfo] target(s) in 97.91 secs Installing ~/.cargo/bin/rg ``` The last line of the output shows the location and the name of the installed binary, which in the case of `ripgrep` is named `rg`. As long as the installation directory is in our `$PATH` as mentioned above, we can then run `rg --help` and start using a faster, rustier tool for searching files! ## Extending Cargo with Custom Commands Cargo is designed to be extensible with new subcommands without having to modify Cargo itself. If a binary in your `$PATH` is named `cargo-something`, you can run it as if it were a Cargo subcommand by running `cargo something`. Custom commands like this are also listed when you run `cargo --list`. It's convenient to `cargo install` extensions to Cargo then be able to run them just like the built-in Cargo tools! ## Summary Sharing code with Cargo and crates.io is part of what makes the Rust ecosystem useful for many different tasks. Rust's standard library is small and stable, but crates are easy to share, use, and improve on a different timeline than the language itself. Don't be shy about sharing code that's useful to you on crates.io; it's likely that it will be useful to someone else as well!