New upstream version 1.22.1+dfsg1

[rustc.git] / src / doc / book / second-edition / nostarch / chapter14.md

[TOC]

# More about Cargo and Crates.io

So far we’ve used only the most basic features of Cargo to build, run, and test
our code, but it can do a lot more. Here we’ll go over some of its other, more
advanced features to show you how to:

* Customize your build through release profiles
* Publish libraries on crates.io
* Organize larger projects with workspaces
* Install binaries from crates.io
* Extend Cargo with your own custom commands

Cargo can do even more than what we can cover in this chapter too, so for a
full explanation, see its documentation at *https://doc.rust-lang.org/cargo/*.

## Customizing Builds with Release Profiles

In Rust *release profiles* are pre-defined, and customizable, profiles with
different configurations, to allow the programmer more control over various
options for compiling your code. Each profile is configured independently of
the others.

Cargo has two main profiles you should know about: the `dev` profile Cargo uses
when you run `cargo build`, and the `release` profile Cargo uses when you run
`cargo build --release`. The `dev` profile is defined with good defaults for
developing, and likewise the `release` profile has good defaults for release
builds.

These names may be familiar from the output of your builds, which shows the
profile used in the build:

```
$ cargo build
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
$ cargo build --release
    Finished release [optimized] target(s) in 0.0 secs
```

The “dev” and “release” notifications here indicate that the compiler is using
different profiles.

### Customizing Release Profiles

Cargo has default settings for each of the profiles that apply when there
aren’t any `[profile.*]` sections in the project’s *Cargo.toml* file. By adding
`[profile.*]` sections for any profile we want to customize, we can choose to
override any subset of the default settings. For example, here are the default
values for the `opt-level` setting for the `dev` and `release` profiles:

Filename: Cargo.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, with a range of zero to three. Applying more optimizations makes
compilation take longer, so if you’re in development and compiling very often,
you’d want compiling to be fast at the expense of the resulting code running
slower. That’s why the default `opt-level` for `dev` is `0`. When you’re ready
to release, it’s better to spend more time compiling. You’ll only be compiling
in release mode once, and running the compiled program many times, so release
mode trades longer compile time for code that runs faster. That’s why the
default `opt-level` for the `release` profile is `3`.

We can choose to override any default setting by adding a different value for
them in *Cargo.toml*. If we wanted to use optimization level 1 in the
development profile, for example, we can add these two lines to our project’s
*Cargo.toml*:

Filename: Cargo.toml

```
[profile.dev]
opt-level = 1
```

This overrides the default setting of `0`. Now when we run `cargo build`, Cargo
will use the defaults for the `dev` profile plus our customization to
`opt-level`. Because we set `opt-level` to `1`, Cargo will apply more
optimizations than the default, but not as many as a release build.

For the full list of configuration options and defaults for each profile, see
Cargo’s documentation at *https://doc.rust-lang.org/cargo/*.

## Publishing a Crate to Crates.io

We’ve used packages from crates.io as dependencies of our project, but you can
also share your code for other people to use by publishing your own packages.
Crates.io distributes the source code of your packages, so it primarily hosts
code that’s open source.

Rust and Cargo have features that help 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.

### Making Useful Documentation Comments

Accurately documenting your packages will help other users know how and when to
use them, so it’s worth spending some time to write documentation. In Chapter
3, we discussed how to comment Rust code with `//`. Rust also has particular
kind of comment for documentation, known conveniently as *documentation
comments*, that will generate HTML documentation. The HTML displays the
contents of documentation comments for public API items, intended for
programmers interested in knowing how to *use* your crate, as opposed to how
your crate is *implemented*.

Documentation comments use `///` instead of `//` and support Markdown notation
for formatting the text if you’d like. You place documentation comments just
before the item they are documenting. Listing 14-1 shows documentation comments
for an `add_one` function in a crate named `my_crate`:

Filename: src/lib.rs

```
/// Adds one to the number given.
///
/// # Examples
///
/// ```
/// let five = 5;
///
/// assert_eq!(6, my_crate::add_one(5));
/// ```
pub fn add_one(x: i32) -> i32 {
    x + 1
}
```

Listing 14-1: A documentation comment for a function

Here, we give a description of what the `add_one` function does, then start a
section with the heading “Examples”, and code that demonstrates how to use the
`add_one` function. We can generate the HTML documentation from this
documentation comment by running `cargo doc`. This command runs the `rustdoc`
tool distributed with Rust and puts the generated HTML documentation in the
*target/doc* directory.

For convenience, running `cargo doc --open` will build the HTML for your
current crate’s documentation (as well as the documentation for all of your
crate’s dependencies) and open the result in a web browser. Navigate to the
`add_one` function and you’ll see how the text in the documentation comments
gets rendered, shown here in Figure 14-2:

<img alt="Rendered HTML documentation for the `add_one` function of `my_crate`" src="img/trpl14-03.png" class="center" />

Figure 14-2: HTML documentation for the `add_one` function

#### Commonly Used Sections

We used the `# Examples` markdown heading in Listing 14-1 to create a section
in the HTML with the title “Examples”. Some other sections that crate authors
commonly use in their documentation include:

* **Panics**: The scenarios in which this function could `panic!`. Callers of
  this function who don’t want their programs to panic should make sure that
  they don’t call this function in these situations.
* **Errors**: If this function returns a `Result`, describing the kinds of
  errors that might occur and what conditions might cause those errors to be
  returned can be helpful to callers so that they can write code to handle the
  different kinds of errors in different ways.
* **Safety**: If this function is `unsafe` to call (we will discuss unsafety in
  Chapter 19), there should be a section explaining why the function is unsafe
  and covering the invariants that this function expects callers to uphold.

Most documentation comment sections don’t need all of these sections, but this
is a good list to check to remind you of the kinds of things that people
calling your code will be interested in knowing about.

#### Documentation Comments as Tests

Adding examples in code blocks in your documentation comments is a way to
clearly demonstrate how to use your library, but it has an additional bonus:
running `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 like in Listing 14-1; you should see a section in
the test results like this:

```
   Doc-tests my_crate

running 1 test
test src/lib.rs - add_one (line 5) ... ok

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

Now try changing either the function or the example so that the `assert_eq!` in
the example will panic. Run `cargo test` again, and you’ll see that the doc
tests catch that the example and the code are out of sync from one another!

#### Commenting Contained Items

There’s another style of doc comment, `//!`, that adds documentation to the
item that contains the comments, rather than adding documentation to the items
following the comments. These are typically used inside the crate root file
(*src/lib.rs* by convention) or inside a module to document the crate or the
module as a whole.

For example, if we wanted to add documentation that described the purpose of
the `my_crate` crate that contains the `add_one` function, we can add
documentation comments that start with `//!` to the beginning of *src/lib.rs*
as shown in Listing 14-3:

Filename: src/lib.rs

```
//! # My Crate
//!
//! `my_crate` is a collection of utilities to make performing certain
//! calculations more convenient.

/// Adds one to the number given.
// ...snip...
```

Listing 14-3: Documentation for the `my_crate` crate as a whole

Notice there isn’t any code after the last line that begins with `//!`. Because
we started the comments with `//!` instead of `///`, we’re documenting the item
that contains this comment rather than an item that follows this comment. In
this case, the item that contains this comment is the *src/lib.rs* file, which
is the crate root. These comments describe the entire crate.

If we run `cargo doc --open`, we’ll see these comments displayed on the front
page of the documentation for `my_crate` above the list of public items in the
crate, as shown in Figure 14-4:

<img alt="Rendered HTML documentation with a comment for the crate as a whole" src="img/trpl14-05.png" class="center" />

Figure 14-4: Rendered documentation for `my_crate` including the comment
describing the crate as a whole

Documentation comments within items are useful for describing crates and
modules especially. Use them to talk about the purpose of the container overall
to help users of your crate understand your organization.

#### 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. The structure that makes sense to
you while you’re developing a crate may not be very convenient for your users,
however. You may wish to organize your structs in a hierarchy containing
multiple levels, but people that want to use a type you’ve defined deep in the
hierarchy might have trouble finding out that those types exist. They might
also be annoyed at having to type `use
my_crate::some_module::another_module::UsefulType;` rather than `use
my_crate::UsefulType;`.

The structure of your public API is a major consideration when publishing a
crate. People who use your crate are less familiar with the structure than you
are, and might have trouble finding the pieces they want to use if the module
hierarchy is large.

The good news is that, if the structure *isn’t* convenient for others to use
from another library, you don’t have to rearrange your internal organization:
you can choose to re-export items to make a public structure that’s different
to your private structure, using `pub use`. Re-exporting takes a public item in
one location and makes it public in another location as if it was defined in
the other location instead.

For example, say we made a library named `art` for modeling artistic concepts.
Within this library is a `kinds` module containing two enums named
`PrimaryColor` and `SecondaryColor` and a `utils` module containing a function
named `mix` as shown in Listing 14-5:

Filename: src/lib.rs

```
//! # 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 14-5: An `art` library with items organized into `kinds` and `utils`
modules

The front page of the documentation for this crate generated by `cargo doc`
would look like Figure 14-6:

<img alt="Rendered documentation for the `art` crate that lists the `kinds` and `utils` modules" src="img/trpl14-07.png" class="center" />

Figure 14-6: Front page of the documentation for `art` that lists the `kinds`
and `utils` modules

Note that the `PrimaryColor` and `SecondaryColor` types aren’t listed on the
front page, nor is the `mix` function. We have to click on `kinds` and `utils`
in order to see them.

Another crate depending on this library would need `use` statements that import
the items from `art` including specifying the module structure that’s currently
defined. Listing 14-7 shows an example of a crate that uses the `PrimaryColor`
and `mix` items from the `art` crate:

Filename: src/main.rs

```
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 14-7: A crate using the `art` crate’s items with its internal structure
exported

The author of the code in Listing 14-7 that uses the `art` crate had to figure
out that `PrimaryColor` is in the `kinds` module and `mix` is in the `utils`
module. The module structure of the `art` crate is more relevant to developers
working on the `art` crate than developers using the `art` crate. The internal
structure that organizes parts of the crate into the `kinds` module and the
`utils` module doesn’t add any useful information to someone trying to
understand how to use the `art` crate. The `art` crate’s module structure adds
confusion in having to figure out where to look and inconvenience in having to
specify the module names in the `use` statements.

To remove the internal organization from the public API, we can take the `art`
crate code from Listing 14-5 and add `pub use` statements to re-export the
items at the top level, as shown in Listing 14-8:

Filename: src/lib.rs

```
//! # Art
//!
//! A library for modeling artistic concepts.

pub use kinds::PrimaryColor;
pub use kinds::SecondaryColor;
pub use utils::mix;

pub mod kinds {
    // ...snip...
}

pub mod utils {
    // ...snip...
}
```

Listing 14-8: Adding `pub use` statements to re-export items

The API documentation generated with `cargo doc` for this crate will now list
and link re-exports on the front page as shown in Figure 14-9, which makes
these types easier to find.

<img alt="Rendered documentation for the `art` crate with the re-exports on the front page" src="img/trpl14-10.png" class="center" />

Figure 14-9: Front page of the documentation for `art` that lists the
re-exports

Users of the `art` crate can still see and choose to use the internal structure
as in Listing 14-7, or they can use the more convenient structure from Listing
14-8, as shown in Listing 14-10:

Filename: src/main.rs

```
extern crate art;

use art::PrimaryColor;
use art::mix;

fn main() {
    // ...snip...
}
```

Listing 14-10: A program using the re-exported items from the `art` crate

In cases where there are many nested modules, re-exporting the types at the top
level with `pub use` can make a big difference in the experience of people who
use the crate.

Creating a useful public API structure is more of an art than a science, and
you can iterate to find the API that works best for your users. Choosing `pub
use` gives you flexibility in how you structure your crate internally, and
decouples that internal structure with what you present to your 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.

### Setting up a Crates.io Account

Before you can publish any crates, you need to create an account on 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—the GitHub account is a requirement for now, but
the site may support other ways of creating an account in the future. Once
you’re logged in, visit your account settings at *https://crates.io/me* and
retrieve your API key. Then run the `cargo login` command with your API key,
like this:

```
$ cargo login abcdefghijklmnopqrstuvwxyz012345
```

This command will inform Cargo of your API token and store it locally in
*~/.cargo/credentials*. Note that this token is a *secret* and should not be
shared with anyone else. If it is shared with anyone for any reason, you should
revoke it and generate a new token on Crates.io.

### Before Publishing a New Crate

Now you have an account, and let’s say you already have a crate you want to
publish. Before publishing, you’ll need to add some metadata to your crate by
adding it to the `[package]` section of the crate’s *Cargo.toml*.

Your crate will first need a unique name. While you’re working on a crate
locally, you may name a crate whatever you’d like. However, crate names on
Crates.io are allocated on a first-come-first-serve basis. Once a crate name is
taken, no one else may publish a crate with that name. Search for the name
you’d like to use on the site to find out if it has been taken. If it hasn’t,
edit the name in *Cargo.toml* under `[package]` to have the name you want to
use for publishing like so:

Filename: Cargo.toml

```
[package]
name = "guessing_game"
```

Even if you’ve chosen a unique name, if you try to run `cargo publish` to
publish the crate at this point, you’ll get a warning and then an error:

```
$ 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.
```

This is because we’re missing some crucial information: a description and
license are required so that people will know what your crate does and under
what terms they may use it. To rectify this error, we need to include this
information in *Cargo.toml*.

Make a description that’s just a sentence or two, as it will appear with your
crate in search results and on your crate’s page. For the `license` field, you
need to give a *license identifier value*. The Linux Foundation’s Software
Package Data Exchange (SPDX) at *http://spdx.org/licenses/* lists the
identifiers you can use for this value. For example, to specify that you’ve
licensed your crate using the MIT License, add the `MIT` identifier:

Filename: Cargo.toml

```
[package]
name = "guessing_game"
license = "MIT"
```

If you want to use a license that doesn’t appear in the SPDX, you need to place
the text of that license in a file, include the file in your project, then use
`license-file` to specify the name of that file instead of using the `license`
key.

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`—this
demonstrates that you can also specify multiple license identifiers separated
by a slash.

So, with a unique name, the version, and author details that `cargo new` added
when you created the crate, your description, and the license you chose added,
the *Cargo.toml* for a project that’s ready to publish might look like this:

Filename: Cargo.toml

```
[package]
name = "guessing_game"
version = "0.1.0"
authors = ["Your Name <you@example.com>"]
description = "A fun game where you guess what number the computer has chosen."
license = "MIT/Apache-2.0"

[dependencies]
```

Cargo’s documentation at *https://doc.rust-lang.org/cargo/* describes other
metadata you can specify to ensure your crate can be discovered and used more
easily!

### Publishing to Crates.io

Now that you’ve created an account, saved your API token, chosen a name for
your crate, and specified the required metadata, you’re ready to publish!
Publishing a crate uploads a specific version to crates.io for others to use.

Take care when publishing a crate, because a publish is *permanent*. The
version can never be overwritten, and the code cannot be deleted. One major
goal of Crates.io is to act as a permanent archive of code so that builds of
all projects that depend on crates from Crates.io will continue to work.
Allowing deletion of versions would make fulfilling that goal impossible.
However, there is no limit to the number of versions of a crate you can publish.

Let’s run the `cargo publish` command again. It should succeed now:

```
$ 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 dev [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 of 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,
you change the `version` value specified in your *Cargo.toml* and republish.
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`

While you can’t remove previous versions of a crate, you can prevent any future
projects from adding them as a new dependency. This is useful when a version of
a crate ends up being broken for one reason or another. For situations such as
this, Cargo supports *yanking* a version of a crate.

Yanking a version prevents new projects from starting to depend on that version
while allowing all existing projects that depend on it to continue to download
and depend on that version. 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.

To yank a version of a crate, run `cargo yank` and specify which version you
want to yank:

```
$ 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:

```
$ cargo yank --vers 1.0.1 --undo
```

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.

## Cargo Workspaces

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:

Filename: Cargo.toml

```
[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.

### Specifying Workspace Dependencies

By default, Cargo will include all transitive path dependencies. A *path
dependency* is when any crate, whether in a workspace or not, specifies 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, or if the crate is itself part of a workspace, 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:

Filename: Cargo.toml

```
[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.

### Creating the Second Crate in the Workspace

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

```
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 14-11:

Filename: src/main.rs

```
extern crate add_one;

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

Listing 14-11: 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.

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

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.

#### 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

```
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.

## 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 that have binary targets can be installed. A binary
target is the runnable program that gets created if the crate has a
*src/main.rs* or another file specified as a binary, as opposed to a library
target that isn’t runnable on its own but is suitable for including within
other programs. Usually, crates have information in the *README* file about
whether a crate is a library, has a binary target, or both.

All binaries from `cargo install` are put into the installation root’s *bin*
folder. If you installed Rust using *rustup.rs* and don’t have any custom
configurations, this will be `$HOME/.cargo/bin`. Ensure that directory is in
your `$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:

```
$ 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 `rg`. As long as the installation
directory is in your `$PATH` as mentioned above, you can then run `rg --help`
and start using a faster, rustier tool for searching files!

## Extending Cargo with Custom Commands

Cargo is designed so you can extend it 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`. Being
able to `cargo install` extensions and then run them just like the built-in
Cargo tools is a super convenient benefit of Cargo’s design!

## 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 timeline different from 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!