What is rustdoc?

The standard Rust distribution ships with a tool called rustdoc. Its job is to generate documentation for Rust projects. On a fundamental level, Rustdoc takes as an argument either a crate root or a Markdown file, and produces HTML, CSS, and JavaScript.

Basic usage

Let's give it a try! Create a new project with Cargo:

$ cargo new docs --lib
$ cd docs

In src/lib.rs, Cargo has generated some sample code. Delete it and replace it with this:

#![allow(unused)]
fn main() {
/// foo is a function
fn foo() {}
}

Let's run rustdoc on our code. To do so, we can call it with the path to our crate root like this:

$ rustdoc src/lib.rs

This will create a new directory, doc, with a website inside! In our case, the main page is located in doc/lib/index.html. If you open that up in a web browser, you will see a page with a search bar, and "Crate lib" at the top, with no contents.

Configuring rustdoc

There are two problems with this: first, why does it think that our package is named "lib"? Second, why does it not have any contents?

The first problem is due to rustdoc trying to be helpful; like rustc, it assumes that our crate's name is the name of the file for the crate root. To fix this, we can pass in a command-line flag:

$ rustdoc src/lib.rs --crate-name docs

Now, doc/docs/index.html will be generated, and the page says "Crate docs."

For the second issue, it is because our function foo is not public; rustdoc defaults to generating documentation for only public functions. If we change our code...

#![allow(unused)]
fn main() {
/// foo is a function
pub fn foo() {}
}

... and then re-run rustdoc:

$ rustdoc src/lib.rs --crate-name docs

We now have some generated documentation. Open up doc/docs/index.html and check it out! It should show a link to the foo function's page, which is located at doc/docs/fn.foo.html. On that page, you'll see the "foo is a function" we put inside the documentation comment in our crate.

Using rustdoc with Cargo

Cargo also has integration with rustdoc to make it easier to generate docs. Instead of the rustdoc command, we could have done this:

$ cargo doc

Internally, this calls out to rustdoc like this:

$ rustdoc --crate-name docs src/lib.rs -o <path>/docs/target/doc -L
dependency=<path>/docs/target/debug/deps

You can see this with cargo doc --verbose.

It generates the correct --crate-name for us, as well as pointing to src/lib.rs. But what about those other arguments?

-o controls the output of our docs. Instead of a top-level doc directory, notice that Cargo puts generated documentation under target. That is the idiomatic place for generated files in Cargo projects.
-L flag helps rustdoc find the dependencies your code relies on. If our project used dependencies, we would get documentation for them as well!

Outer and inner documentation

The /// syntax is used to document the item present after it. That's why it is called an outer documentation. There is another syntax: //!, which is used to document the item it is present inside. It is called an inner documentation. It is often used when documenting the entire crate, because nothing comes before it: it is the root of the crate. So in order to document an entire crate, you need to use //! syntax. For example:

#![allow(unused)]
fn main() {
//! This is my first rust crate
}

When used in the crate root, it documents the item it is inside, which is the crate itself.

For more information about the //! syntax, see the Book.

Using standalone Markdown files

rustdoc can also generate HTML from standalone Markdown files. Let' s give it a try: create a README.md file with these contents:

# Docs

This is a project to test out `rustdoc`.

[Here is a link!](https://www.rust-lang.org)

## Example

```rust
fn foo() -> i32 {
    1 + 1
}
```

And call rustdoc on it:

$ rustdoc README.md

You will find an HTML file in docs/doc/README.html generated from its Markdown contents.

Cargo currently does not understand standalone Markdown files, unfortunately.

Summary

This covers the simplest use-cases of rustdoc. The rest of this book will explain all of the options that rustdoc has, and how to use them.

The rustdoc book