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.

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

$ cargo new docs
$ cd docs

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

# #![allow(unused_variables)]
#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'll see a page with a search bar, and "Crate lib" at the top, with no contents. There's 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's because our function foo is not public; rustdoc defaults to generating documentation for only public functions. If we change our code...

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

... and then re-run rustdoc:

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

We'll 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.

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 srclib.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, you'll notice that Cargo puts generated documentation under target. That's the idiomatic place for generated files in Cargo projects. Also, it passes -L, a flag that helps rustdoc find the dependencies your code relies on. If our project used dependencies, we'd get documentation for them as well!

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)

    ## Subheading

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

And call rustdoc on it:

$ rustdoc README.md

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

Cargo currently does not understand standalone Markdown files, unfortunately.

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.