3 Documentation is an important part of any software project, and it's
4 first-class in Rust. Let's talk about the tooling Rust gives you to
9 The Rust distribution includes a tool, `rustdoc`, that generates documentation.
10 `rustdoc` is also used by Cargo through `cargo doc`.
12 Documentation can be generated in two ways: from source code, and from
13 standalone Markdown files.
15 ## Documenting source code
17 The primary way of documenting a Rust project is through annotating the source
18 code. You can use documentation comments for this purpose:
21 /// Constructs a new `Rc<T>`.
28 /// let five = Rc::new(5);
30 pub fn new(value: T) -> Rc<T> {
31 // implementation goes here
35 This code generates documentation that looks [like this][rc-new]. I've left the
36 implementation out, with a regular comment in its place.
38 The first thing to notice about this annotation is that it uses
39 `///` instead of `//`. The triple slash
40 indicates a documentation comment.
42 Documentation comments are written in Markdown.
44 Rust keeps track of these comments, and uses them when generating
45 documentation. This is important when documenting things like enums:
48 /// The `Option` type. See [the module level documentation](index.html) for more.
57 The above works, but this does not:
60 /// The `Option` type. See [the module level documentation](index.html) for more.
63 Some(T), /// Some value `T`
70 hello.rs:4:1: 4:2 error: expected ident, found `}`
75 This [unfortunate error](https://github.com/rust-lang/rust/issues/22547) is
76 correct; documentation comments apply to the thing after them, and there's
77 nothing after that last comment.
79 [rc-new]: https://doc.rust-lang.org/nightly/std/rc/struct.Rc.html#method.new
81 ### Writing documentation comments
83 Anyway, let's cover each part of this comment in detail:
86 /// Constructs a new `Rc<T>`.
90 The first line of a documentation comment should be a short summary of its
91 functionality. One sentence. Just the basics. High level.
95 /// Other details about constructing `Rc<T>`s, maybe describing complicated
96 /// semantics, maybe additional options, all kinds of stuff.
101 Our original example had just a summary line, but if we had more things to say,
102 we could have added more explanation in a new paragraph.
104 #### Special sections
106 Next, are special sections. These are indicated with a header, `#`. There
107 are four kinds of headers that are commonly used. They aren't special syntax,
108 just convention, for now.
115 Unrecoverable misuses of a function (i.e. programming errors) in Rust are
116 usually indicated by panics, which kill the whole current thread at the very
117 least. If your function has a non-trivial contract like this, that is
118 detected/enforced by panics, documenting it is very important.
125 If your function or method returns a `Result<T, E>`, then describing the
126 conditions under which it returns `Err(E)` is a nice thing to do. This is
127 slightly less important than `Panics`, because failure is encoded into the type
128 system, but it's still a good thing to do.
135 If your function is `unsafe`, you should explain which invariants the caller is
136 responsible for upholding.
144 /// let five = Rc::new(5);
149 Fourth, `Examples`. Include one or more examples of using your function or
150 method, and your users will love you for it. These examples go inside of
151 code block annotations, which we'll talk about in a moment, and can have
152 more than one section:
157 /// Simple `&str` patterns:
160 /// let v: Vec<&str> = "Mary had a little lamb".split(' ').collect();
161 /// assert_eq!(v, vec!["Mary", "had", "a", "little", "lamb"]);
164 /// More complex patterns with a lambda:
167 /// let v: Vec<&str> = "abc1def2ghi".split(|c: char| c.is_numeric()).collect();
168 /// assert_eq!(v, vec!["abc", "def", "ghi"]);
173 Let's discuss the details of these code blocks.
175 #### Code block annotations
177 To write some Rust code in a comment, use the triple graves:
181 /// println!("Hello, world");
186 If you want something that's not Rust code, you can add an annotation:
190 /// printf("Hello, world\n");
195 This will highlight according to whatever language you're showing off.
196 If you're only showing plain text, choose `text`.
198 It's important to choose the correct annotation here, because `rustdoc` uses it
199 in an interesting way: It can be used to actually test your examples in a
200 library crate, so that they don't get out of date. If you have some C code but
201 `rustdoc` thinks it's Rust because you left off the annotation, `rustdoc` will
202 complain when trying to generate the documentation.
204 ## Documentation as tests
206 Let's discuss our sample example documentation:
210 /// println!("Hello, world");
215 You'll notice that you don't need a `fn main()` or anything here. `rustdoc` will
216 automatically add a `main()` wrapper around your code, using heuristics to attempt
217 to put it in the right place. For example:
223 /// let five = Rc::new(5);
228 This will end up testing:
233 let five = Rc::new(5);
237 Here's the full algorithm rustdoc uses to preprocess examples:
239 1. Any leading `#![foo]` attributes are left intact as crate attributes.
240 2. Some common `allow` attributes are inserted, including
241 `unused_variables`, `unused_assignments`, `unused_mut`,
242 `unused_attributes`, and `dead_code`. Small examples often trigger
244 3. If the example does not contain `extern crate`, then `extern crate
245 <mycrate>;` is inserted (note the lack of `#[macro_use]`).
246 4. Finally, if the example does not contain `fn main`, the remainder of the
247 text is wrapped in `fn main() { your_code }`.
249 This generated `fn main` can be a problem! If you have `extern crate` or a `mod`
250 statements in the example code that are referred to by `use` statements, they will
251 fail to resolve unless you include at least `fn main() {}` to inhibit step 4.
252 `#[macro_use] extern crate` also does not work except at the crate root, so when
253 testing macros an explicit `main` is always required. It doesn't have to clutter
254 up your docs, though -- keep reading!
256 Sometimes this algorithm isn't enough, though. For example, all of these code samples
257 with `///` we've been talking about? The raw text:
260 /// Some documentation.
264 looks different than the output:
267 /// Some documentation.
271 Yes, that's right: you can add lines that start with `# `, and they will
272 be hidden from the output, but will be used when compiling your code. You
273 can use this to your advantage. In this case, documentation comments need
274 to apply to some kind of function, so if I want to show you just a
275 documentation comment, I need to add a little function definition below
276 it. At the same time, it's only there to satisfy the compiler, so hiding
277 it makes the example more clear. You can use this technique to explain
278 longer examples in detail, while still preserving the testability of your
281 For example, imagine that we wanted to document this code:
286 println!("{}", x + y);
289 We might want the documentation to end up looking like this:
291 > First, we set `x` to five:
296 > # println!("{}", x + y);
299 > Next, we set `y` to six:
304 > # println!("{}", x + y);
307 > Finally, we print the sum of `x` and `y`:
312 > println!("{}", x + y);
315 To keep each code block testable, we want the whole program in each block, but
316 we don't want the reader to see every line every time. Here's what we put in
320 First, we set `x` to five:
325 # println!("{}", x + y);
328 Next, we set `y` to six:
333 # println!("{}", x + y);
336 Finally, we print the sum of `x` and `y`:
341 println!("{}", x + y);
345 By repeating all parts of the example, you can ensure that your example still
346 compiles, while only showing the parts that are relevant to that part of your
349 ### Documenting macros
351 Here’s an example of documenting a macro:
354 /// Panic with a given message unless an expression evaluates to true.
359 /// # #[macro_use] extern crate foo;
361 /// panic_unless!(1 + 1 == 2, “Math is broken.”);
366 /// # #[macro_use] extern crate foo;
368 /// panic_unless!(true == false, “I’m broken.”);
372 macro_rules! panic_unless {
373 ($condition:expr, $($rest:expr),+) => ({ if ! $condition { panic!($($rest),+); } });
378 You’ll note three things: we need to add our own `extern crate` line, so that
379 we can add the `#[macro_use]` attribute. Second, we’ll need to add our own
380 `main()` as well (for reasons discussed above). Finally, a judicious use of
381 `#` to comment out those two things, so they don’t show up in the output.
383 Another case where the use of `#` is handy is when you want to ignore
384 error handling. Lets say you want the following,
388 /// let mut input = String::new();
389 /// try!(io::stdin().read_line(&mut input));
392 The problem is that `try!` returns a `Result<T, E>` and test functions
393 don't return anything so this will give a mismatched types error.
396 /// A doc test using try!
400 /// # fn foo() -> io::Result<()> {
401 /// let mut input = String::new();
402 /// try!(io::stdin().read_line(&mut input));
409 You can get around this by wrapping the code in a function. This catches
410 and swallows the `Result<T, E>` when running tests on the docs. This
411 pattern appears regularly in the standard library.
413 ### Running documentation tests
415 To run the tests, either:
418 $ rustdoc --test path/to/my/crate/root.rs
423 That's right, `cargo test` tests embedded documentation too. **However,
424 `cargo test` will not test binary crates, only library ones.** This is
425 due to the way `rustdoc` works: it links against the library to be tested,
426 but with a binary, there’s nothing to link to.
428 There are a few more annotations that are useful to help `rustdoc` do the right
429 thing when testing your code:
438 The `ignore` directive tells Rust to ignore your code. This is almost never
439 what you want, as it's the most generic. Instead, consider annotating it
440 with `text` if it's not code, or using `#`s to get a working example that
441 only shows the part you care about.
450 `should_panic` tells `rustdoc` that the code should compile correctly, but
451 not actually pass as a test.
456 /// println!("Hello, world");
462 The `no_run` attribute will compile your code, but not run it. This is
463 important for examples such as "Here's how to start up a network service,"
464 which you would want to make sure compile, but might run in an infinite loop!
466 ### Documenting modules
468 Rust has another kind of doc comment, `//!`. This comment doesn't document the next item, but the enclosing item. In other words:
472 //! This is documentation for the `foo` module.
480 This is where you'll see `//!` used most often: for module documentation. If
481 you have a module in `foo.rs`, you'll often open its code and see this:
484 //! A module for using `foo`s.
486 //! The `foo` module contains a lot of useful functionality blah blah blah
489 ### Documentation comment style
491 Check out [RFC 505][rfc505] for full conventions around the style and format of
494 [rfc505]: https://github.com/rust-lang/rfcs/blob/master/text/0505-api-comment-conventions.md
496 ## Other documentation
498 All of this behavior works in non-Rust source files too. Because comments
499 are written in Markdown, they're often `.md` files.
501 When you write documentation in Markdown files, you don't need to prefix
502 the documentation with comments. For example:
510 /// let five = Rc::new(5);
523 let five = Rc::new(5);
527 when it's in a Markdown file. There is one wrinkle though: Markdown files need
528 to have a title like this:
533 This is the example documentation.
536 This `%` line needs to be the very first line of the file.
540 At a deeper level, documentation comments are syntactic sugar for documentation
551 are the same, as are these:
559 You won't often see this attribute used for writing documentation, but it
560 can be useful when changing some options, or when writing a macro.
564 `rustdoc` will show the documentation for a public re-export in both places:
572 This will create documentation for `bar` both inside the documentation for the
573 crate `foo`, as well as the documentation for your crate. It will use the same
574 documentation in both places.
576 This behavior can be suppressed with `no_inline`:
585 ## Missing documentation
587 Sometimes you want to make sure that every single public thing in your project
588 is documented, especially when you are working on a library. Rust allows you to
589 to generate warnings or errors, when an item is missing documentation.
590 To generate warnings you use `warn`:
593 #![warn(missing_docs)]
596 And to generate errors you use `deny`:
599 #![deny(missing_docs)]
602 There are cases where you want to disable these warnings/errors to explicitly
603 leave something undocumented. This is done by using `allow`:
606 #[allow(missing_docs)]
610 You might even want to hide items from the documentation completely:
619 You can control a few aspects of the HTML that `rustdoc` generates through the
620 `#![doc]` version of the attribute:
623 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
624 html_favicon_url = "https://www.rust-lang.org/favicon.ico",
625 html_root_url = "https://doc.rust-lang.org/")]
628 This sets a few different options, with a logo, favicon, and a root URL.
630 ### Configuring documentation tests
632 You can also configure the way that `rustdoc` tests your documentation examples
633 through the `#![doc(test(..))]` attribute.
636 #![doc(test(attr(allow(unused_variables), deny(warnings))))]
639 This allows unused variables within the examples, but will fail the test for any
640 other lint warning thrown.
642 ## Generation options
644 `rustdoc` also contains a few other options on the command line, for further customization:
646 - `--html-in-header FILE`: includes the contents of FILE at the end of the
647 `<head>...</head>` section.
648 - `--html-before-content FILE`: includes the contents of FILE directly after
649 `<body>`, before the rendered content (including the search bar).
650 - `--html-after-content FILE`: includes the contents of FILE after all the rendered content.
654 The Markdown in documentation comments is placed without processing into
655 the final webpage. Be careful with literal HTML:
658 /// <script>alert(document.cookie)</script>