3 `rustdoc` supports executing your documentation examples as tests. This makes sure
4 that examples within your documentation are up to date and working.
6 The basic idea is this:
16 The triple backticks start and end code blocks. If this were in a file named `foo.rs`,
17 running `rustdoc --test foo.rs` will extract this example, and then run it as a test.
19 Please note that by default, if no language is set for the block code, `rustdoc`
20 assumes it is `Rust` code. So the following:
28 is strictly equivalent to:
36 There's some subtlety though! Read on for more details.
38 ## Passing or failing a doctest
40 Like regular unit tests, regular doctests are considered to "pass"
41 if they compile and run without panicking.
42 So if you want to demonstrate that some computation gives a certain result,
43 the `assert!` family of macros works the same as other Rust code:
48 assert_eq!(foo, "foo");
51 This way, if the computation ever returns something different,
52 the code panics and the doctest fails.
54 ## Pre-processing examples
56 In the example above, you'll note something strange: there's no `main`
57 function! Forcing you to write `main` for every example, no matter how small,
58 adds friction. So `rustdoc` processes your examples slightly before
59 running them. Here's the full algorithm rustdoc uses to preprocess examples:
61 1. Some common `allow` attributes are inserted, including
62 `unused_variables`, `unused_assignments`, `unused_mut`,
63 `unused_attributes`, and `dead_code`. Small examples often trigger
65 2. Any attributes specified with `#![doc(test(attr(...)))]` are added.
66 3. Any leading `#![foo]` attributes are left intact as crate attributes.
67 4. If the example does not contain `extern crate`, and
68 `#![doc(test(no_crate_inject))]` was not specified, then `extern crate
69 <mycrate>;` is inserted (note the lack of `#[macro_use]`).
70 5. Finally, if the example does not contain `fn main`, the remainder of the
71 text is wrapped in `fn main() { your_code }`.
73 For more about that caveat in rule 4, see "Documenting Macros" below.
75 ## Hiding portions of the example
77 Sometimes, you need some setup code, or other things that would distract
78 from your example, but are important to make the tests work. Consider
79 an example block that looks like this:
82 /// /// Some documentation.
83 /// # fn foo() {} // this function will be hidden
84 /// println!("Hello, World!");
87 It will render like this:
90 /// Some documentation.
92 println!("Hello, World!");
95 Yes, that's right: you can add lines that start with `# `, and they will
96 be hidden from the output, but will be used when compiling your code. You
97 can use this to your advantage. In this case, documentation comments need
98 to apply to some kind of function, so if I want to show you just a
99 documentation comment, I need to add a little function definition below
100 it. At the same time, it's only there to satisfy the compiler, so hiding
101 it makes the example more clear. You can use this technique to explain
102 longer examples in detail, while still preserving the testability of your
105 For example, imagine that we wanted to document this code:
110 println!("{}", x + y);
113 We might want the documentation to end up looking like this:
115 > First, we set `x` to five:
120 > # println!("{}", x + y);
123 > Next, we set `y` to six:
128 > # println!("{}", x + y);
131 > Finally, we print the sum of `x` and `y`:
136 > println!("{}", x + y);
139 To keep each code block testable, we want the whole program in each block, but
140 we don't want the reader to see every line every time. Here's what we put in
144 First, we set `x` to five:
149 # println!("{}", x + y);
152 Next, we set `y` to six:
157 # println!("{}", x + y);
160 Finally, we print the sum of `x` and `y`:
165 println!("{}", x + y);
169 By repeating all parts of the example, you can ensure that your example still
170 compiles, while only showing the parts that are relevant to that part of your
173 The `#`-hiding of lines can be prevented by using two consecutive hashes
174 `##`. This only needs to be done with the first `#` which would've
175 otherwise caused hiding. If we have a string literal like the following,
176 which has a line that starts with a `#`:
183 We can document it by escaping the initial `#`:
191 ## Using `?` in doc tests
193 When writing an example, it is rarely useful to include a complete error
194 handling, as it would add significant amounts of boilerplate code. Instead, you
195 may want the following:
200 /// let mut input = String::new();
201 /// io::stdin().read_line(&mut input)?;
205 The problem is that `?` returns a `Result<T, E>` and test functions don't
206 return anything, so this will give a mismatched types error.
208 You can get around this limitation by manually adding a `main` that returns
209 `Result<T, E>`, because `Result<T, E>` implements the `Termination` trait:
212 /// A doc test using ?
217 /// fn main() -> io::Result<()> {
218 /// let mut input = String::new();
219 /// io::stdin().read_line(&mut input)?;
225 Together with the `# ` from the section above, you arrive at a solution that
226 appears to the reader as the initial idea but works with doc tests:
231 /// # fn main() -> io::Result<()> {
232 /// let mut input = String::new();
233 /// io::stdin().read_line(&mut input)?;
239 As of version 1.34.0, one can also omit the `fn main()`, but you will have to
240 disambiguate the error type:
245 /// let mut input = String::new();
246 /// io::stdin().read_line(&mut input)?;
247 /// # Ok::<(), io::Error>(())
251 This is an unfortunate consequence of the `?` operator adding an implicit
252 conversion, so type inference fails because the type is not unique. Please note
253 that you must write the `(())` in one sequence without intermediate whitespace
254 so that rustdoc understands you want an implicit `Result`-returning function.
256 ## Documenting macros
258 Here’s an example of documenting a macro:
261 /// Panic with a given message unless an expression evaluates to true.
266 /// # #[macro_use] extern crate foo;
268 /// panic_unless!(1 + 1 == 2, “Math is broken.”);
273 /// # #[macro_use] extern crate foo;
275 /// panic_unless!(true == false, “I’m broken.”);
279 macro_rules! panic_unless {
280 ($condition:expr, $($rest:expr),+) => ({ if ! $condition { panic!($($rest),+); } });
285 You’ll note three things: we need to add our own `extern crate` line, so that
286 we can add the `#[macro_use]` attribute. Second, we’ll need to add our own
287 `main()` as well (for reasons discussed above). Finally, a judicious use of
288 `#` to comment out those two things, so they don’t show up in the output.
292 There are a few annotations that are useful to help `rustdoc` do the right
293 thing when testing your code:
302 The `ignore` directive tells Rust to ignore your code. This is almost never
303 what you want, as it's the most generic. Instead, consider annotating it
304 with `text` if it's not code, or using `#`s to get a working example that
305 only shows the part you care about.
314 `should_panic` tells `rustdoc` that the code should compile correctly, but
315 not actually pass as a test.
320 /// println!("Hello, world");
326 The `no_run` attribute will compile your code, but not run it. This is
327 important for examples such as "Here's how to retrieve a web page,"
328 which you would want to ensure compiles, but might be run in a test
329 environment that has no network access.
334 /// x += 2; // shouldn't compile!
338 `compile_fail` tells `rustdoc` that the compilation should fail. If it
339 compiles, then the test will fail. However please note that code failing
340 with the current Rust release may work in a future release, as new features
344 /// Only runs on the 2018 edition.
347 /// let result: Result<i32, ParseIntError> = try {
348 /// "1".parse::<i32>()?
349 /// + "2".parse::<i32>()?
350 /// + "3".parse::<i32>()?
355 `edition2018` tells `rustdoc` that the code sample should be compiled using
356 the 2018 edition of Rust. Similarly, you can specify `edition2015` to compile
357 the code with the 2015 edition.
361 The *exact* syntax for code blocks, including the edge cases, can be found
362 in the [Fenced Code Blocks](https://spec.commonmark.org/0.28/#fenced-code-blocks)
363 section of the CommonMark specification.
365 Rustdoc also accepts *indented* code blocks as an alternative to fenced
366 code blocks: instead of surrounding your code with three backticks, you
367 can indent each line by four or more spaces.
371 assert_eq!(foo, "foo");
374 These, too, are documented in the CommonMark specification, in the
375 [Indented Code Blocks](https://spec.commonmark.org/0.28/#indented-code-blocks)
378 However, it's preferable to use fenced code blocks over indented code blocks.
379 Not only are fenced code blocks considered more idiomatic for Rust code,
380 but there is no way to use directives such as `ignore` or `should_panic` with
381 indented code blocks.
383 ### Include items only when collecting doctests
385 Rustdoc's documentation tests can do some things that regular unit tests can't, so it can
386 sometimes be useful to extend your doctests with samples that wouldn't otherwise need to be in
387 documentation. To this end, Rustdoc allows you to have certain items only appear when it's
388 collecting doctests, so you can utilize doctest functionality without forcing the test to appear in
389 docs, or to find an arbitrary private item to include it on.
391 When compiling a crate for use in doctests (with `--test` option), rustdoc will set `cfg(doctest)`.
392 Note that they will still link against only the public items of your crate; if you need to test
393 private items, you need to write a unit test.
395 In this example, we're adding doctests that we know won't compile, to verify that our struct can
396 only take in valid data:
399 /// We have a struct here. Remember it doesn't accept negative numbers!
400 pub struct MyStruct(pub usize);
403 /// let x = my_crate::MyStruct(-5);
406 pub struct MyStructOnlyTakesUsize;
409 Note that the struct `MyStructOnlyTakesUsize` here isn't actually part of your public crate
410 API. The use of `#[cfg(doctest)]` makes sure that this struct only exists while rustdoc is
411 collecting doctests. This means that its doctest is executed when `--test` is passed to rustdoc,
412 but is hidden from the public documentation.
414 Another possible use of `cfg(doctest)` is to test doctests that are included in your README file
415 without including it in your main documentation. For example, you could write this into your
416 `lib.rs` to test your README as part of your doctests:
419 #![feature(external_doc)]
421 #[doc(include="../README.md")]
423 pub struct ReadmeDoctests;
426 This will include your README as documentation on the hidden struct `ReadmeDoctests`, which will
427 then be tested alongside the rest of your doctests.