rustdoc supports executing your documentation examples as tests. This makes sure that your tests are up to date and working.

The basic idea is this:

/// # Examples
///
/// ```
/// let x = 5;
/// ```

The triple backticks start and end code blocks. If this were in a file named foo.rs, running rustdoc --test foo.rs will extract this example, and then run it as a test.

Please note that by default, if no language is set for the block code, rustdoc assumes it is Rust code. So the following:

```rust
let x = 5;
```

is strictly equivalent to:

```
let x = 5;
```

There's some subtlety though! Read on for more details.

Like regular unit tests, regular doctests are considered to "pass" if they compile and run without panicking. So if you want to demonstrate that some computation gives a certain result, the assert! family of macros works the same as other Rust code:

# #![allow(unused_variables)]
#fn main() {
let foo = "foo";

assert_eq!(foo, "foo");
#}

This way, if the computation ever returns something different, the code panics and the doctest fails.

In the example above, you'll note something strange: there's no main function! Forcing you to write main for every example, no matter how small, adds friction. So rustdoc processes your examples slightly before running them. Here's the full algorithm rustdoc uses to preprocess examples:

1. Some common allow attributes are inserted, including unused_variables, unused_assignments, unused_mut, unused_attributes, and dead_code. Small examples often trigger these lints.
2. Any attributes specified with #![doc(test(attr(...)))] are added.
3. Any leading #![foo] attributes are left intact as crate attributes.
4. If the example does not contain extern crate, and #![doc(test(no_crate_inject))] was not specified, then extern crate <mycrate>; is inserted (note the lack of #[macro_use]).
5. Finally, if the example does not contain fn main, the remainder of the text is wrapped in fn main() { your_code }.

For more about that caveat in rule 4, see "Documenting Macros" below.

Sometimes, you need some setup code, or other things that would distract from your example, but are important to make the tests work. Consider an example block that looks like this:

/// /// Some documentation.
/// # fn foo() {} // this function will be hidden
/// println!("Hello, World!");

It will render like this:

# #![allow(unused_variables)]
#fn main() {
/// Some documentation.
# fn foo() {}
println!("Hello, World!");
#}

Yes, that's right: you can add lines that start with #, and they will be hidden from the output, but will be used when compiling your code. You can use this to your advantage. In this case, documentation comments need to apply to some kind of function, so if I want to show you just a documentation comment, I need to add a little function definition below it. At the same time, it's only there to satisfy the compiler, so hiding it makes the example more clear. You can use this technique to explain longer examples in detail, while still preserving the testability of your documentation.

For example, imagine that we wanted to document this code:

# #![allow(unused_variables)]
#fn main() {
let x = 5;
let y = 6;
println!("{}", x + y);
#}

We might want the documentation to end up looking like this:

First, we set x to five:

# #![allow(unused_variables)]
#fn main() {
let x = 5;
# let y = 6;
# println!("{}", x + y);
#}

Next, we set y to six:

# #![allow(unused_variables)]
#fn main() {
# let x = 5;
let y = 6;
# println!("{}", x + y);
#}

Finally, we print the sum of x and y:

# #![allow(unused_variables)]
#fn main() {
# let x = 5;
# let y = 6;
println!("{}", x + y);
#}

To keep each code block testable, we want the whole program in each block, but we don't want the reader to see every line every time. Here's what we put in our source code:

First, we set `x` to five:

```
let x = 5;
# let y = 6;
# println!("{}", x + y);
```

Next, we set `y` to six:

```
# let x = 5;
let y = 6;
# println!("{}", x + y);
```

Finally, we print the sum of `x` and `y`:

```
# let x = 5;
# let y = 6;
println!("{}", x + y);
```

By repeating all parts of the example, you can ensure that your example still compiles, while only showing the parts that are relevant to that part of your explanation.

The #-hiding of lines can be prevented by using two consecutive hashes ##. This only needs to be done with the first # which would've otherwise caused hiding. If we have a string literal like the following, which has a line that starts with a #:

# #![allow(unused_variables)]
#fn main() {
let s = "foo
## bar # baz";
#}

We can document it by escaping the initial #:

/// let s = "foo
/// ## bar # baz";

When writing an example, it is rarely useful to include a complete error handling, as it would add significant amounts of boilerplate code. Instead, you may want the following:

/// ```
/// use std::io;
/// let mut input = String::new();
/// io::stdin().read_line(&mut input)?;
/// ```

The problem is that ? returns a Result<T, E> and test functions don't return anything, so this will give a mismatched types error.

You can get around this limitation by manually adding a main that returns Result<T, E>, because Result<T, E> implements the Termination trait:

/// A doc test using ?
///
/// ```
/// use std::io;
///
/// fn main() -> io::Result<()> {
///     let mut input = String::new();
///     io::stdin().read_line(&mut input)?;
///     Ok(())
/// }
/// ```

Together with the # from the section above, you arrive at a solution that appears to the reader as the initial idea but works with doc tests:

/// ```
/// use std::io;
/// # fn main() -> io::Result<()> {
/// let mut input = String::new();
/// io::stdin().read_line(&mut input)?;
/// # Ok(())
/// # }
/// ```

As of version 1.34.0, one can also omit the fn main(), but you will have to disambiguate the error type:

/// ```
/// use std::io;
/// let mut input = String::new();
/// io::stdin().read_line(&mut input)?;
/// # Ok::<(), io::Error>(())
/// ```

This is an unfortunate consequence of the ? operator adding an implicit conversion, so type inference fails because the type is not unique. Please note that you must write the (()) in one sequence without intermediate whitespace so that rustdoc understands you want an implicit Result-returning function.

As of version 1.37.0, this simplification also works with Options, which can be handy to test e.g. iterators or checked arithmetic, for example:

/// ```
/// let _ = &[].iter().next()?;
///# Some(())
/// ```

Note that the result must be a Some(()) and this has to be written in one go. In this case disambiguating the result isn't required.

Here’s an example of documenting a macro:

/// Panic with a given message unless an expression evaluates to true.
///
/// # Examples
///
/// ```
/// # #[macro_use] extern crate foo;
/// # fn main() {
/// panic_unless!(1 + 1 == 2, “Math is broken.”);
/// # }
/// ```
///
/// ```should_panic
/// # #[macro_use] extern crate foo;
/// # fn main() {
/// panic_unless!(true == false, “I’m broken.”);
/// # }
/// ```
#[macro_export]
macro_rules! panic_unless {
    ($condition:expr, $($rest:expr),+) => ({ if ! $condition { panic!($($rest),+); } });
}
# fn main() {}

You’ll note three things: we need to add our own extern crate line, so that we can add the #[macro_use] attribute. Second, we’ll need to add our own main() as well (for reasons discussed above). Finally, a judicious use of # to comment out those two things, so they don’t show up in the output.

There are a few annotations that are useful to help rustdoc do the right thing when testing your code:

# #![allow(unused_variables)]
#fn main() {
/// ```ignore
/// fn foo() {
/// ```
# fn foo() {}
#}

The ignore directive tells Rust to ignore your code. This is almost never what you want, as it's the most generic. Instead, consider annotating it with text if it's not code, or using #s to get a working example that only shows the part you care about.

# #![allow(unused_variables)]
#fn main() {
/// ```should_panic
/// assert!(false);
/// ```
# fn foo() {}
#}

should_panic tells rustdoc that the code should compile correctly, but not actually pass as a test.

/// ```no_run
/// loop {
///     println!("Hello, world");
/// }
/// ```
# fn foo() {}

The no_run attribute will compile your code, but not run it. This is important for examples such as "Here's how to retrieve a web page," which you would want to ensure compiles, but might be run in a test environment that has no network access.

/// ```compile_fail
/// let x = 5;
/// x += 2; // shouldn't compile!
/// ```

compile_fail tells rustdoc that the compilation should fail. If it compiles, then the test will fail. However please note that code failing with the current Rust release may work in a future release, as new features are added.

/// Only runs on the 2018 edition.
///
/// ```edition2018
/// let result: Result<i32, ParseIntError> = try {
///     "1".parse::<i32>()?
///         + "2".parse::<i32>()?
///         + "3".parse::<i32>()?
/// };
/// ```

edition2018 tells rustdoc that the code sample should be compiled the 2018 edition of Rust. Similarly, you can specify edition2015 to compile the code with the 2015 edition.

The exact syntax for code blocks, including the edge cases, can be found in the Fenced Code Blocks section of the CommonMark specification.

Rustdoc also accepts indented code blocks as an alternative to fenced code blocks: instead of surrounding your code with three backticks, you can indent each line by four or more spaces.

    let foo = "foo";
    assert_eq!(foo, "foo");

These, too, are documented in the CommonMark specification, in the Indented Code Blocks section.

However, it's preferable to use fenced code blocks over indented code blocks. Not only are fenced code blocks considered more idiomatic for Rust code, but there is no way to use directives such as ignore or should_panic with indented code blocks.