Feature Name: More API Documentation Conventions
Start Date: 2016-03-31
RFC PR: rust-lang/rfcs#1574
Rust Issue: N/A

Summary

RFC 505 introduced certain conventions around documenting Rust projects. This RFC augments that one, and a full text of the older one combined with these modifications is provided below.

Motivation

Documentation is an extremely important part of any project. It’s important that we have consistency in our documentation.

For the most part, the RFC proposes guidelines that are already followed today, but it tries to motivate and clarify them.

Detailed design

English

This section applies to rustc and the standard library.

Using Markdown

The updated list of common headings is:

Examples
Panics
Errors
Safety
Aborts
Undefined Behavior

RFC 505 suggests that one should always use the rust formatting directive:

```rust
println!("Hello, world!");
```

```ruby
puts "Hello"
```

But, in API documentation, feel free to rely on the default being ‘rust’:

/// For example:
///
/// ```
/// let x = 5;
/// ```

Other places do not know how to highlight this anyway, so it’s not important to be explicit.

RFC 505 suggests that references and citation should be linked ‘reference style.’ This is still recommended, but prefer to leave off the second []:

[Rust website]

[Rust website]: http://www.rust-lang.org

[Rust website][website]

[website]: http://www.rust-lang.org

But, if the text is very long, it is okay to use this form.

Examples in API docs

Everything should have examples. Here is an example of how to do examples:

/// # Examples
///
/// ```
/// use op;
///
/// let s = "foo";
/// let answer = op::compare(s, "bar");
/// ```
///
/// Passing a closure to compare with, rather than a string:
///
/// ```
/// use op;
///
/// let s = "foo";
/// let answer = op::compare(s, |a| a.chars().is_whitespace().all());
/// ```

When talking about a type, use its full name. In other words, if the type is generic, say Option<T>, not Option. An exception to this is bounds. Write Cow<'a, B> rather than Cow<'a, B> where B: 'a + ToOwned + ?Sized.

Another possibility is to write in lower case using a more generic term. In other words, ‘string’ can refer to a String or an &str, and ‘an option’ can be ‘an Option<T>’.

Link all the things

A major drawback of Markdown is that it cannot automatically link types in API documentation. Do this yourself with the reference-style syntax, for ease of reading:

/// The [`String`] passed in lorum ipsum...
///
/// [`String`]: ../string/struct.String.html

Module-level vs type-level docs

There has often been a tension between module-level and type-level documentation. For example, in today’s standard library, the various *Cell docs say, in the pages for each type, to “refer to the module-level documentation for more details.”

Instead, module-level documentation should show a high-level summary of everything in the module, and each type should document itself fully. It is okay if there is some small amount of duplication here. Module-level documentation should be broad and not go into a lot of detail. That is left to the type’s documentation.

Example

Below is a full crate, with documentation following these rules. I am loosely basing this off of my ref_slice crate, because it’s small, but I’m not claiming the code is good here. It’s about the docs, not the code.

In lib.rs:

//! Turning references into slices
//!
//! This crate contains several utility functions for taking various kinds
//! of references and producing slices out of them. In this case, only full
//! slices, not ranges for sub-slices.
//!
//! # Layout
//!
//! At the top level, we have functions for working with references, `&T`.
//! There are two submodules for dealing with other types: `option`, for
//! &[`Option<T>`], and `mut`, for `&mut T`.
//!
//! [`Option<T>`]: http://doc.rust-lang.org/std/option/enum.Option.html

pub mod option;

/// Converts a reference to `T` into a slice of length 1.
///
/// This will not copy the data, only create the new slice.
///
/// # Panics
///
/// In this case, the code won’t panic, but if it did, the circumstances
/// in which it would would be included here.
///
/// # Examples
///
/// ```
/// extern crate ref_slice;
/// use ref_slice::ref_slice;
/// 
/// let x = &5;
///
/// let slice = ref_slice(x);
///
/// assert_eq!(&[5], slice);
/// ```
///
/// A more complex example. In this case, it’s the same example, because this
/// is a pretty trivial function, but use your imagination.
///
/// ```
/// extern crate ref_slice;
/// use ref_slice::ref_slice;
/// 
/// let x = &5;
///
/// let slice = ref_slice(x);
///
/// assert_eq!(&[5], slice);
/// ```
pub fn ref_slice<T>(s: &T) -> &[T] {
    unimplemented!()
}

/// Functions that operate on mutable references.
///
/// This submodule mirrors the parent module, but instead of dealing with `&T`,
/// they’re for `&mut T`.
mod mut {
    /// Converts a reference to `&mut T` into a mutable slice of length 1.
    ///
    /// This will not copy the data, only create the new slice.
    ///
    /// # Safety
    ///
    /// In this case, the code doesn’t need to be marked as unsafe, but if it
    /// did, the invariants you’re expected to uphold would be documented here.
    ///
    /// # Examples
    ///
    /// ```
    /// extern crate ref_slice;
    /// use ref_slice::mut;
    /// 
    /// let x = &mut 5;
    ///
    /// let slice = mut::ref_slice(x);
    ///
    /// assert_eq!(&mut [5], slice);
    /// ```
    pub fn ref_slice<T>(s: &mut T) -> &mut [T] {
        unimplemented!()
    }
}

in option.rs:

//! Functions that operate on references to [`Option<T>`]s.
//!
//! This submodule mirrors the parent module, but instead of dealing with `&T`,
//! they’re for `&`[`Option<T>`].
//!
//! [`Option<T>`]: http://doc.rust-lang.org/std/option/enum.Option.html

/// Converts a reference to `Option<T>` into a slice of length 0 or 1.
///
/// [`Option<T>`]: http://doc.rust-lang.org/std/option/enum.Option.html
///
/// This will not copy the data, only create the new slice.
///
/// # Examples
///
/// ```
/// extern crate ref_slice;
/// use ref_slice::option;
/// 
/// let x = &Some(5);
///
/// let slice = option::ref_slice(x);
///
/// assert_eq!(&[5], slice);
/// ```
///
/// `None` will result in an empty slice:
///
/// ```
/// extern crate ref_slice;
/// use ref_slice::option;
/// 
/// let x: &Option<i32> = &None;
///
/// let slice = option::ref_slice(x);
///
/// assert_eq!(&[], slice);
/// ```
pub fn ref_slice<T>(opt: &Option<T>) -> &[T] {
    unimplemented!()
}

Drawbacks

It’s possible that RFC 505 went far enough, and something this detailed is inappropriate.

Alternatives

We could stick with the more minimal conventions of the previous RFC.

Unresolved questions

None.

Appendix A: Full conventions text

Below is a combination of RFC 505 + this RFC’s modifications, for convenience.

Summary sentence

In API documentation, the first line should be a single-line short sentence providing a summary of the code. This line is used as a summary description throughout Rustdoc’s output, so it’s a good idea to keep it short.

The summary line should be written in third person singular present indicative form. Basically, this means write ‘Returns’ instead of ‘Return’.

English

This section applies to rustc and the standard library.

All documentation for the standard library is standardized on American English, with regards to spelling, grammar, and punctuation conventions. Language changes over time, so this doesn’t mean that there is always a correct answer to every grammar question, but there is often some kind of formal consensus.

Use line comments

Avoid block comments. Use line comments instead:

// Wait for the main task to return, and set the process error code
// appropriately.

Instead of:

/*
 * Wait for the main task to return, and set the process error code
 * appropriately.
 */

Only use inner doc comments //! to write crate and module-level documentation, nothing else. When using mod blocks, prefer /// outside of the block:

/// This module contains tests
mod tests {
    // ...
}

over

mod tests {
    //! This module contains tests

    // ...
}

Using Markdown

Within doc comments, use Markdown to format your documentation.

Use top level headings (#) to indicate sections within your comment. Common headings:

Examples
Panics
Errors
Safety
Aborts
Undefined Behavior

An example:

/// # Examples

Even if you only include one example, use the plural form: ‘Examples’ rather than ‘Example’. Future tooling is easier this way.

Use backticks (`) to denote a code fragment within a sentence.

Use triple backticks (```) to write longer examples, like this:

This code does something cool.

```rust
let x = foo();

x.bar();
```

When appropriate, make use of Rustdoc’s modifiers. Annotate triple backtick blocks with the appropriate formatting directive.

```rust
println!("Hello, world!");
```

```ruby
puts "Hello"
```

In API documentation, feel free to rely on the default being ‘rust’:

/// For example:
///
/// ```
/// let x = 5;
/// ```

In long-form documentation, always be explicit:

For example:

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

This will highlight syntax in places that do not default to ‘rust’, like GitHub.

Rustdoc is able to test all Rust examples embedded inside of documentation, so it’s important to mark what is not Rust so your tests don’t fail.

References and citation should be linked ‘reference style.’ Prefer

[Rust website]

[Rust website]: http://www.rust-lang.org

[Rust website](http://www.rust-lang.org)

If the text is very long, feel free to use the shortened form:

This link [is very long and links to the Rust website][website].

[website]: http://www.rust-lang.org