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Coding Guidelines

This document describes how to write Rust code in the kernel.

Style & formatting

The code should be formatted using rustfmt. In this way, a person contributing from time to time to the kernel does not need to learn and remember one more style guide. More importantly, reviewers and maintainers do not need to spend time pointing out style issues anymore, and thus less patch roundtrips may be needed to land a change.

Note

Conventions on comments and documentation are not checked by rustfmt. Thus those are still needed to be taken care of.

The default settings of rustfmt are used. This means the idiomatic Rust style is followed. For instance, 4 spaces are used for indentation rather than tabs.

It is convenient to instruct editors/IDEs to format while typing, when saving or at commit time. However, if for some reason reformatting the entire kernel Rust sources is needed at some point, the following can be run:

make LLVM=1 rustfmt

It is also possible to check if everything is formatted (printing a diff otherwise), for instance for a CI, with:

make LLVM=1 rustfmtcheck

Like clang-format for the rest of the kernel, rustfmt works on individual files, and does not require a kernel configuration. Sometimes it may even work with broken code.

Comments

“Normal” comments (i.e. //, rather than code documentation which starts with /// or //!) are written in Markdown the same way as documentation comments are, even though they will not be rendered. This improves consistency, simplifies the rules and allows to move content between the two kinds of comments more easily. For instance:

// `object` is ready to be handled now.
f(object);

Furthermore, just like documentation, comments are capitalized at the beginning of a sentence and ended with a period (even if it is a single sentence). This includes // SAFETY:, // TODO: and other “tagged” comments, e.g.:

// FIXME: The error should be handled properly.

Comments should not be used for documentation purposes: comments are intended for implementation details, not users. This distinction is useful even if the reader of the source file is both an implementor and a user of an API. In fact, sometimes it is useful to use both comments and documentation at the same time. For instance, for a TODO list or to comment on the documentation itself. For the latter case, comments can be inserted in the middle; that is, closer to the line of documentation to be commented. For any other case, comments are written after the documentation, e.g.:

/// Returns a new [`Foo`].
///
/// # Examples
///
// TODO: Find a better example.
/// ```
/// let foo = f(42);
/// ```
// FIXME: Use fallible approach.
pub fn f(x: i32) -> Foo {
    // ...
}

One special kind of comments are the // SAFETY: comments. These must appear before every unsafe block, and they explain why the code inside the block is correct/sound, i.e. why it cannot trigger undefined behavior in any case, e.g.:

// SAFETY: `p` is valid by the safety requirements.
unsafe { *p = 0; }

// SAFETY: comments are not to be confused with the # Safety sections in code documentation. # Safety sections specify the contract that callers (for functions) or implementors (for traits) need to abide by. // SAFETY: comments show why a call (for functions) or implementation (for traits) actually respects the preconditions stated in a # Safety section or the language reference.

Code documentation

Rust kernel code is not documented like C kernel code (i.e. via kernel-doc). Instead, the usual system for documenting Rust code is used: the rustdoc tool, which uses Markdown (a lightweight markup language).

To learn Markdown, there are many guides available out there. For instance, the one at:

This is how a well-documented Rust function may look like:

/// Returns the contained [`Some`] value, consuming the `self` value,
/// without checking that the value is not [`None`].
///
/// # Safety
///
/// Calling this method on [`None`] is *[undefined behavior]*.
///
/// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
///
/// # Examples
///
/// ```
/// let x = Some("air");
/// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
/// ```
pub unsafe fn unwrap_unchecked(self) -> T {
    match self {
        Some(val) => val,

        // SAFETY: The safety contract must be upheld by the caller.
        None => unsafe { hint::unreachable_unchecked() },
    }
}

This example showcases a few rustdoc features and some conventions followed in the kernel:

  • The first paragraph must be a single sentence briefly describing what the documented item does. Further explanations must go in extra paragraphs.

  • Unsafe functions must document their safety preconditions under a # Safety section.

  • While not shown here, if a function may panic, the conditions under which that happens must be described under a # Panics section.

    Please note that panicking should be very rare and used only with a good reason. In almost all cases, a fallible approach should be used, typically returning a Result.

  • If providing examples of usage would help readers, they must be written in a section called # Examples.

  • Rust items (functions, types, constants...) must be linked appropriately (rustdoc will create a link automatically).

  • Any unsafe block must be preceded by a // SAFETY: comment describing why the code inside is sound.

    While sometimes the reason might look trivial and therefore unneeded, writing these comments is not just a good way of documenting what has been taken into account, but most importantly, it provides a way to know that there are no extra implicit constraints.

To learn more about how to write documentation for Rust and extra features, please take a look at the rustdoc book at:

In addition, the kernel supports creating links relative to the source tree by prefixing the link destination with srctree/. For instance:

//! C header: [`include/linux/printk.h`](srctree/include/linux/printk.h)

or:

/// [`struct mutex`]: srctree/include/linux/mutex.h

Naming

Rust kernel code follows the usual Rust naming conventions:

When existing C concepts (e.g. macros, functions, objects...) are wrapped into a Rust abstraction, a name as close as reasonably possible to the C side should be used in order to avoid confusion and to improve readability when switching back and forth between the C and Rust sides. For instance, macros such as pr_info from C are named the same in the Rust side.

Having said that, casing should be adjusted to follow the Rust naming conventions, and namespacing introduced by modules and types should not be repeated in the item names. For instance, when wrapping constants like:

#define GPIO_LINE_DIRECTION_IN  0
#define GPIO_LINE_DIRECTION_OUT 1

The equivalent in Rust may look like (ignoring documentation):

pub mod gpio {
    pub enum LineDirection {
        In = bindings::GPIO_LINE_DIRECTION_IN as _,
        Out = bindings::GPIO_LINE_DIRECTION_OUT as _,
    }
}

That is, the equivalent of GPIO_LINE_DIRECTION_IN would be referred to as gpio::LineDirection::In. In particular, it should not be named gpio::gpio_line_direction::GPIO_LINE_DIRECTION_IN.

Lints

In Rust, it is possible to allow particular warnings (diagnostics, lints) locally, making the compiler ignore instances of a given warning within a given function, module, block, etc.

It is similar to #pragma GCC diagnostic push + ignored + pop in C [1]:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"
static void f(void) {}
#pragma GCC diagnostic pop

But way less verbose:

#[allow(dead_code)]
fn f() {}

By that virtue, it makes it possible to comfortably enable more diagnostics by default (i.e. outside W= levels). In particular, those that may have some false positives but that are otherwise quite useful to keep enabled to catch potential mistakes.

On top of that, Rust provides the expect attribute which takes this further. It makes the compiler warn if the warning was not produced. For instance, the following will ensure that, when f() is called somewhere, we will have to remove the attribute:

#[expect(dead_code)]
fn f() {}

If we do not, we get a warning from the compiler:

warning: this lint expectation is unfulfilled
 --> x.rs:3:10
  |
3 | #[expect(dead_code)]
  |          ^^^^^^^^^
  |
  = note: `#[warn(unfulfilled_lint_expectations)]` on by default

This means that expects do not get forgotten when they are not needed, which may happen in several situations, e.g.:

  • Temporary attributes added while developing.

  • Improvements in lints in the compiler, Clippy or custom tools which may remove a false positive.

  • When the lint is not needed anymore because it was expected that it would be removed at some point, such as the dead_code example above.

It also increases the visibility of the remaining allows and reduces the chance of misapplying one.

Thus prefer expect over allow unless:

  • Conditional compilation triggers the warning in some cases but not others.

    If there are only a few cases where the warning triggers (or does not trigger) compared to the total number of cases, then one may consider using a conditional expect (i.e. cfg_attr(..., expect(...))). Otherwise, it is likely simpler to just use allow.

  • Inside macros, when the different invocations may create expanded code that triggers the warning in some cases but not in others.

  • When code may trigger a warning for some architectures but not others, such as an as cast to a C FFI type.

As a more developed example, consider for instance this program:

fn g() {}

fn main() {
    #[cfg(CONFIG_X)]
    g();
}

Here, function g() is dead code if CONFIG_X is not set. Can we use expect here?

#[expect(dead_code)]
fn g() {}

fn main() {
    #[cfg(CONFIG_X)]
    g();
}

This would emit a lint if CONFIG_X is set, since it is not dead code in that configuration. Therefore, in cases like this, we cannot use expect as-is.

A simple possibility is using allow:

#[allow(dead_code)]
fn g() {}

fn main() {
    #[cfg(CONFIG_X)]
    g();
}

An alternative would be using a conditional expect:

#[cfg_attr(not(CONFIG_X), expect(dead_code))]
fn g() {}

fn main() {
    #[cfg(CONFIG_X)]
    g();
}

This would ensure that, if someone introduces another call to g() somewhere (e.g. unconditionally), then it would be spotted that it is not dead code anymore. However, the cfg_attr is more complex than a simple allow.

Therefore, it is likely that it is not worth using conditional expects when more than one or two configurations are involved or when the lint may be triggered due to non-local changes (such as dead_code).

For more information about diagnostics in Rust, please see: