.. SPDX-License-Identifier: GPL-2.0 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 personcontributing from time to time to the kernel does not need to learn andremember one more style guide. More importantly, reviewers and maintainersdo not need to spend time pointing out style issues anymore, and thusless 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 Ruststyle is followed. For instance, 4 spaces are used for indentation ratherthan tabs. It is convenient to instruct editors/IDEs to format while typing,when saving or at commit time. However, if for some reason reformattingthe entire kernel Rust sources is needed at some point, the following can berun:: 	make LLVM=1 rustfmt It is also possible to check if everything is formatted (printing a diffotherwise), for instance for a CI, with:: 	make LLVM=1 rustfmtcheck Like ``clang-format`` for the rest of the kernel, ``rustfmt`` works onindividual files, and does not require a kernel configuration. Sometimes it mayeven work with broken code.  Comments-------- "Normal" comments (i.e. ``//``, rather than code documentation which startswith ``///`` or ``//!``) are written in Markdown the same way as documentationcomments are, even though they will not be rendered. This improves consistency,simplifies the rules and allows to move content between the two kinds ofcomments more easily. For instance: .. code-block:: rust 	// `object` is ready to be handled now.	f(object); Furthermore, just like documentation, comments are capitalized at the beginningof a sentence and ended with a period (even if it is a single sentence). Thisincludes ``// SAFETY:``, ``// TODO:`` and other "tagged" comments, e.g.: .. code-block:: rust 	// FIXME: The error should be handled properly. Comments should not be used for documentation purposes: comments are intendedfor implementation details, not users. This distinction is useful even if thereader 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 tothe line of documentation to be commented. For any other case, comments arewritten after the documentation, e.g.: .. code-block:: rust 	/// 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 appearbefore every ``unsafe`` block, and they explain why the code inside the block iscorrect/sound, i.e. why it cannot trigger undefined behavior in any case, e.g.: .. code-block:: rust 	// SAFETY: `p` is valid by the safety requirements.	unsafe { *p = 0; } ``// SAFETY:`` comments are not to be confused with the ``# Safety`` sectionsin 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) actuallyrespects the preconditions stated in a ``# Safety`` section or the languagereference.  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: 	https://commonmark.org/help/ This is how a well-documented Rust function may look like: .. code-block:: rust 	/// 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 followedin 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: 	https://doc.rust-lang.org/rustdoc/how-to-write-documentation.html In addition, the kernel supports creating links relative to the source tree byprefixing the link destination with ``srctree/``. For instance: .. code-block:: rust 	//! C header: [`include/linux/printk.h`](srctree/include/linux/printk.h) or: .. code-block:: rust 	/// [`struct mutex`]: srctree/include/linux/mutex.h  Naming------ Rust kernel code follows the usual Rust naming conventions: 	https://rust-lang.github.io/api-guidelines/naming.html When existing C concepts (e.g. macros, functions, objects...) are wrapped intoa Rust abstraction, a name as close as reasonably possible to the C side shouldbe used in order to avoid confusion and to improve readability when switchingback 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 namingconventions, and namespacing introduced by modules and types should not berepeated in the item names. For instance, when wrapping constants like: .. code-block:: c 	#define GPIO_LINE_DIRECTION_IN	0	#define GPIO_LINE_DIRECTION_OUT	1 The equivalent in Rust may look like (ignoring documentation): .. code-block:: rust 	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 givenfunction, module, block, etc. It is similar to ``#pragma GCC diagnostic push`` + ``ignored`` + ``pop`` in C[#]_: .. code-block:: c 	#pragma GCC diagnostic push	#pragma GCC diagnostic ignored "-Wunused-function"	static void f(void) {}	#pragma GCC diagnostic pop .. [#] In this particular case, the kernel's ``__{always,maybe}_unused``       attributes (C23's ``[[maybe_unused]]``) may be used; however, the example       is meant to reflect the equivalent lint in Rust discussed afterwards. But way less verbose: .. code-block:: rust 	#[allow(dead_code)]	fn f() {} By that virtue, it makes it possible to comfortably enable more diagnostics bydefault (i.e. outside ``W=`` levels). In particular, those that may have somefalse positives but that are otherwise quite useful to keep enabled to catchpotential 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, thefollowing will ensure that, when ``f()`` is called somewhere, we will have toremove the attribute: .. code-block:: rust 	#[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 ``expect``\ s do not get forgotten when they are not needed, whichmay 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 ``allow``\ s and reduces thechance 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: .. code-block:: rust 	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? .. code-block:: rust 	#[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 thatconfiguration. Therefore, in cases like this, we cannot use ``expect`` as-is. A simple possibility is using ``allow``: .. code-block:: rust 	#[allow(dead_code)]	fn g() {} 	fn main() {	    #[cfg(CONFIG_X)]	    g();	} An alternative would be using a conditional ``expect``: .. code-block:: rust 	#[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 codeanymore. However, the ``cfg_attr`` is more complex than a simple ``allow``. Therefore, it is likely that it is not worth using conditional ``expect``\ s whenmore than one or two configurations are involved or when the lint may betriggered due to non-local changes (such as ``dead_code``). For more information about diagnostics in Rust, please see: 	https://doc.rust-lang.org/stable/reference/attributes/diagnostics.html