Documentation/rust/coding-guidelines.rst - linux - Git at Google

 .. 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 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:

 .. code-block:: rust

 	// `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.:

 .. code-block:: rust

 	// 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.:

 .. 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 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.:

 .. code-block:: rust

 	// 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:

 	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 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:

 	https://doc.rust-lang.org/rustdoc/how-to-write-documentation.html


 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 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:

 .. 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``.
	.. 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 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:

	.. code-block:: rust

	// `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.:

	.. code-block:: rust

	// 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.:

	.. 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 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.:

	.. code-block:: rust

	// 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:

	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 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:

	https://doc.rust-lang.org/rustdoc/how-to-write-documentation.html


	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 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:

	.. 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``.