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1.. SPDX-License-Identifier: GPL-2.0
2
3Coding Guidelines
4=================
5
6This document describes how to write Rust code in the kernel.
7
8
9Style & formatting
10------------------
11
12The code should be formatted using ``rustfmt``. In this way, a person
13contributing from time to time to the kernel does not need to learn and
14remember one more style guide. More importantly, reviewers and maintainers
15do not need to spend time pointing out style issues anymore, and thus
16less patch roundtrips may be needed to land a change.
17
18.. note:: Conventions on comments and documentation are not checked by
19 ``rustfmt``. Thus those are still needed to be taken care of.
20
21The default settings of ``rustfmt`` are used. This means the idiomatic Rust
22style is followed. For instance, 4 spaces are used for indentation rather
23than tabs.
24
25It is convenient to instruct editors/IDEs to format while typing,
26when saving or at commit time. However, if for some reason reformatting
27the entire kernel Rust sources is needed at some point, the following can be
28run::
29
30 make LLVM=1 rustfmt
31
32It is also possible to check if everything is formatted (printing a diff
33otherwise), for instance for a CI, with::
34
35 make LLVM=1 rustfmtcheck
36
37Like ``clang-format`` for the rest of the kernel, ``rustfmt`` works on
38individual files, and does not require a kernel configuration. Sometimes it may
39even work with broken code.
40
41
42Comments
43--------
44
45"Normal" comments (i.e. ``//``, rather than code documentation which starts
46with ``///`` or ``//!``) are written in Markdown the same way as documentation
47comments are, even though they will not be rendered. This improves consistency,
48simplifies the rules and allows to move content between the two kinds of
49comments more easily. For instance:
50
51.. code-block:: rust
52
53 // `object` is ready to be handled now.
54 f(object);
55
56Furthermore, just like documentation, comments are capitalized at the beginning
57of a sentence and ended with a period (even if it is a single sentence). This
58includes ``// SAFETY:``, ``// TODO:`` and other "tagged" comments, e.g.:
59
60.. code-block:: rust
61
62 // FIXME: The error should be handled properly.
63
64Comments should not be used for documentation purposes: comments are intended
65for implementation details, not users. This distinction is useful even if the
66reader of the source file is both an implementor and a user of an API. In fact,
67sometimes it is useful to use both comments and documentation at the same time.
68For instance, for a ``TODO`` list or to comment on the documentation itself.
69For the latter case, comments can be inserted in the middle; that is, closer to
70the line of documentation to be commented. For any other case, comments are
71written after the documentation, e.g.:
72
73.. code-block:: rust
74
75 /// Returns a new [`Foo`].
76 ///
77 /// # Examples
78 ///
79 // TODO: Find a better example.
80 /// ```
81 /// let foo = f(42);
82 /// ```
83 // FIXME: Use fallible approach.
84 pub fn f(x: i32) -> Foo {
85 // ...
86 }
87
88One special kind of comments are the ``// SAFETY:`` comments. These must appear
89before every ``unsafe`` block, and they explain why the code inside the block is
90correct/sound, i.e. why it cannot trigger undefined behavior in any case, e.g.:
91
92.. code-block:: rust
93
94 // SAFETY: `p` is valid by the safety requirements.
95 unsafe { *p = 0; }
96
97``// SAFETY:`` comments are not to be confused with the ``# Safety`` sections
98in code documentation. ``# Safety`` sections specify the contract that callers
99(for functions) or implementors (for traits) need to abide by. ``// SAFETY:``
100comments show why a call (for functions) or implementation (for traits) actually
101respects the preconditions stated in a ``# Safety`` section or the language
102reference.
103
104
105Code documentation
106------------------
107
108Rust kernel code is not documented like C kernel code (i.e. via kernel-doc).
109Instead, the usual system for documenting Rust code is used: the ``rustdoc``
110tool, which uses Markdown (a lightweight markup language).
111
112To learn Markdown, there are many guides available out there. For instance,
113the one at:
114
115 https://commonmark.org/help/
116
117This is how a well-documented Rust function may look like:
118
119.. code-block:: rust
120
121 /// Returns the contained [`Some`] value, consuming the `self` value,
122 /// without checking that the value is not [`None`].
123 ///
124 /// # Safety
125 ///
126 /// Calling this method on [`None`] is *[undefined behavior]*.
127 ///
128 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
129 ///
130 /// # Examples
131 ///
132 /// ```
133 /// let x = Some("air");
134 /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
135 /// ```
136 pub unsafe fn unwrap_unchecked(self) -> T {
137 match self {
138 Some(val) => val,
139
140 // SAFETY: The safety contract must be upheld by the caller.
141 None => unsafe { hint::unreachable_unchecked() },
142 }
143 }
144
145This example showcases a few ``rustdoc`` features and some conventions followed
146in the kernel:
147
148 - The first paragraph must be a single sentence briefly describing what
149 the documented item does. Further explanations must go in extra paragraphs.
150
151 - Unsafe functions must document their safety preconditions under
152 a ``# Safety`` section.
153
154 - While not shown here, if a function may panic, the conditions under which
155 that happens must be described under a ``# Panics`` section.
156
157 Please note that panicking should be very rare and used only with a good
158 reason. In almost all cases, a fallible approach should be used, typically
159 returning a ``Result``.
160
161 - If providing examples of usage would help readers, they must be written in
162 a section called ``# Examples``.
163
164 - Rust items (functions, types, constants...) must be linked appropriately
165 (``rustdoc`` will create a link automatically).
166
167 - Any ``unsafe`` block must be preceded by a ``// SAFETY:`` comment
168 describing why the code inside is sound.
169
170 While sometimes the reason might look trivial and therefore unneeded,
171 writing these comments is not just a good way of documenting what has been
172 taken into account, but most importantly, it provides a way to know that
173 there are no *extra* implicit constraints.
174
175To learn more about how to write documentation for Rust and extra features,
176please take a look at the ``rustdoc`` book at:
177
178 https://doc.rust-lang.org/rustdoc/how-to-write-documentation.html
179
180
181Naming
182------
183
184Rust kernel code follows the usual Rust naming conventions:
185
186 https://rust-lang.github.io/api-guidelines/naming.html
187
188When existing C concepts (e.g. macros, functions, objects...) are wrapped into
189a Rust abstraction, a name as close as reasonably possible to the C side should
190be used in order to avoid confusion and to improve readability when switching
191back and forth between the C and Rust sides. For instance, macros such as
192``pr_info`` from C are named the same in the Rust side.
193
194Having said that, casing should be adjusted to follow the Rust naming
195conventions, and namespacing introduced by modules and types should not be
196repeated in the item names. For instance, when wrapping constants like:
197
198.. code-block:: c
199
200 #define GPIO_LINE_DIRECTION_IN 0
201 #define GPIO_LINE_DIRECTION_OUT 1
202
203The equivalent in Rust may look like (ignoring documentation):
204
205.. code-block:: rust
206
207 pub mod gpio {
208 pub enum LineDirection {
209 In = bindings::GPIO_LINE_DIRECTION_IN as _,
210 Out = bindings::GPIO_LINE_DIRECTION_OUT as _,
211 }
212 }
213
214That is, the equivalent of ``GPIO_LINE_DIRECTION_IN`` would be referred to as
215``gpio::LineDirection::In``. In particular, it should not be named
216``gpio::gpio_line_direction::GPIO_LINE_DIRECTION_IN``.