`rustfmt`, by default, formats imports in a way that is prone to conflicts
while merging and rebasing, since in some cases it condenses several
items into the same line.
For instance, Linus mentioned [1] that the following case:
use crate::{
fmt,
page::AsPageIter,
};
is compressed by `rustfmt` into:
use crate::{fmt, page::AsPageIter};
which is undesirable.
Similarly, `rustfmt` may put several items in the same line even if the
braces span already multiple lines, e.g.:
use kernel::{
acpi, c_str,
device::{property, Core},
of, platform,
};
The options that control the formatting behavior around imports are
generally unstable, and `rustfmt` releases do not allow to use nightly
features, unlike the compiler and other Rust tooling [2].
For the moment, we can introduce a workaround to prevent `rustfmt`
from compressing the example above -- the "trailing empty comment":
use crate::{
fmt,
page::AsPageIter, //
};
which is reminiscent of the trailing comma behavior in other formatters.
We already used empty comments for formatting purposes in the past,
e.g. in commit b9b701fce4 ("rust: clarify the language unstable features
in use").
In addition, `rustfmt` actually reformats with a vertical layout (i.e. it
does not put two items in the same line) when seeing such a comment,
i.e. it doesn't just preserve the formatting, which is good in the sense
that we can use it to easily reformat some imports, since it matches
the style we generally want to have.
A Git merge driver would help (suggested by Gary and Wedson), though
maintainers would need to set it up, the diffs would still be larger
and the formatting rules for imports would remain hard to predict.
Thus document the style that we will follow in the coding guidelines
by introducing a new section and explain how the trailing empty comment
works there too.
We discussed the issue with upstream Rust in our usual Rust <-> Rust
for Linux meeting [3], and there have also been a few other discussions
in parallel in issues [4][5] and Zulip [6]. We will see what happens,
but upstream Rust has already created a subteam of `rustfmt` to try
to overcome the bandwidth issue [7], which is a good signal, and some
organization work has already started (e.g. tracking issues). We will
continue our discussions with them about it.
Cc: Caleb Cartwright <caleb.cartwright@outlook.com>
Cc: Yacin Tmimi <yacintmimi@gmail.com>
Cc: Manish Goregaokar <manishsmail@gmail.com>
Cc: Deadbeef <ent3rm4n@gmail.com>
Cc: Cameron Steffen <cam.steffen94@gmail.com>
Cc: Jieyou Xu <jieyouxu@outlook.com>
Link: https://lore.kernel.org/all/CAHk-=wgO7S_FZUSBbngG5vtejWOpzDfTTBkVvP3_yjJmFddbzA@mail.gmail.com/ [1]
Link: https://github.com/rust-lang/rustfmt/issues/4884 [2]
Link: https://hackmd.io/iSCyY3JTTz-g8YM-nnzTTA [3]
Link: https://github.com/rust-lang/rustfmt/issues/4991 [4]
Link: https://github.com/rust-lang/rustfmt/issues/3361 [5]
Link: https://rust-lang.zulipchat.com/#narrow/channel/392734-council/topic/rustfmt.20maintenance/near/543815381 [6]
Link: https://github.com/rust-lang/team/pull/2017 [7]
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Rust kernel code is supposed to use the custom mapping of C FFI types,
i.e. those from the `ffi` crate, rather than the ones coming from `core`.
Thus, to minimize mistakes and to simplify the code everywhere, just
provide them in the `kernel` prelude and ask in the Coding Guidelines
to use them directly, i.e. as a single segment path.
After this lands, we can start cleaning up the existing users.
Ideally, we would use something like Clippy's `disallowed-types` to
prevent the use of the `core` ones, but that one sees through aliases.
Link: https://lore.kernel.org/rust-for-linux/CANiq72kc4gzfieD-FjuWfELRDXXD2vLgPv4wqk3nt4pjdPQ=qg@mail.gmail.com/
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250413005650.1745894-1-ojeda@kernel.org
[ Reworded content of the documentation to focus on how to use the
aliases first. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Sometimes kernel developers use `//` for documenting private items,
since those do not get rendered at the moment.
That is reasonable, but the intention behind `///` (and `//!`) vs. `//`
is to convey the distinction between documentation and other kinds of
comments, such as implementation details or TODOs.
It also increases consistency with the public items and thus e.g. allows
to change visibility of an item with less changes involved.
It is not just useful for human readers, but also tooling. For instance,
we may want to eventually generate documentation for private items
(perhaps as a toggle in the HTML UI). On top of that, `rustdoc` lints
as usual for those, too, so we may want to do it even if we do not use
the result.
Thus document this explicitly.
Link: https://lore.kernel.org/rust-for-linux/CANiq72n_C7exSOMe5yf-7jKKnhSCv+a9QcD=OE2B_Q2UFBL3Xg@mail.gmail.com/
Link: https://github.com/Rust-for-Linux/linux/issues/1157
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20250416112454.2503872-1-ojeda@kernel.org
[ Fixed typo. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This list started as a "when to prefer `expect`" list, but at some point
during writing I changed it to a "prefer `expect` unless..." one. However,
the first bullet remained, which does not make sense anymore.
Thus remove it. In addition, fix nearby typo.
Fixes: 04866494e9 ("Documentation: rust: discuss `#[expect(...)]` in the guidelines")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20241117133127.473937-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Discuss `#[expect(...)]` in the Lints sections of the coding guidelines
document, which is an upcoming feature in Rust 1.81.0, and explain that
it is generally to be preferred over `allow` unless there is a reason
not to use it (e.g. conditional compilation being involved).
Tested-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240904204347.168520-19-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
In the C side, disabling diagnostics locally, i.e. within the source code,
is rare (at least in the kernel). Sometimes warnings are manipulated
via the flags at the translation unit level, but that is about it.
In Rust, it is easier to change locally the "level" of lints
(e.g. allowing them locally). In turn, this means it is easier to
globally enable more lints that may trigger a few false positives here
and there that need to be allowed locally, but that generally can spot
issues or bugs.
Thus document this.
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240904204347.168520-17-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
