rust: introduce .clippy.toml

Some Clippy lints can be configured/tweaked. We will use these knobs to
our advantage in later commits.

This is done via a configuration file, `.clippy.toml` [1]. The file is
currently unstable. This may be a problem in the future, but we can adapt
as needed. In addition, we proposed adding Clippy to the Rust CI's RFL
job [2], so we should be able to catch issues pre-merge.

Thus introduce the file.

Link: https://doc.rust-lang.org/clippy/configuration.html [1]
Link: rust-lang/rust#128928 [2]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Tested-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240904204347.168520-12-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

.clippy.toml

@@ -0,0 +1 @@
+# SPDX-License-Identifier: GPL-2.0

.gitignore

@@ -103,6 +103,7 @@ modules.order
 # We don't want to ignore the following even if they are dot-files
 #
 !.clang-format
+!.clippy.toml
 !.cocciconfig
 !.editorconfig
 !.get_maintainer.ignore

MAINTAINERS

@@ -20191,6 +20191,7 @@ B:	https://github.com/Rust-for-Linux/linux/issues
 C:	zulip://rust-for-linux.zulipchat.com
 P:	https://rust-for-linux.com/contributing
 T:	git https://github.com/Rust-for-Linux/linux.git rust-next
+F:	.clippy.toml
 F:	Documentation/rust/
 F:	rust/
 F:	samples/rust/

Makefile

@@ -587,6 +587,9 @@ endif
 # Allows the usage of unstable features in stable compilers.
 export RUSTC_BOOTSTRAP := 1
 
+# Allows finding `.clippy.toml` in out-of-srctree builds.
+export CLIPPY_CONF_DIR := $(srctree)
+
 export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE LD CC HOSTPKG_CONFIG
 export RUSTC RUSTDOC RUSTFMT RUSTC_OR_CLIPPY_QUIET RUSTC_OR_CLIPPY BINDGEN
 export HOSTRUSTC KBUILD_HOSTRUSTFLAGS