rust: move 'rust_common_flags' to a separate configuration file

Currently, 'rustc_common_flags' lives in the top-level 'Makefile'.
This means that we need to touch this whenever adjustments to Rust's CLI
configuration are desired, such as changing what lints Clippy should
emit.

This patch moves these flags to a separate file within 'rust/' so that
necessary changes can be made without affecting 'Makefile'. It copies
the behavior currently used with 'bindgen_parameters', which accepts
comments.

Additionally, 'let_unit_value` is no longer specifically named as it is
now part of the 'style' group [1].

[1]: rust-lang/rust-clippy#8563

Signed-off-by: Trevor Gross <[email protected]>

Author: tgross35

Makefile

@@ -454,19 +454,7 @@ KBUILD_USERLDFLAGS := $(USERLDFLAGS)
 
 # These flags apply to all Rust code in the tree, including the kernel and
 # host programs.
-export rust_common_flags := --edition=2021 \
-			    -Zbinary_dep_depinfo=y \
-			    -Dunsafe_op_in_unsafe_fn -Drust_2018_idioms \
-			    -Dunreachable_pub -Dnon_ascii_idents \
-			    -Wmissing_docs \
-			    -Drustdoc::missing_crate_level_docs \
-			    -Dclippy::correctness -Dclippy::style \
-			    -Dclippy::suspicious -Dclippy::complexity \
-			    -Dclippy::perf \
-			    -Dclippy::let_unit_value -Dclippy::mut_mut \
-			    -Dclippy::needless_bitwise_bool \
-			    -Dclippy::needless_continue \
-			    -Wclippy::dbg_macro
+export rust_common_flags := $(shell grep -v '^#\|^$$' $(srctree)/rust/rust_common_flags)
 
 KBUILD_HOSTCFLAGS   := $(KBUILD_USERHOSTCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTCFLAGS)
 KBUILD_HOSTCXXFLAGS := -Wall -O2 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS)

rust/rust_common_flags

@@ -0,0 +1,32 @@
+# SPDX-License-Identifier: GPL-2.0
+# Parameters passed to rust tools (`rustc`, `clippy`, `rustdoc`) whenever they
+# are run.
+
+# Standard configuration
+--edition=2021
+-Zbinary_dep_depinfo=y
+
+# Standard rust lints
+-Dnon_ascii_idents
+-Drust_2018_idioms
+-Dunreachable_pub
+-Dunsafe_op_in_unsafe_fn
+
+# Documentation lints
+-Wmissing_docs
+-Drustdoc::missing_crate_level_docs
+
+# Clippy groups
+-Dclippy::complexity
+-Dclippy::correctness
+-Dclippy::perf
+-Dclippy::style
+-Dclippy::suspicious
+
+# Clippy lints from pedantic
+-Dclippy::mut_mut
+-Dclippy::needless_bitwise_bool
+-Dclippy::needless_continue
+
+# Clippy lints that we only warn on
+-Wclippy::dbg_macro