rust: build: move Rust configuration to 'scripts/Makefile.rust'

Currently, all Rust flags live in the top level 'Makefile'. This patch
moves these flags to a new file at 'scripts/Makefile.rust' so that
wanted adjustments to Rust's CLI configuration can be applied without
touching the root 'Makefile'. This change is intended to reduce noise
and the potential for merge conflicts.

Making this change will simplify the goals of (1) adding additional
lints over time, and (2) changing from specifying lint groups (e.g.
'correctness', 'perf') to naming individual lints for better version
compatibility. These changes are space consuming (>100 lines) and may
have a few adjustments per cycle, so extracting these arguments out of
the shared 'Makefile' should be much less noisy for everyone.

Signed-off-by: Trevor Gross <tmgross@umich.edu>

Makefile

@@ -452,26 +452,12 @@ KBUILD_USERHOSTCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
 KBUILD_USERCFLAGS  := $(KBUILD_USERHOSTCFLAGS) $(USERCFLAGS)
 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
+# Include configuration for building with Rust
+include $(srctree)/scripts/Makefile.rust
 
 KBUILD_HOSTCFLAGS   := $(KBUILD_USERHOSTCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTCFLAGS)
 KBUILD_HOSTCXXFLAGS := -Wall -O2 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS)
-KBUILD_HOSTRUSTFLAGS := $(rust_common_flags) -O -Cstrip=debuginfo \
-			-Zallow-features= $(HOSTRUSTFLAGS)
+KBUILD_HOSTRUSTFLAGS := $(rust_kbuild_host_flags)
 KBUILD_HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
 KBUILD_HOSTLDLIBS   := $(HOST_LFS_LIBS) $(HOSTLDLIBS)
 
@@ -560,14 +546,7 @@ KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
 		   -Werror=return-type -Wno-format-security -funsigned-char \
 		   -std=gnu11
 KBUILD_CPPFLAGS := -D__KERNEL__
-KBUILD_RUSTFLAGS := $(rust_common_flags) \
-		    --target=$(objtree)/scripts/target.json \
-		    -Cpanic=abort -Cembed-bitcode=n -Clto=n \
-		    -Cforce-unwind-tables=n -Ccodegen-units=1 \
-		    -Csymbol-mangling-version=v0 \
-		    -Crelocation-model=static \
-		    -Zfunction-sections=n \
-		    -Dclippy::float_arithmetic
+KBUILD_RUSTFLAGS := $(rust_kbuild_flags)
 
 KBUILD_AFLAGS_KERNEL :=
 KBUILD_CFLAGS_KERNEL :=

scripts/Makefile.rust

@@ -0,0 +1,30 @@
+# Configuration of flags specific to Rust support
+
+# 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_kbuild_host_flags := $(rust_common_flags) \
+				 -O -Cstrip=debuginfo \
+				 -Zallow-features= $(HOSTRUSTFLAGS)
+
+export rust_kbuild_flags := $(rust_common_flags) \
+			    --target=$(objtree)/scripts/target.json \
+			    -Cpanic=abort -Cembed-bitcode=n -Clto=n \
+			    -Cforce-unwind-tables=n -Ccodegen-units=1 \
+			    -Csymbol-mangling-version=v0 \
+			    -Crelocation-model=static \
+			    -Zfunction-sections=n \
+			    -Dclippy::float_arithmetic