Makefile: move ifdef block inside rust/Makefile

This also makes it so that we enter `rust` on `clean`.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

Makefile

@@ -1162,11 +1162,7 @@ export MODULES_NSDEPS := $(extmod_prefix)modules.nsdeps
 ifeq ($(KBUILD_EXTMOD),)
 core-y			+= kernel/ certs/ mm/ fs/ ipc/ security/ crypto/
 core-$(CONFIG_BLOCK)	+= block/
-
-# Keep this one as an `ifdef` block since its `Makefile` runs `rustc`.
-ifdef CONFIG_RUST
-core-y			+= rust/
-endif
+core-$(CONFIG_RUST)	+= rust/
 
 vmlinux-dirs	:= $(patsubst %/,%,$(filter %/, \
 		     $(core-y) $(core-m) $(drivers-y) $(drivers-m) \
@@ -1584,7 +1580,7 @@ MRPROPER_FILES += include/config include/generated          \
 		  certs/x509.genkey \
 		  vmlinux-gdb.py \
 		  *.spec \
-		  rust/*_generated.h rust/*_generated.rs rust/libmacros.so
+		  rust/libmacros.so
 
 # clean - Delete most, but leave enough to build external modules
 #

rust/Makefile

@@ -17,6 +17,9 @@ endif
 
 obj-$(CONFIG_RUST) += exports.o
 
+# Avoids running `$(RUSTC)` for the sysroot when it may not be available.
+ifdef CONFIG_RUST
+
 ifeq ($(quiet),silent_)
 cargo_quiet=-q
 rust_test_quiet=-q
@@ -346,3 +349,5 @@ $(objtree)/rust/core.o: $$(RUST_LIB_SRC)/core/src/lib.rs FORCE
 rustdoc-core: private rustc_target_flags = $(core-cfgs)
 rustdoc-core: $$(RUST_LIB_SRC)/core/src/lib.rs FORCE
 	$(call if_changed,rustdoc)
+
+endif # CONFIG_RUST