rust: ensure libmodule.so gets recompiled when rustc version changes

Procedural macros can only be used with the rustc that compiles it.

When rustc version changes, the corresponding libcore source code also
changes, so adding `core.o` to the dependency of `libmodule.so` can
force a recompilation of `libmodule.so`.

Signed-off-by: Gary Guo <gary@garyguo.net>

Author: nbdd0121

rust/Makefile

@@ -126,7 +126,11 @@ quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
 	mv $(objtree)/rust/$(patsubst lib%.so,%,$(notdir $@)).d $(depfile); \
 	sed -i '/^\#/d' $(depfile)
 
-$(objtree)/rust/libmodule.so: $(srctree)/rust/module.rs FORCE
+# Procedural macros can only be used with the `rustc` that compiled it.
+# Therefore, to get `libmodule.so` automatically recompiled when the compiler
+# version changes, we add `core.o` as a dependency (even if it is not needed).
+$(objtree)/rust/libmodule.so: $(srctree)/rust/module.rs \
+	$(objtree)/rust/core.o FORCE
 	$(call if_changed_dep,rustc_procmacro)
 
 quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L $@