rust: kbuild: add proc macro library support

Add the proc macro library rule that produces `.rlib` files to be used
by proc macros such as the `macros` crate.

Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>
Tested-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://patch.msgid.link/20251124151837.2184382-4-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

.gitignore

@@ -41,6 +41,7 @@
 *.o.*
 *.patch
 *.pyc
+*.rlib
 *.rmeta
 *.rpm
 *.rsi

rust/Makefile

@@ -421,6 +421,16 @@ $(obj)/exports_bindings_generated.h: $(obj)/bindings.o FORCE
 $(obj)/exports_kernel_generated.h: $(obj)/kernel.o FORCE
 	$(call if_changed,exports)
 
+quiet_cmd_rustc_procmacrolibrary = $(RUSTC_OR_CLIPPY_QUIET) PL $@
+      cmd_rustc_procmacrolibrary = \
+	$(if $(skip_clippy),$(RUSTC),$(RUSTC_OR_CLIPPY)) \
+		$(filter-out $(skip_flags),$(rust_common_flags) $(rustc_target_flags)) \
+		--emit=dep-info,link --crate-type rlib -O \
+		--out-dir $(objtree)/$(obj) -L$(objtree)/$(obj) \
+		--crate-name $(patsubst lib%.rlib,%,$(notdir $@)) $<; \
+	mv $(objtree)/$(obj)/$(patsubst lib%.rlib,%,$(notdir $@)).d $(depfile); \
+	sed -i '/^\#/d' $(depfile)
+
 quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
       cmd_rustc_procmacro = \
 	$(RUSTC_OR_CLIPPY) $(rust_common_flags) $(rustc_target_flags) \