[PATCH] rust: add `rustcheck` make target for check-only builds

["Onur Özkan" <work@onurozkan.dev>]

Adds a new `rustcheck` make target to run a check-only build
similar to `cargo check`. This allows us to verify that the Rust
sources can build without building/linking final artifacts,
which speeds up the iteration (a lot) during development.

The target supports the same flags as other Rust build rules, so
it can also be used with `CLIPPY=1` (e.g., `make LLVM=1 rustcheck
CLIPPY=1) to run Clippy in a faster way.

Also, unlike `make LLVM=1`, it doesn't compile large amounts of C
code (on a fresh checkout) when the goal is only to check that
Rust builds are not broken after some changes.

Suggested-by: Benno Losin <lossin@kernel.org>
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089/topic/x/near/539103602
Signed-off-by: Onur Özkan <work@onurozkan.dev>
---
 Makefile      |  7 +++++
 rust/Makefile | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 80 insertions(+)

diff --git a/Makefile b/Makefile
index cf37b9407821..7812cdc72938 100644
--- a/Makefile
+++ b/Makefile
@@ -1716,6 +1716,8 @@ help:
 	@echo  '		    is formatted, printing a diff otherwise.'
 	@echo  '  rustdoc	  - Generate Rust documentation'
 	@echo  '		    (requires kernel .config)'
+	@echo  '  rustcheck       - Check that the Rust code builds'
+	@echo  '                    (requires kernel .config)'
 	@echo  '  rusttest        - Runs the Rust tests'
 	@echo  '                    (requires kernel .config; downloads external repos)'
 	@echo  '  rust-analyzer	  - Generate rust-project.json rust-analyzer support file'
@@ -1821,6 +1823,11 @@ PHONY += rustdoc
 rustdoc: prepare
 	$(Q)$(MAKE) $(build)=rust $@
 
+# Checking Rust sources.
+PHONY += rustcheck
+rustcheck: prepare0
+	$(Q)$(MAKE) $(build)=rust $@
+
 # Testing target
 PHONY += rusttest
 rusttest: prepare
diff --git a/rust/Makefile b/rust/Makefile
index bfa915b0e588..c148ef4a8c08 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -265,6 +265,79 @@ rusttest-kernel: $(src)/kernel/lib.rs rusttestlib-ffi rusttestlib-kernel \
     rusttestlib-uapi rusttestlib-pin_init FORCE
 	+$(call if_changed,rustc_test)
 
+## Check-only compilation (similar to `cargo check`)
+quiet_cmd_rustc_check_library = $(RUSTC_OR_CLIPPY_QUIET) CHECK $<
+      cmd_rustc_check_library = \
+        OBJTREE=$(abspath $(objtree)) \
+        $(RUSTC_OR_CLIPPY) $(rust_common_flags) \
+                @$(objtree)/include/generated/rustc_cfg $(rustc_target_flags) \
+                --crate-type $(if $(rustc_check_library_proc),proc-macro,rlib) \
+                $(if $(rustc_check_library_proc),,--emit=metadata) \
+                --out-dir $(objtree)/$(obj)/check -L$(objtree)/$(obj)/check \
+                --crate-name $(if $(rustc_check_crate_name),$(rustc_check_crate_name), \
+				$(subst rustcheck-,,$(subst rustchecklib-,,$@))) $<
+
+rustcheck: rustchecklib-build_error rustchecklib-ffi rustchecklib-macros \
+	rustchecklib-compiler_builtins rustchecklib-pin_init_internal \
+	rustchecklib-pin_init rustchecklib-bindings rustchecklib-uapi \
+	rustchecklib-kernel
+
+rustchecklib-build_error: $(src)/build_error.rs FORCE
+	+$(call if_changed,rustc_check_library)
+
+rustchecklib-ffi: $(src)/ffi.rs FORCE
+	+$(call if_changed,rustc_check_library)
+
+rustchecklib-macros: private rustc_target_flags = --extern proc_macro
+rustchecklib-macros: private rustc_check_library_proc = yes
+rustchecklib-macros: $(src)/macros/lib.rs FORCE
+	+$(call if_changed,rustc_check_library)
+
+rustchecklib-compiler_builtins: private rustc_check_crate_name = compiler_builtins_kernel
+rustchecklib-compiler_builtins: $(src)/compiler_builtins.rs FORCE
+	+$(call if_changed,rustc_check_library)
+
+rustchecklib-pin_init_internal: private rustc_target_flags = --cfg kernel \
+	--extern proc_macro
+rustchecklib-pin_init_internal: private rustc_check_library_proc = yes
+rustchecklib-pin_init_internal: $(src)/pin-init/internal/src/lib.rs FORCE
+	+$(call if_changed,rustc_check_library)
+
+rustchecklib-pin_init: private rustc_target_flags = --extern pin_init_internal \
+	--extern macros --cfg kernel
+rustchecklib-pin_init: $(src)/pin-init/src/lib.rs rustchecklib-macros \
+	rustchecklib-pin_init_internal FORCE
+	+$(call if_changed,rustc_check_library)
+
+rustchecklib-bindings: private rustc_target_flags = --extern ffi
+rustchecklib-bindings: $(src)/bindings/lib.rs \
+    $(obj)/bindings/bindings_generated.rs \
+    $(obj)/bindings/bindings_helpers_generated.rs \
+    rustchecklib-ffi FORCE
+	+$(call if_changed,rustc_check_library)
+
+rustchecklib-uapi: private rustc_target_flags = --extern ffi
+rustchecklib-uapi: $(src)/uapi/lib.rs $(obj)/uapi/uapi_generated.rs \
+    rustchecklib-ffi FORCE
+	+$(call if_changed,rustc_check_library)
+
+ifdef CONFIG_JUMP_LABEL
+rustchecklib-kernel: $(obj)/kernel/generated_arch_static_branch_asm.rs FORCE
+endif
+ifndef CONFIG_UML
+ifdef CONFIG_BUG
+rustchecklib-kernel: $(obj)/kernel/generated_arch_warn_asm.rs \
+	$(obj)/kernel/generated_arch_reachable_asm.rs FORCE
+endif
+endif
+
+rustchecklib-kernel: private rustc_target_flags = --extern ffi --extern pin_init \
+	--extern build_error --extern macros --extern bindings --extern uapi
+rustchecklib-kernel: $(src)/kernel/lib.rs rustchecklib-ffi rustchecklib-pin_init \
+	rustchecklib-build_error rustchecklib-macros rustchecklib-bindings \
+	rustchecklib-uapi FORCE
+	+$(call if_changed,rustc_check_library)
+
 ifdef CONFIG_CC_IS_CLANG
 bindgen_c_flags = $(c_flags)
 else
-- 
2.51.0