Kbuild: add single target support for Rust (.{o,i,s,ll})

`.i` maps to macro expansion, which is somewhat similar to what
is done for C and the preprocessor.

The output is automatically reformatted if `rustfmt` is installed,
which mimics what `cargo expand` does by default. The user can
override `RUSTFMT` to avoid it in the odd case is required.

We could consider adding other outputs such as `.mir`/`.hir` if they
happen to be useful in the future.

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

Author: ojeda

scripts/Makefile.build

@@ -333,23 +333,61 @@ rust_cross_flags := --target=$(realpath $(KBUILD_RUST_TARGET))
 
 rust_allowed_features := allocator_api,bench_black_box,concat_idents,generic_associated_types,global_asm
 
+rust_common_cmd = \
+	RUST_MODFILE=$(modfile) $(RUSTC_OR_CLIPPY) \
+	$(rust_flags) $(rust_cross_flags) \
+	-Zallow-features=$(rust_allowed_features) \
+	-Zcrate-attr=no_std \
+	-Zcrate-attr='feature($(rust_allowed_features))' \
+	--extern alloc --extern kernel \
+	--crate-type rlib --out-dir $(obj) -L $(objtree)/rust/ \
+	--crate-name $(basename $(notdir $@))
+
+rust_handle_depfile = \
+	mv $(obj)/$(basename $(notdir $@)).d $(depfile); \
+	sed -i '/^\#/d' $(depfile)
+
+# `--emit=obj`, `--emit=asm` and `--emit=llvm-ir` imply a single codegen unit
+# will be used. We explicitly request `-Ccodegen-units=1` in any case, and
+# the compiler shows a warning if it is not 1. However, if we ever stop
+# requesting it explicitly and we start using some other `--emit` that does not
+# imply it (and for which codegen is performed), then we would be out of sync,
+# i.e. the outputs we would get for the different single targets (e.g. `.ll`)
+# would not match each other.
+
 quiet_cmd_rustc_o_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
       cmd_rustc_o_rs = \
-	RUST_MODFILE=$(modfile) \
-	$(RUSTC_OR_CLIPPY) $(rust_flags) $(rust_cross_flags) \
-		--emit=dep-info,obj,metadata \
-		-Zallow-features=$(rust_allowed_features) \
-		-Zcrate-attr=no_std \
-		-Zcrate-attr='feature($(rust_allowed_features))' \
-		--extern alloc --extern kernel \
-		--crate-type rlib --out-dir $(obj) -L $(objtree)/rust/ \
-		--crate-name $(patsubst %.o,%,$(notdir $@)) $<; \
-	mv $(obj)/$(subst .o,,$(notdir $@)).d $(depfile); \
-	sed -i '/^\#/d' $(depfile)
+	$(rust_common_cmd) --emit=dep-info,obj,metadata $<; \
+	$(rust_handle_depfile)
 
 $(obj)/%.o: $(src)/%.rs FORCE
 	$(call if_changed_dep,rustc_o_rs)
 
+quiet_cmd_rustc_i_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
+      cmd_rustc_i_rs = \
+	$(rust_common_cmd) --emit=dep-info -Zunpretty=expanded $< >$@; \
+	command -v $(RUSTFMT) >/dev/null && $(RUSTFMT) $@; \
+	$(rust_handle_depfile)
+
+$(obj)/%.i: $(src)/%.rs FORCE
+	$(call if_changed_dep,rustc_i_rs)
+
+quiet_cmd_rustc_s_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
+      cmd_rustc_s_rs = \
+	$(rust_common_cmd) --emit=dep-info,asm $<; \
+	$(rust_handle_depfile)
+
+$(obj)/%.s: $(src)/%.rs FORCE
+	$(call if_changed_dep,rustc_s_rs)
+
+quiet_cmd_rustc_ll_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
+      cmd_rustc_ll_rs = \
+	$(rust_common_cmd) --emit=dep-info,llvm-ir $<; \
+	$(rust_handle_depfile)
+
+$(obj)/%.ll: $(src)/%.rs FORCE
+	$(call if_changed_dep,rustc_ll_rs)
+
 # Compile assembler sources (.S)
 # ---------------------------------------------------------------------------