Makefile: introduce infrastructure to build internal Rust library

Introduce infrastructure to build the internal Rust library. This
mirrors the infrastructure we have added to Meson in the preceding
commit. Developers can enable the infrastructure by passing the new
`WITH_RUST` build toggle.

Inspired-by: Ezekiel Newren <ezekielnewren@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitignore

@@ -1,4 +1,6 @@
 /fuzz_corpora
+/target/
+/Cargo.lock
 /GIT-BUILD-DIR
 /GIT-BUILD-OPTIONS
 /GIT-CFLAGS

Makefile

@@ -483,6 +483,14 @@ include shared.mak
 # Define LIBPCREDIR=/foo/bar if your PCRE header and library files are
 # in /foo/bar/include and /foo/bar/lib directories.
 #
+# == Optional Rust support ==
+#
+# Define WITH_RUST if you want to include features and subsystems written in
+# Rust into Git. For now, Rust is still an optional feature of the build
+# process. With Git 3.0 though, Rust will always be enabled.
+#
+# Building Rust code requires Cargo.
+#
 # == SHA-1 and SHA-256 defines ==
 #
 # === SHA-1 backend ===
@@ -683,6 +691,7 @@ OBJECTS =
 OTHER_PROGRAMS =
 PROGRAM_OBJS =
 PROGRAMS =
+RUST_SOURCES =
 EXCLUDED_PROGRAMS =
 SCRIPT_PERL =
 SCRIPT_PYTHON =
@@ -918,6 +927,11 @@ TEST_SHELL_PATH = $(SHELL_PATH)
 LIB_FILE = libgit.a
 XDIFF_LIB = xdiff/lib.a
 REFTABLE_LIB = reftable/libreftable.a
+ifdef DEBUG
+RUST_LIB = target/debug/libgitcore.a
+else
+RUST_LIB = target/release/libgitcore.a
+endif
 
 # xdiff and reftable libs may in turn depend on what is in libgit.a
 GITLIBS = common-main.o $(LIB_FILE) $(XDIFF_LIB) $(REFTABLE_LIB) $(LIB_FILE)
@@ -943,6 +957,15 @@ BASIC_LDFLAGS =
 ARFLAGS = rcs
 PTHREAD_CFLAGS =
 
+# Rust flags
+CARGO_ARGS =
+ifndef V
+CARGO_ARGS += --quiet
+endif
+ifndef DEBUG
+CARGO_ARGS += --release
+endif
+
 # For the 'sparse' target
 SPARSE_FLAGS ?= -std=gnu99 -D__STDC_NO_VLA__
 SP_EXTRA_FLAGS =
@@ -1475,6 +1498,8 @@ CLAR_TEST_OBJS += $(UNIT_TEST_DIR)/unit-test.o
 
 UNIT_TEST_OBJS += $(UNIT_TEST_DIR)/test-lib.o
 
+RUST_SOURCES += src/lib.rs
+
 GIT-VERSION-FILE: FORCE
 	@OLD=$$(cat $@ 2>/dev/null || :) && \
 	$(call version_gen,"$(shell pwd)",GIT-VERSION-FILE.in,$@) && \
@@ -1504,6 +1529,11 @@ endif
 ALL_CFLAGS = $(DEVELOPER_CFLAGS) $(CPPFLAGS) $(CFLAGS) $(CFLAGS_APPEND)
 ALL_LDFLAGS = $(LDFLAGS) $(LDFLAGS_APPEND)
 
+ifdef WITH_RUST
+BASIC_CFLAGS += -DWITH_RUST
+GITLIBS += $(RUST_LIB)
+endif
+
 ifdef SANITIZE
 SANITIZERS := $(foreach flag,$(subst $(comma),$(space),$(SANITIZE)),$(flag))
 BASIC_CFLAGS += -fsanitize=$(SANITIZE) -fno-sanitize-recover=$(SANITIZE)
@@ -2918,6 +2948,12 @@ scalar$X: scalar.o GIT-LDFLAGS $(GITLIBS)
 $(LIB_FILE): $(LIB_OBJS)
 	$(QUIET_AR)$(RM) $@ && $(AR) $(ARFLAGS) $@ $^
 
+$(RUST_LIB): Cargo.toml $(RUST_SOURCES)
+	$(QUIET_CARGO)cargo build $(CARGO_ARGS)
+
+.PHONY: rust
+rust: $(RUST_LIB)
+
 $(XDIFF_LIB): $(XDIFF_OBJS)
 	$(QUIET_AR)$(RM) $@ && $(AR) $(ARFLAGS) $@ $^
 
@@ -3768,6 +3804,7 @@ clean: profile-clean coverage-clean cocciclean
 	$(RM) $(FUZZ_PROGRAMS)
 	$(RM) $(SP_OBJ)
 	$(RM) $(HCC)
+	$(RM) -r Cargo.lock target/
 	$(RM) version-def.h
 	$(RM) -r $(dep_dirs) $(compdb_dir) compile_commands.json
 	$(RM) $(test_bindir_programs)

shared.mak

@@ -56,6 +56,7 @@ ifndef V
 	QUIET_MKDIR_P_PARENT  = @echo '   ' MKDIR -p $(@D);
 
 ## Used in "Makefile"
+	QUIET_CARGO    = @echo '   ' CARGO $@;
 	QUIET_CC       = @echo '   ' CC $@;
 	QUIET_AR       = @echo '   ' AR $@;
 	QUIET_LINK     = @echo '   ' LINK $@;