[PATCH 02/15] xdiff: introduce rust

["Ezekiel Newren via GitGitGadget" <gitgitgadget@gmail.com>]

From: Ezekiel Newren <ezekielnewren@gmail.com>

Upcoming patches will simplify xdiff, while also porting parts of it to
Rust. In preparation, add some stubs and setup the Rust build. For now,
it is easier to let cargo build rust and have make or meson merely link
against the static library that cargo builds. In line with ongoing
libification efforts, use multiple crates to allow more modularity on
the Rust side. xdiff is the crate that this series will focus on, but
we also introduce the interop crate for future patch series.

In order to facilitate interoperability between C and Rust, introduce C
definitions for Rust primitive types in git-compat-util.h.

Signed-off-by: Ezekiel Newren <ezekielnewren@gmail.com>
---
 .gitignore              |  3 +++
 Makefile                | 53 ++++++++++++++++++++++++++++----------
 build_rust.sh           | 57 +++++++++++++++++++++++++++++++++++++++++
 git-compat-util.h       | 17 ++++++++++++
 meson.build             | 52 +++++++++++++++++++++++++++++++------
 rust/Cargo.toml         |  6 +++++
 rust/interop/Cargo.toml | 14 ++++++++++
 rust/interop/src/lib.rs |  0
 rust/xdiff/Cargo.toml   | 15 +++++++++++
 rust/xdiff/src/lib.rs   |  0
 10 files changed, 196 insertions(+), 21 deletions(-)
 create mode 100755 build_rust.sh
 create mode 100644 rust/Cargo.toml
 create mode 100644 rust/interop/Cargo.toml
 create mode 100644 rust/interop/src/lib.rs
 create mode 100644 rust/xdiff/Cargo.toml
 create mode 100644 rust/xdiff/src/lib.rs

diff --git a/.gitignore b/.gitignore
index 04c444404e..ff81e3580c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -254,3 +254,6 @@ Release/
 /contrib/buildsystems/out
 /contrib/libgit-rs/target
 /contrib/libgit-sys/target
+/.idea/
+/rust/target/
+/rust/Cargo.lock
diff --git a/Makefile b/Makefile
index 70d1543b6b..1ec0c1ee66 100644
--- a/Makefile
+++ b/Makefile
@@ -919,6 +919,29 @@ TEST_SHELL_PATH = $(SHELL_PATH)
 
 LIB_FILE = libgit.a
 XDIFF_LIB = xdiff/lib.a
+
+EXTLIBS =
+
+ifeq ($(DEBUG), 1)
+  RUST_BUILD_MODE = debug
+else
+  RUST_BUILD_MODE = release
+endif
+
+RUST_TARGET_DIR = rust/target/$(RUST_BUILD_MODE)
+RUST_FLAGS_FOR_C = -L$(RUST_TARGET_DIR)
+
+.PHONY: compile_rust
+compile_rust:
+	./build_rust.sh . $(RUST_BUILD_MODE) xdiff
+
+EXTLIBS += ./$(RUST_TARGET_DIR)/libxdiff.a
+
+UNAME_S := $(shell uname -s)
+ifeq ($(UNAME_S),Linux)
+  EXTLIBS += -ldl
+endif
+
 REFTABLE_LIB = reftable/libreftable.a
 
 GENERATED_H += command-list.h
@@ -1390,7 +1413,7 @@ UNIT_TEST_OBJS += $(UNIT_TEST_DIR)/lib-reftable.o
 
 # 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)
-EXTLIBS =
+
 
 GIT_USER_AGENT = git/$(GIT_VERSION)
 
@@ -2541,7 +2564,7 @@ git.sp git.s git.o: EXTRA_CPPFLAGS = \
 	'-DGIT_MAN_PATH="$(mandir_relative_SQ)"' \
 	'-DGIT_INFO_PATH="$(infodir_relative_SQ)"'
 
-git$X: git.o GIT-LDFLAGS $(BUILTIN_OBJS) $(GITLIBS)
+git$X: git.o GIT-LDFLAGS $(BUILTIN_OBJS) $(GITLIBS) compile_rust
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) \
 		$(filter %.o,$^) $(LIBS)
 
@@ -2891,17 +2914,17 @@ headless-git.o: compat/win32/headless.c GIT-CFLAGS
 headless-git$X: headless-git.o git.res GIT-LDFLAGS
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) $(ALL_LDFLAGS) -mwindows -o $@ $< git.res
 
-git-%$X: %.o GIT-LDFLAGS $(GITLIBS)
+git-%$X: %.o GIT-LDFLAGS $(GITLIBS) compile_rust
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) $(LIBS)
 
-git-imap-send$X: imap-send.o $(IMAP_SEND_BUILDDEPS) GIT-LDFLAGS $(GITLIBS)
+git-imap-send$X: imap-send.o $(IMAP_SEND_BUILDDEPS) GIT-LDFLAGS $(GITLIBS) compile_rust
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(IMAP_SEND_LDFLAGS) $(LIBS)
 
-git-http-fetch$X: http.o http-walker.o http-fetch.o GIT-LDFLAGS $(GITLIBS)
+git-http-fetch$X: http.o http-walker.o http-fetch.o GIT-LDFLAGS $(GITLIBS) compile_rust
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(CURL_LIBCURL) $(LIBS)
-git-http-push$X: http.o http-push.o GIT-LDFLAGS $(GITLIBS)
+git-http-push$X: http.o http-push.o GIT-LDFLAGS $(GITLIBS) compile_rust
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(CURL_LIBCURL) $(EXPAT_LIBEXPAT) $(LIBS)
 
@@ -2911,11 +2934,11 @@ $(REMOTE_CURL_ALIASES): $(REMOTE_CURL_PRIMARY)
 	ln -s $< $@ 2>/dev/null || \
 	cp $< $@
 
-$(REMOTE_CURL_PRIMARY): remote-curl.o http.o http-walker.o GIT-LDFLAGS $(GITLIBS)
+$(REMOTE_CURL_PRIMARY): remote-curl.o http.o http-walker.o GIT-LDFLAGS $(GITLIBS) compile_rust
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(CURL_LIBCURL) $(EXPAT_LIBEXPAT) $(LIBS)
 
-scalar$X: scalar.o GIT-LDFLAGS $(GITLIBS)
+scalar$X: scalar.o GIT-LDFLAGS $(GITLIBS) compile_rust
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) \
 		$(filter %.o,$^) $(LIBS)
 
@@ -2925,6 +2948,7 @@ $(LIB_FILE): $(LIB_OBJS)
 $(XDIFF_LIB): $(XDIFF_OBJS)
 	$(QUIET_AR)$(RM) $@ && $(AR) $(ARFLAGS) $@ $^
 
+
 $(REFTABLE_LIB): $(REFTABLE_OBJS)
 	$(QUIET_AR)$(RM) $@ && $(AR) $(ARFLAGS) $@ $^
 
@@ -3294,7 +3318,7 @@ perf: all
 
 t/helper/test-tool$X: $(patsubst %,t/helper/%,$(TEST_BUILTINS_OBJS)) $(UNIT_TEST_DIR)/test-lib.o
 
-t/helper/test-%$X: t/helper/test-%.o GIT-LDFLAGS $(GITLIBS)
+t/helper/test-%$X: t/helper/test-%.o GIT-LDFLAGS $(GITLIBS) compile_rust
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) $(filter %.a,$^) $(LIBS)
 
 check-sha1:: t/helper/test-tool$X
@@ -3756,7 +3780,10 @@ cocciclean:
 	$(RM) -r .build/contrib/coccinelle
 	$(RM) contrib/coccinelle/*.cocci.patch
 
-clean: profile-clean coverage-clean cocciclean
+rustclean:
+	cd rust && cargo clean
+
+clean: profile-clean coverage-clean cocciclean rustclean
 	$(RM) -r .build $(UNIT_TEST_BIN)
 	$(RM) GIT-TEST-SUITES
 	$(RM) po/git.pot po/git-core.pot
@@ -3911,13 +3938,13 @@ FUZZ_CXXFLAGS ?= $(ALL_CFLAGS)
 .PHONY: fuzz-all
 fuzz-all: $(FUZZ_PROGRAMS)
 
-$(FUZZ_PROGRAMS): %: %.o oss-fuzz/dummy-cmd-main.o $(GITLIBS) GIT-LDFLAGS
+$(FUZZ_PROGRAMS): %: %.o oss-fuzz/dummy-cmd-main.o $(GITLIBS) GIT-LDFLAGS compile_rust
 	$(QUIET_LINK)$(FUZZ_CXX) $(FUZZ_CXXFLAGS) -o $@ $(ALL_LDFLAGS) \
 		-Wl,--allow-multiple-definition \
 		$(filter %.o,$^) $(filter %.a,$^) $(LIBS) $(LIB_FUZZING_ENGINE)
 
 $(UNIT_TEST_PROGS): $(UNIT_TEST_BIN)/%$X: $(UNIT_TEST_DIR)/%.o $(UNIT_TEST_OBJS) \
-	$(GITLIBS) GIT-LDFLAGS
+	$(GITLIBS) GIT-LDFLAGS compile_rust
 	$(call mkdir_p_parent_template)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) \
 		$(filter %.o,$^) $(filter %.a,$^) $(LIBS)
@@ -3936,7 +3963,7 @@ $(UNIT_TEST_DIR)/clar.suite: $(UNIT_TEST_DIR)/clar-decls.h $(UNIT_TEST_DIR)/gene
 $(UNIT_TEST_DIR)/clar/clar.o: $(UNIT_TEST_DIR)/clar.suite
 $(CLAR_TEST_OBJS): $(UNIT_TEST_DIR)/clar-decls.h
 $(CLAR_TEST_OBJS): EXTRA_CPPFLAGS = -I$(UNIT_TEST_DIR)
-$(CLAR_TEST_PROG): $(UNIT_TEST_DIR)/clar.suite $(CLAR_TEST_OBJS) $(GITLIBS) GIT-LDFLAGS
+$(CLAR_TEST_PROG): $(UNIT_TEST_DIR)/clar.suite $(CLAR_TEST_OBJS) $(GITLIBS) GIT-LDFLAGS compile_rust
 	$(call mkdir_p_parent_template)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) $(LIBS)
 
diff --git a/build_rust.sh b/build_rust.sh
new file mode 100755
index 0000000000..192385a1d9
--- /dev/null
+++ b/build_rust.sh
@@ -0,0 +1,57 @@
+#!/bin/sh
+
+
+rustc -vV || exit $?
+cargo --version || exit $?
+
+dir_git_root=${0%/*}
+dir_build=$1
+rust_build_profile=$2
+crate=$3
+
+dir_rust=$dir_git_root/rust
+
+if [ "$dir_git_root" = "" ]; then
+  echo "did not specify the directory for the root of git"
+  exit 1
+fi
+
+if [ "$dir_build" = "" ]; then
+  echo "did not specify the build directory"
+  exit 1
+fi
+
+if [ "$rust_build_profile" = "" ]; then
+  echo "did not specify the rust_build_profile"
+  exit 1
+fi
+
+if [ "$rust_build_profile" = "release" ]; then
+  rust_args="--release"
+  export RUSTFLAGS=''
+elif [ "$rust_build_profile" = "debug" ]; then
+  rust_args=""
+  export RUSTFLAGS='-C debuginfo=2 -C opt-level=1 -C force-frame-pointers=yes'
+else
+  echo "illegal rust_build_profile value $rust_build_profile"
+  exit 1
+fi
+
+cd $dir_rust && cargo clean && pwd && cargo build -p $crate $rust_args; cd $dir_git_root
+
+libfile="lib${crate}.a"
+if rustup show active-toolchain | grep windows-msvc; then
+  libfile="${crate}.lib"
+fi
+dst=$dir_build/$libfile
+
+if [ "$dir_git_root" != "$dir_build" ]; then
+  src=$dir_rust/target/$rust_build_profile/$libfile
+  if [ ! -f $src ]; then
+    echo >&2 "::error:: cannot find path of static library $src is not a file or does not exist"
+    exit 5
+  fi
+
+  rm $dst 2>/dev/null
+  mv $src $dst
+fi
diff --git a/git-compat-util.h b/git-compat-util.h
index 4678e21c4c..82dc99764a 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -196,6 +196,23 @@ static inline int is_xplatform_dir_sep(int c)
 #include "compat/msvc.h"
 #endif
 
+/* rust types */
+typedef uint8_t   u8;
+typedef uint16_t  u16;
+typedef uint32_t  u32;
+typedef uint64_t  u64;
+
+typedef int8_t    i8;
+typedef int16_t   i16;
+typedef int32_t   i32;
+typedef int64_t   i64;
+
+typedef float     f32;
+typedef double    f64;
+
+typedef size_t    usize;
+typedef ptrdiff_t isize;
+
 /* used on Mac OS X */
 #ifdef PRECOMPOSE_UNICODE
 #include "compat/precompose_utf8.h"
diff --git a/meson.build b/meson.build
index 596f5ac711..324f968338 100644
--- a/meson.build
+++ b/meson.build
@@ -267,6 +267,40 @@ version_gen_environment.set('GIT_DATE', get_option('build_date'))
 version_gen_environment.set('GIT_USER_AGENT', get_option('user_agent'))
 version_gen_environment.set('GIT_VERSION', get_option('version'))
 
+if get_option('optimization') in ['2', '3', 's', 'z']
+  rust_build_profile = 'release'
+else
+  rust_build_profile = 'debug'
+endif
+
+# Run `rustup show active-toolchain` and capture output
+rustup_out = run_command('rustup', 'show', 'active-toolchain',
+                         check: true).stdout().strip()
+
+rust_crates = ['xdiff']
+rust_builds = []
+
+foreach crate : rust_crates
+  if rustup_out.contains('windows-msvc')
+    libfile = crate + '.lib'
+  else
+    libfile = 'lib' + crate + '.a'
+  endif
+
+  rust_builds += custom_target(
+    'rust_build_'+crate,
+    output: libfile,
+    build_by_default: true,
+    build_always_stale: true,
+    command: [
+      meson.project_source_root() / 'build_rust.sh',
+      meson.current_build_dir(), rust_build_profile, crate,
+    ],
+    install: false,
+  )
+endforeach
+
+
 compiler = meson.get_compiler('c')
 
 libgit_sources = [
@@ -1678,14 +1712,16 @@ version_def_h = custom_target(
 libgit_sources += version_def_h
 
 libgit = declare_dependency(
-  link_with: static_library('git',
-    sources: libgit_sources,
-    c_args: libgit_c_args + [
-      '-DGIT_VERSION_H="' + version_def_h.full_path() + '"',
-    ],
-    dependencies: libgit_dependencies,
-    include_directories: libgit_include_directories,
-  ),
+  link_with: [
+    static_library('git',
+      sources: libgit_sources,
+      c_args: libgit_c_args + [
+        '-DGIT_VERSION_H="' + version_def_h.full_path() + '"',
+      ],
+      dependencies: libgit_dependencies,
+      include_directories: libgit_include_directories,
+    ),
+  ] + rust_builds,
   compile_args: libgit_c_args,
   dependencies: libgit_dependencies,
   include_directories: libgit_include_directories,
diff --git a/rust/Cargo.toml b/rust/Cargo.toml
new file mode 100644
index 0000000000..ed3d79d7f8
--- /dev/null
+++ b/rust/Cargo.toml
@@ -0,0 +1,6 @@
+[workspace]
+members = [
+    "xdiff",
+    "interop",
+]
+resolver = "2"
diff --git a/rust/interop/Cargo.toml b/rust/interop/Cargo.toml
new file mode 100644
index 0000000000..045e3b01cf
--- /dev/null
+++ b/rust/interop/Cargo.toml
@@ -0,0 +1,14 @@
+[package]
+name = "interop"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+name = "interop"
+path = "src/lib.rs"
+## staticlib to generate xdiff.a for use by gcc
+## cdylib (optional) to generate xdiff.so for use by gcc
+## rlib is required by the rust unit tests
+crate-type = ["staticlib", "rlib"]
+
+[dependencies]
diff --git a/rust/interop/src/lib.rs b/rust/interop/src/lib.rs
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/rust/xdiff/Cargo.toml b/rust/xdiff/Cargo.toml
new file mode 100644
index 0000000000..eb7966aada
--- /dev/null
+++ b/rust/xdiff/Cargo.toml
@@ -0,0 +1,15 @@
+[package]
+name = "xdiff"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+name = "xdiff"
+path = "src/lib.rs"
+## staticlib to generate xdiff.a for use by gcc
+## cdylib (optional) to generate xdiff.so for use by gcc
+## rlib is required by the rust unit tests
+crate-type = ["staticlib", "rlib"]
+
+[dependencies]
+interop = { path = "../interop" }
diff --git a/rust/xdiff/src/lib.rs b/rust/xdiff/src/lib.rs
new file mode 100644
index 0000000000..e69de29bb2
-- 
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