rust: add a ObjectID struct

We'd like to be able to write some Rust code that can work with object
IDs.  Add a structure here that's identical to struct object_id in C,
for easy use in sharing across the FFI boundary.  We will use this
structure in several places in hot paths, such as index-pack or
pack-objects when converting between algorithms, so prioritize efficient
interchange over a more idiomatic Rust approach.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1528,6 +1528,7 @@ CLAR_TEST_OBJS += $(UNIT_TEST_DIR)/unit-test.o
 
 UNIT_TEST_OBJS += $(UNIT_TEST_DIR)/test-lib.o
 
+RUST_SOURCES += src/hash.rs
 RUST_SOURCES += src/lib.rs
 RUST_SOURCES += src/varint.rs
 

src/hash.rs

@@ -0,0 +1,21 @@
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation: version 2 of the License, dated June 1991.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program; if not, see <https://www.gnu.org/licenses/>.
+
+pub const GIT_MAX_RAWSZ: usize = 32;
+
+/// A binary object ID.
+#[repr(C)]
+#[derive(Debug, Clone, Ord, PartialOrd, Eq, PartialEq)]
+pub struct ObjectID {
+    pub hash: [u8; GIT_MAX_RAWSZ],
+    pub algo: u32,
+}

src/lib.rs

@@ -1 +1,2 @@
+pub mod hash;
 pub mod varint;

src/meson.build

@@ -1,4 +1,5 @@
 libgit_rs_sources = [
+  'hash.rs',
   'lib.rs',
   'varint.rs',
 ]