Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/BUCK

@@ -0,0 +1,57 @@
+load("//tools/buck:rust_cxx_bridge.bzl", "rust_cxx_bridge")
+
+rust_test(
+    name = "test",
+    srcs = ["test.rs"],
+    edition = "2018",
+    deps = [
+        ":ffi",
+        "//:cxx",
+    ],
+)
+
+rust_library(
+    name = "ffi",
+    srcs = [
+        "ffi/cast.rs",
+        "ffi/lib.rs",
+        "ffi/module.rs",
+    ],
+    crate = "cxx_test_suite",
+    edition = "2018",
+    deps = [
+        ":impl",
+        "//:cxx",
+    ],
+)
+
+cxx_library(
+    name = "impl",
+    srcs = [
+        "ffi/tests.cc",
+        ":bridge/source",
+        ":module/source",
+    ],
+    exported_deps = ["//:core"],
+    exported_headers = {
+        "ffi/lib.rs.h": ":bridge/header",
+        "ffi/module.rs.h": ":module/header",
+        "ffi/tests.h": "ffi/tests.h",
+    },
+)
+
+rust_cxx_bridge(
+    name = "bridge",
+    src = "ffi/lib.rs",
+    deps = [
+        ":impl",
+    ],
+)
+
+rust_cxx_bridge(
+    name = "module",
+    src = "ffi/module.rs",
+    deps = [
+        ":impl",
+    ],
+)