Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/BUILD

@@ -0,0 +1,55 @@
+load("@rules_cc//cc:defs.bzl", "cc_library")
+load("@rules_rust//rust:defs.bzl", "rust_library", "rust_test")
+load("//tools/bazel:rust_cxx_bridge.bzl", "rust_cxx_bridge")
+
+rust_test(
+    name = "test",
+    size = "small",
+    srcs = ["test.rs"],
+    edition = "2018",
+    deps = [
+        ":cxx_test_suite",
+        "//:cxx",
+    ],
+)
+
+rust_library(
+    name = "cxx_test_suite",
+    srcs = [
+        "ffi/cast.rs",
+        "ffi/lib.rs",
+        "ffi/module.rs",
+    ],
+    edition = "2018",
+    deps = [
+        ":impl",
+        "//:cxx",
+    ],
+)
+
+cc_library(
+    name = "impl",
+    srcs = [
+        "ffi/tests.cc",
+        ":bridge/source",
+        ":module/source",
+    ],
+    hdrs = ["ffi/tests.h"],
+    deps = [
+        ":bridge/include",
+        ":module/include",
+        "//:core",
+    ],
+)
+
+rust_cxx_bridge(
+    name = "bridge",
+    src = "ffi/lib.rs",
+    deps = [":impl"],
+)
+
+rust_cxx_bridge(
+    name = "module",
+    src = "ffi/module.rs",
+    deps = [":impl"],
+)