Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/apple/xcode.bzl

@@ -0,0 +1,62 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under both the MIT license found in the
+# LICENSE-MIT file in the root directory of this source tree and the Apache
+# License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+# of this source tree.
+
+load("@prelude//apple:apple_sdk.bzl", "get_apple_sdk_name")
+load("@prelude//apple:apple_target_sdk_version.bzl", "get_min_deployment_version_for_node")
+load("@prelude//apple:apple_utility.bzl", "has_apple_toolchain")
+load(
+    "@prelude//cxx:compile.bzl",
+    "CxxSrcWithFlags",  # @unused Used as a type
+)
+load("@prelude//cxx:xcode.bzl", "cxx_populate_xcode_attributes")
+load("@prelude//utils:utils.bzl", "expect")
+
+def apple_populate_xcode_attributes(
+        ctx,
+        srcs: [CxxSrcWithFlags.type],
+        argsfiles_by_ext: {str.type: "artifact"},
+        product_name: str.type) -> {str.type: ""}:
+    data = cxx_populate_xcode_attributes(ctx = ctx, srcs = srcs, argsfiles_by_ext = argsfiles_by_ext, product_name = product_name)
+
+    if has_apple_toolchain(ctx):
+        data["sdk"] = get_apple_sdk_name(ctx)
+        data["deployment_version"] = get_min_deployment_version_for_node(ctx)
+
+    if hasattr(ctx.attrs, "swift_version"):
+        swift_version = ctx.attrs.swift_version
+        if swift_version != None:
+            data["swift_version"] = swift_version
+
+    apple_xcode_data_add_xctoolchain(ctx, data)
+    return data
+
+def apple_xcode_data_add_xctoolchain(ctx: "context", data: {str.type: ""}):
+    _add_label_for_attr(ctx, "_apple_xctoolchain_bundle_id", "xctoolchain_bundle_id_target", data)
+    _add_output_for_attr(ctx, "_apple_xctoolchain_bundle_id", "xctoolchain_bundle_id", data)
+    _add_label_for_attr(ctx, "_apple_xctoolchain", "xctoolchain_bundle_target", data)
+
+def _add_label_for_attr(ctx: "context", attr_name: str.type, field_name: str.type, data: {str.type: ""}):
+    xctoolchain_dep = _get_attribute_with_output(ctx, attr_name)
+    if xctoolchain_dep:
+        data[field_name] = xctoolchain_dep.label
+
+def _add_output_for_attr(ctx: "context", attr_name: str.type, field_name: str.type, data: {str.type: ""}):
+    xctoolchain_dep = _get_attribute_with_output(ctx, attr_name)
+    if xctoolchain_dep:
+        default_info = xctoolchain_dep[DefaultInfo]
+        expect(len(default_info.default_outputs) == 1, "Expected only one output, got {}", len(default_info.default_outputs))
+        data[field_name] = default_info.default_outputs[0]
+
+def _get_attribute_with_output(ctx: "context", attr_name: str.type) -> ["dependency", None]:
+    if hasattr(ctx.attrs, attr_name):
+        dep = getattr(ctx.attrs, attr_name)
+        default_info = dep[DefaultInfo]
+        if len(default_info.default_outputs) > 0:
+            # When there's no xctoolchain, there will be an empty `DefaultInfo`.
+            # So, an empty `DefaultInfo` basically signifies that there's no xctoolchain.
+            return dep
+    return None