Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/toolchains/rust.bzl

@@ -0,0 +1,60 @@
+# 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//rust:rust_toolchain.bzl", "RustPlatformInfo", "RustToolchainInfo")
+
+_DEFAULT_TRIPLE = select({
+    "ovr_config//os:linux": select({
+        "ovr_config//cpu:arm64": "aarch64-unknown-linux-gnu",
+        "ovr_config//cpu:x86_64": "x86_64-unknown-linux-gnu",
+    }),
+    "ovr_config//os:macos": select({
+        "ovr_config//cpu:arm64": "aarch64-apple-darwin",
+        "ovr_config//cpu:x86_64": "x86_64-apple-darwin",
+    }),
+    "ovr_config//os:windows": select({
+        # FIXME: rustup's default ABI for the host on Windows is MSVC, not GNU.
+        # When you do `rustup install stable` that's the one you get. It makes
+        # you opt in to GNU by `rustup install stable-gnu`. We should match that
+        # default when we're able; but for now buck2 doesn't work with the MSVC
+        # toolchain yet.
+        "ovr_config//cpu:arm64": "aarch64-pc-windows-gnu",
+        "ovr_config//cpu:x86_64": "x86_64-pc-windows-gnu",
+    }),
+})
+
+def _system_rust_toolchain_impl(ctx):
+    return [
+        DefaultInfo(),
+        RustToolchainInfo(
+            clippy_driver = "clippy-driver",
+            compiler = "rustc",
+            failure_filter_action = ctx.attrs.failure_filter_action[RunInfo],
+            rustc_action = ctx.attrs.rustc_action[RunInfo],
+            default_edition = ctx.attrs.default_edition,
+            rustc_target_triple = ctx.attrs.rustc_target_triple,
+            rustdoc = "rustdoc",
+            rustc_flags = ctx.attrs.rustc_flags,
+            rustdoc_flags = ctx.attrs.rustdoc_flags,
+        ),
+        RustPlatformInfo(
+            name = "x86_64",
+        ),
+    ]
+
+system_rust_toolchain = rule(
+    impl = _system_rust_toolchain_impl,
+    attrs = {
+        "default_edition": attrs.option(attrs.string(), default = None),
+        "failure_filter_action": attrs.default_only(attrs.dep(providers = [RunInfo], default = "prelude//rust/tools:failure_filter_action")),
+        "rustc_action": attrs.default_only(attrs.dep(providers = [RunInfo], default = "prelude//rust/tools:rustc_action")),
+        "rustc_flags": attrs.list(attrs.string(), default = []),
+        "rustc_target_triple": attrs.string(default = _DEFAULT_TRIPLE),
+        "rustdoc_flags": attrs.list(attrs.string(), default = []),
+    },
+    is_toolchain_rule = True,
+)