Vendor dependencies

Let's see how I like this workflow.

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

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+# 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.
+
+# @unsorted-dict-items
+_rust_toolchain_attrs = {
+    # Report unused dependencies
+    "report_unused_deps": False,
+    # Rustc target triple to use
+    # https://doc.rust-lang.org/rustc/platform-support.html
+    "rustc_target_triple": None,
+    # Baseline compiler config
+    "rustc_flags": [],
+    # Extra flags when building binaries
+    "rustc_binary_flags": [],
+    # Extra flags for doing check builds
+    "rustc_check_flags": [],
+    # Extra flags for doing building tests
+    "rustc_test_flags": [],
+    # Extra flags for rustdoc invocations
+    "rustdoc_flags": [],
+    # Use rmeta for lib->lib dependencies, and only block
+    # linking on rlib crates. The hope is that rmeta builds
+    # are quick and this increases effective parallelism.
+    # Currently blocked by https://github.com/rust-lang/rust/issues/85401
+    "pipelined": False,
+    # Filter out failures when we just need diagnostics. That is,
+    # a rule which fails with a compilation failure will report
+    # success as an RE action, but a "failure filter" action will
+    # report the failure if some downstream action needs one of the
+    # artifacts. If all you need is diagnostics, then it will report
+    # success. This doubles the number of actions, so it should only
+    # be explicitly enabled when needed.
+    "failure_filter": False,
+    # The Rust compiler (rustc)
+    "compiler": None,
+    # Rust documentation extractor (rustdoc)
+    "rustdoc": None,
+    # Clippy (linter) version of the compiler
+    "clippy_driver": None,
+    # Wrapper for rustc in actions
+    "rustc_action": None,
+    # Failure filter action
+    "failure_filter_action": None,
+    # The default edition to use, if not specified.
+    "default_edition": None,
+    # Lints
+    "allow_lints": [],
+    "deny_lints": [],
+    "warn_lints": [],
+    # Prefix (/intern/rustdoc in our case) where fbcode crates' docs are hosted.
+    # Used for linking types in signatures to their definition in another crate.
+    "extern_html_root_url_prefix": "",
+}
+
+RustToolchainInfo = provider(fields = _rust_toolchain_attrs.keys())
+
+# Stores "platform"/flavor name used to resolve *platform_* arguments
+RustPlatformInfo = provider(fields = [
+    "name",
+])
+
+def ctx_toolchain_info(ctx: "context") -> "RustToolchainInfo":
+    toolchain_info = ctx.attrs._rust_toolchain[RustToolchainInfo]
+
+    attrs = dict()
+    for k, default in _rust_toolchain_attrs.items():
+        v = getattr(toolchain_info, k)
+        attrs[k] = default if v == None else v
+
+    return RustToolchainInfo(**attrs)