cargo: Do not download platform2 by default

Cargo tries to download dependencies even if it's optional.
Stub it to stop download the platform2 repository.

See also upstream bug:
rust-lang/cargo#4544

BUG=None
TEST=audio-qv

Change-Id: If5e66313d3ed688a940843b6bff627ed23232bf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/adhd/+/4622503
Commit-Queue: Li-Yu Yu <aaronyu@google.com>
Tested-by: ChromeOS Audio Quick Verifier <audio-qv@chromeos-audio-qv.iam.gserviceaccount.com>
Tested-by: chromeos-cop-builder@chromeos-cop.iam.gserviceaccount.com <chromeos-cop-builder@chromeos-cop.iam.gserviceaccount.com>
Reviewed-by: Chih-Yang Hsia <paulhsia@chromium.org>

Cargo.Bazel.lock

@@ -1,5 +1,5 @@
 {
-  "checksum": "6a8dda70d6281ab460a2fe29586969fe860154c9960e091625a16084dbf11694",
+  "checksum": "94543ee1647ad50697832b07a6b3ec89bda67c736fa2d45100aa019e9fbb4487",
   "crates": {
     "aho-corasick 0.7.20": {
       "name": "aho-corasick",

Cargo.toml

@@ -17,6 +17,12 @@ audio_processor = { path = "audio_processor" }
 cras = { path = "cras/src/server/rust" }
 cras_dlc = { path = "cras/src/server/rust/cras_dlc" }
 
-system_api = { git = "https://chromium.googlesource.com/chromiumos/platform2", version = ">=0.24.53" } # ignored by ebuild
-# Replace the above with the below when making testing in-repo changes in chroot.
-#system_api = { path = "../../platform2/system_api" } # ignored by ebuild
+# When building out of ChromiumOS, we do not actually use system_api.
+# Stub it to avoid downloading the platform2 repository.
+# See also: https://github.com/rust-lang/cargo/issues/4544
+system_api = { path = "repositories/system_api_stub" } # ignored by ebuild
+# Replace the above with the below when testing with ChromiumOS features.
+# For out of chroot using TOT platform2:
+# system_api = { git = "https://chromium.googlesource.com/chromiumos/platform2", version = ">=0.24.53" } # ignored by ebuild
+# For testing in-repo changes in chroot:
+# system_api = { path = "../../platform2/system_api" } # ignored by ebuild

repositories/system_api_stub/Cargo.toml

@@ -0,0 +1,5 @@
+[package]
+name = "system_api"
+version = "0.24.53"
+authors = ["The ChromiumOS Authors"]
+edition = "2021"

repositories/system_api_stub/src/lib.rs

@@ -0,0 +1,3 @@
+// Copyright 2023 The ChromiumOS Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.