aconfig: add support for cargo

Officially, aconfig is build using the Android tool-chain. However, to
speed up the local development cycle, add support for building with
cargo.

While it is possible to tell cargo to place the build artifacts outside
the source tree, there is no way to tell it to not generate the cargo
lock file in the same directory as Cargo.toml. Add a .gitignore to
ignore Cargo.lock and the target directory.

The way the Android build system and cargo generates code from the
protobuf files is slightly different. Tell cargo to enable the "cargo"
feature and introduce src/protos.rs to hide this difference from the
rest of the aconfig source.

Bug: 279485059
Test: m aconfig && aconfig
Test: atest aconfig.test
Test: cargo build
Test: cargo test
Change-Id: I85741f58cadae353ed95c124f566e4f4a7484186

tools/aconfig/.gitignore

@@ -0,0 +1,2 @@
+/Cargo.lock
+/target

tools/aconfig/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "aconfig"
+version = "0.1.0"
+edition = "2021"
+build = "build.rs"
+
+[features]
+default = ["cargo"]
+cargo = []
+
+[dependencies]
+protobuf = "3.2.0"
+
+[build-dependencies]
+protobuf-codegen = "3.2.0"

tools/aconfig/build.rs

@@ -0,0 +1,17 @@
+use protobuf_codegen::Codegen;
+
+fn main() {
+    let proto_files = vec!["protos/aconfig.proto"];
+
+    // tell cargo to only re-run the build script if any of the proto files has changed
+    for path in &proto_files {
+        println!("cargo:rerun-if-changed={}", path);
+    }
+
+    Codegen::new()
+        .pure()
+        .include("protos")
+        .inputs(proto_files)
+        .cargo_out_dir("aconfig_proto")
+        .run_from_script();
+}

tools/aconfig/src/main.rs

@@ -16,9 +16,11 @@
 
 //! `aconfig` is a build time tool to manage build time configurations, such as feature flags.
 
-use aconfig_protos::aconfig::Placeholder;
 use protobuf::text_format::{parse_from_str, ParseError};
 
+mod protos;
+use protos::Placeholder;
+
 fn foo() -> Result<String, ParseError> {
     let placeholder = parse_from_str::<Placeholder>(r#"name: "aconfig""#)?;
     Ok(placeholder.name)

tools/aconfig/src/protos.rs

@@ -0,0 +1,38 @@
+/*
+ * Copyright (C) 2023 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// When building with the Android tool-chain
+//
+//   - an external crate `aconfig_protos` will be generated
+//   - the feature "cargo" will be disabled
+//
+// When building with cargo
+//
+//   - a local sub-module will be generated in OUT_DIR and included in this file
+//   - the feature "cargo" will be enabled
+//
+// This module hides these differences from the rest of aconfig.
+
+// When building with the Android tool-chain:
+#[cfg(not(feature = "cargo"))]
+pub use aconfig_protos::aconfig::Placeholder;
+
+// When building with cargo:
+#[cfg(feature = "cargo")]
+include!(concat!(env!("OUT_DIR"), "/aconfig_proto/mod.rs"));
+
+#[cfg(feature = "cargo")]
+pub use aconfig::Placeholder;