aconfig: add project scaffolding

Introduce a new tool, aconfig, to manage build time configurations, such
as feature flags.

This CL adds a project skeleton. Later CLs will add the actual
implementation.

Note: there is no need for an explicit TEST_MAPPING file;
rust_test_host are automatically included in host-unit-tests and
explicitly adding these tests via TEST_MAPPING will cause an error.

Bug: 279485059
Test: m aconfig && aconfig
Test: atest aconfig.test
Change-Id: I94047a19a55ae4d45b4fd7a0c8105a91ddcdfd79
diff --git a/tools/aconfig/src/main.rs b/tools/aconfig/src/main.rs
new file mode 100644
index 0000000..2f7255e
--- /dev/null
+++ b/tools/aconfig/src/main.rs
@@ -0,0 +1,53 @@
+/*
+ * 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.
+ */
+
+//! `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};
+
+fn foo() -> Result<String, ParseError> {
+    let placeholder = parse_from_str::<Placeholder>(r#"name: "aconfig""#)?;
+    Ok(placeholder.name)
+}
+
+fn main() {
+    println!("{:?}", foo());
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_foo() {
+        assert_eq!("aconfig", foo().unwrap());
+    }
+
+    #[test]
+    fn test_binary_protobuf() {
+        use protobuf::Message;
+        let mut buffer = Vec::new();
+
+        let mut original = Placeholder::new();
+        original.name = "test".to_owned();
+        original.write_to_writer(&mut buffer).unwrap();
+
+        let copy = Placeholder::parse_from_reader(&mut buffer.as_slice()).unwrap();
+
+        assert_eq!(original, copy);
+    }
+}