Automatically derive Eq and Hash for Label (#676)

Ref #630

oak/server/rust/oak_abi/build.rs

@@ -25,5 +25,8 @@ fn main() {
     // https://doc.rust-lang.org/cargo/reference/build-scripts.html#rerun-if-changed
     println!("cargo:rerun-if-changed={}", oak_api_path.to_str().unwrap());
 
-    prost_build::compile_protos(&[oak_api_path, policy_path], &[proto_dir]).unwrap();
+    prost_build::Config::new()
+        .type_attribute(".oak.policy", "#[derive(Eq,Hash)]")
+        .compile_protos(&[oak_api_path, policy_path], &[proto_dir])
+        .unwrap();
 }

oak/server/rust/oak_abi/src/label.rs

@@ -40,21 +40,6 @@ pub trait Label: Sized {
     fn can_flow_to(&self, other: &Self) -> bool;
 }
 
-// Implement the necessary traits on `Tag` so that it may be used in `HashSet`.
-
-impl Eq for crate::proto::policy::Tag {}
-
-#[allow(clippy::derive_hash_xor_eq)]
-impl std::hash::Hash for crate::proto::policy::Tag {
-    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
-        // As a quick solution, we simply serialize the message and hash the resulting `Vec`.
-        let mut bytes = Vec::new();
-        self.encode(&mut bytes)
-            .expect("could not serialize to bytes");
-        bytes.hash(state);
-    }
-}
-
 impl Label for crate::proto::policy::Label {
     fn serialize(&self) -> Vec<u8> {
         let mut bytes = Vec::new();