Add annotation processor for generating keep rules for types mentioned only inside generics

Otherwise they will be removed by R8 since it cannot determine whether we are doing reflection on them.

Old issue below:

---

Using com.squareup.retrofit2:retrofit:2.9.0

After upgrading gradle plugin from

com.android.tools.build:gradle:4.1.3

to

com.android.tools.build:gradle:4.2.2

I’m getting following error when running with minifyEnabled true:

java.lang.IllegalArgumentException: Method return type must not include a type variable or wildcard

Here’s how looks the method, which generates the error:

    @FormUrlEncoded
    @POST("view_ad") //NON-NLS
    Call<GenericDataResponse<EmptyData>> viewAd(
            @Field("session_id") String sessionId, //NON-NLS
            @Field("id") int id //NON-NLS
    );

    public class EmptyData {

        public EmptyData(){}

    }

    public class GenericDataResponse<T> {

        @SerializedName("result") //NON-NLS
        public String result;

        @SerializedName("data") //NON-NLS
        public T data;

        @SerializedName("mess") //NON-NLS
        public String[] messages;
    }

In order to fix this I have to either:

• downgrade to the previous gradle plugin version
• add proguard rule: -keep class com.example.app.EmptyData
• disable proguard/r8

I’ve read the docs about gradle 4.2 changes and nothing seems to be related to proguard directly. Can someone explain what’s going on?

[ulmaxy]

On a class, the signature I’ve got is similar to this: @GET("endpoint") fun getData(): Single<List<SomeClass>>

And adding the @Keep annotation to SomeClass helped

[JakeWharton]

When running R8 in full mode all generic types are removed unless they are explicitly kept. Retrofit automatically keeps the generic part for users of Call, but if you are using Rx or Kotlin or some other library as your adapter it will not work (and it’s wasteful to keep the generic on all usages of those types). The only way is to explicitly keep the body types, hence the annotation processor.