FieldOptions.wire_name to dodge name collisions on options

We’ve got a problem where Wire generates broken code when the same option name is used for different targets in the same package.

extend google.protobuf.MethodOptions {
  optional float objective = 26101;
}

extend google.protobuf.ServiceOptions {
  optional float objective = 26101;
}

This generates two annotations that potentially collide.

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface ObjectiveOption {
  float value();
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface ObjectiveOption {
  float value();
}

I really like the name ObjectiveOption here and am reluctant to qualify the name with its target ObjectiveMethodOption because this collision is just so rare. Instead I’d like to introduce a mechanism where developers who introduce such collisions in the .proto can choose disambiguating names.

We’ll create a new extension:

extend google.protobuf.FieldOptions {
  /**
   * Sets the name used for this field in generated code.
   *
   * This may only be applied to extension fields, and only impacts the generated names for option
   * annotations.
   *
   * It is an error to apply this option to regular message fields and oneof fields.
   *
   * This doesn't impact the name used an any encoding or decoding, such as JSON encoding.
   */
  optional string wire_name = 9999; // TODO
}

And users would use it like so:

extend google.protobuf.MethodOptions {
  optional float objective = 26101 [(wire.wire_name) = "rpc_objective"];
}

extend google.protobuf.ServiceOptions {
  optional float objective = 26101 [(wire.wire_name) = "service_objective"];
}

Start by locating the code that defines Wire's protobuf extensions and the generator for option annotations. Trace how extension field names become generated annotation names, then add coverage for the two colliding options and for rejecting the option on regular and oneof fields. Done means the declared wire_name values disambiguate generated annotations without changing encoding or decoding names.