I'd like to add the ability for cbindgen to generate a stringifier for enums and structs. It could be controlled by a derive-ostream option similar to derive-eq and friends. For enums it would generate something like:

std::ostream& operator<<(std::ostream& aStream, const MyEnum& aEnum) {
  switch (aEnum) {
    case One:
      aStream << "One";
      break;
    ...
  }
  return aStream;
}

and for structs it would do something like this:

struct MyStruct {
  int x;
  int y;
  ...
  friend std::ostream& operator<<(std::ostream& aStream, const MyStruct& aStruct) {
    return aStream << "{ x=" << aStruct.x << ", y=" << aStruct.y << " }";
  }
  ...
};

Any objections/suggestions? The goal here is to remove the hand-written stuff that we have in m-c as part of bug the cleanup I'm doing in bug 1666802 and friends.