Hi,

In a project I'm working on, it would be really useful if MapStruct supported collection mapping methods with multiple source parameters.

Such methods would only be supported when all of the following conditions are met. Otherwise, MapStruct should raise an error, in the same way it does when no suitable mapping method can be found.

1. The target type is an iterable or array type.
2. The first source parameter is an iterable or array type.
3. Any subsequent source parameters are non-iterable/non-array types.
4. A per-element mapping method exists with the same parameter list, except that the iterable/array source parameter is replaced by its element type and the iterable/array target type is replaced by its element type.

Such a mapper could look like this:

@Mapper
public interface AddressMapper {
    @Mapping(target = "description", source = "person.description")
    @Mapping(target = "houseNumber", source = "address.houseNo")
    DeliveryAddressDto personAndAddressToDeliveryAddressDto(Person person, Address address);

    List<DeliveryAddressDto> inhabitantsToDeliveryAddressDto(List<Person> persons, Address address);
}

and the generated code could look something like:

@Override
public List<DeliveryAddressDto> inhabitantsToDeliveryAddressDto(List<Person> persons, Address address) {
    if (persons == null) {
        return null;
    }
    List<DeliveryAddressDto> list = new ArrayList<>( persons.size() );
    for ( Person person : persons ) {
        list.add( personAndAddressToDeliveryAddressDto(person, address) );
    }
    return list;
}

This seems like a natural extension of MapStruct's existing collection mapping support, with additional source parameters simply forwarded unchanged to the element-mapping method.

Would this be something the project would be interested in supporting? If so, I'd be happy to work on an implementation and submit a pull request.