• Setup HTTP mocking for in-tree client tests.
• Automatic test generation to try deserializing random elements from the MusicBrainz database and find potential errors in our deserialization code more easily.

Currently only a select number of them is hard coded when fetching an entity, however to be really useful, there should be a way to query other relationships too. The API for this is not really written yet.

https://musicbrainz.org/doc/Cover_Art_Archive/API

https://musicbrainz.org/ws/2/artist/93165d52-de77-4286-b9c7-3910aa894165

So far I have only considered that either day, or day and month are missing but not this combination. Apparently we will have to consider, that anything can be missing...

This is mostly a boring data entry job, so if it's only done for the most relevant types for searching, I guess this would be fine also.
See the file src/search/fields.rs for more information.

Due to limitations in generics I decided to go with the current approach since it allows for a type safe API preventing one to add fields to a query which are not available, however this has the caveat that a different method has to be called depending on the entity that is to be queried, which might make some API using code more ugly than it had to be.

(The alternative would probably involve code generation using macros, so it's questionable whether it's really worth it.)

The current query builder only links everything together using AND. Lucene syntax allows to group together things using brackets, so we might want a query builder which allows (in a type-safe way) to specify AND / OR with proper precedence and user defined groups ("brackets").