Firestore for Rust

Load modes, and why listings need preloading

• PreloadNone (.collection(name)): don’t preload anything, just fill the cache while working;
• PreloadAllDocs (.collection_with(name, |c| c.preload_all())): preload all documents in the collection;
• PreloadAllIfEmpty (.collection_with(name, |c| c.preload_all_if_empty())): preload all documents only if the cache is empty. This is useful for the persistent cache; for the memory cache it is the same as PreloadAllDocs, since an in-memory cache always starts empty.

.preloaded_collection(name) picks the appropriate preloading mode for the backend.

A lazily filled collection holds only the documents that happened to be read through it. Answering a list or query from it would return a subset that looks like a complete answer, so the library refuses to do so: read_through_cache quietly falls back to Firestore, and read_cached_only returns an error naming the collection. If partial results are genuinely acceptable, opt in with .incomplete_collection_policy(FirestoreCacheIncompleteCollectionPolicy::PartialResults).