The Rust bson crate supports all of these features:

• en/decoding bson values from byte buffers
• en/decoding bson values from extjson values
• [de]serializing arbitrary values from byte buffers containing encoded bson
• [de]serializing arbitrary values from bson values
• [de]serializing bson values from arbitrary formats

The en/decoding functionality is core to bson; [de]serialization is Rust-specific integration with serde and ideally would be implemented in terms of the core en/decoding and be optional behind a feature flag.

Unfortunately, right now both the public API and internal implementation have badly conflated en/decoding and [de]serialization. Fixing this will give more consistent behavior and API and make the library code much more maintainable.