Decoder API makes no sense for formats like JSON or MessagPack about rustc-serialize

[BurntSushi]

@jpastuszek I think "useless" is probably too strong of a word. With that said, you have absolutely described a limitation. One way around this is to use a Json type explicitly in the parts of the structure that are unknown, decode into that, do your case analysis and decode from the Json type into your appropriate type. The problem is that decoding into Json isn't possible with the current Decoder API (although you can decode from a Json type). I think @erickt has fixed that limitation in rustc-serialize's expect successor: https://github.com/erickt/rust-serde

[jpastuszek]

With this sort of limitation this API style is more in pair with https://crates.io/crates/bincode format rather than JSON. Presenting JSON as main supported (in tree) format for this API without explicitly pointing out this sort of limitations and then asking users to use implementation detail to work around it is perhaps little disappointing IMHO.
Thank you for pointing me to rust-serde, the README file sums this up very well.