Decoder::read_seq requires infinite lookahead

[oli-obk]

Implementing Decoder for a representation that does not know the length of sequences ahead of time requires infinite lookahead, since the length needs to be known before the first element is parsed.

For example: if Json were not parsed to the Json enum as an intermediate, when a sequence is expected, the read_seq function would need to parse ahead until it finds the corresponding ].

Also, all the collection implementations in collection_impls.rs do not require the length to be known beforehand. They only use it as an optimization to reduce the number of allocations.

[alexcrichton]

I'm going to close this now that this crate is deprecated in favor of serde. We're discontinuing feature development in rustc-serialize but will still continue to merge bug fixes if they arise.