Actually the last two `.iter()` aren't needed at all because of IntoIter. All three can all be removed if you use Itertools' izip!

    let codes: Vec<_> =
        izip!(&data_table, code_lengths, code_table)
            .map(|(&value, length, code)| {
                HuffmanCode {
                    length: length,
                    code: code,
                    value: value,
                }
            })
            .collect();

(If HuffmanCode was a tuple type, this could even be

    #![feature(fn_traits)]

    let codes: Vec<_> =
        izip!(data_table.iter().cloned(), code_lengths, code_table)
            .map(|args| (&HuffmanCode).call(args))
            .collect();

but now I'm just playing around.)