proposal: encoding/json: add access to the underlying data causing UnmarshalTypeError

[buyology]

Currently one has to maintain a copy of the data being decoded by json.Decoder to retrieve the underlying data yielding an UnmarshalTypeError when decoding a json stream.

Making the data in decodeState.data directly accessible in the returned error:

type UnmarshalTypeError struct {
    …
    Data    []byte
}

would make it easy to enable improved diagnostics for faulty inputs.

(Somewhat related to #8254 and #9693 which are also about error diagnostics in encoding/json.)

[nathany]

#6716 is also related and implemented

[rsc]

On hold until next big encoding/json sweep.