For strict json parsing using encoding/json, one has to write the following quite often:

var mystruct Mystruct
decoder := json.NewDecoder(request.Body)
decoder.DisallowUnknownFields()
err := decoder.Decode(&mystruct)

Instead, being able to chain these calls makes the API more pleasant to use:

var mystruct Mystruct
err := json.NewDecoder(request.Body)
    .DisallowUnknownFields()
    .Decode(&mystruct)

It's also less error prone, as the decoder cannot be reused.

Technically this is a breaking change, since the DisallowUnknownFields might by passed as a function pointer, although I cannot imagine any use for that. Alternatively, We could add a Strict method to the decoder, to avoid breaking existing code.