Can you give a short detailed example showing how this would be useful? Thanks.

If you're given data like...

{
    "foo": 123,
    "bar": null,
    "baz": null
}

...then there's a pretty good chance that "bar" and "baz" can have values. Being able to log their values without having to write custom marshal/unmarshal methods would be convenient.

For example, you could write something like this...

type Test struct {
    KnownField int `json:"foo"`
    UnknownFieldA interface{} `json:"bar,debug"`
    UnknownFieldB json.RawMessage `json:"baz,debug"`
}

... (showing use of either interface{} or json.RawMessage) and you would get the output...

json: wrote to field `UnknownFieldA`: `nil`
json: wrote to field `UnknownFieldB`: `null`

... or, if new data was provided...

json: wrote to field `UnknownFieldA`: `map[a:3.14]`
json: wrote to field `UnknownFieldB`: `{"a": 3.14}`

Why can't you just, you know, unmarshal into a variable of type Test and print the value? You already have to edit the code to add the ,debug tag anyhow.

I agree with Ian. If what you want to do is debug json's internals, then editing the json package itself to add debug prints is always possible. We could possibly make that easier with a global const debug = false, but flipping that would require editing the json package anyway.

In general we don't add these kinds of debug/trace hooks to our packages. What if you wanted to debug the execution of math.Sin on a particular input? And so on. The answer is to copy and modify the code to check what you want to check.

Ah yeah, sorry I forgot to close this. Trivial idea, looking back it's not really useful 😁