encoding/json: clarify Marshal behavior for string keys that implement encoding.TextMarshaler

[reinerRubin]

What version of Go are you using (go version)?

go version go1.10.3 linux/amd64

Does this issue reproduce with the latest release?

yes

What did you do?

https://play.golang.org/p/uBmsW13USTc

tldr:
Before checking for encoding.TextMarshaler implementation the resolve method makes this check

if w.v.Kind() == reflect.String {

so if out type of key is something like:

type MyEnumString string

resolve ignores the encoding.TextMarshaler implementation.

go/src/encoding/json/encode.go

Line 864 in 969b9d8

func (w *reflectWithString) resolve() error {

What did you expect to see?

encoding.TextMarshaler is called for string types.

What did you see instead?

Just base string value.

/I have made small proof for myself:
https://play.golang.org/p/CFRVA4yDUlA

[reinerRubin]

If it's a valid bug, I can fix it. I think about or checking for interface implementation beforehand, or mv Kind() to Type(). But the second variant seems quite dangerous.

[bontibon]

cc @rsc

[bontibon]

See the discussion when TextMarshaler support was added for map keys: #12146. There was concerns about backwards compatibility for existing kind strings that implement TextMarshaler.

This may have to be a Go2 request.

[reinerRubin]

Oh, I have tried to find discussion by "TextMarshaler" keyword, but failed. Thanks!

So what shoud I do now? Add a go2 request explicitly?

[bcmills]

This is mentioned in the documentation (https://tip.golang.org/pkg/encoding/json/#Marshal):

The map keys are sorted and used as JSON object keys by applying the following rules, subject to the UTF-8 coercion described for string values above:

• string keys are used directly
• encoding.TextMarshalers are marshaled
• integer keys are converted to strings

We could certainly be more explicit about the fact that “string keys” means “keys of any string type” rather than “keys of type string exactly”.

[reinerRubin]

I think it would help. Because basically you can not put your string based type instead a string without an explicit conversion.