regexp: support encoding.TextMarshaler

[flimzy]

What did you do?

Marshaling a compiled regular expression, for example as part of a JSON object, results in an empty object. Example:

re := regexp.MustCompile("foo?")
data, _ := json.Marshal(map[string]interface{}{"foo":re})
fmt.Println(string(data))

Produces:

{"foo":{}}

(Playground)

What did you expect to see?

It would seem more intuitive for a compiled regexp to marshal to its string representation, as if .String() were called. Example:

{"foo":"foo?"}

I propose we implement the encoding.TextMarshaler interface to return the value produced by the String() method.

Strictly speaking, this would be a breaking change, so may need to be targeted to Go 2. On the other hand, in a practical sense, the current behavior is completely useless, so anyone depending on the current behavior probably already has a bug in their program. 🤷‍♂️

[heschi]

cc @rsc

[mvdan]

This would also be nice for #45754. cc @dsnet

[icholy]

@mvdan for this to be useful with #45754 it would need to also implement encoding.TextUnmarshaler.

[dsnet]

Regardless of #45754, I'd be surprised as a user to find a type that implements only either encoding.TextMarshaler or encoding.TextUnmarshaler but not both.

[mvdan]

I share @dsnet's sentiment, though I should also note that in the case of regexp, marshal/unmarshal wouldn't be really symmetric: marshal would just show the original expression string, whereas unmarshal would do a comparatively more expensive regexp compile. A compile also takes options, so I imagine those wouldn't be exposed via TextUnmarshaler.

[icholy]

Losing the options feels like a deal-breaker.

re := regexp.MustCompile(".*")
re.Longest()
data, _ := re.MarshalText()

// Longest() call is not preserved
_ = re.UnmarshalText(data)

[dsnet]

This seems to suggest that MarshalText should not emit the same thing as String, but emits sufficient information to preserve options and whether it was constructed with Compile or CompilePOSIX.

That said, I would expect MarshalText to be identical to String for the common case where no options are involved and its built with Compile (assuming that's the more frequently used one).