use encoding.TextMarshaler if a value implements it

[kazegusuri]

I would like to use a type provided by 3rd party package. When the type does not implement neither graphql.Marshaler nor json.Marshaler, the result of MarshalJSON generates an unexpected string in some cases.

For example, assuming using github.com/google/uuid.uuid.UUID in a variable,

This is an example schema.

type Query {
    uuidToString(uuid: UUID!): String!
}

scalar UUID

Bind the scalar to in gqlgenc.yaml

models:
  UUID:
    model: github.com/google/uuid.UUID

Then, gqlgenc generates a code to marshal a variable like this:

	vars := map[string]any{
		"uuid":   uuid.New(),
	}
	b, err := clientv2.MarshalJSON(vars)
        ...
        fmt.Printf("json: %s\n", b)

This generates a json value,

json: {"uuid":[173,99,127,52,45,165,69,156,136,157,99,98,166,115,160,109]} // unexpected

However, if we use json.Marshal directly instead of MarshalJSON, they generate different json values.

	vars := map[string]any{
		"uuid":   uuid.New(),
	}
	b, err := clientv2.MarshalJSON(vars)
        ...
        fmt.Printf("json: %s\n", b)
	b2, err := json.Marshal(vars)
        ...
        fmt.Printf("json: %s\n", b2)

json {"uuid":[140,251,26,253,170,172,74,61,164,123,188,187,190,136,237,190]}
json {"uuid":"8cfb1afd-aaac-4a3d-a47b-bcbbbe88edbe"}

This is because json.Marshal's encoder supports encoding.TextMarshaler.
The description: https://cs.opensource.google/go/go/+/master:src/encoding/json/encode.go;l=32-40
The implementation: https://cs.opensource.google/go/go/+/master:src/encoding/json/encode.go;l=379-395

So I think it's better to respect encoding.TextMarshaler in MarshalJSON too. The current implementation of MarshalJSON calls json.Marshaler.MarshalJSON if a value implements json.Marshaler interface.

[Yamashou]

@kazegusuri LGTM!
Thank you for addressing a pattern I hadn’t noticed.