Detect erroneous JSON(B) encoding

JSON(B) automatically marshals any value. Avoid marshalling values of
pgtype.JSON and pgtype.JSONB. The caller certainly meant to call on a
pointer.

See https://github.com/jackc/pgx/issues/350 for discussion.

refs #350

pgtype/json.go

@@ -33,6 +33,15 @@ func (dst *JSON) Set(src interface{}) error {
 		} else {
 			*dst = JSON{Bytes: value, Status: Present}
 		}
+	// Encode* methods are defined on *JSON. If JSON is passed directly then the
+	// struct itself would be encoded instead of Bytes. This is clearly a footgun
+	// so detect and return an error. See https://github.com/jackc/pgx/issues/350.
+	case JSON:
+		return errors.New("use pointer to pgtype.JSON instead of value")
+	// Same as above but for JSONB (because they share implementation)
+	case JSONB:
+		return errors.New("use pointer to pgtype.JSONB instead of value")
+
 	default:
 		buf, err := json.Marshal(value)
 		if err != nil {