Custom Marshaller can't be for the file

[joe-kimmel-vmw]

Hi -

I have this code:

package main

import (
	"bytes"
	"fmt"

	"github.com/BurntSushi/toml"
)

type Noun struct{ S string }

func (n Noun) MarshalTOML() ([]byte, error) {
	return []byte("\"" + n.S + "\""), nil
}

func main() {
	var buf bytes.Buffer
	type x struct {
		N Noun
	}

	thing := Noun{S: "hello everyone"}

	if err := toml.NewEncoder(&buf).Encode(x{N: thing}); err != nil {
		fmt.Println("got err: ", err)
	} else {
		fmt.Println("successfully marshalled a struct containing a struct with a custom marshaller")
		fmt.Println(buf.String())
	}

	var b2 bytes.Buffer
	if err := toml.NewEncoder(&b2).Encode(thing); err != nil {
		fmt.Println("but you can't directly marshall a struct with a custom marshaller and i don't know why")
		fmt.Println("got err: ", err)
	} else {
		fmt.Println("successfully marshalled a struct containing a struct with a custom marshaller")
		fmt.Println(b2.String())
	}

}

Maybe obviously, I'm trying to make a custom marshaller, but basically the logic for that marshaller is at the level of a file -- I want to put a version check in for the file "if they were using v 0.3 use this marshaller, if 0.4 use this other marshalling logic" but I can't see how to do that without running into the ErrNoKey errors.