Ability to create positional errors after unmarshaling

[xypwn]

Since TOML is generally more user-facing, I think having the ability to generate custom error messages can be extremely helpful. Unfortunately, the only way I'm aware to do this right now is through implementing a custom TextMarshaler - and that can reach its limits very quickly, e.g. when the error occurs at a step that already requires the full document to be unmarshaled. I would really appreciate it if this library had some sort of way to trace back where a specific value came from, ideally with the ability to display a nicely formatted error message.

I implemented a crude version of this myself: xypwn@997b507, but it's definitely not ideal. The way it works is that there's a FieldPosition type, which you can put after a struct field to record the original field's position into it. You can then use a Decoder method to acquire a function that will generate custom errors with a FieldPosition's info.

Example:

var v struct {
	Foo    string
	FooPos toml.FieldPosition
}
dec := toml.NewDecoder(strings.NewReader("Foo = 'baz'"))
makeErr := dec.ErrorMaker()
err := dec.Decode(&v)
if err != nil {
	panic(err)
}

if v.Foo != "bar" {
	fmt.Println(makeErr(v.FooPos, "Foo must equal 'bar'").(*toml.DecodeError).String())
}
// Output:
// 1| Foo = 'baz'
//  |        ~~~ foo must equal 'bar'

I'm not very familiar with this library and its codebase, so maybe someone else has a better idea of how this could be done in a more simple/reliable/performant way.