fix: allow marshalling structs via interface{}

[yan]

This PR allows providing structs (and struct pointers) that are defined as interface{}. e.g., this previously failed with: call of reflect.Value.NumField on interface Value

type Tree struct {
  Objs []interface{} `hcl:"objs,block"`
}

type Object struct {
  Attr string `hcl:"attr"`
}

...
t := Tree{}
t.Objs = append(x.Objs, &Object{ Attr: "hello", })
hcl.MarshalToAST(&t)

[alecthomas]

The problem is there's no way to unmarshal interfaces. At unmarshal time, there's no type information about what concrete type to unmarshal into.

[yan]

The asymmetry is indeed not great, but being able to marshal erased values is really useful. I believe hashi's hcl supports this. What do you think about either returning error on unmarshal, or produce a map[string]interface{} which is what stdlib does, iirc?

[alecthomas]

I think if the only interface supported is the empty one (any/interface{}), then this seems reasonable. If the type is an object it would unmarshal into a map[string]any, if it's a slice it would unmarshal into []any, a string into string, and so on.

[yan]

@alecthomas wdyt about the above solution? i worked on a generic way to unmarshal blocks into any, but that seems like a bad idea. everything else you mentioned above should be supported.

[alecthomas]

Looks awesome, thanks Yan.