I would like to be able to build the target struct that decode maps into across different contributing pieces that are only stitched together at runtime. My approach for doing this is something like this where Bar extends a config stack by adding a Bar element around the wrapped config contained within:

type Wrapped interface{}

func (c *Bar) Stack(in interface{}) interface{} {
	out = struct {
		Wrapped `mapstructure:",squash"`
		Bar     string
	}{
		Wrapped: in,
	}
	return out
}

I'm using an anonymous interface as there is no anonymous struct. Unfortunately, mapstructure cannot decode squashed interfaces containing structs:

* Wrapped: unsupported type for squash: interface

Source code is there:

Is that something that could be changed? Would be happy to look into a PR here.