I have a use case where I compare very nested protobuf code generated structures. The behaviour is that if a structure has an Equals() method, the library stops recursing. To bypass that I convert the protobuf structures to json and back into very very nested map[string]interface{}`s. When I report a particular Diff I would like it to appear with a key - rootType.Level1Field.Level2Field.Level3Field.Leaf or something similar. I cannot do that because the String() implementation does not work with nested maps or slices. I have to resort to fmt.Sprintf("%#v", path) which for nested code generated structures is very verbose. E.g. root["period"].(map[string]interface {})["endsAt"].(map[string]interface {})["value"].(string), this is an actual output for 3 levels of nesting (which isn't that much nesting).

https://github.com/google/go-cmp/blob/master/cmp/path.go#L93

// String returns the simplified path to a node.
// The simplified path only contains struct field accesses.
//
// For example:
//
//	MyMap.MySlices.MyField
func (pa Path) String() string {
	var ss []string
	for _, s := range pa {
		if _, ok := s.(StructField); ok {
			ss = append(ss, s.String())
		}
	}
	return strings.TrimPrefix(strings.Join(ss, ""), ".")
}

There's a type assertion that only allows struct field accesses, for a reason unknown to me. If that's necessary at the very least the documentation is misleading because it suggests that it works on maps and slices - // MyMap.MySlices.MyField

The way I bypassed this is by implementing a custom stringer func

func pathStringer(p cmp.Path) string {
	var ss []string

	if len(p) > 0 { // the root node
		ss = append(ss, "{"+p[0].Type().String()+"}")
	}

	for _, step := range p {
		switch step.(type) {
		case cmp.StructField, cmp.SliceIndex, cmp.Transform, cmp.MapIndex:
			ss = append(ss, step.String())
		}
	}
	return strings.Join(ss, "")
}

Ideally I shouldn't resort to that and I would like it to just work as described in the doc string.