cmp/report_reflect.go
func formatMapKey(v reflect.Value) string {
	var opts formatOptions
	opts.TypeMode = elideType
	opts.AvoidStringer = true
	opts.ShallowPointers = true
	s := opts.FormatValue(v, visitedPointers{}).String()
	return strings.TrimSpace(s)
}
func formatMapKey(v reflect.Value) string {
	var opts formatOptions
	opts.TypeMode = elideType
	opts.AvoidStringer = true
	opts.ShallowPointers = true
	s := opts.FormatValue(v, visitedPointers{}).String()
	return strings.TrimSpace(s)
}
// formatMapKey formats v as if it were a map key.
// The result is guaranteed to be a single line.