Metadata doesn't seem to populate when Decoding from a Struct into a map

[dakojohn]

Problem:

The metadata struct doesn't seem to populate when Decoding a Struct into a map.

Example:

https://go.dev/play/p/tNdbyQI4GE3

package main

import (
	"fmt"

	"github.com/go-viper/mapstructure/v2"
)

type ExampleStruct struct {
	ID          string `mapstructure:"id"`
	Name        string `mapstructure:"name"`
	Description string `mapstructure:"description"`
	Hidden      string `mapstructure:"-"`
	unexported  string
}

func main() {
	p := &ExampleStruct{
		ID:          "123",
		Name:        "test",
		Description: "desc",
		Hidden:      "hidden",
		unexported:  "unexported value",
	}

	// Use the struct as input to create a map as output
	var resultMap map[string]any
	var metadata mapstructure.Metadata

	// Decode FROM the struct TO a map
	err := mapstructure.DecodeMetadata(p, &resultMap, &metadata)
	if err != nil {
		fmt.Printf("Error decoding: %v\n", err)
		return
	}

	fmt.Printf("Original struct: %+v \n", p)
	fmt.Println("Decode results: ", resultMap)
	fmt.Println("Metadata Keys:  ", metadata.Keys)
	fmt.Println("Metadata Unused:", metadata.Unused)
	fmt.Println("Metadata Unset: ", metadata.Unset)
}

Output:

Original struct: &{ID:123 Name:test Description:desc Hidden:hidden unexported:unexported value} 
Decode results:  map[description:desc id:123 name:test]
Metadata Keys:   []
Metadata Unused: []
Metadata Unset:  []

Program exited.

Expected:

Original struct: &{ID:123 Name:test Description:desc Hidden:hidden unexported:unexported value} 
Decode results:  map[description:desc id:123 name:test]
Metadata Keys:   [id name description]
Metadata Unused: []
Metadata Unset:  [unexported -]

Program exited.