Tag rename missing is encoding

[jsumners]

If I'm not mistaken, the main.go below should output:

sub_config:
    first_children:
        - name: parent
          grand_children:
            - name: foo
            - name: bar

Instead, it is outputting:

sub_config:
    first_children:
        - name: parent
          children:
            - name: foo
            - name: bar

Notice that the second slice is intended to be named grand_children but is really being named children as it is actually named in the struct.

If I step through the mapstructure.Decode function, I can see the tag get read and build an appropriate fieldName as sub_config.first_children[0].grand_children, but that at some point gets shuffled back to children.

We can see in this screenshot the result of the Decode prior to rendering the yaml:

main.go

package main

import (
	"fmt"
	"os"

	"github.com/mitchellh/mapstructure"
	"gopkg.in/yaml.v3"
)

type Config struct {
	Subconfig SubSection `mapstructure:"sub_config"`
}

type SubSection struct {
	Children []Child `mapstructure:"first_children"`
}

type Child struct {
	Name     string
	Children []Grandchild `mapstructure:"grand_children"`
}

type Grandchild struct {
	Name string
}

func main() {
	config := &Config{
		Subconfig: SubSection{
			Children: []Child{
				{Name: "parent", Children: []Grandchild{
					{Name: "foo"},
					{Name: "bar"},
				}},
			},
		},
	}

	encodedData := map[string]interface{}{}
	err := mapstructure.Decode(config, &encodedData)
	if err != nil {
		err = fmt.Errorf("unable to encode configuration: %w", err)
		fmt.Println(err)
		os.Exit(1)
	}

	data, err := yaml.Marshal(encodedData)
	if err != nil {
		err = fmt.Errorf("unable to marshal to yaml: %w", err)
		fmt.Println(err)
		os.Exit(1)
	}

	fmt.Println(string(data))
}

go.mod

module foo

go 1.19

require (
	github.com/mitchellh/mapstructure v1.5.0 // indirect
	gopkg.in/yaml.v3 v3.0.1 // indirect
)