Custom Unmarshaller's receive no data for nested structs

[wrouesnel]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce

package main

import (
	"bytes"
	"fmt"

	"github.com/goccy/go-yaml"
)

type Outside struct {
	X      string `yaml:"x"`
	Inside `yaml:",inline"`
}

type Inside struct {
	Y string `yaml:"y"`
	Z string `yaml:"z"`
}

const data = `x: x-value
y: y-value
z: z-value
`

func main() {
	o := Outside{}

	yaml.RegisterCustomUnmarshaler(func(t *Inside, i []byte) error {
		return yaml.Unmarshal(i, t)
	})

	yaml.NewDecoder(bytes.NewReader([]byte(data))).Decode(&o)
	fmt.Println(o)
}

Please provide a minimum yaml content that can be reproduced.
We are more than happy to use Go Playground

Expected behavior
The above should produce:

{x-value { y-value z-value }}

but instead produces:

{x-value { }}

Version Variables

• Go version: 1.25.
• go-yaml's Version: v1.19.2

Additional context

This looks to be a problem with the processing of inline structs which starts here:

go-yaml/decode.go

Line 1382 in 92bc79c

mapNode := ast.Mapping(nil, false)

where a synthetic node map is built without token references, and then when the unmarshal selector logic runs here:

go-yaml/decode.go

Line 699 in 92bc79c

func (d *Decoder) unmarshalableDocument(node ast.Node) ([]byte, error) {

FormatNodeWithResolved alias is called against the synthetic node tree and jumps here:

go-yaml/internal/format/format.go

Line 10 in 92bc79c

func FormatNodeWithResolvedAlias(n ast.Node, anchorNodeMap map[string]ast.Node) string {

Which then immediately tries to get the first token, won't find any (because none is set) and returns an empty string instead.

[vnykmshr]

Traced through the same path you described. The synthetic node map built at decode.go:1382 has no tokens attached, so FormatNodeWithResolvedAlias returns empty bytes to the custom unmarshaler. Your analysis looks right.