encoding/json: fmt.Print representation of json.RawMessage changed in Go 1.27

[adamroyjones]

Go version

go version go1.27.0 linux/amd64

Output of go env in your module/workspace:

go env

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN='/home/adamroyjones/.local/share/go/bin'
GOCACHE='/home/adamroyjones/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/adamroyjones/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build19368907=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/tmp/reprex/go.mod'
GOMODCACHE='/home/adamroyjones/.local/share/go/pkg/mod'
GONOPROXY='github.com/unravelin'
GONOSUMDB='github.com/unravelin'
GOOS='linux'
GOPACKAGESDRIVER=''
GOPATH='/home/adamroyjones/.local/share/go'
GOPRIVATE='github.com/unravelin'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/adamroyjones/.local/share/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.27.0.linux-amd64'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/adamroyjones/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='go1.27.0'
GOTOOLDIR='/home/adamroyjones/.local/share/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.27.0.linux-amd64/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.27.0'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Consider the following program:

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
)

type T struct {
	X json.RawMessage `json:"x"`
	Y []byte          `json:"y"`
}

func main() {
	if err := run(); err != nil {
		panic(err)
	}
}

func run() error {
	j := `{}`
	dec := json.NewDecoder(bytes.NewBufferString(j))

	var t T
	if err := dec.Decode(&t); err != nil {
		return err
	}

	fmt.Printf("%+v\n", t)
	return nil
}

What did you see happen?

$ GOTOOLCHAIN=go1.26.7 go run .
{X:[] Y:[]}

$ GOTOOLCHAIN=go1.27.0 go run .
{X:null Y:[]}

What did you expect to see?

$ GOTOOLCHAIN=go1.26.7 go run .
{X:[] Y:[]}

$ GOTOOLCHAIN=go1.27.0 go run .
{X:[] Y:[]}

I'm not necessarily suggesting that encoding/json ought to change; it is just a minuscule difference between encoding/json before and after the release of Go 1.27 that I thought was worth mentioning.

[gabyhelp]

Related Issues

• encoding/json: Unmarshaling json.RawMessage leads to an unexpected result #46821 (closed)
• encoding/json: Change of behaviour when decoding json null values into RawMessage #11937 (closed)
• json.Rawmessage decodes `null` as `"null"` #40495 (closed)
• encoding/json: changes in gotip(1.27) without any changes #79770 (closed)
• encoding/json: `format:emitempty` error with v1 `json.Marshal` #80962 (closed)
• encoding/json: fields with "inline" tag and unsupported type are silently dropped #79921 (closed)
• encoding/json: unmarshaling into a function value returns a syntax error #79948 (closed)
• encoding/json/v2: missing io.ErrUnexpectedEOF from v1 Decoder.Token #80999 (closed)
• omitempty not omiting structs that encode to an empty JSON object #80733 (closed)
• encode/json: UnmarshalJSON receives modified []byte compared to the one provided to json.Unmarshal #77007 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[dsnet]

The cause of this is because json.RawMessage is now an alias to jsontext.Value, which now has a String method. For a nil, it prints null instead of [], which seems like right behavior. The representation of json.RawMessage via fmt.Print is generally not covered by the Go compatibility agreement, so my inclination is that this change in behavior is acceptable to keep moving forward.

For prior precedence where String methods are okay to add, see #80024.