go/printer: output differs to "go fmt" for consts and structs

[ceving]

Go version

go version go1.23.2 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/szi/.cache/go-build'
GOENV='/home/szi/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/szi/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/szi/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/go1.23.2.linux-amd64'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/go1.23.2.linux-amd64/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23.2'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/szi/.config/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build736226514=/tmp/go-build -gno-record-gcc-switches'

What did you do?

Source

package main

import (
	"fmt"
	"go/parser"
	"go/printer"
	"go/token"
	"log"
	"os"
	"os/exec"
	"path/filepath"
	"runtime"
)

const (
	FirstConst = iota
	SecondConst
)

type FirstStruct struct {
	FirstProperty  string
	SecondProperty string
}

func main() {
	// Get source filename
	var err error
	_, InputName, _, ok := runtime.Caller(0)
	if !ok {
		log.Fatalf("Can not read source.")
	}

	// Parse source file
	Fileset := token.NewFileSet()
	Source, err := parser.ParseFile(Fileset, InputName, nil, parser.ParseComments)
	if err != nil {
		log.Fatal(err)
	}

	// Write source
	OutputName := fmt.Sprintf("printer-%s", filepath.Base(InputName))
	Output, err := os.Create(OutputName)
	if err != nil {
		log.Fatal(err)
	}
	err = printer.Fprint(Output, Fileset, Source)
	if err != nil {
		log.Fatal(err)
	}
	Output.Close()

	// Compare
	cmp := exec.Command("cmp", InputName, OutputName)
	err = cmp.Run()
	if err != nil {
		log.Fatal(err)
	}
}

What did you see happen?

$ go fmt inconsistency.go
$ go run inconsistency.go
2024/10/11 09:58:41 exit status 1
exit status 1
$ diff inconsistency.go printer-inconsistency.go 
16c16
<       FirstConst = iota
---
>       FirstConst      = iota
21,22c21,22
<       FirstProperty  string
<       SecondProperty string
---
>       FirstProperty   string
>       SecondProperty  string

The command go fmt and the package go/print do something different for consts and structs.

What did you expect to see?

No difference

[gabyhelp]

Related Issues and Documentation

• go/printer: Fprintf uses different settings than go fmt #16963 (closed)
• gccgo: reflection-based type printing inconsistency #23620 (closed)
• gccgo: inconsistent behaviour of %s fmt verb #23642 (closed)
• fmt: Inconsistent behaviour between fmt.Print() and fmt.Println() when supplying multiple arguments #41798 (closed)
• Detected a bug while using Print function in fmt package #34740 (closed)
• fmt.Printf("%p", &a) changes result of pointer of empty struct comparison #68701 (closed)
• go/src/bufio: fmt.Print files differently for windows/amd64 and linux/amd64 #30880 (closed)
• Empty struct comparison behavior is inconsistent #43755 (closed)
• go/format: any input to format.Source panics #42300 (closed)
• x/tools/cmd/goimports: inconsistency with go fmt #37584 (closed)

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

[mateusz834]

This is working as intended, go fmt uses a different configuration for the printer:

go/src/cmd/gofmt/gofmt.go

Lines 48 to 49 in d20a4c2

	tabWidth = 8
	printerMode = printer.UseSpaces | printer.TabIndent | printerNormalizeNumbers

go/src/cmd/gofmt/gofmt.go

Line 262 in d20a4c2

res, err := format(fileSet, file, sourceAdj, indentAdj, src, printer.Config{Mode: printerMode, Tabwidth: tabWidth})

than the go/printer defaults:

go/src/go/printer/printer.go

Line 1432 in d20a4c2

return (&Config{Tabwidth: 8}).Fprint(output, fset, node)

You should probably use the go/format package instead, it has the same defaults as go fmt