Golang fmt: %x format should not call .String() before hex for [...]byte or []byte

Go version

go 1.22.4

Output of go env in your module/workspace:

set GO111MODULE=on
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\Trim21\AppData\Local\go-build
set GOENV=C:\Users\Trim21\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\Trim21\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\Trim21\go
set GOPRIVATE=
set GOPROXY=https://goproxy.cn,direct
set GOROOT=C:/Users/Trim21/.g/versions/1.22.4
set GOSUMDB=sum.golang.org
set GOTMPDIR=C:\Users\Trim21\AppData\Local\Temp\go
set GOTOOLCHAIN=local
set GOTOOLDIR=C:\Users\Trim21\.g\versions\1.22.4\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.22.4
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=C:\Users\Trim21\proj\app\go.mod
set GOWORK=
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=C:\Users\Trim21\AppData\Local\Temp\go\go-build1991184521=/tmp/go-build -gno-record-gcc-switches

What did you do?

package main

import (
    "crypto/sha1"
    "encoding/hex"
    "fmt"
)

type WithString []byte

func (h WithString) String() string {
    return hex.EncodeToString(h[:])
}

type WithOutString []byte

func main() {
    v := sha1.Sum([]byte("hello world"))

    var id WithString = v[:]
    var id2 WithOutString = v[:]

    // 32616165366333356339346663666234313564626539356634303862396365393165653834366564
    fmt.Printf("%X\n", id)

    // 32616165366333356339346663666234313564626539356634303862396365393165653834366564
    fmt.Printf("%x\n", id)

    // 2aae6c35c94fcfb415dbe95f408b9ce91ee846ed
    fmt.Printf("%s\n", id)

    fmt.Printf("%X\n", id2) // 2AAE6C35C94FCFB415DBE95F408B9CE91EE846ED
    fmt.Printf("%x\n", id2) // 2aae6c35c94fcfb415dbe95f408b9ce91ee846ed
    fmt.Printf("%s\n", id2) // *�l5�Oϴ���_@�����F�
}

What did you see happen?

you can see %x for WithString is double hexed here, which is not expected

32616165366333356339346663666234313564626539356634303862396365393165653834366564
32616165366333356339346663666234313564626539356634303862396365393165653834366564
2aae6c35c94fcfb415dbe95f408b9ce91ee846ed
2AAE6C35C94FCFB415DBE95F408B9CE91EE846ED
2aae6c35c94fcfb415dbe95f408b9ce91ee846ed
*�l5�Oϴ��_@����F�

What did you expect to see?

should have same result with hex.EncodeToString here

Comment From: gabyhelp

Similar Issues

• fmt does not format byte slices nicely #34916
• fmt: inconsistency on #v printing of byte array #44391
• fmt: %#x on string prints 0x for every byte #8080

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

Comment From: seankhliao

Duplicate of #21535