x/tools/go/analysis/passes/modernize: fmtappendf produces uncompilable code with multi-line []byte type cast #77821
Description
Go version
go version go1.26.0 linux/amd64
Output of go env in your module/workspace:
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=''
GOCACHE='/home/joe/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/joe/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build730289841=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/joe/gofixbug/go.mod'
GOMODCACHE='/home/joe/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/joe/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/joe/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.26.0'
GOWORK=''
PKG_CONFIG='pkg-config'What did you do?
Run go fix on the example:
package t
import "fmt"
type T struct {
S string
}
func (t *T) Bytes() []byte {
return []byte(
fmt.Sprintf("S=%s", t.S),
)
}What did you see happen?
The return statement is placed above the rewrite of fmt.Sprintf producing a naked return that will not compile:
package t
import "fmt"
type T struct {
S string
}
func (t *T) Bytes() []byte {
return
fmt.Appendf(nil, "S=%s", t.S)
}$> go build t.go
# command-line-arguments
./t.go:10:2: not enough return values
have ()
want ([]byte)
./t.go:12:1: missing returnWhat did you expect to see?
The rewrite should produce the return alongside the new fmt.Appendf call.
Compacting the []byte() cast to one line:
package t
import "fmt"
type T struct {
S string
}
func (t *T) Bytes() []byte {
return []byte(fmt.Sprintf("S=%s", t.S))
}And running go fix produces compiling code:
package t
import "fmt"
type T struct {
S string
}
func (t *T) Bytes() []byte {
return fmt.Appendf(nil, "S=%s", t.S)
}This output should also be produced for multi-line type casts.