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cmd/fix, x/tools/go/analysis/passes/modernize: stringsbuilder breaks code on self-referencing code #78465

@csilvers

Description

@csilvers

Go version

go1.26.1 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/csilvers/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/csilvers/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2900690068=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/csilvers/khan/webapp/go.mod'
GOMODCACHE='/home/csilvers/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/csilvers/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/csilvers/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.26.1.linux-amd64'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/csilvers/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/csilvers/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.26.1.linux-amd64/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.26.1'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Consider this example:

package main

func f() string {
	a := "12"
	for range 2 {
		a += a
	}
	return a
}

This is using x/tools version v0.42.0.

What did you see happen?

Output from running go run golang.org/x/tools/go/analysis/passes/modernize/cmd/modernize@latest -fix ./... results in non compiling code:

package main

import "strings"

func f() string {
	var a strings.Builder
	a.WriteString("12")
	for range 2 {
		a.WriteString(a).String()
	}
	return a.String()
}

What did you expect to see?

The line within the loop should be a.WriteString(a.String()).

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BugReportIssues describing a possible bug in the Go implementation.NeedsInvestigationSomeone must examine and confirm this is a valid issue and not a duplicate of an existing one.

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