x/tools/cmd/goimports: line directive can cause file corruption

[kortschak]

What version of Go are you using (go version)?

$ go version
go version go1.17.6 linux/amd64

goimports version v0.1.9..

Does this issue reproduce with the latest release?

Yes, also on playground.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN="/home/user/bin"
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/user/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/user"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/user/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/user/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.6"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/user/thinking/linedirective/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build229616347=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Run goimports on the following program.

package main

import (
	"fmt"
	"os"
)

//line :4

// Comment
func main() {
	fmt.Fprintln(os.Stderr, "Hello world!")
}

What did you expect to see?

No change.

What did you see instead?

package main

import (
	"fmt"
	"os" //line :4
	// Comment
)

func main() {
	fmt.Fprintln(os.Stderr, "Hello world!")
}

This can be reproduced here by clicking the format button.

[cherrymui]

cc @heschi @bradfitz

[heschi]

Yikes. A delightful combination of line directives, which I only half understand the implementation of, and comment manipulation in the AST, which is broken.

I'm not going to have the time to investigate this any time soon. Given that the file doesn't need any changes, there may be an easy fix to just echo out the incoming bytes rather than doing whatever formatting it's doing. That might break regrouping though. Dunno.

[kortschak]

It's worth noting that neither gofmt nor gofumpt have this problem.