x/tools/cmd/goimports: import grouping depends on which imports were already there

[cespare]

Please answer these questions before submitting your issue. Thanks!

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

Go 1.8

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

linux/amd64

What did you do?

This is on the latest version of goimports (golang/tools@8e779ee).

Consider this test program:

package main

import (
	"time"
)

func main() {
	_ = snappy.Encode
	_ = unix.Mmap
	_ = time.Parse
}

It's missing two imports. If we run goimports -w fmt.go, we'll get

package main

import (
	"time"

	"github.com/golang/snappy"
	"golang.org/x/sys/unix"
)

func main() {
	_ = snappy.Encode
	_ = unix.Mmap
	_ = time.Parse
}

This is what I expect. However, if we delete the unix import:

package main

import (
	"time"

	"github.com/golang/snappy"
)

func main() {
	_ = snappy.Encode
	_ = unix.Mmap
	_ = time.Parse
}

and then run goimports again, we get this grouping:

package main

import (
	"time"

	"golang.org/x/sys/unix"

	"github.com/golang/snappy"
)

func main() {
	_ = snappy.Encode
	_ = unix.Mmap
	_ = time.Parse
}

In this second case, I'd expect the file to be the same as the first case instead of introducing a new grouping.

/cc @bradfitz

[bradfitz]

I don't have time to work on this myself.

[cespare]

/cc @haya14busa, who did some recent goimports work, in case they'd like to look into it.

[haya14busa]

Thank you for cc. I think I can work on it.

[gopherbot]

CL https://golang.org/cl/37552 mentions this issue.

[haya14busa]

I explained what was the problem and how I fixed it in cl/37552, but the fix is a bit heuristic... It works better for most cases though.

cl/37552 works good as far as I know and the change is small, but I welcome better solution if you find more elegant solution.