Repository: go
Issue: x/tools/gopls: allowing automatic go.{mod,sum} modifications when unimported completions are selected
Labels: NeedsInvestigation,FeatureRequest,gopls,Tools

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### What version of Go are you using (`go version`)?

<pre>
$ go version
go version devel +2b50ab2aee Tue Mar 2 06:38:07 2021 +0000 linux/amd64
$ go list -m golang.org/x/tools
golang.org/x/tools v0.1.1-0.20210227203037-f5a4005dda57
$ go list -m golang.org/x/tools/gopls
golang.org/x/tools/gopls v0.0.0-20210227203037-f5a4005dda57
</pre>

### Does this issue reproduce with the latest release?

Yes

### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/myitcv/.cache/go-build"
GOENV="/home/myitcv/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/myitcv/gostuff/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/myitcv/gostuff"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/myitcv/gos"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/myitcv/gos/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="devel +2b50ab2aee Tue Mar 2 06:38:07 2021 +0000"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/myitcv/.vim/plugged/govim/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-build1158509768=/tmp/go-build -gno-record-gcc-switches"
</pre></details>

### What did you do?

Start by adding `github.com/kr/pretty` to your module cache:

```
cd $(mktemp -d)
go mod init blah.com
go get github.com/kr/pretty
```

Then separately create the following setup:

```
-- go.mod --
module blah.com

go 1.16
-- main.go --
package main

func main() {

}
```

Edit `main.go` as follows:

```
package main

func main() {
        pretty.Spr_
}
```

and trigger completion twice (see #44764) with the cursor in the position shown by `_`.

Accept the completion result for `github.com/kr/pretty.Sprint`


### What did you expect to see?

No error diagnostics.

### What did you see instead?

```
could not import github.com/kr/pretty (no required module provides package \"github.com/kr/pretty\")
```

This issue is a follow up to the discussions that were started in https://github.com/golang/go/issues/44008. Specifically it is a proposal/suggestion to relax the constraint that a `go.{mod,sum}` should not be updated under any circumstances other than the user specifically invoking a `go get` command, to instead allow updates when the user accepts an unimported completion.

The logic/reasoning behind the shift to the new default is sound. But one of the main reasons that I personally still use the `AllowModfileModifications` option is that unimported completions in new projects are a regular part of my workflow. So having to manually invoke a command to effectively run `go get $pkg` seems like a superfluous step when I just selected a very specific unimported completion.

---

cc @stamblerre @heschik 

FYI @leitzler