x/tools/gopls: allowing automatic go.{mod,sum} modifications when unimported completions are selected

[myitcv]

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

$ 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

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ 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"

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 #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