cmd/go: env should populate GOBIN as effective install target directory for main packages

[myitcv]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.11.1 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/gopher/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/gopher/gopath"
GOPROXY=""
GORACE=""
GOROOT="/go"
GOTMPDIR=""
GOTOOLDIR="/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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-build992263907=/tmp/go-build -gno-record-gcc-switches"

What did you do?

$ mkdir main
$ cd main
$ go mod init example.com/main
go: creating new go.mod: module example.com/main
$ cat <<EOD >main.go
package main

func main() {}
EOD

There is currently no simple way to see the directory go install will use as the target for this main package. go list gives us the final target:

$ go list -f "{{.Target}}" .
/home/gopher/gopath/bin/main

go env simply gives us the current environment values of GOBIN and GOPATH:

$ go env | grep 'GOBIN\|GOPATH'
GOBIN=""
GOPATH="/home/gopher/gopath"

If we then set GOBIN:

$ export GOBIN=/tmp/blah
$ go list -f "{{.Target}}" .
/tmp/blah/main
$ go env | grep 'GOBIN\|GOPATH'
GOBIN="/tmp/blah"
GOPATH="/home/gopher/gopath"

I'd like to propose that go env GOBIN be the derived directory that go install will use, following the precedence logic of GOBIN, GOPATH and os.UserHomeDir():

// pseudo code
func effectiveGOBIN() string {
	if gb := os.Getenv("GOBIN"); gb != "" {
		return gb
	}
	if gp := os.Getenv("GOPATH"); gp != "" {
		return filepath.Join(filepath.SplitList(gp)[0], "bin")
	}
	return filepath.Join(os.UserHomeDir(), "go", "bin")
}

What did you expect to see?

The output of go env giving the target directory for go install

What did you see instead?

No output of go env giving the target directory for go install

[rsc]

This would only make sense to do if we resolve #23439 as being "always use GOPATH/bin as the default GOBIN even outside GOPATH". I'll add a note there and close this issue.