cmd/go: don't warn about GOROOT equal to GOPATH when both are the empty string

As of Go 1.19, runtime.GOROOT() reports the empty string if the binary
was built with -trimpath. cmd/go/internal/cfg uses the path of the go
command executable to reverse-engineer the correct GOROOT setting,
but that means that cmd/go's "GOPATH set to GOROOT" warning needs to
use cfg.GOROOT instead of runtime.GOROOT().

In addition, if we fail to find the GOROOT then there is no point in
complaining about GOPATH also being empty: the missing GOROOT will stop
everything right away anyway, so there is no point confusing the user
with an additional warning about GOPATH.

Updates golang#51461.
Updates golang#18678.
Updates golang#3207.

Change-Id: Id7d0f4dc2f229c202dfda4e6e8af5dea909bb16f
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/543955
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: Bryan C. Mills

src/cmd/go/main.go

@@ -16,7 +16,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"runtime"
 	rtrace "runtime/trace"
 	"slices"
 	"strings"
@@ -107,10 +106,19 @@ func main() {
 		return
 	}
 
+	if cfg.GOROOT == "" {
+		fmt.Fprintf(os.Stderr, "go: cannot find GOROOT directory: 'go' binary is trimmed and GOROOT is not set\n")
+		os.Exit(2)
+	}
+	if fi, err := os.Stat(cfg.GOROOT); err != nil || !fi.IsDir() {
+		fmt.Fprintf(os.Stderr, "go: cannot find GOROOT directory: %v\n", cfg.GOROOT)
+		os.Exit(2)
+	}
+
 	// Diagnose common mistake: GOPATH==GOROOT.
 	// This setting is equivalent to not setting GOPATH at all,
 	// which is not what most people want when they do it.
-	if gopath := cfg.BuildContext.GOPATH; filepath.Clean(gopath) == filepath.Clean(runtime.GOROOT()) {
+	if gopath := cfg.BuildContext.GOPATH; filepath.Clean(gopath) == filepath.Clean(cfg.GOROOT) {
 		fmt.Fprintf(os.Stderr, "warning: GOPATH set to GOROOT (%s) has no effect\n", gopath)
 	} else {
 		for _, p := range filepath.SplitList(gopath) {
@@ -139,15 +147,6 @@ func main() {
 		}
 	}
 
-	if cfg.GOROOT == "" {
-		fmt.Fprintf(os.Stderr, "go: cannot find GOROOT directory: 'go' binary is trimmed and GOROOT is not set\n")
-		os.Exit(2)
-	}
-	if fi, err := os.Stat(cfg.GOROOT); err != nil || !fi.IsDir() {
-		fmt.Fprintf(os.Stderr, "go: cannot find GOROOT directory: %v\n", cfg.GOROOT)
-		os.Exit(2)
-	}
-
 	cmd, used := lookupCmd(args)
 	cfg.CmdName = strings.Join(args[:used], " ")
 	if len(cmd.Commands) > 0 {

src/cmd/go/testdata/script/goroot_executable_trimpath.txt

@@ -29,12 +29,20 @@ wait
 env TESTGOROOT=$GOROOT
 env GOROOT=
 
+# Unset GOPATH and any variables that its default may be derived from,
+# so that we can check for a spurious warning.
+env GOPATH=
+env HOME=''
+env USERPROFILE=''
+env home=''
+
 # Relocated Executable
 # Since we built with -trimpath and the binary isn't installed in a
 # normal-looking GOROOT, this command should fail.
 
 ! exec $WORK/new/bin/go$GOEXE env GOROOT
 stderr '^go: cannot find GOROOT directory: ''go'' binary is trimmed and GOROOT is not set$'
+! stderr 'GOPATH set to GOROOT'
 
 # Cross-compiled binaries in cmd are installed to a ${GOOS}_${GOARCH} subdirectory,
 # so we also want to try a copy there.
@@ -44,13 +52,15 @@ stderr '^go: cannot find GOROOT directory: ''go'' binary is trimmed and GOROOT i
 cp $WORK/new/bin/go$GOEXE $WORK/new/bin/${GOOS}_${GOARCH}/go$GOEXE
 ! exec $WORK/new/bin/${GOOS}_${GOARCH}/go$GOEXE env GOROOT
 stderr '^go: cannot find GOROOT directory: ''go'' binary is trimmed and GOROOT is not set$'
+! stderr 'GOPATH set to GOROOT'
 
 # Relocated Tree:
 # If the binary is sitting in a bin dir next to ../pkg/tool, that counts as a GOROOT,
 # so it should find the new tree.
 mkdir $WORK/new/pkg/tool
 exec $WORK/bin/check$GOEXE $WORK/new/bin/go$GOEXE $WORK/new
 exec $WORK/bin/check$GOEXE $WORK/new/bin/${GOOS}_${GOARCH}/go$GOEXE $WORK/new
+! stderr 'GOPATH set to GOROOT'
 
 -- check.go --
 package main