cmd/go: default to GO111MODULE=on

Fixes golang#41330

Change-Id: Ib66087ac5e1eb827694915f21c32c4d88e1b9a7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/255052
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modload/init.go

@@ -120,9 +120,9 @@ func Init() {
 	switch env {
 	default:
 		base.Fatalf("go: unknown environment setting GO111MODULE=%s", env)
-	case "auto", "":
+	case "auto":
 		mustUseModules = ForceUseModules
-	case "on":
+	case "on", "":
 		mustUseModules = true
 	case "off":
 		if ForceUseModules {
@@ -257,9 +257,9 @@ func WillBeEnabled() bool {
 	// exits, so it can't call this function directly.
 	env := cfg.Getenv("GO111MODULE")
 	switch env {
-	case "on":
+	case "on", "":
 		return true
-	case "auto", "":
+	case "auto":
 		break
 	default:
 		return false

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

@@ -25,7 +25,7 @@ cd $GOPATH/foo/bar/baz
 go env GOMOD
 stdout foo[/\\]go.mod
 
-# GO111MODULE unset should be equivalent to auto.
+# GO111MODULE unset should be equivalent to on.
 env GO111MODULE=
 
 cd $GOPATH/src/x/y/z
@@ -34,7 +34,7 @@ stdout $GOPATH[/\\]src[/\\]x[/\\]y[/\\]z[/\\]go.mod
 
 cd $GOPATH/src/x/y
 go env GOMOD
-! stdout .
+stdout 'NUL|/dev/null'
 
 # GO111MODULE=on should trigger everywhere
 env GO111MODULE=on

src/go/build/build.go

@@ -1072,9 +1072,9 @@ func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode)
 		}
 	}
 
-	// Unless GO111MODULE=on, look to see if there is a go.mod.
+	// If GO111MODULE=auto, look to see if there is a go.mod.
 	// Since go1.13, it doesn't matter if we're inside GOPATH.
-	if go111Module != "on" {
+	if go111Module == "auto" {
 		var (
 			parent string
 			err    error