cmd/go: default to GO111MODULE=auto and make it trigger in GOPATH/src

Fixes #31857

Change-Id: Ib0b791376acb7ee1cdc163f808b8ecf77dbdaf06
Reviewed-on: https://go-review.googlesource.com/c/go/+/176580
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/cfg/cfg.go

@@ -128,11 +128,6 @@ var (
 	// in module-aware mode (as opposed to GOPATH mode).
 	// It is equal to modload.Enabled, but not all packages can import modload.
 	ModulesEnabled bool
-
-	// GoModInGOPATH records whether we've found a go.mod in GOPATH/src
-	// in GO111MODULE=auto mode. In that case, we don't use modules
-	// but people might expect us to, so 'go get' warns.
-	GoModInGOPATH string
 )
 
 func exeSuffix() string {

src/cmd/go/internal/get/get.go

@@ -118,11 +118,6 @@ func runGet(cmd *base.Command, args []string) {
 		// Should not happen: main.go should install the separate module-enabled get code.
 		base.Fatalf("go get: modules not implemented")
 	}
-	if cfg.GoModInGOPATH != "" {
-		// Warn about not using modules with GO111MODULE=auto when go.mod exists.
-		// To silence the warning, users can set GO111MODULE=off.
-		fmt.Fprintf(os.Stderr, "go get: warning: modules disabled by GO111MODULE=auto in GOPATH/src;\n\tignoring %s;\n\tsee 'go help modules'\n", base.ShortPath(cfg.GoModInGOPATH))
-	}
 
 	work.BuildInit()
 

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

@@ -34,7 +34,7 @@ import (
 
 var (
 	cwd            string // TODO(bcmills): Is this redundant with base.Cwd?
-	mustUseModules = true
+	mustUseModules = false
 	initialized    bool
 
 	modRoot     string
@@ -79,8 +79,6 @@ func BinDir() string {
 	return filepath.Join(gopath, "bin")
 }
 
-var inGOPATH bool // running in GOPATH/src
-
 // Init determines whether module mode is enabled, locates the root of the
 // current module (if any), sets environment variables for Git subprocesses, and
 // configures the cfg, codehost, load, modfetch, and search packages for use
@@ -95,9 +93,9 @@ func Init() {
 	switch env {
 	default:
 		base.Fatalf("go: unknown environment setting GO111MODULE=%s", env)
-	case "auto":
+	case "auto", "":
 		mustUseModules = false
-	case "on", "":
+	case "on":
 		mustUseModules = true
 	case "off":
 		mustUseModules = false
@@ -137,28 +135,6 @@ func Init() {
 		base.Fatalf("go: %v", err)
 	}
 
-	inGOPATH = false
-	for _, gopath := range filepath.SplitList(cfg.BuildContext.GOPATH) {
-		if gopath == "" {
-			continue
-		}
-		if search.InDir(cwd, filepath.Join(gopath, "src")) != "" {
-			inGOPATH = true
-			break
-		}
-	}
-
-	if inGOPATH && !mustUseModules {
-		if CmdModInit {
-			die() // Don't init a module that we're just going to ignore.
-		}
-		// No automatic enabling in GOPATH.
-		if root := findModuleRoot(cwd); root != "" {
-			cfg.GoModInGOPATH = filepath.Join(root, "go.mod")
-		}
-		return
-	}
-
 	if CmdModInit {
 		// Running 'go mod init': go.mod will be created in current directory.
 		modRoot = cwd
@@ -300,9 +276,6 @@ func die() {
 	if cfg.Getenv("GO111MODULE") == "off" {
 		base.Fatalf("go: modules disabled by GO111MODULE=off; see 'go help modules'")
 	}
-	if inGOPATH && !mustUseModules {
-		base.Fatalf("go: modules disabled inside GOPATH/src by GO111MODULE=auto; see 'go help modules'")
-	}
 	if cwd != "" {
 		if dir, name := findAltConfig(cwd); dir != "" {
 			rel, err := filepath.Rel(cwd, dir)

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

@@ -2,6 +2,7 @@
 
 [!net] skip
 
+env GO111MODULE=on
 env GOPROXY=
 
 ! go get -d vcs-test.golang.org/insecure/go/insecure

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

-# GO111MODULE=auto should only trigger outside GOPATH/src
+# GO111MODULE=auto should trigger any time a go.mod exists in a parent directory.
 env GO111MODULE=auto
 
 cd $GOPATH/src/x/y/z
 go env GOMOD
-! stdout . # no non-empty lines
-! go list -m -f {{.GoMod}}
-stderr 'not using modules'
+stdout $GOPATH[/\\]src[/\\]x[/\\]y[/\\]z[/\\]go.mod
+go list -m -f {{.GoMod}}
+stdout $GOPATH[/\\]src[/\\]x[/\\]y[/\\]z[/\\]go.mod
 
 cd $GOPATH/src/x/y/z/w
 go env GOMOD
-! stdout .
+stdout $GOPATH[/\\]src[/\\]x[/\\]y[/\\]z[/\\]go.mod
 
 cd $GOPATH/src/x/y
 go env GOMOD
@@ -25,6 +25,17 @@ cd $GOPATH/foo/bar/baz
 go env GOMOD
 stdout foo[/\\]go.mod
 
+# GO111MODULE unset should be equivalent to auto.
+env GO111MODULE=
+
+cd $GOPATH/src/x/y/z
+go env GOMOD
+stdout $GOPATH[/\\]src[/\\]x[/\\]y[/\\]z[/\\]go.mod
+
+cd $GOPATH/src/x/y
+go env GOMOD
+! stdout .
+
 # GO111MODULE=on should trigger everywhere
 env GO111MODULE=on
 

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

-env GO111MODULE=auto
+env GO111MODULE=on
 
 # Derive module path from import comment.
 cd $WORK/x
@@ -12,14 +12,8 @@ addcrlf x.go
 go mod init
 stderr 'module x'
 
-# go mod should die in GOPATH if modules are not enabled for GOPATH
-cd $GOPATH/src/example.com/x/y
-! go mod init
-stderr 'go: modules disabled inside GOPATH/src by GO111MODULE=auto; see ''go help modules'''
-
-env GO111MODULE=
-
 # Derive module path from location inside GOPATH.
+# 'go mod init' should succeed if modules are not explicitly disabled.
 cd $GOPATH/src/example.com/x/y
 go mod init
 stderr 'module example.com/x/y$'

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

-# go get in GO111MODULE=auto should warn when not using modules and go.mod exists
-
-env GO111MODULE=auto
-mkdir z
-cd z
-! go get # fails because no code in directory, not the warning
-stderr 'go get: warning: modules disabled by GO111MODULE=auto in GOPATH/src;\n\tignoring ..[/\\]go.mod;\n\tsee ''go help modules'''
-
--- go.mod --
-module x

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

@@ -8,7 +8,7 @@ env GO111MODULE=off
 
 # GO111MODULE=auto in GOPATH/src
 env GO111MODULE=auto
-! exec $WORK/testimport.exe x/y/z/w .
+exec $WORK/testimport.exe x/y/z/w .
 
 # GO111MODULE=auto outside GOPATH/src
 cd $GOPATH/other

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

+# 'go mod init' should refuse to initialize a module if it will be
+# ignored anyway due to GO111MODULE=off.
 env GO111MODULE=off
-
-# This script tests that running go mod init with
-# GO111MODULE=off when outside of GOPATH will fatal
-# with an error message.
 ! go mod init
 stderr 'go mod init: modules disabled by GO111MODULE=off; see ''go help modules'''