cmd/go: warn about non-use of go.mod in legacy go get

It's important for a smooth transition for non-module users
not to change operation in GOPATH/src by default in Go 1.11,
even if go.mod exists in a downloaded dependency.

Even so, users create go.mod and then are confused about
why 'go get' commands seem to behave oddly, when in fact
they are getting the old 'go get'.

Try to split the difference by printing a warning in 'go get'
when run in a tree that would normally be considered a
module if only it were outside GOPATH/src.

Change-Id: I55a1cbef127f3f36de54a8d7b93e1fc64bf0a708
Reviewed-on: https://go-review.googlesource.com/124859Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

@@ -73,6 +73,11 @@ 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 init() {

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

@@ -118,6 +118,11 @@ 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

@@ -153,6 +153,9 @@ func Init() {
 	} else {
 		if inGOPATH && !MustUseModules {
 			// No automatic enabling in GOPATH.
+			if root, _ := FindModuleRoot(cwd, "", false); root != "" {
+				cfg.GoModInGOPATH = filepath.Join(root, "go.mod")
+			}
 			return
 		}
 		root, _ := FindModuleRoot(cwd, "", MustUseModules)

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