cmd/go: only add a 'go' directive on 'go mod tidy' or when a conversion occurs

If the go.mod file exists and is empty, we initialize it from any of
various formats supported by legacy dependency-management tools.

We also initialize the 'go' directive at that point: we know that the
go.mod file is incomplete, because it does not reflect the information
in the legacy configuration file, and since we know that the go.mod
file is incomplete, we should complete it with as much information as
we have — including the version of the language currently in use.

However, if there is no legacy configuration file present, then we
cannot infer that the go.mod file is incomplete: it may correctly
specify a module without external dependencies. In that case, we
should not initialize the 'go' directive either: the user will not be
expecting unnecessary edits to the go.mod file, and we generally do
not make unnecessary-but-helpful edits unless 'go mod tidy' is invoked
explicitly.

Fixes #30790
Fixes #31100

Change-Id: I05a7872bce54a917c10d910cd9a616cab52e2730
Reviewed-on: https://go-review.googlesource.com/c/go/+/169877
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/modcmd/tidy.go

@@ -64,6 +64,7 @@ func runTidy(cmd *base.Command, args []string) {
 		}
 	}
 	modload.SetBuildList(keep)
+	modload.AddGoStmt()
 	modTidyGoSum() // updates memory copy; WriteGoMod on next line flushes it out
 	modload.WriteGoMod()
 }

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

@@ -408,10 +408,9 @@ func legacyModInit() {
 		fmt.Fprintf(os.Stderr, "go: creating new go.mod: module %s\n", path)
 		modFile = new(modfile.File)
 		modFile.AddModuleStmt(path)
+		AddGoStmt()
 	}
 
-	addGoStmt()
-
 	for _, name := range altConfigs {
 		cfg := filepath.Join(modRoot, name)
 		data, err := ioutil.ReadFile(cfg)
@@ -420,6 +419,7 @@ func legacyModInit() {
 			if convert == nil {
 				return
 			}
+			AddGoStmt()
 			fmt.Fprintf(os.Stderr, "go: copying requirements from %s\n", base.ShortPath(cfg))
 			cfg = filepath.ToSlash(cfg)
 			if err := modconv.ConvertLegacyConfig(modFile, cfg, data); err != nil {
@@ -434,8 +434,12 @@ func legacyModInit() {
 	}
 }
 
-// addGoStmt adds a go statement referring to the current version.
+// AddGoStmt adds a go directive to the go.mod file if it does not already include one.
-func addGoStmt() {
+// The 'go' version added, if any, is the latest version supported by this toolchain.
+func AddGoStmt() {
+	if modFile.Go != nil && modFile.Go.Version != "" {
+		return
+	}
 	tags := build.Default.ReleaseTags
 	version := tags[len(tags)-1]
 	if !strings.HasPrefix(version, "go") || !modfile.GoVersionRE.MatchString(version[2:]) {

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

@@ -0,0 +1,16 @@
+env GO111MODULE=on
+
+env GOPATH=$devnull
+
+go list -m
+stdout '^example.com$'
+
+go list
+stdout '^example.com$'
+
+-- go.mod --
+module example.com
+-- main.go --
+package main
+
+func main() {}

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

@@ -20,7 +20,7 @@ stdout ^vendor/golang.org/x/crypto # dep of .TestImports
 # Modules outside the standard library should not use the packages vendored there...
 cd broken
 ! go build -mod=readonly
-stderr 'updates to go.mod needed'
+stderr 'disabled by -mod=readonly'
 ! go build -mod=vendor
 stderr 'cannot find package'
 stderr 'hpack'