cmd/go: add go statement when initializing go.mod

When creating a go.mod file, add a go statement mentioning the current
Go version. We can be reasonably confident that the current version is
able to build the module. This is as described in the language
transition proposal at https://golang.org/issue/28221.

Updates #28221

Change-Id: I70a99b3a53f4b6c0288da07473c5a71bb28cd86f
Reviewed-on: https://go-review.googlesource.com/c/147281
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modfile/rule.go

@@ -154,7 +154,7 @@ func parseToFile(file string, data []byte, fix VersionFixer, strict bool) (*File
 	return f, nil
 }
 
-var goVersionRE = regexp.MustCompile(`([1-9][0-9]*)\.(0|[1-9][0-9]*)`)
+var GoVersionRE = regexp.MustCompile(`([1-9][0-9]*)\.(0|[1-9][0-9]*)`)
 
 func (f *File) add(errs *bytes.Buffer, line *Line, verb string, args []string, fix VersionFixer, strict bool) {
 	// If strict is false, this module is a dependency.
@@ -181,7 +181,7 @@ func (f *File) add(errs *bytes.Buffer, line *Line, verb string, args []string, f
 			fmt.Fprintf(errs, "%s:%d: repeated go statement\n", f.Syntax.Name, line.Start.Line)
 			return
 		}
-		if len(args) != 1 || !goVersionRE.MatchString(args[0]) {
+		if len(args) != 1 || !GoVersionRE.MatchString(args[0]) {
 			fmt.Fprintf(errs, "%s:%d: usage: go 1.23\n", f.Syntax.Name, line.Start.Line)
 			return
 		}
@@ -477,6 +477,22 @@ func (f *File) Cleanup() {
 	f.Syntax.Cleanup()
 }
 
+func (f *File) AddGoStmt(version string) error {
+	if !GoVersionRE.MatchString(version) {
+		return fmt.Errorf("invalid language version string %q", version)
+	}
+	if f.Go == nil {
+		f.Go = &Go{
+			Version: version,
+			Syntax:  f.Syntax.addLine(nil, "go", version),
+		}
+	} else {
+		f.Go.Version = version
+		f.Syntax.updateLine(f.Go.Syntax, "go", version)
+	}
+	return nil
+}
+
 func (f *File) AddRequire(path, vers string) error {
 	need := true
 	for _, r := range f.Require {

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

@@ -19,6 +19,7 @@ import (
 	"cmd/go/internal/search"
 	"encoding/json"
 	"fmt"
+	"go/build"
 	"io/ioutil"
 	"os"
 	"path"
@@ -335,6 +336,8 @@ func legacyModInit() {
 		modFile.AddModuleStmt(path)
 	}
 
+	addGoStmt()
+
 	for _, name := range altConfigs {
 		cfg := filepath.Join(ModRoot, name)
 		data, err := ioutil.ReadFile(cfg)
@@ -357,6 +360,25 @@ func legacyModInit() {
 	}
 }
 
+// InitGoStmt adds a go statement, unless there already is one.
+func InitGoStmt() {
+	if modFile.Go == nil {
+		addGoStmt()
+	}
+}
+
+// addGoStmt adds a go statement referring to the current version.
+func addGoStmt() {
+	tags := build.Default.ReleaseTags
+	version := tags[len(tags)-1]
+	if !strings.HasPrefix(version, "go") || !modfile.GoVersionRE.MatchString(version[2:]) {
+		base.Fatalf("go: unrecognized default version %q", version)
+	}
+	if err := modFile.AddGoStmt(version[2:]); err != nil {
+		base.Fatalf("go: internal error: %v", err)
+	}
+}
+
 var altConfigs = []string{
 	"Gopkg.lock",
 

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

@@ -10,16 +10,16 @@ stderr 'cannot determine module path'
 
 go mod init x.x/y/z
 stderr 'creating new go.mod: module x.x/y/z'
-cmp go.mod $WORK/go.mod.init
+cmpenv go.mod $WORK/go.mod.init
 
 ! go mod init
-cmp go.mod $WORK/go.mod.init
+cmpenv go.mod $WORK/go.mod.init
 
 # go mod edits
 go mod edit -droprequire=x.1 -require=x.1@v1.0.0 -require=x.2@v1.1.0 -droprequire=x.2 -exclude='x.1 @ v1.2.0' -exclude=x.1@v1.2.1 -replace=x.1@v1.3.0=y.1@v1.4.0 -replace='x.1@v1.4.0 = ../z'
-cmp go.mod $WORK/go.mod.edit1
+cmpenv go.mod $WORK/go.mod.edit1
 go mod edit -droprequire=x.1 -dropexclude=x.1@v1.2.1 -dropreplace=x.1@v1.3.0 -require=x.3@v1.99.0
-cmp go.mod $WORK/go.mod.edit2
+cmpenv go.mod $WORK/go.mod.edit2
 
 # go mod edit -json
 go mod edit -json
@@ -27,20 +27,20 @@ cmp stdout $WORK/go.mod.json
 
 # go mod edit -replace
 go mod edit -replace=x.1@v1.3.0=y.1/v2@v2.3.5 -replace=x.1@v1.4.0=y.1/v2@v2.3.5
-cmp go.mod $WORK/go.mod.edit3
+cmpenv go.mod $WORK/go.mod.edit3
 go mod edit -replace=x.1=y.1/v2@v2.3.6
-cmp go.mod $WORK/go.mod.edit4
+cmpenv go.mod $WORK/go.mod.edit4
 go mod edit -dropreplace=x.1
-cmp go.mod $WORK/go.mod.edit5
+cmpenv go.mod $WORK/go.mod.edit5
 
 # go mod edit -fmt
 cp $WORK/go.mod.badfmt go.mod
 go mod edit -fmt -print # -print should avoid writing file
-cmp stdout $WORK/go.mod.edit4
+cmpenv stdout $WORK/go.mod.edit6
 cmp go.mod $WORK/go.mod.badfmt
 go mod edit -fmt # without -print, should write file (and nothing to stdout)
 ! stdout .
-cmp go.mod $WORK/go.mod.edit4
+cmpenv go.mod $WORK/go.mod.edit6
 
 -- x.go --
 package x
@@ -50,9 +50,13 @@ package w
 
 -- $WORK/go.mod.init --
 module x.x/y/z
+
+go $goversion
 -- $WORK/go.mod.edit1 --
 module x.x/y/z
 
+go $goversion
+
 require x.1 v1.0.0
 
 exclude (
@@ -67,6 +71,8 @@ replace (
 -- $WORK/go.mod.edit2 --
 module x.x/y/z
 
+go $goversion
+
 exclude x.1 v1.2.0
 
 replace x.1 v1.4.0 => ../z
@@ -104,6 +110,8 @@ require x.3 v1.99.0
 -- $WORK/go.mod.edit3 --
 module x.x/y/z
 
+go $goversion
+
 exclude x.1 v1.2.0
 
 replace (
@@ -115,6 +123,8 @@ require x.3 v1.99.0
 -- $WORK/go.mod.edit4 --
 module x.x/y/z
 
+go $goversion
+
 exclude x.1 v1.2.0
 
 replace x.1 => y.1/v2 v2.3.6
@@ -123,12 +133,26 @@ require x.3 v1.99.0
 -- $WORK/go.mod.edit5 --
 module x.x/y/z
 
+go $goversion
+
 exclude x.1 v1.2.0
 
+require x.3 v1.99.0
+-- $WORK/go.mod.edit6 --
+module x.x/y/z
+
+go 1.10
+
+exclude x.1 v1.2.0
+
+replace x.1 => y.1/v2 v2.3.6
+
 require x.3 v1.99.0
 -- $WORK/go.mod.badfmt --
 module     x.x/y/z
 
+go 1.10
+
 exclude x.1     v1.2.0
 
 replace x.1    =>   y.1/v2 v2.3.6

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

@@ -37,6 +37,8 @@ cmp go.mod go.mod.inconsistent
 -- go.mod --
 module m
 
+go 1.20
+
 -- x.go --
 package x
 import _ "rsc.io/quote"

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

@@ -5,6 +5,9 @@ go mod tidy -v
 stderr '^unused y.1'
 ! stderr '^unused [^y]'
 
+# tidy should not touch existing go line
+grep 'go 1.10' go.mod
+
 go list -m all
 ! stdout '^y'
 stdout '^w.1 v1.2.0'
@@ -12,11 +15,17 @@ stdout '^z.1 v1.2.0'
 
 # empty tidy should not crash
 cd triv
+! grep 'go ' go.mod
 go mod tidy
 
+# tidy should add missing go line
+grep 'go ' go.mod
+
 -- go.mod --
 module m
 
+go 1.10
+
 require (
 	x.1 v1.0.0
 	y.1 v1.0.0