cmd/go: add go mod edit -go flag

It can be used to set the Go language version used by the module.

RELNOTES=yes

Updates #28221

Change-Id: Ief0dd185c01195a17be20dff8627c80943c436e7
Reviewed-on: https://go-review.googlesource.com/c/147282
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/alldocs.go

@@ -972,6 +972,8 @@
 // and -dropreplace editing flags may be repeated, and the changes
 // are applied in the order given.
 //
+// The -go=version flag sets the expected Go language version.
+//
 // The -print flag prints the final go.mod in its text format instead of
 // writing it back to go.mod.
 //
@@ -985,6 +987,7 @@
 //
 // 	type GoMod struct {
 // 		Module  Module
+// 		Go      string
 // 		Require []Require
 // 		Exclude []Module
 // 		Replace []Replace

src/cmd/go/internal/modcmd/edit.go

@@ -62,6 +62,8 @@ The -require, -droprequire, -exclude, -dropexclude, -replace,
 and -dropreplace editing flags may be repeated, and the changes
 are applied in the order given.
 
+The -go=version flag sets the expected Go language version.
+
 The -print flag prints the final go.mod in its text format instead of
 writing it back to go.mod.
 
@@ -75,6 +77,7 @@ writing it back to go.mod. The JSON output corresponds to these Go types:
 
 	type GoMod struct {
 		Module  Module
+		Go      string
 		Require []Require
 		Exclude []Module
 		Replace []Replace
@@ -103,7 +106,7 @@ by invoking 'go mod edit' with -require, -exclude, and so on.
 
 var (
 	editFmt    = cmdEdit.Flag.Bool("fmt", false, "")
-	// editGo     = cmdEdit.Flag.String("go", "", "")
+	editGo     = cmdEdit.Flag.String("go", "", "")
 	editJSON   = cmdEdit.Flag.Bool("json", false, "")
 	editPrint  = cmdEdit.Flag.Bool("print", false, "")
 	editModule = cmdEdit.Flag.String("module", "", "")
@@ -131,6 +134,7 @@ func init() {
 func runEdit(cmd *base.Command, args []string) {
 	anyFlags :=
 		*editModule != "" ||
+			*editGo != "" ||
 			*editJSON ||
 			*editPrint ||
 			*editFmt ||
@@ -161,7 +165,11 @@ func runEdit(cmd *base.Command, args []string) {
 		}
 	}
 
-	// TODO(rsc): Implement -go= once we start advertising it.
+	if *editGo != "" {
+		if !modfile.GoVersionRE.MatchString(*editGo) {
+			base.Fatalf(`go mod: invalid -go option; expecting something like "-go 1.12"`)
+		}
+	}
 
 	data, err := ioutil.ReadFile(gomod)
 	if err != nil {
@@ -177,6 +185,12 @@ func runEdit(cmd *base.Command, args []string) {
 		modFile.AddModuleStmt(modload.CmdModModule)
 	}
 
+	if *editGo != "" {
+		if err := modFile.AddGoStmt(*editGo); err != nil {
+			base.Fatalf("go: internal error: %v", err)
+		}
+	}
+
 	if len(edits) > 0 {
 		for _, edit := range edits {
 			edit(modFile)
@@ -344,6 +358,7 @@ func flagDropReplace(arg string) {
 // fileJSON is the -json output data structure.
 type fileJSON struct {
 	Module  module.Version
+	Go      string `json:",omitempty"`
 	Require []requireJSON
 	Exclude []module.Version
 	Replace []replaceJSON
@@ -364,6 +379,9 @@ type replaceJSON struct {
 func editPrintJSON(modFile *modfile.File) {
 	var f fileJSON
 	f.Module = modFile.Module.Mod
+	if modFile.Go != nil {
+		f.Go = modFile.Go.Version
+	}
 	for _, r := range modFile.Require {
 		f.Require = append(f.Require, requireJSON{Path: r.Mod.Path, Version: r.Mod.Version, Indirect: r.Indirect})
 	}

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

@@ -23,7 +23,7 @@ cmpenv go.mod $WORK/go.mod.edit2
 
 # go mod edit -json
 go mod edit -json
-cmp stdout $WORK/go.mod.json
+cmpenv 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
@@ -83,6 +83,7 @@ require x.3 v1.99.0
 	"Module": {
 		"Path": "x.x/y/z"
 	},
+	"Go": "$goversion",
 	"Require": [
 		{
 			"Path": "x.3",

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

+# Test support for go mod -edit to set language version.
+
+env GO111MODULE=on
+! go build
+stderr 'type aliases only supported as of'
+go mod edit -go=1.9
+grep 'go 1.9' go.mod
+go build
+
+-- go.mod --
+module m
+go 1.8
+
+-- alias.go --
+package alias
+type T = int