cmd/go/internal/modcmd: factor out a type for flags whose arguments are Go versions

For #46141
Updates #45094

Change-Id: I6553600c69273762a81795ef021c66f4e0872b6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/321069
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

6 files changed, 52 insertions, 20 deletions

diff --git a/src/cmd/go/internal/modcmd/edit.go b/src/cmd/go/internal/modcmd/edit.go
index 79a93ca44b..e856e7c630 100644
--- a/src/cmd/go/internal/modcmd/edit.go
+++ b/src/cmd/go/internal/modcmd/edit.go
@@ -196,7 +196,7 @@ func runEdit(ctx context.Context, cmd *base.Command, args []string) {
 
 	if *editGo != "" {
 		if !modfile.GoVersionRE.MatchString(*editGo) {
-			base.Fatalf(`go mod: invalid -go option; expecting something like "-go 1.12"`)
+			base.Fatalf(`go mod: invalid -go option; expecting something like "-go %s"`, modload.LatestGoVersion())
 		}
 	}
 
diff --git a/src/cmd/go/internal/modcmd/tidy.go b/src/cmd/go/internal/modcmd/tidy.go
index c72ec30a57..9af624028a 100644
--- a/src/cmd/go/internal/modcmd/tidy.go
+++ b/src/cmd/go/internal/modcmd/tidy.go
@@ -12,8 +12,10 @@ import (
 	"cmd/go/internal/imports"
 	"cmd/go/internal/modload"
 	"context"
+	"fmt"
 
 	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/semver"
 )
 
 var cmdTidy = &base.Command{
@@ -44,28 +46,48 @@ See https://golang.org/ref/mod#go-mod-tidy for more about 'go mod tidy'.
 }
 
 var (
-	tidyE  bool   // if true, report errors but proceed anyway.
-	tidyGo string // go version to write to the tidied go.mod file (toggles lazy loading)
+	tidyE  bool          // if true, report errors but proceed anyway.
+	tidyGo goVersionFlag // go version to write to the tidied go.mod file (toggles lazy loading)
 )
 
 func init() {
 	cmdTidy.Flag.BoolVar(&cfg.BuildV, "v", false, "")
 	cmdTidy.Flag.BoolVar(&tidyE, "e", false, "")
-	cmdTidy.Flag.StringVar(&tidyGo, "go", "", "")
+	cmdTidy.Flag.Var(&tidyGo, "go", "")
 	base.AddModCommonFlags(&cmdTidy.Flag)
 }
 
+// A goVersionFlag is a flag.Value representing a supported Go version.
+//
+// (Note that the -go argument to 'go mod edit' is *not* a goVersionFlag.
+// It intentionally allows newer-than-supported versions as arguments.)
+type goVersionFlag struct {
+	v string
+}
+
+func (f *goVersionFlag) String() string   { return f.v }
+func (f *goVersionFlag) Get() interface{} { return f.v }
+
+func (f *goVersionFlag) Set(s string) error {
+	if s != "" {
+		latest := modload.LatestGoVersion()
+		if !modfile.GoVersionRE.MatchString(s) {
+			return fmt.Errorf("expecting a Go version like %q", latest)
+		}
+		if semver.Compare("v"+s, "v"+latest) > 0 {
+			return fmt.Errorf("maximum supported Go version is %s", latest)
+		}
+	}
+
+	f.v = s
+	return nil
+}
+
 func runTidy(ctx context.Context, cmd *base.Command, args []string) {
 	if len(args) > 0 {
 		base.Fatalf("go mod tidy: no arguments allowed")
 	}
 
-	if tidyGo != "" {
-		if !modfile.GoVersionRE.MatchString(tidyGo) {
-			base.Fatalf(`go mod: invalid -go option %q; expecting something like "-go 1.17"`, tidyGo)
-		}
-	}
-
 	// Tidy aims to make 'go test' reproducible for any package in 'all', so we
 	// need to include test dependencies. For modules that specify go 1.15 or
 	// earlier this is a no-op (because 'all' saturates transitive test
@@ -80,7 +102,7 @@ func runTidy(ctx context.Context, cmd *base.Command, args []string) {
 	modload.RootMode = modload.NeedRoot
 
 	modload.LoadPackages(ctx, modload.PackageOpts{
-		GoVersion:                tidyGo,
+		GoVersion:                tidyGo.String(),
 		Tags:                     imports.AnyTags(),
 		Tidy:                     true,
 		VendorModulesInGOROOTSrc: true,
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index 5cdea12cd3..e358230e74 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -405,7 +405,7 @@ func loadModFile(ctx context.Context) (rs *Requirements, needCommit bool) {
 	if modRoot == "" {
 		Target = module.Version{Path: "command-line-arguments"}
 		targetPrefix = "command-line-arguments"
-		goVersion := latestGoVersion()
+		goVersion := LatestGoVersion()
 		rawGoVersion.Store(Target, goVersion)
 		requirements = newRequirements(modDepthFromGoVersion(goVersion), nil, nil)
 		return requirements, false
@@ -448,7 +448,7 @@ func loadModFile(ctx context.Context) (rs *Requirements, needCommit bool) {
 		// TODO(#45551): Do something more principled instead of checking
 		// cfg.CmdName directly here.
 		if cfg.BuildMod == "mod" && cfg.CmdName != "mod graph" && cfg.CmdName != "mod why" {
-			addGoStmt(latestGoVersion())
+			addGoStmt(LatestGoVersion())
 			if go117EnableLazyLoading {
 				// We need to add a 'go' version to the go.mod file, but we must assume
 				// that its existing contents match something between Go 1.11 and 1.16.
@@ -500,7 +500,7 @@ func CreateModFile(ctx context.Context, modPath string) {
 	modFile = new(modfile.File)
 	modFile.AddModuleStmt(modPath)
 	initTarget(modFile.Module.Mod)
-	addGoStmt(latestGoVersion()) // Add the go directive before converted module requirements.
+	addGoStmt(LatestGoVersion()) // Add the go directive before converted module requirements.
 
 	convertedFrom, err := convertLegacyConfig(modPath)
 	if convertedFrom != "" {
@@ -793,9 +793,9 @@ func addGoStmt(v string) {
 	rawGoVersion.Store(Target, v)
 }
 
-// latestGoVersion returns the latest version of the Go language supported by
+// LatestGoVersion returns the latest version of the Go language supported by
 // this toolchain, like "1.17".
-func latestGoVersion() string {
+func LatestGoVersion() string {
 	tags := build.Default.ReleaseTags
 	version := tags[len(tags)-1]
 	if !strings.HasPrefix(version, "go") || !modfile.GoVersionRE.MatchString(version[2:]) {
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index 83fc7c09c3..23ee3824f3 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -931,8 +931,8 @@ func loadFromRoots(ctx context.Context, params loaderParams) *loader {
 			ld.allClosesOverTests = true
 		}
 
-		if ld.Tidy && semver.Compare(goVersionV, "v"+latestGoVersion()) > 0 {
-			ld.errorf("go mod tidy: go.mod file indicates go %s, but maximum supported version is %s\n", params.GoVersion, latestGoVersion())
+		if ld.Tidy && semver.Compare(goVersionV, "v"+LatestGoVersion()) > 0 {
+			ld.errorf("go mod tidy: go.mod file indicates go %s, but maximum supported version is %s\n", params.GoVersion, LatestGoVersion())
 			base.ExitIfErrors()
 		}
 
diff --git a/src/cmd/go/internal/modload/modfile.go b/src/cmd/go/internal/modload/modfile.go
index bafff3e080..a9c3a91d35 100644
--- a/src/cmd/go/internal/modload/modfile.go
+++ b/src/cmd/go/internal/modload/modfile.go
@@ -55,7 +55,7 @@ var modFile *modfile.File
 // in modFile are intepreted, or the latest Go version if modFile is nil.
 func modFileGoVersion() string {
 	if modFile == nil {
-		return latestGoVersion()
+		return LatestGoVersion()
 	}
 	if modFile.Go == nil || modFile.Go.Version == "" {
 		// The main module necessarily has a go.mod file, and that file lacks a
diff --git a/src/cmd/go/testdata/script/mod_tidy_version.txt b/src/cmd/go/testdata/script/mod_tidy_version.txt
index 5441d9cc06..eaa6ee7b0d 100644
--- a/src/cmd/go/testdata/script/mod_tidy_version.txt
+++ b/src/cmd/go/testdata/script/mod_tidy_version.txt
@@ -32,12 +32,22 @@
 
 cp go.mod go.mod.orig
 
+
 # An invalid argument should be rejected.
 
 ! go mod tidy -go=bananas
-stderr '^go mod: invalid -go option "bananas"; expecting something like "-go 1.17"$'
+stderr '^invalid value "bananas" for flag -go: expecting a Go version like "'$goversion'"$'
 cmp go.mod go.mod.orig
 
+! go mod tidy -go=0.9
+stderr '^invalid value "0.9" for flag -go: expecting a Go version like "'$goversion'"$'
+
+! go mod tidy -go=2000.0
+stderr '^invalid value "2000.0" for flag -go: maximum supported Go version is '$goversion'$'
+
+
+# Supported versions should change the go.mod file to be tidy according to the
+# indicated version.
 
 go mod tidy -go=1.15
 cmp go.mod go.mod.115