cmd/go: make commands other than 'tidy' prune go.mod less agressively

Updates #31870
Updates #33326
Fixes #34822

Change-Id: I1337f171133c20800eacc6b0955ede5a394ea7eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/204878
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

6 files changed, 439 insertions, 120 deletions

diff --git a/src/cmd/go/internal/modcmd/tidy.go b/src/cmd/go/internal/modcmd/tidy.go
index 584a432d66..af2b04c0c2 100644
--- a/src/cmd/go/internal/modcmd/tidy.go
+++ b/src/cmd/go/internal/modcmd/tidy.go
@@ -7,9 +7,6 @@
 package modcmd
 
 import (
-	"fmt"
-	"os"
-
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/modfetch"
@@ -45,28 +42,8 @@ func runTidy(cmd *base.Command, args []string) {
 		base.Fatalf("go mod tidy: no arguments allowed")
 	}
 
-	// LoadALL adds missing modules.
-	// Remove unused modules.
-	used := make(map[module.Version]bool)
-	for _, pkg := range modload.LoadALL() {
-		used[modload.PackageModule(pkg)] = true
-	}
-	used[modload.Target] = true // note: LoadALL initializes Target
-
-	inGoMod := make(map[string]bool)
-	for _, r := range modload.ModFile().Require {
-		inGoMod[r.Mod.Path] = true
-	}
-
-	var keep []module.Version
-	for _, m := range modload.BuildList() {
-		if used[m] {
-			keep = append(keep, m)
-		} else if cfg.BuildV && inGoMod[m.Path] {
-			fmt.Fprintf(os.Stderr, "unused %s\n", m.Path)
-		}
-	}
-	modload.SetBuildList(keep)
+	modload.LoadALL()
+	modload.TidyBuildList()
 	modTidyGoSum() // updates memory copy; WriteGoMod on next line flushes it out
 	modload.WriteGoMod()
 }
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index 82ec62ea08..cbf3b0575a 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -38,11 +38,8 @@ var (
 	mustUseModules = false
 	initialized    bool
 
-	modRoot     string
-	modFile     *modfile.File
-	modFileData []byte
-	excluded    map[module.Version]bool
-	Target      module.Version
+	modRoot string
+	Target  module.Version
 
 	// targetPrefix is the path prefix for packages in Target, without a trailing
 	// slash. For most modules, targetPrefix is just Target.Path, but the
@@ -61,6 +58,27 @@ var (
 	allowMissingModuleImports bool
 )
 
+var modFile *modfile.File
+
+// A modFileIndex is an index of data corresponding to a modFile
+// at a specific point in time.
+type modFileIndex struct {
+	data         []byte
+	dataNeedsFix bool // true if fixVersion applied a change while parsing data
+	module       module.Version
+	goVersion    string
+	require      map[module.Version]requireMeta
+	replace      map[module.Version]module.Version
+	exclude      map[module.Version]bool
+}
+
+// index is the index of the go.mod file as of when it was last read or written.
+var index *modFileIndex
+
+type requireMeta struct {
+	indirect bool
+}
+
 // ModFile returns the parsed go.mod file.
 //
 // Note that after calling ImportPaths or LoadBuildList,
@@ -383,13 +401,14 @@ func InitMod() {
 		base.Fatalf("go: %v", err)
 	}
 
-	f, err := modfile.Parse(gomod, data, fixVersion)
+	var fixed bool
+	f, err := modfile.Parse(gomod, data, fixVersion(&fixed))
 	if err != nil {
 		// Errors returned by modfile.Parse begin with file:line.
 		base.Fatalf("go: errors parsing go.mod:\n%s\n", err)
 	}
 	modFile = f
-	modFileData = data
+	index = indexModFile(data, f, fixed)
 
 	if len(f.Syntax.Stmt) == 0 || f.Module == nil {
 		// Empty mod file. Must add module path.
@@ -406,10 +425,6 @@ func InitMod() {
 		legacyModInit()
 	}
 
-	excluded = make(map[module.Version]bool)
-	for _, x := range f.Exclude {
-		excluded[x.Mod] = true
-	}
 	modFileToBuildList()
 	setDefaultBuildMod()
 	if cfg.BuildMod == "vendor" {
@@ -421,6 +436,53 @@ func InitMod() {
 	}
 }
 
+// fixVersion returns a modfile.VersionFixer implemented using the Query function.
+//
+// It resolves commit hashes and branch names to versions,
+// canonicalizes verisons that appeared in early vgo drafts,
+// and does nothing for versions that already appear to be canonical.
+//
+// The VersionFixer sets 'fixed' if it ever returns a non-canonical version.
+func fixVersion(fixed *bool) modfile.VersionFixer {
+	return func(path, vers string) (resolved string, err error) {
+		defer func() {
+			if err == nil && resolved != vers {
+				*fixed = true
+			}
+		}()
+
+		// Special case: remove the old -gopkgin- hack.
+		if strings.HasPrefix(path, "gopkg.in/") && strings.Contains(vers, "-gopkgin-") {
+			vers = vers[strings.Index(vers, "-gopkgin-")+len("-gopkgin-"):]
+		}
+
+		// fixVersion is called speculatively on every
+		// module, version pair from every go.mod file.
+		// Avoid the query if it looks OK.
+		_, pathMajor, ok := module.SplitPathVersion(path)
+		if !ok {
+			return "", &module.ModuleError{
+				Path: path,
+				Err: &module.InvalidVersionError{
+					Version: vers,
+					Err:     fmt.Errorf("malformed module path %q", path),
+				},
+			}
+		}
+		if vers != "" && module.CanonicalVersion(vers) == vers {
+			if err := module.CheckPathMajor(vers, pathMajor); err == nil {
+				return vers, nil
+			}
+		}
+
+		info, err := Query(path, vers, "", nil)
+		if err != nil {
+			return "", err
+		}
+		return info.Version, nil
+	}
+}
+
 // AllowMissingModuleImports allows import paths to be resolved to modules
 // when there is no module root. Normally, this is forbidden because it's slow
 // and there's no way to make the result reproducible, but some commands
@@ -466,15 +528,15 @@ func setDefaultBuildMod() {
 
 	if fi, err := os.Stat(filepath.Join(modRoot, "vendor")); err == nil && fi.IsDir() {
 		modGo := "unspecified"
-		if modFile.Go != nil {
-			if semver.Compare("v"+modFile.Go.Version, "v1.14") >= 0 {
+		if index.goVersion != "" {
+			if semver.Compare("v"+index.goVersion, "v1.14") >= 0 {
 				// The Go version is at least 1.14, and a vendor directory exists.
 				// Set -mod=vendor by default.
 				cfg.BuildMod = "vendor"
 				cfg.BuildModReason = "Go version in go.mod is at least 1.14 and vendor directory exists."
 				return
 			} else {
-				modGo = modFile.Go.Version
+				modGo = index.goVersion
 			}
 		}
 
@@ -516,9 +578,7 @@ func checkVendorConsistency() {
 		}
 	}
 
-	explicitInGoMod := make(map[module.Version]bool, len(modFile.Require))
 	for _, r := range modFile.Require {
-		explicitInGoMod[r.Mod] = true
 		if !vendorMeta[r.Mod].Explicit {
 			if pre114 {
 				// Before 1.14, modules.txt did not indicate whether modules were listed
@@ -545,9 +605,7 @@ func checkVendorConsistency() {
 	// don't directly apply to any module in the vendor list, the replacement
 	// go.mod file can affect the selected versions of other (transitive)
 	// dependencies
-	goModReplacement := make(map[module.Version]module.Version, len(modFile.Replace))
 	for _, r := range modFile.Replace {
-		goModReplacement[r.Old] = r.New
 		vr := vendorMeta[r.Old].Replacement
 		if vr == (module.Version{}) {
 			if pre114 && (r.Old.Version == "" || vendorVersion[r.Old.Path] != r.Old.Version) {
@@ -563,17 +621,16 @@ func checkVendorConsistency() {
 
 	for _, mod := range vendorList {
 		meta := vendorMeta[mod]
-		if meta.Explicit && !explicitInGoMod[mod] {
-			vendErrorf(mod, "is marked as explicit in vendor/modules.txt, but not explicitly required in go.mod")
+		if meta.Explicit {
+			if _, inGoMod := index.require[mod]; !inGoMod {
+				vendErrorf(mod, "is marked as explicit in vendor/modules.txt, but not explicitly required in go.mod")
+			}
 		}
 	}
 
 	for _, mod := range vendorReplaced {
-		r, ok := goModReplacement[mod]
-		if !ok {
-			r, ok = goModReplacement[module.Version{Path: mod.Path}]
-		}
-		if !ok {
+		r := Replacement(mod)
+		if r == (module.Version{}) {
 			vendErrorf(mod, "is marked as replaced in vendor/modules.txt, but not replaced in go.mod")
 			continue
 		}
@@ -589,7 +646,7 @@ func checkVendorConsistency() {
 
 // Allowed reports whether module m is allowed (not excluded) by the main module's go.mod.
 func Allowed(m module.Version) bool {
-	return !excluded[m]
+	return index == nil || !index.exclude[m]
 }
 
 func legacyModInit() {
@@ -811,16 +868,17 @@ func AllowWriteGoMod() {
 	allowWriteGoMod = true
 }
 
-// MinReqs returns a Reqs with minimal dependencies of Target,
+// MinReqs returns a Reqs with minimal additional dependencies of Target,
 // as will be written to go.mod.
 func MinReqs() mvs.Reqs {
-	var direct []string
+	var retain []string
 	for _, m := range buildList[1:] {
-		if loaded.direct[m.Path] {
-			direct = append(direct, m.Path)
+		_, explicit := index.require[m]
+		if explicit || loaded.direct[m.Path] {
+			retain = append(retain, m.Path)
 		}
 	}
-	min, err := mvs.Req(Target, direct, Reqs())
+	min, err := mvs.Req(Target, retain, Reqs())
 	if err != nil {
 		base.Fatalf("go: %v", err)
 	}
@@ -841,7 +899,9 @@ func WriteGoMod() {
 		return
 	}
 
-	addGoStmt()
+	if cfg.BuildMod != "readonly" {
+		addGoStmt()
+	}
 
 	if loaded != nil {
 		reqs := MinReqs()
@@ -858,14 +918,9 @@ func WriteGoMod() {
 		}
 		modFile.SetRequire(list)
 	}
+	modFile.Cleanup()
 
-	modFile.Cleanup() // clean file after edits
-	new, err := modFile.Format()
-	if err != nil {
-		base.Fatalf("go: %v", err)
-	}
-
-	dirty := !bytes.Equal(new, modFileData)
+	dirty := index.modFileIsDirty(modFile)
 	if dirty && cfg.BuildMod == "readonly" {
 		// If we're about to fail due to -mod=readonly,
 		// prefer to report a dirty go.mod over a dirty go.sum
@@ -879,23 +934,34 @@ func WriteGoMod() {
 	// downloaded modules that we didn't have before.
 	modfetch.WriteGoSum()
 
-	if !dirty {
-		// We don't need to modify go.mod from what we read previously.
+	if !dirty && cfg.CmdName != "mod tidy" {
+		// The go.mod file has the same semantic content that it had before
+		// (but not necessarily the same exact bytes).
 		// Ignore any intervening edits.
 		return
 	}
 
+	new, err := modFile.Format()
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+	defer func() {
+		// At this point we have determined to make the go.mod file on disk equal to new.
+		index = indexModFile(new, modFile, false)
+	}()
+
 	unlock := modfetch.SideLock()
 	defer unlock()
 
 	file := ModFilePath()
 	old, err := renameio.ReadFile(file)
-	if !bytes.Equal(old, modFileData) {
-		if bytes.Equal(old, new) {
-			// Some other process wrote the same go.mod file that we were about to write.
-			modFileData = new
-			return
-		}
+	if bytes.Equal(old, new) {
+		// The go.mod file is already equal to new, possibly as the result of some
+		// other process.
+		return
+	}
+
+	if index != nil && !bytes.Equal(old, index.data) {
 		if err != nil {
 			base.Fatalf("go: can't determine whether go.mod has changed: %v", err)
 		}
@@ -911,37 +977,114 @@ func WriteGoMod() {
 	if err := renameio.WriteFile(file, new, 0666); err != nil {
 		base.Fatalf("error writing go.mod: %v", err)
 	}
-	modFileData = new
 }
 
-func fixVersion(path, vers string) (string, error) {
-	// Special case: remove the old -gopkgin- hack.
-	if strings.HasPrefix(path, "gopkg.in/") && strings.Contains(vers, "-gopkgin-") {
-		vers = vers[strings.Index(vers, "-gopkgin-")+len("-gopkgin-"):]
+// indexModFile rebuilds the index of modFile.
+// If modFile has been changed since it was first read,
+// modFile.Cleanup must be called before indexModFile.
+func indexModFile(data []byte, modFile *modfile.File, needsFix bool) *modFileIndex {
+	i := new(modFileIndex)
+	i.data = data
+	i.dataNeedsFix = needsFix
+
+	i.module = module.Version{}
+	if modFile.Module != nil {
+		i.module = modFile.Module.Mod
 	}
 
-	// fixVersion is called speculatively on every
-	// module, version pair from every go.mod file.
-	// Avoid the query if it looks OK.
-	_, pathMajor, ok := module.SplitPathVersion(path)
-	if !ok {
-		return "", &module.ModuleError{
-			Path: path,
-			Err: &module.InvalidVersionError{
-				Version: vers,
-				Err:     fmt.Errorf("malformed module path %q", path),
-			},
+	i.goVersion = ""
+	if modFile.Go != nil {
+		i.goVersion = modFile.Go.Version
+	}
+
+	i.require = make(map[module.Version]requireMeta, len(modFile.Require))
+	for _, r := range modFile.Require {
+		i.require[r.Mod] = requireMeta{indirect: r.Indirect}
+	}
+
+	i.replace = make(map[module.Version]module.Version, len(modFile.Replace))
+	for _, r := range modFile.Replace {
+		if prev, dup := i.replace[r.Old]; dup && prev != r.New {
+			base.Fatalf("go: conflicting replacements for %v:\n\t%v\n\t%v", r.Old, prev, r.New)
 		}
+		i.replace[r.Old] = r.New
+	}
+
+	i.exclude = make(map[module.Version]bool, len(modFile.Exclude))
+	for _, x := range modFile.Exclude {
+		i.exclude[x.Mod] = true
 	}
-	if vers != "" && module.CanonicalVersion(vers) == vers {
-		if err := module.CheckPathMajor(vers, pathMajor); err == nil {
-			return vers, nil
+
+	return i
+}
+
+// modFileIsDirty reports whether the go.mod file differs meaningfully
+// from what was indexed.
+// If modFile has been changed (even cosmetically) since it was first read,
+// modFile.Cleanup must be called before modFileIsDirty.
+func (i *modFileIndex) modFileIsDirty(modFile *modfile.File) bool {
+	if i == nil {
+		return modFile != nil
+	}
+
+	if i.dataNeedsFix {
+		return true
+	}
+
+	if modFile.Module == nil {
+		if i.module != (module.Version{}) {
+			return true
 		}
+	} else if modFile.Module.Mod != i.module {
+		return true
 	}
 
-	info, err := Query(path, vers, "", nil)
-	if err != nil {
-		return "", err
+	if modFile.Go == nil {
+		if i.goVersion != "" {
+			return true
+		}
+	} else if modFile.Go.Version != i.goVersion {
+		if i.goVersion == "" && cfg.BuildMod == "readonly" {
+			// go.mod files did not always require a 'go' version, so do not error out
+			// if one is missing — we may be inside an older module in the module
+			// cache, and should bias toward providing useful behavior.
+		} else {
+			return true
+		}
+	}
+
+	if len(modFile.Require) != len(i.require) ||
+		len(modFile.Replace) != len(i.replace) ||
+		len(modFile.Exclude) != len(i.exclude) {
+		return true
+	}
+
+	for _, r := range modFile.Require {
+		if meta, ok := i.require[r.Mod]; !ok {
+			return true
+		} else if r.Indirect != meta.indirect {
+			if cfg.BuildMod == "readonly" {
+				// The module's requirements are consistent; only the "// indirect"
+				// comments that are wrong. But those are only guaranteed to be accurate
+				// after a "go mod tidy" — it's a good idea to run those before
+				// committing a change, but it's certainly not mandatory.
+			} else {
+				return true
+			}
+		}
+	}
+
+	for _, r := range modFile.Replace {
+		if r.New != i.replace[r.Old] {
+			return true
+		}
 	}
-	return info.Version, nil
+
+	for _, x := range modFile.Exclude {
+		if !i.exclude[x.Mod] {
+			return true
+		}
+	}
+
+	return false
 }
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index 2172f81797..ca6c260f45 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -428,6 +428,37 @@ func SetBuildList(list []module.Version) {
 	buildList = append([]module.Version{}, list...)
 }
 
+// TidyBuildList trims the build list to the minimal requirements needed to
+// retain the same versions of all packages from the preceding Load* or
+// ImportPaths* call.
+func TidyBuildList() {
+	used := map[module.Version]bool{Target: true}
+	for _, pkg := range loaded.pkgs {
+		used[pkg.mod] = true
+	}
+
+	keep := []module.Version{Target}
+	var direct []string
+	for _, m := range buildList[1:] {
+		if used[m] {
+			keep = append(keep, m)
+			if loaded.direct[m.Path] {
+				direct = append(direct, m.Path)
+			}
+		} else if cfg.BuildV {
+			if _, ok := index.require[m]; ok {
+				fmt.Fprintf(os.Stderr, "unused %s\n", m.Path)
+			}
+		}
+	}
+
+	min, err := mvs.Req(Target, direct, &mvsReqs{buildList: keep})
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+	buildList = append([]module.Version{Target}, min...)
+}
+
 // ImportMap returns the actual package import path
 // for an import path found in source code.
 // If the given import path does not appear in the source code
@@ -966,21 +997,15 @@ func WhyDepth(path string) int {
 // If there is no replacement for mod, Replacement returns
 // a module.Version with Path == "".
 func Replacement(mod module.Version) module.Version {
-	if modFile == nil {
-		// Happens during testing and if invoking 'go get' or 'go list' outside a module.
-		return module.Version{}
-	}
-
-	var found *modfile.Replace
-	for _, r := range modFile.Replace {
-		if r.Old.Path == mod.Path && (r.Old.Version == "" || r.Old.Version == mod.Version) {
-			found = r // keep going
+	if index != nil {
+		if r, ok := index.replace[mod]; ok {
+			return r
+		}
+		if r, ok := index.replace[module.Version{Path: mod.Path}]; ok {
+			return r
 		}
 	}
-	if found == nil {
-		return module.Version{}
-	}
-	return found.New
+	return module.Version{}
 }
 
 // mvsReqs implements mvs.Reqs for module semantic versions,
@@ -1013,15 +1038,17 @@ func (r *mvsReqs) Required(mod module.Version) ([]module.Version, error) {
 			return cached{nil, err}
 		}
 		for i, mv := range list {
-			for excluded[mv] {
-				mv1, err := r.next(mv)
-				if err != nil {
-					return cached{nil, err}
-				}
-				if mv1.Version == "none" {
-					return cached{nil, fmt.Errorf("%s(%s) depends on excluded %s(%s) with no newer version available", mod.Path, mod.Version, mv.Path, mv.Version)}
+			if index != nil {
+				for index.exclude[mv] {
+					mv1, err := r.next(mv)
+					if err != nil {
+						return cached{nil, err}
+					}
+					if mv1.Version == "none" {
+						return cached{nil, fmt.Errorf("%s(%s) depends on excluded %s(%s) with no newer version available", mod.Path, mod.Version, mv.Path, mv.Version)}
+					}
+					mv = mv1
 				}
-				mv = mv1
 			}
 			list[i] = mv
 		}
diff --git a/src/cmd/go/testdata/script/mod_readonly.txt b/src/cmd/go/testdata/script/mod_readonly.txt
index 942a8663f6..1c89b49f51 100644
--- a/src/cmd/go/testdata/script/mod_readonly.txt
+++ b/src/cmd/go/testdata/script/mod_readonly.txt
@@ -2,9 +2,6 @@ env GO111MODULE=on
 [short] skip
 
 # -mod=readonly must not resolve missing modules nor update go.mod
-#
-# TODO(bcmills): 'go list' should suffice, but today it does not fail due to
-# unresolved imports. When that is fixed, use 'go list' instead of 'go list all'.
 env GOFLAGS=-mod=readonly
 go mod edit -fmt
 cp go.mod go.mod.empty
@@ -27,6 +24,7 @@ grep rsc.io/quote go.mod
 # update go.mod - go mod tidy allowed
 cp go.mod.empty go.mod
 go mod tidy
+cp go.mod go.mod.tidy
 
 # -mod=readonly must succeed once go.mod is up-to-date...
 go list all
@@ -43,6 +41,19 @@ cp go.mod go.mod.inconsistent
 stderr 'go: updates to go.mod needed, disabled by -mod=readonly'
 cmp go.mod go.mod.inconsistent
 
+# However, it should not reject files missing a 'go' directive,
+# since that was not always required.
+cp go.mod.nogo go.mod
+go list all
+
+# Nor should it reject files with redundant (not incorrect)
+# requirements.
+cp go.mod.redundant go.mod
+go list all
+
+cp go.mod.indirect go.mod
+go list all
+
 -- go.mod --
 module m
 
@@ -51,3 +62,30 @@ go 1.20
 -- x.go --
 package x
 import _ "rsc.io/quote"
+-- go.mod.nogo --
+module m
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- go.mod.redundant --
+module m
+
+go 1.20
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/sampler v1.3.0 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- go.mod.indirect --
+module m
+
+go 1.20
+
+require (
+	rsc.io/quote v1.5.2 // indirect
+	rsc.io/sampler v1.3.0 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
diff --git a/src/cmd/go/testdata/script/mod_retention.txt b/src/cmd/go/testdata/script/mod_retention.txt
new file mode 100644
index 0000000000..bff4142ad8
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_retention.txt
@@ -0,0 +1,135 @@
+# Regression test for golang.org/issue/34822: the 'go' command should prefer not
+# to update the go.mod file if the changes only affect formatting, and should only
+# remove redundant requirements in 'go mod tidy'.
+
+env GO111MODULE=on
+[short] skip
+
+# Control case: verify that go.mod.tidy is actually tidy.
+cp go.mod.tidy go.mod
+go list all
+cmp go.mod go.mod.tidy
+
+
+# If the only difference in the go.mod file is the line endings,
+# it should not be overwritten automatically.
+cp go.mod.crlf go.mod
+go list all
+cmp go.mod go.mod.crlf
+
+# However, 'go mod tidy' should fix whitespace even if there are no other changes.
+go mod tidy
+cmp go.mod go.mod.tidy
+
+
+# Out-of-order requirements should not be overwritten automatically...
+cp go.mod.unsorted go.mod
+go list all
+cmp go.mod go.mod.unsorted
+
+# ...but 'go mod edit -fmt' should sort them.
+go mod edit -fmt
+cmp go.mod go.mod.tidy
+
+
+# "// indirect" comments should be removed if direct dependencies are seen.
+# changes.
+cp go.mod.indirect go.mod
+go list all
+cmp go.mod go.mod.tidy
+
+# "// indirect" comments should be added if appropriate.
+cp go.mod.toodirect go.mod
+go list all
+cmp go.mod go.mod.toodirect
+go mod vendor # loads everything, so adds "// indirect" comments.
+cmp go.mod go.mod.tidy
+rm -r vendor
+
+
+# Redundant requirements should be preserved...
+cp go.mod.redundant go.mod
+go list all
+cmp go.mod go.mod.redundant
+go mod vendor
+cmp go.mod go.mod.redundant
+rm -r vendor
+
+# ...except by 'go mod tidy'.
+go mod tidy
+cmp go.mod go.mod.tidy
+
+
+# A missing "go" version directive should be added.
+# However, that should not remove other redundant requirements.
+cp go.mod.nogo go.mod
+go list all
+cmp go.mod go.mod.redundant
+
+
+-- go.mod.tidy --
+module m
+
+go 1.14
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- x.go --
+package x
+import _ "rsc.io/quote"
+-- go.mod.crlf --
+module m

+

+go 1.14

+

+require (

+	rsc.io/quote v1.5.2

+	rsc.io/testonly v1.0.0 // indirect

+)

+-- go.mod.unsorted --
+module m
+
+go 1.14
+
+require (
+	rsc.io/testonly v1.0.0 // indirect
+	rsc.io/quote v1.5.2
+)
+-- go.mod.indirect --
+module m
+
+go 1.14
+
+require (
+	rsc.io/quote v1.5.2 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- go.mod.toodirect --
+module m
+
+go 1.14
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/testonly v1.0.0
+)
+-- go.mod.redundant --
+module m
+
+go 1.14
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/sampler v1.3.0 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- go.mod.nogo --
+module m
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/sampler v1.3.0 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
diff --git a/src/cmd/go/testdata/script/mod_tidy.txt b/src/cmd/go/testdata/script/mod_tidy.txt
index de3b52e2c0..b1d9371217 100644
--- a/src/cmd/go/testdata/script/mod_tidy.txt
+++ b/src/cmd/go/testdata/script/mod_tidy.txt
@@ -5,7 +5,6 @@ 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