cmd/go: add -changed to query for non-defaults in the env

Fixes #34208

Change-Id: I8ec2d96262dcd7cbf870f6173690143c54190722
GitHub-Last-Rev: 6543df4784cff1ba5751dc9885ef502e69679118
GitHub-Pull-Request: golang/go#65655
Reviewed-on: https://go-review.googlesource.com/c/go/+/563137
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

15 files changed, 268 insertions, 129 deletions

diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index ad34b8dfcc..9263be5a6b 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -456,7 +456,7 @@
 //
 // Usage:
 //
-//	go env [-json] [-u] [-w] [var ...]
+//	go env [-json] [-changed] [-u] [-w] [var ...]
 //
 // Env prints Go environment information.
 //
@@ -476,6 +476,10 @@
 // form NAME=VALUE and changes the default settings
 // of the named environment variables to the given values.
 //
+// The -changed flag prints only those settings whose effective
+// value differs from the default value that would be obtained in
+// an empty environment with no prior uses of the -w flag.
+//
 // For more about environment variables, see 'go help environment'.
 //
 // # Update packages to use new APIs
diff --git a/src/cmd/go/go_test.go b/src/cmd/go/go_test.go
index a5ce22c0c3..3a3383b271 100644
--- a/src/cmd/go/go_test.go
+++ b/src/cmd/go/go_test.go
@@ -196,7 +196,7 @@ func TestMain(m *testing.M) {
 		defer removeAll(testTmpDir)
 	}
 
-	testGOCACHE = cache.DefaultDir()
+	testGOCACHE, _ = cache.DefaultDir()
 	if testenv.HasGoBuild() {
 		testBin = filepath.Join(testTmpDir, "testbin")
 		if err := os.Mkdir(testBin, 0777); err != nil {
diff --git a/src/cmd/go/internal/bug/bug.go b/src/cmd/go/internal/bug/bug.go
index ed1813605e..d3f9065d3d 100644
--- a/src/cmd/go/internal/bug/bug.go
+++ b/src/cmd/go/internal/bug/bug.go
@@ -106,7 +106,7 @@ func printGoEnv(w io.Writer) {
 	env := envcmd.MkEnv()
 	env = append(env, envcmd.ExtraEnvVars()...)
 	env = append(env, envcmd.ExtraEnvVarsCostly()...)
-	envcmd.PrintEnv(w, env)
+	envcmd.PrintEnv(w, env, false)
 }
 
 func printGoDetails(w io.Writer) {
diff --git a/src/cmd/go/internal/cache/default.go b/src/cmd/go/internal/cache/default.go
index b5650eac66..5430d9651e 100644
--- a/src/cmd/go/internal/cache/default.go
+++ b/src/cmd/go/internal/cache/default.go
@@ -39,7 +39,7 @@ See golang.org to learn more about Go.
 // initDefaultCache does the work of finding the default cache
 // the first time Default is called.
 func initDefaultCache() {
-	dir := DefaultDir()
+	dir, _ := DefaultDir()
 	if dir == "off" {
 		if defaultDirErr != nil {
 			base.Fatalf("build cache is required, but could not be located: %v", defaultDirErr)
@@ -67,14 +67,16 @@ func initDefaultCache() {
 }
 
 var (
-	defaultDirOnce sync.Once
-	defaultDir     string
-	defaultDirErr  error
+	defaultDirOnce    sync.Once
+	defaultDir        string
+	defaultDirChanged bool // effective value differs from $GOCACHE
+	defaultDirErr     error
 )
 
 // DefaultDir returns the effective GOCACHE setting.
-// It returns "off" if the cache is disabled.
-func DefaultDir() string {
+// It returns "off" if the cache is disabled,
+// and reports whether the effective value differs from GOCACHE.
+func DefaultDir() (string, bool) {
 	// Save the result of the first call to DefaultDir for later use in
 	// initDefaultCache. cmd/go/main.go explicitly sets GOCACHE so that
 	// subprocesses will inherit it, but that means initDefaultCache can't
@@ -82,10 +84,11 @@ func DefaultDir() string {
 
 	defaultDirOnce.Do(func() {
 		defaultDir = cfg.Getenv("GOCACHE")
-		if filepath.IsAbs(defaultDir) || defaultDir == "off" {
-			return
-		}
 		if defaultDir != "" {
+			defaultDirChanged = true
+			if filepath.IsAbs(defaultDir) || defaultDir == "off" {
+				return
+			}
 			defaultDir = "off"
 			defaultDirErr = fmt.Errorf("GOCACHE is not an absolute path")
 			return
@@ -95,11 +98,12 @@ func DefaultDir() string {
 		dir, err := os.UserCacheDir()
 		if err != nil {
 			defaultDir = "off"
+			defaultDirChanged = true
 			defaultDirErr = fmt.Errorf("GOCACHE is not defined and %v", err)
 			return
 		}
 		defaultDir = filepath.Join(dir, "go-build")
 	})
 
-	return defaultDir
+	return defaultDir, defaultDirChanged
 }
diff --git a/src/cmd/go/internal/cfg/cfg.go b/src/cmd/go/internal/cfg/cfg.go
index afb595a0c6..002d0006ed 100644
--- a/src/cmd/go/internal/cfg/cfg.go
+++ b/src/cmd/go/internal/cfg/cfg.go
@@ -101,7 +101,9 @@ var (
 
 	// GoPathError is set when GOPATH is not set. it contains an
 	// explanation why GOPATH is unset.
-	GoPathError string
+	GoPathError   string
+	GOPATHChanged bool
+	CGOChanged    bool
 )
 
 func defaultContext() build.Context {
@@ -111,7 +113,7 @@ func defaultContext() build.Context {
 
 	// Override defaults computed in go/build with defaults
 	// from go environment configuration file, if known.
-	ctxt.GOPATH = envOr("GOPATH", gopath(ctxt))
+	ctxt.GOPATH, GOPATHChanged = EnvOrAndChanged("GOPATH", gopath(ctxt))
 	ctxt.GOOS = Goos
 	ctxt.GOARCH = Goarch
 
@@ -125,14 +127,16 @@ func defaultContext() build.Context {
 	ctxt.ToolTags = save
 
 	// The go/build rule for whether cgo is enabled is:
-	//	1. If $CGO_ENABLED is set, respect it.
-	//	2. Otherwise, if this is a cross-compile, disable cgo.
-	//	3. Otherwise, use built-in default for GOOS/GOARCH.
+	//  1. If $CGO_ENABLED is set, respect it.
+	//  2. Otherwise, if this is a cross-compile, disable cgo.
+	//  3. Otherwise, use built-in default for GOOS/GOARCH.
+	//
 	// Recreate that logic here with the new GOOS/GOARCH setting.
-	if v := Getenv("CGO_ENABLED"); v == "0" || v == "1" {
-		ctxt.CgoEnabled = v[0] == '1'
-	} else if ctxt.GOOS != runtime.GOOS || ctxt.GOARCH != runtime.GOARCH {
-		ctxt.CgoEnabled = false
+	// We need to run steps 2 and 3 to determine what the default value
+	// of CgoEnabled would be for computing CGOChanged.
+	defaultCgoEnabled := ctxt.CgoEnabled
+	if ctxt.GOOS != runtime.GOOS || ctxt.GOARCH != runtime.GOARCH {
+		defaultCgoEnabled = false
 	} else {
 		// Use built-in default cgo setting for GOOS/GOARCH.
 		// Note that ctxt.GOOS/GOARCH are derived from the preference list
@@ -159,11 +163,16 @@ func defaultContext() build.Context {
 			if os.Getenv("CC") == "" {
 				cc := DefaultCC(ctxt.GOOS, ctxt.GOARCH)
 				if _, err := LookPath(cc); err != nil {
-					ctxt.CgoEnabled = false
+					defaultCgoEnabled = false
 				}
 			}
 		}
 	}
+	ctxt.CgoEnabled = defaultCgoEnabled
+	if v := Getenv("CGO_ENABLED"); v == "0" || v == "1" {
+		ctxt.CgoEnabled = v[0] == '1'
+	}
+	CGOChanged = ctxt.CgoEnabled != defaultCgoEnabled
 
 	ctxt.OpenFile = func(path string) (io.ReadCloser, error) {
 		return fsys.Open(path)
@@ -262,8 +271,9 @@ func init() {
 
 // An EnvVar is an environment variable Name=Value.
 type EnvVar struct {
-	Name  string
-	Value string
+	Name    string
+	Value   string
+	Changed bool // effective Value differs from default
 }
 
 // OrigEnv is the original environment of the program at startup.
@@ -279,27 +289,28 @@ var envCache struct {
 	m    map[string]string
 }
 
-// EnvFile returns the name of the Go environment configuration file.
-func EnvFile() (string, error) {
+// EnvFile returns the name of the Go environment configuration file,
+// and reports whether the effective value differs from the default.
+func EnvFile() (string, bool, error) {
 	if file := os.Getenv("GOENV"); file != "" {
 		if file == "off" {
-			return "", fmt.Errorf("GOENV=off")
+			return "", false, fmt.Errorf("GOENV=off")
 		}
-		return file, nil
+		return file, true, nil
 	}
 	dir, err := os.UserConfigDir()
 	if err != nil {
-		return "", err
+		return "", false, err
 	}
 	if dir == "" {
-		return "", fmt.Errorf("missing user-config dir")
+		return "", false, fmt.Errorf("missing user-config dir")
 	}
-	return filepath.Join(dir, "go/env"), nil
+	return filepath.Join(dir, "go/env"), false, nil
 }
 
 func initEnvCache() {
 	envCache.m = make(map[string]string)
-	if file, _ := EnvFile(); file != "" {
+	if file, _, _ := EnvFile(); file != "" {
 		readEnvFile(file, "user")
 	}
 	goroot := findGOROOT(envCache.m["GOROOT"])
@@ -397,57 +408,67 @@ var (
 	GOROOTpkg string
 	GOROOTsrc string
 
-	GOBIN      = Getenv("GOBIN")
-	GOMODCACHE = envOr("GOMODCACHE", gopathDir("pkg/mod"))
+	GOBIN                         = Getenv("GOBIN")
+	GOMODCACHE, GOMODCACHEChanged = EnvOrAndChanged("GOMODCACHE", gopathDir("pkg/mod"))
 
 	// Used in envcmd.MkEnv and build ID computations.
-	GOARM     = envOr("GOARM", fmt.Sprint(buildcfg.GOARM))
-	GOARM64   = envOr("GOARM64", fmt.Sprint(buildcfg.GOARM64))
-	GO386     = envOr("GO386", buildcfg.GO386)
-	GOAMD64   = envOr("GOAMD64", fmt.Sprintf("%s%d", "v", buildcfg.GOAMD64))
-	GOMIPS    = envOr("GOMIPS", buildcfg.GOMIPS)
-	GOMIPS64  = envOr("GOMIPS64", buildcfg.GOMIPS64)
-	GOPPC64   = envOr("GOPPC64", fmt.Sprintf("%s%d", "power", buildcfg.GOPPC64))
-	GORISCV64 = envOr("GORISCV64", fmt.Sprintf("rva%du64", buildcfg.GORISCV64))
-	GOWASM    = envOr("GOWASM", fmt.Sprint(buildcfg.GOWASM))
-
-	GOPROXY    = envOr("GOPROXY", "")
-	GOSUMDB    = envOr("GOSUMDB", "")
-	GOPRIVATE  = Getenv("GOPRIVATE")
-	GONOPROXY  = envOr("GONOPROXY", GOPRIVATE)
-	GONOSUMDB  = envOr("GONOSUMDB", GOPRIVATE)
-	GOINSECURE = Getenv("GOINSECURE")
-	GOVCS      = Getenv("GOVCS")
+	GOARM64, goARM64Changed     = EnvOrAndChanged("GOARM64", fmt.Sprint(buildcfg.GOARM64))
+	GOARM, goARMChanged         = EnvOrAndChanged("GOARM", fmt.Sprint(buildcfg.GOARM))
+	GO386, go386Changed         = EnvOrAndChanged("GO386", buildcfg.GO386)
+	GOAMD64, goAMD64Changed     = EnvOrAndChanged("GOAMD64", fmt.Sprintf("%s%d", "v", buildcfg.GOAMD64))
+	GOMIPS, goMIPSChanged       = EnvOrAndChanged("GOMIPS", buildcfg.GOMIPS)
+	GOMIPS64, goMIPS64Changed   = EnvOrAndChanged("GOMIPS64", buildcfg.GOMIPS64)
+	GOPPC64, goPPC64Changed     = EnvOrAndChanged("GOPPC64", fmt.Sprintf("%s%d", "power", buildcfg.GOPPC64))
+	GORISCV64, goRISCV64Changed = EnvOrAndChanged("GORISCV64", fmt.Sprintf("rva%du64", buildcfg.GORISCV64))
+	GOWASM, goWASMChanged       = EnvOrAndChanged("GOWASM", fmt.Sprint(buildcfg.GOWASM))
+
+	GOPROXY, GOPROXYChanged     = EnvOrAndChanged("GOPROXY", "")
+	GOSUMDB, GOSUMDBChanged     = EnvOrAndChanged("GOSUMDB", "")
+	GOPRIVATE                   = Getenv("GOPRIVATE")
+	GONOPROXY, GONOPROXYChanged = EnvOrAndChanged("GONOPROXY", GOPRIVATE)
+	GONOSUMDB, GONOSUMDBChanged = EnvOrAndChanged("GONOSUMDB", GOPRIVATE)
+	GOINSECURE                  = Getenv("GOINSECURE")
+	GOVCS                       = Getenv("GOVCS")
 )
 
+// EnvOrAndChanged returns the environment variable value
+// and reports whether it differs from the default value.
+func EnvOrAndChanged(name, def string) (string, bool) {
+	val := Getenv(name)
+	if val != "" {
+		return val, val != def
+	}
+	return def, false
+}
+
 var SumdbDir = gopathDir("pkg/sumdb")
 
 // GetArchEnv returns the name and setting of the
 // GOARCH-specific architecture environment variable.
 // If the current architecture has no GOARCH-specific variable,
 // GetArchEnv returns empty key and value.
-func GetArchEnv() (key, val string) {
+func GetArchEnv() (key, val string, changed bool) {
 	switch Goarch {
 	case "arm":
-		return "GOARM", GOARM
+		return "GOARM", GOARM, goARMChanged
 	case "arm64":
-		return "GOARM64", GOARM64
+		return "GOARM64", GOARM64, goARM64Changed
 	case "386":
-		return "GO386", GO386
+		return "GO386", GO386, go386Changed
 	case "amd64":
-		return "GOAMD64", GOAMD64
+		return "GOAMD64", GOAMD64, goAMD64Changed
 	case "mips", "mipsle":
-		return "GOMIPS", GOMIPS
+		return "GOMIPS", GOMIPS, goMIPSChanged
 	case "mips64", "mips64le":
-		return "GOMIPS64", GOMIPS64
+		return "GOMIPS64", GOMIPS64, goMIPS64Changed
 	case "ppc64", "ppc64le":
-		return "GOPPC64", GOPPC64
+		return "GOPPC64", GOPPC64, goPPC64Changed
 	case "riscv64":
-		return "GORISCV64", GORISCV64
+		return "GORISCV64", GORISCV64, goRISCV64Changed
 	case "wasm":
-		return "GOWASM", GOWASM
+		return "GOWASM", GOWASM, goWASMChanged
 	}
-	return "", ""
+	return "", "", false
 }
 
 // envOr returns Getenv(key) if set, or else def.
@@ -565,6 +586,7 @@ func gopathDir(rel string) string {
 	return filepath.Join(list[0], rel)
 }
 
+// Keep consistent with go/build.defaultGOPATH.
 func gopath(ctxt build.Context) string {
 	if len(ctxt.GOPATH) > 0 {
 		return ctxt.GOPATH
diff --git a/src/cmd/go/internal/clean/clean.go b/src/cmd/go/internal/clean/clean.go
index b021b784da..de2ef9dcb9 100644
--- a/src/cmd/go/internal/clean/clean.go
+++ b/src/cmd/go/internal/clean/clean.go
@@ -153,7 +153,7 @@ func runClean(ctx context.Context, cmd *base.Command, args []string) {
 	sh := work.NewShell("", fmt.Print)
 
 	if cleanCache {
-		dir := cache.DefaultDir()
+		dir, _ := cache.DefaultDir()
 		if dir != "off" {
 			// Remove the cache subdirectories but not the top cache directory.
 			// The top cache directory may have been created with special permissions
@@ -180,7 +180,7 @@ func runClean(ctx context.Context, cmd *base.Command, args []string) {
 		// Instead of walking through the entire cache looking for test results,
 		// we write a file to the cache indicating that all test results from before
 		// right now are to be ignored.
-		dir := cache.DefaultDir()
+		dir, _ := cache.DefaultDir()
 		if dir != "off" {
 			f, err := lockedfile.Edit(filepath.Join(dir, "testexpire.txt"))
 			if err == nil {
diff --git a/src/cmd/go/internal/envcmd/env.go b/src/cmd/go/internal/envcmd/env.go
index bff3fe5d55..b3838a75e2 100644
--- a/src/cmd/go/internal/envcmd/env.go
+++ b/src/cmd/go/internal/envcmd/env.go
@@ -32,7 +32,7 @@ import (
 )
 
 var CmdEnv = &base.Command{
-	UsageLine: "go env [-json] [-u] [-w] [var ...]",
+	UsageLine: "go env [-json] [-changed] [-u] [-w] [var ...]",
 	Short:     "print Go environment information",
 	Long: `
 Env prints Go environment information.
@@ -53,6 +53,10 @@ The -w flag requires one or more arguments of the
 form NAME=VALUE and changes the default settings
 of the named environment variables to the given values.
 
+The -changed flag prints only those settings whose effective
+value differs from the default value that would be obtained in
+an empty environment with no prior uses of the -w flag.
+
 For more about environment variables, see 'go help environment'.
 	`,
 }
@@ -64,19 +68,20 @@ func init() {
 }
 
 var (
-	envJson = CmdEnv.Flag.Bool("json", false, "")
-	envU    = CmdEnv.Flag.Bool("u", false, "")
-	envW    = CmdEnv.Flag.Bool("w", false, "")
+	envJson    = CmdEnv.Flag.Bool("json", false, "")
+	envU       = CmdEnv.Flag.Bool("u", false, "")
+	envW       = CmdEnv.Flag.Bool("w", false, "")
+	envChanged = CmdEnv.Flag.Bool("changed", false, "")
 )
 
 func MkEnv() []cfg.EnvVar {
-	envFile, _ := cfg.EnvFile()
+	envFile, envFileChanged, _ := cfg.EnvFile()
 	env := []cfg.EnvVar{
 		{Name: "GO111MODULE", Value: cfg.Getenv("GO111MODULE")},
-		{Name: "GOARCH", Value: cfg.Goarch},
+		{Name: "GOARCH", Value: cfg.Goarch, Changed: cfg.Goarch != runtime.GOARCH},
 		{Name: "GOBIN", Value: cfg.GOBIN},
-		{Name: "GOCACHE", Value: cache.DefaultDir()},
-		{Name: "GOENV", Value: envFile},
+		{Name: "GOCACHE"},
+		{Name: "GOENV", Value: envFile, Changed: envFileChanged},
 		{Name: "GOEXE", Value: cfg.ExeSuffix},
 
 		// List the raw value of GOEXPERIMENT, not the cleaned one.
@@ -90,63 +95,82 @@ func MkEnv() []cfg.EnvVar {
 		{Name: "GOHOSTARCH", Value: runtime.GOARCH},
 		{Name: "GOHOSTOS", Value: runtime.GOOS},
 		{Name: "GOINSECURE", Value: cfg.GOINSECURE},
-		{Name: "GOMODCACHE", Value: cfg.GOMODCACHE},
-		{Name: "GONOPROXY", Value: cfg.GONOPROXY},
-		{Name: "GONOSUMDB", Value: cfg.GONOSUMDB},
-		{Name: "GOOS", Value: cfg.Goos},
-		{Name: "GOPATH", Value: cfg.BuildContext.GOPATH},
+		{Name: "GOMODCACHE", Value: cfg.GOMODCACHE, Changed: cfg.GOMODCACHEChanged},
+		{Name: "GONOPROXY", Value: cfg.GONOPROXY, Changed: cfg.GONOPROXYChanged},
+		{Name: "GONOSUMDB", Value: cfg.GONOSUMDB, Changed: cfg.GONOSUMDBChanged},
+		{Name: "GOOS", Value: cfg.Goos, Changed: cfg.Goos != runtime.GOOS},
+		{Name: "GOPATH", Value: cfg.BuildContext.GOPATH, Changed: cfg.GOPATHChanged},
 		{Name: "GOPRIVATE", Value: cfg.GOPRIVATE},
-		{Name: "GOPROXY", Value: cfg.GOPROXY},
+		{Name: "GOPROXY", Value: cfg.GOPROXY, Changed: cfg.GOPROXYChanged},
 		{Name: "GOROOT", Value: cfg.GOROOT},
-		{Name: "GOSUMDB", Value: cfg.GOSUMDB},
+		{Name: "GOSUMDB", Value: cfg.GOSUMDB, Changed: cfg.GOSUMDBChanged},
 		{Name: "GOTMPDIR", Value: cfg.Getenv("GOTMPDIR")},
 		{Name: "GOTOOLCHAIN", Value: cfg.Getenv("GOTOOLCHAIN")},
 		{Name: "GOTOOLDIR", Value: build.ToolDir},
 		{Name: "GOVCS", Value: cfg.GOVCS},
 		{Name: "GOVERSION", Value: runtime.Version()},
-		{Name: "GODEBUG", Value: os.Getenv("GODEBUG")},
+		{Name: "GODEBUG"},
+	}
+
+	for i := range env {
+		switch env[i].Name {
+		case "GO111MODULE":
+			if env[i].Value != "on" && env[i].Value != "" {
+				env[i].Changed = true
+			}
+		case "GOBIN", "GOEXPERIMENT", "GOFLAGS", "GOINSECURE", "GOPRIVATE", "GOTMPDIR", "GOVCS":
+			if env[i].Value != "" {
+				env[i].Changed = true
+			}
+		case "GOCACHE":
+			env[i].Value, env[i].Changed = cache.DefaultDir()
+		case "GOTOOLCHAIN":
+			if env[i].Value != "auto" {
+				env[i].Changed = true
+			}
+		case "GODEBUG":
+			env[i].Value = os.Getenv("GODEBUG")
+			env[i].Changed = env[i].Value != ""
+		}
 	}
 
 	if work.GccgoBin != "" {
-		env = append(env, cfg.EnvVar{Name: "GCCGO", Value: work.GccgoBin})
+		env = append(env, cfg.EnvVar{Name: "GCCGO", Value: work.GccgoBin, Changed: true})
 	} else {
 		env = append(env, cfg.EnvVar{Name: "GCCGO", Value: work.GccgoName})
 	}
 
-	key, val := cfg.GetArchEnv()
-	if key != "" {
-		env = append(env, cfg.EnvVar{Name: key, Value: val})
+	goarch, val, changed := cfg.GetArchEnv()
+	if goarch != "" {
+		env = append(env, cfg.EnvVar{Name: goarch, Value: val, Changed: changed})
 	}
 
 	cc := cfg.Getenv("CC")
+	ccChanged := true
 	if cc == "" {
+		ccChanged = false
 		cc = cfg.DefaultCC(cfg.Goos, cfg.Goarch)
 	}
 	cxx := cfg.Getenv("CXX")
+	cxxChanged := true
 	if cxx == "" {
+		cxxChanged = false
 		cxx = cfg.DefaultCXX(cfg.Goos, cfg.Goarch)
 	}
-	env = append(env, cfg.EnvVar{Name: "AR", Value: envOr("AR", "ar")})
-	env = append(env, cfg.EnvVar{Name: "CC", Value: cc})
-	env = append(env, cfg.EnvVar{Name: "CXX", Value: cxx})
+	ar, arChanged := cfg.EnvOrAndChanged("AR", "ar")
+	env = append(env, cfg.EnvVar{Name: "AR", Value: ar, Changed: arChanged})
+	env = append(env, cfg.EnvVar{Name: "CC", Value: cc, Changed: ccChanged})
+	env = append(env, cfg.EnvVar{Name: "CXX", Value: cxx, Changed: cxxChanged})
 
 	if cfg.BuildContext.CgoEnabled {
-		env = append(env, cfg.EnvVar{Name: "CGO_ENABLED", Value: "1"})
+		env = append(env, cfg.EnvVar{Name: "CGO_ENABLED", Value: "1", Changed: cfg.CGOChanged})
 	} else {
-		env = append(env, cfg.EnvVar{Name: "CGO_ENABLED", Value: "0"})
+		env = append(env, cfg.EnvVar{Name: "CGO_ENABLED", Value: "0", Changed: cfg.CGOChanged})
 	}
 
 	return env
 }
 
-func envOr(name, def string) string {
-	val := cfg.Getenv(name)
-	if val != "" {
-		return val
-	}
-	return def
-}
-
 func findEnv(env []cfg.EnvVar, name string) string {
 	for _, e := range env {
 		if e.Name == name {
@@ -206,7 +230,7 @@ func ExtraEnvVarsCostly() []cfg.EnvVar {
 		return q
 	}
 
-	return []cfg.EnvVar{
+	ret := []cfg.EnvVar{
 		// Note: Update the switch in runEnv below when adding to this list.
 		{Name: "CGO_CFLAGS", Value: join(cflags)},
 		{Name: "CGO_CPPFLAGS", Value: join(cppflags)},
@@ -216,6 +240,21 @@ func ExtraEnvVarsCostly() []cfg.EnvVar {
 		{Name: "PKG_CONFIG", Value: b.PkgconfigCmd()},
 		{Name: "GOGCCFLAGS", Value: join(cmd[3:])},
 	}
+
+	for i := range ret {
+		ev := &ret[i]
+		switch ev.Name {
+		case "GOGCCFLAGS": // GOGCCFLAGS cannot be modified
+		case "CGO_CPPFLAGS":
+			ev.Changed = ev.Value != ""
+		case "PKG_CONFIG":
+			ev.Changed = ev.Value != cfg.DefaultPkgConfig
+		case "CGO_CXXFLAGS", "CGO_CFLAGS", "CGO_FFLAGS", "GGO_LDFLAGS":
+			ev.Changed = ev.Value != work.DefaultCFlags
+		}
+	}
+
+	return ret
 }
 
 // argKey returns the KEY part of the arg KEY=VAL, or else arg itself.
@@ -297,27 +336,43 @@ func runEnv(ctx context.Context, cmd *base.Command, args []string) {
 	}
 
 	if len(args) > 0 {
-		if *envJson {
-			var es []cfg.EnvVar
-			for _, name := range args {
-				e := cfg.EnvVar{Name: name, Value: findEnv(env, name)}
-				es = append(es, e)
+		// Show only the named vars.
+		if !*envChanged {
+			if *envJson {
+				var es []cfg.EnvVar
+				for _, name := range args {
+					e := cfg.EnvVar{Name: name, Value: findEnv(env, name)}
+					es = append(es, e)
+				}
+				env = es
+			} else {
+				// Print just the values, without names.
+				for _, name := range args {
+					fmt.Printf("%s\n", findEnv(env, name))
+				}
+				return
 			}
-			printEnvAsJSON(es)
 		} else {
+			// Show only the changed, named vars.
+			var es []cfg.EnvVar
 			for _, name := range args {
-				fmt.Printf("%s\n", findEnv(env, name))
+				for _, e := range env {
+					if e.Name == name {
+						es = append(es, e)
+						break
+					}
+				}
 			}
+			env = es
 		}
-		return
 	}
 
+	// print
 	if *envJson {
-		printEnvAsJSON(env)
-		return
+		printEnvAsJSON(env, *envChanged)
+	} else {
+		PrintEnv(os.Stdout, env, *envChanged)
 	}
-
-	PrintEnv(os.Stdout, env)
 }
 
 func runEnvW(args []string) {
@@ -423,12 +478,15 @@ func checkBuildConfig(add map[string]string, del map[string]bool) error {
 }
 
 // PrintEnv prints the environment variables to w.
-func PrintEnv(w io.Writer, env []cfg.EnvVar) {
+func PrintEnv(w io.Writer, env []cfg.EnvVar, onlyChanged bool) {
 	for _, e := range env {
 		if e.Name != "TERM" {
 			if runtime.GOOS != "plan9" && bytes.Contains([]byte(e.Value), []byte{0}) {
 				base.Fatalf("go: internal error: encountered null byte in environment variable %s on non-plan9 platform", e.Name)
 			}
+			if onlyChanged && !e.Changed {
+				continue
+			}
 			switch runtime.GOOS {
 			default:
 				fmt.Fprintf(w, "%s=%s\n", e.Name, shellQuote(e.Value))
@@ -503,12 +561,15 @@ func batchEscape(s string) string {
 	return b.String()
 }
 
-func printEnvAsJSON(env []cfg.EnvVar) {
+func printEnvAsJSON(env []cfg.EnvVar, onlyChanged bool) {
 	m := make(map[string]string)
 	for _, e := range env {
 		if e.Name == "TERM" {
 			continue
 		}
+		if onlyChanged && !e.Changed {
+			continue
+		}
 		m[e.Name] = e.Value
 	}
 	enc := json.NewEncoder(os.Stdout)
@@ -591,7 +652,7 @@ func checkEnvWrite(key, val string) error {
 }
 
 func readEnvFileLines(mustExist bool) []string {
-	file, err := cfg.EnvFile()
+	file, _, err := cfg.EnvFile()
 	if file == "" {
 		if mustExist {
 			base.Fatalf("go: cannot find go env config: %v", err)
@@ -655,7 +716,7 @@ func updateEnvFile(add map[string]string, del map[string]bool) {
 		}
 	}
 
-	file, err := cfg.EnvFile()
+	file, _, err := cfg.EnvFile()
 	if file == "" {
 		base.Fatalf("go: cannot find go env config: %v", err)
 	}
diff --git a/src/cmd/go/internal/envcmd/env_test.go b/src/cmd/go/internal/envcmd/env_test.go
index 7419cf3fc2..2f6470b871 100644
--- a/src/cmd/go/internal/envcmd/env_test.go
+++ b/src/cmd/go/internal/envcmd/env_test.go
@@ -56,7 +56,7 @@ func FuzzPrintEnvEscape(f *testing.F) {
 		if runtime.GOOS == "windows" {
 			b.WriteString("@echo off\n")
 		}
-		PrintEnv(&b, []cfg.EnvVar{{Name: "var", Value: s}})
+		PrintEnv(&b, []cfg.EnvVar{{Name: "var", Value: s}}, false)
 		var want string
 		if runtime.GOOS == "windows" {
 			fmt.Fprintf(&b, "echo \"%%var%%\"\n")
diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
index 58acd4dc34..4bf2f381dd 100644
--- a/src/cmd/go/internal/load/pkg.go
+++ b/src/cmd/go/internal/load/pkg.go
@@ -2392,7 +2392,7 @@ func (p *Package) setBuildInfo(ctx context.Context, autoVCS bool) {
 		appendSetting("GOEXPERIMENT", cfg.RawGOEXPERIMENT)
 	}
 	appendSetting("GOOS", cfg.BuildContext.GOOS)
-	if key, val := cfg.GetArchEnv(); key != "" && val != "" {
+	if key, val, _ := cfg.GetArchEnv(); key != "" && val != "" {
 		appendSetting(key, val)
 	}
 
diff --git a/src/cmd/go/internal/modindex/read.go b/src/cmd/go/internal/modindex/read.go
index bda3fb4338..9d8c48f2b0 100644
--- a/src/cmd/go/internal/modindex/read.go
+++ b/src/cmd/go/internal/modindex/read.go
@@ -147,7 +147,8 @@ func GetPackage(modroot, pkgdir string) (*IndexPackage, error) {
 // using the index, for instance because the index is disabled, or the package
 // is not in a module.
 func GetModule(modroot string) (*Module, error) {
-	if !enabled || cache.DefaultDir() == "off" {
+	dir, _ := cache.DefaultDir()
+	if !enabled || dir == "off" {
 		return nil, errDisabled
 	}
 	if modroot == "" {
diff --git a/src/cmd/go/internal/test/test.go b/src/cmd/go/internal/test/test.go
index ac9d2721f5..a13070a91e 100644
--- a/src/cmd/go/internal/test/test.go
+++ b/src/cmd/go/internal/test/test.go
@@ -837,7 +837,7 @@ func runTest(ctx context.Context, cmd *base.Command, args []string) {
 	// Read testcache expiration time, if present.
 	// (We implement go clean -testcache by writing an expiration date
 	// instead of searching out and deleting test result cache entries.)
-	if dir := cache.DefaultDir(); dir != "off" {
+	if dir, _ := cache.DefaultDir(); dir != "off" {
 		if data, _ := lockedfile.Read(filepath.Join(dir, "testexpire.txt")); len(data) > 0 && data[len(data)-1] == '\n' {
 			if t, err := strconv.ParseInt(string(data[:len(data)-1]), 10, 64); err == nil {
 				testCacheExpire = time.Unix(0, t)
diff --git a/src/cmd/go/internal/work/exec.go b/src/cmd/go/internal/work/exec.go
index 04459d0990..90c61a9c30 100644
--- a/src/cmd/go/internal/work/exec.go
+++ b/src/cmd/go/internal/work/exec.go
@@ -46,7 +46,7 @@ import (
 	"cmd/internal/sys"
 )
 
-const defaultCFlags = "-O2 -g"
+const DefaultCFlags = "-O2 -g"
 
 // actionList returns the list of actions in the dag rooted at root
 // as visited in a depth-first post-order traversal.
@@ -337,7 +337,7 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 		}
 
 		// GOARM, GOMIPS, etc.
-		key, val := cfg.GetArchEnv()
+		key, val, _ := cfg.GetArchEnv()
 		fmt.Fprintf(h, "%s=%s\n", key, val)
 
 		if cfg.CleanGOEXPERIMENT != "" {
@@ -1419,7 +1419,7 @@ func (b *Builder) printLinkerConfig(h io.Writer, p *load.Package) {
 		}
 
 		// GOARM, GOMIPS, etc.
-		key, val := cfg.GetArchEnv()
+		key, val, _ := cfg.GetArchEnv()
 		fmt.Fprintf(h, "%s=%s\n", key, val)
 
 		if cfg.CleanGOEXPERIMENT != "" {
@@ -2460,13 +2460,13 @@ func (b *Builder) gccSupportsFlag(compiler []string, flag string) bool {
 
 	cmdArgs := str.StringList(compiler, flag)
 	if strings.HasPrefix(flag, "-Wl,") /* linker flag */ {
-		ldflags, err := buildFlags("LDFLAGS", defaultCFlags, nil, checkLinkerFlags)
+		ldflags, err := buildFlags("LDFLAGS", DefaultCFlags, nil, checkLinkerFlags)
 		if err != nil {
 			return false
 		}
 		cmdArgs = append(cmdArgs, ldflags...)
 	} else { /* compiler flag, add "-c" */
-		cflags, err := buildFlags("CFLAGS", defaultCFlags, nil, checkCompilerFlags)
+		cflags, err := buildFlags("CFLAGS", DefaultCFlags, nil, checkCompilerFlags)
 		if err != nil {
 			return false
 		}
@@ -2707,16 +2707,16 @@ func (b *Builder) CFlags(p *load.Package) (cppflags, cflags, cxxflags, fflags, l
 	if cppflags, err = buildFlags("CPPFLAGS", "", p.CgoCPPFLAGS, checkCompilerFlags); err != nil {
 		return
 	}
-	if cflags, err = buildFlags("CFLAGS", defaultCFlags, p.CgoCFLAGS, checkCompilerFlags); err != nil {
+	if cflags, err = buildFlags("CFLAGS", DefaultCFlags, p.CgoCFLAGS, checkCompilerFlags); err != nil {
 		return
 	}
-	if cxxflags, err = buildFlags("CXXFLAGS", defaultCFlags, p.CgoCXXFLAGS, checkCompilerFlags); err != nil {
+	if cxxflags, err = buildFlags("CXXFLAGS", DefaultCFlags, p.CgoCXXFLAGS, checkCompilerFlags); err != nil {
 		return
 	}
-	if fflags, err = buildFlags("FFLAGS", defaultCFlags, p.CgoFFLAGS, checkCompilerFlags); err != nil {
+	if fflags, err = buildFlags("FFLAGS", DefaultCFlags, p.CgoFFLAGS, checkCompilerFlags); err != nil {
 		return
 	}
-	if ldflags, err = buildFlags("LDFLAGS", defaultCFlags, p.CgoLDFLAGS, checkLinkerFlags); err != nil {
+	if ldflags, err = buildFlags("LDFLAGS", DefaultCFlags, p.CgoLDFLAGS, checkLinkerFlags); err != nil {
 		return
 	}
 
diff --git a/src/cmd/go/internal/work/shell.go b/src/cmd/go/internal/work/shell.go
index 60817d9c3b..1fac8e3a45 100644
--- a/src/cmd/go/internal/work/shell.go
+++ b/src/cmd/go/internal/work/shell.go
@@ -114,7 +114,8 @@ func (sh *Shell) moveOrCopyFile(dst, src string, perm fs.FileMode, force bool) e
 	// Otherwise fall back to standard copy.
 
 	// If the source is in the build cache, we need to copy it.
-	if strings.HasPrefix(src, cache.DefaultDir()) {
+	dir, _ := cache.DefaultDir()
+	if strings.HasPrefix(src, dir) {
 		return sh.CopyFile(dst, src, perm, force)
 	}
 
diff --git a/src/cmd/go/testdata/script/env_changed.txt b/src/cmd/go/testdata/script/env_changed.txt
new file mode 100644
index 0000000000..7b7b154dae
--- /dev/null
+++ b/src/cmd/go/testdata/script/env_changed.txt
@@ -0,0 +1,45 @@
+# Test query for non-defaults in the env
+
+env GOTOOLCHAIN=local
+env GOSUMDB=nodefault
+env GOPROXY=nodefault
+env GO111MODULE=auto
+env CGO_CFLAGS=nodefault
+env CGO_CPPFLAGS=nodefault
+
+go env -changed
+# linux output like GOTOOLCHAIN='local'
+# windows output like GOTOOLCHAIN=local
+stdout 'GOTOOLCHAIN=''?local''?'
+stdout 'GOSUMDB=''?nodefault''?'
+stdout 'GOPROXY=''?nodefault''?'
+stdout 'GO111MODULE=''?auto''?'
+stdout 'CGO_CFLAGS=''?nodefault''?'
+stdout 'CGO_CPPFLAGS=''?nodefault''?'
+
+go env -changed -json
+stdout '"GOTOOLCHAIN": "local"'
+stdout '"GOSUMDB": "nodefault"'
+stdout '"GOPROXY": "nodefault"'
+stdout '"GO111MODULE": "auto"'
+stdout '"CGO_CFLAGS": "nodefault"'
+stdout '"CGO_CPPFLAGS": "nodefault"'
+
+[GOOS:windows] env GOOS=linux
+[!GOOS:windows] env GOOS=windows
+[GOARCH:amd64] env GOARCH=arm64
+[!GOARCH:amd64] env GOARCH=amd64
+
+go env -changed GOOS
+[GOOS:windows] stdout 'set GOOS=linux'
+[!GOOS:windows] stdout 'GOOS=''windows'''
+go env -changed GOARCH
+[GOARCH:amd64] stdout 'set GOARCH=arm64|GOARCH=''arm64'''
+[!GOARCH:amd64] stdout 'set GOARCH=amd64|GOARCH=''amd64'''
+
+go env -changed -json GOOS
+[GOOS:windows] stdout '"GOOS": "linux"'
+[!GOOS:windows] stdout '"GOOS": "windows"'
+go env -changed -json GOARCH
+[GOARCH:amd64] stdout '"GOARCH": "arm64"'
+[!GOARCH:amd64] stdout '"GOARCH": "amd64"'
diff --git a/src/go/build/build.go b/src/go/build/build.go
index 9ce3700dc4..43c74cb99a 100644
--- a/src/go/build/build.go
+++ b/src/go/build/build.go
@@ -286,6 +286,7 @@ func (ctxt *Context) SrcDirs() []string {
 // if set, or else the compiled code's GOARCH, GOOS, and GOROOT.
 var Default Context = defaultContext()
 
+// Keep consistent with cmd/go/internal/cfg.defaultGOPATH.
 func defaultGOPATH() string {
 	env := "HOME"
 	if runtime.GOOS == "windows" {