fastwalk: remove use of go1.21

fastwalk: remove use of go1.21 features to fix fzf build

Author: charlievieth

fastwalk.go

@@ -88,29 +88,40 @@ func DefaultNumWorkers() int {
 	return numCPU
 }
 
+var underWSL struct {
+	once sync.Once
+	wsl  bool
+}
+
 // runningUnderWSL returns if we're a Widows executable running in WSL.
 // See [DefaultToSlash] for an explanation of the heuristics used here.
-var runningUnderWSL = sync.OnceValue(func() bool {
+func runningUnderWSL() bool {
 	if runtime.GOOS != "windows" {
 		return false
 	}
-	// Best check (but not super fast)
-	if _, err := os.Lstat("/proc/sys/fs/binfmt_misc/WSLInterop"); err == nil {
-		return true
-	}
-	// Fast check, but could provide a false positive if the user sets
-	// this on the Windows side.
-	if os.Getenv("WSL_DISTRO_NAME") != "" {
-		return true
-	}
-	// If the binary is compiled for Windows and we're running under Linux
-	// then honestly just the presence of "/proc/version" should be enough
-	// to determine that we're running under WSL, but check the version
-	// string just to be pedantic.
-	data, _ := os.ReadFile("/proc/version")
-	return bytes.Contains(data, []byte("microsoft")) ||
-		bytes.Contains(data, []byte("Microsoft"))
-})
+	w := &underWSL
+	w.once.Do(func() {
+		w.wsl = func() bool {
+			// Best check (but not super fast)
+			if _, err := os.Lstat("/proc/sys/fs/binfmt_misc/WSLInterop"); err == nil {
+				return true
+			}
+			// Fast check, but could provide a false positive if the user sets
+			// this on the Windows side.
+			if os.Getenv("WSL_DISTRO_NAME") != "" {
+				return true
+			}
+			// If the binary is compiled for Windows and we're running under Linux
+			// then honestly just the presence of "/proc/version" should be enough
+			// to determine that we're running under WSL, but check the version
+			// string just to be pedantic.
+			data, _ := os.ReadFile("/proc/version")
+			return bytes.Contains(data, []byte("microsoft")) ||
+				bytes.Contains(data, []byte("Microsoft"))
+		}()
+	})
+	return w.wsl
+}
 
 // DefaultToSlash returns the default ToSlash value used by the global config
 // and only applies to Windows. For any other OS this function is a no-op.
@@ -136,10 +147,7 @@ var runningUnderWSL = sync.OnceValue(func() bool {
 // Additionally, the result of this function is cached the cached value will be
 // returned on all subsequent calls.
 func DefaultToSlash() bool {
-	if runtime.GOOS != "windows" {
-		return false
-	}
-	return runningUnderWSL()
+	return runtime.GOOS == "windows" && runningUnderWSL()
 }
 
 // DefaultConfig is the default Config used when none is supplied.