add test, use filepath.Separator

Author: motemen

src/cmd/go/go_test.go

@@ -5921,3 +5921,11 @@ echo $* >>`+tg.path("pkg-config.out"))
 		t.Errorf("got %q want %q", out, want)
 	}
 }
+
+// Issue 23982
+func TestFilepathUnderCwdFormat(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.run("test", "-x", "-cover", "log")
+	tg.grepStderrNot(`\.log\.cover\.go`, "-x output should contain correctly formatted filepath under cwd")
+}

src/cmd/go/internal/work/exec.go

@@ -1365,8 +1365,8 @@ func (b *Builder) fmtcmd(dir string, format string, args ...interface{}) string
 	cmd := fmt.Sprintf(format, args...)
 	if dir != "" && dir != "/" {
 		dot := " ."
-		if dir[len(dir)-1] == '/' {
-			dot += "/"
+		if dir[len(dir)-1] == filepath.Separator {
+			dot += string(filepath.Separator)
 		}
 		cmd = strings.Replace(" "+cmd, " "+dir, dot, -1)[1:]
 		if b.scriptDir != dir {