env: reject program with --null, error stderr

tests/common/util.rs

@@ -606,7 +606,7 @@ impl UCommand {
     }
 
     /// Spawns the command, feeds the stdin if any, waits for the result,
-    /// asserts success, and returns a command result.
+    /// asserts failure, and returns a command result.
     pub fn fails(&mut self) -> CmdResult {
         let cmd_result = self.run();
         cmd_result.failure();

tests/test_env.rs

@@ -81,3 +81,9 @@ fn test_unset_variable() {
 
     assert_eq!(out.lines().any(|line| line.starts_with("HOME=")), false);
 }
+
+#[test]
+fn test_fail_null_with_program() {
+    let out = new_ucmd!().arg("--null").arg("cd").fails().stderr;
+    assert!(out.contains("cannot specify --null (-0) with command"));
+}