address review comments

src/uu/du/src/du.rs

@@ -588,6 +588,7 @@ pub fn div_ceil(a: u64, b: u64) -> u64 {
     (a + b - 1) / b
 }
 
+// Read file paths from the specified file, separated by null characters
 fn read_files_from(file_name: &str) -> Result<Vec<PathBuf>, std::io::Error> {
     let reader: Box<dyn BufRead> = if file_name == "-" {
         // Read from standard input
@@ -656,7 +657,6 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
             .into());
         }
 
-        // Read file paths from the specified file, separated by null characters
         read_files_from(file_from)?
     } else {
         match matches.get_one::<String>(options::FILE) {

tests/by-util/test_du.rs

@@ -992,3 +992,16 @@ fn test_du_files0_from_dir() {
     let result = ts.ucmd().arg("--files0-from=dir").fails();
     assert_eq!(result.stderr_str(), "du: dir: read error: Is a directory\n");
 }
+
+#[test]
+fn test_du_files0_from_combined() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+
+    at.mkdir("dir");
+
+    let result = ts.ucmd().arg("--files0-from=-").arg("foo").fails();
+    let stderr = result.stderr_str();
+
+    assert!(stderr.contains("file operands cannot be combined with --files0-from"));
+}