diff: report both files when -N is given and neither exists

src/diff.rs

@@ -99,10 +99,20 @@ pub fn compare_files(params: &Params, header: Option<&[u8]>) -> Outcome {
     let mut io_error = false;
     let mut from_missing = false;
     let mut to_missing = false;
-    let from_content = match read_file_contents(&params.from) {
+    let from_result = read_file_contents(&params.from);
+    let to_result = read_file_contents(&params.to);
+    // Like GNU diff, -N and --unidirectional-new-file only treat a missing
+    // operand as empty when the other one exists, so two missing files are
+    // still reported as errors.
+    let is_missing =
+        |r: &io::Result<Vec<u8>>| matches!(r, Err(e) if e.kind() == io::ErrorKind::NotFound);
+    let both_missing = is_missing(&from_result) && is_missing(&to_result);
+    let from_content = match from_result {
         Ok(from_content) => from_content,
         Err(e)
-            if e.kind() == io::ErrorKind::NotFound && params.missing_file != MissingFile::Error =>
+            if e.kind() == io::ErrorKind::NotFound
+                && params.missing_file != MissingFile::Error
+                && !both_missing =>
         {
             from_missing = true;
             vec![]
@@ -113,10 +123,12 @@ pub fn compare_files(params: &Params, header: Option<&[u8]>) -> Outcome {
             vec![]
         }
     };
-    let to_content = match read_file_contents(&params.to) {
+    let to_content = match to_result {
         Ok(to_content) => to_content,
         Err(e)
-            if e.kind() == io::ErrorKind::NotFound && params.missing_file == MissingFile::Empty =>
+            if e.kind() == io::ErrorKind::NotFound
+                && params.missing_file == MissingFile::Empty
+                && !both_missing =>
         {
             to_missing = true;
             vec![]

tests/by-util/test_diff.rs

@@ -364,6 +364,34 @@ fn missing_file_as_empty_keeps_stdin_timestamp() -> Result<(), Box<dyn std::erro
     Ok(())
 }
 
+#[test]
+fn new_file_both_missing_is_an_error() -> Result<(), Box<dyn std::error::Error>> {
+    let tmp_dir = tempdir()?;
+    let missing1 = tmp_dir.path().join("missing1");
+    let missing2 = tmp_dir.path().join("missing2");
+
+    #[cfg(not(windows))]
+    let error_message = "No such file or directory";
+    #[cfg(windows)]
+    let error_message = "The system cannot find the file specified.";
+
+    for option in ["-N", "--new-file", "--unidirectional-new-file"] {
+        let mut cmd = cargo_bin_cmd!("diffutils");
+        cmd.arg("diff").arg(option).arg(&missing1).arg(&missing2);
+        cmd.assert()
+            .code(predicate::eq(2))
+            .failure()
+            .stdout(predicate::str::is_empty())
+            .stderr(predicate::str::contains(format!(
+                "missing1: {error_message}"
+            )))
+            .stderr(predicate::str::contains(format!(
+                "missing2: {error_message}"
+            )));
+    }
+    Ok(())
+}
+
 #[test]
 fn binary_files() -> Result<(), Box<dyn std::error::Error>> {
     let tmp_dir = tempdir()?;