Print warning when we cannot print good diff

pretty_assertions/src/printer.rs

@@ -85,6 +85,17 @@ pub(crate) fn write_lines<TWrite: fmt::Write>(
 ) -> fmt::Result {
     let diff = ::diff::lines(left, right);
 
+    if left != right
+        && diff
+            .iter()
+            .all(|result| matches!(result, ::diff::Result::Both(..)))
+    {
+        // If left and right are not the same and all changes are both, then we cannot print good diff.
+        // We should print the warning here instead.
+        writeln!(f, "Warning: values were different, but no line diff was generated. Do your values have the same line endings?")?;
+        return Ok(());
+    }
+
     let mut changes = diff.into_iter().peekable();
     let mut previous_deletion = LatentDeletion::default();
 
@@ -249,6 +260,19 @@ mod test {
         assert_eq!(actual, expected);
     }
 
+    /// Test diffing newlines from different conventions.
+    ///
+    /// See: https://github.com/colin-kiegel/rust-pretty-assertions/issues/100
+    #[test]
+    fn different_newlines() {
+        // The below inputs caused an output with no visible diff
+        let left = "\r\n";
+        let right = "\n";
+        let expected = String::from("Warning: values were different, but no line diff was generated. Do your values have the same line endings?\n");
+
+        check_printer(write_lines, left, right, &expected);
+    }
+
     #[test]
     fn write_inline_diff_empty() {
         let left = "";