Print warning when we cannot print good diff

pretty_assertions/src/lib.rs

@@ -205,8 +205,22 @@ where
     TRight: AsRef<str> + ?Sized,
 {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        printer::write_header(f)?;
-        printer::write_lines(f, self.left.as_ref(), self.right.as_ref())
+        let left = self.left.as_ref();
+        let right = self.right.as_ref();
+        if left != right
+            && ::diff::lines(left, right)
+                .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 and fallback to the debug output.
+            writeln!(f, "Warning: values were different, but no line diff was generated. It seems like these values only differ in line endings - falling back to debug output:")?;
+            printer::write_header(f)?;
+            printer::write_lines_debug_output_fallback(f, left, right)
+        } else {
+            printer::write_header(f)?;
+            printer::write_lines(f, left, right)
+        }
     }
 }
 

pretty_assertions/src/printer.rs

@@ -77,7 +77,7 @@ impl<'a> LatentDeletion<'a> {
 // https://github.com/johannhof/difference.rs/blob/c5749ad7d82aa3d480c15cb61af9f6baa08f116f/examples/github-style.rs
 // Credits johannhof (MIT License)
 
-/// Present the diff output for two mutliline strings in a pretty, colorised manner.
+/// Present the diff output for two multiline strings in a pretty, colorised manner.
 pub(crate) fn write_lines<TWrite: fmt::Write>(
     f: &mut TWrite,
     left: &str,
@@ -123,6 +123,20 @@ pub(crate) fn write_lines<TWrite: fmt::Write>(
     Ok(())
 }
 
+/// Present the diff output for two multiline strings by debug output.
+pub(crate) fn write_lines_debug_output_fallback<TWrite: fmt::Write>(
+    f: &mut TWrite,
+    left: &str,
+    right: &str,
+) -> fmt::Result {
+    writeln!(
+        f,
+        r#" left: `{:?}`,
+right: `{:?}`"#,
+        left, right,
+    )
+}
+
 /// Group character styling for an inline diff, to prevent wrapping each single
 /// character in terminal styling codes.
 ///
@@ -249,6 +263,22 @@ 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 = "Foo\r\n";
+        let right = "Foo\n";
+        let expected = r#" left: `"Foo\r\n"`,
+right: `"Foo\n"`
+"#
+        .to_string();
+
+        check_printer(write_lines_debug_output_fallback, left, right, &expected);
+    }
+
     #[test]
     fn write_inline_diff_empty() {
         let left = "";