Check for newline terminators in summarize_diff when line diff sees no difference.

The line differ wasn't sensitive to when actual and expected string were different only based on the final line terminator, so we add some extra logic to detect this condition and report accordingly.

PiperOrigin-RevId: 698448355

Author: Googler

googletest/src/matcher_support/summarize_diff.rs

@@ -38,8 +38,27 @@ pub(crate) fn create_diff(
         // line-by-line diff.
         return "".into();
     }
+
     match edit_distance::edit_list(actual_debug.lines(), expected_debug.lines(), diff_mode) {
-        edit_distance::Difference::Equal => "No difference found between debug strings.".into(),
+        edit_distance::Difference::Equal => {
+            // str.lines() is oblivious to the last newline in a
+            // string, so we need to check this to make sure we don't spuriously
+            // claim that 'hello' and 'hello\n' are identical debug strings.
+            //
+            // Although we would have liked to resolve by replacing
+            // str::lines() with str::split('\n'), the potentially
+            // empty last element interferes with good diff output for
+            // "contains" checks.
+            let actual_newline_terminated = actual_debug.ends_with('\n');
+            let expected_newline_terminated = expected_debug.ends_with('\n');
+            if actual_newline_terminated && !expected_newline_terminated {
+                "Actual includes a terminating newline that is absent from expected.".into()
+            } else if !actual_newline_terminated && expected_newline_terminated {
+                "Actual omits a terminating newline that is present in expected.".into()
+            } else {
+                "No difference found between debug strings.".into()
+            }
+        }
         edit_distance::Difference::Editable(edit_list) => {
             format!("{}{}", summary_header(), edit_list.into_iter().collect::<BufferedSummary>(),)
                 .into()
@@ -553,4 +572,20 @@ mod tests {
             })
         )
     }
+
+    #[test]
+    fn create_diff_line_termination_diff() -> Result<()> {
+        verify_that!(
+            create_diff("1\n2\n3", "1\n2\n3\n", Mode::Exact),
+            eq("Actual omits a terminating newline that is present in expected.")
+        )?;
+        verify_that!(
+            create_diff("1\n2\n3\n", "1\n2\n3", Mode::Exact),
+            eq("Actual includes a terminating newline that is absent from expected.")
+        )?;
+        verify_that!(
+            create_diff("1\n2\n3\n", "1\n2\n3\n", Mode::Exact),
+            eq("No difference found between debug strings.")
+        )
+    }
 }

integration_tests/src/integration_tests.rs

@@ -895,6 +895,21 @@ mod tests {
         verify_eq!(vec![1, 2], {2, 1,},)
     }
 
+    #[gtest]
+    fn verify_eq_reports_diff_in_trailing_newline() -> Result<()> {
+        let result = verify_eq!("hello\nworld\n", "hello\nworld");
+        verify_that!(
+            result,
+            err(displays_as(contains_substring(indoc! {r#"
+                    Expected: is equal to "hello\nworld"
+                    Actual: "hello\nworld\n",
+                      which isn't equal to "hello\nworld"
+                      
+                      Actual includes a terminating newline that is absent from expected.
+                "#})))
+        )
+    }
+
     #[gtest]
     fn verify_eq_when_not_equal_returns_error() -> Result<()> {
         let output =