Adjust newlines between the description and diff to improve matcher output consistency.

There are two slight inconsistencies that make output a little less consistent than ideal:

* When constructing the summarize_diff message, some branches return the empty string, another a string with a leading newline, and yet another a string without a leading newline.
* The explanation+diff formatting behavior between str_matcher and eq_matcher was not consistent: str_matcher added a newline between them, and eq_matcher did not, so depending on diff output, messages could be glued without a separator, making it harder to read.

We adjust the formatting code to try to improve consistency.  We drop the leading newline from summarize_diff, and make it the responsibility of callers to conditionally add or omit newlines to separate (possibly empty) diff output from the leading description.

As a result, `eq_matcher` output aligns more closely with `str_matcher`.

PiperOrigin-RevId: 696923930

Author: Googler

googletest/src/matcher_support/summarize_diff.rs

@@ -41,7 +41,7 @@ pub(crate) fn create_diff(
     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::Editable(edit_list) => {
-            format!("\n{}{}", summary_header(), edit_list.into_iter().collect::<BufferedSummary>(),)
+            format!("{}{}", summary_header(), edit_list.into_iter().collect::<BufferedSummary>(),)
                 .into()
         }
         edit_distance::Difference::Unrelated => "".into(),
@@ -73,7 +73,7 @@ pub(crate) fn create_diff_reversed(
         edit_distance::Difference::Equal => "No difference found between debug strings.".into(),
         edit_distance::Difference::Editable(mut edit_list) => {
             edit_list.reverse();
-            format!("\n{}{}", summary_header(), edit_list.into_iter().collect::<BufferedSummary>(),)
+            format!("{}{}", summary_header(), edit_list.into_iter().collect::<BufferedSummary>(),)
                 .into()
         }
         edit_distance::Difference::Unrelated => "".into(),
@@ -470,7 +470,6 @@ mod tests {
             create_diff(expected, actual, Mode::Exact),
             eq(indoc!(
                 "
-
                 Difference(-actual / +expected):
                  prefix
                 -Actual#1
@@ -494,7 +493,6 @@ mod tests {
             create_diff(&build_text(1..50), &build_text(1..51), Mode::Exact),
             eq(indoc! {
                 "
-
                 Difference(-actual / +expected):
                  1
                  2
@@ -514,7 +512,6 @@ mod tests {
             create_diff(&build_text(1..50), &build_text(1..51), Mode::Exact),
             eq(indoc! {
                 "
-
                 Difference(-\x1B[1;31mactual\x1B[0m / +\x1B[1;32mexpected\x1B[0m):
                  1
                  2
@@ -546,7 +543,6 @@ mod tests {
             create_diff(actual, expected, Mode::Exact),
             eq(indoc! {
                 "
-
                 Difference(-\x1B[1;31mactual\x1B[0m / +\x1B[1;32mexpected\x1B[0m):
                 -\x1B[31mThere is a ho\x1B[0m\x1B[1;31mm\x1B[0m\x1B[31me in N\x1B[0m\x1B[1;31mouv\x1B[0m\x1B[31me\x1B[0m\x1B[1;31mlle\x1B[0m\x1B[31m Orleans\x1B[0m
                 +\x1B[32mThere is a ho\x1B[0m\x1B[1;32mus\x1B[0m\x1B[32me \x1B[0m\x1B[1;32mway down \x1B[0m\x1B[32min Ne\x1B[0m\x1B[1;32mw\x1B[0m\x1B[32m Orleans\x1B[0m

googletest/src/matchers/display_matcher.rs

@@ -111,6 +111,7 @@ mod tests {
                 "
                   Actual: \"123\\n234\",
                     which displays as \"123\\n234\" which isn't equal to \"123\\n345\"
+                    
                     Difference(-actual / +expected):
                      123
                     -234

googletest/src/matchers/eq_matcher.rs

@@ -116,7 +116,11 @@ impl<T: Debug, A: Debug + Copy + PartialEq<T>> Matcher<A> for EqMatcher<T> {
             create_diff(&actual_debug, &expected_debug, edit_distance::Mode::Exact)
         };
 
-        format!("which {description}{diff}").into()
+        if diff.is_empty() {
+            format!("which {description}").into()
+        } else {
+            format!("which {description}\n\n{diff}").into()
+        }
     }
 }
 
@@ -176,6 +180,7 @@ mod tests {
             "
             Actual: Strukt { int: 123, string: \"something\" },
               which isn't equal to Strukt { int: 321, string: \"someone\" }
+              
               Difference(-actual / +expected):
                Strukt {
               -    int: 123,
@@ -198,6 +203,7 @@ mod tests {
             Expected: is equal to [1, 3, 4]
             Actual: [1, 2, 3],
               which isn't equal to [1, 3, 4]
+              
               Difference(-actual / +expected):
                [
                    1,
@@ -220,6 +226,7 @@ mod tests {
             Expected: is equal to [1, 3, 5]
             Actual: [1, 2, 3, 4, 5],
               which isn't equal to [1, 3, 5]
+              
               Difference(-actual / +expected):
                [
                    1,
@@ -241,6 +248,7 @@ mod tests {
             "
             ],
               which isn't equal to [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51]
+              
               Difference(-actual / +expected):
                [
               -    1,
@@ -265,6 +273,7 @@ mod tests {
             "
             Actual: [1, 2, 3, 4, 5, 6, 7],
               which isn't equal to [3, 4, 5, 6, 7, 8, 9]
+              
               Difference(-actual / +expected):
                [
               -    1,
@@ -289,6 +298,7 @@ mod tests {
             "
             ],
               which isn't equal to [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51]
+              
               Difference(-actual / +expected):
                [
                    1,
@@ -310,6 +320,7 @@ mod tests {
             "
             ],
               which isn't equal to [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51]
+              
               Difference(-actual / +expected):
                [
               -    1,

googletest/src/matchers/str_matcher.rs

@@ -533,7 +533,11 @@ impl Configuration {
             MatchMode::EndsWith => create_diff_reversed(actual, expected, self.mode.to_diff_mode()),
         };
 
-        format!("{default_explanation}\n{diff}").into()
+        if diff.is_empty() {
+            format!("{default_explanation}").into()
+        } else {
+            format!("{default_explanation}\n\n{diff}").into()
+        }
     }
 
     fn ignoring_leading_whitespace(self) -> Self {