From e3055e5aafef9ea4300aaa0adc1500a5e0768fa8 Mon Sep 17 00:00:00 2001
From: Yuya Nishihara <yuya@tcha.org>
Date: Sun, 14 Jul 2024 19:39:44 +0900
Subject: [PATCH] diff: do not emit unified diff header on absent/empty
 transitions

---/+++ lines are part of unified diff hunks, not Git diff header.
---
 cli/src/diff_util.rs           | 26 ++++++++++++++++++--------
 cli/tests/test_diff_command.rs |  6 ------
 2 files changed, 18 insertions(+), 14 deletions(-)

diff --git a/cli/src/diff_util.rs b/cli/src/diff_util.rs
index 5ab89d385..a61d7a495 100644
--- a/cli/src/diff_util.rs
+++ b/cli/src/diff_util.rs
@@ -1037,14 +1037,10 @@ pub fn show_git_diff(
                     (None, Some(right_mode)) => {
                         writeln!(formatter, "new file mode {right_mode}")?;
                         writeln!(formatter, "index {left_hash}..{right_hash}")?;
-                        writeln!(formatter, "--- /dev/null")?;
-                        writeln!(formatter, "+++ b/{path_string}")?;
                     }
                     (Some(left_mode), None) => {
                         writeln!(formatter, "deleted file mode {left_mode}")?;
                         writeln!(formatter, "index {left_hash}..{right_hash}")?;
-                        writeln!(formatter, "--- a/{path_string}")?;
-                        writeln!(formatter, "+++ /dev/null")?;
                     }
                     (Some(left_mode), Some(right_mode)) => {
                         if left_mode != right_mode {
@@ -1056,15 +1052,29 @@ pub fn show_git_diff(
                         } else if left_hash != right_hash {
                             writeln!(formatter, "index {left_hash}..{right_hash} {left_mode}")?;
                         }
-                        if left_part.content != right_part.content {
-                            writeln!(formatter, "--- a/{path_string}")?;
-                            writeln!(formatter, "+++ b/{path_string}")?;
-                        }
                     }
                     (None, None) => panic!("either left or right part should be present"),
                 }
                 Ok(())
             })?;
+
+            if left_part.content == right_part.content {
+                continue; // no content hunks
+            }
+
+            let left_path = match left_part.mode {
+                Some(_) => format!("a/{path_string}"),
+                None => "/dev/null".to_owned(),
+            };
+            let right_path = match right_part.mode {
+                Some(_) => format!("b/{path_string}"),
+                None => "/dev/null".to_owned(),
+            };
+            formatter.with_label("file_header", |formatter| {
+                writeln!(formatter, "--- {left_path}")?;
+                writeln!(formatter, "+++ {right_path}")?;
+                Ok(())
+            })?;
             show_unified_diff_hunks(
                 formatter,
                 &left_part.content,
diff --git a/cli/tests/test_diff_command.rs b/cli/tests/test_diff_command.rs
index ac9d03bf3..acc6208dd 100644
--- a/cli/tests/test_diff_command.rs
+++ b/cli/tests/test_diff_command.rs
@@ -335,8 +335,6 @@ fn test_diff_file_mode() {
     diff --git a/file1 b/file1
     new file mode 100755
     index 0000000000..e69de29bb2
-    --- /dev/null
-    +++ b/file1
     diff --git a/file2 b/file2
     new file mode 100755
     index 0000000000..d00491fd7e
@@ -354,8 +352,6 @@ fn test_diff_file_mode() {
     diff --git a/file4 b/file4
     new file mode 100644
     index 0000000000..e69de29bb2
-    --- /dev/null
-    +++ b/file4
     "###);
     let stdout = test_env.jj_cmd_success(&repo_path, &["diff", "-r@-", "--git"]);
     insta::assert_snapshot!(stdout, @r###"
@@ -409,8 +405,6 @@ fn test_diff_file_mode() {
     diff --git a/file4 b/file4
     deleted file mode 100755
     index e69de29bb2..0000000000
-    --- a/file4
-    +++ /dev/null
     "###);
 }