Change bg color of whitespace-only hunks

With the default color-words diff formatting, if a diff hunk (a
sequence of removed and/or added characters) consists only of whitespace
characters, it will not be visible on a terminal because the default
configuration only changes their foreground color (which has no effect
because all these characters are whitespace).

Therefore, change the background color of these diff hunks. The user
may further customize their appearance by configuring the "removed
whitespace" and "added whitespace" formats.

CHANGELOG.md

@@ -55,6 +55,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * New command `jj branch move` let you update branches by name pattern or source
   revision.
 
+* In diffs, whitespace-only hunks are marked with a background color. (If a
+  hunk contains printable characters, it is displayed with a foreground color
+  as usual.)
+
 ### Fixed bugs
 
 ## [0.18.0] - 2024-06-05

cli/src/config/colors.toml

@@ -79,7 +79,9 @@
 "diff file_header" = { bold = true }
 "diff hunk_header" = "cyan"
 "diff removed" = "red"
+"diff removed whitespace" = { bg = "red" }
 "diff added" = "green"
+"diff added whitespace" = { bg = "green" }
 "diff modified" = "cyan"
 "diff access-denied" = { bg = "red" }
 

cli/src/diff_util.rs

@@ -387,13 +387,19 @@ fn show_color_words_diff_line(
                 formatter.write_all(data)?;
             }
             DiffHunk::Different(data) => {
-                let before = data[0];
-                let after = data[1];
-                if !before.is_empty() {
-                    formatter.with_label("removed", |formatter| formatter.write_all(before))?;
-                }
-                if !after.is_empty() {
-                    formatter.with_label("added", |formatter| formatter.write_all(after))?;
+                let labels = ["removed", "added"];
+                for (index, label) in labels.iter().enumerate() {
+                    let hunk_part = data[index];
+                    if !hunk_part.is_empty() {
+                        formatter.with_label(label, |formatter| {
+                            if hunk_part.iter().all(|byte| u8::is_ascii_whitespace(byte)) {
+                                formatter
+                                    .with_label("whitespace", |formatter| formatter.write_all(hunk_part))
+                            } else {
+                                formatter.write_all(hunk_part)
+                            }
+                        })?;
+                    }
                 }
             }
         }

cli/tests/test_diff_command.rs

@@ -193,6 +193,35 @@ fn test_diff_basic() {
     insta::assert_snapshot!(stderr, @"");
 }
 
+#[test]
+fn test_diff_whitespace_only_label() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    std::fs::write(
+        repo_path.join("file"),
+        "  first  \n  second  \n  third  \nfourth\n",
+    )
+    .unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["new"]);
+    std::fs::write(
+        repo_path.join("file"),
+        "first  \n    second    \n  third\n\nfifth\n",
+    )
+    .unwrap();
+
+    let stdout = test_env.jj_cmd_success(&repo_path, &["--color=debug", "diff"]);
+    insta::assert_snapshot!(stdout, @r###"
+    <<diff header::Modified regular file>><<diff header:: >><<diff header::file>><<diff header:::>>
+    <<diff removed:: >><<diff removed:: >><<diff removed:: >><<diff removed::1>><<diff:: >><<diff added:: >><<diff added:: >><<diff added:: >><<diff added::1>><<diff::: >><<diff removed whitespace::  >><<diff::first  >>
+    <<diff removed:: >><<diff removed:: >><<diff removed:: >><<diff removed::2>><<diff:: >><<diff added:: >><<diff added:: >><<diff added:: >><<diff added::2>><<diff::: >><<diff::  >><<diff added whitespace::  >><<diff::second  >><<diff added whitespace::  >><<diff::>>
+    <<diff removed:: >><<diff removed:: >><<diff removed:: >><<diff removed::3>><<diff:: >><<diff added:: >><<diff added:: >><<diff added:: >><<diff added::3>><<diff::: >><<diff::  third>><<diff removed whitespace::  >><<diff::>>
+    <<diff removed:: >><<diff removed:: >><<diff removed:: >><<diff removed::4>><<diff:: >><<diff added:: >><<diff added:: >><<diff added:: >><<diff added::4>><<diff::: >><<diff removed::fourth>><<diff::>>
+    <<diff::     >><<diff added:: >><<diff added:: >><<diff added:: >><<diff added::5>><<diff::: >><<diff added::fifth>>
+    "###);
+}
+
 #[test]
 fn test_diff_empty() {
     let test_env = TestEnvironment::default();