revset: add subject() predicate that matches first line of descriptions

It's generally useful, and we can get by without introducing weird special case
about newline terminator.

Closes #5227

CHANGELOG.md

@@ -107,6 +107,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * New `author_name`/`author_email`/`committer_name`/`committer_email(pattern)`
   revset functions to match either name or email field explicitly.
 
+* New `subject(pattern)` revset function that matches first line of commit
+  descriptions.
+
 ### Fixed bugs
 
 * Fixed diff selection by external tools with `jj split`/`commit -i FILESETS`.

docs/revsets.md

@@ -267,6 +267,14 @@ revsets (expressions) as arguments.
 * `description(pattern)`: Commits that have a description matching the given
   [string pattern](#string-patterns).
 
+  A non-empty description is usually terminated with newline character. For
+  example, `description(exact:"")` matches commits without description, and
+  `description(exact:"foo\n")` matches commits with description `"foo\n"`.
+
+* `subject(pattern)`: Commits that have a subject matching the given [string
+  pattern](#string-patterns). A subject is the first line of the description
+  (without newline character.)
+
 * `author(pattern)`: Commits with the author's name or email matching the given
   [string pattern](#string-patterns). Equivalent to `author_name(pattern) |
   author_email(pattern)`.

lib/src/default_index/revset_engine.rs

@@ -1161,6 +1161,14 @@ fn build_predicate_fn(
                 Ok(pattern.matches(commit.description()))
             })
         }
+        RevsetFilterPredicate::Subject(pattern) => {
+            let pattern = pattern.clone();
+            box_pure_predicate_fn(move |index, pos| {
+                let entry = index.entry_by_pos(pos);
+                let commit = store.get_commit(&entry.commit_id())?;
+                Ok(pattern.matches(commit.description().lines().next().unwrap_or_default()))
+            })
+        }
         RevsetFilterPredicate::AuthorName(pattern) => {
             let pattern = pattern.clone();
             box_pure_predicate_fn(move |index, pos| {

lib/src/revset.rs

@@ -156,6 +156,8 @@ pub enum RevsetFilterPredicate {
     ParentCount(Range<u32>),
     /// Commits with description matching the pattern.
     Description(StringPattern),
+    /// Commits with first line of the description matching the pattern.
+    Subject(StringPattern),
     /// Commits with author name matching the pattern.
     AuthorName(StringPattern),
     /// Commits with author email matching the pattern.
@@ -834,6 +836,12 @@ static BUILTIN_FUNCTION_MAP: Lazy<HashMap<&'static str, RevsetFunction>> = Lazy:
             RevsetFilterPredicate::Description(pattern),
         ))
     });
+    map.insert("subject", |diagnostics, function, _context| {
+        let [arg] = function.expect_exact_arguments()?;
+        let pattern = expect_string_pattern(diagnostics, arg)?;
+        let predicate = RevsetFilterPredicate::Subject(pattern);
+        Ok(RevsetExpression::filter(predicate))
+    });
     map.insert("author", |diagnostics, function, _context| {
         let [arg] = function.expect_exact_arguments()?;
         let pattern = expect_string_pattern(diagnostics, arg)?;

lib/tests/test_revset.rs

@@ -2654,17 +2654,17 @@ fn test_evaluate_expression_description() {
     let mut_repo = tx.repo_mut();
 
     let commit1 = create_random_commit(mut_repo)
-        .set_description("commit 1")
+        .set_description("commit 1\n")
         .write()
         .unwrap();
     let commit2 = create_random_commit(mut_repo)
         .set_parents(vec![commit1.id().clone()])
-        .set_description("commit 2")
+        .set_description("commit 2\n\nblah blah...\n")
         .write()
         .unwrap();
     let commit3 = create_random_commit(mut_repo)
         .set_parents(vec![commit2.id().clone()])
-        .set_description("commit 3")
+        .set_description("commit 3\n")
         .write()
         .unwrap();
 
@@ -2687,6 +2687,40 @@ fn test_evaluate_expression_description() {
         resolve_commit_ids(mut_repo, "visible_heads() & description(\"commit 2\")"),
         vec![]
     );
+
+    // Exact match
+    assert_eq!(
+        resolve_commit_ids(mut_repo, r#"description(exact:"commit 1\n")"#),
+        vec![commit1.id().clone()]
+    );
+    assert_eq!(
+        resolve_commit_ids(mut_repo, r#"description(exact:"commit 2\n")"#),
+        vec![]
+    );
+    assert_eq!(
+        resolve_commit_ids(mut_repo, "description(exact:'')"),
+        vec![mut_repo.store().root_commit_id().clone()]
+    );
+
+    // Match subject line
+    assert_eq!(
+        resolve_commit_ids(mut_repo, "subject(glob:'commit ?')"),
+        vec![
+            commit3.id().clone(),
+            commit2.id().clone(),
+            commit1.id().clone(),
+        ]
+    );
+    assert_eq!(resolve_commit_ids(mut_repo, "subject('blah')"), vec![]);
+    assert_eq!(
+        resolve_commit_ids(mut_repo, "subject(exact:'commit 2')"),
+        vec![commit2.id().clone()]
+    );
+    // Empty description should have empty subject line
+    assert_eq!(
+        resolve_commit_ids(mut_repo, "subject(exact:'')"),
+        vec![mut_repo.store().root_commit_id().clone()]
+    );
 }
 
 #[test]