Stop lash add prepending above the H1 when the Tasks section is empty

crates/lash-cli/tests/add_command_test.rs

@@ -366,3 +366,194 @@ fn test_add_preserves_multiline_agent_note_through_reindex() {
         );
     }
 }
+
+// ---------------------------------------------------------------------
+// Appending to a file whose `## Tasks` section is empty
+//
+// `resolve_append` used to return the magic line number 0 whenever the
+// target file held no tasks, meaning "this is a brand-new file, write at
+// offset 0". For a file that already existed with an empty `## Tasks`
+// section that was catastrophically wrong: the checkbox was prepended
+// above the H1, so the parser never saw it as a task, `lash index` found
+// nothing, and `lash lint` still passed. The task was on disk and
+// invisible.
+// ---------------------------------------------------------------------
+
+/// Line the task line lands on, 1-indexed.
+fn task_line_number(content: &str, needle: &str) -> usize {
+    content
+        .lines()
+        .position(|line| line.contains(needle))
+        .map(|idx| idx + 1)
+        .unwrap_or_else(|| panic!("'{needle}' not found in:\n{content}"))
+}
+
+/// Run `lash add`, returning the command's stdout.
+fn add_task(project: &TestProject, title: &str, file: &str) -> String {
+    let output = run_lash_command()
+        .arg("--root")
+        .arg(project.path())
+        .arg("add")
+        .arg(title)
+        .arg("--file")
+        .arg(file)
+        .assert()
+        .success()
+        .get_output()
+        .stdout
+        .clone();
+    String::from_utf8(output).unwrap()
+}
+
+#[test]
+fn test_add_to_empty_tasks_section_lands_below_the_header() {
+    let project = TestProject::builder()
+        .with_index("test-project", "Test Project")
+        .with_file(
+            "empty.md",
+            r#"# Empty
+
+@id: empty
+
+## Tasks
+"#,
+        )
+        .build();
+    index(&project);
+
+    let stdout = add_task(&project, "Ship it", "empty.md");
+
+    let content = fs::read_to_string(project.file_path("empty.md")).unwrap();
+
+    assert_eq!(
+        content.lines().next(),
+        Some("# Empty"),
+        "the H1 must still be the first line:\n{content}"
+    );
+    // The old sentinel surfaced here too, as a literal `empty.md:0`.
+    assert!(
+        stdout.contains(&format!(
+            "empty.md:{}",
+            task_line_number(&content, "- [ ] Ship it")
+        )),
+        "reported line does not match where the task was written:\n{stdout}"
+    );
+    assert!(
+        task_line_number(&content, "- [ ] Ship it") > task_line_number(&content, "## Tasks"),
+        "task must land inside the Tasks section:\n{content}"
+    );
+}
+
+#[test]
+fn test_add_to_empty_tasks_section_is_indexed() {
+    // The destructive part of the bug: the task was written to a place the
+    // parser does not read, so it existed on disk and nowhere else.
+    let project = TestProject::builder()
+        .with_index("test-project", "Test Project")
+        .with_file(
+            "empty.md",
+            r#"# Empty
+
+@id: empty
+
+## Tasks
+"#,
+        )
+        .build();
+    index(&project);
+
+    add_task(&project, "Ship it", "empty.md");
+    index(&project);
+
+    run_lash_command()
+        .arg("--root")
+        .arg(project.path())
+        .arg("list")
+        .assert()
+        .success()
+        .stdout(predicate::str::contains("Ship it"));
+}
+
+#[test]
+fn test_add_to_empty_tasks_section_keeps_following_sections_intact() {
+    let project = TestProject::builder()
+        .with_index("test-project", "Test Project")
+        .with_file(
+            "sections.md",
+            r#"# Sections
+
+@id: sections
+
+## Tasks
+
+## Notes
+
+Prose that must survive.
+"#,
+        )
+        .build();
+    index(&project);
+
+    add_task(&project, "Ship it", "sections.md");
+
+    let content = fs::read_to_string(project.file_path("sections.md")).unwrap();
+
+    assert_eq!(
+        content.lines().next(),
+        Some("# Sections"),
+        "the H1 must still be the first line:\n{content}"
+    );
+    assert!(
+        content.contains("Prose that must survive."),
+        "trailing section content was destroyed:\n{content}"
+    );
+
+    let task = task_line_number(&content, "- [ ] Ship it");
+    assert!(
+        task > task_line_number(&content, "## Tasks")
+            && task < task_line_number(&content, "## Notes"),
+        "task escaped the Tasks section:\n{content}"
+    );
+}
+
+#[test]
+fn test_add_creating_a_new_file_still_writes_a_well_formed_file() {
+    // Guards the other half of the split: the old code used one sentinel for
+    // both "new file" and "existing file with an empty section".
+    let project = TestProject::builder()
+        .with_index("test-project", "Test Project")
+        .build();
+    index(&project);
+
+    add_task(&project, "Ship it", "fresh.md");
+
+    let content = fs::read_to_string(project.file_path("fresh.md")).unwrap();
+
+    assert!(
+        task_line_number(&content, "- [ ] Ship it") > task_line_number(&content, "## Tasks"),
+        "task must land inside the Tasks section:\n{content}"
+    );
+
+    index(&project);
+    run_lash_command()
+        .arg("--root")
+        .arg(project.path())
+        .arg("list")
+        .assert()
+        .success()
+        .stdout(predicate::str::contains("Ship it"));
+}
+
+#[test]
+fn test_add_leaves_the_file_ending_in_a_newline() {
+    let project = project_with_tasks_file();
+    index(&project);
+
+    add_task(&project, "Second task", "tasks.md");
+
+    let content = fs::read_to_string(project.file_path("tasks.md")).unwrap();
+    assert!(
+        content.ends_with('\n'),
+        "file ends mid-line, every later diff shows '\\ No newline at end of file':\n{content}"
+    );
+}