reject a comment containing line breaks in Item.comment and add_line

tests/test_items.py

@@ -515,6 +515,13 @@ def test_array_add_line() -> None:
     )
 
 
+def test_array_add_line_multiline_comment_is_rejected() -> None:
+    t = api.array()
+    with pytest.raises(ValueError, match="line breaks"):
+        t.add_line(1, 2, 3, comment="first line\nsecond line")
+    assert t.as_string() == "[]"
+
+
 def test_array_add_line_invalid_value() -> None:
     t = api.array()
     with pytest.raises(ValueError, match="is not allowed"):
@@ -887,6 +894,21 @@ def test_trim_comments_when_building_inline_table() -> None:
     assert table.as_string() == '{foo = "bar", baz = "foobaz"}'
 
 
+def test_comment_method_multiline_comment_is_rejected() -> None:
+    doc = api.document()
+    doc["x"] = 1
+    with pytest.raises(ValueError, match="line breaks"):
+        doc["x"].comment("first line\nsecond line")
+    assert doc.as_string() == "x = 1\n"
+
+
+def test_comment_method_keeps_existing_hash_prefix() -> None:
+    doc = api.document()
+    doc["x"] = 1
+    doc["x"].comment("# already a comment")
+    assert doc.as_string() == "x = 1 # already a comment\n"
+
+
 def test_append_table_to_inline_table_raises() -> None:
     table = api.table()
     table.append("a", 1)

tomlkit/items.py

@@ -500,6 +500,8 @@ def unwrap(self) -> Any:
 
     def comment(self, comment: str) -> Item:
         """Attach a comment to this item"""
+        if "\n" in comment or "\r" in comment:
+            raise ValueError("Comment cannot contain line breaks")
         if not comment.strip().startswith("#"):
             comment = "# " + comment
 
@@ -1521,6 +1523,8 @@ def add_line(
             4, 5, 6,
         ]
         """
+        if comment and ("\n" in comment or "\r" in comment):
+            raise ValueError("Comment cannot contain line breaks")
         new_values: list[Item] = []
         first_indent = f"\n{indent}" if newline else indent
         if first_indent: