Fix comments getting removed from inside parenthesized strings (#3909)

Since the id of the old leaf may be
the key to comments, the new leaf
must adopt the old comments

CHANGES.md

@@ -17,6 +17,8 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Fix comments getting removed from inside parenthesized strings (#3909)
+
 ### Configuration
 
 <!-- Changes to how Black can be configured -->

src/black/trans.py

@@ -942,6 +942,9 @@ def _transform_to_new_line(
                 LL[lpar_or_rpar_idx].remove()  # Remove lpar.
                 replace_child(LL[idx], string_leaf)
                 new_line.append(string_leaf)
+                # replace comments
+                old_comments = new_line.comments.pop(id(LL[idx]), [])
+                new_line.comments.setdefault(id(string_leaf), []).extend(old_comments)
             else:
                 LL[lpar_or_rpar_idx].remove()  # This is a rpar.
 

tests/data/preview/comments7.py

@@ -131,6 +131,18 @@ def test_fails_invalid_post_data(
 
 square = Square(4) # type: Optional[Square]
 
+# Regression test for https://github.com/psf/black/issues/3756.
+[
+    (
+        "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"  # aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+    ),
+]
+[
+    (  # aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+        "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"  # aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+    ),
+]
+
 # output
 
 from .config import (
@@ -282,3 +294,15 @@ def test_fails_invalid_post_data(
 
 
 square = Square(4)  # type: Optional[Square]
+
+# Regression test for https://github.com/psf/black/issues/3756.
+[
+    (  # aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+        "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+    ),
+]
+[
+    (  # aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+        "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"  # aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+    ),
+]