Fix a string merging/split issue caused by standalone comments.

Fixes psf#2734: a standalone comment causes strings to be merged into one far too long (and requiring two passes to do so).

CHANGES.md

@@ -26,6 +26,8 @@
   normalized as expected (#3168)
 - When using `--skip-magic-trailing-comma` or `-C`, trailing commas are stripped from
   subscript expressions with more than 1 element (#3209)
+- Fix a string merging/split issue when a comment is present in the middle of
+  implicitly concatenated strings on its own line (#3227)
 
 ### _Blackd_
 

src/black/trans.py

@@ -553,6 +553,9 @@ def make_naked(string: str, string_prefix: str) -> str:
 
             next_str_idx += 1
 
+        # Take a note on the index of the non-STRING leaf.
+        non_string_idx = next_str_idx
+
         S_leaf = Leaf(token.STRING, S)
         if self.normalize_strings:
             S_leaf.value = normalize_string_quotes(S_leaf.value)
@@ -572,7 +575,19 @@ def make_naked(string: str, string_prefix: str) -> str:
         string_leaf = Leaf(token.STRING, S_leaf.value.replace(BREAK_MARK, ""))
 
         if atom_node is not None:
-            replace_child(atom_node, string_leaf)
+            # If not all children of the atom node are merged...
+            if non_string_idx - string_idx < len(atom_node.children):
+                # We need to replace the old STRING leaves with the new string leaf.
+                first_child_idx = None
+                for idx in range(string_idx, non_string_idx):
+                    child_idx = LL[idx].remove()
+                    if first_child_idx is None:
+                        first_child_idx = child_idx
+                if first_child_idx is not None:
+                    atom_node.insert_child(first_child_idx, string_leaf)
+            else:
+                # Else replace the atom node with the new string leaf.
+                replace_child(atom_node, string_leaf)
 
         # Build the final line ('new_line') that this method will later return.
         new_line = line.clone()

tests/data/preview/long_strings.py

@@ -72,6 +72,25 @@
     zzz,
 )
 
+inline_comments_func1(
+    "if there are inline "
+    "comments in the middle "
+    # Here is the standard alone comment.
+    "of the implicitly concatenated "
+    "string, we should handle "
+    "them correctly",
+    xxx,
+)
+
+inline_comments_func2(
+    "what if the string is very very very very very very very very very very long and this part does "
+    "not fit into a single line? "
+    # Here is the standard alone comment.
+    "then the string should still be properly handled by merging and splitting "
+    "it into parts that fit in line length.",
+    xxx,
+)
+
 raw_string = r"This is a long raw string. When re-formatting this string, black needs to make sure it prepends the 'r' onto the new string."
 
 fmt_string1 = "We also need to be sure to preserve any and all {} which may or may not be attached to the string in question.".format("method calls")
@@ -395,6 +414,22 @@ def foo():
     zzz,
 )
 
+inline_comments_func1(
+    "if there are inline comments in the middle "
+    # Here is the standard alone comment.
+    "of the implicitly concatenated string, we should handle them correctly",
+    xxx,
+)
+
+inline_comments_func2(
+    "what if the string is very very very very very very very very very very long and"
+    " this part does not fit into a single line? "
+    # Here is the standard alone comment.
+    "then the string should still be properly handled by merging and splitting "
+    "it into parts that fit in line length.",
+    xxx,
+)
+
 raw_string = (
     r"This is a long raw string. When re-formatting this string, black needs to make"
     r" sure it prepends the 'r' onto the new string."