Fix a crash in preview style with assert + parenthesized string. (#3415)

The bug is in the `get_leaves_inside_matching_brackets` on the third line below:

```python
assert xxxxxxxxx.xxxxxxxxx.xxxxxxxxx(
    xxxxxxxxx
).xxxxxxxxxxxxxxxxxx(), (
    "xxx {xxxxxxxxx} xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
)
```

Including the invisible paren, third line is `).xxxxxxxxxxxxxxxxxx()), (`, that it has a matched pair then an unmatched closing paren afterwards. This PR ensures the returned leaves are actually matched.

Fixes #3414.

CHANGES.md

@@ -14,6 +14,8 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Fix a crash in preview style with assert + parenthesized string (#3415)
+
 ### Configuration
 
 <!-- Changes to how Black can be configured -->

src/black/brackets.py

@@ -349,26 +349,23 @@ def get_leaves_inside_matching_brackets(leaves: Sequence[Leaf]) -> Set[LeafID]:
     Matching brackets are included.
     """
     try:
-        # Only track brackets from the first opening bracket to the last closing
-        # bracket.
+        # Start with the first opening bracket and ignore closing brackets before.
         start_index = next(
             i for i, l in enumerate(leaves) if l.type in OPENING_BRACKETS
         )
-        end_index = next(
-            len(leaves) - i
-            for i, l in enumerate(reversed(leaves))
-            if l.type in CLOSING_BRACKETS
-        )
     except StopIteration:
         return set()
+    bracket_stack = []
     ids = set()
-    depth = 0
-    for i in range(end_index, start_index - 1, -1):
+    for i in range(start_index, len(leaves)):
         leaf = leaves[i]
-        if leaf.type in CLOSING_BRACKETS:
-            depth += 1
-        if depth > 0:
-            ids.add(id(leaf))
         if leaf.type in OPENING_BRACKETS:
-            depth -= 1
+            bracket_stack.append((BRACKET[leaf.type], i))
+        if leaf.type in CLOSING_BRACKETS:
+            if bracket_stack and leaf.type == bracket_stack[-1][0]:
+                _, start = bracket_stack.pop()
+                for j in range(start, i + 1):
+                    ids.add(id(leaves[j]))
+            else:
+                break
     return ids

tests/data/preview/trailing_commas_in_leading_parts.py

@@ -25,6 +25,13 @@ def refresh_token(self, device_family, refresh_token, api_key):
     == long_module.long_class.long_func()["some_key"].another_func(arg1)
 )
 
+# Regression test for https://github.com/psf/black/issues/3414.
+assert xxxxxxxxx.xxxxxxxxx.xxxxxxxxx(
+    xxxxxxxxx
+).xxxxxxxxxxxxxxxxxx(), (
+    "xxx {xxxxxxxxx} xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+)
+
 
 # output
 
@@ -72,3 +79,10 @@ def refresh_token(self, device_family, refresh_token, api_key):
     long_module.long_class.long_func().another_func()
     == long_module.long_class.long_func()["some_key"].another_func(arg1)
 )
+
+# Regression test for https://github.com/psf/black/issues/3414.
+assert xxxxxxxxx.xxxxxxxxx.xxxxxxxxx(
+    xxxxxxxxx
+).xxxxxxxxxxxxxxxxxx(), (
+    "xxx {xxxxxxxxx} xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+)