Move hug_parens_with_braces_and_square_brackets into the unstable style

Primarily because of psf#4036 (a crash) but also because of the feedback
in psf#4098 and psf#4099.

CHANGES.md

@@ -14,6 +14,9 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Move the `hug_parens_with_braces_and_square_brackets` feature to the unstable style
+  due to an outstanding crash and proposed formatting tweaks (#4198)
+
 ### Configuration
 
 <!-- Changes to how Black can be configured -->

docs/the_black_code_style/future_style.md

@@ -22,8 +22,6 @@ Currently, the following features are included in the preview style:
   strings
 - `unify_docstring_detection`: fix inconsistencies in whether certain strings are
   detected as docstrings
-- `hug_parens_with_braces_and_square_brackets`: more compact formatting of nested
-  brackets ([see below](labels/hug-parens))
 - `no_normalize_fmt_skip_whitespace`: whitespace before `# fmt: skip` comments is no
   longer normalized
 - `typed_params_trailing_comma`: consistently add trailing commas to typed function
@@ -39,6 +37,8 @@ The unstable style additionally includes the following features:
   ([see below](labels/wrap-long-dict-values))
 - `multiline_string_handling`: more compact formatting of expressions involving
   multiline strings ([see below](labels/multiline-string-handling))
+- `hug_parens_with_braces_and_square_brackets`: more compact formatting of nested
+  brackets ([see below](labels/hug-parens))
 
 (labels/hug-parens)=
 

src/black/mode.py

@@ -179,13 +179,17 @@ class Preview(Enum):
     typed_params_trailing_comma = auto()
 
 
+# See https://github.com/psf/black/issues?q=is%3Aissue+is%3Aopen+label%3A%22C%3A+preview+style%22
+# for all open issues with the preview and unstable style.
 UNSTABLE_FEATURES: Set[Preview] = {
     # Many issues, see summary in https://github.com/psf/black/issues/4042
     Preview.string_processing,
     # See issues #3452 and #4158
     Preview.wrap_long_dict_values_in_parens,
     # See issue #4159
     Preview.multiline_string_handling,
+    # See issue #4036 (crash), #4098, #4099 (proposed tweaks)
+    Preview.hug_parens_with_braces_and_square_brackets,
 }