Treat raw strings like other docstrings

Fixes psf#3944

CHANGES.md

@@ -20,6 +20,7 @@
   multiple lines (#3899)
 - Magic trailing commas are now respected in return types. (#3916)
 - Require one empty line after module-level docstrings. (#3932)
+- Treat raw triple-quoted strings as docstrings (#3941)
 
 ### Configuration
 

src/black/lines.py

@@ -193,11 +193,16 @@ def is_class_paren_empty(self) -> bool:
     @property
     def is_triple_quoted_string(self) -> bool:
         """Is the line a triple quoted string?"""
-        return (
-            bool(self)
-            and self.leaves[0].type == token.STRING
-            and self.leaves[0].value.startswith(('"""', "'''"))
-        )
+        if not self or self.leaves[0].type != token.STRING:
+            return False
+        value = self.leaves[0].value
+        if value.startswith(('"""', "'''")):
+            return True
+        if Preview.accept_raw_docstrings in self.mode and value.startswith(
+            ("r'''", 'r"""', "R'''", 'R"""')
+        ):
+            return True
+        return False
 
     @property
     def opens_block(self) -> bool:

src/black/mode.py

@@ -188,6 +188,7 @@ class Preview(Enum):
     dummy_implementations = auto()
     walrus_subscript = auto()
     module_docstring_newlines = auto()
+    accept_raw_docstrings = auto()
 
 
 class Deprecated(UserWarning):

tests/data/raw_docstring.py

@@ -0,0 +1,32 @@
+# flags: --preview --skip-string-normalization
+class C:
+
+    r"""Raw"""
+
+def f():
+
+    r"""Raw"""
+
+class SingleQuotes:
+
+
+    r'''Raw'''
+
+class UpperCaseR:
+    R"""Raw"""
+
+# output
+class C:
+    r"""Raw"""
+
+
+def f():
+    r"""Raw"""
+
+
+class SingleQuotes:
+    r'''Raw'''
+
+
+class UpperCaseR:
+    R"""Raw"""