Don't (ever) put a single-char closing docstring quote on a new line

CHANGES.md

@@ -14,6 +14,9 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Single-character closing docstring quotes are no longer moved to their own line as
+  this is invalid. This was a bug introduced in version 22.6.0. (#3165)
+
 ### _Blackd_
 
 <!-- Changes to blackd -->

src/black/linegen.py

@@ -330,13 +330,14 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
             # We could enforce triple quotes at this point.
             quote = quote_char * quote_len
 
-            if Preview.long_docstring_quotes_on_newline in self.mode:
+            # It's invalid to put closing single-character quotes on a new line.
+            if Preview.long_docstring_quotes_on_newline in self.mode and quote_len == 3:
                 # We need to find the length of the last line of the docstring
                 # to find if we can add the closing quotes to the line without
                 # exceeding the maximum line length.
                 # If docstring is one line, then we need to add the length
-                # of the indent, prefix, and starting quotes. Ending quote are
-                # handled later
+                # of the indent, prefix, and starting quotes. Ending quotes are
+                # handled later.
                 lines = docstring.splitlines()
                 last_line_length = len(lines[-1]) if docstring else 0
 

tests/data/preview/docstring_preview.py

@@ -42,6 +42,14 @@ def multiline_docstring_at_line_limit_with_prefix():
     second line----------------------------------------------------------------------"""
 
 
+def single_quote_docstring_over_line_limit():
+    "We do not want to put the closing quote on a new line as that is invalid (see GH-3141)."
+
+
+def single_quote_docstring_over_line_limit2():
+    'We do not want to put the closing quote on a new line as that is invalid (see GH-3141).'
+
+
 # output
 
 
@@ -87,3 +95,11 @@ def multiline_docstring_at_line_limit_with_prefix():
     f"""first line----------------------------------------------------------------------
 
     second line----------------------------------------------------------------------"""
+
+
+def single_quote_docstring_over_line_limit():
+    "We do not want to put the closing quote on a new line as that is invalid (see GH-3141)."
+
+
+def single_quote_docstring_over_line_limit2():
+    "We do not want to put the closing quote on a new line as that is invalid (see GH-3141)."