Add changelog / some nits

CHANGES.md

@@ -24,6 +24,8 @@
   at least one pre-existing blank line (#2736)
 - Verbose mode also now describes how a project root was discovered and which paths will
   be formatted. (#2526)
+- Speed-up the new backtracking parser about 4X in general (enabled when
+  `--target-version` is set to 3.10 and higher). (#2728)
 
 ### Packaging
 

src/blib2to3/pgen2/parse.py

@@ -73,7 +73,7 @@ def ilabels(self) -> Set[int]:
 
     @contextmanager
     def switch_to(self, ilabel: int) -> Iterator[None]:
-        with self.patch():
+        with self.backtrack():
             self.parser.stack = self._points[ilabel]
             try:
                 yield
@@ -83,12 +83,11 @@ def switch_to(self, ilabel: int) -> Iterator[None]:
                 self.parser.stack = self._start_point
 
     @contextmanager
-    def patch(self) -> Iterator[None]:
+    def backtrack(self) -> Iterator[None]:
         """
-        Patch basic state operations (push/pop/shift) with node-level
-        immutable variants. These still will operate on the stack; but
-        they won't create any new nodes, or modify the contents of any
-        other existing nodes.
+        Use the node-level invariant ones for basic parsing operations (push/pop/shift).
+        These still will operate on the stack; but they won't create any new nodes, or
+        modify the contents of any other existing nodes.
 
         This saves us a ton of time when we are backtracking, since we
         want to restore to the initial state as quick as possible, which
@@ -349,8 +348,6 @@ def classify(self, type: int, value: Text, context: Context) -> List[int]:
             raise ParseError("bad token", type, value, context)
         return [ilabel]
 
-    STATE_OPERATIONS = ["shift", "push", "pop"]
-
     def shift(self, type: int, value: Text, newstate: int, context: Context) -> None:
         """Shift a token.  (Internal)"""
         if self.is_backtracking: