Avoid the use of a context manager in hot code path (#14331)

Mypyc can't optimize context managers yet, so it's best to avoid them in
hot code paths.

This sacrifices some code quality for a considerable perf gain. This
improved self-check performance by 4%.

mypy/checkexpr.py

@@ -1583,21 +1583,21 @@ def infer_arg_types_in_empty_context(self, args: list[Expression]) -> list[Type]
                 res.append(arg_type)
         return res
 
-    @contextmanager
-    def allow_unions(self, type_context: Type) -> Iterator[None]:
-        # This is a hack to better support inference for recursive types.
-        # When the outer context for a function call is known to be recursive,
-        # we solve type constraints inferred from arguments using unions instead
-        # of joins. This is a bit arbitrary, but in practice it works for most
-        # cases. A cleaner alternative would be to switch to single bin type
-        # inference, but this is a lot of work.
+    def infer_more_unions_for_recursive_type(self, type_context: Type) -> bool:
+        """Adjust type inference of unions if type context has a recursive type.
+
+        Return the old state. The caller must assign it to type_state.infer_unions
+        afterwards.
+
+        This is a hack to better support inference for recursive types.
+
+        Note: This is performance-sensitive and must not be a context manager
+        until mypyc supports them better.
+        """
         old = type_state.infer_unions
         if has_recursive_types(type_context):
             type_state.infer_unions = True
-        try:
-            yield
-        finally:
-            type_state.infer_unions = old
+        return old
 
     def infer_arg_types_in_context(
         self,
@@ -1618,8 +1618,16 @@ def infer_arg_types_in_context(
         for i, actuals in enumerate(formal_to_actual):
             for ai in actuals:
                 if not arg_kinds[ai].is_star():
-                    with self.allow_unions(callee.arg_types[i]):
-                        res[ai] = self.accept(args[ai], callee.arg_types[i])
+                    arg_type = callee.arg_types[i]
+                    # When the outer context for a function call is known to be recursive,
+                    # we solve type constraints inferred from arguments using unions instead
+                    # of joins. This is a bit arbitrary, but in practice it works for most
+                    # cases. A cleaner alternative would be to switch to single bin type
+                    # inference, but this is a lot of work.
+                    old = self.infer_more_unions_for_recursive_type(arg_type)
+                    res[ai] = self.accept(args[ai], arg_type)
+                    # We need to manually restore union inference state, ugh.
+                    type_state.infer_unions = old
 
         # Fill in the rest of the argument types.
         for i, t in enumerate(res):