Couple obvious optimizations for type aliases (#8099)

This PR:
* Removes three unused slots
* Makes recursivity test once per alias
* Documents an existing optimization (accidentally made before)

The effect of this it at the limit of precision on my laptop, but it looks like this gives around 1% speed-up (for cold runs).

mypy/nodes.py

@@ -2741,7 +2741,7 @@ def f(x: B[T]) -> T: ...  # without T, Any would be used here
     line and column: Line an column on the original alias definition.
     """
     __slots__ = ('target', '_fullname', 'alias_tvars', 'no_args', 'normalized',
-                 'line', 'column', 'assuming', 'assuming_proper', 'inferring')
+                 'line', 'column', '_is_recursive')
 
     def __init__(self, target: 'mypy.types.Type', fullname: str, line: int, column: int,
                  *,
@@ -2755,6 +2755,9 @@ def __init__(self, target: 'mypy.types.Type', fullname: str, line: int, column:
         self.alias_tvars = alias_tvars
         self.no_args = no_args
         self.normalized = normalized
+        # This attribute is manipulated by TypeAliasType. If non-None,
+        # it is the cached value.
+        self._is_recursive = None  # type: Optional[bool]
         super().__init__(line, column)
 
     @property

mypy/semanal.py

@@ -2514,6 +2514,9 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> bool:
         # so we need to replace it with non-explicit Anys.
         if not has_placeholder(res):
             res = make_any_non_explicit(res)
+        # Note: with the new (lazy) type alias representation we only need to set no_args to True
+        # if the expected number of arguments is non-zero, so that aliases like A = List work.
+        # However, eagerly expanding aliases like Text = str is a nice performance optimization.
         no_args = isinstance(res, Instance) and not res.args  # type: ignore
         fix_instance_types(res, self.fail, self.note)
         alias_node = TypeAlias(res, self.qualified_name(lvalue.name), s.line, s.column,

mypy/types.py

@@ -166,14 +166,13 @@ class Node:
     can be represented in a tree-like manner.
     """
 
-    __slots__ = ('alias', 'args', 'line', 'column', 'type_ref', '_is_recursive')
+    __slots__ = ('alias', 'args', 'line', 'column', 'type_ref')
 
     def __init__(self, alias: Optional[mypy.nodes.TypeAlias], args: List[Type],
                  line: int = -1, column: int = -1) -> None:
         self.alias = alias
         self.args = args
         self.type_ref = None  # type: Optional[str]
-        self._is_recursive = None  # type: Optional[bool]
         super().__init__(line, column)
 
     def _expand_once(self) -> Type:
@@ -212,10 +211,13 @@ def expand_all_if_possible(self) -> Optional['ProperType']:
 
     @property
     def is_recursive(self) -> bool:
-        if self._is_recursive is not None:
-            return self._is_recursive
-        is_recursive = self.expand_all_if_possible() is None
-        self._is_recursive = is_recursive
+        assert self.alias is not None, 'Unfixed type alias'
+        is_recursive = self.alias._is_recursive
+        if is_recursive is None:
+            is_recursive = self.expand_all_if_possible() is None
+            # We cache the value on the underlying TypeAlias node as an optimization,
+            # since the value is the same for all instances of the same alias.
+            self.alias._is_recursive = is_recursive
         return is_recursive
 
     def can_be_true_default(self) -> bool: