checkexpr: cache type of container literals when possible (#12707)

When a container (list, set, tuple, or dict) literal expression is
used as an argument to an overloaded function it will get repeatedly
typechecked. This becomes particularly problematic when the expression
is somewhat large, as seen in #9427

To avoid repeated work, add a new cache in ExprChecker, mapping the AST
node to the resolved type of the expression. Right now the cache is
only used in the fast path, although it could conceivably be leveraged
for the slow path as well in a follow-up commit.

To further reduce duplicate work, when the fast-path doesn't work, we
use the cache to make a note of that, to avoid repeatedly attempting to
take the fast path.

Fixes #9427

mypy/checker.py

@@ -293,6 +293,7 @@ def reset(self) -> None:
         self._type_maps[1:] = []
         self._type_maps[0].clear()
         self.temp_type_map = None
+        self.expr_checker.reset()
 
         assert self.inferred_attribute_types is None
         assert self.partial_types == []

mypy/checkexpr.py

@@ -177,6 +177,9 @@ class ExpressionChecker(ExpressionVisitor[Type]):
     # Type context for type inference
     type_context: List[Optional[Type]]
 
+    # cache resolved types in some cases
+    resolved_type: Dict[Expression, ProperType]
+
     strfrm_checker: StringFormatterChecker
     plugin: Plugin
 
@@ -197,6 +200,11 @@ def __init__(self,
         self.type_overrides: Dict[Expression, Type] = {}
         self.strfrm_checker = StringFormatterChecker(self, self.chk, self.msg)
 
+        self.resolved_type = {}
+
+    def reset(self) -> None:
+        self.resolved_type = {}
+
     def visit_name_expr(self, e: NameExpr) -> Type:
         """Type check a name expression.
 
@@ -3269,13 +3277,13 @@ def apply_type_arguments_to_callable(
 
     def visit_list_expr(self, e: ListExpr) -> Type:
         """Type check a list expression [...]."""
-        return self.check_lst_expr(e.items, 'builtins.list', '<list>', e)
+        return self.check_lst_expr(e, 'builtins.list', '<list>')
 
     def visit_set_expr(self, e: SetExpr) -> Type:
-        return self.check_lst_expr(e.items, 'builtins.set', '<set>', e)
+        return self.check_lst_expr(e, 'builtins.set', '<set>')
 
     def fast_container_type(
-            self, items: List[Expression], container_fullname: str
+            self, e: Union[ListExpr, SetExpr, TupleExpr], container_fullname: str
     ) -> Optional[Type]:
         """
         Fast path to determine the type of a list or set literal,
@@ -3290,21 +3298,28 @@ def fast_container_type(
         ctx = self.type_context[-1]
         if ctx:
             return None
+        rt = self.resolved_type.get(e, None)
+        if rt is not None:
+            return rt if isinstance(rt, Instance) else None
         values: List[Type] = []
-        for item in items:
+        for item in e.items:
             if isinstance(item, StarExpr):
                 # fallback to slow path
+                self.resolved_type[e] = NoneType()
                 return None
             values.append(self.accept(item))
         vt = join.join_type_list(values)
         if not allow_fast_container_literal(vt):
+            self.resolved_type[e] = NoneType()
             return None
-        return self.chk.named_generic_type(container_fullname, [vt])
+        ct = self.chk.named_generic_type(container_fullname, [vt])
+        self.resolved_type[e] = ct
+        return ct
 
-    def check_lst_expr(self, items: List[Expression], fullname: str,
-                       tag: str, context: Context) -> Type:
+    def check_lst_expr(self, e: Union[ListExpr, SetExpr, TupleExpr], fullname: str,
+                       tag: str) -> Type:
         # fast path
-        t = self.fast_container_type(items, fullname)
+        t = self.fast_container_type(e, fullname)
         if t:
             return t
 
@@ -3323,10 +3338,10 @@ def check_lst_expr(self, items: List[Expression], fullname: str,
             variables=[tv])
         out = self.check_call(constructor,
                               [(i.expr if isinstance(i, StarExpr) else i)
-                               for i in items],
+                               for i in e.items],
                               [(nodes.ARG_STAR if isinstance(i, StarExpr) else nodes.ARG_POS)
-                               for i in items],
-                              context)[0]
+                               for i in e.items],
+                              e)[0]
         return remove_instance_last_known_values(out)
 
     def visit_tuple_expr(self, e: TupleExpr) -> Type:
@@ -3376,7 +3391,7 @@ def visit_tuple_expr(self, e: TupleExpr) -> Type:
                 else:
                     # A star expression that's not a Tuple.
                     # Treat the whole thing as a variable-length tuple.
-                    return self.check_lst_expr(e.items, 'builtins.tuple', '<tuple>', e)
+                    return self.check_lst_expr(e, 'builtins.tuple', '<tuple>')
             else:
                 if not type_context_items or j >= len(type_context_items):
                     tt = self.accept(item)
@@ -3402,6 +3417,9 @@ def fast_dict_type(self, e: DictExpr) -> Optional[Type]:
         ctx = self.type_context[-1]
         if ctx:
             return None
+        rt = self.resolved_type.get(e, None)
+        if rt is not None:
+            return rt if isinstance(rt, Instance) else None
         keys: List[Type] = []
         values: List[Type] = []
         stargs: Optional[Tuple[Type, Type]] = None
@@ -3415,17 +3433,22 @@ def fast_dict_type(self, e: DictExpr) -> Optional[Type]:
                 ):
                     stargs = (st.args[0], st.args[1])
                 else:
+                    self.resolved_type[e] = NoneType()
                     return None
             else:
                 keys.append(self.accept(key))
                 values.append(self.accept(value))
         kt = join.join_type_list(keys)
         vt = join.join_type_list(values)
         if not (allow_fast_container_literal(kt) and allow_fast_container_literal(vt)):
+            self.resolved_type[e] = NoneType()
             return None
         if stargs and (stargs[0] != kt or stargs[1] != vt):
+            self.resolved_type[e] = NoneType()
             return None
-        return self.chk.named_generic_type('builtins.dict', [kt, vt])
+        dt = self.chk.named_generic_type('builtins.dict', [kt, vt])
+        self.resolved_type[e] = dt
+        return dt
 
     def visit_dict_expr(self, e: DictExpr) -> Type:
         """Type check a dict expression.