A few miscellaneous micro-optimizations (#14366)

These are part of the changes that collectively bring a 6% performance
improvement. These are all pretty minor.

Author: JukkaL

mypy/checker.py

@@ -378,10 +378,6 @@ def __init__(
         self.path = path
         self.msg = MessageBuilder(errors, modules)
         self.plugin = plugin
-        self.expr_checker = mypy.checkexpr.ExpressionChecker(
-            self, self.msg, self.plugin, per_line_checking_time_ns
-        )
-        self.pattern_checker = PatternChecker(self, self.msg, self.plugin)
         self.tscope = Scope()
         self.scope = CheckerScope(tree)
         self.binder = ConditionalTypeBinder()
@@ -419,6 +415,12 @@ def __init__(
         # example when type-checking class decorators.
         self.allow_abstract_call = False
 
+        # Child checker objects for specific AST node types
+        self.expr_checker = mypy.checkexpr.ExpressionChecker(
+            self, self.msg, self.plugin, per_line_checking_time_ns
+        )
+        self.pattern_checker = PatternChecker(self, self.msg, self.plugin)
+
     @property
     def type_context(self) -> list[Type | None]:
         return self.expr_checker.type_context

mypy/checkexpr.py

@@ -276,7 +276,7 @@ def __init__(
         self.msg = msg
         self.plugin = plugin
         self.per_line_checking_time_ns = per_line_checking_time_ns
-        self.collect_line_checking_stats = self.chk.options.line_checking_stats is not None
+        self.collect_line_checking_stats = chk.options.line_checking_stats is not None
         # Are we already visiting some expression? This is used to avoid double counting
         # time for nested expressions.
         self.in_expression = False

mypy/types.py

@@ -2623,13 +2623,17 @@ def __init__(
         # We must keep this false to avoid crashes during semantic analysis.
         # TODO: maybe switch this to True during type-checking pass?
         self.items = flatten_nested_unions(items, handle_type_alias_type=False)
-        self.can_be_true = any(item.can_be_true for item in items)
-        self.can_be_false = any(item.can_be_false for item in items)
         # is_evaluated should be set to false for type comments and string literals
         self.is_evaluated = is_evaluated
         # uses_pep604_syntax is True if Union uses OR syntax (X | Y)
         self.uses_pep604_syntax = uses_pep604_syntax
 
+    def can_be_true_default(self) -> bool:
+        return any(item.can_be_true for item in self.items)
+
+    def can_be_false_default(self) -> bool:
+        return any(item.can_be_false for item in self.items)
+
     def __hash__(self) -> int:
         return hash(frozenset(self.items))