[mypyc] Allow per-class free list to be used with inheritance (#19790)

It's still unsupported if interpreted subclasses are allowed.

Author: JukkaL

mypyc/codegen/emitclass.py

@@ -205,10 +205,6 @@ def generate_class_reuse(
     TODO: Generalize to support a free list with up to N objects.
     """
     assert cl.reuse_freed_instance
-
-    # The free list implementation doesn't support class hierarchies
-    assert cl.is_final_class or cl.children == []
-
     context = c_emitter.context
     name = cl.name_prefix(c_emitter.names) + "_free_instance"
     struct_name = cl.struct_name(c_emitter.names)

mypyc/irbuild/prepare.py

@@ -363,6 +363,12 @@ def prepare_class_def(
         line = attrs_lines["free_list_len"]
         if ir.is_trait:
             errors.error('"free_list_len" can\'t be used with traits', path, line)
+        if ir.allow_interpreted_subclasses:
+            errors.error(
+                '"free_list_len" can\'t be used in a class that allows interpreted subclasses',
+                path,
+                line,
+            )
         if free_list_len == 1:
             ir.reuse_freed_instance = True
         else:

mypyc/test-data/irbuild-classes.test

@@ -1754,3 +1754,7 @@ class FreeListError:
 @mypyc_attr(free_list_len=1)  # E: "free_list_len" can't be used with traits
 class NonNative:
     pass
+
+@mypyc_attr(free_list_len=1, allow_interpreted_subclasses=True)  # E: "free_list_len" can't be used in a class that allows interpreted subclasses
+class InterpSub:
+    pass

mypyc/test-data/run-classes.test

@@ -3802,7 +3802,7 @@ from mypy_extensions import mypyc_attr
 
 a = []
 
-@mypyc_attr(free_list=1)
+@mypyc_attr(free_list_len=1)
 class Foo:
     def __init__(self, x: int) -> None:
         self.x = x
@@ -3830,3 +3830,64 @@ def test_alloc() -> None:
     y = Foo(5)
     assert x.x == 4
     assert y.x == 5
+
+@mypyc_attr(free_list_len=1)
+class Base:
+    def __init__(self, x: str) -> None:
+        self.x = x
+
+class Deriv(Base):
+    def __init__(self, x: str, y: str) -> None:
+        super().__init__(x)
+        self.y = y
+
+@mypyc_attr(free_list_len=1)
+class Deriv2(Base):
+    def __init__(self, x: str, y: str) -> None:
+        super().__init__(x)
+        self.y = y
+
+def test_inheritance() -> None:
+    x: Base | None
+    y: Base | None
+    x = Base('x' + str())
+    y = Base('y' + str())
+    y = None
+    d = Deriv('a' + str(), 'b' + str())
+    assert type(d) is Deriv
+    assert d.x == 'a'
+    assert d.y == 'b'
+    assert x.x == 'x'
+    y = Base('z' + str())
+    assert d.x == 'a'
+    assert d.y == 'b'
+    assert y.x == 'z'
+    x = None
+    y = None
+
+def test_inheritance_2() -> None:
+    x: Base | None
+    y: Base | None
+    d: Deriv2 | None
+    x = Base('x' + str())
+    y = Base('y' + str())
+    y = None
+    d = Deriv2('a' + str(), 'b' + str())
+    assert type(d) is Deriv2
+    assert d.x == 'a'
+    assert d.y == 'b'
+    assert x.x == 'x'
+    d = None
+    d = Deriv2('c' + str(), 'd' + str())
+    assert type(d) is Deriv2
+    assert d.x == 'c'
+    assert d.y == 'd'
+    assert x.x == 'x'
+    y = Base('z' + str())
+    assert type(y) is Base
+    assert d.x == 'c'
+    assert d.y == 'd'
+    assert y.x == 'z'
+    x = None
+    y = None
+    d = None