Add support for BaseManager.from_queryset() (#251)

* add support for BaseManager.from_queryset()

* cleanups

* lint fixes

mypy_django_plugin/transformers/managers.py

@@ -0,0 +1,71 @@
+from mypy.nodes import (
+    GDEF, FuncDef, MemberExpr, NameExpr, StrExpr, SymbolTableNode, TypeInfo,
+)
+from mypy.plugin import ClassDefContext, DynamicClassDefContext
+from mypy.types import AnyType, Instance, TypeOfAny
+
+from mypy_django_plugin.lib import helpers
+
+
+def create_new_manager_class_from_from_queryset_method(ctx: DynamicClassDefContext) -> None:
+    semanal_api = helpers.get_semanal_api(ctx)
+
+    assert isinstance(ctx.call.callee, MemberExpr)
+    assert isinstance(ctx.call.callee.expr, NameExpr)
+    base_manager_info = ctx.call.callee.expr.node
+    if base_manager_info is None:
+        if not semanal_api.final_iteration:
+            semanal_api.defer()
+        return
+
+    assert isinstance(base_manager_info, TypeInfo)
+    new_manager_info = semanal_api.basic_new_typeinfo(ctx.name,
+                                                      basetype_or_fallback=Instance(base_manager_info,
+                                                                                    [AnyType(TypeOfAny.unannotated)]))
+    new_manager_info.line = ctx.call.line
+    new_manager_info.defn.line = ctx.call.line
+    new_manager_info.metaclass_type = new_manager_info.calculate_metaclass_type()
+
+    current_module = semanal_api.cur_mod_node
+    current_module.names[ctx.name] = SymbolTableNode(GDEF, new_manager_info,
+                                                     plugin_generated=True)
+    passed_queryset = ctx.call.args[0]
+    assert isinstance(passed_queryset, NameExpr)
+
+    derived_queryset_fullname = passed_queryset.fullname
+    assert derived_queryset_fullname is not None
+
+    sym = semanal_api.lookup_fully_qualified_or_none(derived_queryset_fullname)
+    assert sym is not None
+    if sym.node is None:
+        if not semanal_api.final_iteration:
+            semanal_api.defer()
+        else:
+            # inherit from Any to prevent false-positives, if queryset class cannot be resolved
+            new_manager_info.fallback_to_any = True
+        return
+
+    derived_queryset_info = sym.node
+    assert isinstance(derived_queryset_info, TypeInfo)
+
+    if len(ctx.call.args) > 1:
+        expr = ctx.call.args[1]
+        assert isinstance(expr, StrExpr)
+        custom_manager_generated_name = expr.value
+    else:
+        custom_manager_generated_name = base_manager_info.name + 'From' + derived_queryset_info.name
+
+    custom_manager_generated_fullname = '.'.join(['django.db.models.manager', custom_manager_generated_name])
+    if 'from_queryset_managers' not in base_manager_info.metadata:
+        base_manager_info.metadata['from_queryset_managers'] = {}
+    base_manager_info.metadata['from_queryset_managers'][custom_manager_generated_fullname] = new_manager_info.fullname
+
+    class_def_context = ClassDefContext(cls=new_manager_info.defn,
+                                        reason=ctx.call, api=semanal_api)
+    self_type = Instance(new_manager_info, [])
+    for name, sym in derived_queryset_info.names.items():
+        if isinstance(sym.node, FuncDef):
+            helpers.copy_method_to_another_class(class_def_context,
+                                                 self_type,
+                                                 new_method_name=name,
+                                                 method_node=sym.node)