Add no_gc directive to fully disable GC

Useful when one can guarantee the class cannot participate in
cycles, but the compiler cannot prove it on its own

Cython/Compiler/Options.py

@@ -116,6 +116,7 @@ directive_defaults = {
     'internal' : False,
     'profile': False,
     'no_gc_clear': False,
+    'no_gc': False,
     'linetrace': False,
     'emit_code_comments': True,  # copy original source code into C code comments
     'annotation_typing': False,  # read type declarations from Python function annotations
@@ -246,6 +247,7 @@ directive_scopes = { # defaults to available everywhere
     'inline' : ('function',),
     'staticmethod' : ('function',),  # FIXME: analysis currently lacks more specific function scope
     'no_gc_clear' : ('cclass',),
+    'no_gc' : ('cclass',),
     'internal' : ('cclass',),
     'autotestdict' : ('module',),
     'autotestdict.all' : ('module',),

Cython/Compiler/Symtab.py

@@ -1889,7 +1889,7 @@ class CClassScope(ClassScope):
     def needs_gc(self):
         # If the type or any of its base types have Python-valued
         # C attributes, then it needs to participate in GC.
-        if self.has_cyclic_pyobject_attrs:
+        if self.has_cyclic_pyobject_attrs and not self.directives.get('no_gc', False):
             return True
         base_type = self.parent_type.base_type
         if base_type and base_type.scope is not None:

Cython/Shadow.py

@@ -118,7 +118,7 @@ optimization = _Optimization()
 overflowcheck.fold = optimization.use_switch = \
     optimization.unpack_method_calls = lambda arg: _EmptyDecoratorAndManager()
 
-final = internal = type_version_tag = no_gc_clear = _empty_decorator
+final = internal = type_version_tag = no_gc_clear = no_gc = _empty_decorator
 
 
 def inline(f, *args, **kwds):

tests/run/no_gc.pyx

+"""
+Check that the @cython.no_gc decorator disables generation of the
+tp_clear and tp_traverse slots, that is, disables cycle collection.
+"""
+
+cimport cython
+from cpython.ref cimport PyObject, Py_TYPE
+
+# Force non-gc'd PyTypeObject when safety is guaranteed by user but not provable
+
+cdef extern from *:
+    ctypedef struct PyTypeObject:
+        void (*tp_clear)(object)
+        void (*tp_traverse)(object)
+
+
+def is_tp_clear_null(obj):
+    return (<PyTypeObject*>Py_TYPE(obj)).tp_clear is NULL
+
+def is_tp_traverse_null(obj):
+    return (<PyTypeObject*>Py_TYPE(obj)).tp_traverse is NULL
+
+
+@cython.no_gc
+cdef class DisableGC:
+    """
+    An extension type that has a tp_clear method generated to test that it
+    actually clears the references to NULL.
+
+    >>> uut = DisableGC()
+    >>> is_tp_clear_null(uut)
+    True
+    >>> is_tp_traverse_null(uut)
+    True
+    """
+
+    cdef public object requires_cleanup
+
+    def __cinit__(self):
+        self.requires_cleanup = (
+                "Tuples to strings don't really need cleanup, cannot take part of cycles",)
+