Add structref as a mutable struct that is pass-by-ref

.flake8

@@ -60,7 +60,6 @@ exclude =
     numba/core/types/abstract.py
     numba/core/types/functions.py
     numba/core/types/misc.py
-    numba/core/types/containers.py
     numba/core/types/npytypes.py
     numba/core/types/common.py
     numba/core/types/iterators.py

docs/source/developer/repomap.rst

@@ -82,6 +82,7 @@ These define aspects of the public Numba interface.
   regular functions on the CPU
 - :ghfile:`numba/core/extending.py` - Public decorators for extending Numba
   (``overload``, ``intrinsic``, etc)
+  - :ghfile:`numba/experimental/structref.py` - Public API for defining a mutable struct
 - :ghfile:`numba/core/ccallback.py` - ``@cfunc`` decorator for compiling
   functions to a fixed C signature.  Used to make callbacks.
 - :ghfile:`numba/np/ufunc/decorators.py` - ufunc/gufunc compilation

numba/core/datamodel/models.py

@@ -1342,3 +1342,25 @@ def _actual_model(self):
     def traverse(self, builder):
         return [(self.actual_fe_type,
                  lambda value: builder.extract_value(value, [0]))]
+
+
+@register_default(types.StructRefPayload)
+class StructPayloadModel(StructModel):
+    """Model for the payload of a mutable struct
+    """
+    def __init__(self, dmm, fe_typ):
+        members = tuple(fe_typ.field_dict.items())
+        super().__init__(dmm, fe_typ, members)
+
+
+class StructRefModel(StructModel):
+    """Model for a mutable struct.
+    A reference to the payload
+    """
+    def __init__(self, dmm, fe_typ):
+        dtype = fe_typ.get_data_type()
+        members = [
+            ("meminfo", types.MemInfoPointer(dtype)),
+        ]
+        super().__init__(dmm, fe_typ, members)
+