make field extraction protocl runtime-checkable. fix extraction of trivial index dimensions.

src/pystencilssfg/ir/postprocessing.py

@@ -232,6 +232,9 @@ class SfgDeferredFieldMapping(SfgDeferredNode):
             self._field.strides
         )
 
+        index_dims_are_trivial = self._field.index_shape == (1,)
+        rank = len(self._field.shape)
+
         for param in ppc.live_variables:
             if isinstance(param, SfgKernelParamVar):
                 for prop in param.wrapped.properties:
@@ -280,6 +283,9 @@ class SfgDeferredFieldMapping(SfgDeferredNode):
             expr = self._extraction._extract_size(coord)
 
             if expr is None:
+                if index_dims_are_trivial and coord == rank - 1:
+                    expr = AugExpr.format("1")
+                else:
                     raise SfgException(
                         f"Cannot extract shape in coordinate {coord} from {self._extraction}"
                     )
@@ -300,6 +306,9 @@ class SfgDeferredFieldMapping(SfgDeferredNode):
             expr = self._extraction._extract_stride(coord)
 
             if expr is None:
+                if index_dims_are_trivial and coord == rank - 1:
+                    expr = AugExpr.format("1")
+                else:
                     raise SfgException(
                         f"Cannot extract stride in coordinate {coord} from {self._extraction}"
                     )

src/pystencilssfg/lang/extractions.py

 from __future__ import annotations
-from typing import Protocol
+from typing import Protocol, runtime_checkable
 from abc import abstractmethod
 
 from .expressions import AugExpr
 
 
+@runtime_checkable
 class SupportsFieldExtraction(Protocol):
     """Protocol for field pointer and indexing extraction.