Implement RFC 42: Const from shape-castable.

amaranth/hdl/_ast.py

@@ -839,8 +839,15 @@ class Value(metaclass=ABCMeta):
         raise NotImplementedError # :nocov:
 
 
+class _ConstMeta(ABCMeta):
+    def __call__(cls, value, shape=None, src_loc_at=0, **kwargs):
+        if isinstance(shape, ShapeCastable):
+            return shape.const(value)
+        return super().__call__(value, shape, **kwargs, src_loc_at=src_loc_at + 1)
+
+
 @final
-class Const(Value):
+class Const(Value, metaclass=_ConstMeta):
     """A constant, literal integer value.
 
     Parameters
@@ -898,7 +905,7 @@ class Const(Value):
                             "shape {!r}; this is likely an off-by-one error"
                             .format(self.value, shape),
                     category=SyntaxWarning,
-                    stacklevel=2)
+                    stacklevel=3)
             shape = Shape.cast(shape, src_loc_at=1 + src_loc_at)
         self.width  = shape.width
         self.signed = shape.signed

tests/test_hdl_ast.py

@@ -470,6 +470,25 @@ class ConstTestCase(FHDLTestCase):
         with self.assertRaises(TypeError):
             hash(Const(0))
 
+    def test_shape_castable(self):
+        class MockConstValue:
+            def __init__(self, value):
+                self.value = value
+
+        class MockConstShape(ShapeCastable):
+            def as_shape(self):
+                return unsigned(8)
+
+            def __call__(self, value):
+                return value
+
+            def const(self, init):
+                return MockConstValue(init)
+
+        s = Const(10, MockConstShape())
+        self.assertIsInstance(s, MockConstValue)
+        self.assertEqual(s.value, 10)
+
 
 class OperatorTestCase(FHDLTestCase):
     def test_bool(self):