lib.data: accept data.Const in *.const().

amaranth/lib/data.py

@@ -208,6 +208,11 @@ class Layout(ShapeCastable, metaclass=ABCMeta):
             an all-zero value and had every field assigned to the corresponding value in the order
             in which they appear in :py:`init`.
         """
+        if isinstance(init, Const):
+            if Layout.cast(init.shape()) != self:
+                raise ValueError(f"Const layout {init.shape()!r} differs from shape layout "
+                                 f"{self!r}")
+            return init
         if init is None:
             iterator = iter(())
         elif isinstance(init, Mapping):
@@ -1139,6 +1144,11 @@ class _AggregateMeta(ShapeCastable, type):
         return super().__call__(cls, target)
 
     def const(cls, init):
+        if isinstance(init, Const):
+            if Layout.cast(init.shape()) != Layout.cast(cls.__layout):
+                raise ValueError(f"Const layout {init.shape()!r} differs from shape layout "
+                                 f"{cls.__layout!r}")
+            return init
         if cls.__layout_cls is UnionLayout:
             if init is not None and len(init) > 1:
                 raise ValueError("Initializer for at most one field can be provided for "

tests/test_lib_data.py

@@ -387,6 +387,7 @@ class LayoutTestCase(FHDLTestCase):
         })
         self.assertRepr(sl.const(None).as_value(), "(const 3'd0)")
         self.assertRepr(sl.const({"a": 0b1, "b": 0b10}).as_value(), "(const 3'd5)")
+        self.assertRepr(sl.const(sl.const({"a": 0b1, "b": 0b10})).as_value(), "(const 3'd5)")
 
         fl = data.FlexibleLayout(2, {
             "a": data.Field(unsigned(1), 0),
@@ -408,6 +409,10 @@ class LayoutTestCase(FHDLTestCase):
                 r"^Layout constant initializer must be a mapping or a sequence, not "
                 r"<.+?object.+?>$"):
             sl.const(object())
+        sl2 = data.StructLayout({"f": unsigned(2)})
+        with self.assertRaisesRegex(ValueError,
+                r"^Const layout StructLayout.* differs from shape layout StructLayout.*$"):
+            sl2.const(sl.const({}))
 
     def test_const_field_shape_castable(self):
         class CastableFromHex(ShapeCastable):
@@ -1013,6 +1018,21 @@ class StructTestCase(FHDLTestCase):
         self.assertEqual(v2.as_value().init, 0b010011)
         v3 = Signal(S, init=dict(p=0b0011, q=0b00))
         self.assertEqual(v3.as_value().init, 0b000011)
+        v3 = Signal(S, init=S.const({"p": 0b0011, "q": 0b00}))
+        self.assertEqual(v3.as_value().init, 0b000011)
+
+    def test_const_wrong(self):
+        class S(data.Struct):
+            p: 4
+            q: 2 = 1
+
+        class S2(data.Struct):
+            p: 2
+            q: 4
+
+        with self.assertRaisesRegex(ValueError,
+                f"^Const layout StructLayout.* differs from shape layout StructLayout.*$"):
+            S.const(S2.const({"p": 0b11, "q": 0b0000}))
 
     def test_shape_undefined_wrong(self):
         class S(data.Struct):