hdl.ast: deprecate UserValue in favor of ValueCastable.

Closes #527.

nmigen/hdl/ast.py

@@ -1237,6 +1237,7 @@ class ArrayProxy(Value):
         return "(proxy (array [{}]) {!r})".format(", ".join(map(repr, self.elems)), self.index)
 
 
+# TODO(nmigen-0.4): remove
 class UserValue(Value):
     """Value with custom lowering.
 
@@ -1257,6 +1258,7 @@ class UserValue(Value):
         * Indexing or iterating through individual bits;
         * Adding an assignment to the value to a ``Module`` using ``m.d.<domain> +=``.
     """
+    @deprecated("instead of `UserValue`, use `ValueCastable`", stacklevel=3)
     def __init__(self, *, src_loc_at=0):
         super().__init__(src_loc_at=1 + src_loc_at)
         self.__lowered = None
@@ -1292,7 +1294,7 @@ class ValueCastable:
     providing fixed semantics upfront, it is kept abstract for as long as possible, only being
     cast to a concrete nMigen value when required.
 
-    Note that it is necessary to ensure that nMigen's view of representation of all values stays 
+    Note that it is necessary to ensure that nMigen's view of representation of all values stays
     internally consistent. The class deriving from ``ValueCastable`` must decorate the ``as_value``
     method with the ``lowermethod`` decorator, which ensures that all calls to ``as_value``return the
     same ``Value`` representation. If the class deriving from ``ValueCastable`` is mutable, it is

tests/test_hdl_ast.py

@@ -1009,20 +1009,24 @@ class MockUserValue(UserValue):
 
 class UserValueTestCase(FHDLTestCase):
     def test_shape(self):
-        uv = MockUserValue(1)
-        self.assertEqual(uv.shape(), unsigned(1))
-        self.assertIsInstance(uv.shape(), Shape)
-        uv.lowered = 2
-        self.assertEqual(uv.shape(), unsigned(1))
-        self.assertEqual(uv.lower_count, 1)
+        with warnings.catch_warnings():
+            warnings.filterwarnings(action="ignore", category=DeprecationWarning)
+            uv = MockUserValue(1)
+            self.assertEqual(uv.shape(), unsigned(1))
+            self.assertIsInstance(uv.shape(), Shape)
+            uv.lowered = 2
+            self.assertEqual(uv.shape(), unsigned(1))
+            self.assertEqual(uv.lower_count, 1)
 
     def test_lower_to_user_value(self):
-        uv = MockUserValue(MockUserValue(1))
-        self.assertEqual(uv.shape(), unsigned(1))
-        self.assertIsInstance(uv.shape(), Shape)
-        uv.lowered = MockUserValue(2)
-        self.assertEqual(uv.shape(), unsigned(1))
-        self.assertEqual(uv.lower_count, 1)
+        with warnings.catch_warnings():
+            warnings.filterwarnings(action="ignore", category=DeprecationWarning)
+            uv = MockUserValue(MockUserValue(1))
+            self.assertEqual(uv.shape(), unsigned(1))
+            self.assertIsInstance(uv.shape(), Shape)
+            uv.lowered = MockUserValue(2)
+            self.assertEqual(uv.shape(), unsigned(1))
+            self.assertEqual(uv.lower_count, 1)
 
 
 class MockValueCastableChanges(ValueCastable):

tests/test_hdl_xfrm.py

@@ -1,5 +1,7 @@
 # nmigen: UnusedElaboratable=no
 
+import warnings
+
 from nmigen.hdl.ast import *
 from nmigen.hdl.cd import *
 from nmigen.hdl.ir import *
@@ -614,7 +616,9 @@ class UserValueTestCase(FHDLTestCase):
     def setUp(self):
         self.s  = Signal()
         self.c  = Signal()
-        self.uv = MockUserValue(self.s)
+        with warnings.catch_warnings():
+            warnings.filterwarnings(action="ignore", category=DeprecationWarning)
+            self.uv = MockUserValue(self.s)
 
     def test_lower(self):
         sync = ClockDomain()
@@ -645,6 +649,8 @@ class UserValueRecursiveTestCase(UserValueTestCase):
     def setUp(self):
         self.s = Signal()
         self.c = Signal()
-        self.uv = MockUserValue(MockUserValue(self.s))
+        with warnings.catch_warnings():
+            warnings.filterwarnings(action="ignore", category=DeprecationWarning)
+            self.uv = MockUserValue(MockUserValue(self.s))
 
     # inherit the test_lower method from UserValueTestCase because the checks are the same