Implement RFC 46: Change Shape.cast(range(1)) to unsigned(0).

amaranth/hdl/_ast.py

@@ -93,8 +93,8 @@ class Shape:
 
         * a :class:`Shape`, where the result is itself;
         * an :class:`int`, where the result is :func:`unsigned(obj) <unsigned>`;
-        * a :class:`range`, where the result is wide enough to represent any element of the range,
-          and is signed if any element of the range is signed;
+        * a :class:`range`, where the result has minimal width required to represent all elements
+          of the range, and is signed if any element of the range is signed;
         * an :class:`enum.Enum` whose members are all :ref:`constant-castable <lang-constcasting>`
           or :class:`enum.IntEnum`, where the result is wide enough to represent any member of
           the enumeration, and is signed if any member of the enumeration is signed;
@@ -121,6 +121,8 @@ class Shape:
                 signed = obj[0] < 0 or obj[-1] < 0
                 width  = max(bits_for(obj[0], signed),
                              bits_for(obj[-1], signed))
+                if obj[0] == obj[-1] == 0:
+                    width = 0
                 return Shape(width, signed)
             elif isinstance(obj, type) and issubclass(obj, Enum):
                 # For compatibility with third party enumerations, handle them as if they were

docs/changes.rst

@@ -36,9 +36,11 @@ Implemented RFCs
 
 .. _RFC 17: https://amaranth-lang.org/rfcs/0017-remove-log2-int.html
 .. _RFC 39: https://amaranth-lang.org/rfcs/0039-empty-case.html
+.. _RFC 46: https://amaranth-lang.org/rfcs/0046-shape-range-1.html
 
 * `RFC 17`_: Remove ``log2_int``
 * `RFC 39`_: Change semantics of no-argument ``m.Case()``
+* `RFC 46`_: Change ``Shape.cast(range(1))`` to ``unsigned(0)``
 
 
 Language changes
@@ -52,6 +54,7 @@ Language changes
 * Changed: ``Value.matches()`` with no patterns is ``Const(0)`` instead of ``Const(1)``. (`RFC 39`_)
 * Changed: ``Signal(range(stop), reset=stop)`` warning has been changed into a hard error and made to trigger on any out-of range value.
 * Changed: ``Signal(range(0))`` is now valid without a warning.
+* Changed: ``Shape.cast(range(1))`` is now ``unsigned(0)``. (`RFC 46`_)
 * Deprecated: :func:`amaranth.utils.log2_int`. (`RFC 17`_)
 * Removed: (deprecated in 0.4) :meth:`Const.normalize`. (`RFC 5`_)
 * Removed: (deprecated in 0.4) :class:`Repl`. (`RFC 10`_)

tests/test_hdl_ast.py

@@ -115,7 +115,7 @@ class ShapeTestCase(FHDLTestCase):
         self.assertEqual(s3.width, 4)
         self.assertEqual(s3.signed, True)
         s4 = Shape.cast(range(0, 1))
-        self.assertEqual(s4.width, 1)
+        self.assertEqual(s4.width, 0)
         self.assertEqual(s4.signed, False)
         s5 = Shape.cast(range(-1, 0))
         self.assertEqual(s5.width, 1)
@@ -129,6 +129,9 @@ class ShapeTestCase(FHDLTestCase):
         s8 = Shape.cast(range(0, 10, 3))
         self.assertEqual(s8.width, 4)
         self.assertEqual(s8.signed, False)
+        s9 = Shape.cast(range(0, 3, 3))
+        self.assertEqual(s9.width, 0)
+        self.assertEqual(s9.signed, False)
 
     def test_cast_enum(self):
         s1 = Shape.cast(UnsignedEnum)
@@ -1088,7 +1091,7 @@ class SignalTestCase(FHDLTestCase):
         s10 = Signal(range(0))
         self.assertEqual(s10.shape(), unsigned(0))
         s11 = Signal(range(1))
-        self.assertEqual(s11.shape(), unsigned(1))
+        self.assertEqual(s11.shape(), unsigned(0))
 
     def test_shape_wrong(self):
         with self.assertRaisesRegex(TypeError,