From 0849e1af0bbecc629b1331ee257c58bf931b241a Mon Sep 17 00:00:00 2001
From: Wanda <wanda@phinode.net>
Date: Sat, 30 Dec 2023 12:10:29 +0100
Subject: [PATCH] hdl.ast: make `Slice` const-castable.

Fixes #1006.
---
 amaranth/hdl/ast.py   |  3 +++
 docs/changes.rst      |  1 +
 docs/lang.rst         |  1 +
 tests/test_hdl_ast.py | 16 ++++++++++++++++
 4 files changed, 21 insertions(+)

diff --git a/amaranth/hdl/ast.py b/amaranth/hdl/ast.py
index e0a0c4f..70e4763 100644
--- a/amaranth/hdl/ast.py
+++ b/amaranth/hdl/ast.py
@@ -731,6 +731,9 @@ class Const(Value):
                 value |= part_value << width
                 width += len(const)
             return Const(value, width)
+        elif type(obj) is Slice:
+            value = Const.cast(obj.value)
+            return Const(value.value >> obj.start, unsigned(obj.stop - obj.start))
         else:
             raise TypeError(f"Value {obj!r} cannot be converted to an Amaranth constant")
 
diff --git a/docs/changes.rst b/docs/changes.rst
index 6b7e752..34a251c 100644
--- a/docs/changes.rst
+++ b/docs/changes.rst
@@ -15,6 +15,7 @@ Language changes
 
 .. currentmodule:: amaranth.hdl
 
+* Added: `class:ast.Slice` objects have been made const-castable.
 * Removed: (deprecated in 0.4) :meth:`Const.normalize`. (`RFC 5`_)
 * Removed: (deprecated in 0.4) :class:`ast.Sample`, :class:`ast.Past`, :class:`ast.Stable`, :class:`ast.Rose`, :class:`ast.Fell`.
 
diff --git a/docs/lang.rst b/docs/lang.rst
index fdd197b..d238050 100644
--- a/docs/lang.rst
+++ b/docs/lang.rst
@@ -325,6 +325,7 @@ They may also be provided as a pattern to the :ref:`match operator <lang-matchop
 
    * :class:`Const`
    * :class:`Cat`
+   * :class:`Slice`
 
    This list will be expanded in the future.
 
diff --git a/tests/test_hdl_ast.py b/tests/test_hdl_ast.py
index be28c7a..b97b2a1 100644
--- a/tests/test_hdl_ast.py
+++ b/tests/test_hdl_ast.py
@@ -799,6 +799,16 @@ class SliceTestCase(FHDLTestCase):
         s1 = Const(10)[2]
         self.assertEqual(repr(s1), "(slice (const 4'd10) 2:3)")
 
+    def test_const(self):
+        a = Const.cast(Const(0x1234, 16)[4:12])
+        self.assertEqual(a.value, 0x23)
+        self.assertEqual(a.width, 8)
+        self.assertEqual(a.signed, False)
+        a = Const.cast(Const(-4, signed(8))[1:6])
+        self.assertEqual(a.value, 0x1e)
+        self.assertEqual(a.width, 5)
+        self.assertEqual(a.signed, False)
+
 
 class BitSelectTestCase(FHDLTestCase):
     def setUp(self):
@@ -1197,6 +1207,12 @@ class SignalTestCase(FHDLTestCase):
         self.assertEqual(s2.shape(), signed(2))
         self.assertEqual(s2.decoder(SignedEnum.FOO), "FOO/-1")
 
+    def test_const_wrong(self):
+        s1 = Signal()
+        with self.assertRaisesRegex(TypeError,
+                r"^Value \(sig s1\) cannot be converted to an Amaranth constant$"):
+            Const.cast(s1)
+
 
 class ClockSignalTestCase(FHDLTestCase):
     def test_domain(self):