lib.data: implement equality for View, reject all other operators.

amaranth/lib/data.py

@@ -580,6 +580,10 @@ class View(ValueCastable):
     an Amaranth value instead of a constant integer. The returned element is chosen dynamically
     in that case.
 
+    A view can only be compared for equality with another view of the same layout,
+    returning a single-bit value. No other operators are supported on views. If required,
+    a view can be converted back to its underlying value via :meth:`as_value`.
+
     Custom view classes
     ###################
 
@@ -609,7 +613,7 @@ class View(ValueCastable):
                 warnings.warn("View layout includes a field {!r} that will be shadowed by "
                               "the view attribute '{}.{}.{}'"
                               .format(name, type(self).__module__, type(self).__qualname__, name),
-                              SyntaxWarning, stacklevel=1)
+                              SyntaxWarning, stacklevel=2)
         self.__orig_layout = layout
         self.__layout = cast_layout
         self.__target = cast_target
@@ -732,6 +736,49 @@ class View(ValueCastable):
                                  .format(self.__target, name))
         return item
 
+    def __eq__(self, other):
+        if not isinstance(other, View) or self.__layout != other.__layout:
+            raise TypeError(f"View of {self.__layout!r} can only be compared to another view of the same layout, not {other!r}")
+        return self.__target == other.__target
+
+    def __ne__(self, other):
+        if not isinstance(other, View) or self.__layout != other.__layout:
+            raise TypeError(f"View of {self.__layout!r} can only be compared to another view of the same layout, not {other!r}")
+        return self.__target != other.__target
+
+    def __add__(self, other):
+        raise TypeError("Cannot perform arithmetic operations on a View")
+
+    __radd__ = __add__
+    __sub__ = __add__
+    __rsub__ = __add__
+    __mul__ = __add__
+    __rmul__ = __add__
+    __floordiv__ = __add__
+    __rfloordiv__ = __add__
+    __mod__ = __add__
+    __rmod__ = __add__
+    __lshift__ = __add__
+    __rlshift__ = __add__
+    __rshift__ = __add__
+    __rrshift__ = __add__
+    __lt__ = __add__
+    __le__ = __add__
+    __gt__ = __add__
+    __ge__ = __add__
+
+    def __and__(self, other):
+        raise TypeError("Cannot perform bitwise operations on a View")
+
+    __rand__ = __and__
+    __or__ = __and__
+    __ror__ = __and__
+    __xor__ = __and__
+    __rxor__ = __and__
+
+    def __repr__(self):
+        return f"{self.__class__.__name__}({self.__layout!r}, {self.__target!r})"
+
 
 class _AggregateMeta(ShapeCastable, type):
     def __new__(metacls, name, bases, namespace):

tests/test_lib_data.py

@@ -1,4 +1,5 @@
 from enum import Enum
+import operator
 from unittest import TestCase
 
 from amaranth.hdl import *
@@ -632,6 +633,63 @@ class ViewTestCase(FHDLTestCase):
                 r"^View of \(sig \$signal\) with an array layout does not have fields$"):
             Signal(ArrayLayout(unsigned(1), 1), reset=[0]).reset
 
+    def test_eq(self):
+        s1 = Signal(StructLayout({"a": unsigned(2)}))
+        s2 = Signal(StructLayout({"a": unsigned(2)}))
+        s3 = Signal(StructLayout({"a": unsigned(1), "b": unsigned(1)}))
+        self.assertRepr(s1 == s2, "(== (sig s1) (sig s2))")
+        self.assertRepr(s1 != s2, "(!= (sig s1) (sig s2))")
+        with self.assertRaisesRegex(TypeError,
+                r"^View of .* can only be compared to another view of the same layout, not .*$"):
+            s1 == s3
+        with self.assertRaisesRegex(TypeError,
+                r"^View of .* can only be compared to another view of the same layout, not .*$"):
+            s1 != s3
+        with self.assertRaisesRegex(TypeError,
+                r"^View of .* can only be compared to another view of the same layout, not .*$"):
+            s1 == Const(0, 2)
+        with self.assertRaisesRegex(TypeError,
+                r"^View of .* can only be compared to another view of the same layout, not .*$"):
+            s1 != Const(0, 2)
+
+    def test_operator(self):
+        s1 = Signal(StructLayout({"a": unsigned(2)}))
+        s2 = Signal(unsigned(2))
+        for op in [
+            operator.__add__,
+            operator.__sub__,
+            operator.__mul__,
+            operator.__floordiv__,
+            operator.__mod__,
+            operator.__lshift__,
+            operator.__rshift__,
+            operator.__lt__,
+            operator.__le__,
+            operator.__gt__,
+            operator.__ge__,
+        ]:
+            with self.assertRaisesRegex(TypeError,
+                    r"^Cannot perform arithmetic operations on a View$"):
+                op(s1, s2)
+            with self.assertRaisesRegex(TypeError,
+                    r"^Cannot perform arithmetic operations on a View$"):
+                op(s2, s1)
+        for op in [
+            operator.__and__,
+            operator.__or__,
+            operator.__xor__,
+        ]:
+            with self.assertRaisesRegex(TypeError,
+                    r"^Cannot perform bitwise operations on a View$"):
+                op(s1, s2)
+            with self.assertRaisesRegex(TypeError,
+                    r"^Cannot perform bitwise operations on a View$"):
+                op(s2, s1)
+
+    def test_repr(self):
+        s1 = Signal(StructLayout({"a": unsigned(2)}))
+        self.assertRepr(s1, "View(StructLayout({'a': unsigned(2)}), (sig s1))")
+
 
 class StructTestCase(FHDLTestCase):
     def test_construct(self):