lib.data: make Field() immutable.

Mutability of Field isn't specified by the RFC and can cause issues
if the objects stored in Layout subclasses are mutated. There isn't
any reason to do that (the subclasses themselves are mutable and
handle that correctly), so disallow it.

amaranth/lib/data.py

@@ -13,33 +13,25 @@ __all__ = [
 
 class Field:
     def __init__(self, shape, offset):
-        self.shape  = shape
-        self.offset = offset
-
-    @property
-    def shape(self):
-        return self._shape
-
-    @shape.setter
-    def shape(self, shape):
         try:
             Shape.cast(shape)
         except TypeError as e:
             raise TypeError("Field shape must be a shape-castable object, not {!r}"
                             .format(shape)) from e
-        self._shape = shape
+        if not isinstance(offset, int) or offset < 0:
+            raise TypeError("Field offset must be a non-negative integer, not {!r}"
+                            .format(offset))
+        self._shape  = shape
+        self._offset = offset
+
+    @property
+    def shape(self):
+        return self._shape
 
     @property
     def offset(self):
         return self._offset
 
-    @offset.setter
-    def offset(self, offset):
-        if not isinstance(offset, int) or offset < 0:
-            raise TypeError("Field offset must be a non-negative integer, not {!r}"
-                            .format(offset))
-        self._offset = offset
-
     @property
     def width(self):
         return Shape.cast(self.shape).width

tests/test_lib_data.py

@@ -59,6 +59,12 @@ class FieldTestCase(TestCase):
                 r"^Field offset must be a non-negative integer, not -1$"):
             Field(unsigned(2), -1)
 
+    def test_immutable(self):
+        with self.assertRaises(AttributeError):
+            Field(1, 0).shape = unsigned(2)
+        with self.assertRaises(AttributeError):
+            Field(1, 0).offset = 1
+
 
 class StructLayoutTestCase(TestCase):
     def test_construct(self):