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.

Author: whitequark

amaranth/lib/data.py

@@ -13,33 +13,25 @@
 
 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
-
-    @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._shape  = shape
         self._offset = offset
 
+    @property
+    def shape(self):
+        return self._shape
+
+    @property
+    def offset(self):
+        return self._offset
+
     @property
     def width(self):
         return Shape.cast(self.shape).width

tests/test_lib_data.py

@@ -59,6 +59,12 @@ def test_offset_wrong(self):
                 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):