Implement RFC 22: Add ValueCastable.shape().

Fixes #794.
Closes #876.

amaranth/hdl/ast.py

@@ -1110,7 +1110,11 @@ class Signal(Value, DUID, metaclass=_SignalMeta):
             new_name = other.name + str(name_suffix)
         else:
             new_name = tracer.get_var_name(depth=2 + src_loc_at, default="$like")
-        kw = dict(shape=Value.cast(other).shape(), name=new_name)
+        if isinstance(other, ValueCastable):
+            shape = other.shape()
+        else:
+            shape = Value.cast(other).shape()
+        kw = dict(shape=shape, name=new_name)
         if isinstance(other, Signal):
             kw.update(reset=other.reset, reset_less=other.reset_less,
                       attrs=other.attrs, decoder=other.decoder)
@@ -1363,6 +1367,9 @@ class ValueCastable:
         if not hasattr(cls, "as_value"):
             raise TypeError(f"Class '{cls.__name__}' deriving from `ValueCastable` must override "
                             "the `as_value` method")
+        if not hasattr(cls, "shape"):
+            raise TypeError(f"Class '{cls.__name__}' deriving from `ValueCastable` must override "
+                            "the `shape` method")
         if not hasattr(cls.as_value, "_ValueCastable__memoized"):
             raise TypeError(f"Class '{cls.__name__}' deriving from `ValueCastable` must decorate "
                             "the `as_value` method with the `ValueCastable.lowermethod` decorator")

amaranth/lib/data.py

@@ -115,20 +115,6 @@ class Layout(ShapeCastable, metaclass=ABCMeta):
         raise TypeError("Object {!r} cannot be converted to a data layout"
                         .format(obj))
 
-    @staticmethod
-    def of(obj):
-        """Extract the layout that was used to create a view.
-
-        Raises
-        ------
-        TypeError
-            If ``obj`` is not a :class:`View` instance.
-        """
-        if not isinstance(obj, View):
-            raise TypeError("Object {!r} is not a data view"
-                            .format(obj))
-        return obj._View__orig_layout
-
     @abstractmethod
     def __iter__(self):
         """Iterate fields in the layout.
@@ -611,6 +597,16 @@ class View(ValueCastable):
         self.__layout = cast_layout
         self.__target = cast_target
 
+    def shape(self):
+        """Get layout of this view.
+
+        Returns
+        -------
+        :class:`Layout`
+            The ``layout`` provided when constructing the view.
+        """
+        return self.__orig_layout
+
     @ValueCastable.lowermethod
     def as_value(self):
         """Get underlying value.