hdl.rec: make Record inherit from UserValue.

Closes #354.

nmigen/hdl/ast.py

@@ -1188,7 +1188,10 @@ class UserValue(Value):
 
     def _lazy_lower(self):
         if self.__lowered is None:
-            self.__lowered = Value.cast(self.lower())
+            lowered = self.lower()
+            if isinstance(lowered, UserValue):
+                lowered = lowered._lazy_lower()
+            self.__lowered = Value.cast(lowered)
         return self.__lowered
 
     def shape(self):

nmigen/hdl/rec.py

@@ -85,7 +85,7 @@ class Layout:
 
 
 # Unlike most Values, Record *can* be subclassed.
-class Record(Value):
+class Record(UserValue):
     @staticmethod
     def like(other, *, name=None, name_suffix=None, src_loc_at=0):
         if name is not None:
@@ -113,6 +113,8 @@ class Record(Value):
         return Record(other.layout, name=new_name, fields=fields, src_loc_at=1)
 
     def __init__(self, layout, *, name=None, fields=None, src_loc_at=0):
+        super().__init__(src_loc_at=src_loc_at)
+
         if name is None:
             name = tracer.get_var_name(depth=2 + src_loc_at, default=None)
 
@@ -165,8 +167,8 @@ class Record(Value):
         else:
             return super().__getitem__(item)
 
-    def shape(self):
-        return Shape(sum(len(f) for f in self.fields.values()))
+    def lower(self):
+        return Cat(self.fields.values())
 
     def _lhs_signals(self):
         return union((f._lhs_signals() for f in self.fields.values()), start=SignalSet())

nmigen/hdl/xfrm.py

@@ -38,10 +38,6 @@ class ValueVisitor(metaclass=ABCMeta):
     def on_Signal(self, value):
         pass # :nocov:
 
-    @abstractmethod
-    def on_Record(self, value):
-        pass # :nocov:
-
     @abstractmethod
     def on_ClockSignal(self, value):
         pass # :nocov:
@@ -98,9 +94,6 @@ class ValueVisitor(metaclass=ABCMeta):
         elif isinstance(value, Signal):
             # Uses `isinstance()` and not `type() is` because nmigen.compat requires it.
             new_value = self.on_Signal(value)
-        elif isinstance(value, Record):
-            # Uses `isinstance()` and not `type() is` to allow inheriting from Record.
-            new_value = self.on_Record(value)
         elif type(value) is ClockSignal:
             new_value = self.on_ClockSignal(value)
         elif type(value) is ResetSignal:
@@ -147,9 +140,6 @@ class ValueTransformer(ValueVisitor):
     def on_Signal(self, value):
         return value
 
-    def on_Record(self, value):
-        return value
-
     def on_ClockSignal(self, value):
         return value
 
@@ -372,8 +362,6 @@ class DomainCollector(ValueVisitor, StatementVisitor):
     def on_ResetSignal(self, value):
         self._add_used_domain(value.domain)
 
-    on_Record = on_ignore
-
     def on_Operator(self, value):
         for o in value.operands:
             self.on_value(o)