hdl.ast: recognize a Enum used as decoder and format it better.

nmigen/test/test_hdl_ast.py

@@ -1,3 +1,5 @@
+from enum import Enum
+
 from ..hdl.ast import *
 from .tools import *
 
@@ -486,6 +488,14 @@ class SignalTestCase(FHDLTestCase):
         s8 = Signal.like(s1, name_suffix="_ff")
         self.assertEqual(s8.name, "s1_ff")
 
+    def test_decoder(self):
+        class Color(Enum):
+            RED  = 1
+            BLUE = 2
+        s = Signal(decoder=Color)
+        self.assertEqual(s.decoder(1), "RED/1")
+        self.assertEqual(s.decoder(3), "3")
+
 
 class ClockSignalTestCase(FHDLTestCase):
     def test_domain(self):