hdl.rec: use a helpful error on unknown field reference.

nmigen/hdl/rec.py

@@ -87,10 +87,14 @@ class Record(Value):
                 self.fields[field_name] = Signal(field_shape, name=concat(name, field_name))
 
     def __getattr__(self, name):
-        return self.fields[name]
+        return self[name]
 
     def __getitem__(self, name):
+        try:
             return self.fields[name]
+        except KeyError:
+            raise NameError("Record does not have a field '{}'. Did you mean one of: {}?"
+                            .format(name, ", ".join(self.fields))) from None
 
     def shape(self):
         return sum(len(f) for f in self.fields.values()), False

nmigen/test/test_hdl_rec.py

@@ -78,3 +78,12 @@ class RecordTestCase(FHDLTestCase):
 
         self.assertEqual(repr(r), "(rec <unnamed> stb)")
         self.assertEqual(r.stb.name, "stb")
+
+    def test_wrong_field(self):
+        r = Record([
+            ("stb", 1),
+            ("ack", 1),
+        ])
+        with self.assertRaises(NameError,
+                msg="Record does not have a field 'en'. Did you mean one of: stb, ack?"):
+            r.en