hdl.{ast,cd,dsl,xfrm}: reject inappropriately used comb domain.

Fixes #125.

nmigen/test/test_hdl_ast.py

@@ -515,6 +515,11 @@ class ClockSignalTestCase(FHDLTestCase):
         s1 = ClockSignal()
         self.assertEqual(repr(s1), "(clk sync)")
 
+    def test_wrong_name_comb(self):
+        with self.assertRaises(ValueError,
+                msg="Domain 'comb' does not have a clock"):
+            ClockSignal("comb")
+
 
 class ResetSignalTestCase(FHDLTestCase):
     def test_domain(self):
@@ -534,6 +539,11 @@ class ResetSignalTestCase(FHDLTestCase):
         s1 = ResetSignal()
         self.assertEqual(repr(s1), "(rst sync)")
 
+    def test_wrong_name_comb(self):
+        with self.assertRaises(ValueError,
+                msg="Domain 'comb' does not have a reset"):
+            ResetSignal("comb")
+
 
 class MockUserValue(UserValue):
     def __init__(self, lowered):

nmigen/test/test_hdl_cd.py

@@ -55,3 +55,8 @@ class ClockDomainTestCase(FHDLTestCase):
         sync.rename("pix")
         self.assertEqual(sync.name, "pix")
         self.assertEqual(sync.clk.name, "pix_clk")
+
+    def test_wrong_name_comb(self):
+        with self.assertRaises(ValueError,
+                msg="Domain 'comb' may not be clocked"):
+            comb = ClockDomain()

nmigen/test/test_hdl_dsl.py

@@ -458,6 +458,13 @@ class DSLTestCase(FHDLTestCase):
         ()
         """)
 
+    def test_FSM_wrong_domain(self):
+        m = Module()
+        with self.assertRaises(ValueError,
+                msg="FSM may not be driven by the 'comb' domain"):
+            with m.FSM(domain="comb"):
+                pass
+
     def test_FSM_wrong_redefined(self):
         m = Module()
         with m.FSM():

nmigen/test/test_hdl_xfrm.py

@@ -88,6 +88,16 @@ class DomainRenamerTestCase(FHDLTestCase):
             "pix": cd_pix,
         })
 
+    def test_rename_wrong_to_comb(self):
+        with self.assertRaises(ValueError,
+                msg="Domain 'sync' may not be renamed to 'comb'"):
+            DomainRenamer("comb")
+
+    def test_rename_wrong_from_comb(self):
+        with self.assertRaises(ValueError,
+                msg="Domain 'comb' may not be renamed"):
+            DomainRenamer({"comb": "sync"})
+
 
 class DomainLowererTestCase(FHDLTestCase):
     def setUp(self):