From d410f0dcfe976216f093535e294d1eab7c8af7a8 Mon Sep 17 00:00:00 2001
From: whitequark <cz@m-labs.hk>
Date: Sat, 29 Dec 2018 15:02:04 +0000
Subject: [PATCH] back.pysim: warn if simulation is not run.

This would have prevented 3ea35b85.
---
 nmigen/back/pysim.py    | 10 ++++++++++
 nmigen/test/test_sim.py |  6 ++++++
 2 files changed, 16 insertions(+)

diff --git a/nmigen/back/pysim.py b/nmigen/back/pysim.py
index 7a5639c..0bcb0b3 100644
--- a/nmigen/back/pysim.py
+++ b/nmigen/back/pysim.py
@@ -1,5 +1,6 @@
 import math
 import inspect
+import warnings
 from contextlib import contextmanager
 from bitarray import bitarray
 from vcd import VCDWriter
@@ -365,6 +366,8 @@ class Simulator:
         self._gtkw_file       = gtkw_file
         self._traces          = traces
 
+        self._run_called      = False
+
         while not isinstance(self._fragment, Fragment):
             self._fragment = self._fragment.get_fragment(platform=None)
 
@@ -755,10 +758,14 @@ class Simulator:
         return False
 
     def run(self):
+        self._run_called = True
+
         while self.step():
             pass
 
     def run_until(self, deadline, run_passive=False):
+        self._run_called = True
+
         while self._timestamp < deadline:
             if not self.step(run_passive):
                 return False
@@ -766,6 +773,9 @@ class Simulator:
         return True
 
     def __exit__(self, *args):
+        if not self._run_called:
+            warnings.warn("Simulation created, but not run", UserWarning)
+
         if self._vcd_writer:
             vcd_timestamp = (self._timestamp + self._delta) / self._epsilon
             self._vcd_writer.close(vcd_timestamp)
diff --git a/nmigen/test/test_sim.py b/nmigen/test/test_sim.py
index cdd83d7..5b77222 100644
--- a/nmigen/test/test_sim.py
+++ b/nmigen/test/test_sim.py
@@ -530,3 +530,9 @@ class SimulatorIntegrationTestCase(FHDLTestCase):
                 self.assertEqual((yield self.rdport.data), 0x33)
             sim.add_clock(1e-6)
             sim.add_process(process)
+
+    def test_wrong_not_run(self):
+        with self.assertWarns(UserWarning,
+                msg="Simulation created, but not run"):
+            with Simulator(Fragment()) as sim:
+                pass
-- 
2.30.2