Heuristic Sampling Improvements

src/scenic/domains/driving/roads.py

@@ -57,7 +57,7 @@ def _toVector(thing: Vectorlike) -> Vector:
 
 
 def _rejectSample(message):
-    if veneer.isActive():
+    if not veneer.allowSampleRejection():
         raise InvalidScenarioError(message)
     else:
         raise RejectionException(message)

src/scenic/syntax/veneer.py

@@ -299,6 +299,7 @@
 ### Internals
 
 activity = 0
+heuristicSampling = 0
 currentScenario = None
 scenarioStack = []
 scenarios = []
@@ -331,6 +332,25 @@ def isActive():
     return activity > 0
 
 
+def allowSampleRejection():
+    """Should a RejectionException() be allowed
+
+    Returning True means a RejectionException is valid. Returning False indicates
+    a RejectionException() means the scenario is invalid.
+    """
+    return isActive() or heuristicSampling
+
+
+@contextmanager
+def enableHeuristicSampling():
+    global heuristicSampling
+    heuristicSampling += 1
+    try:
+        yield
+    finally:
+        heuristicSampling -= 1
+
+
 def activate(options, namespace=None):
     """Activate the veneer when beginning to compile a Scenic module."""
     global activity, _globalParameters, lockedParameters, lockedModel, currentScenario
@@ -1517,7 +1537,8 @@ def alwaysProvidesOrientation(region):
         return True
     else:  # TODO improve somehow!
         try:
-            sample = region.sample()
+            with enableHeuristicSampling():
+                sample = region.sample()
             return sample.orientation is not None or sample is nowhere
         except RejectionException:
             return False