Disable explain mode for DeadlineExceeded

For two reasons: our analysis is just not relevant to deadline exceeded failures, and running under a tracer adds some non-representative slowdowns.

Note that this code handles the case where we first discover an unrelated bug, start shrinking, and then find a deadline-exceeding input.

hypothesis-python/src/hypothesis/core.py

@@ -529,6 +529,7 @@ def __init__(
 
         self.files_to_propagate = set()
         self.failed_normally = False
+        self.failed_over_deadline = False
 
         self.explain_traces = defaultdict(set)
 
@@ -668,6 +669,7 @@ def _execute_once_for_engine(self, data):
             trace = frozenset()
             if (
                 self.failed_normally
+                and not self.failed_over_deadline
                 and Phase.explain in self.settings.phases
                 and sys.gettrace() is None
                 and not PYPY
@@ -736,6 +738,9 @@ def _execute_once_for_engine(self, data):
                 if trace:  # pragma: no cover
                     # Trace collection is explicitly disabled under coverage.
                     self.explain_traces[interesting_origin].add(trace)
+                if interesting_origin[0] == DeadlineExceeded:
+                    self.failed_over_deadline = True
+                    self.explain_traces.clear()
                 data.mark_interesting(interesting_origin)
 
     def run_engine(self):

hypothesis-python/tests/nocover/test_scrutineer.py

@@ -25,6 +25,10 @@
 pytestmark = pytest.mark.skipif(PYPY or sys.gettrace(), reason="See comment")
 
 BUG_MARKER = "# BUG"
+DEADLINE_PRELUDE = """
+from datetime import timedelta
+from hypothesis.errors import DeadlineExceeded
+"""
 PRELUDE = """
 from hypothesis import Phase, given, settings, strategies as st
 
@@ -70,3 +74,10 @@ def test_explanations(code, testdir):
     assert len(expected) == code.count(BUG_MARKER)
     for report in expected:
         assert report in pytest_stdout
+
+
+@pytest.mark.parametrize("code", FRAGMENTS)
+def test_no_explanations_if_deadline_exceeded(code, testdir):
+    code = code.replace("AssertionError", "DeadlineExceeded(timedelta(), timedelta())")
+    pytest_stdout, _ = get_reports(DEADLINE_PRELUDE + PRELUDE + code, testdir=testdir)
+    assert "Explanation:" not in pytest_stdout