debug: improve the assert firing in #1896

Author: nedbat

coverage/lcovreport.py

@@ -121,7 +121,9 @@ def lcov_arcs(
             # this probably means 'line' was never executed at all.
             # Cross-check with the line stats.
             assert len(executed_arcs[line]) == 0
-            assert line in analysis.missing
+            assert line in analysis.missing, (
+                f"In {fr.filename}: {line=} should be in analysis.missing={sorted(analysis.missing)}"
+            )
             destinations = [
                 (dst, "-") for dst in missing_arcs[line]
             ]