PEP 479 wraps a StopIteration raised from a generator/coroutine into a
RuntimeError. CPython sets both __cause__ and __context__ of the new
RuntimeError to the original StopIteration (the same object). RustPython's
wrapping sites set __cause__ only, leaving __context__ as None.
set___cause__ already toggles __suppress_context__ to true, so the
user-facing traceback is unchanged — only assertions that inspect
__context__ directly observe the gap.
Three sites all wrap StopIteration / StopAsyncIteration into RuntimeError
for PEP 479. Set __context__ to the same exception instance before
setting __cause__ at each:
- frame.rs::StopIterationError intrinsic (generator yield-expression path)
- coroutine.rs generator __next__ StopIteration branch
- coroutine.rs async-generator __anext__ StopAsyncIteration branch
PyBaseExceptionRef is Arc-backed, so e.clone() is a refcount bump;
'cause is context' holds, matching CPython.
Unmasks:
- test_generator_stop.TestPEP479.test_stopiteration_wrapping_context
- test_yield_from.TestInterestingEdgeCases.test_close_and_throw_work
- test_yield_from.TestInterestingEdgeCases.test_close_and_throw_raise_stop_iteration
- test_yield_from.TestInterestingEdgeCases.test_close_and_throw_yield
