[Python-Dev] No longer enable Py_TRACE_REFS by default in debug build

[Michael Sullivan]

> The main question is if anyone ever used Py_TRACE_REFS? Does someone
> use sys.getobjects() or PYTHONDUMPREFS environment variable?

I used sys.getobjects() today to track down a memory leak in the
mypyc-compiled version of mypy.

We were leaking memory badly but no sign of the leak was showing up in
mypy's gc.get_objects() based profiler. Using a debug build and switching
to sys.getobjects() showed that we were badly leaking int objects. A quick
inspection of the values in question (large and random looking) suggested
we were leaking hash values, and that quickly pointed me to
https://github.com/mypyc/mypyc/pull/562.

I don't have any strong feelings about whether to keep it in the "default"
debug build, though. I was using a debug build that I built myself with
every debug feature that seemed potentially useful.

-sully
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