Auto-implementation of ToPyObject allows PyObject to outlive object

[athre0z]

🌍 Environment

• Your operating system and version: macOS 14.2
• Your python version: Python 3.6
• How did you install python (e.g. apt or pyenv)? Did you use a virtualenv?: macports, yes
• Your rust version (rustc --version): rustc 1.32.0-nightly (14997d56a 2018-12-05)
• Are you using the latest pyo3 version? -- Yes, I'm tested this on master

💥 Reproducing

I PRed two tests that reliably reproduce the issue in #307.

[athre0z]

It doesn't appear to be specific to IntoPyTuple, it also crashes when constructing the tuple from an iterator (I added two more tests to the PR).

Edit: Alright, turns out that I just did the object allocation wrong. For code that still causes a crash, please check this commit. Nevertheless, it should not be possible to cause a SIGSEGV without invoking unsafe code. I suspect the issue essentially runs down to any pyclass auto-implementing ToPyObject with code that transforms a non-mut reference into a PyObject, allowing the resulting object to live longer than the original reference.