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As per the comment on #797 we should probably remove Python::assume_gil_acquired(), because it can give you a Python<'static> thanks to the lifetime inference. This is almost certainly unsound.
I think it probably explains this crash #653
See this playground for a minimal example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=213173005786e15199cf0312079c3d72
A function assume_gil_acquired() would still be useful, but I think it needs to return a type without a lifetime, so that there is no risk of incorrect inference. For example, something like this:
struct AssumeGILAcquired(Unsendable);
fn assume_gil_acquired() -> AssumeGilAcquired { ... }
impl AssumeGILAcquired {
fn py(&self) -> Python { ... }
}
/// later on:
let gil = assume_gil_acquired();
let py = gil.py();
In the above example, the Python lifetime is tied exactly to the lifetime of the AssumeGILAcquired token, so we can avoid 'static lifetime problems.