view README.rst @ 44207:e960c30d7e50

rust-cpython: mark all PyLeaked methods as unsafe

Unfortunately, these methods can be abused to obtain the inner 'static
reference. The simplest (pseudo-code) example is:

  let leaked: PyLeaked<&'static _> = shared.leak_immutable();
  let static_ref: &'static _ = &*leaked.try_borrow(py)?;
      // PyLeakedRef::deref() tries to bound the lifetime to itself, but
      // the underlying data is a &'static reference, so the returned
      // reference can be &'static.

This problem can be easily fixed by coercing the lifetime, but there are
many other ways to achieve that, and there wouldn't be a generic solution:

  let leaked: PyLeaked<&'static [_]> = shared.leak_immutable();
  let leaked_iter: PyLeaked<slice::Iter<'static, _>>
      = unsafe { leaked.map(|v| v.iter()) };
  let static_slice: &'static [_] = leaked_iter.try_borrow(py)?.as_slice();

So basically I failed to design the safe borrowing interface. Maybe we'll
instead have to add much more restricted interface on top of the unsafe
PyLeaked methods? For instance, Iterator::next() could be implemented if
its Item type is not &'a (where 'a may be cheated.)

Anyway, this seems not an easy issue, so it's probably better to leave the
current interface as unsafe, and get broader comments while upstreaming this
feature.

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