Recently, @withoutboats challenged me to demonstrate a hypothetical version of Pin with different guarantees, and show how it would be sound.  However, while working on that, I stumbled upon unsoundness in the actual Pin.  I haven't seen this reported before, though I could have missed it.  Playground link: proof of concept (methods 1 and 2)  This code demonstrates two related exploits to obtain a Pin<&mut T> from &mut T, ultimately resulting in a segfault.  It compiles on stable and has no unsa...

Self-referential generators violate LLVM's expectations for noalias due to the overlapping bounds of the interior references and the &mut self argument to the Generator::resume func...