Explain safety of unsync::OnceCell::get(&self) in more detail

[Enselic]

Background: I'm working on improving my unsafe Rust skills, and figuring out how once_cell works is one way to do that. By more elaborately explaining why unsync::OnceCell::get(&self) is safely implemented, I hope to help others increase their understanding of unsafe Rust as well.

(It was not obvious to me at first why unsync::OnceCell::get(&self) was safe. But I think I figured it out now.)

[matklad]

bors r+

Thanks!

[bors]

Build succeeded!

And happy new year! 🎉

• Rust