[Sean1708]

WRT your edit:

  #[lang = "unsafe_cell"]

This tells you that UnsafeCell is a Language Item[1], which basically means that the compiler does have knowledge of it.

[1]: https://doc.rust-lang.org/beta/unstable-book/language-featur...

[estebank]

This annotation let's the compiler treat their items in special ways, but it is usually for either easier way to refer to them (I want to desugar this type to a cell of what I already have) or for diagnostics. I think box is one of the few "magical" things in the language.