[simias]

I assume the parent meant that there was no compiler magic at play, as far as I know UnsafeCell is written in pure Rust, just using unsafe code: https://doc.rust-lang.org/src/core/cell.rs.html#1486-1488

So basically if Rust's stdlib didn't provide it, you could reimplement it yourself from scratch.

EDIT: actually, reading the comments, it looks like I'm wrong about that:

> If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language feature to work around the restriction that `&T` may not be mutated.

[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.

[jrvidal]

I'm not sure that's correct. `UnsafeCell` is a Rust lang item that might disable some optimizations, I think?

https://doc.rust-lang.org/src/core/cell.rs.html#1483

[simias]

You're right, I just saw that. Well I least I learned something new about Rust today!

[aratakareigen]

Yep, you can't implement UnsafeCell yourself without running into undefined behavior.