Hacker News Reader

Comment by jrvidal | Hacker News Reader

> The nice thing is that RefCell is not magic

I'd like to point out that `RefCell` does contain a bit of magic, since it is based on `UnsafeCell`, which is _the_ core "primitive" of Rust that enables interior mutability:

https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html

(Not trying to be pedantic, just to clarify things for less knowledgeable readers).

View on HN

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.

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...

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.

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

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

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

Sorry, I should have been clearer about that. You are completely right that at some point things have to rely on some primitives that are 'magic' (in this case UnsafeCell). But I meant that RefCell itself uses standard constructs to guarantee the 'single mutable xor multiple immutable' invariant.

Reply on news.ycombinator.com