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> 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 _th

> 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 https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html (Not trying to be pedantic, just to clarify things for less knowledgeable readers).

simias 7y ago
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 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.
- jrvidal 7y ago
  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 https://doc.rust-lang.org/src/core/cell.rs.html#1483
  - simias 7y ago
    You're right, I just saw that. Well I least I learned something new about Rust today!
  - aratakareigen 7y ago
    Yep, you can't implement UnsafeCell yourself without running into undefined behavior.
- Sean1708 7y ago
  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-features/lang-items.html https://doc.rust-lang.org/beta/unstable-book/language-featur...
  - estebank 7y ago
    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.
danieldk 7y ago
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.

simias 7y ago

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 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.

jrvidal 7y ago
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 https://doc.rust-lang.org/src/core/cell.rs.html#1483
- simias 7y ago
  You're right, I just saw that. Well I least I learned something new about Rust today!
- aratakareigen 7y ago
  Yep, you can't implement UnsafeCell yourself without running into undefined behavior.
Sean1708 7y ago
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-features/lang-items.html https://doc.rust-lang.org/beta/unstable-book/language-featur...
- estebank 7y ago
  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.

danieldk 7y ago

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.