Purpose

This crate is another attempt at the ghost-cell / qcell saga of cell crates. This provides an alternative to std::cell::RefCell that can allow interior mutability checked at compile time, rather than runtime. Because Rust doesn't allow for unlimited creation of invariant generics, this always comes with a rather large complexity cost. Whereas ghost-cell uses invariant lifetimes and qcell can use either invariant lifetimes or newtypes, this crate instead uses const generic usizes.

Pros

As with other *cell crates, this model provides interior mutability checked at compile time. Unlike ghost-cell's model, this crate doesn't require all of your borrows to exist in a closure and, unlike qcell::TCell, this crate allows for more than three simultaneous borrows.

Cons

First, any item that contains a Cell or Token must be generic over const ID: usize. You may choose to get rid of this if you are sure that, for example, a certain instance of a struct will always have ID = 3

Second, in order to provide a safe API, Tokens must be built using a TokenBuilder struct that ensures IDs are unique. There may in the future be a way around this, but don't hold your breath!

Third, the use of explicit usize discriminants makes passing a Cell or Token to outside crates inherently unsafe. It is recommended to instead send raw values, e.g., with Cell::into_inner().

Example

use frankencell::*;
let (token1, next) = first().unwrap().token();
let (token2, _) = next.token();

let a = Cell::new('a');
let b = Cell::new('b');

println!("{}", a.borrow(&token1));
println!("{}", b.borrow(&token2));

// The following fails to compile:
println!("{}", a.borrow(&token2));
println!("{}", b.borrow(&token1));

Future improvements

Currently because of how const works, it is impossible for a const fn to return different values on different calls. In order to generate unique IDs however, the following would have to be possible:

const fn inc() -> usize {
    // Insert magic here
}

#[test]
fn test_inc() {
    assert_eq!(inc(), 0);
    assert_eq!(inc(), 1);
    assert_eq!(inc(), 2);

    // user-facing API is now significantly better
    let token_3: Token<3> = Token::next();
    let token_4: Token<4> = Token::next();
}

This may become possible when/if heap allocations are allowed in const contexts, but even then this pattern will likely never be officially endorsed by the Rust compiler.

It may also be possible with macros when/if macros are allowed to keep a local state (rust-lang/rust issue 44034).

Should I use this?

Probably not. At the moment this is really more of a proof-of-concept. There's still a lot of work that needs to go into the compiler and, even then, this may not be a viable solution.

If you're simply looking for something that's more ergonomic than ghost-cell and qcell, the cell-family crate seems to have a good approach. cell-family` crate seems to have a good approach.