Torrencem/fixed_vec

fixed_vec

READ THIS:

Unfortunately (and somewhat fortunately), this crate was found to be unsound in a fundamental way. From this reddit post:

fn unsound_fixedvec_example() {
    let mut v = vec![0];
    let mut idx_opt = None;
    
    loop {
        let v = std::mem::take(&mut v);
        let v = name!(v);
        let v = FixedVec::fix(v);
        if let Some(bad_idx) = idx_opt {
            println!("Bad: {}", v.get(bad_idx));
        } else {
            idx_opt = Some(v.check_index(0).unwrap());
        }
    }
}

fn unsound_fixedvec_example2(opt: Option<&dyn std::any::Any>) {
    let v = if opt.is_some() { Vec::new() } else { vec![666] };
    let v = name!(v);
    let v = FixedVec::fix(v);

    if let Some(idx_any) = opt {
        if let Some(bad_idx) = idx_any.downcast_ref() {
            println!("Bad: {}", v.get(*bad_idx));
        }
    } else {
        let idx = v.check_index(0).unwrap();
        unsound_fixedvec_example2(Some(&idx));
    }
}

both segfault with only safe rust :(. It was still a fun idea though! The problem is that the type of Name is only unique between macro invocations, and not between values. Dang.

Bounds check indices only once, instead of over and over if the indices will be re-used:

use fixed_vec::{name, FixedVec};

let v = vec![0u32; 10];
let v = name!(v);
let mut v = FixedVec::fix(v);

// Perform the two index checks here:
let index_a = v.check_index(...).unwrap();
let index_b = v.check_index(...).unwrap();

for _ in 0..100 {
    // These do *not* perform bounds checks!
    // At compile time, v and index_a must match
    *v.get_mut(index_a) += 5;
    *v.get_mut(index_b) += 10;
}

let v = v.unfix();

// continue using v...