Const Guards [docs.rs]

With const_guards you can express certain compile time constraints on rust's const_generics using the unstable generic_const_exprs feature.

Documentation

For documentation visit docs.rs.

Motivation

Consider the following usage of the first method on slices from the standard library:

let slice: [(); 1] = [(); 1];
let head: Option<&()> = slice.first();

Would it be nice if we could just write

let head: &() = slice.first();

since the compiler should know this slice has length 1 at this point.
With const guards we can express such as follows:

#[guard(N > 0)]
fn first<'a, T, const N: usize>(slice: &'a [T; N]) -> &'a T {
    &slice[0]
}

The index call on the slice &slice[0] cannot possible fail because we enforced the length of the slice to be > 0 at compile time. We could now call it as follows

let slice: [(); 1] = [(); 1];
let head: &() = first(&slice);

while the case where the slice is actually empty would fail to compile:

let slice: [(); 0] = [(); 0];
let head: &() = first(&slice);

Finally we could even express this as a trait to make it more accessable:

trait SliceHead<'a, T, const N: usize> {
    #[guard(<const N: usize> { N > 0 })]
    fn head(&self) -> &'a T;
}

impl<'a, T, const N: usize> SliceHead<'a, T, N> for &'a [T; N] {
    fn head(&self) -> &'a T {
        &self[0]
    }
}

fn main() {
    let slice: &[(); 1] = &[(); 1];
    let head: &() = slice.head();
}

Though, as you can see, we need to introduce generics not introduced by the guarded item explicitly.

Implementation

Consider this simple example of a const guard:

fn main() {
    f::<0>()
}

#[guard(N > 0)]
fn f<const N: usize>() {
    todo!()
}

and have a look at the expanded form:

struct Guard<const U: ()>;

trait Protect {}
impl Protect for Guard<true> {}

fn main() {
    f::<0>()
}

fn f<const N: usize>()
where
    Guard<{
        const fn _f_guard<const N: usize>() -> bool {
            if !N > 0 {
                panic!("guard evaluated to false")
            }
            true
        }
        _f_guard::<N>()
    }>: Protect,
{
    todo!()
}