Cleanup: use associated const rather than const generic to specify Aggregator verify key length

[branlwyd]

Currently, the length of an Aggregator's verify key length is defined by the const generic L parameter on the Aggregator trait:

libprio-rs/src/vdaf.rs

Line 173 in 7b8a9bc

pub trait Aggregator<const L: usize>: Vdaf

This works fine, but requires generic code working with the Aggregator to specify L as an explicit const generic in functions that work with an Aggregator<L>; the Rust compiler is not yet clever enough to infer L.

I think we could work around this by instead specifying the verify key length as an associated constant, something like:

pub trait Aggregator: Vdaf {
  const VERIFY_KEY_LEN: usize;
}

Each implementation of the Aggregator trait would then define its verify key length. Concrete VDAF implementations which want to support multiple verify key lengths (which I think is common) can keep the concrete struct implementing Aggregator generic on L, as it already is for e.g. Prio3:

libprio-rs/src/vdaf/prio3.rs

Line 315 in 7b8a9bc

pub struct Prio3<T, P, const L: usize>

Example Rust playground showing this technique: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e4507e8cb4677ffef618304f3ec9d0e4

This suggestion would be a breaking change, so it should be implemented as part of a major version cutover.

[branlwyd]

I tried this out in 7b5ae53 and unfortunately got the following error:

error: generic parameters may not be used in const operations
   --> src/vdaf.rs:200:27
    |
200 |         verify_key: &[u8; Self::VERIFY_KEY_LEN],
    |                           ^^^^^^^^^^^^^^^^^^^^ cannot perform const operation using `Self`
    |
    = note: type parameters may not be used in const expressions

Googling finds rust-lang/rust#60551 -- it doesn't look like there's any reason this code couldn't work, the Rust maintainers simply haven't gotten around to stabilizing it yet. :-(

[branlwyd]

(closing as infeasible for now, we might try this cleanup again once the above-referenced bug is fixed)