const+ everywhere

TL;DR

• Write const to mean "can only do const things"
• Write const+ to mean "can only directly do const things, but may do non-const things via trait bounds"

Design axioms

• We will want a lower floor than const eventually
  • What we've learned from the ?Sized experience is that it's best to have people "ask for the things they need".
• You should be able to declare a maybe-const function without understanding effects.
  • Therefore, we prefer a const-keyword-based syntax.

Default trait and impl

Equivalent of default trait and impl:

#![allow(unused)]
fn main() {
const+ trait Default {
    const+ fn default() -> Self;
}

struct Wrapper<T> {
    value: T
}

impl<T> const+ Default for Wrapper<T>
where
    T: const+ Default,
{
    const+ fn default() -> Self {
        Wrapper {
            value: T::default()
        }
    }
}

const+ fn get_me<T>(x: Option<Wrapper<T>>) -> Wrapper<T>
where
    T: const+ Default,
{
    match x {
        None => <Wrapper<T>>::default(), // <-- do we need something here?
        Some(v) => v,
    }
}
}