do bounds

This is roughly what is described in the RFC.

TL;DR

• You can think of the do keyword as saying "plus some other effects"
• If you write do fn, then, it is "default + some more"
• If you write const do

Design axioms

• We will want a "lower floor" than const
  • ...and therefore we have people write const do fn

Default trait and impl

Equivalent of default trait and impl:

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

struct Wrapper<T> {
    value: T
}

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

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