~const fns, ~const methods

TL;DR

• Traits can be declared as const, in which case all methods are "maybe-const" (depending on the impl)
• Impl can decide whether it is implementing const Trait (const or maybe-const methods)

Design axioms

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,
{
    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,
    }
}
}