const and const?

Summary

• Declare the "floor" on the item
  • const
  • do -- declares default effects
• For bounds, declare "maybe" bounds with a ?, e.g., const? or do?
• In a trait method, declare "maybe" items with a ? to tie them to the trait mode

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,
{
    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,
    }
}
}

Observations

• Pretty confusing that you write const? fn in the trait definition but const fn outside of it