How can I cast `&self` to a trait object in default method?

[stebalien]

They aren't special. That one works because () is sized. See my thread here:

DST Receivers/Idempotent unsize

When trying to make something object safe, I ran into the following problem: trait Trait {} struct View<'a>(&'a mut Wrapper<Trait + 'a>); struct Wrapper<A: ?Sized + Trait>(A); impl<T: ?Sized + Trait> Wrapper<T> { // I want this to work on both `Wrapper<SomeType>` and `Wrapper<Trait>`. fn view<'a>(&'a mut self) -> View<'a> { // FIXME: Not sized so we can't do this. View(self as &mut Wrapper<Trait + 'a>) } } fn main() {} However, I fell there should be some way to…

And a related RFC Issue:

Unsized receivers

With the unsize feature, you could write:

#![feature(unsize)]
use std::marker::Unsize;

trait T {
    fn foo<'a>(&'a self) where Self: 'a + Unsize<T + 'a> {
        bar(self)
    }
}

fn bar(x: &T) {}

fn baz(x: Box<T>) {
    x.foo()
}

fn biz<V: T>(x: V) {
    x.foo()
}

But it's rather messy, unstable, and has some weird edge cases.