Trait bound for ZST (zero sized types)

help
1

Do we have any means to detect ZST at the current moment or in future(RFC)?

It is interesting topic once trait specialization will be in place, but as of now I do not see any easy way to do it. We cannot really trait bound on something like mem::size_of::<T> == 0 I think?

2

Here's a way with const generics. (Might be possible with just one trait, I didn't play with it too long.)

#![feature(const_generics)]
#![feature(const_evaluatable_checked)]

pub enum HasSize<const SIZE: usize> {}
pub trait IsZeroSized {}
impl IsZeroSized for HasSize<0> {}

pub trait ZeroSized {}
impl<T: Sized> ZeroSized for T where HasSize<{ std::mem::size_of::<T>() }>: IsZeroSized {}

fn check<T: ZeroSized>(_: T) {
    println!("Emptiness is loneliness")
}

fn main() {
    check(());
    // mismatched types: expected `4_usize`, found `0_usize`
    // check(1);
}
3

And a less comprehensible way that only uses bounds, no helper traits or types.

#![feature(const_generics)]
#![feature(const_evaluatable_checked)]

fn check<T>(_: T)
where
    [(); 1 / ((std::mem::size_of::<T>() == 0) as usize)]: ,
{
    println!("Emptiness is loneliness")
}

fn main() {
    check(());
    // evaluation of `check::<i32>::{constant#0}` failed:
    //    attempt to divide `1_usize` by zero
    //check(1);
}
4

Thanks.

I'll subscribe to tracking issue for const_evaluatable_checked, hopefully it will work with specialization

5

:grinning_face_with_smiling_eyes:

but is it? Let's not mix the inhabited ZSTs, which are lonely, with the uninhabited ones, which are empty :upside_down_face:

6

It does work with specialization Rust Playground

7

Yeah yeah I know.
But it is still nightly feature and I'm not going to use it until stabilization, which may change things

8

const_evaluatable_unchecked is also unstable.