Code shared from the Rust Playground

playground.rs

pub mod zero_sized {
    use std::marker::PhantomData;
    
    pub struct ZeroSizeProof<T>(PhantomData<T>, ());
    
    pub trait ZeroSized: Sized {
        #[deny(const_err)]
        const I_AM_ZERO_SIZED: ();
        fn proof_zero_size(&self) -> ZeroSizeProof<Self>;
    }

    impl<T: Sized> ZeroSized for T {
        const I_AM_ZERO_SIZED: ()  = [()][std::mem::size_of::<Self>()];
        fn proof_zero_size(&self) -> ZeroSizeProof<Self> {
            ZeroSizeProof(PhantomData, Self::I_AM_ZERO_SIZED)
        }
    }
}
        
pub mod foo {
    use crate::zero_sized::*;

    pub trait Foo: ZeroSized {
        fn requires_zero_size(self, _proof: ZeroSizeProof<Self>) {
            println!("requires_zero_size called");
        }
    }
    
    pub trait FooExt: Foo {
        fn requires_zero_size(self);
    }
    
    impl<T: Foo> FooExt for T {
        fn requires_zero_size(self) {
            let proof = self.proof_zero_size();
            <Self as Foo>::requires_zero_size(self, proof)
        }
    }
}

use crate::zero_sized::ZeroSizeProof;

impl foo::Foo for () {}
impl foo::Foo for u32 {
    fn requires_zero_size(self, _proof: ZeroSizeProof<Self>) {
        println!("requires_zero_size was called, but Self is not zero-sized, bwa-ha-ha-ha!");
    }
}

fn main() {
    use foo::FooExt;
    ().requires_zero_size();
    42_u32.requires_zero_size();
}