An externally implementable item is not compatible with its declaration.

Erroneous code example:

#![feature(extern_item_impls)]

#[eii(foo)]
fn x();

#[foo]
fn y(a: u64) -> u64 {
//~^ ERROR E0806
    a
}


fn main() {}

The error here is caused by the fact that y implements the externally implementable item foo. It can only do so if the signature of the implementation y matches that of the declaration of foo. So, to fix this, y's signature must be changed to match that of x:

#![feature(extern_item_impls)]

#[eii(foo)]
fn x();

#[foo]
fn y() {}


fn main() {}

One common way this can be triggered is by using the wrong signature for #[panic_handler]. The signature is provided by core.

#![no_std]

#[panic_handler]
fn on_panic() -> ! {
//~^ ERROR E0806

    loop {}
}

fn main() {}

Should be:

#![no_std]

#[panic_handler]
fn on_panic(info: &core::panic::PanicInfo<'_>) -> ! {
    loop {}
}

fn main() {}