E0806 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0806

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() {}