# Error code E0756

The `ffi_const` attribute was used on something other than a foreign function declaration.

Erroneous code example:

    #![feature(ffi_const)]

    #[ffi_const] // error!
    pub fn foo() {}
    fn main() {}

The `ffi_const` attribute can only be used on foreign function declarations which have no side effects except for their return value:

    #![feature(ffi_const)]

    extern "C" {
        #[ffi_const] // ok!
        pub fn strlen(s: *const i8) -> i32;
    }
    fn main() {}

You can get more information about it in the unstable Rust Book.