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.