Error code E0755

The ffi_pure attribute was used on a non-foreign function.

Erroneous code example:

#![feature(ffi_pure)]

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

The ffi_pure attribute can only be used on foreign functions which do not have side effects or infinite loops:

#![feature(ffi_pure)]

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

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