Error code E0757

A function was given both the ffi_const and ffi_pure attributes.

Erroneous code example:

#![allow(unused)] #![feature(ffi_const, ffi_pure)] fn main() { extern "C" { #[ffi_const] #[ffi_pure] // error: `#[ffi_const]` function cannot be `#[ffi_pure]` pub fn square(num: i32) -> i32; } }

As ffi_const provides stronger guarantees than ffi_pure, remove the ffi_pure attribute:

#![allow(unused)] #![feature(ffi_const)] fn main() { extern "C" { #[ffi_const] pub fn square(num: i32) -> i32; } }

You can get more information about const and pure in the GCC documentation on Common Function Attributes. The unstable Rust Book has more information about ffi_const and ffi_pure.