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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" { #[unsafe(ffi_const)] #[unsafe(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" { #[unsafe(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.