Understanding Rust Error E0724

Rust Error E0724

Rust Error E0724 occurs when the #[ffi_returns_twice] attribute is used on something other than a foreign function declaration.

Erroneous Code Example:

#![allow(unused)]
#![feature(ffi_returns_twice)]
#![crate_type = "lib"]

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

In this code example, the #[ffi_returns_twice] attribute is incorrectly applied to the function foo(), which is not a foreign function declaration. This results in Error E0724.

Correcting the Error:

To resolve Rust Error E0724, the #[ffi_returns_twice] attribute should only be used on foreign function declarations. This can be achieved by declaring the function inside an extern block.

Corrected Code Example:

#![allow(unused)]
#![feature(ffi_returns_twice)]

fn main() {
  extern "C" {
    #[ffi_returns_twice] // ok!
    pub fn foo();
  }
}

In the corrected code example, the #[ffi_returns_twice] attribute is appropriately applied to the foo() function within the extern block, avoiding Error E0724.

Importance of #[ffi_returns_twice]: