#[ffi_returns_twice] was used on something other than a foreign function
declaration.
Erroneous code example:
#![feature(ffi_returns_twice)]
#![crate_type = "lib"]
#[ffi_returns_twice] // error!
pub fn foo() {}#[ffi_returns_twice] can only be used on foreign function declarations.
For example, we might correct the previous example by declaring
the function inside of an extern block.
#![feature(ffi_returns_twice)]
extern "C" {
#[ffi_returns_twice] // ok!
pub fn foo();
}