allow calling unsafe function and ffi function safely:)

[Neutron3529]

2 macros are provided to ensure unsafe rust program will be called safely.

One of them is ffi, which allow calling ffi in safe rust:

// create safe printf function
ffi!{
    extern "C" fn printf(fmt: *const i8)->i32
}
fn main(){
    printf(c"Calling FFI in a safe way:)\n".as_ptr())
}

Another is a safe macro, which converts the unsafe function into the safe version. But sadly, you have to provide the signature about the old unsafe function (to ensure some safetyness..)

use r#unsafe::r#unsafe;
safe!{
    fn r#unsafe()->i32 := very_safe
}
fn main(){
    // then you can use `very_safe()` to call `r#unsafe::r#unsafe` safely:)
}

[workingjubilee]

Needs a test which demonstrates that this works even if you deny(unsafe_code) in the crate it is expanded in (otherwise, it's not actually making the call safe!)

[workingjubilee]

Oh it does have one, neat.