Best way to keep track of state in detoured function


I'm playing with detouring functions using the detour-rs crate. I'm now thinking of a situation where I want to keep track of some state in the function I detour to. So far the only thing have found that works is using static mutable variables. I have modified a detour-rs example below to illustrate what I want to do (replace add5 with add_extra that instead adds 5 + something extra).

Another option I considered was passing state as a mutable borrow to the add_extra function. The hooked close then however becomes FnMut while the static detour initialization requires it to be Fn.

I am aware that the solution I got to work is not safe. And now I'm wondering if there is a better/safer solution, or is this the best I can achieve?

use std::error::Error;
use detour::static_detour;

static_detour! {
  static Test: /* extern "X" */ fn(i32) -> i32;

static mut EXTRA_SUM: i32 = 0;

fn add5(val: i32) -> i32 {
  val + 5

fn add_extra_static(val: i32) -> i32 {
  unsafe { EXTRA_SUM += 1} ; + unsafe { EXTRA_SUM }

fn main() -> Result<(), Box<dyn Error>> {
  let hooked = |val| add_extra_static(val);

  unsafe { Test.initialize(add5, hooked)?.enable()? };

  assert_eq!(add5(1), 7);
  assert_eq!(add5(1), 8);

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