let mut state = 1;
let mut fat_closure = || state += 1;
let (fnptr, userdata) = make_trampoline(&mut &mut fat_closure);
unsafe {
fnptr(userdata);
}
assert_eq!(state, 2);
use std::ffi::c_void;
fn make_trampoline<C: FnMut()>(closure: &mut &mut C) -> (unsafe fn(*mut c_void), *mut c_void) {
let fnptr = |userdata: *mut c_void| {
let closure: *mut &mut C = userdata.cast();
(unsafe { &mut *closure })()
};
(fnptr, closure as *mut _ as *mut c_void)
}
This isn’t fully accurate. In your example, `&mut C` actually has the same layout as usize. It’s not a fat pointer. `C` is a concrete type and essentially just an anonymous struct with FnMut implemented for it.
You’re probably thinking of `&mut dyn FnMut` which is a fat pointer that pairs a pointer to the data with a pointer to a VTable.
So in your specific example, the double indirection is unnecessary.
The following passes miri: https://play.rust-lang.org/?version=nightly&mode=debug&editi...
(did this on mobile, so please excuse any messiness).
Well, capturing closures that are implemented like C++ lambdas or Rust closures anyway. The executable stack crimes do make a thin fn-ptr with state.
Unfortunately a lot of existing C APIs won't have the user arg in the place you need it, it's a mix of first, last, and sometimes even middle.
The only unsafe here is to demonstrate it works with C/C++ FFI (where void* userdata is actually not type safe)
I have a case where I need to create a static templated lambda to be passed to C as a pointer. Such thing is impossible in Rust, which I considered at first.