Call a C callback from a Send closure - rust

Question

In Rust I have a callback defined as Box<dyn Fn(MyType) + Send>. I want to be able to convert a C callback provided by FFI into that signature.
To do that, I created a Rust closure that calls the C callback with a context pointer:
let rust_closure = move |data: u32| {
callback(obj, data);
};
The problem is obj in this case is a c_void. According to the docs (https://doc.rust-lang.org/core/ffi/enum.c_void.html) it should have auto-implemented Send and Sync. However the compiler tells me:
the trait Send is not implemented for *const c_void
How can I convince Rust that my c_void is indeed Send? I've tried wrapping it in a unit struct and impl'ing Send for the struct, but that did not change the error.
I've created a minimal example in the playground.
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=5737c7449b925ab83e35d2404a6bd9f8

Answer 1

Pointers are automatically !Send so the solution is to wrap the pointer in a custom type, that hopefully better describes its use and reasons to guarantee that the pointer is safe to use across threads. I'm assuming you made a solution similar to this:
struct UserObj(*const c_void);
// SAFETY: Any user data object must be safe to send between threads.
unsafe impl Send for UserObj {}
let obj = UserObj(obj);
let rust_closure = move |data: u32| {
callback(obj.0, data);
};
ch.add_callback(Box::new(rust_closure));
Which unfortunately, does yield the same error:
error[E0277]: `*const c_void` cannot be sent between threads safely
--> src/lib.rs:33:25
|
29 | let rust_closure = move |data: u32| {
| ____________________________-
30 | | callback(obj.0, data);
31 | | };
| |_________- within this `[closure#src/lib.rs:29:28: 31:10]`
32 |
33 | ch.add_callback(Box::new(rust_closure));
| ^^^^^^^^^^^^^^^^^^^^^^ `*const c_void` cannot be sent between threads safely
|
= help: within `[closure#src/lib.rs:29:28: 31:10]`, the trait `Send` is not implemented for `*const c_void`
= note: required because it appears within the type `[closure#src/lib.rs:29:28: 31:10]`
= note: required for the cast to the object type `dyn Fn(u32) + Send`
But why? We're moving a UserObj into the closure and we've asserted to the compiler that it is Send. Why does it still complain about *const c_void?
As of Rust 2021 edition, closures can capture disjoint fields rather than the whole object, meaning the above closure would capture obj.0 instead of obj itself, causing the error. My suggestion would be to add a method to get the pointer instead of using field access in this case:
struct UserObj(*const c_void);
impl UserObj {
fn as_ptr(&self) -> *const c_void {
self.0
}
}
// SAFETY: Any user data object must be safe to send between threads.
unsafe impl Send for UserObj {}
let obj = UserObj(obj);
let rust_closure = move |data: u32| {
callback(obj.as_ptr(), data);
};
ch.add_callback(Box::new(rust_closure));
Playground