Rust 如何通过原始指针将闭包作为参数传递给C函数

我在Rust中使用WinAPI，有些函数(比如EnumWindows())需要回调.回调通常接受一个附加参数(LPARAM类型，i64的别名)，您可以使用它向回调传递一些自定义数据.

我已经向WinAPI回调发送了Vec<T>个对象作为LPRAM，效果很好.例如，在我的例子中，将lparam值"解包"为Vec<RECT>如下所示:

unsafe extern "system" fn enumerate_callback(hwnd: HWND, lparam: LPARAM) -> BOOL {
    let rects = lparam as *mut Vec<RECT>;
}

我现在必须传递一个闭包，而不是传递一个向量.我不能使用函数指针，因为我的闭包必须捕获一些变量，如果我使用函数，这些变量将无法访问.在C++中，我将用std::function<>来完成特定任务，我认为在RIST中，相应的抽象是一个闭包.

我的解包代码如下所示:

unsafe extern "system" fn enumerate_callback(hwnd: HWND, lparam: LPARAM) -> BOOL {
    let cb: &mut FnMut(HWND) -> bool = &mut *(lparam as *mut c_void as *mut FnMut(HWND) -> bool);
    // ...
}

SSCCE:

use std::os::raw::c_void;

fn enum_wnd_proc(some_value: i32, lparam: i32) {
    let closure: &mut FnMut(i32) -> bool =
        unsafe { (&mut *(lparam as *mut c_void as *mut FnMut(i32) -> bool)) };

    println!("predicate() executed and returned: {}", closure(some_value));
}

fn main() {
    let sum = 0;
    let mut closure = |some_value: i32| -> bool {
        sum += some_value;
        sum >= 100
    };

    let lparam = (&mut closure as *mut c_void as *mut FnMut(i32) -> bool) as i32;
    enum_wnd_proc(20, lparam);
}

(Playground)

我发现了以下错误:

error[E0277]: expected a `std::ops::FnMut<(i32,)>` closure, found `std::ffi::c_void`
 --> src/main.rs:5:26
  |
5 |         unsafe { (&mut *(lparam as *mut c_void as *mut FnMut(i32) -> bool)) };
  |                          ^^^^^^^^^^^^^^^^^^^^^ expected an `FnMut<(i32,)>` closure, found `std::ffi::c_void`
  |
  = help: the trait `std::ops::FnMut<(i32,)>` is not implemented for `std::ffi::c_void`
  = note: required for the cast to the object type `dyn std::ops::FnMut(i32) -> bool`

error[E0606]: casting `&mut [closure@src/main.rs:12:23: 15:6 sum:_]` as `*mut std::ffi::c_void` is invalid
  --> src/main.rs:17:19
   |
17 |     let lparam = (&mut closure as *mut c_void as *mut FnMut(i32) -> bool) as i32;
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0606]: casting `*mut dyn std::ops::FnMut(i32) -> bool` as `i32` is invalid
  --> src/main.rs:17:18
   |
17 |     let lparam = (&mut closure as *mut c_void as *mut FnMut(i32) -> bool) as i32;
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: cast through a thin pointer first

error[E0277]: expected a `std::ops::FnMut<(i32,)>` closure, found `std::ffi::c_void`
  --> src/main.rs:17:19
   |
17 |     let lparam = (&mut closure as *mut c_void as *mut FnMut(i32) -> bool) as i32;
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an `FnMut<(i32,)>` closure, found `std::ffi::c_void`
   |
   = help: the trait `std::ops::FnMut<(i32,)>` is not implemented for `std::ffi::c_void`
   = note: required for the cast to the object type `dyn std::ops::FnMut(i32) -> bool`

我想知道:

1. 有没有办法将一个函数/闭包传递给另一个函数并执行那些"类似C"的强制转换？
2. 将闭包转换为i64值以将其传递给回调的正确方法是什么？

我用的是稳定版的Rust.