Rust 提取指向特征函数的原始指针

Rust的vtable struct 不能从Rust代码访问.

如果你只是想了解这是如何工作的，那么我们可以使用一些不安全的魔法(but none of this is guaranteed!):

fn what_data(a: &dyn Trait) -> u32 {
    unsafe {
        // `&dyn Trait` is stored as two references, the data pointer coming first and then the vtable
        // pointer. But again, this is not guaranteed!
        // On nightly we can extract the metadata with `std::ptr::metadata()`, but unfortunately it doesn't
        // give access to the full vtable, only to an opaque struct representing it :(
        let raw_dyn = std::mem::transmute::<&dyn Trait, [*const (); 2]>(a);
        // This is how the vtable is structured in memory (again, *currently*. Nothing is guaranteed!):
        #[repr(C)]
        struct Vtable {
            // First, comes the function to execute the drop glue. This is necessary for e.g. `Box<dyn Trait>`
            // to be able to drop the data correctly.
            _drop_in_place: fn(*mut ()),
            // Then, the struct size (`std::mem::size_of_val(&data)`). This is necessary for e.g. `Box<dyn Trait>`
            // to be able to correctly *deallocate* the data.
            _size: usize,
            // Then the alignment (`std::mem::align_of_val(&data)`) for the same purpose as the size.
            _align: usize,
            // And here, if you enable the unstable trait upcasting feature, comes the pointers to the supertraits'
            // vtables, so you can upcast. But we don't have supertraits, so this is empty.
            // And finally, the pointers to the methods: first supertrait methods, then the methods of the trait itself.
            // We only have one method and no supertraits, so this is easy.
            get_data: fn(*const ()) -> u32,
        }
        ((*raw_dyn[1].cast::<Vtable>()).get_data)(raw_dyn[0])
    }
}

Playground.

但是如果你真的想在你的某个项目中为此编写代码，那么可以使用@Finomennis和@prog-fh建议的闭包.