是否有可能创建一个由`*const Any`类型键入的`HashMap`？

Question

这段代码

use std::any::Any;
use std::collections::HashMap;

fn main() {
    let x: HashMap<*const Any, i32> = HashMap::new();
}

给我以下错误:

error: the trait `core::marker::Sized` is not implemented for the type `core::any::Any` [E0277]

let x: HashMap<*const Any, i32> = HashMap::new();
                                  ^~~~~~~~~~~~

首先,我不明白为什么它是抱怨core::any::Any,当键是类型*const core::any::Any.*const _无论它指向什么,都不应该是大小？为了测试这个,我试过:

use std::any::Any;
use std::mem::size_of;

fn main() {
    println!("size_of(*const Any) = {}", size_of::<*const Any>());
}

正如预期的那样,产生:

size_of(*const Any) = 16

Answer 1

这不是最漂亮的解决方案，但这是我想出的：

use std::any::Any;
use std::collections::HashMap;
use std::hash::{Hasher, Hash};
use std::cmp;

struct Wrapper {
    v: *const Any,
}

impl Wrapper {
    fn get_addr(&self) -> usize {
        self.v as *const usize as usize
    }
}

impl Hash for Wrapper {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.get_addr().hash(state)
    }
}

impl cmp::PartialEq for Wrapper {
    fn eq(&self, other: &Self) -> bool {
        self.get_addr() == other.get_addr()
    }
}

impl cmp::Eq for Wrapper {}

fn main() {
    let x: HashMap<Wrapper, i32> = HashMap::new();
}