如何通过通道发送哈希图？

Question

我想通过一个频道发送哈希图：

use std::collections::HashMap;
use std::sync::mpsc;
use std::thread;

pub fn frequency(input: &[&str]) {
    let handles = vec![];
    let (tx, rx) = mpsc::channel();
    for txts in input {
        let handle = thread::spawn(|| {
            tx.send(HashMap::new());
        });
        handles.push(handle);
    }
}

但是编译器不会让我：

error[E0277]: `Sender<HashMap<_, _>>` cannot be shared between threads safely
   --> src/lib.rs:9:22
    |
9   |         let handle = thread::spawn(|| {
    |                      ^^^^^^^^^^^^^ `Sender<HashMap<_, _>>` cannot be shared between threads safely
    |
    = help: the trait `Sync` is not implemented for `Sender<HashMap<_, _>>`
    = note: required because of the requirements on the impl of `Send` for `&Sender<HashMap<_, _>>`
    = note: required because it appears within the type `[closure@src/lib.rs:9:36: 11:10]`
note: required by a bound in `spawn`

即使 Hashmap 有发送和同步实现。

如何通过频道发送哈希图？