折叠字符串以在rust中构建hashmap char计数器，但给出两阶段借用错误

Question

我正在尝试构建一个哈希图，该哈希图计算字符串中的字符频率。我的方法是将字符串中的字符折叠起来，在每次迭代时增加当前字符的计数。不幸的是，铁锈告诉我我在借贷上做错了。

使用clone()或将hm.get(&c)分解为单独的一行均无效。

我看不出如何避免有关两阶段借阅的错误。 （relevant rust-lang github issue）

use std::collections::HashMap;

fn main() {
    let some_str = "some string";

    let hm = some_str.chars().fold(HashMap::new(), |mut hm, c| {
        match hm.get(&c) {
            None => hm.insert(c, 1),
            Some(i) => hm.insert(c, i + 1),
        };
        hm
    });

    for (key, val) in hm.iter() {
        println!("{}: {}", key, val);
    }
}

给出此错误

warning: cannot borrow `hm` as mutable because it is also borrowed as immutable
 --> dank.rs:9:24
  |
7 |         match hm.get(&c) {
  |               -- immutable borrow occurs here
8 |             None => hm.insert(c, 1),
9 |             Some(i) => hm.insert(c, i+1)
  |                        ^^           - immutable borrow later used here
  |                        |
  |                        mutable borrow occurs here
  |
  = note: `#[warn(mutable_borrow_reservation_conflict)]` on by default
  = warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
  = note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>

Answer 1

向@Stargateur提供我的问题的答案