rust - 如何创建一个以函数引用为值的 lazy_static HashMap？

Question

我尝试创建一个 HashMap 并将函数作为值:

#[macro_use]
extern crate lazy_static;

use std::collections::HashMap;

lazy_static! {
    static ref HASHES: HashMap<&'static str, &'static Fn([u8])> = {
        let mut m = HashMap::new();
        m.insert("md5", &md5);
        m
    };
}

fn md5(bytes: &[u8]) -> String {
    String::default()
}

编译器给我一个错误:

error[E0277]: the trait bound `std::ops::Fn([u8]) + 'static: std::marker::Sync` is not satisfied in `&'static std::ops::Fn([u8]) + 'static`
  --> src/main.rs:6:1
   |
6  |   lazy_static! {
   |  _^ starting here...
7  | |     static ref HASHES: HashMap<&'static str, &'static Fn([u8])> = {
8  | |         let mut m = HashMap::new();
9  | |         m.insert("md5", &md5);
10 | |         m
11 | |     };
12 | | }
   | |_^ ...ending here: within `&'static std::ops::Fn([u8]) + 'static`, the trait `std::marker::Sync` is not implemented for `std::ops::Fn([u8]) + 'static`
   |
   = note: `std::ops::Fn([u8]) + 'static` cannot be shared between threads safely
   = note: required because it appears within the type `&'static std::ops::Fn([u8]) + 'static`
   = note: required because of the requirements on the impl of `std::marker::Sync` for `std::collections::hash::table::RawTable<&'static str, &'static std::ops::Fn([u8]) + 'static>`
   = note: required because it appears within the type `std::collections::HashMap<&'static str, &'static std::ops::Fn([u8]) + 'static>`
   = note: required by `lazy_static::lazy::Lazy`
   = note: this error originates in a macro outside of the current crate

我不明白我应该怎么做才能修复这个错误，我不知道创建这样一个 HashMap 的任何其他方法。

Answer 1

您的代码有多个问题。编译器出现的错误告诉您您的代码将允许内存不安全:

`std::ops::Fn([u8]) + 'static` cannot be shared between threads safely

您存储在 HashMap 中的类型不能保证它可以被共享。

您可以通过将您的值类型更改为 &'static (Fn([u8]) + Sync) 来指定这样的界限来“修复”这个问题。由于您的函数签名不匹配，这将解锁下一个错误:

expected type `std::collections::HashMap<&'static str, &'static std::ops::Fn([u8]) + std::marker::Sync + 'static>`
   found type `std::collections::HashMap<&str, &fn(&[u8]) -> std::string::String {md5}>`

“修复”&'static (Fn(&[u8]) -> String + Sync) 会导致深奥的生命周期错误:

expected type `std::collections::HashMap<&'static str, &'static for<'r> std::ops::Fn(&'r [u8]) -> std::string::String + std::marker::Sync + 'static>`
   found type `std::collections::HashMap<&str, &fn(&[u8]) -> std::string::String {md5}>`

这可以通过将带有 &md5 的函数转换为 &'static (Fn(&[u8]) -> String + Sync)) 来“修复”，这会导致

note: borrowed value must be valid for the static lifetime...
note: consider using a `let` binding to increase its lifetime

这是因为 the reference you've made is to a temporary value that doesn't live outside of the scope 触底反弹.

---

我将 fix 放在引号中，因为这不是真正正确的解决方案。正确的做法是只使用函数指针:

lazy_static! {
    static ref HASHES: HashMap<&'static str, fn(&[u8]) -> String> = {
        let mut m = HashMap::new();
        m.insert("md5", md5 as fn(&[u8]) -> std::string::String);
        m
    };
}

老实说，我会说 HashMap 可能有点矫枉过正；我会使用一个数组。小型数组可能比小型 HashMap 更快:

type HashFn = fn(&[u8]) -> String;

static HASHES: &'static [(&'static str, HashFn)] = &[
    ("md5", md5),
];

您可以从遍历列表开始，或者花点时间将其按字母顺序排列，然后使用 binary_search当它变大时。