rust - 减去 UniCase HashSet

Question

我正在尝试运行以下代码:

extern crate unicase;

use unicase::UniCase; 
use std::collections::HashSet;

fn main() {
    let a = UniCase("a".to_owned());
    let b = UniCase("b".to_owned());
    let s1: HashSet<UniCase<String>> = [a].iter().cloned().collect();
    let s2: HashSet<UniCase<String>> = [a, b].iter().cloned().collect();
    let s3 = s2 - s1; 
}

( Playground )

并得到这个错误:

error[E0369]: binary operation `-` cannot be applied to type `std::collections::HashSet<unicase::UniCase<std::string::String>>`

据我所知，HashSets 之间Sub 的要求是包含的类型实现Eq + Hash + Clone，它UniCase 似乎可以。任何指针？

Answer 1

从the documentation可以看出, Sub 是为 references HashMaps 实现的:

impl<'a, 'b, T, S> Sub<&'b HashSet<T, S>> for &'a HashSet<T, S> 
    where T: Eq + Hash + Clone,
          S: BuildHasher + Default,

采用显式引用有效:

extern crate unicase;

use unicase::UniCase;
use std::collections::HashSet;

fn main() {
    let a = UniCase("a".to_owned());
    let b = UniCase("b".to_owned());
    let s1: HashSet<_> = [a.clone()].iter().cloned().collect();
    let s2: HashSet<_> = [a, b].iter().cloned().collect();
    let s3 = &s2 - &s1;
    println!("{:?}", s3);
}

无需指定 s1 或 s2 的内部类型，但您必须克隆 a，因为它已移入数组。