use std::collections::HashSet;

fn main() {
    let mut hsets = vec![];
    hsets.push(HashSet::new());
    hsets[0].insert("a1");
    hsets[0].insert("a2");
    hsets.push(HashSet::new());
    hsets[1].insert("b1");
    hsets[1].insert("b2");

    // 临时变量，存放 [a1, a2]
    let mut arr = vec![];

    for v in &hsets[0] {
        arr.push(v);
    }

    for v2 in arr {
        hsets[1].insert(v2);
    }
}

提示错误：

error[E0502]: cannot borrow `hsets` as mutable because it is also borrowed as immutable
  --> src/main.rs:18:9
   |
13 |     for v in &hsets[0] {
   |               ----- immutable borrow occurs here
...
17 |     for v2 in arr {
   |               --- immutable borrow later used here
18 |         hsets[1].insert(v2);
   |         ^^^^^ mutable borrow occurs here

error: aborting due to previous error