rust - 预期类型 String，当我明确将其设为 String 时发现类型 str

Question

use std::collections::{HashMap, HashSet};

type Snapshot = HashMap<String, HashSet<String>>;

fn compare_snapshots(snapshot1: Snapshot, snapshot2: Snapshot, entry1: String, entry2: String) {}

fn main() {
    let snapshot1 = HashMap::new();
    let snapshot2 = HashMap::new();

    let entries1: HashSet<String> = snapshot1.keys().cloned().collect();
    let entries2: HashSet<String> = snapshot2.keys().cloned().collect();
    let entries: Vec<String> = entries1.union(&entries2).cloned().collect();

    for e1 in entries {
        for e2 in entries {
            if e1 < e2 {
                compare_snapshots(snapshot1, snapshot2, *e1.to_string(), *e2.to_string());
            }
        }
    }
}

以及以下错误:

error[E0308]: mismatched types
  --> src/main.rs:18:57
   |
18 |                 compare_snapshots(snapshot1, snapshot2, *e1.to_string(), *e2.to_string());
   |                                                         ^^^^^^^^^^^^^^^ expected struct `std::string::String`, found str
   |
   = note: expected type `std::string::String`
              found type `str`

error[E0308]: mismatched types
  --> src/main.rs:18:74
   |
18 |                 compare_snapshots(snapshot1, snapshot2, *e1.to_string(), *e2.to_string());
   |                                                                          ^^^^^^^^^^^^^^^ expected struct `std::string::String`, found str
   |
   = note: expected type `std::string::String`
              found type `str`

既然我明确地给出了 entries 为什么我会得到它们一个Vec<String>类型？

Answer 1

你的情况可以减少到

fn repro(_: String) {}

fn main() {
    repro(*"".to_string());
}

出于某种原因，您正在取消引用 String .字符串实现 Deref<Target = str> , 所以你已经明确地尝试制作一个 str .这最终是行不通的，因为 str未调整大小，但您首先遇到类型不匹配。

去掉星号:

fn repro(_: String) {}

fn main() {
    repro("".to_string());
}

另见:

• What are Rust's exact auto-dereferencing rules?