rust - 调用返回字符串文字数组的函数，错误为 "cannot return value referencing local variable"

Question

parse_winning_hand 函数的目标是接受 5 个 i32 的向量，然后将相应的字母附加到 return_val< 的第 i 个索引(从 0 到 5)。 deal 是一个驱动程序函数，它在运行结束时调用 parse_winning_hand:

fn parse_winning_hand(hand: &Vec<i32>) -> [&str; 5] {
    let mut temp_hand = hand.to_vec();
    let mut return_val = ["12", "12", "12", "12", "12"];
    for i in 0..5 {
        let popped = temp_hand.pop().unwrap();
        let mut suit = "X";
        if popped < 14 {
            suit = "C";
        } else if popped < 27 {
            suit = "D";
        } else if popped < 40 {
            suit = "H";
        } else {
            suit = "S";
        }
        return_val[i] = suit;
    }
    return return_val;
}

fn deal(arr: &[i32]) -> [&'static str; 5] {
    // ...
    let decided_winner = decide_winner(hand_one_score, hand_two_score);
    if decided_winner == 1 {
        let ret = parse_winning_hand(&hand_one);
        return ret;
    } else {
        let ret = parse_winning_hand(&hand_two);
        return ret;
    }
}

我在编译过程中遇到的错误是:

error[E0515]: cannot return value referencing local variable `hand_one`
   --> Poker.rs:276:10
    |
275 |         let ret = parse_winning_hand(&hand_one);
    |                                      --------- `hand_one` is borrowed here
276 |         return ret;
    |                ^^^ returns a value referencing data owned by the current function

error[E0515]: cannot return value referencing local variable `hand_two`
   --> Poker.rs:279:10
    |
278 |         let ret = parse_winning_hand(&hand_two);
    |                                      --------- `hand_two` is borrowed here
279 |         return ret;
    |                ^^^ returns a value referencing data owned by the current function

我已经搜索了此问题的解决方案，但要么该解决方案不适用于我的需求，要么由于我缺乏知识而无法理解发布的解决方案。为什么我收到错误？我该如何解决？

Answer 1

你的例子可以是reduced对此:

fn parse_winning_hand(_: &[i32]) -> [&str; 1] {
    ["0"]
}

fn deal() -> [&'static str; 1] {
    parse_winning_hand(&vec![])
}

error[E0515]: cannot return value referencing temporary value
 --> src/lib.rs:6:5
  |
6 |     parse_winning_hand(&vec![])
  |     ^^^^^^^^^^^^^^^^^^^^------^
  |     |                   |
  |     |                   temporary value created here
  |     returns a value referencing data owned by the current function
  |
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

这是因为您忘记在parse_winning_hand 的返回类型上指定'static 生命周期:

fn parse_winning_hand(_: &[i32]) -> [&'static str; 1]
//                                    ^~~~~~~

没有'static，lifetime elision导致函数签名将返回值的生命周期与输入值的生命周期联系起来:

fn parse_winning_hand<'a>(_: &'a [i32]) -> [&'a str; 1]

这意味着返回值不可能超过传入的 Vec。

另见:

• What is lifetime elision in very simple terms?
• Why is it discouraged to accept a reference to a String (&String), Vec (&Vec), or Box (&Box) as a function argument?