從矢量返回元素時出錯E0495

Question

我一直試圖在Rust中創建一個簡單的解釋器。這是一段代碼片段。

use std::vec::Vec; 
use std::option::Option; 
use std::borrow::Borrow; 

trait Data {} 

trait Instruction { 
    fn run(&self, stack: &mut Vec<Box<Data>>) -> Option<&Data>; 
} 

struct Get { 
    stack_index: usize, 
} 

impl Instruction for Get { 
    fn run(&self, stack: &mut Vec<Box<Data>>) -> Option<&Data> { 
     Some(stack[self.stack_index].borrow()) 
    } 
} 

fn main() {} 

以上包含一個簡單的Get指令。它有一個run方法，它只是從給定的堆棧Data返回一個值。 Data是一種抽象特徵，代表真正的任何類型的數據。我還沒有實施Data。

但是編譯代碼生成錯誤代碼E0495

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in function call due to conflicting requirements 
    --> <anon>:17:14 
    | 
17 |   Some(stack[self.stack_index].borrow()) 
    |    ^^^^^^^^^^^^^^^^^^^^^^^ 
    | 
note: first, the lifetime cannot outlive the anonymous lifetime #2 defined on the body at 16:63... 
    --> <anon>:16:64 
    | 
16 |  fn run(&self, stack: &mut Vec<Box<Data>>) -> Option<&Data> { 
    | ________________________________________________________________^ starting here... 
17 | |   Some(stack[self.stack_index].borrow()) 
18 | |  } 
    | |_____^ ...ending here 
note: ...so that reference does not outlive borrowed content 
    --> <anon>:17:14 
    | 
17 |   Some(stack[self.stack_index].borrow()) 
    |    ^^^^^ 
note: but, the lifetime must be valid for the anonymous lifetime #1 defined on the body at 16:63... 
    --> <anon>:16:64 
    | 
16 |  fn run(&self, stack: &mut Vec<Box<Data>>) -> Option<&Data> { 
    | ________________________________________________________________^ starting here... 
17 | |   Some(stack[self.stack_index].borrow()) 
18 | |  } 
    | |_____^ ...ending here 
note: ...so that expression is assignable (expected std::option::Option<&Data>, found std::option::Option<&Data>) 
    --> <anon>:17:9 
    | 
17 |   Some(stack[self.stack_index].borrow()) 
    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 

我缺少什麼？

Answer 1

如在What is the return type of the indexing operation on a slice?中所解釋的，stack[self.stack_index]正在通過值返回切片中的值。然後，您正在嘗試返回對局部變量的引用，這是不允許的，如Is there any way to return a reference to a variable created in a function?中所述。

真的，你會想要做Some(&*stack[self.stack_index])，它取消參考Box去Data，然後重新引用它。因爲您的實現不符合規則lifetime elision，你需要明確的壽命添加到您的特質方法和實現：

fn run<'a>(&self, stack: &'a mut Vec<Box<Data>>) -> Option<&'a Data> 

我會用get和map雖然可能實現它：

stack.get(self.stack_index).map(Box::as_ref)