rust - 在同一个函数中多次使用 self

Question

我和借用检查员发生了争执。我的问题有点复杂，但对于这种情况，我使用的是类似缓冲区的结构。我的缓冲区有一个函数 safe_write_to_slot，它首先检索第一个空元素(返回 Ok(location) 或 Err(error message) 的结果)，然后将值写入该检索到的位置。然而，问题是，当我将检索到的位置分配给一个值时，rust 提示我在几行之后再次使用 self 。我如何首先调用返回结果的 (self) 函数，然后继续使用 self 执行某些操作？

use std::result::Result;

struct Elems {
    pub elems : Vec<int>,
}

impl Elems {
    pub fn new() -> Elems {
        Elems{elems: vec![0,0,0,0,0,0]}
    }

    pub fn safe_write_to_slot(&mut self, elem : uint) -> Result<(), &str> {
        let loc = try!(self.get_slot());

        self.unsafe_write_to_slot(loc);

        Ok(())
    }

    pub fn get_slot(&self) -> Result<uint, &str>{
        let mut loc = -1i;

        for x in range(0, self.elems.len()) {
            if *self.elems.get(x) == 0 {
                loc = x as int;
            }
        }

        if loc != -1 { Ok(loc as uint) } else { Err("No free slots") }
    }

    fn unsafe_write_to_slot(&mut self, elem : uint) {
        self.elems[elem] = 1;
    }
}

我得到的错误是:

   Compiling borrow v0.0.1 (file:///borrow)
main.rs:19:9: 19:13 error: cannot borrow `*self` as mutable because it is also borrowed as immutable
main.rs:19         self.unsafe_write_to_slot(loc);
                   ^~~~
main.rs:17:24: 17:28 note: previous borrow of `*self` occurs here; the immutable borrow prevents subsequent moves or mutable borrows of `*self` until the borrow ends
main.rs:17         let loc = try!(self.get_slot());
                                  ^~~~
/main.rs:17:19: 17:41 note: expansion site
main.rs:22:6: 22:6 note: previous borrow ends here
main.rs:16     pub fn safe_write_to_slot(&mut self, elem : uint) -> Result<(), &str> {
/main.rs:22     }
               ^
main.rs:37:9: 37:29 error: cannot assign to immutable dereference (dereference is implicit, due to indexing)
main.rs:37         self.elems[elem] = 1;
                   ^~~~~~~~~~~~~~~~~~~~
error: aborting due to 2 previous errors
Could not compile `borrow`.

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Answer 1

生命周期推断导致了这里的问题。

get_slot 方法解释为:

pub fn get_slot<'a>(&'a self) -> Result<uint, &'a str> {

结果被绑定(bind)到与 self 相同的生命周期，这导致 self 保持卡住状态，直到结果被删除。但是，您不想将 self 的生命周期链接到 &str，因为您只返回字符串文字。通过在 get_slot 和 safe_write_to_slot 中将 &str 更改为 &'static str，您将不会再收到错误，因为 self 在调用 unsafe_write_to_slot 时将不再被认为是借用的。