rust - 为什么对已删除对象的可变引用仍算作可变引用？

Question

这是一个简化的例子:

struct Connection {}

impl Connection {
    fn transaction(&mut self) -> Transaction {
        Transaction { conn: self }
    }
}

struct Transaction<'conn> {
    conn: &'conn Connection,
}

impl<'conn> Transaction<'conn> {
    fn commit(mut self) {}
}

fn main() {
    let mut db_conn = Connection {};

    let mut trans = db_conn.transaction(); //1
    let mut records_without_sync = 0_usize;
    const MAX_RECORDS_WITHOUT_SYNC: usize = 100;
    loop {
        //do something
        records_without_sync += 1;
        if records_without_sync >= MAX_RECORDS_WITHOUT_SYNC {
            trans.commit();
            records_without_sync = 0;
            trans = db_conn.transaction(); //2
        }
    }
}

编译器报告在 1 和 2 处有两个可变借用，但事实并非如此。由于 trans.commit() 按值获取 self，trans 被丢弃，所以到点 2 应该有没有可变引用。

1. 为什么编译器看不到 2 处没有可变引用？
2. 如何修复代码，保留相同的逻辑？

Answer 1

有一个可变引用。

如果你改变transaction对此:

fn transaction(&mut self) -> Transaction {
    let _: () = self;
    Transaction{conn: self}
}

您会看到编译器错误:

 = note: expected type `()`
 = note:    found type `&mut Connection`

所以 self类型为 &mut Connection ...一个可变的引用。然后将其传递给 Transaction从此函数返回的实例。

这意味着您的可变借用在 trans 的生命周期内存在(我添加的花括号表示借用的范围):

let mut trans = db_conn.transaction();
{ // <-------------------- Borrow starts here
    let mut records_without_sync = 0_usize;
    const MAX_RECORDS_WITHOUT_SYNC: usize = 100;
    loop {
        //do something
        records_without_sync += 1;
        if records_without_sync >= MAX_RECORDS_WITHOUT_SYNC {
            trans.commit();
            records_without_sync = 0;
            trans = db_conn.transaction();// <--- ####### D'oh! Still mutably borrowed
        }
    }
} // <-------------------- Borrow ends here

如果您正在寻找这种 parent-><-child设置，我想你必须达到Rc<RefCell> .

具体来说，一个 Rc引用计数你传递连接的次数和RefCell在运行时而不是编译时跟踪借用。是的，这确实意味着如果您设法尝试在运行时可变地借用它两次，您会感到 panic 。在不了解您的架构的情况下，很难说这是否合适。

Here is my solution anyway :

use std::cell::RefCell;
use std::rc::Rc;

struct Connection {}

impl Connection {
    fn do_something_mutable(&mut self) {
        println!("Did something mutable");
    }
}

type Conn = Rc<RefCell<Connection>>;

struct Transaction {
    conn: Conn,
}

impl Transaction {
    fn new(connection: Conn) -> Transaction {
        Transaction { conn: connection }
    }

    fn commit(mut self) {
        self.conn.borrow_mut().do_something_mutable();
    }
}

fn main() {
    let db_conn = Rc::new(RefCell::new(Connection {}));

    let mut trans = Transaction::new(db_conn.clone());
    let mut records_without_sync = 0_usize;
    const MAX_RECORDS_WITHOUT_SYNC: usize = 100;
    loop {
        //do something
        records_without_sync += 1;
        if records_without_sync >= MAX_RECORDS_WITHOUT_SYNC {
            trans.commit();
            records_without_sync = 0;
            trans = Transaction::new(db_conn.clone());
            break; // Used to stop the loop crashing the playground
        }
    }
}