提供统一单线程代码和基于RwLocks代码的接口。

这个特质的副作用是，当借检查器被激活以检查当前作用域中引入的死锁时，表现得很好。这是因为现在只能在对可变引用的调用中使用 Self::write。

示例

use parking_lot::RwLock;
use read_write_api::{
    DowngradableWriteGuard,
    RwApi,
    RwApiWrapper,
    RwApiWrapperOwned,
    UpgradableReadGuard,
};

fn do_something(mut x: impl RwApi<Target=u64>) -> u64 {
    let guard = x.upgradable_read();
    if *guard == 1 {
        let mut guard = guard.upgrade_to_downgradable();
        *guard = 2;
        *guard.downgrade()
    } else {
        *guard
    }
}

assert_eq!(do_something(RwApiWrapperOwned(1)), 2);
assert_eq!(do_something(RwApiWrapperOwned(3)), 3);
assert_eq!(do_something(&mut RwApiWrapperOwned(1)), 2);
assert_eq!(do_something(&mut RwApiWrapperOwned(3)), 3);

assert_eq!(do_something(RwLock::new(1)), 2);
assert_eq!(do_something(RwLock::new(3)), 3);
assert_eq!(do_something(&RwLock::new(1)), 2);
assert_eq!(do_something(&RwLock::new(3)), 3);
assert_eq!(do_something(&mut RwLock::new(1)), 2);
assert_eq!(do_something(&mut RwLock::new(3)), 3);

fn do_something_ref<'a>(mut x: impl RwApi<Target=&'a mut u64>) -> u64 {
    if **x.read() == 1 {
        **x.write() = 2;
        **x.read()
    } else {
        **x.read()
    }
}

assert_eq!(do_something_ref(RwApiWrapper(&mut 1)), 2);
assert_eq!(do_something_ref(RwApiWrapper(&mut 3)), 3);
assert_eq!(do_something_ref(&mut RwApiWrapper(&mut 1)), 2);
assert_eq!(do_something_ref(&mut RwApiWrapper(&mut 3)), 3);

assert_eq!(do_something_ref(RwLock::new(&mut 1)), 2);
assert_eq!(do_something_ref(RwLock::new(&mut 3)), 3);
assert_eq!(do_something_ref(&RwLock::new(&mut 1)), 2);
assert_eq!(do_something_ref(&RwLock::new(&mut 3)), 3);
assert_eq!(do_something_ref(&mut RwLock::new(&mut 1)), 2);
assert_eq!(do_something_ref(&mut RwLock::new(&mut 3)), 3);