Read Only Lock.
This is a wrapper around Arc<RwLock<T>> that only implements RwLock::read() operations.
Create a normal Arc<RwLock<T>> in thread_1, send a RoLock to thread_2:
```rust
let rw = Arc::new(RwLock::new(0)); // Regular Arc
assert!(*rw.read().unwrap() == 0); // This can read... *rw.write().unwrap() = 1; // and write.
std::thread::spawn(move|| {
assert!(*ro.read().unwrap() == 1); // This one can only read.
});
``
-thread1still has full read/write control
-thread2can onlyRoLock::read()`
This type guarantees at compile time that you cannot write because the function doesn't even exist: ```rust let rw = Arc::new(RwLock::new(0)); let ro = RoLock::new(&rw);
ro.write(); // Compile error! ```
Since the inner field of RoLock (self.0) is private, you can't call RwLock::write directly either:
```rust
let rw = Arc::new(RwLock::new(0));
let ro = RoLock::new(&rw);
ro.0.write(); // Compile error! ```