Make `RwLockReadGuard` covariant over its type parameter

[Kestrer]

Currently RwLockReadGuard is invariant over its type parameter, meaning that for instance this sound code:

use std::sync::{RwLock, RwLockReadGuard};

fn do_stuff<'a>(_: RwLockReadGuard<'_, &'a i32>, _: &'a i32) {}

fn main() {
    let j: i32 = 5;
    let lock = RwLock::new(&j);
    {
        let i = 6;
        do_stuff(lock.read().unwrap(), &i);
    }
    drop(lock);
}

Will not compile. However, since RwLockReadGuard does not allow for mutation of the inner value, it would be sound to make it covariant, and it would be useful in some circumstances.

See also the same issue in parking_lot.

[sobolevn]

I would love to give this a try! 👍
This is my first contribution to rust, so I might ask some questions 🙂

[sobolevn]

First steps:

• RwLockReadGuard's second type argument is defined here:

  rust/library/std/src/sync/rwlock.rs

  Lines 102 to 104 in 5d6ee0d

  	pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
  	lock: &'a RwLock<T>,
  	}

  It is only used for lock: &'a RwLock<T>
• RwLock is defined here:

  rust/library/std/src/sync/rwlock.rs

  Lines 78 to 82 in 5d6ee0d

  	pub struct RwLock<T: ?Sized> {
  	inner: sys::MovableRWLock,
  	poison: poison::Flag,
  	data: UnsafeCell<T>,
  	}

  Its type argument is only used for data: UnsafeCell<T> which is the source of invariance according to these rules