Rollup merge of #143191 - connortsui20:stabilize-rwlock-downgrade, r=tgross35

Stabilize `rwlock_downgrade` library feature

Tracking Issue: #128203

Method to be stabilized:

```rust
impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
    pub fn downgrade(s: Self) -> RwLockReadGuard<'a, T> {}
}
```

~~I would like to point out that my documentation comment is longer than ideal, but at the same time I don't really know how else to show why `downgrade` is actually necessary (instead of just unlocking and relocking). If anyone has ideas for making this more concise that would be great!~~ I have made the documentation a bit more clear.

Stabilization report: #128203 (comment)

Author: matthiaskrgr

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

@@ -700,7 +700,6 @@ impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
     ///
     /// ```
     /// #![feature(nonpoison_rwlock)]
-    /// #![feature(rwlock_downgrade)]
     ///
     /// use std::sync::nonpoison::{RwLock, RwLockWriteGuard};
     ///
@@ -719,7 +718,6 @@ impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
     ///
     /// ```
     /// #![feature(nonpoison_rwlock)]
-    /// #![feature(rwlock_downgrade)]
     ///
     /// use std::sync::Arc;
     /// use std::sync::nonpoison::{RwLock, RwLockWriteGuard};
@@ -752,8 +750,7 @@ impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
     /// # let final_check = rw.read();
     /// # assert_eq!(*final_check, 3);
     /// ```
-    #[unstable(feature = "rwlock_downgrade", issue = "128203")]
-    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
     pub fn downgrade(s: Self) -> RwLockReadGuard<'rwlock, T> {
         let lock = s.lock;
 

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

@@ -813,8 +813,6 @@ impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
     /// `downgrade` takes ownership of the `RwLockWriteGuard` and returns a [`RwLockReadGuard`].
     ///
     /// ```
-    /// #![feature(rwlock_downgrade)]
-    ///
     /// use std::sync::{RwLock, RwLockWriteGuard};
     ///
     /// let rw = RwLock::new(0);
@@ -831,8 +829,6 @@ impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
     /// thread calling `downgrade` and any reads it performs after downgrading.
     ///
     /// ```
-    /// #![feature(rwlock_downgrade)]
-    ///
     /// use std::sync::{Arc, RwLock, RwLockWriteGuard};
     ///
     /// let rw = Arc::new(RwLock::new(1));
@@ -863,7 +859,7 @@ impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
     /// # let final_check = rw.read().unwrap();
     /// # assert_eq!(*final_check, 3);
     /// ```
-    #[unstable(feature = "rwlock_downgrade", issue = "128203")]
+    #[stable(feature = "rwlock_downgrade", since = "CURRENT_RUSTC_VERSION")]
     pub fn downgrade(s: Self) -> RwLockReadGuard<'rwlock, T> {
         let lock = s.lock;
 

library/std/tests/sync/lib.rs

@@ -4,7 +4,6 @@
 #![feature(once_cell_try)]
 #![feature(lock_value_accessors)]
 #![feature(reentrant_lock)]
-#![feature(rwlock_downgrade)]
 #![feature(std_internals)]
 #![feature(sync_nonpoison)]
 #![feature(nonpoison_condvar)]