Rollup merge of #44797 - lucasem:master, r=frewsxcv

docs improvement std::sync::{PoisonError, TryLockError}

Addresses the `PoisonError` and `TryLockError` parts of #29377.
Adds examples and links.

r? @steveklabnik

src/libstd/sys_common/poison.rs

@@ -65,6 +65,31 @@ pub struct Guard {
 /// each lock, but once a lock is poisoned then all future acquisitions will
 /// return this error.
 ///
+/// # Examples
+///
+/// ```
+/// use std::sync::{Arc, Mutex};
+/// use std::thread;
+///
+/// let mutex = Arc::new(Mutex::new(1));
+///
+/// // poison the mutex
+/// let c_mutex = mutex.clone();
+/// let _ = thread::spawn(move || {
+///     let mut data = c_mutex.lock().unwrap();
+///     *data = 2;
+///     panic!();
+/// }).join();
+///
+/// match mutex.lock() {
+///     Ok(_) => unreachable!(),
+///     Err(p_err) => {
+///         let data = p_err.get_ref();
+///         println!("recovered: {}", data);
+///     }
+/// };
+/// ```
+///
 /// [`Mutex`]: ../../std/sync/struct.Mutex.html
 /// [`RwLock`]: ../../std/sync/struct.RwLock.html
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -72,10 +97,16 @@ pub struct PoisonError<T> {
     guard: T,
 }
 
-/// An enumeration of possible errors which can occur while calling the
-/// [`try_lock`] method.
+/// An enumeration of possible errors associated with a [`TryLockResult`] which
+/// can occur while trying to aquire a lock, from the [`try_lock`] method on a
+/// [`Mutex`] or the [`try_read`] and [`try_write`] methods on an [`RwLock`].
 ///
+/// [`Mutex`]: struct.Mutex.html
+/// [`RwLock`]: struct.RwLock.html
+/// [`TryLockResult`]: type.TryLockResult.html
 /// [`try_lock`]: struct.Mutex.html#method.try_lock
+/// [`try_read`]: struct.RwLock.html#method.try_read
+/// [`try_write`]: struct.RwLock.html#method.try_write
 #[stable(feature = "rust1", since = "1.0.0")]
 pub enum TryLockError<T> {
     /// The lock could not be acquired because another thread failed while holding
@@ -148,6 +179,28 @@ impl<T> PoisonError<T> {
 
     /// Consumes this error indicating that a lock is poisoned, returning the
     /// underlying guard to allow access regardless.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::HashSet;
+    /// use std::sync::{Arc, Mutex};
+    /// use std::thread;
+    ///
+    /// let mutex = Arc::new(Mutex::new(HashSet::new()));
+    ///
+    /// // poison the mutex
+    /// let c_mutex = mutex.clone();
+    /// let _ = thread::spawn(move || {
+    ///     let mut data = c_mutex.lock().unwrap();
+    ///     data.insert(10);
+    ///     panic!();
+    /// }).join();
+    ///
+    /// let p_err = mutex.lock().unwrap_err();
+    /// let data = p_err.into_inner();
+    /// println!("recovered {} items", data.len());
+    /// ```
     #[stable(feature = "sync_poison", since = "1.2.0")]
     pub fn into_inner(self) -> T { self.guard }