OnceLock: Rework example, statics aren't dropped

Author: ianrrees

library/std/src/sync/once_lock.rs

@@ -17,25 +17,28 @@ use crate::sync::Once;
 /// ‘lazy static’ or ‘memoizing’):
 ///
 /// ```
-/// use std::collections::HashMap;
 /// use std::sync::OnceLock;
-///
-/// fn hash_map() -> &'static HashMap<u32, char> {
-///     static HASHMAP: OnceLock<HashMap<u32, char>> = OnceLock::new();
-///     HASHMAP.get_or_init(|| {
-///         let mut m = HashMap::new();
-///         m.insert(0, 'a');
-///         m.insert(1, 'b');
-///         m.insert(2, 'c');
-///         m
-///     })
+/// 
+/// struct Computation {}
+/// 
+/// impl Computation {
+///     fn new() -> Self {
+///         // Do some slow/expensive computation here
+///         Self {}
+///     }
 /// }
-///
-/// // The `HashMap` is built, stored in the `OnceLock`, and returned.
-/// let _ = hash_map();
-///
-/// // The `HashMap` is retrieved from the `OnceLock` and returned.
-/// let _ = hash_map();
+/// 
+/// fn computation() -> &'static Computation {
+///     // n.b. static items do not call [`Drop`] on program termination, so if
+///     // [`Computation`] impls Drop, that will not be used for this instance.
+///     static COMPUTATION: OnceLock<Computation> = OnceLock::new();
+///     COMPUTATION.get_or_init(|| Computation::new())
+/// }
+/// 
+/// // The `Computation` is built, stored in the `OnceLock`, and returned.
+/// let _ = computation();
+/// // The `Computation` is retrieved from the `OnceLock` and returned.
+/// let _ = computation();
 /// ```
 ///
 /// Writing to a `OnceLock` from a separate thread: