IteratorRandom::sample: allow large "amount" (#1695)

Author: dhardy

CHANGELOG.md

@@ -19,13 +19,15 @@ You may also find the [Upgrade Guide](https://rust-random.github.io/book/update.
 - Remove fns `SeedableRng::from_os_rng`, `try_from_os_rng` (#1674)
 - Remove `Clone` support for `StdRng`, `ReseedingRng` (#1677)
 - Use `postcard` instead of `bincode` to test the serde feature (#1693)
+- Avoid excessive allocation in `IteratorRandom::sample` when `amount` is much larger than iterator size ([#1695])
 - Rename `os_rng` -> `sys_rng`, `OsRng` -> `SysRng`, `OsError` -> `SysError` ([#1697])
 
 ### Additions
 - Add fns `IndexedRandom::choose_iter`, `choose_weighted_iter` (#1632)
 - Pub export `Xoshiro128PlusPlus`, `Xoshiro256PlusPlus` prngs (#1649)
 - Pub export `ChaCha8Rng`, `ChaCha12Rng`, `ChaCha20Rng` behind `chacha` feature (#1659)
 
+[#1695]: https://github.com/rust-random/rand/pull/1695
 [#1697]: https://github.com/rust-random/rand/pull/1697
 
 ## [0.9.2] - 2025-07-20

src/seq/iterator.rs

@@ -245,8 +245,7 @@ pub trait IteratorRandom: Iterator + Sized {
     where
         R: Rng + ?Sized,
     {
-        let mut reservoir = Vec::with_capacity(amount);
-        reservoir.extend(self.by_ref().take(amount));
+        let mut reservoir = Vec::from_iter(self.by_ref().take(amount));
 
         // Continue unless the iterator was exhausted
         //
@@ -259,10 +258,6 @@ pub trait IteratorRandom: Iterator + Sized {
                     *slot = elem;
                 }
             }
-        } else {
-            // Don't hang onto extra memory. There is a corner case where
-            // `amount` was much less than `self.len()`.
-            reservoir.shrink_to_fit();
         }
         reservoir
     }