From e800d5a0ec18e8296980737665880e97ad97db9a Mon Sep 17 00:00:00 2001
From: Stiopa Koltsov <stepan.koltsov@gmail.com>
Date: Fri, 3 Feb 2023 00:13:50 +0000
Subject: [PATCH] Specify behavior of HashSet::insert

`HashSet::insert` does not replace the value with equal value.

Fixes #107581.
---
 library/std/src/collections/hash/set.rs       |  4 +++-
 library/std/src/collections/hash/set/tests.rs | 20 +++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/library/std/src/collections/hash/set.rs b/library/std/src/collections/hash/set.rs
index b59f89d321c47..ac7acf999a6ee 100644
--- a/library/std/src/collections/hash/set.rs
+++ b/library/std/src/collections/hash/set.rs
@@ -875,7 +875,9 @@ where
     /// Returns whether the value was newly inserted. That is:
     ///
     /// - If the set did not previously contain this value, `true` is returned.
-    /// - If the set already contained this value, `false` is returned.
+    /// - If the set already contained this value, `false` is returned,
+    ///   and the set is not modified: original value is not replaced,
+    ///   and the value passed as argument is dropped.
     ///
     /// # Examples
     ///
diff --git a/library/std/src/collections/hash/set/tests.rs b/library/std/src/collections/hash/set/tests.rs
index 941a0450cc770..022ca68ffcfbb 100644
--- a/library/std/src/collections/hash/set/tests.rs
+++ b/library/std/src/collections/hash/set/tests.rs
@@ -3,6 +3,7 @@ use super::HashSet;
 
 use crate::panic::{catch_unwind, AssertUnwindSafe};
 use crate::sync::atomic::{AtomicU32, Ordering};
+use crate::sync::Arc;
 
 #[test]
 fn test_zero_capacities() {
@@ -502,3 +503,22 @@ fn const_with_hasher() {
     const X: HashSet<(), ()> = HashSet::with_hasher(());
     assert_eq!(X.len(), 0);
 }
+
+#[test]
+fn test_insert_does_not_overwrite_the_value() {
+    let first_value = Arc::new(17);
+    let second_value = Arc::new(17);
+
+    let mut set = HashSet::new();
+    let inserted = set.insert(first_value.clone());
+    assert!(inserted);
+
+    let inserted = set.insert(second_value);
+    assert!(!inserted);
+
+    assert!(
+        Arc::ptr_eq(set.iter().next().unwrap(), &first_value),
+        "Insert must not overwrite the value, so the contained value pointer \
+            must be the same as first value pointer we inserted"
+    );
+}