Rollup merge of rust-lang#67358 - cuviper:get_or_insert_owned, r=LukasKalbertodt

Add HashSet::get_or_insert_owned

This is an extension for tracking issue rust-lang#60896. The more-general `get_or_insert_with` has potential for misuse, so we might remove it, but I think `get_or_insert_owned` covers most use cases.

Author: JohnTitor

src/libstd/collections/hash/set.rs

@@ -631,6 +631,38 @@ where
         self.map.raw_entry_mut().from_key(&value).or_insert(value, ()).0
     }
 
+    /// Inserts an owned copy of the given `value` into the set if it is not
+    /// present, then returns a reference to the value in the set.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(hash_set_entry)]
+    ///
+    /// use std::collections::HashSet;
+    ///
+    /// let mut set: HashSet<String> = ["cat", "dog", "horse"]
+    ///     .iter().map(|&pet| pet.to_owned()).collect();
+    ///
+    /// assert_eq!(set.len(), 3);
+    /// for &pet in &["cat", "dog", "fish"] {
+    ///     let value = set.get_or_insert_owned(pet);
+    ///     assert_eq!(value, pet);
+    /// }
+    /// assert_eq!(set.len(), 4); // a new "fish" was inserted
+    /// ```
+    #[inline]
+    #[unstable(feature = "hash_set_entry", issue = "60896")]
+    pub fn get_or_insert_owned<Q: ?Sized>(&mut self, value: &Q) -> &T
+    where
+        T: Borrow<Q>,
+        Q: Hash + Eq + ToOwned<Owned = T>,
+    {
+        // Although the raw entry gives us `&mut T`, we only return `&T` to be consistent with
+        // `get`. Key mutation is "raw" because you're not supposed to affect `Eq` or `Hash`.
+        self.map.raw_entry_mut().from_key(value).or_insert_with(|| (value.to_owned(), ())).0
+    }
+
     /// Inserts a value computed from `f` into the set if the given `value` is
     /// not present, then returns a reference to the value in the set.
     ///