insert_unique_unchecked operation

Sometimes a map is constructed when it is known that all keys are
unique (e. e. if keys are coming from another map or from a
sorted/deduplicated iterator). In this case we can make insertion
faster by skipping a check that a key already exists in the map.

`insert_unique_unchecked` is guaranteed to be memory-safe, but does
not guarantee anything beyond that: if inserted key is not unique,
`HashMap` can panic, loop forever, return incorrect entry etc.

Added simple benchmark. `insert_unique_unchecked` is about 30%
faster than `insert`.  Your mileage may vary of course.

Similar PR was
[added to `indexmap` crate](https://github.com/bluss/indexmap/pull/200)
and they asked to discuss the name of the operation with `hashbrown`
crate owners to come to the same naming convention (if `hashbrown`
is willing to have the same operation).

benches/insert_unique_unchecked.rs

@@ -0,0 +1,32 @@
+//! Compare `insert` and `insert_unique_unchecked` operations performance.
+
+#![feature(test)]
+
+extern crate test;
+
+use hashbrown::HashMap;
+use test::Bencher;
+
+#[bench]
+fn insert(b: &mut Bencher) {
+    let keys: Vec<String> = (0..1000).map(|i| format!("xxxx{}yyyy", i)).collect();
+    b.iter(|| {
+        let mut m = HashMap::with_capacity(1000);
+        for k in &keys {
+            m.insert(k, k);
+        }
+        m
+    });
+}
+
+#[bench]
+fn insert_unique_unchecked(b: &mut Bencher) {
+    let keys: Vec<String> = (0..1000).map(|i| format!("xxxx{}yyyy", i)).collect();
+    b.iter(|| {
+        let mut m = HashMap::with_capacity(1000);
+        for k in &keys {
+            m.insert_unique_unchecked(k, k);
+        }
+        m
+    });
+}

src/map.rs

@@ -1278,6 +1278,31 @@ where
         }
     }
 
+    /// Insert a key-value pair into the map without checking
+    /// if the key already exists in the map.
+    ///
+    /// This operation is safe if a key does not exist in the map.
+    ///
+    /// However, if a key exists in the map already, the behavior is unspecified:
+    /// this operation may panic, loop forever, or any following operation with the map
+    /// may panic, loop forever or return arbitrary result.
+    ///
+    /// That said, this operation (and following operations) are guaranteed to
+    /// not violate memory safety.
+    ///
+    /// This operation is faster than regular insert, because it does not perform
+    /// lookup before insertion.
+    ///
+    /// This operation is useful during initial population of the map.
+    /// For example, when constructing a map from another map, we know
+    /// that keys are unique.
+    #[cfg_attr(feature = "inline-more", inline)]
+    pub fn insert_unique_unchecked(&mut self, k: K, v: V) {
+        let hash = make_insert_hash::<K, S>(&self.hash_builder, &k);
+        self.table
+            .insert(hash, (k, v), make_hasher::<K, _, V, S>(&self.hash_builder));
+    }
+
     /// Tries to insert a key-value pair into the map, and returns
     /// a mutable reference to the value in the entry.
     ///
@@ -3898,6 +3923,16 @@ mod test_map {
         assert_eq!(*m.get(&5).unwrap(), 3);
     }
 
+    #[test]
+    fn test_insert_unique_unchecked() {
+        let mut map = HashMap::new();
+        map.insert_unique_unchecked(10, 11);
+        map.insert_unique_unchecked(20, 21);
+        assert_eq!(Some(&11), map.get(&10));
+        assert_eq!(Some(&21), map.get(&20));
+        assert_eq!(None, map.get(&30));
+    }
+
     #[test]
     fn test_is_empty() {
         let mut m = HashMap::with_capacity(4);

src/set.rs

@@ -991,6 +991,28 @@ where
         self.map.insert(value, ()).is_none()
     }
 
+    /// Insert a value the set without checking if the value already exists in the set.
+    ///
+    /// This operation is safe if a value does not exist in the set.
+    ///
+    /// However, if a value exists in the set already, the behavior is unspecified:
+    /// this operation may panic, loop forever, or any following operation with the set
+    /// may panic, loop forever or return arbitrary result.
+    ///
+    /// That said, this operation (and following operations) are guaranteed to
+    /// not violate memory safety.
+    ///
+    /// This operation is faster than regular insert, because it does not perform
+    /// lookup before insertion.
+    ///
+    /// This operation is useful during initial population of the set.
+    /// For example, when constructing a set from another set, we know
+    /// that values are unique.
+    #[cfg_attr(feature = "inline-more", inline)]
+    pub fn insert_unique_unchecked(&mut self, value: T) {
+        self.map.insert_unique_unchecked(value, ());
+    }
+
     /// Adds a value to the set, replacing the existing value, if any, that is equal to the given
     /// one. Returns the replaced value.
     ///