IndexMap::insert_no_overwrite_unchecked

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, or any following operation
with the map may panic or return arbitrary result.

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.

Simple benchmark of `insert` vs `insert_no_overwrite_unchecked` included:

```
test insert                        ... bench:      14,112 ns/iter (+/- 1,653)
test insert_no_overwrite_unchecked ... bench:      11,099 ns/iter (+/- 1,100)
```

benches/insert_no_overwrite_unchecked.rs

@@ -0,0 +1,29 @@
+#![feature(test)]
+
+extern crate test;
+
+use test::Bencher;
+
+use indexmap::IndexMap;
+
+#[bench]
+fn insert(b: &mut Bencher) {
+    b.iter(|| {
+        let mut m = IndexMap::with_capacity(1000);
+        for i in 0..1000 {
+            m.insert(i, i);
+        }
+        m
+    });
+}
+
+#[bench]
+fn insert_no_overwrite_unchecked(b: &mut Bencher) {
+    b.iter(|| {
+        let mut m = IndexMap::with_capacity(1000);
+        for i in 0..1000 {
+            m.insert_no_overwrite_unchecked(i, i);
+        }
+        m
+    });
+}

src/map.rs

@@ -373,6 +373,26 @@ where
         self.core.insert_full(hash, key, value)
     }
 
+    /// 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, or any following operation with the map may panic
+    /// or return arbitrary result.
+    ///
+    /// 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.
+    pub fn insert_no_overwrite_unchecked(&mut self, key: K, value: V) {
+        let hash = self.hash(&key);
+        self.core.push(hash, key, value);
+    }
+
     /// Get the given key’s corresponding entry in the map for insertion and/or
     /// in-place manipulation.
     ///
@@ -1834,4 +1854,14 @@ mod tests {
         assert!(values.contains(&'b'));
         assert!(values.contains(&'c'));
     }
+
+    #[test]
+    fn insert_no_overwrite_unchecked() {
+        let mut map = IndexMap::new();
+        map.insert_no_overwrite_unchecked(10, 11);
+        map.insert_no_overwrite_unchecked(20, 21);
+        assert_eq!(Some(&11), map.get(&10));
+        assert_eq!(Some(&21), map.get(&20));
+        assert_eq!(None, map.get(&30));
+    }
 }

src/map/core.rs

@@ -209,7 +209,7 @@ impl<K, V> IndexMapCore<K, V> {
 
     /// Append a key-value pair, *without* checking whether it already exists,
     /// and return the pair's new index.
-    fn push(&mut self, hash: HashValue, key: K, value: V) -> usize {
+    pub(crate) fn push(&mut self, hash: HashValue, key: K, value: V) -> usize {
         let i = self.entries.len();
         self.indices.insert(hash.get(), i, get_hash(&self.entries));
         if i == self.entries.capacity() {