Add a new constructor to IndexSet and IndexMap respectively: 'just' c…

…reating a collection from a single element.

src/map.rs

@@ -1295,6 +1295,17 @@ impl<K, V, S> IndexMut<usize> for IndexMap<K, V, S> {
     }
 }
 
+impl<K, V, S> IndexMap<K, V, S>
+where
+    K: Hash + Eq,
+    S: BuildHasher + Default,
+{
+    /// Create an `IndexMap` from the key-value pair.
+    pub fn just(pair: (K, V)) -> Self {
+        Self::from_iter([pair])
+    }
+}
+
 impl<K, V, S> FromIterator<(K, V)> for IndexMap<K, V, S>
 where
     K: Hash + Eq,

src/set.rs

@@ -996,6 +996,17 @@ where
     }
 }
 
+impl<T, S> IndexSet<T, S>
+where
+    T: Hash + Eq,
+    S: BuildHasher + Default,
+{
+    /// Create a new set with the single `element`
+    pub fn just(element: T) -> Self {
+        Self::from_iter([element])
+    }
+}
+
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 impl<T, const N: usize> From<[T; N]> for IndexSet<T, RandomState>

tests/tests.rs

@@ -1,4 +1,16 @@
-use indexmap::{indexmap, indexset};
+use indexmap::{indexmap, indexset, IndexMap, IndexSet};
+
+#[test]
+fn test_set_just() {
+    let s = IndexSet::<u8>::just(42);
+    assert_eq!(s, IndexSet::<_>::from_iter([42]));
+}
+
+#[test]
+fn test_map_just() {
+    let m = IndexMap::<u8, u16>::just((42, 1337));
+    assert_eq!(m.get(&42), Some(&1337));
+}
 
 #[test]
 fn test_sort() {