Add an explicit bounds check in move_index

It did already panic as expected, but with a confusing message if the
`to` index was out of bounds. Now we have a direct bounds check for that
at the start, just as there already was for the `from` index.

src/map/core.rs

@@ -480,6 +480,7 @@ impl<K, V> IndexMapCore<K, V> {
 
     pub(super) fn move_index(&mut self, from: usize, to: usize) {
         let from_hash = self.entries[from].hash;
+        let _ = self.entries[to]; // explicit bounds check
         if from != to {
             // Use a sentinel index so other indices don't collide.
             update_index(&mut self.indices, from_hash, from, usize::MAX);

src/map/tests.rs

@@ -813,3 +813,18 @@ fn test_partition_point() {
     assert_eq!(b.partition_point(|_, &x| x < 7), 4);
     assert_eq!(b.partition_point(|_, &x| x < 8), 5);
 }
+
+macro_rules! move_index_oob {
+    ($test:ident, $from:expr, $to:expr) => {
+        #[test]
+        #[should_panic(expected = "index out of bounds")]
+        fn $test() {
+            let mut map: IndexMap<i32, ()> = (0..10).map(|k| (k, ())).collect();
+            map.move_index($from, $to);
+        }
+    }
+}
+move_index_oob!(test_move_index_out_of_bounds_0_10, 0, 10);
+move_index_oob!(test_move_index_out_of_bounds_0_max, 0, usize::MAX);
+move_index_oob!(test_move_index_out_of_bounds_10_0, 10, 0);
+move_index_oob!(test_move_index_out_of_bounds_max_0, usize::MAX, 0);