Grow rather than reserve in insert to optimize out the checked_add call

Author: nipunn1313

lib.rs

@@ -303,7 +303,10 @@ impl<A: Array> SmallVec<A> {
     }
 
     pub fn insert(&mut self, index: usize, element: A::Item) {
-        self.reserve(1);
+        let cap = self.capacity();
+        if self.len == cap {
+            self.grow(cmp::max(cap * 2, 1))
+        }
 
         let len = self.len;
         assert!(index <= len);