Optimize the += operation for a collection when it's an object array (#23901)

src/System.Management.Automation/engine/parser/Compiler.cs

@@ -39,6 +39,9 @@ namespace System.Management.Automation.Language
         internal static readonly MethodInfo ObjectList_ToArray =
             typeof(List<object>).GetMethod(nameof(List<object>.ToArray), Type.EmptyTypes);
 
+        internal static readonly MethodInfo ArrayOps_AddObject =
+            typeof(ArrayOps).GetMethod(nameof(ArrayOps.AddObjectArray), StaticFlags);
+
         internal static readonly MethodInfo ArrayOps_GetMDArrayValue =
             typeof(ArrayOps).GetMethod(nameof(ArrayOps.GetMDArrayValue), StaticFlags);
 

src/System.Management.Automation/engine/runtime/Binding/Binders.cs

@@ -2717,6 +2717,14 @@ namespace System.Management.Automation.Language
                                            lhsEnumerator.Expression.Cast(typeof(IEnumerator)),
                                            rhsEnumerator.Expression.Cast(typeof(IEnumerator)));
                 }
+                else if (target.Value is object[] targetArray)
+                {
+                    // Adding 1 item to an object[]
+                    // This is an optimisation over the default EnumerableOps_AddObject.
+                    call = Expression.Call(CachedReflectionInfo.ArrayOps_AddObject,
+                                           target.Expression.Cast(typeof(object[])),
+                                           arg.Expression.Cast(typeof(object)));
+                }
                 else
                 {
                     // Adding 1 item to a list

src/System.Management.Automation/engine/runtime/Operations/ArrayOps.cs

@@ -12,6 +12,15 @@ namespace System.Management.Automation
 {
     internal static class ArrayOps
     {
+        internal static object AddObjectArray(object[] lhs, object rhs)
+        {
+            int newIdx = lhs.Length;
+            Array.Resize(ref lhs, newIdx + 1);
+            lhs[newIdx] = rhs;
+
+            return lhs;
+        }
+
         internal static object[] SlicingIndex(object target, object[] indexes, Func<object, object, object> indexer)
         {
             var result = new object[indexes.Length];