Avoid some smaller allocations in ItemGroupintrinsicTask

src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/ItemGroupIntrinsicTask.cs

@@ -482,23 +482,56 @@ private List<ProjectItemInstance> ExpandItemIntoItems(
             }
 
             // Filter the metadata as appropriate
+            List<string> metadataToRemove = null;
             if (keepMetadata != null)
             {
-                foreach (var item in items)
+                foreach (ProjectItemInstance item in items)
                 {
-                    var metadataToRemove = item.MetadataNames.Where(name => !keepMetadata.Contains(name));
-                    foreach (var metadataName in metadataToRemove)
+                    if (metadataToRemove == null)
+                    {
+                        metadataToRemove = new List<string>();
+                    }
+                    else
+                    {
+                        metadataToRemove.Clear();
+                    }
+
+                    foreach (string metadataName in item.EnumerableMetadataNames)
+                    {
+                        if (!keepMetadata.Contains(metadataName))
+                        {
+                            metadataToRemove.Add(metadataName);
+                        }
+                    }
+
+                    foreach(string metadataName in metadataToRemove)
                     {
                         item.RemoveMetadata(metadataName);
                     }
                 }
             }
             else if (removeMetadata != null)
             {
-                foreach (var item in items)
+                foreach (ProjectItemInstance item in items)
                 {
-                    var metadataToRemove = item.MetadataNames.Where(name => removeMetadata.Contains(name));
-                    foreach (var metadataName in metadataToRemove)
+                    if (metadataToRemove == null)
+                    {
+                        metadataToRemove = new List<string>();
+                    }
+                    else
+                    {
+                        metadataToRemove.Clear();
+                    }
+
+                    foreach (string metadataName in item.EnumerableMetadataNames)
+                    {
+                        if (removeMetadata.Contains(metadataName))
+                        {
+                            metadataToRemove.Add(metadataName);
+                        }
+                    }
+
+                    foreach (string metadataName in metadataToRemove)
                     {
                         item.RemoveMetadata(metadataName);
                     }
@@ -517,7 +550,7 @@ private List<ProjectItemInstance> ExpandItemIntoItems(
         /// <returns>A list of matching items</returns>
         private HashSet<string> EvaluateExcludePaths(IReadOnlyList<string> excludes, ElementLocation excludeLocation)
         {
-            HashSet<string> excludesUnescapedForComparison = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+            HashSet<string> excludesUnescapedForComparison = new HashSet<string>(excludes.Count, StringComparer.OrdinalIgnoreCase);
             foreach (string excludeSplit in excludes)
             {
                 string[] excludeSplitFiles = EngineFileUtilities.GetFileListUnescaped(

src/Build/Instance/ProjectInstance.cs

@@ -827,7 +827,7 @@ private ProjectInstance(ProjectInstance that, bool isImmutable, RequestedProject
                                 // any extant metadata.
                                 // UNDONE: This could be achieved at lower GC cost by applying
                                 // the metadata filter at DeepClone time above.
-                                foreach (var metadataName in filteredItem.MetadataNames)
+                                foreach (var metadataName in filteredItem.EnumerableMetadataNames)
                                 {
                                     if (!itemFilter.Value.Contains(metadataName, StringComparer.OrdinalIgnoreCase))
                                     {

src/Build/Instance/ProjectItemInstance.cs

@@ -285,6 +285,8 @@
             get { return new ReadOnlyCollection<string>(_taskItem.MetadataNames.Cast<string>()); }
         }
 
+        internal TaskItem.MetadataNamesEnumerable EnumerableMetadataNames => _taskItem.EnumerableMetadatNames;
+
         /// <summary>
         /// ITaskItem implementation
         /// </summary>
@@ -957,6 +959,8 @@
                 }
             }
 
+            public MetadataNamesEnumerable EnumerableMetadataNames => new MetadataNamesEnumerable(this);
+
             /// <summary>
             /// Gets the number of metadata set on the item.
             /// Computed, not necessarily fast.
@@ -1961,6 +1965,58 @@
                 return null;
             }
 
+            internal readonly struct MetadataNamesEnumerable
+            {
+                private readonly TaskItem _item;
+
+                public MetadataNamesEnumerable(TaskItem taskItem) => _item = taskItem;
+
+                public readonly MetadataNamesEnumerator GetEnumerator() => new MetadataNamesEnumerator(_item.MetadataCollection);
+            }
+
+            internal struct MetadataNamesEnumerator
+            {
+                private readonly IEnumerator<ProjectMetadataInstance> _metadataCollectionEnumerator;
+                private bool _metadataNamesEnumerated;
+                private int _itemSpecModifiersIndex;
+
+                internal MetadataNamesEnumerator(ICopyOnWritePropertyDictionary<ProjectMetadataInstance> metadataCollection)
+                {
+                    _metadataCollectionEnumerator = ((IEnumerable<ProjectMetadataInstance>)metadataCollection).GetEnumerator();
+                    _metadataNamesEnumerated = false;
+                    _itemSpecModifiersIndex = 0;
+                }
+
+                public string Current { get; private set; }
+
+                public bool MoveNext()
+                {
+                    if (!_metadataNamesEnumerated)
+                    {
+                        if (_metadataCollectionEnumerator.MoveNext())
+                        {
+                            Current = _metadataCollectionEnumerator.Current.Name;
+
+                            return true;
+                        }
+                        else
+                        {
+                            _metadataNamesEnumerated = true;
+                        }
+                    }
+
+                    if (_itemSpecModifiersIndex < FileUtilities.ItemSpecModifiers.All.Length)
+                    {
+                        Current = FileUtilities.ItemSpecModifiers.All[_itemSpecModifiersIndex];
+                        ++_itemSpecModifiersIndex;
+
+                        return true;
+                    }
+
+                    return false;
+                }
+            }
+
             /// <summary>
             /// A class factory for instance model items.
             /// </summary>