Improve Annotation and SpanContext performance. (dotnet/razor#1881)

The razor perf test shows about 70 ms CPU of WithSpanContext is in allocation. GetAnnotation similarly is showing about 60 ms in allocation (of which this only partly improves)\n\nCommit migrated from dotnet/razor@a060f12

Author: ToddGrun

src/Razor/Microsoft.AspNetCore.Razor.Language/src/Legacy/LegacySyntaxNodeExtensions.cs

@@ -67,17 +67,29 @@ public static TNode WithSpanContext<TNode>(this TNode node, SpanContext spanCont
 
             var newAnnotation = new SyntaxAnnotation(SyntaxConstants.SpanContextKind, spanContext);
 
-            var newAnnotations = new List<SyntaxAnnotation>();
-            newAnnotations.Add(newAnnotation);
-            foreach (var annotation in node.GetAnnotations())
-            {
-                if (annotation.Kind != newAnnotation.Kind)
-                {
-                    newAnnotations.Add(annotation);
+            List<SyntaxAnnotation> newAnnotations = null;
+            if (node.ContainsAnnotations)
+            {
+                var existingNodeAnnotations = node.GetAnnotations();
+                for (int i = 0; i < existingNodeAnnotations.Length; i++)
+                {
+                    var annotation = existingNodeAnnotations[i];
+                    if (annotation.Kind != newAnnotation.Kind)
+                    {
+                        if (newAnnotations == null)
+                        {
+                            newAnnotations = new List<SyntaxAnnotation>();
+                            newAnnotations.Add(newAnnotation);
+                        }
+
+                        newAnnotations.Add(annotation);
+                    }
                 }
             }
+
+            var newAnnotationsArray = newAnnotations == null ? new[] { newAnnotation } : newAnnotations.ToArray();
 
-            return node.WithAnnotations(newAnnotations.ToArray());
+            return node.WithAnnotations(newAnnotationsArray);
         }
 
         public static SyntaxNode LocateOwner(this SyntaxNode node, SourceChange change)

src/Razor/Microsoft.AspNetCore.Razor.Language/src/Syntax/SyntaxNodeExtensions.cs

@@ -28,6 +28,11 @@ public static object GetAnnotationValue<TNode>(this TNode node, string key) wher
                 throw new ArgumentNullException(nameof(node));
             }
 
+            if (!node.ContainsAnnotations)
+            {
+                return null;
+            }
+
             var annotation = node.GetAnnotations().FirstOrDefault(n => n.Kind == key);
             return annotation?.Data;
         }