Reduce allocations when creating sytem items.

src/Compilers/CSharp/Portable/Syntax/SyntaxFactory.cs

@@ -1366,17 +1366,8 @@ public static SyntaxTriviaList TriviaList(params SyntaxTrivia[] trivias)
         /// <param name="trivias">A sequence of trivia.</param>
         public static SyntaxTriviaList TriviaList(IEnumerable<SyntaxTrivia> trivias)
         {
-            if (trivias == null)
-            {
-                return default(SyntaxTriviaList);
-            }
-
-            SyntaxTriviaListBuilder builder = SyntaxTriviaListBuilder.Create();
-            foreach (SyntaxTrivia trivia in trivias)
-            {
-                builder.Add(trivia);
-            }
-
+            var builder = SyntaxTriviaListBuilder.Create();
+            builder.AddRange(trivias);
             return builder.ToList();
         }
 

src/Compilers/Core/Portable/Syntax/SyntaxTriviaList.cs

@@ -3,11 +3,12 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
-using System.Linq;
 using System.Diagnostics;
+using System.Linq;
+using System.Runtime.InteropServices;
+using Microsoft.CodeAnalysis.Syntax;
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Utilities;
-using System.Runtime.InteropServices;
 
 namespace Microsoft.CodeAnalysis
 {
@@ -248,21 +249,31 @@ public SyntaxTriviaList InsertRange(int index, IEnumerable<SyntaxTrivia> trivia)
                 throw new ArgumentOutOfRangeException(nameof(index));
             }
 
-            var items = trivia.ToList();
-            if (items.Count == 0)
+            if (trivia == null)
             {
-                return this;
+                throw new ArgumentNullException(nameof(trivia));
             }
 
-            var list = this.ToList();
-            list.InsertRange(index, items);
+            var builder = SyntaxTriviaListBuilder.Create();
+
+            var current = 0;
+            foreach (var thisTrivia in this)
+            {
+                if (current == index)
+                {
+                    builder.AddRange(trivia);
+                }
+
+                builder.Add(thisTrivia);
+                current++;
+            }
 
-            if (list.Count == 0)
+            if (current == index)
             {
-                return this;
+                builder.AddRange(trivia);
             }
 
-            return new SyntaxTriviaList(default(SyntaxToken), list[0].UnderlyingNode.CreateList(list.Select(n => n.UnderlyingNode)), 0, 0);
+            return builder.ToList();
         }
 
         /// <summary>

src/Compilers/Core/Portable/Syntax/SyntaxTriviaListBuilder.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System;
+using System.Collections;
+using System.Collections.Generic;
 
 namespace Microsoft.CodeAnalysis.Syntax
 {
@@ -42,6 +44,17 @@ public void Clear()
             }
         }
 
+        public void AddRange(IEnumerable<SyntaxTrivia> items)
+        {
+            if (items != null)
+            {
+                foreach (var item in items)
+                {
+                    this.Add(item);
+                }
+            }
+        }
+
         public SyntaxTriviaListBuilder Add(SyntaxTrivia item)
         {
             if (_nodes == null || _count >= _nodes.Length)

src/Compilers/VisualBasic/Portable/Syntax/SyntaxExtensions.vb

@@ -15,10 +15,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
 
         <Extension()>
         Public Function ToSyntaxTriviaList(sequence As IEnumerable(Of SyntaxTrivia)) As SyntaxTriviaList
-            Return SyntaxFactory.TriviaList(sequence.Aggregate(New List(Of SyntaxTrivia), Function(list, trivia)
-                                                                                              list.Add(trivia)
-                                                                                              Return list
-                                                                                          End Function))
+            Return SyntaxFactory.TriviaList(sequence)
         End Function
 
         <Extension()>

src/Compilers/VisualBasic/Portable/Syntax/SyntaxNodeFactories.vb

@@ -825,14 +825,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         ''' </summary>
         ''' <param name="trivias">A sequence of trivia.</param>
         Public Shared Function TriviaList(trivias As IEnumerable(Of SyntaxTrivia)) As SyntaxTriviaList
-            If trivias IsNot Nothing Then
-                Dim builder = SyntaxTriviaListBuilder.Create()
-                For Each t In trivias
-                    builder.Add(t)
-                Next
-                Return builder.ToList
-            End If
-            Return New SyntaxTriviaList()
+            Dim builder = SyntaxTriviaListBuilder.Create()
+            builder.AddRange(trivias)
+            Return builder.ToList()
         End Function
 
         ''' <summary>