Use a pooled StringBuilder in Dependency.GetID

Traces showed this taking upward of 1% of allocations of opening a solution.

Fixes dotnet#2918.

src/Microsoft.VisualStudio.ProjectSystem.Managed.VS/ProjectSystem/VS/Tree/Dependencies/Snapshot/Dependency.cs

@@ -1,17 +1,22 @@
 // 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.Concurrent;
 using System.Collections.Immutable;
 using System.Globalization;
 using System.Linq;
+using System.Text;
 using Microsoft.VisualStudio.Imaging.Interop;
 using Microsoft.VisualStudio.ProjectSystem.VS.Tree.Dependencies.CrossTarget;
 using Microsoft.VisualStudio.ProjectSystem.VS.Utilities;
+using Microsoft.VisualStudio.Text;
 
 namespace Microsoft.VisualStudio.ProjectSystem.VS.Tree.Dependencies.Snapshot
 {
     internal class Dependency : IDependency
     {
+        private static ConcurrentBag<StringBuilder> s_builderPool = new ConcurrentBag<StringBuilder>();
+
         // These priorities are for graph nodes only and are used to group graph nodes 
         // appropriatelly in order groups predefined order instead of alphabetically.
         // Order is not changed for top dependency nodes only for grpah hierarchies.
@@ -331,7 +336,25 @@ public static string GetID(ITargetFramework targetFramework, string providerType
             Requires.NotNullOrEmpty(providerType, nameof(providerType));
             Requires.NotNullOrEmpty(modelId, nameof(modelId));
 
-            return $"{targetFramework.ShortName}\\{providerType}\\{Normalize(modelId)}".TrimEnd(CommonConstants.BackSlashDelimiter);
+            StringBuilder sb = null;
+            try
+            {
+                int length = targetFramework.ShortName.Length + providerType.Length + 2;
+                if (!s_builderPool.TryTake(out sb))
+                {
+                    sb = new StringBuilder(length);
+                }
+
+                sb.Append(targetFramework.ShortName).Append('\\');
+                sb.Append(providerType).Append('\\');
+                sb.Append(Normalize(modelId));
+                sb.TrimEnd(CommonConstants.BackSlashDelimiter);
+                return sb.ToString();
+            }
+            finally
+            {
+                s_builderPool.Add(sb);
+            }
         }
 
         private static string GetFullPath(string originalItemSpec, string containingProjectPath)

src/Microsoft.VisualStudio.ProjectSystem.Managed/Text/StringBuilderExtensions.cs

@@ -46,5 +46,29 @@ public static void AppendFormat(this StringBuilder builder, StringFormat format)
                 builder.AppendFormat(format.Format, format.Arguments);
             }
         }
+
+        public static StringBuilder TrimEnd(this StringBuilder builder, params char[] trimChars)
+        {
+            while (builder.Length > 0)
+            {
+                var match = false;
+                foreach (var c in trimChars)
+                {
+                    if (builder[builder.Length - 1] == c)
+                    {
+                        match = true;
+                        builder.Length--;
+                        break;
+                    }
+                }
+
+                if (!match)
+                {
+                    return builder;
+                }
+            }
+
+            return builder;
+        }
     }
 }