Improve CSharpLanguageCharacteristics.MapKeyword performance (dotnet/razor#1879)

* Improve CSharpLanguageCharacteristics.MapKeyword performance

The razor typing perf test profile I'm looking at has 156 ms of CPU cycles spent in this method, mostly in Enum.ToString()
\n\nCommit migrated from dotnet/razor@e821a46

Author: ToddGrun

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

@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. 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.Generic;
 using System.Diagnostics;
 using Microsoft.AspNetCore.Razor.Language.Syntax.InternalSyntax;
@@ -9,8 +10,6 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
 {
     internal class CSharpLanguageCharacteristics : LanguageCharacteristics<CSharpTokenizer>
     {
-        private static readonly CSharpLanguageCharacteristics _instance = new CSharpLanguageCharacteristics();
-
         private static Dictionary<SyntaxKind, string> _tokenSamples = new Dictionary<SyntaxKind, string>()
         {
             { SyntaxKind.Arrow, "->" },
@@ -64,8 +63,108 @@ internal class CSharpLanguageCharacteristics : LanguageCharacteristics<CSharpTok
             { SyntaxKind.Transition, "@" },
         };
 
+        // Allows performance optimization of GetKeyword such that it need not do Enum.ToString
+        private static IReadOnlyDictionary<CSharpKeyword, string> _keywordNames = new Dictionary<CSharpKeyword, string>()
+        {
+            { CSharpKeyword.Await, "await" },
+            { CSharpKeyword.Abstract, "abstract" },
+            { CSharpKeyword.Byte, "byte" },
+            { CSharpKeyword.Class, "class" },
+            { CSharpKeyword.Delegate, "delegate" },
+            { CSharpKeyword.Event, "event" },
+            { CSharpKeyword.Fixed, "fixed" },
+            { CSharpKeyword.If, "if" },
+            { CSharpKeyword.Internal, "internal" },
+            { CSharpKeyword.New, "new" },
+            { CSharpKeyword.Override, "override" },
+            { CSharpKeyword.Readonly, "readonly" },
+            { CSharpKeyword.Short, "short" },
+            { CSharpKeyword.Struct, "struct" },
+            { CSharpKeyword.Try, "try" },
+            { CSharpKeyword.Unsafe, "unsafe" },
+            { CSharpKeyword.Volatile, "volatile" },
+            { CSharpKeyword.As, "as" },
+            { CSharpKeyword.Do, "do" },
+            { CSharpKeyword.Is, "is" },
+            { CSharpKeyword.Params, "params" },
+            { CSharpKeyword.Ref, "ref" },
+            { CSharpKeyword.Switch, "switch" },
+            { CSharpKeyword.Ushort, "ushort" },
+            { CSharpKeyword.While, "while" },
+            { CSharpKeyword.Case, "case" },
+            { CSharpKeyword.Const, "const" },
+            { CSharpKeyword.Explicit, "explicit" },
+            { CSharpKeyword.Float, "float" },
+            { CSharpKeyword.Null, "null" },
+            { CSharpKeyword.Sizeof, "sizeof" },
+            { CSharpKeyword.Typeof, "typeof" },
+            { CSharpKeyword.Implicit, "implicit" },
+            { CSharpKeyword.Private, "private" },
+            { CSharpKeyword.This, "this" },
+            { CSharpKeyword.Using, "using" },
+            { CSharpKeyword.Extern, "extern" },
+            { CSharpKeyword.Return, "return" },
+            { CSharpKeyword.Stackalloc, "stackalloc" },
+            { CSharpKeyword.Uint, "uint" },
+            { CSharpKeyword.Base, "base" },
+            { CSharpKeyword.Catch, "catch" },
+            { CSharpKeyword.Continue, "continue" },
+            { CSharpKeyword.Double, "double" },
+            { CSharpKeyword.For, "for" },
+            { CSharpKeyword.In, "in" },
+            { CSharpKeyword.Lock, "lock" },
+            { CSharpKeyword.Object, "object" },
+            { CSharpKeyword.Protected, "protected" },
+            { CSharpKeyword.Static, "static" },
+            { CSharpKeyword.False, "false" },
+            { CSharpKeyword.Public, "public" },
+            { CSharpKeyword.Sbyte, "sbyte" },
+            { CSharpKeyword.Throw, "throw" },
+            { CSharpKeyword.Virtual, "virtual" },
+            { CSharpKeyword.Decimal, "decimal" },
+            { CSharpKeyword.Else, "else" },
+            { CSharpKeyword.Operator, "operator" },
+            { CSharpKeyword.String, "string" },
+            { CSharpKeyword.Ulong, "ulong" },
+            { CSharpKeyword.Bool, "bool" },
+            { CSharpKeyword.Char, "char" },
+            { CSharpKeyword.Default, "default" },
+            { CSharpKeyword.Foreach, "foreach" },
+            { CSharpKeyword.Long, "long" },
+            { CSharpKeyword.Void, "void" },
+            { CSharpKeyword.Enum, "enum" },
+            { CSharpKeyword.Finally, "finally" },
+            { CSharpKeyword.Int, "int" },
+            { CSharpKeyword.Out, "out" },
+            { CSharpKeyword.Sealed, "sealed" },
+            { CSharpKeyword.True, "true" },
+            { CSharpKeyword.Goto, "goto" },
+            { CSharpKeyword.Unchecked, "unchecked" },
+            { CSharpKeyword.Interface, "interface" },
+            { CSharpKeyword.Break, "break" },
+            { CSharpKeyword.Checked, "checked" },
+            { CSharpKeyword.Namespace, "namespace" },
+            { CSharpKeyword.When,  "when" },
+        };
+
+        private static readonly CSharpLanguageCharacteristics _instance = new CSharpLanguageCharacteristics();
+
         protected CSharpLanguageCharacteristics()
         {
+#if DEBUG
+            var values = Enum.GetValues(typeof(CSharpKeyword));
+
+            Debug.Assert(values.Length == _keywordNames.Count, "_keywordNames and CSharpKeyword are out of sync");
+            for (var i = 0; i < values.Length; i++)
+            {
+                var keyword = (CSharpKeyword) values.GetValue(i);
+
+                var expectedValue = keyword.ToString().ToLowerInvariant();
+                var actualValue = _keywordNames[keyword];
+
+                Debug.Assert(expectedValue == actualValue, "_keywordNames and CSharpKeyword are out of sync for " + expectedValue);
+            }
+#endif
         }
 
         public static CSharpLanguageCharacteristics Instance => _instance;
@@ -170,7 +269,12 @@ public override SyntaxKind FlipBracket(SyntaxKind bracket)
 
         public static string GetKeyword(CSharpKeyword keyword)
         {
-            return keyword.ToString().ToLowerInvariant();
+            if (!_keywordNames.TryGetValue(keyword, out var value))
+            {
+                value = keyword.ToString().ToLowerInvariant();
+            }
+
+            return value;
         }
     }
 }