Eliminate extra List<RazorError> copies

Changing to IReadOnlyList since we always want to support indexing.
Replacing ToArray() with non-linq when needed, and with a static
EmptyArray when not needed.

Eliminates 50mb of list copies.

src/Microsoft.AspNet.Razor/Parser/CSharpLanguageCharacteristics.cs

@@ -80,7 +80,7 @@ namespace Microsoft.AspNet.Razor.Parser
             return new CSharpTokenizer(source);
         }
 
-        protected override CSharpSymbol CreateSymbol(SourceLocation location, string content, CSharpSymbolType type, IEnumerable<RazorError> errors)
+        protected override CSharpSymbol CreateSymbol(SourceLocation location, string content, CSharpSymbolType type, IReadOnlyList<RazorError> errors)
         {
             return new CSharpSymbol(location, content, type, errors);
         }

src/Microsoft.AspNet.Razor/Parser/HtmlLanguageCharacteristics.cs

@@ -129,7 +129,7 @@ namespace Microsoft.AspNet.Razor.Parser
             }
         }
 
-        protected override HtmlSymbol CreateSymbol(SourceLocation location, string content, HtmlSymbolType type, IEnumerable<RazorError> errors)
+        protected override HtmlSymbol CreateSymbol(SourceLocation location, string content, HtmlSymbolType type, IReadOnlyList<RazorError> errors)
         {
             return new HtmlSymbol(location, content, type, errors);
         }

src/Microsoft.AspNet.Razor/Parser/LanguageCharacteristics.cs

@@ -91,7 +91,7 @@ namespace Microsoft.AspNet.Razor.Parser
 
         public virtual Tuple<TSymbol, TSymbol> SplitSymbol(TSymbol symbol, int splitAt, TSymbolType leftType)
         {
-            var left = CreateSymbol(symbol.Start, symbol.Content.Substring(0, splitAt), leftType, Enumerable.Empty<RazorError>());
+            var left = CreateSymbol(symbol.Start, symbol.Content.Substring(0, splitAt), leftType, RazorError.EmptyArray);
             TSymbol right = null;
             if (splitAt < symbol.Content.Length)
             {
@@ -107,6 +107,6 @@ namespace Microsoft.AspNet.Razor.Parser
             return type == KnownSymbolType.Unknown || !Equals(GetKnownSymbolType(type), GetKnownSymbolType(KnownSymbolType.Unknown));
         }
 
-        protected abstract TSymbol CreateSymbol(SourceLocation location, string content, TSymbolType type, IEnumerable<RazorError> errors);
+        protected abstract TSymbol CreateSymbol(SourceLocation location, string content, TSymbolType type, IReadOnlyList<RazorError> errors);
     }
 }

src/Microsoft.AspNet.Razor/RazorError.cs

@@ -9,6 +9,8 @@ namespace Microsoft.AspNet.Razor
 {
     public class RazorError : IEquatable<RazorError>
     {
+        internal static readonly RazorError[] EmptyArray = new RazorError[0];
+
         internal const int DefaultErrorLength = 1;
 
         /// <summary>

src/Microsoft.AspNet.Razor/Tokenizer/CSharpTokenizer.cs

@@ -72,7 +72,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer
             get { return CSharpSymbolType.RazorCommentStar; }
         }
 
-        protected override CSharpSymbol CreateSymbol(SourceLocation start, string content, CSharpSymbolType type, IEnumerable<RazorError> errors)
+        protected override CSharpSymbol CreateSymbol(SourceLocation start, string content, CSharpSymbolType type, IReadOnlyList<RazorError> errors)
         {
             return new CSharpSymbol(start, content, type, errors);
         }

src/Microsoft.AspNet.Razor/Tokenizer/HtmlTokenizer.cs

@@ -59,7 +59,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer
             }
         }
 
-        protected override HtmlSymbol CreateSymbol(SourceLocation start, string content, HtmlSymbolType type, IEnumerable<RazorError> errors)
+        protected override HtmlSymbol CreateSymbol(SourceLocation start, string content, HtmlSymbolType type, IReadOnlyList<RazorError> errors)
         {
             return new HtmlSymbol(start, content, type, errors);
         }

src/Microsoft.AspNet.Razor/Tokenizer/Symbols/CSharpSymbol.cs

@@ -11,7 +11,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer.Symbols
     {
         // Helper constructor
         public CSharpSymbol(int offset, int line, int column, string content, CSharpSymbolType type)
-            : this(new SourceLocation(offset, line, column), content, type, Enumerable.Empty<RazorError>())
+            : this(new SourceLocation(offset, line, column), content, type, RazorError.EmptyArray)
         {
             if (content == null)
             {
@@ -20,7 +20,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer.Symbols
         }
 
         public CSharpSymbol(SourceLocation start, string content, CSharpSymbolType type)
-            : this(start, content, type, Enumerable.Empty<RazorError>())
+            : this(start, content, type, RazorError.EmptyArray)
         {
             if (content == null)
             {
@@ -34,7 +34,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer.Symbols
             int column,
             string content,
             CSharpSymbolType type,
-            IEnumerable<RazorError> errors)
+            IReadOnlyList<RazorError> errors)
             : base(new SourceLocation(offset, line, column), content, type, errors)
         {
             if (content == null)
@@ -47,7 +47,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer.Symbols
             SourceLocation start,
             string content,
             CSharpSymbolType type,
-            IEnumerable<RazorError> errors)
+            IReadOnlyList<RazorError> errors)
             : base(start, content, type, errors)
         {
             if (content == null)

src/Microsoft.AspNet.Razor/Tokenizer/Symbols/HtmlSymbol.cs

@@ -11,7 +11,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer.Symbols
     {
         // Helper constructor
         public HtmlSymbol(int offset, int line, int column, string content, HtmlSymbolType type)
-            : this(new SourceLocation(offset, line, column), content, type, Enumerable.Empty<RazorError>())
+            : this(new SourceLocation(offset, line, column), content, type, RazorError.EmptyArray)
         {
             if (content == null)
             {
@@ -20,7 +20,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer.Symbols
         }
 
         public HtmlSymbol(SourceLocation start, string content, HtmlSymbolType type)
-            : base(start, content, type, Enumerable.Empty<RazorError>())
+            : base(start, content, type, RazorError.EmptyArray)
         {
             if (content == null)
             {
@@ -34,7 +34,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer.Symbols
             int column,
             string content,
             HtmlSymbolType type,
-            IEnumerable<RazorError> errors)
+            IReadOnlyList<RazorError> errors)
             : base(new SourceLocation(offset, line, column), content, type, errors)
         {
             if (content == null)
@@ -47,7 +47,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer.Symbols
             SourceLocation start,
             string content,
             HtmlSymbolType type,
-            IEnumerable<RazorError> errors)
+            IReadOnlyList<RazorError> errors)
             : base(start, content, type, errors)
         {
             if (content == null)

src/Microsoft.AspNet.Razor/Tokenizer/Symbols/SymbolBase.cs

@@ -15,7 +15,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer.Symbols
             SourceLocation start,
             string content,
             TType type,
-            IEnumerable<RazorError> errors)
+            IReadOnlyList<RazorError> errors)
         {
             if (content == null)
             {
@@ -32,7 +32,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer.Symbols
 
         public string Content { get; }
 
-        public IEnumerable<RazorError> Errors { get; }
+        public IReadOnlyList<RazorError> Errors { get; }
 
         public TType Type { get; }
 

src/Microsoft.AspNet.Razor/Tokenizer/Tokenizer.cs

@@ -90,7 +90,7 @@ namespace Microsoft.AspNet.Razor.Tokenizer
             CurrentState = StartState;
         }
 
-        protected abstract TSymbol CreateSymbol(SourceLocation start, string content, TSymbolType type, IEnumerable<RazorError> errors);
+        protected abstract TSymbol CreateSymbol(SourceLocation start, string content, TSymbolType type, IReadOnlyList<RazorError> errors);
 
         protected TSymbol Single(TSymbolType type)
         {
@@ -115,7 +115,14 @@ namespace Microsoft.AspNet.Razor.Tokenizer
             TSymbol sym = null;
             if (HaveContent)
             {
-                sym = CreateSymbol(start, Buffer.ToString(), type, CurrentErrors.ToArray());
+                // Perf: Don't allocate a new errors array unless necessary.
+                var errors = CurrentErrors.Count == 0 ? RazorError.EmptyArray : new RazorError[CurrentErrors.Count];
+                for (var i = 0; i < CurrentErrors.Count; i++)
+                {
+                    errors[i] = CurrentErrors[i];
+                }
+
+                sym = CreateSymbol(start, Buffer.ToString(), type, errors);
             }
             StartSymbol();
             return sym;

test/Microsoft.AspNet.Razor.Test/Tokenizer/TokenizerLookaheadTest.cs

@@ -138,7 +138,7 @@ namespace Microsoft.AspNet.Razor.Test.Tokenizer
                 SourceLocation start,
                 string content,
                 CSharpSymbolType type,
-                IEnumerable<RazorError> errors)
+                IReadOnlyList<RazorError> errors)
             {
                 throw new NotImplementedException();
             }