Add @removetaghelper directive.

- Added parsing recognition of the @removetaghelper directive.
- Added TagHelperDescriptor handling: @removetaghelper will cause the system to ignore TagHelpers that are added via @addtaghelper.
- Added Chunk generation, this involved building a CodeGenerator (soon to be named ChunkGenerator) that pulled out the stringified @removetaghelper lookup text which it then uses to construct a RemoveTagHelperChunk.
- Modified CodeVisitors to now understand RemoveTagHelperChunk's.
- Added code generation, this involved creating a dummy string in order to give the @removetaghelper "..." coloring.

#112

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CSharp/Visitors/CSharpDesignTimeHelpersVisitor.cs

@@ -59,6 +59,16 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
         }
 
         protected override void Visit(AddTagHelperChunk chunk)
+        {
+            VisitAddOrRemoveTagHelperChunk(chunk.LookupText, chunk);
+        }
+
+        protected override void Visit(RemoveTagHelperChunk chunk)
+        {
+            VisitAddOrRemoveTagHelperChunk(chunk.LookupText, chunk);
+        }
+
+        private void VisitAddOrRemoveTagHelperChunk(string lookupText, Chunk chunk)
         {
             // We should always be in design time mode because of the calling AcceptTree method verification.
             Debug.Assert(Context.Host.DesignTimeMode);
@@ -71,12 +81,12 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
 
             Writer.WriteStartAssignment(TagHelperDirectiveSyntaxHelper);
 
-            // The parsing mechanism for the AddTagHelperChunk (CSharpCodeParser.TagHelperDirective()) removes quotes
+            // The parsing mechanism for the add/remove TagHelper chunk (CSharpCodeParser.TagHelperDirective()) 
-            // that surround the chunk.LookupText.
+            // removes quotes that surround the lookupText.
             _csharpCodeVisitor.CreateExpressionCodeMapping(
                 string.Format(
                     CultureInfo.InvariantCulture,
-                    "\"{0}\"", chunk.LookupText),
+                    "\"{0}\"", lookupText),
                 chunk);
 
             Writer.WriteLine(";");

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/ChunkVisitor.cs

@@ -61,6 +61,10 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler
             {
                 Visit((AddTagHelperChunk)chunk);
             }
+            else if (chunk is RemoveTagHelperChunk)
+            {
+                Visit((RemoveTagHelperChunk)chunk);
+            }
             else if(chunk is SetLayoutChunk)
             {
                 Visit((SetLayoutChunk)chunk);
@@ -120,6 +124,7 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler
         protected abstract void Visit(StatementChunk chunk);
         protected abstract void Visit(TagHelperChunk chunk);
         protected abstract void Visit(AddTagHelperChunk chunk);
+        protected abstract void Visit(RemoveTagHelperChunk chunk);
         protected abstract void Visit(UsingChunk chunk);
         protected abstract void Visit(ChunkBlock chunk);
         protected abstract void Visit(DynamicCodeAttributeChunk chunk);

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CodeVisitor.cs

@@ -35,6 +35,9 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler
         protected override void Visit(AddTagHelperChunk chunk)
         {
         }
+        protected override void Visit(RemoveTagHelperChunk chunk)
+        {
+        }
         protected override void Visit(LiteralCodeAttributeChunk chunk)
         {
         }

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeTree/Chunks/TagHelpers/RemoveTagHelperChunk.cs

@@ -0,0 +1,18 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNet.Razor.Generator.Compiler
+{
+    /// <summary>
+    /// A <see cref="Chunk"/> used to look up <see cref="TagHelpers.TagHelperDescriptor"/>s that should be ignored 
+    /// within the Razor page.
+    /// </summary>
+    public class RemoveTagHelperChunk : Chunk
+    {
+        /// <summary>
+        /// Text used to look up <see cref="TagHelpers.TagHelperDescriptor"/>s that should be ignored within the Razor
+        /// page.
+        /// </summary>
+        public string LookupText { get; set; }
+    }
+}

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeTree/CodeTreeBuilder.cs

@@ -45,6 +45,14 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler
             }, association, topLevel: true);
         }
 
+        public void AddRemoveTagHelperChunk(string lookupText, SyntaxTreeNode association)
+        {
+            AddChunk(new RemoveTagHelperChunk
+            {
+                LookupText = lookupText
+            }, association, topLevel: true);
+        }
+
         public void AddLiteralChunk(string literal, SyntaxTreeNode association)
         {
             // If the previous chunk was also a LiteralChunk, append the content of the current node to the previous one.

src/Microsoft.AspNet.Razor/Generator/TagHelpers/AddOrRemoveTagHelperCodeGenerator.cs

@@ -0,0 +1,60 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNet.Razor.Parser.SyntaxTree;
+
+namespace Microsoft.AspNet.Razor.Generator
+{
+    /// <summary>
+    /// A <see cref="SpanCodeGenerator"/> responsible for generating <see cref="Compiler.AddTagHelperChunk"/>s and
+    /// <see cref="Compiler.RemoveTagHelperChunk"/>s.
+    /// </summary>
+    public class AddOrRemoveTagHelperCodeGenerator : SpanCodeGenerator
+    {
+        /// <summary>
+        /// Instantiates a new <see cref="AddOrRemoveTagHelperCodeGenerator"/>.
+        /// </summary>
+        /// <param name="lookupText">
+        /// Text used to look up <see cref="TagHelpers.TagHelperDescriptor"/>s that should be added or removed.
+        /// </param>
+        public AddOrRemoveTagHelperCodeGenerator(bool removeTagHelperDescriptors, string lookupText)
+        {
+            RemoveTagHelperDescriptors = removeTagHelperDescriptors;
+            LookupText = lookupText;
+        }
+
+        /// <summary>
+        /// Gets the text used to look up <see cref="TagHelpers.TagHelperDescriptor"/>s that should be added to or 
+        /// removed from the Razor page.
+        /// </summary>
+        public string LookupText { get; }
+
+        /// <summary>
+        /// Whether we want to remove <see cref="TagHelpers.TagHelperDescriptor"/>s from the Razor page.
+        /// </summary>
+        /// <remarks>If <c>true</c> <see cref="GenerateCode"/> generates <see cref="Compiler.AddTagHelperChunk"/>s,
+        /// <see cref="Compiler.RemoveTagHelperChunk"/>s otherwise.</remarks>
+        public bool RemoveTagHelperDescriptors { get; }
+
+        /// <summary>
+        /// Generates <see cref="Compiler.AddTagHelperChunk"/>s if <see cref="RemoveTagHelperDescriptors"/> is 
+        /// <c>true</c>, otherwise <see cref="Compiler.RemoveTagHelperChunk"/>s are generated.
+        /// </summary>
+        /// <param name="target">
+        /// The <see cref="Span"/> responsible for this <see cref="AddOrRemoveTagHelperCodeGenerator"/>.
+        /// </param>
+        /// <param name="context">A <see cref="CodeGeneratorContext"/> instance that contains information about 
+        /// the current code generation process.</param>
+        public override void GenerateCode(Span target, CodeGeneratorContext context)
+        {
+            if (RemoveTagHelperDescriptors)
+            {
+                context.CodeTreeBuilder.AddRemoveTagHelperChunk(LookupText, target);
+            }
+            else
+            {
+                context.CodeTreeBuilder.AddAddTagHelperChunk(LookupText, target);
+            }
+        }
+    }
+}

src/Microsoft.AspNet.Razor/Generator/TagHelpers/AddTagHelperCodeGenerator.cs

@@ -1,42 +0,0 @@
-// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
-// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
-
-using Microsoft.AspNet.Razor.Parser.SyntaxTree;
-
-namespace Microsoft.AspNet.Razor.Generator
-{
-    /// <summary>
-    /// A <see cref="SpanCodeGenerator"/> responsible for generating <see cref="Compiler.AddTagHelperChunk"/>s.
-    /// </summary>
-    public class AddTagHelperCodeGenerator : SpanCodeGenerator
-    {
-        /// <summary>
-        /// Instantiates a new <see cref="AddTagHelperCodeGenerator"/>.
-        /// </summary>
-        /// <param name="lookupText">
-        /// Text used to look up <see cref="TagHelpers.TagHelperDescriptor"/>s.
-        /// </param>
-        public AddTagHelperCodeGenerator(string lookupText)
-        {
-            LookupText = lookupText;
-        }
-
-        /// <summary>
-        /// Text used to look up <see cref="TagHelpers.TagHelperDescriptor"/>s.
-        /// </summary>
-        public string LookupText { get; private set; }
-
-        /// <summary>
-        /// Generates a <see cref="Compiler.AddTagHelperChunk"/>.
-        /// </summary>
-        /// <param name="target">
-        /// The <see cref="Span"/> responsible for this <see cref="AddTagHelperCodeGenerator"/>.
-        /// </param>
-        /// <param name="context">A <see cref="CodeGeneratorContext"/> instance that contains information about 
-        /// the current code generation process.</param>
-        public override void GenerateCode(Span target, CodeGeneratorContext context)
-        {
-            context.CodeTreeBuilder.AddAddTagHelperChunk(LookupText, target);
-        }
-    }
-}

src/Microsoft.AspNet.Razor/Parser/CSharpCodeParser.Directives.cs

@@ -20,6 +20,7 @@ namespace Microsoft.AspNet.Razor.Parser
         private void SetupDirectives()
         {
             MapDirectives(AddTagHelperDirective, SyntaxConstants.CSharp.AddTagHelperKeyword);
+            MapDirectives(RemoveTagHelperDirective, SyntaxConstants.CSharp.RemoveTagHelperKeyword);
             MapDirectives(InheritsDirective, SyntaxConstants.CSharp.InheritsKeyword);
             MapDirectives(FunctionsDirective, SyntaxConstants.CSharp.FunctionsKeyword);
             MapDirectives(SectionDirective, SyntaxConstants.CSharp.SectionKeyword);
@@ -30,10 +31,12 @@ namespace Microsoft.AspNet.Razor.Parser
 
         protected virtual void AddTagHelperDirective()
         {
-            TagHelperDirective(SyntaxConstants.CSharp.AddTagHelperKeyword, (lookupText) =>
+            TagHelperDirective(SyntaxConstants.CSharp.AddTagHelperKeyword, removeTagHelperDescriptors: false);
-            {
+        }
-                return new AddTagHelperCodeGenerator(lookupText);
+
-            });
+        protected virtual void RemoveTagHelperDirective()
+        {
+            TagHelperDirective(SyntaxConstants.CSharp.RemoveTagHelperKeyword, removeTagHelperDescriptors: true);
         }
 
         protected virtual void LayoutDirective()
@@ -508,7 +511,7 @@ namespace Microsoft.AspNet.Razor.Parser
             Output(SpanKind.Code);
         }
 
-        private void TagHelperDirective(string keyword, Func<string, SpanCodeGenerator> codeGeneratorBuilder)
+        private void TagHelperDirective(string keyword, bool removeTagHelperDescriptors)
         {
             AssertDirective(keyword);
 
@@ -550,7 +553,8 @@ namespace Microsoft.AspNet.Razor.Parser
                     // renders the C# to colorize the user provided value. We trim the quotes around the user's value
                     // so when we render the code we can project the users value into double quotes to not invoke C# 
                     // IntelliSense. 
-                    Span.CodeGenerator = codeGeneratorBuilder(rawValue.Trim('"'));
+                    Span.CodeGenerator = 
+                        new AddOrRemoveTagHelperCodeGenerator(removeTagHelperDescriptors, rawValue.Trim('"'));
                 }
 
                 // We expect the directive to be surrounded in quotes.

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

@@ -210,8 +210,8 @@ namespace Microsoft.AspNet.Razor.Parser
         /// <returns></returns>
         protected virtual IEnumerable<TagHelperDescriptor> GetTagHelperDescriptors([NotNull] Block documentRoot)
         {
-            var tagHelperRegistrationVisitor = new TagHelperRegistrationVisitor(TagHelperDescriptorResolver);
+            var addOrRemoveTagHelperSpanVisitor = new AddOrRemoveTagHelperSpanVisitor(TagHelperDescriptorResolver);
-            return tagHelperRegistrationVisitor.GetDescriptors(documentRoot);
+            return addOrRemoveTagHelperSpanVisitor.GetDescriptors(documentRoot);
         }
 
         private static IEnumerable<ISyntaxTreeRewriter> GetDefaultRewriters(ParserBase markupParser)

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

@@ -19,6 +19,7 @@ namespace Microsoft.AspNet.Razor.Parser
         {
             public static readonly int UsingKeywordLength = 5;
             public static readonly string AddTagHelperKeyword = "addtaghelper";
+            public static readonly string RemoveTagHelperKeyword = "removetaghelper";
             public static readonly string InheritsKeyword = "inherits";
             public static readonly string FunctionsKeyword = "functions";
             public static readonly string SectionKeyword = "section";

src/Microsoft.AspNet.Razor/Parser/TagHelpers/AddOrRemoveTagHelperSpanVisitor.cs

@@ -0,0 +1,77 @@
+// Copyright (c) Microsoft Open Technologies, Inc. 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.Linq;
+using Microsoft.AspNet.Razor.Generator;
+using Microsoft.AspNet.Razor.Parser.SyntaxTree;
+using Microsoft.AspNet.Razor.TagHelpers;
+
+namespace Microsoft.AspNet.Razor.Parser.TagHelpers
+{
+    /// <summary>
+    /// A <see cref="ParserVisitor"/> that generates <see cref="TagHelperDescriptor"/>s from
+    /// tag helper code generators in a Razor document.
+    /// </summary>
+    public class AddOrRemoveTagHelperSpanVisitor : ParserVisitor
+    {
+        private readonly ITagHelperDescriptorResolver _descriptorResolver;
+
+        private List<TagHelperDescriptor> _descriptors;
+
+        public AddOrRemoveTagHelperSpanVisitor(ITagHelperDescriptorResolver descriptorResolver)
+        {
+            _descriptorResolver = descriptorResolver;
+        }
+
+        public IEnumerable<TagHelperDescriptor> GetDescriptors([NotNull] Block root)
+        {
+            _descriptors = new List<TagHelperDescriptor>();
+
+            // This will recurse through the syntax tree.
+            VisitBlock(root);
+
+            return _descriptors;
+        }
+
+        public override void VisitSpan(Span span)
+        {
+            // We're only interested in spans with an AddOrRemoveTagHelperCodeGenerator.
+
+            if (span.CodeGenerator is AddOrRemoveTagHelperCodeGenerator)
+            {
+                var codeGenerator = (AddOrRemoveTagHelperCodeGenerator)span.CodeGenerator;
+
+                if (_descriptorResolver == null)
+                {
+                    var directive = codeGenerator.RemoveTagHelperDescriptors ?
+                                  SyntaxConstants.CSharp.RemoveTagHelperKeyword :
+                                  SyntaxConstants.CSharp.AddTagHelperKeyword;
+
+                    throw new InvalidOperationException(
+                        RazorResources.FormatTagHelpers_CannotUseDirectiveWithNoTagHelperDescriptorResolver(
+                            directive, typeof(ITagHelperDescriptorResolver).FullName, typeof(RazorParser).FullName));
+                }
+
+                // Look up all the descriptors associated with the "LookupText".
+                var descriptors = _descriptorResolver.Resolve(codeGenerator.LookupText);
+
+                if (codeGenerator.RemoveTagHelperDescriptors)
+                {
+                    var evaluatedDescriptors = 
+                        new HashSet<TagHelperDescriptor>(descriptors, TagHelperDescriptorComparer.Default);
+
+                    // We remove all found descriptors from the descriptor list to ignore the associated TagHelpers on the 
+                    // Razor page.
+                    _descriptors.RemoveAll(descriptor => evaluatedDescriptors.Contains(descriptor));
+                }
+                else
+                {
+                    // Add all the found descriptors to our list.
+                    _descriptors.AddRange(descriptors);
+                }
+            }
+        }
+    }
+}

src/Microsoft.AspNet.Razor/Parser/TagHelpers/TagHelperRegistrationVisitor.cs

@@ -1,61 +0,0 @@
-// Copyright (c) Microsoft Open Technologies, Inc. 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 Microsoft.AspNet.Razor.Generator;
-using Microsoft.AspNet.Razor.Parser.SyntaxTree;
-using Microsoft.AspNet.Razor.TagHelpers;
-
-namespace Microsoft.AspNet.Razor.Parser.TagHelpers
-{
-    /// <summary>
-    /// A <see cref="ParserVisitor"/> that generates <see cref="TagHelperDescriptor"/>s from
-    /// tag helper code generators in a Razor document.
-    /// </summary>
-    public class TagHelperRegistrationVisitor : ParserVisitor
-    {
-        private readonly ITagHelperDescriptorResolver _descriptorResolver;
-
-        private List<TagHelperDescriptor> _descriptors;
-
-        public TagHelperRegistrationVisitor(ITagHelperDescriptorResolver descriptorResolver)
-        {
-            _descriptorResolver = descriptorResolver;
-        }
-
-        public IEnumerable<TagHelperDescriptor> GetDescriptors([NotNull] Block root)
-        {
-            _descriptors = new List<TagHelperDescriptor>();
-
-            // This will recurse through the syntax tree.
-            VisitBlock(root);
-
-            return _descriptors;
-        }
-
-        public override void VisitSpan(Span span)
-        {
-            // We're only interested in spans with an AddTagHelperCodeGenerator.
-            if (span.CodeGenerator is AddTagHelperCodeGenerator)
-            {
-                if (_descriptorResolver == null)
-                {
-                    throw new InvalidOperationException(
-                        RazorResources.FormatTagHelpers_CannotUseDirectiveWithNoTagHelperDescriptorResolver(
-                            SyntaxConstants.CSharp.AddTagHelperKeyword,
-                            nameof(ITagHelperDescriptorResolver),
-                            nameof(RazorParser)));
-                }
-
-                var addGenerator = (AddTagHelperCodeGenerator)span.CodeGenerator;
-
-                // Look up all the descriptors associated with the "LookupText".
-                var descriptors = _descriptorResolver.Resolve(addGenerator.LookupText);
-
-                // Add all the found descriptors to our HashSet.
-                _descriptors.AddRange(descriptors);
-            }
-        }
-    }
-}