Surface TagHelper resolution errors.

- Decided to not expose the resolutions errors in the Razor extension. If we feel that it's good debug information we can add it later.
- Added a `TagHelperResolutionResult` type to Razor.Workspaces so it can be used in the language and remote service.

#1014

src/Microsoft.CodeAnalysis.Razor.Workspaces/DefaultTagHelperResolver.cs

@@ -17,15 +17,23 @@ namespace Microsoft.CodeAnalysis.Razor
 
         public bool DesignTime { get; }
 
-        public override IReadOnlyList<TagHelperDescriptor> GetTagHelpers(Compilation compilation)
+        public override TagHelperResolutionResult GetTagHelpers(Compilation compilation)
         {
-            var results = new List<TagHelperDescriptor>();
+            var descriptors = new List<TagHelperDescriptor>();
             var errors = new ErrorSink();
 
-            VisitTagHelpers(compilation, results, errors);
-            VisitViewComponents(compilation, results, errors);
+            VisitTagHelpers(compilation, descriptors, errors);
+            VisitViewComponents(compilation, descriptors, errors);
 
-            return results;
+            var diagnostics = new List<RazorDiagnostic>();
+            for (var i = 0; i < errors.Errors.Count; i++)
+            {
+                var diagnostic = RazorDiagnostic.Create(errors.Errors[i]);
+                diagnostics.Add(diagnostic);
+            }
+            var resolutionResult = new TagHelperResolutionResult(descriptors, diagnostics);
+
+            return resolutionResult;
         }
 
         private void VisitTagHelpers(Compilation compilation, List<TagHelperDescriptor> results, ErrorSink errors)

src/Microsoft.CodeAnalysis.Razor.Workspaces/TagHelperResolutionResult.cs

@@ -0,0 +1,21 @@
+// 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.Collections.Generic;
+using Microsoft.AspNetCore.Razor.Evolution;
+
+namespace Microsoft.CodeAnalysis.Razor
+{
+    public sealed class TagHelperResolutionResult
+    {
+        public TagHelperResolutionResult(IReadOnlyList<TagHelperDescriptor> descriptors, IReadOnlyList<RazorDiagnostic> diagnostics)
+        {
+            Descriptors = descriptors;
+            Diagnostics = diagnostics;
+        }
+
+        public IReadOnlyList<TagHelperDescriptor> Descriptors { get; }
+
+        public IReadOnlyList<RazorDiagnostic> Diagnostics { get; }
+    }
+}

src/Microsoft.CodeAnalysis.Razor.Workspaces/TagHelperResolver.cs

@@ -1,19 +1,17 @@
 // 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.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Razor.Evolution;
 using Microsoft.CodeAnalysis.Host;
 
 namespace Microsoft.CodeAnalysis.Razor
 {
     internal abstract class TagHelperResolver : ILanguageService
     {
-        public abstract IReadOnlyList<TagHelperDescriptor> GetTagHelpers(Compilation compilation);
+        public abstract TagHelperResolutionResult GetTagHelpers(Compilation compilation);
 
-        public virtual async Task<IReadOnlyList<TagHelperDescriptor>> GetTagHelpersAsync(
+        public virtual async Task<TagHelperResolutionResult> GetTagHelpersAsync(
             Project project,
             CancellationToken cancellationToken = default(CancellationToken))
         {

src/Microsoft.CodeAnalysis.Remote.Razor/RazorLanguageService.cs

@@ -20,7 +20,7 @@ namespace Microsoft.CodeAnalysis.Remote.Razor
         {
         }
 
-        public async Task<IEnumerable<TagHelperDescriptor>> GetTagHelpersAsync(Guid projectIdBytes, string projectDebugName, CancellationToken cancellationToken = default(CancellationToken))
+        public async Task<TagHelperResolutionResult> GetTagHelpersAsync(Guid projectIdBytes, string projectDebugName, CancellationToken cancellationToken = default(CancellationToken))
         {
             var projectId = ProjectId.CreateFromSerialized(projectIdBytes, projectDebugName);
 
@@ -28,9 +28,9 @@ namespace Microsoft.CodeAnalysis.Remote.Razor
             var project = solution.GetProject(projectId);
 
             var resolver = new DefaultTagHelperResolver(designTime: true);
-            var results = await resolver.GetTagHelpersAsync(project, cancellationToken).ConfigureAwait(false);
+            var result = await resolver.GetTagHelpersAsync(project, cancellationToken).ConfigureAwait(false);
 
-            return results;
+            return result;
         }
 
         public Task<IEnumerable<DirectiveDescriptor>> GetDirectivesAsync(Guid projectIdBytes, string projectDebugName, CancellationToken cancellationToken = default(CancellationToken))

src/Microsoft.VisualStudio.LanguageServices.Razor/DefaultTagHelperResolver.cs

@@ -8,6 +8,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Razor.Evolution;
 using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Razor;
 
 namespace Microsoft.VisualStudio.LanguageServices.Razor
 {
@@ -17,7 +18,7 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
         [Import]
         public VisualStudioWorkspace Workspace { get; set; }
 
-        public async Task<IEnumerable<TagHelperDescriptor>> GetTagHelpersAsync(Project project)
+        public async Task<TagHelperResolutionResult> GetTagHelpersAsync(Project project)
         {
             try
             {
@@ -26,13 +27,14 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
                 {
                     // The OOP host is turned off, so let's do this in process.
                     var resolver = new CodeAnalysis.Razor.DefaultTagHelperResolver(designTime: true);
-                    return await resolver.GetTagHelpersAsync(project, CancellationToken.None).ConfigureAwait(false);
+                    var result =  await resolver.GetTagHelpersAsync(project, CancellationToken.None).ConfigureAwait(false);
+                    return result;
                 }
 
                 using (var session = await client.CreateSessionAsync(project.Solution))
                 {
-                    var results = await session.InvokeAsync<IEnumerable<TagHelperDescriptor>>("GetTagHelpersAsync", new object[] { project.Id.Id, "Foo", }).ConfigureAwait(false);
-                    return results;
+                    var result = await session.InvokeAsync<TagHelperResolutionResult>("GetTagHelpersAsync", new object[] { project.Id.Id, "Foo", }).ConfigureAwait(false);
+                    return result;
                 }
             }
             catch (Exception exception)

src/Microsoft.VisualStudio.LanguageServices.Razor/ITagHelperResolver.cs

@@ -1,15 +1,14 @@
 // 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.Collections.Generic;
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Razor.Evolution;
 using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Razor;
 
 namespace Microsoft.VisualStudio.LanguageServices.Razor
 {
     public interface ITagHelperResolver
     {
-        Task<IEnumerable<TagHelperDescriptor>> GetTagHelpersAsync(Project project);
+        Task<TagHelperResolutionResult> GetTagHelpersAsync(Project project);
     }
 }

tooling/Microsoft.VisualStudio.RazorExtension/RazorInfo/RazorInfoViewModel.cs

@@ -169,7 +169,7 @@ namespace Microsoft.VisualStudio.RazorExtension.RazorInfo
                 var assemblies = await _assemblyResolver.GetRazorEngineAssembliesAsync(project);
 
                 var directives = await _directiveResolver.GetRazorEngineDirectivesAsync(_workspace, project);
-                var tagHelpers = await _tagHelperResolver.GetTagHelpersAsync(project);
+                var resolutionResult = await _tagHelperResolver.GetTagHelpersAsync(project);
 
                 var files = GetCshtmlDocuments(project);
 
@@ -178,7 +178,7 @@ namespace Microsoft.VisualStudio.RazorExtension.RazorInfo
                     Assemblies = new ObservableCollection<AssemblyViewModel>(assemblies.Select(a => new AssemblyViewModel(a))),
                     Directives = new ObservableCollection<DirectiveViewModel>(directives.Select(d => new DirectiveViewModel(d))),
                     Documents = new ObservableCollection<DocumentViewModel>(documents.Select(d => new DocumentViewModel(d))),
-                    TagHelpers = new ObservableCollection<TagHelperViewModel>(tagHelpers.Select(t => new TagHelperViewModel(t))),
+                    TagHelpers = new ObservableCollection<TagHelperViewModel>(resolutionResult.Descriptors.Select(t => new TagHelperViewModel(t))),
                 };
             }
             catch (Exception ex)