Create endpoint and remote service interface

src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Protocol/DocumentHighlight/RemoteDocumentHighlight.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the MIT license. See License.txt in the project root for license information.
+
+using System.Runtime.Serialization;
+using Microsoft.CodeAnalysis.Razor.Workspaces;
+using Microsoft.CodeAnalysis.Text;
+using Microsoft.VisualStudio.LanguageServer.Protocol;
+
+namespace Microsoft.CodeAnalysis.Razor.Protocol.DocumentHighlight;
+
+using DocumentHighlight = VisualStudio.LanguageServer.Protocol.DocumentHighlight;
+
+[DataContract]
+internal readonly record struct RemoteDocumentHighlight(
+    [property: DataMember(Order = 0)] LinePositionSpan Position,
+    [property: DataMember(Order = 1)] DocumentHighlightKind Kind)
+{
+    public static RemoteDocumentHighlight FromLspDocumentHighlight(DocumentHighlight h)
+        => new RemoteDocumentHighlight(h.Range.ToLinePositionSpan(), h.Kind);
+
+    public static DocumentHighlight ToLspDocumentHighlight(RemoteDocumentHighlight r)
+        => new DocumentHighlight
+        {
+            Range = r.Position.ToRange(),
+            Kind = r.Kind
+        };
+}

src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Remote/IRemoteDocumentHighlightService.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the MIT license. See License.txt in the project root for license information.
+
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.ExternalAccess.Razor;
+using Microsoft.CodeAnalysis.Razor.Protocol.DocumentHighlight;
+using Microsoft.CodeAnalysis.Text;
+
+namespace Microsoft.CodeAnalysis.Razor.Remote;
+
+internal interface IRemoteDocumentHighlightService
+{
+    ValueTask<RemoteDocumentHighlight[]?> GetHighlightsAsync(RazorPinnedSolutionInfoWrapper solutionInfo, DocumentId documentId, LinePosition position, CancellationToken cancellationToken);
+}

src/Razor/src/Microsoft.VisualStudio.LanguageServices.Razor/LanguageClient/Cohost/CohostDocumentHighlightPresentationEndpoint.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the MIT license. See License.txt in the project root for license information.
+
+using System.Composition;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Razor;
+using Microsoft.CodeAnalysis.ExternalAccess.Razor.Cohost;
+using Microsoft.CodeAnalysis.Razor.Protocol.DocumentHighlight;
+using Microsoft.CodeAnalysis.Razor.Remote;
+using Microsoft.CodeAnalysis.Razor.Workspaces;
+using Microsoft.VisualStudio.LanguageServer.ContainedLanguage;
+using Microsoft.VisualStudio.LanguageServer.Protocol;
+using Microsoft.VisualStudio.Razor.LanguageClient.Extensions;
+
+namespace Microsoft.VisualStudio.Razor.LanguageClient.Cohost;
+
+#pragma warning disable RS0030 // Do not use banned APIs
+[Shared]
+[CohostEndpoint(Methods.TextDocumentDocumentHighlightName)]
+[Export(typeof(IDynamicRegistrationProvider))]
+[ExportCohostStatelessLspService(typeof(CohostDocumentHighlightEndpoint))]
+[method: ImportingConstructor]
+#pragma warning restore RS0030 // Do not use banned APIs
+internal class CohostDocumentHighlightEndpoint(
+    IRemoteServiceInvoker remoteServiceInvoker,
+    IHtmlDocumentSynchronizer htmlDocumentSynchronizer,
+    LSPRequestInvoker requestInvoker)
+    : AbstractRazorCohostDocumentRequestHandler<DocumentHighlightParams, DocumentHighlight[]?>, IDynamicRegistrationProvider
+{
+    private readonly IRemoteServiceInvoker _remoteServiceInvoker = remoteServiceInvoker;
+    private readonly IHtmlDocumentSynchronizer _htmlDocumentSynchronizer = htmlDocumentSynchronizer;
+    private readonly LSPRequestInvoker _requestInvoker = requestInvoker;
+
+    protected override bool MutatesSolutionState => false;
+
+    protected override bool RequiresLSPSolution => true;
+
+    public Registration? GetRegistration(VSInternalClientCapabilities clientCapabilities, DocumentFilter[] filter, RazorCohostRequestContext requestContext)
+    {
+        if (clientCapabilities.SupportsVisualStudioExtensions)
+        {
+            return new Registration
+            {
+                Method = Methods.TextDocumentDocumentHighlightName,
+                RegisterOptions = new DocumentHighlightRegistrationOptions()
+                {
+                    DocumentSelector = filter
+                }
+            };
+        }
+
+        return null;
+    }
+
+    protected override RazorTextDocumentIdentifier? GetRazorTextDocumentIdentifier(DocumentHighlightParams request)
+        => request.TextDocument.ToRazorTextDocumentIdentifier();
+
+    protected override async Task<DocumentHighlight[]?> HandleRequestAsync(DocumentHighlightParams request, RazorCohostRequestContext context, CancellationToken cancellationToken)
+    {
+        var razorDocument = context.TextDocument.AssumeNotNull();
+
+        var csharpResult = await _remoteServiceInvoker.TryInvokeAsync<IRemoteDocumentHighlightService, RemoteDocumentHighlight[]?>(
+            razorDocument.Project.Solution,
+            (service, solutionInfo, cancellationToken) => service.GetHighlightsAsync(solutionInfo, razorDocument.Id, request.Position.ToLinePosition(), cancellationToken),
+            cancellationToken).ConfigureAwait(false);
+
+        // If we got a response back, then either Razor or C# wants to do something with this, so we're good to go
+        if (csharpResult is not null)
+        {
+            return csharpResult.Select(RemoteDocumentHighlight.ToLspDocumentHighlight).ToArray();
+        }
+
+        // If we didn't get anything from Razor or Roslyn, lets ask Html what they want to do
+        var htmlDocument = await _htmlDocumentSynchronizer.TryGetSynchronizedHtmlDocumentAsync(razorDocument, cancellationToken).ConfigureAwait(false);
+        if (htmlDocument is null)
+        {
+            return null;
+        }
+
+        request.TextDocument = request.TextDocument.WithUri(htmlDocument.Uri);
+
+        var result = await _requestInvoker.ReinvokeRequestOnServerAsync<DocumentHighlightParams, DocumentHighlight[]?>(
+            htmlDocument.Buffer,
+            Methods.TextDocumentDocumentHighlightName,
+            RazorLSPConstants.HtmlLanguageServerName,
+            request,
+            cancellationToken).ConfigureAwait(false);
+
+        // Since we don't need to map positions in Html, and document highlight results don't have Uri's, we can return these results directly
+        return result?.Response;
+    }
+}