Don't switch runtime / design time solutions if cohosting is on (#80065)

Author: chsienki

src/Features/Core/Portable/Options/LegacyRazorOptions.cs

@@ -0,0 +1,10 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace Microsoft.CodeAnalysis.Options;
+
+internal class LegacyRazorOptions
+{
+    public static readonly Option2<bool> UseCohosting = new("razor_use_cohosting", defaultValue: false);
+}

src/Features/Core/Portable/Workspace/CompileTimeSolutionProvider.cs

@@ -14,6 +14,7 @@
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.Collections;
 using Microsoft.CodeAnalysis.Host.Mef;
+using Microsoft.CodeAnalysis.Options;
 using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.Shared.Extensions;
 using Roslyn.Utilities;
@@ -28,17 +29,13 @@ namespace Microsoft.CodeAnalysis.Host;
 internal sealed class CompileTimeSolutionProvider : ICompileTimeSolutionProvider
 {
     [ExportWorkspaceServiceFactory(typeof(ICompileTimeSolutionProvider), [WorkspaceKind.Host]), Shared]
-    private sealed class Factory : IWorkspaceServiceFactory
+    [method: ImportingConstructor]
+    [method: Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
+    private sealed class Factory(IGlobalOptionService globalOptions) : IWorkspaceServiceFactory
     {
-        [ImportingConstructor]
-        [Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
-        public Factory()
-        {
-        }
-
         [Obsolete(MefConstruction.FactoryMethodMessage, error: true)]
         public IWorkspaceService CreateService(HostWorkspaceServices workspaceServices)
-            => new CompileTimeSolutionProvider(workspaceServices.Workspace);
+            => new CompileTimeSolutionProvider(workspaceServices.Workspace, globalOptions);
     }
 
     private const string RazorEncConfigFileName = "RazorSourceGenerator.razorencconfig";
@@ -52,6 +49,8 @@ public IWorkspaceService CreateService(HostWorkspaceServices workspaceServices)
     private static readonly ImmutableArray<string> s_razorSourceGeneratorFileNamePrefixes = s_razorSourceGeneratorAssemblyNames
         .SelectAsArray(static assemblyName => Path.Combine(assemblyName, RazorSourceGeneratorTypeName));
 
+    private readonly bool _useCohosting;
+
     private readonly object _gate = new();
 
     /// <summary>
@@ -65,8 +64,9 @@ public IWorkspaceService CreateService(HostWorkspaceServices workspaceServices)
 
     private Solution? _lastCompileTimeSolution;
 
-    public CompileTimeSolutionProvider(Workspace workspace)
+    public CompileTimeSolutionProvider(Workspace workspace, IGlobalOptionService globalOptions)
     {
+        _useCohosting = globalOptions.GetOption(LegacyRazorOptions.UseCohosting);
         _ = workspace.RegisterWorkspaceChangedHandler((e) =>
         {
             if (e.Kind is WorkspaceChangeKind.SolutionCleared or WorkspaceChangeKind.SolutionRemoved)
@@ -89,6 +89,12 @@ private static bool IsRazorAnalyzerConfig(TextDocumentState documentState)
 
     public Solution GetCompileTimeSolution(Solution designTimeSolution)
     {
+        if (_useCohosting)
+        {
+            // when cohosting is on, the design time and runtime solutions are the same
+            return designTimeSolution;
+        }
+
         lock (_gate)
         {
             _designTimeToCompileTimeSolution.TryGetValue(designTimeSolution, out var cachedCompileTimeSolution);

src/VisualStudio/Core/Def/Options/VisualStudioOptionStorage.cs

@@ -420,5 +420,6 @@ public bool TryFetch(LocalUserRegistryOptionPersister persister, OptionKey2 opti
         {"dotnet_validate_compilation_tracker_states", new FeatureFlagStorage(@"Roslyn.ValidateCompilationTrackerStates")},
         {"dotnet_source_generator_execution", new RoamingProfileStorage("TextEditor.Roslyn.Specific.SourceGeneratorExecution")},
         {"xaml_enable_lsp_intellisense", new FeatureFlagStorage(@"Xaml.EnableLspIntelliSense")},
+        {"razor_use_cohosting", new FeatureFlagStorage("Razor.LSP.UseRazorCohostServer")},
     };
 }