Don't generate code for closed files that are part of fallback projects (#9503)

Fixes a regression from #9486

Author: davidwengier

src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/ProjectSystem/FallbackHostProject.cs

@@ -0,0 +1,14 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the MIT license. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Razor.Language;
+
+namespace Microsoft.CodeAnalysis.Razor.ProjectSystem;
+
+internal class FallbackHostProject : HostProject
+{
+    public FallbackHostProject(string projectFilePath, string intermediateOutputPath, RazorConfiguration razorConfiguration, string rootNamespace, string displayName)
+        : base(projectFilePath, intermediateOutputPath, razorConfiguration, rootNamespace, displayName)
+    {
+    }
+}

src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/ProjectSystem/FallbackProjectManager.cs

@@ -82,7 +82,12 @@ private void AddFallbackProject(ProjectId projectId, string filePath)
         }
 
         var rootNamespace = project.DefaultNamespace ?? "ASP";
-        var hostProject = new HostProject(project.FilePath, intermediateOutputPath, FallbackRazorConfiguration.Latest, rootNamespace, project.Name);
+
+        // We create this as a fallback project so that other parts of the system can reason about them - eg we don't do code
+        // generation for closed files for documents in these projects. If these projects become "real", either because capabilities
+        // change or simply a timing difference between Roslyn and our CPS components, the HostProject instance associated with
+        // the project will be updated, and it will no longer be a fallback project.
+        var hostProject = new FallbackHostProject(project.FilePath, intermediateOutputPath, FallbackRazorConfiguration.Latest, rootNamespace, project.Name);
 
         _projectSnapshotManagerAccessor.Instance.ProjectAdded(hostProject);
 

src/Razor/src/Microsoft.VisualStudio.Editor.Razor/BackgroundDocumentGenerator.cs

@@ -134,6 +134,12 @@ public void Enqueue(IProjectSnapshot project, IDocumentSnapshot document)
             throw new ArgumentNullException(nameof(document));
         }
 
+        if (project is ProjectSnapshot { HostProject: FallbackHostProject })
+        {
+            // We don't support closed file code generation for fallback projects
+            return;
+        }
+
         _dispatcher.AssertDispatcherThread();
 
         lock (Work)

src/Razor/test/Microsoft.CodeAnalysis.Razor.Workspaces.Test/ProjectSystem/FallbackProjectManagerTest.cs

@@ -67,10 +67,34 @@ public void DynamicFileAdded_UnknownProject_Adds()
         var project = Assert.Single(_projectSnapshotManager.GetProjects());
         Assert.Equal("RootNamespace", project.RootNamespace);
 
+        Assert.IsType<FallbackHostProject>(((ProjectSnapshot)project).HostProject);
+
         var documentFilePath = Assert.Single(project.DocumentFilePaths);
         Assert.Equal(SomeProjectFile1.FilePath, documentFilePath);
     }
 
+    [Fact]
+    public void DynamicFileAdded_UnknownToKnownProject_NotFallbackHostProject()
+    {
+        var projectId = ProjectId.CreateNewId();
+        var projectInfo = ProjectInfo.Create(projectId, VersionStamp.Default, "DisplayName", "AssemblyName", LanguageNames.CSharp, filePath: SomeProject.FilePath)
+            .WithCompilationOutputInfo(new CompilationOutputInfo().WithAssemblyPath(Path.Combine(SomeProject.IntermediateOutputPath, "SomeProject.dll")))
+            .WithDefaultNamespace("RootNamespace");
+
+        Workspace.TryApplyChanges(Workspace.CurrentSolution.AddProject(projectInfo));
+
+        _fallbackProjectManger.DynamicFileAdded(projectId, SomeProject.Key, SomeProject.FilePath, SomeProjectFile1.FilePath);
+
+        var project = Assert.Single(_projectSnapshotManager.GetProjects());
+        Assert.IsType<FallbackHostProject>(((ProjectSnapshot)project).HostProject);
+
+        var hostProject = new HostProject(SomeProject.FilePath, SomeProject.IntermediateOutputPath, RazorConfiguration.Default, "RootNamespace", "DisplayName");
+        _projectSnapshotManager.ProjectConfigurationChanged(hostProject);
+
+        project = Assert.Single(_projectSnapshotManager.GetProjects());
+        Assert.IsNotType<FallbackHostProject>(((ProjectSnapshot)project).HostProject);
+    }
+
     [Fact]
     public void DynamicFileAdded_TrackedProject_AddsDocuments()
     {