Workaround for MSbuildProjectLoader.LoadProjectInfoAsync throwing on unrecognized project language (#44928)

Author: tmat

src/BuiltInTools/dotnet-watch/EnvironmentVariablesBuilder.cs

@@ -60,7 +60,7 @@ public void SetVariable(string name, string value)
 
         public void ConfigureProcess(ProcessSpec processSpec)
         {
-            processSpec.Arguments = [.. GetCommandLineDirectives(), .. processSpec.Arguments];
+            processSpec.Arguments = [.. GetCommandLineDirectives(), .. processSpec.Arguments ?? []];
             AddToEnvironment(processSpec.EnvironmentVariables);
         }
 

src/BuiltInTools/dotnet-watch/HotReload/IncrementalMSBuildWorkspace.cs

@@ -38,7 +38,17 @@ public async Task UpdateProjectConeAsync(string rootProjectPath, CancellationTok
 
         var loader = new MSBuildProjectLoader(this);
         var projectMap = ProjectMap.Create();
-        var projectInfos = await loader.LoadProjectInfoAsync(rootProjectPath, projectMap, progress: null, msbuildLogger: null, cancellationToken).ConfigureAwait(false);
+
+        ImmutableArray<ProjectInfo> projectInfos;
+        try
+        {
+            projectInfos = await loader.LoadProjectInfoAsync(rootProjectPath, projectMap, progress: null, msbuildLogger: null, cancellationToken).ConfigureAwait(false);
+        }
+        catch (InvalidOperationException)
+        {
+            // TODO: workaround for https://github.com/dotnet/roslyn/issues/75956
+            projectInfos = [];
+        }
 
         var oldProjectIdsByPath = oldSolution.Projects.ToDictionary(keySelector: static p => p.FilePath!, elementSelector: static p => p.Id);
 

src/BuiltInTools/dotnet-watch/HotReloadDotNetWatcher.cs

@@ -91,6 +91,7 @@ public override async Task WatchAsync(CancellationToken shutdownCancellationToke
 
                     var rootProjectOptions = Context.RootProjectOptions;
                     var rootProjectNode = Context.ProjectGraph.GraphRoots.Single();
+                    var rootProjectCapabilities = rootProjectNode.GetCapabilities();
 
                     await using var runtimeProcessLauncher = _runtimeProcessLauncherFactory?.TryCreate(rootProjectNode, projectLauncher, rootProjectOptions.BuildProperties);
                     if (runtimeProcessLauncher != null)
@@ -211,6 +212,16 @@ public override async Task WatchAsync(CancellationToken shutdownCancellationToke
                             await compilationHandler.Workspace.UpdateFileContentAsync(changedFiles, iterationCancellationToken);
                         }
 
+                        if (!rootProjectCapabilities.Contains("SupportsHotReload"))
+                        {
+                            Context.Reporter.Warn($"Project '{rootProjectNode.GetDisplayName()}' does not support Hot Reload and must be rebuilt.");
+
+                            // file change already detected
+                            waitForFileChangeBeforeRestarting = false;
+                            iterationCancellationSource.Cancel();
+                            break;
+                        }
+
                         HotReloadEventSource.Log.HotReloadStart(HotReloadEventSource.StartType.Main);
                         var stopwatch = Stopwatch.StartNew();
 

test/dotnet-watch.Tests/HotReload/ApplyDeltaTests.cs

@@ -68,6 +68,60 @@ public static void Print()
             await App.AssertOutputLineStartsWith("Changed!");
         }
 
+        [Fact]
+        public async Task ChangeFileInFSharpProject()
+        {
+            var testAsset = TestAssets.CopyTestAsset("FSharpTestAppSimple")
+                .WithSource();
+
+            App.Start(testAsset, []);
+
+            await App.AssertOutputLineStartsWith(MessageDescriptor.WaitingForFileChangeBeforeRestarting);
+
+            UpdateSourceFile(Path.Combine(testAsset.Path, "Program.fs"), content => content.Replace("Hello World!", "<Updated>"));
+
+            await App.AssertOutputLineStartsWith("<Updated>");
+        }
+
+        [Fact]
+        public async Task ChangeFileInFSharpProjectWithLoop()
+        {
+            var testAsset = TestAssets.CopyTestAsset("FSharpTestAppSimple")
+                .WithSource();
+
+            var source = """
+            module ConsoleApplication.Program
+
+            open System
+            open System.Threading
+
+            [<EntryPoint>]
+            let main argv =
+                while true do
+                    printfn "Waiting"
+                    Thread.Sleep(200)
+                0
+            """;
+
+            var sourcePath = Path.Combine(testAsset.Path, "Program.fs");
+
+            File.WriteAllText(sourcePath, source);
+
+            App.Start(testAsset, []);
+
+            await App.AssertOutputLineStartsWith(MessageDescriptor.WaitingForChanges);
+
+            UpdateSourceFile(sourcePath, content => content.Replace("Waiting", "<Updated>"));
+
+            await App.AssertOutputLineStartsWith(MessageDescriptor.WaitingForChanges, failure: _ => false);
+            await App.AssertOutputLineStartsWith("<Updated>");
+
+            UpdateSourceFile(sourcePath, content => content.Replace("<Updated>", "<Updated2>"));
+
+            await App.AssertOutputLineStartsWith(MessageDescriptor.WaitingForChanges, failure: _ => false);
+            await App.AssertOutputLineStartsWith("<Updated2>");
+        }
+
         // Test is timing out on .NET Framework: https://github.com/dotnet/sdk/issues/41669
         [CoreMSBuildOnlyFact]
         public async Task HandleTypeLoadFailure()