If the last generator is removed that was generating, correctly update

If the last generator was removed, and it was generating trees, we would
have potentially left that tree in the Compilation that was returned.
I believe this would have been a transient issue -- any later change
to the project would have created a new CompilationTracker with an
InProgress state; the code that processes an InProgress state into
the final state would have correctly seen we no longer had a generator
and would have dropped the old compilation at that point.

src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.CompilationTracker.cs

@@ -824,7 +824,12 @@ private async Task<CompilationInfo> FinalizeCompilationAsync(
                     {
                         using var generatedDocumentsBuilder = new TemporaryArray<SourceGeneratedDocumentState>();
 
-                        if (ProjectState.SourceGenerators.Any())
+                        if (!ProjectState.SourceGenerators.Any())
+                        {
+                            // We don't have any generators, so if we have a compilation from a previous run with generated files, we definitely can't use it anymore
+                            compilationWithStaleGeneratedTrees = null;
+                        }
+                        else // we have a generator
                         {
                             // If we don't already have a generator driver, we'll have to create one from scratch
                             if (generatorInfo.Driver == null)

src/Workspaces/CoreTest/SolutionTests/SolutionWithSourceGeneratorTests.cs

@@ -87,6 +87,11 @@ public async Task WithReferencesMethodCorrectlyUpdatesWithEqualReferences()
             project = project.WithAnalyzerReferences(new[] { analyzerReference2 });
 
             Assert.Single((await project.GetRequiredCompilationAsync(CancellationToken.None)).SyntaxTrees);
+
+            // Now remove and confirm that we don't have any files
+            project = project.WithAnalyzerReferences(SpecializedCollections.EmptyEnumerable<AnalyzerReference>());
+
+            Assert.Empty((await project.GetRequiredCompilationAsync(CancellationToken.None)).SyntaxTrees);
         }
 
         private class TestGeneratorReferenceWithFilePathEquality : TestGeneratorReference, IEquatable<AnalyzerReference>