Fix cohost override setting

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/SourceGenerators/RazorSourceGenerator.cs

@@ -38,8 +38,11 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
             var parseOptions = context.ParseOptionsProvider;
             var compilation = context.CompilationProvider;
 
+            // We might get initialized before razor tooling has a chance to set this, so check it any time something changes. It's extremely cheap and we'll stop propagating directly below if it didn't changed.
+            var useRazorCohostServer = context.AdditionalTextsProvider.Collect().Combine(context.CompilationProvider).Select((_, _) => RazorCohostingOptions.UseRazorCohostServer);
+
             // determine if we should suppress this run and filter out all the additional files and references if so
-            var isGeneratorSuppressed = analyzerConfigOptions.CheckGlobalFlagSet("SuppressRazorSourceGenerator").Select((suppress, _) => !RazorCohostingOptions.UseRazorCohostServer && suppress);
+            var isGeneratorSuppressed = analyzerConfigOptions.CheckGlobalFlagSet("SuppressRazorSourceGenerator").Combine(useRazorCohostServer).Select((suppress, _) => !suppress.Right && suppress.Left);
             var additionalTexts = context.AdditionalTextsProvider.EmptyOrCachedWhen(isGeneratorSuppressed, true);
             var metadataRefs = context.MetadataReferencesProvider.EmptyOrCachedWhen(isGeneratorSuppressed, true);
 

src/Compiler/test/Microsoft.NET.Sdk.Razor.SourceGenerators.Tests/RazorSourceGeneratorTests.cs

@@ -3447,5 +3447,66 @@ protected override void BuildRenderTree(global::Microsoft.AspNetCore.Components.
 
                 """");
         }
+
+        [Fact]
+        public async Task UseRazorCohostServer_CanOverride_Suppression()
+        {
+            using var eventListener = new RazorEventListener();
+            var project = CreateTestProject(new()
+            {
+                ["Component.Razor"] = "<h1>Hello world</h1>",
+            });
+            var compilation = await project.GetCompilationAsync();
+
+            // Start with the generator suppressed
+            var (driver, additionalTexts, optionsProvider) = await GetDriverWithAdditionalTextAndProviderAsync(project, configureGlobalOptions: (o) =>
+            {
+                o.TestGlobalOptions["build_property.SuppressRazorSourceGenerator"] = "true";
+            });
+
+            // results are empty, and no recorded steps run
+            var result = RunGenerator(compilation!, ref driver).VerifyPageOutput();
+            Assert.DoesNotContain("DocumentsWithSuppression", (IReadOnlyDictionary<string, ImmutableArray<IncrementalGeneratorRunStep>>)result.TrackedSteps);
+            Assert.DoesNotContain("ImplementationSourceOutput", (IReadOnlyDictionary<string, ImmutableArray<IncrementalGeneratorRunStep>>)result.TrackedOutputSteps);
+
+            // now enable UseRazorCohostServer
+            RazorCohostingOptions.UseRazorCohostServer = true;
+
+            // unfortunately if we re-run we'll still be in the suppressed mode. Because nothing in the compilation has changed
+            // there is no event that the source generator can 'trigger' on to re-evaluate.
+            result = RunGenerator(compilation!, ref driver).VerifyPageOutput();
+
+            // but if we make a trivial change to the compilation, we will re-run and get results
+            compilation = compilation!.WithAssemblyName(compilation.AssemblyName);
+
+            result = RunGenerator(compilation!, ref driver).VerifyPageOutput(@"
+#pragma checksum ""Component.razor"" ""{ff1816ec-aa5e-4d10-87f7-6f4963833460}"" ""6b5db227a6aa2228c777b0771108b184b1fc5df3""
+// <auto-generated/>
+#pragma warning disable 1591
+namespace MyApp
+{
+    #line default
+    using global::System;
+    using global::System.Collections.Generic;
+    using global::System.Linq;
+    using global::System.Threading.Tasks;
+    using global::Microsoft.AspNetCore.Components;
+    #line default
+    #line hidden
+    #nullable restore
+    public partial class Component : global::Microsoft.AspNetCore.Components.ComponentBase
+    #nullable disable
+    {
+        #pragma warning disable 1998
+        protected override void BuildRenderTree(global::Microsoft.AspNetCore.Components.Rendering.RenderTreeBuilder __builder)
+        {
+            __builder.AddMarkupContent(0, ""<h1>Hello world</h1>"");
+        }
+        #pragma warning restore 1998
+    }
+}
+#pragma warning restore 1591
+");
+        }
     }
 }