[AOT] Audit dynamic code flags and add comments (#45946)

Author: JamesNK

src/Hosting/Hosting/src/GenericHost/GenericWebHostBuilder.cs

@@ -341,6 +341,7 @@ static void InvokeContainer(GenericWebHostBuilder genericWebHostBuilder, Configu
         {
             var containerType = configureContainerBuilder.GetContainerType();
 
+            // Configure container uses MakeGenericType with the container type. MakeGenericType + struct container type requires IsDynamicCodeSupported.
             if (containerType.IsValueType && !RuntimeFeature.IsDynamicCodeSupported)
             {
                 throw new InvalidOperationException("A ValueType TContainerBuilder isn't supported with AOT.");

src/Hosting/Hosting/src/Internal/StartupLoader.cs

@@ -68,6 +68,7 @@ public static StartupMethods LoadMethods(IServiceProvider hostingServiceProvider
             Justification = "There is a runtime check for ValueType startup container. It's unlikely anyone will use a ValueType here.")]
         static Type CreateConfigureServicesDelegateBuilder(Type type)
         {
+            // Configure container uses MakeGenericType with the container type. MakeGenericType + struct container type requires IsDynamicCodeSupported.
             if (type.IsValueType && !RuntimeFeature.IsDynamicCodeSupported)
             {
                 throw new InvalidOperationException("ValueType startup container isn't supported with AOT.");

src/Http/Http.Abstractions/src/Extensions/UseMiddlewareExtensions.cs

@@ -109,7 +109,9 @@ public static IApplicationBuilder UseMiddleware(
                 return (RequestDelegate)invokeMethod.CreateDelegate(typeof(RequestDelegate), instance);
             }
 
-            var factory = RuntimeFeature.IsDynamicCodeSupported
+            // Performance optimization: Use compiled expressions to invoke middleware with services injected in Invoke.
+            // If IsDynamicCodeCompiled is false then use standard reflection to avoid overhead of interpreting expressions.
+            var factory = RuntimeFeature.IsDynamicCodeCompiled
                 ? CompileExpression<object>(invokeMethod, parameters)
                 : ReflectionFallback<object>(invokeMethod, parameters);
 

src/Http/Http.Extensions/src/RequestDelegateFactory.cs

@@ -422,6 +422,7 @@ private static Expression[] CreateArgumentsAndInferMetadata(MethodInfo methodInf
 
         for (var i = 0; i < argTypes.Length; i++)
         {
+            // MakeGenericMethod + value type requires IsDynamicCodeSupported to be true.
             if (RuntimeFeature.IsDynamicCodeSupported)
             {
                 // Register expressions containing the boxed and unboxed variants
@@ -560,7 +561,7 @@ private static Expression CreateEndpointFilterInvocationContextBase(RequestDeleg
             DefaultEndpointFilterInvocationContextConstructor,
             new Expression[] { HttpContextExpr, paramArray });
 
-        if (!RuntimeFeature.IsDynamicCodeCompiled)
+        if (!RuntimeFeature.IsDynamicCodeSupported)
         {
             // For AOT platforms it's not possible to support the closed generic arguments that are based on the
             // parameter arguments dynamically (for value types). In that case, fallback to boxing the argument list.

src/Http/Routing/src/Matching/JumpTableBuilder.cs

@@ -92,6 +92,8 @@ public static JumpTable Build(int defaultDestination, int exitDestination, (stri
         [UnconditionalSuppressMessage("AOT", "IL3050", Justification = "Guarded by IsDynamicCodeCompiled")]
         static JumpTable MakeILEmitTrieJumpTableIfSupported(int defaultDestination, int exitDestination, (string text, int destination)[] pathEntries, JumpTable fallback)
         {
+            // ILEmitTrieJumpTable use IL emit to generate a custom, high-performance jump table.
+            // EL emit requires IsDynamicCodeCompiled to be true. Fallback to another jump table implementation if not available.
             return RuntimeFeature.IsDynamicCodeCompiled
                 ? new ILEmitTrieJumpTable(defaultDestination, exitDestination, pathEntries, vectorize: null, fallback)
                 : fallback;

src/Shared/PropertyHelper/PropertyHelper.cs

@@ -211,6 +211,7 @@ public static PropertyHelper[] GetVisibleProperties(
         Debug.Assert(!getMethod.IsStatic);
         Debug.Assert(getMethod.GetParameters().Length == 0);
 
+        // MakeGenericMethod + value type requires IsDynamicCodeSupported to be true.
         if (RuntimeFeature.IsDynamicCodeSupported)
         {
             // TODO: Remove disable when https://github.com/dotnet/linker/issues/2715 is complete.
@@ -285,6 +286,7 @@ public static PropertyHelper[] GetVisibleProperties(
         var parameters = setMethod.GetParameters();
         Debug.Assert(parameters.Length == 1);
 
+        // MakeGenericMethod + value type requires IsDynamicCodeSupported to be true.
         if (RuntimeFeature.IsDynamicCodeSupported)
         {
             // TODO: Remove disable when https://github.com/dotnet/linker/issues/2715 is complete.