Clean up some DI code

src/libraries/Microsoft.Extensions.DependencyInjection/src/CallSiteJsonFormatter.cs

@@ -81,11 +81,6 @@ protected override object VisitServiceProvider(ServiceProviderCallSite servicePr
             return null;
         }
 
-        protected override object VisitServiceScopeFactory(ServiceScopeFactoryCallSite serviceScopeFactoryCallSite, CallSiteFormatterContext argument)
-        {
-            return null;
-        }
-
         protected override object VisitIEnumerable(IEnumerableCallSite enumerableCallSite, CallSiteFormatterContext argument)
         {
             argument.WriteProperty("itemType", enumerableCallSite.ItemType);

src/libraries/Microsoft.Extensions.DependencyInjection/src/ServiceLookup/CallSiteKind.cs

@@ -19,10 +19,6 @@ internal enum CallSiteKind
 
         Transient,
 
-        CreateInstance,
-
-        ServiceScopeFactory,
-
         Singleton
     }
 }

src/libraries/Microsoft.Extensions.DependencyInjection/src/ServiceLookup/CallSiteRuntimeResolver.cs

@@ -20,6 +20,12 @@ private CallSiteRuntimeResolver()
 
         public object Resolve(ServiceCallSite callSite, ServiceProviderEngineScope scope)
         {
+            // Fast path to avoid virtual calls if we already have the cached value in the root scope
+            if (scope.IsRootScope && callSite.Value is object cached)
+            {
+                return cached;
+            }
+
             return VisitCallSite(callSite, new RuntimeResolverContext
             {
                 Scope = scope
@@ -153,11 +159,6 @@ protected override object VisitServiceProvider(ServiceProviderCallSite servicePr
             return context.Scope;
         }
 
-        protected override object VisitServiceScopeFactory(ServiceScopeFactoryCallSite serviceScopeFactoryCallSite, RuntimeResolverContext context)
-        {
-            return serviceScopeFactoryCallSite.Value;
-        }
-
         protected override object VisitIEnumerable(IEnumerableCallSite enumerableCallSite, RuntimeResolverContext context)
         {
             var array = Array.CreateInstance(

src/libraries/Microsoft.Extensions.DependencyInjection/src/ServiceLookup/CallSiteValidator.cs

@@ -78,7 +78,7 @@ protected override Type VisitRootCache(ServiceCallSite singletonCallSite, CallSi
         protected override Type VisitScopeCache(ServiceCallSite scopedCallSite, CallSiteValidatorState state)
         {
             // We are fine with having ServiceScopeService requested by singletons
-            if (scopedCallSite is ServiceScopeFactoryCallSite)
+            if (scopedCallSite.ServiceType == typeof(IServiceScopeFactory))
             {
                 return null;
             }
@@ -100,8 +100,6 @@ protected override Type VisitScopeCache(ServiceCallSite scopedCallSite, CallSite
 
         protected override Type VisitServiceProvider(ServiceProviderCallSite serviceProviderCallSite, CallSiteValidatorState state) => null;
 
-        protected override Type VisitServiceScopeFactory(ServiceScopeFactoryCallSite serviceScopeFactoryCallSite, CallSiteValidatorState state) => null;
-
         protected override Type VisitFactory(FactoryCallSite factoryCallSite, CallSiteValidatorState state) => null;
 
         internal struct CallSiteValidatorState

src/libraries/Microsoft.Extensions.DependencyInjection/src/ServiceLookup/CallSiteVisitor.cs

@@ -2,7 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
-using System.Runtime.CompilerServices;
 
 namespace Microsoft.Extensions.DependencyInjection.ServiceLookup
 {
@@ -51,8 +50,6 @@ protected virtual TResult VisitCallSiteMain(ServiceCallSite callSite, TArgument
                     return VisitConstant((ConstantCallSite)callSite, argument);
                 case CallSiteKind.ServiceProvider:
                     return VisitServiceProvider((ServiceProviderCallSite)callSite, argument);
-                case CallSiteKind.ServiceScopeFactory:
-                    return VisitServiceScopeFactory((ServiceScopeFactoryCallSite)callSite, argument);
                 default:
                     throw new NotSupportedException(SR.Format(SR.CallSiteTypeNotSupported, callSite.GetType()));
             }
@@ -84,8 +81,6 @@ protected virtual TResult VisitScopeCache(ServiceCallSite callSite, TArgument ar
 
         protected abstract TResult VisitServiceProvider(ServiceProviderCallSite serviceProviderCallSite, TArgument argument);
 
-        protected abstract TResult VisitServiceScopeFactory(ServiceScopeFactoryCallSite serviceScopeFactoryCallSite, TArgument argument);
-
         protected abstract TResult VisitIEnumerable(IEnumerableCallSite enumerableCallSite, TArgument argument);
 
         protected abstract TResult VisitFactory(FactoryCallSite factoryCallSite, TArgument argument);

src/libraries/Microsoft.Extensions.DependencyInjection/src/ServiceLookup/ConstantCallSite.cs

@@ -21,7 +21,7 @@ public ConstantCallSite(Type serviceType, object defaultValue): base(ResultCache
             Value = defaultValue;
         }
 
-        public override Type ServiceType => DefaultValue?.GetType() ?? _serviceType;
+        public override Type ServiceType => _serviceType;
         public override Type ImplementationType => DefaultValue?.GetType() ?? _serviceType;
         public override CallSiteKind Kind { get; } = CallSiteKind.Constant;
     }

src/libraries/Microsoft.Extensions.DependencyInjection/src/ServiceLookup/Expressions/ExpressionResolverBuilder.cs

@@ -117,11 +117,6 @@ protected override Expression VisitServiceProvider(ServiceProviderCallSite servi
             return ScopeParameter;
         }
 
-        protected override Expression VisitServiceScopeFactory(ServiceScopeFactoryCallSite serviceScopeFactoryCallSite, object context)
-        {
-            return Expression.Constant(serviceScopeFactoryCallSite.Value);
-        }
-
         protected override Expression VisitFactory(FactoryCallSite factoryCallSite, object context)
         {
             return Expression.Invoke(Expression.Constant(factoryCallSite.Factory), ScopeParameter);

src/libraries/Microsoft.Extensions.DependencyInjection/src/ServiceLookup/ILEmit/ILEmitResolverBuilder.cs

@@ -194,12 +194,6 @@ protected override object VisitServiceProvider(ServiceProviderCallSite servicePr
             return null;
         }
 
-        protected override object VisitServiceScopeFactory(ServiceScopeFactoryCallSite serviceScopeFactoryCallSite, ILEmitResolverBuilderContext argument)
-        {
-            AddConstant(argument, serviceScopeFactoryCallSite.Value);
-            return null;
-        }
-
         protected override object VisitIEnumerable(IEnumerableCallSite enumerableCallSite, ILEmitResolverBuilderContext argument)
         {
             if (enumerableCallSite.ServiceCallSites.Length == 0)

src/libraries/Microsoft.Extensions.DependencyInjection/src/ServiceProvider.cs

@@ -41,7 +41,7 @@ internal ServiceProvider(IEnumerable<ServiceDescriptor> serviceDescriptors, Serv
             // The list of built in services that aren't part of the list of service descriptors
             // keep this in sync with CallSiteFactory.IsService
             CallSiteFactory.Add(typeof(IServiceProvider), new ServiceProviderCallSite());
-            CallSiteFactory.Add(typeof(IServiceScopeFactory), new ServiceScopeFactoryCallSite(Root));
+            CallSiteFactory.Add(typeof(IServiceScopeFactory), new ConstantCallSite(typeof(IServiceScopeFactory), Root));
             CallSiteFactory.Add(typeof(IServiceProviderIsService), new ConstantCallSite(typeof(IServiceProviderIsService), CallSiteFactory));
 
             if (options.ValidateScopes)