EndpointInit.cs

    public class EndpointInit
    {
        private readonly IServiceProvider provider;
        private readonly IActionDescriptorCollectionProvider collectionProvider;
        private readonly IActionInvokerFactory invokerFactory;
        private readonly PageLoader pageLoader;

        public EndpointInit(IServiceProvider provider, IActionDescriptorCollectionProvider collectionProvider, IActionInvokerFactory invokerFactory, PageLoader pageLoader)
        {
            this.provider = provider;
            this.collectionProvider = collectionProvider;
            this.invokerFactory = invokerFactory;
            this.pageLoader = pageLoader;
        }

        public async Task PreCompileAsync()
        {
            var actions = collectionProvider.ActionDescriptors;

            using var scope = provider.CreateScope();
            var httpContext = new DefaultHttpContext()
            {
                RequestServices = scope.ServiceProvider
            };

            foreach (var a in actions.Items)
            {
                // roughly based on seeing this in the stack trace from a bind provider;
                // https://github.com/dotnet/aspnetcore/blob/v5.0.3/src/Mvc/Mvc.Core/src/Routing/ActionEndpointFactory.cs#L420

                var route = new RouteData();
                foreach (var pair in a.RouteValues)
                    route.Values.Add(pair.Key, pair.Value);

                var descriptor = a;

                if (descriptor is PageActionDescriptor page)
                    descriptor = await pageLoader.LoadAsync(page);

                var context = new ActionContext(httpContext, route, descriptor);
                invokerFactory.CreateInvoker(context);
            }
        }
    }