Scaffolding: Avoid recompiling T4 templates

This dramatically improves the performance of the commands.

Part of dotnet#27598

src/EFCore.Design/Scaffolding/Internal/TextTemplatingModelGenerator.cs

@@ -5,7 +5,8 @@
 using System.Text;
 using Microsoft.EntityFrameworkCore.Design.Internal;
 using Microsoft.EntityFrameworkCore.Internal;
-using Engine = Mono.TextTemplating.TemplatingEngine;
+using Microsoft.VisualStudio.TextTemplating;
+using Mono.TextTemplating;
 
 namespace Microsoft.EntityFrameworkCore.Scaffolding.Internal;
 
@@ -23,7 +24,7 @@ public class TextTemplatingModelGenerator : TemplatedModelGenerator
 
     private readonly IOperationReporter _reporter;
     private readonly IServiceProvider _serviceProvider;
-    private Engine? _engine;
+    private TemplatingEngine? _engine;
 
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
@@ -47,8 +48,8 @@ public TextTemplatingModelGenerator(
     ///     any release. You should only use it directly in your code with extreme caution and knowing that
     ///     doing so can result in application failures when updating to a new Entity Framework Core release.
     /// </summary>
-    protected virtual Engine Engine
-        => _engine ??= new Engine();
+    protected virtual TemplatingEngine Engine
+        => _engine ??= new TemplatingEngine();
 
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
@@ -151,6 +152,8 @@ public override ScaffoldedModel GenerateModel(IModel model, ModelCodeGenerationO
         var entityTypeTemplate = Path.Combine(options.ProjectDir!, TemplatesDirectory, EntityTypeTemplate);
         if (File.Exists(entityTypeTemplate))
         {
+            var compiledEntityTypeTemplate = Engine.CompileTemplate(File.ReadAllText(entityTypeTemplate), host);
+
             host.TemplateFile = entityTypeTemplate;
 
             foreach (var entityType in model.GetEntityTypes())
@@ -161,7 +164,10 @@ public override ScaffoldedModel GenerateModel(IModel model, ModelCodeGenerationO
                 host.Session.Add("NamespaceHint", options.ModelNamespace);
                 host.Session.Add("ProjectDefaultNamespace", options.RootNamespace);
 
-                generatedCode = ProcessTemplate(entityTypeTemplate, host);
+                UpdateSession(compiledEntityTypeTemplate, host.Session);
+                generatedCode = compiledEntityTypeTemplate.Process();
+                HandleErrors(host);
+
                 if (string.IsNullOrWhiteSpace(generatedCode))
                 {
                     continue;
@@ -176,6 +182,8 @@ public override ScaffoldedModel GenerateModel(IModel model, ModelCodeGenerationO
         var configurationTemplate = Path.Combine(options.ProjectDir!, TemplatesDirectory, EntityTypeConfigurationTemplate);
         if (File.Exists(configurationTemplate))
         {
+            var compiledConfigurationTemplate = Engine.CompileTemplate(File.ReadAllText(configurationTemplate), host);
+
             host.TemplateFile = configurationTemplate;
 
             foreach (var entityType in model.GetEntityTypes())
@@ -186,7 +194,10 @@ public override ScaffoldedModel GenerateModel(IModel model, ModelCodeGenerationO
                 host.Session.Add("NamespaceHint", options.ContextNamespace ?? options.ModelNamespace);
                 host.Session.Add("ProjectDefaultNamespace", options.RootNamespace);
 
-                generatedCode = ProcessTemplate(configurationTemplate, host);
+                UpdateSession(compiledConfigurationTemplate, host.Session);
+                generatedCode = compiledConfigurationTemplate.Process();
+                HandleErrors(host);
+
                 if (string.IsNullOrWhiteSpace(generatedCode))
                 {
                     continue;
@@ -205,12 +216,29 @@ public override ScaffoldedModel GenerateModel(IModel model, ModelCodeGenerationO
         }
 
         return resultingFiles;
+
+        // HACK: CompiledTemplate holds on to the previous session
+        static void UpdateSession(CompiledTemplate compiledTemplate, ITextTemplatingSession session)
+        {
+            var textTransformation = typeof(CompiledTemplate)
+                .GetField("textTransformation", BindingFlags.Instance | BindingFlags.NonPublic)!
+                .GetValue(compiledTemplate)!;
+            textTransformation.GetType()
+                .GetProperty("Session")!
+                .SetValue(textTransformation, session);
+        }
     }
 
     private string ProcessTemplate(string inputFile, TextTemplatingEngineHost host)
     {
         var output = Engine.ProcessTemplate(File.ReadAllText(inputFile), host);
+        HandleErrors(host);
 
+        return output;
+    }
+
+    private void HandleErrors(TextTemplatingEngineHost host)
+    {
         foreach (CompilerError error in host.Errors)
         {
             _reporter.Write(error);
@@ -223,9 +251,7 @@ private string ProcessTemplate(string inputFile, TextTemplatingEngineHost host)
 
         if (host.Errors.HasErrors)
         {
-            throw new OperationException(DesignStrings.ErrorGeneratingOutput(inputFile));
+            throw new OperationException(DesignStrings.ErrorGeneratingOutput(host.TemplateFile));
         }
-
-        return output;
     }
 }