[XSG] avoid generating CB twice (#32242)

After some measurment, XSG took 25s on Xaml.UnitTests. code behind file
were generated twice. generating once only shave 10s (40%) of XSG time

Author: StephaneDelcroix

src/Controls/src/SourceGen/CodeBehindGenerator.cs

@@ -7,6 +7,7 @@
 using System.Xml;
 
 using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.Text;
 
 using Microsoft.Maui.Controls.Xaml;
@@ -32,6 +33,10 @@ public void Initialize(IncrementalGeneratorInitializationContext initContext)
 			.WithComparer(new CompilationReferencesComparer())
 			.WithTrackingName(TrackingNames.ReferenceCompilationProvider);
 
+		var referenceTypeCacheProvider = referenceCompilationProvider
+			.Select(GetTypeCache)
+			.WithTrackingName(TrackingNames.ReferenceTypeCacheProvider);
+
 		var xmlnsDefinitionsProvider = referenceCompilationProvider
 			.Select(GetAssemblyAttributes)
 			.WithTrackingName(TrackingNames.XmlnsDefinitionsProvider);
@@ -57,27 +62,27 @@ public void Initialize(IncrementalGeneratorInitializationContext initContext)
 			.Where(static p => p?.Kind == "Css")
 			.WithTrackingName(TrackingNames.CssProjectItemProvider);
 
-		var referenceTypeCacheProvider = referenceCompilationProvider
-			.Select(GetTypeCache)
-			.WithTrackingName(TrackingNames.ReferenceTypeCacheProvider);
-
 		var xamlSourceProviderForCB = xamlProjectItemProviderForCB
 			.Combine(xmlnsDefinitionsProvider, referenceTypeCacheProvider, referenceCompilationProvider)
+			.Select(GetSource)
 			.WithTrackingName(TrackingNames.XamlSourceProviderForCB);
 
 		var compilationWithCodeBehindProvider = xamlSourceProviderForCB
-			.Select(GetSyntaxTree)
 			.Collect()
 			.Combine(referenceCompilationProvider)
 			.Select(static (t, ct) =>
 			{
 				var compilation = t.Right;
-				foreach (var tree in t.Left)
+				var options = compilation.SyntaxTrees.FirstOrDefault()?.Options as CSharpParseOptions;
+				foreach (var (source, xamlItem, diagnostics) in t.Left)
 				{
 					ct.ThrowIfCancellationRequested();
 
-					if (tree is not null)
+					if (source is not null)
+					{
+						var tree = CSharpSyntaxTree.ParseText(source, options: options, cancellationToken: ct);
 						compilation = compilation.AddSyntaxTrees(tree);
+					}
 				}
 				return compilation;
 			})
@@ -95,12 +100,15 @@ public void Initialize(IncrementalGeneratorInitializationContext initContext)
 		// Register the XAML pipeline for CodeBehind
 		initContext.RegisterSourceOutput(xamlSourceProviderForCB, static (sourceProductionContext, provider) =>
 		{
-			var (xamlItem, xmlnsCache, typeCache, compilation) = provider;
+			var (source, xamlItem, diagnostics) = provider;
 
 			try
 			{
-				var code = CodeBehindCodeWriter.GenerateXamlCodeBehind(xamlItem, compilation, sourceProductionContext.ReportDiagnostic, sourceProductionContext.CancellationToken, xmlnsCache, typeCache);
-				sourceProductionContext.AddSource(GetHintName(xamlItem?.ProjectItem, "sg"), code);
+				if (diagnostics != null)
+					foreach (var diag in diagnostics)
+						sourceProductionContext.ReportDiagnostic(diag);
+				if (source != null)
+					sourceProductionContext.AddSource(GetHintName(xamlItem?.ProjectItem, "sg"), source);
 			}
 			catch (Exception e)
 			{

src/Controls/src/SourceGen/GeneratorHelpers.cs

@@ -365,19 +365,25 @@ static void SimplifyOnPlatform(XmlNode node, string? targetFramework, XmlNamespa
 
 	public static IDictionary<XmlType, ITypeSymbol> GetTypeCache(Compilation compilation, CancellationToken cancellationToken) => new Dictionary<XmlType, ITypeSymbol>();
 
-	public static SyntaxTree? GetSyntaxTree((XamlProjectItemForCB? xamlItem, AssemblyCaches xmlnsCache, IDictionary<XmlType, ITypeSymbol> typeCache, Compilation compilation) tuple, CancellationToken cancellationToken)
+	public static (string? source, XamlProjectItemForCB? xamlItem, IList<Diagnostic>? diagnostics) GetSource((XamlProjectItemForCB? xamlItem, AssemblyCaches xmlnsCache, IDictionary<XmlType, ITypeSymbol> typeCache, Compilation compilation) tuple, CancellationToken cancellationToken)
 	{
-		var options = tuple.compilation.SyntaxTrees.FirstOrDefault()?.Options as CSharpParseOptions;
+		cancellationToken.ThrowIfCancellationRequested();
+		var (xamlItem, xmlnsCache, typeCache, compilation) = tuple;
+
+		string? code = null;
+		List<Diagnostic>? diagnostics = null;
+		void reportDiagnostic(Diagnostic diagnostic) => (diagnostics ??= new List<Diagnostic>()).Add(diagnostic);
 		try
 		{
-			var code = CodeBehindCodeWriter.GenerateXamlCodeBehind(tuple.xamlItem, tuple.compilation, null, cancellationToken, tuple.xmlnsCache, tuple.typeCache);
-			return CSharpSyntaxTree.ParseText(code, options: options, cancellationToken: cancellationToken);
+			code = CodeBehindCodeWriter.GenerateXamlCodeBehind(xamlItem, compilation, reportDiagnostic, cancellationToken, xmlnsCache, typeCache);
 		}
-		catch (Exception)
+		catch (Exception e)
 		{
+			var location = xamlItem?.ProjectItem?.RelativePath is not null ? Location.Create(xamlItem.ProjectItem.RelativePath, new TextSpan(), new LinePositionSpan()) : null;
+			reportDiagnostic(Diagnostic.Create(Descriptors.XamlParserError, location, e.Message));
 		}
-		return null;
-	}
+		return (code, xamlItem, diagnostics);
+    }
 
 	/// <summary>
 	/// Formats a value as a culture-independent C# literal for source generation.

src/Controls/tests/Xaml.UnitTests/Controls.Xaml.UnitTests.csproj

@@ -11,6 +11,7 @@
     <DisableMSBuildAssemblyCopyCheck>true</DisableMSBuildAssemblyCopyCheck>
     <MauiEnableXamlCBindingWithSourceCompilation>true</MauiEnableXamlCBindingWithSourceCompilation>
     <DefineConstants>$(DefineConstants);MauiAllowImplicitXmlnsDeclaration</DefineConstants>
+    <ReportAnalyzer>true</ReportAnalyzer>
     <!-- We can't yet disable the namescope generation, ConstraintTypeConverter and ReferenceTypeConverter need to be ported to sourcegen
     <DefineConstants>$(DefineConstants);_MAUIXAML_SG_NAMESCOPE_DISABLE</DefineConstants>
     -->