add support for CreateSourceInfo in design window loader (#21972)

* feat: add support for CreateSourceInfo in design window loader

* Move XAML SourceInfo default to RuntimeXamlLoaderConfiguration

Applies to all designer/runtime load paths now; newer VS and VS Code designers do not use DesignWindowLoader.

---------

Co-authored-by: Kim Henrik <KimHenrik@outlook.de>
Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

src/Avalonia.Build.Tasks/XamlCompilerTaskExecutor.cs

@@ -157,6 +157,12 @@ namespace Avalonia.Build.Tasks
                 var arg1 = new CustomAttributeArgument(strType, "AvaloniaUseCompiledBindingsByDefault");
                 var arg2 = new CustomAttributeArgument(strType, defaultCompileBindings.ToString());
                 asm.CustomAttributes.Add(new CustomAttribute(ctor) { ConstructorArguments = { arg1, arg2 } });
+                if (createSourceInfo)
+                {
+                    var arg3 = new CustomAttributeArgument(strType, "AvaloniaXamlCreateSourceInfo");
+                    var arg4 = new CustomAttributeArgument(strType, createSourceInfo.ToString());
+                    asm.CustomAttributes.Add(new CustomAttribute(ctor) { ConstructorArguments = { arg3, arg4 } });
+                }
             }
 
             TypeDefinition AddClass(string name, TypeAttributes extraAttributes = 0)

src/Markup/Avalonia.Markup.Xaml/RuntimeXamlLoaderConfiguration.cs

@@ -1,10 +1,16 @@
 using System;
+using System.Linq;
 using System.Reflection;
 
 namespace Avalonia.Markup.Xaml;
 
 public class RuntimeXamlLoaderConfiguration
 {
+    /// <summary>
+    /// When enabled, the XAML compiler embeds SourceInfo metadata (file path, line, and column) into generated code.
+    /// </summary>
+    private bool? _createSourceInfo = null;
+
     /// <summary>
     /// Default assembly for clr-namespace:.
     /// </summary>
@@ -24,9 +30,24 @@ public class RuntimeXamlLoaderConfiguration
 
     /// <summary>
     /// When enabled, the XAML compiler embeds SourceInfo metadata (file path, line, and column) into generated code.
-    /// Default is 'false'.
+    /// When not explicitly set, the 'AvaloniaXamlCreateSourceInfo' from the LocalAssembly will be used to determine the value.
     /// </summary>
-    public bool CreateSourceInfo { get; set; } = false;
+    public bool CreateSourceInfo 
+    { 
+        get {
+            if(_createSourceInfo == null)
+            {
+                if(LocalAssembly is {} asm)
+                {
+                    var createSourceInfo = asm.GetCustomAttributes<AssemblyMetadataAttribute>()
+                        .FirstOrDefault(a => a.Key == "AvaloniaXamlCreateSourceInfo")?.Value;
+                    return bool.TryParse(createSourceInfo, out var parsedCreateSourceInfo) && parsedCreateSourceInfo;
+                }
+            }   
+            return _createSourceInfo ?? false;
+        }
+        set => _createSourceInfo = value; 
+    }
 
     /// <summary>
     /// XAML diagnostics handler.