Support synthetic Source property.

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/CompiledBindingExtension.cs

@@ -50,6 +50,13 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
 
         protected override ExpressionObserver CreateExpressionObserver(IAvaloniaObject target, AvaloniaProperty targetProperty, object anchor, bool enableDataValidation)
         {
+            if (Path.RawSource != null)
+            {
+                return CreateSourceObserver(
+                    Path.RawSource,
+                    Path.BuildExpression(enableDataValidation));
+            }
+
             if (Path.SourceMode == SourceMode.Data)
             {
                 return CreateDataContextObserver(

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/CompiledBindings/CompiledBindingPath.cs

@@ -14,9 +14,10 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
 
         public CompiledBindingPath() { }
 
-        internal CompiledBindingPath(IEnumerable<ICompiledBindingPathElement> bindingPath)
+        internal CompiledBindingPath(IEnumerable<ICompiledBindingPathElement> bindingPath, object rawSource)
         {
             _elements = new List<ICompiledBindingPathElement>(bindingPath);
+            RawSource = rawSource;
         }
 
         public ExpressionNode BuildExpression(bool enableValidation)
@@ -63,10 +64,13 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
         }
 
         internal SourceMode SourceMode => _elements.Count > 0 && _elements[0] is IControlSourceBindingPathElement ? SourceMode.Control : SourceMode.Data;
+
+        internal object RawSource { get; }
     }
 
     public class CompiledBindingPathBuilder
     {
+        private object _rawSource;
         private List<ICompiledBindingPathElement> _elements = new List<ICompiledBindingPathElement>();
 
         public CompiledBindingPathBuilder Not()
@@ -122,7 +126,13 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
             return this;
         }
 
-        public CompiledBindingPath Build() => new CompiledBindingPath(_elements);
+        public CompiledBindingPathBuilder SetRawSource(object rawSource)
+        {
+            _rawSource = rawSource;
+            return this;
+        }
+
+        public CompiledBindingPath Build() => new CompiledBindingPath(_elements, _rawSource);
     }
 
     public interface ICompiledBindingPathElement

src/Markup/Avalonia.Markup.Xaml/XamlIl/CompilerExtensions/Transformers/AvaloniaXamlIlBindingPathParser.cs

@@ -70,6 +70,11 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
                     v.Property is AvaloniaSyntheticCompiledBindingProperty prop
                     && prop.Name == SyntheticCompiledBindingPropertyName.ElementName);
 
+            var sourceProperty = syntheticCompiledBindingProperties
+                .FirstOrDefault(v =>
+                    v.Property is AvaloniaSyntheticCompiledBindingProperty prop
+                    && prop.Name == SyntheticCompiledBindingPropertyName.Source);
+
             var relativeSourceProperty = syntheticCompiledBindingProperties
                 .FirstOrDefault(v =>
                     v.Property is AvaloniaSyntheticCompiledBindingProperty prop
@@ -84,6 +89,16 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
                 throw new XamlIlParseException($"Invalid ElementName '{elementNameProperty.Values[0]}'.", elementNameProperty.Values[0]);
             }
 
+            if (sourceProperty?.Values[0] != null)
+            {
+                if (convertedNode != null)
+                {
+                    throw new XamlIlParseException("Only one of ElementName, Source, or RelativeSource specified as a binding source. Only one property is allowed.", binding);
+                }
+
+                convertedNode = new RawSourceBindingExpressionNode(sourceProperty?.Values[0]);
+            }
+
             if (GetRelativeSourceObjectFromAssignment(
                 context,
                 relativeSourceProperty,
@@ -91,7 +106,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
             {
                 if (convertedNode != null)
                 {
-                    throw new XamlIlParseException("Both ElementName and RelativeSource specified as a binding source. Only one property is allowed.", binding);
+                    throw new XamlIlParseException("Only one of ElementName, Source, or RelativeSource specified as a binding source. Only one property is allowed.", binding);
                 }
 
                 var mode = relativeSourceObject.Children
@@ -206,6 +221,10 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
             {
                 binding.Children.Remove(elementNameProperty);
             }
+            if (sourceProperty != null)
+            {
+                binding.Children.Remove(sourceProperty);
+            }
             if (relativeSourceProperty != null)
             {
                 binding.Children.Remove(relativeSourceProperty);
@@ -263,6 +282,17 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
         public IXamlIlAstTypeReference Type { get; }
 
         public IList<BindingExpressionGrammar.INode> Path { get; }
+
+        public override void VisitChildren(IXamlIlAstVisitor visitor)
+        {
+            for (int i = 0; i < Path.Count; i++)
+            {
+                if (Path[i] is IXamlIlAstNode ast)
+                {
+                    Path[i] = (BindingExpressionGrammar.INode)ast.Visit(visitor);
+                }
+            }
+        }
     }
 
     class VisualAncestorBindingExpressionNode : BindingExpressionGrammar.INode
@@ -281,4 +311,20 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
     {
         public IXamlIlType Type { get; set; }
     }
+
+    class RawSourceBindingExpressionNode : XamlIlAstNode, BindingExpressionGrammar.INode
+    {
+        public RawSourceBindingExpressionNode(IXamlIlAstValueNode rawSource)
+            : base(rawSource)
+        {
+            RawSource = rawSource;
+        }
+
+        public IXamlIlAstValueNode RawSource { get; private set; }
+
+        public override void VisitChildren(IXamlIlAstVisitor visitor)
+        {
+            RawSource = (IXamlIlAstValueNode)RawSource.Visit(visitor);
+        }
+    }
 }

src/Markup/Avalonia.Markup.Xaml/XamlIl/CompilerExtensions/Transformers/AvaloniaXamlIlTransformSyntheticCompiledBindingMembers.cs

@@ -25,6 +25,11 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
                     return new AvaloniaSyntheticCompiledBindingProperty(node,
                         SyntheticCompiledBindingPropertyName.RelativeSource);
                 }
+                else if (prop.Name == "Source")
+                {
+                    return new AvaloniaSyntheticCompiledBindingProperty(node,
+                        SyntheticCompiledBindingPropertyName.Source);
+                }
             }
 
             return node;
@@ -34,7 +39,8 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
     enum SyntheticCompiledBindingPropertyName
     {
         ElementName,
-        RelativeSource
+        RelativeSource,
+        Source
     }
 
     class AvaloniaSyntheticCompiledBindingProperty : XamlIlAstNode, IXamlIlAstPropertyReference

src/Markup/Avalonia.Markup.Xaml/XamlIl/CompilerExtensions/XamlIlBindingPathHelper.cs

@@ -235,6 +235,9 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
                         }
                         nodes.Add(new ElementNamePathElementNode(elementName.Name, elementType));
                         break;
+                    case RawSourceBindingExpressionNode rawSource:
+                        nodes.Add(new RawSourcePathElementNode(rawSource.RawSource));
+                        break;
                 }
             }
 
@@ -563,6 +566,28 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
             public IXamlIlType Type => _arrayType.ArrayElementType;
         }
 
+        class RawSourcePathElementNode : XamlIlAstNode, IXamlIlBindingPathElementNode
+        {
+            private readonly IXamlIlAstValueNode _rawSource;
+
+            public RawSourcePathElementNode(IXamlIlAstValueNode rawSource)
+                :base(rawSource)
+            {
+                _rawSource = rawSource;
+                
+            }
+
+            public IXamlIlType Type => _rawSource.Type.GetClrType();
+
+            public void Emit(XamlIlEmitContext context, IXamlIlEmitter codeGen)
+            {
+                context.Emit(_rawSource, codeGen, Type);
+                codeGen
+                    .EmitCall(context.GetAvaloniaTypes()
+                    .CompiledBindingPathBuilder.FindMethod(m => m.Name == "SetRawSource"));
+            }
+        }
+
         class XamlIlBindingPathNode : XamlIlAstNode, IXamlIlBindingPathNode, IXamlIlAstEmitableNode
         {
             private readonly List<IXamlIlBindingPathElementNode> _transformElements;
@@ -603,6 +628,24 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
                 codeGen.EmitCall(types.CompiledBindingPathBuilder.FindMethod(m => m.Name == "Build"));
                 return XamlIlNodeEmitResult.Type(0, types.CompiledBindingPath);
             }
+
+            public override void VisitChildren(IXamlIlAstVisitor visitor)
+            {
+                for (int i = 0; i < _transformElements.Count; i++)
+                {
+                    if (_transformElements[i] is IXamlIlAstNode ast)
+                    {
+                        _transformElements[i] = (IXamlIlBindingPathElementNode)ast.Visit(visitor);
+                    }
+                }
+                for (int i = 0; i < _elements.Count; i++)
+                {
+                    if (_elements[i] is IXamlIlAstNode ast)
+                    {
+                        _elements[i] = (IXamlIlBindingPathElementNode)ast.Visit(visitor);
+                    }
+                }
+            }
         }
     }
 

tests/Avalonia.Markup.Xaml.UnitTests/MarkupExtensions/CompiledBindingExtensionTests.cs

@@ -494,6 +494,31 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
                 Assert.Equal("test", target.Text);
             }
         }
+
+        [Fact]
+        public void ResolvesSourceBindingLongForm()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:local='clr-namespace:Avalonia.Markup.Xaml.UnitTests.MarkupExtensions;assembly=Avalonia.Markup.Xaml.UnitTests'
+        x:DataType='local:TestDataContext'
+        Title='test'>
+    <TextBlock Text='{CompiledBinding Length, Source=Test}' x:Name='text'/>
+</Window>";
+                var loader = new AvaloniaXamlLoader();
+                var window = (Window)loader.Load(xaml);
+                var target = window.FindControl<TextBlock>("text");
+
+                window.ApplyTemplate();
+                window.Presenter.ApplyTemplate();
+                target.ApplyTemplate();
+
+                Assert.Equal("Test".Length.ToString(), target.Text);
+            }
+        }
     }
 
     public class TestDataContext