Tighten AST child-access API and trim output-path overhead

GetChildByRole<T> returned default! while it can legitimately yield null (empty slot
or a kind the node does not declare); it now returns T?. Callers that already null-checked
are unchanged; the few that rely on a structural guarantee (name tokens, label identifier,
property accessors) now state it with an explicit !, and MethodDeclaration drops a redundant
cast that the lie required.

Also removes two members left dead by the Role removal: the AddAnnotation override that
only forwarded to the base, and GetCSharpNodeBefore, which ignored 'this' and had no callers.

On the output path, GetChildNodes() allocates a snapshot List per call; the read-only
.Children traversals that never mutate the tree now walk the live FirstChild/NextSibling chain
(O(n), allocation-free, since child indices are cached), an EmptyStatement child check uses
HasChildren, and the label-statement sibling scan breaks on the first match instead of walking
to the end of the block. The generated GetChildNodes snapshot is kept where traversal may mutate.

Assisted-by: Claude:claude-opus-4-8:Claude Code

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -1352,11 +1352,11 @@ namespace ICSharpCode.Decompiler.CSharp
 					RemoveAttribute(prop, KnownAttribute.ExtensionMarker);
 					if (propDef.Getter != null)
 					{
-						RemoveAttribute(prop.GetChildByRole<Accessor>(SlotKind.Getter), KnownAttribute.ExtensionMarker);
+						RemoveAttribute(prop.GetChildByRole<Accessor>(SlotKind.Getter)!, KnownAttribute.ExtensionMarker);
 					}
 					if (propDef.Setter != null)
 					{
-						RemoveAttribute(prop.GetChildByRole<Accessor>(SlotKind.Setter), KnownAttribute.ExtensionMarker);
+						RemoveAttribute(prop.GetChildByRole<Accessor>(SlotKind.Setter)!, KnownAttribute.ExtensionMarker);
 					}
 					RunTransforms(syntaxTree, decompileRun, new SimpleTypeResolveContext(propDef.DeclaringTypeDefinition));
 					break;
@@ -1851,11 +1851,11 @@ namespace ICSharpCode.Decompiler.CSharp
 							RemoveAttribute(prop, KnownAttribute.ExtensionMarker);
 							if (p.Getter != null)
 							{
-								RemoveAttribute(prop.GetChildByRole<Accessor>(SlotKind.Getter), KnownAttribute.ExtensionMarker);
+								RemoveAttribute(prop.GetChildByRole<Accessor>(SlotKind.Getter)!, KnownAttribute.ExtensionMarker);
 							}
 							if (p.Setter != null)
 							{
-								RemoveAttribute(prop.GetChildByRole<Accessor>(SlotKind.Setter), KnownAttribute.ExtensionMarker);
+								RemoveAttribute(prop.GetChildByRole<Accessor>(SlotKind.Setter)!, KnownAttribute.ExtensionMarker);
 							}
 							extMemberDecl = prop;
 							break;

ICSharpCode.Decompiler/CSharp/OutputVisitor/CSharpOutputVisitor.cs

@@ -1903,7 +1903,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		public virtual void VisitLabelStatement(LabelStatement labelStatement)
 		{
 			StartNode(labelStatement);
-			WriteIdentifier(labelStatement.GetChildByRole<Identifier>(SlotKind.Identifier));
+			WriteIdentifier(labelStatement.GetChildByRole<Identifier>(SlotKind.Identifier)!);
 			WriteToken(Roles.Colon);
 			bool foundLabelledStatement = false;
 			for (AstNode? tmp = labelStatement.NextSibling; tmp != null; tmp = tmp.NextSibling)
@@ -1911,6 +1911,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				if (tmp.Slot?.Kind == labelStatement.Slot?.Kind)
 				{
 					foundLabelledStatement = true;
+					break;
 				}
 			}
 			if (!foundLabelledStatement)
@@ -2428,7 +2429,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			WriteIdentifier(customEventDeclaration.NameToken);
 			OpenBrace(policy.EventBraceStyle);
 			// output add/remove in their original order
-			foreach (AstNode node in customEventDeclaration.Children)
+			for (AstNode? node = customEventDeclaration.FirstChild; node != null; node = node.NextSibling)
 			{
 				if (node.Slot?.Kind == SlotKind.AddAccessor || node.Slot?.Kind == SlotKind.RemoveAccessor)
 				{
@@ -2505,7 +2506,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				if (isSingleLine)
 					Space();
 				// output get/set in their original order
-				foreach (AstNode node in indexerDeclaration.Children)
+				for (AstNode? node = indexerDeclaration.FirstChild; node != null; node = node.NextSibling)
 				{
 					if (node.Slot?.Kind == SlotKind.Getter || node.Slot?.Kind == SlotKind.Setter)
 					{
@@ -2667,7 +2668,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				if (isSingleLine)
 					Space();
 				// output get/set in their original order
-				foreach (AstNode node in propertyDeclaration.Children)
+				for (AstNode? node = propertyDeclaration.FirstChild; node != null; node = node.NextSibling)
 				{
 					if (node.Slot?.Kind == SlotKind.Getter || node.Slot?.Kind == SlotKind.Setter)
 					{
@@ -2735,7 +2736,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			foreach (var trivia in syntaxTree.LeadingTrivia)
 				trivia.AcceptVisitor(this);
 			// don't do node tracking as we visit all children directly
-			foreach (AstNode node in syntaxTree.Children)
+			for (AstNode? node = syntaxTree.FirstChild; node != null; node = node.NextSibling)
 			{
 				node.AcceptVisitor(this);
 				MaybeNewLinesAfterUsings(node);

ICSharpCode.Decompiler/CSharp/OutputVisitor/InsertParenthesesVisitor.cs

@@ -420,7 +420,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 				if (node is CastExpression cast)
 					return InterpolationNeedsParenthesis(cast.Expression);
 
-				foreach (var child in node.Children)
+				for (AstNode? child = node.FirstChild; child != null; child = child.NextSibling)
 				{
 					if (InterpolationNeedsParenthesis(child))
 						return true;

ICSharpCode.Decompiler/CSharp/Syntax/AstNode.cs

@@ -297,21 +297,20 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		}
 
 		/// <summary>
-		/// Gets the first child with the specified role.
-		/// Returns null if the child is not found.
+		/// Gets the first child with the specified role, or null if this node has no such child (the
+		/// slot is empty or the node declares no slot of that kind).
 		/// </summary>
-		public T GetChildByRole<T>(SlotKind kind) where T : AstNode?
+		public T? GetChildByRole<T>(SlotKind kind) where T : AstNode
 		{
 			int count = GetChildCount();
 			for (int i = 0; i < count; i++)
 			{
 				if (GetChildSlotInfo(i).Kind == kind)
 				{
-					AstNode? c = GetChild(i);
-					return c != null ? (T)c : default!;
+					return GetChild(i) as T;
 				}
 			}
-			return default!;
+			return null;
 		}
 
 		public T? GetParent<T>() where T : AstNode
@@ -731,11 +730,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				prev = prev.GetPrevNode();
 			return prev;
 		}
-		// Comments and directives are trivia, not sibling nodes, so the previous sibling is always a C# node.
-		public AstNode? GetCSharpNodeBefore(AstNode node)
-		{
-			return node.PrevSibling;
-		}
 
 		/// <summary>
 		/// Gets the next sibling which fullfills a given predicate
@@ -825,11 +819,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return this.StartLocation <= location && location <= this.EndLocation;
 		}
 
-		public override void AddAnnotation(object annotation)
-		{
-			base.AddAnnotation(annotation);
-		}
-
 		internal string DebugToString()
 		{
 			string text = ToString();

ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/EntityDeclaration.cs

@@ -48,12 +48,12 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		}
 
 		public virtual Identifier NameToken {
-			get { return GetChildByRole<Identifier>(SlotKind.Identifier); }
+			get { return GetChildByRole<Identifier>(SlotKind.Identifier)!; }
 			set { SetChildByRole(SlotKind.Identifier, value); }
 		}
 
 		public virtual AstType ReturnType {
-			get { return GetChildByRole<AstType>(SlotKind.Type); }
+			get { return GetChildByRole<AstType>(SlotKind.Type)!; }
 			set { SetChildByRole(SlotKind.Type, value); }
 		}
 

ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/MethodDeclaration.cs

@@ -72,7 +72,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		public bool IsExtensionMethod {
 			get {
-				ParameterDeclaration pd = (ParameterDeclaration)GetChildByRole<ParameterDeclaration>(SlotKind.Parameter);
+				ParameterDeclaration? pd = GetChildByRole<ParameterDeclaration>(SlotKind.Parameter);
 				return pd != null && pd.HasThisModifier;
 			}
 		}

ICSharpCode.Decompiler/CSharp/Transforms/FixNameCollisions.cs

@@ -69,7 +69,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					ISymbol? symbol = node.GetSymbol();
 					if (symbol != null && renamedSymbols.TryGetValue(symbol, out string? newName))
 					{
-						node.GetChildByRole<Identifier>(SlotKind.Identifier).Name = newName;
+						// An IdentifierExpression / MemberReferenceExpression always carries its name identifier.
+						node.GetChildByRole<Identifier>(SlotKind.Identifier)!.Name = newName;
 					}
 				}
 			}

ICSharpCode.Decompiler/CSharp/Transforms/NormalizeBlockStatements.cs

@@ -126,7 +126,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			{
 				var b = new BlockStatement();
 				statement.ReplaceWith(b);
-				if (statement is EmptyStatement && !statement.Children.Any())
+				if (statement is EmptyStatement && !statement.HasChildren)
 				{
 					b.CopyAnnotationsFrom(statement);
 				}