Bridge only the debug-step marker in NodeLookup

NodeLookup.AddNode indexed every annotation of every rendered node by reference
identity, but the debug-step highlighter only ever looks up the DebugStepMarker;
the rest were dead keys, and a shared annotation (ResolveResult and friends,
copied across nodes) would resolve to whichever node rendered last. Make
DebugStepMarker public and bridge only it -- behaviour-preserving for resolution
while dropping the per-annotation dictionary churn on every rendered node.

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

ICSharpCode.Decompiler/CSharp/Transforms/TransformContext.cs

@@ -144,9 +144,15 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			else
 				step.ModifiedNodeCandidates.Add(candidate);
 		}
+	}
 
-		sealed class DebugStepMarker
-		{
-		}
+	/// <summary>
+	/// Annotation attached to a step's modified node so the debug-step highlighter can still locate
+	/// that node's rendered range after a later transform copies its annotations onto a replacement
+	/// (via CopyAnnotationsFrom). This is the only annotation <c>NodeLookup</c> bridges to a text
+	/// range; indexing every annotation would add dead keys and let shared ones collide by identity.
+	/// </summary>
+	public sealed class DebugStepMarker
+	{
 	}
 }

ILSpy.Tests/Views/DebugStepsTests.cs

@@ -32,6 +32,7 @@ using AwesomeAssertions;
 
 using ICSharpCode.Decompiler.CSharp;
 using ICSharpCode.Decompiler.CSharp.Syntax;
+using ICSharpCode.Decompiler.CSharp.Transforms;
 
 using ICSharpCode.ILSpy;
 using ICSharpCode.ILSpy.AppEnv;
@@ -275,7 +276,7 @@ public class DebugStepsTests
 	[AvaloniaTest]
 	public Task NodeLookup_Resolves_Copied_Ast_Annotations()
 	{
-		var marker = new object();
+		var marker = new DebugStepMarker();
 		var original = new IdentifierExpression("old");
 		original.AddAnnotation(marker);
 		var replacement = new IdentifierExpression("new").CopyAnnotationsFrom(original);
@@ -287,6 +288,15 @@ public class DebugStepsTests
 			"C# debug-step markers copied by AST replacements must still resolve to emitted text");
 		range.Start.Should().Be(12);
 		range.Length.Should().Be(3);
+
+		// A non-marker annotation is not bridged: only the debug-step marker is queried by the
+		// resolver, and indexing arbitrary shared annotations would collide by reference identity.
+		var otherAnnotation = new object();
+		var otherNode = new IdentifierExpression("x");
+		otherNode.AddAnnotation(otherAnnotation);
+		lookup.AddNode(otherNode, 30, 1);
+		lookup.TryGetRange(otherAnnotation, out _).Should().BeFalse(
+			"only DebugStepMarker annotations are bridged to a text range");
 		return Task.CompletedTask;
 	}
 

ILSpy/TextView/NodeLookup.cs

@@ -19,6 +19,7 @@
 using System.Collections.Generic;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
+using ICSharpCode.Decompiler.CSharp.Transforms;
 
 namespace ICSharpCode.ILSpy.TextView
 {
@@ -34,15 +35,20 @@ namespace ICSharpCode.ILSpy.TextView
 
 		public void AddNode(object node, int start, int length)
 		{
-			if (length > 0)
+			if (length <= 0)
+				return;
+			var range = new TextRange(start, length);
+			nodes[node] = range;
+			// Bridge only the debug-step marker: it is the sole annotation the resolver ever looks up
+			// (it rides CopyAnnotationsFrom so a replaced node's step still resolves to the emitted
+			// text). Indexing every annotation would add dead keys never queried, and would let a
+			// shared annotation resolve to whichever node rendered last.
+			if (node is IAnnotatable annotatable)
 			{
-				nodes[node] = new TextRange(start, length);
-				if (node is IAnnotatable annotatable)
+				foreach (var annotation in annotatable.Annotations)
 				{
-					foreach (var annotation in annotatable.Annotations)
-					{
-						nodes[annotation] = new TextRange(start, length);
-					}
+					if (annotation is DebugStepMarker)
+						nodes[annotation] = range;
 				}
 			}
 		}