Address review: keep while-loop instead of headless for for ref locals

Per @siegfriedpammer: rather than rescue the hoisted for-initializer in
DeclareVariables, don't form the for-loop at all. TransformFor now bails when the
loop variable is a by-ref-like local used after the loop, leaving the while-loop
(matching source); the ref decl keeps its initializer, no CS8174. Reverts the
DeclareVariables change; test now expects the while form.

Assisted-by: Copilot:claude-opus-4.8:GitHub Copilot CLI

ICSharpCode.Decompiler.Tests/TestCases/Pretty/RefLocalsAndReturns.cs

@@ -168,17 +168,18 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static Issue3827Node RefLocalUsedAfterForLoop(Issue3827Node[] buckets, int hash, Issue3827Node newNode)
 		{
-			// The ref local is used after the loop, so its declaration is hoisted in front of the
-			// for-statement; the for-initializer ref-assignment must stay on the declaration, because
-			// a ref local cannot be declared without an initializer.
+			// The ref local is used after the loop, so its declaration is hoisted in front of the loop.
+			// The loop is kept as a while-loop (rather than converted to a headless for-loop) so the
+			// ref-assignment stays on the declaration -- a ref local cannot be declared without an initializer.
 			ref Issue3827Node reference = ref buckets[hash & 3];
-			for (; reference != null; reference = ref reference.next)
+			while (reference != null)
 			{
 				if (reference.value == hash)
 				{
 					reference = newNode;
 					break;
 				}
+				reference = ref reference.next;
 			}
 			if (reference == null)
 			{

ICSharpCode.Decompiler/CSharp/Transforms/DeclareVariables.cs

@@ -588,39 +588,6 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				&& identExpr.TypeArguments.Count == 0;
 		}
 
-		/// <summary>
-		/// Matches the case where the variable's only initialization is the first initializer of a
-		/// for-statement (`for (v = expr; ...)`) and the declaration has to be placed in front of the
-		/// for-statement (e.g. because the variable is used after the loop). Used to keep a ref local's
-		/// initializer on its declaration (a ref local may not be declared without one).
-		/// </summary>
-		bool IsMatchingForInitializerAssignment(VariableToDeclare v, [NotNullWhen(true)] out ForStatement? forStatement,
-			[NotNullWhen(true)] out Statement? initializerStatement, [NotNullWhen(true)] out AssignmentExpression? assignment)
-		{
-			forStatement = null;
-			initializerStatement = null;
-			assignment = null;
-
-			if (v.InsertionPoint.nextNode is not ForStatement { Parent: BlockStatement } fs)
-				return false;
-
-			if (fs.Initializers.FirstOrDefault() is not ExpressionStatement { Expression: AssignmentExpression a } firstInit)
-				return false;
-
-			if (a.Operator != AssignmentOperatorType.Assign
-				|| a.Left is not IdentifierExpression identExpr
-				|| identExpr.Identifier != v.Name
-				|| identExpr.TypeArguments.Count != 0)
-			{
-				return false;
-			}
-
-			forStatement = fs;
-			initializerStatement = firstInit;
-			assignment = a;
-			return true;
-		}
-
 		bool CombineDeclarationAndInitializer(VariableToDeclare v, TransformContext context)
 		{
 			if (v.Type.IsByRefLike)
@@ -674,32 +641,6 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 						return vds;
 					}, "Combine variable declaration with initializer"));
 				}
-				else if (v.Type.IsByRefLike && IsMatchingForInitializerAssignment(v, out var forStatement, out var forInitializerStatement, out var forInitAssignment))
-				{
-					// A by-ref-like local that is used after the loop has its declaration hoisted in
-					// front of the for-statement, but its only initialization is the for-initializer
-					// ref-assignment. A ref local must be declared with an initializer (CS8174), so move
-					// the ref-assignment's value up into the declaration and drop the for-initializer.
-					AstType type = context.TypeSystemAstBuilder.ConvertType(v.Type);
-					if (v.ILVariable.IsRefReadOnly && type is ComposedType { HasRefSpecifier: true } composedType)
-					{
-						composedType.HasReadOnlySpecifier = true;
-					}
-
-					var vds = new VariableDeclarationStatement(type, v.Name, forInitAssignment.Right.Detach());
-					var init = vds.Variables.Single();
-					init.AddAnnotation(forInitAssignment.Left.GetResolveResult());
-					foreach (object annotation in forInitAssignment.Left.Annotations.Concat(forInitAssignment.Annotations))
-					{
-						if (annotation is not ResolveResult)
-						{
-							init.AddAnnotation(annotation);
-						}
-					}
-
-					forInitializerStatement.Remove();
-					forStatement.Parent!.InsertChildBefore(forStatement, vds, Slots.Statement);
-				}
 				else if (CanBeDeclaredAsOutVariable(v, out var dirExpr))
 				{
 					// 'T v; SomeCall(out v);' can be combined to 'SomeCall(out T v);'

ICSharpCode.Decompiler/CSharp/Transforms/PatternStatementTransform.cs

@@ -203,6 +203,12 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (variable != m3.Get<IdentifierExpression>("ident").Single().GetILVariable())
 				return null;
 			WhileStatement loop = (WhileStatement)next;
+			// Cannot convert to for loop, if the iteration variable is a ref local used after the loop: its
+			// declaration is hoisted in front, leaving a headless `for (; cond; v = ref ...)` whose only
+			// initialization is the for-initializer ref-assignment -- which can't be split from a ref local
+			// (CS8174). Keeping it a while-loop matches the source and keeps the initializer on the decl.
+			if (variable != null && variable.Type.IsByRefLike && IsVariableUsedAfter(loop, variable))
+				return null;
 			// Cannot convert to for loop, if any variable that is used in the "iterator" part of the pattern,
 			// will be declared in the body of the while-loop.
 			var iteratorStatement = m3.Get<Statement>("iterator").Single();
@@ -247,6 +253,16 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			return false;
 		}
 
+		bool IsVariableUsedAfter(Statement loop, IL.ILVariable variable)
+		{
+			for (AstNode? sibling = loop.NextSibling; sibling != null; sibling = sibling.NextSibling)
+			{
+				if (sibling.DescendantsAndSelf.OfType<IdentifierExpression>().Any(ie => ie.GetILVariable() == variable))
+					return true;
+			}
+			return false;
+		}
+
 		bool IteratorVariablesDeclaredInsideLoopBody(Statement iteratorStatement)
 		{
 			foreach (var id in iteratorStatement.DescendantsAndSelf.OfType<IdentifierExpression>())