Fix #3827: keep the while-loop for a ref local used after the loop (avoid an uninitialized hoisted ref decl)

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

@@ -45,6 +45,13 @@ public static void Test()
 			}
 		}
 
+		public class Issue3827Node
+		{
+			public Issue3827Node next;
+
+			public int value;
+		}
+
 		public delegate ref T RefFunc<T>();
 		public delegate ref readonly T ReadOnlyRefFunc<T>();
 		public delegate ref TReturn RefFunc<T1, TReturn>(T1 param1);
@@ -159,6 +166,28 @@ public void RefReadonlyCallVirt(RefLocalsAndReturns provider)
 
 		private static int DefaultInt = 0;
 
+		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 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];
+			while (reference != null)
+			{
+				if (reference.value == hash)
+				{
+					reference = newNode;
+					break;
+				}
+				reference = ref reference.next;
+			}
+			if (reference == null)
+			{
+				reference = newNode;
+			}
+			return reference;
+		}
+
 		public static ref T GetRef<T>()
 		{
 			throw new NotImplementedException();

ICSharpCode.Decompiler/CSharp/Transforms/PatternStatementTransform.cs

@@ -202,6 +202,12 @@ public override AstNode VisitTryCatchStatement(TryCatchStatement tryCatchStateme
 			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();
@@ -244,6 +250,16 @@ bool ForStatementUsesVariable(ForStatement statement, IL.ILVariable? variable)
 			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>())

ICSharpCode.Decompiler/IL/Transforms/HighLevelLoopTransform.cs

@@ -395,6 +395,8 @@ bool MatchForLoop(BlockContainer loop, IfInstruction whileCondition, Block while
 				// - increment block
 				if (incrementBlock.Instructions.Count <= 1 || loop.Blocks.Count < 3)
 					return false;
+				if (StoresRefLocalUsedAfterLoop(loop, incrementBlock))
+					return false;
 				context.Step("Transform to for loop: " + loop.EntryPoint.Label, loop);
 				// move the block to the end of the loop:
 				loop.Blocks.MoveElementToEnd(incrementBlock);
@@ -467,6 +469,27 @@ bool MatchForLoop(BlockContainer loop, IfInstruction whileCondition, Block while
 			return true;
 		}
 
+		static bool StoresRefLocalUsedAfterLoop(BlockContainer loop, Block incrementBlock)
+		{
+			foreach (var store in incrementBlock.Instructions.SkipLast(1).SelectMany(inst => inst.Descendants).OfType<StLoc>())
+			{
+				var variable = store.Variable;
+				if (variable.Type.IsByRefLike && IsVariableUsedAfterLoop(loop, variable))
+					return true;
+			}
+			return false;
+		}
+
+		static bool IsVariableUsedAfterLoop(BlockContainer loop, ILVariable variable)
+		{
+			foreach (var use in variable.LoadInstructions.Concat<ILInstruction>(variable.AddressInstructions).Concat(variable.StoreInstructions.Cast<ILInstruction>()))
+			{
+				if (loop.GetCommonParent(use) is Block { Kind: BlockKind.ControlFlow } && loop.IsBefore(use))
+					return true;
+			}
+			return false;
+		}
+
 		bool IsAssignment(ILInstruction inst)
 		{
 			if (inst is StLoc)