Remove useless stack variables.

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -1068,8 +1068,8 @@ namespace ICSharpCode.Decompiler.CSharp
 				if (!parameter.Type.ContainsAnonymousType())
 					arguments[i] = arguments[i].ConvertTo(parameter.Type, this);
 				
-				if (parameter.IsOut && arguments[i].Expression is DirectionExpression) {
+				if (parameter.IsOut && arguments[i].Expression is DirectionExpression dirExpr) {
-					((DirectionExpression)arguments[i].Expression).FieldDirection = FieldDirection.Out;
+					dirExpr.FieldDirection = FieldDirection.Out;
 				}
 			}
 			

ICSharpCode.Decompiler/CSharp/StatementBuilder.cs

@@ -25,6 +25,7 @@ using ICSharpCode.Decompiler.Semantics;
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.Decompiler.Util;
 using ICSharpCode.Decompiler.IL.Patterns;
+using System;
 
 namespace ICSharpCode.Decompiler.CSharp
 {
@@ -60,7 +61,11 @@ namespace ICSharpCode.Decompiler.CSharp
 		
 		protected internal override Statement VisitNop(Nop inst)
 		{
-			return new EmptyStatement();
+			var stmt = new EmptyStatement();
+			if (inst.Comment != null) {
+				stmt.AddChild(new Comment(inst.Comment), Roles.Comment);
+			}
+			return stmt;
 		}
 		
 		protected internal override Statement VisitIfInstruction(IfInstruction inst)

ICSharpCode.Decompiler/CSharp/Transforms/DeclareVariables.cs

@@ -108,6 +108,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		{
 			try {
 				this.context = context;
+				EnsureExpressionStatementsAreValid(rootNode);
 				FindInsertionPoints(rootNode, 0);
 				ResolveCollisions();
 				InsertVariableDeclarations();
@@ -117,6 +118,45 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 		}
 
+		#region EnsureExpressionStatementsAreValid
+		void EnsureExpressionStatementsAreValid(AstNode rootNode)
+		{
+			foreach (var stmt in rootNode.DescendantsAndSelf.OfType<ExpressionStatement>()) {
+				if (!IsValidInStatementExpression(stmt.Expression)) {
+					// assign result to dummy variable
+					var v = new ILVariable(VariableKind.StackSlot, stmt.Expression.GetResolveResult().Type, 0);
+					v.Name = "_";
+					stmt.Expression = new AssignmentExpression(
+						new IdentifierExpression(v.Name).WithRR(new ILVariableResolveResult(v, v.Type)),
+						stmt.Expression.Detach());
+				}
+			}
+		}
+
+		private static bool IsValidInStatementExpression(Expression expr)
+		{
+			switch (expr) {
+				case InvocationExpression ie:
+				case ObjectCreateExpression oce:
+				case AssignmentExpression ae:
+					return true;
+				case UnaryOperatorExpression uoe:
+					switch (uoe.Operator) {
+						case UnaryOperatorType.PostIncrement:
+						case UnaryOperatorType.PostDecrement:
+						case UnaryOperatorType.Increment:
+						case UnaryOperatorType.Decrement:
+						case UnaryOperatorType.Await:
+							return true;
+						default:
+							return false;
+					}
+				default:
+					return false;
+			}
+		}
+		#endregion
+
 		#region FindInsertionPoints
 		List<(InsertionPoint InsertionPoint, BlockContainer Loop)> loopTracking = new List<(InsertionPoint, BlockContainer)>();
 

ICSharpCode.Decompiler/IL/Transforms/CopyPropagation.cs

@@ -47,7 +47,16 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				ILVariable v;
 				ILInstruction copiedExpr;
 				if (block.Instructions[i].MatchStLoc(out v, out copiedExpr)) {
-					if (v.IsSingleDefinition && CanPerformCopyPropagation(v, copiedExpr)) {
+					if (v.IsSingleDefinition && v.LoadCount == 0 && v.Kind == VariableKind.StackSlot) {
+						// dead store to stack
+						if (copiedExpr.Flags == InstructionFlags.None) {
+							// no-op -> delete
+							block.Instructions.RemoveAt(i--);
+						} else {
+							// evaluate the value for its side-effects
+							block.Instructions[i] = copiedExpr;
+						}
+					} else if (v.IsSingleDefinition && CanPerformCopyPropagation(v, copiedExpr)) {
 						DoPropagate(v, copiedExpr, block, ref i, context);
 					}
 				}

ICSharpCode.Decompiler/IL/Transforms/ILInlining.cs

@@ -336,7 +336,6 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		static bool IsSafeForInlineOver(ILInstruction expr, ILInstruction expressionBeingMoved)
 		{
 			return SemanticHelper.MayReorder(expressionBeingMoved, expr);
-
 		}
 	}
 }