Implemented support for string concatenation compound assignments involving ReadOnlySpan<char>.

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

@@ -4719,12 +4719,14 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		}
 #endif
 
-		private static void StringPropertyCompoundAssign()
+		private static void StringPropertyCompoundAssign(char c)
 		{
 			StaticStringProperty += "a";
 			StaticStringProperty += 1;
+			StaticStringProperty += c;
 			new CustomClass().StringProp += "a";
 			new CustomClass().StringProp += 1;
+			new CustomClass().StringProp += c;
 		}
 
 		public uint PreIncrementIndexer(string name)

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -247,15 +247,11 @@ namespace ICSharpCode.Decompiler.CSharp
 			return result;
 		}
 
-		private bool IsSpanBasedStringConcat(CallInstruction call, out List<(ILInstruction, KnownTypeCode)> operands)
+		static bool IsSpanBasedStringConcat(CallInstruction call, out List<(ILInstruction, KnownTypeCode)> operands)
 		{
 			operands = null;
 
-			if (call.Method is not { Name: "Concat", IsStatic: true })
+			if (!IsSpanBasedStringConcat(call.Method))
-			{
-				return false;
-			}
-			if (!call.Method.DeclaringType.IsKnownType(KnownTypeCode.String))
 			{
 				return false;
 			}
@@ -283,7 +279,29 @@ namespace ICSharpCode.Decompiler.CSharp
 			return call.Arguments.Count >= 2 && firstStringArgumentIndex <= 1;
 		}
 
-		private bool IsStringToReadOnlySpanCharImplicitConversion(IMethod method)
+		internal static bool IsSpanBasedStringConcat(IMethod method)
+		{
+			if (method is not { Name: "Concat", IsStatic: true })
+			{
+				return false;
+			}
+			if (!method.DeclaringType.IsKnownType(KnownTypeCode.String))
+			{
+				return false;
+			}
+
+			foreach (var p in method.Parameters)
+			{
+				if (!p.Type.IsKnownType(KnownTypeCode.ReadOnlySpanOfT))
+					return false;
+				if (!p.Type.TypeArguments[0].IsKnownType(KnownTypeCode.Char))
+					return false;
+			}
+
+			return true;
+		}
+
+		internal static bool IsStringToReadOnlySpanCharImplicitConversion(IMethod method)
 		{
 			return method.IsOperator
 				&& method.Name == "op_Implicit"

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -1799,7 +1799,17 @@ namespace ICSharpCode.Decompiler.CSharp
 
 		protected internal override TranslatedExpression VisitUserDefinedCompoundAssign(UserDefinedCompoundAssign inst, TranslationContext context)
 		{
-			IType loadType = inst.Method.Parameters[0].Type;
+			IType loadType;
+			bool isSpanBasedStringConcat = CallBuilder.IsSpanBasedStringConcat(inst.Method);
+			if (isSpanBasedStringConcat)
+			{
+				loadType = typeSystem.FindType(KnownTypeCode.String);
+			}
+			else
+			{
+				loadType = inst.Method.Parameters[0].Type;
+			}
+
 			ExpressionWithResolveResult target;
 			if (inst.TargetKind == CompoundTargetKind.Address)
 			{
@@ -1821,11 +1831,24 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (UserDefinedCompoundAssign.IsStringConcat(inst.Method))
 			{
 				Debug.Assert(inst.Method.Parameters.Count == 2);
-				var value = Translate(inst.Value).ConvertTo(inst.Method.Parameters[1].Type, this, allowImplicitConversion: true);
+				Expression valueExpr;
-				var valueExpr = ReplaceMethodCallsWithOperators.RemoveRedundantToStringInConcat(value, inst.Method, isLastArgument: true).Detach();
+				ResolveResult valueResolveResult;
+				if (isSpanBasedStringConcat && inst.Value is NewObj { Arguments: [AddressOf addressOf] })
+				{
+					IType charType = typeSystem.FindType(KnownTypeCode.Char);
+					var value = Translate(addressOf.Value, charType).ConvertTo(charType, this);
+					valueExpr = value.Expression;
+					valueResolveResult = value.ResolveResult;
+				}
+				else
+				{
+					var value = Translate(inst.Value).ConvertTo(inst.Method.Parameters[1].Type, this, allowImplicitConversion: true);
+					valueExpr = ReplaceMethodCallsWithOperators.RemoveRedundantToStringInConcat(value, inst.Method, isLastArgument: true).Detach();
+					valueResolveResult = value.ResolveResult;
+				}
 				return new AssignmentExpression(target, AssignmentOperatorType.Add, valueExpr)
 					.WithILInstruction(inst)
-					.WithRR(new OperatorResolveResult(inst.Method.ReturnType, ExpressionType.AddAssign, inst.Method, inst.IsLifted, new[] { target.ResolveResult, value.ResolveResult }));
+					.WithRR(new OperatorResolveResult(inst.Method.ReturnType, ExpressionType.AddAssign, inst.Method, inst.IsLifted, new[] { target.ResolveResult, valueResolveResult }));
 			}
 			else if (inst.Method.Parameters.Count == 2)
 			{

ICSharpCode.Decompiler/IL/Transforms/TransformAssignment.cs

@@ -21,6 +21,7 @@ using System.Diagnostics;
 using System.Linq;
 using System.Linq.Expressions;
 
+using ICSharpCode.Decompiler.CSharp;
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.Decompiler.Util;
 
@@ -452,7 +453,17 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					return false; // for now we only support binary compound assignments
 				if (!targetType.IsKnownType(KnownTypeCode.String))
 					return false;
-				if (!IsMatchingCompoundLoad(concatCall.Arguments[0], compoundStore, out var target, out var targetKind, out var finalizeMatch, forbiddenVariable: storeInSetter?.Variable))
+				var arg = concatCall.Arguments[0];
+				if (arg is Call call && CallBuilder.IsStringToReadOnlySpanCharImplicitConversion(call.Method))
+				{
+					arg = call.Arguments[0];
+					if (!(concatCall.Arguments[1] is NewObj { Arguments: [AddressOf addressOf] } newObj) || !ILInlining.IsReadOnlySpanCharCtor(newObj.Method))
+					{
+						return false;
+					}
+				}
+
+				if (!IsMatchingCompoundLoad(arg, compoundStore, out var target, out var targetKind, out var finalizeMatch, forbiddenVariable: storeInSetter?.Variable))
 					return false;
 				context.Step($"Compound assignment (string concatenation)", compoundStore);
 				finalizeMatch?.Invoke(context);