ILReader: handle implicit O->Ref conversions

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -1236,8 +1236,9 @@ namespace ICSharpCode.Decompiler.CSharp
 						var pointerType = new PointerType(((ByReferenceType)inputType).ElementType);
 						return arg.ConvertTo(pointerType, this).WithILInstruction(inst);
 					} else {
-						Debug.Fail("ConversionKind.StopGCTracking should only be used with managed references");
-						goto default;
+						// ConversionKind.StopGCTracking should only be used with managed references,
+						// but it's possible that we're supposed to stop tracking something we just started to track.
+						return arg;
 					}
 				case ConversionKind.SignExtend:
 					// We just need to ensure the input type before the conversion is signed.

ICSharpCode.Decompiler/IL/ILReader.cs

@@ -129,7 +129,7 @@ namespace ICSharpCode.Decompiler.IL
 
 		ILVariable CreateILVariable(Cil.VariableDefinition v)
 		{
-			VariableKind kind = v.IsPinned ? VariableKind.PinnedLocal : VariableKind.Local;
+			VariableKind kind = IsPinned(v.VariableType) ? VariableKind.PinnedLocal : VariableKind.Local;
 			ILVariable ilVar = new ILVariable(kind, typeSystem.Resolve(v.VariableType), v.Index);
 			if (!UseDebugSymbols || debugInfo == null || !debugInfo.TryGetName(v, out string name)) {
 				ilVar.Name = "V_" + v.Index;
@@ -143,6 +143,14 @@ namespace ICSharpCode.Decompiler.IL
 			return ilVar;
 		}
 
+		bool IsPinned(TypeReference type)
+		{
+			while (type is OptionalModifierType || type is RequiredModifierType) {
+				type = ((TypeSpecification)type).ElementType;
+			}
+			return type is PinnedType;
+		}
+
 		ILVariable CreateILVariable(ParameterDefinition p)
 		{
 			IType parameterType;
@@ -1028,8 +1036,8 @@ namespace ICSharpCode.Decompiler.IL
 					// to a method expecting a native pointer.
 					inst = new Conv(inst, PrimitiveType.I, false, Sign.None);
 				} else if (expectedType == StackType.Ref) {
-					// implicitly start GC tracking
-					if (!inst.ResultType.IsIntegerType()) {
+					// implicitly start GC tracking / object to interior
+					if (!inst.ResultType.IsIntegerType() && inst.ResultType != StackType.O) {
 						// We also handle the invalid to-ref cases here because the else case
 						// below uses expectedType.ToKnownTypeCode(), which doesn't work for Ref.
 						Warn($"Expected {expectedType}, but got {inst.ResultType}");
@@ -1332,6 +1340,12 @@ namespace ICSharpCode.Decompiler.IL
 						negate ? ComparisonKind.Equality : ComparisonKind.Inequality,
 						Sign.None, condition, new LdcI8(0));
 					break;
+				case StackType.Ref:
+					// introduce explicit comparison with null ref
+					condition = new Comp(
+						negate ? ComparisonKind.Equality : ComparisonKind.Inequality,
+						Sign.None, new Conv(condition, PrimitiveType.I, false, Sign.None), new Conv(new LdcI4(0), PrimitiveType.I, false, Sign.None));
+					break;
 				default:
 					if (negate) {
 						condition = Comp.LogicNot(condition);

ICSharpCode.Decompiler/IL/Instructions/Conv.cs

@@ -79,7 +79,15 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Used to convert unmanaged pointers to managed references.
 		/// </summary>
-		StartGCTracking
+		StartGCTracking,
+		/// <summary>
+		/// Converts from an object reference (O) to an interior pointer (Ref) pointing to the start of the object.
+		/// </summary>
+		/// <remarks>
+		/// C++/CLI emits "ldarg.1; stloc.0" where arg1 is a string and loc0 is "ref byte" (e.g. as part of the PtrToStringChars codegen);
+		/// we represent this type conversion explicitly in the ILAst.
+		/// </remarks>
+		ObjectInterior
 	}
 	
 	partial class Conv : UnaryInstruction, ILiftableInstruction
@@ -250,11 +258,15 @@ namespace ICSharpCode.Decompiler.IL
 							return ConversionKind.Invalid;
 					}
 				case PrimitiveType.Ref:
+					// There's no "conv.ref" in IL, but IL allows these conversions implicitly,
+					// whereas we represent them explicitly in the ILAst.
 					switch (inputType) {
 						case StackType.I4:
 						case StackType.I:
 						case StackType.I8:
 							return ConversionKind.StartGCTracking;
+						case StackType.O:
+							return ConversionKind.ObjectInterior;
 						default:
 							return ConversionKind.Invalid;
 					}

ICSharpCode.Decompiler/TypeSystem/DecompilerTypeSystem.cs

@@ -132,6 +132,11 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		{
 			if (typeReference == null)
 				return SpecialType.UnknownType;
+			// We need to skip SentinelType and PinnedType.
+			// But PinnedType can be nested within modopt, so we'll also skip those.
+			while (typeReference is OptionalModifierType || typeReference is RequiredModifierType) {
+				typeReference = ((TypeSpecification)typeReference).ElementType;
+			}
 			if (typeReference is SentinelType || typeReference is PinnedType) {
 				typeReference = ((TypeSpecification)typeReference).ElementType;
 			}