Add IMethod.ThisIsRefReadOnly

ICSharpCode.Decompiler/IL/ILReader.cs

@@ -191,7 +191,7 @@ namespace ICSharpCode.Decompiler.IL
 					declaringType = new ParameterizedType(declaringType, declaringType.TypeParameters);
 				}
 				ILVariable ilVar = CreateILVariable(-1, declaringType, "this");
-				ilVar.IsRefReadOnly = declaringType.GetDefinition()?.IsReadOnly == true;
+				ilVar.IsRefReadOnly = method.ThisIsRefReadOnly;
 				parameterVariables[paramIndex++] = ilVar;
 			}
 			while (paramIndex < parameterVariables.Length) {

ICSharpCode.Decompiler/IL/Transforms/ILInlining.cs

@@ -272,8 +272,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			var type = method.DeclaringType;
 			if (type.IsReferenceType == true)
 				return false; // reference types are never implicitly copied
-			if (type.GetDefinition()?.IsReadOnly == true)
-				return false; // readonly structs are never implicitly copied
+			if (method.ThisIsRefReadOnly)
+				return false; // no copies for calls on readonly structs
 			return true;
 		}
 

ICSharpCode.Decompiler/TypeSystem/IMethod.cs

@@ -40,6 +40,11 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// </summary>
 		bool ReturnTypeIsRefReadOnly { get; }
 
+		/// <summary>
+		/// Gets whether the method is readonly (C# 8): accepts the 'this' reference as ref readonly
+		/// </summary>
+		bool ThisIsRefReadOnly { get; }
+
 		/// <summary>
 		/// Gets the type parameters of this method; or an empty list if the method is not generic.
 		/// </summary>

ICSharpCode.Decompiler/TypeSystem/Implementation/FakeMember.cs

@@ -133,6 +133,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		
 		IEnumerable<IAttribute> IMethod.GetReturnTypeAttributes() => EmptyList<IAttribute>.Instance;
 		bool IMethod.ReturnTypeIsRefReadOnly => false;
+		bool IMethod.ThisIsRefReadOnly => false;
 
 		public IReadOnlyList<ITypeParameter> TypeParameters { get; set; } = EmptyList<ITypeParameter>.Instance;
 

ICSharpCode.Decompiler/TypeSystem/Implementation/LocalFunctionMethod.cs

@@ -117,6 +117,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		IEnumerable<IAttribute> IEntity.GetAttributes() => baseMethod.GetAttributes();
 		IEnumerable<IAttribute> IMethod.GetReturnTypeAttributes() => baseMethod.GetReturnTypeAttributes();
 		bool IMethod.ReturnTypeIsRefReadOnly => baseMethod.ReturnTypeIsRefReadOnly;
+		bool IMethod.ThisIsRefReadOnly => baseMethod.ThisIsRefReadOnly;
 		/// <summary>
 		/// We consider local functions as always static, because they do not have a "this parameter".
 		/// Even local functions in instance methods capture this.

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataMethod.cs

@@ -426,6 +426,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		}
 		#endregion
 
+		public bool ThisIsRefReadOnly => DeclaringTypeDefinition?.IsReadOnly ?? false;
+
 		public Accessibility Accessibility => GetAccessibility(attributes);
 		
 		internal static Accessibility GetAccessibility(MethodAttributes attr)

ICSharpCode.Decompiler/TypeSystem/Implementation/SpecializedMethod.cs

@@ -97,6 +97,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		public IEnumerable<IAttribute> GetReturnTypeAttributes() => methodDefinition.GetReturnTypeAttributes();
 		public bool ReturnTypeIsRefReadOnly => methodDefinition.ReturnTypeIsRefReadOnly;
 
+		bool IMethod.ThisIsRefReadOnly => methodDefinition.ThisIsRefReadOnly;
+
 		public IReadOnlyList<ITypeParameter> TypeParameters {
 			get {
 				return specializedTypeParameters ?? methodDefinition.TypeParameters;

ICSharpCode.Decompiler/TypeSystem/VarArgInstanceMethod.cs

@@ -114,6 +114,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		IEnumerable<IAttribute> IEntity.GetAttributes() => baseMethod.GetAttributes();
 		IEnumerable<IAttribute> IMethod.GetReturnTypeAttributes() => baseMethod.GetReturnTypeAttributes();
 		bool IMethod.ReturnTypeIsRefReadOnly => baseMethod.ReturnTypeIsRefReadOnly;
+		bool IMethod.ThisIsRefReadOnly => baseMethod.ThisIsRefReadOnly;
 
 		public IReadOnlyList<ITypeParameter> TypeParameters {
 			get { return baseMethod.TypeParameters; }