Merge pull request #42961 from jnm2/simplify_interpolation_ref_structs

Do not offer to simplify ToString on ref structs

src/EditorFeatures/CSharpTest/SimplifyInterpolation/SimplifyInterpolationTests.cs

@@ -894,6 +894,45 @@ struct TypeNotImplementingIFormattable
 struct TypeNotImplementingIFormattable
 {
     public string ToString(string format) => ""A"";
+}");
+        }
+
+        [Fact, WorkItem(42936, "https://github.com/dotnet/roslyn/issues/42936")]
+        public async Task ToStringSimplificationIsNotOfferedOnRefStruct()
+        {
+            await TestMissingInRegularAndScriptAsync(
+@"class C
+{
+    string M(RefStruct someValue) => $""Test: {someValue[||].ToString()}"";
+}
+
+ref struct RefStruct
+{
+    public override string ToString() => ""A"";
+}");
+        }
+
+        [Fact, WorkItem(42936, "https://github.com/dotnet/roslyn/issues/42936")]
+        public async Task PadLeftSimplificationIsStillOfferedOnRefStruct()
+        {
+            await TestInRegularAndScriptAsync(
+@"class C
+{
+    string M(RefStruct someValue) => $""Test: {someValue.ToString()[||].PadLeft(10)}"";
+}
+
+ref struct RefStruct
+{
+    public override string ToString() => ""A"";
+}",
+@"class C
+{
+    string M(RefStruct someValue) => $""Test: {someValue.ToString(),10}"";
+}
+
+ref struct RefStruct
+{
+    public override string ToString() => ""A"";
 }");
         }
     }

src/Features/Core/Portable/SimplifyInterpolation/Helpers.cs

@@ -73,7 +73,8 @@ private static IOperation Unwrap(IOperation expression)
         {
             if (expression is IInvocationOperation { TargetMethod: { Name: nameof(ToString) } } invocation &&
                 HasNonImplicitInstance(invocation) &&
-                !syntaxFacts.IsBaseExpression(invocation.Instance.Syntax))
+                !syntaxFacts.IsBaseExpression(invocation.Instance.Syntax) &&
+                !invocation.Instance.Type.IsRefLikeType)
             {
                 if (invocation.Arguments.Length == 1 &&
                     invocation.Arguments[0].Value is ILiteralOperation { ConstantValue: { HasValue: true, Value: string value } } literal &&