Fix LibraryImportDiagnosticsAnalyzer and DownlevelLibraryImportDiagnosticsAnalyzer missing diagnostics when StringMarshalling is set

src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/Diagnostics.cs

@@ -413,6 +413,60 @@ await VerifyCS.VerifyAnalyzerAsync(source,
                     .WithArguments("Method", "Test"));
         }
 
+        [Theory]
+        [InlineData("StringMarshalling = StringMarshalling.Utf16")]
+        [InlineData("StringMarshalling = StringMarshalling.Utf8")]
+        [InlineData("")]
+        public async Task StringBuilderNotSupported_ReportsDiagnostic(string stringMarshallingArg)
+        {
+            string marshallingPart = string.IsNullOrEmpty(stringMarshallingArg)
+                ? ""
+                : $", {stringMarshallingArg}";
+
+            // StringBuilder as a simple parameter
+            string source = $$"""
+
+                using System.Runtime.InteropServices;
+                using System.Text;
+                partial class Test
+                {
+                    [LibraryImport("DoesNotExist"{{marshallingPart}})]
+                    public static partial void Method(StringBuilder {|#0:sb|});
+                }
+                """;
+
+            await VerifyCS.VerifyAnalyzerAsync(source,
+                VerifyCS.Diagnostic(GeneratorDiagnostics.ParameterTypeNotSupported)
+                    .WithLocation(0)
+                    .WithArguments("System.Text.StringBuilder", "sb"));
+        }
+
+        [Theory]
+        [InlineData("StringMarshalling = StringMarshalling.Utf16")]
+        [InlineData("StringMarshalling = StringMarshalling.Utf8")]
+        public async Task StringBuilderNotSupported_WithStringParam_ReportsDiagnostic(string stringMarshallingArg)
+        {
+            // StringBuilder with [Out] alongside a string parameter (repro from dotnet/runtime#126687)
+            string source = $$"""
+
+                using System.Runtime.InteropServices;
+                using System.Text;
+                partial class Test
+                {
+                    [LibraryImport("DoesNotExist", {{stringMarshallingArg}})]
+                    internal static partial int Method(
+                        string volumeMountPoint,
+                        [Out] StringBuilder {|#0:volumeName|},
+                        int bufferLength);
+                }
+                """;
+
+            await VerifyCS.VerifyAnalyzerAsync(source,
+                VerifyCS.Diagnostic(GeneratorDiagnostics.ParameterTypeNotSupported)
+                    .WithLocation(0)
+                    .WithArguments("System.Text.StringBuilder", "volumeName"));
+        }
+
         private static void VerifyDiagnostics(DiagnosticResult[] expectedDiagnostics, Diagnostic[] actualDiagnostics)
         {
             Assert.True(expectedDiagnostics.Length == actualDiagnostics.Length,