Error when ref is used on a parameter or return type of an UnmanagedCallersOnly method

docs/compilers/CSharp/Compiler Breaking Changes - DotNet 6.md

@@ -108,4 +108,3 @@ These are _function_type_conversions_.
         }
     }
     ```
-

docs/compilers/CSharp/Compiler Breaking Changes - DotNet 7.md

@@ -34,3 +34,25 @@
         public CustomHandler(int literalLength, int formattedCount, C c) {}
     }
     ```
+
+
+3. In Visual Studio 17.1, `ref`/`ref readonly`/`in`/`out` are not allowed to be used on return/parameters of a method attributed with `UnmanagedCallersOnly`.  
+https://github.com/dotnet/roslyn/issues/57025
+
+    ```cs
+    using System.Runtime.InteropServices;
+    [UnmanagedCallersOnly]
+    static ref int M1() => throw null; // error CS8977: Cannot use 'ref', 'in', or 'out' in a method attributed with 'UnmanagedCallersOnly'.
+
+    [UnmanagedCallersOnly]
+    static ref readonly int M2() => throw null; // error CS8977: Cannot use 'ref', 'in', or 'out' in a method attributed with 'UnmanagedCallersOnly'.
+
+    [UnmanagedCallersOnly]
+    static void M3(ref int o) => throw null; // error CS8977: Cannot use 'ref', 'in', or 'out' in a method attributed with 'UnmanagedCallersOnly'.
+
+    [UnmanagedCallersOnly]
+    static void M4(in int o) => throw null; // error CS8977: Cannot use 'ref', 'in', or 'out' in a method attributed with 'UnmanagedCallersOnly'.
+
+    [UnmanagedCallersOnly]
+    static void M5(out int o) => throw null; // error CS8977: Cannot use 'ref', 'in', or 'out' in a method attributed with 'UnmanagedCallersOnly'.
+    ```

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6875,6 +6875,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_CompileTimeCheckedOverflow_Title" xml:space="preserve">
     <value>The operation may overflow at runtime (use 'unchecked' syntax to override)</value>
   </data>
+  <data name="ERR_CannotUseRefInUnmanagedCallersOnly" xml:space="preserve">
+    <value>Cannot use 'ref', 'in', or 'out' in the signature of a method attributed with 'UnmanagedCallersOnly'.</value>
+  </data>
   <data name="IDS_FeatureNewLinesInInterpolations" xml:space="preserve">
     <value>newlines in interpolations</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2003,6 +2003,10 @@ internal enum ErrorCode
         ERR_LambdaExplicitReturnTypeVar = 8975,
         ERR_InterpolatedStringsReferencingInstanceCannotBeInObjectInitializers = 8976,
 
+        // Added in VS 17.1. Technically a breaking change, but the code it breaks was already guaranteed to throw
+        // at runtime.
+        ERR_CannotUseRefInUnmanagedCallersOnly = 8977,
+
         #endregion
 
         // Note: you will need to re-generate compiler code after adding warnings (eng\generate-compiler-code.cmd)

src/Compilers/CSharp/Portable/Symbols/Source/SourceMethodSymbolWithAttributes.cs

@@ -918,14 +918,19 @@ private void DecodeUnmanagedCallersOnlyAttribute(ref DecodeWellKnownAttributeArg
                 return;
             }
 
-            checkAndReportManagedTypes(ReturnType, returnTypeSyntax, isParam: false, diagnostics);
+            checkAndReportManagedTypes(ReturnType, this.RefKind, returnTypeSyntax, isParam: false, diagnostics);
             foreach (var param in Parameters)
             {
-                checkAndReportManagedTypes(param.Type, param.GetNonNullSyntaxNode(), isParam: true, diagnostics);
+                checkAndReportManagedTypes(param.Type, param.RefKind, param.GetNonNullSyntaxNode(), isParam: true, diagnostics);
             }
 
-            static void checkAndReportManagedTypes(TypeSymbol type, SyntaxNode syntax, bool isParam, BindingDiagnosticBag diagnostics)
+            static void checkAndReportManagedTypes(TypeSymbol type, RefKind refKind, SyntaxNode syntax, bool isParam, BindingDiagnosticBag diagnostics)
             {
+                if (refKind != RefKind.None)
+                {
+                    diagnostics.Add(ErrorCode.ERR_CannotUseRefInUnmanagedCallersOnly, syntax.Location);
+                }
+
                 // use-site diagnostics will be reported at actual parameter declaration site, we're only interested
                 // in reporting managed types being used
                 switch (type.ManagedKindNoUseSiteDiagnostics)