Require SizeConst for UnmanagedType.ByValArray

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7574,6 +7574,12 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_AddressOfInAsync_Title" xml:space="preserve">
     <value>The '&amp;' operator should not be used on parameters or local variables in async methods.</value>
   </data>
+  <data name="WRN_ByValArraySizeConstRequired" xml:space="preserve">
+    <value>Attribute parameter 'SizeConst' must be specified.</value>
+  </data>
+  <data name="WRN_ByValArraySizeConstRequired_Title" xml:space="preserve">
+    <value>Attribute parameter 'SizeConst' must be specified.</value>
+  </data>
   <data name="ERR_BadStaticAfterUnsafe" xml:space="preserve">
     <value>'static' modifier must precede 'unsafe' modifier.</value>
   </data>

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

@@ -2183,6 +2183,7 @@ internal enum ErrorCode
         ERR_UnexpectedParameterList = 9122,
         WRN_AddressOfInAsync = 9123,
         WRN_CapturedPrimaryConstructorParameterInFieldInitializer = 9124,
+        WRN_ByValArraySizeConstRequired = 9125,
 
         ERR_BadRefInUsingAlias = 9130,
         ERR_BadUnsafeInUsingDirective = 9131,

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

@@ -209,6 +209,7 @@ internal static int GetWarningLevel(ErrorCode code)
             switch (code)
             {
                 case ErrorCode.WRN_AddressOfInAsync:
+                case ErrorCode.WRN_ByValArraySizeConstRequired:
                     // Warning level 8 is exclusively for warnings introduced in the compiler
                     // shipped with dotnet 8 (C# 12) and that can be reported for pre-existing code.
                     return 8;
@@ -2394,6 +2395,7 @@ internal static bool IsBuildOnlyDiagnostic(ErrorCode code)
                 case ErrorCode.WRN_TargetDifferentRefness:
                 case ErrorCode.ERR_OutAttrOnRefReadonlyParam:
                 case ErrorCode.WRN_RefReadonlyParameterDefaultValue:
+                case ErrorCode.WRN_ByValArraySizeConstRequired:
                     return false;
                 default:
                     // NOTE: All error codes must be explicitly handled in this switch statement

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

@@ -309,5 +309,7 @@ protected override void ReportAttributeParameterRequired(DiagnosticBag diagnosti
         }
 
         public override int ERR_BadAssemblyName => (int)ErrorCode.ERR_BadAssemblyName;
+
+        public override int? WRN_ByValArraySizeConstRequired => (int)ErrorCode.WRN_ByValArraySizeConstRequired;
     }
 }