Unsafe evolution: add warning for unsafe delegates

docs/compilers/CSharp/Warnversion Warning Waves.md

@@ -20,6 +20,7 @@ The compiler shipped with .NET 11 (the C# 15 compiler) contains the following wa
 | Warning ID | Description |
 |------------|-------------|
 | CS9368 | [RequiresUnsafeAttribute is only valid under the updated memory safety rules](https://github.com/dotnet/csharplang/issues/9704) |
+| CS9377 | [The 'unsafe' modifier does not have any effect here under the current rules](https://github.com/dotnet/csharplang/issues/9704) |
 
 ## Warning level 10
 

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -8276,6 +8276,14 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
     <value>An unsafe context is required for constructor '{0}' marked as 'RequiresUnsafe' or 'extern' to satisfy the 'new()' constraint of type parameter '{1}' in '{2}'</value>
     <comment>'RequiresUnsafe', 'extern', and 'new()' should not be localized. {2} is the generic method or type.</comment>
   </data>
+  <data name="WRN_UnsafeMeaningless" xml:space="preserve">
+    <value>The 'unsafe' modifier does not have any effect here under the current rules.</value>
+    <comment>'unsafe' is a keyword, should not be localized.</comment>
+  </data>
+  <data name="WRN_UnsafeMeaningless_Title" xml:space="preserve">
+    <value>The 'unsafe' modifier does not have any effect here under the current rules.</value>
+    <comment>'unsafe' is a keyword, should not be localized.</comment>
+  </data>
   <data name="ERR_NoAwaitOnAsyncEnumerable" xml:space="preserve">
     <value>'{0}' does not contain a definition for '{1}' and no accessible extension method '{1}' accepting a first argument of type '{0}' could be found (did you mean to iterate over the async collection with 'await foreach' instead?)</value>
     <comment>'await foreach' is not localizable</comment>

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

@@ -2482,6 +2482,7 @@ internal enum ErrorCode
         ERR_UnionConstructorCallsDefaultConstructor = 9375,
 
         ERR_UnsafeConstructorConstraint = 9376,
+        WRN_UnsafeMeaningless = 9377,
 
         // Note: you will need to do the following after adding errors:
         //  1) Update ErrorFacts.IsBuildOnlyDiagnostic (src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs)

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

@@ -211,6 +211,7 @@ internal static int GetWarningLevel(ErrorCode code)
             switch (code)
             {
                 case ErrorCode.WRN_RequiresUnsafeAttributeLegacyRules:
+                case ErrorCode.WRN_UnsafeMeaningless:
                     // Warning level 11 is exclusively for warnings introduced in the compiler
                     // shipped with dotnet 11 (C# 15) and that can be reported for pre-existing code.
                     return 11;
@@ -2587,6 +2588,7 @@ or ErrorCode.ERR_InstanceFieldInUnion
                 or ErrorCode.ERR_InstanceCtorWithOneParameterInUnion
                 or ErrorCode.ERR_UnionConstructorCallsDefaultConstructor
                 or ErrorCode.ERR_UnsafeConstructorConstraint
+                or ErrorCode.WRN_UnsafeMeaningless
                     => false,
             };
 #pragma warning restore CS8524 // The switch expression does not handle some values of its input type (it is not exhaustive) involving an unnamed enum value.

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

@@ -375,6 +375,14 @@ private DeclarationModifiers MakeModifiers(TypeKind typeKind, BindingDiagnosticB
                 diagnostics.Add(ErrorCode.ERR_SealedStaticClass, GetFirstLocation(), this);
             }
 
+            if (!modifierErrors &&
+                typeKind == TypeKind.Delegate &&
+                (mods & DeclarationModifiers.Unsafe) == DeclarationModifiers.Unsafe &&
+                this.ContainingModule.UseUpdatedMemorySafetyRules)
+            {
+                diagnostics.Add(ErrorCode.WRN_UnsafeMeaningless, GetFirstLocation());
+            }
+
             switch (typeKind)
             {
                 case TypeKind.Interface: