Complete warnings for AOT unsafe interop

[jkotas]

Warnings for AOT unsafe interop are incomplete best effort currently: #74630 (comment)

• Generate warnings for unsafe interop in marshalled structs
• Generate warnings for AOT unsafe marshallers like AsAny and LayoutClass
• Use actual marshaller kind to produce the warning instead of the approximation at

  runtime/src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/Dataflow/ReflectionMethodBodyScanner.cs

  Lines 566 to 643 in 2201016

  	static bool IsComInterop(MarshalAsDescriptor? marshalInfoProvider, TypeDesc parameterType)
  	{
  	// This is best effort. One can likely find ways how to get COM without triggering these alarms.
  	// AsAny marshalling of a struct with an object-typed field would be one, for example.
  	// This logic roughly corresponds to MarshalInfo::MarshalInfo in CoreCLR,
  	// not trying to handle invalid cases and distinctions that are not interesting wrt
  	// "is this COM?" question.
  	NativeTypeKind nativeType = NativeTypeKind.Default;
  	if (marshalInfoProvider != null)
  	{
  	nativeType = marshalInfoProvider.Type;
  	}
  	if (nativeType == NativeTypeKind.IUnknown || nativeType == NativeTypeKind.IDispatch || nativeType == NativeTypeKind.Intf)
  	{
  	// This is COM by definition
  	return true;
  	}
  	if (nativeType == NativeTypeKind.Default)
  	{
  	TypeSystemContext context = parameterType.Context;
  	if (parameterType.IsPointer)
  	return false;
  	while (parameterType.IsParameterizedType)
  	parameterType = ((ParameterizedType)parameterType).ParameterType;
  	if (parameterType.IsWellKnownType(Internal.TypeSystem.WellKnownType.Array))
  	{
  	// System.Array marshals as IUnknown by default
  	return true;
  	}
  	else if (parameterType.IsWellKnownType(Internal.TypeSystem.WellKnownType.String) ||
  	InteropTypes.IsStringBuilder(context, parameterType))
  	{
  	// String and StringBuilder are special cased by interop
  	return false;
  	}
  	if (parameterType.IsValueType)
  	{
  	// Value types don't marshal as COM
  	return false;
  	}
  	else if (parameterType.IsInterface)
  	{
  	// Interface types marshal as COM by default
  	return true;
  	}
  	else if (parameterType.IsDelegate || parameterType.IsWellKnownType(Internal.TypeSystem.WellKnownType.MulticastDelegate)
  	|| parameterType == context.GetWellKnownType(Internal.TypeSystem.WellKnownType.MulticastDelegate).BaseType)
  	{
  	// Delegates are special cased by interop
  	return false;
  	}
  	else if (InteropTypes.IsCriticalHandle(context, parameterType))
  	{
  	// Subclasses of CriticalHandle are special cased by interop
  	return false;
  	}
  	else if (InteropTypes.IsSafeHandle(context, parameterType))
  	{
  	// Subclasses of SafeHandle are special cased by interop
  	return false;
  	}
  	else if (parameterType is MetadataType mdType && !mdType.IsSequentialLayout && !mdType.IsExplicitLayout)
  	{
  	// Rest of classes that don't have layout marshal as COM
  	return true;
  	}
  	}
  	return false;
  	}

• Steer people towards P/Invoke source generator by the warning text

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[MichalStrehovsky]

Steer people towards P/Invoke source generator by the warning text

If we tell people to use the source generator for AsAny/LayoutClass/System.Delegate marshalling, the source generator will then tell them it's unsupported. It will shift the blame, but the end result for the user is the same if we don't send them on a hike first.

[jkotas]

Yes, it is the case with retrofitting existing AOT unsafe code.

Steering people towards source generated interop is more interesting for new code. Source generated interop has natural AOT safety guiderails and prevents the AOT unsafe code to be written in the first place.

[hez2010]

towards P/Invoke source generator

Do source generated P/Invokes support direct P/Invoke and static linking?

If not then I think it's not a replacement of current P/Invokes.

[jkotas]

Do source generated P/Invokes support direct P/Invoke and static linking?

Yes. This aspect is not different between source generated and built-in marshalling.

All P/Invokes (except a few outliers) in this repo have been switched to be source generated, and native aot publishing uses direct P/Invokes and static linking extensively.

[MichalStrehovsky]

Use actual marshaller kind to produce the warning instead of the approximation at

This logic is shared with IL Linker and IL Linker doesn't have access to this unfortunately.