[DllImportGenerator] Don't generate a full stub body when the body does nothing

[jkoritzinsky]

Generate the partial function as a DllImport when a full-body generated stub would be a no-op wrapper around an inner DllImport.

Fixes #60596

[AaronRobinsonMSFT]

This is great. I need this for the "unsupported framework" fallback. I have it all set up except the forwarder to DllImport logic.

[am11]

Crossgen2 considers Boolean and Char as billitable types:

runtime/src/coreclr/tools/aot/ILCompiler.TypeSystem.ReadyToRun.Tests/MarshalUtilsTests.cs

Lines 25 to 56 in ba01c59

	[Theory]
	[InlineData(WellKnownType.Void)]
	[InlineData(WellKnownType.Boolean)]
	[InlineData(WellKnownType.Char)]
	[InlineData(WellKnownType.SByte)]
	[InlineData(WellKnownType.Byte)]
	[InlineData(WellKnownType.Int16)]
	[InlineData(WellKnownType.UInt16)]
	[InlineData(WellKnownType.Int32)]
	[InlineData(WellKnownType.UInt32)]
	[InlineData(WellKnownType.Int64)]
	[InlineData(WellKnownType.UInt64)]
	[InlineData(WellKnownType.IntPtr)]
	[InlineData(WellKnownType.UIntPtr)]
	[InlineData(WellKnownType.Single)]
	[InlineData(WellKnownType.Double)]
	[InlineData(WellKnownType.RuntimeFieldHandle)]
	[InlineData(WellKnownType.RuntimeTypeHandle)]
	[InlineData(WellKnownType.RuntimeMethodHandle)]
	public void IsBlittableType_BilittableWellKnownTypes_ReturnsTrue(WellKnownType type) =>
	Assert.True(MarshalUtils.IsBlittableType(_context.GetWellKnownType(type)));
	[Theory]
	[InlineData(WellKnownType.String)]
	[InlineData(WellKnownType.ValueType)]
	[InlineData(WellKnownType.Enum)]
	[InlineData(WellKnownType.Array)]
	[InlineData(WellKnownType.MulticastDelegate)]
	[InlineData(WellKnownType.Exception)]
	[InlineData(WellKnownType.Object)]
	public void IsBlittableType_NonBilittableWellKnownTypes_ReturnsFalse(WellKnownType type) =>
	Assert.False(MarshalUtils.IsBlittableType(_context.GetWellKnownType(type)));

but TypeSymbolExtensions doesn't:

runtime/src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/TypeSymbolExtensions.cs

Lines 54 to 69 in b201a16

	private static bool IsSpecialTypeBlittable(SpecialType specialType)
	=> specialType switch
	{
	SpecialType.System_Void
	or SpecialType.System_SByte
	or SpecialType.System_Byte
	or SpecialType.System_Int16
	or SpecialType.System_UInt16
	or SpecialType.System_Int32
	or SpecialType.System_UInt32
	or SpecialType.System_Int64
	or SpecialType.System_UInt64
	or SpecialType.System_Single
	or SpecialType.System_Double
	or SpecialType.System_IntPtr
	or SpecialType.System_UIntPtr => true,

should they match?

[jkoritzinsky]

In this case, crossgen2 is incorrect. Bool and Char are not blittable in the current system (which is the whole reason blittable != unmanaged)

[am11]

Ah right. The test was added as part of addressing #46653 (comment), which suggests crossgen2 case is different (crossgen2 is not involved in Marshal.SizeOf).

[am11]

Perhaps we can split IsBlittable and RequiresSpecialMarshalling methods in crossgen2 to avoid confusion. cc @MichalStrehovsky.

[MichalStrehovsky]

Crossgen2 considers Boolean and Char as billitable types:

It doesn't. The unit test just uses a weird base class library where Bool and Char have no fields:

runtime/src/coreclr/tools/aot/ILCompiler.TypeSystem.ReadyToRun.Tests/CoreTestAssembly/Platform.cs

Lines 34 to 35 in ba01c59

	public struct Boolean { }
	public struct Char { }

If you zoom into IsBlittableType implementation:

runtime/src/coreclr/tools/Common/TypeSystem/Interop/IL/MarshalUtils.cs

Lines 14 to 65 in 15be630

	public static bool IsBlittableType(TypeDesc type)
	{
	if (!type.IsDefType)
	{
	return false;
	}
	TypeDesc baseType = type.BaseType;
	bool hasNonTrivialParent = baseType != null
	&& !baseType.IsWellKnownType(WellKnownType.Object)
	&& !baseType.IsWellKnownType(WellKnownType.ValueType);
	if (hasNonTrivialParent && !IsBlittableType(baseType))
	{
	return false;
	}
	var mdType = (MetadataType)type;
	if (!mdType.IsSequentialLayout && !mdType.IsExplicitLayout)
	{
	return false;
	}
	foreach (FieldDesc field in type.GetFields())
	{
	if (field.IsStatic)
	{
	continue;
	}
	MarshallerKind marshallerKind = MarshalHelpers.GetMarshallerKind(
	field.FieldType,
	parameterIndex : null,
	customModifierData: null,
	field.GetMarshalAsDescriptor(),
	isReturn: false,
	isAnsi: mdType.PInvokeStringFormat == PInvokeStringFormat.AnsiClass,
	MarshallerType.Field,
	elementMarshallerKind: out var _);
	if (marshallerKind != MarshallerKind.Enum
	&& marshallerKind != MarshallerKind.BlittableValue
	&& marshallerKind != MarshallerKind.BlittableStruct
	&& marshallerKind != MarshallerKind.UnicodeChar)
	{
	return false;
	}
	}
	return true;
	}

You can see why we would end up saying the struct is blittable if there's no fields.

Maybe the code in MarshalUtils should check for primitive types first, but as things are now I think it kind of works for real core libraries. I think we would see bugs if it didn't work.

[jkoritzinsky]

We definitely would definitely see bugs if this was broken in real scenarios. I was very confused about how it was broken.