MarshalingPInvokeScanner detects structs as non-blittable

[rolfbjarne]

Description

MarshalingPInvokeScanner says all structs as non-blittable.

Reproduction Steps

• Clone rolfbjarne/marshalling-scanner (including submodules)
• Run make customstruct

Expected behavior

Test case should succeed.

Actual behavior

Fails with:

[...]
Assembly /Users/rolf/test/dotnet/marshalling-scanner/customstruct/bin/Debug/net9.0/customstruct.dll requires marshal-ilgen for method .Class1:DoSomething (first pass).
❌ Failed, because some assemblies are incompatible
make: *** [customstruct] Error 1

Regression?

No

Known Workarounds

No response

Configuration

No response

Other information

This happens because there's no code here handling structs:

runtime/src/tasks/MonoTargetsTasks/MarshalingPInvokeScanner/MinimalMarshalingTypeCompatibilityProvider.cs

Lines 108 to 114 in 035d268

	if (baseTypeHandle.Kind == HandleKind.TypeReference)
	{
	TypeReference baseType = reader.GetTypeReference((TypeReferenceHandle)baseTypeHandle);
	if (reader.GetString(typeDef.Namespace) == "System" &&
	reader.GetString(baseType.Name) == "Enum")
	return Compatibility.Compatible;
	}

The code should look at all instance fields of a struct, and if they're all blittable, then the struct is blittable too.

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