[mono] Support UnsafeAccessorAttribute method calls with custom modifiers

[lambdageek]

Enable these disabled tests:

• Verify_IgnoreCustomModifier
• Verify_UnmanagedCallConvBitAreTreatedAsCustomModifiersAndIgnored
• Verify_ManagedUnmanagedFunctionPointersDontMatch and
• Verify_InvalidTargetUnsafeAccessorAmbiguousMatch

Part of #86040

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Issue Details

---

Enable these disabled tests:

• Verify_IgnoreCustomModifier
• Verify_UnmanagedCallConvBitAreTreatedAsCustomModifiersAndIgnored
• Verify_ManagedUnmanagedFunctionPointersDontMatch and
• Verify_InvalidTargetUnsafeAccessorAmbiguousMatch

Author:	lambdageek
Assignees:	-
Labels:	untriaged, area-AssemblyLoader-mono, area-VM-meta-mono
Milestone:	-

[lambdageek]

The CoreCLR algorithm is

runtime/src/coreclr/vm/prestub.cpp

Lines 1244 to 1300 in a41f655

	bool TrySetTargetMethod(
	GenerationContext& cxt,
	LPCUTF8 methodName,
	bool ignoreCustomModifiers = true)
	{
	STANDARD_VM_CONTRACT;
	_ASSERTE(methodName != NULL);
	_ASSERTE(cxt.Kind == UnsafeAccessorKind::Constructor
	|| cxt.Kind == UnsafeAccessorKind::Method
	|| cxt.Kind == UnsafeAccessorKind::StaticMethod);
	TypeHandle targetType = cxt.TargetType;
	_ASSERTE(!targetType.IsTypeDesc());
	MethodDesc* targetMaybe = NULL;
	// Following the iteration pattern found in MemberLoader::FindMethod().
	// Reverse order is recommended - see comments in MemberLoader::FindMethod().
	MethodTable::MethodIterator iter(targetType.AsMethodTable());
	iter.MoveToEnd();
	for (; iter.IsValid(); iter.Prev())
	{
	MethodDesc* curr = iter.GetDeclMethodDesc();
	// Check the target and current method match static/instance state.
	if (cxt.IsTargetStatic != (!!curr->IsStatic()))
	continue;
	// Check for matching name
	if (strcmp(methodName, curr->GetNameThrowing()) != 0)
	continue;
	// Check signature
	MetaSig::CompareState state{};
	state.IgnoreCustomModifiers = ignoreCustomModifiers;
	if (!DoesMethodMatchUnsafeAccessorDeclaration(cxt, curr, state))
	continue;
	// Check if there is some ambiguity.
	if (targetMaybe != NULL)
	{
	if (ignoreCustomModifiers)
	{
	// We have detected ambiguity when ignoring custom modifiers.
	// Start over, but look for a match requiring custom modifiers
	// to match precisely.
	if (TrySetTargetMethod(cxt, methodName, false /* ignoreCustomModifiers */))
	return true;
	}
	COMPlusThrow(kAmbiguousMatchException, W("Arg_AmbiguousMatchException_UnsafeAccessor"));
	}
	targetMaybe = curr;
	}
	cxt.TargetMethod = targetMaybe;
	return cxt.TargetMethod != NULL;
	}

(https://discord.com/channels/@me/769298323270008832/1130894020411924571)

Right now in Mono we piggyback on find_method_in_class in loader.c, but actually we should write a separate implementation to use in mono_unsafe_accessor_find_ctor and mono_unsafe_accessor_find_method that does the same two pass algorithm as CoreCLR that first tries ignoring custom modifiers and then doing an exact match.

MethodTable::MethodIterator iter(targetType.AsMethodTable()) in CoreCLR doesn't have an exact corresponding mechanism in Mono, I think (I think it traverses all the methods in the class hierarchy - is that right?) so we would need a loop using mono_class_get_methods() inside a second loop going up the hierarchy

[lambdageek]

We will need to add a variant of mono_metadata_signature_equal that passes a new flag to signature_equiv: SIG_EQUIV_FLAG_IGNORE_CMODS and propagate that all the way down into the type equivalence code in metadata.c