SweepStep doesn't process all type references for forwarded types

[rolfbjarne]

Test case:
linkertestcase-aedcbaa.zip

Extract and execute testcase.sh (not test.sh):

$ ./testcase.sh
++ dirname ./testcase.sh
+ cd .
+ LINKER=/usr/local/share/dotnet/sdk/5.0.100-rc.2.20480.7/Sdks/Microsoft.NET.ILLink.Tasks/tools/netcoreapp3.0/illink.dll
+ /usr/local/share/dotnet/dotnet /usr/local/share/dotnet/sdk/5.0.100-rc.2.20480.7/Sdks/Microsoft.NET.ILLink.Tasks/tools/netcoreapp3.0/illink.dll @testcase.rsp --custom-step -LoadReferencesStep:Xamarin.SetupStep,/Users/rolf/test/linkertestcase/dotnet-linker.dll --custom-data LinkerOptionsFile=/Users/rolf/test/linkertestcase/custom-linker-options.txt
[...]
+ monodis './obj/iPhoneSimulator/Debug/net5.0-ios/ios-x64/linked/link all.dll'
+ grep BROKEN
	IL_0020:  call instance void [Xamarin.iOS]ObjCRuntime.BlockLiteral::SetupBlockImpl(<BROKEN CLASS token_ 1000024 due to Could not resolve type with token 01000024 from typeref (expected class 'System.Delegate' in assembly 'System.Collections, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a') assembly:System.Collections, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a type:System.Delegate member:(null)>, <BROKEN CLASS token_ 1000024 due to Could not resolve type with token 01000024 from typeref (expected class 'System.Delegate' in assembly 'System.Collections, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a') assembly:System.Collections, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a type:System.Delegate member:(null)>, bool, string)
	IL_0020:  call instance void [Xamarin.iOS]ObjCRuntime.BlockLiteral::SetupBlockImpl(<BROKEN CLASS token_ 1000024 due to Could not resolve type with token 01000024 from typeref (expected class 'System.Delegate' in assembly 'System.Collections, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a') assembly:System.Collections, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a type:System.Delegate member:(null)>, <BROKEN CLASS token_ 1000024 due to Could not resolve type with token 01000024 from typeref (expected class 'System.Delegate' in assembly 'System.Collections, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a') assembly:System.Collections, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a type:System.Delegate member:(null)>, bool, string)
[...]
+ echo '❌ FAILED'
❌ FAILED
+ exit 1

The problem occurs when one of our custom linker step rewrites IL to call a different method: https://github.com/xamarin/xamarin-macios/blob/699c1f98fbfb9ac1c5d790a99812aa2f2b1a001b/tools/linker/CoreOptimizeGeneratedCode.cs#L911-L918

The different method is imported into the module in question here: https://github.com/xamarin/xamarin-macios/blob/699c1f98fbfb9ac1c5d790a99812aa2f2b1a001b/tools/linker/CoreOptimizeGeneratedCode.cs#L1058-L1073

The problem occurs because the imported method has a parameter whose type is System.Delegate, from System.Runtime.dll, and when System.Runtime.dll is linked away, the corresponding TypeReference in this method's parameter's ParameterType is not updated in SweepStep to reference System.Private.CoreLib.dll instead.

The end result is an invalid assembly (as can be seen from the monodis output) where the imported method's parameter's type is apparently System.Delegate from System.Collections.dll.

Here is an ugly hack that fixes the problem: https://gist.github.com/rolfbjarne/45e8e6d001f5fe9c9c1cf4036ad212f6