Use the .NET 8 linker only for .NET 8

[rolfbjarne]

Currently .NET's linker is implemented so that if you're using .NET 7 to build a project, you'll get the .NET 7 linker, even if the project is building for .NET 6 (say using the net6.0-ios TargetFramework).

This is unfortunate for a couple of reasons:

• The .NET 6 codebase (sdk, ref, runtime packs) was not tested with the .NET 7 linker (we'd need a time machine for that). Testing the .NET 7 linker with .NET 6 projects is more feasible, except that it increases our testing matrix quite a lot (will we have to test the .NET 8 linker with .NET 6 + .NET 7 + .NET 8 projects?) In any case our tests won't cover every scenario, and it's entirely possible that customers will run into issues with the .NET 7 linker that isn't covered by our tests.
• In order to support building from Windows, we poke into the internals of the linker's MSBuild logic:
  

  xamarin-macios/msbuild/Xamarin.iOS.Tasks.Windows/Xamarin.iOS.Common.After.targets

  Lines 307 to 370 in 0a78bba

  	<!-- Overrides Core SDK target to remote the ILLink execution -->
  	<!-- https://github.com/dotnet/sdk/blob/cdf57465e1cba9e44b5c9a76a403d41b1b8f178c/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.ILLink.targets#L76-L132 -->
  	<Target Name="_RunILLink"
  	DependsOnTargets="_ComputeManagedAssemblyToLink;PrepareForILLink"
  	Inputs="$(MSBuildAllProjects);@(ManagedAssemblyToLink);@(TrimmerRootDescriptor);@(ReferencePath)"
  	Outputs="$(_LinkSemaphore)">
  	<PropertyGroup>
  	<!-- We need to use netX.Y because when building from VS it sets MSBuildRuntimeType to Core and will pick net472 (which doesn't contain the illink assembly) -->
  	<_RemoteILLinkPath>$(ILLinkTasksAssembly.Replace('$(NetCoreRoot)', '$(_DotNetRootRemoteDirectory)').Replace('net472', 'net$(BundledNETCoreAppTargetFrameworkVersion)').Replace('$([System.IO.Path]::GetFileName('$(ILLinkTasksAssembly)'))', 'illink.dll'))</_RemoteILLinkPath>
  	</PropertyGroup>
  	<!-- Include Debug symbols as input so those are copied to the server -->
  	<ItemGroup>
  	<!-- @(_PDBToLink) comes from the _ComputeManagedAssemblyToLink target, which is a dependency of this target and is included in Microsoft.NET.ILLink.targets -->
  	<!-- @(_PDBToLink) should contain the current assembly pdb and the project reference pdbs -->
  	<_ILLinkDebugSymbols Include="@(_PDBToLink)" />
  	</ItemGroup>
  	<Delete SessionId="$(BuildSessionId)" Files="@(_LinkedResolvedFileToPublishCandidate)" />
  	<Xamarin.iOS.Tasks.ILLink
  	SessionId="$(BuildSessionId)"
  	AssemblyPaths="@(ManagedAssemblyToLink)"
  	ReferenceAssemblyPaths="@(ReferencePath)"
  	RootAssemblyNames="@(TrimmerRootAssembly)"
  	TrimMode="$(TrimMode)"
  	DefaultAction="$(TrimmerDefaultAction)"
  	RemoveSymbols="$(TrimmerRemoveSymbols)"
  	FeatureSettings="@(_TrimmerFeatureSettings)"
  	CustomData="@(_TrimmerCustomData)"
  	BeforeFieldInit="$(_TrimmerBeforeFieldInit)"
  	OverrideRemoval="$(_TrimmerOverrideRemoval)"
  	UnreachableBodies="$(_TrimmerUnreachableBodies)"
  	UnusedInterfaces="$(_TrimmerUnusedInterfaces)"
  	IPConstProp="$(_TrimmerIPConstProp)"
  	Sealer="$(_TrimmerSealer)"
  	Warn="$(ILLinkWarningLevel)"
  	NoWarn="$(NoWarn)"
  	TreatWarningsAsErrors="$(ILLinkTreatWarningsAsErrors)"
  	WarningsAsErrors="$(WarningsAsErrors)"
  	WarningsNotAsErrors="$(WarningsNotAsErrors)"
  	SingleWarn="$(TrimmerSingleWarn)"
  	CustomSteps="@(_TrimmerCustomSteps)"
  	RootDescriptorFiles="@(TrimmerRootDescriptor)"
  	OutputDirectory="$(IntermediateLinkDir)"
  	DumpDependencies="$(_TrimmerDumpDependencies)"
  	ExtraArgs="$(_ExtraTrimmerArgs)"
  	ToolExe="$(_DotNetHostFileName)"
  	ToolPath="$(_DotNetHostDirectory)"
  	ILLinkPath="$(_RemoteILLinkPath)"
  	LinkerItemsDirectory="$(_LinkerItemsDirectory)"
  	DebugSymbols="@(_ILLinkDebugSymbols)"
  	ContinueOnError="ErrorAndContinue">
  	<Output TaskParameter="ExitCode" PropertyName="_ILLinkExitCode" />
  	<Output TaskParameter="LinkedItems" ItemName="_XamarinLinkedItems" />
  	<Output TaskParameter="LinkerOutputItems" ItemName="_XamarinLinkerItems" />
  	</Xamarin.iOS.Tasks.ILLink>
  	<Touch Files="$(_LinkSemaphore)" AlwaysCreate="true" Condition=" '$(_ILLinkExitCode)' == '0' " />
  	</Target>

  
  This leads to bugs when the linker's internals changes in .NET 7, and we're still using poking logic from .NET 6 (because we still haven't invented a time machine). Example: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1621047?src=WorkItemMention&src-action=artifact_link

I believe Android found an issue with the .NET 7 linker as well (iirc Cecil fixed a bug that turned out to not be backwards bug-compatible).

The fix seems easy: we should look into whether the linker can be used from the target .NET version (i.e. when building a net6.0-ios app use the .NET 6 linker). This is something the linker/runtime team should do, so this issue is just a tracking issue for our side.

[marek-safar]

This should be address in linker with dotnet/linker#3029

[rolfbjarne]

This has been addressed in the linker.