From d2f2ca014cbcf750aab004d9808375a39d57a67b Mon Sep 17 00:00:00 2001
From: Vincent Dondain <vidondai@microsoft.com>
Date: Tue, 27 Mar 2018 22:39:02 -0400
Subject: [PATCH] [msbuild] Skip reference assemblies passed to mtouch (#3791)

- Fixes issue #3199: Could not AOT the assembly System.Runtime.CompilerServices.Unsafe.dll (MT3001)
  (https://github.com/xamarin/xamarin-macios/issues/3199)
- Test case: https://www.dropbox.com/s/kxt3isgzn74nq35/Issue3199.zip?dl=0

### Problem

Given a Nuget Package added via the "package reference" mechanism and the said package having netstandard 'lib' **and** 'ref' folders;
MTouch was getting reference assemblies and was trying to AOT them. This resulted in an error as those assemblies are only facades.

### Solution

'ResolveNuGetPackageAssets' computes the NuGet packages using their json asset file, adds the full assemblies to 'ReferenceCopyLocalPaths' and outputs the resolved references via '_ReferencesFromNuGetPackages'.

This is exactly what "Microsoft.NuGet.Build.Tasks"'s 'ResolveNuGetPackageAssets' target does.
---
 msbuild/Xamarin.iOS.Tasks.Core/Xamarin.iOS.Common.targets | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/msbuild/Xamarin.iOS.Tasks.Core/Xamarin.iOS.Common.targets b/msbuild/Xamarin.iOS.Tasks.Core/Xamarin.iOS.Common.targets
index 094c23ccfcb1..8e2010f726ae 100644
--- a/msbuild/Xamarin.iOS.Tasks.Core/Xamarin.iOS.Common.targets
+++ b/msbuild/Xamarin.iOS.Tasks.Core/Xamarin.iOS.Common.targets
@@ -780,6 +780,11 @@ Copyright (C) 2013-2016 Xamarin. All rights reserved.
 		Outputs="$(_NativeExecutable);$(DeviceSpecificOutputPath)mtouch.stamp">
 
 		<ItemGroup>
+			<!-- Skip the reference assemblies. There is currently no scenario where it is valid to pass them to 'mtouch'. They are impossible to AOT. Fixes: https://github.com/xamarin/xamarin-macios/issues/3199 -->
+			<!-- Remove exact references, such as if a package had a framework reference to 'System' that we already have -->
+			<!-- This is exactly what "Microsoft.NuGet.Build.Tasks"'s 'ResolveNuGetPackageAssets' target is doing -->
+			<!-- Effectively 'ResolveNuGetPackageAssets' computes the NuGet packages using their json asset file, adds the full assemblies to 'ReferenceCopyLocalPaths' and outputs the resolved references via '_ReferencesFromNuGetPackages' -->
+			<ReferencePath Remove="@(_ReferencesFromNuGetPackages)" />
 			<MTouchReferencePath Include="@(ReferenceCopyLocalPaths)" Condition="'%(Extension)' == '.dll'" />
 		</ItemGroup>