Add a test for dotnet/linker#2358 (dotnet/linker#2369)

Currently this causes a crash in the linker due to null location scope.


Commit migrated from dotnet/linker@da412ca

src/tools/illink/test/Mono.Linker.Tests.Cases/LinkAttributes/Dependencies/LinkerAttributeRemovalAndPreserveAssembly_Lib.Descriptor.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<linker>
+  <assembly fullname="test" />
+</linker>

src/tools/illink/test/Mono.Linker.Tests.Cases/LinkAttributes/Dependencies/LinkerAttributeRemovalAndPreserveAssembly_Lib.cs

@@ -0,0 +1,10 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Mono.Linker.Tests.Cases.LinkAttributes.Dependencies
+{
+	public static class Used
+	{
+		public static void Use () { }
+	}
+}

src/tools/illink/test/Mono.Linker.Tests.Cases/LinkAttributes/LinkerAttributeRemovalAndPreserveAssembly.LinkAttributes.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<linker>
+  <assembly fullname="*">
+    <type fullname="Mono.Linker.Tests.Cases.LinkAttributes.AttributeToRemoveAttribute">
+      <attribute internal="RemoveAttributeInstances"/>
+    </type>
+  </assembly>
+</linker>

src/tools/illink/test/Mono.Linker.Tests.Cases/LinkAttributes/LinkerAttributeRemovalAndPreserveAssembly.cs

@@ -0,0 +1,48 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using Mono.Linker.Tests.Cases.Expectations.Assertions;
+using Mono.Linker.Tests.Cases.Expectations.Metadata;
+using Mono.Linker.Tests.Cases.LinkAttributes.Dependencies;
+
+namespace Mono.Linker.Tests.Cases.LinkAttributes
+{
+	[IgnoreDescriptors (false)]
+
+	// The test verifies that removed attributes which are later on kept due to descriptors correctly warn.
+	// The setup is:
+	//  - test assembly with the AttributeToRemoveAttribute type
+	//  - link attributes.xml which marks the attribute for removal (applied early, in this case via command line, but could be a embedded in the test assembly)
+	//  - the attribute is used by the test assembly
+	//  - another assembly lib.dll is added and is referenced (after the attribute is used/marked)
+	//  - This new assembly has a descriptor which marks the entire test assembly (note that it marks the TEST assembly)
+	//  - This marking causes the warning, as it's an explicit request to keep the attribute which was supposed to be removed
+
+	[SetupLinkAttributesFile ("LinkerAttributeRemovalAndPreserveAssembly.LinkAttributes.xml")]
+
+	[SetupCompileBefore (
+		"lib.dll",
+		new[] { "Dependencies/LinkerAttributeRemovalAndPreserveAssembly_Lib.cs" })]
+	// https://github.com/dotnet/linker/issues/2358 - adding the descriptor currently causes nullref in the linker
+	// resources: new object[] { new string[] { "Dependencies/LinkerAttributeRemovalAndPreserveAssembly_Lib.Descriptor.xml", "ILLink.Descriptors.xml" } })]
+
+	[ExpectedNoWarnings]
+
+	class LinkerAttributeRemovalAndPreserveAssembly
+	{
+		public static void Main ()
+		{
+			TestAttributeRemoval ();
+		}
+
+		[AttributeToRemoveAttribute]
+		[Kept]
+		static void TestAttributeRemoval ()
+		{
+			Used.Use ();
+		}
+	}
+
+	public class AttributeToRemoveAttribute : Attribute { }
+}