Add tests cover using statement difference between mcs and csc.

Add new [KeptReferencesInAssembly] attribute for asserting the expected
references in an assembly

linker/Tests/Mono.Linker.Tests.Cases.Expectations/Assertions/KeptReferencesInAssemblyAttribute.cs

@@ -0,0 +1,15 @@
+using System;
+
+namespace Mono.Linker.Tests.Cases.Expectations.Assertions {
+	[AttributeUsage (AttributeTargets.Class, AllowMultiple = true, Inherited = false)]
+	public class KeptReferencesInAssemblyAttribute : BaseInAssemblyAttribute {
+		public KeptReferencesInAssemblyAttribute (string assemblyFileName, string [] expectedReferenceAssemblyNames)
+		{
+			if (string.IsNullOrEmpty (assemblyFileName))
+				throw new ArgumentNullException (nameof (assemblyFileName));
+
+			if (expectedReferenceAssemblyNames == null)
+				throw new ArgumentNullException (nameof (expectedReferenceAssemblyNames));
+		}
+	}
+}

linker/Tests/Mono.Linker.Tests.Cases.Expectations/Mono.Linker.Tests.Cases.Expectations.csproj

@@ -61,6 +61,7 @@
     <Compile Include="Assertions\KeptMemberAttribute.cs" />
     <Compile Include="Assertions\KeptMemberInAssemblyAttribute.cs" />
     <Compile Include="Assertions\KeptReferenceAttribute.cs" />
+    <Compile Include="Assertions\KeptReferencesInAssemblyAttribute.cs" />
     <Compile Include="Assertions\KeptResourceAttribute.cs" />
     <Compile Include="Assertions\KeptResourceInAssemblyAttribute.cs" />
     <Compile Include="Assertions\KeptSecurityAttribute.cs" />

linker/Tests/Mono.Linker.Tests.Cases/Mono.Linker.Tests.Cases.csproj

@@ -297,6 +297,10 @@
     <Compile Include="PreserveDependencies\PreserveDependencyOnUnusedMethodInNonReferencedAssembly.cs" />
     <Compile Include="PreserveDependencies\PreserveDependencyOnUnusedMethodInNonReferencedAssemblyWithCopyUsedAction.cs" />
     <Compile Include="PreserveDependencies\PreserveDependencyOnUnusedMethodInNonReferencedAssemblyWithEmbeddedXml.cs" />
+    <Compile Include="References\AssemblyOnlyUsedByUsingWithCsc.cs" />
+    <Compile Include="References\AssemblyOnlyUsedByUsingWithMcs.cs" />
+    <Compile Include="References\Dependencies\AssemblyOnlyUsedByUsing_Copied.cs" />
+    <Compile Include="References\Dependencies\AssemblyOnlyUsedByUsing_Lib.cs" />
     <Compile Include="References\Individual\CanSkipUnresolved.cs" />
     <Compile Include="References\Individual\Dependencies\CanSkipUnresolved_Library.cs" />
     <Compile Include="References\Dependencies\UserAssembliesAreLinkedByDefault_Library1.cs" />

linker/Tests/Mono.Linker.Tests.Cases/References/AssemblyOnlyUsedByUsingWithCsc.cs

@@ -0,0 +1,29 @@
+using Mono.Linker.Tests.Cases.Expectations.Assertions;
+using Mono.Linker.Tests.Cases.Expectations.Metadata;
+using Mono.Linker.Tests.Cases.References.Dependencies;
+
+namespace Mono.Linker.Tests.Cases.References {
+	/// <summary>
+	/// We can't detect the using usage in the assembly.  As a result, nothing in `library` is going to be marked and that assembly will be deleted.
+	/// Because of that, `copied` needs to have it's reference to `library` removed even though we specified an assembly action of `copy`
+	/// </summary>
+	[SetupLinkerAction ("copy", "copied")]
+	[SetupCompileBefore ("library.dll", new [] {"Dependencies/AssemblyOnlyUsedByUsing_Lib.cs"})]
+
+	// When csc is used, `copied.dll` will have a reference to `library.dll`
+	[SetupCompileBefore ("copied.dll", new [] {"Dependencies/AssemblyOnlyUsedByUsing_Copied.cs"}, new [] {"library.dll"}, compilerToUse: "csc")]
+
+	// Here to assert that the test is setup correctly to copy the copied assembly.  This is an important aspect of the bug
+	[KeptMemberInAssembly ("copied.dll", typeof (AssemblyOnlyUsedByUsing_Copied), "Unused()")]
+
+	// We library should be gone.  The `using` statement leaves no traces in the IL so nothing in `library` will be marked
+	[RemovedAssembly ("library.dll")]
+	[KeptReferencesInAssembly ("copied.dll", new [] {"mscorlib"})]
+	public class AssemblyOnlyUsedByUsingWithCsc {
+		public static void Main ()
+		{
+			// Use something to keep the reference at compile time
+			AssemblyOnlyUsedByUsing_Copied.UsedToKeepReference ();
+		}
+	}
+}

linker/Tests/Mono.Linker.Tests.Cases/References/AssemblyOnlyUsedByUsingWithMcs.cs

@@ -0,0 +1,24 @@
+using Mono.Linker.Tests.Cases.Expectations.Assertions;
+using Mono.Linker.Tests.Cases.Expectations.Metadata;
+using Mono.Linker.Tests.Cases.References.Dependencies;
+
+namespace Mono.Linker.Tests.Cases.References {
+	[SetupLinkerAction ("copy", "copied")]
+	[SetupCompileBefore ("library.dll", new [] {"Dependencies/AssemblyOnlyUsedByUsing_Lib.cs"})]
+
+	// When mcs is used, `copied.dll` will not have a reference to `library.dll`
+	[SetupCompileBefore ("copied.dll", new [] {"Dependencies/AssemblyOnlyUsedByUsing_Copied.cs"}, new [] {"library.dll"}, compilerToUse: "mcs")]
+
+	// Here to assert that the test is setup correctly to copy the copied assembly.  This is an important aspect of the bug
+	[KeptMemberInAssembly ("copied.dll", typeof (AssemblyOnlyUsedByUsing_Copied), "Unused()")]
+
+	[RemovedAssembly ("library.dll")]
+	[KeptReferencesInAssembly ("copied.dll", new [] {"mscorlib"})]
+	public class AssemblyOnlyUsedByUsingWithMcs {
+		public static void Main ()
+		{
+			// Use something to keep the reference at compile time
+			AssemblyOnlyUsedByUsing_Copied.UsedToKeepReference ();
+		}
+	}
+}

linker/Tests/Mono.Linker.Tests.Cases/References/Dependencies/AssemblyOnlyUsedByUsing_Copied.cs

@@ -0,0 +1,16 @@
+
+// This is what triggers the behavior difference between Roslyn and mcs.  Roslyn will keep the reference
+// to this assembly because of this whereas mcs will not
+using ImportantForBug = Mono.Linker.Tests.Cases.References.Dependencies.AssemblyOnlyUsedByUsing_Lib;
+
+namespace Mono.Linker.Tests.Cases.References.Dependencies {
+	public class AssemblyOnlyUsedByUsing_Copied {
+		public static void UsedToKeepReference ()
+		{
+		}
+
+		private static void Unused ()
+		{
+		}
+	}
+}

linker/Tests/Mono.Linker.Tests.Cases/References/Dependencies/AssemblyOnlyUsedByUsing_Lib.cs

@@ -0,0 +1,4 @@
+namespace Mono.Linker.Tests.Cases.References.Dependencies {
+	public class AssemblyOnlyUsedByUsing_Lib {
+	}
+}

linker/Tests/TestCasesRunner/ResultChecker.cs

@@ -244,6 +244,9 @@ void VerifyLinkingOfOtherAssemblies (AssemblyDefinition original)
 							case nameof (RemovedResourceInAssemblyAttribute):
 								VerifyRemovedResourceInAssembly (checkAttrInAssembly);
 								break;
+							case nameof (KeptReferencesInAssemblyAttribute):
+								VerifyKeptReferencesInAssembly (checkAttrInAssembly);
+								break;
 							default:
 								UnhandledOtherAssemblyAssertion (expectedTypeName, checkAttrInAssembly, linkedType);
 								break;
@@ -506,6 +509,13 @@ protected virtual bool TryVerifyKeptMemberInAssemblyAsMethod (string memberName,
 			return false;
 		}
 
+		void VerifyKeptReferencesInAssembly (CustomAttribute inAssemblyAttribute)
+		{
+			var assembly = ResolveLinkedAssembly (inAssemblyAttribute.ConstructorArguments [0].Value.ToString ());
+			var expectedReferenceNames = ((CustomAttributeArgument []) inAssemblyAttribute.ConstructorArguments [1].Value).Select (attr => (string) attr.Value);
+			Assert.That (assembly.MainModule.AssemblyReferences.Select (asm => asm.Name), Is.EquivalentTo (expectedReferenceNames));
+		}
+
 		void VerifyKeptResourceInAssembly (CustomAttribute inAssemblyAttribute)
 		{
 			var assembly = ResolveLinkedAssembly (inAssemblyAttribute.ConstructorArguments [0].Value.ToString ());