What is the expected behavior when a reference is missing?

src/tools/illink/test/ILLink.RoslynAnalyzer.Tests/generated/ILLink.RoslynAnalyzer.Tests.Generator/ILLink.RoslynAnalyzer.Tests.TestCaseGenerator/ReferencesTests.g.cs

@@ -57,6 +57,18 @@ public Task CopyWithLinkedWillHaveMethodDepsKept ()
 			return RunTest (allowMissingWarnings: true);
 		}
 
+		[Fact]
+		public Task MissingReferenceInUnusedCodePath ()
+		{
+			return RunTest (allowMissingWarnings: true);
+		}
+
+		[Fact]
+		public Task MissingReferenceInUsedCodePath ()
+		{
+			return RunTest (allowMissingWarnings: true);
+		}
+
 		[Fact]
 		public Task ReferencesAreRemovedWhenAllUsagesAreRemoved ()
 		{

src/tools/illink/test/Mono.Linker.Tests.Cases.Expectations/Metadata/DeleteBeforeAttribute.cs

@@ -0,0 +1,17 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+
+namespace Mono.Linker.Tests.Cases.Expectations.Metadata;
+
+/// <summary>
+/// Deletes a file before the linker is ran
+/// </summary>
+[AttributeUsage (AttributeTargets.Class)]
+public class DeleteBeforeAttribute : BaseMetadataAttribute
+{
+	public DeleteBeforeAttribute (string fileName)
+	{
+	}
+}

src/tools/illink/test/Mono.Linker.Tests.Cases/References/Dependencies/MissingAssembly.cs

@@ -0,0 +1,9 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace Mono.Linker.Tests.Cases.References.Dependencies;
+
+public class MissingAssembly
+{
+
+}

src/tools/illink/test/Mono.Linker.Tests.Cases/References/MissingReferenceInUnusedCodePath.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+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;
+
+[ExpectNonZeroExitCode (1)]
+[NoLinkedOutput]
+[SetupCompileBefore ("missing.dll", new[] { "Dependencies/MissingAssembly.cs" })]
+[DeleteBefore ("missing.dll")]
+public class MissingReferenceInUnusedCodePath
+{
+	public static void Main ()
+	{
+	}
+
+	private static void UnusedPath ()
+	{
+		typeof (MissingAssembly).ToString ();
+	}
+}

src/tools/illink/test/Mono.Linker.Tests.Cases/References/MissingReferenceInUsedCodePath.cs

@@ -0,0 +1,20 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+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;
+
+[ExpectNonZeroExitCode (1)]
+[NoLinkedOutput]
+[SetupCompileBefore ("missing.dll", new[] { "Dependencies/MissingAssembly.cs" })]
+[DeleteBefore ("missing.dll")]
+public class MissingReferenceInUsedCodePath
+{
+	public static void Main ()
+	{
+		typeof (MissingAssembly).ToString ();
+	}
+}

src/tools/illink/test/Mono.Linker.Tests/TestCasesRunner/TestCaseMetadataProvider.cs

@@ -152,6 +152,13 @@ public virtual IEnumerable<SourceAndDestinationPair> GetLinkAttributesFiles ()
 				.Select (GetSourceAndRelativeDestinationValue);
 		}
 
+		public IEnumerable<string> GetDeleteBefore ()
+		{
+			return _testCaseTypeDefinition.CustomAttributes
+				.Where (attr => attr.AttributeType.Name == nameof (DeleteBeforeAttribute))
+				.Select (attr => (string)attr.ConstructorArguments[0].Value);
+		}
+
 		public virtual IEnumerable<NPath> GetExtraLinkerReferences ()
 		{
 			var netcoreappDir = Path.GetDirectoryName (typeof (object).Assembly.Location);

src/tools/illink/test/Mono.Linker.Tests/TestCasesRunner/TestCaseSandbox.cs

@@ -159,6 +159,9 @@ public virtual void PopulateFromExpectations (TestCaseMetadataProvider metadataP
 			foreach (var res in metadataProvider.GetLinkAttributesFiles ()) {
 				res.Source.FileMustExist ().Copy (InputDirectory.Combine (res.DestinationFileName));
 			}
+
+			foreach (var delete in metadataProvider.GetDeleteBefore ())
+				InputDirectory.Combine (delete).FileMustExist ().Delete ();
 		}
 
 		private static NPath GetExpectationsAssemblyPath ()