[mtouch] Show warnings when we can't find referenced assemblies.

This would have helped track down dotnet#4235.

docs/website/mmp-errors.md

@@ -156,6 +156,8 @@ Alternatively, enable the managed [linker](https://docs.microsoft.com/xamarin/ma
 
 As a last-straw solution, use an older version of Xamarin.Mac that does not require these new SDKs to be present during the build process.
 
+<!-- 0136 and 0137 used by mtouch -->
+
 # MM1xxx: file copy / symlinks (project related)
 
 ### <a name="MM1034">MM1034: Could not create symlink '{file}' -> '{target}': error {number}

docs/website/mtouch-errors.md

@@ -683,6 +683,22 @@ Alternatively, enable the managed [linker](https://docs.microsoft.com/en-us/xama
 
 As a last-straw solution, use an older version of Xamarin.iOS that does not require these new SDKs to be present during the build process.
 
+### <a name="MT0136"/>MT0136: Cannot find {assembly} referenced from {assembly}.
+
+This warning occurs when an assembly passed to mtouch contains a reference to
+another assembly that can't be found.
+
+mtouch may in certain cases still find references at a later point, which
+means that if the build otherwise succeeds, this warning can be ignored.
+
+### <a name="MT0137"/>MT0137: Cannot find {assembly}, referenced by an attribute in {assembly}.
+
+This warning occurs when an attribute contains a reference to another assembly
+that can't be found.
+
+mtouch may in certain cases still find references at a later point, which
+means that if the build otherwise succeeds, this warning can be ignored.
+
 # MT1xxx: Project related error messages
 
 ### MT10xx: Installer / mtouch

tests/mtouch/MTouch.cs

@@ -1716,6 +1716,62 @@ public void MT0132 ()
 			}
 		}
 
+		[Test]
+		public void MT0136 ()
+		{
+			using (var mtouch = new MTouchTool ()) {
+				var tmpdir = mtouch.CreateTemporaryDirectory ();
+				mtouch.CreateTemporaryCacheDirectory ();
+
+				var codeDll = @"public class A {}";
+				var codeExe = @"public class B : A {}";
+
+				var dllPath = CompileTestAppLibrary (tmpdir, codeDll, profile: Profile.iOS, appName: "A");
+
+				mtouch.CreateTemporaryApp (extraCode: codeExe, extraArg: $"-r:{StringUtils.Quote (dllPath)}");
+				mtouch.Debug = false;
+				mtouch.Linker = MTouchLinker.DontLink;
+				File.Delete (dllPath);
+				mtouch.AssertExecuteFailure (MTouchAction.BuildSim, "build");
+				mtouch.AssertWarningPattern (136, "Cannot find 'A, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' referenced from '.*/testApp.exe'.");
+				mtouch.AssertError (2002, "Failed to resolve assembly: 'A, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'");
+				mtouch.AssertErrorCount (1);
+				mtouch.AssertWarningCount (1);
+			}
+		}
+
+		[Test]
+		public void MT0137 ()
+		{
+			using (var mtouch = new MTouchTool ()) {
+				var tmpdir = mtouch.CreateTemporaryDirectory ();
+				mtouch.CreateTemporaryCacheDirectory ();
+
+				var codeDll = @"public class A {}";
+				var codeExe = @"
+public class TAttribute : System.Attribute
+{
+	public TAttribute (System.Type type) {}
+}
+
+[T (typeof (A))]
+public class B
+{
+}
+";
+
+				var dllPath = CompileTestAppLibrary (tmpdir, codeDll, profile: Profile.iOS, appName: "A");
+
+				mtouch.CreateTemporaryApp (extraCode: codeExe, extraArg: $"-r:{StringUtils.Quote (dllPath)}");
+				mtouch.Debug = false;
+				mtouch.Linker = MTouchLinker.DontLink;
+				File.Delete (dllPath);
+				mtouch.AssertExecute (MTouchAction.BuildSim, "build");
+				mtouch.AssertWarningPattern (137, "Cannot find 'A, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null', referenced by an attribute in 'testApp.exe'.");
+				mtouch.AssertWarningCount (1);
+			}
+		}
+
 		[Test]
 		[TestCase ("all")]
 		[TestCase ("-all")]

tools/mtouch/Target.cs

@@ -393,6 +393,10 @@ void ComputeListOfAssemblies (HashSet<string> assemblies, AssemblyDefinition ass
 				}
 
 				var reference_assembly = ManifestResolver.Resolve (reference);
+				if (reference_assembly == null) {
+					ErrorHelper.Warning (136, "Cannot find '{0}' referenced from '{1}'.", reference.FullName, main.FileName);
+					continue;
+				}
 				ComputeListOfAssemblies (assemblies, reference_assembly, exceptions);
 			}
 
@@ -402,46 +406,50 @@ void ComputeListOfAssemblies (HashSet<string> assemblies, AssemblyDefinition ass
 			// Custom Attribute metadata can include references to other assemblies, e.g. [X (typeof (Y)], 
 			// but it is not reflected in AssemblyReferences :-( ref: #37611
 			// so we must scan every custom attribute to look for System.Type
-			GetCustomAttributeReferences (assembly, assemblies, exceptions);
-			GetCustomAttributeReferences (main, assemblies, exceptions);
+			GetCustomAttributeReferences (main, assembly, assemblies, exceptions);
+			GetCustomAttributeReferences (main, main, assemblies, exceptions);
 			if (main.HasTypes) {
 				foreach (var ca in main.GetCustomAttributes ())
-					GetCustomAttributeReferences (ca, assemblies, exceptions);
+					GetCustomAttributeReferences (main, ca, assemblies, exceptions);
 			}
 		}
 
-		void GetCustomAttributeReferences (ICustomAttributeProvider cap, HashSet<string> assemblies, List<Exception> exceptions)
+		void GetCustomAttributeReferences (ModuleDefinition main, ICustomAttributeProvider cap, HashSet<string> assemblies, List<Exception> exceptions)
 		{
 			if (!cap.HasCustomAttributes)
 				return;
 			foreach (var ca in cap.CustomAttributes)
-				GetCustomAttributeReferences (ca, assemblies, exceptions);
+				GetCustomAttributeReferences (main, ca, assemblies, exceptions);
 		}
 
-		void GetCustomAttributeReferences (CustomAttribute ca, HashSet<string> assemblies, List<Exception> exceptions)
+		void GetCustomAttributeReferences (ModuleDefinition main, CustomAttribute ca, HashSet<string> assemblies, List<Exception> exceptions)
 		{
 			if (ca.HasConstructorArguments) {
 				foreach (var arg in ca.ConstructorArguments)
-					GetCustomAttributeArgumentReference (arg, assemblies, exceptions);
+					GetCustomAttributeArgumentReference (main, arg, assemblies, exceptions);
 			}
 			if (ca.HasFields) {
 				foreach (var arg in ca.Fields)
-					GetCustomAttributeArgumentReference (arg.Argument, assemblies, exceptions);
+					GetCustomAttributeArgumentReference (main, arg.Argument, assemblies, exceptions);
 			}
 			if (ca.HasProperties) {
 				foreach (var arg in ca.Properties)
-					GetCustomAttributeArgumentReference (arg.Argument, assemblies, exceptions);
+					GetCustomAttributeArgumentReference (main, arg.Argument, assemblies, exceptions);
 			}
 		}
 
-		void GetCustomAttributeArgumentReference (CustomAttributeArgument arg, HashSet<string> assemblies, List<Exception> exceptions)
+		void GetCustomAttributeArgumentReference (ModuleDefinition main, CustomAttributeArgument arg, HashSet<string> assemblies, List<Exception> exceptions)
 		{
 			if (!arg.Type.Is ("System", "Type"))
 				return;
 			var ar = (arg.Value as TypeReference)?.Scope as AssemblyNameReference;
 			if (ar == null)
 				return;
 			var reference_assembly = ManifestResolver.Resolve (ar);
+			if (reference_assembly == null) {
+				ErrorHelper.Warning (137, "Cannot find '{0}', referenced by an attribute in '{1}'.", ar.FullName, main.Name);
+				return;
+			}
 			ComputeListOfAssemblies (assemblies, reference_assembly, exceptions);
 		}