Always show IL2026 for IsTrimmable assemblies

src/linker/Linker/LinkContext.cs

@@ -72,6 +72,7 @@ public class LinkContext : IMetadataResolver, IDisposable
 		readonly CustomAttributeSource _customAttributes;
 		readonly List<MessageContainer> _cachedWarningMessageContainers;
 		readonly ILogger _logger;
+		readonly Dictionary<AssemblyDefinition, bool> _isTrimmable;
 
 		public Pipeline Pipeline {
 			get { return _pipeline; }
@@ -210,6 +211,7 @@ public LinkContext (Pipeline pipeline, ILogger logger)
 			_parameters = new Dictionary<string, string> (StringComparer.Ordinal);
 			_customAttributes = new CustomAttributeSource (this);
 			_cachedWarningMessageContainers = new List<MessageContainer> ();
+			_isTrimmable = new Dictionary<AssemblyDefinition, bool> ();
 			FeatureSettings = new Dictionary<string, bool> (StringComparer.Ordinal);
 
 			SymbolReaderProvider = new DefaultSymbolReaderProvider (false);
@@ -424,12 +426,15 @@ static bool IsCPPCLIAssembly (ModuleDefinition module)
 			}
 		}
 
-		bool IsTrimmable (AssemblyDefinition assembly)
+		public bool IsTrimmable (AssemblyDefinition assembly)
 		{
-			if (!assembly.HasCustomAttributes)
-				return false;
+			if (_isTrimmable.TryGetValue (assembly, out bool isTrimmable))
+				return isTrimmable;
 
-			bool isTrimmable = false;
+			if (!assembly.HasCustomAttributes) {
+				_isTrimmable.Add (assembly, false);
+				return false;
+			}
 
 			foreach (var ca in assembly.CustomAttributes) {
 				if (!ca.AttributeType.IsTypeOf<AssemblyMetadataAttribute> ())
@@ -450,6 +455,7 @@ bool IsTrimmable (AssemblyDefinition assembly)
 				isTrimmable = true;
 			}
 
+			_isTrimmable.Add (assembly, isTrimmable);
 			return isTrimmable;
 		}
 

src/linker/Linker/MessageContainer.cs

@@ -132,24 +132,43 @@ private static MessageContainer CreateWarningMessageContainer (LinkContext conte
 			if (version > context.WarnVersion)
 				return Empty;
 
-			if (subcategory == MessageSubCategory.TrimAnalysis) {
-				Debug.Assert (origin.MemberDefinition != null);
-				var declaringType = origin.MemberDefinition?.DeclaringType ?? (origin.MemberDefinition as TypeDefinition);
-				var assembly = declaringType.Module.Assembly;
-				var assemblyName = assembly?.Name.Name;
-				if (assemblyName != null && context.IsSingleWarn (assemblyName)) {
-					if (context.AssembliesWithGeneratedSingleWarning.Add (assemblyName))
-						context.LogWarning ($"Assembly '{assemblyName}' produced trim warnings. For more information see https://aka.ms/dotnet-illink/libraries", 2104, context.GetAssemblyLocation (assembly));
-					return Empty;
-				}
-			}
+			if (TryLogSingleWarning (context, code, origin, subcategory))
+				return Empty;
 
 			if (context.IsWarningAsError (code))
 				return new MessageContainer (MessageCategory.WarningAsError, text, code, subcategory, origin);
 
 			return new MessageContainer (MessageCategory.Warning, text, code, subcategory, origin);
 		}
 
+		static bool TryLogSingleWarning (LinkContext context, int code, MessageOrigin origin, string subcategory)
+		{
+			if (subcategory != MessageSubCategory.TrimAnalysis)
+				return false;
+
+			Debug.Assert (origin.MemberDefinition != null);
+			var declaringType = origin.MemberDefinition?.DeclaringType ?? (origin.MemberDefinition as TypeDefinition);
+			var assembly = declaringType.Module.Assembly;
+
+			Debug.Assert (assembly != null);
+			if (assembly == null)
+				return false;
+
+			// Any IL2026 warnings left in an assembly with an IsTrimmable attribute are considered intentional
+			// and should not be collapsed, so that the user-visible RUC message gets printed.
+			if (code == 2026 && context.IsTrimmable (assembly))
+				return false;
+
+			var assemblyName = assembly.Name.Name;
+			if (!context.IsSingleWarn (assemblyName))
+				return false;
+
+			if (context.AssembliesWithGeneratedSingleWarning.Add (assemblyName))
+				context.LogWarning ($"Assembly '{assemblyName}' produced trim warnings. For more information see https://aka.ms/dotnet-illink/libraries", 2104, context.GetAssemblyLocation (assembly));
+
+			return true;
+		}
+
 		/// <summary>
 		/// Create a info message.
 		/// </summary>

test/Mono.Linker.Tests.Cases/Warnings/Dependencies/TriggerWarnings_TrimmableLib.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
+
+[assembly: AssemblyMetadata ("IsTrimmable", "True")]
+
+namespace Mono.Linker.Tests.Cases.Warnings.Dependencies
+{
+	public class TriggerWarnings_TrimmableLib
+	{
+		public static void Main ()
+		{
+			RequirePublicMethods (UnknownType ());
+			RUCIntentional ();
+		}
+
+		static Type UnknownType () => null;
+
+		static void RequirePublicMethods ([DynamicallyAccessedMembers (DynamicallyAccessedMemberTypes.PublicMethods)] Type type) { }
+
+		[RequiresUnreferencedCode ("RUC warning left in the trimmable assembly.")]
+		static void RUCIntentional () { }
+	}
+}

test/Mono.Linker.Tests.Cases/Warnings/WarningsFromTrimmableAssembliesCanSurviveSingleWarn.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using Mono.Linker.Tests.Cases.Expectations.Assertions;
+using Mono.Linker.Tests.Cases.Expectations.Metadata;
+using Mono.Linker.Tests.Cases.Warnings.Dependencies;
+
+namespace Mono.Linker.Tests.Cases.Warnings
+{
+	[SkipKeptItemsValidation]
+	[SetupCompileBefore ("library.dll", new[] { typeof (TriggerWarnings_TrimmableLib) })]
+	[SetupLinkerArgument ("--singlewarn")]
+	[LogContains (".*warning IL2026: .*TriggerWarnings_TrimmableLib.RUCIntentional.*RUC warning left in the trimmable assembly.*", regexMatch: true)]
+	[LogDoesNotContain ("IL2072")]
+	[LogContains ("warning IL2104: Assembly 'library' produced trim warnings")]
+	[LogDoesNotContain ("IL2026")]
+	public class WarningsFromTrimmableAssembliesCanSurviveSingleWarn
+	{
+		public static void Main ()
+		{
+			TriggerWarnings_TrimmableLib.Main ();
+		}
+	}
+}