[Trimming] Fix startup crash when TypeMapAssemblyTarget attribute survives but target assembly is fully trimmed

src/tools/illink/src/linker/Linker.Steps/MarkStep.cs

@@ -1488,6 +1488,9 @@ public virtual void MarkAssembly(AssemblyDefinition assembly, DependencyInfo rea
             if (CheckProcessed(assembly))
                 return;
 
+            // Flush any TypeMapAssemblyTarget attributes that were waiting for this assembly to be marked.
+            _typeMapHandler.TriggerPendingAssemblyTargets(assembly);
+
             var assemblyOrigin = new MessageOrigin(assembly);
 
             EmbeddedXmlInfo.ProcessDescriptors(assembly, Context);

src/tools/illink/src/linker/Linker/TypeMapHandler.cs

@@ -28,7 +28,16 @@ sealed class TypeMapHandler
         // [type map group: custom attributes]
         Dictionary<TypeReference, List<CustomAttributeWithOrigin>> _pendingExternalTypeMapEntries = null!;
         Dictionary<TypeReference, List<CustomAttributeWithOrigin>> _pendingProxyTypeMapEntries = null!;
-        Dictionary<TypeReference, List<CustomAttributeWithOrigin>> _pendingAssemblyTargets = null!;
+
+        // [type map group: (custom attribute, resolved target assembly)]
+        // The resolved target assembly is null if it could not be found. In that case, the attribute will not be marked.
+        Dictionary<TypeReference, List<(CustomAttributeWithOrigin Attr, AssemblyDefinition? TargetAssembly)>> _pendingAssemblyTargets = null!;
+
+        // [target assembly: (type map group, custom attribute, calling method)]
+        // When a type map group is seen, assembly targets whose referenced assembly has not yet been marked are moved here.
+        // When the referenced assembly is eventually marked (due to a TypeMap entry being marked), all pending entries for
+        // it are also marked.
+        Dictionary<AssemblyDefinition, List<(TypeReference Group, CustomAttributeWithOrigin Attr, MethodDefinition? CallingMethod)>> _pendingAssemblyTargetsByAssembly = null!;
 
         HashSet<TypeReference> _referencedExternalTypeMaps = null!;
         HashSet<TypeReference> _referencedProxyTypeMaps = null!;
@@ -52,6 +61,7 @@ public void Initialize(LinkContext context, MarkStep markStep, AssemblyDefinitio
             _pendingExternalTypeMapEntries = new(typeReferenceEqualityComparer);
             _pendingProxyTypeMapEntries = new(typeReferenceEqualityComparer);
             _pendingAssemblyTargets = new(typeReferenceEqualityComparer);
+            _pendingAssemblyTargetsByAssembly = [];
             _referencedExternalTypeMaps = new(typeReferenceEqualityComparer);
             _referencedProxyTypeMaps = new(typeReferenceEqualityComparer);
             var typeMapResolver = new TypeMapResolver(entryPointAssembly);
@@ -70,12 +80,11 @@ public void ProcessExternalTypeMapGroupSeen(MethodDefinition callingMethod, Type
                     MarkTypeMapAttribute(entry, new DependencyInfo(DependencyKind.TypeMapEntry, callingMethod));
                 }
             }
-            if (_pendingAssemblyTargets.Remove(typeMapGroup, out List<CustomAttributeWithOrigin>? assemblyTargets))
+            if (_pendingAssemblyTargets.Remove(typeMapGroup, out List<(CustomAttributeWithOrigin Attr, AssemblyDefinition? TargetAssembly)>? assemblyTargets))
             {
-                foreach (var entry in assemblyTargets)
+                foreach (var (entry, targetAssembly) in assemblyTargets)
                 {
-                    var info = new DependencyInfo(DependencyKind.TypeMapAssemblyTarget, callingMethod);
-                    MarkTypeMapAttribute(entry, info);
+                    MarkAssemblyTargetIfReady(typeMapGroup, entry, targetAssembly, callingMethod);
                 }
             }
         }
@@ -91,12 +100,11 @@ public void ProcessProxyTypeMapGroupSeen(MethodDefinition callingMethod, TypeRef
                     MarkTypeMapAttribute(entry, new DependencyInfo(DependencyKind.TypeMapEntry, callingMethod));
                 }
             }
-            if (_pendingAssemblyTargets.Remove(typeMapGroup, out List<CustomAttributeWithOrigin>? assemblyTargets))
+            if (_pendingAssemblyTargets.Remove(typeMapGroup, out List<(CustomAttributeWithOrigin Attr, AssemblyDefinition? TargetAssembly)>? assemblyTargets))
             {
-                foreach (var entry in assemblyTargets)
+                foreach (var (entry, targetAssembly) in assemblyTargets)
                 {
-                    var info = new DependencyInfo(DependencyKind.TypeMapAssemblyTarget, callingMethod);
-                    MarkTypeMapAttribute(entry, info);
+                    MarkAssemblyTargetIfReady(typeMapGroup, entry, targetAssembly, callingMethod);
                 }
             }
         }
@@ -112,6 +120,41 @@ void MarkTypeMapAttribute(CustomAttributeWithOrigin entry, DependencyInfo info)
                 _markStep.MarkRequirementsForInstantiatedTypes(targetTypeDef);
         }
 
+        void MarkAssemblyTargetIfReady(TypeReference typeMapGroup, CustomAttributeWithOrigin entry, AssemblyDefinition? targetAssembly, MethodDefinition? callingMethod)
+        {
+            // If the target assembly could not be resolved (it is not present in the linker input),
+            // the attribute cannot safely be kept: at runtime Assembly.Load would throw
+            // FileNotFoundException. Drop it silently; the assembly simply does not participate
+            // in the type map.
+            if (targetAssembly is null)
+                return;
+
+            if (_context.Annotations.IsMarked(targetAssembly))
+            {
+                // Target assembly is already marked. Ideally we would only keep the attribute when the
+                // assembly has at least one surviving TypeMap/TypeMapAssociation entry, but checking that
+                // here would add complexity for a narrow case. We accept this slight over-approximation.
+                MarkTypeMapAttribute(entry, new DependencyInfo(DependencyKind.TypeMapAssemblyTarget, callingMethod));
+            }
+            else
+            {
+                // Target assembly is not yet marked. Defer: mark when (if) the assembly eventually gets marked.
+                _pendingAssemblyTargetsByAssembly.AddToList(targetAssembly, (typeMapGroup, entry, callingMethod));
+            }
+        }
+
+        // Called from MarkStep.MarkAssembly whenever an assembly is first marked (for any reason).
+        // This ensures that pending TypeMapAssemblyTarget attributes are flushed regardless of
+        // the order in which assemblies are visited.
+        internal void TriggerPendingAssemblyTargets(AssemblyDefinition newlyMarkedAssembly)
+        {
+            if (!_pendingAssemblyTargetsByAssembly.Remove(newlyMarkedAssembly, out List<(TypeReference Group, CustomAttributeWithOrigin Attr, MethodDefinition? CallingMethod)>? waiting))
+                return;
+
+            foreach (var (_, attr, callingMethod) in waiting)
+                MarkTypeMapAttribute(attr, new DependencyInfo(DependencyKind.TypeMapAssemblyTarget, callingMethod));
+        }
+
         public void ProcessType(TypeDefinition definition)
         {
             EnsureInitialized();
@@ -192,21 +235,16 @@ static TypeReference UnwrapToResolvableType(TypeReference type)
             return type;
         }
 
-        private void AddAssemblyTarget(TypeReference typeMapGroup, CustomAttributeWithOrigin attr)
+        private void AddAssemblyTarget(TypeReference typeMapGroup, CustomAttributeWithOrigin attr, AssemblyDefinition? resolvedTargetAssembly)
         {
             // Validate attribute
             if (attr.Attribute.ConstructorArguments is not ([{ Value: string }]))
                 return;
 
-            // If the type map group has been seen, mark the attribute immediately
-            if (_referencedExternalTypeMaps.Contains(typeMapGroup) || _referencedProxyTypeMaps.Contains(typeMapGroup))
-            {
-                _markStep.MarkCustomAttribute(attr.Attribute, new DependencyInfo(DependencyKind.TypeMapEntry, null), new MessageOrigin(attr.Origin));
-                return;
-            }
-
-            // Otherwise, it's pending until the type map group is seen
-            _pendingAssemblyTargets.AddToList(typeMapGroup, attr);
+            // Otherwise, it's pending until the type map group is seen.
+            // Note: resolvedTargetAssembly may be null if the assembly could not be resolved (e.g., it doesn't
+            // exist in the input). In that case the attribute will be dropped (not marked) when the group is seen.
+            _pendingAssemblyTargets.AddToList(typeMapGroup, (attr, resolvedTargetAssembly));
         }
 
 
@@ -297,18 +335,20 @@ public void Resolve(LinkContext context, TypeMapHandler manager)
                         }
                         else if (attr.AttributeType.Name is "TypeMapAssemblyTargetAttribute`1")
                         {
-                            manager.AddAssemblyTarget(typeMapGroup, (attr, assembly));
+                            AssemblyDefinition? resolvedTargetAssembly = null;
                             if (attr.ConstructorArguments[0].Value is string str)
                             {
                                 var nextAssemblyName = AssemblyNameReference.Parse(str);
                                 if (context.TryResolve(nextAssemblyName) is AssemblyDefinition nextAssembly)
                                 {
+                                    resolvedTargetAssembly = nextAssembly;
                                     if (seen.Add(nextAssembly))
                                     {
                                         toVisit.Enqueue(nextAssembly);
                                     }
                                 }
                             }
+                            manager.AddAssemblyTarget(typeMapGroup, (attr, assembly), resolvedTargetAssembly);
                         }
                     }
                 }

src/tools/illink/test/Mono.Linker.Tests.Cases/Reflection/Dependencies/TypeMapAllConditionalEntriesDep.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.
+
+// All TypeMap entries in this assembly are conditional (3-argument form).
+// The trim target (AllConditionalTrimTarget) is never referenced by the test assembly,
+// so ILLink drops the entry, which leaves this assembly with no surviving TypeMap entries.
+// The test verifies that the TypeMapAssemblyTarget attribute pointing here is also removed.
+using System.Runtime.InteropServices;
+using Mono.Linker.Tests.Cases.Reflection.Dependencies;
+
+[assembly: TypeMap<AllConditionalGroupType>("ConditionalEntry", typeof(AllConditionalTarget), typeof(AllConditionalTrimTarget))]
+
+namespace Mono.Linker.Tests.Cases.Reflection.Dependencies
+{
+    public class AllConditionalTarget;
+    public class AllConditionalTrimTarget;
+}

src/tools/illink/test/Mono.Linker.Tests.Cases/Reflection/Dependencies/TypeMapAllConditionalGroupDep.cs

@@ -0,0 +1,10 @@
+// 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.Reflection.Dependencies
+{
+    // Group marker type used by TypeMapAssemblyTargetRemovedWhenAllEntriesTrimmed.
+    // Lives in its own assembly so the test assembly's TypeMapAssemblyTarget<AllConditionalGroupType>
+    // generic argument does not create a compile-time reference to the conditional.dll dependency.
+    public class AllConditionalGroupType;
+}

src/tools/illink/test/Mono.Linker.Tests.Cases/Reflection/TypeMapAssemblyTargetRemovedWhenAllEntriesTrimmed.cs

@@ -0,0 +1,37 @@
+// 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.Runtime.InteropServices;
+using Mono.Linker.Tests.Cases.Expectations.Assertions;
+using Mono.Linker.Tests.Cases.Expectations.Metadata;
+using Mono.Linker.Tests.Cases.Reflection.Dependencies;
+
+// The test assembly references "conditional.dll" only by the assembly name string.
+// All TypeMap entries in conditional.dll are conditional on AllConditionalTrimTarget,
+// which is never marked. So conditional.dll ends up with no surviving TypeMap entries.
+// The fix should cause this TypeMapAssemblyTarget attribute to be removed from the linked output.
+[assembly: TypeMapAssemblyTarget<AllConditionalGroupType>("conditional")]
+
+namespace Mono.Linker.Tests.Cases.Reflection
+{
+    // Compile the group-type assembly first so both the test assembly and conditional.dll can reference it.
+    // Compile conditional.dll second with addAsReference:false so the test assembly has no compile-time
+    // dependency on it (only the string reference in TypeMapAssemblyTarget).
+    [SetupCompileBefore("allconditionalgroup.dll", new[] { "Dependencies/TypeMapAllConditionalGroupDep.cs" })]
+    [SetupCompileBefore("conditional.dll", new[] { "Dependencies/TypeMapAllConditionalEntriesDep.cs" },
+        references: new[] { "allconditionalgroup.dll" }, addAsReference: false)]
+    [SetupLinkerAction("link", "System.Private.CoreLib")] // Needed to apply embedded XML (RemoveAttributeInstances)
+    [SetupLinkerArgument("--ignore-link-attributes", "false")]
+    [RemovedAssembly("conditional.dll")]
+    [RemovedAttributeInAssembly("test", typeof(TypeMapAssemblyTargetAttribute<AllConditionalGroupType>))]
+    [Kept]
+    class TypeMapAssemblyTargetRemovedWhenAllEntriesTrimmed
+    {
+        [Kept]
+        static void Main()
+        {
+            // Use the group so the trimmer processes the TypeMapAssemblyTarget attribute.
+            _ = TypeMapping.GetOrCreateExternalTypeMapping<AllConditionalGroupType>();
+        }
+    }
+}