Allow ExternalTypeMap to have duplicates entries with identical mapping but different trimTarget

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/TypeMapLazyDictionary.cs

@@ -265,6 +265,20 @@ private unsafe struct TypeNameUtf8
         {
             public required void* Utf8TypeName { get; init; }
             public required int Utf8TypeNameLen { get; init; }
+
+            public bool Equals(TypeNameUtf8 other)
+            {
+                if (Utf8TypeNameLen != other.Utf8TypeNameLen)
+                    return false;
+                byte* thisChars = (byte*)Utf8TypeName;
+                byte* otherChars = (byte*)other.Utf8TypeName;
+                for (int i = 0; i < Utf8TypeNameLen; i++)
+                {
+                    if (thisChars[i] != otherChars[i])
+                        return false;
+                }
+                return true;
+            }
         }
 
         [RequiresUnreferencedCode("Lazy TypeMap isn't supported for Trimmer scenarios")]
@@ -282,6 +296,8 @@ public DelayedType(TypeNameUtf8 typeNameUtf8, RuntimeAssembly fallbackAssembly)
                 _type = null;
             }
 
+            public TypeNameUtf8 TypeName => _typeNameUtf8;
+
             public unsafe Type GetOrLoadType()
             {
                 if (_type is null)
@@ -328,7 +344,9 @@ protected override bool TryGetOrLoadType(string key, [NotNullWhen(true)] out Typ
             public void Add(string key, TypeNameUtf8 targetType, RuntimeAssembly fallbackAssembly)
             {
                 int hash = ComputeHashCode(key);
-                if (_lazyData.ContainsKey(hash))
+                // Allow duplicates that have the same string -> mapping. They may have different trimTargets.
+                // Warn if the mapping conflicts with an existing mapping.
+                if (_lazyData.TryGetValue(hash, out DelayedType? existing) && existing.TypeName.Equals(targetType))
                 {
                     ThrowHelper.ThrowAddingDuplicateWithKeyArgumentException(key);
                 }

src/libraries/System.Runtime.InteropServices/tests/TrimmingTests/TypeMap.cs

@@ -8,6 +8,8 @@
 
 [assembly: TypeMap<UsedTypeMap>("TrimTargetIsTarget", typeof(TargetAndTrimTarget), typeof(TargetAndTrimTarget))]
 [assembly: TypeMap<UsedTypeMap>("TrimTargetIsUnrelated", typeof(TargetType), typeof(TrimTarget))]
+[assembly: TypeMap<UsedTypeMap>("DuplicateMappingWithDifferentTrimTargets", typeof(TargetType2), typeof(TrimTarget2))]
+[assembly: TypeMap<UsedTypeMap>("DuplicateMappingWithDifferentTrimTargets", typeof(TargetType2), typeof(TrimTarget3))]
 [assembly: TypeMap<UsedTypeMap>("TrimTargetIsUnreferenced", typeof(UnreferencedTargetType), typeof(UnreferencedTrimTarget))]
 [assembly: TypeMapAssociation<UsedTypeMap>(typeof(SourceClass), typeof(ProxyType))]
 
@@ -34,7 +36,7 @@
 
 IReadOnlyDictionary<string, Type> usedTypeMap = TypeMapping.GetOrCreateExternalTypeMapping<UsedTypeMap>();
 
-if (!usedTypeMap.TryGetValue("TrimTargetIsTarget", out Type targetAndTrimTargetType))
+if (!usedTypeMap.TryGetValue("TrimTargetIsTarget", out Type? targetAndTrimTargetType))
 {
     Console.WriteLine("TrimTargetIsTarget not found in used type map.");
     return 1;
@@ -46,12 +48,13 @@
     return 2;
 }
 
-if (!usedTypeMap.TryGetValue("TrimTargetIsUnrelated", out Type targetType))
+if (!usedTypeMap.TryGetValue("TrimTargetIsUnrelated", out Type? targetType))
 {
     Console.WriteLine("TrimTargetIsUnrelated not found in used type map.");
     return 3;
 }
 
+
 if (targetType != GetTypeWithoutTrimAnalysis(nameof(TargetType)))
 {
     Console.WriteLine("TrimTargetIsUnrelated type does not match expected type.");
@@ -71,7 +74,7 @@
 }
 
 IReadOnlyDictionary<Type, Type> usedProxyTypeMap = TypeMapping.GetOrCreateProxyTypeMapping<UsedTypeMap>();
-if (!usedProxyTypeMap.TryGetValue(typeof(SourceClass), out Type proxyType))
+if (!usedProxyTypeMap.TryGetValue(typeof(SourceClass), out Type? proxyType))
 {
     Console.WriteLine("SourceClass not found in used proxy type map.");
     return 7;
@@ -95,6 +98,12 @@
     return 10;
 }
 
+if (!usedTypeMap.TryGetValue("DuplicateMappingWithDifferentTrimTargets", out Type? duplicatedTarget))
+{
+    Console.WriteLine("Could not find duplicated target type");
+    return 11;
+}
+
 return 100;
 
 [MethodImpl(MethodImplOptions.NoInlining)]
@@ -106,7 +115,10 @@ static Type GetTypeWithoutTrimAnalysis(string typeName)
 class UsedTypeMap;
 class TargetAndTrimTarget;
 class TargetType;
+class TargetType1;
 class TrimTarget;
+class TrimTarget1;
+class TrimTarget2;
 class UnreferencedTargetType;
 class UnreferencedTrimTarget;
 class SourceClass;