Warn on trimming with COM hosting (built-in COM support) enabled (#72493)

Author: elinor-fung

eng/testing/linker/SupportFiles/Directory.Build.props

@@ -3,7 +3,6 @@
     <SkipConfigureTrimming>true</SkipConfigureTrimming>
     <PublishTrimmed>true</PublishTrimmed>
     <SkipImportRepoLinkerTargets>true</SkipImportRepoLinkerTargets>
-    <SuppressTrimAnalysisWarnings>true</SuppressTrimAnalysisWarnings>
     <TrimmerDefaultAction>link</TrimmerDefaultAction>
     <TrimMode>link</TrimMode>
     <TrimmerRemoveSymbols>false</TrimmerRemoveSymbols>

src/coreclr/System.Private.CoreLib/src/ILLink/ILLink.Suppressions.LibraryBuild.xml

@@ -8,6 +8,13 @@
       <property name="Target">M:System.StartupHookProvider.ProcessStartupHooks()</property>
       <property name="Justification">This warning is left in the product so developers get an ILLink warning when trimming an app with System.StartupHookProvider.IsSupported=true.</property>
     </attribute>
+    <attribute fullname="System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute">
+      <argument>ILLink</argument>
+      <argument>IL2026</argument>
+      <property name="Scope">member</property>
+      <property name="Target">M:Internal.Runtime.InteropServices.ComActivator.GetClassFactoryForTypeInternal(Internal.Runtime.InteropServices.ComActivationContextInternal*)</property>
+      <property name="Justification">This warning is left in the product so developers get an ILLink warning when trimming an app with System.Runtime.InteropServices.BuiltInComInterop.IsSupported.IsSupported=true.</property>
+    </attribute>
     <attribute fullname="System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute">
       <argument>ILLink</argument>
       <argument>IL2026</argument>

src/coreclr/System.Private.CoreLib/src/Internal/Runtime/InteropServices/ComActivator.cs

@@ -59,7 +59,6 @@ void CreateInstanceLic(
 
     internal partial struct ComActivationContext
     {
-        [RequiresUnreferencedCode("Built-in COM support is not trim compatible", Url = "https://aka.ms/dotnet-illink/com")]
         public static unsafe ComActivationContext Create(ref ComActivationContextInternal cxtInt)
         {
             if (!Marshal.IsBuiltInComSupported)
@@ -217,7 +216,6 @@ private static void ClassRegistrationScenarioForType(ComActivationContext cxt, b
         /// Internal entry point for unmanaged COM activation API from native code
         /// </summary>
         /// <param name="pCxtInt">Pointer to a <see cref="ComActivationContextInternal"/> instance</param>
-        [RequiresUnreferencedCode("Built-in COM support is not trim compatible", Url = "https://aka.ms/dotnet-illink/com")]
         [UnmanagedCallersOnly]
         private static unsafe int GetClassFactoryForTypeInternal(ComActivationContextInternal* pCxtInt)
         {
@@ -243,7 +241,9 @@ private static unsafe int GetClassFactoryForTypeInternal(ComActivationContextInt
             try
             {
                 var cxt = ComActivationContext.Create(ref cxtInt);
+#pragma warning disable IL2026 // suppressed in ILLink.Suppressions.LibraryBuild.xml
                 object cf = GetClassFactoryForType(cxt);
+#pragma warning restore IL2026
                 IntPtr nativeIUnknown = Marshal.GetIUnknownForObject(cf);
                 Marshal.WriteIntPtr(cxtInt.ClassFactoryDest, nativeIUnknown);
             }
@@ -259,7 +259,6 @@ private static unsafe int GetClassFactoryForTypeInternal(ComActivationContextInt
         /// Internal entry point for registering a managed COM server API from native code
         /// </summary>
         /// <param name="pCxtInt">Pointer to a <see cref="ComActivationContextInternal"/> instance</param>
-        [RequiresUnreferencedCode("Built-in COM support is not trim compatible", Url = "https://aka.ms/dotnet-illink/com")]
         [UnmanagedCallersOnly]
         private static unsafe int RegisterClassForTypeInternal(ComActivationContextInternal* pCxtInt)
         {
@@ -291,20 +290,24 @@ private static unsafe int RegisterClassForTypeInternal(ComActivationContextInter
             try
             {
                 var cxt = ComActivationContext.Create(ref cxtInt);
-                ClassRegistrationScenarioForType(cxt, register: true);
+                ClassRegistrationScenarioForTypeLocal(cxt, register: true);
             }
             catch (Exception e)
             {
                 return e.HResult;
             }
 
             return 0;
+
+            // Use a local function for a targeted suppression of the requires unreferenced code warning
+            [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2026:RequiresUnreferencedCode",
+                Justification = "The same feature switch applies to GetClassFactoryForTypeInternal and this function. We rely on the warning from GetClassFactoryForTypeInternal.")]
+            static void ClassRegistrationScenarioForTypeLocal(ComActivationContext cxt, bool register) => ClassRegistrationScenarioForType(cxt, register);
         }
 
         /// <summary>
         /// Internal entry point for unregistering a managed COM server API from native code
         /// </summary>
-        [RequiresUnreferencedCode("Built-in COM support is not trim compatible", Url = "https://aka.ms/dotnet-illink/com")]
         [UnmanagedCallersOnly]
         private static unsafe int UnregisterClassForTypeInternal(ComActivationContextInternal* pCxtInt)
         {
@@ -336,14 +339,19 @@ private static unsafe int UnregisterClassForTypeInternal(ComActivationContextInt
             try
             {
                 var cxt = ComActivationContext.Create(ref cxtInt);
-                ClassRegistrationScenarioForType(cxt, register: false);
+                ClassRegistrationScenarioForTypeLocal(cxt, register: false);
             }
             catch (Exception e)
             {
                 return e.HResult;
             }
 
             return 0;
+
+            // Use a local function for a targeted suppression of the requires unreferenced code warning
+            [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2026:RequiresUnreferencedCode",
+                Justification = "The same feature switch applies to GetClassFactoryForTypeInternal and this function. We rely on the warning from GetClassFactoryForTypeInternal.")]
+            static void ClassRegistrationScenarioForTypeLocal(ComActivationContext cxt, bool register) => ClassRegistrationScenarioForType(cxt, register);
         }
 
         private static bool IsLoggingEnabled()