[net10.0] [dotnet] Rework aggressive attribute trimming. Fixes #20906. (

#21314)

The aggressive attribute trimming as exposed by ILLink is undocumented and has
problematic behavior, so disable it (it can't be statically validated) -
ILLink already removed support for it anyway.

However, this has a somewhat significant size increase for our apps, and it
would affect pretty much every app, so re-implement the aggressive attribute
trimming ourselves.

This is accomplished by copying ILLink's list of attributes to remove into our
own xml definition, which we then pass to ILLink.

I tested a release build of monotouch-test for all platforms, and the final
size is almost entirely unchanged, except for a minimal size decrease.

Here's the results for iOS: https://gist.github.com/rolfbjarne/dac3e08620a3ae499ad1fc134765e0d2

Fixes #20906.

docs/build-apps/build-properties.md

@@ -30,3 +30,85 @@ Example:
 ```
 
 This property was introduced in .NET 9.
+
+## MobileAggressiveAttributeTrimming
+
+This property determines whether numerous attributes that are very rarely
+needed at runtime should be trimmed away.
+
+This is enabled by default.
+
+Note that while the attributes that are removed are very rarely used, it's
+technically possible that the removal can change runtime behavior.
+
+For example, System.Xml.Serialization will behave differently if a constructor
+has the `[Obsolete]` attribute (which is one of the attributes that are
+removed). This is low enough risk to justify removing these attributes by
+default because of the size savings.
+
+The list of attributes that are removed may change in the future, but at the
+time of this writing (for .NET 10), these are the attributes:
+
+* Microsoft.CodeAnalysis.EmbeddedAttribute
+* System.CLSCompliantAttribute
+* System.CodeDom.Compiler.GeneratedCodeAttribute
+* System.ComponentModel.EditorBrowsableAttribute
+* System.Diagnostics.CodeAnalysis.DoesNotReturnAttribute
+* System.Diagnostics.CodeAnalysis.DoesNotReturnIfAttribute
+* System.Diagnostics.CodeAnalysis.DynamicallyAccessedMembersAttribute
+* System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverageAttribute
+* System.Diagnostics.CodeAnalysis.ExperimentalAttribute
+* System.Diagnostics.CodeAnalysis.FeatureGuardAttribute
+* System.Diagnostics.CodeAnalysis.FeatureSwitchDefinitionAttribute
+* System.Diagnostics.CodeAnalysis.MemberNotNullAttribute
+* System.Diagnostics.CodeAnalysis.MemberNotNullWhenAttribute
+* System.Diagnostics.CodeAnalysis.NotNullIfNotNullAttribute
+* System.Diagnostics.CodeAnalysis.NotNullWhenAttribute
+* System.Diagnostics.CodeAnalysis.RequiresAssemblyFilesAttribute
+* System.Diagnostics.CodeAnalysis.RequiresDynamicCodeAttribute
+* System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute
+* System.Diagnostics.CodeAnalysis.StringSyntaxAttribute
+* System.Diagnostics.CodeAnalysis.SuppressMessageAttribute
+* System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute
+* System.Diagnostics.CodeAnalysis.UnscopedRefAttribute
+* System.ObsoleteAttribute
+* System.Reflection.AssemblyCompanyAttribute
+* System.Reflection.AssemblyConfigurationAttribute
+* System.Reflection.AssemblyCopyrightAttribute
+* System.Reflection.AssemblyDefaultAliasAttribute
+* System.Reflection.AssemblyDescriptionAttribute
+* System.Reflection.AssemblyMetadataAttribute
+* System.Reflection.AssemblyProductAttribute
+* System.Reflection.AssemblyTitleAttribute
+* System.Runtime.CompilerServices.AsyncMethodBuilderAttribute
+* System.Runtime.CompilerServices.CallerArgumentExpressionAttribute
+* System.Runtime.CompilerServices.CallerFilePathAttribute
+* System.Runtime.CompilerServices.CallerLineNumberAttribute
+* System.Runtime.CompilerServices.CallerMemberNameAttribute
+* System.Runtime.CompilerServices.CompilerFeatureRequiredAttribute
+* System.Runtime.CompilerServices.CompilerGlobalScopeAttribute
+* System.Runtime.CompilerServices.EnumeratorCancellationAttribute
+* System.Runtime.CompilerServices.ExtensionAttribute
+* System.Runtime.CompilerServices.InterpolatedStringHandlerArgumentAttribute
+* System.Runtime.CompilerServices.InterpolatedStringHandlerAttribute
+* System.Runtime.CompilerServices.IntrinsicAttribute
+* System.Runtime.CompilerServices.IsReadOnlyAttribute
+* System.Runtime.CompilerServices.IsUnmanagedAttribute
+* System.Runtime.CompilerServices.NativeIntegerAttribute
+* System.Runtime.CompilerServices.RefSafetyRulesAttribute
+* System.Runtime.CompilerServices.ScopedRefAttribute
+* System.Runtime.CompilerServices.SkipLocalsInitAttribute
+* System.Runtime.CompilerServices.TupleElementNamesAttribute
+* System.Runtime.InteropServices.LibraryImportAttribute
+* System.Runtime.InteropServices.Marshalling.ContiguousCollectionMarshallerAttribute
+* System.Runtime.InteropServices.Marshalling.CustomMarshallerAttribute
+* System.Runtime.InteropServices.Marshalling.MarshalUsingAttribute
+* System.Runtime.InteropServices.Marshalling.NativeMarshallingAttribute
+* System.Runtime.Versioning.NonVersionableAttribute
+* System.Runtime.Versioning.ObsoletedOSPlatformAttribute
+* System.Runtime.Versioning.RequiresPreviewFeaturesAttribute
+* System.Runtime.Versioning.SupportedOSPlatformAttribute
+* System.Runtime.Versioning.SupportedOSPlatformGuardAttribute
+* System.Runtime.Versioning.TargetPlatformAttribute
+* System.Runtime.Versioning.UnsupportedOSPlatformAttribute
+* System.Runtime.Versioning.UnsupportedOSPlatformGuardAttribute

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -151,15 +151,6 @@
 		<!-- We don't need to generate reference assemblies for apps or app extensions -->
 		<ProduceReferenceAssembly Condition="'$(ProduceReferenceAssembly)' == '' And ('$(OutputType)' == 'Exe' Or '$(IsAppExtension)' == 'true')">false</ProduceReferenceAssembly>
 
-		<!-- 
-			With NativeAOT we want to prevent ILLink from removing attributes like `IsTrimmable` so further trimming can be done by the NativeAOT toolchain.
-			For this reason, in case of NativeAOT, we set _AggressiveAttributeTrimming to false by default and store the overwriten default in 
-			_OriginalAggressiveAttributeTrimming property, which is later used to properly configure NativeAOT trimming.
-		-->
-		<_OriginalAggressiveAttributeTrimming>$(_AggressiveAttributeTrimming)</_OriginalAggressiveAttributeTrimming>
-		<_AggressiveAttributeTrimming Condition="'$(_UseNativeAot)' == 'true'">false</_AggressiveAttributeTrimming>
-		<_AggressiveAttributeTrimming Condition="'$(_AggressiveAttributeTrimming)' == ''">true</_AggressiveAttributeTrimming>
-
 		<!-- This property is also defined in Xamarin.Shared.props, but that file is imported too late for us to use the definition, and we can't remove it from Xamarin.Shared.props quite yet, because that would break legacy
 			 (in other words: once we drop support for legacy Xamarin, we can remove the other definition) -->
 		<_CanOutputAppBundle Condition="'$(_CanOutputAppBundle)' == '' And ('$(OutputType)' == 'Exe' Or '$(IsAppExtension)' == 'true' Or '$(IsWatchApp)' == 'true')">true</_CanOutputAppBundle>
@@ -679,10 +670,16 @@
 			<!-- Linker -->
 			<_AdditionalTaskAssemblyDirectory>$(_XamarinSdkRootDirectoryOnMac)tools/dotnet-linker/</_AdditionalTaskAssemblyDirectory>
 			<_AdditionalTaskAssembly>$(_AdditionalTaskAssemblyDirectory)dotnet-linker.dll</_AdditionalTaskAssembly>
+
+			<MobileAggressiveAttributeTrimming Condition="'$(MobileAggressiveAttributeTrimming)' == ''">true</MobileAggressiveAttributeTrimming>
+			<_TrimAttributesLinkDescriptionPath>$(_XamarinSdkRootDirectory)tools/TrimAttributes.LinkDescription.xml</_TrimAttributesLinkDescriptionPath>
+			<_ExtraTrimmerArgs>$(_ExtraTrimmerArgs) --link-attributes $(_TrimAttributesLinkDescriptionPath)</_ExtraTrimmerArgs>
 		</PropertyGroup>
 
 		<ItemGroup>
 			<!-- Configure linker features -->
+			<RuntimeHostConfigurationOption Include="ObjCRuntime.AggressiveAttributeTrimming" Value="true" Trim="true" Condition="'$(MobileAggressiveAttributeTrimming)' == 'true'" />
+			<RuntimeHostConfigurationOption Include="ObjCRuntime.AggressiveAttributeTrimmingMono" Value="true" Trim="true" Condition="'$(MobileAggressiveAttributeTrimming)' == 'true' And '$(_XamarinRuntime)' == 'MonoVM'" />
 			<RuntimeHostConfigurationOption Include="ObjCRuntime.Runtime.Arch.IsSimulator" Value="$(_IsSimulatorFeature)" Trim="true" />
 			<RuntimeHostConfigurationOption Include="ObjCRuntime.Runtime.IsManagedStaticRegistrar" Value="$(_IsManagedStaticRegistrarFeature)" Trim="true" />
 			<RuntimeHostConfigurationOption Include="ObjCRuntime.Runtime.IsNativeAOT" Value="$(_IsNativeAOTFeature)" Trim="true" />
@@ -1342,16 +1339,6 @@
 
 			<!-- Link in the output from ILC -->
 			<_NativeExecutableObjectFiles Include="$(NativeObject)" />
-
-			<!-- By default perform aggressive attribute trimming, except when explicitly disabled -->
-			<_TrimmerFeatureSettings Condition="'$(_OriginalAggressiveAttributeTrimming)' != 'false' and '%(Identity)' == 'System.AggressiveAttributeTrimming'">
-				<Value>true</Value>
-				<Trim>true</Trim>
-			</_TrimmerFeatureSettings>
-			<RuntimeHostConfigurationOption Condition="'$(_OriginalAggressiveAttributeTrimming)' != 'false' and '%(Identity)' == 'System.AggressiveAttributeTrimming'">
-				<Value>true</Value>
-				<Trim>true</Trim>
-			</RuntimeHostConfigurationOption>
 		</ItemGroup>
 
 		<ItemGroup>

src/Makefile

@@ -510,6 +510,17 @@ endef
 #            BuildDotNetPlatformAssembly (platform, platform.ToUpper (), platform.ToLower (), bitness)
 $(foreach platform,$(DOTNET_PLATFORMS),$(foreach bitness, $(DOTNET_$(shell echo $(platform) | tr '[:lower:]' '[:upper:]')_BITNESSES), $(eval $(call BuildDotNetPlatformAssembly,$(platform),$(shell echo $(platform) | tr '[:lower:]' '[:upper:]'),$(shell echo $(platform) | tr '[:upper:]' '[:lower:]'),$(bitness)))))
 
+define DefineTargets
+$(1)_NUGET_TARGETS = \
+	$(DOTNET_DESTDIR)/$($(1)_NUGET_SDK_NAME)/tools/TrimAttributes.LinkDescription.xml \
+
+$(DOTNET_DESTDIR)/$($(1)_NUGET_SDK_NAME)/tools/TrimAttributes.LinkDescription.xml: TrimAttributes.LinkDescription.xml
+	$$(Q) $(CP) $$< $$@
+
+DOTNET_TARGETS += $$($(1)_NUGET_TARGETS)
+endef
+$(foreach platform,$(DOTNET_PLATFORMS),$(eval $(call DefineTargets,$(platform))))
+
 #
 # Global targets
 #