NativeAOT: Disable AggressiveAttributeTrimming with ILLink (xamarin#1…

…8545)

In the current setup with NativeAOT, during app build we run both ILLink
and ILCompiler in cascade.
When `_AggressiveAttributeTrimming` feature switch is set to `true`
ILLink removes `IsTrimmable` attribute from assemblies, which means that
when we are also running in `TrimMode=partial` (which translates to
`--defaultrooting` ILC command-line arguments) ILC trimming is disabled
completely.

This PR disables `_AggressiveAttributeTrimming` in the first pass ie
during trimming by ILLink and enables it in the second trimming pass
performed by ILCompiler.

Additionally, to workaround ILCompiler incompatibility with
`Microsoft.iOS` (and friends) this platform assembly is explicitly
rooted when passed to ILCompiler for trimming (this will be fixed once
dotnet/runtime#86649 is resolved).

Estimated savings: This change reduces the size of the application
bundle by `0,58Mb` (or ~4,3% compared to the baseline)

| MAUI ios app | Base | This PR | diff (%) |
|--------------|-----------|-----------|----------|
| SOD (Mb)     | 41,93 | 40,5      | -3,4% |
| .ipa (Mb)    | 13,43  | 12,85    | -4,3% |


Fixes: xamarin#18479

---------

Co-authored-by: Alex Soto <alex@alexsoto.me>

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -96,7 +96,10 @@
 
 		<ProjectCapability Condition="'$(_KeepLaunchProfiles)' != 'true'" Remove="LaunchProfiles" />
 	</ItemGroup>
-
+
+	<!-- Default item includes (globs and implicit references) -->
+	<Import Project="Xamarin.Shared.Sdk.DefaultItems.targets" />
+
 	<PropertyGroup>
 		<!-- Add a property that specifies the name of the platform assembly for each platform -->
 		<_PlatformAssemblyName>Microsoft.$(_PlatformName)</_PlatformAssemblyName>
@@ -132,7 +135,6 @@
 		<UseSystemResourceKeys Condition="'$(UseSystemResourceKeys)' == '' And '$(_BundlerDebug)' == 'true'">false</UseSystemResourceKeys>
 		<UseNativeHttpHandler Condition="'$(_PlatformName)' != 'macOS' And '$(UseNativeHttpHandler)' == ''">true</UseNativeHttpHandler>
 		<!-- AutoreleasePoolSupport needs to be set earlier, so that illink doesn't override it - https://github.com/dotnet/runtime/pull/86753 - so it's set in Xamarin.Shared.Sdk.props -->
-		<_AggressiveAttributeTrimming Condition="'$(_AggressiveAttributeTrimming)' == ''">true</_AggressiveAttributeTrimming>		
 		<NullabilityInfoContextSupport Condition="'$(NullabilityInfoContextSupport)' == ''">false</NullabilityInfoContextSupport>
 		<BuiltInComInteropSupport Condition="'$(BuiltInComInteropSupport)' == ''">false</BuiltInComInteropSupport>
 		<!-- Verify DI trimmability at development-time, but turn the validation off for production/trimmed builds. -->
@@ -141,6 +143,15 @@
 
 		<!-- 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>
 	</PropertyGroup>
 
 	<PropertyGroup>
@@ -152,9 +163,6 @@
 		<TargetPlatformSupported Condition=" '$(TargetPlatformIdentifier)' == '$(_PlatformName)' ">true</TargetPlatformSupported>
 	</PropertyGroup>
 
-	<!-- Default item includes (globs and implicit references) -->
-	<Import Project="Xamarin.Shared.Sdk.DefaultItems.targets" />
-
 	<PropertyGroup Condition="'$(_GlobalizationDataFileLocation)' == ''">
 		<_GlobalizationDataFileLocation Condition="'$(_UseNativeAot)' == 'true'">Resource</_GlobalizationDataFileLocation>
 		<_GlobalizationDataFileLocation Condition="'$(_UseNativeAot)' != 'true'">Assembly</_GlobalizationDataFileLocation>
@@ -1216,6 +1224,19 @@
 
 			<!-- 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="'$(_OriginalAggressiveAttributeTrimming)' != 'false' and '%(Identity)' == 'System.AggressiveAttributeTrimming'">
+				<Value>true</Value>
+				<Trim>true</Trim>
+			</RuntimeHostConfigurationOption>
+
+			<!-- Explicitly root the framework assembly due to NativeAOT trimming incompatibility tracked: https://github.com/dotnet/runtime/issues/86649 -->
+			<TrimmerRootAssembly Include="$(_PlatformAssemblyName)" />
 		</ItemGroup>
 
 		<ItemGroup>