NativeAOT: Consider enabling trim/AOT/single-file warnings by default

[ivanpovazan]

Since NativeAOT is much more restrictive in terms of trimming and AOT compatibility of compiled applications (and used frameworks) compared to Mono, we should enable trimming, AOT and single-file warnings by default. Especially for the fact that NativeAOT does not have a safety - fallback mechanism like Mono's interpreter.

Additionally, since we will release NativeAOT on iOS as an experimental feature, enabling these warnings (and treating them as errors) would let us detect issues with our frameworks and compiled applications, which will enable us to address them early and improve the AOT experience for our customers.

[MichalStrehovsky]

If we don't do anything to break this, adding <PublishAot>true</PublishAot> to the project should already enable these analyzers (we should validate).

To validate, one should be able to create a new MAUI project in VS and add:

public class Class1
{
    static Type t;

    static void Frob()
        => t.MakeGenericType(Array.Empty<Type>());
}

To the .cs somewhere. You should see IL3050 and IL2055 warnings when you build (and green squiggles in VS).

[ivanpovazan]

If we don't do anything to break this, adding true to the project should already enable these analyzers (we should validate).

If I did not miss anything, there are two places/properties which affect what can be currently seen in the build output:

• Xamarin suppresses trim warnings by default:
  https://github.com/xamarin/xamarin-macios/blob/d1d3dcdf68928303f8a00719a26dc1844b8567d9/dotnet/targets/Xamarin.Shared.Sdk.props#L23
• ILLink produces a single warning per assembly https://github.com/dotnet/runtime/blob/2c4b4c0831d1d9f505ff5d3055c38bb41646021b/src/tools/illink/src/ILLink.Tasks/build/Microsoft.NET.ILLink.targets#L225

Building a MAUI template application with how the analysers are currently configured (from above) the build log does not include precise information:

/Users/ivan/repos/xamarin/xamarin-macios/builds/downloads/dotnet-sdk-8.0.100-preview.7.23360.1/MauiTest/obj/Release/net8.0-ios/ios-arm64/linked/Microsoft.iOS.dll : warning IL3053: Assembly 'Microsoft.iOS' produced AOT analysis warnings. [/Users/ivan/repos/xamarin/xamarin-macios/builds/downloads/dotnet-sdk-8.0.100-preview.7.23360.1/MauiTest/MauiTest.csproj]
/Users/ivan/repos/xamarin/xamarin-macios/builds/downloads/dotnet-sdk-8.0.100-preview.7.23360.1/MauiTest/obj/Release/net8.0-ios/ios-arm64/linked/Microsoft.Maui.Controls.Xaml.dll : warning IL3053: Assembly 'Microsoft.Maui.Controls.Xaml' produced AOT analysis warnings. [/Users/ivan/repos/xamarin/xamarin-macios/builds/downloads/dotnet-sdk-8.0.100-preview.7.23360.1/MauiTest/MauiTest.csproj]
/Users/ivan/repos/xamarin/xamarin-macios/builds/downloads/dotnet-sdk-8.0.100-preview.7.23360.1/MauiTest/obj/Release/net8.0-ios/ios-arm64/linked/Microsoft.Maui.dll : warning IL3053: Assembly 'Microsoft.Maui' produced AOT analysis warnings. [/Users/ivan/repos/xamarin/xamarin-macios/builds/downloads/dotnet-sdk-8.0.100-preview.7.23360.1/MauiTest/MauiTest.csproj]
/Users/ivan/repos/xamarin/xamarin-macios/builds/downloads/dotnet-sdk-8.0.100-preview.7.23360.1/MauiTest/obj/Release/net8.0-ios/ios-arm64/linked/System.Linq.Expressions.dll : warning IL3053: Assembly 'System.Linq.Expressions' produced AOT analysis warnings. [/Users/ivan/repos/xamarin/xamarin-macios/builds/downloads/dotnet-sdk-8.0.100-preview.7.23360.1/MauiTest/MauiTest.csproj]

To get the full analysis log, we would have to pass -p:SuppressTrimAnalysisWarnings=false -p:TrimmerSingleWarn=false

Another thing worth considering is enabling these properties only for the ILC pass, and not for ILLink, to avoid getting duplicate warnings from the two trimming tools, or even incorrect warnings due to ILLink-ILC compatibility and difference in trimming capabilities.

/cc: @vitek-karas

[ivanpovazan]

Here are the list of warnings for two different apps built with Xamarin+NativeAOT with analyzers enabled:

• dotnet new ios app: https://gist.github.com/ivanpovazan/930ccc4dd4199c449f0987a210e89a8f
• dotnet new maui app: https://gist.github.com/ivanpovazan/640891a7fe619fac3f4be982a928bdfc

Apps were built with the current state of: https://github.com/xamarin/xamarin-macios/tree/release/8.0.1xx-preview7

[MichalStrehovsky]

Would it be possible to at least address the ones in new ios?

MAUI is probably a lost cause for .NET 8 and there will be warnings to tell people they're not getting a great experience. It would be nice if we could address at least those that are in an empty app but I don't know if it's possible.

To get the full analysis log, we would have to pass -p:SuppressTrimAnalysisWarnings=false -p:TrimmerSingleWarn=false

We should disable warning suppression in the SDK. TrimmerSingleWarn is useful to turn off for us, but brings no value to customers (customer knowing a MAUI assembly is not trim safe is better than spamming them with a dozen warnings they can do nothing about because they don't have MAUI source code they can edit).

[vitek-karas]

+1 on leaving TrimmerSingleWarn=true, it's a much better experience.
+10 for trying to make the default template for ios warning free, but it might be too difficult

In general I think it would make sense to suppress all trim analysis warnings from the linker, not only due to duplicates but also due to possible differences (the tools will run with different input and settings). For correctness those from the NativeAOT should be sufficient (and more precise).

[rolfbjarne]

We have these two issues already:

• Audit reflection usage in our assemblies #10405
• Don't suppress trim analysis warnings when linking all assemblies #11246