[Xamarin.Android.Build.Tasks] emit XA1040 for experimental runtimes

[jonathanpeppers]

MonoVM is currently the default (and supported!) runtime for .NET for Android. Emit XA1040 when the user opts into using the experimental runtimes, such as:

• $(UseMonoRuntime)=false CoreCLR
• $(PublishAot)=true NativeAOT

If desired, you can opt out of the warning completely with $(AndroidAllowExperimentalRuntime)=true, or use existing MSBuild mechanisms to suppress it.

[jpobst]

We could also require the user to set this like the runtime does for experimental features:

<PropertyGroup>
    <EnablePreviewFeatures>true</EnablePreviewFeatures>
</PropertyGroup>

[jonathanpeppers]

Right now we have the same Mono.Android.dll for all runtimes, so we can't really use [RequiresPreviewFeatures]:

• https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca2252#how-to-fix-violations

Are you saying to just use the same property to opt out of the warning?

[jpobst]

Are you saying to just use the same property to opt out of the warning?

Yeah, it feels like the accepted ".NET way" of doing this would be to always throw this message as an error unless the user sets $(EnablePreviewFeatures).

I realize technically this is a bit different because it's not an API/assembly decorated with [RequiresPreviewFeatures], but it feels like from a user point of view that isn't a meaningful distinction.

The .NET documented standard is $(EnablePreviewFeatures).

Granted, this is bikeshedding over naming, so treat this just as a suggestion. 😉

[jonpryor]

I like the $(EnablePreviewFeatures) suggestion. Change made, let's see how CI goes.