[illink] set $(StartupHookSupport)=false *only* for Release mode

[jonathanpeppers]

Context: https://github.com/dotnet/runtime/blob/242f7b23752599f22157268de41fee91cb97ef6c/docs/design/features/host-startup-hook.md

.NET has a concept of a "startup hook", which appears to work fine
on Mono if you set:

env.txt
    DOTNET_STARTUP_HOOKS=StartupHook
(MSBuild)
    <StartupHookSupport>true</StartupHookSupport>
    <RuntimeHostConfigurationOption Include="STARTUP_HOOKS" Value="StartupHook" />

The startup hook is a managed assembly with a static method, in the
"global" namespace:

class StartupHook
{
    public static void Initialize()
    {
        // ...
    }
}

When the runtime starts, it will call StartupHook.Initialize() before
any other managed code is executed.

dotnet watch (Hot Reload) relies on this feature.

I added a test to Mono.Android-Tests to verify that the startup hook
was called.

This is not yet working on CoreCLR on Android, investigation ongoing.

[jonathanpeppers]

@rolfbjarne do you know if this works on iOS/Catalyst?

[rolfbjarne]

@rolfbjarne do you know if this works on iOS/Catalyst?

I did a quick test, and it doesn't seem to work, so I filed an issue: dotnet/macios#24492

[jonathanpeppers]

The one test failure is a flaky test, we can ignore:

[rolfbjarne]

Would it make sense to use the [UnsafeAccessor] attribute to avoid using reflection here?

[jonathanpeppers]

It looks like you can only do that if a type is public, and it's internal in System.Private.CoreLib.

[rolfbjarne]

Do you have a test to ensure this method is really trimmed away for a Release build (or when startuphook support is not enabled)?

[jonathanpeppers]

Yes, we run the new test in both trimmed, AOT, all runtimes, etc.

They have a line in dotnet/runtime to preserve it, too:

https://github.com/dotnet/runtime/blob/eaf635955da0410b94cb8607f6e598c51245f830/src/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml#L656-L663