Enable runtime-async feature switch for NativeAOT System.Private.CoreLib

[Copilot]

Description

Enables the runtime-async compiler feature for the core libraries when building for Native AOT.

Changes

• Added <Features>$(Features);runtime-async=on</Features> to NativeAOT System.Private.CoreLib csproj
• Suppressed CA2252 (preview features usage) since runtime-async=on causes the compiler to inject AsyncHelpers.Await calls which are marked [RequiresPreviewFeatures]

<!-- Enable runtime async for Native AOT -->
<PropertyGroup>
  <Features>$(Features);runtime-async=on</Features>
  <NoWarn>$(NoWarn);CA2252</NoWarn>
</PropertyGroup>

Notes

The CA2252 suppression is project-scoped because runtime-async=on transforms all async methods via compiler-generated calls—individual method suppression isn't practical or meaningful here. This is consistent with how other runtime-async-enabled projects handle it (e.g., tests disable analyzers entirely).

Original prompt

I'd like to enable the runtimeasync feature switch for the core libraries if and only if we're building for native aot. I think we might need to plumb down another msbuild switch.

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[VSadov]

I had to suppress <NoWarn>$(NoWarn);SYSLIB5007</NoWarn> in #119432 as well. Possibly just a different way to suppress the same build errors?

I wonder what is the scenario when we see AsyncHelpers.Await marked as preview features?
I think they are no longer marked as experimental.

[VSadov]

My theory was that we were building something against net10, perhaps temporarily for some build/codeflow related reasons.

But maybe the reason is something else.

I did not look deeply, just disabled the warnings to get them out of the way, but I kind of wonder why we need to do this.

[MichalStrehovsky]

Isn't this because we mark AsyncHelpers as RequiresPreviewFeatures?

runtime/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncHelpers.cs

Lines 72 to 73 in 84263e2

	[RequiresPreviewFeatures]
	public static T Await<T>(Task<T> task)

[VSadov]

Ah. There is another marker.

@agocke - perhaps it is time to remove [RequiresPreviewFeatures]?
I assume we do not plan to ship with this?