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namespace System.Runtime.InteropServices;

public static class ImmutableCollectionsMarshal
{
    public static ImmutableArray<T> AsImmutableArray<T>(T[]? array);
    public static T[]? AsArray<T>(ImmutableArray<T> array);
}

Thanks!

Can you please also delete https://github.com/dotnet/runtime/blob/0c612cf7c36f53651e646d62d3bb362823880c00/src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/ImmutableArrayFactory.cs and fix up the uses to use the new public API instead?

Ooh, missed that one, thank you! Done in 3f53f2c.
Also removed all unsafe tricks since I just realized internal members are available now that the API lives here 😄