Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

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namespace System
{
    public static partial class Nullable
    {
        public static ref readonly T GetValueRefOrDefaultRef<T>(in T? nullable) where T : struct;
    }
}

Note: ideally the unit tests would compare the actual address, but they can't directly access internals from corelib and I'm not sure what the recommended approach is for this other than using a dynamic method, which seems a bit overkill and also potentially problematic for eg. NativeAOT? I'd have loved to use the APIs from #23716 if they had been implemented already 😄

Let me know if the included tests are enough or if any changes are recommended in that area 👍

Thank you @stephentoub and @tannergooding for helping push this through! 😄
I'll be adding a .NET 7 target to the HighPerformance package soon to leverage this too 😋