Relax CS4012 on ROS with the scoped keyword

[RenderMichael]

Currently, the line

async void MyMethod()
{
    ReadOnlySpan<char> sessionId = "myString".AsSpan();
}

throws a compiler error CS4012: Parameters or locals of type 'ReadOnlySpan<char>' cannot be declared in async methods or lambda expressions

I propose that this be relaxed with the scoped keyword

async void MyMethod()
{
    scoped ReadOnlySpan<char> sessionId = "myString".AsSpan();
}

[svick]

It's not clear to me what is the proposed behavior when it comes to crossing awaits or what is the reason for the scoped keyword, but this looks like a duplicate of #4687.

[RenderMichael]

This is for the happy-path scenario of uncrossed awaits.

Being accessed on the other side of an await would force it onto the heap, using scoped here overloads the concept to refer to "not accessed outside of this region". Much like how scoped is normally used, it's just to ensure the span is being used properly. But it is strictly unnecessary.

Thanks for pointing me to #4687, except for my proposed solution this is a duplicate