C# 13: Allow ref and unsafe in iterators and async

C# 13 might get a new feature soon that allows ref and unsafe in iterators and async methods.

Motivation

In the current world you can not do this:

async Task MyMethodAsync()
{
    await AnAsyncMethod();
    ref int x = ref GetRef();
    DoSomething(ref x);
    await AnohterAsnycMethod();
}

The problem with await and ref is that the compiler can not guarantee that the reference is still valid after the await. But in the given case above, that shouldn't be an issue, as x is only used between two await calls, where the reference is still valid.

The same applies to ref structs like Span<T> or ReadOnlySpan<T>. You can't use them in iterators (yield) or async methods.

The proposal would exactly allow that:

async Task MyMethodAsync()
{
    var result = await AnAsyncMethod();
    ReadOnlySpan<char> span = result.AsSpan();
    DoSomething(span);
    await AnohterAsnycMethod();
}

The read more about the proposal, see the original proposal.

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