Proposal: Allow unsafe code in iterators

[gafter]

@stephentoub commented on Tue Apr 21 2015

Today we disallow any usage of pointers in iterators, due to the issues involved with those pointers and the associated state moving to the heap. But we allow pointers in async methods (which have similar constraints) with the restriction that awaits can't be used in an unsafe context, e.g. this is allowed:

unsafe (...)
{
    ... // no awaits
}
await ...;
unsafe (...)
{
    ... // no awaits
}

but this is not allowed:

unsafe (...)
{
    await ...;
}

We could allow unsafe code in iterators, but with a similar constraint: yield is not allowed in unsafe contexts, e.g. this would be allowed:

unsafe (...)
{
    ... // no yields
}
yield return ...;
unsafe (...)
{
    ... // no yields
}

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@paulomorgado commented on Tue Apr 28 2015

That makes sense!

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@whoisj commented on Mon May 11 2015

how do I thumbs up this?

er... +1!

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@agocke commented on Fri May 13 2016

@stephentoub Seems fine to me.

More broadly, do you know of any CLR restrictions about spilling unmanaged pointers across the state machine? AFAIK, that seems the be the main restriction around async and unsafe, so theoretically the compiler could support spilling unmanaged pointers, as long as there are no issues with doing this in the CLR.

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@stephentoub commented on Fri May 13 2016

More broadly, do you know of any CLR restrictions about spilling unmanaged pointers across the state machine?

Are you asking about enabling not just:

unsafe
{
    ... // no yields
}
yield return ...;
unsafe
{
    ... // no yields
}

but also:

unsafe
{
    ...
    yield return ...;
    ...
}

?

If so, even if it could be made to work (e.g. storing the pointers into the state machine), I think it would be dangerous, as it'd be incredibly easy to write bad code, e.g.

unsafe
{
    int* state = stackalloc int[1];
    state[0] = 42;
    yield return ...;
    Use(state[0]);
}

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@agocke commented on Fri May 13 2016

@stephentoub Yup, very dangerous, but it is unsafe, to be fair.

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@agocke commented on Fri May 13 2016

Certain things, like fixed, would still have to be prohibited outright, obviously.