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by quotemstr 244 days ago
It's not an implementation thing. It's fundamental. A GC can't move anything it finds in a conservative root. You can build partly precise hybrid GCs (I've built a few) but the mere possibility of conservative roots complicates implementation and limits compaction potential.

If, OTOH, Alloy is handle based, then maybe there's hope. Still a weird choice to use Rust this way.
1 comments
ltratt 244 days ago
We don't exactly want Alloy to have to be conservative, but Rust's semantics allow pointers to be converted to usizes (in safe mode) and back again (in unsafe mode), and this is something code really does. So if we wanted to provide an Rc-like API -- and we found reasonable code really does need it -- there wasn't much choice.

I don't think Rust's design in this regard is ideal, but then again what language is perfect? I designed languages for a long while and made far more, and much more egregious, mistakes! FWIW, I have written up my general thoughts on static integer types, because it's a surprisingly twisty subject for new languages https://tratt.net/laurie/blog/2021/static_integer_types.html

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quotemstr 244 days ago
> We don't exactly want Alloy to have to be conservative, but Rust's semantics allow pointers to be converted to usizes (in safe mode) and back again (in unsafe mode), and this is something code really does. So if we wanted to provide an Rc-like API -- and we found reasonable code really does need it -- there wasn't much choice.

You can define a set of objects for which this transformation is illegal --- use something like pin projection to enforce it.

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ltratt 244 days ago
The only way to forbid it would be to forbid creating pointers from `Gc<T>`. That would, for example, preclude a slew of tricks that high performance language VMs need. That's an acceptable trade-off for some, of course, but not all.
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quotemstr 244 days ago
Not necessarily. It would just require that deriving these pointers be done using an explicit lease that would temporarily defer GC or lock an object in place during one. You'd still be able to escape from the tyranny of conservative scanning everything.
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