Which one is reason and which one is the result? It seems to say, if &T is Send, then T is Sync; totally confused. Why is not: if T is Sync, then &T is Send. if T is not Sync, then &T never be Send?

"If and only if" means that both of these are true: if &T is Send, then T is Sync if T is Sync, then &T is Send Therefore both of these are also true: if T is not Sync, then &T is not Send if &T is not Send, then T is not Sync You can define Sync in terms of Send this way: If &T is Send then…

The current implementation does have a causal direction: unsafe impl<T: Sync + ?Sized> Send for &T {} i.e the property of T influences the property of &T, and not vice-versa. One could imagine it the other way though: // core: unsafe impl<T: ?Sized> Sync for T where &T: Send {} // your crate: u…

There is difference between the two. unsafe impl<T: Sync + ?Sized> Send for &T {} will not make YourType Sync when I write unsafe impl Send for &YourType {} But the later one does

Yes, they are different, and we can't change that, so the alternative is just a thought experiment. With the blanket Send for &T we have now, you're responsible for impl Sync for YourType. If we had chosen the other way with blanket Sync for T, you would write impl Send for &YourType instead. Most …

[image] cuviper: you would write impl Send for &YourType instead I think this doesn’t actually work though , even without the blanket impl in place. I’m not 100% sure though why that error exists. (Well, I do know that it’s pretty important in order to prevent something like impl Unpin for dy…

Interesting -- I wonder if that's actually an important restriction of auto traits overall, or just a limitation in how the compiler is currently able to reason about them? In the world where we would be writing manual Send for &YourType, this also implies that we'd want it automatic most of the ti…

[image] zylthinking: Which one is reason and which one is the result? I recommend this post about the relationships between Send and Sync.

T is Sync if and only if &T is Send?

help

quinedot August 27, 2021, 7:19pm 8

I recommend this post about the relationships between Send and Sync.

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T is Sync if and only if &T is Send?

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