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by rspeer 3399 days ago
Okay, once again I try to find an example from my code, and once again it turns out that Rust makes it simple in the simple case, but the more complex cases are still confusing.

If I change the test value Some("zh".to_string()) into Some("zh"), it points to that line and tells me:

    expected struct `std::string::String`, found &str
Sure, it's a different situation because the value is wrapped in an Option. But if String and &str are truly compatible, I would never expect to see that error message.
1 comments
steveklabnik 3399 days ago
Ah yes. So this is an area where the diagnostic is _slightly_ misleading; it's trying to point out that you have two different types, and that that's the difference between the two of them. It's not that they can't be compared. Maybe a bug should be filed...

String and &str can normally be compared because of Deref, that is, &String derefs to &str. Option, on the other hand, does not implement Deref, and so no coercion happens. Rust doesn't do a lot of coercions, but Deref is one of the bigger ones.

Back to the _actual_ topic at hand, I can see how this can be a pain point until you know the rules, though. :/

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JoshTriplett 3399 days ago
> String and &str can normally be compared because of Deref, that is, &String derefs to &str. Option, on the other hand, does not implement Deref, and so no coercion happens.

For this particular case, any particular reason we couldn't add an impl of PartialEq? In fact, once we have specialization, couldn't we have a general impl of PartialEq for Options of Deref types?

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steveklabnik 3399 days ago 

  impl<T> PartialEq<Option<T>> for Option<T> where T: PartialEq<T>
does exist, but I'm not 100% sure why Deref doesn't kick in here, I just know that it doesn't.

> couldn't we have a general impl of PartialEq for Options of Deref types?

I think the issue is None. You'd get a null pointer, which doesn't make any sense in safe rust.

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JoshTriplett 3399 days ago
> I think the issue is None. You'd get a null pointer, which doesn't make any sense in safe rust.

No, I don't mean an impl between Option<T> and T, I mean a bidirectional impl of PartialEq between Option<T> and Option<U> where U is T's Deref::Target.

In that case, None doesn't cause an issue; None == None, and Some(t) == Some(u) iff *t == u

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steveklabnik 3399 days ago
Ah right, yes.
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