[GitHub] [arrow-rs] carols10cents commented on issue #655: Allow `collect`ing from Values as well as `Option`

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carols10cents commented on issue #655:
URL: https://github.com/apache/arrow-rs/issues/655#issuecomment-899917303


   So this doesn't work because specialization isn't done yet... the problem is that if these two trait implementations:
   
   ```
   impl<T: ArrowPrimitiveType, Ptr: Borrow<Option<<T as ArrowPrimitiveType>::Native>>>
       FromIterator<Ptr> for PrimitiveArray<T>
   { ... }
   
   impl<T: ArrowPrimitiveType, Ptr: Borrow<<T as ArrowPrimitiveType>::Native>>
       FromIterator<Ptr> for PrimitiveArray<T>
   { ... }
   ```
   
   were allowed, then someone could come along and add:
   
   ```
   impl Borrow<Option<i32>> for Integer32 { ... }
   impl Borrow<i32> for Integer32 { ... }
   ```
   
   and then Rust wouldn't know which `FromIterator` implementation to use. 
   
   @shepmaster helped with this answer; I only got as far as "I think it's specialization related, but I don't know why" ;) 
   
   He also pointed me to https://stackoverflow.com/questions/37347311/how-is-there-a-conflicting-implementation-of-from-when-using-a-generic-type, perhaps that will be of some help to someone.
   
   I'm working with @shepmaster on a way around this but we might not be able to finish it until Wednesday.


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