Option to implement `std::convert::AsRef` for `Field`?

[chmp]

I am maintaining the serde_arrow crate. Initially I used &[Field] in my signatures, but more recently switched to the more idiomatic &[FieldRef]. I could cover both signatures with a generic F: AsRef<Field>, if arrow were to implement std::convert::AsRef<Field> for Field.

I recognize that this is a convenience feature for a single crate, but as it is easy to implement, I wanted to give it a shot :)

Required impl:

impl std::convert::AsRef<Field> for Field{
    fn as_ref(&self) -> &Field{
        self
    }
}

Context: the Rust docs contain a section on implementing AsRef for you own types.

[tustvold]

I think this is fairly harmless, I'd be apprehensive about adding it everywhere, but if it is just for one or two types I don't see an issue

[chmp]

Once thing I realized though, this change would break code that uses a blanket impl for impl <T: AsRef<Field>> Trait for T with an impl for Field directly impl Trait for Field.

[tustvold]

Will the orphan rules let you write such a blanket definition in a foreign crate?

[chmp]

I thought so. But the orphan rules explicitly prevent this scenario, as you suspected :)

use arrow::datatypes::Field;

trait Example { }

impl Example for Field { }

impl<T: AsRef<Field>> Example for T { }

   |
27 |     impl Example for Field { }
   |     ---------------------- first implementation here
28 |
29 |     impl<T: AsRef<Field>> Example for T { }
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `arrow_schema::Field`
   |
   = note: upstream crates may add a new impl of trait `std::convert::AsRef<arrow_schema::Field>` for type `arrow_schema::Field` in future versions

[findepi]

I tried to workaround lack of AsRef provided by Field.
Of course, rust won't allow me to just add impl AsRef<Field> for Field in downstream project, so I tried to get away with my own trait as a replacement for AsRef

// workaround for the fact that Field does not implement AsRef<Field>
trait AsFieldRef {
    fn as_ref(&self) -> &Field;
}

impl AsFieldRef for Field {
    fn as_ref(&self) -> &Field {
        self
    }
}

impl<T> AsFieldRef for T
where
    T: AsRef<Field>,
{
    fn as_ref(&self) -> &Field {
        self.as_ref()
    }
}

Unfortunately, this won't work either. Rust reports duplicate implementation because upstream may add impl AsRef<Field> for Field in the future.

[alamb]

There is a proposal PR here:

• Implement AsRef for Schema and Field #8417