Implement AsRef for Schema and Field (apache#8417)

# Which issue does this PR close?

- closes apache#5819

# Rationale for this change

This allows writing generic code that accepts `&Field`, `Arc<Field>` or
`&Arc<Field>`, or iterators of these. Same for `Schema`.

# What changes are included in this PR?

`impl AsRef<Field> for Field`
`impl AsRef<Schema> for Schema`

as suggested by `AsRef` own docs

# Are these changes tested?

yes

# Are there any user-facing changes?

it does not break API. Downstream crates cannot implement `AsRef<Field>
for Field`

Downstream crates cannot workaround these either -
apache#5819 (comment)

Author: findepi

arrow-schema/src/field.rs

@@ -172,6 +172,12 @@ impl Hash for Field {
     }
 }
 
+impl AsRef<Field> for Field {
+    fn as_ref(&self) -> &Field {
+        self
+    }
+}
+
 impl Field {
     /// Default list member field name
     pub const LIST_FIELD_DEFAULT_NAME: &'static str = "item";
@@ -1259,6 +1265,36 @@ mod test {
         assert!(f1.cmp(&f3).is_lt());
     }
 
+    #[test]
+    #[expect(clippy::needless_borrows_for_generic_args)] // intentional to exercise various references
+    fn test_field_as_ref() {
+        let field = || Field::new("x", DataType::Binary, false);
+
+        // AsRef can be used in a function accepting a field.
+        // However, this case actually works a bit better when function takes `&Field`
+        fn accept_ref(_: impl AsRef<Field>) {}
+
+        accept_ref(field());
+        accept_ref(&field());
+        accept_ref(&&field());
+        accept_ref(Arc::new(field()));
+        accept_ref(&Arc::new(field()));
+        accept_ref(&&Arc::new(field()));
+
+        // AsRef can be used in a function accepting a collection of fields in any form,
+        // such as &[Field], or &[Arc<Field>]
+        fn accept_refs(_: impl IntoIterator<Item: AsRef<Field>>) {}
+
+        accept_refs(vec![field()]);
+        accept_refs(vec![&field()]);
+        accept_refs(vec![Arc::new(field())]);
+        accept_refs(vec![&Arc::new(field())]);
+        accept_refs(&vec![field()]);
+        accept_refs(&vec![&field()]);
+        accept_refs(&vec![Arc::new(field())]);
+        accept_refs(&vec![&Arc::new(field())]);
+    }
+
     #[test]
     fn test_contains_reflexivity() {
         let mut field = Field::new("field1", DataType::Float16, false);

arrow-schema/src/schema.rs

@@ -546,13 +546,38 @@ impl Hash for Schema {
     }
 }
 
+impl AsRef<Schema> for Schema {
+    fn as_ref(&self) -> &Schema {
+        self
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use crate::datatype::DataType;
     use crate::{TimeUnit, UnionMode};
 
     use super::*;
 
+    #[test]
+    #[expect(clippy::needless_borrows_for_generic_args)] // intentional to exercise various references
+    fn test_schema_as_ref() {
+        fn accept_ref(_: impl AsRef<Schema>) {}
+
+        let schema = Schema::new(vec![
+            Field::new("name", DataType::Utf8, false),
+            Field::new("address", DataType::Utf8, false),
+            Field::new("priority", DataType::UInt8, false),
+        ]);
+
+        accept_ref(schema.clone());
+        accept_ref(&schema.clone());
+        accept_ref(&&schema.clone());
+        accept_ref(Arc::new(schema.clone()));
+        accept_ref(&Arc::new(schema.clone()));
+        accept_ref(&&Arc::new(schema.clone()));
+    }
+
     #[test]
     #[cfg(feature = "serde")]
     fn test_ser_de_metadata() {