Chore: use try_new for struct array into_arrow (#5159)

plus some comments

---------

Signed-off-by: Connor Tsui <[email protected]>

Author: connortsui20

vortex-compute/src/arrow/struct_.rs

@@ -6,29 +6,43 @@ use std::sync::Arc;
 use arrow_array::{ArrayRef, StructArray};
 use arrow_schema::{Field, Fields};
 use vortex_error::VortexResult;
-use vortex_vector::StructVector;
+use vortex_vector::{StructVector, VectorOps};
 
 use crate::arrow::IntoArrow;
 
 impl IntoArrow<ArrayRef> for StructVector {
     fn into_arrow(self) -> VortexResult<ArrayRef> {
+        let len = self.len();
         let (fields, validity) = self.into_parts();
         let arrow_fields = fields
             .iter()
             .map(|field| field.clone().into_arrow())
             .collect::<VortexResult<Vec<ArrayRef>>>()?;
 
-        // We need to make up the field names since vectors are unnamed.
+        // We need to make up the field names since vectors are unnamed, so we just use the field
+        // indices.
         let fields = Fields::from(
             (0..arrow_fields.len())
-                .map(|i| Field::new(i.to_string(), arrow_fields[i].data_type().clone(), true))
+                .map(|i| {
+                    Field::new(
+                        i.to_string(),
+                        arrow_fields[i].data_type().clone(),
+                        true, // Vectors are always nullable.
+                    )
+                })
                 .collect::<Vec<Field>>(),
         );
 
-        Ok(Arc::new(StructArray::new(
-            fields,
-            arrow_fields,
-            validity.into_arrow()?,
-        )))
+        // SAFETY: Since all of these components came from a valid `StructVector`, we know that all
+        // of the lengths of the vectors are correct. Additionally, all extra metadata is directly
+        // derived from the existing components so all invariants are upheld.
+        Ok(Arc::new(unsafe {
+            StructArray::new_unchecked_with_length(
+                fields,
+                arrow_fields,
+                validity.into_arrow()?,
+                len,
+            )
+        }))
     }
 }