Fix struct cast panic in rule and add tests (#6962)

## Summary

Closes: #6933

## Testing

Adds a regression test as well as a few other tests to confirm this is
correct.

Signed-off-by: Connor Tsui <connor.tsui20@gmail.com>

Author: connortsui20

vortex-array/src/arrays/struct_/compute/rules.rs

@@ -51,7 +51,9 @@ impl ArrayParentReduceRule<Struct> for StructCastPushDownRule {
         parent: ScalarFnArrayView<Cast>,
         _child_idx: usize,
     ) -> VortexResult<Option<ArrayRef>> {
-        let target_fields = parent.options.as_struct_fields();
+        let Some(target_fields) = parent.options.as_struct_fields_opt() else {
+            return Ok(None);
+        };
         let mut new_fields = Vec::with_capacity(target_fields.nfields());
 
         for (target_name, target_dtype) in target_fields.names().iter().zip(target_fields.fields())
@@ -136,6 +138,8 @@ impl ArrayParentReduceRule<Struct> for StructGetItemRule {
 
 #[cfg(test)]
 mod tests {
+    use vortex_buffer::buffer;
+
     use crate::IntoArray;
     use crate::arrays::StructArray;
     use crate::arrays::VarBinViewArray;
@@ -146,6 +150,7 @@ mod tests {
     use crate::dtype::DType;
     use crate::dtype::FieldNames;
     use crate::dtype::Nullability;
+    use crate::dtype::PType;
     use crate::dtype::StructFields;
     use crate::scalar::Scalar;
     use crate::validity::Validity;
@@ -173,7 +178,8 @@ mod tests {
             Nullability::NonNullable,
         );
 
-        // Use ArrayBuiltins::cast which goes through the optimizer and applies StructCastPushDownRule
+        // Use `ArrayBuiltins::cast` which goes through the optimizer and applies
+        // `StructCastPushDownRule`.
         let result = source.into_array().cast(target).unwrap().to_struct();
         assert_arrays_eq!(
             result.unmasked_field_by_name("a").unwrap(),
@@ -188,4 +194,124 @@ mod tests {
             ConstantArray::new(Scalar::null(utf8_null), 1)
         );
     }
+
+    /// Regression test: casting a struct to a non-struct DType must not panic. Previously,
+    /// `StructCastPushDownRule` called `as_struct_fields()` which panics on non-struct types.
+    #[test]
+    fn cast_struct_to_non_struct_does_not_panic() {
+        let source = StructArray::try_new(
+            FieldNames::from(["x"]),
+            vec![buffer![1i32, 2, 3].into_array()],
+            3,
+            Validity::NonNullable,
+        )
+        .unwrap();
+
+        // Casting a struct to a primitive type should not panic. Before the fix,
+        // `StructCastPushDownRule` would panic via `as_struct_fields()` on the non-struct target.
+        let result = source
+            .into_array()
+            .cast(DType::Primitive(PType::I32, Nullability::NonNullable));
+        // Whether this errors or succeeds depends on execution, but the key invariant is that the
+        // optimizer rule does not panic.
+        if let Ok(arr) = &result {
+            assert_eq!(
+                arr.dtype(),
+                &DType::Primitive(PType::I32, Nullability::NonNullable)
+            );
+        }
+    }
+
+    #[test]
+    fn cast_struct_drop_field() {
+        // Casting to a struct with a subset of fields should succeed.
+        let source = StructArray::try_new(
+            FieldNames::from(["a", "b", "c"]),
+            vec![
+                buffer![1i32, 2, 3].into_array(),
+                buffer![10i64, 20, 30].into_array(),
+                buffer![100u8, 200, 255].into_array(),
+            ],
+            3,
+            Validity::NonNullable,
+        )
+        .unwrap();
+
+        let target = DType::Struct(
+            StructFields::new(
+                FieldNames::from(["a", "c"]),
+                vec![
+                    DType::Primitive(PType::I32, Nullability::NonNullable),
+                    DType::Primitive(PType::U8, Nullability::NonNullable),
+                ],
+            ),
+            Nullability::NonNullable,
+        );
+
+        let result = source.into_array().cast(target).unwrap().to_struct();
+        assert_eq!(result.unmasked_fields().len(), 2);
+        assert_arrays_eq!(
+            result.unmasked_field_by_name("a").unwrap(),
+            buffer![1i32, 2, 3].into_array()
+        );
+        assert_arrays_eq!(
+            result.unmasked_field_by_name("c").unwrap(),
+            buffer![100u8, 200, 255].into_array()
+        );
+    }
+
+    #[test]
+    fn cast_struct_field_type_widening() {
+        // Casting struct fields to wider types (i32 -> i64).
+        let source = StructArray::try_new(
+            FieldNames::from(["val"]),
+            vec![buffer![1i32, 2, 3].into_array()],
+            3,
+            Validity::NonNullable,
+        )
+        .unwrap();
+
+        let target = DType::Struct(
+            StructFields::new(
+                FieldNames::from(["val"]),
+                vec![DType::Primitive(PType::I64, Nullability::NonNullable)],
+            ),
+            Nullability::NonNullable,
+        );
+
+        let result = source.into_array().cast(target).unwrap().to_struct();
+        assert_eq!(
+            result.unmasked_field_by_name("val").unwrap().dtype(),
+            &DType::Primitive(PType::I64, Nullability::NonNullable)
+        );
+        assert_arrays_eq!(
+            result.unmasked_field_by_name("val").unwrap(),
+            buffer![1i64, 2, 3].into_array()
+        );
+    }
+
+    #[test]
+    fn cast_struct_add_non_nullable_field_fails() {
+        // Adding a non-nullable field via cast should fail.
+        let source = StructArray::try_new(
+            FieldNames::from(["a"]),
+            vec![buffer![1i32].into_array()],
+            1,
+            Validity::NonNullable,
+        )
+        .unwrap();
+
+        let target = DType::Struct(
+            StructFields::new(
+                FieldNames::from(["a", "b"]),
+                vec![
+                    DType::Primitive(PType::I32, Nullability::NonNullable),
+                    DType::Primitive(PType::I32, Nullability::NonNullable),
+                ],
+            ),
+            Nullability::NonNullable,
+        );
+
+        assert!(source.into_array().cast(target).is_err());
+    }
 }