mbutrovich commented on issue #10947:
URL: https://github.com/apache/arrow-rs/issues/10947#issuecomment-6201501401
The same panic also happens when every valid value fits the declared
precision and only the data under a null slot does not:
```rust
use std::sync::Arc;
use arrow_array::{ArrayRef, Decimal128Array};
use arrow_buffer::{NullBuffer, ScalarBuffer};
use arrow_cast::cast::{cast_with_options, CastOptions};
use arrow_schema::DataType;
fn main() {
// [1, null], where the null slot holds a value that does not fit i64
let values = ScalarBuffer::from(vec![1i128, i128::MAX]);
let nulls = NullBuffer::from(vec![true, false]);
let a: ArrayRef = Arc::new(Decimal128Array::new(values,
Some(nulls)).with_precision_and_scale(5, -2).unwrap());
let options = CastOptions { safe: true, ..Default::default() };
let _ = cast_with_options(&a, &DataType::Decimal64(18, 0), &options);
// panics: called `Option::unwrap()` on a `None` value
}
```
`make_upscaler` chooses the infallible closure when `input_precision +
delta_scale <= output_precision`, and that closure unwraps `from_decimal` for
every slot
([decimal.rs](https://github.com/apache/arrow-rs/blob/72887ed91c80002bb7ec755ce1e964bab07bebea/arrow-cast/src/cast/decimal.rs#L186-L194)).
`unary` applies it to null slots too, whose values the spec leaves unspecified
([Columnar.rst](https://github.com/apache/arrow/blob/fcf8f3014227cafe24deeb044c2d51f5fc9902e5/docs/source/format/Columnar.rst?plain=1#L394)).
Should the infallible path be taken only when the array has no nulls, or apply
the fallible closure to null slots?
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