neilconway opened a new issue, #10946:
URL: https://github.com/apache/arrow-rs/issues/10946
### Describe the bug
Casting decimal -> decimal with a much larger scale can produce incorrect
values, including panics in debug mode.
### To Reproduce
```rust
use arrow_array::{Array, Decimal256Array};
use arrow_buffer::i256;
use arrow_cast::{cast_with_options, CastOptions};
use arrow_schema::DataType;
let array = Decimal256Array::from(vec![
i256::from_i128(1),
i256::from_i128(10_i128.pow(16)),
i256::from_i128(10_i128.pow(17)),
])
.with_precision_and_scale(76, 0)
.unwrap();
// Strict mode; safe mode returns the same values (no nulls, no error)
let options = CastOptions { safe: false, ..Default::default() };
let result = cast_with_options(&array, &DataType::Decimal256(76, 60),
&options).unwrap();
```
Results:
```
1 ->
1000000000000000000000000000000000000000000000000000000000000
10000000000000000 ->
10000000000000000000000000000000000000000000000000000000000000000000000000000
100000000000000000 ->
-15792089237316195423570985008687907853269984665640564039457584007913129639936
```
The first result is correct.
The second result exceeds the maximum value representable at precision 76
but is returned without error.
The third result is just wrong.
### Expected behavior
_No response_
### Additional context
_No response_
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