neilconway opened a new issue, #10947:
URL: https://github.com/apache/arrow-rs/issues/10947
### Describe the bug
A decimal can hold values that exceed its declared precision. Casting such a
value to another decimal type either panics — even with safe = true, which is
documented to replace unrepresentable values with null — or silently returns a
wrapped value in both safe and strict modes, depending on the target type.
### To Reproduce
```rust
use arrow_array::Decimal128Array;
use arrow_cast::{cast_with_options, CastOptions};
use arrow_schema::DataType;
let array = Decimal128Array::from(vec![10_i128.pow(37)])
.with_precision_and_scale(5, 0)
.unwrap();
let result = cast_with_options(&array, &DataType::Decimal128(9, 2),
&options).unwrap();
// safe=true and safe=false both return incorrect raw value
// -20847100762815390390123822295304634368
// and do not indicate an error
```
Related / similar:
```rust
use arrow_array::Decimal128Array;
use arrow_cast::{cast_with_options, CastOptions};
use arrow_schema::DataType;
// Declared precision 2; the stored value has 31 digits
let array = Decimal128Array::from(vec![10_i128.pow(30)])
.with_precision_and_scale(2, 1)
.unwrap();
let result = cast_with_options(
&array,
&DataType::Decimal32(9, 2),
&CastOptions { safe: true, ..Default::default() },
);
```
with `safe=true`, this results in
```
thread 'main' panicked at arrow-cast/src/cast/decimal.rs:193:56:
called `Option::unwrap()` on a `None` value
```
### Expected behavior
Values that cannot be represented in the target type become null in safe
mode and an error with safe = false.
### Additional context
_No response_
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