yongster commented on code in PR #10707:
URL: https://github.com/apache/arrow-rs/pull/10707#discussion_r3794814682
##########
arrow-cast/src/cast/mod.rs:
##########
@@ -348,6 +348,19 @@ pub fn cast(array: &dyn Array, to_type: &DataType) ->
Result<ArrayRef, ArrowErro
cast_with_options(array, to_type, &CastOptions::default())
}
+/// Convert an integer to a decimal native value without wrapping.
+///
+/// `AsPrimitive` / `as` silently truncates when the source is wider than `M`
+/// (for example `5_000_000_000i64 as i32`). All integer sources fit in `i128`,
+/// so go through that and then use [`DecimalCast`] which is range-checked.
+fn integer_to_decimal_native<I, M>(value: I) -> Option<M>
+where
+ I: NumCast,
+ M: DecimalCast,
+{
+ num_cast::<I, i128>(value).and_then(M::from_decimal)
+}
Review Comment:
Yes, the `i128` intermediate is intentional.
This helper is shared by casts to Decimal32, Decimal64, Decimal128, and
Decimal256, so the target native type `M` can be `i32`, `i64`, `i128`, or
`i256`. A direct generic `num_cast::<I, M>` is not available for all of these
targets, and `DecimalCast::from_decimal` provides the checked final
conversion.
All integer input types dispatched to this helper fit losslessly in `i128`.
I agree that the current `num_cast` expression obscures that invariant, so I
will replace it with an explicit lossless `Into<i128>` conversion.
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