Box ErrorKind inside Error

[emilk]

Related

• Closes Box quiver::Error (or its ErrorKind) so it can be embedded without boxing at every call site #28

What

Error was 96 bytes. It rides in the Err arm of every Result in the crate, and embedders assert on the size of error enums that hold it. kind is now a Box<ErrorKind>, taking Error to 24 bytes. The allocation only happens on the cold error path.

The two three-field variants set the old size: WrongDatatype and WrongArrayType each hold column: String + expected: String + actual: DataType = 72 bytes, plus the discriminant. Arrow's DataType is only 24 bytes, so it was never the culprit.

• Error::new(record_type, kind) boxes for you, so no construction site names Box.
• ErrorKind and all its variants are unchanged.
• A const _: () = assert!(size_of::<Error>() <= 24, …) keeps it from growing back.

Breaking: Error { kind } can no longer be pattern-matched through. Match *err.kind (or &*err.kind) instead, as the tests here now do.

Testing

Existing tests, updated for the new match shape. The compile-time assert is the regression test; I checked it fires by tightening it to 16.

Compatibility

Breaking for anyone destructuring Error, which is why it goes into 0.6.

🤖 Generated with Claude Code

[emilk]

Post-merge review, as part of reviewing the 0.6 stack for #30. Clippy and tests are clean at the tip of release-0.6. 96 → 24 bytes for a type that rides in every Err arm is worth the break, and the compile-time size assert is the right regression test.

Error::source() is still None, and the From<Error> for ArrowError doc claims otherwise. kind has no #[source], so the chain stops dead at Error. Verified at the tip of release-0.6 with a try_newtype_datatype! failure: err.source().is_none() is true, and the NotEven cause that ErrorKind::Conversion carefully stores as #[source] Box<dyn Error + Send + Sync> is unreachable programmatically — only its Display text survives, twice, through ArrowError::ExternalError. Meanwhile the doc says "The error is preserved (including its source chain)". #[source] pub kind: Box<ErrorKind> fixes the gap and the doc in one line (thiserror allows a field to be both interpolated and the source).

Box is now part of the public API. The break was unavoidable; the box being permanent wasn't. A private field with kind(&self) -> &ErrorKind / into_kind(self) gets the same 24 bytes, reads better at match sites, and lets the representation change again without a third breaking release. You were already spending the break. It would also collapse the tests, where matches!(*err.kind, …) now runs past 120 characters at ~15 call sites.

— Claude