[quangvdao]

#[derive(KernelSlots)] now rejects a malformed PrepareKernel slot at its declaration instead of skipping it.

Why

The derive emitted one PrepareKernel<F, R> impl per field of type exactly Box<dyn PrepareKernel<F, R>> and skipped every other field. A slot written with an extra object bound (+ Send), a wrapper (Option<Box<…>>) or the wrong number of type arguments was skipped too, so the registry silently lost that impl. The error then appeared as an unsatisfied PrepareKernel bound at whichever stage impl needed the slot, far from the field that caused it.

What

• jolt-kernels-derive: a field whose type mentions the identifier PrepareKernel anywhere but is not exactly Box<dyn PrepareKernel<F, R>> is a derive error spanned at that field's type: "a KernelSlots slot must be exactly Box<dyn PrepareKernel<F, R>>". All such fields are reported in one pass (syn::Error::combine). Fields that never mention PrepareKernel (commit, joint_opening, the fronts) are still skipped. The emitted impls for valid slots are unchanged.
• The check is syntactic. A slot spelled through a type alias or a renamed import never mentions PrepareKernel, so it is still skipped and still surfaces as a missing bound. The crate docs, the PrepareKernel trait docs and specs/prover-stage-drivers.md now say this.

Tests

crates/jolt-kernels-derive/src/tests.rs (new) calls expand directly. It checks that an extra object bound, an Option wrapper and a one-element tuple are each rejected with the message above, and that a struct with one valid slot, a three-argument slot, a bare PrepareKernel field and an ordinary field reports exactly two errors. It also checks that a valid slot next to a &'static str field and a bespoke boxed trait field expands to exactly one PrepareKernel<F, R> impl.

Verification

cargo fmt --all --check; cargo clippy -p jolt-kernels-derive -p jolt-kernels -p jolt-prover --all-targets -- -D warnings (JoltBackend in jolt-kernels uses the derive); cargo nextest run -p jolt-kernels-derive (5 passed) and -p jolt-kernels (152 passed).

[github-actions]

📏 PR diff

Note

Actual code changed: 61 lines

🟢 +44 added    🔴 −17 removed

Tests, docs, fixtures, and helper scripts excluded.

Category	🟢 Added	🔴 Removed
⌨️ Actual code	+44	−17
🧪 Tests	+88	0
📚 Docs	+20	−15
📦 Fixtures	0	0
🔧 Helper scripts	0	0
Total diff	+152	−32

_{Every changed line is classified once.}