[Immanuel-Fowler]

What

Arrays and structs cross the sandbox boundary, under the rules the C ABI adopted in 0.14.0:

• int[], float[], bool[] cross as a pointer and a count into guest memory.
• A struct whose fields are numbers, booleans, or such structs crosses as its #[repr(C)] bytes, written field by field at offset_of! offsets with padding zero, so neither side reads padding or depends on the other's endianness.
• What the host writes for a call, the host frees after it; what the guest returns, the host copies and frees.
• The guest allocator is 8-byte aligned. Handler state may be of these types, and a handler may return it.

orchestrate check rejects a handler type that cannot cross, by name, as the compiler's own error. It used to surface from the guest crate's cargo build.

Also fixed on the way

• Every string argument leaked guest memory. The host allocated it in the guest and never freed it. Under an 8mb cap, 20,000 one-kilobyte calls failed four times, each failure resetting the guest's state. A regression test sends those calls.
• A handler could not return its own array or string state on any boundary: return seen moved the value out of the actor, the child, or the guest and rustc refused (E0382 in-process, E0507 in the guest). A returned state binding is now a copy. Snapshots for the secret child and the sandbox guest change by one .clone() each.

What still does not cross

An array of strings or structs, or a struct holding one, crosses the in-process, secret, and landline boundaries but not the sandbox. LIMITATIONS-notes.md (new, at the root) carries the per-boundary table and everything else found on this branch, for the paper's limitations section.

Tests

• runtime_sandbox_serverlet_carries_arrays_and_structs: arrays and nested structs both ways, array state, a string beside an array in one call, an empty array.
• runtime_sandbox_frees_what_the_host_writes: 20,000 string and array calls under an 8mb cap.
• runtime_serverlet_handler_returns_its_state: in-process, secret, and sandboxed serverlets returning array and string state.
• tests/error_cases/sandbox_unsupported_type.orch: the check-time rejection.
• Snapshots regenerated and reviewed: sandbox_guest (codec and helpers), secret_serverlet (one .clone()).

Changelog entries are under [Unreleased]. No version bump.

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