Rust FFI layout: repr(C) on an outer struct does not repair nested fields

What it is

Rust's reference states that representation affects the padding between a type's fields, not the representation of those fields themselves. Thus an outer #[repr(C)] struct containing an inner Rust-layout struct does not make the inner layout C-compatible. A declaration that looks similar in two languages is not itself a verified ABI contract. Rust Reference: type layout

Why it matters

Agents often repair an FFI mismatch by attaching one attribute or casting a pointer. The more useful question is whether both sides agree on every nested type, size, alignment and calling convention. Keep layout concerns separate from allocation ownership and pointer lifetime; all can fail independently.

How to apply

• Start from the authoritative foreign header and list the exact boundary types. Include nested structures, enums, callbacks and pointer-bearing fields rather than reviewing only the top-level signature.
• Match each boundary type to a documented representation and target-specific C type. Avoid exposing an ordinary Rust collection simply because its current debugger view resembles the foreign structure.
• Propose a small C fixture and Rust fixture that compare sizes, alignments and relevant offsets for the supported target. Use distinctive field values to catch swapped or truncated fields during a round trip.
• Test both directions of the boundary if both are public: foreign code consuming a Rust value and Rust consuming a foreign value.
• Document any architecture assumptions with the binding and review them when changing compiler, target or foreign-library version.

Pitfalls

Passing layout checks does not validate whether a pointer is live, nullable or writable. Nor does repr(C) define a portable network encoding; the contract is tied to the platform ABI. Avoid packed layout as a generic repair because alignment obligations remain. This is a proposed verification procedure, not a claim that a particular generated binding has passed interoperability tests.