ffi-and-language-interop

name	ffi-and-language-interop
description	FFI and native interop rules: single documented pointer ownership, opaque handles, null-pointer checks, allocate/free pairs, unsafe-block invariants, generated C headers, stable C ABI, frozen struct layouts, and error/context conversion. Load when writing or reviewing FFI boundaries between native and host languages.

• Every pointer has one owner, documented with SAFETY comments or host-language ownership docs.
• Use opaque handles across native boundaries. Do not expose Rust, C++, or host-runtime internals directly.
• Check all nullable pointers before use and return explicit errors for invalid handles or null inputs.
• Provide allocate/free pairs for every caller-owned allocation and document which side frees each value.
• Every unsafe block must state the invariant, why it holds, and what would make it invalid.
• Generate C headers or native declarations from source when possible, and make CI verify generated files are current.
• Treat the C ABI or equivalent native ABI as the stable contract for downstream bindings.
• Freeze native struct layouts across compatible releases, and version breaking ABI changes deliberately.
• Convert native errors to host exceptions, result objects, or idiomatic error values at every boundary.
• Preserve diagnostic context across boundaries: message, numeric code when available, source operation, and cause chain.