Reproducible builds

Crubit can emit bindings for Rust or C++ libraries so that they can be called from the other language, but these bindings require strict ABI compatibility. To accomplish this, Crubit uses clang and rustc as libraries to discover the public APIs of the wrapped libraries, and the ABI that they expose when built (e.g. their function calling conventions, and the memory layout of their structs).

To ensure that the generated bindings are ABI-compatible with the built libraries, Crubit needs to see exactly what the real compiler sees:

• Build the wrapped libraries using the same version of the compiler that Crubit uses, with the same compilation settings (flags, environment variables, etc.). For example, an extra -DSOME_DEFINE or a new version of the compiler can completely change the ABI, or even the API, of a given library. If the compiler sees it, but Crubit does not, the generated bindings will be incorrect.
• Do not store Crubit outputs (e.g. ..._cc_api.h or ..._rs_api.rs) in a source code repository, as they will go out of date. Crubit comes with Bazel integration support to invoke it at build time.
• The ABI must be deterministic and reproducible (i.e. depend only on compilation settings, the compiler version, and the wrapped libraries themselves). For example, the -Zrandomize-layout flag cannot be used without a fixed seed.