> If Rust call out to C code and they were using incompatible understanding of an array length due to mismatching build rules (like preprocessor definitions), y

> If Rust call out to C code and they were using incompatible understanding of an array length due to mismatching build rules (like preprocessor definitions), you have a soundness bug it won't show up obviously in any interfaces anywhere. In that scenario, I'd say that the build rules are a poorly-documented part of the C code's interface, and by calling out to it with mismatching build rules the Rust code is unsound. In general, C interfaces tend to have a lot of poorly-documented or undocumented preconditions, which is one of the things that make Rust-to-C FFI so tricky. One of Rust's big ideas is to make all these preconditions fully explicit, either through typestate or through "Safety" sections on unsafe functions and traits.