Comment by dathinab

If I remember correctly there is a bit of difference between explicit `extern "rust"` and no explicit calling convention but I'm not so sure.

Anyway at least when not using explicit rust representation Rust doesn't even guarantee that the layout of a struct is the same for two repeated build _with the same compiler and code_. That is very intentionally and I think there is no intend to change that "in general" (but various subsets might be standarized, like `Option<&T> where T: Sized` mapping `None` to a null pointer allowing you to use it in C-FFI is already a de facto standard). Which as far as I remember is where explicit extern rust comes in to make sure that we can have a prebuild libstd, it still can change with _any_ compiler version including patch versions. E.g. a hypothetical 1.100 and 1.100.1 might not have the same unstable rust calling convention.