You can choose in Rust to explain the representation you want for your data type. Unlike C or C++ that's not a non-portable vendor extension it's just part of the language, look at the repr documentation: https://doc.rust-lang.org/nomicon/other-reprs.html

So if you want "what C does" you can just repr(C) and that's what you get. For most people that's not a good trade unless they're doing FFI with a language that shares this representational choice.