Why is rust allowed to reorder fields? If I know that fields are going to be generally accessed together, this prevents me from ordering them so they fit in cache lines.
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.
Rusts tradeoff is awful. People organize struct members in the logical ways in which they will be accessed. Rust just decides to do things different so they can say they do things different and "better" than the old C. The featured article is an extreme case but other normal cases would have the same issue. Packing structs should be the exception.
