This strikes me as likely. Bitwise compatibility with machine ABI layout rules has powerful compatibility advantages even in places where it might make code slo

This strikes me as likely. Bitwise compatibility with machine ABI layout rules has powerful compatibility advantages even in places where it might make code slower. (And, for the large majority of code, slower doesn't matter anyway.) Of course C and C++ themselves have to keep to machine ABI layout rules for backward compatibility to code built when those rules were (still) thought a good idea. Compilers offer annotations to dictate packing for specified types, and the Rust compiler certainly also offers such a choice. So, maybe such annotations should just be used a lot more in Rust, C, and C++. This is not unlike the need to write "const" everywhere in C and C++ because the inherited default (from before it existed) was arguably wrong. We just need to get used to ignoring the annotation clutter. But there is no doubt there are lots of people who think padding to alignment boundaries is faster. And, there can be other reasons to align even more strictly than the machine ABI says, even knowing what it costs.