It makes sense that rav1d would need a bunch of repr(C) because the initial conversion is just mindlessly derived from the C, so if there's C which just bit-copies part of a data structure this cannot possibly work in Rust unless we explicitly say "This structure needs to be laid out the same as in C".

It also sounds as though at least some of this "speed up" was assuming dav1d doesn't need big integers where actually it does, but only for larger images. I can well imagine that DVD resolution works fine with 16-bit variables where the Ultra HD blows up nastily.