That's not my experience. Yes casts are annoying but in Rust usize generally means something is an index to a Vec or arena (generally speaking, it means the id of some entity). You generally should not do math with indices in your business logic but rather use some higher level interface that does the math for you, because if you mess up you will have an out of bounds bug (which in Rust is not UB, but it's still a panic). If you keep your usizes to be just indices, they will not pose a portability problem.

When I talk about a higher level interface it may be something like this https://crates.io/crates/typed-generational-arena those sort of things are very common in Rust. In here the index is an usize, but you don't directly do math with it (also you have a generation number to guard against index reuse), so you never experience any `as usize` thing.

Of course that's not applicable to everything. Some domains will naturally require doing arithmetic with indices, which is annoying in Rust. Hopefully you enclose this into a library and try to forget the horrors (rather than using usize everywhere, which is objectively the wrong thing to do)

Then you have libraries like petgraph that does use u32 indices by default, to save memory https://docs.rs/petgraph/latest/petgraph/graph/type.DefaultI...