--each-feature and --feature-powerset are great! My only complaint is that they are slow. bpytop shows that I'm never using more than one core when running cargo-hack. It would be awesome if cargo hack could run multiple cargo processes in parallel, one for each feature combination. Normally Cargo doesn't allow two processes to share the same build directory at the same time, so you could set CARGO_TARGET_DIR to a separate value for each.

There is a little contention for the crate cache, but my crude benchmark shows that I can build targets in parallel in about a quarter of the time it takes to build them sequentially.