No idea if copy prop is still slow, but I found this gem: https://github.com/rust-lang/rust/issues/36673
in MIR terms this issue is ancient
Yeah, looking at the code, it's still slow. I tried improving that sometime last year, adding a rather crude hack to update the def-use chains instead of recreating them (which is what makes this pass so slow). But it's also limited in the cases it can handle which made the gains from running the pass rather marginal and I didn't have the time to finish the work I started in a way that's suitable for a PR.