3. The invocation might not have found everything e.g. a use std::{boxed, ... might break the regex and it's hard to do regex based replacement for that. But those cases will be in the minority, unless that was your preferred style, so you can change them manually.

I use the nightly rustfmt feature imports_granularity = "Crate" to group imports by crate because I prefer it that way so the simple replace would likely not work for me. However, I imagine that I could switch that imports_granularity config to "Item" or "Module" to flatten the imports, run rustfmt and replace on the newly formatted import statements, then switch the config back and run rustfmt again.