e4bf113027
Because `PassMode::Cast` is by far the largest variant, but is relatively rare. This requires making `PassMode` not impl `Copy`, and `Clone` is no longer necessary. This causes lots of sigil adjusting, but nothing very notable.
rustc_target contains some very low-level details that are
specific to different compilation targets and so forth.
For more information about how rustc works, see the rustc dev guide.