These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 1.03 s (backtrack: 0/20).

[ebuild  N     ] acct-group/plugdev-0-r3::gentoo  0 KiB
[ebuild  N     ] dev-libs/libusb-1.0.30:1::gentoo  USE="-debug -doc -examples -static-libs -test -udev -verify-sig" 641 KiB
[ebuild  N     ] virtual/libusb-1-r2:1::gentoo  USE="-udev" 0 KiB
[ebuild  N     ] dev-libs/hidapi-0.15.0::gentoo  USE="-doc" 356 KiB
[ebuild  N     ] app-crypt/libu2f-host-1.1.10-r1::gentoo  USE="(-systemd)" 461 KiB

Total: 5 packages (5 new), Size of downloads: 1457 KiB