Wayland and Weston are now provided by ArchLinux, this means the
hawaii repository is not going to provide them anymore.
ArchLinux wiki has more information about it.
Remove previously installed weston-git
pacman -R weston-git
pacman -Syu cairo
Remove a previously installed mesa-full-wayland
If you previously installed
mesa-full-wayland you have to install
sudo pacman --needed -Syu mesa libdrm libgl libglapi libegl libgles intel-dri nouveau-dri libgbm khrplatform-devel
Install Wayland and Weston
pacman -Syu wayland weston