SUBSYSTEM=="tty", ACTION=="add", ATTRS{idVendor}=="1d50", ATTRS{idProduct}=="6018", ENV{ID_USB_INTERFACE_NUM}=="00", MODE="0666", SYMLINK+="ttyBMP0"

This works, but ATTRS{bInterfaceNumber} doesn't work despite it being listed in udevadm info --attribute-walk --name=/dev/ttyACM0. Why? Who knows.

udev? no, u dev