Apple's XNU kernel includes sources for the nullfs filesystem. On other OSs this might be referred to as a bind or loopback filesystem. The source for said filesystem is available here in Apple's public kernel sources.
Unfortunately this filesystem is hidden from users both through the lack of an available mount_nullfs binary as well as behind the com.apple.private.nullfs_allow entitlement on modern versions of macOS. This means that in normal development scenarios nullfs cannot be used by any program that isn't signed with an Apple codesigning certificate.
This program allows users to create nullfs mounts.
The use of this program requires disabling both SIP and AMFI. This cripples many of the modern security measures built into macOS.
SIP and AMFI can be disabled by rebooting into recovery mode by holding CMD-R on reboot and running the following commands:
sudo csrutil disable
sudo nvram boot-args=“amfi_get_out_of_my_way=0x1”If disabling SIP and AMFI sounds bad to you (and it should) then you should probably be using either bindfs or unionfs-fuse. Both are FUSE based filesystems that emulate loopback and unionfs (which is a loopback when only one branch is specified) filesystems respectively that run perfectly fine on modern macOS.
A formula for bindfs is available in standard homebrew repositories. A formula for unionfs-fuse is available to tap here.
Alternatively you can use local NFS mounts:
mount localhost:target_fs mount_pointmount_nullfs [-o options] target_fs mount_point