Mounting the nix store on /nix/ fails but /nix works

[Atemu]

Describe the bug
A clear and concise description of what the bug is.

I have a separate /nix/ ZFS dataset whose mountpoint I declare in my NixOS config.

However when I try to boot that system, it mounts the root dataset, tries to read something out of the nix store (before it's mounted) and fails, prompting me to reboot or ignore.

  fileSystems."/nix" = {
    device = "Rpool/R/nix";
    fsType = "zfs";
  };

works but

  fileSystems."/nix/" = {
    device = "Rpool/R/nix";
    fsType = "zfs";
  };

doesn't.

To Reproduce
Steps to reproduce the behavior:

1. Have a separate ZFS dataset for /nix/
2. Try to mount it on /nix and /nix/ respectively
3. Observe that /nix gets mounted before anything is read from the nix store while /nix/ isn't

Expected behavior
A clear and concise description of what you expected to happen.

It should behave the same, no matter the trailing slash.

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.

Metadata
Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

 - system: `"x86_64-linux"`
 - host os: `Linux 4.19.98, NixOS, 19.09.1970.6d9a4a615ee (Loris)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.2`
 - channels(root): `"nixos-19.09.1970.6d9a4a615ee"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module:

[danbst]

I think this is because of NixOS/systemd@49e323d

/nix is handled specially, in particular, it is not unmounted. But /nix/ isn't matched, so it gets unmounted, which breaks everything (because everything lives in /nix/store).

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