nixos-rebuild-ng: tab completes filenames for arg1 on fish

[mweinelt]

Nixpkgs version

• Unstable (25.11)

Describe the bug

Tab completing nixos-rebuild(-ng) shows me files, not flags, when run on fish shell

Steps to reproduce

cd $(mktemp -d)
touch a b c d e f g h i j k l m n o p q r s t u v w x y z

Then in your terminal type nixos-rebuild and press <TAB>.

❯ nixos-rebuild 
a  f  k  p  u  z
b  g  l  q  v  
c  h  m  r  w  
d  i  n  s  x  
e  j  o  t  y  

Expected behaviour

Show me the switches, just like on bash

Screenshots

n/a

Relevant log output

n/a

Additional context

n/a

System metadata

• system: "x86_64-linux"
• host os: Linux 6.12.33, NixOS, 25.11 (Xantusia), 25.11.20250616.809998a
• multi-user?: yes
• sandbox: yes
• version: nix-env (Lix, like Nix) 2.94.0-devpre20250617_87d99da System type: x86_64-linux Additional system types: i686-linux, x86_64-v1-linux, x86_64-v2-linux, x86_64-v3-linux Features: gc, signed-caches System configuration file: /etc/nix/nix.conf User configuration files: /home/hexa/.config/nix/nix.conf:/etc/xdg/nix/nix.conf:/home/hexa/.nix-profile/etc/xdg/nix/nix.conf:/nix/profile/etc/xdg/nix/nix.conf:/home/hexa/.local/state/nix/profile/etc/xdg/nix/nix.conf:/etc/profiles/per-user/hexa/etc/xdg/nix/nix.conf:/nix/var/nix/profiles/default/etc/xdg/nix/nix.conf:/run/current-system/sw/etc/xdg/nix/nix.conf Store directory: /nix/store State directory: /nix/var/nix Data directory: /nix/store/fysypr2nk56y1x9nhxlsmyajniyqlr6s-lix-2.94.0-devpre20250617_87d99da/share
• channels(root): "nixos, nixos-unstable"
• nixpkgs: /nix/store/axym8ar735sa522cf0zhg78cc96cq820-source

Notify maintainers

@thiagokokada

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[thiagokokada]

I am not too familiar with how fish generates completion. Right now we use https://github.com/iterative/shtab project to generate shell completions, but we don't generate it for fish since there is no support yet: tqdm/shtab#174.

So I am not sure why fish is doing that. Maybe it is trying to use the bash completions wrongly? Or maybe by default it just complete with files?