/usr/share/fish/completions/env.fish is in fish-common 2.4.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 | complete -c env -a "(set -n)=" -x --description "Redefine variable"
complete -c env -a "(__fish_complete_subcommand -- -u --unset)" --description "Command"
complete -c env -s i -l ignore-environment --description "Start with an empty environment"
complete -c env -s u -l unset --description "Remove variable from the environment" -x -a "(set -n)"
complete -c env -l help --description "Display help and exit"
complete -c env -l version --description "Display version and exit"
# Since env runs subcommands, it can accept any switches
complete -c env -u
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