/usr/share/fish/completions/builtin.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 | complete -c builtin -s h -l help --description 'Display help and exit'
complete -c builtin -s n -l names --description 'Print names of all existing builtins'
complete -c builtin -xa '(builtin -n)'
complete -c builtin -n '__fish_use_subcommand' -xa '__fish_complete_subcommand'
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