mount -t completions fail on FreeBSD

[asomers]

• Have you checked if problem occurs with fish 2.5.0?
• Tried fish without third-party customizations (check sh -c 'env HOME=$(mktemp -d) fish')?

fish version installed (fish --version):
2.5.0

OS/terminal used:
FreeBSD/any

Talk about the the issue here.
share/functions/__fish_print_filesystems.fish contains a Linuxism. The mount.* helpers are Linux specific. On FreeBSD, they're called mount_*, a convention that dates to 386BSD if not older.

Reproduction steps

1. type "mount -t" and press tab.
2. Various filesystems should be listed. Instead, an error appears.

No matches for wildcard '$PATH/mount.'.
/usr/local/share/fish/functions/__fish_print_filesystems.fish (line 1): string replace -ra "./mount." "" -- $PATH/mount.*
^
in command substitution
called on line 1 of file /usr/local/share/fish/functions/__fish_print_filesystems.fish

in function '__fish_print_filesystems'
called on standard input

in command substitution
called on standard input

For now, I've worked around the issue by patching that file in FreeBSD's build system. It's effective if inelegant, but the same bug likely affects other operating systems. It would be better to fix the bug here, but I can't find any other examples of fish function or completion files that get modified during the build.
Downstream bug: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=216032

[krader1961]

I happened to be investigating a different problem on FreeBSD 12.0 when this was opened and can confirm this is a bug.

@asomers: We would love to see you submit a patch for review and merging. You're obviously someone who cares about making fish behave better on FreeBSD and we would love to see someone like yourself contributing FreeBSD platform specific improvements.