Allow setting prefix to remove before completing directories?

[yedayak]

Hello, I'm trying to write custom completions for skopeo, and I ran into something that I don't know if it's possible to do using the builtin cobra completions.

I'm trying to make this complete directories:

skopeo copy dir:<TAB>

I tried returning nil, cobra.ShellCompDirectiveFilterDirs | cobra.ShellCompDirectiveNoFileComp, but this doesn't complete anything. It seems to be because _filedir -d looks at $cur when completing (when it eventually calls compgen here), and since cur is dir: it doesn't find any completions. I manually edited the completion file and it did manage to complete directories like I wanted:

            cur=${cur#"dir:"}
            _filedir -d

Is there some way to solve this issue with cobra? The PR I created for skopeo: https://github.com/containers/skopeo/pull/2524

I only tested this with bash Bash version: 5.1.16(1)-release