Bash v2.04+ does something similar, in that it lazy-loads completions. As soon as you type `git ` <kbd>Tab</kbd>, it runs `_completion_loader git`, which determines what file to source, in this case `/usr/share/bash-completion/completions/git`.
I've been on the lookout for something like this. I just clocked in all my Bash startup files at 4858 lines, so I sure could use it.
