Re: unload-feature questions and thoughts

[Davis Herring]

> Yes, that's what I've been proposing. It should have to be a new hook
> for compatibility, I think.

I'm not sure the compatibility is worth the complexity.  The current
behavior is so badly documented and random (auto-mode-alist being treated
as a hook) that I would just call it a bug fix.  To check this, I went
through all the unload hooks in the sources:

delsel.el seems to think that its pre-command-hook entry will get cleared
for it, and the `remove-hook's it already does wouldn't hurt anything with
the change.

Regardless of the change, server.el needs to clear its `kill-emacs-hook'
entry (loadhist.el wouldn't, even without `server-unload-hook', because
`server-mode' is autoloaded).  It also needs to kill its C-x # binding,
suggesting that loadhist.el should perhaps sweep keymaps.  With the
change, server.el wouldn't have to do any of its extant `remove-hook's,
but they too would do no harm.

shadowfile.el needs to kill its C-x 4 s binding (again, should sweep
keymaps).  Its `remove-hook's would also be harmlessly redundant.

strokes.el wouldn't have to get rid of its `kill-emacs-query-functions'
entry itself.

whitespace.el has to have its unload function, because the functions it
adds to hooks are autoloaded.

emacs-lisp/cl.el and emacs-lisp/elp.el have meaningful unload functions too.

eshell/*.el make their unload functions customizable for some reason. 
eshell/esh-mode.el and eshell/esh-module.el are particularly odd: the
former has a default value of nil, so it just suppresses loadhist.el's
hook/a-m-a action, and the latter recursively unloads features -- it
should document that those features cannot `require' 'esh-module because
`unload-feature' would then refuse to unload it because of them even
though they would get unloaded too.

gnus/gnus-registry.el and gnus/spam-stat.el each have an interactive
unload hook that isn't necessary as an unload hook; it should be renamed
to "...-uninstall-hooks" to match its counterpart.

url/url-vars.el has a similarly needless unload hook.

So I think that the evidence is for the intuitively obvious statement that
no one knows how to use the hook.  I found no examples where the normal
heuristics would break anything, although neither did I find any examples
where the unload-hook actually broke things by suppressing the "normal
heuristics".  So I think treating it as a bug is probably the right thing
to do for simplicity: just do the heuristics regardless, and let the rare
hooks that really need to suppress them kill the variable bound for that
purpose.

Davis

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