Re: master 09b5f00613: ; Fix calls to treesit functions

[Stefan Monnier]

>> I think this can be done simply by listing which functions are always
>> defined.
>
> No, this is not enough, IMO.  What is needed in addition is to
> document how to know, with each of these always-defined functions,
> whether tree-sitter can be used (most probably, by examining the
> returned value).

That should be an obvious consequence of the behavior of the function
without needing to document it specifically.

Like with `treesit-parser-list`: if the list is empty, then it means we
don't have any parser at hand and that's it.  If the callers additionally
needs to know whether it's because Tree-sitter is absent or for some
other reason, they'll need to call additional functions like
`treesit-available-p`.

>> Then we just have to make sure that those functions can be
>> implemented correctly even without Tree-sitter.
> The implementation without tree-sitter will probably be trivial, like
> return nil or something like that.

Yes, but for some functions this is not an option because we can't
provide a correct behavior without using Tree-sitter.  So we have to
choose the set of always-defined functions based on whether they *can*
be correctly implemented without Tree-sitter.

`treesit-available-p`, `treesit-parser-list`, and `treesit-node-at` are
functions we can implement without Tree-sitter, because nil is already
the correct answer for those, without having to add any special
case in their docstring for that.


        Stefan