bug#37951: 27.0.50; octave completion-at-point for fieldnames

[noah]

This is relevant to octave.el (should I be sending to a different
list or tagging it somehow?)

The completion-at-point functions in both octave source buffers
and inferior octave buffers don't complete for fieldnames of
eg. structs. For example, it would be nice to get completion
candidates after '.' in these cases

    octave> s(1) = struct('field1', [], 'field2', [])
    octave> s(2) = struct('field1', [], 'field2', [])
    octave> s.         # case 1
    octave> s(1).      # case 2
    
This is simply a matter of not capturing the correct bounds of
objects at point in `octave-completion-at-point` and
`inferior-octave-completion-at-point`.

The octave function `completion_matches` already returns the correct
completion candidates ("s.field1" "s.field2") when passed the full
"s." or "s(1)." strings in the above example.

The following modification to retrieve the bounds of the object
at point seems to work in both source and inferior buffers.

    (defun my-octave-bounds-of-object-at-point (&optional lim)
      (let ((beg (save-excursion
                   (skip-syntax-backward "w_" lim)
                   ;; just this extra check
                   (when (eq ?. (char-before))
                     (forward-char -1)
                     (and (eq ?\) (char-before)) ; struct(i).
                          (ignore-errors (backward-sexp)))
                     (skip-syntax-backward "w_" lim))
                   (point)))
            (end (point)))
        (when (< beg end)                   ; extends region past point
          (save-excursion
            (skip-syntax-forward "w_")
            (setq end (point))))
        (when (> end beg)
          (cons beg end))))

However, being pretty new to octave, I'm not sure about all the cases
where this completion would be applicable.


In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2019-10-27 built on noah-M51AC
Repository revision: 0e4dd67aae8b10032317a29a6bd99d2d4a64c897
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Ubuntu 18.04.3 LTS

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