Debugging SMIE for sh-script.el

[Kévin Le Gouguec]

Hello,

I am having a hard time understanding the mechanics of SMIE in the
context of sh-script.el; for starters, I am stumped trying to debug a
call to smie-indent-calculate.

Take this two-line shell snippet:

#+begin_src sh
${foo}/bar --baz \
      --quux
#+end_src

With point on "--quux", evaluate (smie-indent-calculate): this yields 6.
Looking at this function, I see that it simply funcalls everything in
smie-indent-functions, which in a shell buffer is
(sh-smie--indent-continuation t).

Instrumenting sh-smie--indent-continuation with C-M-x, I see that we
first go into the catch-all branch of the cond (";; Just make sure a
line-continuation is indented deeper."), which let-binds "indent" to:

#+begin_src elisp
(let ((sh-indent-after-continuation nil))
  (smie-indent-calculate))
#+end_src

Stepping through this recursive call with Edebug, I see that we go into
the first branch of sh-smie--indent-continuation's opening cond, which
returns nil.  Continuing into the parent call of
sh-smie--indent-continuation… "indent" is set to 6 for some reason??

I struggle to understand whether I am missing something, or whether this
binding is bogus.  On the one hand, run-hook-wrapped makes no promises
as to its return value when all functions return nil; on the other hand,
"indent" gets consistently assigned to whatever column "/bar" begins at
(e.g. 6 here, or 9 with "${foobar}/bar"), so there is a method to the
madness…

What am I missing?