bug#8711: 24.0.50; binding _ to unused values with lexical-binding

[Helmut Eller <eller.helmut@gmail.com>]

* Stefan Monnier [2011-05-23 14:24] writes:

> The two warnings come from different analyzes:
> - The warning for `bar' comes from cconv.el which is intended to check
>   whether the variable is syntactically used, rather than semantically.
> - The warning for `foo' checks whether pure functions are not called for
>   their side-effects and it's applied after optimizations so in the
>   above code, the code generated by destructuring-bind ends up optimized
>   to something that calls `car' without binding the result to _ because
>   it figured that _ is not used and just got rid of it without warning.
>
> So most likely the answers I give here aren't satisfactory to the OP,
> since his real problem is probably different than (destructuring-bind
> (_) x) and the solution for that problem is probably going to be
> yet different.

My problem is basically that I have a macro "destructure-case" that
expands to destructuring-bind, e.g.:

(destructure-case location
  ((:error _) nil)) ; do nothing

expands to

(ecase (car location)
  (:error (destructuring-bind (_) (cdr location)
            (ignore _)
            nil)))

The macro inserts the (ignore _) to suppress the "value returned from
(car --cl-rest--) is unused" warning.  But that trick only works for the
non-lexical-binding case.

I think that rewriting (let ((_ foo))) to (progn foo nil) is not quite
right because that loses the information that the value that flows to
the variable _ is intentionally ignored, but I guess that is somewhat
hard to fix this.  (Maybe byte-optimize-letX could call
byte-optimize-form-code-walker directly but instead of specifying that
the form is evaluated "for-effect" somehow say that the value is
"ignorable".)

It's perhaps easier to teach destructuring-bind the _ convention so that
it can produce more direct code.


There is also a somewhat related problem with loop:

;; -*- lexical-binding: t -*-
(defun foo (alist) (loop for (_key . value) in alist collect value))

produces a "variable `_key' not left unused" warning.

Helmut