Subject: Re: bug#65344: 28.2; Unable to Edebug cl-flet form which uses argument destructuring

From: Stefan Monnier <[email-masked]>
Date: Sat, 02 Sep 2023 15:27:51 -0400

>> In this case, the code for F will be run in the body of the
>> flet. Doesn't that qualify as being run later, as you describe above, 
>> ignoring the "non-instrumented" part, maybe?
>
> No, in the above case the `def-form` is
>
>     (lambda () (list 1 2))
>
> which will be "run" right when we enter the `cl-flet` ("run" is an
> exaggeration here since this lambda is a constant so it'll just
> self-evaluate) and not when `f` is called.

More specifically, the annotated code of

    (defun sm-foo (x)
      (cl-flet ((f (prog1 (lambda (y) (+ x y)) 0)))
        (f 5)))

stored into (symbol-function 'sm-foo) now looks like:

    (closure (t) (x)
     (edebug-enter 'sm-foo (list x)
      #'(lambda nil :closure-dont-trim-context
          (edebug-after (edebug-before 0) 3
           (let* ((--cl-f--
                   (edebug-enter 'f@cl-flet@4 nil
                    #'(lambda nil :closure-dont-trim-context
                        (edebug-after (edebug-before 0) 1
                         (prog1
                             #'(lambda (y)
                                (edebug-enter 'edebug-anon5 (list y)
                                 #'(lambda nil :closure-dont-trim-context
                                    (edebug-after (edebug-before 0) 3
                                     (+ (edebug-after 0 1 x)
                                      (edebug-after 0 2 y))))))
                           0))))))
             (progn
               (edebug-after (edebug-before 1) 2
                (funcall --cl-f-- 5))))))))

As you can see for `sm-foo` itself, the

    (edebug-enter 'sm-foo (list x)
     #'(lambda nil :closure-dont-trim-context

is (correctly) placed at the very beginning of the body of the function,
so that code coverage can track whether `sm-foo` was called or not.

In contrast the

    (edebug-enter 'f@cl-flet@4 nil
     #'(lambda nil :closure-dont-trim-context

is placed around the code which will compute&return the `f@cl-flet@4`
function, but not inside its body, which instead gets

    (edebug-enter 'edebug-anon5 (list y)
     #'(lambda nil :closure-dont-trim-context

It's actually difficult (and in general impossible) to associate the
name `f@cl-flet@4` with the corresponding `lambda`, so the use of
`edebug-anon5` is largely unavoidable here.  But making the code-coverage
say that `f@cl-flet@4` is called just because we computed that function
(regardless if it's been called) is a problem.

        Stefan