Misleading location for warnings related to inlining

[Gbury]

Consider the following code:

let[@inline] foo f x =
  (f[@inlined]) x + 1

let bar x =
  let[@inline] aux y = y + 1 in
  (foo[@inlined]) aux x

Trying to compile it with a max_inlining_depth of 1 results in the following warnings:

File "/home/guigui/tmp/warn.ml", line 3, characters 2-17:
3 |   (f[@inlined]) x + 1
      ^^^^^^^^^^^^^^^
Warning 55: Cannot inline: [@inlined] attribute was not used on this function application (the optimizer did not know what function was being applied)
File "/home/guigui/tmp/warn.ml", line 7, characters 2-23:
7 |   (foo[@inlined]) aux x
      ^^^^^^^^^^^^^^^^^^^^^
Warning 55: Cannot inline: [@inlined] attribute was not used on this function application{Do_not_inline}

Whereas the first warning is correct (an indirect and unknown call cannot be inlined), the second warning is much more puzzling and misleading: indeed, the foo function has been inlined (as can be seen by looking at the -dflambda output or the inlining report), and it is in fact the call to f inside the body of the inlined foo which was not inlined, because of the maximum inlining depth.

There are two things that should probably be fixed in that situation:

• the location of the warning (particularly now that there is source code quoting in the error message)
• the contents of the warning message should be clearer about why the inlining did not take place (the message is generated there :

  ocaml/middle_end/flambda/simplify/simplify_apply_expr.ml

  Lines 132 to 141 in f7d8fcc

  	| Do_not_inline ->
  	(* emission of the warning at this point should not happen,
  	if it does, then that means that
  	{Inlining_decision.make_decision_for_call_site}
  	did not honour the attributes on the call site *)
  	if not (DA.do_not_rebuild_terms dacc) then begin
  	warn_not_inlined_if_needed apply
  	"[@inlined] attribute was not used on this function application\
  	{Do_not_inline}"
  	end;

  )