[m-rev.] for review: implement --warn-suspicious-recursion

[Julien Fischer]

On Wed, 1 May 2019, Zoltan Somogyi wrote:

> For review by anyone, but I would like the person who last worked on
> to_char_list in string.m to review the diff to string.m, even if he does not
> look at the rest of the diff.

A couple more comments in addition to those from Peter:

> Implement warnings for suspicious recursion.
> 
> compiler/simplify_goal_call.m:
>     If the --warn-suspicious-recursion option is set, and if the warning
>     isn't disable, generate warnings for two different kinds of suspicious

s/disable/disabled/

>     recursion. They are both related to, but separate from, the warning
>     we have long generated for infinite recursion, which occurs when
>     the input args of a recursive call are the same as the corresponding
>     args in the clause head.
>
>     Both kinds suspicious recursion look at the input args of a recursive call
>     that are NOT the same as the corresponding args in the clause head.
>     Both require these args to have non-unique modes.  (If they have unique
>     modes, then the depth of the recursion may be controlled by state outside
>     the view of the Mercury compiler, which means that a warning would be
>     likely to be misleading.)

Unless the unique arguments *only* occur in the recursive calls, in which case
you could issue the warning.  That may be a case worth including here since
it's not unheard of for users to thread the I/O (or other unique) state through
a predicate and then not use it.

Julien.