bug#61730: 30.0.50; Compiler warnings for delq and delete

[Mattias Engdegård]

> I think we should add compiler warnings

You rang?

> for `delete' and `delq' function
> calls whose return values are unused.

Let's experiment: warning about for-effect calls to

  mapcar mapcan mapconcat
  delq delete delete-dups delete-consecutive-dups
  cl-delete cl-delete-if cl-delete-if-not cl-delete-duplicates
  sort

results in 34 such calls found on master, most of them about `delq` and 
`delete`, but `delete-dups`, `cl-delete`, `mapconcat` and `sort` are also 
represented.

Some of these are no doubt safe, a few of them knowingly so, but it's 
definitely not obvious from a quick look at the code. It's poor style in any 
case.

Thus such a warning definitely falls on the beneficial side. Let's do it.

> I think the warnings could be added in a similar way as the "mapcar
> called for effect" warnings work.

That's probably the best place to start (although we prefer warnings to be 
emitted by the front-end and not in codegen).

We may want to change the warning text from "called for effect" because it's 
written from the perspective of the compiler; the programmer thinks of it as 
'not using the return value'.

For that matter, `with-suppressed-warnings` doesn't work for suppressing this 
warning very well; we may want to do something about that. This is true for the 
existing `mapcar` warning as well.

> Adding the same kind of warning for `remq' and `remove' would probably
> also be useful.  This will probably not occur that often but it still
> would be useful I think.

The compiler should already warn about those two since they are declared 
side-effect-free. Unless somehow `byte-compile-delete-errors` is set during 
compilation, which can happen if the code messes about with (optimise (safety 
...)) -- it's a bit unfortunate. Please tell us if you observe anomalies in 
this regard.