bug#14883: warning about unused returned value from some ert shoulds

bug#14883: warning about unused returned value from some ert shoulds

[Glenn Morris]

Package: emacs
Version: 24.3
Severity: minor

Compiling a file with contents:

    ;; -*- lexical-binding: t; -*-
    (require 'ert)
    (require 'cl-lib)
    (defvar foo-test 1)
    (should foo-test)
    (cl-assert foo-test)

results in a warning (for some reason, only when lexical-binding is t):

   foo.el:5:1:Warning: value returned from (car value-94600) is unused

The warning is from the `should'. Obviously it is a bogus warning
because the whole point of the should is to test the return value.
The similar cl-assert does not trigger a warning, because internally
cl-assert is written differently.

This is the same issue as http://debbugs.gnu.org/10969, for what is now
cl-pop.

byte-optimize-form-code-walker has special handling for the standard
`pop' macro so as not to warn about unused return values from (pop foo).
It would be nice if there was a general way to get the same result for
arbitrary macros (sort of the opposite of side-effect-free?), but I
don't see how.

bug#14883: warning about unused returned value from some ert shoulds

[Stefan Monnier]

> This is the same issue as http://debbugs.gnu.org/10969, for what is now
> cl-pop.

There's no `cl-pop'.  Instead `pop' is now always like CL's `pop'.

> byte-optimize-form-code-walker has special handling for the standard
> `pop' macro so as not to warn about unused return values from (pop foo).
> It would be nice if there was a general way to get the same result for
> arbitrary macros (sort of the opposite of side-effect-free?), but I
> don't see how.

I think the way to do it is to introduce a new function (call it
`ignorable') which works like `identity' and then make pop use it around
the code that returns the value.

Then byte-optimize-form-code-walker can be modified to recognize this
function and just throw away the whole call when `for-effect' is nil.
We can then get rid of this ad-hoc handling of `pop'.


        Stefan

bug#14883: warning about unused returned value from some ert shoulds

[Stefan Monnier]

> Compiling a file with contents:
>     ;; -*- lexical-binding: t; -*-
>     (require 'ert)
>     (require 'cl-lib)
>     (defvar foo-test 1)
>     (should foo-test)
>     (cl-assert foo-test)
> results in a warning (for some reason, only when lexical-binding is t):
>    foo.el:5:1:Warning: value returned from (car value-94600) is unused

This `car' came from the closure conversion.  I believe it is now fixed
in the trunk.  Thank you.


        Stefan