Detector for unused variables

[Jonathan M Davis]

On Wednesday, August 01, 2012 17:07:31 Timon Gehr wrote:
> On 08/01/2012 12:38 PM, bearophile wrote:
> > Regan Heath:
> >> Indeed. IIRC Walter's rationale on things like this has always been
> >> that they belong in 3rd party tools.
> > 
> > Walter is not always right.
> > 
> >> It's why the DMD front end is available for use, so people can create
> >> tools like this, syntax highlighters, re-formatters, dependency tree
> >> diagrams, or... you name it.
> > 
> > Detecting unused variables is a core feature,
> 
> No. This mustn't be part of the language.
> 
> http://d.puremagic.com/issues/show_bug.cgi?id=7989
> 
> > it belongs in the
> > compiler, like compile-time array bound tests and other errors currently
> > detected by DMD. This allows everyone to use it with minimum work, so
> > everyone enjoys it, and it requires less work to be implemented because
> > the compiler already does lot of analysis.
> 
> It is not an error. Making things that are not errors illegal is prone
> to introduce bugs into code using __traits(compiles).
> 
> This can be a warning at best.

And you can't have anything be a warning if it's not something that should 
always be fixed, because it's never good practice to leave warnings in your 
code (especially since if you do, it becomes difficult to see the ones that 
actually  matter). The fact that D has -w makes it that much critical that a 
warning be _guaranteed_ to be something that should be fixed.

And unused variables have too many valid use cases in D: template constraints, 
generic code, RAII, etc. You can't make them either an error, and if you can't 
make them an error, you can't make them a warning. The only real difference 
between an error and a warning as far as dmd goes is if it's something that 
must _always_ be fixed before your code can compile. It's not something that 
should be left in your source code in either case, and given how many people 
compile with -w, there's often effectively zero difference between warnings and 
errors when using dmd.

- Jonathan M Davis