[dmd-internals] How not to report unused identifiers

[Jonathan M Davis via dmd-internals]

On Tuesday, March 29, 2016 21:11:09 cy via dmd-internals wrote:
> Well, Module in dmodule.d has a list called "members" returned
> from parsing the code, each item a symbol that's supposedly a
> member of that module. But, the list also includes template
> specializations and package names, which are apparantly never
> searched for?
>
> In dmd, I made a bool[const(char)*] searched array in the Module
> class, and added to that a symbol's "toChars" representation
> every time it was searched for in that module. Then, in the third
> pass (was that a mistake?) I compared every item in
> Module.members to see if its toChars representation was in that
> array. If not, it warns about an unused identifier.
>
> So, it searched for like 5 symbols, and there were 2000 symbols
> give or take in the members array, resulting in a wall of text of
> unused identifiers, that are actually used I'm sure, but I have
> no idea how to determine that, if they're never looked up.

I don't know all that much about the compiler internals, so I don't know
quite what you're looking at, but I would point out that any attempt to warn
about "unused identifiers" would have to be done very carefully. Aside from
the normal issues with RAII and the like where something is declared and
then not used but still does something, it's _very_ common in D code to
declare stuff and then not actually use it when dealing with template
constraints and the traits tested in them, because it's testing for
compilation, not actually running the code. And even if all of that is
handled correctly, if the compiler is very aggressive about warning about
unused identifers, templates and string mixins could run into trouble
depending on the exact arguments used with them and how complicated some of
their static if-else logic is.

So, any attempt to warn about unused identifiers is going to have to be
_very_ careful, or it's going to cause problems for perfectly legitimate
code - especially if you consider that if -w is used, then warnings are
treated as errors. And the fact that that affects conditional compilation
makes it even worse. The compiler can't ever warn about any unused
identifiers which should actually stay in the code, or it's going to be a
big problem.

- Jonathan M Davis