[cfe-dev] [Patch] Diagnose unused declaration

[Neil Booth]

Ted Kremenek wrote:-

> 
> On Jan 3, 2009, at 11:14 AM, EdB wrote:
> 
> >> Looks good. There seems to be a whitespace issue in
> >> ActOnEndOfTranslationUnit.
> >
> > Pleased find attached a new version:
> > It fixed some crash.
> > Unused warning is no longer actived by default. You should enable it
> > with -Wunused_declaration
> 
> I'm not certain that the approach here will work.  Since an  
> IdentifierInfo is not unique to a declaration using a bit in  
> IdentifierInfo to perform this check will be ineffective in many  
> cases.  For example:
> 
> void f() {
>    int x = ...
>    x++;
> }
> 
> void g() {
>     int x;   // no warning will be issued
> }
> 
> The other thing that bothers me about this is putting this bit in  
> IdentifierInfo in the first place.  It seems like a very invasive  
> thing to do for a simple check like this, especially one that isn't  
> always used.  Generally we like to keep such information "on the side"  
> unless there is a good reason not to.  Why not use a DenseSet to keep  
> track (on the side) of what declarations are used?  You can then do  
> this on a declaration based (i.e., for a particular NamedDecl) rather  
> than using IdentifierInfo's in this conflated way.

Further, in standard C90

  int z; sizeof z;

z is generally not considered "used".  C99 adds more complexity.
In C++ the rules are vastly more complicated.

Neil.