[cfe-dev] [Patch] Diagnose unused declaration

[Sebastian Redl]

Douglas Gregor wrote:
> Hello,
>
> On Jan 4, 2009, at 7:59 PM, EdB wrote:
>   
>> There is place  for improvement but I want to be sure that  
>> information are at
>> the right place before.
>>     
>
> A few comments on this iteration:
>
> In Sema::PopDeclContext(), the warning-generation code needs to be  
> much more picky about which declarations it complains about. For  
> example, I tried this C++ example:
>
> namespace N {
>    int x;
> }
>
> namespace N {
>    int f() {
>      return x;
>    }
> }
>
> and Clang produced 4 warnings: one each for x, f, __builtin_va_list,  
> and N:
>
> This option should not produce warnings about any of these, because:
>
> 	- x is the definition of a variable that can be referred to by other  
> translation units, so even if it isn't used within this translation  
> unit, we should not warn about it. Plus, "x" is used later on in the  
> program (in a different DeclContext for the namespace N). Note that  
> GCC's -Wunused-variable documentation explicitly says that it only  
> warns about local variables and non-constant static variables; we  
> should do something similar.
>
> 	- f is the definition of a function, which could also be referenced  
> by other translation units. Note that GCC's -Wunused-function only  
> complains about static functions (and, presumably, functions in  
> anonymous namespaces) because those aren't visible outside of the  
> translation unit.
>
> 	- __builtin_va_list wasn't written by the user, so we shouldn't  
> complain about it. Besides, do we even want to complain about unused  
> typedefs? Especially in C++, there are certainly reasons to write  
> typedefs even if they aren't used in the translation unit.
>
> 	- N is a namespace; do we ever want to warn about those?
>
> I see that UsedDeclaration grows throughout the processing of the  
> translation unit, because nothing is ever removed from the set (even  
> when it is no longer visible through name lookup). We'd like to avoid  
> that.
>   
I think both problems could be solved together by, instead of keeping
track of declarations that are used, keeping track of those that are unused.
In other words, the UsedDeclarations becomes UnusedDeclarations. When a
new declaration is encountered, it is added to the set if
1) it is function-local or TU-local and
2) it does not have the unused attribute.
When a declaration is used, it is removed from the set. When a scope is
left, we check which of the declarations now going out of scope are
still in the set and warn about those.

Sebastian