Using core.reflect to check for unused parameters

[Stefan Koch]

On Monday, 30 August 2021 at 18:13:59 UTC, Stefan Koch wrote:
> Hi,
>
> I've just extended core.reflect's reflection to function bodies.
>
> Which means you can now do fun things such as checking for 
> unused parameters at compile-time.
>

Ah the bugs are biting again.

if you have played with the code I posted you will have seen that 
it didn't count parameters which were used as variable initializer

i.e.
```d
void func(int p)
{
   auto x = p;
}
```

would have complained about unused parameters.
The reason is that we didn't visit the `_init` field of the 
`VariableDeclaration`.


luckily it's easily fixed.
```d
     class Marker : TransitiveVisitor
     {
         bool[const(VariableDeclaration)]* parameterUsageList;

         this(bool[const(VariableDeclaration)] *parameterUsageList)
         {
             this.parameterUsageList = parameterUsageList;
         }

         alias visit = TransitiveVisitor.visit;
         override void visit(VariableDeclaration vd)
         {
             if (auto b = vd in *parameterUsageList)
             {
                 *b = true;
             }
             vd._init.accept(this);
         }
     }
```

now this visitor will reliably detect usage of parameters.