Docs: Section on local variables

[bearophile]

Timon Gehr:

> Andrej Mitrovic:
>> Keeping variables clean
>> is the responsibility of the programmer and not the compiler.
>>
>> If it doesn't affect the semantics of code the compiler should 
>> shut up. Please don't turn the compiler into a reincarnation 
>> of Clippy.
>
> +1.

I think currently the D compiler doesn't shut up in some cases.

Comparing the unused variable warning with Clippy is not good. 
Clippy gives suggestions, while here the compiler is giving 
something more like an error message.


> Another thing: It might not be unused in every static code path.
>
> Even more important:
>
> template isInputRange(R)
> {
>     enum bool isInputRange = is(typeof(
>     {
>         R r;              // can define a range object
>         if (r.empty) {}   // can test for empty
>         r.popFront();     // can invoke popFront()
>         auto h = r.front; // can declare an unused variable
>     }()));
> }

If the unused variable is a warning, and I use "-wi" that code 
compiles.

The warning for unused variables helps me clean up my C code and 
has avoided me more than one bug. So I'd like this optional 
warning in the D front-end. I'd even like a warning for variables 
assigned and then later never read again.

Bye,
bearophile