First Draft: ref For Variable Declarations

[Nick Treleaven]

On Wednesday, 17 April 2024 at 14:29:17 UTC, Dukc wrote:
> On Wednesday, 17 April 2024 at 14:07:04 UTC, Richard (Rikki) 
> Andrew Cattermole wrote:
>>
>> Currently the DIP only shows aliasing of variables, 
>> expressions like the above do not appear to be supported.
>
> If that was the case this DIP would be completely useless, as 
> you could already do everything it enabled with `alias`.
>
> I'm sure the intended restriction is the same as with `ref` 
> function parameters: the expression must be an lvalue.

The behaviour could perhaps be defined as follows - if this would 
compile:
```d
(ref p) { CODE }(lvalue);
```
Then, with some restrictions on `CODE`, you can write:
```d
ref p = lvalue;
CODE
```
And the effects will be equivalent. `CODE` cannot contain 
`return` statements or `__traits(parameters)`, 
`__traits(parent)`, `is(T PS == __parameters)` expressions. Have 
I missed anything else?