Let's get the semantic around closure fixed.

[TheGag96]

On Thursday, 20 May 2021 at 12:31:00 UTC, Steven Schveighoffer 
wrote:
> On 5/19/21 9:02 AM, Steven Schveighoffer wrote:
>
>> Of course, with Walter's chosen fix, only allowing capture of 
>> non-scoped variables, all of this is moot. I kind of feel like 
>> that's a much simpler (even if less convenient) solution.
>
> After reading a lot of this discussion, I have changed my mind. 
> We should implement the "correct" thing even if it performs 
> poorly. While Walter's solution gets the compiler out of 
> responsibility, it doesn't square with the fact that closures 
> are already hidden allocations, so consistency dictates we deal 
> with inner allocations the same way.
>
> We need one heap block per scope that has captured variables. 
> Expensive, but I don't see a way around it. Hopefully 
> optimizers and scope delegates can alleviate performance issues.
>
> -Steve

Yeah... Honestly, that getting-around-immutable thing seems like 
the nail in the coffin for the current behavior. Hopefully making 
it work "correctly" won't be too painful...

The delegate-related thing I really want improved is being able 
to capture local variables in places like:

```d
int i = 3;
someRange.map!(x => x.thing == i).each!writeln;
```

...without needing the GC, since we "know" that `i` doesn't 
escape. Dunno if that's a pipe dream, though.