Let's get the semantic around closure fixed.

[Paul Backus]

On Friday, 28 May 2021 at 13:47:32 UTC, deadalnix wrote:
> On Friday, 28 May 2021 at 02:47:41 UTC, Adam D. Ruppe wrote:
>> On Friday, 28 May 2021 at 02:41:27 UTC, Q. Schroll wrote:
>>> I was a little surprised that it needs the GC in the first 
>>> place. It's a template parameter after all.
>>
>> It isn't the lambda that allocates per se, it is the 
>> `MapResult` struct that cannot necessarily be scope since it 
>> doesn't know if the range will be stored or returned or 
>> whatever.
>>
>> Though perhaps if the returned map result inherited the 
>> lifetime of the captured variables it could work, just that 
>> gets complicated.
>
> I suspect the compiler should be able to see through it after a 
> few rounds of inlining.
>
> If there are so many layers that it doesn't, I suspect one 
> allocation isn't going to be your bottleneck.

`ldc -O` is able to elide the allocation: 
<https://d.godbolt.org/z/9aoMo9hbe>

However, the code still does not qualify as `@nogc`, because 
`@nogc` analysis is done prior to optimization.