Prevent struct going into heap memory

[Richard (Rikki) Andrew Cattermole]

On 21/07/2025 12:00 PM, Paul Backus wrote:
> On Sunday, 20 July 2025 at 17:04:11 UTC, Richard (Rikki) Andrew 
> Cattermole wrote:
>> In response to some concerns around the situation for RAII structs 
>> going into closures, I am proposing a resolution to this that will be 
>> a language-wide guarantee, not an ultra-specific, not-a-guarantee 
>> solution.
>>
>> https://github.com/dlang/dmd/issues/18704
>>
>> If a struct destructor is annotated with an attribute ``@stackonly`` 
>> it may only be called if the ``this`` pointer is allocated on the 
>> stack. It does not overload.
> 
> Isn't the obvious solution to this bug (and related ones like [issue 
> #19119][1]) to simply skip the end-of-scope destructor call for structs 
> that are allocated in heap closures? Why introduce this weird, special- 
> purpose hack instead of fixing the root cause of the problem?
> 
> [1]: https://github.com/dlang/dmd/issues/19119

Ah see, I am arguing that the GC should handle cleanup.

Problem is, some people believe it shouldn't. Hence this proposal to 
give those that want that "reliability" control to make it so.