Truly @nogc Exceptions?

[Atila Neves]

On Thursday, 20 September 2018 at 12:48:13 UTC, Steven 
Schveighoffer wrote:
> On 9/20/18 6:48 AM, Atila Neves wrote:
>> On Wednesday, 19 September 2018 at 21:16:00 UTC, Steven 
>> Schveighoffer wrote:
>>> [...]
>> 
>> <snip>
>> 
>> I expressed my concern for DIP1008 and the `msg` field when it 
>> was first announced. I think the fix is easy and a one line 
>> change to dmd. I also expressed this on that thread but was 
>> apparently ignored. What's the fix? Have the compiler insert a 
>> call to the exception's destructor at the end of the 
>> `catch(scope Exception)` block. That's it. The `msg` field is 
>> just a slice, point it to RAII managed memory and you're good 
>> to go.
>> 
>> Give me deterministic destruction of exceptions caught by 
>> scope when using dip1008 and I'll give you @nogc exception 
>> throwing immediately. I've even already written the code!
>
> I thought it already did that? How is the exception destroyed 
> when dip1008 is enabled?

I've had a 2 week holiday since I was working on this so I don't
quite remember. I'll try and find out the specifics for you later.
What I do remember is that the destructor didn't get called.

> But this means you still have to build msg when throwing the 
> error/exception. It's not needed until you print it, and 
> there's no reason anyway to make it allocate, even with RAII.

Maybe, but it's a lot easier to just allocate and have the 
destructor take care of it.

> For some reason D forces msg to be built, but it does't e.g. 
> build the entire stack trace string before hand, or build the 
> string that shows the exception class name or the file/line 
> beforehand.

Good points.