Truly @nogc Exceptions?

[Steven Schveighoffer]

On 9/20/18 6:48 AM, Atila Neves wrote:
> On Wednesday, 19 September 2018 at 21:16:00 UTC, Steven Schveighoffer 
> wrote:
>> Given dip1008, we now can throw exceptions inside @nogc code! This is 
>> really cool, and helps make code that uses exceptions or errors @nogc. 
>> Except...
>>
>> The mechanism to report what actually went wrong for an exception is a 
>> string passed to the exception during *construction*. Given that you 
>> likely want to make such an exception inside a @nogc function, you are 
>> limited to passing a compile-time-generated string (either a literal 
>> or one generated via CTFE).
> 
> <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?

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. 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.

-Steve