Error handling meeting summary

[Richard Andrew Cattermole (Rikki)]

Hello all!

We've had a meeting on how error handling should work in D.

Approved work:

- Finalisers are brought back to the baseline behaviour. They 
will not be rewritten to sequences in the frontend. 
https://github.com/dlang/dmd/blob/3d06a911ac442e9cde5fd5340624339a23af6eb8/compiler/src/dmd/statementsem.d#L3428

- A CLI switch may be offered that performs rewriting of finally 
statements to sequences if an ``Exception`` is not thrown within 
the try body and finally statements will be rewritten to 
``catch(Exception)`` statements. This is Walter's approach to 
error handling that he recommends. It cannot be made the default 
as it will break language features.

- A filter method is added to the ``Thread`` class hierarchy; 
this filter method, by default, will call a grave digger global 
function pointer if set. Otherwise, all ``Throwable``'s will 
propagate like they do today.

     It was considered that perhaps it would be a good idea to a 
flag on ``Thread`` creation to determine if you want to kill the 
process if ``Error`` is seen. No decision is made but I see no 
reason a PR wouldn't be accepted if it only changes the default 
behaviour of the filter method.

I'm happy to do finally statement being brought back to baseline, 
and filter method. However, it will be in the next two months, 
not now. If someone wants to get to it before me they may.

Walter will have to do the CLI switch stuff, due to intersecting 
with dmd's glue code.

Approved work for someone who wants to do it:

- A pragma within a function that disables unwinding. Requires a 
DIP. Requires Iain&Martin's consultation.

- Contracts being called in the caller, not callee. Walter wants 
a DIP document to cover the current state (does not need to go 
into the queue, it only has to exist). Timon offered to do it. 
Anyone can implement as long as we get the document.

- Null deref read barrier aka null check. CLI switch, throw Error 
with a hook function. Noted that this is useful for platforms 
like web assembly and debugging CI's where no stack trace is 
present.

Now for the trouble-making question, is there something that 
people need that I haven't covered in these lists?