[SAoC 2021] Replace druntime Hooks with Templates: Milestone 3, Week 5

[Teodor Dutu]

Hi,

This report also contains the first 3 days of milestone 4, due to 
the misalignment between weeks and months in the callendar.

This week I finished the lowering of the new hook for 
`_d_delstruct`. My first approach for outputing the appropriate 
errors caused by the `delete` expression was to return a 
`CommaExp` made up of both the original expression and the 
lowered call to `_d_arraydtor`. This was followed by the call 
when it came to errors and CTFE and ignoring the `delete` in 
e2ir.d.

However, this was an inelegant solution and Razvan suggested to 
me that I only return the lowering and gag errors when analysing 
it semantically. At the same time, `_d_delstruct` is both 
`nothrow` and `nogc` because of a hack that was necessary in 
order for the lowering to be possible. As a result, when checking 
the new hook for `nothrow` and `nogc`, only the dtor of the 
parameter is checked. Now the code is working without keeping the 
original `DeleteExp` all the way to e2ir.d.

I also inspected `_d_arrayappendcTX` a bit more. One solution for 
my problem from the previous week can be to hack the hook's 
attribute and lie to the compiler that it's `pure`. Purity caused 
the error in question, but this hack might also be necessary for 
`nothrow` and `@safe`. However, I can't give a verdict on this 
yet and still need to explore more options.

Thanks,\
Teodor