[SAoC 2021] Replace druntime Hooks with Templates: Milestone 2, Week 4

[Teodor Dutu]

Hi,

This week I focused on finishing the replacement of the 
`_d_arrayctor` hook with a template.
After receiving the advice to void-initialise the array created 
by `_d_arrayctor`, I changed its signature so that lowerings to 
it now make use of its return value, via [this 
PR](https://github.com/dlang/druntime/pull/3611).

The reasoning behind this change was outlined in [this forum 
post](https://forum.dlang.org/post/simesvkancmscrtsciwq@forum.dlang.org). Long story short, `_d_arrayctor` was and still is strongly pure. Because expressions such as `T[4] a = b` were lowered to `_d_arrayctor(a[], b[])`, this meant that the lowering was a call to a strongly pure function whose return value was ignored. This made the compiler issue some warnings, while also running the risk of having the compiler remove the call to `_d_arrayctor` altogether.

Next, I changed the aforementioned lowering to `a = 
_d_arrayctor(!typeof(a))(b[])`.
This worked, except for [this 
test](https://cirrus-ci.com/task/4617199160655872?logs=test_druntime#L1548), where I noticed that the compiler inserts a `catch` block at the end of `_d_arrayctor`. This block catches the exception thrown [here](https://github.com/dlang/druntime/blob/1a2f2158345ece9fc3eec098a55519644bf6dfd0/src/core/internal/array/construction.d#L73), calls `__ArrayDtor(to)` and then re-throws the exception. But the first elements of the array have already been destroyed by `_d_arrayctor` itself.
This creates something like a double free error. In addition, 
`__ArrayDtor` calls the destructor of each element, including the 
final ones, which have not been initialised.
I am currently looking for ways to remove this final catch block 
without negatively impacting use cases when this block really is 
needed.

Thanks,
Teodor