[SAoC 2023] Replace DRuntime Hooks with Templates Weekly Update #8

[Teodor Dutu]

Hi,

Last week I fixed [this 
bug](https://issues.dlang.org/show_bug.cgi?id=24159) introduced 
by `_d_arrayappendcTX`. The fix meant first storing the lowering 
to a new field inside `CatAssignExp` instead of modifying the 
original array.

This in turn caused `vthis`s for some functions because of the 
inliner. The solution was to [inline the 
lowering](https://github.com/teodutu/dmd/blob/71518bee8b2779a9cd6064537470582dae4c29c1/compiler/src/dmd/optimize.d#L931-L937) when present.

After this, I ran into a familiar error: the lowering needs no 
codegen at CTFE. In fact, the lowering shouldn't even be 
introduced during semantic if the `CatAssignExp` is only needed 
at CTFE. However, this problem is familiar because I ran into it 
in the past. The same restrictions hold for `CatExp`s, but 
because telling if an expression needs codegen or not is a mess 
during semantic, I couldn't figure out when not to perform the 
lowering to `_d_arraycatnTX`. The solution in both cases was to 
handle this case during IR generation. Therefore, unfortunately, 
both lowerings are made even in some situations when they're not 
required and then `e2ir.d` checks of using the GC is enabled. If 
not, [it outputs an 
error](https://github.com/dlang/dmd/blob/3d552df287d0b836861f760701b16569311e4dd7/compiler/src/dmd/e2ir.d#L2796-L2802). I'd love to do this more nicely in the future and I'm looking for advice on how to figure out when to not generate lowerings during semantic analysis. If you have any advice on the topic, please let me know.

Furthermore, I'm almost done converting `_d_newarray{mTX, miTX, 
Op}` to a single template. I created [this draft 
PR](https://github.com/dlang/dmd/pull/15819) with my changes. I 
still need to fix some final tests in dmd and Phobos before 
turning it into a legit PR. I need to say that the previous 
implementations of `_d_newarraym{,i}TX` were really messy because 
they called `_d_newarrayOp` with a funcion argument with which to 
initialise the arrays (one of `_d_newarray{,i}T`). But now that 
[I've converted these 2 to a single 
template](https://github.com/dlang/dmd/pull/15299) as well, I was 
able to coalesce all hooks for multi-dimensional arrays into a 
single one much more smoothly.

Thanks,
Teodor