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Hi,
This week I finished converting _d_newThrowable to a template, as well as its lowering in the compiler. The PRs for these are:
Also this week, I fixed the bug when compiling the new lowering for a ~= b to either _d_arrayappendcTX or _d_arrayappendT. It turned out it was because of how $ is evaluated by the compiler. The initial lowering when b was a single element looked like this:
__ctfe ? a ~= b : _d_arrayappendcTX(a, 1), a[$ - 1] = b, a;
When a chain of concatenations (like (a ~= b) ~= c) was lowered, in order for (a ~= b) to not be executed 3 times, I saved the result in a temporary variable, before concatenating c to it.
When this kind of expression was inlined, the $ was mishandled by the compiler, which resulted in an incorrect ArrayIndexError. To solve this issue, I used a.length instead of $. Now the lowering looks like this:
__ctfe ? a ~= b : _d_arrayappendcTX(a, 1), a[a.length - 1] = b, a;
// Note that the assignment above is actually a construction.
The PR that changes the lowering is: https://github.com/dlang/dmd/pull/13495.
A point of contention is the fact that I lower a ~= b to a CondExp. As I explained here, I believe this is the more elegant approach, despite being more complex.
Another issue was that because the current template _d_arrayappendcTX uses a temporary implementation that calls the old hook using a mixin.
For this reason, the new hook is always impure and has to be declared pure so it can pass semantic analysis from pure contexts.
_d_arrayappendT also calls _d_arrayappendcTX and, hence, is declared pure as well. However, _d_arrayappendT also calls copyEmplace, which can sometimes be impure. This causes an error when trying to instantiate _d_arrayappendT. To bypass this, I created an impure private function in DRuntime, which is called _d_arrayappendT after being made pure via a cast. This way, _d_arrayappendT appears pure when it needs to be, but is also able to call an impure copyEmplace when it has to.
The implementation of this hack is here: https://github.com/dlang/druntime/pull/3662.
Keep in mind that this is a hack for a stop-gap hook. In the future, I intend to change the implementation of _d_arrayappendcTX so that it won't call the old hook anymore and thus allow attributes such as pure, nothrow etc to be inferred by the compiler.
Thanks,
Teodor