Request for Comment assert(__ctfe)

[Johannes Pfau]

Am Wed, 08 Apr 2020 09:26:36 -0400 schrieb Steven Schveighoffer:

> 
>> My primary fear here is that the current implementation checks the
>> function too late in semantic and therefore such checks (or the "ensure
>> no non-ctfe function calls ctfe-only functions" check are much more
>> difficult to implement than they should be. Using a pragma or attribute
>> would easily solve this.
> 
> This is a legitimate concern. Indeed, it might not fix this issue just
> to not emit the code to the binary.
> 
> -Steve


I've posted a proof-of-concept PR using pragma(ctfe) here:
https://github.com/dlang/dmd/pull/11014

Unlike the `assert(__ctfe)` check, this is done very early in the 
semantic analysis. I added support to bypass all -betterC checks I could 
find in the DMD source code. If one is missing, it should be trivial to 
add. I've also added examples to the tests in the PR.

I also tried to backport the commit which introduces the betterC bypass 
to Stefan's PR. But as expected it does not work, as the assert detection 
is done too late. I'm not saying that we must go for pragma(ctfe), but 
whatever implementation we merge, it should at least support those tests 
as well. Especially considering the pragma implementation is also way 
less complex.


Another thing we could think about is how to handle this code:

@nogc:

pragma(ctfe)
string mixinHelper2(string a)
{
     return "int " ~ a ~ ";";
}

void main() @nogc
{
     mixin(mixinHelper("a"));
}


I'd probably disable nogc checks in ctfe-only functions for convenience. 
But I guess that is debatable.

-- 
Johannes