Request for Comment assert(__ctfe)

[Jacob Carlborg]

On 2020-04-05 14:11, Stefan Koch wrote:
> Hi Guys,
> 
> I am currently working on speeding up dmd. In the presence of templates 
> and ctfe.
> and one thing that stood out is that we do codegen for symbols which are 
> never used.
> (and hopefully eliminated by a smart linker but eh ...)
> 
> I've seen the following pattern in the wild.
> 
> 
> string GenerateMixin(string[] params)
> {
>      assert(__ctfe);
>      .... more code ....
> }
> 
> that means if we see assert(__ctfe) inside the outermost scope of a 
> function body ( not in a branch) then we know that this function would 
> assert if called at runtime.
> And therefore no correct code can be calling it.
> which also means we can forgo having code-gen or optimization for the 
> function body.
> and reduce the count of names which has to get mangled.
> 
> They only downside to this, is that giving assert(__ctfe) the special 
> meaning to skip codegen might be confusing to some people .... then 
> again you wouldn't use assert(__ctfe) unless you expect that function to 
> not be available at run-time
> 
> What do you guys think?

For many years I wanted a feature that could indicate a function is only 
used at compile time.

BTW, this is a breaking change. Consider this code:

import core.exception;

void foo()
{
     assert(__ctfe);
}

void main() @system
{
     try
         foo();
     catch (AssertError)
     {}
}

-- 
/Jacob Carlborg