cast(function qualifier)

[Jonathan M Davis]

On Wednesday, May 1, 2024 7:51:45 PM MDT Timon Gehr via dip.ideas wrote:
> The status quo is that it is annoying to type-pun function pointers to
> stronger function attributes as the entire type has to be specified:
>
> ```d
> (cast(int function(int)pure)&f)(2);
> ```
>
> This is also error prone as the return type and/or argument types could
> go out of sync with the actual type of `f`.
>
>
> `cast` could simply allow a list of function attributes and cast a
> function pointer or delegate to a type that additionally has those
> attributes:
>
> ```d
> (cast(pure)&f)(2);
> ```
>
> This simple syntactic sugar would make it a bit easier to work with
> attributes in `@system`/`@trusted` code. Of course, safety checks would
> still be performed, as for the more explicit cast syntax.

It certainly would make the code that has to cast functions pointers to
alter their attributes much tcleaner. Of course, there's always the argument
that making it clean instead of ugly will make it so that programmers are
more willing to do it when such code should be quite rare, but ultimately,
if someone is determined to be stupid with casting, they'll do it
regardless.

So, ultimately, it's probably a good idea, and the syntax makes sense.

That being said, one concern that I can think of is that it arguably makes
cast() inconsistent. Right now, casting with an attribute only works with
type qualifiers, so it makes sense that cast() would remove all type
qualifiers. However, if stuff like cast(pure) or cast(nothrow) is legal,
then I could see someone expecting that cast() would remove those as well,
which seems like it would be a bad idea in practice (as well as being a
breaking change). But if we don't make cast() remove pure, nothrow, etc.,
then it could be argued that that's inconsistent. I'm not sure that that
inconsistency is worth worrying about, but I could see it coming up at some
point.

In any case, I do agree that making cast work with function attributes on
function pointers and delegates would be a good addition.

- Jonathan M Davis