[css-mixins-1] Mixins can be called (but not defined) without params

[andruud]

It's mildly annoying that you can invoke a mixin without a parameter list (@apply --m), but defining a parameter-less mixin requires an explicitly empty list (@mixin --m()).

It seem more civilized to either make the parameter list optional on the defining side as well, or require () when applying a mixin with no parameters.

If mixins are indeed going to gain a stronger underlying connection to custom functions (#12927), then I'd slightly prefer to just always require (), but I'm happy enough as long as we achieve some "symmetry" between @apply and @mixin.

[tabatkins]

I made the "arg-less calling" syntax to match what Sass can do, because it seemed nice, but you're right, Sass also lets you omit the arglist from the definition if you don't want any arguments. (see https://sass-lang.com/documentation/at-rules/mixin/)

So I'm fine with allowing the arglist to be omitted at the definition site if you want.

[sorvell]

I think it's valuable to be able to describe and explain mixins as functions that operate at the style rule level. Calling and defining them similarly helps reinforce that idea. For those with coding experience the () can help reinforce that this is a "call that returns a result."

For those reasons I like the idea of consistently defining and calling mixins with ().