Placeholder existence check

Coming from https://github.com/nex3/sass/pull/821, can placeholder existance checks be added? Checking existence, especially within the current (@media) scope, would be useful.

[robwierzbowski]

BTW, I no longer have a need for this feature. Removing myself from the discussion 👋🏼

[nex3]

The problem with trying to do the same thing as we do with variables is that style rules fundamentally don’t work the same way Sass variables do. They’re declarative, they cascade, and they apply selectively based on the structure of the document which isn’t known at compile-time. All of this makes the tools we use for dealing with variables inapplicable to selectors. Put another way, the question “has Sass compiled a style rule that contains this simple selector yet” doesn’t map to any semantic insight about the way the document is being styled.

To put this in perspective, I’m building quite a complicated project: a game-engine using only CSS and HTML. It’s coming along nicely, and I’m finding work-arounds all over the place. But having detection for these place-holders would mean that a user of the engine could override defaults rather than importing the defaults they need. Perhaps I could walk through with you my problem at some point and you could possibly come up with some better solutions? It’s very possible I’ve missed something I could use instead.

Style rules have a built-in mechanism for overriding defaults: the cascade. I recommend you use that, possibly wrapped with mixins if you want to control the specifics of what people can override.