1882379 - Generate warning for useless @scope selectors

Status: NEW

(Core :: CSS Parsing and Computation, enhancement)

Description

[David Shin[:dshin]]

@scope defines its in-scope regions strictly in terms of ancestor/descendant relationship:

An element is in scope if:

• It is an inclusive descendant of the scoping root, and
• It is not an inclusive descendant of a scoping limit.

But it also has :scope resolved to the scoping root. This means that it's possible to write some no-effect selectors:

• Scope-end that escapes the ancestor/descendant relationship, like: @scope (.scope-start) to (:scope ~ .scope-end /* ... */) { /*...*/ } == @scope (.scope-start)
• Inner rules that will never match, like:

@scope (.scope-start) to (.scope-end) {
  /* ... */ :scope ~ * { /* ... */ }
}

Note that & may resolve to the scoping root, but it's not always:

The & selector is defined to represent the selector representing the scoping root (the <scope-start> selector), or else :scope if no selector was specified.

So & ~ * { /**/ } isn't guaranteed to be useless. On the other hand, relative selector style specification ~ * { /* ... */ } is anchored on :scope so it is useless.