Crash when @importing an upstream file with a top level @include that outputs a declaration only when that upstream file @uses another file

[Goodwine]

Example (HRX)

<==> input.scss
.foo {
  @import 'upstream';
}

<==> upstream.scss
@use 'other';

@mixin a() {
  b: c;
}

@include a;

<==> _other.scss
// empty, it doesn't matter

This crashes because evaluate.dart checks for the declaration's "parent's parent". And I guess the reference isn't there.

dart-sass/lib/src/visitor/evaluate.dart

Line 1345 in a42380f

var siblings = _parent.parent!.children;

Interestingly, this doesn't crash if we...:

• remove the @use rule
• replace @use for a built-in module like @use 'sass:math'
• wrap @include a with &{}
• remove the wrapping rule .foo{} (it does fail because the delcarataion ends up without a container, but it at least it doesn't crash)

[Goodwine]

I think I found out why. There is an if() which replaces _parent with _root when the user declares any @use that isn't a built-in module

dart-sass/lib/src/visitor/evaluate.dart

Lines 1904 to 1909 in a42380f

	if (loadsUserDefinedModules) {
	_root = ModifiableCssStylesheet(stylesheet.span);
	_parent = _root;
	_endOfImports = 0;
	_outOfOrderImports = null;
	}

I guess we need to detect that. But I'm not sure whether this should throw an error or not 🤔

[nex3]

The @include a should be an error. We should probably explicitly check that _parent != root when evaluating a declaration.

[Goodwine]

discussed this offline a bit more. In this case @include a shouldn't have thrown an error because the @import was nested within a style rule. It should (and it does) throw an error when a top level @include produces a declaration. We just need to be able to catch this specific case