<===> input.scss
@mixin bar {
  @at-root @bar {a: b}
}

.foo {
  @include bar;
}
<===> error
Error: Invalid CSS after "": expected selector, was "@bar"
        on line 2 of input.scss, in `bar'
        from line 6 of input.scss
  Use --trace for backtrace.

<===> error-dart-sass
Error: expected selector.
  ,
2 |   @at-root @bar{a: b}
  |            ^
  '
  input.scss 2:12  bar()
  input.scss 6:3   root stylesheet