<===> input.scss
@mixin bar() {
  @at-root { @content; }
}

.test {
  @include bar() {
    color: yellow;
    .nest2 {
      color: green;
    }
  }
}

<===> error
Error: Properties are only allowed within rules, directives, mixin includes, or other properties.
        on line 7 of input.scss
  Use --trace for backtrace.

<===> error-dart-sass
Error: Declarations may only be used within style rules.
  ,
7 |     color: yellow;
  |     ^^^^^^^^^^^^^
  '
  input.scss 7:5   @content
  input.scss 2:14  bar()
  input.scss 6:3   root stylesheet