<===> input.scss
@mixin parent {
  @mixin nested {
    foo: bar;
  }

  @include nested;
}


test {
  @include parent;
}

<===> error
Error: Mixins may not be defined within control directives or other mixins.
        on line 2 of input.scss
  Use --trace for backtrace.

<===> error-dart-sass
Error: Mixins may not contain mixin declarations.
  ,
2 |   @mixin nested {
  |   ^^^^^^^^^^^^^^
  '
  input.scss 2:3  root stylesheet