<===> input.scss
a {
  b: {
    @warn test;
    c: d;
  }
}

<===> error
Error: Illegal nesting: Only properties may be nested beneath properties.
        on line 3 of input.scss
  Use --trace for backtrace.

<===> output-dart-sass.css
a {
  b-c: d;
}

<===> warning-dart-sass
WARNING: test
    input.scss 3:5  root stylesheet