<===> input.scss
@mixin c() {
  @warn test;
  c: d;
}

a {
  b: {
    @include c();
  }
}

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

<===> warning
WARNING: test
         on line 2 of input.scss, in `c'
         from line 8 of input.scss

<===> warning-dart-sass
WARNING: test
    input.scss 2:3  c()
    input.scss 8:5  root stylesheet