<===> input.scss
.test .nest {
  length: length(&);
  @each $list in & {
    list: $list;
    length: length($list);
  }
}

.test, .other {
  length: length(&);
  @each $list in & {
    list: $list;
    length: length($list);
  }
}

<===> output.css
.test .nest {
  length: 1;
  list: .test .nest;
  length: 2;
}

.test, .other {
  length: 2;
  list: .test;
  length: 1;
  list: .other;
  length: 1;
}