<===> input.scss
@function test($pos) {
  @return test-#{$pos};
}

.foo {
  content: test(str-slice('scale-0', 7));   // Nope
  content: test-#{str-slice('scale-0', 7)}; // Yep
}
<===> output.css
.foo {
  content: test-0;
  content: test-0;
}
