<===> input.scss
@function aFunction() {
    @return 1em;
}

@media (max-width: 1em) {
    %placeholder {
        color: red;
    }
}

@media (max-width: aFunction()) {
    .test {
        @extend %placeholder;
    }
}
<===> output.css
@media (max-width: 1em) {
  .test {
    color: red;
  }
}