<===> input.scss
@function issues-warning($a) {
  @warn "From function: #{inspect($a)}";
  @return $a;
}

@mixin calls-function-that-warns($a) {
  warned: issues-warning($a);
}

.test {
  @include calls-function-that-warns(testing);
}

<===> output.css
.test {
  warned: testing;
}

<===> warning
WARNING: From function: testing
         on line 2 of input.scss, in `issues-warning'
         from line 7 of input.scss, in `calls-function-that-warns'
         from line 11 of input.scss

<===> warning-dart-sass
WARNING: From function: testing
    input.scss 2:3   issues-warning()
    input.scss 7:11  calls-function-that-warns()
    input.scss 11:3  root stylesheet