Generated by SassMeister.com, the Sass playground.

SassMeister-input.scss

// automatic adjacent selector generation

$heading-blocks: h1 h2 h3 h4 h5 h6;
$other-blocks: p, ul, ol;

@mixin adjacent($list:()) {
    @if $list == () { // self-adjacent, if no list provided
        & + & { @content; }
    } @else { // takes space or comma separated list of single elements
        $arr: unquote("#{nth($list, 1)} + &");
        @for $n from 2 through length($list) {
            $arr: append($arr, unquote(", #{nth($list,$n)} + &"));    
        }
        #{$arr} { @content; }
    }
}

.test {
    @include adjacent {
        color: blue;    
    }
    @include adjacent($heading-blocks) {
        color: green;    
    }
    @include adjacent($other-blocks) { 
        color: red; 
    }
}

SassMeister-output.css

.test + .test {
  color: blue;
}
h1 + .test, h2 + .test, h3 + .test, h4 + .test, h5 + .test, h6 + .test {
  color: green;
}
p + .test, ul + .test, ol + .test {
  color: red;
}