Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.7)
// Compass (v1.0.1)
// ----

// path e.g. 'c.B'
$z-stack: (
  a:  'just a comment', //  8
  b:  2,                //  7 @warn
  c: (                  //  4
    A: auto,            //  6
    B: auto,            //  5
  ),
  d:  3,                //  3
  e:  auto              //  1
);

// split string
@function split-path($path) {
  @if 'string' == type-of($path) {
    $index: str-index($path, '.');
    @if $index {
      @return str-slice($path, 1, $index - 1), str-slice($path, $index + 1);
    }
  }
  @return $path, null;
}

@function z-index($path, $stack: $z-stack, $from: 1, $parent: '') {
  $path:  split-path($path);
  $name:  nth($path, 1);
  $child: nth($path, 2);
  $min:   $from;
  $z:     null;

  // unknown layer
  @if not map-has-key($stack, $name) {
    @warn 'Unknown layer `#{$parent}#{$name}`';
  }
  @else {
    $names: map-keys($stack);
    $index: index($names, unquote($name)) or index($names, red);

    // loop from back layer to current
    @for $i from length($names) through $index {
      $l: nth($names, $i);
      $z: map-get($stack, $l);
      
      @if 'map' == type-of($z) {
        // targeted map
        @if $name == $l {
          @if $child {
            // get child z-index
            $z: z-index($child, $z, $min + 1, #{$name}#{'.'});
            @if $z == null {
              @return null;
            }
          }
          @else {
            // the map itself has an index just in case
            $z: $min;
          }
        }
        @else {
          // get higher nested index
          $z: z-index(nth(map-keys($z), 1), $z, $min + 1, #{$name}#{'.'});
        }
      }
      // set min as auto
      @else if auto == $z or 'number' != type-of($z) {
        $z: $min;
      }
      // set min for unexpected value
      @else if $z < $min {
        @warn 'Wrong z-index for layer `#{$parent}#{$l}`';
        $z: $min;
      }
      
      $min: $z + 1;
    }
  }

  @return $z;
};

@mixin demo($stack, $parent: '') {
  @each $name, $index in $stack {
    #{$name}  {
      @if 'map' == type-of($index) {
        /* generated for the map */
        @include demo($index, #{$parent}#{$name}#{'.'});
      }
      @else {
        /* #{$index} */
      }
      z-index: z-index(#{$parent}#{$name}); 
    }
  }
}

@include demo($z-stack);

SassMeister-output.css

a {
  /* just a comment */
  z-index: 8;
}

b {
  /* 2 */
  z-index: 7;
}

c {
  /* generated for the map */
  z-index: 4;
}
c A {
  /* auto */
  z-index: 6;
}
c B {
  /* auto */
  z-index: 5;
}

d {
  /* 3 */
  z-index: 3;
}

e {
  /* auto */
  z-index: 1;
}

This is great. Is there a way to adapt this so that it can be used in plain Sass? Which of the functions depend on Compass?

Compass is used by default on SassMeister but this gist doesn't depend on it ;)