Ensure component modifiers have a set base. @extend in Sass can get out of hand. Manually setting both base and modifiers can trim down the compiled styles.

base-debug.css

// Find modifer without base.
// .[base--]modifier     - yes
// .[base-s]ubelement    - no
// .foo-[base--]modifier - no
// .bar[base--]modifier  - uhh.. Just name your classes correctly.
[class*="base--"]:not([class*="-base--"]) {
  outline: 2px solid red;
}
// Base exists, cancel.
[class*="base--"]:not([class*="-base--"]).base {
  outline: none;
}

debug.scss

$debug-enable-modifier-has-base: false !default;
$debug-base-component-list: (
  'base'
);

$debug-enable-modifier-has-base: true;

@if ($debug-enable-modifier-has-base) {
  @each $base in $debug-base-component-list {
    [class*="#{$base}--"]:not([class*="-#{$base}--"]) {
      outline: 2px solid red;
      &.#{$base} {
        outline: none;
      }
    }
  }
}