twbs--bootstrap/scss/mixins/_nav-divider.scss

// Horizontal dividers
//
// Dividers (basically an hr) within dropdowns and nav lists

@mixin nav-divider($color: $nav-divider-color, $margin-y: $nav-divider-margin-y) {
  height: 0;
  margin: $margin-y 0;
  overflow: hidden;
  border-top: 1px solid $color;
}