<===> input.scss
@mixin selector-access {
  mixin-sees: &;
}

@function function-access() {
  @return &;
}

.foo {
  @include selector-access;
  function-sees: function-access();
}

.bar a {
  @include selector-access;
  function-sees: function-access();
}

.bar,
.baz {
  @include selector-access;
  function-sees: function-access();
}

.qux {
  &.waldo {
    .where & {
      .final {
        @include selector-access;
        function-sees: function-access();
      }
    }
  }
}
<===> output.css
.foo {
  mixin-sees: .foo;
  function-sees: .foo;
}

.bar a {
  mixin-sees: .bar a;
  function-sees: .bar a;
}

.bar,
.baz {
  mixin-sees: .bar, .baz;
  function-sees: .bar, .baz;
}

.where .qux.waldo .final {
  mixin-sees: .where .qux.waldo .final;
  function-sees: .where .qux.waldo .final;
}