Mixin Call Interpolation

[Spedwards]

Currently Variable Interpolation doesn't work with Mixins and I'd like that to change. I'm actually surprised that it doesn't work.

.flair.flair-one { color: red; }
.flair.flair-two { color: green; }
.flair.flair-three { color: blue; }

.custom(@a; @b; @c) {
  &::before { .flair.flair-@{a}; }
  .flair.flair-@{b};
  &::after { .flair.flair-@{c}; }
}

.this-flair {
  .custom(one; two; three);
}

Should result in the following css:

.flair.flair-one { color: red; }
.flair.flair-two { color: green; }
.flair.flair-three { color: blue; }

.this-flair {
    color: green;
}
.this-flair::before {
    color: red;
}
.this-flair::after {
    color: blue;
}

[seven-phases-max]

As a feature request this would be a duplicate of #1338 (with #1485 as a better alternative).
As for a question I suggested one of possible solutions in the corresponding SO topic.