Mixins: Inconsistent errors when not passing required params

[dwighthouse]

Related to: #76

There are two ways to avoid passing a required param to a mixin: empty params (@include some-mixin();) and no params (@include some-mixin;). However, they behave differently.

Empty Params Example:

/* Mixin with no defaults */
@mixin heading-text($font-size) {
    font-size: $font-size;
}

/* Using empty param form results in empty value instead of error */
h1 {
    @include heading-text();
}

Output:

/* Mixin with no defaults */
/* Using empty param form results in empty value instead of error */
h1 {
    font-size: ; /* <= ERROR */
}

This is invalid CSS.

---

No Params Example:

/* Mixin with no defaults */
@mixin heading-text($font-size) {
    font-size: $font-size;
}

/* Using no param form results in PreCSS throwing */
h1 {
    @include heading-text;
}

Output (throws):

Message:
    precss: /.../main.css:3:5: Could not resolve the variable "$font-size" within "$font-size"

  1 | /* Mixin with no defaults */
  2 | @mixin heading-text($font-size) {
> 3 |     font-size: $font-size;
    |     ^
  4 | }
  5 | 

This is confusing because the issue is at the site of the @include, not at the @mixin. ("What do you mean you can't resolve the variable?! It's RIGHT THERE!")

---

I do not know what the correct answer should be, only that it should be consistent. I see two reasonable options:

1. Output the CSS without the result of the mixin's failed operation (and maybe a warning about what happened).

   h1 {}

2. Throw with a more specific error, like:

   Message:
       precss: /.../main.css:8:5: Included mixin without required variable "$font-size"
   
     6 | /* Using empty param form results in empty value instead of error */
     7 | h1 {
   > 8 |     @include heading-text;
       |     ^
     9 | }