<===> input.scss
@mixin foo() {
    foo: &;
}

foo {
  @include foo { bar: baz }
}

<===> error
Error: Mixin "foo" does not accept a content block.
        on line 6 of input.scss, in `foo'
        from line 6 of input.scss
  Use --trace for backtrace.

<===> error-dart-sass
Error: Mixin doesn't accept a content block.
    ,
1   | @mixin foo() {
    |        ===== declaration
... |
6   |   @include foo { bar: baz }
    |   ^^^^^^^^^^^^ invocation
    '
  input.scss 6:3  root stylesheet