<===> input.scss
.nesting-characters {
  --parens: (foo; (bar: baz;) bang!);
  --curly: {foo; {bar: baz;} bang!};
  --square: [foo; [bar: baz;] bang!];
  --multiple: [({{([])}})];

  // Nested properties aren't supported in custom properties.
  --nested-props: {foo: bar;};

  // A property that's ambiguous with a nested selector is interpreted as a
  // custom property.
  --ambiguous:foo {bar: baz;};
}

<===> output.css
.nesting-characters {
  --parens: (foo; (bar: baz;) bang!);
  --curly: {foo; {bar: baz;} bang!};
  --square: [foo; [bar: baz;] bang!];
  --multiple: [({{([])}})];
  --nested-props: {foo: bar;};
  --ambiguous:foo {bar: baz;};
}