<===> input.scss
@function _test1() {
  @return 'hello';
}

@function -test2() {
  @return 'hello';
}

@function test() {
  @return 'world';
}

@mixin _test1() {
  mixin: true;
}

@mixin -test2() {
  mixin: true;
}

@mixin test() {
  mixin: true;
}

$-test1: true;
$_test2: true;
$test: true;

.test {
  function: function-exists('_test1');
  function: function-exists('-test1');
  function: function-exists('_test2');
  function: function-exists('-test2');
  function: function-exists('test1');
  function: function-exists('test2');
  function: function-exists('test');
  mixin: mixin-exists('_test1');
  mixin: mixin-exists('-test1');
  mixin: mixin-exists('_test2');
  mixin: mixin-exists('-test2');
  mixin: mixin-exists('test1');
  mixin: mixin-exists('test2');
  mixin: mixin-exists('test');
  variable: variable-exists('_test1');
  variable: variable-exists('-test1');
  variable: variable-exists('_test2');
  variable: variable-exists('-test2');
  variable: variable-exists('test1');
  variable: variable-exists('test2');
  variable: variable-exists('test');
  global-variable: global-variable-exists('_test1');
  global-variable: global-variable-exists('-test1');
  global-variable: global-variable-exists('_test2');
  global-variable: global-variable-exists('-test2');
  global-variable: global-variable-exists('test1');
  global-variable: global-variable-exists('test2');
  global-variable: global-variable-exists('test');
}

<===> output.css
.test {
  function: true;
  function: true;
  function: true;
  function: true;
  function: false;
  function: false;
  function: true;
  mixin: true;
  mixin: true;
  mixin: true;
  mixin: true;
  mixin: false;
  mixin: false;
  mixin: true;
  variable: true;
  variable: true;
  variable: true;
  variable: true;
  variable: false;
  variable: false;
  variable: true;
  global-variable: true;
  global-variable: true;
  global-variable: true;
  global-variable: true;
  global-variable: false;
  global-variable: false;
  global-variable: true;
}