Stylus selector combinator mixin: http://codepen.io/thybzi/pen/GobVEX

combinate.styl

//**
//* Put CSS combinator between current selector and subj -- in direct and reversed order
//* Apply CSS rules block to resulting selector
//* Example output: .parent .curr + .subj, .parent .subj + .curr { color: red; }
//* Limitation: current selector must have a separent level in selector tree (e.g. & > .curr)
//* @param {string} subj Another selector to combinate with
//* @param {string='+'} comb CSS combinator (can be: '+', '~', '>' or ' ')
//*/
combinate(subj, comb='+')
  scurr = selectors()[-1]
  pcomb = ' '
  for c in ('>' '+' '~')
    if match('^& & ' + c + ' ', scurr)
      pcomb = c
  if comb != ' '
    comb = ' ' + comb + ' '
  if pcomb != ' '
    pcomb = ' ' + pcomb + ' '
  sdir = comb +  subj
  srev = '../' + pcomb + subj + comb + '^[-1..-1]'
  {join(', ', (sdir srev))}
    {block}
  
 
// Let the fun begin
.wrapper
  
  /*!* Test one (parent with rule '>', combinate with '+' by default) */
  & > .item
    +combinate('*')
      margin-top: 15px

  /*!* Test two (parent rule with ' ', combinate with '>') */
  .test
    +combinate('.fest', '>')
      vertical-align: bottom
      color: green

  /*!* Test three (parent rule with'~', combinate with ' ') */
  & ~ .foobar
    +combinate('.qux', ' ')
      padding-left: 45px
     
  /*!* Test four (many pre-parents, direct parent rule with '~', combinate with '+' */    
  & > .bar ~ .baz .qux + .whistle
    & ~ .andro
      +combinate('figure', '+')
        padding-left: 45px