+= extension for Lua 5.3

lparser.c

static void assignment (LexState *ls, struct LHS_assign *lh, int nvars) {
  expdesc e;
  check_condition(ls, vkisvar(lh->v.k), "syntax error");

  if (testnext(ls, ',')) {  /* assignment -> ',' suffixedexp assignment */
    struct LHS_assign nv;
    nv.prev = lh;
    suffixedexp(ls, &nv.v);
    if (!vkisindexed(nv.v.k))
      check_conflict(ls, lh, &nv.v);
    checklimit(ls->fs, nvars + ls->L->nCcalls, LUAI_MAXCCALLS,
                    "C levels");
    assignment(ls, &nv, nvars+1);
  }
  // Language extension begins here
  
  // This is a extra token which matches '+='
  else if (testnext(ls, TK_PLUSEQ)) {
    // Get the expression  
    expr(ls, &e);

    // Get the value
    expdesc infix = lh->v;
    
    // Sum
    luaK_infix(ls->fs, OPR_ADD, &infix);
    luaK_posfix(ls->fs, OPR_ADD, &infix, &e, ls->linenumber);

    // Store the result in the variable itself
    luaK_storevar(ls->fs, &lh->v, &infix);

    return;  /* avoid default */
  }
  
  ...

This works for the simple case x += 1 and x.y += 1 but not for:

local x = {
  x = {
    x = 0
  }
}

x.x.x += 1

which fails with: attempt to index a number value.