doc/manual/var_t_return

Memory safety for returning by var T is ensured by a simple borrowing rule: If result does not refer to a location pointing to the heap (that is in result = X the X involves a ptr or ref access) then it has to be derived from the routine's first parameter:

proc forward[T](x: var T): var T =
  result = x # ok, derived from the first parameter.

proc p(param: var int): var int =
  var x: int
  # we know 'forward' provides a view into the location derived from
  # its first argument 'x'.
  result = forward(x) # Error: location is derived from `x`
                      # which is not p's first parameter and lives
                      # on the stack.

In other words, the lifetime of what result points to is attached to the lifetime of the first parameter and that is enough knowledge to verify memory safety at the call site.