Nim/doc/manual/var_t_return.rst

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 deviated by the routine's first parameter:

.. code-block:: nim
  proc forward[T](x: var T): var T =
    result = x # ok, deviated from the first parameter.

  proc p(param: var int): var int =
    var x: int
    # we know 'forward' provides a view into the location deviated by
    # 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 callsite.