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.. default-role:: code
.. include:: ../rstcommon.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 derived from the routine's first parameter:
```nim
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.
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