Allow variable decleration with inout and borrowed keywords.

[yinonburgansky]

Currently there is an asymmetry in the language, only function arguments can be declared as owned, inout, and borrowed but variable declarations are always owned (var).

I think completing the symmetry, allowing variable declaration as inout and borrowed would be nice.

declaring variable with borrowed and inout:

var my_list = List(1, 2, 3)

var x = my_list[0]  # copy since `var` behaves like `owned`
x = 2  # doesn't change the list

borrowed y = my_list[0]  # immutable reference to the first element
my_list[0] = 7 
print(y)  # prints 7

inout z = my_list[0]  # mutable reference to the first element
z = 8  # changes the first element in the list
print(y)  # prints 8

# my_list is destroyed here since there are no more live references (y, z)

In for loop:

var my_list = List(1, 2, 3)
for inout x in my_list:
    x += 1
print(my_list)  # prints [2, 3, 4]

---

On a related note,
return type is also always owned and might benefit from inout and borrowed,
but not sure about that, as it's very similar to Reference and maybe returning a mutable/immutable reference would allow the following functionality?

struct List[T]:
    ...
    fn __getitem__(self, i: Int) -> inout[life_of(self)] T: # or Reference[..]
         ...

fn first_element(my_list: List[Int]) -> inout[life_of(my_list)] Int: # or Reference[..]
    inout first = my_list[0]
    return first

### now we can use `first_element(my_list)` instead of `my_list[0]` 
var my_list = List(1, 2, 3)

var x = first_element(my_list)  # copy since `var` behaves like `owned`
x = 2  # doesn't change the list

borrowed y = first_element(my_list)  # immutable reference to the first element
first_element(my_list) = 7  # allowed to be on the left side since it returns `inout`?
print(y)  # prints 7

inout z = first_element(my_list)  # mutable reference to the first element
z = 8  # changes the first element in the list
print(y)  # prints 8

# my_list is destroyed here since there are no more live references (y, z)

[yinonburgansky]

Closing as this seems to be discussed already in lifetimes-keyword-renaming