Casting &i32 as usize - rust

Question

I have a function that is meant to make a move in connect 4, this function takes 3 parameters. The main issue is that the square variable is out of scope in the for loop so it must be borrowed however I cannot cast an &i32 to usize.
fn make_move<'playing>(board: &'playing Vec<&str>, column: i32, turn: &'playing i32) -> &'playing Vec<&'playing str> {
let mut square = column;
let new_board = board;
for i in 0..6 {
if new_board[(&square) as usize] == " " {
square = &square + 7;
} else {
if turn % 2 == 0 {
new_board[(&square - 7) as usize] = "●";
} else {
new_board[(&square - 7) as usize] = "○";
}
}
}
return new_board;
}
error[E0606]: casting `&i32` as `usize` is invalid
--> src/lib.rs:6:22
|
6 | if new_board[(&square) as usize] == " " {
| ---------^^^^^^^^^
| |
| cannot cast `&i32` as `usize`
| help: dereference the expression: `*(&square)`
I also just started using Rust so I am sure my logic is flawed and maybe this is a dumb mistake but help would be much appreciated.

Answer 1

In this case, you don't need the ampersands at all before square. square is an i32, which is an integer type, and you want to turn it into another integer type, so you don't need to borrow it at all. Since i32 is Copy, you can make cheap copies of square without a problem.
Additionally, new_board is making a copy of your reference to board, not copying the actual board. Vec is not Copy, since copies are not cheap, so you'll need to use the clone method to create a new board and then return the actual Vec, not a reference. With the above change and this one, the result would look like this:
fn make_move<'playing>(
board: &'playing Vec<&str>,
column: i32,
turn: &'playing i32,
) -> Vec<&'playing str> {
let mut square = column;
let mut new_board = board.clone();
for i in 0..6 {
if new_board[(square) as usize] == " " {
square = square + 7;
} else {
if turn % 2 == 0 {
new_board[(square - 7) as usize] = "●";
} else {
new_board[(square - 7) as usize] = "○";
}
}
}
return new_board;
}
Alternatively, you can make board a mutable reference, in which case you will modify both the old and new board, but you can continue to return a reference. That would look like this:
fn make_move<'playing>(
board: &'playing mut Vec<&str>,
column: i32,
turn: &'playing i32,
) -> &'playing Vec<&'playing str> {
let mut square = column;
let new_board = board;
for i in 0..6 {
if new_board[(square) as usize] == " " {
square = square + 7;
} else {
if turn % 2 == 0 {
new_board[(square - 7) as usize] = "●";
} else {
new_board[(square - 7) as usize] = "○";
}
}
}
return new_board;
}