How to pass a mutable function parameter as argument to another function?

Question

Why does Rust prevent this code from compiling, with the error: "cannot borrow immutable local variable arr as mutable"? How to pass the vector into another function as mutable reference?

let mut vec = vec![0];

fn bar(vec: &mut Vec<i32>) {
    // some code here
}

fn foo(vec: &mut Vec<i32>) {
    bar(&mut vec);
}

foo(&mut vec);

related: github.com/rust-lang/rust/issues/36542 -- my error was cannot borrow as mutable, consider changing this to be mutable — milahu
– milahu, Commented Dec 4, 2020 at 8:08

antoyo · Accepted Answer · 2015-11-21 15:04:16Z

19

You don't need to use &mut in this case:

let mut vec = vec![0];

fn bar(vec: &mut Vec<i32>) {
    // some code here
}

fn foo(vec: &mut Vec<i32>) {
    bar(vec);
}

foo(&mut vec);

because vec is already a &mut Vec<i32>.

answered Nov 21, 2015 at 15:04

antoyo

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