A variable already borrowed as immutable was borrowed as mutable.
Erroneous code example:
fn bar(x: &mut i32) {}
fn foo(a: &mut i32) {
let y = &a; // a is borrowed as immutable.
bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed
// as immutable
println!("{}", y);
}To fix this error, ensure that you don’t have any other references to the variable before trying to access it mutably:
fn bar(x: &mut i32) {}
fn foo(a: &mut i32) {
bar(a);
let y = &a; // ok!
println!("{}", y);
}For more information on Rust’s ownership system, take a look at the References & Borrowing section of the Book.