A variable already borrowed as immutable was borrowed as mutable.
Erroneous code example:
fn bar(x: &mut i32) {} fn foo(a: &mut i32) { let ref 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 ref y = a; // ok! println!("{}", y); }
For more information on Rust's ownership system, take a look at the References & Borrowing section of the Book.