1 This error occurs when an attempt is made to assign to a borrowed value.
3 Erroneous code example:
11 let mut fancy_num = FancyNum { num: 5 };
12 let fancy_ref = &fancy_num;
13 fancy_num = FancyNum { num: 6 };
14 // error: cannot assign to `fancy_num` because it is borrowed
16 println!("Num: {}, Ref: {}", fancy_num.num, fancy_ref.num);
20 Because `fancy_ref` still holds a reference to `fancy_num`, `fancy_num` can't
21 be assigned to a new value as it would invalidate the reference.
23 Alternatively, we can move out of `fancy_num` into a second `fancy_num`:
31 let mut fancy_num = FancyNum { num: 5 };
32 let moved_num = fancy_num;
33 fancy_num = FancyNum { num: 6 };
35 println!("Num: {}, Moved num: {}", fancy_num.num, moved_num.num);
39 If the value has to be borrowed, try limiting the lifetime of the borrow using
48 let mut fancy_num = FancyNum { num: 5 };
51 let fancy_ref = &fancy_num;
52 println!("Ref: {}", fancy_ref.num);
55 // Works because `fancy_ref` is no longer in scope
56 fancy_num = FancyNum { num: 6 };
57 println!("Num: {}", fancy_num.num);
61 Or by moving the reference into a function:
69 let mut fancy_num = FancyNum { num: 5 };
71 print_fancy_ref(&fancy_num);
73 // Works because function borrow has ended
74 fancy_num = FancyNum { num: 6 };
75 println!("Num: {}", fancy_num.num);
78 fn print_fancy_ref(fancy_ref: &FancyNum){
79 println!("Ref: {}", fancy_ref.num);