This error occurs when an attempt is made to assign to a borrowed value.
Erroneous code example:
struct FancyNum { num: u8, } fn main() { let mut fancy_num = FancyNum { num: 5 }; let fancy_ref = &fancy_num; fancy_num = FancyNum { num: 6 }; // error: cannot assign to `fancy_num` because it is borrowed println!("Num: {}, Ref: {}", fancy_num.num, fancy_ref.num); }
Because fancy_ref
still holds a reference to fancy_num
, fancy_num
can't be assigned to a new value as it would invalidate the reference.
Alternatively, we can move out of fancy_num
into a second fancy_num
:
struct FancyNum { num: u8, } fn main() { let mut fancy_num = FancyNum { num: 5 }; let moved_num = fancy_num; fancy_num = FancyNum { num: 6 }; println!("Num: {}, Moved num: {}", fancy_num.num, moved_num.num); }
If the value has to be borrowed, try limiting the lifetime of the borrow using a scoped block:
struct FancyNum { num: u8, } fn main() { let mut fancy_num = FancyNum { num: 5 }; { let fancy_ref = &fancy_num; println!("Ref: {}", fancy_ref.num); } // Works because `fancy_ref` is no longer in scope fancy_num = FancyNum { num: 6 }; println!("Num: {}", fancy_num.num); }
Or by moving the reference into a function:
struct FancyNum { num: u8, } fn main() { let mut fancy_num = FancyNum { num: 5 }; print_fancy_ref(&fancy_num); // Works because function borrow has ended fancy_num = FancyNum { num: 6 }; println!("Num: {}", fancy_num.num); } fn print_fancy_ref(fancy_ref: &FancyNum){ println!("Ref: {}", fancy_ref.num); }