An attempt was made to mutate data using a non-mutable reference. This
commonly occurs when attempting to assign to a non-mutable reference of a
mutable reference (&(&mut T)).
Erroneous code example:
struct FancyNum {
num: u8,
}
fn main() {
let mut fancy = FancyNum{ num: 5 };
let fancy_ref = &(&mut fancy);
fancy_ref.num = 6; // error: cannot assign to data in a `&` reference
println!("{}", fancy_ref.num);
}Here, &mut fancy is mutable, but &(&mut fancy) is not. Creating an
immutable reference to a value borrows it immutably. There can be multiple
references of type &(&mut T) that point to the same value, so they must be
immutable to prevent multiple mutable references to the same value.
To fix this, either remove the outer reference:
struct FancyNum {
num: u8,
}
fn main() {
let mut fancy = FancyNum{ num: 5 };
let fancy_ref = &mut fancy;
// `fancy_ref` is now &mut FancyNum, rather than &(&mut FancyNum)
fancy_ref.num = 6; // No error!
println!("{}", fancy_ref.num);
}Or make the outer reference mutable:
struct FancyNum {
num: u8
}
fn main() {
let mut fancy = FancyNum{ num: 5 };
let fancy_ref = &mut (&mut fancy);
// `fancy_ref` is now &mut(&mut FancyNum), rather than &(&mut FancyNum)
fancy_ref.num = 6; // No error!
println!("{}", fancy_ref.num);
}