blob: 9f064e44c820902ade4810905f20b1cfaa3c02f6 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
|
#### Note: this error code is no longer emitted by the compiler.
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:
```compile_fail
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);
}
```
|
