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-rw-r--r-- | compiler/rustc_error_codes/src/error_codes/E0389.md | 63 |
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diff --git a/compiler/rustc_error_codes/src/error_codes/E0389.md b/compiler/rustc_error_codes/src/error_codes/E0389.md new file mode 100644 index 000000000..9f064e44c --- /dev/null +++ b/compiler/rustc_error_codes/src/error_codes/E0389.md @@ -0,0 +1,63 @@ +#### 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); +} +``` |