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Diffstat (limited to 'compiler/rustc_error_codes/src/error_codes/E0506.md')
-rw-r--r-- | compiler/rustc_error_codes/src/error_codes/E0506.md | 73 |
1 files changed, 73 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0506.md b/compiler/rustc_error_codes/src/error_codes/E0506.md new file mode 100644 index 000000000..c312a0460 --- /dev/null +++ b/compiler/rustc_error_codes/src/error_codes/E0506.md @@ -0,0 +1,73 @@ +An attempt was made to assign to a borrowed value. + +Erroneous code example: + +```compile_fail,E0506 +struct FancyNum { + num: u8, +} + +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, +} + +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, +} + +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 print_fancy_ref(fancy_ref: &FancyNum){ + println!("Ref: {}", fancy_ref.num); +} + +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); +``` |