Error code E0509
This error occurs when an attempt is made to move out of a value whose type
implements the Drop
trait.
Erroneous code example:
struct FancyNum { num: usize } struct DropStruct { fancy: FancyNum } impl Drop for DropStruct { fn drop(&mut self) { // Destruct DropStruct, possibly using FancyNum } } fn main() { let drop_struct = DropStruct{fancy: FancyNum{num: 5}}; let fancy_field = drop_struct.fancy; // Error E0509 println!("Fancy: {}", fancy_field.num); // implicit call to `drop_struct.drop()` as drop_struct goes out of scope }
Here, we tried to move a field out of a struct of type DropStruct
which
implements the Drop
trait. However, a struct cannot be dropped if one or
more of its fields have been moved.
Structs implementing the Drop
trait have an implicit destructor that gets
called when they go out of scope. This destructor may use the fields of the
struct, so moving out of the struct could make it impossible to run the
destructor. Therefore, we must think of all values whose type implements the
Drop
trait as single units whose fields cannot be moved.
This error can be fixed by creating a reference to the fields of a struct,
enum, or tuple using the ref
keyword:
struct FancyNum { num: usize } struct DropStruct { fancy: FancyNum } impl Drop for DropStruct { fn drop(&mut self) { // Destruct DropStruct, possibly using FancyNum } } fn main() { let drop_struct = DropStruct{fancy: FancyNum{num: 5}}; let ref fancy_field = drop_struct.fancy; // No more errors! println!("Fancy: {}", fancy_field.num); // implicit call to `drop_struct.drop()` as drop_struct goes out of scope }
Note that this technique can also be used in the arms of a match expression:
struct FancyNum { num: usize } enum DropEnum { Fancy(FancyNum) } impl Drop for DropEnum { fn drop(&mut self) { // Destruct DropEnum, possibly using FancyNum } } fn main() { // Creates and enum of type `DropEnum`, which implements `Drop` let drop_enum = DropEnum::Fancy(FancyNum{num: 10}); match drop_enum { // Creates a reference to the inside of `DropEnum::Fancy` DropEnum::Fancy(ref fancy_field) => // No error! println!("It was fancy-- {}!", fancy_field.num), } // implicit call to `drop_enum.drop()` as drop_enum goes out of scope }