Resolving Rust Error E0505: Value Moved While Borrowed

Understanding Rust Error E0505

Rust Error E0505 occurs when a value is moved out of a variable while it's still borrowed. Rust enforces strict ownership rules to prevent data races and ensure memory safety. To fix this error, you can avoid moving the variable, release the borrow before the move, or implement the Copy trait on the type.

E0505 Error Example

struct Value {}

fn borrow(val: &Value) {}

fn eat(val: Value) {}

fn main() {
    let x = Value{};
    let _ref_to_val: &Value = &x;
    eat(x);
    borrow(_ref_to_val);
}

In the erroneous code example above, the function 'eat' takes ownership of 'x'. However, 'x' cannot be moved because the borrow to '_ref_to_val' needs to last till the function 'borrow'.

Possible Solutions for E0505

Solution 1: Pass by reference

If it's possible to change the function signature of 'eat' to accept a reference instead of taking ownership, you can pass 'x' by reference:

struct Value {}

fn borrow(val: &Value) {}

fn eat(val: &Value) {}

fn main() {
    let x = Value{};
    let ref_to_val: &Value = &x;
    eat(&x);
    borrow(ref_to_val);
}

Solution 2: Release borrow before move

You can ensure that the borrowed value '_ref_to_val' is no longer used before the move:

struct Value {}

fn borrow(val: &Value) {}

fn eat(val: Value) {}

fn main() {
    let x = Value{};
    let ref_to_val: &Value = &x;
    borrow(ref_to_val);
    eat(x);
}

Solution 3: Implement the Copy trait

You can implement the Copy trait for the 'Value' struct. This way, the 'x' variable will be copied instead of being moved:

#[derive(Clone, Copy)]
struct Value {}

fn borrow(val: &Value) {}

fn eat(val: Value) {}

fn main() {
    let x = Value{};
    let ref_to_val: &Value = &x;
    eat(x);
    borrow(ref_to_val);
}

To further enhance your understanding of Rust's ownership system, refer to the 'References & Borrowing' section of the Rust book.

Recommended Reading

• References & Borrowing