Understanding Rust Error E0161: Cannot Move a Value with Unknown Size

Introduction to Rust Error E0161

Rust Error E0161 occurs when a value is moved in the code whose size is not known at compile time. This type of error prevents arbitrary-sized data from being moved, as data with unknown sizes can cause issues with memory management and thread safety.

Error E0161 Example

Consider the erroneous code example below, which tries to call the method 'f()' on a trait object 'dyn Bar' without knowing its size at compile time.

trait Bar {
    fn f(self);
}

impl Bar for i32 {
    fn f(self) {}
}

fn main() {
    let b: Box<dyn Bar> = Box::new(0i32);
    b.f();
    // error: cannot move a value of type dyn Bar: the size of dyn Bar cannot
    //        be statically determined
}

How to Fix Error E0161

To resolve this error, you need to wrap the value with a reference. This allows the value to be moved as usual, since a reference has a known fixed size. Update the trait signature and the implementation as follows:

trait Bar {
    fn f(&self);
}

impl Bar for i32 {
    fn f(&self) {}
}

fn main() {
    let b: Box<dyn Bar> = Box::new(0i32);
    b.f();
    // ok!
}

In this fixed example, the 'f()' function of the 'Bar' trait uses '&self' instead of 'self'. Hence, it is more memory-efficient, and the size of the moved value is known at compile time.

Conclusion

Rust Error E0161 is a common issue developers may face when working with traits that use methods with 'self' in their signatures. By changing 'self' to a reference, either '&self' or '&mut self', the value can be safely and accurately moved at compile time, hence resolving the error.